Suppression of Near-interface Oxidation in Thermally-evaporated BaSi 2 Films and Its Effects on Preferred Orientation an
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.31
Suppression of Near-interface Oxidation in Thermally-evaporated BaSi2 Films and Its Effects on Preferred Orientation and the Rectification Behavior of n-BaSi2/p+-Si Diodes Kosuke O. Hara1, Keisuke Arimoto1, Junji Yamanaka2, Kiyokazu Nakagawa1 and Noritaka Usami3 1 Center for Crystal Science and Technology, University of Yamanashi, 7-32 Miyamae, Kofu, Yamanashi 400-8511, Japan
2 Center for Instrumental Analysis, University of Yamanashi, 4-4-37 Takeda, Kofu, Yamanashi 400-8510, Japan
3
Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
ABSTRACT
Thermal evaporation is a simple method to fabricate a BaSi 2 film, a new solar cell material consisting of earth-abundant elements. In this study, we optimized the evaporation process and suppressed near-interface oxidation in evaporated BaSi2 films on Si(100) substrates, which has been detected in previous studies. Composition depth profiles determined by Auger electron spectroscopy show the decrease of oxygen concentration near the interface to the background level by optimizing the source pre-melting condition. By reducing oxygen concentration, the BaSi2 film becomes more preferentially oriented toward [100] as long as the deposition rate is not changed, as evidenced by X-ray diffraction. It is also shown that the rectification behavior of n-BaSi2/p+-Si diodes improves by suppressing the near-interface oxidation.
INTRODUCTION New active-layer materials with earth-abundant elements are desired for largescale deployment of high-efficiency thin-film solar cells. BaSi2 is a promising candidate because of excellent optoelectronic properties, simple film-fabrication routes, and the abundance of constituent elements in the Earth’s crust [1]. BaSi2 epitaxial films with (100) orientation can be grown on either Si(111) or Si(100) surface by molecular beam epitaxy (MBE) [2,3], which has been extensively used for fundamental studies as well as solar cell applications of BaSi2. The band gap (Eg) of BaSi2 is 1.3 eV [4,5], suitable for single-junction solar cells. Optical absorption coefficients exceed 10 4 cm−1 for photon energies over Eg [4,5], which are similar to the materials of thin-film solar cells. In
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addition, long minority-carrier lifetime (up to 11 μs [6–8]) and diffusion length (10 μm [9,10]) have been observed. Solar cells with p-type BaSi2/n-type Si(111) heterojunction fabricated by MBE have recently reached an efficiency of 9.9% [11,12]. Thermal evaporation is a simple technique for fabricating BaSi2 films with a high deposition rate up to 3 μm/min [13–15]. The conductivity type of evaporated BaSi2 films is n-type and electron density can be controlled by deposition rate [15]. A long carrier l
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