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1
CHARACTERIZATION OF DEEP LEVEL DEFECTS IN
N-TYPE ZINC OXIDE LAYERS GROWN BY
HYDROTHERMAL TECHNIQUE
A dissertation submitted in partial fulfillment of the requirement for the degree
of Doctor of Philosophy
in Physics
By
HADIA NOOR
Department of Physics
The Islamia University of Bahawalpur Pakistan
2012
2
Abstract
Zinc oxide (ZnO) is a promising wide-bandgap semiconductor due to its favorable
properties for a variety of demanding device applications such as UV light
emitters/detectors, high-power and high-temperature devices. The presence of defects
in the material can considerably change the electrical properties of the
semiconductors. However, recently it has been found that the terminated face of the
material significantly alter the characteristics of such devices. The defects in ZnO
have been studied in last decades, but no clear consensus has been made. This
dissertation investigates the electrical properties of defects in ZnO grown by
hydrothermal and molecular beam epitaxy techniques using deep level transient
spectroscopy (DLTS). Among the growth techniques available to grow the thin film,
the hydrothermal is one of the most cheap and user friendly technique. DLTS
provides a sensitive method for identifying defects and for determining their
parameters. The main findings are as follow:
A. Several circular Schottky contacts (1mm diameter) with Pd metal on the Zn-
face and O-face on n-type ZnO grown by hydrothermal and Ohmic contact of
nickel-gold on the backside were deposited by e-beam technique. The as-
obtained samples were labeled as group A and B samples, respectively. The
present literature on n-type ZnO has highlighted a defect, labeled as E3
irrespective of growth technique, which is also studied thoroughly in this
research project. The respective summary of each group A and B of samples
is explained below:
DLTS has been carried out on the group A samples to study deep level
defects. Its result showed two electron trap level E1 having activation
energy Ec-0.22 ±0.02 eV and E2 with activation energy Ec-0.49 ±0.05
3
eV. E1 level has time-delayed transformation of shallow donor defects
zincinterstitial and vacancyoxygen (Zni-VO) complex. It is observed through
X-ray differaction that the preferred direction of ZnO growth is along
(1010) plane i.e. VO-Zni complex, assuming that under favourable
condition (Zni-VO) complex is transformed into a zinc antisite (ZnO).
Consequently, the trap concentration increases with decreasing free
carrier concentration. Hence, the ZnO is correlated to E1 level
demonstrating the increase in concentration.
Several renowned research groups have revealed different points
defects in bulk ZnO like naming oxygen vacancy, zinc interstitial,
and/or zinc antisite. These defects having activation energy (free
carrier concentration) in the range of 0.32–0.22 eV (1014
-1017
cm-3
)
below conduction band. The results of group A and B samples also
showed activation energy (free carrier concentration) as observed by
other renowned research groups. This result is due to activation energy
of the level while it is not conceivable by with Vincent et al.,[ J. Appl.
Phys. 50 (1979) 5484]. They believed that data should be carefully
interpreted obtaining by capacitance transient measurement of diodes
having carrier concentration greater than 1015
cm-3
. Thus the influence
of background free-carrier concentration, ND induced field on the
emission rate signatures of an electron point defect in ZnO Schottky
devices has been studied by using deep level transient spectroscopy.
Many theoretical models were tested on the experimental data to
understand the mechanism. Our findings were supported by Poole-
Frenkel model based on Coulomb potential. It is revealed by
4
investigation that Zn related charged impurities were found to be
responsible for electron trap. Results were also tested through
qualitative measurements like current-voltage and capacitance-voltage
measurements.
B. Several Schottky contacts of 1mm diameter with silver were prepared on ZnO
grown by molecular beam epitaxy. These samples were labeled as group C
samples, DLTS measurements revealed a hole trap exhibiting metastability
effect in the emission rates of trap with storage time. We determined that hole
trap transfers from one configuration to other with storage time. As a result the
activation energy of the acceptor level varied in the range of 0.31 eV to 0.49
eV above the valance band at different measurement time. Impurities cannot
be removed in the growth procedure. SIMS results showed the presence of
nitrogen. During the growth process nitrogen occupies O site and produces
Zn-N complex. But Zn-N bond is not stable because of its large bonding
energy and consequently results into metastable nature of the defect. All
experimental findings and available literature support the conclusion that the
observed hole trap arise from Zn-N complex.
C. The ZnO nanorods were grown on glass substrate coated with different metal
(Ni, Al, Ag and Au) by aqueous chemical growth. These samples were labeled
as D, E, F and G, respectively. The structural properties of ZnO nanorods were
investigated by X-Ray diffraction (XRD) and scanning electron microscopy
(SEM). The intensity of ZnO (0 0 2) diffraction peak in X-ray diffraction
pattern is maximum of sample D because of nucleation of Ni metal coated on
substrate. SEM measurements strongly support our observation that thin layer
Ni metal increases the growth of nanorods.
5
Declaration
I, Ms. Hadia Noor, PhD student in the subject of Physics session (2007 – 2012)
hereby declare that the material produced in this dissertation titled “Characterization
of deep level defects in n- type zinc oxide layers grown by hydrothermal technique” is
my own work and has not been submitted in as a whole or in part for any degree at
this or any other university.
Hadia Noor
6
Certificate
It is certified that the work presented in this dissertation entitled “Characterization of
deep level defects in n-type zinc oxide layers grown by hydrothermal technique” by
Hadia Noor under my supervision at the Department of Physics, The Islamia
University of Bahawalpur, Pakistan.
Supervisor
Prof. Dr. M. Asghar Hashmi
Chairman
Prof. Dr. Sh. Aftab Ahmad
Department of Physics
7
8
Acknowledgement
All praises and thanks to Almighty ALLAH, the most Beneficent and the
Merciful, The Creator of The Universe, Who enabled me to complete my research
work successfully. I would like to send my humble salutation to the Holy Prophet
Hazrat Muhammad (peace be upon him), Who is a source of guidance and knowledge
for humanity.
Now when I just finished this dissertation, all the memories of the past years
are flashing back in my mind. Many people who helped me slowly came into my
memory one by one, even though my gratitude is beyond words.
First of all, I am cordially thankful to the most inspiring character in this
research i.e my supervisor Prof. Dr. Muhammad Asghar Hashmi, whose
encouragement, guidance and support from the initial to the final level enabled me to
surpass all the obstacles in the completion of this research work. Without his great co-
operation I would not be able to complete my research work.
I am extremely appreciative and thankful to Prof. Magnus Willander, ITN,
Linköping University, Norrköping Campus, Norrköping Sweden, for his great co-
operation, excellent guidance and providing me full access to the research laboratories
at ITN, Linköping University during my six months stay in Sweden. My gratitude and
obligations are due to Prof. Dr. Omer Nur, Dr. Peter Kalson, and Dr. Q. Wahab for
helping me during research work. I appreciate the cooperation of Ms. Sadia Faraz in
co-authoring the related articles. In addition, I would like to thank my lab fellows Dr.
Adnan Ali and Dr. M. Imran Arshad for their cooperation during all these years. I am
very grateful to Higher Education Commission for providing me financial support for
completing my Ph.D studies.
9
Last but not the least I wish to express my nice feelings towards my loving
and sweet family and friends. Words cannot describe my immense feelings of
appreciation for them. Special thanks for their prayers, encouragement and
unforgettable sacrifices with patience throughout my life and Ph.D studies.
Sincerly
Hadia Noor
10
Contents
Abstract 2
Declaration 5
Certificate 6
Acknowledgement 8
List of Publications 13
List of Figures 14
List of Tables 18
CHAPTER 1 19
INTRODUCTION 20
1.1 Semiconductor Materials 20
1.1.1 Wide Bandgap Semiconductor Materials 23
1.2 Motivation 25
1.3 Dissertation Outline 29
CHAPTER 2 31
PROPERTIES OF ZnO 32
2.1 Crystal Structure 32
2.2 Point Defects 37
2.2.1 Types of Point Defects 37
2.2.2 Point Defects in ZnO 38
2.3 Contacts to Zinc Oxide 44
2.3.1 Schottky Contact to ZnO 44
2.3.2 Ohmic Contacts to ZnO 46
CHAPTER 3 49
GROWTH TECHNIQUES 50
3.1 Hydrothermal technique 50
3.1.1 Experimental Setup of Hydrothermal Method 50
3.1.2 Hydrothermal Synthesis of Zinc Oxide 51
3.2 Molecular Beam Epitaxy 54
3.3 Aqueous Chemical Growth 56
CHAPTER 4 59
CHARACTERIZATION TECHNIQUES 60
4.1 Current-Voltage Measurements (I-V) 60
11
4.2 Capacitance-Voltage Measurements (C-V) 63
4.3 Deep Level Transient Spectroscopy (DLTS) 64
4.3.1 Carrier Kinetics in Semiconductors with Deep Level- Shockley-Read-Hall
Theory 65
4.3.2 Basic Principle of DLTS 66
4.3.3 Measurement of Defect Parameters by DLTS 71
4.4 X-Ray Diffraction (XRD) 75
4.4.1 Hexagonal System 76
CHAPTER 5 79
EXPERIMENTAL DETAILS 80
5.1 Group A and B Samples 80
5.2 Group C Samples 81
5.3 Samples D, E, F and G 82
5.3.1 Preparation of Seed Solution 82
5.3.2 Pretreatment of Substrate 82
5.3.3 Chemical Bath Deposition Growth 83
CHAPTER 6 84
RESULTS AND DISCUSSION 85
Section- I 85
6.1 Current-Voltage Measurements 85
6.2 Capacitance-Voltage Measurements 88
6.3 Deep Level Transient Spectroscopy Measurements 93
6.4 X-Ray Diffraction 96
6.5 Trap Identification 99
6.5.1 Electron Level E1 99
6.5.2 Electron Level E2 103
Section- II 104
6.6 Current-Voltage Measurements 104
6.7 Capacitance-Voltage Measurements 106
6.8 Deep Level Transient Spectroscopy Measurements 110
6.9 Trap Identification 113
6.9.1 Electron Level E1 113
6.9.2 Electron Level E2 119
12
Section- III 120
6.10 Hall Measurement 120
6.11 SIMS Measurement 122
6.12 Current-Voltage Measurement 123
6.13 Capacitance-voltage Measurement 124
6.14 Deep Level Transient Spectroscopy Measurements 125
6.15 Trap Identification 127
Section- IV 130
6.16 XRD Measurements 130
6.17 SEM Measurements 133
CHAPTER 7 136
CONCLUSIONS AND FUTURE PLAN 137
7.1 Group A Samples 137
7.2 Group B Samples 137
7.3 Group C Samples 138
7.4 Samples D, E, F, & G 139
7.5 Future Plan 139
REFERENCES 140
13
List of Publications
1. Influence of background concentration induced field on the emission
ratesignatures of an electron trap in zinc oxide Schottky devices. Hadia Noor, P.
Klasan, S. M. Faraz, O. Nur, Q.Wahab, M. Asghar, and M. Willander, J. Appl.
Phys. 107 (2010) 103717.
2. Time-delayed transformation of defects in zinc oxide layers grown along
the zinc-face using a hydrothermal technique. Hadia Noor, P. Klasan, O. Nur,
Q.Wahab, M. Asghar, and M. Willander, J.Appl. Phys.105 (2009) 123510.
3. Modeling and simulations of Pd/n-ZnO Schottky diode and its comparison with
measurements. S. Faraz, Hadia Noor, M.Asghar, M. Willander, Q. Wahab,
Advanced Materials Research. 79-82 (2009) 1317-1320.
4. Post-annealing modification in structural properties of ZnO thin films on p-type Si
substrate deposited by evaporation. M. Asghar, Hadia Noor, M.S. Awan, S.
Naseem and M.-A Hasan, Mater. Sci. in Semicond Processing, 11 (2008) 30-35.
5. Characterization of ZnO Thin Film Deposited by RF Operated Thermal
Evaporation. M. Asghar, I. Hussain, Hadia Noor, M. S. Awan and M.-A. Hasan,
Proceeding IEEE Regional Symposium on Microelectronics, Penang, Malaysia
(2007) 160.
6. Growth and Characterization of Single Crystalline Cubic SiC on porous Si using
low pressure chemical vapour deposition technique. M. Asghar, Payam Shoghi,
Hadia Noor, and M.-A. Hasan, NAM Proceedings (2007) 59.
14
List of Figures
Figure 1.1 The block diagram of semiconductor by conductivity. 21
Figure 1.2 The block diagram of semiconductor by structure. 21
Figure 1.3 The block diagram of semiconductor by composition. 22
Figure 1.4 The block diagram of semiconductor by bandgap. 23
Figure 2.1 Unit cell of ZnO and neighbouring atoms, viewing direction approx
parallel to c. Small spheres: O-2
, big spheres: Zn+2
. 33
Figure 2.2 Temperature dependence of PL spectrum for Zn-polar face and O-
polar face in the temperature range of 10 K– 300K. 36
Figure 2.3 Schematic representation of defects in semiconductors. 38
Figure 2.4 Electron energy diagram in equilibrium (1) and in the presence of an
electric field (2) showing field-enhanced electron emission: (a)
Poole-Frenkel emission, (b) phonon-assisted tunneling. 43
Figure 2.5 Metal-semiconductor contacts according to the simple Schottky
model . 44
Figure 2.6 Energy band diagrams for Ohmic contact. 46
Figure 3.1 Schematic drawing of a hydrothermal growth system. 53
Figure 3.2 The diagram of MBE system [36] 56
Figure 2.3 The chemicals for growth solution of ZnO. 57
Figure 3.4 The stirring of growth solution for ZnO. 58
Figure 3.5 Growth solution container placed in oven. 58
Figure 4.1 Band diagram of the intimate contact between metal and
semiconductor (n-type) for a rectifying junction. 60
Figure 4.2 Electron energy band diagram for a semiconductor with deep a level
trap. 65
Figure 4.3 The metal-semiconductor contact and the depletion layer . 66
Figure 4.4 The schematic illustration of a majority injection pulse sequence and
energy band bending. (a) bias time, (b) capacitance-time, (c) and (d)
the energy band bending during the pulse and after pulse. 67
Figure 4.5 XRD measurement of the Zn. 77
Figure 5.1 ZnO wafer grown by hydrothermal. 80
Figure 5.2 ZnO samples grown by MBE. 81
15
Figure 6.1 I-V characteristics of the Pd-Schottky contact on the Zn face of the
ZnO. 86
Figure 6.2 I-V characteristics of the Pd-Schottky contact on the Zn face of the
ZnO at 120 K. 87
Figure 6.3 I-V characteristics of the Pd-Schottky contact on the Zn face of the
ZnO at 340 K. 87
Figure 6.4 Plot between Ideality factor and 1000/T indicates the To- effect for
the Zn face Pd/ZnO Schottky diode. 88
Figure 6.5 C-V measurements of the Pd-Schottky contact on the Zn face of the
ZnO. 89
Figure 6.6 Graph between applied bias and inverse squared capacitance. 90
Figure 6.7 Depth profile of free carrier concentration of Pd/ZnO. 90
Figure 6.8 C-V measurements of the Zn face of the ZnO at different
temperatures. 91
Figure 6.9 Depth profile of free carrier concentration of Zn-face ZnO at
different temperatures. 91
Figure 6.10 DLTS spectrum displaying two electron deep level defects below
conduction band of ZnO. 94
Figure 6.11 The DLTS spectrum of levels E1 in ZnO. 94
Figure 6.12 The Arrhenius plot of levels E1 and E2 in ZnO. 95
Figure 6.13 Trap concentrations of levels E1 in Zn-face ZnO 95
Figure 6.14 Typical XRD pattern of the Zn-face ZnO layer exhibiting the Zni-VO
complex as the preferential direction of growth. Peaks other than
ZnO are seen because the XRD measurements were performed on
Pd\ZnO–Zn\Au–Cr mounted on an alumina substrate by silver paste. 97
Figure 6.15 C-V measurements indicate decrease in amplitude. 100
Figure 6.16 DLTS measurements indicate increase in amplitude. 101
Figure 6.17 Demonstration of time-delayed transformation phenomenon of
defects in ZnO layer. 101
Figure 6.18 Representative I-V measurements of group B samples. 105
Figure 6.19 Representative C-V measurements of group B samples. 105
Figure 6.20 Schottky behavior of the sample B is demonstrated in 1/C2-V, filled
squares represent the experimental data and the line corresponds to
16
the theoretical fit of the data, extrapolated to x-axis yield built-in
potential. 107
Figure 6.21 The uniform spatial distribution of the free-carriers in the as-
deposited ZnO material. 107
Figure 6.22 C-V measurements of the O face of the ZnO at different temperature. 109
Figure 6.23 Depth profile of free carrier concentration of O-face ZnO at different
temperatures. 109
Figure 6.24 Representative DLTS scans of group A and B samples to show the
variation in peak position of E1 level even measured under same
measuring setup. 111
Figure 6.25 The DLTS spectra measured at different frequencies for Arrhenius
plot of levels E1 in sample B. 112
Figure 6.26 The Arrhenius plot of levels E1 in samples A and B. 112
Figure 6.27 Depth profile of trap concentration of levels E1 in O-face ZnO. 113
Figure 6.28 Influence of background concentration ND on activation energy of E1
level. Data 1 and 2 are ours and rest of the data is taken from Refs.
