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Applied Surface Science 253 (2007) 5179–5183

Structural, electrical and optical properties of Gd doped and undoped

ZnO:Al (ZAO) thin films prepared by RF magnetron sputtering

Wei Lin, Ruixin Ma *, Wei Shao, Bin Liu

Department of Non-Ferrous Metallurgy, University of Science and

Technology Beijing, Beijing 100083, People’s Republic of China

Received 14 June 2006; received in revised form 11 October 2006; accepted 21 November 2006

Available online 12 December 2006

Abstract

The influence of the gadolinium doping on the structural features and opto-electrical properties of ZnO:Al (ZAO) films deposited by radio

frequency (RF) magnetron sputtering method onto glass substrates was investigated. X-ray analysis showed that the films were polycrystalline

fitting well with a hexagonal wurtzite structure and have preferred orientation in [0 0 2] direction. The Gd doped ZAO film with a thickness of

140 nm showed a high visible region transmittance of 90%. The optical band gap was found to be 3.38 eV for pure ZnO film and 3.58 eV for ZAO

films while a drop in optical band gap of ZAO film was observed by Gd doping. The lowest resistivities of 8.4 � 10�3 and 10.6 � 10�3 V cm were

observed for Gd doped and undoped ZAO films, respectively, which were deposited at room temperature and annealed at 150 8C.

# 2006 Elsevier B.V. All rights reserved.

PACS : 81.05.�t; 81.05.Je; 81.15.Cd

Keywords: Zinc oxide; RF magnetron sputtering; Transparent conductive thin film; Optical and electronic properties

1. Introduction

Zinc oxide (ZnO) has a wide direct band gap of 3.37 eV and

a relatively large exciton binding energy of 60 meV that makes

it attractive for a variety of electronic and electro-optical

applications [1]. For the band gap of ZnO is well above the

visible range, prepared films are electrically conductive, which

is leading to applications for transparent conducting oxides

(TCO) thin films such as transparent electrodes for liquid

crystal displays, solar cell, photo-thermal conversion system

and so on [2–4]. The high quality of ITO films deposited by

sputtering of oxide targets has already been successfully

achieved on a commercialized production scale. However, both

ITO target and its film contain a very high concentration of

indium (90% In2O3 + 10% SnO2 in weight) that is a very

expensive material. ZnO is considered as one of the most

* Corresponding author at: Mailbox HaiPu, University of Science and Tech-

nology Beijing, Xueyuan Road, Haidian District, Beijing, People’s Republic of

China. Tel.: +86 10 8238 4751; fax: +86 10 8238 4751.

E-mail address: mrx_601@yahoo.com.cn (R. Ma).

0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2006.11.032

promising materials by taking the advantages of abundance in

nature, non-toxicity and good stability in hydrogen plasma

process. Moreover, though some dopant is very expensive, only

a small amount of dopant can make ZnO film’s electrical and

optical properties comparable to ITO film. This means that the

cost price of doped ZnO film is still lower than that of ITO film.

Based on the viewpoint that ZnO films are more potential than

ITO films, we believe that it’s valuable for our further

investigation about ZnO films [5,6]. The electrical conductivity

and optical transmittance of ZnO are related to the free carrier

density generated by dopant atom substitution of Zn atom

(giving out one extra electron), and oxygen vacancies (acting as

two electron donors).

A lot of studies have focused on changing various technics

parameters and the dopant concentration to improve the

performance of ZnO:Al (ZAO). Chung et al. [7] have

researched working pressure effect on the physical properties

of ZAO films. However, only a few studies on the effects of

doping on the electrical properties and their dependence with

the film structure have been reported. It is generally

considered that the ion radii of dopants for transparent and

conductive ZnO films should be small and close to that of

W. Lin et al. / Applied Surface Science 253 (2007) 5179–51835180

Zn2+. But the co-doping effect of a big ion radius rare earth

impurity and Al3+ has not been reported before. Gd2O3 has

certain and useful physical property in optical applications,

which was selected as dopant and expected to improve the

transmittance for ZAO film. The aim of this work is to

comparatively establish structural, optical and electrical

property comparative relations between the undoped and

Gd doped ZAO films.

2. Experimental

2.1. Ceramic targets preparation

The ceramic sputtering targets, of which raw materials were

mechanical mixed powders, were fabricated by sintering in the

air after die pressing and cold isostatic pressing. The following

compositions were chosen: 97% ZnO + 3% Al2O3 and 97%

ZnO + 2% Al2O3 + 1% Gd2O3 (in weight). Raw materials were

commercially available ZnO, Al2O3 and Gd2O3 (99.99% in

purity) powders. The linear shrinkage ratios of sintered targets

reached to about 20%, as well as the relative density was over

90% (highest is 97.63%). The sintered circular ceramic targets

with a diameter of 65 mm were used in thin films’ preparation.

