World Journal of Condensed Matter Physics, 2012, 2, 10-15

doi:10.4236/wjcmp.2012.21002 Published Online February 2012 (http://www.SciRP.org/journal/wjcmp)

Simple Preparation and Characterization of

Nano-Crystalline Zinc Oxide Thin Films by Sol-Gel
Method on Glass Substrate
Muhammad Saleem1*, Liang Fang1,2, Aneela Wakeel1, M. Rashad1, C. Y. Kong3
1
Department of Applied Physics, Chongqing University, Chongqing, China; 2Key Laboratory of Optoelectronic Technology and Systems of
the Education Ministry of China, Chongqing University, Chongqing, China; 3Department of Applied Physics, Chongqing Normal Uni-
versity, Chongqing, China.
Email: *saleem.malikape@gmail.com

Received September 5th, 2011; revised October 5th, 2011; accepted October 15th, 2011

ABSTRACT
Nanocrystalline ZnO thin films have been fabricated by a multi-step sol-gel method using spin coating technique. Zinc
acetate dihydrate, 2-methoxyethanol and monoethanolamine were used as a starting material, solvent and stabilizer,
respectively. X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) were employed to characterize struc-
ture and morphologies of the as-deposited samples. The results of XRD and SEM showed that the heat treatment condi-
tions, final rotational (spinning) speed, fume exhaust and precise control of concentration of reactants (precursor and
solvent used) strongly affect the crystallographic orientation and morphology of the resultant ZnO films. The XRD pat-
tern showed that the ZnO films formed the preferred orientation along c-axis and the grain size is 16nm for the samples.
Only one peak corresponding to the (002) plane at 2θ = 34.34˚ appears on the diffractograms. The as-deposited films
had a transparency of greater than 80% in the visible-near IR region from 400 nm - 800 nm. The optical band gap ener-
gy and thickness were calculated to be 3.296 eV and 266 nm respectively.

Keywords: Sol-Gel Method; Spin Coating; XRD; SEM; EDS; Optical Properties

1. Introduction metal-organic chemical vapour deposition (MOCVD) [18],

molecular beam epitexy (MBE) [19], pulsed laser depo-
ZnO is an inexpensive n-type of II-VI semiconductor
compound, which has technical applications such as pho- sition (PLD) [20] and the sol-gel process [21-27]. Sol-gel
to-catalysts [1], thin film gas sensors [2], varistors [3], light method is widely adopted for the fabrication of transpa-
emitting diodes [4], spintronic devices [5], nanolasers [6]. rent and conducting oxide due to its simplicity, safety, no
ZnO thin films have also been widely used as surface need of costly vacuum system and hence cheap method
acoustic wave (SAW) device and film bulk acoustic reso- for large area coating. The sol-gel process also offers other
nator (FBAR) because of its excellent piezoelectric pro- advantages such as high surface morphology at low cry-
perties [7,8]. Its stability in hydrogen plasma [9] culmi- stallizing temperature, the easy control of chemical com-
nated in its exhaustive use as window layer for polycrys- ponents and fabrication of thin film at low cast for eluci-
talline solar cells [10-12] and silicon thin film solar cells dating the structural and optical properties of ZnO thin film.
[13-15]. Moreover, ZnO has large band gap 3.37 eV, lar- The fabrication of doped and un-doped ZnO thin films by
ge excitonic binding energy 60 meV and high carrier mo- the sol-gel process has already been reported by many re-
bility at room temperature. ZnO is composed of hexago- searchers [24-27]. However, up to date, multiple deposi-
nal wurtzite crystal structure with unit cell a = 3.253 Å and tion steps have been generally necessary to produce thin
c = 5.215 Å due to their unique optical, electrical and films of high-quality. In order to obtained high-quality
semi-conducting properties, ZnO thin films form the films, a typical multi-step deposition process has been
mainstay of the electronics industry and the cornerstone demonstrated.
of modern technology. In the present work, nanoycrystalline ZnO films have
Nanostructures of ZnO are fabricated using various thin been produced by the sol-gel method using zinc acetate pre-
film techniques as sputtering [16], spray pyrolysis [17], cursor and their surface morphologies, preferential orien-
*
Corresponding author. tation and optical properties were studied in detail.

