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Proc. of Microelectronics & Nanotechnology (2014) Received 2 July 2013; accepted 7 October 2013
43
Morphological and Structural Properties of Zinc Oxide Films
by Sol-gel Spin Coating Technique C.A. Norhidayah1,*, N. Sarip1, S.A Kamaruddin1, N. Nafarizal1, S.N.M. Tawil2,
M.Z. Sahdan1, 1Microelectronics and Nanotechnology Shamsuddin Research Centre
Universiti Tun Hussein Onn Malaysia 86400 Parit Raja, Batu Pahat, Johor, Malaysia 2Department of Electrical and Electronic Engineering
Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur, Malaysia
1. Introduction
Zinc oxide (ZnO) is n-type II-VI semiconductor
compounds, which have technical applications such as thin film gas sensors [1], transistors [2], light emitting
diodes [3] and solar cells. [4] ZnO film was used as a
surface acoustic wave (SAW) devices and film bulk
acoustic resonator (FBAR) due to its excellent
piezoelectric properties [5]. Moreover, ZnO has a large
band gap (3.37 eV) and large exciton binding energy (60
meV) and it consists of a hexagonal wurtzite structure
with unit cell shape = 0.325 nm and c = 0.521 nm because
of their unique properties of semiconductors, ZnO films
bone behind the base of the electronics industry and
modern technology.
Various synthesis techniques have been used to synthesize ZnO films such as chemical vapour deposition
(CVD) [6], molecular beam epitaxy (MBE) [7], pulsed
laser deposition (PLD) and sol-gel method [8]. Among the methods, sol-gel method is widely adopted for the
fabrication of ZnO films due to it is an inherently low-
cost and simple liquid-phase deposition method. In
addition, the chemicals required for sol-gel is relatively
inexpensive, and easy to prepare. On the other hand, synthesis techniques play a significant role in controlling
the properties of ZnO films. In fact, the electrical and
optical properties of the films strongly depend on the structure and morphology of the films. In order to produce high quality ZnO film, it is necessary to study
the parameters that can affect the properties of the films,
such as preheating and annealing temperature, thickness,
sol concentration and etc.
In this present work, ZnO films have been prepared
by the sol-gel spin coating using different
monoethanolamine (MEA) volume and their surface
morphologies and structural properties were studied in
detail.
2. Experimental procedures
2.1 Films Preparation
ZnO films were prepared by sol-gel spin coating onto
glass substrates. Fig. 1 shows a flow chart showing the procedure to prepare ZnO films. Zinc acetate dehydrate
[Zn (COOCH3)2.2H2O, ZAD], isopropanol [C3H8O, IPA]
and monoethanolamine [C2H7NO, MEA] were used as a
precursor, solvent and stabilizer, respectively. Three ZnO
solutions were prepared by dissolving ZAD in IPA mixed with different MEA volume at room temperature
condition. The molar ratio of MEA to ZAD was varied
from 0.2 M to 0.4 M and the concentration of zinc acetate
dehydrate fixed to 0.4 M. The solution was stirred on a
magnetic stirrer for 24 hour until a clear and
homogeneous solution was obtained. Before the
fabrication process, three substrates were cleaned with
acetone and then with deionized water using ultrasonic
cleaner to remove dust or organic contaminants. All substrate were subsequently dried by a stream of nitrogen
gas. Then, each of the solution was spin-coated onto glass substrate using 3 step programs (1000 r.p.m for 5s, 3000
r.p.m. for 30s and 1000 r.p.m. for 5s). After depositing by
spin coating the film was preheated at 280°C for 3
minutes to evaporate the solvent and remove residual organic material. After that, the same coating process was
Abstract: In this work, zinc oxide (ZnO) films were prepared by sol-gel spin coating technique and the effect of
stabilizer volume on the morphological and structural properties of films was studied. Monoethanolamine (MEA)
was used as the stabilizer material. X-ray diffractometer (XRD, Bruker D8 Advance) was employed to examine the structural properties of the ZnO films. Furthermore, the morphology of the ZnO films was characterized using field
emission scanning electron microscope (FE-SEM, JEOL JSM-7600F).. The experimental results reveal that the
volume of stabilizer in the sol–gel spin coating technique exert a strong influence on the properties of the ZnO
films. The detail explanation on the mechanism will be discussed in this paper.
