Page 1
Chapter 3
UNDOPED ZnO THIN FILMS
The contents of this chapter is published in
1. Nanda Shakti, P.S. Gupta, Structural and Optical properties of sol-gel
prepared ZnO thin film, Applied Physics Research 2(1), ( 2010) 19.
2. Nanda Shakti, P.S.Gupta, Structural and photoluminescence of ZnO thin
film and nanowire, Optoelectronic and Advanced Material - Rapid
Communication, 4(5), (2010) 662.
3. Nanda Shakti, P.S. Gupta, Study of Carrier concentration and mobility with
multilayer ZnO film, Processing and Fabrication of Advanced Materials-
XVII, I.I.T. Delhi, Vol II, (2008) 346.
Page 2
CHAPTER.3: UNDOPED ZnO THIN FILMS
59
3. Introduction
Zinc oxide is an inexpensive n-type semiconductor having direct band gap of 3.3 eV
which crystallizes in hexagonal wurtzite structure (c = 5.025 Å and a = 3.249 Å) [1].
Due to large exciton binding energy of 60 meV , they have potential applications in
Optoelectronic devices such as in solar cells [2],Optical wave guide [3], Light emitting
diodes (LED) [4]. Zinc oxide thin films are applied in Thin Film Transistors (TFT) [5]
and have been recognized as spintronic material [6].Various gas, chemical and
biological sensors were based on ZnO thin film [7]. Thin films of Zinc oxide can be
prepared by various techniques; among them are Sputtering [8], Chemical Vapor
Deposition (CVD) [9], Laser ablation [10], Sol-gel [11], Spray pyrolysis [12]. M.
Inoguchi et al. [13] have studied structural & optical properties of nanocrystalline ZnO
thin films derived from clear emulsion of monodispersed ZnO nanocrystals.
Properties of ZnO thin films show dependence on the synthesis technique used. Apart
from doping, to increase the functionality of ZnO thin film, the effect of preparation
conditions on the properties have to be considered for its effective technological
applications. Relatively few works [14-15] have been done in this direction for ZnO
film prepared by sol-gel process. In this chapter we have studied the synthesis and
characterization of undoped ZnO thin films by sol-gel spin coating process [16]. Zinc
acetate dihydrate was used as the precursor material. The Sol-gel process has the
advantages of controllability of compositions, simplicity in processing and is cost
effective. We have studied the effect of annealing the ZnO thin films at three different
temperatures (400oC, 500oC and 6000C) on its structural, optical and electrical
properties. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) with
Eenergy dispersive scattering (EDS) attachment were used for structural and
morphological characterization respectively. UV-VIS-NIR spectrometry,
Photoluminescence and spectroscopic ellipsometry were used for optical
characterization. The electrical characterization of the films was done using four-probe
method.
Page 3
CHAPTER.3: UNDOPED ZnO THIN FILMS
60
The second part of the chapter deals with the investigation of quantum confinement
effect in sol-gel prepared ZnO thin films. Till now an exclusive investigation on
quantum confinement effect in sol-gel prepared ZnO thin films has not been reported.
We have prepared ZnO thin film by sol-gel method with annealing the samples at five
different temperatures viz. 250ºC, 300ºC, 350ºC, 400ºC and 450ºC.The shift of optical
band gap with annealing temperature have been observed.
3.1 Synthesis and characterization of ZnO thin film by sol-gel
spin coating method
3.1.1 Experimental method
ZnO thin films were prepared on fused quartz substrates (2.5×2.5 Cm2) by sol-gel spin
coating method. The quartz substrates were cleaned with soap solution followed by
ultrasonication in acetone for 5 min. Then it was degreased in ethanol for final cleaning.
For sol preparation 10% solution of zinc acetate dihydrate Zn(CH3COO)2.2H2O was
prepared in boiling iso-propanol. The turbid solution was cleared by adding 10 drops of
diethanolamine by a 5ml dropper. The clear sol was further boiled for 30 min. The sol
was then allowed to cool to room temperature. For film preparation, the quartz substrate
was spin coated by a spin coater (SCU 2005 Apex Inst.Co.) while spinning rate was
kept at 3000 rpm. The wet films were dried at 100oC for 10 min and subsequently
annealed for 1 hour. The process was repeated to obtain the workable thickness of the
film. Multilayer films post annealed at 400oC, 500oC and 600oC for 1hour were
prepared to study the effect of annealing.
