P type Doping of ZnO

GROWTH OF ZINC OXIDE NANOSTRUCTURES AND FILMS

AND P-DOPING OF FILMS

IN AQUEOUS SOLUTION

TAY CHUAN BENG

B. Eng (Hons.), M. Eng

A THESIS SUBMITTED

FOR THE DEGREE OF DOCTOR OF PHILOSOPHY

DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING

NATIONAL UNIVERSITY OF SINGAPORE

2009

i

ACKNOWLEDGEMENTS

First and foremost, I would like to express my sincere appreciation to both of my

supervisors, Prof Chua Soo Jin and Prof Loh Kian Ping, whose patience, guidance and

insights are crucial to this body of work.

I would also like to express my thanks to:

• Dr S. Tripathy, Dr C.B. Soh, Dr H.Q. Le and Dr H.F. Liu from IMRE, whose instructions

and guidance were important lifelines during the early stages of my research,

• H. Musni and B. H. Tan from Centre for Optoelectronics, NUS whose experience,

skill and time helped to keep the lab equipments and experiments running

smoothly and properly,

• Wang Miao and Haryono from Singapore MIT-Alliance, as well as Lin Fen, Huang

Leihua, Tian Feng, Mantavya Sinha, Vivek Dixit for all the good memories,

• Liu Minghui, Deng Suzi, Zhong Yulin from Chemistry Dept, NUS, for opening up the

world of chemistry to me,

• and all the others at NUS and IMRE that have helped me one way or another.

Finally, and most importantly, my profound gratitute goes to my Dad, Mom, Chuan

Hock, MIchelle, Benjamin and Matthew. Without your constant support, motivation

and love, I would not have been able to finish this work. Thank you for everything.

ii

TABLE OF CONTENTS

1 Introduction ............................................................................................................... 1

1.1 Introduction......................................................................................................... 1

1.2 Background .......................................................................................................... 1

1.3 Crystal Structure .................................................................................................. 2

1.4 ZnO Growth Techniques ...................................................................................... 3

1.4.1 Vapor phase transport ................................................................................. 3

1.4.2 Chemical vapor deposition (CVD) and metal-organic chemical vapor

deposition (MOCVD) .................................................................................................. 4

1.4.3 Molecular beam epitaxy (MBE) ................................................................... 4

1.4.4 Aqueous solution-based synthesis .............................................................. 5

1.4.5 Comparison of gas and solution phase growth methods ............................ 5

1.5 Doping in ZnO ...................................................................................................... 8

1.6 Motivation and objectives ................................................................................ 13

1.7 Organization of the thesis ................................................................................. 14

2 Aqueous solution growth of ZnO ............................................................................. 16

2.1 Introduction....................................................................................................... 16

2.2 Basic terminologies and concepts ..................................................................... 16

2.3 Temperature-dependent ionic equilibrium of ZnAc2 and NH3 ......................... 19

2.4 Nucleation and growth ...................................................................................... 26

2.4.1 Homogeneous nucleation .......................................................................... 26

2.4.2 Heterogeneous nucleation ........................................................................ 28

2.4.3 Crystal growth ............................................................................................ 29

2.5 Effect of pH on ZnO surface .............................................................................. 33

2.6 Conclusion ......................................................................................................... 35

iii

3 Experimental methods for growth and characterization of ZnO............................ 36

3.1 Introduction....................................................................................................... 36

3.2 Growth procedure and apparatus .................................................................... 36

3.2.1 Pre-coating of substrate with ZnO seeds ................................................... 36

3.2.2 ZnO growth in solution .............................................................................. 38

3.3 Characterization tools ....................................................................................... 40

3.4 Field-emission scanning electron microscopy (FESEM) .................................... 40

3.5 Photoluminescence spectroscopy .................................................................... 41

3.6 Raman spectroscopy ......................................................................................... 45

3.7 Secondary ion mass spectrometry (SIMS) ........................................................ 49

3.8 Hall effect measurement ................................................................................... 50

3.9 Conclusion ......................................................................................................... 54

4 Prediction of Length and Density of ZnO Nanorods on GaN Substrate .................. 55

4.1 Introduction....................................................................................................... 55

4.2 Experimental Procedure .................................................................................... 57

4.3 Results ............................................................................................................... 58

4.4 Discussion .......................................................................................................... 61

4.5 Effect of Solubility of Zinc on Density and Length of ZnO Nanorod Arrays ...... 62

4.6 Effect of Temperature on Density and Length .................................................. 66

4.7 ZnO Nanorod Length and Density Maps ........................................................... 68

4.8 Limitations of Model ......................................................................................... 69

4.9 Conclusion ......................................................................................................... 71

5 Growth and Defects of ZnO Nanorods Grown from a ZnO Seed Layer ................... 72

5.1 Introduction....................................................................................................... 72

5.2 Experimental Procedure .................................................................................... 73

5.3 Results ............................................................................................................... 74

iv

5.4 Discussion .......................................................................................................... 80

5.4.1 Role of solubility in growth morphology ................................................... 80

5.5 Role of interfacial properties in aqueous solution ............................................ 85

5.6 Defects and the growth mechanism ................................................................. 87

5.7 Conclusion ......................................................................................................... 88

6 Growth of p-ZnO film using multiple growth cycles ............................................... 90

6.1 Introduction....................................................................................................... 90

6.2 Experiment ........................................................................................................ 92

6.3 Results and discussion ....................................................................................... 94

6.3.1 Evolution of film morphology using a multi-step growth approach ......... 94

6.4 Role of K as a dopant for ZnO films ................................................................... 96

6.5 Effect of electric field on the growth and doping of ZnO films in solution ....... 98

6.6 Effect of annealing in nitrogen ambient on p-type doping by K ..................... 103

6.7 Fabrication of p-ZnO / n-GaN LED ................................................................... 105

6.8 Conclusion ....................................................................................................... 106

7 Conclusions and Recommendations ...................................................................... 108

7.1 Conclusions...................................................................................................... 108

7.2 Recommendations .......................................................................................... 111

8 Bibliography ........................................................................................................... 113

v

SUMMARY

ZnO is a wide bandgap material with a large exciton binding energy (60 meV) and highly

polar surfaces which promote anisotropic growth of many interesting nanostructures.

Due to its multifunctional properties, ZnO has been proposed for a wide variety of

applications such as transparent conducting electrodes, gas sensors, piezoelectric

sensors and generator, acoustic wave devices, light emitting diodes and solar cells.

This work studies the growth of ZnO nanorods and films in aqueous solution using zinc

acetate and ammonium hydroxide in detail. Regardless of the type of substrate used,

the solubility of zinc (SZn), interface properties of the substrate and growth

temperature emerged as the main factors determining the growth rate and

morphology of the nanorods. For GaN substrates, the activation energies for density

and length of nanorods are -2.11 and 0.77 eV respectively. An empirical growth map

for growth prediction of the density and length of nanorods is generated. For

substrates with a pre-coated layer of ZnO nanoparticles, a uniform coverage of

nanorods is obtained when SZn < 0.88 mM, and large clustered rods are obtained when

SZn > 1.56 mM. For values of SZn that lies in between, both nanorods and large clustered

rods can be obtained.

Using photoluminescence and Raman spectroscopy, the native defects were identified

and associated with the growth conditions. When growth pH < PZC, the growth rate is

very slow and hydrogen defects are the major defects with very strong UV emissions.

When growth pH > PZC, the growth rate is fast and the major defects are interstitial

oxygen, interstitial zinc and zinc vacancies with strong visible emissions. Interstitial zinc

and zinc vacancies contributes to the green emission while interstitial oxygen, the red

component.

Next, ZnO films were grown and doped with potassium using a new growth strategy

which can be applied to any substrate, regardless of its lattice matching. The p-type

conductivity in ZnO:K films is confirmed using Hall effect, SIMS and XPS measurements.

An optimum hole concentration of 3.8 x 1017 cm-3 is obtained at 0.07 M KAc without

any applied bias and 3.98 x 1017 cm-3 when -0.4 V is applied. To the best of our

vi

knowledge, this is the first report of p-type doping of ZnO films in aqueous solution at

low temperatures using potassium from group I as a p-dopant.

Annealing above 400°C activates the hydrogen defects and converts the film to n-type

with electron concentrations to 1 x 1019 cm-3. By extending the annealing time beyond

30 min at 800°C, the hydrogen defects can be reduced and the p-type conductivity can

be recovered.

Finally, a p-ZnO / n-GaN junction is fabricated with a rectifying I-V characteristic and a

weak orange electroluminescence at a forward bias current of 75.9 to 98.3 mA. The

reverse bias leakage current ranges from 1.3 to 1.5 mA at 3 V.

vii

LIST OF TABLES

Table 1.1. Summary of intrinsic doping levels of undoped ZnO polycrystalline films and

single crystals which have been grown using various methods. ....................................... 8

Table 1.2. Summary of various group III elements as well as their corresponding growth

methods and levels of n-doping. ....................................................................................... 9

Table 1.3. Calculated bond lengths and the defect energy levels in ZnO for group I and

V dopants. Ideal ZnO bond length (ro) is 1.93 Å. Taken from [32]. .................................. 10

Table 1.4. Summary of p-type mono-doping of ZnO using group V elements. .............. 11

Table 2.1. List of Enthalpy Values [58-60]. Enthalpy alues with an asterisk * denotes

calculated values of enthalpy of formation from tabulated enthalpy of reaction. ......... 21

Table 3.1. Lattice parameters of various substrate materials for ZnO growth [69]. ...... 37

Table 3.2. Frequency and symmetry of the fundamental optical modes in ZnO ............ 48

Table 4.1. Summary of different results and methods for aqueous solution growth. ... 56

Table 4.2. Summary of effects of temperature and reactant concentrations on density

and length of ZnO nanorods. ........................................................................................... 62

Table 5.1. Summary of observed growth behavior with solution pH ............................. 81

Table 6.1. Summary of reported investigators, precursors, growth temperature and

substrates for epitaxial ZnO growth in aqueous solution. .............................................. 91

Table 6.2. Summary of carrier parameters obtained from Hall effect measurements for

samples grown without KAc and with 0.07 and 0.24 M KAc. The film thickness is

obtained from the SEM image of the cross-section of the film. ..................................... 97

Table 6.3. Summary of carrier parameters obtained from Hall effect measurements for

samples grown with 0.24 M KAc at different bias voltages. The film thickness is

obtained from the SEM image of the cross-section of the film. ..................................... 99

Table 6.4. Summary of percentage atomic concentrations from quantifation of the

fitted components of Zn 2p, O 1s and K 2s in the XPS survey spectra. The relative

viii

sensitivity factors (RSF) that were used for quantification are indicated beside the

element in parenthesis. ................................................................................................. 101

ix

LIST OF FIGURES

Figure 1.1. Schematic diagram of wurtzite crystal structure of ZnO and its common

surface planes. ................................................................................................................... 2

Figure 1.2. Schematic showing the free energy of the precursors in gaseous and

hydrated states and the final ZnO product........................................................................ 6

Figure 1.3. Carrier concentrations as a function of the preservation period after

deposition. A very stable p-type conductivity is obtained when Li-N codoping method is

used. Graph was taken from [45]. ................................................................................... 12

Figure 2.1. Equilibrium complex concentrations and solubility of zinc as a function of pH

at 300K. The pH is increased by adding more NH3 while keeping the mass of ZnAc2

constant at 0.016 M. Curves show the equilibrium concentrations of (a) zinc acetate

complexes, (b) Zn2+ ions, (c) zinc ammine complexes, (d) zinc hydroxide complexes and

(e) total zinc ion concentration respectively. .................................................................. 22

Figure 2.2. Variation of solubility of zinc with pH. The solubility of zinc was calculated

using Eq. (2.15). The data for each curve is obtained by keeping the concentration of

ZnAc2 fixed while varying the concentration of NH3. The concentrations of ZnAc2 are

indicated on each curve. .................................................................................................. 25

Figure 2.3. Variation of solubility of zinc and pH when the concentration of NH3 is

varied while ZnAc2 is kept constant at 0.02 M. The solubility of zinc was calculated using

Eq. (2.15). ......................................................................................................................... 25

Figure 2.4. The Gibbs free energy of nucleation with respect to embryo radius. The

critical radius r* and energy ∆G* depends on the balance between the surface and

volume energy of the growing embryo. .......................................................................... 28

Figure 2.5. Processes involved in heterogeneous nucleation on a substrate surface. ... 28

Figure 2.6. Hydrolysis of hydrated metal ions in aqueous solution. The positively

charged metal ion attracts the electrons away from the O-H bond, leading to the

breakage of the O-H bond and release of the H+ ion into the solution........................... 30

Figure 2.7. (A) Aggregation and (B) coalescence of individual particles. ........................ 32

x

Figure 2.8. A model for adsorption of Zn2+ on ZnO surface. ........................................... 34

Figure 2.9. Adsorption of Zn2+ ions depends on the pH of the solution. Highest rate of

adsorption when the pH is higher than the PZC of ZnO. ................................................. 34

Figure 3.1 TEM image of the ZnO nanoparticles that are grown by refluxing 0.02 M KOH

and 0.01 M ZnAc2 in methanol for 2 h. The diameter of the nanoparticles range from 10

to 20 nm. Agglomeration of the nanoparticles can be clearly seen. ............................... 38

Figure 3.2. Apparatus for growth of ZnO on a substrate. ............................................... 39

Figure 3.3 (a) Band structure and symmetries of wurtzite ZnO. The splitting into three

valence bands (A, B and C) is caused by field and spin-orbit splitting [75]. (b) Schematic

drawing of the exciton states. (c) Summary of various optical transitions near the band

gap and their corresponding energy and wavelength ranges. ........................................ 43

Figure 3.4. Comparison of the low-temperature PL (4 K) spectra from (a) a bulk single

crystal ZnO grown by VPT [77] and (b) ZnO nanorods grown using a solution containing

zinc nitrate, HMT and PEI [78] on a pre-coated Si substrate which had been pre-coated

using ZnAc2 solution. Both samples have been annealed in forming gas at 600°C. ....... 44

Figure 3.5. Rayleigh and Raman scattering ..................................................................... 46

Figure 3.6. Depth profiling using a dual beam technique. .............................................. 49

Figure 3.7. Schematic of the Hall effect in a long, thin bar of semiconductor with four

ohmic contacts. The direction of the magnetic field B is along the z-axis and the sample

has a finite thickness d. .................................................................................................... 50

Figure 3.8. Schematic of a van der Pauw configuration used in the determination of the

two characteristic resistances RA and RB. ........................................................................ 52

Figure 3.9. Schematic of a van der Pauw configuration used in the determination of the

Hall voltage VH. ................................................................................................................. 53

Figure 4.1. SEM images of ZnO nanorods grown at temperatures (a) 60°C, (b) 80°C, (c)

100°C and (d) 150°C in solutions containing 0.016 M Zn(Ac)2 and 0.173 M NH4OH. ..... 59

Figure 4.2. The effect of growth temperature on (a) length and (b) density of ZnO

nanorods. ......................................................................................................................... 59

xi

Figure 4.3. SEM images of ZnO nanorods with different molar ratios: (a) 0.016 M

Zn(Ac)2, 0.1 M NH4OH, (b) 0.016 M Zn(Ac)2, 0.143 M NH4OH, (c) 0.016 M Zn(Ac)2, 0.204

M NH4OH and (d) 0.016 M Zn(Ac)2, 0.306 M NH4OH. ..................................................... 60

Figure 4.4. Effect of molar ratio on (a) length and (b) density of ZnO nanorods. The

concentration of Zn(Ac)2 is kept constant at 0.016 M and the concentration of NH4OH is

varied from 0.1 M to 0.4 M to increase the molar ratio. ................................................ 60

Figure 4.5. Effect of increasing concentration of precursors while maintaining a

constant molar ratio on (a) length and (b) density of ZnO nanorods. Zn(Ac)2 is increased

from 0.01 to 0.03 M , and concentration of ammonia by a proportional amount to

maintain a constant molar ratio of 6.27. ......................................................................... 61

Figure 4.6. Logarithm of ZnO nanorods lengths plotted against the total concentration

of zinc ions in the precursor solution. � represents the data points when the Zn(Ac)2

concentration is kept constant at 0.016 M and the NH4OH concentration is varied from

0.1 to 0.4 M while � represents the data points when concentration of Zn(Ac)2 is

increased from 0.01 to 0.033 M with a constant molar ratio [NH4+]/[Zn2+]. The growth

temperature is kept constant at 373 K. ........................................................................... 63

Figure 4.7. Logarithm of rod density (cm-2) plotted against the total concentration of

zinc ions in the precursor solution. The inset shows the corresponding initial degree of

supersaturation of zinc in the precursor solution at the growth temperature 373K. The

data points for varying the ratio of reactant concentrations are represented by �,

while the increasing reactant concentrations with a constant ratio by �. .................... 64

Figure 4.8. Plot of (a) Y = ( )[ ]n

ZnB SAB 1ln and (b) Y= ( )[ ]m

ZnL SAL 1ln against 1/T. � and �

represent the density and length data points respectively when temperature is varied

from 60 to 150°C. The inset shows the degree of supersaturation of zinc, S, against

temperature for a precursor solution containing 0.016 M Zn(Ac)2 and 0.173 M NH4OH.

.......................................................................................................................................... 67

Figure 4.9. Black lines show the contour plot of (a) log[B(cm-2)] and (b) length (nm) for

various concentrations of ZnAc2 and NH4OH. The validity limits for pH between 9.7 and

10.6, and degree of supersaturation of zinc between 20 and 60 are shown in red and

blue lines respectively. ..................................................................................................... 70

xii

Figure 5.1. SEM morphology of ZnO nanorods grown on Si substrates with a pre-coat of

ZnO nanoparticles using growth solutions with 0.02 M ZnAc2 and (a) 0.02 M, (b) 0.04 M,

(c) 0.1 M, (d) 0.3 M, (e) 0.4 M and (f) 1.1 M NH4OH. The concentration of NH4OH and

the corresponding initial solution pH values in square parentheses are indicated on the

top left corner. Scale bar shows 1 µm. ............................................................................ 75

Figure 5.2. SEM image showing the morphologies of ZnO nanorods grown in various

concentrations of ZnAc2 and NH4OH. (a), (b) and (c) were grown with 0.4 M, 0.8 M and

1.1 M NH4OH respectively while keeping ZnAc2 fixed at 0.01 M. (d), (e) and (f) were

grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively while keeping ZnAc2 fixed at

0.02 M. (g), (h) and (i) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively

while keeping ZnAc2 fixed at 0.03 M. The scale bar is 1 µm and all images were taken

with the same magnification. .......................................................................................... 75

Figure 5.3. The Raman spectra measured from samples grown with 0.4, 0.8 and 1.1 M

NH4OH on a glass substrate. Inset shows the shift of the E2H peak to higher frequencies

as concentration of NH4OH is increased. ......................................................................... 77

Figure 5.4. Photoluminescence spectra recorded from samples grown in 0.02 M (black

line), 0.04 M (blue line), 0.3 M (green line) and 1.1 M NH4OH (red line) while the

concentration of ZnAc2 is kept constant at 0.02 M. ........................................................ 77

Figure 5.5. PL spectra of sample grown in high pH (10.7) after annealing at various

temperatures in (a) air and (b) nitrogen ambient, as well as low pH sample (7) annealed

in (c) air and (d) nitrogen ambient. The sharp peak at 650 nm is due to the doubling of

the 325 nm laser line and should be ignored. ................................................................. 78

Figure 5.6. Plot showing the solubility of zinc, SZn, against the concentration of NH4OH

for 0.006 M (black dotted line), 0.01 M (blue line), 0.02 M (green line) and 0.03 M (red

line) of ZnAc2. The SZn data points which are labeled (a) to (i) corresponds to the SEM

images in Figs 5.2 (a) to (i) respectively which have been reproduced here for ease of

comparison. The value of SZn when 0.006 M ZnAc2 and 0.4 M NH4OH is marked with a

square (�) and the corresponding SEM image is shown in Fig 5.7. Growth in region 1

produces uniform nanorods, region II a mixed morphology of nanorods and large rods

and region III only large rods. .......................................................................................... 82

xiii

Figure 5.7. SEM image showing the top and cross-sectional view of a sample grown in

0.006 M ZnAc2 and 0.4 M NH4OH. The mixed morphology confirms the dependence of

SZn which shown in Fig 5.6. .............................................................................................. 83

Figure 5.8. Plot of solubility of zinc against pH for 0.006 M (black dotted line), 0.01 M

(blue line), 0.02 M (green line) and 0.03 M (red line) of ZnAc2. The corresponding SEM

images from Fig 5.2 are shown here for ease of comparison. ........................................ 85

Figure 5.9. SEM image showing the top view of a sample grown in (a) 0.02 M and (b)

1.1 M NH4OH. The concentration of ZnAc2 is kept constant at 0.02 M. The scale bar

shows 1 µm. ..................................................................................................................... 86

Figure 6.1. Modified growth setup to study the effect of internal electric field on the

growth and doping of ZnO films. ..................................................................................... 93

Figure 6.2. Morphology evolution from the seed layer to the film layer growth on n-

Si(100). ............................................................................................................................. 94

Figure 6.3. Morphology evolution from the seed layer to the film layer growth on n-

GaN epilayer. .................................................................................................................... 95

Figure 6.4. PL spectra of as-grown seed layer (black) and the subsequent film growth

layers (blue for 30 min, green for 90 min and red for 180 min). The film layer growth

step significantly enhances UV emission while slightly reducing the visible emissions.. 95

Figure 6.5. SEM image shows the top view of the ZnO film after one cycle of seed layer

growth followed by two cycles of film layer growth for the samples that are grown (a)

without KAc and with (b) 0.07 and (c) 0.24 M KAc. ......................................................... 96

Figure 6.6. SIMS depth profile for Zn, O and K. ............................................................... 97

Figure 6.7. SIMS depth profile for Zn, O and K for samples grown in the presence of

0.24 M KAc and varying bias voltages from 0, -0.1, -0.4 to -0.9 V. .................................. 99

Figure 6.8. Typical XPS survey spectrum of a ZnO sample doped with K. The peak

positions of Zn, O, K and C have been marked. Au peaks are from the calibration

reference. ....................................................................................................................... 100

Figure 6.9. Typical component fitting of Zn, O and K using Zn 2p3/2, O 1s and K 2s peaks

for atomic concentration quantification. Synthetic peaks were fitted to the measured

xiv

peaks using a Shirley background and Gaussian-Lorentian distributions. Quantification

was performed using the fitted synthetic peaks to improve estimation accuracy. ...... 100

Figure 6.10. XPS valence band spectra of Zn 3d for samples grown with (a) -0.9V, (b) -

0.1V and (c) -0.4V. The corresponding hole concentrations in cm-3 has been indication

in the legend. A larger core level shift is observed for a higher hole concentration. ... 101

Figure 6.11. Plot of core energy level Zn 2p3/2 against the hole concentration as

measured using Hall effect. The as-measured as well as the C 1s and O 1s calibrated

peak values are shown. A line is fitted to show the increasing binding energy with hole

concentration. ................................................................................................................ 102

Figure 6.12. Effect of anneal temperatures on the carrier concentration and mobility

for ZnO films grown (a) without any KAc, and with (b) 0.07 and (c) 0.24 M KAc. (d) The

effect of anneal duration at 800°C for sample grown in 0.24 M KAc. Annealing for all

samples were done in a nitrogen ambient. Data points for as-grown samples were

represented at 100 °C. The electron concentrations and mobilities are marked by ● and

● respecQvely, while the hole concentraQon and mobility by ○ and ○ respecQvely. ... 103

Figure 6.13. I-V characteristic plotted in (a) logarithmic and (b) linear scale. Each line

shows the I-V from measured from a different device. Inset of (a) shows a schematic

diagram of the device while the inset of (b) confirm the ohmic behavior of the top and

bottom contacts after annealing at 700°C 1 h. .............................................................. 107

Figure 6.14. The electroluminescence spectra at various current injection levels from

20 mA to 70 mA. ............................................................................................................ 107

1

1 Introduction

1.1 Introduction

This chapter begins with a brief historical background and some basic properties of ZnO.

