射频磁控溅射法制备氧化锌薄膜及其特性的研究
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摘要
ZnO是一种新型的Ⅱ-Ⅵ族直接带隙化合物半导体，禁带宽度为3.37eV，激子束缚能为60meV，具有六方纤锌矿结构。ZnO薄膜具有广泛的应用，如ZnO薄膜可以制成表面声波谐振器、压电器件、压敏器件、透明电极、气敏传感器、导电膜等。近年来，随着短波器件的广泛应用，直接宽带半导体材料的研究越来越受人们的重视，对氧化锌薄膜材料的研究和开发在国内外科学界及工业部门引起了极大的关注和兴趣。
ZnO薄膜制备的主要方法有：磁控溅射、金属有机化学气相沉积、脉冲激光沉积、分子束外延、电子束蒸发沉积、喷雾热分解、溶胶-凝胶法、薄膜氧化法等。各种方法各有优缺点。根据需要制备相应的高质量的薄膜是ZnO薄膜应用的关键，同时制备成本也是必须考虑的重要因素。通常认为理想ZnO薄膜具有高的c轴择优取向。磁控溅射在最佳条件下可以得到均匀、致密、有良好的c轴取向性和可见光波段透明性好等优点的薄膜，使得它成为在ZnO制备中研究最多并且使用最广泛的方法。本课题采用RF反应磁控溅射制备了ZnO薄膜并对其进行探索性研究。研究内容主要包括：射频磁控溅射工艺条件对ZnO薄膜结构特性和表面形貌的影响、对ZnO薄膜光学特性的影响，以及ZnO发光机理的探讨。薄膜的结构特性用XRD进行了分析，薄膜的表面形貌通过原子力显微镜进行表征，薄膜的透射光谱用紫外-可见双光束分光光度计进行测量，发光性质用光栅光谱仪进行了分析。
研究结果表明利用射频磁控溅射法工艺，在功率为100W，真空度为2×10~(-4)Pa，靶基距为70mm左右，溅射时间为60分钟，基片为单面抛光的(100)硅片，基片温度200℃、氩氧比为2∶3的条件下得到了结晶质量良好的ZnO薄膜；通过退火可以使薄膜应力得到驰豫，降低缺陷浓度，改善薄膜的结构特性。本实验采用射频磁控溅射的方法，探索出制备ZnO薄膜的最佳工艺条件，最终在(100)硅衬底基片上制备出了较高c轴取向的ZnO薄膜。但是由于试验条件限制，在发光试验的测试中，我们只观察到了在350nm处的一个明显的发射峰和在480nm处的微弱的峰，对此本文也进行了探索性分析。
Zinc oxide, one of the most interestingⅡ-Ⅳcompound semiconductors, is a direct semiconductor of wurtzite structure. It is a wide-band-gap of 3.37eV and has a large excitation binding energy of 60meV. ZnO thin films have many potential applications in various domains, such as surface and bulk acoustic wave devices(SAW) and piezoelectric parts, sense organ and transparent conducting films/electrodes, and gas sensor. In recent years, wide-band-gap semiconductor compounds have attracted a great deal of attention because of the intense commercial interest in developing practical short-wavelength semiconductor diode lasers for the huge market needs, the researches and developments of ZnO films have attracted great attention and interest from researchers and the industry.
To achieve the requirements of different applications, a number of techniques have been used for fabrication of ZnO thin films, including magnetron sputtering, metal organic chemical vapor deposition, pulsed laser deposition, molecular beam epitaxy, spray pyrolysis, and sol-gel process, reactive deposition and thermal oxidation of Zn films. Each of these techniques has its merits and demerits. The key to the application of ZnO films is high quality for the purpose, and the cost must be considered. For various applications, highly preferred c-axis orientation of ZnO films is usually important. Uniform and compact ZnO films with the highly preferred c-axis orientation and the high transmittance in visible region can be prepared by magnetron sputtering under optimized condition. It is the most investigated and widely used method. This thesis studied how to obtain outstanding ZnO Films by RF Magnetron sputtering. The content include: Influences of technical conditions on the structure, surface morphology, and light emitting of ZnO films by RF Magnetron Sputtering, luminescence mechanism of ZnO. Structural properties of ZnO are studied By X-ray Diffraction and Atomic Force Microscopy(AFM), light Emitting characteristic is tested by photoluminescence experimentation.