The structural properties of the prepared films were studied by x-ray diffraction
measurements (Panalytical Xpert pro, with CuKα radiation (λ= 1.54059 Å)). Scanning
electron microscope (Jeol; JSM-6390LV) was used to record Energy dispersive
scattering (EDS) and surface morphology.
Page 4
CHAPTER.3: UNDOPED ZnO THIN FILMS
61
UV-VIS-NIR Spectrophotometer (HR 4000 Ocean Optics) was used to record the
transmission spectrum of the films in the wavelength range from 250 nm to 600 nm.
Ellipsometer (Nano View; SE MG-1000UV) was used to measure refractive index n,
extinction coefficient k and thickness of the films. Photoluminescence of the films was
measured by Spectroflurophotometer (Hitachi).
3.1.2 Structural Properties
The crystal structure of ZnO films was investigated through X-ray diffraction (XRD).
The X-ray diffraction spectrum of ZnO film annealed at 400ºC, 500ºC and 600ºC with
prominent reflection planes is shown in figure 3.1.
The peaks in the XRD spectrum correspond to those of the bulk ZnO patterns from the
JCPDS data (Powder Diffraction File, Card no: 36-1451) [17], having hexagonal
wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å.The presence of
prominent peaks shows that the film is polycrystalline in nature. The lattice constants
‘a’ and ‘c’ of the Wurtzite structure of ZnO can be calculated using the relations (3.1) &
(3.2) given below [18].
a = θ
λ
sin3
1 (3.1)
c = θ
λ
sin (3.2)
For (002) plane calculated values are a= 3.15 and c= 5.29 which agrees with the JCPDS
data.
Page 5
CHAPTER.3: UNDOPED ZnO THIN FILMS
62
20 30 40 50 60 70 80
(101)
(002)
(100)
(201)
(112)
(103)
(110)
(102)
600oC
500oC
400oC
Inte
nsity
2θ (degrees)
Fig. 3.1 XRD spectrum of ZnO thin film annealed at 400ºC, 500ºC and 600ºC.
Page 6
CHAPTER.3: UNDOPED ZnO THIN FILMS
63
Since the total breadth of a XRD peak can be separated into contributions from the
finite crystallite size and the presence of strain in the crystal. The Williamson-Hall plots
[19] for the Gaussian peak shape have been used to determine crystallite size and strain
in the ZnO thin films.
Bt(2θ).cosθ = 0.9λ/D + 4ε.sinθ (3.3)
where λ = 1.54059Å, is the wavelength of X-ray used, D is the crystallite size, ε is the
microstrain and Bt(2θ) is the full width at half maximum, at diffraction angle 2θ in
radians. A straight line fit in the plot of Bt(2θ).cosθ and sinθ yields the values of
crystallite size as well as microstrain. This is plotted for the samples as shown in figure
3.2.
0.25 0.30 0.35 0.40 0.45 0.50
0.0040
0.0045
0.0050
0.0055
0.0060
0.0065
0.0070
400oC
Bt(2θθ θθ).
Co
sθθ θθ
Sinθθθθ
y= 0.00323 + 0.00496 x
0.25 0.30 0.35 0.40 0.45 0.50
0.0035
0.0040
0.0045
0.0050
0.0055
y= 0.0017 + 0.00803 x
Bt(2θθ θθ).
Co
sθθ θθ
Sinθθθθ
500oC
0.25 0.30 0.35 0.40 0.45 0.50
0.0034
0.0036
0.0038
0.0040
0.0042
0.0044
0.0046
0.0048
0.0050
0.0052
y = 0.00174 + 0.00677 x
600oC
Sinθθθθ
Bt(2θθ θθ).
Co
sθθ θθ
Fig. 3.2 Williamson-Hall plots for ZnO film at different annealing temperatures.
Page 7
CHAPTER.3: UNDOPED ZnO THIN FILMS
64
Table 3.1(a) and 3.1(b) gives the values of crystallite size and microstrain respectively,
as obtained from the Williamson-Hall plots (figure 3.2) for ZnO films annealed at
different temperatures.