Various growth techniques and their achievements in p- and n-type doping are then

presented. This is followed the motivation and objectives of this thesis. Finally, the

organization of this thesis is presented.

1.2 Background

Zinc oxide (ZnO) is classified as a IIb-VI compound semiconductor, which comprises of

the binary compounds of Zn, Cd, and Hg with O, S, Se and Te and their ternary or

quaternary alloys. ZnO is a wide band gap semiconductor with a direct gap of around

3.4 eV and crystallizes preferentially in the hexagonal wurtzite-type structure. It occurs

naturally as zincite and usually contains some manganese, iron and other elements.

The color of zincite ranges from yellow to red, depending on the type of impurities.

Synthetically-grown pure ZnO, however, is colourless and clear due its large band gap.

ZnO is not a new material despite the recent surge in research work. Work on ZnO had

begun gradually in the 1930s and peaked around the end of the 1970s and the early

1980s. The focus of research was on bulk ZnO covering topics such as growth, doping,

transport, deep centres, band structure, excitons, bulk and surface polaritons,

luminescence, high excitation effects and lasing. Several reviews [1-3] cover the work

done during this early period. The research interest faded for several reasons: difficulty

in obtaining a p-type ZnO and a shift in interest to lower dimension structures such as

quantum wells which were exclusively based on GaAs/Al1-xGaxAs. A revival in ZnO

research began in mid-1990s based on the possibility to grow epitaxial layers, quantum

wells, nanorods or quantum dots and its possible applications in blue/UV

optoelectronics, radiation hard electronic devices, visible

semiconductor spintronics and transparent conducting oxides.

reviews covering the current progress have been

1.3 Crystal Structure

ZnO has a hexagonal wurtzite structure. Part of its

The Zn2+ and O2- sublattices exhibit

other along the c-axis. The lattice parameters are

density of 5.605 g cm-3 [7]

Figure 1.1. Schematic diagram of wurtzite crystal structure of ZnO and its common surface planes.

In the wurtzite structure, each

versa. This tetrahedral coordination characterizes covalent bonds with sp

It is known that when moving from the group IV over the III

semiconductors, the bonds will show an increasing amount of ionic bon

ZnO shows a substantial amount of ionic bonding and lies at the borderline between

being classed as a covalent and ionic compound. The bottom of the conduction band is

formed essentially from the 4s levels of

2p levels of O2-. The band gap between the conduction and valence bands is about

3.437 eV at low temperatures of about 4 K

adiation hard electronic devices, visible-blind electronic circuits,

semiconductor spintronics and transparent conducting oxides. Several excellent

current progress have been published [4-6].

Crystal Structure

ZnO has a hexagonal wurtzite structure. Part of its wurtzite structure is shown in

sublattices exhibit hexagonal close packing and interpen

axis. The lattice parameters are a = 3.2495 Å and c = 5.2069 Å with a

[7].

Schematic diagram of wurtzite crystal structure of ZnO and its common surface planes.

In the wurtzite structure, each Zn2+ is surrounded tetrahedrally by four O

versa. This tetrahedral coordination characterizes covalent bonds with sp

It is known that when moving from the group IV over the III-V and II-VI to the I

semiconductors, the bonds will show an increasing amount of ionic bon



formed essentially from the 4s levels of Zn2+ and the top of the valence

. The band gap between the conduction and valence bands is about

3.437 eV at low temperatures of about 4 K [8].

2

blind electronic circuits,

Several excellent

wurtzite structure is shown in Fig 1.1.

hexagonal close packing and interpenetrate each

= 5.2069 Å with a

Schematic diagram of wurtzite crystal structure of ZnO and its

is surrounded tetrahedrally by four O2- and vice

versa. This tetrahedral coordination characterizes covalent bonds with sp3 hybridisation.

VI to the I-VII

semiconductors, the bonds will show an increasing amount of ionic bonding. As such,



and the top of the valence band from the

. The band gap between the conduction and valence bands is about

3

Furthermore, the tetrahedral coordination gives a polar symmetry along the c-axis. This

polarity is responsible for its piezoelectricity, spontaneous polarization, anisotropic

crystal growth habit, etching behaviour and defect generation.

Fig 1.1 also shows the common polar and non-polar planes in the wurtzite structure.

Common polar face terminations of wurtzite ZnO are the Zn-terminated (0001) and O-

terminated (000-1) faces which are both c-axis oriented. The common non-polar faces

are (11-20) which is a-axis oriented, (10-10) and (1-102) faces, which both have equal

number of Zn and O atoms.

1.4 ZnO Growth Techniques

ZnO is a versatile material with a rich chemistry. It can be grown using a wide variety of

methods, ranging from simple thermal evaporation to more sophisticated state-of-the-

art epitaxial growth techniques. Vapor phase transport growth methods are most

commonly used. They consist of thermal evaporation, ion sputtering, pulsed laser

deposition, CVD, MOCVD and MBE. An alternative method which has not gained wide

spread adoption is the aqueous chemical growth method. These techniques will be

described briefly in the sections below.

1.4.1 Vapor phase transport

In vapor phase transport, material is vaporized from a ZnO solid source, typically in

powder form, and transported onto a substrate where it condenses and deposits. ZnO

source can be vaporized by thermal evaporation, laser ablation, sputtering, or electron

beam.

High temperatures are required for vaporization of ZnO powder as its melting point is

about 1975°C. In thermal evaporation, for example, ZnO powders are heated to a

temperature range of 1100 to 1400°C in order produce Zn vapors. The Zn vapors are

transported by a carrier gas and deposited as ZnO on a substrate placed downstream of

the carrier gas [9, 10].

4

Lower growth temperatures can be achieved by using sub-oxides of zinc (ZnOx, 0 ≤ x <

1) which have a melting point of about 419°C. ZnOx can be obtained by reduction of

ZnO using graphite [10, 11] as shown in the reactions (1.1) and (1.2) below:

22

1

2

1COZnCZnO +→+ (1.1)

2)1()1( COxZnOCOxZnO x −+→−+ , where 0 ≤ x < 1 (1.2)

Reduction can also be achieved using hydrogen [12], or reduction of zinc salts such as

ZnS [13].

1.4.2 Chemical vapor deposition (CVD) and metal-organic chemical vapor

deposition (MOCVD)

The use of volatile Zn sources in CVD and MOCVD methods allows even lower

vaporization temperatures to be applied. In CVD, zinc acetylacetonate hydrate (hereon

denoted as Zn(acac)2), with vaporization temperatures between 130 and 140°C, is

typically used as a source. Upon vaporization, Zn2+ vapor is transported by nitrogen for

reaction with oxgen at temperatures ranging from 500 to 600°C.

ZnOZnOHacacZnCOC → →⋅ °−+° 600500,2160

222)(

In MOCVD, a metal-organic source, typically dimethyl zinc or diethyl zinc with

vaporization temperatures ranging from 117 to 130°C, is used. The metal-organic

source is decomposed to form Zn vapor and then transported into the reaction

chamber using inert gas argon where it reacts with oxygen to form ZnO. This reaction

typically takes place at temperatures ranging from 300 to 500°C [14, 15].

ZnOZnDeZnCOC → → °−+°− 500300,2130117 2

1.4.3 Molecular beam epitaxy (MBE)

In MBE, high purity Zn metal (melting point 420°C) is thermally evaporated in a

Knudsen effusion cell. Under ultrahigh vacuum conditions (< 10-8 Pa), Zn vapor is

directed onto the substrate which typically has a thin layer of Ag as a catalyst. In the

5

presence of O2 and a growth temperature of 300 to 500°C, growth of ZnO on the

substrate can be achieved [16].

1.4.4 Aqueous solution-based synthesis

In general, oxides are particularly suited for growth in solution. Literature is rich with

reports of nanostructures fabricated in chemical solutions. The ease of ZnO growth in

solution is reflected in the low growth temperatures of 60 to 90°C. Growth precursors

in aqueous solution generally consists of a zinc salt, such as zinc acetate, zinc nitrate or

zinc chloride, and a base such as sodium hydroxide and aqueous ammonia.

Occasionally a surfactant is added to influence the growth habit. In water, hydration of

the zinc salt leads to free Zn2+ ions which undergoes hydrolysis and condensation to

give ZnO. The growth method and mechanisms will be explored in detail in Chapter 2.

Growth of ZnO in aqueous solution is an attractive alternative to MOCVD because it is a

simple, cheap, non-toxic and low temperature method. Large-scale processing has also

been demonstrated [17].

1.4.5 Comparison of gas and solution phase growth methods

Growth of ZnO is more readily achieved with precursors in gaseous state than in

aqueous state. Obviously, the higher free energy of growth units in gaseous state

results in a large driving force and a lower activation energy barrier as shown in Fig 1.2.

Since the growth units have sufficient energy for diffusion, adsorption, surface

reactions, nucleation and growth, growth can be achieved over a wide range of

conditions and precursor concentrations.

The opposite is true for aqueous chemical growth methods which have a small driving

force and high activation energy barrier as shown in Fig 1.2. While formation of ZnO

can be encouraged by shifting the chemical equilibrium to favor hydrolysis and

condensation of ZnO, the growth species have much lower free energy due to low

growth temperatures. Careful control of precursor concentrations and zinc solubility is

needed to achieve growth of ZnO. As such, understanding of chemical equilibriums is

essential in controlling the growth process.

6

Natural growth processes in nature has shown that it is possible to grow perfectly

crystalline structures at ambient temperatures and pressure. One example is the

growth of single crystal calcium carbonate by sea urchins. A recent paper described the

conversion of amorphous calcium carbonate to single crystal calcium carbonate

through a secondary nucleation mechanism at an ambient temperature of 15°C [18]. It

is possible that an organic catalyst exists to lower the activation energy and aid the

dissolution and secondary nucleation process. Unraveling this process and applying it

to the case of ZnO will help to establish aqueous chemical growth as a viable

alternative to gas phase methods.

Figure 1.2. Schematic showing the free energy of the precursors in gaseous and hydrated states and the final ZnO product.

When compared in terms of energy, material and processing, aqueous chemical growth

methods have clear advantages over gas phase methods. Gas phase methods generally

waste a large amount of energy and material. A huge amount of energy is needed to

convert the solid state Zn source to free Zn2+ ions in vapor state as growth precursors.

Zn, O precursor atoms, ions cluster molecules or complexes in fluid (air

or solution)

ZnO in solid phase

DiffusionAdsorption

Surface reactionNucleation and Growth

Driving force

Precursors in

gaseous state

Precursors in

hydrated state

ZnO (s)

Activation energy (∆∆∆∆G*)

∆∆∆∆Gg

∆∆∆∆Gaq

∆∆∆∆Gg*

∆∆∆∆Gaq*

Fre

e E

ne

rgy,

G

7

Upon condensation of the solid ZnO, this excess energy is simply discarded into the

environment. Furthermore, recycling waste of material is uneconomical because the

exhaust gases are emitted in large diluted volumes, especially when high vacuum

systems are used.

In contrast, very little extra energy is needed in aqueous chemical growth methods to

break the lattice bonds of the solid Zn source to form free Zn2+ ions. This is because the

energy needed to dissolve the zinc salt and break the lattice bonds are provided by the

hydration energy in water at room temperature. Upon dissolution, growth proceeds by

hydrolysis and condensation which can be induced by manipulating the chemical

equilibrium of precursors. The growth system is typically a closed system which allows

easy separation and recycling of materials. As such, wastage of energy and material are

minimized.

In solution phase, growth precursors has higher concentration and better homogeneity

than the gas phase, especially when high vacuum growth conditions are used. It follows

that aqueous chemical growth methods should give high homogeneity and faster

growth rates than that of the gas phase. However, it is noted that due to much lower

growth temperatures typically less than 100 °C in solution methods, growth units may

not have enough kinetic energy to diffuse across the surface to obtain a smooth film

layer growth.

Finally, growing in solution is a low cost, safe and simple process. Basic equipment

consists of a growth vessel, water bath or convection oven is sufficient. In comparison,

gas phase methods will need a special setup in order to operate at high temperatures

and vacuum conditions. In the case of CVD or MOCVD, the growth precursors are

hazardous and additional safety systems are needed.

In summary, the additional complexity in understanding the growth process and

mechanism of aqueous chemical growth methods are more than compensated by its

energy, materials and processing advantages over gas phase methods.

8

1.5 Doping in ZnO

For a growth method to gain wide acceptance and application in device fabrication, it

must be able to produce ZnO with a low density of intrinsic defects as well as

demonstrate reliable p- and n-doping beyond 1019 cm-3. Reliable p- and n- doping partly

depends on the ability to produce ZnO with a low density of intrinsic defects such as

oxygen vacancies, zinc interstitials and hydrogen donor impurities. These intrinsic

defects typically render the undoped ZnO as n-type.

Table 1.1. Summary of intrinsic doping levels of undoped ZnO polycrystalline films and single crystals which have been grown using various methods.

Type of film Growth method

Intrinsic electron conc. (cm-3)

Substrate Ref.

Polycrystalline PLD 1018 - 1020 fused silica and Si [19]

Polycrystalline Magnetron sputtering

1019 glass and sapphire [20]

Polycrystalline MOCVD 1017 - 1018 sapphire [21]

Polycrystalline Aqueous solution

1019 MgAl2O4 (111) [22]

Single crystal hydrothermal at 300-400°C

1013 to 1014 ZnO seed [23]

Single crystal vapor phase transport

1014-1015 Not reported [24]

A summary of reported intrinsic doping in undoped ZnO polycrystalline films and single

crystals which have been grown using various methods are summarized in Table 1.1.

Two important points can be drawn from the table:

• Firstly, the intrinsic doping concentrations in undoped ZnO single crystals is

about five orders of magnitude less than those in polycrystalline ZnO. This large

difference of intrinsic doping concentrations shows that there is plenty of room

to reduce the concentration of intrinsic defects.

9

• Secondly, a comparable intrinsic defect density in polycrystalline ZnO film is

obtained using both gas phase and solution phase growth methods. This

suggests that solution phase growth methods are capable of growing the same

quality of ZnO as gas phase methods. Together with the advantage of being able

to minimize energy and material wastages, solution phase methods are indeed

a promising growth method for ZnO.

By substituting Zn with group III elements such as Ga [25, 26], Al [27, 28] and In [29],

high levels of n-doping beyond 1020 cm-3 have been achieved. Success in n-doping was

not limited to gas phase methods. Aqueous chemical growth methods have also

achieved electron concentrations of approximately 1019 cm-3 using Al [28] and In [30].

Table 1.2 summarizes the various dopants and methods that have been reported.

Table 1.2. Summary of various group III elements as well as their corresponding growth methods and levels of n-doping.

Dopant Electron conc. (cm-3) Growth method Substrate Ref.

Ga 1018 - 1020 MBE ScAlMgO4 [26]

Ga > 1020 PLD glass [25]

Al > 1021 Filtered cathodic vacuum arc

p-doped 4H-SiC [27]

Al > 1021 Aqueous solution glass [28]

In > 1020 Magnetron sputtering

glass [29, 31]

In 1019 Hydrothermal ZnO seed [30]

In contrast, p-type doping is more difficult to achieve. This difficulty is due to the high

densities of intrinsic defects as shown in table 1.1 as well as low solubility of dopant

species in ZnO and tendencies of dopants to form deep level instead of shallow level

acceptor states [8]. There are two groups of candidates for p-type dopants: group I

elements which substitute Zn atoms and group V elements which substitute O atoms.

10

The calculated bond lengths and defect energy levels for various dopants from group I

and V are shown in Table 1.3. In terms of strain energy, Li and N are the best

candidates as they have almost the same bond length as the ideal ZnO. Also, all group I

elements have a shallower energy level compared to group V elements. Therefore, in

theory, it appears that group I elements, particularly Li, will be promising candidates as

p-dopants.

Table 1.3. Calculated bond lengths and the defect energy levels in ZnO for group I and V dopants. Ideal ZnO bond length (ro) is 1.93 Å. Taken from [32].

Element Bond lengths

r (Å)

Strain (%)

100×−

=o

o

r

rrε

Defect energy level

E (eV)

Li 2.03 5 0.09

Na 2.10 9 0.17

K 2.42 25 0.32

N 1.88 -3 0.40

P 2.18 13 0.93

As 2.23 16 1.15

However, experimental results show otherwise. Hydrothermally-grown bulk ZnO

crystals are typically grown in a high concentrations of KOH or LiOH bases as

mineralizers. As such, they typically have a high concentration of K or Li incorporated.

The high doping concentration of Li did not give a good p-type conductivity and the

ZnO crystals were highly resistive. This contradiction is explained by Li occupation of

interstitial sites where it acts as a donor [33] and compensates the acceptor

contributions. This may also be the reason why hydrothermally-grown LEO films were

n-type instead of p-type, despite the presence of Na in the growth solution [34].

Contrary to theoretical predictions, reports have shown that group V elements are

more promising in achieving p-type doping. Among group V elements, N is considered

as the most ideal p-type dopant since its bond length is closest to the ideal Zn-O bond

length and it has the shallowest acceptor energy level as seen from Table 1.3.

11

Therefore, it is not surprising that most reports on doping with a group V element

focused on N as a shallow acceptor dopant.

Table 1.4 summarizes the p-type doping that has been achieved using various dopants

from group V. It can be seen from the table that the level of p-doping appears to be

comparable to the intrinsic n-doping concentrations shown in Table 1.1. Therefore, it is

not surprising that the p-type conductivity from doping with an element from group V

is unstable and may disappear with time [35, 36].

Table 1.4. Summary of p-type mono-doping of ZnO using group V elements.

Dopant Hole concentration (cm-3) Method Substrate Reference

N 2 x 1016 MBE ScAlMgO4 [37]

P 1.9 x 1016 – 3.8 x 1019 RF sputtering Glass, n-Si [38]

As 2.5 x 1017 – 1.2 x 1018 PLD c-sapphire [39]

Sb 1 x 1016 MBE p-Si (111) [40]

One way to improve the stability of p-type conductivity is to increase the solubility of

nitrogen and lower its ionization energy though a donor-acceptor codoping method

[41]. Typically the acceptor concentration is higher than the donor concentration in

order to obtain a p-type layer. This method has been successfully demonstrated using

Al-N [42], In-N [43] and Ga-N [44] combinations using magnetron sputtering. Reported

hole concentrations range from 1017 to 1018 cm-3.

Another successful method was the dual-acceptor codoping method using Li and N [45].

This combination has achieved very reproducible and stable hole concentrations of

about 1019 cm-3. As shown in Fig 1.3, the hole concentrations arising from Li or N

monodoping schemes deteriorate and disappear completely after 3 months. In contrast,

the Li-N scheme gives a very stable hole concentration. The mechanism leading to the

enhanced p-type stability is still unclear.

12

Figure 1.3. Carrier concentrations as a function of the preservation period after deposition. A very stable p-type conductivity is obtained when Li-N codoping method is used. Graph was taken from [45].

It is interesting to note in Fig 1.3 that in a mono-doping scheme, Li does give a

comparable, or better, doping results than N, although it is unstable and disappears

after 3 months. Very recently, Lin et al successfully fabricated Na-doped ZnO film using

pulsed laser deposition on glass and quartz substrates and obtained stable p-type

conductivity in the range of 1016 to 1018 cm-3. Although the doping levels are low, they

appear to be stable. Fig 1.3, Lin’s result and theoretical calculations point to the

possibility of using group I elements as p-dopants despite earlier difficulties.

Looking back at Tables 1.2 and 1.4, gas phase methods, such as magnetron sputtering

and pulsed laser deposition, appear to be the method of choice for growth and in-situ

p- and n-type doping. There are only a few reports employing solution methods for n-

type ZnO and none for p-type ZnO. This is surprising because:

• aqueous chemical growth methods offer a comparable intrinsic defect density

to other gas phase methods.

13

• the dopant concentrations in aqueous solution are very much higher than that

in gaseous state, and this leads to a more homogeneous dopant distribution

and a higher level of dopant incorporation in ZnO.

In fact, Chapter 6 of this thesis will describe how the aqueous solution growth method

is employed to grow and dope a p-type ZnO film with potassium from group I as the p-

dopant. To the best of our knowledge, this is the first report of extrinsic p-type doping

using aqueous solution growth methods. This successful demonstration shows that

aqueous chemical growth methods have an important role to play in the growth and

doping of ZnO.

1.6 Motivation and objectives

Gas phase growth methods have emerged as the preferred growth method due to

ability to grow and dope ZnO films and nanostructures although these methods are

expensive and not environmentally-friendly. Solution methods offer an alternative

processing route that is environmentally-friendly, are low cost, non-toxic and suitable

for large scale processing. Due to lack of understanding of underlying growth

mechanisms as well as difficulty in growing and doping ZnO epitaxial layers, solution

methods have not been accepted as one of the mainstream growth methods.

The understanding of growth mechanisms of ZnO in solution has been lacking because

of the wide variety of precursors and growth methodology that are available in the

literature. The current focus on achieving functional devices based on ZnO has not

helped to encourage further research on growth mechanism in aqueous solution.

Therefore, it is an important objective of this thesis to understand the growth

mechanisms for one particular growth solution, namely the aqueous system based on

zinc acetate and ammonia. This system is chosen because the materials are readily

available in the laboratory. It is also believed that other growth systems, as long as they

consist of a zinc salt and base, will behave in a similar manner.

A further objective of this thesis is to study how ZnO thin films, with a thickness range

of 1 to 5 µm, can be formed from ZnO nanostructures in aqueous solution. This

14

objective is motivated by the ease of growth of nanostructures in solution and the

ready application of ZnO films in device fabrication, such as transparent conductive

oxides for LEDs and optoelectronic devices.

As mentioned earlier, doping is important challenge to be overcome. There are only a

few reports of n-type doping and none of p-type using aqueous chemical growth

methods. Current achievements in p-type doping have mainly focused on group V

elements as well as codoping using group I and V elements using gas phase methods.

Considering that theory favors group I elements as acceptors, and the recent report of

successful p-type doping using Na using PLD [46] and the processing advantages of

doping in solution phase, it is another objective of this thesis to investigate p-doping of

ZnO using group I elements using aqueous chemical growth methods.

1.7 Organization of the thesis

In this section, the layout of this thesis is described.

The first chapter introduces the material properties of ZnO, its growth methods and

doping while the second chapter covers the aqueous solution growth method in more

detail. The third chapter describes the experimental setup and characterization

methods that will be regularly referenced by the experimental chapters that follows.

Chapters four and five presents the experimental results and discussions on the growth

of nanostructures on various substrates. In particular, chapter four looks into the

growth factors of ZnO nanorods on GaN substrates and chapter five extends this

understanding to other substrates that have a larger lattice mismatch compared to ZnO,

where growth is initiated from ZnO nanoparticles that have been spincoated onto the

substrate.