We got good crystallization ZnO films according to the process conditions: sputtering power is about 100W, the pressure is 2×10-4pa, the substate is Si (100) substate-to-target separation is 60mm, deposition time is 60 min, the substrate temperature is 200℃and Ar: 02 =2:3. Thermal annealing can improve the structural properties of ZnO, Highly c-axis oriented ZnO films on (100) silicon substrate were obtained by RF Magnetron sputtering. But in light Emitting experimentation, we just observed a obvious 350 nm photoluminescence(PL) peak and two feeble PL peaks in 480 nm, we discuss this result and clarify bur viewpoint.
ZnO薄膜制备的主要方法有：磁控溅射、金属有机化学气相沉积、脉冲激光沉积、分子束外延、电子束蒸发沉积、喷雾热分解、溶胶-凝胶法、薄膜氧化法等。各种方法各有优缺点。根据需要制备相应的高质量的薄膜是ZnO薄膜应用的关键，同时制备成本也是必须考虑的重要因素。通常认为理想ZnO薄膜具有高的c轴择优取向。磁控溅射在最佳条件下可以得到均匀、致密、有良好的c轴取向性和可见光波段透明性好等优点的薄膜，使得它成为在ZnO制备中研究最多并且使用最广泛的方法。本课题采用RF反应磁控溅射制备了ZnO薄膜并对其进行探索性研究。研究内容主要包括：射频磁控溅射工艺条件对ZnO薄膜结构特性和表面形貌的影响、对ZnO薄膜光学特性的影响，以及ZnO发光机理的探讨。薄膜的结构特性用XRD进行了分析，薄膜的表面形貌通过原子力显微镜进行表征，薄膜的透射光谱用紫外-可见双光束分光光度计进行测量，发光性质用光栅光谱仪进行了分析。
研究结果表明利用射频磁控溅射法工艺，在功率为100W，真空度为2×10~(-4)Pa，靶基距为70mm左右，溅射时间为60分钟，基片为单面抛光的(100)硅片，基片温度200℃、氩氧比为2∶3的条件下得到了结晶质量良好的ZnO薄膜；通过退火可以使薄膜应力得到驰豫，降低缺陷浓度，改善薄膜的结构特性。本实验采用射频磁控溅射的方法，探索出制备ZnO薄膜的最佳工艺条件，最终在(100)硅衬底基片上制备出了较高c轴取向的ZnO薄膜。但是由于试验条件限制，在发光试验的测试中，我们只观察到了在350nm处的一个明显的发射峰和在480nm处的微弱的峰，对此本文也进行了探索性分析。
Zinc oxide, one of the most interestingⅡ-Ⅳcompound semiconductors, is a direct semiconductor of wurtzite structure. It is a wide-band-gap of 3.37eV and has a large excitation binding energy of 60meV. ZnO thin films have many potential applications in various domains, such as surface and bulk acoustic wave devices(SAW) and piezoelectric parts, sense organ and transparent conducting films/electrodes, and gas sensor. In recent years, wide-band-gap semiconductor compounds have attracted a great deal of attention because of the intense commercial interest in developing practical short-wavelength semiconductor diode lasers for the huge market needs, the researches and developments of ZnO films have attracted great attention and interest from researchers and the industry.
To achieve the requirements of different applications, a number of techniques have been used for fabrication of ZnO thin films, including magnetron sputtering, metal organic chemical vapor deposition, pulsed laser deposition, molecular beam epitaxy, spray pyrolysis, and sol-gel process, reactive deposition and thermal oxidation of Zn films. Each of these techniques has its merits and demerits. The key to the application of ZnO films is high quality for the purpose, and the cost must be considered. For various applications, highly preferred c-axis orientation of ZnO films is usually important. Uniform and compact ZnO films with the highly preferred c-axis orientation and the high transmittance in visible region can be prepared by magnetron sputtering under optimized condition. It is the most investigated and widely used method. This thesis studied how to obtain outstanding ZnO Films by RF Magnetron sputtering. The content include: Influences of technical conditions on the structure, surface morphology, and light emitting of ZnO films by RF Magnetron Sputtering, luminescence mechanism of ZnO. Structural properties of ZnO are studied By X-ray Diffraction and Atomic Force Microscopy(AFM), light Emitting characteristic is tested by photoluminescence experimentation.
