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Applied Surface Science 252 (2006) 8657–8661

Fabrication and vacuum annealing of transparent conductive Ga-doped

Zn0.9Mg0.1O thin films prepared by pulsed laser deposition technique

Zhiqiang Chen, Guojia Fang *, Chun Li, Su Sheng, Guanwen Jie, Xing-Zhong Zhao

Department of Physics and Center of Nanoscience and Nanotechnology Research, Wuhan University, Wuhan 430072, China

Received 5 June 2005; received in revised form 22 October 2005; accepted 2 December 2005

Available online 18 January 2006

Abstract

In this study, highly transparent conductive Ga-doped Zn0.9Mg0.1O (ZMO:Ga) thin films have been deposited on glass substrates by pulsed laser

deposition (PLD) technique. The effects of substrate temperature and post-deposition vacuum annealing on structural, electrical and optical

properties of ZMO:Ga thin films were investigated. The properties of the films have been characterized through Hall effect, double beam

spectrophotometer and X-ray diffraction. The experimental results show that the electrical resistivity of film deposited at 200 8C is

8.12 � 10�4 V cm, and can be further decreased to 4.74 � 10�4 V cm with post-deposition annealing at 400 8C for 2 h under 3 � 10�3 Pa.

In the meantime, its band gap energy can be increased to 3.90 eV from 3.83 eV. The annealing process leads to improvement of (0 0 2) orientation,

wider band gap, increased carrier concentration and blue-shift of absorption edge in the transmission spectra of ZMO:Ga thin films.

# 2005 Elsevier B.V. All rights reserved.

Keywords: ZMO:Ga films; Pulsed laser deposition (PLD); Vacuum annealing; Substrate temperature; Band gap energy

1. Introduction

Transparent conducting oxides (TCOs) are characterized by

a unique combination of low electrical resistivity and high

optical transparency. Tin-doped indium oxide (ITO) is a widely

used material for TCO applications though indium is rare and

expensive and its supply may be limited by the availability of

natural resources. However, in recent years, ZnO films have

attracted interest as a transparent conductive coating material,

because the materials: consist of cheap and abundant element;

are readily produced for large-scale coating; allow tailoring of

ultraviolet absorption; have a high stability in hydrogen plasma;

and have low growth temperature [1,2], and their potential

applications as solar cells, gas sensors, piezoelectric transdu-

cers and ultrasonic oscillators [3].

Intrinsic ZnO has a band gap energy of 3.4 eV, relatively

large among transparent conducting oxide materials, and when

doping with Mg compositions the band gap energy can increase

from 3.4 eV to 7.8 eV [4–6]. ZnO and MgO have wurtzite and

rock salt structures, respectively. Since they have different

* Corresponding author. Tel.: +86 27 87642784; fax: +86 27 68752569.

E-mail address: gjfang@whu.edu.cn (G. Fang).

0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2005.12.018

coordination number and crystal symmetry, but similar ionic

radii (Mg2+, 0.66 A and Zn2+, 0.74 A), it is expected that there

can be some replacement in either structure without changing

the original structure when alloying [7]. There have been

plentiful reports of the depositions of Zn1 � xMgxO (ZMO) thin

films [8–14]. But without any intentional doping, the ZMO film

is n-type with a resistivity above 105 V cm [8,14]. As Ga is well

known as an effective n-type dopant in ZnO [15–18], we expect

that Ga-doping could decrease the resistivity of ZMO thin

films. To date there has been little research on Ga-doped

Zn0.9Mg0.1O layers and in our knowledge there is no report

about Ga-doped Zn0.9Mg0.1O films deposit by PLD system and

with post-deposition vacuum annealing. In this study we

investigated the structural, electrical and optical properties of

the Ga-doped ZMO films.

2. Experimental

Ga-doped ZnMgO (ZMO:Ga) films were deposited on glass

substrates by plused laser deposition (PLD) system using a KrF

excimer laser as an ablation source. Ga-doped ZnMgO target

was prepared by mixing ZnO (90 at%; purity: 99.99%), MgO

(10 at%; purity: 99.99%) and Ga2O3 (3 at% relative to

Zn0.9Mg0.1O; purity: 99.99%) powders. The mixed powder

Z. Chen et al. / Applied Surface Science 252 (2006) 8657–86618658

Fig. 1. (a) XRD spectra of ZMO:Ga films deposited at different temperatures

(100–500 8C) and (b) XRD spectra of ZMO:Ga films deposited at 200 8C and

post-deposition annealed at 400 8C and 500 8C for 2 h in vacuum.

was ground, pressed into a pellet, and sintered at 1400 8C in air.