Table 3.1: Crystallite size (3.1a) and microstrain values (3.1b) for ZnO films annealed
at 400°C, 500°C and 600°C.
Table 3.1a:
Annealing temperature (OC) Crystallite size (nm) 400 42.9 500 81.6 600 79.6
Table 3.1b:
Annealing temperature (OC) microstrain 400 0.0012 500 0.0020 600 0.0017
From the table 3.1 we observe that for film annealed at 500°C, the crystallite size is
maximum and on further increase of annealing temperature to 600°C the crystallite size
decreases. This may be due to the fact that on heat treatment, the flow of fluid near the
substrate interface coagulates, to reorganize the grain size to increase. The coagulation
process discontinues on further increasing the heat treatment temperature with threshold
around 500°C [20]. Further the microstrain value corresponding to the annealing
temperature of 500 °C is also maximum indicating the increased tensile strain in the
crystallite of the film sample.
Page 8
CHAPTER.3: UNDOPED ZnO THIN FILMS
65
3.1.3 Surface Morphological Characterization
The micro structural analysis of the ZnO film annealed at 400ºC was carried out using
Scanning electron microscope (SEM) with energy dispersive scattering (EDS)
attachment. Figure 3.3 shows the surface morphology of the ZnO film annealed at
400ºC.
Fig. 3.3 SEM micrograph of ZnO film annealed at 400ºC.
Page 9
CHAPTER.3: UNDOPED ZnO THIN FILMS
66
Fig. 3.4 EDS of ZnO film annealed at 400ºC
The film surface is granular in nature with grain size of the order of nm. The EDS
analysis of the film (Fig 3.4) shows the presence of Zinc, Oxygen, Silicon and Gold.
The Gold in the ZnO film is from the Gold coating of the sample ZnO film for SEM
analysis.
Page 10
CHAPTER.3: UNDOPED ZnO THIN FILMS
67
3.1.4 Optical Properties
Figure 3.5 shows the transmission spectra of the ZnO films deposited at different
annealing temperatures.
250 300 350 400 450 500
0
20
40
60
80
100
Tra
ns
mit
tan
ce
%
Wavelength (nm)
400OC
500OC
600OC
Fig. 3.5 Transmission spectra of ZnO thin film annealed at 400ºC, 500ºC and 600ºC.
The spectra show interference fringes which has its origin in the interference of light
reflected between air-film and film-substrate interface. The appearance of interference
fringes indicates smooth reflecting surface of the film and low scattering loss at the
surface. The films annealed at 400ºC and 600ºC exhibit good transparency in visible
region (>90%).From the spectrometric Ellipsometry, the thicknesses of the ZnO films
annealed at 400ºC, 500ºC and 600ºC are measured to be 120.4 nm, 163.3 nm and 64.7
nm respectively. The variation of refractive index n and extinction coefficient k with
Page 11
CHAPTER.3: UNDOPED ZnO THIN FILMS
68
photon energy for ZnO films annealed at different temperatures is shown in figures 3.6
and 3.7.
1.5 2.0 2.5 3.0 3.5
1.85
1.90
1.95
2.00
2.05
2.10
2.15
2.20
2.25
2.30
600oC
500oC
n
hν (eV)
400oC
Fig. 3.6 Plot of refractive index n as a function of Photon energy for ZnO
film at different annealing temperature.
Page 12
CHAPTER.3: UNDOPED ZnO THIN FILMS
69
1.5 2.0 2.5 3.0 3.5
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
500oC
600oC
k
hν (eV)
4000C
Fig. 3.7 Plot of extinction coefficient k as a function of Photon energy for ZnO
films at different annealing temperature.
Page 13
CHAPTER.3: UNDOPED ZnO THIN FILMS
70
In figure 3.6 the refractive index increases with increasing photon energy, with peak at
about 3.25eV. This can be attributed to the band gap of ZnO ~ 3.3eV.It may be noted
that the refractive index n is almost unaffected by the variation in the annealing
temperature upto the peak, that is, 3.25 eV, after which it shows a small dependence on
the annealing temperature. Figure 3.7 shows k to build up only after 3 eV and attains a
peak at 3.4 eV. It also shows a dependence on annealing temperature above 3 eV.
