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Low-temperature deposited Titanium-doped zinc oxide thin films on the flexible PET substrate by DC magnetron sputtering
Vacuum 86 (2011) 483e486
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Low-temperature deposited Titanium-doped zinc oxide thin films on the flexible PET substrate by DC magnetron sputtering Hanfa Liu*, Chengxin Lei School of Science, Shandong University of Technology, 12 Zhangzhou Road, Zibo, 255049 Shandong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2011 Received in revised form 4 October 2011 Accepted 5 October 2011
Transparent conducting Titanium-doped zinc oxide thin films (TZO) with high transparency and relatively low resistivity were firstly deposited on water-cooled polyethylene terephthalate (PET) substrates at room temperature by DC magnetron sputtering. The microstructure, optical and electrical properties of the deposited films were investigated and discussed. The XRD patterns show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity decreases when the sputtering power increases from 45 W to 60 W. However, as the puttering power continue increases from 60 W to 90 W, the electrical resistivity increases rapidly. When the puttering power is 60 W, the films deposited on PET substrate have the lowest resistivity of 4.72 104 U cm and a relatively high transmittance of above 92% in the visible range. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Titanium-doped zinc oxide thin films Thin films Magnetron sputtering Stress
1. Introduction Flexible substrates transparent conducting oxide (TCO) thin films are widely studied for applications such as in liquid crystal displays (LCD), thin film solar cells, personal digital assistants (PDA), cell phone and digital camer, etc [1,2]. In recent years, Sndoped In2O3(ITO) and Al-doped ZnO (ZnO:Al) on flexible substrates have been extensively studied [3,4]. But in our knowledge, no report was published for TZO films prepared on flexible substrates up to now. Comparing with ITO and ZnO:Al films, TZO films have the lower best film forming temperature, low cost, innocuity, good stability (in plasma) and low chemical reactivity. In our work, TZO thin films were firstly deposited on PET substrates by DC magnetron sputtering at room temperature. The dependence of the properties of TZO thin films on sputtering power has been studied. 2. Experimental 2.1. Sample preparation The TZO films on PET substrates were deposited in a GJP500C2 model DC magnetron sputtering system at room temperature. The * Corresponding author. Tel.: þ86 15965521318. E-mail address: [email protected] (H. Liu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.006
vacuum chamber was evacuated to a base pressure of 4.5 104 Pa at room temperature. A sintered ceramic with a mixture of ZnO (99.99% purity) and TiO2 (99.99% purity) was employed as source material. The content of TiO2 added to the ZnO target was 2.1 wt.%. The diameter of the target was 75 mm and the thickness of target was 3 mm. The targetesubstrate distance was about 60 mm. During the process of deposition, the sputtering time and the Ar pressure were controlled at 25 min and 4.0 Pa, respectively. Prior to the deposition, the PET substrates were ultrasonically cleaned in acetone for 10 min, marinated in alcohol for 10 min and washed by purified water. In order to investigate the effect of sputtering power on the properties of TZO films, the sputtering power was varied from 45 W to 90 W. 2.2. Sample testing We used a D8 ADVANCE XRD using CuKa1 radiation (l ¼ 0.15406 nm) to analyse the structural properties of the TZO films. A Sirion 200 scanning electronmicroscope (SEM) was used to investigate the surface morphologies of the deposited films. The sheet resistivities (R) were measured with a four-probe instrument. The thickness (l) of the films was measured using an SGC-10 thin film thickness tester. The optical transmittance were measured with UVevis spectrophotometers at room temperature. The resistivity of TZO films was evaluated using the formula r ¼ Rl.
484
H. Liu, C. Lei / Vacuum 86 (2011) 483e486
50000
Intensity/(a.u.)
