Preparation of Plate-Like Bulk Beta Iron-Disilicide Crystals Using Metal to Semiconductor Phase Transition by Heat Treat
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Preparation of Plate-Like Bulk Beta Iron-Disilicide Crystals Using Metal to Semiconductor Phase Transition by Heat Treatment Masato Osamuraa,b, Hidetaka Ishiharac, Zhengxin Liua,b, Hisao Tanoueb, Shirou. Sakuragid, Yasushi Hoshinoc, Yasuhiko Nakayamab,e, Yunosuke Makitab a
System Engineering Corporation, Yamato, Kanagawa, 242-0001 Japan National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba Central 2, Umezono1-1-1, Tsukuba, 305-8568 Japan c Nippon Institute of Technology, Miyashiromachi, Minami-saitamagun, Saitama, 345-8501 Japan d Union Material Co. Ltd., Tonemachi, Hita-soumagun, Ibaraki, 300-1602 Japan e Japan Science and Technology Corporation (JST), 16-1 Onogawa, Tsukuba, 305-8569 Japan b
ABSTRACT Plate-like β-FeSi2 bulk crystals with size of 10x10 mm2 and thickness of 1 mm were fabricated by annealing CVT (chemical vapor transport)-grown plate-like α-Fe2Si5 at 800°C in Ar atmosphere. Before annealing, α-Fe2Si5 crystals were characterized by x-ray diffraction (XRD) and scanning electron microscopy (SEM) to be single crystals with flat surfaces. XRD measurements of β-FeSi2 crystals subjected to annealing showed that they had a polycrystalline structure. The mean Fe/Si composition ratio of β-FeSi2 crystal measured by energy dispersive x-ray spectroscopy (EDX) was 31/69 and it was the same as that of α-Fe2Si5 bulk crystal before annealing. SEM, Raman scattering and electron probe micro-analysis (EPMA) measurements identified that there existed small Si precipitates mixed in the matrix of β-FeSi2 crystals. At annealing temperature of 800°C, the plate-like β-FeSi2 bulk was obtained even the annealing duration time was as short as 5 hours. INTRODUCTION Recently, semiconductor β-FeSi2 has become attractive for applications in optoelectronic devices [1-3]. The advantages arise from its direct band gap (around 0.85 eV), high optical adsorption coefficient (above 105 cm-1) and relatively high thermoelectric power (of the order of 10-4 K-1) [4,5]. Additional interesting features are the stability at high temperature due to the low vapor pressure, non-toxicity and the abundant occurrence in the earth’s crust that are inherent to constituent Fe and Si atoms. In order to use β-FeSi2 for the fabrication of optoelectronic devices such as photo-sensors and light emitting diodes (LED) operating at 1.5 µm for optical communication systems, epitaxial growth technology of β-FeSi2 films with high quality should be necessarily developed to prepare junction structures. However, until now, attempts for β-FeSi2 film growth have been concentrated on the usage of Si as substrates. In most cases the obtained films are polycrystalline with a high pinhole density and rough surface morphology. This is because that the lattice constants and thermal expansion coefficient of β-FeSi2 are much larger than those of Si. For this reason, homo-epitaxial growth is most desirable in which lattice mismatching is essentially not produced by using bulk β-FeSi2 crystal as a substrate. Till now, only small bulk cryst
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