During the deposition, the glass substrates were placed parallel

to the target surface with a 5 cm distance and heated from

100 8C to 500 8C by a lamp heater located behind the substrate

holder with the base chamber pressure below 3 � 10�3 Pa.

Films were deposited in oxygen ambient at a pressure of

1 � 10�2 Pa. The laser was operated at a pulsed rate of 5 Hz.

The laser beam was focused through a 60 cm focal lens onto the

rotating target at a 458 angle of incidence. The energy density of

laser beam at target surface was maintained at 2 mJ/cm2. The

films deposited at 200 8C were annealed at 400 8C and 500 8Cfor 2 h under 3 � 10�3 Pa, respectively.

The structural, electrical and optical properties of ZMO:Ga

thin films were characterized by various techniques. The film

thicknesses were all less than 450 nm measured by the surface

profile measuring system (TALYSURF SERLES II). The

surface morphology of the films was observed using SPM-

9500JS atomic force microscope (AFM). The structure was

determined by a D/MAX-rB diffractometer with Cu Ka-source

and Ni filter X-ray diffraction (XRD). The optical transmission

measurements were carried out in a VARIAN Cary 5000 double

beam spectrophotometer. The spectral region used in this work

was 290–800 nm. The films were cut to 1 cm � 1 cm pieces

and indium dots were made on the surfaces for a Van der Pauw

configuration. The electrical resistivity and mobility were

measured by Van der Pauw method at room temperature.

3. Results and discussion

Fig. 1a shows the X-ray diffraction patterns for ZMO:Ga

films grown at different temperatures under 1 � 10�2 Pa of

oxygen pressure. The XRD spectrum of films deposited at

100 8C shows no obvious peaks. The XRD spectra of films

deposited at higher temperatures are dominated by the (0 0 2)

reflection for all films confirming the strong (0 0 2) textures.

The films, deposited at 200 8C and 300 8C, exhibit only the

(0 0 2) peak. When substrate temperature increases to 400 8Cand 500 8C, the film is also dominated by (0 0 2) peak with

slight (0 0 4) peak. The intensity value of the (0 0 2) peak is

increasing and the full width at half maximum (FWHM) value

decreases from 0.958 to 0.38 when the substrate temperature

shifts from 200 8C to 500 8C, which is indicative of high quality

single crystal films at higher deposition temperature. The

diffraction peak position of (0 0 2) peak shifts from 33.888 to

34.248 with the increase of substrate temperature from 200 8Cto 500 8C. X-ray diffraction patterns for ZMO:Ga films grown

at 200 8C and post-deposition annealed at 400 8C and 500 8Cfor 2 h under 3 � 10�3 Pa are shown in Fig. 1b. Films are also

dominated by (0 0 2) peak. The diffraction peak position of

(0 0 2) peak shifts from 33.888 to 34.228, and the FWHM value

decreases from 0.958 to 0.448 when film annealed at 500 8Cafter deposited at 200 8C. By using Scherrer’s formula [19]:

d = 0.9l/b cos u, where l is the X-ray wavelength, b the

corrected peak width and u is the Bragg diffraction angle, the

calculated grain size of ZMO:Ga films increases from 15 nm at

200 8C to 48 nm at 500 8C. Post-deposition annealing also can

make grain size larger. The increase of 2u value of the (0 0 2)

peak may be related to oxygen deficiency [20] and stresses due

to thermal expansion coefficient mismatch between the film

(4 � 10�6 K�1) and substrate (9 � 10�6 K�1) [21]. According

to Chang et al. [22], the residual stress of the film can be

reduced by annealing process that resulted in a peak shift of

XRD patterns toward high angle, as obtained in this study. In

addition, based on the fact that the ion sizes of Ga and Mg are

smaller than that of Zn, it is possibly that the Ga and Mg ions

successfully substitute the Zn sites as increasing growth

temperature but there are still interstitially positioned Ga and

Mg ions in crystal because the lattice parameter grown at

500 8C is larger than that the JCPDS value. These interstitially

positioned ions also may have some contributions to the peak

shift of ZnO(0 0 2).