19, 33, and 34. 115
Figure 6.29 The ND-induced field effect on the thermal energy data of the level.
Data 1 and 2 are ours and rest of the data is taken from Refs. 19, 33,
and 34. 116
Figure 6.30 Qualitative evidence of the Poole–Frenkel mechanism on the ND-
induced variation in emission rate signatures of E1 level. 117
Figure 6.31 Theoretical fitting of the ND-induced field emission rates (filled
circles) obeying Poole–Frenkel mechanism associated with Coulomb
potential (curve C), while square well potential (r = 4.8 nm) is not
consistent (curve S). 118
Figure 6.32 Representative temperature dependent Hall measurements of group
C samples. The upper part, middle part, and lower part of Figure
display mobility, carrier concentrations and resistivity, respectively. 121
Figure 6.33 SIMS depth profiles of O, Zn and N elements in group C samples. 122
Figure 6.34 Representative I-V measurements of group C samples. 123
Figure 6.35 Schottky behavior of group C sample is demonstrated in A2/C
2-V,
filled squares represent the experimental data and the line
corresponds to the theoretical fit of the data. 124
17
Figure 6.36 Depth profile of free carriers of group C sample. 125
Figure 6.37 Representative DLTS spectrum displaying one hole trap of group C
samples. 126
Figure 6.38 Typical DLTS spectra of levels H measured at different frequencies
of group C samples. 126
Figure 6.39 The Arrhenius plot of hole level in group C samples. 127
Figure 6.40 Metastability behavior of hole trap H with respect to time. 129
Figure 6.41 The Arrhenius plot of hole level in group C samples with passage of
time. 129
Figure 6.42 XRD patterns of four samples (D, E, F, & G). 131
Figure 6.43 SEM image of sample D grown by ACG. 134
Figure 6.44 SEM image of sample D grown by ACG. 134
Figure 6.45 SEM image of sample E grown by ACG. 135
Figure 6.46 SEM image of sample E grown by ACG. 135
18
List of Tables
Table 1.1 III-V compound semiconductors 20
Table 1.2 II-VI compound semiconductors 22
Table 1.3 Electrical parameters such as activation energy capture cross section
and defect concentration of different defects in ZnO. 26
Table 2.1 Physical properties of Zinc oxide 34
Table 2.2 2θ, intensity, and Miller indices of peaks of Zn-polar face and O-polar
face samples calculated from XRD measurements . 36
Table 3.1 The growth conditions optimized by Sakagami for zinc oxide crystals. 52
Table 4.1 Calculated miller indices (hkl) of hexagonal system when l = 0. 77
Table 4.2 Calculated miller indices (hkl) of hexagonal system 78
Table 6.1 Electrical parameters of Zn face ZnO calculated from C-V
measurements. 91
Table 6.2 Details of electrical parameters such as activation energy, capture
cross section measured via indirect and direct methods and trap
concentration of defects observed in the DLTS spectrum of Zn-face
ZnO. 96
Table 6.3 2θ, intensity, Miller indices, and sources of peaks measured from
XRD data in ZnO layers listed. 98
Table 6.4 Electrical parameters of O face ZnO calculated from C-V
measurements 108
Table 6.5 2θ, intensityand Miller indices from XRD data of sample D, E, F
and G. 132
19
CHAPTER 1
20
1 INTRODUCTION
This chapter presents an introduction about the importance of wide bandgap
semiconductor materials in the development of electronics industry. Among the wide
bandgap materials special focus is given to zinc oxide because of its potential
applications. A brief comparison of zinc oxide attractive properties with other wide
bandgap materials is given. Motivation and dissertation outline are also presented.
1.1 Semiconductor Materials
The importance of electronic industry cannot be denied in the progress of
world. Semiconductors are one of the basic and major materials of this industry.
Semiconductors can be classified in various ways. Four significant ways are given
below:
Conductivity
Structure
Composition
Bandgap
The block diagrams of semiconductor classification by foresaid ways are shown
in Figures 1.1, 1.2, 1.3 and 1.4.
Table 1.1 III-V compound semiconductors [1]
Group III A Group V A
N P As Sb
B BN BP BAs BSb
Al AlN AlP AlAs AlSb
Ga GaN GaP GaAs GaSb
21
In InN InP InAs InSb
Figure 1.1 The block diagram of semiconductor by conductivity.
Figure 1.2 The block diagram of semiconductor by structure.
22
Figure 1.3 The block diagram of semiconductor by composition.
III-V and II-VI compound semiconductors are formed by the combination of
elements of respective group in Periodic Table. These III-V and II-VI compound
semiconductors are listed in Tables 1.1 and 1.2, respectively. SiC, SiGe and GeSn are
the examples of IV-IV compound semiconductors.
Table 1.2 II-VI compound semiconductors [1]
Group II B Group VI A
O S Se Te
Zn ZnO ZnS ZnSe ZnTe
Cd CdO CdS CdSe CdTe
Hg HgO HgS HgSe HgTe
23
Figure 1.4 The block diagram of semiconductor by bandgap.
Bandgaps are direct and in-direct for various compound semiconductors.
1.1.1 Wide Bandgap Semiconductor Materials
Wide bandgap semiconductors like GaN, ZnO and SiC got significant
attention due to their importance in optoelectronics and microelectronic devices. Wide
bandgap semiconductors are related to the emission/absorption wavelength of optical
devices. Light emitting diodes, laser diodes, photodiodes, photoconductive sensors,
electro-modulation devices are the examples of optical devices. Among the wide
bandgap semiconductors there is an increased interest in ZnO because of the
following advantages over other wide bandgap materials;
ZnO is a low cost material as compared to other wide bandgap materials such
as SiC and GaN.
Quality films of ZnO can be grown by a number of cost effective methods like
aqueous chemical growth method and hydrothermal growth technique etc.,
whereas quality GaN and SiC film growth require comparatively much
expensive techniques.
Semiconductor: Bandgap
Narrow bandgap
(0.1-1.5 eV)
Moderate bandgap
(1.5-3.0 eV)
wide bandgap
(3.0-6.0 eV)
24
ZnO is more suitable material for wet chemical etching which is most
essential in the device design and fabrication [2].
ZnO is an attractive material with high luminescence efficiency as compared
to GaN and SiC [3].
The growth of large single crystals of ZnO is possible on the other hand which
is still an issue for GaN and SiC.
It is easy to make nanostructures from ZnO which emit light and sense charge
transfer efficiently [4].
ZnO is a fascinating material due to piezoelectric properties and potential uses
in electronics [5], optoelectronics [6], energy conversion [7], and biosensors
[8].
It has great tolerance for radiation [9,10].
A brief preview of ZnO applications is presented in the following: zinc oxide
(ZnO) is a II-VI direct wide bandgap (3.37 eV) semiconductor with a large exciton
binding energy (60 meV). This material is famous for its photonic and electronic
applications such as UV light emitters/detectors and as high-power and high-
temperature devices [3,11]. ZnO nanostructures have significant device applications
for example surface acoustic wave filters [12], photodetectors [13], photonic crystals
[14], light emitting diodes [15], gas sensors [16], photodiodes [17], optical modulator
waveguides [18], varistors [19], solar cells [20, 21] and nanowire, nanolasers,
biosensors and field emission devices [22]. It has superior physical parameters for
electronic applications including a high breakdown electric field strength, high
thermal conductivity, high electron saturation velocity and high radiation tolerance. In
addition to its valuable optoelectronic properties, it is a candidate for the fabrication
of a dilute magnetic semiconductor with a Curie temperature higher than room
25
temperature [23]. Piezoelectric properties of the material are being explored for
fabrication of various pressure transducers, acoustic- and opto-acoustic devices [24].
Because of these characteristics, ZnO is now considered to be in the line of traditional
semiconductors such as Si and GaAs, and it is also compatible with wide-bandgap
semiconductors such as SiC and GaN [25].
1.2 Motivation
The electronic device applications as meantioned in previous section are
linked with the in house defects chemistry and thereafter, electronic structure of the
material. Defects have detrimental effects on the working of the devices, they are
known to degrade the lifetime and efficiency of the devices. However sometimes
defects are blessings, for example, several research groups [26-31] have suggested
that oxygen vacancies (defects) are the source of green luminescence in ZnO.
Therefore, an understanding of defects in the materials is essential for improving the
material quality and device performance. The defect chemistry and electronic
structure of the material have been the subjects of recent theoretical and experimental
studies. To understand the true nature and cause of defects in ZnO, it is important to
review the previous history of the defects in the material. A comprehensive study of
defects in ZnO grown by various techniques reported in the literature is prepared in
the form of a Table 1.3.
26
Table 1.3 Electrical parameters such as activation energy capture cross section and
defect concentration of different defects in ZnO.
Defect
identificati-
on
Defect
activation
energy
(eV)
Capture cross
section
(cm2)
Defect
concentration
(cm-3
)
Growth
technique
Research
group
Oxygen
vacancy
0.1 (1.2±0.5)×10-13
(14±2) ×1014
Seeded chemical
vapor transport
Wenckstern
et al. [11] 0.11 (1.2±0.5)×10
-13 (14±2) ×10
14
Pulsed-laser
deposition
0.12 (1.3±0.5)×10-13
(14±2) ×1014
Pressurized melt
growth
0.22 5×10-13
-
Hydrothermal
Grown
Vines et al.
[32]
0.27
1.6×10-16
for Pd-
SBD
1.2×10-16
for Au-
SBD
(4-5)×1014
Vapor-phase
grown
Fang et al.
[33]
0.27 0.3×10-14
2.1×1012
Hydrothermal
grown
Simpson et
al. [34]
0.29 1.5×10-15
1016
Pressurized melt
growth Auret et al.
[35] 0.29 - 10
14
Seeded chemical
vapor transport
0.29 2.8×10-14
5.1×1012
Hydrothermal
grown
Simpson et
al. [34]
0.3
-
-
Hydrothermal
grown
Kuriyama et
al. [36]
0.3 4×10-16
1 ×1015
Pressurized melt
growth
Ling et al.
[37]
Conted….
27
Conted….
Defect
identificati-
on
Defect
activation
energy
(eV)
Capture
cross
section
(cm2)
Defect
concentration
(cm-3
)
Growth
technique
Research
group
Oxygen
vacancy
0.31 5×10-13
-
Hydrothermal
grown
Vines et al.
[38]
0.32 2.5×10-14
8.3×1012
Hydrothermal
grown
Simpson et
al. [34]
0.53 4×10-15
1×1015
Vapor
transport
process
Hofmann
et.al [39]
0.57 1.5×10-16
-
Hydrothermal
grown
Vines et al.
[32]
Zinc
interstitial
0.1 - - Solid state
method
Chattopadh-
yay et al.
[40]
0.27
1.6×10-16
for Pd-
SBD
1.2×10-16
for Au-
SBD
(4-5)×1014
Vapor-phase
grown
Fang et al.
[33]
0.32 -
1014
to 1016
Pulsed-laser
deposition
Frenzel et.
al [41]
Zinc antisite
0.22 8.22×10
-
17
110×1014
Hydrothermal
technique
Noor et. al
[42]
28
Defect
identificati
-on
Defect
activatio
n energy
(eV)
Capture
cross
section
(cm2)
Defect
concentratio
n
(cm-3
)
Growth
technique
Research
group
Intrinsic
Defect
0.29 (6.2±0.7)
×10-16
(62±7)×10
14
Pulsed-laser
deposition
Wenckster
n
et al. [11]
0.3 (2±0.4)×10
-
16
(80±4)×1014
Pressurized
melt growth
0.3 (6.2±0.7)×10
-16
(2.2±0.4)×101
4
Seeded
chemical
vapor
transport
Zn-related
defect
0.26 11.16×10-17
-
Hydrotherma
l
technique
Noor et. al
[43]
0.31 1×10-15
1.2×10
14
Vapor-phase
Frank et al.
[44] -ve U-
centre 0.54 2×10
-13
from 1015
to
1017
Donor like
defect
0.01
-
-
Pulsed laser
injection
Seghier et
al.
[45]
0.03 - 1017
Seeded
chemical
vapor
transport Wenckster
n
et al. [11] 0.05 - 5×1016
Pressurized
melt growth
0.07 - 5.7×1016
Pulsed-laser
deposition
Surface
related
defect
0.49
3.4×10-14
for
Pd-SBD
3.5×10-15
for
Au-SBD
- Vapor-phase
grown
Fang et al.
[33]
0.49 1.18×10-14
2.01×1014
Hydrotherma
l
technique
Noor et. al
[42]
H diffusion
in ZnO
0.17 &
0.37
-
-
Dc
magnetron
sputtering
Nickel [46]
29
However, the following few of the unclear results listed in Table 1.3 are given to
justify the present work:
The fluctuating activation energy of so called oxygen vacancy ( Vo) defects
varies from 0.1eV to 0.75eV [32-39] but the defects parameters should be same like
fingerprints. Similarly, defect parameters like capture cross section and defect
concentration of oxygen vacancy also vary from one to other. The defect parameters
(activation energy and trap concentration) of zinc interstitial are not clear [33, 40, 41].
In the same way activation energy and trap concentration of donor like defect are not
the same [11,45]. Identification of an electron defect level having activation energy
0.3 eV is not clear [36-38]. In addition, a number of controversial interpretations of
defects are discussed in the recent literature [47-54].
This ambiguous knowledge about defect parameters and their origin motivated us
for this study. For getting better quality, good performance, high efficiency and long
working lifetime of ZnO-based devices, it is necessary to carry out a comprehensive
study of defects in ZnO.
1.3 Dissertation Outline
This dissertation is organized into seven chapters. Chapter 1 covers motivation
and dissertation outline about the current research. Chapter 2 highlights the details of
material (ZnO) such as crystal structure, physical properties and contacts to n-type
ZnO. Chapter 3 explains the growth techniques such as hydrothermal method,
molecular beam epitaxy and aqueous chemical growth. Chapter 4 presents various
characterization techniques such as current-voltage (I-V), capacitance-voltage (C-V),
deep level transient spectroscopy (DLTS) and x-ray diffraction (XRD) used for
characterization of defects in semiconductors. Chapter 5 explains the experimental
30
details of growth techniques such as hydrothermal method, molecular beam epitaxy,
and aqueous chemical growth. Chapter 6 explains the experimental results and
discussions of current experiments. Chapter 7 describes the conclusions and future
plan.
31
CHAPTER 2
32
PROPERTIES OF ZnO
This chapter briefly explains crystal structure of ZnO. A detail review about the
different defects in ZnO is provided. Furthermore, the influence of electric field on
deep level defects is enlightened which is helpful to identify the defects. The progress
of Schottky and Ohmic contacts to ZnO are also described because quality of ZnO
based devices depends upon better quality contacts (Ohmic and Schottky).
2.1 Crystal Structure
The crystal structure of ZnO is hexagonal (Wurtzite) as shown in Figure 2.1.
