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21
CHAPTER 2
GROWTH AND CHARACTERIZATION OF VERTICALLY
ALIGNED ZnO NANORODS
2.1 INTRODUCTION
In the past few years, wide band gap semiconductor nanostructures
such as nanorods, nanowires, and nanobelts have been of growing interest due
to their importance in both scientific research and potential applications,
including nano&electronics, nano&optoelectronics, nano&piezotronics,
gas/chemical sensors, surface acoustic wave devices (SAW) and field
emission device applications. Considerable effort has been devoted to the
synthesis and study of ZnO nanostructures of various morphologies.
ZnO nanomaterials have been synthesized by various techniques such as
chemical vapor deposition (Wu and Liu 2002), physical vapor deposition
(Kong et al 2001), molecular beam epitaxy (Heo et al 2002) and a simple
method that just involves heating a Zn powder containing catalyst
nanoparticles (Wang et al 2004). All these approaches apply the VLS
mechanism for nanowire growth, in which a metal or oxide catalyst is
necessary to dissolve feeding source atoms in the molten state to initiate the
growth of nanostructures. The UV stimulated emission from optically pumped
nanowires at room temperature has also been reported (Yang et al 2002). ZnO
nanobelts have been successfully synthesized by simply evaporating ZnO
powders without the presence of catalyst (Pan et al 2001). Nevertheless, a
high temperature, as high as the melting point of bulk ZnO, is needed in such
method. Among the various techniques, pulsed laser ablation is a simple
22
method compared with well&established techniques like metal organic
chemical vapor deposition and molecular beam Epitaxy. The PLD has been
widely used for the synthesis of nanostructures and thin films.
2.2 PULSED LASER DEPOSITION (PLD)
In the pulsed laser deposition technique, also called laser ablation,
high power laser pulses are used to evaporate material from a target surface
such that the stoichiometry of the material is preserved in the interaction. As a
result, intense “plume of particles” is generated from the target surface. The
plume expands away from the target with a strong forward directed velocity
and condenses on the substrate placed opposite to the target. A schematic
diagram of the typical PLD system is shown in Figure 2.1.
Figure 2.1 Schematic diagram of pulsed laser deposition setup
When the laser radiation is absorbed by a solid surface,
electromagnetic energy is first converted into electronic excitation and then
into thermal, chemical and even mechanical energy to cause evaporation,
substrate holder
Vacuum chamber ∼∼∼∼10-6
mbar
23
ablation, excitation, plasma formation and exfoliation. The ablated plume
contains a mixture of energetic species including atoms, molecules, ions,
clusters and micron&sized solid particulates. The collisional mean free path
inside the dense plume is very short. As a result, immediately after the laser
ablation, the plume rapidly expands into the vacuum from the target to form a
nozzle jet with hydrodynamic flow characteristics. PLD has several attractive
features, including stoichiometric transfer of material from the target,
generation of energetic species, hyperthermal reaction between the ablated
cation and molecular oxygen in the ablation plasma, and compatibility with
background gas pressure ranging from UHV to 100 Torr. This process
attributes to many advantages as well as disadvantages. The advantages are
flexibility, fast response, energetic evaporants and congruent evaporation.
It can be used for the synthesis of various materials including organic crystals,
semiconductors, dielectric materials and refractory materials. PLD is simpler,
easier and suitable technique for some complex and high melting point
compounds. It has been successfully used in carbon nanotube synthesis, with
high yield (Eason 2007). The disadvantages are the presence of micro&size
particulates and the narrow forward angular distribution that makes
large&area&scale&up a very difficult task.
One of the important advantages of PLD technique is that it can be
operated from ultra high vacuum to 100 Torr of ambient pressure.
The growth at low oxygen pressure leads to either the formation of thin film
or randomly aligned nanostructures (Yan et al 2003). The low pressure PLD
can be defined as if the oxygen pressure is in the range of 1 × 10&5 to 0.5 Torr.
The kinetic energy of ablated species is shown to have approximately several
100 eV. These energies are sufficiently high to create defects in the growing
thin films in vacuum or at low background pressure. The kinetic energy of the
ablated species can be reduced by increasing the ambient gas pressures.