Surface scanning was done for films fabricated at different

conditions using AFM. Samples deposited at 200 8C have an

average grain size of 33 nm with arithmetic mean roughness

about 5 nm (Fig. 2a). Fig. 2b shows an AFM micrograph of the

surface morphology of the film deposited at 400 8C. Most

crystallites formed under this condition are found to have grain

Z. Chen et al. / Applied Surface Science 252 (2006) 8657–8661 8659

Fig. 2. AFM pictures of ZMO:Ga films deposited at different temperatures: (a) 200 8C; (b) 400 8C; (c) 200 8C deposited and annealed at 400 8C for 2 h in vacuum; (d)

200 8C deposited and annealed at 500 8C for 2 h in vacuum.

Fig. 3. Electrical resistivity (~), carrier concentration (&) and mobility (*) of

ZMO:Ga thin films vs. deposition temperature.

size between 45 nm and 50 nm with arithmetic mean roughness

smaller than 1 nm. Fig. 2c shows the AFM results for the

surface morphology of the film deposited at 200 8C and post-

deposition annealed at 400 8C for 2 h, the film has an average

grain size of 38 nm with arithmetic mean roughness about

2.2 nm. Fig. 2d shows the AFM results for the surface

morphology of the film deposited at 200 8C and post-deposition

annealed at 500 8C for 2 h, the film has an average grain size of

50 nm with arithmetic mean roughness about 2.1 nm. After

annealing, the average grain size is increased as compared with

that of as-deposited one, also the annealing makes the film

surface smoother.

Fig. 3 shows the resistivity, carrier concentration and Hall

mobility of ZMO:Ga thin films as a function of the substrate

temperatures. All of the films showed n-type conductivity as

determined from the signature of Hall-coefficient. The

resistivity of ZMO:Ga films decreased from 3.5 � 10�3 V cm

cm to 9.5 � 10�4 V cm with increase of substrate temperature

from 100 8C to 300 8C. However, further increase of substrate

temperature above 300 8C, the resistivity shows significantly

increase. The highest carrier concentration of 1.42 � 1021 cm�3

was obtained for film deposited at 200 8C, which was reduced to

3.72 � 1020 cm�3 for films grown at higher deposition

temperature of 500 8C. Mobility value of 5.4 cm2/V s was

obtained for film deposited at 200 8C, which decreased to

2.3 cm2/V s for film grown at 500 8C. The mobility of impurity-

doped ZnO films with carrier concentration of 1019–1020 cm�3 is

determined by both the ionized impurity scattering and grain

boundary scattering; however, with carrier concentration of

Z. Chen et al. / Applied Surface Science 252 (2006) 8657–86618660

Table 1

Electrical properties and figure of merit of ZMO:Ga films deposited at 200 8Cand further annealed at 400 8C and 500 8C for 2 h in vacuum

Sample r (V cm) m (cm2/V s) N (cm�3) FTC (V�1)

As-deposited 8.12 � 10�4 5.40 1.42 � 1021 0.33

Annealed

at 400 8C4.74 � 10�4 7.18 1.84 � 1021 0.43

Annealed

at 500 8C6.96 � 10�4 6.02 1.51 � 1021 0.49

Fig. 5. (ahn)2 vs. photon energy (hn) of ZMO:Ga films deposited at (a) 100 8C(&), (b) 200 8C (*), (c) 400 8C (~), 500 8C (!), (d) 200 8C and (e) annealed

at 400 8C, (b) for 2 h in vacuum.

1020–1021 cm�3, the mobility is mainly dominated by ionized

impurity scattering [1,14]. The decrease in mobility of films may

occur due to the increasing ionized impurity (Zn, Mg and Ga

interstitially positioned ions) scattering at higher deposition

temperatures. In the meantime, the carrier concentration would

decrease with the decrease of substitute Ga ions in films

deposited at higher deposition temperatures [23]. All these lead

to the increase of resistivity. The resistivity, carrier concentration

and mobility change of films deposited at 200 8C and annealed at

400 8C and 500 8C are shown in Table 1. As a result, a

considerable decrease in resistivity was obtained, which can be

attributed to an increase in intrinsic donors by annealing.