2.2. Thin films preparation

Gd doped and undoped ZnO thin films on Corning eagle

2000 glass substrates were deposited at room temperature by

ratio frequency (RF) magnetron sputtering. Glass substrates

were thoroughly cleaned using acetone, ethanol absolute and

water as solvents and ultrasonic technology, and then dried

before loading it in the deposition chamber. The base pressure

in sputtering chamber was about 2 � 10�4 Pa. A sputter-etch of

20 min was used to remove the target surface contamination. At

last, argon was introduced, and the gas pressure was adjusted.

The deposition conditions are shown in Table 1.

The thin film’s thickness was measured by elliptical

polarization Instrument (Jobin-Yvon). Electrical properties of

the films were measured by a dc four-point probe technique.

The optical transmittances of the studied films were determined

at room temperature for photon wavelengths ranging from 320

to 800 nm, using a UV-250 spectrophotometer. Crystallo-

graphic and phase structures were determined by an X-ray

diffractometer (Rigaku) with Cu Ka radiation. The surface

morphologies and thickness of the deposited films were

investigated by a Cambridge S-360 scanning electron micro-

scope (SEM).

Table 1

Deposition conditions of Gd doped and undoped ZAO films

Parameters Value

Power mode RF

Substrate temperature Room temperature

Deposition time (min) 17

Partial pressure of O2 (Pa) 0.45

Target–substrate distance (mm) 119

Pressure (Pa) 10�4

3. Results and discussions

3.1. Structural characterization

Fig. 1 shows the X-ray diffraction patterns for the Gd doped

and undoped ZAO thin films deposited at room temperature.

The (0 0 2), (1 0 2), (1 0 3) peaks are observed. The results of

the films were in good agreement with those reported in the

PDF for ZnO (PDF75-576, a = 3.24270 A, b = c = 5.19480 A),

and the peaks correspond to a hexagonal wurtzite structure

with no additional peaks of gadolinium and aluminum phases

[8]. The (0 0 2) reflex for both films is dominating in the XRD

spectra. For both films, the crystalline granule grew along its

c-axis and perpendicular to the surface of the substrate. There

was no any peak shift of Gd doped film as compared with ZAO

film, which indicates that gadolinium atoms doping did not

distort the original crystalline structure [9,10]. It was same

with the report by Lin et al. [11] who studied heavily Al doped

ZnO films by simultaneous rf and dc magnetron sputtering

methods. The crystal size g can be estimated using Scherrer

formula:

g ¼ 0:94l

Dð2uÞ cos u; (1)

where l is the X-ray wavelength, u the Bragg diffraction angle

in degrees and D(2u) is the FWHM in radian [12–14]. The

crystallite size is in the 28–42 nm range.

Fig. 2 illustrates the surface morphology of Gd doped and

undoped ZAO thin films deposited at room temperature. The

films show different morphology of surface grains, which are

dependent on the deposition temperature and addition of

dopants [15]. For the obtained ZAO thin films, uniform and

dense surface with small grain size is observed. For gadolinium

oxide doping, the surface morphology shows a noticeable

variation of the film surface with appearing of some big grains.

It indicates that, during deposition of Gd doped ZAO film by

sputtering onto substrates the growth has taken place by

Fig. 1. The XRD patterns of Gd doped and undoped ZAO thin films deposited

at room temperature.

Fig. 2. SEM micrographs of undoped (a) and doped (b) ZAO thin films deposited on glass substrates annealed at 150 8C.

W. Lin et al. / Applied Surface Science 253 (2007) 5179–5183 5181

nucleation and coalescence process. Randomly distributed

(0 0 2) oriented nuclei may have first formed and these nuclei

then have grown to form observable islands. As islands increase

their size by further deposition and come closer to each other,

the larger ones appeared to grow by coalescence of smaller one.

Flattening of islands to give increased surface coverage of the

clusters follows this step [8,16].

3.2. Optical and electrical properties

The transmittance of the Gd doped and undoped ZAO films

of thickness 170 nm is depicted in Fig. 3. The transmittance of

all deposited films at l = 550 nm is better than 87.0%. In the

visible region, the undoped ZAO film shows the transmittance

of the order of 87% while that of Gd doped ZAO film was of the

order of 92%. Al doping affects the transmittance of ZnO films

Fig. 3. Variation of absorption (at) and transmittance (%T) with wavelength (l)

of Gd doped and undoped ZAO thin films.

a little. It’s noticed that the additional Gd doping improves the

optical properties. This attributed to decrease in free carrier

absorption due to the elevated carrier mobility of the film [17].

