International Journal of Physical Sciences Vol. 7(23), pp.
2971-2979, 15 June, 2012
Available online at http://www.academicjournals.org/IJPS
DOI: 10.5897/IJPS12.219
ISSN 1992-1950 ©2012 Academic Journals
Full Length Research Paper
Effect of zinc acetate concentration on the structural
and optical properties of ZnO thin films deposited by
Sol-Gel method
M. Saleem1, L. Fang1*, H. B. Ruan1,2, F. Wu1, Q. L. Huang1, C. L. Xu1 and C. Y. Kong2
1
Department of Applied Physics, Chongqing University, Chongqing 400044, China.
2
Optical Engineering Key Laboratory, Chongqing Normal University, Chongqing 400030, China.
Accepted 03 May, 2012
The transparent Zinc Oxide (ZnO) thin films were deposited by multi-step sol-gel method and the effect
of sol concentration on structural, morphological and optical properties were investigated. X-ray
diffraction (XRD) and scanning electron microscopy (SEM) were employed to characterize structures
and morphologies of the as-deposited films. The crystallographic orientation of the ZnO films shows the
preferred orientation of (002) that is along c-axis direction and the films were uniform and well adherent
to the substrates. The grain size is in the range of 15.3~19.7 nm and the thickness in the range of
266~295 nm, both increase with zinc acetate concentration. It is found that the transmittance of the films
is enhanced from 83 to 95% in the visible near- IR region in the range from 400 to 800 nm by increasing
the concentration. The optical band gap energy attenuates from 3.307 to 3.227 eV and whereas the
Urbach energies of the films increase from 68.2 to 82.4 meV with increasing the concentration from 0.35
M to 0.65 M.
Key words: Sol-Gel method, spin coating, structural properties, optical properties.
INTRODUCTION
Zinc oxide is a II-VI group, n-type semiconductor com- transparent conductive film and the solar window
pound which has technical applications such as photo- because of the high optical transmittance in the visible
catalysts by Chakrabarti et al. (2004), solar cells by light region.
Pradhan et al. (2007) and Baxter et al. (2006), thin film Diverse techniques have been employed to fabricate
gas sensors (Shishiyanu et al., 2005), varistors (Suvaci ZnO thin film including RF sputtering (Tadatsugu et al.,
and Ozer 2005), light emitting diodes (Saito et al., 2002), 1984), spray pyrolysis (Krunks and Mellikov 1995), metal-
spintronic devices (Meron and Markovich 2005), and organic chemical vapour deposition (MOCVD) (Tominaga
nanolasers (Huang et al., 2001). ZnO thin films have also et al., 2002), molecular beam epitexy (MBE) (Look et al.,
been widely used as surface acoustic wave (SAW) 2002), pulsed laser deposition (PLD) (Naghavi et al.,
device and film bulk acoustic resonator (FBAR) because 2000) and the sol-gel process (Natsume and Sakata
of its excellent piezoelectric properties by Webb et al. 2000; Luna-Arredondo et al., 2005; Farley et al., 2004;
(1981) and Kang et al. (2005). Moreover, ZnO has large Mandalapu et al., 2006; Viart et al., 2003; Ohyama et al.,
band gap 3.37 eV and large excitonic binding energy 60 1997; Gonzalez et al., 1998). Sol-gel method has widely
meV at room temperature and its composed of hexagonal been adopted for the fabrication of transparent and
wurtzite crystal structure with lattice parameters a = conducting oxide due to its simplicity, safety, no need of
3.248Å and c = 5.205Å. ZnO thin film is applied as the costly vacuum system and hence cheap method for large
area coating. The sol-gel process also offers other
advantages such as high surface morphology at low
crystallizing temperature, the easy control of chemical
*Corresponding author. E-mail: fangliangcqu@yahoo.com.cn. components and fabrication of thin films at low cost to
Tel: +86-23-65678369. investigate structural and optical properties of ZnO thin
2972 Int. J. Phys. Sci.
At 60°C for 30 min
At 60°C for 90 min
2000 rpm, 30 s
200°C, 10 min
400°C, 1 hour
Figure 1. The flow chart showing the procedure for preparing ZnO films.
Film (Natsume and Sakata 2002). The deposition of (ZAD), 2-methoxyethanol [(CH3O(CH2)2OH), chemical purity
doped and undoped ZnO thin films by using the sol-gel 99.95%] (2-ME) and monoethanolamine [((HOCH2CH2)NH2),
chemical purity 99.95 %] (MEA).
route has been already reported (Mandalapu et al., 2006;
Viart et al., 2003; Ohyama et al., 1997; Gonzalez et al.,
1998). However, up to date, multiple deposition steps ZnO sol-gel preparation
(concentration of the sol-gel, heat treatment condition,
substrates used etc.) have been generally necessary to The sol-gel synthesis and thin film process is outlined in Figure 1.