Chapter six focuses on the growth and p-type doping of the ZnO film. A new growth

film growth strategy is presented for substrates that are closely lattice-matched, such

as GaN, as well as substrates with large lattice mismatches, such as glass, sapphire and

Si. The p-doping of ZnO film using potassium from group I is explored using two setups:

the traditional closed vessel without any applied voltage bias, and a new growth setup

15

with an applied voltage bias. The electrical properties of the unintentionally-doped and

potassium-doped ZnO films are studied and compared. The effects of thermal

annealing on the electrical properties are also presented. This is followed by the

description of the fabrication and characterization of a p-ZnO / n-GaN light emitting

diode.

Finally, chapter eight summarizes and concludes the work done and presents the

future directions for further work.

16

2 Aqueous solution growth of ZnO

2.1 Introduction

Aqueous solution growth has been widely used to grow highly oriented ZnO nanowires

and other nanostructures. The first report was by Andres-Verges et al [47] in 1990

where he reported on the formation of ZnO rods in aqueous solutions containing zinc

nitrate, zinc chloride and hexamethylenetetramine. About ten years later, Vayssieres et

al. [48] used this method to grow nanorods on conducting glass and Si substrates using

a seed layer. Mende et al[49] and Govender et al [50] has reviewed the various growth

precursors in solution and the resulting nanostructures. In addition, Le et. al has

studied the growth of ZnO nanorods on GaN substrates using zinc acetate (ZnAc2) and

ammonium hydroxide (NH3) [51]. This thesis will focus mainly on the growth solution

consisting of ZnAc2 and NH3.

In this chapter, a brief introduction to the chemical principles related to aqueous

solution growth of ZnO is provided. First, the basic terminologies and concepts such as

concentration, supersaturation, pH, solubility product and complexation are introduced.

Then the calculation ionic equilibrium of the ZnAc2 and NH3 system is discussed. Finally,

nucleation and growth processes are described in terms of homogeneous and

heterogeneous nucleation and crystal growth. Further details on the principles

described herein can be readily obtained from several excellent authors [52-54].

2.2 Basic terminologies and concepts

Growth solutions usually consist of at least two components such as zinc acetate

dihydrate (Zn(CH3COO)2.2H2O), hereon denoted as ZnAc2 for brevity, and aqueous

ammonia (NH3). The concentration of any component in the solution is typically

expressed in molar (M), i.e. the number of moles of solute per litre of the solution, and

17

is denoted by square brackets. For example, [Zn2+] represents the concentration of Zn2+

ions. When 0.3 g of ZnAc2 powder is dissolved in 42 ml of water, and assuming all the

powder dissolves in the water, then the number of moles of ZnAc2 in water is

mmol 1.367or mol 001367.05.219

3.0

ZnAcof massmolar

added ZnAcof mass)(

2

22 ===ZnAcn

(2.1)

Assuming complete dissociation of ZnAc2, then 1.367 mmol of ZnAc2 will dissociate to

give 1.367 mmol of Zn2+ ions and 2.734 mmol of CH3COO- ions. Then the concentration

of Zn2+ ions in the solution is then given by

( )

M 0325.0100042

001367.0

litresin water of volume

ZnAcof moles ofnumber ][ 22 ===+

Zn (2.2)

The pH of a solution is another important variable in aqueous solution growth. The pH

value of a solution given by the negative logarithm of the hydrogen ion concentration

in the solution:

[ ]+−= HlogpH (2.3)

The pH of pure water is 7. Addition of a base such as ammonia will increase the pH

value while an acid will decrease the pH value.

When ZnAc2 powder is added to an aqueous solution, it dissolves quickly to form a

homogeneous solution. At any given temperature, there is a maximum amount of

ZnAc2 that can dissolve in a given amount of water giving rise to a saturated solution.

The concentration of ZnAc2 required to make a saturated solution is called solubility

concentration.

The growth solution is supersaturated when ZnAc2 concentration exceeds its solubility

concentration. The degree of supersaturation is defined as

ZnS

CS = (2.4)

where C is the actual concentration of ZnAc2 added, and SZn is the solubility

concentration. When S < 1, no growth or nucleation will occur. For low to moderate

18

values of S greater than 1, heterogeneous nucleation on a substrate will occur. When S

is very large, precipitation via homogenous nucleation in the solution will occur in the

solution.

For example, consider the sparingly soluble Zn(OH)2 in equilibrium with its saturated

aqueous solution:

)(2)()()( 22 aqOHaqZnsOHZn

−+ +⇔ (2.5)

Zn(OH)2 dissolves in water to give a small concentration of Zn2+ and OH-. This

concentration is defined by the solubility product Ksp, which is the product of the

concentrations of the dissolved ions:

[ ] [ ] 1622 101 −−+ ×=⋅= OHZnK sp (2.6)

To prevent precipitation of Zn(OH)2, a complexing agent can be added. The complexing

agent reduces the concentration of free Zn2+ ions and helps to prevent rapid bulk

precipitation of the desired product. For example, in equation (2.5), OH- can play the

role of a complexing agent in the precipitation of Zn(OH)2 as it forms complexes with

Zn2+ such as Zn(OH)42-. Formation of Zn complexes removes Zn2+ ions from the solution,

shifts the balance in equation (2.5) to the right, and thus reduces the degree of

precipitation. With sufficiently high concentrations of OH-, Zn(OH)2 precipitate can be

completely dissolved. As such, it is misleading to rely solely on the solubility product of

Zn(OH)2 in equation (2.5) and (2.6) to estimate of the amount of zinc acetate that can

be dissolved in the growth solution before Zn(OH)2 is precipitated. The presence of

zinc complexes in the solution should also be taken into consideration.

As such, a better way to capture the true solubility of zinc in aqueous solution is to

calculate the temperature-dependent ionic equilibrium of the solution. Such a model

will account for all the possible zinc complex species and is very useful in understanding

aqueous solution growth. One such model for the growth system using ZnAc2 and NH3

will be described in next section.

19

2.3 Temperature-dependent ionic equilibrium of ZnAc2

and NH3

As mentioned earlier, a temperature-dependent model of the ionic equilibrium of the

growth solution can provide the concentrations of all the possible zinc complex species.

The equilibrium concentrations of every species can be calculated by taking into

account the various hydroxide, ammine and acetate complex species are formed when

ZnAc2 and NH3 is mixed in an aqueous solution. The reaction equations and the

corresponding rate constants at 298 K are given below.

Hydroxide complex formation [55]

4.4

21

2 10]][[

])([)( ==↔+

−+

++−+

OHZn

OHZnKOHZnOHZn (2.7a)

71.11

22 10]][)([

1)()( ==↓↔+ −+

−+

OHOHZnKOHZnOHOHZn (2.7b)

5.4

2322 10])([)()( −==↓↔ OHZnKOHZnOHZn (2.7c)

71.13432 10

][

])([)()( −

−

−−− ==↔+↓

OH

OHZnKOHZnOHOHZn (2.7d)

61.0

2

2

45

2

42 10][

])([)(2)( −

−

−−− ==↔+↓

OH

OHZnKOHZnOHOHZn (2.7e)

Acetate complex formation [55]

3.1

26

2 10]][[

])([)( −

−+

++−+ ==↔+

AcZn

AcZnKAcZnAcZn (2.8a)

8.0272 10

]][)([

])([)()( −

−+−+ ==↔+

AcAcZn

AcZnKAcZnAcAcZn (2.8b)

Ammine complex formation [56]

20

59.2

3

2

2

38

2

33

2 10]][[

])([)( ==↔+

+

+++

NHZn

NHZnKNHZnNHZn (2.9a)

91.4

2

3

2

2

239

2

233

2 10]][[

])([)(2 ==↔+

+

+++

NHZn

NHZnKNHZnNHZn (2.9b)

92.6

3

3

2

2

3310

2

333

2 10]][[

])([)(3 ==↔+

+

+++

NHZn

NHZnKNHZnNHZn (2.9c)

62.8

4

3

2

2

4311

2

433

2 10]][[

])([)(4 ==↔+

+

+++

NHZn

NHZnKNHZnNHZn (2.9d)

39.4

4

3

12234 10]][[

][==+↔+

−+−+

OHNH

NHKOHNHOHNH (2.9e)

The ionic equilibrium for the aqueous solution can be obtained by solving

simultaneously the reaction equations, the mass and charge balance. The mass balance

equation can be written as

])([2])([][ 2AcZnAcZnAccAc ++= +− (2.10)

∑=

++ ⋅++=4

1

2

334 ])([][][m

mN NHZnmNHNHc (2.11)

where cAc and cN are the total concentrations of acetate and ammine ions.

The charge balance equation equates the positive and negative charges in the aqueous

solution and can be written as

][][])([2])([

])([2][])([])([][2

2

43

4

1

2

34

2

−−−−=

+++++

+++=

++++ ∑OHAcOHZnOHZn

NHZnNHAcZnOHZnZnm

m (2.12)

The temperature dependence [57] of the rate constants in the reaction equations

[2.7a-e], [2.8a-b] and [2.9a-e] is estimated using

21

−

∆=

211

2 11

3.2log

TTR

H

K

Ko

r

T

T (2.13)

where the ideal gas constant R = 8.314 × 10-3 kJ.mol-1K-1, KT1 and KT2 are rate constants

at temperature T1 and T2 respectively, and ∆rH0 is the standard enthalpy of reaction

and is given by

∑ ∑ ∆−∆=∆j i

ifijfjr HnHnH000 (2.14)

where ∆fH0 is the standard enthalpy of formation, i and j specify reactants and products

respectively, and ni and nj are the amounts in moles of each substance in the chemical

reaction. The standard enthalpy values for the product and reactants for reaction

equations (2.7), (2.8) and (2.9) are summarized in Table 2.1.

Table 2.1. List of Enthalpy Values [58-60]. Enthalpy alues with an asterisk * denotes calculated values of enthalpy of formation from tabulated enthalpy of reaction.

Species

∆∆∆∆fH0 (kJ/mol)

Zn2+ -153.64

Zn(OH)+ -388.35

Zn(OH)2 -611.95

Zn(OH)2 ↓↓↓↓ -641.90

Zn(OH)3- -817.97

Zn(OH)42- -1125.64

Zn(NH3)2+ -244.81 *

Zn(NH3)22+ -338.07 *

Zn(NH3)32+ -434.68 *

Zn(NH3)42+ -536.72 *

H20 -285.83

OH- -230

NH3 -80.29

NH4+ -132.5

22

Figure 2.1. Equilibrium complex concentrations and solubility of zinc as a function of pH at 300K. The pH is increased by adding more NH3 while keeping the mass of ZnAc2 constant at 0.016 M. Curves show the equilibrium concentrations of (a) zinc acetate complexes, (b) Zn2+ ions, (c) zinc ammine complexes, (d) zinc hydroxide complexes and (e) total zinc ion concentration respectively.

In the calculation of the ionic equilibrium, it is assumed that the equilibrium

concentrations of zinc acetate complex species are very small compared to the other

species. Thus, in the range of 0 to 150°C, the temperature dependence of K6 and K7 in

equations (2.8a) and (2.8b) is neglected.

Using the model described above, the ionic equilibrium of the ZnAc2 and NH3 in an

aqueous solution can be calculated for various ZnAc2 and NH3 precursor concentrations

and temperature. A detailed description of the procedure to calculate the ionic

equilibrium is provided in the Appendix. Fig 2.1 shows the solubility of zinc and

concentration of the major zinc complexes as a function of pH at 300 K respectively.

The pH is increased by adding more NH3 while keeping the mass of ZnAc2 constant at

0.016 M.

Several important points can be seen from Fig 2.1:

23

• As shown by curve (a) in Fig 2.1, the concentrations of zinc acetate complexes are

small compared to the other zinc complexes. This justifies the omission of

temperature effect on equilibrium constants K6 and K7 in equations (2.8a) and

(2.8b).

• The “true” solubility of zinc is given by the sum of all its zinc complexes, namely the

hydroxides, ammines and acetates. The solubility shows a minimum point in the pH

range of 8.5 to 9.5. Interestingly, this pH range coincides with the point of zero

charge of ZnO surface which will be described later in this chapter.

• At pH values greater than 9.7, the increase in zinc solubility is contributed mainly by

the increasing concentration of zinc ammine complexes. On the other hand, at pH

values lower than 8, the increase is due to Zn2+ ions. In chapter 4, we will see how

these different dominant species play an important role in determining the growth

rate and the structural defects of ZnO.

Proceeding from the equilibrium concentrations, the “true” solubility of zinc and the

degree of supersaturation of zinc, which are related to the nucleation density, can be

calculated:

The solubility of zinc, or the total concentration of zinc ions in the precursor solution is

given by the sum of all the zinc species in the solution:

∑ ∑ ∑= = =

+−++−+ +++=4

1

4

1

2

1

)2(23

)2(2* ])([])([])([][m n p

ppn

mmZn AcZnNHZnOHZnZnC (2.15)

The degree of supersaturation of zinc in the solution in equation (2.4) can be rewritten

as

*/ ZnCCS = (2.16)

where C is the original concentration of ZnAc2 that was added to the growth solution.

Although solubility is the main driving force for nucleation and growth in solution, it is

rarely used as a growth control variable due to measurement difficulties. Instead, pH is

24

the dominant growth control variable. This is reflected in the reported growth

procedures where a final adjustment to the pH range of 10 to 11 is usually practiced.

Using pH as the primary growth control variable presents several drawbacks.

• Firstly, for the same pH value, the solubility of zinc can vary with concentration

of ZnAc2 that is added. This is illustrated in Fig 2.2 which shows the variation of

the zinc solubility with pH. Each curve in Fig 2.2 is obtained by keeping the

ZnAc2 concentration fixed while varying the concentration of NH3 to change the

solution pH. This sequence is similar to typical experimental procedures for

adjustment of growth pH. The variation of the solubility of zinc, due to different

concentrations of ZnAc2, is smallest at pH 7 and increases with pH. At pH 10, the

difference in the solubility of zinc for initial ZnAc2 concentrations of 0.03 M and

0.06 M is large enough to cause differences in growth morphology as will be

shown later in chapter 4.

• Secondly, for a fixed concentration of ZnAc2, large increases in NH3

concentration shifts the pH only slightly while affecting the solubility of zinc

significantly. This is shown in Fig 2.3 where the solubility of zinc and pH is

plotted against the concentration of NH3. In the absence of an accurate pH

titration and measurement, this variation in the solubility of zinc in different

growth batches may produce an inconsistent growth morphologies between

batches.

By employing the calculated solubility of zinc as the primary growth control variable,

these drawbacks can be minimized. In fact, it will be shown in chapters 3 and 4 that

the solubility of zinc can be used a predictor of ZnO nanorod density and length on

GaN substrates, as well as its growth morphology on substrates.

25

Figure 2.2. Variation of solubility of zinc with pH. The solubility of zinc was calculated using Eq. (2.15). The data for each curve is obtained by keeping the concentration of ZnAc2 fixed while varying the concentration of NH3. The concentrations of ZnAc2 are indicated on each curve.

Figure 2.3. Variation of solubility of zinc and pH when the concentration of NH3 is varied while ZnAc2 is kept constant at 0.02 M. The solubility of zinc was calculated using Eq. (2.15).

1.E-04

1.E-03

1.E-02

1.E-01

7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5 11.0

pH

Solu

bili

ty o

f zi

nc

(M)

0.11 M

0.06 M

0.03 M

0.01 M

1.E-04

1.E-03

1.E-02

0 0.2 0.4 0.6 0.8 1 1.2 1.4

Concentration of NH3 [M]

Solu

bili

ty o

f Zn

[M

]

7.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

11.5

12.0

pH

0.02 M ZnAc2

Solubility of Zn

pH

26

2.4 Nucleation and growth

As mentioned in Section 2.3, solubility of zinc and the degree of supersaturation is the

main driver for ZnO growth. Homogeneous nucleation of ZnO will occur when the

degree of supersaturation, S, is much larger than unity. At lower degrees of

supersaturation that is slightly greater than unity, heterogeneous nucleation is the

preferred growth mechanism.

2.4.1 Homogeneous nucleation

The first stage in homogeneous nucleation is the formation of embryos through

collision between individual ions or molecules. Embryos grow by collecting individual

species, most likely ions, that collide with them. Growth by collision between embryos

is also possible when the concentration of embryos is high.

Embryos are unstable thermodynamically because their large surface area to volume

ratio leads to a high surface energy. Many of these embryos will dissolve before they

can grow into stable nuclei. One way to increase the number of nuclei is to grow at a

lower temperature so that the longer lifetime of the embryos increases their chances

of growing to a thermodynamically stable size.

The thermodynamically stable size is determined by the energy balance between the

surface energy required to form the embryo and the energy released due to a phase

transformation from liquid phase to solid phase when a spherical particle is formed. Fig

2.4 shows this energy balance which is known as the Gibbs free energy of nucleation

(∆G) as a function of the embryo radius, r. It is clear that the newly formed embryo is

stable if its radius, r, is greater than r* and it will continue to grow bigger to reduce its

Gibbs free energy. On the other hand, when r < r*, the embryo will preferentially

dissolve into the solution to reduce its Gibbs free energy.

Mathematically, the Gibbs free energy can be expressed [61] as

(2.17) VGrrG ∆+=∆ 32

3

44 πγπ

27

where r is the radius of the nucleus, γ is the interfacial energy and ∆Gv is the change of

Gibbs free energy per unit volume of the solid phase, given by the equation

( )SV

kT

S

C

V

kTG

Zn

V lnln −=

−=∆ (2.18)

where V is the atomic volume, k is the Boltzmann constant, T (K)is the growth

temperature, C [mol/l] is the concentration of dissolved zinc acetate, SZn [mol/l] is the

solubility of zinc and S is the degree of supersaturation of zinc.

By setting d∆G/dr = 0, the critical energy for nucleation to occur, ∆G* is given by,

2

3

22

2*

)(ln3

16

STk

VG

γπ⋅=∆ (2.19)

while the critical radius, r* is given by

)ln(

22*

SkT

V

Gr

V

γγ=

∆−= (2.20)

Equations (2.19) and (2.20) show that both the free energy for nucleation and critical

size of the embryo can be reduced by growing at a higher temperature, a high degree

of supersaturation or by lowering the interfacial energy. The smaller free energy or

critical embryo size will lead to a higher nucleation density.

Figure 2.4. The Gibbs free energy of nucleation with respect to embryo radius. The critical radius r* and energy between the surface and volume energy of the growing embryo.

2.4.2 Heterogeneous nucleation

Besides homogeneous nucleation, another process that takes place in the solution is

heterogeneous nucleation. In heterogeneous nucleation, two processes occur as shown

in Fig 2.5. In the first process, individual ions adsorb onto the substrate surface and

create a nucleus for reaction and further growth.

The second process involves adsorption of sub

adsorption, the contact area bet

the interface energy between the embryo and the substrate surface is usually less than

that between the embryo and the solution, surface adsorption reduces the critical

energy for nucleation and may transf

embryo. Due to a lower energy barrier, heterogeneous nucleation can occur near

equilibrium saturation conditions, i.e. when S has a low to moderate value of greater

than 1.

Figure 2.5. Processes involved in heterogeneous nucleation on a substrate surface.

To encourage heterogeneous nucleation on the substrate surface, the interface energy

between the embryo and the substrate surface can be minimized by reducing the

lattice mismatch between the substrate and the embryo. For example, the lattice

The Gibbs free energy of nucleation with respect to embryo radius. The critical radius r* and energy ∆G* depends on the balancebetween the surface and volume energy of the growing embryo.

Heterogeneous nucleation



2.5. In the first process, individual ions adsorb onto the substrate surface and

create a nucleus for reaction and further growth.

The second process involves adsorption of sub-critical embryos onto the surface. Upon

adsorption, the contact area between the embryo and the solution is reduced. Since



energy for nucleation and may transform a sub-critical embryo into a super



Processes involved in heterogeneous nucleation on a substrate



een the substrate and the embryo. For example, the lattice

28

The Gibbs free energy of nucleation with respect to embryo G* depends on the balance

between the surface and volume energy of the growing embryo.



2.5. In the first process, individual ions adsorb onto the substrate surface and

critical embryos onto the surface. Upon

ween the embryo and the solution is reduced. Since



critical embryo into a super-critical



Processes involved in heterogeneous nucleation on a substrate



een the substrate and the embryo. For example, the lattice

29

constants a of gallium nitride (GaN) and ZnO are 3.189 Å and 3.2495 Å respectively. The

corresponding lattice mismatch works out to be

%9.1018618.02495.3

2495.3189.3−≈−=

−=

−=∆

film

filmsubstrate

a

aaa (2.21)

where the negative value indicates that the ZnO film is compressed. Due to the small

mismatch, it is not surprising that ZnO can nucleate readily on GaN substrates in

solution. Using equation (2.21) to compare ZnO to Si <100> which has a lattice constant

of 5.43 Å, the lattice mismatch is about 0.67103 ≈ 67%. The large tensile strain in ZnO

leads to a high interface energy at the ZnO – Si interface and limits nucleation on Si

<100> substrates surface. It is believed that if growth occurs on the Si surface, it is likely

to originate from surface defect sites on the Si substrate.

The high interface energy from large lattice mismatches can be reduced by applying a

pre-coat of ZnO nanoparticles on the substrate surface. These particles will form an

aggregate film which begins to coalesce when annealed at temperatures above 350°C

[62]. This aggregate ZnO film provides a lattice matched surface for nucleation and

growth on various substrates such as Si, glass, plastic and sapphire. As we will see in

chapter 4, the growth morphologies of ZnO nanorods on the aggregate film can be

predicted from the solubility of zinc in the growth solution, regardless of the underlying

substrate.

2.4.3 Crystal growth

As described earlier, heterogeneous nucleation involves the adsorption of ions or

nuclei onto the substrate. Once a stable nuclei is formed, the nuclei can grow by

several ways. One way is by self-assembly where ionic species from the solution adsorb

onto the nucleus. This process begins with the solvated Zn2+ ions in aqueous solution,

Zn2+ + 6H2O ↔ Zn(H2O)62+ (2.22)

The hydrated Zn2+ ion attracts the electrons from the oxygen of the attached water

molecules. The electron deficient oxygen, in turn attracts electrons from the O-H bonds.

This weakens and makes it easier to break the O

hydrolysis of the hydrated

Figure 2.6. Hydrolysis of hydrated metal ions in aqueous solution. The positively charged metal ion attracts the electrons away from the Oleading to the breakage of the Osolution.

Hydrolysis of the hydrated zinc ion

hydroxide complexes,

Zn(H2O)62+ + nH2O

It is customary to omit the water, thus equation (2.23) can be rewritten as

Zn2+ + nOH- ↔ Zn(OH)

Each hydrolysis step has an equilibrium constant which has been presented earlier in

equations (2.7 a–e).

After hydrolysis, the condensation of the hydrolyzed zinc ions leads to the formation

and growth of ZnO. There are two types of condensation reactions, na

oxolation. In olation, an “ol” bridge is formed by a condensation reaction between a

hydroxo- and aquo-species,

Zn-OH + Zn-OH2 →

In oxolation, an “oxo” bridge is formed by the condensation reaction between two

hydroxo-species,

Zn-OH + Zn-OH →

This weakens and makes it easier to break the O-H bonds. This process is known as

ydrated Zn2+ ion and is schematically shown in Fig 2.6.

Hydrolysis of hydrated metal ions in aqueous solution. The positively charged metal ion attracts the electrons away from the Oleading to the breakage of the O-H bond and release of the H+

Hydrolysis of the hydrated zinc ion is a sequential process and gives a range of zinc

O ↔ Zn(H2O)6-n(OH)n(2-n)+ + nH3O+


Zn(OH)n(2-n)+

hydrolysis step has an equilibrium constant which has been presented earlier in


and growth of ZnO. There are two types of condensation reactions, na


species,

→ Zn-OH-Zn + H2O


Zn-O-Zn + H2O

30

H bonds. This process is known as

2.6.