The space group of this hexagonal lattice is a P63mc. ZnO is a polar material. The
values of some physical properties of ZnO are listed in Table 2.1. The lattice
parameters of ZnO are a = 3.25 Ao and c = 3.18 A
o. There is a strong ionic bond ZnO
due to the large difference in the values of electronegativity (oxygen = 3.44 and Zinc
= 1.65) [1]. ZnO is characterized by two connecting sub lattices of Zn+2
and O.-2
Each Zn ion is surrounded by tetrahedra of O ions and each O ion is surrounded by
tetrahedra of Zn ions. This tetrahedral coordination provides the polar symmetry
along the hexagonal axis. In an ideal Wurtzite crystal, the ratio of lattice parameters
c/a and the u (it is the parameter by which each atom is displaced with respect to the
next along the c-axis) are correlated by the relationship as
8
3
a
uc (2.1)
33
ZnO Unit Cell
Figure 2.1 Unit cell of ZnO and neighbouring atoms, viewing direction approx
parallel to c. Small spheres: O-2
, big spheres: Zn+2
[2].
b a
34
Table 2.1 Physical properties of Zinc oxide [3]
Property Values
Chemical formula
ZnO
Lattice parameters at 300 K
a0
c0
3.24 Ao
5.20 Ao
Density
5.606 gcm-3
Stable phase at 300 K
Wurtzite
Melting point
1975 oC
Boiling point
2360 oC
Linear expansion coefficient
a0 = 6.5×10-6 o
C-1
c0 = 3.0×10-6 o
C-1
Energy bandgap
3.4 eV, direct
Static dielectric constant
8.5
Refractive index
2.008, 2.029
Exciton binding energy
60 meV
Electron effective mass
0.28 mo
Hole effective mass
0.59 mo
Bulk Young’s modulus
111.2 ± 4.7 GPa
Bulk hardness
5.0 ± 0.1 GPa
Electron Hall mobility at 300 K
200 cm2 (Vs)
-1
Hole Hall mobility at 300 K
5-50 cm2 (Vs)
-1
35
The conditions for an ideal crystal are
3
8
a
cand
8
3u (2.2)
ZnO crystals deviate from this ideal arrangement by variation of both of these
values. This deviation occurs in the lattice because tetrahedral distances are roughly
constant. For Wurtzite ZnO, experimentally calculated values of u and c/a were in the
range u=0.3817–0.3856 and c/a=1.593–1.6035 [4 – 6].
Several properties of ZnO such as piezoelectricity and spontaneous
polarization depend on its polarity. Based on termination scheme, the Wurtzite ZnO
has four types (i) polar type terminated at Zn-face along (0001) direction (ii) polar
type terminated at O-face along (0001) direction; (iii) non-polar type along (11 2 0)
direction (iv) non-polar along (1010) direction. The number of Zn and O atoms are
equal in both non-polar (11 2 0) and (1010) faces. Furthermore, the polar surfaces and
non-polar surface (1010) are stable, but the (11 2 0) face is less stable and generally
has a higher level of surface roughness than its counterparts. Each polar face has
unique chemical and physical properties like electronic structure of O-terminated face
is slightly different from Zn-terminated [7]. The optical properties, for example in
photoluminescence, the PL intensities of Zn-polar face are different from O-polar face
because of exciton-phonon coupling strengths, opposite band bending effects and
difference of the adsorbed molecules as shown in Figure 2.2 [8-10]. In the same
manner XRD intensities of Zn-polar face are different from O-polar face as exposed
in Table 2.2.
36
Figure 2.2 Temperature dependence of PL spectrum for Zn-polar face and O-polar
face in the temperature range of 10 K– 300K [10].
Table 2.2 2θ, intensity, and Miller indices of peaks of Zn-polar face and O-polar face
samples calculated from XRD measurements [11].
Peak# Zn-face O-face Identification
(Miller Indices)
2Θ
(degree)
Intensity 2Θ
(degree)
Intensit
y
P1 31.28 1097 31.04 1844 ZnO (1010)
P2 34.48 15744 34.48 812147 ZnO (0002)
P3 36.36 455 Missing ZnO(1011)
P4 38.08 1607 Missing Al(11 1), Ag(111)
P5 40.2 32603 40.2 13862 Zn(1010)
P6 44.76 880 44. 32 90 Ca(531)
P7 Missing 52.72 1334 Unidentify
P8 Missing 57.44 59 ZnO(11 2 0)
P9 64.4 990 64.6 246 Li(211)
P10 65.48 1362 Missing Al(220)
P11 72.64 2208 72.6 49633 ZnO (0004), Zn(11 2 0)
P12 73.72 8260 Missing Ca(331)
P13 77.56 382 Missing Mg(20 2 2)
P14 81.6 181 Missing Al(222), Ag(222)
P15 86.64 1466 86.92 473 Zn (20 2 1)
37
This difference of properties can be due to defects present in Zn and O-polar.
In the following section we describe the Physics of various defects reported in
literature.
2.2 Point Defects
Semiconductors usually have small crystallographic irregularities, also called
as point defects. Point defects are introduced during growth or in post- growth
methods for example annealing or ion implantation. Even if semiconductor materials
are free of point defects and unexpectedly pure, defects in small concentrations do
exist which have remarkable effect on properties of semiconductors. Defects are not
bad in all cases. However, sometimes they may appreciably enhance the performance
of devices instead of reducing it. The defects such as dislocation, edge dislocation and
others are not addressed because this dissertation is focused only on point defects
2.2.1 Types of Point Defects
There are many forms of crystal point defects. Defects involving only the host
atoms are called the intrinsic defects. Several types of defects are shown in Figure 2.3.
A defect wherein a host atom is missing from one of these sites is
known as a vacancy defect.
If an atom is located on a non-lattice site within the crystal, then it
is said to be an interstitial defect.
A defect in which one host atom is located on wrong site (one host
atom occupied the site of another atom), then it is called an antisite
defect.
38
Figure 2.3 Schematic representation of defects in semiconductors [12].
2.2.2 Point Defects in ZnO
There are two types of point defects in bulk ZnO which have been studied in
literature.
2.2.2.1 Intrinsic Defects
In case of ZnO, the oxygen vacancies (VO), zinc vacancies (VZn), oxygen
interstitials (Oi), zinc interstitials (Zni), oxygen antisites (OZn) and zinc antisites (ZnO)
are the intrinsic defects [13]. Similarly, complex defects are the combination of point
defects like VO-Zni in ZnO [14,15]. They are acting as free carrier concentration and
are responsible for n or p-type in bulk ZnO. The electric field induced by intrinsic
defects affects the properties of deep level defects present in bulk ZnO to be discussed
in section 2.2.2.3.
2.2.2.2 Extrinsic Defects
The defects involving the foreign atoms (impurities) are called the extrinsic
defects for example hydrogen interstitial (Hi) and hydrogen on oxygen sites (HO) in
ZnO [16, 17].
39
2.2.2.3 Influence of Electric Field on Deep Level Defects
The activation energy of traps can be lowered in strong electric fields. The
activation energy of traps is calculated from the emission rate. The emission rate of
carriers from trap is most significant parameter of defect. Each defect has a unique
emission rate. According to Martin et. al. [18] the origin and the carrier excitation
phenomenon of the defect are determined from the emission rate. The emission
rateen,p (the subscripts n and p stand for electron and holes carriers) of defect in terms
of mathematical equation is defined as:
kT
GNVge vcthpnpnpn exp.,,, (2.3)
where G ,png ,,
pn, , thV , vcN . , k, and T are the change in Gibb’s free energy at
constant temperature and pressure, degeneracy of deep states, capture cross-section,
the carrier’s average thermal velocity, the effective density of states, the Boltzmann
constant and absolute temperature, respectively. An internal electric field is produced
in depletion region due to the free carrier concentrations.The relation between internal
electric field and free carrier concentration is explained as what follows:
The electric-field is calculated by the following relation [19]:
Electric field = Qdep /εεo (2.4)
Qdep = god EqN 2 (2.5)
where Qdep, q, εo, ε, , Nd, and Eg are charges in the depletion region, electronic charge,
permittivity of free space, relative permittivity of the host crystal, free carrier
concentration and bandgap, respectively. The emission rate of defects is affected by
40
the electric field due to free carrier concentration. The electric field dependence of the
emission rate has to be taken into account, because neglecting this effect may lead to
serious misinterpretations in the determination of deep level parameters. On the other
hand, it can yield a lot of useful information on the nature of deep traps. Interaction of
fields with emission rate of defect has been explained by various models which helps
to identify the nature of defects. Some of famous models are the following:
(i) Poole-Frenkle [20]
(ii) Phonon-assisted tunneling [21, 22]
(iii) Pure tunneling [18]
Vincet et al. and Naz et al. [23, 24] proposed a simple model to distinguish
between phonon-assisted tunneling and Poole-Frenkle mechanisms, i.e to compare the
plots of ln(e) vs F2 and ln(e) vs F0.5, respectively. In the case of phonon-assisted
tunneling the plot of ln(e) vs F2 is expected to provide a good linear fit, while for
Poole-Frenkle the electric field should follow the ln(e) vs F0.5 linear variation.
2.2.2.3.1 The Poole-Frenkel
According to the Poole–Frenkel theory, when electric field is applied, the
electron band diagram is slanted and the barrier depth is reduced. The emission rate
due to the barrier lowering of the Poole-Frenkel effect is written as [20]:
kT
EEEvNe iic
nncn
exp (2.6)
It can be written as:
41
kT
FEeFe i
non
exp (2.7)
The various models like Coulomb potential, square well potential and dipole
potential have been suggested to fit Poole–Frenkel effect on the emission rates of the
carriers from defect level.The mathematical equations for emission rates at electric
field F to Coulomb potential and square well potential are given below [18,23]:
2
111
1
)0( 2
ee
Fe
n
n (2.8)
where kTqqF or // 2
1
2
11
2
1
)0(
e
e
Fe
n
n (2.9)
where kTqFr /
where F and r are the electric field and radius of potential well, respectively. All other
constants bear usual meanings.
2.2.2.3.2 Phonon-Assisted Tunneling
Phonon-assisted tunneling theory states that if the electron has coupling with
the suitable phonon, then electron will tunnel through the barrier because the emission
energy will be reduced. The field dependence emission rate of the carrier to
characterize the phonon-assisted tunneling mechanism is given below [24]:
2
2
exp0
cF
F
e
Fe (2.10)
42
where Fe and 0e are the emission rate at field F and the emission rate at zero
field. The characteristic field cF is calculated as:
3
2
2
*3
q
hmFc (2.11)
where *m , q and 2 are the carrier effective mass, charge of electron, and the tunneling
time. The tunneling time is calculated as:
122
Tk
h
B
( 2.12)
where h and 1 are the Planck’s constant and characteristic time constant of the order
of the inverse local impurity vibration frequency.
The electric field to enhance emission for Poole-Frenkle and Phonon-assisted
tunneling is in the range of 104-106 V/cm. The enhanced emission processes are
illustrated in Figure 2.4. The electric field in pure tunneling is in the range of ≥107
V/cm [20].
43
Figure 2.4 Electron energy diagram in equilibrium (1) and in the presence of an
electric field (2) showing field-enhanced electron emission: (a) Poole-Frenkel
emission, (b) phonon-assisted tunneling [25].
44
2.3 Contacts to Zinc Oxide
For quality devices, suitable metals should be used to make contacts to ZnO
thin films. Device performance depends not only on the materials quality but also on
the type of metals and its contact. There are two types of contacts that are fabricated
on ZnO, namely:
Schottky Contacts [1]
Ohmic Contacts [26]
2.3.1 Schottky Contact to ZnO
The Schottky model of the metal and n-type semiconductor is defined as [24]:
smB
(2.13)
where φm is work function of metal, defined as the potential difference between the
the vacuum level and the Fermi level EF and φB and χs are barrier height and electron
affinity of semiconductor. The electron affinity of semiconductor is defined as the
energy difference between the bottom of the conduction band EC and the vacuum
level at the semiconductor surface.
Figure 2.5 Metal-semiconductor contacts according to the simple Schottky model
[25].
EC EF EV
X S
EF
M
45
The parameters such as barrier height, series resistance, and ideality factor of
the diode are determined after the fabrication of Schottky contacts. The fabrication of
Schottky contacts is difficult due to following factors: (i) the surface states, (ii) the
contaminants, (iii) the defects in the surface layer, and the diffusion of the metal into
the semiconductor. It is fact that high-quality Schottky contacts are serious issue for
ZnO device applications. First time Mead [27] fabricated the Schottky contacts with
different metal on vacuum-cleaved n-type ZnO surfaces in 1965 but he did not
comprehensively study the thermal stability of the Schottky diodes fabricated on ZnO.
The metals such as Au, Ag, Pd, Pt, and Ti are used for Schottky - contact to n-
type ZnO [28-45]. Al has strong reaction with anions (O). Al is supposed to make the
most dissociated cations (Zn) in ZnO. This results in low barrier height and leakage
current. Simpson et al. [46] reported that the thermal stability of Ag Schottky contacts
was better than that of Au Schottky contacts. Some other reports [30,31] also show
that Au Schottky contacts have some serious problems at temperatures more than 340
K. I-Vcharacteristics of as-deposited Au and Ag Schottky contacts on n-type ZnO are
improved by exposure to the plasma at room temperature or cleaning with organic
solvent before the metal deposition [31,47]. Ohashi et al. [33] suggested that the value
of the barrier height was lower because of high donor density. The higher value of
ideality factors may be due to relatively high carrier concentration which leads to the
increase of tunneling current through the junction. Ip et al. [43] observed that the
Schottky barrier height of Pt contacts on P-doped n-type ZnO grown by plused laser
deposition decreased and ideality factor increased with increasing temperature. It is
clear from the literature that the ideality factor for the ZnO Schottky contacts is higher
than unity in the most of the cases. The higher value of ideality factor can be
explained by different phenomenon such as the prevalence of tunneling [48] the
46
presence of an interface layer, surface states [34] increased surface conductivity [47]
and/or the influence of deep recombination centers [31].
2.3.2 Ohmic Contacts to ZnO
Any contact having a linear and symmetric current-voltage relationship for
both positive and negative voltages is called Ohmic contact. An Ohmic contact obeys
Ohm’s law and is demonstrated in Figure 2.6. It is essential for carrying electrical
current into and out of the semiconductor, ideally with no contact resistance. The
contact resistance is one of the main problems for long-lifetime operation of optical
and electrical devices. The high contact resistance between the semiconductor and
metal affects the device performance because of thermal stress and/or contact failure
[49]. It is necessary to attain Ohmic contacts that have low contact resistance and
thermally stable for high-performance ZnO-based optical and electrical devices. This
can be attained by reducing the metal-semiconductor barrier height or increasing the
effective carrier concentration of the surface [50-52].
Figure 2.6 Energy band diagrams for Ohmic contact [53].
EC
EF
EV
qF
qB
Ei
Metal Semiconductor
47
Ohmic-contact metallization is essential for obtaining good electronic device
performance [50]. In case of wide-bandgap semiconductors like ZnO, low resistance
Ohmic contact can be attained by thermal annealing. But surface roughness and
structural degradation of the interface can be produced during the thermal annealing
process [54,55], resulting in poor device performance [56]. The non-alloyed Ohmic
contacts having low specific contact resistance arechosen in Ohmic-contact
technology especially for shallow junction and low voltage devices because they give
smooth metal semiconductor interfaces [52, 57, 58].
The metals such as Al, Au, In, InGa , Pt , Ti, and Ru are often used for
Ohmic-contact to n-type ZnO [59-63]. Some reports are following; Lee et al. [64]
have successfully fabricated low-resistance and non-alloyed Ohmic contacts to
epitaxially grown n-ZnO. Marlow et al. [65] deposited Ohmic-contacts to n-ZnO by
hydrogen plasma treatment by Ti/Au metallization schemes and observed the contact
resistivity decreased by Ar plasma-treated. They suggested that this decrease in
contact resistivity is due to the formation of shallow donor by ion bombardment. Kim
et al.[54] also reported that resistivity of annealed samples decrease two order of
magnitude as compared to the as deposited samples. They proposed that this reduction
in resistivity is caused by the combined effects of the increases in the carrier
concentration near the ZnO layer surface and the contact area. Kim et al. [63]
proposed that Ohmic contact of Ru to n-type ZnO is appropriate for high-temperature
ZnO based devices. The Ohmic-contacts of Pt were characterized as linear if it
deposited to n-ZnO after surface modification [50,66]. Sheng et al. [52] reported an
extensive study of Ohmic-contact to n-ZnO. They found the following results: (i) the
electron concentrations are increased and the specific contact resistivity is decreased
48
because of tunneling and (ii) the specific contact resistivity of heavily doped ZnO is
lowered than unintentionally doped ZnO.
49
CHAPTER 3
50
2 GROWTH TECHNIQUES
Zinc Oxide can be grown by various techniques such as, sol-gel chemistry, spray
pyrolysis, metal organic chemical vapor deposition (MOCVD), molecular beam
epitaxy (MBE), pulsed laser deposition (PLD), hydrothermal, reactive thermal
evaporation, and sputtering [1-7]. ZnO samples used in this study were grown by
hydrothermal technique, MBE and aqueous chemical methods. These three techniques
are hereby described in detail.