The final concentrations of ZAD were 0.35, 0.5 and 0.65 mol/L
produce high-quality thin films. Considering the afore-
respectively, in 30 ml of 2-ME using 250 ml conical flask. After
mentioned factors affecting the cost-of-ownership and the stirring for 30 min at 60°C, MEA was added drop wise under
film quality aspects, a typical multi-step deposition constant stirring. The resultant solutions were stirred for 90 min to
process has been demonstrated. yield colorless, homogeneous and transparent solutions. The
In this article, we report growth of ZnO films with solutions were aged for 72 h at room temperature in order to obtain
different zinc acetate concentrations on glass substrate the optimum viscosity during deposition of the film. The molar ratio
of MEA to ZAD was maintained at 1:1. Prior to the coating of the
by the sol-gel method using spin coating. The aim of this film, the glass substrates were pre-cleaned with detergent, and then
study is to investigate the influence of zinc acetate they were cleaned in methanol and acetone for 15 min each by
concentration on structural, morphological and optical using ultra-sonic water bath. Afterwards, substrates were rinsed
properties. with distilled water and then dried in hot air. The aged solution was
dropped on glass substrates which were rotated at 2000 rpm for 30
s. The as- deposited films were then pre-heated at 200°C for 10
MATERIALS AND METHODS min into a furnace to evaporate the solvent and remove organic
residuals. This spinning to preheating procedure was repeated
Chemicals eight times. After the deposition of final layer, films were post-
heated in air at 400°C for 1 h in order to obtain crystallized ZnO in
For the preparation of ZnO sol, following materials were used: Zinc wurtzite structure. Well-oriented ZnO thin films can be controlled via
acetate dehydrate [(Zn(CH2 COO)2.2H2 O), chemical purity 99.95%] adjusting the preparation parameters, such as precursor
Saleem et al. 2973
Figure 2. XRD spectra of the ZnO thin film with different content.
concentration, pre-heating and post- heating temperature and the samples ameliorates drastically, accompanying a strong
sol aging time. reflex along c-axis that is, (002) plane. This result
suggests that the sol concentration affects the films
Material characterization crystallinity as well as orientation of the crystallites in the
films. The other orientations like (100) and (101) are also
X-ray diffraction (XRD) was used for the crystalline structure of the seen with comparatively lower intensities. It can be seen
ZnO thin films. XRD patterns were obtained with a MRD-Single from Table 1 that the full width at half maximum (FWHM)
Scan diffractometer with Cu Kα (λ=1.54056Å) radiation and of the (002) diffraction peak decreases when zinc
scanning range of 2θ set between 20˚ and 80˚. During the
measurement, the current and the voltage of XRD were maintained
concentration increases up to 0.65M, demonstrating that
at 20 mA and 36 KV respectively, and scan speed was 4˚/min. The the crystalline quality of the film gets better as the
surface morphology of films was evaluated using Scanning Electron concentration becomes higher. Wang et al. (2007)
Microscopy (FEG-SEM, Nova-400). The thickness of the film was suggested the growth mechanism of c-axis oriented ZnO
determined by a surface profilometer (AMBIOS).The transmission thin film is a self-assembly process in which a dipole-
spectra of the films were recorded using a double-beam ultraviolet /
dipole interaction between the polar nanograins plays a
visible (UV-4100) recording spectrophotometer with a wavelength
range 200 to 800 nm and the optical band gap was measured from great role for the crystal growth. They think that after the
the transmission spectra. first layer of ZnO sol film was coated and pre-heated, the
nuclei were formed and gradually grew into crystals.
Since the glass substrate is an amorphous material, the
RESULTS AND DISCUSSION nuclei should be randomly oriented; correspondingly, the
crystals were also randomly oriented. However, the (002)
Structural analysis plane of ZnO has the minimum surface energy (Fujimura
et al., 1993) so most of the crystals grew preferentially
Figure 2 shows the X-ray diffraction (XRD) patterns of along the (002) direction and only a small part of crystals
ZnO films as a function of Zn content. It was observed grew along other directions. After the second layer of
that the as- deposited films were polycrystalline with ZnO sol film was coated and pre-heated, the new crystals
hexagonal wourtzite structure. From the XRD patterns, were formed using the former layer as a growth template;
(100), (002) and (101) diffraction peaks are observed therefore, some new crystals still grew along other
showing the growth of ZnO crystallites in different directions rather than the (002) direction and so on. This
directions. Strong preferential growth is observed along process can explain why sample has the (100) and (101)
c-axis depending on the initial zinc concentration which is peaks besides the (002) peak. However, the XRD results
perpendicular to the surface of the substrate. By in the present study indicate that all the films are
increasing the sol concentration, crystallinity of the polycrystalline and randomly oriented. Although
2974 Int. J. Phys. Sci.
Table 1. Structural parameters of ZnO thin films.