Hydrolysis of hydrated metal ions in aqueous solution. The positively charged metal ion attracts the electrons away from the O-H bond,

ion into the

is a sequential process and gives a range of zinc

(2.23)


(2.24)

hydrolysis step has an equilibrium constant which has been presented earlier in


and growth of ZnO. There are two types of condensation reactions, namely olation and


(2.25)


(2.26)

31

A more detailed discussion on the growth mechanism, as well as crystal habit, based on

the zinc hydroxide complexes as the growth units is provided by Li et al. [63] where

ZnAc2 and NH3 are the growth precursors. Despite the wide range of precursors that

are used in the growth of ZnO, the equations (2.22) to (2.26) represent the typical

hydrolysis and condensation reactions that lead to the growth of ZnO. For example, the

reactions in the popular method of using a solution of zinc nitrate, Zn(NO3)2 and

hexamethyltetramine (HMT) can be written as:

(CH2)6N4 + 6H2O ↔ 6HCHO + 4NH3 (2.27)

NH3 + H2O ↔ NH4+ OH- (2.28)

2OH- + Zn2+ ↔ ZnO + H2O (2.29)

where equation (2.29) incorporates equations (2.24), (2.25) and (2.26).

Besides zinc hydroxide complexes, an alternative mechanism using zinc ammine

complexes as basic growth units, have been proposed by Xu et al [64] and Wei et al

[65], who grew ZnO nanorods and nanotubes, respectively, in solutions containing zinc

chloride and ammonia at a pH of 10. In their proposed mechanism, zinc ammine

complexes undergoes an ligand exchange with H2O before undergoing the typical

condensation reaction in the presence of OH- to form ZnO:

Zn(NH3)42+ + 2OH- ↔ ZnO + 4NH3 ↑ + H2O (2.30)

Two observations led to this alternative mechanism. Firstly, aging the growth solution

converted the nanorods to nanotubes. When the solution is aged, the concentration of

Zn(NH3)42+ is reduced and the equilibrium in equation (2.30) will shift to the left leading

to preferential dissolution of the top (0001) face of ZnO nanords to form nanotubes.

Secondly, a faster growth rate is observed when the sample is placed closer to the top

of the growth solution. During growth, NH3 evaporates out of the solution and fill the

air above the solution inside the sealed bottle. This leads to a lower concentration of

NH3 at the top of the solution and shifts the equilibrium in equation (2.30) to the right,

leading to faster growth rate compared to samples placed at the middle and bottom of

the growth solution.

Aggregation is another route for crystal growth. In the presen

of ZnO particles in the solution, the high probability of collisions between particles and

van der Waals forces cause the particles to stick together to form a large particle, or

stick on the surface of a substrate to form a film a

mode will result in grain boundaries between individual particles. Deposition of

aggregated particles on substrates can be minimized by stirring during growth or

placing the substrate facing downwards to prevent particle

surface. In our experiments, the substrate is always mounted facing downwards to

minimize aggregation on the surface. Detailed description of the experimental details

will be presented in chapter 3.

If the growth temperature is high

atoms and coalescence between the aggregated particles can occur to form a large

particle as shown in Fig 2.7 (b). In aqueous solution growth, coalescence rarely occurs

in-situ due to the low growth tempera

usually used to achieve coalescence. In chapter 4 and 5, the effect of post

treatments in various ambient conditions on the native defects of ZnO, carrier

concentrations and mobility will be discussed.

Figure 2.7. (A) Aggregation and (B) coalescence of individual particles.

growth rate compared to samples placed at the middle and bottom of

Aggregation is another route for crystal growth. In the presence of high concentration



stick on the surface of a substrate to form a film as shown in Fig 2.7 (a). This growth



placing the substrate facing downwards to prevent particles from falling onto the



will be presented in chapter 3.

If the growth temperature is high enough, appreciable surface diffusion of crystal


2.7 (b). In aqueous solution growth, coalescence rarely occurs

situ due to the low growth temperatures. Instead, post-anneal treatments are

usually used to achieve coalescence. In chapter 4 and 5, the effect of post


concentrations and mobility will be discussed.

(A) Aggregation and (B) coalescence of individual particles.

32

growth rate compared to samples placed at the middle and bottom of

ce of high concentration



2.7 (a). This growth



s from falling onto the



enough, appreciable surface diffusion of crystal


2.7 (b). In aqueous solution growth, coalescence rarely occurs

anneal treatments are

usually used to achieve coalescence. In chapter 4 and 5, the effect of post-anneal


(A) Aggregation and (B) coalescence of individual particles.

33

2.5 Effect of pH on ZnO surface

Another important aspect in the growth of ZnO in aqueous solution is the surface

chemistry of the ZnO. Two important points characterize the surface of ZnO: the

surface is electrically charged and highly hydrated [52].

The charge on the oxide surface results from ionization of hydroxyl groups on their

surface upon contact with water. There is a difference in chemical potential of the

constituents of ZnO (namely Zn and O atoms) in liquid and solid phases. Due to the low

mobility of Zn2+ in the solid and low solubility product of ZnO or Zn(OH)2, the migration

of Zn2+ ions towards the liquid phase and the dissolution of ZnO do not occur. Instead,

the difference in chemical potential of oxygen is reduced by adsorption of water and

the dissociation of the adsorbed molecules. This explains the presence of hydroxyl

groups on the ZnO surface. During ZnO growth in solution, hydroxyls groups continue

to be present at the surface since they come from the coordination sphere of the last

Zn2+.

These surface groups (Zn-OH) can ionize in the presence of water:

Zn-O- + H3O+ ↔ Zn-OH + H2O ↔ Zn-OH2+ + OH- (2.31)

The overall charge on the surface groups can be negative, neutral or zero, depending

on the external solution pH. It is important to note that for a particular pH value, the

surface charge is not homogeneously distributed over the entire surface: some sites

have opposite charges or higher density of charges. However, overall surface charge

density is a consistent function of pH. In acidic medium, the overall surface charge is

positive while in basic medium, it is negative. There exist a pH point where the ZnO

surface is neutral. This point corresponds to the point of zero charge (PZC) or the

isoelectric point (IEP). The PZC for ZnO is known to fall in the pH range of 8.7 to 9.7 [66,

67].

Figure 2.8. A model for adsorption of

Figure 2.9. Adsorption of Highest rate of adsorption when the pH is higher than the PZC of ZnO.

Fig 2.8 illustrates a typical model for adsorption on a hydrated ZnO surface

water, Zn2+ ions are hydrated through the adsorption of

in Section 2.4.3, the positive

bond, leading to the first dissociation of

subsequent dissociation of the

acidic solution, H+ ions will prefer to reside on the surface rather than enter into a

solution that is rich in H+. As the pH increases, the concentration of

reduces and it becomes easi

Once the OH- ion loses the

adsorb onto site OH- has dissociated. It is clear from

thus the growth of ZnO, is fastest at high pH as there are many ready sites for

adsorption due to ready dissociation of the

A model for adsorption of Zn2+ on ZnO surface.

Adsorption of Zn2+ ions depends on the pH of the solution. Highest rate of adsorption when the pH is higher than the PZC of ZnO.

8 illustrates a typical model for adsorption on a hydrated ZnO surface

ions are hydrated through the adsorption of H2O molecules

in Section 2.4.3, the positive Zn2+ will lead to a weakening and breakage of the O

bond, leading to the first dissociation of H2O molecule, leaving behind an

subsequent dissociation of the OH- ion depends on the pH of the solution. In a low pH

ions will prefer to reside on the surface rather than enter into a

. As the pH increases, the concentration of H+

reduces and it becomes easier to dissociate H+ from the O-H bonds into the solution.

ion loses the H+ ion, another hydrated Zn2+ ion from the solution can

has dissociated. It is clear from Fig 2.9 that adsorption of

is fastest at high pH as there are many ready sites for

adsorption due to ready dissociation of the OH- ion.

34

ions depends on the pH of the solution. Highest rate of adsorption when the pH is higher than the PZC of ZnO.

8 illustrates a typical model for adsorption on a hydrated ZnO surface [53]. In

molecules. As mentioned

will lead to a weakening and breakage of the O-H

molecule, leaving behind an OH- ion. The

ion depends on the pH of the solution. In a low pH

ions will prefer to reside on the surface rather than enter into a

+ in the solution

H bonds into the solution.

ion from the solution can

2.9 that adsorption of Zn2+, and

is fastest at high pH as there are many ready sites for

35

In chapter 4, the role of the surface charges in determining the growth morphology and

growth rate of ZnO nanorods will be discussed.

2.6 Conclusion

A brief introduction to the basic terminologies and concepts related to aqueous

solution growth of ZnO in a growth system consisting ZnAc2 and NH3 has been

presented. The methodology to calculate the ionic equilibrium of the ZnAc2 and NH3

system has been discussed. Finally, nucleation and growth are described in terms of

homogeneous and heterogeneous nucleation and crystal growth.

36

3 Experimental methods for

growth and characterization of ZnO

3.1 Introduction

This chapter describes the experimental procedures and apparatus that are used for

growing ZnO nanorods in aqueous solution throughout this thesis. The primary

characterization methods are scanning electron microscopy, Raman and

photoluminescence spectroscopy and Hall effect measurements. Each of these

characterization methods will also be discussed in this chapter.

3.2 Growth procedure and apparatus

The growth process of ZnO can be divided into three steps. These steps are the pre-

coating of the substrate with ZnO seeds, growth in aqueous solution, and finally the

post-growth treatment such as thermal annealing under various ambient conditions.

3.2.1 Pre-coating of substrate with ZnO seeds

The first step is usually required for substrates with a large lattice mismatches with ZnO

such as Si, glass or sapphire. Table 3.1 shows the lattice parameters, the corresponding

lattice mismatches and the thermal expansion coefficients along the a- and c-axis for

several common substrates.

The pre-coating solution that is used throughout this thesis consists of ZnO

nanoparticles suspended in ethanol. They were prepared by stirring a solution

containing 0.02 M KOH and 0.01 M ZnAc2, in methanol at 60°C for 2 h, similar to

Pacholski’s method [68]. Fig 3.1 shows a typical TEM image of the ZnO nanoparticles,

which diameter ranges from 10 to 20 nm. Agglomeration of the nanoparticles, which is

37

clearly seen in Fig 3.1, can be minimized by adding surfactants such as

dodecylammonium (DDA) into the refluxing solution.

Table 3.1. Lattice parameters of various substrate materials for ZnO growth [69].

Material Crystal structure

Lattice parameters

a (Å) c (Å)

Lattice mismatch

(%)

Thermal-expansion coefficient ααααa (10-6 K-1) ααααc (10-6 K-1)

ZnO Hexagonal 3.252 5.213

0 2.9 4.75

ScAlMgO4 Hexagonal 3.246 25.195

0.09 Not available

GaN Hexagonal 3.189 5.185

1.8 5.17 4.55

6H-SiC Hexagonal 3.080 15.117

3.5 4.2 4.68

AlN Hexagonal 3.112 4.980

4.5 5.3 4.2

α-Al2O3 Hexagonal 4.757 12.983

18.4 (after 30° in-plane

rotation)

7.3 8.1

Si Cubic 5.43 40.1 3.59 GaAs Cubic 5.652 42.4 6.0 Glass Amorphous Not

applicable Not

applicable Not

applicable

By pre-coating the substrate surface with ZnO nanoparticles, heterogeneous growth of

ZnO on the substrate can be enhanced. As mentioned earlier in chapter 2, when lattice

mismatch of the original bare substrate is minimized through the use of an

intermediate ZnO seed layer, the interfacial energy γ and thus the free energy for

nucleation in Equation (2.19) can be minimized to encourage growth. On the other

hand, when a substrate has a small lattice mismatch with ZnO, such as GaN as shown in

Table 2.1, no pre-coating of ZnO nanoparticles is required for heterogeneous growth of

ZnO to occur.

38

The ZnO nanoparticles are evenly coated on a clean substrate by spin-coating at 3000

rpm for 30 s. The spin-coating step is carried out three times before annealing the

seeded substrate at 400°C in air for 10 min.

Figure 3.1 TEM image of the ZnO nanoparticles that are grown by refluxing 0.02 M KOH and 0.01 M ZnAc2 in methanol for 2 h. The diameter of the nanoparticles range from 10 to 20 nm. Agglomeration of the nanoparticles can be clearly seen.

3.2.2 ZnO growth in solution

The growth of ZnO nanorods on the seeded substrate was carried out in a Schott Duran

laboratory borosilicate glass bottle with an autoclavable polypropylene screw cap. The

Schott Duran bottles were suitable for aqueous solution growth because of the

following properties [70]:

• Borosilicate glass is highly chemical resistant to water, acids, saline solutions,

organic substances and halogens. However, care must be taken to avoid

exposure to strongly alkaline solutions which erodes the glass. When exposed

to 1 M NaOH (pH ~ 14) at 50°C, some glass erosion can occur. Nonetheless, the

erosion is minimal as can be seen from the manufacturer’s datasheet which

estimates that about 1 mm of glass will be eroded after 25 years.

39

• Polypropylene cap has high chemical resistance to alkaline solution, acids and

alcohols.

• High thermal shock resistance with maximum temperature difference of 100°C,

allowing safe immersion of the vessels, which contains growth solutions at

room temperature, directly into hot water baths or convection ovens up to

120°C. For gradual heating applications where the temperature differences is

less than 100°C, the maximum heating temperature of the glass bottles is

limited by the polypropylene caps which have a maximum operating

temperature of 140°C.

The growth solution was prepared by adding a certain amount of ZnAc2 (Merck, GR for

analysis) followed by NH3 (J.T. Baker, 30% w/w) into deionized water at room

temperature. All chemicals were used without any further purification process.

The substrate was suspended, facing downwards, away from the bottom of the bottle

as shown in Fig 3.2. The bottle was sealed tightly and placed into a water bath (or

convection oven), with a fixed growth temperature ranging from 80 to 95°C.

After the growth, the substrate was removed from the bottle, rinsed thoroughly with

deionized water and allowed to dry in air at room temperature.

Figure 3.2. Apparatus for growth of ZnO on a substrate.

sealable growth vessel

substrate

sample

growth

solution

temperature controlled water bath

40

3.3 Characterization tools

The growth method described earlier can be used to grow ZnO nanostructures and well

as thin films. In this work, the structural, optical and electron transport properties of

the resulting structures are examined using field-emission scanning electron

microscopy, micro-photoluminescence spectroscopy, micro-Raman spectroscopy and

Hall measurement. In the following sections, some background theory on the

characterization method as well as relevant ZnO-related characterization results will be

presented. This information will serve as the foundation for understanding the

experimental results that will be discussed in Chapters 4 through 7.

3.4 Field-emission scanning electron microscopy (FESEM)

The feature size of the ZnO nanorods and films, ranging from 100 to 1000 nm, can be

examined using an energetic electron beam in an electron microscope. The resolution

limit is approximately 0.5λ as dictated by Abbe’s Law. Compared to visible light in the

traditional optical microscope, the energetic electron beam has a much shorter de

Broglie wavelength and thus a smaller resolution limit. The resolution limit of SEM

typically ranges from 5 to 20 nm and depends on the specific emission volume arising

from the interaction between the electron beam and the irradiated surface. From this

interaction volume, various secondary signals such as secondary electrons,

backscattered electrons, Auger electrons, characteristic X-rays and photons are

produced [71]. SEM uses the secondary electrons and backscattered electrons to

reconstruct the topography of the surface.

The general morphology of the ZnO nanorods and films, such as the uniformity,

orientation, length, diameter distribution, spatial density and film thickness are

routinely examined using SEM. Two models of SEM are routinely used in this work:

Hitachi model S4100 FESEM located at Centre for IC Failure Analysis and Reliability and

JOEL model JSM6700F FESEM located at Institute for Materials Research and

Engineering. Both machines are operating in secondary electron imaging mode with

the acceleration voltage ranging from 5 to 30 kV.

41

For samples that are grown on insulating substrates such as GaN epilayer on sapphire

and glass substrates, a silver tape is used to create a conductive path from the ZnO

layer to the sample holder, thus preventing sample charging and image distortion.

3.5 Photoluminescence spectroscopy

Another characterization method that is routinely employed is Photoluminescence (PL)

spectroscopy. This is a simple, nondestructive yet sensitive method to characterize the

optical properties of ZnO nanostructure and film. In this method, electrons are excited

to a higher energy states by absorption of laser light with energy larger than the band

gap energy. These excited electrons then relax to their equilibrium state through

various radiative and non-radiative processes. The measured intensity and energy

distribution of this radiative relaxation process yields information on the properties and

dynamic processes in ZnO.

The probe depth of this method depends on the excitation wavelength. When a He-Cd

laser of 325 nm is used, the large absorption coefficient of ZnO leads to a penetration

depth of approximately 50 – 100 nm [72, 73]. Therefore, for a bulk ZnO sample, this

method only probes the upper surface of a thick ZnO film. On the other hand, for

nanorods with a diameter of 100-200 nm, the penetration depth is sufficient to provide

information on the bulk structure.

The characteristics of the PL emission from ZnO are determined by several factors:

• the fundamental band-to-band transition as shown in Fig 3.3 (a). The optically

allowed band-to-band transition is a function of the density of states with

respect to energy, i.e. the absorption coefficient is proportional to the square

root of photon energy for photon energies greater than the band gap energy.

The valence band splits into three bands, namely A, B and C, due to crystal-field

and spin-orbit interactions. A wide range of values ranging from 3.195 to 3.44

eV at room temperature have been reported [74]. As indicated in Fig 3.3 (a), EgA,

EgB and EgC have been reported to be 3.437, 3.4425 and 3.4813 eV respectively

at 4.2 K. The band gap temperature dependence up to 300 K is given by [75]:

42

T

TTETE gg −

×⋅==

−

900

1005.5)0()(

24

(3.1)

• free excitons which are formed from the Coulombic attraction between

electron and hole, leading to a series of hydrogen-like states below the band

gap as shown in Fig 3.3 (b). All exciton series from the A, B and C valence bands

into the conduction bands have an approximate exciton binding energy of 60

meV and an excitonic Bohr radius of 1.8 nm. Typically, the A and B exciton

peaks can be tracked up to 160 K, above which line broadening prevents a

satisfactory distinction between the two peaks. The intensity of A and B free

excitons increases with temperature up to 40 and 80 K respectively, and then

decreases as temperature is increased further. Free excitons are able move

freely through the crystal and scatter upon collision with defects or phonons.

• bound excitons which have no translational degrees of freedom due to

formation of complexes upon binding to a defect centre such as ionized or

neutral donors, or neutral acceptors. As such, bound exciton emission usually

dominate at low cryogenic temperatures, whereas free exciton emission takes

over at higher temperatures. Bound excitons emission usually disappear at 50

to 150 K, depending on the type of donors or acceptors. Donor-bound excitons

(DBE) and acceptor-bound excitons (ABE) have been reported in the range of

3.348 – 3.374 eV and 3.3481 – 3.3564 eV respectively.

• deep centres which are located deep within the band gap. Emission from these

deep centres is usually in the visible light range as compared to the intrinsic

emission in the near UV region. These deep centres can be intrinsic due to

native defects, such as oxygen vacancies or zinc interstitials, or extrinsic defects

such as Cu, Fe, Co or Mn.

Several excellent reviews on ZnO PL have been published [69, 75, 76] and a summary of

the important optical transitions and their corresponding energy as well as wavelength

ranges is provided in Fig 3.3 (c).

43

Figure 3.3 (a) Band structure and symmetries of wurtzite ZnO. The splitting into three valence bands (A, B and C) is caused by field and spin-orbit splitting [75]. (b) Schematic drawing of the exciton states. (c) Summary of various optical transitions near the band gap and their corresponding energy and wavelength ranges.

The number of observed bound exciton peaks in ZnO nanostructures is typically lower

than in ZnO single crystals [76]. This is shown in Fig 3.4 which compares the low-

temperature PL spectra of (a) a bulk single crystal ZnO [77] and (b) ZnO nanorods

grown using a solution containing zinc nitrate, hexamethylenetetramine (HMT) and

44

polyethyleneimine (PEI)1 [78] on a pre-coated Si substrate which had been pre-coated

using ZnAc2 solution. Both samples have been annealed in forming gas at 600°C.

Figure 3.4. Comparison of the low-temperature PL (4 K) spectra from (a) a bulk single crystal ZnO grown by VPT [77] and (b) ZnO nanorods grown using a solution containing zinc nitrate, HMT and PEI [78] on a pre-coated Si substrate which had been pre-coated using ZnAc2 solution. Both samples have been annealed in forming gas at 600°C.

When ZnO nanorods are grown in aqueous solutions, UV emission is usually

accompanied by visible emissions [79, 80]. The visible emissions are usually attributed

to intrinsic defects and consists of orange and green components. Orange emission is

typically related to interstitial oxygen while the assignment of green emission remains

controversial as it has been attributed to various defects such as donor-acceptor pair,

oxygen vacancy, anti-site oxygen, zinc interstitials and zinc vacancies [76]. Both orange

and green emission can be present in the same sample [81]. This suggests a large

variety of defects are present in ZnO nanorods that are grown using solution methods.

1 PEI functions as a surfactant to increase the aspect ratio of the nanorods.

45

Furthermore, by examining the changes in PL spectra after annealing the samples in

various ambient gas and temperatures, the type of intrinsic defects can be deduced.

The quality of a ZnO sample can also be evaluated using its PL spectrum near the band

edge region. One prerequisite is the presence and clear distinction of A and B free

exciton peaks below 160 K. Another measure is the ratio between UV emission and the

visible emission from deep level centres [82, 83]. However, the ratio, being dependent

on the excitation density as well as excitation area [84], is not a good determining

factor of the crystalline quality of ZnO. Nonetheless, it is still useful for comparing the

quality of different samples when excitation conditions, such as the excitation laser

power, objective lens magnification, the detector integration time and neutral density

filter settings, are kept constant. One such application is the comparison of the PL

spectra after various thermal annealing conditions, allowing the deduction of type and

evolution of defects.

PL measurements in this work were carried out using a Renishaw 2000 micro-PL setup

with the 325 nm line from a Kimmon He-Cd laser as excitation source. Since the setup

combines a laser souce with a microscope, a spatial resolution of 1 µm or better can be

achieved. This setup is equipped with neutral density filters to decrease the intensity of

incident laser light and prevent the saturation of the photodetector.

3.6 Raman spectroscopy

Raman spectroscopy is based on the inelastic scattering process that occurs due to the

collision between incident photons and a molecule. As shown in Fig 3.5, most of the

incident photons undergo Rayleigh scattering and do not change their energy after

collision with the molecule while a very small percentage of incident photons undergo

inelastic scattering and exchange energy with the molecules upon collision.

46

Figure 3.5. Rayleigh and Raman scattering

If the incident photon delivers (or receives) an hv quantum of energy to the molecule,

the energy of the scattered photon reduces to hvo - hv (or increases to hvo + hv) where

hvo is the energy of the incident photon. The scattered light having the frequency vo-v

and vo+v are known as Stokes Raman scattering and anti-Stokes Raman scattering

respectively.

Raman spectroscopy is theoretically capable of examining the energy changes that

accompany transition between different electronic, vibrational and rotational energy

levels. However, it has been associated almost exclusively with transitions between the

vibrational energy levels. In this work, Raman spectroscopy is used as a vibrational

spectroscopy method to examine the structural properties of ZnO such as crystallinity,

orientation, composition, strain and defects.