40000 30000 20000 (002)
10000
P=90W P=75W P=60W P=45W Substrate
0 20
30
40
50 2 θ /(°)
60
70
80
90
Fig. 1. XRD spectrum of the TZO films deposited on PET substrate.
the non-crystalline region in grain boundary. From the XRD patterns, one can infer that Ti atoms substitute in the hexagonal lattice or probably segregate to the non-crystalline region in grain boundaries. Fig. 1 also shows that, when the sputtering power increases from 45 W to 90 W, the intensity of (002) peak were 4.9%, 8.6%, 15.1% and 16.3% respectively. This shows that with the sputtering power increases, the (002) peak becomes more intense and sharper. This is due to the polycrystalline of the resulting films being improved and grain size becoming larger when elevating the sputtering power. The average crystallite size for these samples was about 16.25e22.99 nm estimated from the XRD spectrum by using Scherre formula [6]. Fig. 2 shows the SEM images of the surface morphologies of TZO films deposited at different sputtering power. It is clearly observed that the grain size and the surface roughness increase rapidly with sputtering power increasing, which is in good consistent with our XRD results as shown in Fig. 1. 3.2. Stress analysis of the TZO films
3. Results and discussion 3.1. Structural characterization of TZO films Fig. 1 shows the X-ray diffraction spectrum of the PET substrate and the sample TZO films deposited on water-cooled PET substrate at different sputtering power. It can be seen that apart from the XRD patterns of the substrate, only the (002) peak is observed at about 2q ¼ 34.55 for all sample films. These values are very close to that of the standard ZnO crystal (34.45 ). This indicates that all of the obtained films were polycrystalline with the hexagonal wurtzite structure and had a preferred orientation with the c-axis perpendicular to the substrates almost independent of the sputtering power [5]. No Ti2O2 phase was found from the XRD patterns, which implies that Ti atoms replace Zn in the hexagonal lattice or Ti segregate to
Stress of thin films is a very important parameter for the practical application. The total stress in thin films commonly consists of two components. One is the intrinsic stress introduced by impurities, defects and lattice distortions in the crystal, and the other is the extrinsic stress introduced by the lattice mismatch and thermal expansion coefficient mismatch between the film and substrate [7]. For hexagonal crystals, the stress (s) in the plane of the film can be calculated using the formula [7].
s ¼ 453:6 109 ðc c0 Þ=c0
(1)
where “c” is the lattice parameter of the ZnO film calculated from (002) peak of XRD pattern and the “c0” is the lattice parameter for the ZnO bulk (c0 ¼ 0:52065 nm). The estimated values of stress “s” in the films grown at different sputtering power are listed in Table 1
Fig. 2. SEM images of TZO films at different power. (a) 45W (b) 60 W (c) 75 W (d) 90 W.
H. Liu, C. Lei / Vacuum 86 (2011) 483e486 Table 1 The stress and average transmittance of TZO thin films at different sputtering power. 45
60
75
90
Thickness/nm Stress/GPa Average transmittance/%
166.2 0.8172 95.73
262 0.8050 92.85
345.7 2.1990 91.40
412.6 2.7687 78.94
From the Table 1, it is seen that the films are in a state of tensile stress and as the sputtering power increases from 45 to 90 W, the stress of the films deposited on PET substrate first decreased then increase. The lowest stress was about 0.8050 GPa for a thickness of 286 nm at 60 W sputtering power.
50
PET Substrate
40
Resistivity/ (10-4
Sputtering power/W
485
30
20
10
Glass substrate 3.3. Growth rate of TZO films
0
Fig. 3 shows the dependence of the growth rate on the sputtering power. It is observed that a clear increase in growth rate as sputtering power increase. The Growth rate and the sputtering power is basically a linear relationship. This increase indicates that the number of atoms sputtered from the target is proportional to the sputtering power [8]. When the sputtering power increase from 45 W to 90 W, the growth rate increase from 6.648 nm/min to 16.504 nm/min. 3.4. Electrical properties of TZO films Fig. 4 shows the sputtering power dependence of the resistivity for the TZO films deposited on PET and glass substrate. For glass substrate, it is seen that as the sputtering power increases from 45 to 90 W, the resistivities of the films decreased continuously from 10.32 104 U cm to 3.04 104 U cm. However, for PET substrate, the resistivities of the films first decreases and then increases when the sputtering power increases from 45 to 90 W. The resistivities of TZO films deposited on glass and PET substrate are nearly equal when sputtering power increases from 45 W to 60 W, But, the resistivities of TZO films deposited on PET substrate increase quickly when the sputtering power more than 60 W. At low sputtering power, the sputtered species has low surface mobility on the substrate, which result in degraded crystallinity and few Ti substitution, thus the film has low hall mobility (m) and carrier concentration (N). From the relation r ¼ 1/(Nem), get the
40
50
60
70
80
90
Sputtering power/W Fig. 4. Electrical resistivity of TZO films deposited on PET and glass substrate as a function of sputtering power.