The absorption coefficient α and the extinction coefficient k are related by the formula [21]
k = π
αλ
4 (3.4)
The Optical energy gap Eg and absorption coefficient α are related by the equation
α = ( )βυυ
gEhh
k−
(3.5)
where k = a constant
h = Planck’s constant
hν = The incident photon energy and β is a number which characterizes the
nature of electronic transition between valance band and conduction band [22]. For
direct allowed transitions β =1/2 and it is known that ZnO is a direct band gap
semiconductor. Therefore the formula used is
α = ( ) 2/1
gEhh
k−
υ
υ
which gives
( ) ( )gEhCh −= ννα
2 (3.6)
Page 14
CHAPTER.3: UNDOPED ZnO THIN FILMS
71
where C is a constant. The variation of (hνα)2 vs. hν as obtained using equation (3.6) is
shown in figure 3.8 for ZnO films annealed at different temperatures.
Fig. 3.8 Plot of (αhν)2 vs. Photon energy hν for ZnO films at different annealing
temperatures.
1.5 2.0 2.5 3.0 3.5
0.0000
0.0005
0.0010
0.0015
0.0020
0.0025
400oC
500oC
(αhν)
2 n
m-2(e
v)2
hν (eV)
600oC
Page 15
CHAPTER.3: UNDOPED ZnO THIN FILMS
72
The energy gap Eg of the samples was evaluated from the intercept of the linear portion of
the each curve for different annealing temperature with the hν in X-axis. Table 3.2 gives
values of Eg of the films deposited at different annealing temperatures.
Table 3.2: Band gap Eg for ZnO films at different annealing temperature as estimated
from Fig.3.8
The value of Band gap as calculated above agrees nearly with band gap of bulk ZnO (3.37 eV).
It is assumed that the absorption coefficient α near the band edge shows an exponential
dependence on photon energy for many materials. This dependence is given by [23]
α = αoexp
uE
hυ (3.7)
where αo is a constant and Eu is Urbach energy which is the width of the tails of the
localized state associated with the amorphous state in the forbidden band. The plot of
lnα vs. photon energy hν plots for ZnO thin films annealed at 400°C, 500°C and 600°C
is shown in figure 3.9.
Annealing
temperature (ºC)
Band gap Eg (eV)
400 3.216
500 3.212
600 3.216
Page 16
CHAPTER.3: UNDOPED ZnO THIN FILMS
73
1.5 2.0 2.5 3.0 3.5
-12
-11
-10
-9
-8
-7
-6
-5
-4 600oC
500oC
ln α
hv (eV)
400oC
Fig. 3.9 Urbach plot for ZnO film annealed at 400°C, 500°C and 600°C.
Page 17
CHAPTER.3: UNDOPED ZnO THIN FILMS
74
From the plot the value of Urbach energy at the band edge ~3.21 eV for films annealed
at different temperatures is shown in table 3.3.
Table 3.3: Urbach energy for ZnO thin films annealed at different temperatures.
From the table, the increase in Urbach energy between 400°C and 500°C may be
attributed to the increase thermal induced structural disorder of the film within this
temperature range [24].
The room temperature Photoluminescence (PL) of ZnO film annealed at different
temperatures is shown in figure 3.10.
Annealing temperature (°C) Urbach energy Eu (meV)
400 96.6
500 101.0
600 101.0
Page 18
CHAPTER.3: UNDOPED ZnO THIN FILMS
75
350 400 450 500 550 600
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
1.1
1.2
500oC
400oC
Inte
nsity
Wavelength (nm)
600oC
Fig. 3.10 Room temperature Photoluminescence spectrum of Zn0 films
annealed at different temperatures.
Page 19
CHAPTER.3: UNDOPED ZnO THIN FILMS
76
At the excitation wavelength of 315 nm the Photoluminescence spectrum consist of a
prominent peak at about 380 nm responsible for UV emission and suppressed peak at
about 500 nm responsible for green emission. The reason of small intensity at 500 nm
may lie in the fact that with increasing annealing temperature, both the amount of grown
ZnO and the specific surface area of the grains decreases, which jointly weakens the
green emission. The band ~ 380 nm correspond to the band edge emission due to free
exciton while at ~ 500 nm attributed to the presence of defects, non-stoichiometry and
crystal imperfection [14]. From the spectra we note UV emission depends on the
annealing temperature. This type of variation can be attributed to the polycrystalline
nature of ZnO film.
