1.Introduction

Recently transparent conduction oxides (TCO) widely used in field of optoelectronic devices, such as touch screen, thin film solar cells and liquid crystal display, has been received extensively attention[1-2]. Among all the TCOs, indium tin oxide (ITO) has been most widely used because of its high work function, high transmittance and low resistivity[3]. Nevertheless, the application of ITO is limited due to not only the indium is a rare and expensive, but also toxic element. Compared with ITO, zinc oxide (ZnO) is nontoxic, inexpensive and abundant, so it has been actively investigated as an alternative material to ITO[4-5]. Group IIIA elements such as Al, In, Ga and B have been widely used as n-type dopants for ZnO[6-7]. Among these elements, Al is able to lower the electrical resistivity of ZnO efficiently. Now there are many methods to prepare Al-doped ZnO (ZAO) thin films, such as spray pyrolysis[8-10], sol-gel[11], chemical vapor deposition(CVD)[12-13], magnetron sputtering[14-15] and so on. The magnetron sputtering is becoming the most widely used techniques of preparation of all kinds of thin films due to low deposition temperature, simple process, inexpensive equipment and suitable for the growth of large area. There are many reports about the preparation of ZnO thin films by magnetron sputtering in the literature[14-19], most researches have been studied parameters which influences the quality of the film such as target-substrate distance, sputtering power, substrate temperature, amount of Al-doped, annealing temperature and annealing time etc single relationship between process parameters. However, it was rarely reported that the synchronous study on the influence of oxygen partial pressure and substrate temperature on the structures and properties of ZAO thin films deposited by the method of DC magnetron sputtering[20-22].

BDBO主要针对畸形的顶点或者旋转轴的中心位于椎间盘区域的患者，或者重度矢状位畸形需要截骨的范围超过PSO所能达到的最大截骨角度时也可应用BDBO。BDBO不适用于椎间融合及脊柱前柱钙化或AS患者。

“那么试问，古代发行千万枚的钱难道都是纯手工制作的，每一枚都人工打磨，修饰吗？答案是否定的。虽然最初钱币有方孔就是为了方便后期人工打磨，但是并不是一枚一枚的精雕细琢，而是把百十来枚一起串在方形的铁棍上，一同打磨，绝对不会偏爱哪一枚，唯独对你的这几枚多下心血。”老贾继续解释。“所以说孟导演你这批钱大有文章啊，我看应该是清末坊间对照图谱仿造的前朝钱币。”

In this work, a series of Al-doped ZnO (ZAO) thin films were fabricated on glass substrate at different oxygen partial pressures and substrate temperatures by direct current magnetron sputtering. The structures, optical properties and electrical resistivity of ZAO thin films were analyzed in detail by X-ray diffraction, UV visible spectrophotometer and four-point probe measurement system. The results might have potential important applications on the preparation of Al-doped zinc oxide thin films of transparent conductor.

两百多年前，一只好奇的喜鹊叼着一粒红色的果实，从西庄山隔河对岸的山上款款飞来，落在树上歇息，正欲大饱口福时，树下一只狐狸吼叫了一声，喜鹊受了惊吓，松开金口。那颗椭圆形红色的果实随之落到了地面，没有被吞到喜鹊那美丽花白羽毛覆盖着的肚皮中去。随后那颗红色的果实在一堆狐狸粪便旁边萌生，借着自身以及狐狸粪便的营养，我诞生了，从一棵小草般的嫩芽长到枝繁叶茂。现在的我已经有农村烧水用的水壶粗细，用行话说直径也有一尺开外吧，两层楼高的树干，使得我可以傲视山峦和丛林。更绝的是，我的分枝极有层次和分寸，九个分枝在不同的层次，朝不同的侧面展开，每个分枝上长满浓绿的叶子，就像九团绿云飘在空中。

2.Experimental

Fig.1 shows the X-ray diffraction of ZAO thin films deposited at different oxygen partial pressures, it can be observed from Fig.1 that (002) peak is the only observable one for all the films, the location of ZnO (002) peak is in the range 34.2°～34.3°, which implies that all the ZAO thin films grow along c-axis perpendicular to substrate and are of good crystal quality. The intensity of the diffraction increases with the oxygen partial pressure increasing up to 100: 5 and then decreases if further increases. When the oxygen partial pressure is lower, zinc atoms are not fully reacted causes some defects such as zinc interstitial, which affects the crystalline quality of the thin film. With the increase of the oxygen partial pressure the proportion of zinc and oxygen atoms approaches the stoichiometric ratio, the concentration of zinc gap is gradually reduced in the film, so the crystalline quality of the film is improved. With the further increase of the oxygen partial pressure, the concentration of free oxygen in the film also increases, which leads the concentration of grain-boundary increases, then the grain size and crystalline quality decreases. The structural parameters of ZAO thin films deposited at different oxygen partial pressures are given in Table 1.