higher resistivity. With the sputtering power increasing, the species kinetic energy increases, for glass substrate, which improves the film crystallinity and Ti substitution. Lead to the Hall mobility and carrier concentration increase, the resistivity decrease. But for PET substrate, with the sputtering power increasing exceeds 60 W, the large species kinetic energy will make the substrate have a big damage and deformation. The crystalline quality of TZO films deteriorated, resulting in rapid increase of resistivity. The lowest resistivity of TZO films deposited on PET substrate was about 4.72 104 U cm for a thickness of 262 nm (sheet resistance is 18 U/ sq) at 60 W sputtering power. 3.5. Optical properties of TZO films Fig. 5 shows the optical transmittance as a function of the wavelength for the samples deposited on PET substrate at different sputtering power. The estimated values of Average transmittance of the films grown at different sputtering power are listed in Table 1. It can be seen that as the sputtering power increases from 45 to 90 W, the Average transmittance in the range of 400e760 nm of the films decrease from 95.73% to 78.94%. When the puttering power is 60 W, the films deposited on PET substrate which has the lowest resistivity have a high transmittance of above 92% in the visible range.
18 100
80
14
Transmittance / %
Growth rate/nm/min
16
12 10
90W
60
45W
60W
75W
40
8 20
6 40
50
60
70
80
90
Sputtering power/W Fig. 3. Growth rate of TZO films deposited on PET substrate as a function of sputtering power.
0
400
500
600
700
800
900
Wavelength /nm Fig. 5. Optical transmittance as a function of wavelength for TZO films deposited on PET substrate.
486
H. Liu, C. Lei / Vacuum 86 (2011) 483e486
4. Conclusions
References
Highly transparent and conducting TZO thin films were successfully prepared on PET substrate by DC magnetron sputtering at room temperature. The effect of the sputtering power on the structural, morphological, electrical and optical properties of TZO films was investigated in detail. All the deposited films were highly textured along the c-axis and perpendicular to the surface of the substrate. As the sputtering power increases, the electrical resistivity of TZO films first decreases then increase, the Average transmittance in the range of 400e760 nm of the films decrease. When the sputtering power is 60 W, it is obtained that the lowest resistivity of TZO films deposited on PET substrate is 4.72 104 U cm. And it have a high average transmittance of above 92% in the visible range.
[1] Hun Park Ji, Byun Dongjin, Lee Joong Kee. Optical and electrical properties of fluorine-doped galliumetin oxide composite films on PET prepared by ECRMOCVD. Surface & Coatings Technology 2010;205:S144e5. [2] Zhang Huafu, Lei Chengxin, Liu Hanfa, Yuan Changkun. Low-temperature deposition of transparent conducting ZnO: Zr films on PET substrates by DC magnetron sputtering. Vacuum 2010;85:184e6. [3] Yang TL, Zhang DH, Ma J, Ma HL, Chen Y. Transparent conducting ZnO: Al films deposited on organic substrates deposited by r.f. magnetron-sputtering. Thin Solid Films 1998;326:60e2. [4] Hao Lei, Diao Xungang, Xu Huaizhe, Gu Baoxia, Wang Tianmin. Thickness dependence of structural, electrical and optical properties of indium tin oxide (ITO) films deposited on PET substrates. Applied Surface Science 2008;254: 3504e8. [5] Yen WT, Lin YC, Yao PC, Ke JH, Chen YL. Growth characteristics and properties of ZnO: Ga thin films prepared by pulsed DC magnetron sputtering. Applied Surface Science 2010;256:3432e7. [6] Callity B. Element of X-ray diffraction. London: Addison-Wesley; 1959. p. 99. [7] Kumar Rajesh, Khare Neeraj, Kumar Vijay, Bhalla GL. Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering. Applied Surface Science 2008;254:6509e13. [8] Yu Xuhu, Ma Jin, Ji Feng, Wang Yuheng, Zhang Xijian, Cheng Chuanfu, et al. Effects of sputtering power on the properties of ZnO: Ga films deposited by r.f. magnetronsputtering at low temperature. Journal of Crystal Growth 2005;274:474e9.
