Thickness dependent structural and optical
properties of nanocrystalline ZnO (nc-ZnO) films
prepared by sol-gel process
Sanjeev Kumar1, 2, Chetna1, Fouran Singh2 and A.Kapoor1
1
2
Department of Electronic Science, University of Delhi South Campus, New Delhi - 110021, India
Materials Science Group, Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi- 110067, India
*
Corresponding author’s e-mail:sanjeevkumarphy@gmail.com, Tel.: +91-9999106526
Abstract
In this paper we reported thickness dependent
structural, surface morphology and optical properties of
Zinc Oxide (ZnO) films. Zinc acetate dihydrate used as a
precursor, methanol as solvent and monoethanolamine as
a stabilizer. Structural investigations were carried out
using X-ray Diffractometer. The phase transition from
amorphous to crystalline hexagonal wurtizite structure
was observed at low thickness of film. Surface morphology
of films at different thickness is studies by Scanning
Electron Microscopy (SEM). Surface morphology studies
showed that an increase in the films thickness causes an
increase in the grain size. The transmission in UV region
decreased strongly with the increase of film thickness due
to the different thickness of films. As the results, film
thickness affected the properties of ZnO films
significantly.
Keywords: Sol-gel, Zinc Oxide, Thickness and surface
morphology.
Introduction
Submit Zinc Oxide (ZnO) is a II–VI compound
n-type semiconductor with hexagonal wurtzite structure.
ZnO has a direct band gap of 3.37 eV [1] and large
exciton binding energy of 60 meV at room temperature
[2]. The microscopic surface topology and grain
structure of transparent conducting films strongly affect
the performance of solar cells. For such applications,
development of low resistive transparent conducting
oxide (TCO) thin films along with textured surface is
important [3].
This In this work, we attempted to grow ZnO
films on corning glass by sol-gel method. Effect of films
thickness on structural, surface morphology and optical
(transmittance, band gap) properties are reported.
Analysis of thickness induced films stress using X-rd
data.
Experimental
ZnO films on corning substrates were deposited by solgel method using spin coating technique. Precursor
solution of ZnO was prepared by dissolving Zinc acetate
dihydrate [Zn(CH3COO)2.2H2O] (purity 99.95%)
(Merck Extra pure chemical Ind. Ltd, India) into
Ethanol (AR, Merck chemicals, India) and
monoethanolamine [MEA] (Merck, India). The Zn
precursor solution was prepared by dissolving zinc
acetate dihydrate in ethanol so as prepare concentration
of 0.2 mol/l. MEA was then added in the solution. The
molar ratio MEA/Zn was fixed to 1. The mixture was
stirred ultrasonically at 25°C for 2 hours. The clear
transparent and homogenous solution thus obtained was
left for ageing for 72 hours. Substrates were
ultrasonically cleaned using acetone, methanol and
deionized water sequentially for 15 min each. One drop
of solution was dropped onto the substrate, which was
rotated at 2500 rpm for 30 sec by a spin coater. After
deposition by spin coating, the films were dried in air
for 10 min over hot plate to evaporate the solvent and
remove organic residuals. An approximate thickness of
19 nm was obtained for each spin. The above process of
coating and drying was repeated several times to
increase the film thickness. In the present work the
thickness of the films were in range of 90-400 nm. The
films were then annealed in air at temperature range
450°C for 1 hour in a microprocessor controlled furnace
by a heating rate 5°/min.
Crystalline nature of the ZnO films was confirmed by
PAN alytical X’pert PRO diffractometer using the
CuKα radiation having a wavelength 1.5140°A. The
film thickness was measured by surface profiler
ambious XP-1. The surface morphology of the films
was investigated with scanning electron microscopy
(SEM Zeiss EVO–40 EP). The band gap of ZnO films
was measured by optical transmittance using a
Shimadzu
Solid
Spec
3700
double
beam
spectrophotometer.
10
100
Results and discussion
1.6x10
(b)
(h )2 (cm -1 eV)2
80
60 98 nm
40
20
180 nm 366 nm
10
1.2x10
98 nm
180 nm
280 nm
366 nm
9
8.0x10
9
4.0x10
280 nm
0
300
400
500
600
700
0.0
3.1
800
3.2
Wave le ngth (nm)
3.3
h(eV)
3.4
3.34
(c)
3.32
Band gap (eV)
Fig. 1 shows the thickness of the ZnO film as a function
of the number of deposition cycles. The film thickness
increased linearly with increase the number of cycles,
representing a typical characteristic of the sol.gel
technique. It is interesting to see that the linearity of the
thickness change is observed. The XRD and SEM
investigations give the information about the structure
of film. The preferred orientation of grains, grain size,
stress, strain, etc can be obtained from the analysis of
the width, intensity and position of X-rd peaks.
Trans mittanc e (%)
(a)
3.30
3.28
3.26
3.24
3.22
100
The crystallites sizes as determined from the value of
FWHM were calculated using Scherer’s formula
0.94
D
B cos 
(1)
Where, λ, θ and B are the X-ray wavelength, Bragg’s
diffraction angle and FWHM of the ZnO (002)
diffraction peak, respectively. No crystals have been
obtained at low thickness because film has shown
amorphous nature. The crystal size decrease with
increase in the film thickness up to 289 nm after that it
is increase.
Thickness of film (nm)
400
350
300
250
150
200
250
300
350
400
Thicknes s of film (nm)
Fig.2 Effect of film thickness on (a) Transmittance in
the wavelength range 300-800 nm for ZnO films. (b)
Band gap calculation for ZnO films using Tauc’s plot.
(c) Band gap variation of ZnO films.
Transmittance measurements were carried out in 300800 nm range of spectrum. The effect of film thickness
on the optical properties such as transmittance and band
gap of ZnO films is investigated. Figure 5 shows the
transmittance curves of ZnO films with thickness (92642 nm). All the samples showed high and constant
transparency higher than 84% in the visible range (300–
800 nm). Maximum transmittance was found to be 88%
for 280 nm thick film and the minimum transmittance
was found to be ~ 85% for 366 nm thick films as shown
in Fig. 2. The transmittance remains constant for all the
samples, in this range of spectra.
Conclusion
200
150
100
4
6
8
10
12
14
16
18
20
22
Number of deposition cycles
Fig. 1 Graph between thickness of film Vs number of
deposition cycles.
Fig.2 shows the Tauc’s plot for optical band gap as a
function of film thickness for the ZnO films. The band
gap was observed to decrease from 3.33 to 3.24 eV as
the film thickness increase from 98 to 280 nm. Beyond
this thickness (366 nm), increase in the band gap is
observed to be 3.25 eV.
High quality transparent ZnO films were deposited with
different thickness by sol.gel method using spin coating
technique. The quality and the growth pattern of the
film are controlled by its thickness. Minimum stress has
been obtained at film thickness 280 nm. Highly c-axis
oriented crystalline films with an average transmittance
of 88 % were obtained for a film thickness of 280 nm.
Band gap is dependent on thickness induced stress of
films
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