当前IoT系统普遍存在多个管理域，如智慧小区、智慧学校、智慧医院等，出于安全考虑，不同管理域之间是彼此“绝缘”的，如何实现IoT跨域的互联互通及相关安全协议的研究成为新型IoT安全增强技术的重要目标之一。

The structure and crystallinity of the ZAO thin films were investigated by X-ray diffraction (XRD) (Rigaku D/Max-B with Cu kα radiation, λ=0.15408 nm). The optical properties were acquired in a Shimadzu 2500 UV-VIS-NIR double beam spectrophotometer from 200nm to 800nm. The sheet resistance was examined by a four-point probe measurement system at RT.

3. Results and discussion

3.1 The influence of oxygen partial pressure on the structures and properties

Al-doped ZnO thin films were deposited on glass substrates by direct current magnetron sputtering using a Zn∶Al alloy(w/w, 98∶2) target with the purity of 99.99%. The target was a circular disc with a size of 51 mm in diameter and 5 mm in thickness. The distance between the target and the substrate was 130 mm. Before deposition, the chamber was pumped to a base pressure of 2×10-4Pa. The sputtering gas Ar with a purity of 99.9% and the reactive gas O2 with a purity of 99.9% were introduced into the chamber separately and controlled by the standard mass flow controllers, the sputtering current was fixed at 0.15 A. The experiments were divided into two groups: (1) Setting the constants of substrate temperature (400 ℃) and deposition time (30 min), and the ratios of Ar/O2 flow vary from 100∶3, 100∶5, 100∶7, to 100∶9; (2) with the constants of Ar/O2 flow ratio (100∶5) and deposition time of 30 min, and substrate temperatures vary from room temperature (RT), 200℃, 400℃, to 500 ℃.

Fig.1 X-ray diffraction patterns of ZAO thin films deposited at different oxygen partial pressures

Tab.1 The Structural parameters of ZAO thin films depositedat different oxygen partial pressures

Ar/O2ratio2θ(°)FWHM(°)Grainsize(nm)100/334.340.8110.15100/534.210.3027.40100/7///100/934.190.5116.12

Fig.3 shows the dependence of the electrical properties of the ZAO thin films on the different oxygen partial pressures. We found that the sheet resistance first decrease with an increase in the Ar/O2 flow ratio of 100∶5, and then increases rapidly with further increase of the oxygen partial pressures. Moreover, under the same condition, the electrical conductivity of the thin films deposited at 100∶5 are always better than those at the other Ar/O2 flow ratios, and the film with sheet resistance is as low as 80 Ω/sq. The electrical conductivity increase can be attributed to the improvement in crystal quality as crystallite size increases and the carrier lifetime and concentration increase[23-25]. With further increase of the oxygen partial pressure, the concentration of free oxygen in the thin film also increases, the doped Al3+ mostly combines with oxygen to generate aluminum-oxygen compounds, then the probability of aluminum substituting for zinc decreased, which affects the migration rate of the carrier. As a result, the sheet resistance increases.

Fig.2 Optical transmittance spectra of ZAO films with different oxygen partial pressures

The transmittance spectra of ZAO thin films grown at different oxygen partial pressures are shown in Fig.2. The average transmittance is approximately 80% for all the deposited thin films, the transmission spectrum in the visible optical region is transparent, and the transmittance increases with the increase of oxygen partial pressures. Because of fundamental absorption in the vicinity of the band gap, transmittance presented increased to an extreme value and then slightly decreased. The transmittance decreases acutely as the wavelength reaches the ultraviolet range. The absorption edge move to the long wave direction with the increase of oxygen partial pressures, showed a red-shift.

Fig.3 Dependence of the sheet resistance for ZAO films on different oxygen partial pressures

3.2 The influence of deposition temperature on the structures and properties

The transmittance spectra of ZAO films grown at different deposition temperatures are shown in Fig.5 Obviously, the deposition temperature have a significant effect on the optical transmittance of ZAO thin films. The transmittance of the thin film deposited at RT is the lowest, and with the deposition temperature increasing, the transmittance increases in the visible region, and the absorption edge exhibits an obvious blue-shift. This phenomenon can be explained by the energy gap width in the Burstein-Moss effect[26].