ZnO has a wurtzite symmetry and belongs to the C46v space group. There are 4 atoms

per unit cell leading to 12 phonon branches, 9 optical and 3 acoustic. Group theory

predicts that near the center of the Brillouin zone, there are eight sets of phonon

modes: 2E2, 2A1, 2E1 and 2B1. Both E2 sets, one of the A1 set and one of the E1 set are

Raman active while the B1 modes are silent. Furthermore, the A1 and E1 are polar and

split into transverse optical (TO) and longitudinal optical (LO) phonons.

The selection rules on the (0001) surface specifies that the E2 and A1(LO) mode can be

detected when the incident and scattered polarization are parallel, and only the E2

47

mode can be observed when the incident and scattered polarization are perpendicular.

In the case of the {10-10} surfaces, the optical modes E2, A1(TO) and E1(TO) can be

detected, depending on whether the incident and scattered polarization is parallel or

perpendicular [85]. At oblique angles, it is possible to observe the optical mode A1(LO)

[86]. For the case of ZnO nanorods that are randomly oriented, all of the above modes

will be observable. This is the case that will be seen later in Chapter 5 when randomly

oriented ZnO nanorods are grown on Si and glass substrates that have been precoated

with ZnO nanoparticles.

The reported values for the various fundamental and multi-phonon modes are

summarized in Table 3.2. In ZnO nanostructures, the Raman spectra always show a

shift from the bulk phonon frequencies. There are three main mechanisms that can

induce phonon shifts in ZnO nanostructures [87]:

• spatial confinement within nanostructure boundaries that are less than 4 nm

• phonon localization by defects (oxygen deficiency, excess zinc, surface

impurities, etc) leading to a frequency shift of a few cm-1

• laser induced heating in nanostructures leading to a frequency shift of tens of

cm-1

Since spatial confinement will not be applicable for nanorods with diameters of 80 –

200 nm, and if laser induced heating is avoided, the interpretation of frequency shifts

can be focused on structural defects and stresses.

Information on stress and crystalline quality can be extracted from the E2 phonon

frequency and linewidth. An increase in the E2 phonon frequency with respect to the

unstrained ZnO indicates compressive stress, whereas a decrease points to tensile

stress. A narrower linewidth indicates better crystallinity and vice versa. The same

applies to A1(LO) phonon, but with a much smaller magnitude of the frequency and

linewidth variations.

48

In addition to the fundamental modes, several multi-phonon modes and defect-

induced bands have been clearly identified in literature. For example, the 332 cm-1

peak has been ascribed to two E2L phonons from the K-M-Σ around 160 cm-1 and is

denoted as 2E2L(M) [88, 89]. Also, a broad asymmetric band from 540-670 cm-1 has been

assigned as a defect-induced band with strong dependence on oxygen stoichiometry

[90].

In this work, Raman spectra was recorded in a backscattering configuration using a

Jobin Yvon T64000 triple spectrograph micro-Raman setup with the 514.5 nm line of an

Ar ion laser as the excitation source.

By varying the excitation wavelength, selective probing of phonon, either on the

surface or averaged over the bulk, can be achieved. For example, a 514.5 nm (Ar ion

laser) excitation source will return an averaged bulk material properties 325 nm (He-Cd

laser) excitation source only provides information from the top surface layer of

approximately 50-100 nm.

Table 3.2. Frequency and symmetry of the fundamental optical modes in ZnO

Symmetry character

Frequency (cm-1)

Remarks

E2L 101 The lower mode E2L is associated with vibration of heavy Zn sublattice while the higher mode E2H is associated with vibration of O atoms [87]. When compared to the bulk parameters, compressive stress will result in a higher mode frequency while poorer crystal quality gives a larger peak width [85].

E2H 437

E1(TO) 407 Shows strength of polar lattice bonds. A lower mode frequency indicates existence of vacancy point defects which diminishes long range electrostatic forces [91].

A1(TO) 380

E1(LO) 583 Higher crystallinity will result in stronger E1(LO) and A1(LO).

A1(LO) 574

49

3.7 Secondary ion mass spectrometry (SIMS)

SIMS is one of the most sensitive and versatile technique for surface analysis. It is

capable of detecting all elements, including isotopes and molecular species. It can be

used to perform depth profiling, thin film analysis, surface contamination and dopant

diffusion in samples.

In this technique, the ZnO sample is sputtered with a primary ion beam resulting in

destructive removal of material from the sample. The sputtered species are then

analyzed by a mass spectrometer. Only about 1% of the sputtered species, which are

ejected as charged ions, can be detected by the system. The subsequent analysis of the

emitted charged ions provides detailed information on the elemental and molecular

composition of the surface.

Figure 3.6. Depth profiling using a dual beam technique.

Depth profiles of ZnO samples in this work were measured using TOF-SIMS IV from

ION-TOF GmbH. The lateral resolution is typically 100 nm and the depth resolution is 1

nm. A high resolution depth profile is obtained by employing a dual beam technique.

The first beam is a high current Ga ion beam which is used to sputter a crater on the

sample. The second beam is a low current Ar ion beam that progressively analyzes the

bottom of the crater.

50

3.8 Hall effect measurement

Figure 3.7. Schematic of the Hall effect in a long, thin bar of semiconductor with four ohmic contacts. The direction of the magnetic field B is along the z-axis and the sample has a finite thickness d.

The basic physical principle underlying the Hall effect is the Lorentz force. When an

electron moves along a direction perpendicular to an applied magnetic field, it

experiences a force acting normal to both directions and moves in response to this

force and the force effected by the internal electric field.

For an n-type, bar-shaped semiconductor shown in Fig 3.7, the majority of the carriers

is electrons of a bulk density n. When a constant current I flows along the x-axis from

left to right in the presence of a z-directed magnetic field, electrons are subjected to

the Lorentz force and drift towards the negative y-axis, resulting in an excess surface

electrical charge on the side of the sample. This charge results in the Hall voltage (VH), a

potential drop across the two sides of the sample with a magnitude equal to

qnd

IBVH = (3.1)

51

where I is the current, B is the magnetic field, d is the sample thickness, and q is the

elementary charge (1.602 x 10-19 C).

In some cases, it is convenient to use sheet density (ns) instead of bulk density (n). Then,

rearranging Eq. (3.1) gives the equation

H

sVq

IBndn == (3.2)

Thus, by measuring the magnitude of VH and substituting the known values of I, B, and

q, ns can be determined. Furthermore, the type of majority carrier can be determined

by the sign of the VH: negative for n-type semiconductors and positive for p-type

semiconductors.

If the bulk resistivity (ρ) or the sheet resistance (RS) of the semiconductor is known,

then the Hall mobility (µ) can be determined from the following equation:

===

dqn

RqnIBR

V

sSsS

H

ρµ

11 (3.3)

A popular configuration for Hall measurements is the van der Pauw configuration. Its

popularity stems from its ability to measure both resistivity and the Hall voltage of any

arbitrary shape. A schematic of a rectangular van der Pauw configuration is shown in

Fig. 3.8.

52

Figure 3.8. Schematic of a van der Pauw configuration used in the determination of the two characteristic resistances RA and RB.

In resistivity measurements, a direct current I12 is injected into contact 1 and out of

contact 2 and the voltage V43 from contact 4 to contact 3 is measured as shown in Fig.

3.8. Next, a current I23 is injected into contact 2 and out of contact 3 while voltage V14

from contact 1 to contact 4 is measured. The characteristic resistances RA and RB are

calculated by means of the following expressions:

12

43

I

VRA = (3.4a)

23

14

I

VRB = (3.4b)

Van der Pauw has demonstrated that these two characteristic resistances RA and RB,

are related to the sheet resistance RS through the van der Pauw equation

1expexp =

−+

−

S

B

S

A

R

R

R

R ππ (3.5)

53

which can be solved numerically for RS.

Figure 3.9. Schematic of a van der Pauw configuration used in the determination of the Hall voltage VH.

For the Hall voltage measurement, the magnetic field B is kept constant and a constant

current I13 is forced through the opposing pair of contacts 1 and 3 while the VH (= V24) is

measured across contacts 2 and 4. This is shown schematically in Fig. 3.9. Once the Hall

voltage VH is acquired, the sheet carrier density ns can be calculated via Eq. (3.2) from

the known values of I13, B, and q.

To improve accuracy of Hall and resistivity measurements, several precautions are

taken. The lateral dimensions of the sample are large compared to the size of the

contacts (>20X) and the sample thickness (>15X). The contacts should be ohmic within

the measurement range. Ohmic contacts to n-ZnO were made by evaporating Al/Pt or

Ti/Al/Ni/Au while p-ZnO using Ni/Au. The thickness of the sample is kept below 1 mm

with thickness variations of less than 1%. Thermomagnetic effects due to non-uniform

temperature are minimized by injecting a small constant current. Finally,

photoconductive and photovoltaic effects is minimized by measuring in a dark

environment.

54

Hall measurements were done using Accent HL5500. The system applies a permanent

magnetic field of 0.32 T. Measurements are performed in DC mode at room

temperature or over a temperature range from 90 to 300 K. During Hall measurement,

the optimum current is automatically chosen by the system to provide an acceptable

signal to noise ratio, minimize specimen heating and limit potential gradients to 1 V/cm.

3.9 Conclusion

The experimental procedures and apparatus that are used for growing ZnO nanorods in

aqueous solution throughout this thesis have been described. Characterization

techniques such as scanning electron microscopy, Raman and photoluminescence

spectroscopy and Hall effect measurements were discussed. The background theory on

each characterization method as well as relevant ZnO-related characterization results

has been presented.

55

4 Prediction of Length and Density of ZnO

Nanorods on GaN Substrate

4.1 Introduction

The growth of ZnO nanorods in solution on various substrates such as glass and Si, has

been well-documented in various publications. Boyle et al. discussed the influence of

reactants, substrate pretreatment, growth time and temperature on the growth of

flower-like ZnO nanowires [92]. Govender et al compared the different growth

precursors and discussed the influence of reaction conditions, ligands, counter-ions, pH,

ionic strength, deposition time, influence of substrate and seed layers [50]. Tian et al

described the reduction of aspect ratio through the addition of citrate anions [93] while

Zhang et al discussed the influence of pH on growth solution for a zinc acetate, sodium

hydroxide and citric acid growth system [94, 95]. As can be seen in Table 4.1 which

summarizes the growth of ZnO nanostructures in aqueous solution, there is a

bewildering array of possible growth factors. One reason for this is that ZnO

nanostructures can be easily synthesized from a wide variety of precursors and the

growth habit can be modified by a large number of surfactants. Another reason is that

most of the identified factors are the ‘second-order’ growth drivers, which can lead to

many possible combinations.

It is well-known that the density and length of ZnO nanorods depends on the initial

nucleation and subsequent growth, both of which can be thermodynamically modeled

based on the Gibbs free energy. As described in Chapter 2, for a fixed growth system

and substrate surface, the Gibbs free energy depends on the growth temperature, the

interfacial energy and the solubility of zinc. The solubility of zinc indirectly includes the

effect of pH, effects of reactant concentrations and the effect growth temperature on

the speciation of the growth solution. Unlike the solution pH which can be easily

measured experimentally, the solubility can only be obtained by solving the ionic

56

equilibrium of the growth solution. For each different growth precursor, all the possible

chemical species need to be identified and a new ionic equilibrium computed. This

process is tedious and sometimes the required thermodynamic data, and hence the

equilibrium rate constants are not available for computation. Due to these difficulties,

the solubility of zinc is not a popular growth factor even though it may provide a better

prediction of the length and density of the nanorods.

Table 4.1. Summary of different results and methods for aqueous solution growth.

Growth solution Resulting morphology Focus of investigation Zinc nitrate, HMT Nanorods, microtubes On Si and conducting glass

substrates [48] Zinc nitrate, HMT, Nanorods, nanotubes Influence of substrate and

seed layer [96] Zinc nitrate, HMT Aligned nanowire arrays Influence of seed layer [62] Zinc-nitrate, HMT, citrate Oriented nanocolumns,

nanoplates Control of aspect ratio: addition of citrate anions decreases aspect ratio [93]

Zinc nitrate, zinc acetate, HMT

Highly aligned nanorods Influence of substrate and seed layer [97]

Zinc nitrate, triethanolamine, HCl (pH 5)

Ordered nanorods Influence of substrate and counter ions in growth solution [92]

Zinc nitrate, thiourea, ammonium chloride, ammonia

Nanowires, tower-like, flower-like, tube-like

Influence of reactants, substrate pretreatment, and growth time and temperature [98]

Zinc acetate, sodium hydroxide, citric acid

Disk-like, flower-like, nanorods

Influence of pH on growth solution [94, 95]

Comparison of different growth solutions

Star-like,nanorods Influence of reaction conditions: ligand, counter-ions, pH, ionic strength, and deposition time Influence of substrate/seed layer [50]

Zn foil, zinc sulfate, ammonium ions, sodium hydroxide

Nanobelt arrays, ordered nanowires

Influence of temperature and concentration of solutions [99]

Zinc acetate, ammonia Nanorods Influence of temperature, precursor molar ratios [80]

57

In this chapter, we will focus on the ZnAc2-NH4OH growth system because the

experimental data is readily available from Le et al’s work [80] and the ionic

equilibrium model has already been solved in Chapter 2. We will also attempt to

replace the ‘second-order’ growth drivers such as pH and degree of supersaturation

with the more fundamental factors, namely the solubility of zinc and growth

temperature. To do this, the solubility is calculated and compared with the

experimental data by Le et al. who has reported on the trends in the density and length

of hydrothermally-grown ZnO nanorods when precursor concentrations, molar ratios

and growth temperature were varied [80]. Then, a quantitative relationship between

the solubility and temperature is empirically derived for the ZnO nanorods density and

length. Finally a growth process map for length and density prediction is presented.

Although the work in this chapter focuses on GaN, it can readily be applied to any other

lattice-matched substrates.

4.2 Experimental Procedure

The ZnO nanorods were grown on a GaN epilayer. The GaN epilayer, which is 3 µm

thick, was grown on c-plane sapphire at 1020°C using an EMCORE D125 MOCVD.

Trimethylgallium and NH3 were used as the Ga and N source, respectively.

The growth methodology of ZnO nanorods in aqueous solution follows the

experimental procedure as described in Chapter 3 Section 3.2.2. Since the GaN epilayer

has a very low lattice mismatch with ZnO, no pre-coating was performed.

Three groups of experiments were carried out. The first group studies the effect of

growth temperature on the morphology of ZnO nanorods. The next two groups of

experiment aim to study the morphology dependence on reactant concentrations by

systematically varying the concentrations of ZnAc2 and NH4OH.

The three groups of experiments are as follows:

• Varying growth temperature: the growth temperature is varied from 60 to

150°C while maintaining the growth duration at 4 hours and the concentrations

of Zn(Ac)2 and NH4OH at 0.016 and 0.173 M respectively.

58

• Varying the ratio of reactant concentration: [Zn(Ac)2] is maintained at the same

concentration as above at 0.016 M while the [NH4OH] is varied from 0.1 to 0.4

M.

• Increasing reactant concentrations with a constant concentration ratio: the

concentrations of Zn(Ac)2 and NH4OH are increased from 0.01 to 0.033 M in

order to maintain a constant molar ratio [NH4OH]/[Zn(Ac)2] of 6.27.

The growth duration for all three groups of experiments was maintained at 4 h.

After growth, the ZnO nanorods were examined using a field emission scanning

electron microscope. The top and cross-sectional views of the SEM images were used

to obtain the density and length of the ZnO nanorods respectively. The density of the

ZnO is defined as the number of nanorods in an area of 1 cm2. This was calculated by

counting the number of rods in a randomly chosen area of 5 µm2 in the top view

scanning electron micrograph.

4.3 Results

Fig 4.1 shows the SEM images of the ZnO nanorod for the first group of experiments

where the growth temperatures are (a) 60°C, (b) 80°C, (c) 100°C and (d) 150°C. When

the length and density of the nanorods are plotted on a temperature axis, both

parameters show an initial increase up to 95°C followed by a decrease at higher

temperatures, as shown by the curves (a) and (b) in Fig 4.2 respectively.

Figure 4.1. SEM images of ZnO nanorods grown at temperatures (a) 60(b) 80°C, (c) 100°C and (d) 150and 0.173 M NH4OH

Figure 4.2. The effect of growth temperature on (a) length and (b) density of ZnO nanorods.

SEM images of ZnO nanorods grown at temperatures (a) 60C and (d) 150°C in solutions containing 0.016

OH.

The effect of growth temperature on (a) length and (b) density

59

SEM images of ZnO nanorods grown at temperatures (a) 60°C, C in solutions containing 0.016 M Zn(Ac)2

The effect of growth temperature on (a) length and (b) density

Figure 4.3. SEM images of ZnO nanorods with different molar ratios0.016 M Zn(Ac)2, 0.1 M0.016 M Zn(Ac)2, 0.204

Figure 4.4. Effect of molar ratio on (a) length and (b) density of ZnO nanorods. The concentration of Zn(Ac)concentration of NHratio.

Figs 4.3 and 4.4 show the results from

ratio of reactant concentration is varied. From the trend plots of the density and length

in Fig 4.4 (a) and (b) respectively, an increasing ratio of [NH

lower density and longer length

SEM images of ZnO nanorods with different molar ratios0.1 M NH4OH, (b) 0.016 M Zn(Ac)2, 0.143 M

, 0.204 M NH4OH and (d) 0.016 M Zn(Ac)2, 0.306

Effect of molar ratio on (a) length and (b) density of ZnO ds. The concentration of Zn(Ac)2 is kept constant at 0.016

concentration of NH4OH is varied from 0.1 M to 0.4 M to increase the molar

s 4.3 and 4.4 show the results from the second group of experiments where the


4.4 (a) and (b) respectively, an increasing ratio of [NH4OH]/[Zn(Ac)

lower density and longer length of the ZnO nanorods.

60

SEM images of ZnO nanorods with different molar ratios: (a) M NH4OH, (c)

0.306 M NH4OH.

Effect of molar ratio on (a) length and (b) density of ZnO is kept constant at 0.016 M and the

M to increase the molar

the second group of experiments where the


OH]/[Zn(Ac)2] leads to a

61

Finally, Fig 4.5 shows the trend plots of the length and density for the third group of

experiments when the reactant concentrations are increased while maintaining a

constant concentration ratio of 6.27. Fig 4.5 shows that a higher concentration of

precursors increases the length and reduces the density.

Figure 4.5. Effect of increasing concentration of precursors while maintaining a constant molar ratio on (a) length and (b) density of ZnO nanorods. Zn(Ac)2 is increased from 0.01 to 0.03 M , and concentration of ammonia by a proportional amount to maintain a constant molar ratio of 6.27.

4.4 Discussion

The effects of the various growth variables on density and length of ZnO nanorods are

qualitatively summarized in Table 4.2. Table 4.2 shows that that the density and length

appears to be inversely correlated to each other.

To go beyond these qualitative observations, we will need to tease out more

information from the set of experiments. One way is to compute the ionic equilibrium

of the Zn(Ac)2-NH4OH system because the computation results provide the

concentrations of all the possible zinc complex species and the solubility of zinc which

can provide further insight to the roles the major growth species, solubility and

temperature play in the growth and nucleation processes. A detailed description of the

62

chemical equilibriums and the calculations that are required to obtain the equilibrium

concentrations of the various chemical species in the growth solution have been

presented earlier in Section 2.3.

Table 4.2. Summary of effects of temperature and reactant concentrations on density and length of ZnO nanorods.

Growth variable Density Length Temperature Increase up from 60 to

95 °C. Decrease at higher temperatures

Increase up from 60 to 95 °C. Decrease

at higher temperatures

Increase reactant molar ratio [NH4OH]/[ZnAc2]

Decrease Increase

Increase reactant concentrations with a constant ratio of [NH4OH]/[ZnAc2]

Decrease Increase

In the discussion that follows, we will show how the trends of length and density in

Table 4.2 can be explained by just two variables; namely solubility of zinc and

temperature. The discussion will begin by examining the experiments that are

conducted at the same growth temperature in order to isolate the temperature effects

and focus on the solubility effects. Once the relationships between solubility and length

as well as density are established, the temperature effects will be introduced and the

relationships will be modified accordingly.

4.5 Effect of Solubility of Zinc on Density and Length of

ZnO Nanorod Arrays

The length data in Figs 4.4 and 4.5 are combined in Fig 4.6 such that the length is now

plotted against the solubility of zinc for the fixed and varying reactant concentration

ratio experiments. The data points for varying the ratio of reactant concentrations are

represented by �, while the increasing reactant concentrations with a constant ratio

by �. For both group of experiments, the

relationship with the solubility of zinc ions

reservoir of growth units in the solution which in turn leads to longer lengths. When

[ZnAc2] is kept constant, a higher [NH

turn leads to a higher solubility and longer lengths as shown in

hand, when both [NH4OH] and [ZnAc

maintain constant ratio of [NH

and complexing agents which leads to a higher solubility and longer lengths as seen in

Fig 4.5. However, the relationship between the reactant concentrations and solubility

are non-linear. This can be seen in

unchanged when the molar ratio is increased beyond 8 and when [ZnAc

beyond 0.025 M at constant molar ratio.

Figure 4.6. Logarithm of ZnO nanorods lengths plotted against the total concentration of zinc ions in points when the Zn(Ac)2 concentration is kept constant at 0.016NH4OH concentration is varied from 0.1 to 0.4data points when concentration of Zn(Ac)2 is increased from 0.01 M with a constant molar ratio [NH4+]/[Zn2+]. The growth temperature is kept constant at 373

group of experiments, the length of the nanorods shows a

relationship with the solubility of zinc ions. A higher solubility provides a larger


] is kept constant, a higher [NH4OH] provides more complexing agents which in

turn leads to a higher solubility and longer lengths as shown in Fig 4.4. On the other

OH] and [ZnAc2] are increased in such a manner so as to

maintain constant ratio of [NH4OH]:[ZnAc2], there is an increasing concentration of Zn


4.5. However, the relationship between the reactant concentrations and solubility

linear. This can be seen in Figs 4.4 and 4.5 where the length remains relatively

unchanged when the molar ratio is increased beyond 8 and when [ZnAc

beyond 0.025 M at constant molar ratio.

Logarithm of ZnO nanorods lengths plotted against the total concentration of zinc ions in the precursor solution. � represents the data points when the Zn(Ac)2 concentration is kept constant at 0.016NH4OH concentration is varied from 0.1 to 0.4 M while � represents the data points when concentration of Zn(Ac)2 is increased from 0.01 M with a constant molar ratio [NH4+]/[Zn2+]. The growth temperature is kept constant at 373 K.

63

shows a positive

. A higher solubility provides a larger


omplexing agents which in

4.4. On the other

] are increased in such a manner so as to

concentration of Zn2+


4.5. However, the relationship between the reactant concentrations and solubility

e length remains relatively

unchanged when the molar ratio is increased beyond 8 and when [ZnAc2] is increased

Logarithm of ZnO nanorods lengths plotted against the total represents the data

points when the Zn(Ac)2 concentration is kept constant at 0.016 M and the represents the

data points when concentration of Zn(Ac)2 is increased from 0.01 to 0.033 M with a constant molar ratio [NH4+]/[Zn2+]. The growth temperature is

Similarly, the density data in

logarithm of density (B), log(B) is plotted against the solubility

negative correlation with the solubility

nucleation because a lower solubility of zinc

energy change which encourages nucleation and thus increases the den

nanorods.