We got good crystallization ZnO films according to the process conditions: sputtering power is about 100W, the pressure is 2×10-4pa, the substate is Si (100) substate-to-target separation is 60mm, deposition time is 60 min, the substrate temperature is 200℃and Ar: 02 =2:3. Thermal annealing can improve the structural properties of ZnO, Highly c-axis oriented ZnO films on (100) silicon substrate were obtained by RF Magnetron sputtering. But in light Emitting experimentation, we just observed a obvious 350 nm photoluminescence(PL) peak and two feeble PL peaks in 480 nm, we discuss this result and clarify bur viewpoint.
引文
[1] Jin B J, etal. Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition[J]. Mater Sci Eng, 2000, B71: 301—305
[2] Tang Z K, Wong G K L, Yu P, et al. Room—temperature ultraviolet laser emissionfrom self—assembled ZnO microcrystallite thin films[J]. Appl Phys Lett., 1998, 72(25): 3270-3272
[3] Srikant V and D.R.Clarke. On the optical band gap of zinc oxide[J]. J Appl Phys,1998, 83 (10) : 5447-5451
[4] P. Drude, Z. Phys., 1(1900)161
[5] G Frank, E. Kauer and H. Kostlin, Transparent heat-reflecting coatings based on highly doped semiconductors [J]. Thin Solid Films,1981,77:107-118
[6] E. Burstein, J. W. Davisson, Infrared Photoconductivity due to Neutral Impurities in Germanium[J]. Phys.Rev. 1954,93:65-68
[7] Wang J Z, Du G T, Zhang Y T etal. RETRACTED: Luminescence properties of ZnO films annealed in growth ambient and oxygen[J]. J.Cryst.Growth.2004, 263: 269-272
[8] V. K. Jain and A. P. Kulshreshtha, Indium-Tin-Oxide transparent conducting coatings on silicon solar cells and their "figure of merit"[J]. Sol. Energy Mater,1981,4:151-158
[9] H. Angus Macleod, Structure-related optical properties of thin films[J]. J. Vac. Sci. Technol.,1986, A3:418-422
[10] K.Iwata, P.Fons, S.Niki, A.Yamada, K.Matsubara etal. ZnO growth on Si by radical source MBE[J]. J.Cryst.Growth 2000, 214-215: 50-54
[11] H.J.Ko, Y.F.Chen, Z.Zhu, T.Yao, etal. Photoluminescence properties of ZnO epilayers grown on CaF_2(111) by plasma assisted molecular beam epitaxy[J]. Appl.Phys.Lett. 2000,76:1905-1907
[12] M.N.Kamalasanan, S.Chandra, Sol-gel synthesis of ZnO thin films[J]. Thin Solid Films 1996,288:112-115
[13] Y.Natsume, H.Sakata, Zinc oxide films prepared by sol-gel spin-coating[J]. Thin Solid Films 2000,372:30-36
[14] J. F. Chang, M. H. Hon, The effect of deposition temperature on the properties of Al-doped zinc oxide thin films[J]. Thin Solid Films, 2001, 386: 79-86
[15] K. Tominaga, T. Murayama, N. Umezu, et al. Properties of films of multilayered ZnO: A1 and ZnO deposited by an alternating sputtering method[J]. Thin Solid Films, 1999, 343-344: 160-163
[16] K. B. Sundaram, A. Khan, Characterization and optimization of zinc oxide films by r. f. magnetron sputtering[J]. Thin Solid Films 1997, 295: 87-91
[17] A. Yamada, B. Sang, M. Konagai, Atomic layer deposition of ZnO transparent conducting oxides[J]. Appl. Surf. Sci 1997 112: 216-222
[18] H. K. Pulker and E. Jung, Correlation between film structure and sorption behaviour of vapour deposited ZnS, cryolite and MgF_2 films[J]. Thin Solid Films, 1972, 9: 57-66
[19] S. Ogura, Ph. D. Thesis, Newcastle upon Tyne Polytechnic, 1975