3.1.5 Electrical properties
The electrical conductivity of the samples were measured using four-probe method.
Figure 3.11 shows the plot of electrical conductivity of ZnO thin film as a function of
annealing temperature.
The decrease in conductivity of the samples is attributed to the reason that with increase
in annealing temperature the number of imperfections in the atomic lattice structure
increases which hampers electron movement and also due to scattering from grain
boundaries.
Page 20
CHAPTER.3: UNDOPED ZnO THIN FILMS
77
Fig. 3.11 Plot of electrical conductivity vs Annealing temperature of ZnO films.
Page 21
CHAPTER.3: UNDOPED ZnO THIN FILMS
78
3.2 Investigation of Quantum Confinement effect in sol-gel
prepared ZnO thin films
The central theme of nanoscience is, how scale affects the properties of materials.
Quantum confinement effect is one of them. It is the effect of size on energy band gap
of the material. The electronic energy levels and density of states determine the optical
and electronic properties of materials. In nanostructured materials the energy levels and
density of states changes with change in size, resulting in dramatic changes in material
properties. The energy level spacing increases with decreasing dimension and this is
known as Quantum size confinement effect. It can be explained using particle- in- a- box
problem in quantum mechanics. We have prepared ZnO films annealed at 250°C,
300°C, 350°C, 400°C and 450°C by sol-gel spin coating method and investigated the
effect of crystallite size on the band gap of the prepared ZnO films.
3.2.1 Structural properties
The structural properties of the films prepared by varying annealing temperatures were
studied by X-ray diffraction. Figure 3.12 shows the X-ray diffraction of the ZnO films
annealed at 250°C, 300°C, 350°C, 400°C and 450°C. From the diffractogram it is seen
that the intensity of the characteristic peaks of ZnO increases as annealing temperature
increases, showing increasing crystallinity.
Page 22
CHAPTER.3: UNDOPED ZnO THIN FILMS
79
Fig. 3.12 X-ray diffraction of ZnO films annealed at five different temperatures.
Page 23
CHAPTER.3: UNDOPED ZnO THIN FILMS
80
The crystallite size of the ZnO films annealed at different temperatures was also
calculated by using Scherrer’s formula [25].
D = θβ
λ
cos
9.0 (3.8)
Where D is the crystallite size, wavelength of the X-ray used is λ =1.54059Å, β is the
broadening of diffraction line measured at the half of its maximum intensity in radians
and θ is the angle of diffraction.
Table 3.4 gives the average crystallite size of the films with their corresponding
annealing temperature.
Table 3.4: Average crystallite size of ZnO films calculated using Scherrer’s formula .
Annealing Temperature (°C) Average crystallite Size (nm)
250 6.33
300 3.77
350 7.92
400 23.49
450 29.49
From table 3.5 we observe that the crystallite size increases with annealing temperature
of ZnO films, except at 300°C where it decreased from 6.33 nm to 3.77 nm, on heat
treatment the flow of fluid near the substrate interface coagulates, to reorganize and
increase of grain size. The coagulation process discontinues on further increasing the
heat-treatment temperature with a threshold around 300°C. More over the crystallite
size <10 nm for annealing temperature upto 350°C, indicating nanocrystals in
embedded in the amorphous phase.
Page 24
CHAPTER.3: UNDOPED ZnO THIN FILMS
81
3.2.2 Optical properties
Figure 3.13(a) shows the transmittance of ZnO film prepared on glass substrate,
annealed at five different temperatures. The films show interference fringes in the
transmission spectrum due to the interference of light reflected from air-film and film-
substrate interface. This also indicates that the film surface is smooth.
200 400 600 800 1000
0
20
40
60
80
100
Tra
nsm
itta
nce
%
wavelength (nm)
250oC
300oC
350oC
400oC
450oC
Fig. 3.13(a) Transmission spectrum of ZnO films annealed at five different
temperatures.