3.1 Hydrothermal Technique
Hydrothermal technique can be defined as a method of synthesis of crystals
under elevated pressure and temperature conditions in the presence of water. The term
hydrothermal usually refers to any heterogeneous reaction in the presence of aqueous
solvents under high pressure and temperature conditions to dissolve and recrystallize
(recover) materials that are relatively insoluble under ordinary conditions. Today, the
hydrothermal technique has gained a unique place in various branches of modern
science and technology like in fabrication of nanostructures. Hydrothermal method is
preferred one and economical because of its short reaction times and lower energy
requirements. It is very important for its technological efficiency in developing
bigger, pure, and dislocation-free single crystals.
3.1.1 Experimental Setup of Hydrothermal Method
In hydrothermal technique, the growth chamber is capable of containing
corrosive solvent at high temperature and pressure required for growth. The
temperature, pressure, and corrosion resistance of solvent are the important
parameters for selecting a suitable growth chamber. The nutrient is supplied to the
51
growth chamber along with solvent (i.e an aqueous solution of alkali). The
temperature gradient is applied at the opposite ends of the growth chamber. The
nutrient dissolves at the hotter end of chamber and seed crystal are introduced at
cooler end of chamber for additional growth. An ideal growth chamber for
hydrothermal should have the following characteristics
Inertness to acids, bases and oxidizing agents.
Easy to assemble and dissemble.
Sufficient length to obtain a desired temperature gradient.
Leak-proof with unlimited capabilities for the required temperature and
pressure.
Rugged enough to bear high pressure and temperature condition for long
duration, so that no machining or treatment is needed after each experimental
run.
3.1.2 Hydrothermal Synthesis of Zinc Oxide
The hydrothermal technique is the most suitable method for fabricating
isometric (having equal measurement) zinc oxide crystals of good quality [8-13]. In
this method, the aqueous solution of ZnO is heated at suitable temperature in
temperature gradient chamber where ZnO seed crystals are mounted. ZnO vapors are
condensed on the seed crystals and form ZnO wafer.These wafers can be separated
from seed crystals by stress pressure. The schematic diagram of the hydrothermal
growth chamber is shown in Figure 3.1. The hydrothermal apparatus consists of
furnaces, pressure gauge, autoclave, heaters and Pt crucibles. The furnaces and
autoclave are used to increase the temperature. The Pt crucible contains baffle, vessel
of seed crystals in upper part of crucible and vessel of solution in lower part of
52
crucible. The heaters F1 and F2 are used to control the temperature in such a way that
temperature of upper part is less than lower part of Pt crucible.
Hydrothermal growth process is described as following: initially some seed
crystals are hanged in the upper part of Pt crucible by a Pt wire. The sintered ZnO as
nutrient and aqueous solution of alkali (NaOH, KOH, LiOH) or chlorides of desired
molarity as solvent are mixed together to form the solution [14-18]. The solvent
breaks the intra molecular bonds of ZnO due to week Van de Waal forces. This
solution is put in lower part of Pt crucible. A Pt baffle is installed to separate seed
crystals from solution. The crucible is placed into an autoclave after closing it by
welding. The autoclave is then placed into a two-zone vertical furnace. When solution
temperature and pressure of autoclave are increased, water of aqueous solution
converted into steam and moved to upper part of crucible. Then steam returned backin
liquid form to lower part of crucible because of condensation of water molecules.
ZnO is attached to seed crystals in upper part of Pt crucible and seed crystals grow to
bulk. Only ZnO grow in upper part of crucible due to the presence of its mother seed.
Table 3.1 The growth conditions optimized by Sakagami for zinc oxide crystals [30].
Parameters
Values
Growth temperature
370–400 °C
Temperature difference
10–15 °C
Total pressure
700–1000 kg/cm
Partial pressure
10–30 kg/cm
Solvent KOH 3.0 M + LiOH 1.5 M
Oxidizer
H2O2 0.1 M–0.3 M
Nutrient ZnO sintered
Growth run
15–20 days/run
53
Figure 3.1 Schematic drawing of a hydrothermal growth system. F: furnaces (F1,F2),
T: thermocouples for control (T1,T2) and monitor (T3), P: pressure gauge, A:
autoclave, C: Pt crucible, S: seed crystals, N: nutrient, Baffle [19].
In the zinc oxide growth, the oxidation of the chamber material produces
hydrogen due to the presence of alkalis. The amount of hydrogen formed depends on
the temperature, the concentration of the alkali, and the duration of the experiment.
54
Several research groups have successfully grown ZnO nanostructures by
hydrothermal methods for biomaterials applications [20-28]. Sun et. al. [29] had
grown ZnO nanostructures by hydrothermal technique. They obtained the outstanding
variety of ZnO nanostructures having different and controllable morphologies by
varying growth parameters such as precursor chemicals, their concentrations and/or
the growth temperature. Sakagami [30] has grown zinc oxide crystals of high purity
by the hydrothermal method using platinum-lined chamber. The growth conditions
optimized by Sakagami are listed in Table 3.1.
3.2 Molecular Beam Epitaxy
Molecular beam Epitaxy is the most advance technique to grow atom by atom
layers of thin films. There is no substitute to the quality of materials grown by MBE,
however it does have some fundamental limits. An ultra high vacuum is required to
grow the material. Advantages of MBE are the following:
Excellent interface and surface morphology.
It is possible to control the thickness of epilayer precisely.
In-situ characterization techniques (RHEED).
High purity starting materials, easy chemistry.
Low growth temperature which reduces any undesirable thermally
activated processes such as diffusion.
The slow growth rate and high cost are disadvantages of MBE growth. Now
various scientists have successfully grown ZnO by MBE [31-34].
The substrate is placed on sample holder. It is heated to required temperature
and rotated continuously (if necessary) for better quality of growth homogeneity [35].
55
The ultra high vacuum (UHV) in the range of 10-6
to 10-4
mbar is needed for MBE
growth. The O2, CO2, H2O and N2 contamination on the growing surface can be
ignored after outgasing under UHV. The reduced rate down to nm/sec is achieved by
particular growth conditions. In this way precise growth of control thickness is
possible. Prior to growth, substrate is cleaned to avoid the effect of contamination on
the properties of substrate. The basic requirements for MBE growth are following, (i)
pure startng materials are to be used. (ii) Low background pressure in the evaporator
to decrease contamination; (iii) Uniform flux of effusion cell across the substrate. (iv)
The reaction chamber is evacuated to <10-8
mbar and the walls of the chamber cooled
with liquid nitrogen. The UHV is an important characteristic of MBE. The most
common diagnostic techniques to monitor the growth is reflected high-energy electron
diffraction (RHEED). Each monolayer growth can be seen in the intensity and pattern
of the RHEED signal. Thus growth can be controlled precisely at themonolayer
level.The diagram of MBE system is shown in Figure. 3.2.
56
(1) RHEED, (2) growth chamber, (3) baking heaters, (4) ion pump, (5) gate wall, (6)
mechanical pump, (7) preparation chamber, (8) fishing lever, (9) Viewing windows.
Figure 3.2 The diagram of MBE system [36]
3.3 Aqueous Chemical Growth
It is the easiest, cheap and time saving growth technique. A number of ZnO
nanostructures such as nanowires, nanorods and nanotubes have been grown by this
technique [37-40]. In this method, first of all, seed solution of required material is
prepared by the mixture of different chemicals of desired molarity under necessary
condition of temperature and stirring. The seed solution should be transparent. Then
few drops of the seed solution were placed on the substrate and rotated with help of
1 4 3
2
5
6 7 8
9
57
spin coater at desired speed. After this the growth solution is made by the mixture of
chemicals (HMT and Zinc nitrate hexahydrate for ZnO as shown in Figure 3.3) of
preferred molarity in deionized water at room temperature and stirred using magnetic
stirrer apparatus as illustrated in Figure 3.4. The substrate is put in the solution with
face downward towards the solution. Subsequently, solution container is placed into
the oven as shown in Fig.3.5 at temperature less than 100oC for some hours. Then
substrate is removed from the solution, cleaned with deionized water and dried at
room temperature and used for characterization.
Figure 2.3 The chemicals for growth solution of ZnO.
58
Figure 3.4 The stirring of growth solution for ZnO.
Figure 3.5 Growth solution container placed in oven.
59
CHAPTER 4
60
3 CHARACTERIZATION TECHNIQUES
Characterization techniques can generally be placed into two categories i.e. Electrical
and Optical. It is one of the most important steps to study the various properties of
semiconductor materials. This chapter describes the various characterization
techniques like current-voltage (I-V), capacitance-voltage (C-V), deep level transient
spectroscopy (DLTS), and X-ray diffraction (XRD). To perform the electrical
measurements, Schottky and Ohmic contacts are necessary, described in the previous
chapter.
4.1 Current-Voltage Measurements (I-V)
The current-voltage characteristic (I-V) provides the knowledge of
fundamental parameters about the performance of devices. Figure 4.1 shows the
intimate contact between the metal and the n-type semiconductor.
Figure 4.1 Band diagram of the intimate contact between metal and semiconductor (n-
type) for a rectifying junction [1].
61
When a voltage is applied to the metal contact, current flows through a
uniform metal-semiconductor interface. The movement of majority carriers across the
interface such as electrons in n-type material or holes in p-type material generates a
flow of current. Three main current mechanisms can occur, namely, (i) Thermionic
emission (TE), (ii) thermionic field emission (TFE), and (iii) Field emission (FE).
Thermionic emission is dominant for lightly doped semiconductors, for the
intermediate doped semiconductors, the current flows as a result of thermionic field
emission whereas field emission can dominate in heavily doped semiconductors [2].
The current-voltage technique involves a range of voltages, both positive and
negative, to the metal-semiconductor system and the measurement of the resulting
current [3]. When a reverse or negative voltage is applied, the energy levels of an n-
type semiconductor are lowered with respect to the metal Fermi level. The barrier for
electrons to traverse from the semiconductor to the metal is increased as a result of the
increased band bending, associated with the lowering of the Fermi level. In this
scenario, the electrons flow from the semiconductor to the metal is decreased. When
positive or forward voltage is applied, results in the Fermi level of the semiconductor
being raised compared to the metal. In this case, the barrier for electrons to traverse
from the metal to the semiconductor remains the same, but the barrier for electrons to
traverse from the semiconductor to the metal is reduced. Thus, equilibrium conditions
in the current flow are altered, and the flow of current from the metal to the n-type
semiconductor is greater than the opposing current. Therefore the metal-
semiconductor junction with positive voltage has a rectifying behavior. A large
current exists under forward bias than the reverse bias.
62
The ideality factor (n), barrier height (φB)I-V and saturation current (Is) are
measured by forward I-V measurements at given temperature. By plotting the
experimentally measured I-V data, as ln(I) vs V, the intercept of the linear fit will give
saturation current (Is) from which barrier height (φB)I-V could be extracted, (method is
described below) and the slope of the plot will give the value of ideality factor (n),
using following relations[1]:
1exp
nkT
qVII S (4.1)
kT
qTAAI
B
S
exp2* (4.2)
slopekT
qn
(4.3)
dV
Idslope
ln (4.4)
where A is Schottky contact area, A* is the Richardson constant, T is temperature in
Kelvin, k is the Boltzmann’s constant (k = 1.3810-23
J/K or 8.61710-5
eV/K) and q
is the electric charge. Deviation of n from the ideal range 1.01 <n < 1.1 may be
generated by a thick interface layer or substantial recombination in the depletion
region. A non-uniform interface will also produce an increase in n from 1.01- to
higher value [1]. The saturation current is calculated from the intercept of semilog
graph of I-V measurements at different temperatures. In this way we have the values
of saturation current at different temperatures. The slpoe of plot of log (Is/T2
) vs
1000/T gives the barrier height.
63
4.2 Capacitance-Voltage Measurements (C-V)
Capacitance-Voltage (C-V) method is based upon the voltage dependence of
the charge in the depletion region of the diode. The C-V method measures the
electrostatic properties of the Schottky barrier and is not affected by transport
processes such as tunneling and image force lowering.
The capacitance of Schottky diodedecreases asreverse bias increases.In
capacitances-voltage characteristics, capacitance for a Schottky diode is given by
following relation [4]:
VV
qNAC
bi
roD
2
(4.5)
where C, A, ε, q, ND, V, k and T are capacitances, area of contact, dielectric constant,
electric charge free carrier concentration, applied voltage, Boltzmann constant and
temperature, respectively. In C-V measurements, a plot of (A/C)2
vs V will give a
straight line. The slope and x-intercept of straight line are (2/ εqND) and 2Vbi/εqND
respectively.The Vbi (electrostatic potential at equilibrium in semiconductor) is called
built-in potential. The free carrier concentration ND is calculated from the slope using
the following relation:
slopeq
N D
2 (4.6)
The depth profile of free carrier concentration is also found from C-V
measurements by plotting the graph between free carrier concentration and depth. The
depth x is calculated by following relations:
64
dV
dCq
N D 2
2
(4.7)
C
Ax o
(4.8)
The barrier height, (φB) C-V can be calculated from:
VoVbivcB (4.9)
D
C
ON
NIn
q
kTV (4.10)
222
ekTmNC
(4.11)
where VO is the energy difference between the Fermi level and the bottom of the
conduction band, and NC
is the conduction band density of the states, m*
is the
effective mass of material. High densities of impurities or defects with deep energy
levels in the band gap make it difficult to measure the Schottky barrier height (SBH)
by C-V, since these defects change the space-charge region and hence the intercept
voltage. These deep level defects typically cause the SBH (C-V) to be greater than the
SBH (I-V).
4.3 Deep Level Transient Spectroscopy (DLTS)
Deep Level transient spectroscopy is one of the most versatile techniques to
detect impurities and defects in semiconductors. The level of detection, going down to
~1011
cm-3
by this method, is unmatched with other characterization techniques. The
DLTS technique also provides electrical parameters of the levels associated with
impurities or defects. By monitoring capacitance transients produced by pulsing the
65
semiconductor junction at different temperatures, a spectrum is generated which
exhibits a peak for each level. The height of the peak is proportional to trap density
and the position of the peak on the temperature axis leads to the determination of the
fundamental parameters governing thermal emission and capture (activation energy
and cross section).
4.3.1 Carrier Kinetics in Semiconductors with Deep Level- Shockley-
Read-Hall Theory
The carrier kinetics in extrinsic semiconductors can be explained by
Shockley-Read-Hall theory [2]. To support our results, we will describe this theory
qualitatively only. Consider ND, Pt, and Nt which denote the doping concentration, a
hole trap concentration and an electron trap concentration, respectively. Both the
electron and the hole traps are uniformly distributed throughout the semiconductor
with the trap activation energy of ET as shown in Figure. 4.2 Ec, Ev, cn, en,cp and ep
represent the energy of conduction band, energy of valence band, capture of electrons,
emission of electrons, capture of holes and emission of holes, respectively.
Figure 4.2 Electron energy band diagram for a semiconductor with deep a level trap
[2].
66
Figure 4.2 shows a recombination event (a) followed by (c) and a generation
event (b) followed by (d). The impurity is a G-R center and both the conduction and
valence bands take part in recombination and generation. A trap has only two charge
states, a filled state which is occupied by an electron, or an empty state which is
unoccupied. If the capture and emission of electrons are dominating, then the total
electron trap concentration can be expressed as
NT = nt + pt (4.12)
where nt is the concentration of the trapped electrons (occupied states), pt is the
concentration of unoccupied states. Similarly, if the capture and emission of holes are
dominant, then the total hole trap concentration is:
PT = nt + pt (4.13)
where nt and pt , are respectively the concentrations of trapped holes (occupied states)
and neutral traps (unoccupied traps).
4.3.2 Basic Principle of DLTS
In the following, we consider an n-type semiconductor containing only
electron traps. A depletion layer of width x will be formed as show in Figure 4.3.
Figure 4.3 The metal-semiconductor contact and the depletion layer [2].
x
Depletion region
V
67
When bias voltage V is imposed across the junction, the depletion layer
capacitance per unit area is given by:
VV
qNC
bi
roD
2
(4.14)
where Vbi is the built in potential and V is the bias voltage, positive for forward bias
and negative for reverse bias.
Figure 4.4 The schematic illustration of a majority injection pulse sequence and
energy band bending. (a) bias time, (b) capacitance-time, (c) and (d) the energy band
bending during the pulse and after pulse [2].
68
Assuming that a forward bias pulse ∆V is applied, then the capacitance prior to
the application of the forward bias pulse is:
VV
qNC
bi
roD
o
2
(4.15)
When a pulse ∆V<V is applied, electrons will move into the n-side and they will
fill the traps step by step. After a certain period of time, when the traps are filled then
the capacitance will increase.
