Room Temperature Fabrication of (ZnO) x (InN) 1-x films with Step-Terrace Structure by RF Magnetron Sputtering
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Room Temperature Fabrication of (ZnO)x(InN)1-x films with Step-Terrace Structure by RF Magnetron Sputtering Koichi Matsushima1, Tomoaki Ide1, Daisuke Yamashita1, Hyunwoong Seo1, Kazunori Koga1, Masaharu Shiratani1, and Naho Itagaki1 1 Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0385, Japan ABSTRACT We study effects of deposition temperature on growth mode and surface morphology of hetero-epitaxial (ZnO)x(InN)1-x (ZION) films on ZnO templates. ZION films deposited at low temperature of RT-250oC grow two dimensionally, whereas ZION films deposited at high temperature of 350-450oC grow three dimensionally. Growth mode is changed from twodimensional growth mode to three-dimensional one, because the critical thickness where film strain begin to relax decreases with increasing the deposition temperature. At high deposition temperatures, the number of point defects in ZION films decreases because migration of adatoms on the growing surface is enhanced. The strain energy in ZION films increases with increasing the deposition temperature, since the strain energy is not released by point defects. Therefore, lattice relaxation for the higher deposition temperature begins at the smaller film thickness to release the strain energy. As a result, ZION films with atomically-flat surface were obtained even at RT. INTRODUCTION Materials with tunable band gap are required in optoelectronic applications such as photovoltaics and photodiodes [1-3]. As such a tunable band gap material, we recently have reported a new semiconducting material, (ZnO)x(InN)1-x (ZION), which is a pseudo-binary alloy of wurtzite ZnO and wurtzite InN [4-9]. ZION has a potential tunable band gap range from 0.7 eV (InN) to 3.4 eV (ZnO), while we have realized ZION with a band gap range from 1.5 to 3.0 eV. Moreover, ZION shows a high photosensitivity above 103. For device applications of ZION, epitaxial films with atomically smooth surface are important. Such films generally grow on lattice-matched substrates at high deposition temperature where migration of adatoms on the growing surface is enhanced [10]. However, there is no lattice-matched substrate for ZION films. Therefore, we use ZnO templates with 0.7%-lattice-mismatch as substrates for ZION films. Here, we report effects of deposition temperature on growth mode and surface morphology of heteroepitaxial ZION films on ZnO templates. EXPERIMENTAL ZION films were deposited on 0.7%-lattice- mismatched ZnO templates by the following steps. First, epitaxial ZnO templates with atomically-flat surface were deposited on c-plane sapphire substrates by using the nitrogen-mediated crystallization (NMC) method [11-16], which offers fabrication of high-quality epitaxial films on large lattice-mismatched substrates. Then, ZION films were deposited on ZnO templates by RF magnetron sputtering. ZnO and In targets with a purity of 99.99% were used. O2, N2 and Ar gases were supplied to the sputtering chamber at gas flow rates of 3.0, 33.3, and 6.6 sccm, respectively. The total pressure was 0.3 Pa. The RF
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