Hence, the control and reproducibility over the nanostructure growth of oxide
24
and nitride materials can be enhanced in high&pressure PLD compared with
the low&pressure PLD (Eason 2007). Similarly, the high&pressure PLD can be
defined as if the oxygen pressure is in the range of 0.5 Torr to 100 Torr.
The number of collisions between ablated species and gas molecules is
strongly depends on the ambient gas pressure. The high gas pressure leads to
large number of collisions, which in turn results the formation of
nanoparticles in the gas phase (Okada et al 2005).
The nanoparticles act as a nucleation site for the growth of
nanostructures. This approach is called high&pressure PLD or nanoparticle
assisted PLD. Table 2.1 compares the typical growth parameters of high and
low pressure PLD. Both inert and reactive gases e.g, oxygen and argon can be
used as a gas source in PLD technique. In this study, efforts have been made
to optimize the aligned growth of ZnO nanorods by varying the PLD growth
parameters. The effect of laser wavelength, oxygen partial pressure and
substrate temperature on the growth of ZnO nanorods have been analyzed
without using any metal catalyst and templates.
Table 2.1 Comparison of the growth parameters of high and low#pressure
PLD
Growth Parameters High#pressure PLD Low#pressure PLD
Background pressure 0.5 & 100 Torr 10&5 & 0.5 Torr
Morphology Self&assembled ZnO nanorods
Mostly thin films
Growth temperature RT & 800 °C RT & 800 °C
Target & Substrate distance 20 & 40 mm 30 & 60 mm
Substrates Any substrates Any substrates
Material types Oxides and nitrides Oxides and nitrides
25
2.3 LOW#PRESSURE PULSED LASER DEPOSITION
2.3.1 Experimental Details
ZnO thin films were grown on GaN (0001), Al2O3 (0001) and
Si (100) substrates at different laser wavelengths using a pulsed laser
deposition technique. ZnO targets were prepared by grounding commercial
(Alfa Aesar) ZnO powder for 1 hour. Then the powders were mechanically
pressed and sintered at 900 °C for the duration of 8 hours. The Q&switched
Nd:YAG laser was used for ablating ZnO targets in the infrared (IR) and
visible wavelengths. The film deposition was carried out in a stainless steel
vacuum chamber evacuated by a diffusion pump to a base pressure of
1×10&5 mbar. The Q&switched Nd:YAG laser (λ = 1064 nm and 532 nm,
repetition rate of 10 Hz and the pulse duration of 8 ns) was focused by a lens
on the ZnO target at an angle of incidence of 45º. During the deposition, the
laser energy was maintained at above 30 mJ/pulse. The substrate to target
(T&S) distance was varied from 40 to 50 mm. A KrF excimer laser
(λ = 248 nm, laser energy with 200 mJ/pulse and repetition rate of 7 Hz) was
used to ablate ZnO targets in the UV laser wavelength. The stainless steel
chamber evacuated by a turbo molecular pump to a base pressure of
1×10&5 mbar was used for the KrF laser experiments. The film deposition time
was varied from 5&60 minutes. The oxygen background pressure (Po2) was in
the range of 5×10&5 Torr to 100 mTorr. The substrate temperature (Tsub) was
varied from 450 to 700 °C.
2.3.2 Scanning Electron Microscopy (SEM)
The surface morphology of the ZnO films was examined using
scanning electron microscopy (SEM, Stereoscan, LEO&440). ZnO thin films
deposited at different Po2, Tsub and T&S resulted in the randomly aligned ZnO
nanostructures. The diameter/width of the ZnO nanostructures were in the
26
range of 300 & 2000 nm. The SEM images of ZnO films grown on GaN and
Al2O3 substrates in the Po2 of 7.5×10&5 Torr are shown in Figure 2.2 (a) and
(b), respectively.
Figure 2.2 SEM micrographs of a, b) ZnO nanorods and microrods
grown on GaN and Al2O3 substrates at 7.5××××10#5
Torr c, d)
ZnO nanorods and microrods grown on GaN substrate at
7.5××××10#4
Torr and 10 mTorr e) ZnO wire#like structures
grown at 50 mTorr f) ZnO belt#like structures grown at
100 mTorr
27
As evident from the SEM image, irrespective of substrate it has
been observed a random distribution of ZnO nanorods and microrods.
The diameter of the ZnO nanorods grown on GaN substrate varies from
250&300 nm and length varies from 10&20 �m, whereas diameter of the
microrods varies from 1.2&1.6 �m and the length varies from 9&22 �m.