Fig.4 X-ray diffraction patterns of ZAO thin films at different deposition temperatures

Tab.2 The Structural parameters of ZAO thin filmsat different deposition temperatures

Depositiontemperature(℃)2θ(°)FWHM(°)Grainsize(nm)RT34.160.4817.1220034.170.4020.5540034.210.3027.4050034.330.3324.92

Fig.5 Optical transmittance spectra of ZAO films with different deposition temperatures

According to the experimental results above, the ZAO thin film could get the best crystalline quality with the Ar/O2 flow ratio of 100∶5, we choose the Ar/O2 flow ratio of 100∶5 and deposition time of 30 min as fundamental conditions, to study the effect of deposition temperature on the structure and properties of the ZAO thin films. Fig.4 shows the X-ray diffraction of ZAO thin films deposited at different deposition temperatures. It’s obvious that the crystalline quality of the film deposited at RT is poor with three very weak diffraction peaks corresponding ZnO(110),(002),(101)crystal face, respectively. The film gradually showed c-axis perpendicular with the increase of the deposition temperature, the diffraction peak intensity increases markedly with an deposition temperature up to 400℃, and then decreases sharply with further increase of the deposition temperature. This is mainly because with the increase of the deposition temperature, the internal defects of the ZAO thin films is reduced and the film can get a larger energy which makes the grain size increases in the deposition process. The lower the defect density of grain-boundary is, the greater the intensity of the diffraction peak increases. The structural parameters of ZAO thin films deposited at different deposition temperatures are given in Table 2.

Fig.6 shows the change of the resistivity of the films as a function of the deposition temperature for ZAO thin films. As the deposition temperature increases, the resistivity decreases sharply at temperature below 400 ℃, this decrease in resistivity may be related to the decrease of the grain-boundary defect in the thin film and the improvement of the crystalline quality. However, as the deposition temperature further increases, the resistivity increases slightly. This is probably the result of that high temperature can cause the slight damage of the thin film in crystalline quality, which is confirmed by the results of XRD measurements, and then the carrier mobility in the thin film was lowered.

Fig.6 Dependence of the sheet resistance for ZAO films on different deposition temperature

4.Conclusions

In summary, high-quality ZAO thin films were deposited by DC reactive magnetron sputtering. All of the deposited thin films exhibited polycrystalline with a hexagonal structure stacked along the c-axis perpendicular to the substrate. The results show that the structures, optical and electrical properties of ZAO thin films have been significant effect by oxygen partial pressure and deposition temperature. The deposited films with a low sheet resistance of 80 Ω/sq, an average transmittance of 80% in the visible range were obtained with a Ar/O2 flow ratio of 100∶5 and deposition temperature of 400 ℃. The results might have important applications on the preparation of aluminum-doped zinc oxide thin films of transparent conductor with deposition method of DC magnetron sputtering.

The authors appreciatively grateful the financial support by the National Natural Science Foundation of China (No. 21761012).

References

[1] Ma H B, Cho J S, Park C H. A Study of Indium Tin Oxide Thin Film Deposited at Low Temperature Using Facing Target Sputtering System[J]. Surf. Coat. Technol. 2002, 153(2/3):131-137.

[2] Nisha M, Anusha S, Antony A, et al. Effect of Substrate Temperature on the Growth of ITO Thin Films[J]. Appl. Surf. Sci. 2005, 252 (5):1430-1435.

[3] Hamberg I,Granqvist C G.Evaporated Sn Doped In2O3 Films: Basic Optical Properties and Applications to Energy Efficient Windows[J]. J. Appl. Phys. 1986, 60 (11):R123-R160.

[4] Henley S J, Ashfold M N R, Cherns D. The Growth of Transparent Conducting ZnO Films by Pulsed Laser Ablation[J]. Surf. Coat. Technol. 2004, 177/178:271-276.

[5] Ataev B M, Bagamadova A M, Mamedov V V, et al. Highly Conductive and Transparent Thin ZnO Films Prepared in Situ in a Low Pressure System[J]. J. Cryst. Growth. 1999, 198/199:1222-1225.

[6] Hiramatsu M, Imaeda K, Horio N, et al. Transparent Conducting ZnO Thin Films Prepared by XeCl Excimer Laser Ablation[J]. J. Vac. Sci. Technol. A. 1998, 16(2): 669-673.

[7] Chen M, Pei Z L, Sun C, et al. ZAO: An Attractive Potential Substitute for ITO in Flat Display Panels[J]. Mat. Sci. and Eng B. 2001, 85(2/3):212-217.

[8] Chopra K L, Paulson P D, Dutta V. Thin Film Solar Cell: An Overview [J]. Progress in Photovoltaics: Research and Applications. 2004, 12(2/3):69-92.