Page 25
CHAPTER.3: UNDOPED ZnO THIN FILMS
82
Figure 3.13(b) shows the absorbance of ZnO films prepared on glass substrate, annealed
at five different temperatures. From the figure we observe that the edge of the
absorbance peak of ZnO films does not coincide at the same point for different
annealing temperatures. There is a shifting of edges in the absorbance spectrum of the
films.
350 420 490
0.0
0.5
1.0
1.5
Ab
so
rba
nce
Wavelength (nm)
250oC
300oC
350oC
400oC
450oC
Fig. 3.13(b) Absorbance of ZnO film annealed at five different temperatures.
Page 26
CHAPTER.3: UNDOPED ZnO THIN FILMS
83
The band gap of the ZnO films annealed at five different temperatures is estimated by
the variation of (hνα)2 vs. hν as obtained using equation (3.6) is shown in figure 3.14.
3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1
0.0000.000
0.025
0.050
0.0750.075
(αhν)
2 (
nm
-2e
V2)
Photon energy (eV)
250oC
300oC
350oC
400oC
450oC
Fig. 3.14 Plot of (αhν)2 vs. Photon energy hν for ZnO films annealed at five
different temperatures.
Table 3.5 displays the estimated values of band gap of ZnO films corresponding to
their annealing temperatures. We observe that as the annealing temperature increases
the band gap of ZnO film decreases. Indicating the size effect that comes into play.
Page 27
CHAPTER.3: UNDOPED ZnO THIN FILMS
84
Table 3.5: Estimated band gap of ZnO films annealed at five different temperatures.
Annealing temperature (°C) Band gap (eV)
250 3.83
300 3.80
350 3.78
400 3.77
450 3.30
The Photoluminescence (PL) of ZnO films annealed at five different temperatures is
shown in figure 3.15.
375 400 425 450 475 500 525
PL In
tensity (
a.u
.)
Wavelength (nm)
250oC
300oC
350oC
400oC
450oC
Fig. 3.15 Photoluminescence of ZnO films annealed at five different temperatures.
Page 28
CHAPTER.3: UNDOPED ZnO THIN FILMS
85
Figure 3.15 displays the shift in the emission peak of of ZnO film due to band edge
emission towards the longer wavelength side with increase in annealing temperature,
Conforming the narrowing of band gap.
After Gaussian fitting of the PL peaks the band edge emission wavelength of the films
were found and tabulated in table 3.6.
Table 3.6:Band edge emission peaks of ZnO films annealed at five different
temperatures.
Annealing Temperature (°C) Band edge emission peak (eV)
250 3.2219
300 3.2197
350 3.2024
400 3.1831
450 3.1309
From table 3.6, we observe that as annealing temperature increases the band edge
emission energy decreases, clearly indicating the narrowing of band gap.
The emission of the nanocrystallite structured ZnO films will have the same quantum
size effect as the quantum dot and can be described by the following equation,
E����,������ ��� =E����,����� +��ℏ���� × � �
��∗+ �
�!∗" − 0.248E��∗ (3.9)
With The bulk band gap E(gap,bulk) = 3.2 eV
Page 29
CHAPTER.3: UNDOPED ZnO THIN FILMS
86
The bulk exciton binding energy ERy* = 60 meV
The electron and hole effective masses are taken as me*=0.24m0 and mh
*=2.31m0
h is Plank’s constant and R is radius of ZnO nanocrystal [26].
Figure 3.16 shows the plot of nanocrystal band gap E(gap,nanocrystal) versus the nanocrystal
radius R. The solid curve is the theoretical fit of equation (8) while the symbols are the
grain sizes estimated from XRD data and their corresponding band gaps measured from
Photoluminescence spectrum.
Fig. 3.16 Plot of E(gap,nanocrystal) versus the nanocrystal radius R for ZnO films annealed
at different temperatures.
Page 30
CHAPTER.3: UNDOPED ZnO THIN FILMS
87
From figure 3.16 it is observed that the band gaps measured from PL follow the
theoretical curve especially between crystallite size 5nm to 25 nm. It is also observed
that the band gap variation is not much as the crystallite size changes, corresponding to
different annealing temperature. Thus the above curve indicates that the PL emission is
due to the nanocrystals by quantum confinement effect. The decrease in crystallite size
indeed exhibits quantum confinement effect.
