Effects of sputtering gas pressure dependence of surface morphology of ZnO films fabricated via nitrogen mediated crysta
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Effects of sputtering gas pressure dependence of surface morphology of ZnO films fabricated via nitrogen mediated crystallization Kazuya Iwasaki, Koichi Matsushima, Daisuke Yamashita, Hyunwoong Seo, Kazunori Koga, Masaharu Shiratani and Naho Itagaki Kyushu University, Motooka 744, Fukuoka 819-0395, Japan ABSTRACT ZnO films were fabricated by RF magnetron sputtering with nitrogen mediated crystallization (NMC) under various gas pressures. X-ray diffraction measurements show that the NMC-ZnO films are highly crystalline regardless of the gas pressure, and the full width at half maximum values of the (0002) rocking curves range from 0.032 to 0.044. In contrast, atomic force microscopy (AFM) reveals that the gas pressure plays an important role in determining the surface morphology of the films. The root-mean-square (RMS) roughness decreases monotonically from 1.05 to 0.60 nm with increasing pressure from 0.2 to 0.7 Pa. However, the RMS roughness increases with further increases in the pressure, reaching 2.15 nm at 2.1 Pa. The height distribution of the NMC-ZnO films derived from the AFM images is narrowest at 0.7 Pa, indicating that the smooth surface obtained at 0.7 Pa can be attributed to spatially uniform nucleation occurring in a short time period. These results indicate that the sputtering gas pressure is a key parameter for controlling the surface morphology of NMC-ZnO films. INTRODUCTION Zinc oxide (ZnO) has attracted attention as a potential alternative for the gallium nitride (GaN) commonly used in light-emitting diodes (LEDs) due to its wide and direct band gap of 3.37 eV and its large exciton binding energy of 60 meV, which is much larger than that for GaN (28 meV) [1–4]. Moreover, the high abundance and low toxicity of ZnO means that such a replacement would lead to a significant reduction in LED cost and environmental impact. ZnObased LEDs require single-crystalline films with a low defect density; however, lattice-matched epitaxial substrates for ZnO, such as bulk ZnO and ScAlMgO4, are expensive. Therefore, a method of fabricating single-crystalline ZnO films on cost-effective substrates is a prerequisite for the replacement of GaN by ZnO. C-plane sapphire is an attractive epitaxial substrate for growing ZnO films because it is inexpensive and readily available in large area wafers. However, due to the large lattice mismatch of 18% between ZnO and sapphire, ZnO films prepared on sapphire substrates contain a large number of crystal defects, such as threading dislocations and 30° rotated domains [5,6]. Insertion of a buffer layer is one of the most promising methods for reducing crystal defects related to lattice mismatch [7,8]. Our group recently developed a new type of buffer layer fabricated using the nitrogen mediated crystallization (NMC) method, in which nitrogen atoms are used to inhibit crystal growth, resulting in a high concentration of crystal grains and a smooth surface [9–12]. By utilizing the buffer layers fabricated by NMC, hereafter referred to as NMCZnO buffer layers, we have succeeded in
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