[9] Volintiru I, Creatore M, Kniknie B J, et al. Evolution of the Electrical and Structural Properties During the Growth of Al Doped ZnO Films by Remote Plasma-enhanced Metalorganic Chemical Vapor Deposition [J].J. Appl. Phys. 2007, 102(4):043709.

[10] Ma Q B, Ye Z Z, He H P, et al. Structural, Electrical, and Optical Properties of Transparent Conductive ZnO: Ga Films Prepared by DC Reactive Magnetron Sputtering [J]. Journal of Crystal Growth. 2007, 304(1):64-68.

[11] Schuler T, Aegerter M A. Optical, Electrical and Structural Properties of Sol-Gel ZnO: Al Coating [J].Thin Solid Films. 1999, 351(1/2):125-131.

[12] Fa S, Feitknecht L, Schlüchter R, et al. Rough ZnO layers by LP-CVD Processand Their Effect in Improving Performance of Amorphous and Microcryst-alline Silicon Solar Cells[J]. Solar Energy Materials and Solar Cells. 2006, 90(18/19):2960-2967.

[13] Fa S, Steinhauser J, Oliveira N, et al. Opto-Electronic Properties of Rough LP-CVD ZnO: B for Use as TCO in Thin-Film Silicon Solar Cells[J].Thin Solid Films. 2007, 515(24):8558-8561.

[14] Müller J, Schöpe G, Kluth O, et al. Upscaling of Texture-Etched Zinc Oxide Substrates for Silicon Thin Film Solar Cells[J]. Thin Solid Films. 2001, 392(2): 327-333.

[15] Hong R J, Jiang X, Szyszka B, et al. Studies on ZnO:Al Thin Films Deposited by in-Line Reactive Mid-Frequency Magnetron Sputtering[J]. Applied Surface Science. 2003, 207 (1-4):341-350.

[16] Lu F, Xu C H, Wen L S, et al. Analysis on the Process of ZAO Films by DC Magnetron Reactive Sputtering[J]. Science China. 2011, 54 (1):28-32.

[17] Li C P, Yang B H, Chen X M, et al. Effects of Annealing on the Characteristics of ZnO Films Deposited in Various O2/(O2+Ar) Ratios[J]. Optoelectronics Letters. 2010, 16 (4):0284-0287.

[18] Wang J Z, Elangovan E, Franco N, et al. Influence of Oxygen Partial Pressure on Properties of N-doped ZnO Films Deposited by Magnetron Sputtering[J]. Trans. Nonferrous Met. Soc. China. 2010, 20(12):2326-2330.

[19] Tang N, Wang J L, Xu H X, et al. Optical Characterization of ZnO Thin Films Deposited by RF Magnetron Sputtering Method[J]. Science in China Series E: Technological Sciences. 2009, 52 (8):2200-2203.

[20] Zhang H F，Liu R J，Liu H F，et al. Effect of Substrate Temperature on Properties of ZnO:Zr Transparent Conductive Thin Films Deposited by DC Magnetron Sputtering[J]. Journal of Synthetic Crystals. 2010, 39(3):766-770.

[21] Feng C C, Zhou M, Wu C X, et al. Effect of Oxygen Partial Pressure on the Morphology of ZnO Nanostructure Prepared by Chemical Vapor Deposition[J]. Journal of Synthetic Crystals. 2009, 38(3):657-661.

[22] Yan N N，Wang H，Liu J，Zuo Y. Influence of Oxygen Partial Pressure on Properties of ZnO:Al Thin Films Fabricated at Room Temperature[J]. Journal of the Chinese Ceramic Society. 2013, 41(9):1251-1257.

[23] Lv M S, Xiu X W, Pang Z Y, et al. Transparent Conducting Zirconium-doped Zinc Oxide Films Prepared by RF Magnetron Sputtering[J]. Appl. Surf. Sci. 2005, 252 (5):2006-2011.

[24] Hao X T, Ma J, Zhang D H, et al. Thickness Dependence of Structural, Optical and Electrical Properties of ZnO:Al Films Prepared on Flexible Substrates[J]. Appl. Surf. Sci. 2001, 183:(1/2):137-142.

[25] Zhang H F, Liu H F, Zhou A P, et al. Influence of the Distance Between Target and Substrate on the Properties of Transparent Conducting Al-Zr Co-doped Zinc Oxide Thin Films[J]. Journal of Semiconductors. 2009, 30 (11):113002.

[26] Sernelius B E, Berggren K F, Jin Z C, et al. Band-gap Tailoring of ZnO by Means of Heavy Al Doping[J]. Physics Review B. 1988, 37:10244-10248.