The average diameter and length of the ZnO nanorods grown on Al2O3
substrate were 300 nm and 8 �m, respectively. The diameter of the microrods
was about 0.9 �m and the length varies from 7&11 �m. The SEM images in
Figure 2.2 (c) and (d) show the ZnO nanorods and microrods grown on GaN
substrate in the Po2 of 7.5×10&4 and 10 mTorr.
The diameter and length of the nanorods and microrods were
smaller than that observed in relatively low oxygen pressure. ZnO wire&like
structure was obtained for the growth at the Po2 of 50 mTorr as shown in
Figure 2.2 (e). Figure 2.2 (f) shows the SEM micrograph of ZnO belt&like
structure grown at the Po2 of 100 mTorr. In all the above conditions,
randomly aligned ZnO nanostructures were observed. The alignment and
reproducibility of nanostructures were found to be major issues in
low&pressure PLD. From the SEM images, it is evident that features of frozen
droplets are absent at tip of the rods. This implies that the VLS mechanism is
not applicable for the ZnO rods in the present growth. In this work no metal
catalyst was used. This suggests the growth of ZnO rods via VS mechanism
(Roy et al 2003, Conley et al 2005).
2.3.3 Growth Mechanism of ZnO Nanorods and Microrods
Both ZnO nanorods and microrods were obtained on GaN and
Al2O3 substrate at the similar deposition conditions. The possible mechanism
for the growth of nanorods and microrods is depicted as shown in
Figure 2.3.
28
Figure 2.3 Schematic diagram of growth mechanism of ZnO nanorods
and microrods
The absorption coefficient “α” of the ZnO decreases with
increasing wavelength from UV (266 nm) to IR (1064 nm). The penetration
depth of the laser appears to be larger in the near IR region than in the UV
region. Koren et al (1989) reported that the density and size of particulates are
much higher for the YBCO films prepared by Nd:YAG laser at IR wavelength
as compared to one prepared with an excimer laser at UV wavelength.
Bilkova et al (2004) also demonstrated the formation of micrometric and
subµmetric sized particulates in the ZnO films grown using Nd:YAG
(λ=532 nm) laser. It has been shown that the penetration depth of the laser at
532 nm determines the size and density of particulates in the ZnO thin films.
When the experiment is conducted at low oxygen pressure, the number of
collisions between the ejected species and gaseous atoms is less before they
reach the substrate. Consequently, particulates are predominantly formed
from solidified liquid droplets that are expelled from the target (Chrisey and
29
Hubler 1994). These micro and nano size particulates may acts as seed
crystals for the growth of ZnO rods. Wang (2004) reported that the catalyst
droplet directs the growth direction and defines the diameter of the nanowires.
The nucleation can occur at any possible sites on the substrate. This reveals
that the particulates may induce the growth of ZnO nanostructure. Therefore,
it is likely that the diameter of the ZnO rods varies from nanometer to
micrometer size due to the different diameter of the particulates.
Figure 2.4 SEM micrographs of a, b) ZnO films grown on GaN and
Al2O3 substrates at 7.5××××10#5
Torr (λ = 1064 nm) c) ZnO film
grown on GaN substrate at 7××××10#4
Torr (λ = 248 nm)
The ZnO films were deposited at IR and UV wavelengths to further
confirm the particulates effect on the formation of nanorods and microrods.
Figure 2.4 (a) and (b) shows the SEM images of ZnO films grown on GaN
and Al2O3 substrate at the laser wavelength of λ = 1064 nm. The image
30
clearly shows the typical micron size particulates on the film surface. Here the
number of particulates ejected from the target is very high due to the large
penetration depth of the laser. On the other hand, the ejection of large number
of particulates may possibly hinder the growth of the rods. This could be the
reason for the absence of nanorods and microrods at higher laser wavelength.
When the experiment was carried out using KrF excimer laser (λ = 248 nm) a
relatively smooth surface of ZnO was observed, as shown in Figure 2.4 (c).
The probability for the formation of particulates in KrF excimer laser is very
less. Hence, no rods could be observed in the ZnO films. All the ZnO films
grown in IR and UV laser wavelengths do not reveal any rod like
morphology. This implies that the rod like growth may depend on the
penetration depth of the laser.