VVV
nNqC
bi
tDo
2
(4.16)
When all traps are filled with electrons, nt= NT. At the moment when ∆V= 0 at t
= 0 as shown in Figure 4.4 (b), the capacitance C changes to:
VV
NNqtC
bi
tDo
20
(4.17)
After ∆V= 0, the trapped electrons are regularly re-emitted to the conduction
band as shown in Figure. 4.4 (d). This means that nt slowly reduces and finally nt, = 0,
then Eq. (4.17) becomes Eq. (4.15) and C returns to Co. The change of the capacitance
C with time after t = 0 is referred to as the capacitance transient, which can be written
as:
VV
tnNqtC
bi
tDo
2
(4.18)
Eq. (4.18) can be written as:
69
tnNK
VV
tC D
bi
22
1 (4.19)
where oqK 2 , is a constant, by defining S (t) = -dV/d (1/C2), we obtain
tt nnKSS 00 2 (4.20)
Suppose that nt(0) = NT and nt (∞) = 0, then from Eq. (4.20) we have
2
0
K
tStSN tt
T
(4.21)
Thus, from above Eq. (4.21) the deep-level impurity concentration can be
calculated by the DLTS measurements. The emissions of the trapped electrons from
traps change the capacitance with time. Since the capacitance transient reveals the
emission process of trapped electrons. The emission rate depends upon the following
factors: (i) trap activation energy ET; (ii) trap capture cross section σn;
(iii) temperature T. Hence these trap parameters can be calculated by capacitance
transient spectroscopy. The capacitance of a junction due to impurities with a single
level of activation energy can be written as:
eD
to
t
N
nCtC
exp
2
01 (4.22)
where nt (0) the concentration of trapped electrons at t = 0, ND is the doping
concentration which can be considered as the free electron concentration, τe is the
trapped electron emission time constant, which is given by:
70
2
exp
T
kT
EE
nn
TC
e
(4.23)
where
2
3
2
1
T
N
T
v Cthn , Vth is the electron thermal velocity and NC is the election
effective density of states in the conduction band. If the trap concentration is
uniformly distributed in the depletion region and nt (t = 0) = NT, then NT be expressed
as:
o
DTC
CNN
2 (4.24)
By using the Equations (4.22-4.24), we can find out all trap parameters such as
the trap energy level ET, the trapped electron emission time constant τe and trap
concentration NT through a series of proper C-t measurements at various temperatures
[5, 6].
The type of above foresaid technique is isothermal single shot technique (IST).
IST is time domain technique and therefore takes extremely long period to just get a
single data point. It produces data of emission rate of very high quality. But there are
some disadvantages of IST, for instance; (i) IST requires extremely good control over
temperature; (ii) Analysis of data becomes difficult when more than one defect is
present. Thus IST is converted into method commonly known as Deep Level
Transient Spectroscopy (DLTS) by introducing the filtering function. It determines the
temperature where the transient has a certain time constant instead of the time
constant for the transient at a certain temperature as is the case in the single shot
71
transient technique. The change of time domain to temperature domain can be
explained by the following relation:
dttWTtfTSt
)(,)(
(4.25)
where S(T), f(t),W(t) and T are the output DLTS signal, current or capacitance signal,
the filtering function and temperature, respectively. In this technique, the electronic
system responds only within a desired emission rate based on mechanical rate
window, which can be changed if required, while the temperature is increased from a
low value to higher values. The transients due to different deep levels are appeared
into peaks at different temperatures for a single desired rate window. There are many
methods to make rate window for example (i) Dual – Gated Integrator (Double
Boxcar) (ii) Lock in amplifier. The DLTS used in our lab is lock in amplifier. In lock-
in amplifiers DLTS, a square wave weighting function is used whose period set by the
frequency of the lock-in amplifier, the frequency in return yields carriers emission
rate (en) defined as: en = 2.17 × f. When this frequency has the proper relationship
with emission rate of defect, a peak of level is obtained at certain temperature. The
defect parameters like activation energy, the capture cross section and the
concentration of the defect level are calculated as described below.
4.3.3 Measurement of Defect Parameters by DLTS
From DLTS, activation energy and capture cross section of the defect level are
calculated as a result of several temperature scans performed. In temperature scan the
frequency is kept constant while the temperature is traversed. After performing
several measurements with different frequencies referred as (T-scan) or at different
temperatures referred as (F-scan), there are frequency-temperature data pairs
72
belonging to each peak. The obtained f-T i.e. (en,T) data pairs can be illustrated
according to the following equation, which is the logarithmic form of the expression
of the emission rate. The emission of majority carriers from deep levels is a process
which depends exponentially on the inverse temperature and the position in the band
gap. The evaluation of the detailed balance relation is performed on its logarithmic
form. This is the equation:
ktEENve TCCnthn /exp (4.26)
where en, σn are emission rate and capture cross section. The definition of the thermal
velocity according to the Boltzmann velocity distribution:
m
kTvth
3 (4.27)
Since the example refers to n-type material, the effective mass of the electrons
should be put in this equation. The effective density of states in conduction band is:
2/3
2
22
h
kTmNC
(4.28)
After substituting vth and NC the equation (4.26) becomes:
kTEETh
mke TCnn /)(exp{)3(2
2 22/1
2/3
2
2
(4.29)
The constants can be marked with K (the capture cross section and the
effective mass are supposed to be temperature independent, which is a simplification
again):
73
}/)(exp{2 kTEEKTe TCnn (4.30)
Taking the logarithmic form of both sides of the equation:
kTEEKTe TCnn /)()ln()/ln( 2 (4.31)
This relation is the so-called Arrhenius plot. To illustrate this curve, emission
rate-temperature (en-T) data pairs are needed which are characteristic of the deep
level. These data come from the fitting of the DLTS spectra.
kTEKTe actnn /)ln()/ln( 2 (4.32)
where Eact is the activation energy of the level needed to promote electron from the
level to the conduction band. Measuring the emission rate as a function of temperature
and plotting ln (e/T 2) vs 1/T (an Arrhenius plot) will give a straight line, from which
the apparent activation energy, Eact can be extracted from the slope and the apparent
carrier capture cross section, σn from the intersection of the line with the y-axis.
Trap concentration (NT) is calculated using the following formula:
21
2VV
VV
C
CNN RB
DT
(4.33)
where VB,VR, V1 V2 andC
C are the built in potential, reverse bias, and forward biases,
amplitude of peak, respectively.
The capture cross section is a characteristic parameter of the deep level. It can
be investigated by varying the width of the filling pulse and measuring the peak
amplitude. The measurements are frequency scans performed at constant temperature.
74
The list of filling pulse width and peak amplitude data pairs. The capture cross section
is supposed to be an exponential process. The capture time constant can be described
by the following equation:
thD
cvN
1
(4.34)
Supposing that the trap is empty before the filling pulse, the trap concentration at the
end of the pulse is:
)]/exp(1[)( cpTp tNtN (4.35)
This equation has a logarithmic form including the relative occupation n(t) which is
proportional with the peak amplitude at a given filling pulse width:
T
p
pN
tNtn
)()( (4.36)
c
p
p
ttn
)](1ln[ (4.37)
According to this modal, illustrating the left side of the above equation as a function
of the filling pulse the received points can be fitted by straight line. The slope of the
line gives the time constant of the capture process whose reciprocal value is the
capture velocity:
c
cV
1 (4.38)
The capture cross section is calculated according to the equation:
75
thc vDN
1
(4.39)
This series of measurements can be repeated at different temperature. In this way
(known as indirect method) the temperature dependence of the capture cross section
can be investigated.
4.4 X-Ray Diffraction (XRD)
X-ray diffraction is a technique in which the pattern produced by the
diffraction of X-rays through the closely spaced lattice of atoms in a crystal is
recorded and then analyzed to reveal the crystal structure of that lattice. This generally
leads to an understanding of the material and molecular structure of a substance. The
spacing in the crystal lattice can be determined using Bragg’s law.
Miller indices are a symbolic vector representation for the orientation of an
atomic plane in a crystal lattice and are defined as the reciprocals of the fractional
intercepts which the plane makes with the crystallographic axes. Indexing pattern of
any crystal system can be found by analytical methods. Analytical methods of
indexing involve arithmetical manipulation of the observed sin2 θ values in an attempt
to find certain relationships among them. Since each crystal system is characterized
by particular relationships between sin2θ values, recognition of these relationships
identifies the crystal system and leads to a solution of the line indices. Since we are
working with ZnO and its crystal structure is hexagonal. The Miller indices of
hexagonal system are determined as described below.
76
4.4.1 Hexagonal System
For hexagonal crystal system, sin2 θ values are given by [7]:
2222 ClkhkhASin (4.40)
where2
2
3aA
and
2
2
4cC
where λ is wavelength of x-rays, a, and c are lattice constants of material. The
possible values of (h2+hk+k
2) are S = (h
2+hk+k
2) = 1, 3, 4, 9, etc. The indexing
procedure is explained by a particular example (shown in Figure.4.5) such as the
powder pattern of zinc (its structure is hexagonal), the observed sin2θ values are
tabulated in Table 4.1. Initially we suppose l = 0 and divide the sin2θ values by the
integers (S) 1, 3, 4, etc, and put the results in Table as shown in Table 4.1. This
applies to the first line of the pattern. After this we then observe those numbers, which
are equal to one another or equal to one of the observed sin2θ values. In this case, the
two values, 0.112 and 0.111, are almost equal, so we suppose that lines 2 and 5 are
hk0 lines. Afterward we cautiously put A = 0.112 which is equivalent to saying that
line 2 is 100. As the sin2θ value of line 5 is very nearly 3 times that of line 2, line 5
should be 110. We find the value of C by using the equation:
2222 ClkhkhASin (4.41)
We now subtract AS from each sin2 θ value, the values of A = 0.112, 3A =
0.336, 4A = 0.448, etc., and look for remainders (Cl2) which are in the ratio of 1, 4, 9,
16, etc. The resultant numbers are given in Table 4.2. Now the five numbers are
important because these entries (0.024, 0.079, 0.221, and 0.390) are very close in ratio
1, 4, 9, and 16. Therefore we put 0.024 = C (1)2, 0.097 = C (2)
2 , 0.221 = C (3)
2, and
77
0.390 = C(4)2. This gives C = 0.024 and indices of line 1 as 002 and line 6 as 004.
Since line 3 has a sin2 θ value equal to the sum of A and C, its indices must be 101. In
this fashion, the indices of lines 4 and 5 are found to be 102 and 103, respectively.
Similarly, indices are found to all the lines on the pattern. It can be rechecked by a
comparison of observed and calculated sin2 θ values.
Figure 4.5 XRD measurement of the Zn.
Table 4.1 Calculated miller indices (hkl) of hexagonal system when l = 0.
Line sin2θ Sin
2θ/3 sin
2θ/4 Sin
2θ/7 hkl
1 0.097 0.032 0.024 0.014 -
2 0.112* 0.037 0.028 0.016 100
3 0.136 0.045 0.034 0.019 -
4 0.209 0.070 0.052 0.030 -
5 0.332 0.111* 0.083 0.047 110
6 0.390 0.130 0.098 0.056 -
30 40 50 60 70 80
0
50
100
150
200
Inte
nsi
ty (
cp
s)
2 (degree)
78
Table 4.2 Calculated miller indices (hkl) of hexagonal system
Line sin2θ sin
2θ - A sin
2θ – 3A hkl
1 0.097* - - 002
2 0.112 0.000 - 100
3 0.136 0.024* - 101
4 0.209 0.097* - 102
5 0.332 0.221* - 110, 103
6 0.390* 0.278 0.054 004
79
CHAPTER 5
80
4 EXPERIMENTAL DETAILS
ZnO samples used in this study were grown by hydrothermal technique, MBE and
aqueous chemical method.. The samples (“group A” and “group B”) were grown by
hydrothermal technique. The samples (“group C”) were grown by MBE. The other
samples labeled as sample “D, E, F, G” were grown by aqueous chemical method.
The experimental details of all samples are explained in this chapter.
5.1 Group A and B Samples
Hydrothermally grown, wurtzite (0001) single crystal bulk n type ZnO wafers
were purchased from ZnOrdic AB original 10×10×5 mm in size shown in Figure. 5.1,
but were cut into pieces for various characterization purposes. The rocking curve of x-
ray diffraction (XRD) peak at 17.74o was 20-60 arcsec. All the samples had both Zn
and O-face. For the purpose of electrical characterization, metal contacts are
necessary. Schottky contacts of 1mm diameter (thickness ~ 2000Å) with palladium
metal were prepared on the Zn-face and O-face and samples were grouped into
“group A” and “group B”, samples respectively.Ohmic contacts of nickel/gold
(thickness ~ 200/2000Å) were prepared on the relevant backside of group A and B
samples. These samples were provided by our collaborator Prof. Dr. Magnus
Willander.
Figure 5.1 ZnO wafer grown by hydrothermal [1].
81
5.2 Group C Samples
Thin film of ZnO on silicon (111) was grown by molecular beam epitaxity.
Prior to the growth of ZnO, cleaning was performed by the standard procedure:
substrate was cleaned by dipping it in the mixture of 100 ml of H2O2 (deionized
water) and 150 ml of H2SO4 for 20 minutes and subsequently cleaned in the HF
(hydrofluoric acid) for 15 seconds. This cleaning procedure is known as piranha
procedure. After cleaning, substrate was loaded into the chamber. To attain high
quality film, a proper vacuum is required. For this purpose rotary pump and Turbo
molecular pump were used to achieve a pressure of 0.75×10-4
Torr in 4 hrs. To
perform the growth, effusion cell was heated at temperature 286 0C to evaporate zinc
from the cell. Atomic oxygen was produced by an RF-plasma source equipped with
an electrostatic ion trap operated at 300 W. The substrate temperature and chamber
pressure were varied from 382-430o
C and (1- 4) ×10-4
×0.75 Torr, respectively during
the growth period of 24 hours. The samples grown by MBE were categorized as
“group C”. For the electrical measurements again Schottky contacts of different
diameter with silver were prepared as shown in Figure. 5.2. These samples were
grown at University of North Carolina Charlotte, North Carolina, USA.
Figure 5.2 ZnO samples grown by MBE.
82
5.3 Samples D, E, F and G
Samples D, E, F and G were grown by aqueous chemical growth. This growth
procedure consists of three steps [2]:
Preparation of seed solution
Pretreatment of substrate
Chemical bath deposition growth
5.3.1 Preparation of Seed Solution
A solution was made by mixing the 274 mg zinc acetate dehydrate in 125 ml
of methanol, the molarity of this solution was 0.01M (molarity). The solution was
heated up to 60 oC with stirring using magnetic stirrer apparatusuntil the solution
became transparent. Another solution was prepared by mixing 109 mg KOH in 65 ml
of methanol (this will give a 0.03 M concentration) at 60 0C. Similarly this solution
was stirred until it becomes transparent. Then the solution of KOH + methanol was
added to the solution of zinc acetate dehydrate + methnol at 60 oC very slowly, while
stirring was continued. After mixing these two solutions, the resulting solution was
kept at 60 oC for 2 hrs with stirring using magnetic stirrer apparatus.
5.3.2 Pretreatment of Substrate
We used the metal coated glass as substrates: Ni, Al, Au and Ag coated glass
labeled as samples “D, E, F, G, respectively were used to form nanostructures of ZnO.
Finally, few drops of the seed solution were placed on the each metal coated glass
substrate. The substrate was placed on to the spinner that spins at a spin speed of 4000
rpm for 30 seconds [3]. Then metal coated glass substrates were placed into the oven
at 100 oC for 10 minutes.
83
5.3.3 Chemical Bath Deposition Growth
Two equimolar aqueous solutions of Zinc nitrate hexahydrate (ZNH) (Zn
(NO3)2 .6H2O) and hexa-methylene-tetramine (HMT) (C6H12N4) were prepared. Both
solutions have molarity of 0.1M. Then both solutions were mixed at room temperature
with stirring using magnetic stirrer apparatus. The substrates were placed in this
solution with face downward towards the solution. After this solution containers were
put in the oven at a constant temperature 95oC for 7 hrs. After 7 hrs the substrates
were removed from the solution, cleaned with deionized water and dried at room
temperature.
84
CHAPTER 6
5
85
6 RESULTS AND DISCUSSION
This chapter is divided into four sections.
Section- I
The first section presents study of defects in Zn-face ZnO (group A samples) grown
by hydrothermal method. Results described in this section are based in part on paper
[Noor et.al. 2009[1]].
6.1 Current-Voltage Measurements
To study the deep level defects metallic contacts are required (Schottky and
Ohmic). It is necessary, before starting the deep level measurements, to determine the
quality of the contacts. I-V measurements provide a deep and detail characteristics of
the Schottky diode / Schottky contact. A number of measurements were performed on
all the samples (group-A), the representative results are presented in this section.