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 Simple Preparation and Characterization of Nano-Crystalline Zinc Oxide Thin Films by 11
Sol-Gel Method on Glass Substrate

2. Materials and Methods

2.1. ZnO Sol-Gel Preparation
The sol-gel synthesis and thin film process arrangement
is shown schematically in Figure 1. Zinc acetate dehydrate
(Zn(CH2COO)2·2H2O) (ZAD), 2-methoxyethanol (CH3O
(CH2)2OH) (2-ME) and monoethanolamine ((HOCH2CH2)
NH2) (MEA) were used as starting material, solvent and
stabilizer, respectively. The mixture was confected to 0.5
mol·L–1 and stirred magnetically. After stirring for 30 mi-
nutes at 60˚C, MEA was added drop by drop under con-
stant stirring. The resultant solution was stirred for 90
minutes. to yield a colorless, homogeneous and transpa-
rent solution. The solution was aged for 72 hours at room
temperature in order to make it more glutinous. The mo-
lar ratio of MEA to ZAD was maintained at 1:1. Prior to
the coating of the film, the glass substrates were sonica-
ted or washed with detergent, and then cleaned in metha-
nol and acetone for 5 minutes each. Afterwards, substra-
tes were rinsed 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 seconds. The as-depo-
sited films were then pre-heated at 200˚C for 10 minutes
into a furnace to evaporate the solvent and remove orga-
nic residuals. This spinning to preheating procedure was
repeated eight times. After the deposition of final layer,
films were calcined in air at 400˚C for 1hour to ensure
that all organic species were expelled from the films.

2.2. Material Characterization Figure 1. The flow chart showing the procedure for prepar-
X-ray diffraction (XRD) was used for the physical struc- ing ZnO films.
ture (crystalline structure and microstructure) of the ZnO
thin films. XRD patterns were obtained with a MRD-Sin- get a stable homogenous solution. The explanation of the
gle Scan diffractometer with Cu Kα (λ = 1.54056Å) ra- formation of ZnO crystals in [28] enlighten that there are
diation and scanning range of 2θ set between 20˚ and 80˚. two possible ways of describing the growth of ZnO crys-
During the measurement, the current and the voltage of tal; Aggregation and Ostwald Growth (ripening). As soon
XRD were maintained at 20 mA and 36 KV respectively, as the smallest stable molecular clusters (they may be
and scan speed was 4˚/min. The surface morphology of unit cells) are formed, they rapidly combine to give the next
most stable aggregate. The primary aggregates would fur-
films was evaluated using Scanning electron microscopy
ther rapidly combine to give the next most stable second-
(FEG-SEM, Nova-400). The thickness of the film was
dary aggregate and so on. The authors observed that the
measured by a surface profilometer (AMBIOS).
primary clusters were stable aggregates and would be the
The transmission spectra of the films were measured
result of rapid aggregation rather than a result of Ostwald
by a double-beam ultraviolet/visible (UV-4100) spectro-
growth and concluded that the Ostwald mechanism shou-
photometer with a wavelength range 200 nm - 800 nm
ld be considered as only one possible approach to the fo-
and the optical band gap was measured from the trans-
rmation of bulk materials. Tokumoto et al. [29] reported
mission spectra.
that the formation of ZnO colloidal particles in an alco-
holic solvent consists of two stages. During the early
2.3. Growth Mechanisms
stage of phase transformation, small olygomers are conti-
Sol-gel technique basically involves solution preparation nuously formed. At advanced stages, the aggregation of
which usually contains the metal alkoxide compounds M the olygomers leads to crystalline wurtizite, the primary
(OR)X where M is metal and R is alkyl radical or metal colloidal particles. The primary particles then aggregate
carboxylates M (COOR)X dissolved in suitable solvent to and form a third family, the secondary colloidal particles.

Copyright © 2012 SciRes. WJCMP

12 Simple Preparation and Characterization of Nano-Crystalline Zinc Oxide Thin Films by
Sol-Gel Method on Glass Substrate

The growth of the colloidal particles is a stepped, discon-

tinuous process indicating that the predominant mecha-
nism of aggregation is heterogeneous coagulation. This me-
chanism of formation and growth leads to a hierarchical
structure.