2.3.4 X#Ray Photoelectron Spectroscopy (XPS)
The composition of the ZnO nanorods and microrods were
examined using X&ray photoelectron spectroscopy. The XPS measurements
were performed with a VG scientific ESCALAB MK200X Electron
spectroscope for chemical analysis (ESCA) machine using Al&Kα X&ray as a
source (1486.6 eV, width 0.85 eV). The hemispherical analyzer was operated
with pass energy of 20 eV to give analyzer resolution of 0.4 eV. The base
pressure of the chamber was maintained at 1×10&10 mbar. The XPS spectrum
of the ZnO thin films grown on Al2O3 is shown in the Figure 2.5. The peaks
centered at 7 eV, 86.9 eV and 136.6 eV are due to the binding energy of Zn3d,
Zn3p and Zn3s, respectively. Peak centered at 527.8 eV is due to O1s peak
(Yuen et al 2007). The zinc and oxygen were only detected species in the
XPS spectrum. This shows that no other external metal catalyst was involved
in the ZnO rod growth.
31
Figure 2.5 XPS spectrum of ZnO nanorods grown on Al2O3 substrate
at 7.5××××10#5
Torr
2.3.5 X#Ray Diffraction (XRD)
The structural properties of the samples were studied using Philips
X&pert X&Ray Diffraction system using CuKα as a source (λ = 1.54 Å). Figure
2.6 shows XRD spectra of ZnO nanorods and microrods grown on Al2O3 and
GaN substrates at λ = 532 nm. The strong (0002) peak can be attributed to the
hexagonal wurtzite structure of ZnO. The lattice constants of ZnO and GaN
are very close to each other. Hence, the peaks due to (0002) plane of GaN and
ZnO overlap (Wang et al 2005). The FWHM of ZnO films grown in oxygen
ambient are 666 arcsec and 578 arcsec on GaN and Al2O3 substrate,
respectively. The higher FWHM of (0002) peak may be due to the overlap of
(0002) planes of both ZnO and GaN. The biaxial stress was calculated using,
σ = & 453.6 x 109 [(c&co)/ co] (2.1)
32
Figure 2.6 XRD patterns of ZnO nanorods and microrods grown on
GaN and Al2O3 substrates at 7.5××××10#5
Torr
where co = 0.5206 nm is the strain free c&axis lattice constant of ZnO (Oh et al
2005). The “c” is the lattice constant along c&axis of ZnO grown by PLD. The
estimated tensile stress is decreased from 3.48 GPa (on Al2O3) to 2.43 GPa
(on GaN). This shows the relaxation of the tensile stress in the ZnO films
grown on GaN substrate, which may be due to close lattice match.
2.3.6 Photoluminescence (PL)
The photoluminescence spectra were recorded at room temperature
using He&Cd (λ = 325 nm) laser as an excitation source with the excitation
power of 38 mW. Comparison of the PL spectra of ZnO films grown on GaN
and Al2O3 substrates is shown in Figure 2.7.
33
Figure 2.7 PL spectra of ZnO nanorods and microrods grown on GaN
and Al2O3 substrates
The ZnO films show near band edge (NBE) emission at 3.282 eV
and 3.277 eV on GaN and Al2O3 substrates in oxygen ambient, respectively.
The blue shift in the NBE emission of ZnO/GaN film is due to the low tensile
stress; this is in good correlation with XRD results. The PL intensity of ZnO
film grown on GaN in oxygen ambient is twice stronger than that of ZnO
grown on Al2O3. The NBE emission is due to the free exciton&exciton
recombination of ZnO. In addition to this, FWHM of ZnO/GaN film is
smaller (92 meV) than that of the ZnO/Al2O3 (101 meV) film. All the ZnO
films show weak defect related emission in the visible region. The emission
intensity is determined by the radiative and non&radiative transitions.
The non&radiative transition is generally induced by crystal imperfections
such as oxygen vacancies and zinc interstitials (Shan et al 2007) in ZnO thin
films.
34
2.4 HIGH # PRESSURE PULSED LASER DEPOSITION
2.4.1 Experimental Details
In the high&pressure PLD experiments the Po2 was maintained
between 6 and 8 Torr. Before loading into the chamber, the GaN (0001),
Al2O3 (0001) and Si (100) substrates were cleaned sequentially in acetone and
methanol for 3 min. The Si surfaces were etched in HF:H2O in the ratio of
1:10 for 1 min to remove the native oxide layer of the Si (100) substrate. Then
the substrates were rinsed with deionized (DI) water and dried in atmosphere.