Zinc content (mol/L) Planes 2θ (degrees) d (Å) I/I0 FWHM (degrees)
100 31.64 2.8255 48.1 0.523
0.35 002 34.31 2.6115 100 0.539
101 36.17 2.4813 45.2 0.369
100 31.789 2.8126 19.9 0.525
0.5 002 34.22 2.6181 100 0.493
101 36.27 2.4747 19.3 0.351
100 31.9 2.8031 41.9 0.411
0.65 002 34.45 2.6012 100 0.417
101 36.36 2.4688 32.9 0.392
Table 2. Evaluated structural parameters of ZnO thin films.
Zinc content (mol/L) Planes FWHM(β)˚ 2θ˚ D (nm) δ×10-3 (nm)-2 Strain (%) Eg (eV) Eu (meV) Thickness (nm)
0.35 002 0.539 34.31 15.3 4.3 0.346 3.307 68.2 266
0.5 002 0.493 34.22 16.9 3.5 0.599 3.303 77.5 285
0.65 002 0.417 34.45 19.7 2.5 -0.05 3.227 82.4 295
Ohyama et al. (1998) and Natsum et al. (2000) and θ are the X-ray wavelength (=1.5406Ǻ), full length of dislocation lines per unit volume of the
reported c-axis that is (002) plane oriented ZnO width at half maximum (FWHM) and Bragg angle crystal is calculated using the following formula by
films on glass substrate, the random orientation in respectively. By inserting different values from Khan et al. (2010).
this study is due to the difference in the precursor Table 1 in the Scherrer formula, grain sizes of
chemistry and pre- and pos-heating temperatures (002) oriented thin films obtained is presented in δ = 1/D2 (2)
(Fujihara et al., 2001). The structural parameters Table 2. The calculated values of the grain size
of ZnO thin films obtained form the XRD patterns ranged between 15.4 and 19.8 nm which are Strain (εzz) of the thin films is calculated from the
are presented in Table 1. approximately same as found in the literature by c-axis lattice parameter using the following
From the XRD spectrum, grain size (D) of the Kumari et al. (2011). It was observed that grain formula by Ong et al. (2002).
films is calculated using the Debye Scherrer size increased with increasing zinc concentration. Where, c is the lattice parameter of ZnO films
formula by Khan et al. (2010). This trend agrees well with SEM micrograph calculated from XRD data and co = 5.205Ǻ is the
analysis shown in Figure 4, where the ZnO grain unstrained lattice parameter of ZnO. According to
D = k λ / β cos θ (1) size is clearly seen to increase with increasing Zn the above formula, the positive values of εzz
concentration. represent tensile strain while a negative value
where k is a constant to be taken as 0.49 and λ, β, The dislocation density (δ), defined as the represents compressive strain. As the ZnO
Saleem et al. 2975
Figure 3. Variation of TC(hkl) values of ZnO films with zinc content.
c − co
ε zz = ×100% Where I(hkl), IO(hkl) and N are the measured relative
co (3) intensity of a diffraction peak, intensity of the standard
powder diffraction peak and the number of diffraction
content increases the thickness of the film also increases, peaks respectively. If TC (hkl) = 1, for all the (hkl) planes
indicating that as the film grows thicker, the film is relaxed considered, indicates a sample with randomly oriented
by reducing the strain. The film which has thickness 295 crystallite, while values larger than 1, indicate the
nm shows the minimum strain. The evaluated structural abundance of crystallite in a given (hkl) direction. Values
parameters of ZnO thin films using its (002) reflection are 1> TC (hkl) >0 indicate the lack of grains orientated in
presented in Table 2, showing that the strain in sol-gel that direction. The TC (hkl) values calculated for the three
derived ZnO films is tensile and the optimum thickness of main diffraction peaks that is (100), (002) and (101) are
film could be up to around 300 nm for the best structural given in Figure 3. It can be observed that the highest
quality. According to Ghosh et al. (2004), the tensile TC(hkl) value is in (002) plane for all ZnO thin films.
strain might be due to a thermal mismatch between the
ZnO film and glass substrate. From the dislocation
density data, one can clearly observe that the crystalli- Morphological analysis
zation of the films is good because of their small
dislocation density (δ) values which represent the amount Scanning electron microscopy (SEM) micrograph of ZnO
of defects in the film. The larger D and smaller full width thin films are shown in Figure 4 and the films show large
at half maximum (FWHM) values show better number of grain boundaries. It can be seen from Figure 4
crystallization of the films. that grains become more uniform and bigger in size as
Moreover the texture coefficient TC (hkl) is introduced the sol-gel zinc content increases and the small grains
to characterize the preferential crystallite orientation made a smooth surface with a fine structure similar to
along the (hkl) direction defined by Manifacier et al. those observed by Kumari et al. (2011). This can be
(1976). understood from the XRD results. The thickness
measurement data of ZnO thin films are also presented in
I ( hkl ) / I o ( hkl ) Table 2. It was observed that increasing zinc concen-
TC ( hkl ) = tration of prepared sol resulted in an increase in film
N ∑ I (hkl ) / I o (hkl )
−1
thickness from 266 to 295 nm which can be explained as
N (4) follows. As the concentration of solution increases, film
2976 Int. J. Phys. Sci.
Figure 4. The SEM morphology of ZnO thin films with different content.
