さらなる高効率化のためにはまず電気特性の改善必要であり，その方法としてはナノ結 晶相内の塩素の排除および酸素析出体の抑制，キャリア濃度の最適化が考えられる．ナノ結 晶相内の塩素の排除は，SiCl4を用いてプラズマCVDで作製したSiナノ粒子をHClによっ てエッチングする，またはSiCl4の代わりにSiH4を用いて合成することでナノ粒子表面の塩 素を水素に置き換えることができる．また，キャリア濃度は，プラズマCVDでのSiナノ粒 子作製の際に不純物として用いるトリメチルホスフィンの流量によって制御できる．先行 研究と比較して本研究で作製した試料のキャリア濃度は小さく，今後はさらにキャリア濃 度を高くして試料を作製することで性能が向上すると考えられる．
一方，熱伝導率の低減は，ナノ結晶相の粒径の低減によって実現する．本研究では，6 nm のSiナノ粒子が焼結中に30 nmへと粒成長している．電子の平均自由行程は300 Kにおい
44
て20 nm以下である31ので，焼結条件を最適化して粒成長を抑制することで電気特性への
影響を抑えつつ熱伝導率をさらに下げられると考えられる．
45
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