Figure 4.7. Logarithm of rod density (cmconcentration of zinc ions in the precursor solution. The inset shows the corresponding initial degree of supersaturation of zinc in the precursor solution at the growth temperature 373K.ratio of reactant concentrations are represented by reactant concentrations with a constant ratio by

The consistency of data fitting showed by solubility in

reproduced using other commonly quoted growth parameters such as the degree of

supersaturation of zinc and pH.

For example, the inset of Fig

supersaturation and solubility. While the density

correlation, there is no clear correlation between the corresponding degree of

Similarly, the density data in Figs 4.4 and 4.5 are combined in Fig 4.7 where

, log(B) is plotted against the solubility. The density shows a

negative correlation with the solubility. This is expected from the thermodynamics of

lower solubility of zinc provides a more favorable Gibbs free

energy change which encourages nucleation and thus increases the den

Logarithm of rod density (cm-2) plotted against the total concentration of zinc ions in the precursor solution. The inset shows the corresponding initial degree of supersaturation of zinc in the precursor

he growth temperature 373K. The data points for varying the ratio of reactant concentrations are represented by �, while the increasing reactant concentrations with a constant ratio by �.

The consistency of data fitting showed by solubility in Figs 4.6 and 4.7


supersaturation of zinc and pH.

Fig 4.7 shows the relationship between the de

supersaturation and solubility. While the density and solubility shows a clear negative

no clear correlation between the corresponding degree of

64

where the

he density shows a

. This is expected from the thermodynamics of

a more favorable Gibbs free

energy change which encourages nucleation and thus increases the density of the

) plotted against the total concentration of zinc ions in the precursor solution. The inset shows the corresponding initial degree of supersaturation of zinc in the precursor

The data points for varying the , while the increasing

cannot be


degree of

and solubility shows a clear negative

no clear correlation between the corresponding degree of

65

supersaturation and the solubility. The difficulty in applying the degree of

supersaturation as a density predictor arises from the two possible modes of

nucleation: heterogeneous and homogeneous. Heterogeneous nucleation occurs for a

slight degree of supersaturation, i.e. when the initial concentration of ZnAc2 is close to

the solubility limit. Higher levels of supersaturation tend to produce instantaneous

homogeneous nucleation in the solution which quickly brings the degree of

supersaturation down close to the solubility limit. This explains why solubility is much

more successful than the degree of supersaturation in predicting the density, as well as

the length of the nanorods.

Solution pH, by itself, is not a consistent predictor of length and density. In Fig 4.4, the

increasing length with NH4OH suggests that length increases with pH. On the other

hand, in Fig 4.5 where a constant molar ratio is maintained which in turn results in a

relatively constant pH, the length did not remain constant as expected. A similar

contradiction is obtained when Figs 4.4 and 4.5 are examined. Fig 4.4 suggests a

decreasing density with increasing pH while a constant pH in Fig 5 fails to maintain a

constant density.

The ability of solubility to provide a consistent correlation for both length and density

of the nanorods suggests that solubility is a more fundamental parameter for

prediction of length and density. In fact, quantitative expressions can be obtained from

Figs 4.6 and 4.7 to relate the length and density to the solubility of zinc in the growth

solution:

m

ZnL SAL 1= [10]

n

ZnB SAB 1= [11]

where AB1, AL1, m and n are constants. The units for L, B, and SZn are nm, cm-2 and mol/l

respectively. The best fit is obtained when AL1 = 8.710 x 108, AB1 = 1.047 x 10-8, m = 1.81

and n = -5.22 as shown by the solid line in Figs 4.7 and 4.8.

66

4.6 Effect of Temperature on Density and Length

In this section, we will examine how the temperature affects the length and density of

the nanorods. When the growth temperature increases from 333K to 423 K, the

solubility of zinc is expected to increase. Since the concentration of ZnAc2 is kept

constant, the degree of supersaturation of zinc in the solution is expected to decrease

as shown in the inset of Fig 4.9. Based on equations [10] and [11], the length is

expected to increase while the density, decrease. However, experimental data in Fig

4.2 shows otherwise. This suggests that an additional temperature factor needs to be

included.

As mentioned earlier, the critical free energy for nucleation in Equation (2.19) is

dependent on solubility and growth temperature. While the temperature-dependence

of solubility has been explicitly included into the equilibrium rate constants the using

equation (2.13), the growth temperature factor has not yet been included in Equations

(10) and (11). In order to take into account this additional temperature dependence,

equations [10] and [11] are rewritten as

+= 21 expL

aLm

ZnLA

kT

ESAL (12)

+= 21 expB

aBn

ZnBA

kT

ESAB (13)

where k = 8.62×10-5 eV.K-1 is the Boltzmann constant, T is the growth temperature in

Kelvins, and EaL, EaB, m, n, AB1, AB2, AL1 and AL2 are constants to be determined by curve

fitting to experimental data.

Taking the natural logarithm on both sides and rearranging, we arrive at

2

1

ln L

aL

m

ZnL

AkT

E

SA

L+=

[14]

2

1

ln B

aB

n

ZnB

AkT

E

SA

B+=

[15]

Curves (a) and (b) in Fig 4.

plotted against T1 , using the values of

best fit is obtained when E

Figure 4.8. Plot of (a) Y

� and � represent the density and length datatemperature is varied from 60 to 150supersaturation of zinc, S, against temperature for a precursor solution containing 0.016 M Zn(Ac)2

The activation energy for length in Equation (12) is positive while density in Equation

(13) is negative. This implies that higher temperatures lead to a shorter length and

higher density. On the other hand, from ionic equilibrium calculation, we know that

higher temperatures increase the solubility which in turn leads to longer lengths and

lower density. The opposing effects of temperature and solubility gives rise to an

optimal point where maximum density and length is obtained. This optimal point is at

100 °C as shown in Fig 4.2.

A physical understanding of the activation energies of length and density can be

obtained by recalling that a

increases the nucleation rate. A higher nucleation rate directly le

4.8 show ( )[ ]n

ZnB SAB 1ln and ( )[ ]m

ZnL SAL 1ln , respectively,

, using the values of m, n, AB1 and AL1 obtained earlier. The line of

EaL = 0.77 eV, EaB = -2.11eV, AL2=-24.3 and AB2

Plot of (a) Y = ( )[ ]n

ZnB SAB 1ln and (b) Y= ( )[ ]m

ZnL SAL 1ln

represent the density and length data points respectively when temperature is varied from 60 to 150°C. The inset shows the degree of supersaturation of zinc, S, against temperature for a precursor solution

M Zn(Ac)2 and 0.173 M NH4OH.







4.2.


obtained by recalling that a higher temperature reduces the critical radius an

increases the nucleation rate. A higher nucleation rate directly leads to a higher

67

, respectively,

obtained earlier. The line of

B2 = 67.2.

against 1/T.

points respectively when C. The inset shows the degree of

supersaturation of zinc, S, against temperature for a precursor solution








higher temperature reduces the critical radius and

ads to a higher

68

nanorod density. As more zinc ions are nucleated on the substrate surface, less zinc

ions are available in the solution for subsequent growth. Furthermore, the remaining

zinc ions in the solution will be distributed over a larger number nucleation sites. Both

these factors lead to shorter nanorods.

4.7 ZnO Nanorod Length and Density Maps

The length, L, and density, B, defined using equations (12)-(13), the parameters of m, n,

Ea, A1 and A2 as determined by the best fit to the experimental data are summarized

below.

+= 21 expL

aLm

ZnLA

kT

ESAL (4.16)

+= 21 expB

aBn

ZnBA

kT

ESAB (4.17)

where m = 1.81, n = -5.22, AL1 = 8.710 x 108, AL2 = -24.3, AB1=1.047 x 10-8, AB2=67.2, k =

8.62×10-5 eV.K-1, EaL = 0.77eV, EaB = -2.11eV. The units for B, L and SZn are cm-2, nm and

mol/l respectively.

These equations provide a convenient relative measure of length and density for a

given precursor concentration and temperature and is useful for prediction of the

length and density of ZnO nanorods for various growth conditions within the validity

range. In fact, the same approach can be applied for other lattice-matched substrates.

It's worth noting that the constants AB1 and AL1 are interface related constants as we

shall see in the next chapter. If the surface of the substrate is rougher, then the AB1 is

expected to be larger while AL1 smaller, leading to a higher density and shorter length

of nanorods.

Contour plots of log(B) and L at 373 K are shown in Fig 4.9 (a) and (b) respectively. The

validity ranges of pH and S are also indicated on the plots. Several important points can

be obtained from Fig 4.9:

69

1. The maximum achievable density for the ZnAc2-NH3 system is about 109.5 cm-2 with

a corresponding length of less than 1 µm. Higher densities of nanorods can be

obtained by growing in a pH less than 9.7 at the expense of poor substrate

coverage. It is also possible that higher densities of nanorods is likely to result in

coalescence of rods and formation of a film. One way to enhance the density is to

use surfactants such as PEI or AlCl3 to increase the aspect ratio of the rods.

2. The maximum length is about 8 µm with a corresponding density less than 107 cm-2.

Longer lengths can be obtained simply by extending the growth duration beyond 4h

or refreshing the growth solution after 4h. Due to the low density of rods, it is

expected that several growth cycles can be employed to extend the rod lengths.

3. It is clear that the densities and length cannot be maximized simultaneously in a

single growth step. In order to maximize density and length, a two-step process

may be required where the substrate is grown in a “high-density” solution and then

transferred to a second solution to maximize the length of the rods.

4.8 Limitations of Model

The parameters m, n, EaB, EaL, AB1, AB2, AL1 and AL2 are obtained through curve-fitting of

experimental data to the calculated equilibrium concentration of zinc in the solution.

Therefore, the validity of the expressions developed are limited to experimental

conditions, consisting of a temperature range between 333K to 423K, a solution pH

range from 9.7 to 10.6 and a degree of supersaturation between 20 and 60.

Furthermore, this model was based on the growth of ZnO on Ga-face of GaN substrates

grown on sapphire by MOCVD. For other lattice matched substrates or GaN substrates

with different surface roughness, the exact value of the constants will be different

because of different interface properties. However, the trend of density and length of

nanorods based on solubility can still be applied.

70

Figure 4.9. Black lines show the contour plot of (a) log[B(cm-2)] and (b) length (nm) for various concentrations of ZnAc2 and NH4OH. The validity limits for pH between 9.7 and 10.6, and degree of supersaturation of zinc between 20 and 60 are shown in red and blue lines respectively.

71

4.9 Conclusion

In summary, we have shown that solubility and temperature provides a better

prediction than other commonly used parameters such as pH and degree of

supersaturation.

We have also provided an empirical model to predict the density and length of

hydrothermally-grown ZnO nanorods using zinc acetate and ammonium hydroxide as

reagents, on the Ga-face of an unintentionally doped GaN substrate for a growth

temperature ranging from 333 to 423 K. We have identified solubility of zinc ions and

temperature as the main factors affecting the density and lengths of the ZnO nanorods.

The density can be maximized by minimizing the solubility of zinc and increasing the

growth temperature, while the length, by maximizing the solubility and lowering the

growth temperature. The activation energy for density and length is found to be -2.11

eV and 0.77 eV respectively. Due to opposing dependence on solubility and

temperature, it is not possible to maximize density and length of nanorods in a single

growth step. Finally, growth maps indicating the predicted density and length of ZnO

nanorods on GaN substrates have been provided for a range of precursor

concentration.

72

5 Growth and Defects of ZnO Nanorods

Grown from a ZnO Seed Layer

5.1 Introduction

In Chapter 4, we have shown that solubility is a better predictor than pH and

supersaturation for the growth of ZnO nanorods on lattice-matched substrates. Here,

we will extend the application of solubility to predict the morphology and density of

non-lattice-matched substrates that have been pre-coated with a layer of ZnO

nanoparticles. In addition to solubility, we will also show that the interfacial properties

such as the surface roughness and surface charge, and the type of growth species are

important mechanisms that affect the growth habit and growth rate of ZnO in aqueous

solution.

Knowledge of the growth mechanisms provides the key to identifying and reducing the

defects in ZnO. The as-grown ZnO using solution methods have been reported to have

a high concentration of defects [17]. As a result, post-annealing treatments are often

required to improve the crystal quality and remove the defects in the structure.

Studenkin et al showed that orange and green PL emissions are due to oxygen-rich and

oxygen-deficient growth conditions respectively. He further claimed that these defects

are complementary and cannot exist simultaneously [100]. Meanwhile, Djurisic et al.

review the growth and defects in ZnO nanostructures and attributed the yellow PL

emission, that are typically observed in solution-grown nanorods, to oxygen interstitials

and Li impurities [76]. In general, the assignment of orange or yellow emission to

interstitial oxygen defects is widely accepted, regardless of the growth method.

However, the assignment of green emission remains open and controversial. Among

the possible assignments are singly ionized oxygen vacancies, anti-site oxygen, oxygen

73

vacancies, zinc interstitials, Cu impurities, donor-acceptor transitions and zinc

vacancies [69, 76]. The large number of candidates suggests that the green emission is

very sensitive to the fabrication method, growth conditions and the post-growth

treatments.

Contrary to Studenkin’s report, we will show in this chapter that both orange and blue

emission can be present simultaneously when ZnO is grown using solution methods.

We will also show that the high defect density in nanorods is a consequence of growing

in the pH range of 10.5-11 and propose a method to suppress these visible emissions

without any post-annealing treatment. Furthermore, we will attempt to identify the

defects in our samples based on Raman, PL and post-annealing data. Finally we will

relate the origin of the defects to the growth mechanisms based on solubility,

interfacial properties and growth species in aqueous solution.

5.2 Experimental Procedure

The ZnO nanorods were grown on silicon substrates. Prior to growth in solution, the

substrates are pre-coated with a layer of ZnO nanoparticles. The presence of this layer

of ZnO nanoparticles on the substrate surface reduces the significance of the substrate

orientation. The pre-coat and growth procedure follows the experimental procedure as

described in Chapter 3 Sections 3.2.1 and 3.2.2.

The two groups of experiments were carried out. The first group of experiments

explores the growth of ZnO over a wide range of pH by varying the concentration of

NH4OH while keeping the concentration of ZnAc2 fixed. The second group of

experiments systematically investigates the effect of varying the reactant

concentrations over a narrow range of pH. Their descriptions are as follows:

• Growth over a wide pH range: the concentration of NH4OH is varied from 0.02

to 1.1 M while the concentration the ZnAc2 is kept constant at 0.02 M.

• Systematic growth over a narrow pH range from 10.4 to 10.8: the concentration

of ZnAc2 is varied from 0.01 to 0.03 M and NH4OH from 0.4 to 1.1 M.

74

The growth durations for all three groups of experiments were maintained at 3 h at

97°C.

After growth, the ZnO nanorods were examined using a field emission scanning

electron microscope. The density of the ZnO nanorods is estimated in a similar manner

as described in Chapter 4. The crystal quality and defects of the ZnO nanorods were

studied using Raman scattering and PL measurements. Raman spectra were recorded

in a backscattering configuration using a JY-T64000 micro-Raman setup with the 514.5

nm line of an Ar ion laser as the excitation source. PL measurements were carried out

using a Renishaw 2000 micro-PL setup with the 325 nm line from a He–Cd laser as

excitation source. To study the effect of annealing on the defect PL, the samples were

annealed inside a rapid thermal annealing chamber at various temperatures.

5.3 Results

Fig 5.1 shows the morphology of the rods when the concentration of NH4OH is varied

from 0.02 M to 1.1 M, while the concentration of ZnAc2 is kept constant at 0.02 M. The

corresponding initial pH values of the growth solutions are shown in square

parentheses.

Fig 5.2 shows the images from the second set of experiments where the concentrations

of both ZnAc2 and NH4OH were varied systematically over the range of 0.01 to 0.03 M

and 0.4 to 1.1 M respectively.

From Fig 5.2, three distinct types of morphology can be seen:

• uniform coverage of nanorods is observed in the first column with 0.4 M NH4OH,

with an increase in nanorods density when [ZnAc2] is increased from 0.01 to 0.03 M.

• a mixture of small nanorods and large rods in the second column with 0.8 M NH4OH.

• large rods with 1.1 M NH4OH. Coverage improves and density increases when

[ZnAc2] is increased from 0.01 to 0.03 M.

Figure 5.1. SEM morphology of ZnO nanorods grownpre-coat of ZnO nanoparticlesand (a) 0.02 M, (b) 0.04 M, (c) 0.1 M, (d) 0.3 M, (e) 0.4 M NH4OH. The concentration of pH values in square parentheses are indicated on the top left corner. Scale bar shows 1 µm.

Figure 5.2. SEM image showing the morphologies of ZnO nanorods grown in various concentrations of ZnAcwith 0.4 M, 0.8 M and 1.1 M NHat 0.01 M. (d), (e) and (f) were grown with 0.4 M, 0.8 M and 1.1 M NHrespectively while keeping ZnAcwith 0.4 M, 0.8 M and 1.1 M NHat 0.03 M. The scale bar is 1 magnification.

SEM morphology of ZnO nanorods grown on Si substrates with a coat of ZnO nanoparticles using growth solutions with 0.02 M

and (a) 0.02 M, (b) 0.04 M, (c) 0.1 M, (d) 0.3 M, (e) 0.4 M and (f) 1.1 M . The concentration of NH4OH and the corresponding initial solution

pH values in square parentheses are indicated on the top left corner. Scale

SEM image showing the morphologies of ZnO nanorods grown in various concentrations of ZnAc2 and NH4OH. (a), (b) and (c) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH respectively while keeping ZnAcat 0.01 M. (d), (e) and (f) were grown with 0.4 M, 0.8 M and 1.1 M NHrespectively while keeping ZnAc2 fixed at 0.02 M. (g), (h) and (i) were grown

M and 1.1 M NH4OH respectively while keeping ZnAcThe scale bar is 1 µm and all images were taken with the same

75

on Si substrates with a 0.02 M ZnAc2 and (f) 1.1 M

the corresponding initial solution pH values in square parentheses are indicated on the top left corner. Scale

SEM image showing the morphologies of ZnO nanorods grown d (c) were grown

OH respectively while keeping ZnAc2 fixed at 0.01 M. (d), (e) and (f) were grown with 0.4 M, 0.8 M and 1.1 M NH4OH

fixed at 0.02 M. (g), (h) and (i) were grown OH respectively while keeping ZnAc2 fixed

m and all images were taken with the same

76

Fig 5.3 shows the Raman spectra of the ZnO grown in 0.02 M ZnAc2 and 0.4, 0.8 and 1.1

M NH4OH, corresponding to the pH range of 10.6 to 10.8. The corresponding SEM

images of these samples are shown in Figs 5.2(d), (e) and (f). A clean glass substrate

was used so that only the Raman peaks attributable to ZnO will be observed. The

observed peaks at about 332, 380, 413 and 439 cm-1 are assigned to 2E2L(M), A1(TO), E1(TO)

and E2H [86, 101]. In addition, a broad asymmetrical peak at 580 cm-1 has been

attributed to a defect-induced mode and will be discussed later.

The E2H peak position for 0.4, 0.8 and 1.1 M NH4OH are 439.7, 440.5 and 441.1 cm-1

respectively as shown in the inset of Fig 5.3. When compared against the E2H peak of

unstrained bulk ZnO at 439 cm-1, the increasing frequency indicates progressively

higher compressive stresses in ZnO nanorods with higher concentration of NH4OH in

the growth solution.

The width of the E2 peak increases with the concentration of NH4OH, indicating a

poorer crystal quality at higher pH. As all the samples were grown on similar glass

substrates, the increase compressive stress in the structure cannot be fully explained

by lattice mismatch between glass substrate and ZnO. It is likely that the compressive

stress arises from increased concentration of defects, such as zinc or oxygen

interstitials, incorporated into the structure during growth.

The broad asymmetric peak, which is centered at 580 cm-1, is strongest when

crystallinity is poorest at 1.1M NH3. This peak is unlikely to be A1(LO) or E1(LO) as LO

phonons becomes stronger with higher crystallinity. We attribute this peak to a defect-

induced band which have been reported to have a strong dependence of on the oxygen

stoichiometry [90]. In this case, the concentration of defects increases with the

concentration of NH4OH in the growth solution, agreeing with the trend of E2H band.

Finally, the multi-phonon band at 332 cm-1 is ascribed to E2L [102]. The low frequency

E2L is associated with the vibration of the heavy zinc sublattice while the high frequency

E2H mode involves only the oxygen atoms. With the E2H peak normalized, the E2L peak is

seen to increase with the amount of NH4OH in the growth solution. This can be

77

attributed to a more ordered zinc sublattice and/or a more disordered oxygen

sublattice.

Figure 5.3. The Raman spectra measured from samples grown with 0.4, 0.8 and 1.1 M NH4OH on a glass substrate. Inset shows the shift of the E2H peak to higher frequencies as concentration of NH4OH is increased.

Figure 5.4. Photoluminescence spectra recorded from samples grown in 0.02 M (black line), 0.04 M (blue line), 0.3 M (green line) and 1.1 M NH4OH (red line) while the concentration of ZnAc2 is kept constant at 0.02 M.

78

Figure 5.5. PL spectra of sample grown in high pH (10.7) after annealing at various temperatures in (a) air and (b) nitrogen ambient, as well as low pH sample (7) annealed in (c) air and (d) nitrogen ambient. The sharp peak at 650 nm is due to the doubling of the 325 nm laser line and should be ignored.

Fig 5.4 shows the PL spectra for samples grown at various NH4OH concentrations while

the concentration of ZnAc2 is fixed at 0.02 M. The visible defect emission decreases

while the ultraviolet band edge emission increases when the concentration of NH4OH is

reduced. A marked improvement of the ratio between the UV and visible emission can

be seen when the growth solution pH is less than PZC. The ability to suppress the

visible emission without any post-annealing treatment suggests that it is possible to

control the type and concentration of defects by carefully controlling the growth

conditions in aqueous solution.

Fig 5.5 shows the changes in PL spectra when samples are annealed in air and nitrogen

ambient at various temperatures. The spectra consists of two major components, a UV

79

component centered at 380 nm and a broad visible component which can be separated

into green and orange emission bands centered at 500 - 510 nm and 640 - 650 nm

respectively. The intensities are shown in logarithmic scale. The sharp peak at 650 nm

is due to the doubling of the 325 nm laser line and should be ignored. Two samples are

studied: the first sample is grown at a high pH of approximately 10.7, using 0.02 M

ZnAc2 and 0.8 M NH4OH while the second sample is grown at a low pH of

approximately 7, using 0.02 M ZnAc2 and 0.02 M NH4OH.

The variation of the PL of the sample grown in high pH when annealed in air and

nitrogen ambient is shown in Fig 5.5 (a) and (b) respectively. In air, the orange emission

does not change significantly, but in nitrogen, significant reduction was observed above

400°C. This suggests the presence of interstitial oxygen in the sample. In nitrogen,

interstitial oxygen readily diffuses out of the sample above 400°C while in air, the out-

diffusion of oxygen is balanced with adsorption of oxygen from air. This assignment of

interstitial oxygen to the orange emission agrees well with the reported literature [69,

76].

Continual reduction of green emission is observed for both air and nitrogen ambient as

annealing temperature is increased. The green emission is unlikely to be related to an

oxygen defect because its intensity reduces both in air and nitrogen ambient.

Considering the formation energy of native defects arising from growth in an oxygen-

rich environment, the likely point defects are zinc vacancies and interstitial zinc [103].

We believe that a combination of zinc vacancies and interstitial zinc is likely to be

related to the green emission. Interstitial zinc has a low migration energy barrier [103].