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[21] Y. Liu, C. R. Gorla, S. Liang, et al. Ultraviolet detectors based on epitaxial ZnO films grown by MOCVD. J Elec Mater, 2000, 29(1): 60
[22] 丘志仁，俞平，黄锦圣，等．氧化锌薄膜的室温受激紫外激光发射。中山大学学报(自然科学版)，1998，37(1)51—54
[23] Ohyama M, Kozuka H, Yoko T. Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution. Thin Solid Films, 1997, 306: 78—85
[24] Jeong S H, Boo J H. Influnence of target-to-substrate distance on the properties of AZO films grown by RF magnetron sputtering[J]. Thin Solid Films, 2004, 447-448: 105-110
[25] S. Major, Satyendra Kumar, M. Bhatnagar and K. L. Chopra, Effect of hydrogen plasma treatment on transparent conducting oxides[J]. Appl. Phys. Lett. 1986, 7: 394-396
[26] T. K. Subramanyam, B. Srinivasulu Naidu and S. Uthanna, Cryst. Res. Technol., 10, 35(2000)1193
[27] G. Haacke, New figure of merit for transparent conductors[J]. J. Appl. Phys. 1976, 47(9): 4086-4089
[28] Lee J B, Lee M H, Park C K, etal. Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of SAW devices[J]. Thin Solid Films, 2004,447-448:296-301
[29] V. K. Jain and A. P. Kulshreshtha, Indium-Tin-Oxide transparent conducting coatings on silicon solar cells and their "figure of merit"[J].Sol. Energy Mater.l981,4(2):151-158
[30] S. Ogura, Ph. D. Thesis, Newcastle upon Tyne Polytechnic, 1975
[31] M. Harris, H. A. Macleod, S. Ogura, et al., Thin Solid Films, 57 (1979)173
[32] Chen Y F, Hong S K, Ko H J, etal. Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch[J]. Appl Phys Lett,2001,78(21):3352-3354
[33] Koike K, Komuro T, Ogata K, etal. CaF_2 growth as a buffer layer of ZnO/Si heteroepitaxy[J]. Physica E,2004,21:679-683
[34] Nahhas A, Kim H K, Blachere J. Epitaxial growth of ZnO films on Si substrates using an epitaxial GaN buffer[J].Appl Phys Lett,2001,78(11):1511-1513
[35] Ashrafi A A, Ueta A,Kumano H,etal. Role of ZnS buffer layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy [J]. J Cryst Growth, 2000,221(1-4):435-439.
[36] Zhu X F, Lin B X, Liao G H, etal.The effect of Zn buffer layer on growth and luminescence of ZnO films deposited on Si substrates [J]. J Cryst Growth,1998,193(3):316-321.
[37] Bang K H, Hwang D K, Myoung J M. Effects of ZnO buffer layer thickness on properties of ZnO thin films deposited by radio-frequency magnetron sputtering[J]. Appl Surf Sci,2003,207:359-364.
[38] 李剑光,叶志镇,汪雷,等. "用MOVCD 在 ZnO/Al_2O_3 生长 GaN 及其特性", 半导体学报, 1999, 20(10): 862-866
[39] H. fabricius, T. Skettrup, P. Bisgaard, Ultraviolet detectors in thin sputtered ZnO films. Appl Optics, 1986, 25:2764
[40] Y. Liu, C. R. Gorla, S. Liang, et al. Ultraviolet detectors based on epitaxial ZnO films grown by MOCVD, J Elec Mater, 2000, 29:69
[41] S. Liang, H. Sheng, Y. Liu, et al. ZnO Schottky ultraviolet photodetectors, J Cryst Growth,2001, 225:110
[42] Bagnall D M, Chen Y F, Zhu Z, et al. High temperature excitonic stimulated emission from ZnO epitaxial layers. [J]. Appl.Phys.Lett., 1998, 73 (8) : 1038-1040
[43] B. D. Cullity, Elements of X-Ray Diffraction, Addition-Wes-ley, Reading, MA, 1978, pp. 102.
[44] A. Segmuller, M.Murakami, in: K. N. Tu, R. Rosenberg(Eds.), Analytical Techniques for Thin Films, Academic, Boston, 1988, pp. 143.
[45] R. Cebulla, R. Wendt, K, Ellmer, J. Appl. Phys. 83(1998) 1087.