3.2.3 Surface morphological properties
The surface morphology of the ZnO films annealed at five different temperatures are
shown in figures 3.17 to 3.21. The surface of all the films are granular in nature. The
grain size of the films vary between 50-100 nm. This value is much larger than the
crystallite size obtained from XRD measurement. This is due to the fact that these
grains are itself made from still smaller grains which is detected by FESEM. The
surface porosity of the films is also increased as the annealing temperature is increased.
Page 31
CHAPTER.3: UNDOPED
Fig. 3.17 F
: UNDOPED ZnO THIN FILMS
88
ig. 3.17 FESEM images of ZnO film annealed at 250
d at 250oC
Page 32
CHAPTER.3: UNDOPED ZnO THIN FILMS
89
Fig. 3.18 FESEM images of ZnO film annealed at 300°C
Page 33
CHAPTER.3: UNDOPED ZnO THIN FILMS
90
Fig. 3.19 FESEM images of ZnO film annealed at 350°C
Page 34
CHAPTER.3: UNDOPED ZnO THIN FILMS
91
Fig. 3.20 FESEM images of ZnO film annealed at 400°C
Page 35
CHAPTER.3: UNDOPED ZnO THIN FILMS
92
Fig. 3.21 FESEM images of ZnO film annealed at 450°C
Page 36
CHAPTER.3: UNDOPED
The Energy dispersive sca
were recorded by an attac
confirmed and their atomic
the computer generated
films annealed at five diffe
Fig. 3.22 EDS of
: UNDOPED ZnO THIN FILMS
93
ispersive scattering of ZnO film annealed at five differe
by an attachment to FESEM. The presence of Zinc an
their atomic weight percent were measured. Figures 3.22
generated EDS spectrum with quantity of Zinc and oxygen
at five different temperatures recorded using JEOL FESE
EDS of ZnO film annealed at 250°C (S1-eds) and 30
t five different temperatures
Zinc and Oxygen were
Figures 3.22 to 3.26 displays
c and oxygen present in ZnO
g JEOL FESEM.
) and 300°C (S2-eds).
Page 37
CHAPTER.3: UNDOPED ZnO THIN FILMS
94
Fig. 3.23 EDS of ZnO film annealed at 350°C (S3-eds) and 400°C (S4-eds).
Page 38
CHAPTER.3: UNDOPED
Fig. 3.2
From The EDS measurem
follow non uniform varia
induced defects in the crys
ZnO film has highest Oxyg
3.3 Conclusions
We have studied the eff
temperatures (400oC, 500
properties. X-ray diffracti
Eenergy Dispersive Scat
: UNDOPED ZnO THIN FILMS
95
24 EDS of ZnO film annealed at 450°C (S5-eds)
S measurement we see that the film annealed at differe
variation of oxygen or Zinc content, this may
ts in the crystal lattice. However at the highest annealing
highest Oxygen content.
died the effect of annealing the ZnO thin films
C, 500oC and 6000C) on its structural, optica
ray diffraction (XRD) and Scanning electron microsco
persive Scattering (EDS) attachment were used for
eds).
led at different temperatures
this may due to heating
est annealing temperature the
at three different
optical and electrical
ron microscopy (SEM) with
re used for structural and
Page 39
CHAPTER.3: UNDOPED ZnO THIN FILMS
96
morphological characterization respectively. UV-VIS-NIR spectrometry,
Photoluminescence and spectroscopic Ellipsometry were used for optical
characterization. The electrical characterization of the films was done using four-probe
method. The band gap of the films were calculated and were in agreement with bulk
ZnO. It was found that the ZnO films were emitting in Ultra violet region. The
conductivity of film slowly decreased with annealing temperature indicating scattering
of mobile electrons due to heating induced defects.
The investigation of Quantum confinement effect in sol-gel prepared ZnO thin films
were also performed. We prepared ZnO thin film by sol-gel method with annealing the
samples at five different temperatures viz. 250ºC, 300ºC, 350ºC, 400ºC and 450ºC.The
blue shift of optical band gap with annealing temperature have been observed. The
quantum confinement effect in these systems was found.
Page 40
CHAPTER.3: UNDOPED ZnO THIN FILMS
97
References
[1] Chennupati Jagadish, & Stephen J. Pearton, Zinc oxide Bulk, Thin Films and
Nanostructures, China: Elsevier (2007).