This allows the interstitial zinc to diffuse into zinc vacancy sites and thus reduce the

green emission intensity under various annealing conditions. The slight increase in

green emission after annealing at 800°C in nitrogen is attributed to removal of

hydrogen defects as water molecules [81].

These results indicate that when ZnO is grown at high pH, the major defects are zinc

interstitials, oxygen interstitials and zinc vacancies. This is consistent with the Raman

results. Both zinc and oxygen interstitials lead to compressive stress in the crystal

structure. As the pH increases, the concentration of these defects increase, leading to

80

stronger defect emission and higher compressive stresses. These defects are present in

the bulk of the rods and not just on the surface of the rods because the Raman spectra

were recorded using visible excitation wavelength, which penetrates into the bulk of

the rods.

The variations of the PL of the sample grown in low pH when annealed in air and

nitrogen ambient are shown in Fig 5.5 (c) and (d) respectively. The orange emission is

very low in as-grown samples and increases when annealed in air because oxygen from

air adsorbs into the bulk. As expected, the orange emission remains unchanged when

annealed in nitrogen ambient.

The green emission in a low pH sample remains roughly unchanged when annealed in

both air and nitrogen, and is possibly caused by zinc vacancies. The stability of the

green emission in a low pH sample stands in contrast against that of a high pH sample.

It is unclear what led to the different stability of the green emission: possible reasons

are lack of interstitial zinc and stability of hydrogen defects at vacancy sites. When the

sample is annealed at 800°C in nitrogen, the hydrogen defects are removed from the

structure as water molecules resulting in more zinc vacancies and a higher green

emission peak, similar to that observed in the high pH sample.

The UV intensity for both low and high pH samples generally peaks when annealed at

400°C, regardless ambient. This maximum intensity also corresponds with the

maximum electron concentration measured by Hall effect which is shown later in

Chapter 6. We believe that this is due to activation of hydrogen defects at 400°C.

Beyond 400°C, the UV intensity will decrease due to desorption of H from the sample

5.4 Discussion

5.4.1 Role of solubility in growth morphology

Table 1 shows the relationship between the growth pH values and the observed ZnO

grown as seen in Fig 5.1. Uniform growth is observed in the pH range of 7 to 7.5 and

10.6 to 10.8. From Chapter 2, we know that this pH range for uniform growth of

nanorods is far from the PZC of ZnO which has been reported to range from 8.7 to 9.7.

81

We also know that the interfacial energy γ reduces rapidly to a small constant value as

the pH moves away from the PZC.

Table 5.1. Summary of observed growth behavior with solution pH

pH range < 7 7 – 7.5 9 - 10.2 10.6 – 10.8 > 11

ZnO nanorod growth

None Slow and uniform

Poor coverage

Fast and uniform

None

To understand how pH affects the Gibb’s free energy of nucleation, we will need to

examine Eq. (2.19) in Chapter 2. For the convenience of the following discussion, we

reproduce Eq. (2.19) which describes the critical Gibb’s free energy of nucleation.

2

3

22

2*

ln3

16

⋅=∆

ZnS

CTk

VG

γπ (5.1)

where V is the atomic volume, k is the Boltzmann constant defined as 1.38 x 10-23 m2 k

g s-2 K-1, T [K] is the growth temperature, C [mol/l] is the concentration of zinc acetate

used in the experiment, SZn [mol/l] is the solubility of zinc and γ is the interfacial energy.

For constant values of interfacial energy γ, ZnAc2 concentration C and temperature T,

we can rewrite Eq. (5.1) as

2

2

1

2

3

22

2*

lnln3

16

=

⋅=∆

ZnZn S

K

K

S

CTk

VG

γπ (5.2)

where K1 and K2 are constants.

82

Figure 5.6. Plot showing the solubility of zinc, SZn, against the concentration of NH4OH for 0.006 M (black dotted line), 0.01 M (blue line), 0.02 M (green line) and 0.03 M (red line) of ZnAc2. The SZn data points which are labeled (a) to (i) corresponds to the SEM images in Figs 5.2 (a) to (i) respectively which have been reproduced here for ease of comparison. The value of SZn when 0.006 M ZnAc2 and 0.4 M NH4OH is marked with a square (�) and the corresponding SEM image is shown in Fig 5.7. Growth in region 1 produces uniform nanorods, region II a mixed morphology of nanorods and large rods and region III only large rods.

83

Eq. (5.2) is significant because it states that the solubility of zinc is the dominant factor

affecting the Gibbs free energy in the pH range where uniform growth of ZnO nanorods

occur. Since the initial value of SZn can be calculated from the ionic equilibrium of the

growth solution using thermodynamic data, we should be able to see a correlation

between SZn and the morphology of the nanorods.

The dominant role of SZn can be clearly seen when the growth morphologies in Fig 5.2 is

compared against their corresponding value of SZn in Fig 5.6. Fig 5.2 is reproduced

above Fig 5.6 for ease of comparison.

Firstly, when ZnAc2 is kept at 0.02 M to maintain a constant value of C, while NH4OH is

increased from 0.4 to 1.1 M to vary the pH within the range of 10.6 to 10.8, the density

of rods is observed to reduce with higher concentrations of NH4OH as shown in Figs 5.2

(d), (e) and (f). A higher concentration of NH4OH increases the solubility of zinc SZn and

thus, from Eq. (5.2), leads to a higher Gibbs free energy, a lower rate of nucleation and

a lower density of rods. The same behavior can be observed in Figs 5.2 (a), (b) and (c)

as well as Figs 5.2 (g), (h) and (i) when 0.01 and 0.03 M of ZnAc2 are used respectively.

This clearly shows the dominant role of solubility over interfacial energy in pH ranges

that uniform growth of ZnO occurs.

Figure 5.7. SEM image showing the top and cross-sectional view of a sample grown in 0.006 M ZnAc2 and 0.4 M NH4OH. The mixed morphology confirms the dependence of SZn which shown in Fig 5.6.

84

It can also be seen that the morphology of the rods can be predicted from the value of

SZn. There are three distinct regions in Fig 5.6:

• Region I where SZn is less than 0.88 mmol/l, we obtained uniform and dense

array of ZnO nanorods with good surface coverage as shown in Figs 5.6 (a), (d)

and (g).

• Region II where SZn is in between 0.88 and 1.56 mmol/l, a transition region

where both nanorods and large clustered rods exist in Figs 5.6 (b), (e) and (h).

• Region III of Fig 6 where SZn is greater than 1.56 mmol/l, large clustered rods

with poor surface coverage are obtained as shown in Figs 5.6 (c), (f) and (i).

To confirm the morphology dependence on SZn, and not on the concentration of

NH4OH, a sample is grown in a solution of 0.006 M ZnAc2 and 0.4 M NH4OH. The value

of SZn at this point is indicated by a square ‘�’ in Fig 5.6. The corresponding SEM image

in Fig 5.7 shows a mixed morphology which confirms a SZn dependence instead of

[NH4OH] dependence.

As in the case of growth of ZnO on GaN epilayers in Chapter 4, pH by itself is not a good

predictor for morphology. To illustrate this point, Fig 5.8 plots SZn against the pH

instead of [NH4OH] as in Fig 5.6. It can be seen clearly that the three types of

morphologies cannot be grouped using the pH variable alone. A combination of pH and

concentration of ZnAc2 is required to get a good prediction of the morphology. The

solubility of zinc which already incorporates the pH and various reactant

concentrations does a much better job in predicting the nanorods morphology.

Finally, we have observed that the dependence of these morphology trends on the

value of SZn is independent of substrate such as glass, ITO, silicon (111), silicon (100)

and plastic. This is attributed to the presence of a seed layer of ZnO nanoparticles on

the substrate which provides similar interface properties.

85

Figure 5.8. Plot of solubility of zinc against pH for 0.006 M (black dotted line), 0.01 M (blue line), 0.02 M (green line) and 0.03 M (red line) of ZnAc2. The corresponding SEM images from Fig 5.2 are shown here for ease of comparison.

5.5 Role of interfacial properties in aqueous solution

In the previous section, we have shown for a narrow range of pH > PZC where good

growth of nanorods can be achieved, that solubility is a good predictor of the free

Gibbs energy and thus the density and morphology of the nanorods. When the pH

varies over a wider range, however, other factors such as the interfacial energy and the

type and charge of growth species in the solution need to be considered in order to

understand the differences in growth habits and mechanisms. One such example that

we will discuss in this section is differences in the growth rate and habit between the

two ZnO growth zones at the pH ranges of 7-7.5 and 10.6-10.8.

86

When growth pH in the region of 7 to 7.5, the solution pH is less than the PZC of ZnO,

thus H+ ions will tend to reside on the surface of ZnO rather than dissociate into the H+

rich solution. This leads to a positively charged ZnO surface with hydrogen stabilizing

the oxygen sites [104] on the polar surfaces [105]. In the pH range of 7 to 7.5, the

major species in the solution are positively charged Zn2+ and Zn(OH)+. The positively

charged zinc surface and a low concentration of negatively charged growth species

result in a slow growth rate, preferentially along the [0001] direction. As the zinc polar

surfaces are stable with the adsorption of hydrogen [105], the rods appear flat-topped

as shown in Fig 5.9 (a) when 0.02 M NH4OH is used to obtain a solution pH of about 7.

When the growth pH is in the range of 10.6 to 10.8, the solution pH is greater than the

PZC of ZnO and the H+ ions readily enter the solution. This results in a negatively

charged surface of ZnO, except for the polar (0001) surface which remains positively

charged. In the pH range of 10.6 to 10.8, the major species in the solution are

negatively charged Zn(OH)42-. A positively charged polar surface and presence of

negatively charged major growth species leads to a fast growth rate in the [0001]

direction. It is known that crystal faces whose growth rate is fastest will be minimized,

thus the rods appear more tapered and sharp-tipped. The much larger and tapered

rods can be seen in Figs 5.9 (b) when 1.1 M NH4OH is used to obtain a solution pH of

10.8.

Figure 5.9. SEM image showing the top view of a sample grown in (a) 0.02 M and (b) 1.1 M NH4OH. The concentration of ZnAc2 is kept constant at 0.02 M. The scale bar shows 1 µm.

87

In this section, we have seen how the solubility, surface charge and the type of major

growth species affect the ZnO growth rates and growth habits. In the next section, we

will show that these factors are also determines the type of defects in the resulting

nanorods.

5.6 Defects and the growth mechanism

From the PL and Raman results, we know that the concentration of defects and

compressive stresses increase with the concentration of NH4OH. We also know that the

zinc sublattice becomes increasingly ordered at the expense of the oxygen sublattice.

In order to understand how the concentration NH4OH is related to the incorporation of

defects during growth, we will need to look at the interface properties and growth

species which affect the growth habit and growth rate of the nanorods.

As more NH4OH is added, the solution becomes more basic and pH increases. More H+

ions from the surface of ZnO enter into the solution and frees up more sites for

adsorption of zinc ions. This leads to a lower concentration of hydrogen defects as well

as zinc vacancies in the bulk, which improves the zinc sublattice.

As explained earlier for a high pH regime, the growth rate is fast in the polar direction

due to the surface charge and presence of majority negatively-charged growth units.

Increasing the concentration of NH4OH leads to an increasing concentration of

negatively charged growth species and thus a faster growth rate. This faster growth

rate leads to the more native defects such as interstitial zinc and zinc vacancies.

A higher pH also provides a growth environment that is richer in hydroxyl groups. The

excess hydroxyl groups normally leave the structure through dehydration. Fast growth

rates also prevents complete dehydration which in turn, leaves behind some hydroxyl

groups embedded in the structure in the form of interstitial oxygen. This creates a

more disordered oxygen sublattice as seen in the Raman spectra.

A marked improvement of the ratio between the UV and visible emission was seen

when pH is much less than PZC. This can be attributed to the slower growth rate which

88

results in less structural defects, as well as the incorporation of hydrogen in zinc

vacancy sites. When aligned along the c-axis, hydrogen is known form a stable shallow

donor [106] up to 1200°C [107] in hydrothermal samples grown above 300°C and this

contributes to the strong ultraviolet emission. In comparison to hydrothermal methods,

when ZnO is grown at temperatures below 100°C, desorption of H is observed at

temperatures above 400°C similar to the behavior of implanted H defects. The ability to

drive out hydrogen defects and lower the background doping concentration at lower

temperatures is important for p-type doping as will be shown later in Chapter 6.

Furthermore, at low pH, the growth environment is zinc rich as the concentration of

Zn2+ is much higher than that of OH-, leading to low concentrations of interstitial

oxygen and thus weak orange emission.

With these, it appears that our current understanding of the growth drivers consisting

of solubility, the interface properties and the majority growth species in the solution

sufficiently supports the experimental evidence of the growth morphology, density

distribution and intrinsic defects.

5.7 Conclusion

ZnO nanorods have been hydrothermally-grown on substrates pre-coated with ZnO

nanoparticles. When the pH is sufficiently far away from PZC and the interfacial energy

is at the minimum, good growth coverage of ZnO nanorods on the substrate can be

obtained. In this growth regime, we have shown that the solubility of zinc and surface

charges are important factors that determine the growth morphology of the rods.

When SZn < 0.88 mmol/l, a uniform coverage of nanorods is obtained and when SZn >

1.56 mmol/l, large clustered rods are obtained. Finally, when 0.88 < SZn < 1.56 mmol/l,

a transition region where both nanorods and large clustered rods exist.

We have shown that the visible light emissions from defects can be minimized while

the UV emission from band edge transitions can be enhanced simply by growing ZnO in

the regime where 7 < pH <7.5.

89

Finally, we have identified the different type of native defects in the bulk and

associated these defects with the different growth conditions. By growing in the region

where pH is less than PZC, hydrogen defects are the major defects. In this growth

regime, the growth rate is low and the visible defect emissions are minimized while the

UV band edge emissions are enhanced. The high UV emission intensity is attributed to

the shallow H defect states. When the sample is grown in a solution where the pH is

greater than PZC, the growth rate is fast and the major defects are interstitial oxygen,

interstitial zinc and zinc vacancies. In this growth regime, the bulk is rich with defects

and the intensity of the visible defect emissions is stronger than the UV emission. The

green component of the visible emission is attributed to interstitial zinc and zinc

vacancies while the red component is attributed to interstitial oxygen. Presence of

these defects results in a poorer crystal quality and a structure that is in compressive

stress.

90

6 Growth of p-ZnO film using

multiple growth cycles

6.1 Introduction

In this chapter, we will look at the feasibility of growing and doping ZnO films in

aqueous solution at low temperatures.

There are only a few reports on the epitaxial growth of ZnO using solution methods and

these reports can be traced to two main groups: Mader's group at University of Bonn

[108], Germany and Lange's group at Uni. of California, Santa Barbara, USA [22, 109].

Mader's group first reported the epitaxial growth of ZnO films on ScMgAlO4 [108] using

a sol-gel method followed by high temperature annealing at 850°C. This was followed

up by Lange's group who reported on the direct growth [109] as well as lateral epitaxial

growth [22] of ZnO films on MgAl2O4 (111) substrates. Kim et al extended the lateral

epitaxial growth method to grow ZnO thin film on GaN buffered Al2O3 (0001) substrates

[110]. Although Lange's group and Kim et al reported discontinuous film at pH 7.5, Sim

et al managed to obtain a continuous epitaxial film a single step by growing at 150°C

[111]. Since the same growth parameters were used by Lange and Sim, it can be

speculated that the growth duration was extended from 2 h to a much longer duration

to allow coalescence of individual rods and subsequent formation of a continuous film.

Table 6.1 below summarizes the precursors, substrates and growth temperatures used

in these reports.

91

Table 6.1. Summary of reported investigators, precursors, growth temperature and substrates for epitaxial ZnO growth in aqueous solution.

Investigator Precursor Temperature Substrate

1 Wessler et al ZnAc2, monoethanolamine, and 2-methoxyethanol. Solgel method.

850°C ScMgAlO4

2 Andeen et al Zn(NO3)2 and NH4OH. Continuous film at pH 10.5-11.

150 to 200°C MgAl2O4 (111)

3 Andeen et al Zn(NO3)2, NH4OH and NaC3. Two step procedure: (1) pH 7.5 for epitaxial film, (2) LEO at pH 10.9

90°C MgAl2O4 (111)

4 Kim et al Two step procedure: (1) Zn(NO3)2 & NH4OH at pH ~ 7.5 (2) Zn(NO3)2, NH4OH & NaC3 at pH ~ 10.9

90°C GaN-buffered Al2O3 (0001)

5 Sim et al Zn(NO3)2 and NH4OH. Continuous film at pH 7.5

150°C MgAl2O4 (111)

The current state-of-the-art for growing a smooth continuous film requires a seed layer

to be grown at pH 7.5 and a subsequent film growth at pH 10.9 with the help of NaC3 as

a surfactant [22, 110]. Without NaC3, a rough surface arising from island mode growth

is obtained from both pH 7.5 [34] and 10.9 [109]. The rationale for the two-step

approach is as follows:

1. The slow growth at pH 7.5 allows a high density of ZnO islands to populate

substrate surface without introducing a large height difference between the

islands. This simplifies the eventual coalescence into a smooth film.

2. The fast growth at pH 10.9 minimizes the growth time needed to grow and

coalese these islands, as well as to cover the film discontinuity in the earlier

step. By employing NaC3 as a surfactant, lateral growth is enhanced while

vertical growth is retarded. Eventually, this leads to a smooth and continuous

film.

Although this approach succeeds in producing a smooth and continuous film, it fails to

minimize the native defects in the film. As we have shown from the photoluminescence

92

and Raman Scattering spectra in the previous chapter, the crystal quality of ZnO is

better at pH 7.5 than 10.9, although the coverage is poorer and growth rate is much

slower. Since the seed layer is grown at pH 7.5 and the bulk of the film at pH 10.9, it is

expected that the film has a rich density of defects. This can be seen from the low ratio

of UV to visible light intensity in the PL spectra [22].

Based on our earlier results, we propose an opposite process to obtain a high quality

thin film on surface. The initial seed layer of ZnO should be grown at pH 10-11. At this

pH range, a high density of nanorods with a very good coverage of the substrate can be

obtained in a short time. The subsequent film growth should be performed at low pH.

Although the growth rate is slow, it produces a low defect density and a flat-top

morphology which is suitable for coalescence into a smooth film as shown by Sim et al

[34].

In addition, we will look at how the K, a group I element, can be incorporated into the

film during growth. When K substitutes Zn, it contributes a hole and functions as a p-

type dopant. Its role as a p-type dopant will be investigated from the electrical

properties of the film with and without K. Next, we will explore the application of an

electric field in the growth solution to improve the growth rate at pH 7.5.

Finally a p-ZnO / n-GaN heterostructure is fabricated to confirm the p-doping of ZnO

using K.

6.2 Experiment

A thin film of ZnO was grown using a multiple step strategy with the general procedure

as described earlier in Chapter 3.

Firstly, a substrate is precoated with ZnO nanoparticles. For the seed layer growth, the

growth solution consists of 0.03 M ZnAc2 and 0.37 M NH4OH, for 30 min at 90°C while

the film layer growth consists of 0.03 M Zn(Ac)2, for 3 h at 90°C. The film layer growth

step is performed three times, to allow the rods to coalesce and form a continuous film.

93

To study the incorporation of K, the same amount of potassium acetate (KAc) is added

into the growth solution for both steps.

To study the effect of electric field on the doping efficiency, the growth setup was

modified to accommodate two electrodes in the growth vessel shown in Fig 3.2. The

modified growth setup is shown in Fig 6.1. The first electrode is the calomel reference

electrode which provide a reference voltage against the standard hydrogen electrode

potential. The second electrode is the platinum (Pt) counter electrode which closes the

circuit path.

Figure 6.1. Modified growth setup to study the effect of internal electric field on the growth and doping of ZnO films.

For Hall measurements, the ZnO film was grown on Al2O3 (0001) in order to withstand

the high annealing temperatures. Aluminium contacts of 1 µm thickness were

evaporated on the four corners of the sample for Hall measurements. The carrier

concentration and mobility was measured using Accent HL5500 Hall measurement

system after growth and after annealing steps in nitrogen ambient at 200, 400, 600,

800 and 950°C for 10 min. Annealing was performed using ULVAC RTA system. The

94

annealing chamber was pumped down to a vacuum pressure of less than 10-5 mTorr

before nitrogen gas is flowed into the chamber with a flowrate of 100 sccm.

The film morphology was observed using Hitachi S4100 FESEM. XPS measurements

were done using a dual chamber VG ESCA/SIMSLAB system using a Al Kα beam. All the

measured XPS spectra was calibrated using the C 1s peak from a calibration standard.

The PL spectra measured using Renishaw Ramascope 2000 micro-PL with a He-Cd laser

as excitation source. Finally, the SIMS depth profile was obtained using TOF-SIMS IV.

6.3 Results and discussion

6.3.1 Evolution of film morphology using a multi-step growth approach

The evolution of the surface morphology from the first step of seed layer growth, to

the second step of first film layer growth and to the final step of the third film layer

growth is shown in Figs 6.2 and 6.3 on n-Si(100) and n-GaN substrates respectively. The

coalescence into a smooth continuous film is much better for GaN because of the good

lattice match between GaN and ZnO. For non-lattice matched substrates, the rods are

randomly orientated. Due to in-plane rotation among the rods, the rods take a longer

time to coalesce into a continuous film.

Figure 6.2. Morphology evolution from the seed layer to the film layer growth on n-Si(100).

95

Figure 6.3. Morphology evolution from the seed layer to the film layer growth on n-GaN epilayer.

Fig 6.4 shows the development of the PL spectra from a seed layer growth step to the

film layer growth steps. As expected, the seed layer growth step produces a high

density of defects which leads to a low UV emission intensity and a high visible

emission intensity. A continual increase in the UV emission intensity and a slight

decrease in the visible emission intensity is observed as film layer growth proceeds

from 30, 90 to 180 min. This is expected because the top film layer, which has a much

lower density of defects, contributes the strong UV emission and decreases the

intensity of the excitation laser that reaches the underlying seed layer. In order to

minimize the defects from the seed layer growth, the seed layer growth duration is

kept short at 30 min, while the film layer growth is lengthened by growing for three

cycles of 3 h each.

Figure 6.4. PL spectra of as-grown seed layer (black) and the subsequent film growth layers (blue for 30 min, green for 90 min and red for 180 min). The film layer growth step significantly enhances UV emission while slightly reducing the visible emissions.

96

When KAc is added into the growth solutions of the seed and film layer growth steps,

the growth rate of ZnO is enhanced. This effect can be seen in Fig 6.5 which shows the

top view of the film after one cycle of seed layer growth and two cycles of film layer

growth. Increasing the concentration of KAc increases both the rod diameters and

length. After the third cycle of film growth, the film thickness is 1.7, 1.9 and 2.0 µm for

without KAc, 0.07 M and 0.24 M KAc respectively.

Figure 6.5. SEM image shows the top view of the ZnO film after one cycle of seed layer growth followed by two cycles of film layer growth for the samples that are grown (a) without KAc and with (b) 0.07 and (c) 0.24 M KAc.

6.4 Role of K as a dopant for ZnO films

To investigate the incorporation of K in the structure, Hall effect measurements and

SIMS depth profile were obtained for the samples grown without KAc and with 0.07 M

and 0.24 M KAc. The results for Hall effect measurements are summarized in Table 6.1

while the SIMS depth profile is shown in Fig 6.6.