[46] Jim Y J, Kim Y T, Yan H J, Park J C, Han J I, Lee Y E, Kim H J. Epitaxial growth of ZnO thin films on R-plane sapphire substrate by radio frequency magnetron sputtering[J]. J. Vac. Sci. Technol. A. 1997, 15(3): 1103-1107.
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[2] Tang Z K, Wong G K L, Yu P, et al. Room—temperature ultraviolet laser emissionfrom self—assembled ZnO microcrystallite thin films[J]. Appl Phys Lett., 1998, 72(25): 3270-3272
[3] Srikant V and D.R.Clarke. On the optical band gap of zinc oxide[J]. J Appl Phys,1998, 83 (10) : 5447-5451
[4] P. Drude, Z. Phys., 1(1900)161
[5] G Frank, E. Kauer and H. Kostlin, Transparent heat-reflecting coatings based on highly doped semiconductors [J]. Thin Solid Films,1981,77:107-118
[6] E. Burstein, J. W. Davisson, Infrared Photoconductivity due to Neutral Impurities in Germanium[J]. Phys.Rev. 1954,93:65-68
[7] Wang J Z, Du G T, Zhang Y T etal. RETRACTED: Luminescence properties of ZnO films annealed in growth ambient and oxygen[J]. J.Cryst.Growth.2004, 263: 269-272
[8] V. K. Jain and A. P. Kulshreshtha, Indium-Tin-Oxide transparent conducting coatings on silicon solar cells and their "figure of merit"[J]. Sol. Energy Mater,1981,4:151-158
[9] H. Angus Macleod, Structure-related optical properties of thin films[J]. J. Vac. Sci. Technol.,1986, A3:418-422
[10] K.Iwata, P.Fons, S.Niki, A.Yamada, K.Matsubara etal. ZnO growth on Si by radical source MBE[J]. J.Cryst.Growth 2000, 214-215: 50-54
[11] H.J.Ko, Y.F.Chen, Z.Zhu, T.Yao, etal. Photoluminescence properties of ZnO epilayers grown on CaF_2(111) by plasma assisted molecular beam epitaxy[J]. Appl.Phys.Lett. 2000,76:1905-1907
[12] M.N.Kamalasanan, S.Chandra, Sol-gel synthesis of ZnO thin films[J]. Thin Solid Films 1996,288:112-115
[13] Y.Natsume, H.Sakata, Zinc oxide films prepared by sol-gel spin-coating[J]. Thin Solid Films 2000,372:30-36
[14] J. F. Chang, M. H. Hon, The effect of deposition temperature on the properties of Al-doped zinc oxide thin films[J]. Thin Solid Films, 2001, 386: 79-86
[15] K. Tominaga, T. Murayama, N. Umezu, et al. Properties of films of multilayered ZnO: A1 and ZnO deposited by an alternating sputtering method[J]. Thin Solid Films, 1999, 343-344: 160-163
[16] K. B. Sundaram, A. Khan, Characterization and optimization of zinc oxide films by r. f. magnetron sputtering[J]. Thin Solid Films 1997, 295: 87-91
[17] A. Yamada, B. Sang, M. Konagai, Atomic layer deposition of ZnO transparent conducting oxides[J]. Appl. Surf. Sci 1997 112: 216-222
[18] H. K. Pulker and E. Jung, Correlation between film structure and sorption behaviour of vapour deposited ZnS, cryolite and MgF_2 films[J]. Thin Solid Films, 1972, 9: 57-66
[19] S. Ogura, Ph. D. Thesis, Newcastle upon Tyne Polytechnic, 1975
[20] M. Harris, H. A. Macleod, S. Ogura, et al. The relationship between optical inhomogeneity and film structure[J]. Thin Solid Films, 1979, 57: 173-178
[21] Y. Liu, C. R. Gorla, S. Liang, et al. Ultraviolet detectors based on epitaxial ZnO films grown by MOCVD. J Elec Mater, 2000, 29(1): 60
[22] 丘志仁，俞平，黄锦圣，等．氧化锌薄膜的室温受激紫外激光发射。中山大学学报(自然科学版)，1998，37(1)51—54
[23] Ohyama M, Kozuka H, Yoko T. Sol-gel preparation of ZnO films with extremely preferred orientation along (002) plane from zinc acetate solution. Thin Solid Films, 1997, 306: 78—85