The undoped ZnO targets were prepared by mechanically pressing the
commercial ZnO (Alfa Aesar & purity 99.99%) powder. The ZnO targets were
sintered at 900 °C for 8 hours in the atmosphere. The chamber was evacuated
to the base pressure of 8×10&6 Torr after loading the ZnO target and the
substrates. KrF excimer laser (λ = 248 nm, repetition rate of 10 Hz, pulse
duration of 30 ns and laser energy with 230 mJ/pulse) was used to irradiate
the ZnO target. The laser fluence was 3 J/cm2. The ablated species was then
deposited on a Si substrate that was mounted on a substrate heater. During
growth, the substrate temperature was varied from 550 to 700 °C in steps of
50 °C. The target to substrate distance was varied from 25 mm to 30 mm.
2.4.2 Field Emission Scanning Electron Microscopy (FESEM)
The surface morphology of ZnO films was analyzed using FESEM
(Hitachi S4700 system, with a resolution of 1.2 nm at 25 kV).
Figure 2.8 (a – e) shows top view of ZnO films grown on Si at Tsub of
500 °C, 550 °C, 600 °C, 650 °C and 700 °C, respectively. The laser fluence
was 3 J/cm2. Figure 2.9 (a) and (b) shows cross§ional view of ZnO films
grown at Tsub of 650 and 600 °C, respectively.
35
Figure 2.8 FESEM micrographs (top view) of ZnO films grown on
Si substrates at a) 500 °C b) 550 °C c) 600 °C d) 650 °C
e) 700 °C
The film deposited at 500 °C shows sheet&like structure with a
rough surface. The SEM morphology turned into grain like structures with the
average diameter of ~ 1 `m at 550 °C. As the substrate temperature further
increased to 600 °C, the films were covered by near hexagonal crystals and
randomly aligned on the substrate, with the average diameter of ZnO grains as
36
~ 400 nm. ZnO films deposited at 650 °C show the well separated vertically
aligned hexagonal nanorods with tapered tip at the top. The average diameter
of ZnO nanorods was around 300 nm. When the substrate temperature was
increased to 700 °C the diameter of the nanorods further increased and starts
to contact with the neighboring nanorods. ZnO nanorods lost the ordered
alignment and lead to the formation of continuous ZnO film at higher
temperatures. The vertically aligned growth of ZnO nanorods is obvious from
the cross§ional view of the ZnO films deposited at 650 and 600 °C,
respectively as shown in Figure 2.9 (a) & (b). The nanorods grown at 650 °C
show hardly any coalescence between the neighboring nanorods than that
grown at 600 °C.
ZnO films were deposited at the laser fluence of 2 J/cm2 to study its
effect on the growth of ZnO nanorods. Figure 2.9 (c) shows the FESEM
micrographs of ZnO films grown at 2 J/cm2. ZnO rod&like structures with
hexagonal facets are observed with continuous ZnO film in the background.
The density of the ZnO nanorods was low compared to ZnO nanorods grown
at 3 J/cm2. The background film consists of grains with diameter of ~ 300 nm.
This can be attributed to the low ablation rate and growth rate at reduced laser
fluence. The composition of the ZnO nanorods grown at 650 °C (3 J/cm2) was
analyzed using energy dispersive X&ray analysis (EDX). The EDX (Figure 2.9
(d)) confirmed the presence of zinc and oxygen species, the other impurities
were not found. This shows that the as grown nanorods followed the
vapor&solid growth mechanism rather than an impurity assisted vapor&liquid&
solid growth mechanism.
37
Figure 2.9 FESEM micrographs (cross sectional view) of ZnO films
grown on Si substrates with a laser fluence of 3 J/cm2 at
a) 650 °C b) 600 °C c) ZnO films grown at 650 °C with the
laser fluence of 2 J/cm2 and d) EDX spectra of ZnO
nanorods grown at 650 °C
Energy (keV)
Inte
nsi
ty (
cou
nts
)
(d)
38
2.4.3 Growth Mechanism of Aligned ZnO Nanorods
Figure 2.10 shows the schematic diagram for the growth
mechanism of ZnO nanorods. During the laser ablation, a short wavelength
and high power laser beam interact with sintered ZnO target (Step&I).