Figure 6.1 shows the forward and reverse biased I-V characteristics of Pd/ZnO SBD
(Schottky barrier diode) at temperature 306 K. A low leakage current is found ~ 6 μA
at higher reverse bias i.e. -3V. This low leakage current is appropriate for capacitance-
voltage (C-V) and DLTS measurements. I-V measurements were analyzed to
determine the quality parameters ideality factor, and barrier height for group “A”
samples. But results related with barrier height of the devices were not convincing
therefore are not being presented here.
The value for the ideality factor (n) is obtained by following the procedure
described in chapter 4 and by using the equations 4.3 and 4.4. The I-V measurements
were performed in the temperature range 120 – 340K. As a result, the upper and lower
limits of the n appear to be 1.7 at 340 K and 13.5 at 120 K, (Figures. 6.2 and 6.3)
86
respectively. The variation in the values of the ideality factor could be related to the
thermionic transport mechanism that is extremely affected in the low temperature
regime. This phenomenon is known as To-effect [2, 3]. A plot of n versus 103/T as
shown in Fig. 6.4 demonstrates that To-effect is present in the temperature range 335-
250 K while it is not observed in the low temperature range 250-120 K. It could be
connected to the To anomalous effect. The product of n and temperature is almost the
same for the data having the To-effect. According to the literature survey, this constant
product is a sign of a trap assisted carrier transport mechanism [4,5].
Figure 6.1 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO.
-3 -2 -1 0 1 210
-9
10-7
10-5
10-3
10-1
C
urren
t(A
)
Voltage(V)
T = 306K
n = 4.1
87
Figure 6.2 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO at
120 K.
Figure 6.3 I-V characteristics of the Pd-Schottky contact on the Zn face of the ZnO at
340 K.
-3 -2 -1 0 1 2
1E-7
1E-6
1E-5
1E-4
1E-3
Cu
rren
t(A
)
Voltage(V)
T = 340K
n = 1.7
88
Figure 6.4 Plot between Ideality factor and 1000/T indicates the To- effect for the Zn
face Pd/ZnO Schottky diode.
6.2 Capacitance-Voltage Measurements
Figure. 6.5 shows the experimental capacitance–voltage characteristics of the
Pd/ZnO SBD. C–V analysis is done at 1 MHz ac signal as a function of voltage at
room temperature. The depth profile of the apparent free carrier concentration (ND)
from C-V measurement is shown in Figure 6.7. The free carrier concentration (ND)
can be obtained from the plot of (A/C)2 –V (Figure. 6.6) by means of the given
equation:
1
2
22
C
A
dV
d
qN
o
D
(6.1)
The contact area (A), charge of electron (q), permittivity of free space (ε0) and
dielectric constant (ε) are 0.78×10-2
cm2, 1.6×10
-19 C, 8.85×10
-12 Fm-1 and 8.5 for
Pd/ZnO, respectively. The free carrier concentration (ND) is found to be 3.44×1017
cm-3. The depth x is calculated from the depletion capacitance C by using the
following equation:
2 4 6 80
6
12
1000/T (K -1)
n (
idea
lity
fa
cto
r)
89
Rbi
D
VVN
mx
71005.1 (6.2)
where Vbi and VR are built in potential and applied voltage respectively. The built-in
potential Vbi is determined by the linear extrapolation to the intercept on the voltage
axis of the (A/C)2–V. The value of barrier height (φB)C-V is obtained from C–V
measurement by using the equation 4.7. The barrier height (φB)C-V is found to be 0.12
eV.
As can be seen in Figure 6.5 C-V characteristics have an unusual peak in
forward bias region. This peak points out the defects in depletion region that
strengthen our explanation for the observed higher value of the ideality factor. These
features again show the presence of majority carrier traps on the surface and/or in the
bulk of the semiconductor material. The C-V measurements at different temperature
shown in Figure 6.8 and their parameters (built in potential and free carrier
concentration) at different temperatures are listed in Table 6.1. Depth profiles of free
carrier concentration of Zn-face ZnO at different temperatures shown in Figure 6.9.
Figure 6.5 C-V measurements of the Pd-Schottky contact on the Zn face of the ZnO.
-3 -2 -1 0 1150
200
250
300
Applied Bias (V)
Cap
aci
atn
ce (
pF
)
90
Figure 6.6 Graph between applied bias and inverse squared capacitance.
Figure 6.7 Depth profile of free carrier concentration of Pd/ZnO.
-3 -2 -1 0 18.0x10
14
1.0x1015
1.2x1015
1.4x1015
1.6x1015
1.8x1015
2.0x1015
A
2/C
2 (
cm
/F)2
Voltage(V)
200 150 100 50 010
16
1017
1018
ND
(cm
-3)
Depth (nm)
91
Figure 6.8 C-V measurements of the Zn face of the ZnO at different temperatures.
Figure 6.9 Depth profile of free carrier concentration of Zn-face ZnO at different
temperatures.
-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0150
200
250
300
350
400
450
C
ap
aci
atn
ce (
pF
)
Voltage(V)
159 K
199 K
304 K
338 K
240 260 280 300 320 340 360
4
6
N
D (10
17cm
-3)
Depth (nm)
T = 159 K
220 240 260 280 300 320 340
4
6
8
ND
(1
01
7cm
-3)
Depth (nm)
T = 199 K
135 140 145 150 155 160 1652
3
4
5
6
7
8
9
10
N
D (1
01
7cm
-3)
Depth (nm)
T =304 K
200 220 240 260 280 300
6
8
10
12
14
16
N
D (10
17cm
-3)
Depth (nm)
T = 338 K
92
Table 6.1 Electrical parameters of Zn face ZnO calculated from C-V measurements.
Temperature
(K)
Built in
potential (V)
Free carrier
concentration
(cm-3
)×1017
122 7.92 3.13
128 7.95 3.15
140 7.82 3.14
141 8.16 3.19
151 7.94 3.2
152 8.13 3.28
159 7.14 3.28
160 7.75 3.15
169 7.89 3.19
170 7.14 3.3
178 7.11 3.29
180 7.78 3.23
189 7.15 3.32
193 6.86 3.22
200 7.78 3.23
207 6.92 3.28
209 7.87 3.3
240 7.46 3.19
293 7.62 3.01
294 6.62 3.29
295 6.58 3.08
296 6.72 3.43
296 6.34 3.28
296 6.6 3.37
302 6.86 3.55
93
6.3 Deep Level Transient Spectroscopy Measurements
Two electron defect levels labeled as E1 & E2 were observed from the DLTS
scan of group A representative sample having Pd-Schottky contacts of 0.78 mm2 or 1
mm in diameter, shown in Figure 6.10. DLTS spectrum was taken from 150 K to 350
K with digital operating systems. For DLTS measurements, following parameters
were used: UR (reverse bias) = –2 V, Vp (filling pulse) = +2 V, tp (filling time) = 20 μs
and en (emission rate) = 2170 s-1
. During the DLTS scan, the temperature ramping rate
was limited at 0.01 K/sec for the precise temperature control. Representative DLTS
spectra at different frequencies of E1 were shown in Figure 6.11.
The activation energies, i.e., energy level positions in the bandgap (Ec–ET)
and capture cross sections (σ∞) of the defects (E1 & E2) have been obtained from the
temperature dependence of the thermal emission rates by means of Arrhenius plots (
Figure 6.12), defect E1 has activation energy ( Ec-Et ~ 0.22 ± 0.02 eV) with capture
cross sections of about (8.22 ± 0.4)×10-17
cm2
and defect E2 has activation energy ( Ec-
Et ~ 0.49 ± 0.02 eV) with capture cross section of about (1.18 ± 0.5)×10-14
cm2. The
trap concentration (NT) of the defects (E1 & E2) is calculated by using the equation
[12]:
2
22
2p
rp
D
t
W
WW
N
N
C
C
(6.3)
where ND is the free carrier concentration, Wr and Wp the thickness of the depletion
layer at reverse bias and at the pulse voltage, respectively and
D
tFo
Nq
EE2
2
(6.4)
94
where ε, εo, q, EF, and Et consist of usual meanings. The trap concentration (NT) of the
defects (E1) was shown in Figure 6.13. All the parameters of defects levels are listed
in Table 6.2.
Figure 6.10 DLTS spectrum displaying two electron deep level defects below
conduction band of ZnO.
Figure 6.11 The DLTS spectrum of levels E1 in ZnO.
150 200 250 300 350
E2
E1
emission rate = 2170 s
-1 D
LT
S S
ign
al
(a.u
)
Temperature(K)
x10
2.11015
cm-3
100 150 200 250 300 350 400 450
-15
-10
-5
0
DL
S (
mV
)
Temperature(K)
10Hz
100Hz
500Hz
1000Hz
2500Hz
95
Figure 6.12 The Arrhenius plot of levels E1 and E2 in ZnO.
Figure 6.13 Trap concentrations of levels E1 in Zn-face ZnO
0.150 0.155 0.160 0.165 0.170 0.175 0.180 0.185 0.1902
4
6
8
10
12
Nt (
x1
014 c
m-3
)
Depth (m)
4 6
1E-3
0.01
E1
E2
en
T -
2(s
-1K
-2
)
1000/T (K -1)
96
Table 6.2 Details of electrical parameters such as activation energy, capture cross
section measured via indirect and direct methods and trap concentration of defects
observed in the DLTS spectrum of Zn-face ZnO.
Defect
ID
Tmeas
(K) E (eV) σ∞ (cm
2)
σT (cm2)
(× 10-19
)
NT (cm-3
)
(× 1012
)
E1 200 0.22 ± 0.02 (8.22±0.4)×10-17
6.8±0.3
110 ± 50
E2 291 0.49 ±0.02 (1.18±0.5)×10-14
1.5 ±0.5
2.01 ± 0.6
6.4 X-Ray Diffraction
Figure 6.14 displays the representative XRD spectrum of group “A” sample
exhibiting twelve peaks (P1-P12). The corresponding planes of peaks were calculated
by using standard formula explained in “The Elements of X-ray diffraction” by B. D.
Cullity [13]. 2θ, intensity, Miller indices, and sources of peaks are listed in Table. 6.3.
It is clear from the table that P1(1010), P2 (0002), P3(1011), P5 (1010), P9 (0004)
and P12 Zn (20 2 1) were originated from ZnO. P4 Al(111), Ag(111), P6 Ca(531), P8
Al(220), P10 Ca(331) and P11 Mg(20 2 2) were originated from alumina which was
used for back contacts.
97
Figure 6.14 Typical XRD pattern of the Zn-face ZnO layer exhibiting the Zni-VO
complex as the preferential direction of growth. Peaks other than ZnO are seen
because the XRD measurements were performed on Pd\ZnO–Zn\Au–Cr mounted on
an alumina substrate by silver paste.
40 60 80
0.0
2.0x104
4.0x104
p8p12
p11
p10
p9p7p6
p5
p4p3
p2
Inte
nsi
ty (
cp
s)
2 (degree)
p1
98
Table 6.3 2θ, intensity, Miller indices, and sources of peaks measured from XRD
data in ZnO layers listed.
Peak# 2θ
Intensity
Zn-Face
Identification
(Miller Indices)
Source of Peak
P1 31.28 1097 ZnO(Zni-VO) (1010) ZnO layer
P2 34.48 15744 ZnO (0002) ZnO layer
P3 36.36 455 ZnO(1011) ZnO layer
P4 38.08 1607 Al(111), Ag(111) Alumina
P5 40.2 32603 Zn (Zni-VO) (1010) ZnO layer
P6 44.76 880 Ca(531) Alumina
P7 64.4 990 Li(211) Hydrothermal growth of ZnO
layer
P8 65.48 1362 Al(220) Alumina
P9 72.64 2208 ZnO (0004), Zn(11 2 0) ZnO layer
P10 73.72 8260 Ca(331) Alumina
P11 77.56 382 Mg(20 2 2) Alumina
P12 86.64 1466 Zn (20 2 1) ZnO layer
99
6.5 Trap Identification
In the following section, detail discussion of E1 and E2 are given.
6.5.1 Electron Level E1
We focus on E1 level in this section. A deep level defect is defined as defect
level having activation energies ≥ 0.1 Eg and trap concentration ≤ 0.01ND. E1 having
activation energy 0.22 eV below the conduction band edge, may be a shallow dopant
level in ZnO (the energy bandgap Eg of ZnO is 3.37 eV). In literature Kitchill et
al.[14] have given the same justification of Mg-related level in p-GaN (Eg ~ 3.34 eV)
as a shallow acceptor level with activation energy 0.2 eV above the valence band.
Several research groups have observed same electrical properties as our E1 level in
the recent literature. Some well-known papers are as follows: Dong et al. [15]
reported a similar defect level at energy of 0.3 eV, attributed to an oxygen vacancy.
Vineset al. [16] found the trap of energy 0.31eV below the conduction band and they
linked this trap to the oxygen vacancy. An electron trap associated with Zn
interstitials having activation energy (trap concentration) of 0.32 eV (1014
–1016
cm−3
)
below the conduction, was discovered by Frenzel et al. [17]. The intrinsic donor like
defects in ZnO having activation energy in the range of 0.30–0.37 eV, were reported
by Wenckstern et al. [18]. A Zn related defect level having ionization energy of 0.31
eV, was observed by Frank et al. [19]. A similar level at an energy of 0.29 eV,
connected to an oxygen vacancy, was found by Auret et al . [20]. By comparing it
with literature we conclude that research groups were not agreed on the energy of
level (E1) and its origin. After this, we carried out further experiments in order to
resolve this issue.
100
C-V and DLTS measurements were performed after equal interval of time and
displayed in Figures 6.15 and 6.16. It is observed that the free carrier density ND
decreases with increasing trap E1 concentration NT with a passage of time as shown in
Figure 6.17. The possible justification of this phenomenon could be: (a) site
competition [21] (b) lattice relaxation [22] and/or (c) transformation of defect states
[23]. In accordance with the study of ZnO material, we can experimentally consider
the above mechanism with the transformation of defect states as described below.
ZnO has intrinsically n-type conductivity. The Zni, VO and the Zni-VO complex
are common sources of n-type conductivity in ZnO [24, 25].Our ZnO sample had
been grown along Zn face and/or under Zn rich condition. As expected, Zn atoms are
present in large numbers in our samples. Zn atoms may occupy interstitial Zni site or
make complex with oxygen vacancy Zni-VO. To support our expectations XRD
measurement was performed.
Figure 6.15 C-V measurements indicate decrease in amplitude.
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
200
240
280
320
360
400
Cap
aci
atn
ce (
pF
)
Voltage(V)
0 hours
744 hours
2208 hours
2760 hours
101
Figure 6.16 DLTS measurements indicate increase in amplitude.
Figure 6.17 Demonstration of time-delayed transformation phenomenon of defects in
ZnO layer.
0 1000 2000 3000
0.0
0.4
0.8
1.2
0.0
0.4
0.8
1.2
8
16
24
32
ND
(x1
016 c
m-3
)
NT
(x1
016 c
m-3
)
Time (hours)
120 160 200 240 280 320 360-350
-300
-250
-200
-150
-100
-50
0
D
LS
(m
V)
Temperature(K)
0 hours
744 Hours
2208 hour
2760 hours
102
X-ray diffraction measurement (as shown in Figure 6.14) of the sample clearly
reveals that the preferable growth direction is along (1010) plane because the
intensity of peak P5 (1010) is highest. It is interesting to note that this highest peak
originates from Zni-VO complex [26] as mentioned in Table 6.2. From this result of
XRD measurement, we can suppose that our sample has n-type conductivity due to
Zni-VO complex. The presence of Zni in ZnO matrix makes Zni-VO complex unstable
bond [20]. Under certain conditions, the bond (Zni–VO) could break, consequently ND
decreases and Zni may occupy VO site and make a zinc antisite and as a result, the
concentration of E1 will increase. This change in concentration of free carrier ND and
trap Nt with respect to time is shown in Figure. 6.17. In this manner, the relationships
of ND with the Zni-VO complex and ZnO with the level E1 support our explanations. As
a result, the transformation description of defects phenomenon seems to be justified.
Some theoretical work is present in literature about the presence of zinc
antisites and vacancy complexes in the ZnO lattice such as in an electron irradiated
ZnO lattice presence of VO-OZn and VZn-ZnO complexes using the Doppler broadening
annihilation-radiation measurements together with local density approximation
calculations were observed by Chen et al. [27]. Oba et al. [28] also discussed the
existence of multiple charged states of Zn antisites, +2/+1/0 in Zn-rich ZnO crystal.