The absorbance has slightly increased after Gd doping, which is

related to the introduction of the Gd defect states within the

forbidden band leading to absorption of incident photons. In the

ultraviolet region, the optical transmittance of the film falls

sharply due to the onset of the fundamental absorption in this

region. The absorption edge of the Gd doped ZAO film

appeared to shift towards the longer wavelength side. The

optical absorption coefficient provides information on the band

structure. a is given by the formula [18]:

T

1� R¼ expð�atÞ; (2)

where T is the optical transmittance, R the optical reflectance, a

the absorption coefficient and t is the film thickness. The optical

band gap, Eg, can be determined from the experimental spectra

of the absorption coefficient, a, as a function of the photon

energy, hn, using the following equation [19,20]:

ahn ¼ Cðhn� EgÞ1=2; (3)

where C is a constant dependent on the electron–hole mobility.

Plot of ðahnÞ2 as a function of photon energy against hn of

undoped and Gd doped ZAO films is shown in Fig. 4. The

values of the optical energy gap determined by extrapolating

the linear portion of the curves to a = 0 are in agreement with

those reported in the literature [21–23]. The optical band gap

was found to be 3.37 eV for pure ZnO film and it increases to

3.58 eV for ZnO:Al films. The widening of optical band gap

may be attributed to Moss–Burstein shift effect [24]. This effect

is due to that the conduction band filling in highly degenerate

semiconductor makes the Fermi level exceed the conduction

band minimum. The band gap was found to be decreased to

3.45 eV for Gd doped ZAO film (1 wt.% Gd doping concen-

tration), which is similar to the report of Kim and Park [25] who

Fig. 4. Plot of ðahnÞ2 as a function of photon energy against, hn, for pure ZnO,

undoped and Gd doped ZAO films deposited at room temperature.

W. Lin et al. / Applied Surface Science 253 (2007) 5179–51835182

observed the band gap shift from 3.25 to 3.13 eV for Co doped

ZnO film (8% doping concentration).

Resistivity of doped and Gd doped ZAO films deposited at

room temperature and annealed at different temperature is

illustrated in Fig. 5. The lowest resistivities of 8.4 � 10�3 and

10.6 � 10�3 V cm are observed for Gd doped and undoped

ZAO films, respectively, deposited at room temperature and

Fig. 5. Resistivity of Gd doped and undoped ZAO films annealed at 20, 100,

150, 200, 250 and 300 8C.

annealed at 150 8C. Gd doped ZAO films exhibit a sensitivity to

the different annealing temperatures from 20 to 300 8C. For Gd

doped and undoped ZAO films, the electrical resistivities are

decreasing with increasing of the annealing temperature from

20 to 300 8C and exhibit a minimum of resistivity value at

approximately 150 8C. Raising the annealing temperature, the

atoms receive energy and migrate to relative equilibrium

positions. This induces a series of effects, for example, reducing

the lattice strain and the oxygen defect levels, appearing a more

perfect crystallite, pilling up of donor, weakening the grain

boundary scattering and increasing the number of current

carrier [26,27]. However, excessive annealing temperature

causes coarsening of grain, deteriorating of crystal orientation

and bringing overmuch defects at grain boundary, which are

closely related to the increase in resistivity. After doped Gd,

ZAO thin film has an obvious increase in resistivity under the

same deposition condition. This is similar to the results of

Shinde et al. [8] and Han et al. [28].

4. Conclusions

Gd doped and undoped ZAO films on glass substrates grown

by RF magnetron sputtering method at room temperature

annealing at different temperature from 20 to 300 8C. A small

amount of Al and Gd impurities in the films did not alter the

wurtzite structure of ZnO. All of the deposited films

demonstrated a c-axis preferred orientation with grain size

ranging from 28 to 42 nm. Among all the studied films, ZAO

films (3 wt.%) deposited at room temperature annealing at

150 8C have the most interesting and useful properties, which

are equal to the application in solar cell. They have a

transmittance to visible light above 87% and an electrical

resistivity of the order of 8.4 � 10�3 V cm. Moreover, the

electrical properties of Gd doped ZAO films (2 wt.% Al and

1 wt.% Gd) films deposited at room temperature are very

sensitive to the annealing temperature. This feature is not

obviously exhibited by ZAO and pure ZnO films. The optical

band gap of pure ZnO film is 3.37 eVand it increases to 3.58 eV

for ZnO:Al films, which is generally attributed to Burstein–

Moss effect. The optical band gap was found to be decreased

from 3.58 to 3.45 eV by Gd doping for ZAO films. The

electrical conductivity of the studied Gd doped ZAO films has

been correlated to the existence of Gd and Al atoms in the ZnO

matrix and the role of Gd doping effect in the grain growth

process by crystallite aggregation. The grain size coarsening for

the Gd doped ZAO films deposited at room temperature leads to

considerable grain boundary effects, which distributes their

electrical properties.

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