The strong laser&material interaction leads to the evaporation of Zn, O and
neutrals from the ZnO target. The background pressure determines the
number of collision between the ablated species and gaseous atoms. At a high
oxygen gas pressure above 1 Torr, the increased collisions lead to the
condensation of ablated species resulting in the formation of nanoparticles
(Step&II). The nanoparticles then reach the substrate surface and may act as a
nucleation site or seed crystal for the growth of nanostructures (Hartanto et al
2004, Kawakami et al 2003) (Step&III). The nucleation can occur at any
possible site on the substrate. Since ZnO nanoparticles are thermodynamically
favorable site for the growth of ZnO nanostructures, the nanorods growth can
be attributed to the formation of nanoparticles in the gas phase (Step&VI).
Figure 2.10 Schematic diagram represents the growth mechanism of
ZnO nanorods
This implies that nanorods could grow only at such a high gas
pressures whereas thin film growth was obtained when deposition was
39
conducted at low oxygen pressure. The nanoparticles formed in the gas phase
play an important role in the growth of vertically aligned nanorods.
2.4.4 X#Ray Diffraction
Figure 2.11 (a) shows the XRD patterns of ZnO films grown at
different substrate temperatures. All the samples show a pronounced (002)
peak of wurtzite ZnO, indicating the preferentially oriented growth along the
c&axis which is perpendicular to the substrate surface. The XRD peaks match
with the JCPDS card No: 00&003&0888. ZnO sheet&like structures grown at
500 °C exhibits (100), (002), (101), (102), (103), (112) and (004) peaks. This
indicates that the sheet&like structures are randomly aligned on the Si
substrate. The full width at half maximum (FWHM) of (002) peak is 0.19°.
The c&axis lattice constant ‘c’ was obtained using the formula: 2d sinθ = nλ
and the biaxial stress was calculated using,
σ = & 453.6 x 109 [(c&co)/ co] (2.2)
where c0 = 0.5206 is the strain&free lattice constant (Oh et al 2005). These
detailed data points were connected in lines in Figure 2.11 (b). As the Tsub
increases to 550 °C, high temperature provides enough energy for the adatoms
to gain high surface mobility, which promotes the formation of the closely
packed columnar grains oriented along the [002] direction and the (002) peak
was observed at 34.55°. The vertically aligned nanorods were obtained as the
temperature was increased to 600 and 650 °C. The (002) peak was observed at
34.46°, with a shift towards lower angle. The (002) peak position of nanorods
closely matches with (002) peak of bulk ZnO. The peak shifts towards higher
angle at 700 °C and the (002) peak was observed at 34.61°. The position of
(002) peak shifts towards higher angle with increase of Tsub. This may be
attributed to the unstable oxygen adatoms on the ZnO growth surface which
may be easier to desorbs at higher temperature. The FWHM of ZnO films
40
grown at 500, 550, 600, 650 and 700 °C are 0.19, 0.26, 0.26, 0.31 and 0.38°,
respectively. ZnO films grown at 500, 550, 600, 650 and 700 °C have
compressive stress of 1.82, 1.82, 0.6, 0.6 and 2.43 GPa, respectively. This
implies that due to the different thermal expansion coefficients of ZnO and Si,
the compressive stress would be induced parallel to the c&axis. ZnO grown at
higher temperature has high FWHM and large compressive stress compared
with ZnO grown at lower temperatures. The temperature dependence of the
film quality can be interpreted mainly by the mobility of the adatoms on the
substrate surface at different temperatures.
(a) (b)
Figure 2.11 a) XRD patterns of ZnO film deposited at different
temperatures and b) Plot between Tsub versus peak position,
FWHM and stress
During PLD, the kinetics of atomic arrangement is mainly
determined by substrate temperature and the energy of deposition atoms.
Therefore, at a high temperature, adatoms on the surface have high mobility.
41
There should be enough time for the adatoms to move on the surface to look
for the lowest energy sites before these adatoms are covered by the next layer
of atoms (Ye et al 2003). Otherwise, low substrate temperature results in poor
adatom mobility, which results in the degradation of crystallinity. On the
other hand, the decomposition of ZnO was promoted at high Tsub. In the
present work, it has been confirmed that the stress between ZnO film and Si
substrate increases due to the increase of the Tsub and the crystallinity of ZnO
film degrades at higher deposition temperature which is in agreement with
Liu et al (2004).