growth increases, which in turn, increase the film
thickness and the grain size. However, considerable
micropores related to the grain boundaries are observed
as the film grows thicker. Studies by Brinker and Scherer,
(1975) have shown that the micro and nanopores are
usual characteristics of sol-gel derived ZnO film.
Optical properties
Figure 5 depicts the optical transmittance spectra of sol-
gel deposited ZnO thin films in the UV-visible region from
200 to 800 nm. The transmittance was 83 to 95% in the
visible- near IR region from 400 to 800 nm for ZnO
concentration from 0.35 to 0.65 M. It is found that the
transmittance increases with a decrease in solution
concentration. However, the larger transmittance in 0.5
and 0.65 M concentration may be due to the structural
homogeneity and crystallinity as evidenced from Figure 4.
The transmittance of the films increases with the increase
Figure 5. Transmittance spectra of ZnO film with of ZnO concentration as shown in the study of Kim et al.
different content. (2005), who suggested that the transmittance of the films
Saleem et al. 2977
Where α is the absorption coefficient, hυ is the photon
energy, Eg is the optical band gap and A is a constant.
Figure 6 depict the plot of (αhυ)2 versus photon energy
hυ. Extrapolation of linear portion to the energy axis gives
the band gap energy (Eg). The estimated values of Eg
are listed in Table 2. The band gap values of ZnO thin
films are found to be 3.307eV to 3.227eV with the
increase of ZnO content. The band gap of ZnO films
decreases nearly linearly that can be explained with the
increase in crystallinity and the grain size factors. The
decrease in band gap has also been observed by Zelaya-
Angel et al. (1994) for CdS films. Therefore, our results
are in good agreement with the reported values in
literature (Lokhande et al., 2002; Lee et al., 1996).
Furthermore, band gap lowering can occur due to the
presence of defect states, disorder etc. However, such
defects will produce tail in the transmittance spectra
which are clearly observed in the present study (Figure
Figure 6. Plot of (αhυ)2 vs. photon energy (hυ) of ZnO film 5).
with different content.
For many materials, it is assumed that the absorption
coefficient α near the band edge shows an exponential
dependence on photon energy hυ. This dependence is
given by the relation obtained from Urbach (1953).
α = αo exp [hυ/Eu] (8)
Where αo is a constant and Eu is Urbach energy which is
the width of the tails of the localized states associated
with the amorphous state in the forbidden gap. The
logarithm of the absorption coefficient α versus photon
energy hυ is shown in Figure 7. The value of Eu is
calculated by taking the reciprocal of the slope of the
linear portion in the lower photon energy region and the
values obtained from this figure are given in Table 2. The
Eu values change inversely with optical band gape
energy. Ziaul et al. (2010) and Caglar et al. (2009) have
reported approximately same results for the Urbach
energy. The Urbach tail (Eu) is used to describe
exponential dependence of the absorption coefficient at
Figure 7. The plot of ln(α) vs. photon energy (hυ) of ZnO thin the optical absorption edge. The change of Urbach tail
film. (Eu) is caused by disorder in the materials, which leads
to an extension of the parabolic density of states into the
band edge. Disorder generally includes thermal disorder,
which reflects thermal occupation of phonon states, or
increases with the percentage of c-axis orientation, structural disorder, which is associated with impurities
consistent with a reduction in light dispersion in grain and defects in the material (Cody et al., 1981). In this
boundaries as the film microstructure becomes more c- case, the Urbach tail was mainly attributed to the effects
axis oriented. The results indicate that high optical quality of solution concentration on the fundamental optical
ZnO thin films can be successfully achieved via this low absorption.
temperature chemical approach. In the visible region of
solar spectrum, transmission spectrum of 0.65 M film
show sinusoidal behavior; this may be due to the layered Conclusion
structure of thin film. The corresponding optical band gap
of ZnO thin films is estimated by extrapolation of the In this study, we have grown transparent ZnO thin films
2
linear relationship between (αhυ) and hυ according to the on glass substrate by a multi-step sol-gel technique with
equation given by Caglar et al. (2009). 0.35 to 0.65 M zinc acetate concentrations solution. The
structural, morphological and optical properties were
(αhυ)2 = A (hυ-Eg) (7) investigated. It is found that the structural and optical
2978 Int. J. Phys. Sci.
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