Acknowledgements Authors are thankful to Shandong Province Natural Science Foundation for financial support under Contract No. ZR2009GL015.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Short communication
Low-temperature deposited Titanium-doped zinc oxide thin films on the flexible PET substrate by DC magnetron sputtering Hanfa Liu*, Chengxin Lei School of Science, Shandong University of Technology, 12 Zhangzhou Road, Zibo, 255049 Shandong, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 September 2011 Received in revised form 4 October 2011 Accepted 5 October 2011
Transparent conducting Titanium-doped zinc oxide thin films (TZO) with high transparency and relatively low resistivity were firstly deposited on water-cooled polyethylene terephthalate (PET) substrates at room temperature by DC magnetron sputtering. The microstructure, optical and electrical properties of the deposited films were investigated and discussed. The XRD patterns show that all the deposited films are polycrystalline with a hexagonal structure and have a preferred orientation along the c-axis perpendicular to the substrate. The electrical resistivity decreases when the sputtering power increases from 45 W to 60 W. However, as the puttering power continue increases from 60 W to 90 W, the electrical resistivity increases rapidly. When the puttering power is 60 W, the films deposited on PET substrate have the lowest resistivity of 4.72 104 U cm and a relatively high transmittance of above 92% in the visible range. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Titanium-doped zinc oxide thin films Thin films Magnetron sputtering Stress
1. Introduction Flexible substrates transparent conducting oxide (TCO) thin films are widely studied for applications such as in liquid crystal displays (LCD), thin film solar cells, personal digital assistants (PDA), cell phone and digital camer, etc [1,2]. In recent years, Sndoped In2O3(ITO) and Al-doped ZnO (ZnO:Al) on flexible substrates have been extensively studied [3,4]. But in our knowledge, no report was published for TZO films prepared on flexible substrates up to now. Comparing with ITO and ZnO:Al films, TZO films have the lower best film forming temperature, low cost, innocuity, good stability (in plasma) and low chemical reactivity. In our work, TZO thin films were firstly deposited on PET substrates by DC magnetron sputtering at room temperature. The dependence of the properties of TZO thin films on sputtering power has been studied. 2. Experimental 2.1. Sample preparation The TZO films on PET substrates were deposited in a GJP500C2 model DC magnetron sputtering system at room temperature. The * Corresponding author. Tel.: þ86 15965521318. E-mail address: [email protected] (H. Liu). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.10.006
vacuum chamber was evacuated to a base pressure of 4.5 104 Pa at room temperature. A sintered ceramic with a mixture of ZnO (99.99% purity) and TiO2 (99.99% purity) was employed as source material. The content of TiO2 added to the ZnO target was 2.1 wt.%. The diameter of the target was 75 mm and the thickness of target was 3 mm. The targetesubstrate distance was about 60 mm. During the process of deposition, the sputtering time and the Ar pressure were controlled at 25 min and 4.0 Pa, respectively. Prior to the deposition, the PET substrates were ultrasonically cleaned in acetone for 10 min, marinated in alcohol for 10 min and washed by purified water. In order to investigate the effect of sputtering power on the properties of TZO films, the sputtering power was varied from 45 W to 90 W. 2.2. Sample testing We used a D8 ADVANCE XRD using CuKa1 radiation (l ¼ 0.15406 nm) to analyse the structural properties of the TZO films. A Sirion 200 scanning electronmicroscope (SEM) was used to investigate the surface morphologies of the deposited films. The sheet resistivities (R) were measured with a four-probe instrument. The thickness (l) of the films was measured using an SGC-10 thin film thickness tester. The optical transmittance were measured with UVevis spectrophotometers at room temperature. The resistivity of TZO films was evaluated using the formula r ¼ Rl.
484
H. Liu, C. Lei / Vacuum 86 (2011) 483e486
50000
Intensity/(a.u.)
40000 30000 20000 (002)
10000
P=90W P=75W P=60W P=45W Substrate
0 20
30
40
50 2 θ /(°)
60
70
80
90
Fig. 1. XRD spectrum of the TZO films deposited on PET substrate.
