Correlation between crystallinity and mid-infrared optical absorption spectra of silicon supersaturated with sulfur
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Correlation between crystallinity and mid-infrared optical absorption spectra of silicon supersaturated with sulfur Ikurou Umezu1, Katsuki Nagao1, Tatsuya Nakai1, Muneyuki Naito2, Mitsuru Inada3, Tadashi Saitoh3, Tamao Aoki1 and Akira Sugimura1 1 Department of Physics, Konan University, Kobe 658-8501, Japan 2 Department of Chemistry, Konan University, Kobe 658-8501, Japan 3 Department of Pure and Applied Physics, Kansai University, Suita 564-8680, Japan ABSTRACT We prepared silicon hyperdoped with sulfur by ion-implantation followed by pulsed laser melting. Effects of laser fluence during pulsed laser melting and of post-annealing on the silicon hyperdoped with sulfur are investigated. The structure of hyperdoped layer changes from polyto mono-crystal with increasing laser fluence. Interface between sulfur-implanted-layer and single-crystal substrate disappear above 1.1 J/cm2. The spectral intensity of mid-infrared (MIR) optical absorption increases with crystallinity and spectral shape depends on whether the melt depth during pulsed laser melting reaches interface between implanted layer and single-crystal silicon substrate or not. The MIR absorption intensity rapidly decreases with thermal annealing temperature and almost disappears at 750 °C. The activation energy of conductivity decreases with increasing laser fluence and further decreases with increasing post thermal-annealing temperature. The insulator-metal transition is observed for the sample annealed at 750 °C. These results indicate that there is no direct correlation between MIR optical absorption band and insulator-metal transition. INTRODUCTION Silicon based intermediate band semiconductors are candidates of high efficiency solar cells using earth-abundant materials[1]. Hyperdoping with deep-level impurities is one of the possible methods to form intermediate band. Silicon hyperdoped with chalcogen, titanium or gold is considered to form intermediate band[2-4]. However, formation of hyperdoped silicon monocrystal with deep-level impurity is not easy since the maximum concentration of the deeplevel impurities such as sulfur and titanium in silicon crystal under equilibrium condition is less than 1017 cm-3, which is not enough to form the deep-impurity band. Very large concentration, more than 1020 cm-3, is necessary to form deep-impurity band due to the small Bohr radius of deep impurities[5]. Ion-implantation followed by pulsed laser melting is known as a nonequilibrium method to realize formation of hyperdoped crystalline materials[6-8]. Characteristic features such as emergence of very large sub-bandgap optical absorption[8-10], lifetime recovering[11] and the insulator-to-metal transition[5] have been reported for silicon hyperdoped with sulfur[5,8-10] or titanium[6,7,11] prepared by pulsed laser melting. However, the hyperdoping is not easy for most of deep impurities due to lateral segregation called cellular break down[12]. Hyperdoped monocrystal structure can be formed relatively easily for hyperdoping of silicon with sulfur[13]. Furthermore, la
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