Keywords: Zinc Oxide (ZnO), Field Emission Scanning Electron Microscope (FE-SEM) and X-ray Diffraction
(XRD)
Proc. of Microelectronics & Nanotechnology (2014)
44
repeated for ten times to obtain an initial ten layer film
structure. These deposited samples were annealed at 500 oC for 1 hour to obtain the crystalline ZnO films before
cooling in air at room temperature.
2.2 Characterization Techniques
The crystallinity and structural properties of the prepared
films were studied by x-ray diffractometer (XRD, Bruker D8 Advance) with Cu Kα (λ= 1.54059 Å) radiation and
scanning range of 2θ set between 30o to 70o. Furthermore,
the surface morphology of ZnO film was evaluated using
field emission scanning electron microscope (FE-SEM,
JEOL JSM-7600F).
Fig. 1 Procedure for preparing ZnO films
3. Results & Discussion
Fig. 2 shows the XRD spectrum of the ZnO films with
different MEA volume. It was observed that the films
were polycrystalline with hexagonal wurtzite structure.
From XRD spectrum (101), (002), (100) and (110)
diffraction peaks are observed showing the growth of
ZnO crystallites in different direction. The peaks at 2θ angle 34.98°, 34.45° and 34.39° correspond to diffraction
from planes (002) of hexagonal wurtzite structure. In
general, the XRD peak shifted occur for three reasons,
namely changes in the lattice parameters, the presence of
residual stress and defect concentration. Furthermore, the
presence of prominent peaks shows that the film is
polycrystalline in nature. From the XRD spectrum, grain
size (D) of the film can be calculated using Debye
Scherrer’s formula [9].
cos
0.89D (1)
Where , and are the x-ray wavelength (1.54059 Å), the Bragg’s diffraction angle in degrees and the full
width at half maximum (FWHM) of the peak
corresponding to the “θ” value in radians, respectively.
The large D and smaller FHWM values indicate better
crystallization of the film. Table 1 gives the crystallite
size along prominent diffraction planes for films with
different MEA volume. As we can see from the table, the
crystallite size increased from 28.3 to 57.3 nm as the increased of MEA volume from 0.2 M to 0.4 M. According to Ostwald ripening, the increase in the
particle size is due to the merging of the smaller particles
into larger ones and is a result of potential energy
difference between small and large particles and can
occur through solid-state diffusion [10]
In addition, dislocation density represents the amount
of defects in the crystal. These values are given in Table
1. The dislocation density (δ), defined as the length of
dislocation lines per unit volume of crystal structural
using the following formula:
2
1
D (2)
The FE-SEM morphologies of the ZnO films produced at various MEA volumes are shown in Fig. 3
(50 k magnification at 5 kV applied voltage). From the
micrographs, it was observed that nanoparticle structures
were formed in the films, which is a normal characteristic
of films derived from sol-gel. It was observed that the
surface morphology of the films is almost uniformly with
increases MEA volume.
Fig. 2, XRD spectrum of ZnO films with different MEA
volume: (a) 0.2 M, (b) 0.3 M and (c) 0.4 M
Table 1 Evaluated structural parameter of ZnO films MEA
content
(mol)
planes FWHM
(β)0
2θ0 D
(nm)
δ × 10–3
(nm)–2
0.2 002 0.295 34.98 28.3 1.25
0.3 002 0.145 34.45 40.3 3.05
0.4 002 0.206 34.39 57.3 6.16
Stirring for 24 h
Spin coating
Preheating at 280 0C for 3 min
Annealing at 500 0C for
1h
ZnO films
Zinc acetate dehydrate,
monoethanolamine, isopropanol
10
time
s
Proc. of Microelectronics & Nanotechnology (2014)
45
4. Conclusion
ZnO films were successfully grown onto glass substrate
by sol-gel spin coating technique. The effects of MEA
volume on the morphological and structural properties were investigated. Based on XRD measurement, it was
revealed that ZnO films were polycrystalline with
hexagonal wurtzite structure. The grain size is in the
range of 28.3 nm to 57.3 nm due to increase MEA
volume. On the other hand, the FE-SEM analysis
revealed the surface morphology almost uniform with
increasing MEA volume. Based on finding 0.4 M MEA is
optimum for the fabrication of high quality ZnO films.
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(c)
Fig. 3 FE-SEM image of ZnO morphology with different MEA volume:
(a) 0.2 M, (b) 0.3 M and (c) 0.4 M
(a) (b)
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