[2] E. Fortunato, A. Goncalves, A. Marques, A. Viana, H. Aguas, L. Pereira, I. Ferreira,
P. Vilarinho, R. Martins, Surf. Coat. Technol., 180 (2004) 20.
[3] P.Yu, Z.K. Tang, G.K.L.Wong, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa,
J. Cryst. Growth, 184-185 (1998) 601.
[4] A. Tsukazaki, M. Kubota, A. Ohtomo, T. Onuma, K. Ohtani, H. Ohno, S.F.
Chichibu, M. Kawasaki, Jpn. J. Appl. Phys , 44 (2005) L643.
[5] Y.Ohya, T. Niwa, T. Ban, Y. Takahashi, Jpn. J. Appl. Phys, 40 (2001) 297.
[6] Parmanand Sharma, Amita Gupta, J.Frank Owens, Akhisha Inoue, K.V. Rao, J.
Magn. Magn. Mater., 282 (2004) 115.
[7] W.P. Kang, C.K. Kim, Sens. Actuators, B , 14 (1993) 682.
[8] A. Moustaghfir, E.Tomasella, S.Ben Amor, M. Jcquet, J. Cellier, T.Sauvage, Surf.
Coat. Technol., 174-175 (2003)193.
[9] K. Haga, M. Kamidaira, Y. Kashiwaba, T.Sekiguchi, H. Watanabe, J. Cryst.
Growth, 214 (2000) 77.
[10] K.L. Narasimhan, S.P. Pai, V.R. Palkar, R. Pinto, Thin Solid Films, 295 (1997)
104.
[11] Dinguha Bao, Haoshuang Gu, Anxiang Kuang, Thin Solid Films, 132 (1998) 37.
[12] F.D.Paraguay, W.L. Estrada, D.R.N. Acosta, E. Andrade, M. Mikiyoshida, Thin
Solid Films, 350 (1999) 192.
[13] M. Inoguchi, K. Suzuki, N. Tanaka, K. Kageyama, H. Takagi, J. Mater. Res., 24
(2009) 2243.
[14] Pramod Sagar, P.K. Shishodia, R.M. Mehra, H. Okada, Akihiro Wakahara, Akira
Yoshida, J. Lumin., 126 (2007) 800.
Page 41
CHAPTER.3: UNDOPED ZnO THIN FILMS
98
[15] Harish Bahadur, S.B. Samanta, A.K. Srivastava, K.N. Sood, R. Kishore, R.K.
Sharma, A.Basu, Rashmi M.Kar, Prem Pal, Vivekanand Bhatt, Sudhir Chandra, J.
Mat. Sci., 41 (2006) 7562.
[16] C.Jeffry Brinker, George W. Scherer, Sol-gel Science the Physics and Chemistry of
sol-gel processing, San Diego: Academic Press (1990) (Chapter 2).
[17] Joint Committee on Powder Diffraction Standards, Powder Diffraction File, Card
no: 36-1451.
[18] C. Suryanarayana, M.Grant Norton, X-Ray Diffraction A practical approach, New
York: Plenum Press (1998).
[19] Rachana Gupta and Mukul Gupta, Phys.Rev. B, 72, (2005) 024202 .
[20] T.Ghosh, S. Bandopadhyay, K.K Roy., A.K. Maiti, K. Goswami, Cryst. Res.
Technol., 44 (2009) 879.
[21] E.R. Shaaban, J. Appl. Sci., 6(2) (2006) 340.
[22]Ming-Fu. Li, Modern Semiconductor Quantum Physics, Singapore: World
Scientific (2001).
[23] F. Urbach, Phys. Rev., 92 (1953) 1324.
[24] S.W. Xue, X.T. Zu, W.L. Zhou, H.X. Deng, X. Xiang, L. Zhang, H. Deng, J.
Alloys Compd., 448 (2008) 21.
[25] B.D. Cullity, S.R. Stock, Elements of X-Ray diffraction (3rd ed.), Prentice Hall
(2001).
[26] S.T. Tan, B.J. Chen, X.W. Sun, W.J. Fan, J. Appl. Phys. ,98 (2005) 013505.