Materials Sciences and Applications, 2011, 2, 340-345
doi:10.4236/msa.2011.25044 Published Online May 2011 (
Copyright © 2011 SciRes. MSA
Optical and Structural Properties of ZnO Thin
Films Fabricated by Sol-Gel Method
Ziaul Raza Khan1, Mohd Shoeb Khan2, Mohammad Zulfequar1, Mohd Shahid Khan1*
1Department of Physics, Jamia Millia Islamia, New Delhi, India; 2Department of Chemistry, Jamia Millia Islamia, New Delhi, India.
Received January 24th, 2011; revised February 10th, 2011; accepted March 15th, 2011.
Highly oriented and transparent ZnO thin films have been fabricated on ultrasonically cleaned quartz substrates by the
sol-gel technique. X-ray diffraction, UV-VIS, FTIR, photoluminescence and SEM are used to characterize ZnO thin
films. X-ray diffraction study show that all the films prepared in this work have hexagonal wurtzite structure, with lat-
tice constants a = b = 3.260 Å, c = 5.214 Å. The optical band gap energy of the thin films is found to be direct allowed
transition ~3.24 eV. The FTIR spectrum of the film has the characteristics ZnO absorption band at 482 cm1. The pho-
toluminescence spectrum of the samples has an UV emission peak centred at 383 nm with broad band visible emission
centred in the range of 500 - 600 nm.
Keywords: ZnO, XRD, SEM, Photoluminescence, Band Gap
1. Introduction
Significant research efforts have been made in recent
years for developing highly oriented and transparent ZnO
thin films, because of their potential application in trans-
parent electrode in display, window layers in solar cells,
field emitters, ultraviolet laser emission, photodetectors,
piezoelectricity, bio-sensors, short wavelength light emit-
ting diode and information technology [1-8]. A II-VI
group semiconductor material ZnO has wide band gap
(~3.3 eV at room temperature) and large excitonic bind-
ing energy ~60 meV. Due to their unique optical, elec-
trical and semiconducting properties, ZnO thin films are
extensively used in various applications. Despite several
approaches adopted for making these ZnO thin films;
controlling the size, shape, crystallinity and various pa-
rameters affecting the size and shape of these materials
still need to be investigated. Therefore, it is essential to
investigate optimum conditions for fabrication of highly
oriented and transparent ZnO thin films. The main con-
cern of researcher is to get better quality of material
stoichiometry. ZnO thin films are grown by different
techniques such as pulsed laser deposition (PLD),
magnetron sputtering, MOCVD, spray pyrolysis etc
[9-12]. Sol-gel technique is widely adopted due to its
comparatively simple procedure as there is no need of
costly vacuum system and it has a wide-range advantage
of large area deposition and uniformity of the films
thickness. The sol-gel process also offers other advan-
tages for thin film deposition including outstanding con-
trol of the stoichiometry and easy doping in film compo-
sition. The structural and physical properties of ZnO thin
films prepared by sol-gel technique using various inor-
ganic and organic precursors at different deposition con-
ditions have been reported in literature [13,14]. In the
present work, we report growth of ZnO thin films on
quartz substrate by Sol-gel method using zinc acetate
precursor and their structural, optical, vibrational and
photoluminescence properties.
2. Experimental Details
All the reagents used in the present work for the chemi-
cal synthesis were of analytical grade. Zinc acetate dihy-
drate (Zn(CH3COO)2·2H2O) was first dissolved in a
2-methoxyethanol ((CH3)2CHOH) with monoethanola-
mine (MEA: H2NCH2CH2OH) which was used as a sta-
biliser. The molar ratio of MEA to zinc acetate was kept
to 1.0 and concentration of zinc acetate was 0.80 mol/l.
The resultant solution was stirred at 60˚C for 1 h to yield
a clear and homogeneous solution ready for coating. The
coating was performed with freshly prepared solution.
The films on ultrasonically cleaned quartz substrates
were prepared using spin-coating unit which was rotated
at 3000 rpm for 30 s. The films were preheated (baked)
Optical and Structural Properties of ZnO Thin Films Fabricated by Sol-Gel Method
Copyright © 2011 SciRes. MSA
at temperature 250˚C for 5 min in a furnace to evaporate
the solvent and remove organic residuals. The spin-
coating to preheating procedure was repeated ten times.