the non-crystalline region in grain boundary. From the XRD patterns, one can infer that Ti atoms substitute in the hexagonal lattice or probably segregate to the non-crystalline region in grain boundaries. Fig. 1 also shows that, when the sputtering power increases from 45 W to 90 W, the intensity of (002) peak were 4.9%, 8.6%, 15.1% and 16.3% respectively. This shows that with the sputtering power increases, the (002) peak becomes more intense and sharper. This is due to the polycrystalline of the resulting films being improved and grain size becoming larger when elevating the sputtering power. The average crystallite size for these samples was about 16.25e22.99 nm estimated from the XRD spectrum by using Scherre formula [6]. Fig. 2 shows the SEM images of the surface morphologies of TZO films deposited at different sputtering power. It is clearly observed that the grain size and the surface roughness increase rapidly with sputtering power increasing, which is in good consistent with our XRD results as shown in Fig. 1. 3.2. Stress analysis of the TZO films
3. Results and discussion 3.1. Structural characterization of TZO films Fig. 1 shows the X-ray diffraction spectrum of the PET substrate and the sample TZO films deposited on water-cooled PET substrate at different sputtering power. It can be seen that apart from the XRD patterns of the substrate, only the (002) peak is observed at about 2q ¼ 34.55 for all sample films. These values are very close to that of the standard ZnO crystal (34.45 ). This indicates that all of the obtained films were polycrystalline with the hexagonal wurtzite structure and had a preferred orientation with the c-axis perpendicular to the substrates almost independent of the sputtering power [5]. No Ti2O2 phase was found from the XRD patterns, which implies that Ti atoms replace Zn in the hexagonal lattice or Ti segregate to
Stress of thin films is a very important parameter for the practical application. The total stress in thin films commonly consists of two components. One is the intrinsic stress introduced by impurities, defects and lattice distortions in the crystal, and the other is the extrinsic stress introduced by the lattice mismatch and thermal expansion coefficient mismatch between the film and substrate [7]. For hexagonal crystals, the stress (s) in the plane of the film can be calculated using the formula [7].
s ¼ 453:6 109 ðc c0 Þ=c0
(1)
where “c” is the lattice parameter of the ZnO film calculated from (002) peak of XRD pattern and the “c0” is the lattice parameter for the ZnO bulk (c0 ¼ 0:52065 nm). The estimated values of stress “s” in the films grown at different sputtering power are listed in Table 1
Fig. 2. SEM images of TZO films at different power. (a) 45W (b) 60 W (c) 75 W (d) 90 W.
H. Liu, C. Lei / Vacuum 86 (2011) 483e486 Table 1 The stress and average transmittance of TZO thin films at different sputtering power. 45
60
75
90
Thickness/nm Stress/GPa Average transmittance/%
166.2 0.8172 95.73
262 0.8050 92.85
345.7 2.1990 91.40
412.6 2.7687 78.94
From the Table 1, it is seen that the films are in a state of tensile stress and as the sputtering power increases from 45 to 90 W, the stress of the films deposited on PET substrate first decreased then increase. The lowest stress was about 0.8050 GPa for a thickness of 286 nm at 60 W sputtering power.
50
PET Substrate
40
Resistivity/ (10-4
Sputtering power/W
485
30
20
10
Glass substrate 3.3. Growth rate of TZO films
0
Fig. 3 shows the dependence of the growth rate on the sputtering power. It is observed that a clear increase in growth rate as sputtering power increase. The Growth rate and the sputtering power is basically a linear relationship. This increase indicates that the number of atoms sputtered from the target is proportional to the sputtering power [8]. When the sputtering power increase from 45 W to 90 W, the growth rate increase from 6.648 nm/min to 16.504 nm/min. 3.4. Electrical properties of TZO films Fig. 4 shows the sputtering power dependence of the resistivity for the TZO films deposited on PET and glass substrate. For glass substrate, it is seen that as the sputtering power increases from 45 to 90 W, the resistivities of the films decreased continuously from 10.32 104 U cm to 3.04 104 U cm. However, for PET substrate, the resistivities of the films first decreases and then increases when the sputtering power increases from 45 to 90 W. The resistivities of TZO films deposited on glass and PET substrate are nearly equal when sputtering power increases from 45 W to 60 W, But, the resistivities of TZO films deposited on PET substrate increase quickly when the sputtering power more than 60 W. At low sputtering power, the sputtered species has low surface mobility on the substrate, which result in degraded crystallinity and few Ti substitution, thus the film has low hall mobility (m) and carrier concentration (N). From the relation r ¼ 1/(Nem), get the
40
50
60
70
80
90
Sputtering power/W Fig. 4. Electrical resistivity of TZO films deposited on PET and glass substrate as a function of sputtering power.