2.4.5 Micro # Raman spectroscopy
In the wurtzite ZnO, the number of atoms per unit cell is s = 4 and
there are a total of 12 phonon modes, namely, one longitudinal&acoustic (LA),
two transverse&acoustic (TA), three longitudinal&optical (LO), and six
transverse&optical (TO) branches. Raman spectroscopy has been commonly
employed to derive zone¢er and some zone&boundary phonon modes in
ZnO. Because the space group ‘C’ describes the crystalline structure of the
wurtzite ZnO with 2 formula units in the primitive cell, the optical phonons at
the point of the Brillouin zone belong to the following irreducible
representation. The A1and E1 branches are both Raman and infrared active,
the two non&polar E2 branches are Raman active only, and the B1 branches are
inactive silent modes. The each A1 and E1 modes are split into LO and TO
components with different frequencies due to the macroscopic electric fields
associated with the LO phonons. Because the electrostatic forces dominate the
anisotropy in the short&range forces, the TO&LO splitting is larger than the
A1 & E1 splitting. For the lattice vibrations with A1 and E1 symmetries, the
atoms move parallel and perpendicular to the c axis, respectively.
The low&frequency E2 mode is associated with the vibration of the
heavy Zn sublattice, while the high&frequency E2 mode involves only the
42
oxygen atoms. In the case of highly oriented ZnO films, if the incident light is
exactly normal to the surface, only A1 (LO) and E2 modes are observed, and
the other modes are forbidden according to the Raman selection rules (Ozgur
et al 2005, Singamaneni et al 2003, Zhu et al 2009). The vibrational modes of
the as grown ZnO nanostructures were recorded using micro&Raman
spectroscopy. Figure 2.12 shows the micro&Raman spectra of ZnO films
grown on Si substrate at different Tsub of 500, 550, 600, 650 and 700 °C. The
spectra show Raman modes at 331, 665 (multi&phonon), 380 (A1 (TO)) and
439 cm&1 (E2H) (Ozgur et al 2005, Singamaneni et al 2003).
Figure 2.12 a) Micro # Raman spectra of ZnO film deposited at different
Tsub, at fluence of 3 J/cm2 and b) ZnO deposited at the Tsub
of 650 °C with the fluence of 2 J/cm
2
The Raman shifts observed at 304 and 521 cm&1 are correspond to
Si substrate. The E2H mode of ZnO films were observed at 439 cm&1. No shift
in E2H mode was observed for ZnO films grown at different temperatures. The
strong E2H mode can be attributed to low intrinsic defects associated with O
e.g. O vacancies (VO), since E2H mode is only associated with the vibration of
43
O atoms (Singamaneni et al 2003). The low VO of ZnO nanostructures can be
attributed to the high oxygen pressure during PLD growth. In addition to the
E2H mode, ZnO sheet&like structures grown at 500 °C shows additional mode
(AM) at 477 cm&1. The FWHM of ZnO films grown on Si substrates at Tsub of
500, 550, 600, 650 and 700 °C were 5.8, 6.4, 6.4, 6.9 and 7.1 cm&1,
respectively. These results are in good correlation with XRD and FESEM.
ZnO films deposited at 650 °C with the laser fluence of 2 J/cm2 possesses AM
mode at 477 cm&1 in addition to the well defined E2H at 439 cm&1. The decrease
in the laser fluence influences the growth of ZnO nanorods.
2.5 SUMMARY
Vertically aligned ZnO nanorods were grown on Si substrate using
high&pressure pulsed laser deposition. The randomly aligned ZnO nanorods,
microrods, nanowires and nanobelts were obtained in the Po2 of 4×10&4 Torr –
100 mTorr. The controllability and reproducibility on the growth of ZnO
nanostructures were found to be poor in low&pressure PLD.
The vertically aligned ZnO nanorods were realized in the high Po2 of
6 – 8 Torr. The (002) peak in XRD pattern implies the preferred orientation of
ZnO nanostructures along c&axis. The strong E2H mode shows that as grown
ZnO nanorods possesses good crystalline properties. The high pressure PLD
is suitable method to obtain aligned ZnO nanorods.