By comparing the carrier concentrations in Table 6.2 and the depth profiles of K in Fig

6.6, we can establish a positive correlation between the hole concentration and the

concentration of K in the ZnO film. Without KAc, the actual K concentration is negligible

and the film is n-type.

Furthermore, the experimental data shows that that the amount of K incorporated in

the structure cannot be simply increased by increasing the concentration of KAc in the

97

growth solution. The optimum point with the highest K and hole concentration is

obtained with 0.07 M KAc, followed by 0.24 M KAc.

Table 6.2. Summary of carrier parameters obtained from Hall effect measurements for samples grown without KAc and with 0.07 and 0.24 M KAc. The film thickness is obtained from the SEM image of the cross-section of the film.

Conc. Of KAc

(M)

Film thickness

(µµµµm)

Type of majority carrier

Carrier conc. (cm-3)

Carrier mobility

(cm2/V-s)

0 1.7 n 1.4 × 1016 0.45

0.07 1.9 p 3.8 × 1017 0.038

0.24 2.0 p 3.7 × 1014 11.8

Figure 6.6. SIMS depth profile for Zn, O and K.

0.01

0.1

1

10

100

0 20 40 60 80 100 120 140

Time (s)

Inte

nsit

y (

a.u

.)

Zn (No KAc)

Zn (0.07M KAc)

Zn (0.24M KAc)

O (No KAc)

O (0.07M KAc)

O (0.24M KAc)

K (No KAc)

K (0.07M KAc)

K (0.24M KAc)

98

6.5 Effect of electric field on the growth and doping of

ZnO films in solution

The thickness and carrier concentrations for various bias voltages are summarized in

Table 6.3 while the SIMS depth profile is shown in Fig 6.7. The film thickness is

measured from the cross-sectional view of the sample using SEM. The carrier

concentrations and mobility are obtained from Hall effect measurements. Finally the

depth profile is obtained using SIMS.

When ZnO is grown in the presence of an increasing electric field, the growth rate

increases dramatically. For example, at a - 0.9 V bias, the thickness of the film increases

370% to 7.4 µm compared to 2.0 µm without any bias. Without any bias at pH 7.5,

growth is slow because the surface of the substrate and the majority growth units are

positively charged. By applying a negative bias, the positive growth units in the solution

are attracted towards the substrate and the growth rate can be increased significantly.

Unfortunately, the incorporation of K in the film does not increase with the bias voltage.

SIMS depth profile showed that the maximum incorporation of K occurs at a voltage

bias of -0.1 V. In fact, at -0.9V, the concentration of K in the structure is even lower

than that without any bias applied. It appears that there is a trade-off between the

growth rate and amount of K that incorporated inside the structure and the optimum

point is about -0.1 V. Higher negative bias voltages beyond -0.1 V results in thicker film

and lower K incorporation.

Further examination of the hole concentration and the concentration of K in the film

shows that although the level of K incorporation is highest at -0.1 V, it does not lead to

the highest concentration of holes. The highest hole concentration of 3.98 x 1017 cm-3 is

obtained by applying a bias of -0.4 V. One possible explanation of this phenomenon is

the occupation of interstitial sites by K atoms. To confirm this, XPS is performed and

Figs 6.8 and 6.9 shows the typical survey scan spectra and quantification peaks. A

summary of the atomic percentages from quantifying the survey scan, as well as the

99

calculated atomic percentages of K at lattice and interstitial sites, Zn vacancies and the

measured hole concentrations are shown in Table 6.4.

Table 6.3. Summary of carrier parameters obtained from Hall effect measurements for samples grown with 0.24 M KAc at different bias voltages. The film thickness is obtained from the SEM image of the cross-section of the film.

Applied potential

(V)

Film Thickness

(µµµµm)

Type of Majority Carrier

Carrier conc.

(cm-3)

Carrier mobility

(cm2/V-s)

0 2.0 hole 3.70 x 1014 11.8

-0.1 2.6 hole 1.17 x 1016 0.17

-0.4 5.7 hole 3.98 x 1017 0.046

-0.9 7.4 hole 3.21 x 1014 5.5

Figure 6.7. SIMS depth profile for Zn, O and K for samples grown in the presence of 0.24 M KAc and varying bias voltages from 0, -0.1, -0.4 to -0.9 V.

The data in Table 6.4 resolves the discrepancy between the hole concentration and the

concentration of K in the ZnO film. Although the concentration of K for a bias of -0.1 V

0.01

0.1

1

10

100

1000

0 20 40 60 80 100 120 140

Time (s)

Inte

nsity (a.u

.)

Zn (No bias)Zn (-0.1V)Zn (-0.4V)Zn (-0.9V)O (No bias)O (-0.1V)O (-0.4V)O (-0.9V)K (No bias)K (-0.1V)K (-0.4V)K (-0.9V)

100

is the highest, not all the K atoms are located at substitutional sites: only 23.4 at% are

in substitutional lattice sites compared to 23.9 at% at -0.4V. In fact, the bias of -0.1 V

has 11.7 at% which is the highest concentration of K at interstitial sites among the

three samples.

Since the hole concentration is correlated to concentration of K donors at

substitutional sites, the bias of -0.4 which has the highest atomic percentage of K at

lattice sites will have the highest concentration of holes, as measured by Hall effect

measurements.

Figure 6.8. Typical XPS survey spectrum of a ZnO sample doped with K. The peak positions of Zn, O, K and C have been marked. Au peaks are from the calibration reference.

Figure 6.9. Typical component fitting of Zn, O and K using Zn 2p3/2, O 1s and K 2s peaks for atomic concentration quantification. Synthetic peaks were fitted to the measured peaks using a Shirley background and Gaussian-Lorentian distributions. Quantification was performed using the fitted synthetic peaks to improve estimation accuracy.

101

Table 6.4. Summary of percentage atomic concentrations from quantifation of the fitted components of Zn 2p, O 1s and K 2s in the XPS survey spectra. The relative sensitivity factors (RSF) that were used for quantification are indicated beside the element in parenthesis.

Applied potential difference

Element (RSF) units -0.1 V - 0.4 V - 0.9 V

Zn (2p1/2 = 9.29, 2p3/2 = 18) at% 20.7 25.5 31.5

O (2.85) at% 44.1 50.6 55.2

K (1.95) at% 35.1 23.9 13.3

K at lattice sites at% 23.4 23.9 13.3

K at interstitial sites at% 11.7 0 0

Zn vacancy sites at% 0 1.3 10.4

Hole concentration cm-3 1.17 × 1016 3.98 × 1017 3.21 × 1014

Figure 6.10. XPS valence band spectra of Zn 3d for samples grown with (a) -0.9V, (b) -0.1V and (c) -0.4V. The corresponding hole concentrations in cm-3 has been indication in the legend. A larger core level shift is observed for a higher hole concentration.

0

1

3 8 13 18

Binding energy (eV)

Norm

alised inte

nsity (a.u

.)

(a) 3.2E14

(b) 1.2E16

(c) 4.0E17

Zn 3d

12 17Binding energy (eV)

0

1

3 8 13 18

Binding energy (eV)

Norm

alised inte

nsity (a.u

.)

(a) 3.2E14

(b) 1.2E16

(c) 4.0E17

Zn 3d

12 17Binding energy (eV)

102

It is well-known that the hole concentration is related to the position of the Fermi level

(EF) in ZnO, which in turn affects the binding energy of core levels. When ZnO is doped

with p-type dopants, EF will shift away from the conduction band towards the valence

band, giving rise to a higher binding energy of Zn core levels. Thus, by examining the

core level shifts in binding energy, the shift in EF and the relative hole concentration

values can be deduced. Fig 6.10 shows the shift in the Zn 3d core levels in the XPS

spectra. A close-up of the Zn 3d, normalized for ease of comparison, is shown in the

inset of the figure. The shift to higher binding energy is largest for 4 x 1017 cm-3 which is

grown at a bias of -0.4 V, followed by 1.2 x 1016 cm-3 grown at -0.1 V and finally 3.2 x

1014 cm-3 grown at a bias of -0.9 V.

To make an accurate measurement of the core level shift, the Zn 2p3/2 peak was chosen

for comparison because it is relatively intense with minimal overlap with other peaks.

To account for charging, the peak position internally calibrated against the O 1s and C

1s peaks. The binding energy of the Zn 2p3/2 is plotted against its corresponding hole

concentration in Fig 6.11. As expected, the Zn 2p3/2 peak position will shift to a higher

binding energy when its hole concentration increases, confirming the role of K in p-

doping of ZnO.

Figure 6.11. Plot of core energy level Zn 2p3/2 against the hole concentration as measured using Hall effect. The as-measured as well as the C 1s and O 1s calibrated peak values are shown. A line is fitted to show the increasing binding energy with hole concentration.

1021.5

1022.5

1.E+14 1.E+18Hole concentration (cm-3

)

Bin

din

g E

nerg

y o

f Zn 2

p3/2 (eV

) As-measuredC 1s1O 1s1LinefitPower (Linefit)

103

6.6 Effect of annealing in nitrogen ambient on p-type

doping by K

The effect of post-annealing treatments were studied using samples grown without

KAc, and with 0.07 and 0.24 M KAc. Annealing temperatures were varied from 200°C to

800°C in a nitrogen ambient. The rise time from room temperature to the desired

annealing temperature is 30 s. The anneal temperatures were held for 10 mins before

allowing the sample to cool down to room temperature again. The flow of nitrogen gas

was maintained until the sample has cooled down.

Figure 6.12. Effect of anneal temperatures on the carrier concentration and mobility for ZnO films grown (a) without any KAc, and with (b) 0.07 and (c) 0.24 M KAc. (d) The effect of anneal duration at 800°C for sample grown in 0.24 M KAc. Annealing for all samples were done in a nitrogen ambient. Data points for as-grown samples were represented at 100 °C. The electron concentrations and mobilities are marked by ● and ● respectively, while

the hole concentration and mobility by ○ and ○ respectively.

104

Fig 6.12 (a) shows the results for the sample that has been grown without the presence

of KAc. The as-grown ZnO film is n-type with an intrinsic carrier concentration of about

1.4 x 1016 cm-3. The following observations can be seen:

• The film is intrinsically n-type without any extrinsic dopants, possibly due to the

presence of native defects in the structure.

• At 400°C, electron concentration increases to above 1018 cm-3. This sharp rise is

attributed to activation of hydrogen donors. Hydrogen is usually present in

samples grown in aqueous solution due to incomplete dehydration during the

formation of ZnO.

• Above 400°C, electron concentration decreases gradually to about 3 x 1018 cm-3

at 700°C. The gradual decrease can be attributed to desorption of hydrogen

from ZnO.

Fig 6.12 (b) shows the temperature dependence of the doping levels in a sample that is

grown with 0.07 M KAc in the growth solution. The as-grown film is p-type with a

carrier concentration of 3.8 x 1016 cm-3. Below 300°C, the p-type doping is stable with a

concentration range of 1017 to 1018 cm-3. When annealed at 400 °C and above, p-type

switches to n-type. The electron concentration appears to decrease gradually with

higher annealing temperatures, similar to that of undoped ZnO.

Fig 6.12 (c) shows the temperature dependence of a film grown in the presence of 0.24

M KAc. The hole concentration increases from 3.7 × 1014 to 5 × 1018 cm-3 when the as-

grown sample is annealed at 300°C. However, at 400 °C and above, a switch from p-

type to n-type is observed with a gradual decrease of electron concentration with

temperature, similar to the earlier two samples.

The switch from p to n-type at 400°C with an electron concentration of about 1 x 1019

cm-3, followed by a gradual decrease in electron concentration with annealing

temperatures is observed for all samples, regardless of the presence of K. We believe

that this phenomena is caused by hydrogen defects. Hydrogen defects have been

shown to be present in hydrothermal and aqueous solution-based growth methods and

are known to be donors in ZnO when activated. Our results show that these hydrogen

105

defects are activated at 400°C at higher concentrations than the extrinsic p-type doping

by K. This leads to the switch from p to n-type. However, the hydrogen defects are

unstable at high temperatures and can be driven out at high annealing temperatures

above 400°C. This manifests itself as a decreasing electron concentration at higher

annealing temperatures.

Due to the unstable nature of hydrogen defects, it is possible to drive out these

hydrogen defects to a concentration that is below the initial p-type doping and revert

back to p-type conductivity. Fig 6.12 (d) shows the changes in the doping

concentrations for the sample grown in 0.24 M KAc when it is annealed for different

durations at 800 °C. As observed earlier, a 10 min anneal will activate the hydrogen

defects which will lead to the over-compensation of the p-type film and the conversion

to n-type film. This conversion is reversed and the p-type conductivity is recovered

when the sample is annealed at 800°C for another 20 mins or more.

6.7 Fabrication of p-ZnO / n-GaN LED

To confirm the p-type conductivity, a ZnO film is grown using 0.07 M KAc on a n-GaN

epilayer that was grown on sapphire substrate. The n-GaN epilayer was grown by

MOCVD on a sapphire substrate. A window was opened for the growth of ZnO. Then,

Ni (20 nm) / Au (100 nm) and Ti (15 nm) / Al (220 nm) / Ni (40 nm) / Au (50 nm) were

deposited as p and n-contacts respectively. A schematic of the device is shown in the

inset of Fig 6.13 (a). Finally, a thermal anneal at 700 °C in vacuum for 1 h was

performed to form ohmic contacts, reduce the concentration of hydrogen defects and

activate the K dopants in the ZnO layer. Both Ni/Au and Ti/Al/Ni/Au shows ohmic

characteristic on p-ZnO:K and n-GaN respectively, as shown in the inset of Fig 6.13 (b).

The logarithmic and linear I-V plots of four different diodes are shown in Fig 6.13 (a)

and (b) respectively. At 3 V reverse bias, the leakage current ranges from 1.3 to 1.5 mA.

At a forward bias of 6 V, the current ranges from 75.9 to 98.3 mA.

The electroluminescence spectra consisting of a UV and broad yellow-orange emission

is shown in Fig 6.14. The UV emission at 20mA consists of a peak centered at 372 nm

with a shoulder at 378 nm and can be attributed to bound exciton emissions. At 70 mA,

106

these peaks shift to 375 and 386 nm, respectively, possibly due to a higher junction

temperature. A broad yellow-orange luminescence is also observed and is believed to

originate from the deep level defects that were introduced during the seed layer

growth.

Both the I-V characteristic and electroluminescence provides further evidence of p-

doping of the ZnO film.

6.8 Conclusion

We have demonstrated an alternative growth strategy which begins with a seed layer

growth at pH 10-11, followed by successive film layer growth at pH 7.5. This growth

method produces films with a lower defect density as seen from the PL spectra.

We have also doped the ZnO film with K and showed that the incorporation of K in ZnO

leads to p-type conductivity. An optimum doping concentration of 3.8 x 1017 cm-3 is

obtained at 0.07 M KAc without the presence of an electric field. When an electric field

is applied, the optimum bias is found to be -0.4 V which gives a doping concentration of

3.98 x 1017 cm-3. To the best of our knowledge, this is the first report of a p-type doping

with potassium from group I using aqueous solution methods at low temperature.

We have shown that the activation of intrinsic hydrogen defects through thermal

annealing at temperatures higher than 400°C can over-compensate the p-type doping

and convert the film to n-type with an electron concentration of 1 x 1019 cm-3. By

extending the annealing time beyond 30 min to reduce the hydrogen defect and

electron concentrations, the p-type conductivity can be recovered.

Finally, we fabricated a p-ZnO / n-GaN junction. The measured I-V characteristic is

rectifying and a weak orange electroluminescence is obtained.

107

Figure 6.13. I-V characteristic plotted in (a) logarithmic and (b) linear scale. Each line shows the I-V from measured from a different device. Inset of (a) shows a schematic diagram of the device while the inset of (b) confirm the ohmic behavior of the top and bottom contacts after annealing at 700°C 1 h.

Figure 6.14. The electroluminescence spectra at various current injection levels from 20 mA to 70 mA.

108

7 Conclusions and Recommendations

7.1 Conclusions

In this dissertation, ZnO nanorods and films were grown in aqueous solution. Besides

water, there were two other basic growth precursors: ZnAc2 and NH4OH. For p-type

doping, KAc was used as a dopant source.

ZnO nanorods were grown spontaneously on a GaN epilayer, which has a lattice

mismatch of 1.8% with ZnO. We showed that a good prediction of the density and

length of the vertically aligned nanorods can be obtained by using the zinc solubility

and growth temperature as predictors.

Using experimental results and the data from the ionic equilibrium of the solution, it

was shown that the density of nanorods can be increased by reducing the solubility of

zinc and increasing the growth temperature, while the length can be increased by

increasing the solubility and reducing the growth temperature. The activation energy

for density and length is found to be -2.11 eV and 0.77 eV respectively. Due to

opposing dependence of density and length of nanorods on the zinc solubility and

temperature, it is not possible to simultaneously maximize density and length of

nanorods in a single growth step.

Based on these experimental results, we produced an empirical growth map based on

the initial concentrations of ZnAc2 and NH4OH to predict the density and length of ZnO

nanorods that were grown on the Ga-face of an unintentionally doped GaN substrate

for a growth temperature ranging from 333 to 423 K. Although this model was based

on GaN as a substrate, it can be easily extended to any other substrates that have a

good lattice match with ZnO.

109

Next, we studied the growth of ZnO nanorods on substrates pre-coated with ZnO

nanoparticles. Pre-coating is done for substrates with poor lattice matching or where

the structure is not wurtzite. We found that good growth coverage of ZnO nanorods on

the substrate occurs when two conditions are fulfilled: firstly, the pH of the growth

solution is sufficiently far away from point of zero charge (PZC) and secondly, the

interfacial energy is at the minimum. We also showed that in this growth regime when

the two conditions are met, the solubility of zinc (SZn) and surface charges are

important factors that determine the growth morphology of the rods. When SZn < 0.88

mmol/l, a uniform coverage of nanorods is obtained and when SZn > 1.56 mmol/l, large

clustered rods are obtained. Finally, when 0.88 < SZn < 1.56 mmol/l, a transition region

where both nanorods and large clustered rods exist.

For both types of substrate, with and without good lattice matching, the zinc solubility

and growth temperature emerged as better predictors of density and length of ZnO

nanorods compare to other commonly used parameters such as pH and degree of

supersaturation.

The defects in ZnO has been studied using PL and Raman. Our results show that the

visible light emissions from defects can be minimized while the UV emission from band

edge transitions can be enhanced simply by growing ZnO in the regime where 7 < pH

<7.5. This improvement is brought about by the slow growth rate which leads to a

much lower concentration of defects in the structure.

The different types of native defects in the bulk were identified and associated with the

different growth conditions. By growing in the region where pH is less than PZC,

hydrogen defects are the major defects. In this growth regime, the visible defect

emissions are minimized while the UV band edge emissions are enhanced. The high UV

emission intensity is attributed to the shallow H defect states. When the sample is

grown in a solution where the pH is greater than PZC, the major defects are interstitial

oxygen, interstitial zinc and zinc vacancies. In this growth regime, the bulk is rich with

defects and the intensity of the visible defect emissions is stronger than the UV

emission. The green component of the visible emission is attributed to interstitial zinc

and zinc vacancies while the red component is attributed to interstitial oxygen.

110

Presence of these defects result in a poorer crystal quality and a structure that is in

compressive stress.

Based on this understanding of the growth and defect distribution for nanorods, we

proposed an alternative strategy for growing ZnO film in aqueous solution. This

strategy can be applied to a wide range of substrates, regardless of its lattice matching

with ZnO. For substrates with poor lattice matching, only an additional step of pre-

coating the substrate with ZnO nanoparticles will be needed. Our film growth approach

begins with a short seed layer growth at pH 10-11, followed by successive film layer

growth at pH 7.5. This growth method shows lower defect density as demonstrated by

the PL spectra.

We have also succeeded in doping the ZnO film with potassium. Hall effect, SIMS and

XPS measurements showed that the incorporation of K in ZnO leads to p-type

conductivity. An optimum doping concentration of 3.8 x 1017 cm-3 is obtained at 0.07 M

KAc without the presence of an electric field. When an electric field is applied in a

similar setup to a standard three-electrode electrodeposition setup, the optimum bias

is found to be -0.4 V which gives a doping concentration of 3.98 x 1017 cm-3.

When the films are subjected to thermal annealing in a nitrogen ambient, we have

observed a sharp increase in electron concentration to 1 x 1019 cm-3 regardless of

whether the film is doped with K. We attributed this increase to the activation of

intrinsic hydrogen defects which can over-compensate the p-type doping and convert

the film to n-type. By extending the annealing time beyond 30 min at 800°C to drive

out the hydrogen defects, the electron concentration can be reduced and the p-type

conductivity can be recovered. The ability to drive out hydrogen defects by annealing

underlines the importance of the low growth temperatures employed in aqueous

solution growth. Low growth temperatures below 100°C not only prevents the

activation of hydrogen defects in as-grown samples, but it also reduces the stability of

incorporated hydrogen defects and thus makes it easier to remove these defects

through annealing at temperatures above 400°C.

111

Finally, we fabricated a p-ZnO / n-GaN junction. Ni/Au is used for the p-ZnO contact

and Ti/Al/Ni/Au is used for the n-GaN. The measured I-V characteristic is rectifying and

a weak orange electroluminescence is observed at a forward bias current of 75.9 to

98.3 mA. The reverse bias leakage current ranges from 1.3 to 1.5 mA at 3V.

Our results demonstrate the ability and promise of aqueous solution growth methods

to grow and dope ZnO nanorods and films. In order to establish aqueous solution

growth methods as a viable alternative to favorites MOCVD and PLD, much more needs

to be done. Some of the work that lies ahead will be outlined briefly in the next section

where we will provide some recommendations for future work.

7.2 Recommendations

The current growth method can be improved by addressing several drawbacks of the

current growth method which is based on a closed bath. Firstly, the growth rate in a

closed bath is non-linear with the fastest growth rates is achieved between 55 and

65 °C and much slower growth rates for both higher and lower temperatures. Growth

eventually stops when the growth solution reaches its equilibrium at 90°C. Secondly,

the amount of growth that can be achieved is limited by its initial conditions such as

the precursor concentrations and temperature. Continuation of the growth process can

be achieved by refreshing the growth solution as shown in our multi-step film growth

strategy. At each step of changing the growth solution, defects are usually introduced

at the sample surface: for example adsorption of large ZnO powder on the sample

surface. To overcome these drawbacks, a continuous flow bath can be used. It

linearizes the growth rate by constantly introducing "fresh" growth solution and allows

continuous growth to occur with the need of removing the sample to a fresh solution.

This concept is similar to that discussed by Richardson et al for growth of epitaxial ZnO

in aqueous solution [112, 113].

Another area that can be explored is the extrinsic n-doping of ZnO. Although the ZnO

film is intrinsically n-type due to intrinsic defects and hydrogen-related defects, we

have seen that the electron concentrations can change significantly with annealing

temperatures. Heat treatment of ZnO films are usually required to obtain ohmic

112

contacts and this can lead to uncertainties in the final electron concentration. By

obtaining extrinsic doping to a level higher than the intrinsic doping, the temperature

dependence of n-type ZnO can be reduced.

Finally, the performance of metal contacts on both p and n-type ZnO films should be

systematically studied. The ability to obtain high quality ohmic contacts to ZnO films is

an important component for successful device fabrication. Currently, Ni/Au and

Ti/Al/Ni/Au have been successfully used to obtain ohmic contacts on p and n-type ZnO

respectively. The thickness and anneal temperatures have been borrowed from similar

metallization schemes for GaN. It is believed that the performance of the contacts can

be improved by optimizing the thickness and annealing temperatures.

113

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