3. Results and Discussion

3.1. Structural Analysis of Nanoycrystalline
Films
Figure 2 depict the X-ray diffraction (XRD) pattern of
the crystal structure and orientation of the nanocrystalline
ZnO thin film deposited on glass substrate using spin-
coating at 2000rpm, pre-heated at 200˚C and annealed in
air at 400˚C. From the XRD pattern, one can clearly ob- Figure 2. XRD spectrum of the nanocrystalline ZnO thin
serve a diffraction peak at 2θ = 34.34˚. Strong preferen- film.
tial growth is observed along c-axis i.e. (002) plane, sug-
Table 1. Lattice parameters of the ZnO thin film.
gesting that the prepared ZnO nanocrystals have the
wurtzite structure. Sumetha Suwanboon [30] proposed a a (Å) c (Å)
qualitative idea for the formation mechanism of the pre-
ferential oriented thin films could be the minimization of Standard Calculated Standard Calculated
the surface free energy of each crystal plane and usually
3.253 3.013 5.215 5.218
films grows so as to minimize the surface energy. Due to
the minimization of surface energy, heterogeneous nu-
cleation readily happens at the interface of film and sub- Table 2. Structural parameters of thin film.
strate.
The unit cell “a” and “c” of the polycrystalline ZnO plane d (A˚) FWHM (β)˚ 2θ˚ D (nm) δ × 10–3 (nm)–2 ε × 10–2
films with (002) orientation are calculated using the Re-
002 2.6093 0.511 34.34 16 3.9 1.22
lations (1) and (2):
a  1 3 sin (1)
   cos 4 (5)
c   sin (2)
The calculated structural parameters of the thin film
The values obtained for the unit cell a = 3.013Å and c are presented in Table 2.
= 5.218Å are in good agreement with those reported in
the JCPDS standard data (Card no. 80-0074). The calcu- 3.2. Morphological Analysis of
lated lattice parameters are given in Table 1. Nano-Crystalline Films
From the XRD spectrum, grain size (D) of the film is
calculated using the Debye Scherrer formula [31]. The surface topography of thin film is very important
tool to investigate microstructure of the films. Scanning
D  k   cos  (3) Electron Microscopy (SEM) micrograph of ZnO thin film is
where k is a constant to be taken 0.49 [31] and λ, β, and θ shown in Figure 3(a) with presence of tightly packed grains.
are the X-ray wavelength (=1.5406Ǻ), full width at half The nanocrystals are regularly distributed on the glass
maximum (FWHM) and Bragg angle respectively. By in- substrate and crystallite size is approximately 16 nm. It can
serting the different values from Table 2 in the Scherrer be seen from Figure 3(a) that the small grains made a
formula grain size of (002) oriented thin film is 16 nm smooth and transparent surface similar to those observed
which is same as reported in literature [32]. by other author [32]. The average film thickness is about
The dislocation density (δ), which represents the amount 266nm. Figure 3(b) delineated energy dispersive scat-
of defects in the crystal, is estimated from the following tering (EDS) of the above representative film. It may be
equation: seen that besides the characteristics peaks of Zinc and Ox-
ygen, peaks arising out of the substrate Manganese, Gal-
  1 D2 (4)
lium, Silicon and Gold are also available. The Gold in
Strain (ε) of the thin film is determined from the fol- ZnO film is from the Gold coating of the samples for
lowing formula: SEM analysis.

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 Simple Preparation and Characterization of Nano-Crystalline Zinc Oxide Thin Films by 13
Sol-Gel Method on Glass Substrate

(a)

Figure 4. Transmittance spectrum of nanocrystalline ZnO

thin film.

(b)

Figure 3. (a) SEM image of nanocrystaalline ZnO thin film;

(b) EDS image of nanocrystaalline ZnO thin film.

3.3. Optical Properties of Nanocrystalline

Figure 5. Plot of (αhυ)2 vs photon energy hυ of anocrystal-
ZnO Films line ZnO thin film.
Figure 4 shows the optical transmittance spectrum of na-
nocrystalline ZnO thin films annealed at 400˚C in air for eV which is slightly smaller than that of bulk ZnO (3.37
1 hour using UV-visible region from 200 nm - 800 nm. eV).
The transmittance is over 80% in the visible region from This difference is due to the fact that the values of band
400 nm to 800 nm for all the samples [32]. Sharp absorp- gap Eg depend on many factors e.g. the granular struc-
tion edge is located at 380 nm which is due to the fact ture, the nature and concentration of precursors, the
that the ZnO is a direct band gap semiconductor. The structural defects and the crystal structure of the films.
corresponding optical band gap of ZnO thin film is esti- Moreover, departures from stoichometry form lattice de-
mated by extrapolation of the linear relationship between fects and impurity states. Dinghua Bao et al. [34] re-
(αhυ)2 and hυ according to the equation [33]. ported that the band gap difference between the thin film
αhυ = A (hυ-Eg)1/2 (6) and crystal is due to the grain boundaries and imperfect-
tions of the polycrystalline thin films. D. L. Zhang et al.
where α is the absorption coefficient, hυ is the photon
[35] reported that this band gap difference between the
energy, Eg is the optical band gap and A is a constant.
film and bulk ZnO is due to the grain boundary, the
Figure 5 depicts the plot of (αhυ)2 versus photon energy
stress and the interaction potentials between defects and
hυ. The values of the direct optical band gap Eg are cal-
host materials in the films.
culated from the intercept of (αhυ)2 vs hυ curve had also
been plotted. The presence of a single slop in the plot
suggests that the film has direct and allowed transition.
4. Conclusion
The band gap value of ZnO thin film is found to be 3.296 In this study, we have grown nanocrystalline ZnO thin

Copyright © 2012 SciRes. WJCMP

14 Simple Preparation and Characterization of Nano-Crystalline Zinc Oxide Thin Films by
Sol-Gel Method on Glass Substrate

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