The films were then post-heated (annealed) in air at
400˚C for three hour. The phases of ZnO thin film were
determined by X-ray diffraction, using Panalytical Dif-
fractometer type PW3710 with Cu Ka radiation (λ =
0.154 nm). Optical transmittance was observed using
UV-Visible double beam spectrophotometer model (Jas-
co-V570), and the optical band gap energy was calcu-
lated from the data of the optical transmittance and wa-
velength. The IR spectrum was recorded using SHI-
MADZU FTIR-8400S Japan in the range of 400 cm1 to
5000 cm1. Photoluminescence of the samples was
measured using a He-Cd laser as an excitation source,
operating at 325 nm with an output power of 50 mW and
monochromatic (Jovin Youvan) fitted with a Hamamatsu
R928 photomultiplier detector. Surface morphology was
examined by the Scanning Electron Microscope model-
Carl ZEISS EVO-40.
3. Results and Discussion
3.1. X-Ray Diffraction Analysis
The XRD pattern of ZnO thin film fabricated by sol-gel
method on quartz substrates is shown in Figure 1. All the
peaks of the ZnO thin films correspond to the peaks of
standard ZnO (JCPDS S6-314). For all the samples,
(100), (101) and (002) diffraction peaks are observed in
the XRD pattern, showing the growth of ZnO crystallites
along different directions. Strong preferential growth is
observed along (002) plane indicating that the films are
oriented along c-axis [6]. The typical hexagonal wurtzite
structure of thin films is inferred from the XRD pattern.
The crystallites sizes (D) of the films are estimated
using the Scherer formula [15]:
where k is a constant taken to be 0.94, λ is the wave-
length of X-Ray used (λ = 1.54 Å) and β2θ is the full
width at half maximum of (002) peak of XRD pattern,
Bragg angle, 2θ, is around 34.44˚. The average value of
grain size is found to be 20 nm.
The dislocation density (δ), defined as the length of
dislocation lines per unit volume, are estimated using the
Strain (ε) of the thin films is estimated using the equa-
Figure 1. XRD pattern of ZnO thin film.
The evaluated structural parameters of thin films are
presented in Table 1.
The lattice parameters of ZnO thin films evaluated
from XRD data are in good agreement with those re-
ported in (JCPDS S6-314). The calculated lattice para-
meters are given in Table 2.
3.2. Optical Properties
The optical transmission spectrum of the ZnO thin film
grown on quartz substrates is shown in Figure 2. The
average value of transmittance of thin films in the visible
range is found to be 91% - 95%. In the visible region of
solar spectrum, transmission spectra of ZnO thin films
show sinusoidal behaviour; this may be due to the
layered structure of thin films. The value of band gap is
estimated from fundamental absorption edge of the films.
For the direct transitions, the absorption coefficient is
expressed by [16]:
hvA hvE
 (4)
where A is the constant, Eg is the energy gap, ν is the
frequency of the incident radiation and h is Planck’s con-
stant. Figure 3 shows the plot (αhν)2 vs. hν. The Eg val-
ues of thin film are calculated from this plot. The pres-
ence of a single slope in the plot suggests that the films
have direct and allowed transition. The band gap energy
is obtained by extrapolating the straight line portion of
the plot to zero absorption coefficient. The band gap
value of ZnO thin film is found to be 3.24 eV.
The absorption coefficient of ZnO thin films is shown
in Figure 4. The absorption coefficient of ZnO thin films
is found to be zero in forbidden energy region and it is
found to increase rapidly with the decrease in wavelength
beyond energy band gap. Zero absorption coefficients of
ZnO thin films in the visible range of spectrum make
these thin films suitable as window layer in solar cells.
Optical and Structural Properties of ZnO Thin Films Fabricated by Sol-Gel Method
Copyright © 2011 SciRes. MSA
Table 1. Structural parameters of ZnO thin films.
Planes Interplanar spacing(Å) d (002) FWHM(β) (002) (× 103 (rad.))Grain size D (nm)δ (× 1014) (lines/m2) ε (× 103)
(100) 2.8095 8.37 18 30.86 2.01
(002) 2.6016 6.28 24 17.36 1.49
(101) 2.4764 8.37 18 30.86 1.98
Table 2. Lattice parameters of the ZnO thin film.
a (Å) c (Å)
Calculated Standard Calculated Standard
3.260 3.253 5.214 5.215
Figure 2. Transmission spectrum of ZnO thin film.
