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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