higher resistivity. With the sputtering power increasing, the species kinetic energy increases, for glass substrate, which improves the film crystallinity and Ti substitution. Lead to the Hall mobility and carrier concentration increase, the resistivity decrease. But for PET substrate, with the sputtering power increasing exceeds 60 W, the large species kinetic energy will make the substrate have a big damage and deformation. The crystalline quality of TZO films deteriorated, resulting in rapid increase of resistivity. The lowest resistivity of TZO films deposited on PET substrate was about 4.72 104 U cm for a thickness of 262 nm (sheet resistance is 18 U/ sq) at 60 W sputtering power. 3.5. Optical properties of TZO films Fig. 5 shows the optical transmittance as a function of the wavelength for the samples deposited on PET substrate at different sputtering power. The estimated values of Average transmittance of the films grown at different sputtering power are listed in Table 1. It can be seen that as the sputtering power increases from 45 to 90 W, the Average transmittance in the range of 400e760 nm of the films decrease from 95.73% to 78.94%. When the puttering power is 60 W, the films deposited on PET substrate which has the lowest resistivity have a high transmittance of above 92% in the visible range.
18 100
80
14
Transmittance / %
Growth rate/nm/min
16
12 10
90W
60
45W
60W
75W
40
8 20
6 40
50
60
70
80
90
Sputtering power/W Fig. 3. Growth rate of TZO films deposited on PET substrate as a function of sputtering power.
0
400
500
600
700
800
900
Wavelength /nm Fig. 5. Optical transmittance as a function of wavelength for TZO films deposited on PET substrate.
486
H. Liu, C. Lei / Vacuum 86 (2011) 483e486
4. Conclusions
References
Highly transparent and conducting TZO thin films were successfully prepared on PET substrate by DC magnetron sputtering at room temperature. The effect of the sputtering power on the structural, morphological, electrical and optical properties of TZO films was investigated in detail. All the deposited films were highly textured along the c-axis and perpendicular to the surface of the substrate. As the sputtering power increases, the electrical resistivity of TZO films first decreases then increase, the Average transmittance in the range of 400e760 nm of the films decrease. When the sputtering power is 60 W, it is obtained that the lowest resistivity of TZO films deposited on PET substrate is 4.72 104 U cm. And it have a high average transmittance of above 92% in the visible range.
[1] Hun Park Ji, Byun Dongjin, Lee Joong Kee. Optical and electrical properties of fluorine-doped galliumetin oxide composite films on PET prepared by ECRMOCVD. Surface & Coatings Technology 2010;205:S144e5. [2] Zhang Huafu, Lei Chengxin, Liu Hanfa, Yuan Changkun. Low-temperature deposition of transparent conducting ZnO: Zr films on PET substrates by DC magnetron sputtering. Vacuum 2010;85:184e6. [3] Yang TL, Zhang DH, Ma J, Ma HL, Chen Y. Transparent conducting ZnO: Al films deposited on organic substrates deposited by r.f. magnetron-sputtering. Thin Solid Films 1998;326:60e2. [4] Hao Lei, Diao Xungang, Xu Huaizhe, Gu Baoxia, Wang Tianmin. Thickness dependence of structural, electrical and optical properties of indium tin oxide (ITO) films deposited on PET substrates. Applied Surface Science 2008;254: 3504e8. [5] Yen WT, Lin YC, Yao PC, Ke JH, Chen YL. Growth characteristics and properties of ZnO: Ga thin films prepared by pulsed DC magnetron sputtering. Applied Surface Science 2010;256:3432e7. [6] Callity B. Element of X-ray diffraction. London: Addison-Wesley; 1959. p. 99. [7] Kumar Rajesh, Khare Neeraj, Kumar Vijay, Bhalla GL. Effect of intrinsic stress on the optical properties of nanostructured ZnO thin films grown by rf magnetron sputtering. Applied Surface Science 2008;254:6509e13. [8] Yu Xuhu, Ma Jin, Ji Feng, Wang Yuheng, Zhang Xijian, Cheng Chuanfu, et al. Effects of sputtering power on the properties of ZnO: Ga films deposited by r.f. magnetronsputtering at low temperature. Journal of Crystal Growth 2005;274:474e9.
Acknowledgements Authors are thankful to Shandong Province Natural Science Foundation for financial support under Contract No. ZR2009GL015.