[24] Jeong S H, Boo J H. Influnence of target-to-substrate distance on the properties of AZO films grown by RF magnetron sputtering[J]. Thin Solid Films, 2004, 447-448: 105-110
[25] S. Major, Satyendra Kumar, M. Bhatnagar and K. L. Chopra, Effect of hydrogen plasma treatment on transparent conducting oxides[J]. Appl. Phys. Lett. 1986, 7: 394-396
[26] T. K. Subramanyam, B. Srinivasulu Naidu and S. Uthanna, Cryst. Res. Technol., 10, 35(2000)1193
[27] G. Haacke, New figure of merit for transparent conductors[J]. J. Appl. Phys. 1976, 47(9): 4086-4089
[28] Lee J B, Lee M H, Park C K, etal. Effects of lattice mismatches in ZnO/substrate structures on the orientations of ZnO films and characteristics of SAW devices[J]. Thin Solid Films, 2004,447-448:296-301
[29] V. K. Jain and A. P. Kulshreshtha, Indium-Tin-Oxide transparent conducting coatings on silicon solar cells and their "figure of merit"[J].Sol. Energy Mater.l981,4(2):151-158
[30] S. Ogura, Ph. D. Thesis, Newcastle upon Tyne Polytechnic, 1975
[31] M. Harris, H. A. Macleod, S. Ogura, et al., Thin Solid Films, 57 (1979)173
[32] Chen Y F, Hong S K, Ko H J, etal. Effects of an extremely thin buffer on heteroepitaxy with large lattice mismatch[J]. Appl Phys Lett,2001,78(21):3352-3354
[33] Koike K, Komuro T, Ogata K, etal. CaF_2 growth as a buffer layer of ZnO/Si heteroepitaxy[J]. Physica E,2004,21:679-683
[34] Nahhas A, Kim H K, Blachere J. Epitaxial growth of ZnO films on Si substrates using an epitaxial GaN buffer[J].Appl Phys Lett,2001,78(11):1511-1513
[35] Ashrafi A A, Ueta A,Kumano H,etal. Role of ZnS buffer layers in growth of zincblende ZnO on GaAs substrates by metalorganic molecular-beam epitaxy [J]. J Cryst Growth, 2000,221(1-4):435-439.
[36] Zhu X F, Lin B X, Liao G H, etal.The effect of Zn buffer layer on growth and luminescence of ZnO films deposited on Si substrates [J]. J Cryst Growth,1998,193(3):316-321.
[37] Bang K H, Hwang D K, Myoung J M. Effects of ZnO buffer layer thickness on properties of ZnO thin films deposited by radio-frequency magnetron sputtering[J]. Appl Surf Sci,2003,207:359-364.
[38] 李剑光,叶志镇,汪雷,等. "用MOVCD 在 ZnO/Al_2O_3 生长 GaN 及其特性", 半导体学报, 1999, 20(10): 862-866
[39] H. fabricius, T. Skettrup, P. Bisgaard, Ultraviolet detectors in thin sputtered ZnO films. Appl Optics, 1986, 25:2764
[40] Y. Liu, C. R. Gorla, S. Liang, et al. Ultraviolet detectors based on epitaxial ZnO films grown by MOCVD, J Elec Mater, 2000, 29:69
[41] S. Liang, H. Sheng, Y. Liu, et al. ZnO Schottky ultraviolet photodetectors, J Cryst Growth,2001, 225:110
[42] Bagnall D M, Chen Y F, Zhu Z, et al. High temperature excitonic stimulated emission from ZnO epitaxial layers. [J]. Appl.Phys.Lett., 1998, 73 (8) : 1038-1040
[43] B. D. Cullity, Elements of X-Ray Diffraction, Addition-Wes-ley, Reading, MA, 1978, pp. 102.
[44] A. Segmuller, M.Murakami, in: K. N. Tu, R. Rosenberg(Eds.), Analytical Techniques for Thin Films, Academic, Boston, 1988, pp. 143.
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[46] Jim Y J, Kim Y T, Yan H J, Park J C, Han J I, Lee Y E, Kim H J. Epitaxial growth of ZnO thin films on R-plane sapphire substrate by radio frequency magnetron sputtering[J]. J. Vac. Sci. Technol. A. 1997, 15(3): 1103-1107.
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