The optical transmission spectra of ZMO:Ga films deposited

at different temperatures and post-deposition annealed at 400–

500 8C are shown in Fig. 4. An average transmittance of above

85% in the visible range was obtained in all the deposited films.

Compared with that of films deposited at 100 8C, the optical

transmittance of films deposited at temperatures from 200 8C to

400 8C is higher, and there are blue-shift of the absorption

edges which is mainly attributed to the Burstein–Moss effect,

since the absorption edge of a degenerate semiconductor is

shifted to shorter wavelengths with increasing carrier con-

centration. For the film deposited at 200 8C, the absorption edge

of the transmittance shifts to the shortest wavelength, this film

corresponds to the highest carrier concentration in Fig. 3. The

absorption edge of film deposited at 500 8C shifts to the longest

wavelength. When the samples are annealed, shifts in the

Fig. 4. Optical transmission spectra of ZMO:Ga thin films deposited at

different temperatures 100–500 8C (a–e) and 200 8C and annealed at 400 8C(f) and 500 8C (g) for 2 h in vacuum.

absorption edges towards shorter wavelength and increase in

the average transmission are also observed.

Fig. 5 shows plots of (ahn)2 versus hn for ZMO:Ga films

before and after annealing. The ZMO:Ga film has a direct band

gap, the absorption edge for direct interband transition is given

by [24]:

ahn ¼ Cðhn� EgÞ1=2

where C is a constant for a direct transition and a is the optical

absorption coefficient, which is given from dividing absorbance

by film thickness. The optical energy gap (Eg) can then be

obtained from the intercept of (ahn)2 versus hn for direct

allowed transitions. Fig. 5 shows that the Eg of film deposited

at 200 8C is 3.83 eV. When increase the substrate temperature

to 500 8C, the Eg decreased to 3.57 eV, this may be caused by

the increasing defects in films and/or the re-evaporation of Mg

component of films during the deposition with the vacuum

pumping process at high substrate temperature. Fig. 5 also

shows that the Eg of films deposited at 200 8C and annealed at

400 8C for 2 h was increased to 3.90 eV with post-deposition

vacuum annealing. There is no obvious difference of Eg when

the annealing temperature increases from 400 8C to 500 8C. A

band gap energy 3.59 eV of Al-doped ZnO films deposited by

PLD technique and subsequently annealed in argon has been

reported in this group [25]. This work illustrates that the Eg of

ZnO films increases with the alloy of Mg component.

To assist the selection of the ZMO:Ga thin films produced

concerning their application on optoelectronic devices, we

determine the figure of merit (FTC), defined as [26]:

FTC ¼1

ra

where a is the average optical absorption coefficient, which is

given from dividing absorbance from 380 nm to 800 nm by film

thickness. The results obtained for the figure of merit with the

different temperature studied are shown in Fig. 6. It is possible

Z. Chen et al. / Applied Surface Science 252 (2006) 8657–8661 8661

Fig. 6. Figure of merit of ZMO:Ga thin films deposited at different temperatures.

to observe film deposited at 200 8C have high value in the figure

of merit with the temperature. The results obtained for the

figure of merit with film deposited at 200 8C and post-deposi-

tion annealed at different temperature for 2 h under

3 � 10�3 Pa are shown in Table 1. It is possible to observe

an increase in the figure of merit with the increase of annealing

temperature from 400 8C to 500 8C. This behavior is related

with the variation of the electro-optical properties of the

ZMO:Ga thin films with the annealing treatment.

4. Summary and conclusions

Ga-doped Zn0.9Mg0.1O films have been grown by PLD on

glass substrate. The experimental results show that the

electrical resistivity and the band gap energy of films deposited

at 200 8C are 8.12 � 10�4 V cm and 3.83 eV, which can be

further optimized to 4.74 � 10�4 V cm and 3.90 eV, respec-

tively, after vacuum annealing at 400 8C for 2 h. The

transmittance of this film at wavelength l = 330 nm was more

than 50%. Band gap engineering possibilities in the range of

3.57–3.90 eV were demonstrated with resistivities

r < 10�2 V cm. These band gap-modified transparent con-

ductive films may be useful not only as a TCO in the UV region

but also to control the band lineup of multilayered semi-

conductor structures.

Acknowledgement

This work was supported by the National Natural Science

Foundation of China under grant no. 60244003.

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