Figure 3. The plots (αhν)2 vs. photon energy of the ZnO thin
The logarithm of the absorption coefficient α(ν) of
ZnO thin films is plotted as a function of the photon en-
ergy (hν) in Figure 5. The values of the Urbach’ s energy
(Eu) are calculated by taking the reciprocal of the slopes
of the linear portion in the lower photon energy region of
these curves and the value of urbach energy found to be
0.06 eV. Urbach energy Eu is very important tools to
investigate structural disorder in thin films as reported by
M. Caglar et al. in [16].
3.3. FTIR Analysis
FTIR spectroscopy is very useful tools for investigating
Figure 4. The plots α vs. energy of ZnO thin film.
Figure 5. The plots of ln(α) vs. photon energy of ZnO thin
Optical and Structural Properties of ZnO Thin Films Fabricated by Sol-Gel Method
Copyright © 2011 SciRes. MSA
vibrational properties of synthesized materials. The band
positions and absorption peak not only depend on the
chemical composition and structure of the thin films but
on the morphology of thin films also [17]. FTIR spec-
trum of ZnO thin film is shown in Figure 6. The absorp-
tion band observed at 482 cm1 is attributed to the ZnO
stretching vibrations [18]. The broad peak in the range of
3900 to 3800 cm1 is attributed to water molecule present
in thin films. Weak peaks at 1550 cm1 and 1665 cm1
are attributed to symmetric and asymmetric C=O bonds
vibrations respectively. The absorption peaks appearing
at 2380 cm1 is due to the absorption of atmospheric CO2
by metallic cation [19]. The IR frequencies along with
the vibrational assignment for ZnO thin films are given
in Table 3.
3.4. Photoluminescence Spectroscopy
Photoluminescence spectrum of ZnO thin film is shown
in Figure 7. Two emission peaks is observed in photo-
luminescence spectrum. A strong peak centred at 383 nm,
near the band edge due to free exciton emission is ob-
served in photoluminescence spectrum. A weak and
broad peak centred at 550 nm is also observed in photo-
luminescence spectrum [20]. The broad band in the re-
gion of 500 - 600 nm, in photoluminescence spectrum of
ZnO thin films is related to the amount of non-stoi- chi-
ometric intrinsic defects and the same may be due to zinc
vacancy in ZnO films as reported Kim et al. [21]. It
Figure 6. FTIR spectrum of ZnO thin film.
Table 3. IR frequency assignment with the corresponding
Positions (cm1) Intensities Assignments
482 Medium ZnO stretching
3800, 3900 Doublet HOH stretching
1550, 1665 Doublet C=O
2380 Strong O=C=O
is known that pure ZnO can show green or orange visible
luminescence depending on the growth temperature and
availability of oxygen during sample preparation. The
main issue is improving material quality; mostly ZnO
shows the ultraviolet emission with the green emission.
Visible emission in ZnO shows the defects in material
crystal structures. In this thin film we have observed one
visible band, presence of this band due to the stochio-
metric defects occurs during the synthesis of thin films.
3.5. SEM Analysis
Surface morphology of thin films is very important tool
to investigate microstructure of thin films. SEM micro-
graph of ZnO thin films is shown in Figure 8. The
growth of unsymmetrical ZnO rod having large surface
area can be seen in SEM micrograph. From micrograph,
it is observed that growth of small square shape crystal-
lites with the rod shape. Such type of square slabs in ZnO
thin films are also observed by L. Znaidi et al. [12].
4. Conclusions
We have grown ZnO thin films on quartz substrates by
sol-gel technique with 0.80 M zinc acetate solutions.
Films have been characterized using optical and struc-
tural measurements. All the films exhibit high transmit-
tance (91% - 95%) in the range of 400 nm to 800 nm,
thus making the films suitable for optoelectronic devices,
for instance as window layers in solar cells. The films
show a direct transition in the range 3.24 eV. The X-ray
diffraction analysis revealed that all samples have hex-
agonal wurtzite structure. The crystallites sizes as mea-
sured using XRD data are found to be in the range of 18 -
24 nm. The film has the strong emission band at 383 nm
and also a broad emission peak centred at 550 nm visible
Figure 7. Photoluminescence spectrum of ZnO thin film.
Optical and Structural Properties of ZnO Thin Films Fabricated by Sol-Gel Method
Copyright © 2011 SciRes. MSA
Figure 8. SEM micrograph of ZnO thin film.
5. Acknowledgements
The authors are thankful to Material Science Group of
National Physical Laboratory, Delhi, India for extending
photoluminescence facility.
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