Accordingly to Janotti and Vande Walle [25], formation energy of Zn antisite is lower
than the Zn-interstitial, which is 0.57 eV. The foresaid literature study (experimental
and theoretical) clearly supported that the E1 level is zinc antisite.
103
6.5.2 Electron Level E2
By Arrhenius plot, the activation energy and capture cross section of E2 are Ec-
Et ~ 0.49 ± 0.02 eV and (1.18 ± 0.5)×10-14
cm2, respectively. The capture cross section
of E2 calculated by indirect method is (1.5 ±0.5)×10-19
cm2. The trap concentration of
the E2 is (2.01±0.6)×1014
cm-3
. Dong et al. [8] observed an electron trap level having
similar features as observed in our sample. They attributed it to a surface defect in
ZnO.
104
Section- II
The second part presents study of defects in O-face ZnO (group B samples) grown by
hydrothermal method. Results described in this section are based in part on paper
[Noor et.al. 2010[29]].
6.6 Current-Voltage Measurements
The numbers of measurements were performed on all the samples (group-B),
the representative results are presented in this section. Figure 6.18 shows
representative current-voltage (I-V) measurements of group B samples. Schottky
barrier height φB and ideality factor n of the diodes are calculated from the data which
are based on thermionic emission theory. I-V relationship for Schottky diode is
described by the equations 4.1 and 4.2 [30]. By plotting the measured I-V data on
semilog graph, the ideality factor is calculated from the slope of linear fit for forward
bias with the help of the Eq. 4.3. The ideality factor (quality parameter) of the
Schottky device was found to be 3.4.
The ideality factor n is greater than the practical limits i.e. 1-2 (diffusion –
recombination nature of current) [31]. It means fewer amount is recorded as Schottky
current and remaining current followed some other parallel paths. Such paths were
possibly offered by thermionic field emission (TFE), interface/surface states and/or
ND-induced barrier height lowering. The high temperature is a necessary condition for
thermionic field emission as carrier may tunnel through the thinner part of the barrier
[32]. Thus TFE cannot be valid in our case as measurements were carried out at room
temperature. On the other hand, interface and/or surface states cannot be avoided.
Characteristically, these states can act as carrier trap and/or recombination center. As
105
a result, Schottky current is reduced and hence n becomes large. But results related
with barrier height of the devices were not convincing therefore are not being
presented here.
-4 -3 -2 -1 0 1 2
10-6
10-5
10-4
10-3
10-2
T = 300K
n = 3.4 Cu
rren
t(A
)
Voltage(V)
Figure 6.18 Representative I-V measurements of group B samples.
Figure 6.19 Representative C-V measurements of group B samples.
106
6.7 Capacitance-Voltage Measurements
Figure 6.19 shows the representative experimental capacitance–voltage
characteristics of the group B samples. C–V analysis is performed at 1 MHz ac signal
as a function of voltage at room temperature. The depth profile of the apparent free
carrier concentration (ND) from C-V measurement is shown in Figure 6.21. The free
carrier concentration (ND) and depth (x) are calculated from the plot of (A/C) 2
–V
(shown in Figure 6.20) by means of equations 6.1 and 6.2. The free carrier
concentration (ND) is found to be 3×1016
cm-3
.
The built-in potential Vbi is determined by the linear extrapolation to the
intercept on the voltage axis of the (A/C) 2
-V. The built-in potential Vbi is found to be
1.43 V. The value of barrier height (φB) C-V is obtained from C-V measurement by
using the equation 4.7. The barrier height (φB) C-V is found to be 1.56 eV. The plot of
1/C2 versus V was linear, as can be seen in Figur 6.20, it indicates uniform variation in
free carrier concentration as a function of depth (see Figure. 6.21). Similar results in
intrinsically (bulk) n-ZnO Schottky diodes were observed by Dong et al. [8] and Fang
et al. [33]. They suggested that these results are due to the surface defects. In this
way, our results are consistent with the literature. C-V measurements at different
temperature are shown in Figur 6.22. As the carrier freeze out at low temperature,
carrier concentration increased with increasing temperature.The associated parameters
such as built in potential and free carrier concentration measured at different
temperatures are listed in Table 6.4. Free carrier concentration depth profiles of O-
face ZnO at different temperatures are shown in Figure. 6.23.
107
Figure 6.20 Schottky behavior of the sample B is demonstrated in 1/C2-V, filled
squares represent the experimental data and the line corresponds to the theoretical fit
of the data, extrapolated to x-axis yield built-in potential.
80 100 120 140 160 1802
3
4
ND
(10
16cm
-3)
Depth (nm)
Figure 6.21 The uniform spatial distribution of the free-carriers in the as-deposited
ZnO material.
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.50.0
5.0x1014
1.0x1015
1.5x1015
2.0x1015
2.5x1015
3.0x1015
3.5x1015
A
2/C
2 (
cm
/F)2
Voltage(V)
108
Table 6.4 Electrical parameters of O face ZnO calculated from C-V measurements
Temperature
(K)
Built in
potential (V)
Free carrier
concentration
(cm-3
)×1016
133 2.34 2.81
146 2.5 3.15
155 2.53 3.22
158 2.56 3.26
175 2.11 3.38
181 2.51 3.28
187 2 3.24
196 2.06 3.34
206 1.95 3.27
216 2 3.35
225 1.91 3.28
235 1.9 3.3
245 1.84 3.3
255 1.84 3.33
263 1.76 2.99
274 1.62 2.9
284 1.29 2.65
290 1.38 2.99
291 1.96 4.08
295 1.93 3.94
298 2.06 3.94
300 1.73 3.54
302 1.49 2.95
109
Figure 6.22 C-V measurements of the O face of the ZnO at different temperature.
Figure 6.23 Depth profile of free carrier concentration of O-face ZnO at different
temperatures.
200 250 300 350 400 4500
2
4
6
8
10
ND
(10
16cm
-3)
Depth (nm)
T = 133 K
-4.0 -3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5120
140
160
180
200
220
240
260
280
Cap
aci
atn
ce (
pF
)
Voltage(V)
133 K
206 K
298 K
110
6.8 Deep Level Transient Spectroscopy Measurements
We expected that all results of DLTS measurements to be same in both group
of samples because both are n-type ZnO grown by same technique i.e hydrothermal
having Pd-Schottky contacts of 0.78 mm2 or 1mm in diameter. But DLTS results of
group B samples are entirely different from group A samples as can be seen below.
DLTS results of both groups of samples are following: Two majority electron
defect levels labeled as E1 & E2 were observed from the DLTS scans of group A and
B representative samples are shown in Figure 6.24. DLTS spectra were taken from
150 K to 350 K with digital operating systems. For DLTS measurements, following
parameters were used: UR (reverse bias) = –3 V, Vp (filling pulse) = +3 V, tp (filling
time) = 20 μs and en (emission rate) = 2170 s-1
. During DLTS scans, temperature
ramping rate was limited at 0.01 K/sec for the precise temperature control.
Representative DLTS spectra of group B samples measured at different frequencies of
E1are shown in Figure 6.25. DLTS spectra at different frequencies are required for
Arrhenius plots to calculate activation energy and capture cross-section of level. Level
E2 exhibits the same emission rate but E1 appears at different emission rates in
samples A and B (see Figure 6.24). First level E1 has different activation energy and
capture cross-sections in both groups of samples i.e EC-0.22 eV and 8.22×10-17
cm2
for group A samples and EC - 0.26 eV and 11.16 ×10-17
cm2 for group B samples by
means of Arrhenius plots as shown in Figure 6.26. Second level E2 has same
activation energy and capture cross-section that is EC-0.49 eV and 1.18×10-14
cm2 for
both groups A and B samples. Depth profile of trap concentration (NT) of the defects
(E1) in group B samples calculated by using equations 6.3 and 6.4 is shown in Figure
6.27.
111
Level E1 has different emission rates in both groups A and B of the samples. If
we carefully consider the experimental detail of both groups A and B of samples, they
are different by two factors: (i) face difference (ii) free carrier concentration. As face
of material is assumed to cause surface contamination, therefore, we suppose that ND
may be the only factor that affects the emission rates of the level E1.
Figure 6.24 Representative DLTS scans of group A and B samples to show the
variation in peak position of E1 level even measured under same measuring setup.
100 150 200 250 300 350-350
-300
-250
-200
-150
-100
-50
0
Temperature(K)
DL
TS
Sig
na
l (m
V)
2.11015
cm-3
emission rate = 2170 s-1
x15
E2
E1
Sample B
Sample A
112
150 200 250 300
-300
-250
-200
-150
-100
-50
0
DL
TS
Sig
na
l (m
eV
)
Temperature(K)
1 Hz
1000 Hz
100 Hz
2000 Hz
Figure 6.25 The DLTS spectra measured at different frequencies for Arrhenius plot of
levels E1 in sample B.
Figure 6.26 The Arrhenius plot of levels E1 in samples A and B.
4.6 4.8 5.0 5.2 5.4 5.6 5.8 6.0 6.2 6.4 6.6 6.8
10-3
10-2
10-1
Sample B
en
T -
2(s
-1K
-2
)
1000/T (K -1)
Sample A
113
0.24 0.26 0.28 0.30 0.32 0.34 0.36 0.38
0
20
40
60
80
100
120
140
Nt (
x1
014 c
m-3
)
Depth (mm)
Figure 6.27 Depth profile of trap concentration of levels E1 in O-face ZnO.
6.9 Trap Identification
Detail study of trap E1 and E2 are explained in following section.
6.9.1 Electron Level E1
As mentioned earlier, electron level E1 has different activation energy in both
groups A and B of samples. Free carrier concentration ND may be the only factor that
affects the emission rates of the foresaid level. This contention is consistent with the
following reports:
Miyajima et al. [35] discovered two defects in Ga doped ZnSe (1015 - 1018
cm-3)
and defined them as trap A and B. Their results revealed that the activation energy of
trap A did not vary with ND but it was not same in the case for trap B. The activation
energy of trap B changed from 0.4–0.56 eV as a function of ND (1018-1015 cm-3
). They
114
observed the activation energy of trap B increased as free carrier concentration
decreased. They attributed trap B to the complex of Zn vacancy and Ga or the
complex of interstitial Se and Ga. Baber et al [36] reported that activation energy (Ec -
Et) of Ti donor level in InP using DLTS. They calculated activation energy change
from 0.54 ± 0.03 eV to 0.63 ± 0.03 eV. They observed that the specimen with the
largest activation energy (0.63 ± 0.03 eV) having the lowest background free carrier
concentration of 5×1015
cm-3
whereas the specimen attained the lowest activation
energy (0.54 ± 0.03 eV) containing the highest free carrier concentration of (1-
3)×1017
cm-3
. They suggested that emission rate was changed by electric field due to
free carrier concentration. Diaconu et al. [37] reported the similar results. They doped
Co (0.02, 0.20 and 2.00 at %) in ZnO samples labeled as “a, b and c”, respectively.
The free carrier concentrations of samples (a, b and c) were 3.7×1017 cm-3 ,
0.5×1017cm-3and 31.2×1017 cm-3, respectively. They found three defects E1, E2 and E3
in all (a, b and c) samples. The activation energy of E1 was calculated to be 0.29 eV,
0.34 eV and 0.17 eV in samples a, b, and c, respectively. Similarly the activation
energies (0.37 eV, 0.39 eV and 0.30 eV) were calculated for E2 in samples a, b and c,
respectively. They also calculated activation energies (0.45 eV, 0.59 eV and 0.33 eV)
for E3 in all three samples. They found activation energy of defects (E1, E2 and E3)
shifted towards lower value with increasing of free carrier concentration in all
samples (a, b and c).
The above reports qualitatively justify our argument. Some well-known
groups have reported E1 like level in intrinsically n-type ZnO samples having
activation energy in the range of 0.32 to 0.22 eV with background free carrier
concentration (1014
-1017
) cm-3
. They linked it to oxygen vacancy, zinc interstial
and/or zinc antisite. This defect identification is probably dependent upon the
115
activation energy of the level which according to Vincent et al. [38], is not credible.
They proposed that it was necessary to become careful before interpreting the data
obtained by means of capacitance transient measurement of diodes having free carrier
concentration greater than 1015
cm-3
. That is why we plotted the graph between
activation energy of the defect E1 against the background free carrier concentration in
our samples and already reported by other research groups [19, 33,34] as shown in
Figure 6.28. It is evident from the graph that activation energy decreases as the
background free carrier concentration increase. The information from the literature
indicates that the reduction in thermal emission energy of a defect level is linked with
the electric field enhanced emission. Figure 6.29 illustrates the variation of activation
energy as a function of electric-field created in depletion region due to ND calculated
using the equation 2.4.
Figure 6.28 Influence of background concentration ND on activation energy of E1
level. Data 1 and 2 are ours and rest of the data is taken from Refs. 19, 33, and 34.
10-1
100
101
102
103
160
180
200
220
240
260
280
300
320
340
Acti
vati
on
En
eerg
y (
meV
)
Free Carrier Concentration (1015 cm-3)
116
0.1 1
180
240
300
360
Internal Field (105 V/cm)
Acti
va
tio
n E
neerg
y (
meV
)
Figure 6.29 The ND-induced field effect on the thermal energy data of the level. Data
1 and 2 are ours and rest of the data is taken from Refs. 19, 33, and 34.
It is clear from the Figure 6.29 that an electric field has a remarkable effect on
the activation energy; the value of activation energy becomes lower due to the
increase of internal electric field. It means activation energy of defect is sensitive to
the internal electric field. Several models have used to explain the origin of defect as
described in section 2.2.2.3.
Since in our case, ND-induced barrier height lowering causing thermal
emission of the trap to the lower value. The linear relationship of log (en) with
F1/2
(depicted in Figure. 6.30) proves the Poole-Frenkel mechanism [39]. Therefore we
will concentrate only on Poole–Frenkel mechanism for our data. Two models to
evaluate the emission rate for the Poole–Frenkel effect are: (i) Columbic potential (ii)
square well potential [38,40] as explained in section 2.2.2.3.1. The mathematical
117
equations for emission rates at electric field F to Coulomb potential and square well
potential are given below:
2
111
1
)0( 2
ee
Fe
n
n (6.5)
where kTqqF or // 2
1
2
11
2
1
)0(
e
e
Fe
n
n (6.6)
where kTqFr /
Figure 6.36 shows the experimental emission data (filled circles) together with
theoretical emission rates (lines) which were calculated using equations 6.5 and 6.6
for the observed trap E1.
Figure 6.30 Qualitative evidence of the Poole–Frenkel mechanism on the ND-induced
variation in emission rate signatures of E1 level.
200 400 600 800
100
101
102
E
mis
sio
n r
ate
(s-1
)
F 0.5(V/cm)
118
Figure 6.31 Theoretical fitting of the ND-induced field emission rates (filled circles)
obeying Poole–Frenkel mechanism associated with Coulomb potential (curve C),
while square well potential (r = 4.8 nm) is not consistent (curve S).
It is clear from the Figure 6.31 that experimental data is consistent with the
Poole–Frenkel model associated with Coulomb potential. Consequently, the level E1
is identified as a charged impurity. Additionally, in intrinsically n-type ZnO material,
most of the research groups have reported Zn-related electron defects (interstitials and
antisites) showing relatively shallower energy spectrum (0.22–0.32 eV)
[1,17,19,33,34]. As a result, we attribute the foresaid charged impurity to Zn. This
justification is in agreement with the theoretical results that Zn-interstitial is shallower
than O-related defects (interstitials and antisites) in ZnO.
0 10 20 30 40
0
40
80
120
C
Em
issi
on
ra
te (
s-1)
F( 106 V/m)
S
119
6.9.2 Electron Level E2
An electron level having activation energy Ec-0.49 eV was observed by Fang
et al. [33]. They labeled this electron trap as E4 and linked it with surface defects. By
comparing defect parameters (activation energy, capture cross section and built in
potential) of E2 with Fang et al., we attribute E2 to the surface defect.
120
Section- III
Third part describes Hall measurement, current-voltage, capacitance- voltage, deep
level transient spectroscopy, and secondary ion mass spectroscopy (SIMS) of group C
samples grown by MBE.
6.10 Hall Measurement
Figure 6.32 shows representative temperature dependent Hall measurement of
group C samples. The upper part, middle part, and lower part of Figure 6.32 display
mobility, carrier concentrations and resistivity, respectively. As a general rule,
positive and negative data values of carrier concentration correspond to p-type
conductivity and n-type conductivity of the sample under test, respectively.
Accordingly, we can see that as mixed conductivity with respect to the temperature as
shown in middle part Figure 6.32. Data of mobility and carrier concentration Vs
temperature is scattered. Conductivity is not stable with temperature. The values of
mobility become zero as temperatures vary. To explain mobility ( H ) results,
consider the Hall mobility equation given below [41]:
HH R (6.7)
where HR and are the Hall coefficient and conductivity, Hall coefficient is define as
222
np
np
Hnpe
npR
(6.8)
where all parameters bear the usual meanings. Mobility becomes zero when 2
pp =
2
nn .
121
The scatter data of mobility becomes zero at some temperatures when number
of electrons equals to number holes by equation 6.8. The mixed conductivity transits
from one type to another at zero mobility.
Figure 6.32 Representative temperature dependent Hall measurements of group C
samples. The upper part, middle part, and lower part of Figure display mobility,
carrier concentrations and resistivity, respectively.
122
6.11 SIMS Measurement
Figure 6.33 shows typical SIMS measurement of group C samples. SIMS
measurement (Figure 6.33) shows the presence of oxygen, zinc and nitrogen as a
function of film depth. The elements are arranged as oxygen, zinc and nitrogen by
decreasing the contents in the layer, discussion of the results are presented in Sec.
6.15.
Figure 6.33 SIMS depth profiles of O, Zn and N elements in group C samples.
123
6.12 Current-Voltage Measurement
Figure 6.34 shows semilog current-voltage (I-V) measurement of sample C.
According to the thermionic emission theory, ideality factor n is determined from the
slope of the linear region of the forward- bias region of semilog I-V characteristics
through the relation 4.3. Calculated value of the ideality factor was 5.1. The ideality
factor (n) which is equal to 1 for an ideal diode has usually a value greater than unity.
High values of n can be attributed to the presence thermionic field emission, interface
states, and generation – recombination centers.
Figure 6.34 Representative I-V measurements of group C samples.
124
6.13 Capacitance-Voltage Measurement
Capacitance – voltage measurement of group C samples is performed at 1
MHz ac signal as a function of voltage at room temperature. The depth profile of the
apparent free carrier concentration (ND) from C-V measurement is shown in Figure
6.36. The free carrier concentration (ND) and depth (x) are calculated from the plot of
(A/C) 2
–V (shown in Figure 6.35) by means of given equations 5.1 and 5.2. The free
carrier concentration (ND) is found to be 2.84×1015
cm-3
.The built-in potential Vbi is
determined by the linear extrapolation to the intercept on the voltage axis of the (A/C)
2 -V. The built-in potential Vbi was found to be 0.07 V. The low value of built-in
potential is due to majority carrier traps on the surface and/or in the bulk of the
semiconductor materials to be discussed later.
Figure 6.35 Schottky behavior of group C sample is demonstrated in A2/C
2-V, filled
squares represent the experimental data and the line corresponds to the theoretical fit
of the data.
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.50.0
2.0x1015
4.0x1015
6.0x1015
8.0x1015
1.0x1016
1.2x1016
1.4x1016
1.6x1016
1.8x1016
2.0x1016
A2/C
2 (
cm/F
)2
Voltage(V)
125
Figure 6.36 Depth profile of free carriers of group C sample.
6.14 Deep Level Transient Spectroscopy Measurements
A hole defect level labeled as H is observed from the DLTS scans of group C
samples having Ag-Schottky contacts of 0.78 mm2 or 1 mm in diameter is shown in
Figure 6.37. DLTS spectrum was taken from 120 K to 350 K with digital operating
systems. For DLTS measurements, the following parameters were used: UR (reverse
bias) = –1 V, Vp (filling pulse) = +1 V, tp (filling time) = 20 μs and en (emission rate)
= 5425 s-1
. During the DLTS scan, the temperature ramping rate was limited at 0.01
K/sec for the precise temperature control. Representative DLTS spectra of H level
measured at different lock in frequencies are shown in Figure 6.38.
The activation energy, i.e., energy level position in the bandgap (Ev + ET) and
capture cross section (σ∞) of the defect (H) have been obtained from thermal
emission rates by means of Arrhenius plots (shown in Figure 6.39), defect H ( Ev + Et
~ 0.45 eV) with capture cross section of about 3.78×10-17
cm2, respectively. By
comparing this result with class A samples, the defects to be found in class A samples
126
were electron traps but observed in class C samples is the hole trap. The nature of
defects was different in both groups of samples because class A samples were grown
by hydrothermal and class C samples were grown by MBE. But defect labeled as E1
in class A samples and H in class C samples had relationship with time. The trap
concentration of E1 increased with decreasing free carrier concentration with passage
of time.
Figure 6.37 Representative DLTS spectrum displaying one hole trap of group C
samples.
Figure 6.38 Typical DLTS spectra of levels H measured at different frequencies of
group C samples.
100 150 200 250 300 350 400 450
-60
-45
-30
-15
0
D
LS
(m
V)
Temperature(K)
5 Hz
10 Hz
50 Hz
1000 Hz
100 150 200 250 300 350 400 450
-60
-50
-40
-30
-20
-10
0
Temperature(K)
DL
TS
Sig
na
l (m
eV
)
127
Figure 6.39 The Arrhenius plot of hole level in group C samples.
6.15 Trap Identification
In semiconductor, impurities and defects occur as energy states in bandgap. If
a defect is transformed from one structural configuration to another, it is called
metastable defect. The configuration of metastable defect can be explored by
changing the measuring parameters such as electric field, temperature, storage time,
and carrier injection conditions. Several groups have reported the metastable defect by
using the DLTS [42,43]. A metastable hole trap with storage time (shown in Figure.
6.40) is detected in DLTS measurements. This hole trap located at different energy
states is observed by repeating DLTS measurements, while all the measuring
parameters remain same except time. As a result, the activation energy of the acceptor
level varies from 0.31 to 0.49 eV above the valance band (as shown in Figure 6.41)
Our results are in good agreement with available reported literature [44]. Wang et.al.
[44] found the hole trap having energy 0.45 eV above the valance band by generalized
gradient approximation (GGA). They related it to Zn-N complex.
3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1
10-4
10-3
1000/T (K-1
)
Ev+ Et= 0.451 eV
= 3.78 10-17cm2
eT-2
(s-1
K-2
)
128
Impurities introduced during growth process are very hard to eliminate. They
can come from sources, substrate, atmosphere, and walls of growth chamber. Even in
the ultrahigh vacuum atmosphere used in growth. The oxygen used for the production
of oxygen plasma could be the real source of nitrogen. During the deposition of film,
N occupied at oxygen vacancy along with one less electron in the crystal. It is
preferable for N to occupy at O site rather than Zn site because its ionic radius is
comparable to the O. The SIMS measurement (Figure 6.33) shows the presence of
nitrogen which is constant as a function of film depth. This SIMS result supports our
assumption about existence of nitrogen. Zn-N bond is not stable because of excess
amount of its bonding energy and it could be changed with time. This excess amount
of its bonding energy provides the driving force for the change of local bonding.
These changes of local bonding depend upon detailed kinetic factor for example
lattice locations and the activation energy for N displacement [45, 46]. Consequently,
we observed activation energy of hole trap H varying from 0.31 to 0.49 eV above the
valance band. Hence, it indicates that hole trap H consists of more than one
metastable conureuration. Owing to the foresaid properties of Zn-N complex, the
origin of the metastable hole defect H with Zn-N complex is justified.
129
0 100 200 300 400 500 600 700 800
320
340
360
380
400
420
440
460
480
500
Time (hours)
Act
iva
tio
n E
nee
rgy
(m
eV)
Figure 6.40 Metastability behavior of hole trap H with respect to time.
3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.31E-6
1E-5
1E-4
1E-3
0.01
0.1
1
10
100
eT
-2(s
-1K
-2)
1000/T(K-1
)
Figure 6.41 The Arrhenius plot of hole level in group C samples with passage of time.
130
Section- IV
The fourth part presents the measurements of ZnO samples (D, E, F, & G) grown on
glass coated with metals (Ni, Al, Au and Ag) by aqueous chemical growth described
in section 5.3 .
6.16 XRD Measurements
The XRD measurements of four samples (D, E, F, & G) grown by aqueous
chemical growth were performed. The source of XRD machine is copper that has
wavelength 1.54 Ao. XRD patterns of four samples (D, E, F, & G) are shown in
Figure 6.42. In this patterns we observed 8 peaks (P1, P2, P3, P4, P5, P6, P7, and P8)
at angles (2θ) 31.81o, 34.47
o, 36.31
o, 38.19
o, 44.71
o, 47.59
o, 56.55
o, and 62.87
o
respectively. Miller indices were calculated by hexagonal formula which has been
explained in Sec 4.4. The 6 peaks (P1, P2, P3, P6, P7, and P8) are due to hexagonal
structure of ZnO. Miller indices of peaks (P1, P2, P3, P6, P7, and P8) are 100, 002,
101, 102, 110 and 103, respectively. The 2θ, intensity and Miller indices of four
samples (D, E, F, & G) are listed in Table 6.5.
131
Figure 6.42 XRD patterns of four samples (D, E, F, & G).
30 40 50 60
0
200
400
600
800
1000
1200
1400
P6 P8
P4
P2
P3P1
Inte
nsi
ty (
cp
s)
2 (degree)
Sample F
30 40 50 60
0
50
100
150
200
P8P6
P7P5
P4
P3
P2
P1
Sample G
2 (degree)
Inte
nsi
ty (
cp
s)
30 40 50 60
0
100
200
300
400
500
600
P8
P7
P6
P3
P2
P1
Sample D
Inte
nsi
ty (
cp
s)
2 (degree)
30 40 50 60
0
100
200
300
400
P8P6
P3
P2
P1
P1
Sample E
Inte
nsi
ty (
cp
s)
2 (degree)
132
Table 6.5 2θ, intensity and Miller indices from XRD data of sample D, E, F and G.
Peak no 2θ (degree)
Intensity
Miller
indices Sample
D (Ni)
Sample E
(Al)
Sample F
(Au)
Sample G
(Ag)
P1 31.81 76 21 45 79 ZnO (100)
P2 34.47 538 400 228 199 ZnO (002)
P3 36.31 190 102 96 108 ZnO (101)
P4 38.19 - - 1274 92 -
P5 44.71 - - - 18 -
P6 47.59 66 41 24 28 ZnO (102)
P7 56.55 17 - - 17 ZnO (110)
P8 62.87 104 59 36 24 ZnO (103)
It is clear from above results that P2 is the highest peak in all samples except
sample F. P4 is the highest peak in sample F but it is unidentified. However P2 (002)
is the highest intense peak in all samples if we neglect the unidentified peak. The
intensity of the XRD peak indicates the atomic positions within the unit cell [13]. The
strong peak P2 (002) indicates the preferable direction of growth of ZnO is c-axis.
This result is in good agreement with available data reported in literature [47]
Peaks (P1, P2, P3, P6 & P8) originated from hexagonal ZnO are observed
in all four samples (D-G) but intensities of these 5 peaks in all samples are
different.The sharp and narrow diffraction peaks point out that the material has good
crystal quality for characterization as shown in Figure 6.41. In our case, thin layer of
metals (Ni, Al, Au & Ag) coated on substrates behaves as nucleation layer for growth
of ZnO nanostructure. Ni seemed to play a significant role for the nucleation in the
ZnO growth because intensity of peak P2 in sample D is maximum i.e 538 cps than
other samples (E, F and G).To the best of our knowledge, we are the first who used Ni
metal as nucleation in the ZnO growth. Reports about other metals are available in
133
literature for example Zhang et al. [48] reported the nucleation of ZnO nanowires
grown by PLD using thin coating of Au on substrates. Yamabi et al. [49] attained
high-quality heterogeneous nucleation by applying layer of Zn(O2CCH3)2 on
substrates such as glass.
6.17 SEM Measurements
Figure 6.43 and 6.44 show SEM images of sample D with different
magnification level of the equipment. The image shown in Figure 6.43 is magnified
by 1630 times. The image shown in Figure 6.44 is magnified by 46580 times. SEM
images of sample E with different magnification level of the equipment are shown in
Figure 6.45 and 6.46. SEM images shown in Figure 6.45 and 6.46 are magnified by
2200 and 16700 times, respectively. All images have been obtained at electron beam
energy 20 KeV. The growth of nanostructures in samples D is more intense than
sample E. Figures 6.43 and 6.45 clearly exhibit the flower like structure which
consists of nanorods as exposed at higher magnification (shown in Figures 6.44 and
6.45). It is supposed that thin metal layer of Ni enhances the growth. This result is
consistent with XRD measurement.
134
Figure 6.43 SEM image of sample D grown by ACG.
Figure 6.44 SEM image of sample D grown by ACG.
135
Figure 6.45 SEM image of sample E grown by ACG.
Figure 6.46 SEM image of sample E grown by ACG.
136
CHAPTER 7
137
7 CONCLUSION AND FUTURE PLAN
The following sections contain conclusion of each group of samples and future plan.
7.1 Group A Samples
The deep level defects in Pd/n-ZnO sample of Zn-face grown by hydrothermal
technique using DLTS have been studied. DLTS performed under various conditions.
The salient features of the research activity are following:
Two electron defects named as E1 (dominant) and E2 located at 0.22 ± 0.02 eV
and 0.49 ± 0.02 eV below the conduction band minimum, respectively are found in
Zn-face (sample A) ZnO. Level E1 shows time delayed transformations as a function
of free carrier concentration ND of the device during isothermal annealing studies.
Based on the available information in the literature together with the vigilant analysis
of the level E1, the level E1 can be linked to a zinc antisite defect and that ZnO is
intrinsically n-type because of Zni-VO complex. Under certain conditions, zinc
interstitial sits on the VO site when the Zni-VO complex is broken. Consequently, the
trap concentration NT increases and the free carrier concentration ND decrease.
Qualitative measurements such as I-V, C-V, and XRD measurements also support our
conclusion.
7.2 Group B Samples
The deep level defects in Pd/n-ZnO samples of different face (Zn-face & O-
face) grown by hydrothermal technique using DLTS has been studied. DLTS
performed under various conditions. The out standing features of the research activity
are following:
138
The effect of background doping concentration induced field on an electron
defects in ZnO Schottky devices has been studied. We investigated two samples A
(Zn-face) & B (O-face) having doping concentration 3.44 × 1017
cm-3 and 3 × 1016
cm-
3, respectively. Two electron trap levels E1 and E2, having activation energies (0.22 &
0.26) eV and (0.49 & 0.49) eV below the conduction band minimum, of samples A
and B, respectively are observed. Level E2 is due to the surface states. Some famous
research groups observed an electron trap having the activation energy in the range of
0.22- 0.32 eV with free carrier concentration in the range of 1017 - 1014 cm-3 in bulk-
ZnO devices. Therefore, decrease in emission rate of level E1 is connected to the
lowering of ND-induced barrier height. After getting these interesting results, we
applied Poole–Frenkel model carrying Coulomb potentialon the emission rate data
(ours + reported) related with E1 and observed the data to be well fitted with this
model. Zn-interstitials in ZnO are residual shallower donors in ZnO according to the
Look et al. [Phys. Rev. Lett. 82 (1999) 2552] theoretical calculations, hence E1 level
has been attributed as a charged impurity that is originated from Zn.
7.3 Group C Samples
We have studied the hole trap in ZnO grown by molecular beam epitaxy using
DLTS. This hole trap shows metastable behavior which can exist in more than one
configurations, each with distinct electric properties. The configuration of hole
changed with storage time. It is hard to remove the impurities during the growth
process. N sits on the O site during the growth process to form the Zn-N complex.
This Zn-N complex is locally instable because of its excess bonding energy and might
have changed its configuration with time. SIMS measurement shows the presence of
nitrogen which support our conclusion about hole trap.
139
7.4 Samples D, E, F, & G
We have studied the effect of thin layers of different metals (Ni, Al, Ag and
Au coated on substrates) on the fabrication of nanorods grown by aqueous chemical
growth. XRD measurements reveal the presence of ZnO nanorods with c-axis
orientation. The peak (002) observed in sample D has more intense than remaining
three samples. The largest intensity of peak originated from 002 plane of sample D is
associated with nucleation due to Ni metal coated on substrate. SEM measurements
also agree with these results that thin layer Ni metal enhances the production of
nanorods.
7.5 Future Plan
With that detailed investigations of defects contained in ZnO grown by
different growth techniques, fabrication of ZnO based devices having high efficiency
and better lifetime is possible. The study of deep level defects in ZnO grown by
different growth techniques and the influence of annealing on defects is the issue for
future work. The radiation study may be helpful for strengthening the proposed
identification of the observed defects.
140
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141
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