Bipolar Doping and Band-Gap Anomalies in Delafossite Transparent Conductive Oxides

Abstract

Doping wide-gap materials p type is highly desirable but often difficult. This makes the recent discovery of p-type delafossite oxides, ${\mathrm{CuM}}^{\mathrm{III}}{\mathrm{O}}_{2}$, very attractive. The ${\mathrm{CuM}}^{\mathrm{III}}{\mathrm{O}}_{2}$ also show unique and unexplained physical properties: Increasing band gap from ${\mathrm{M}}^{\mathrm{III}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Al},\mathrm{Ga}$, to In, not seen in conventional semiconductors. The largest gap ${\mathrm{CuInO}}_{2}$ can be mysteriously doped both n and p type but not the smaller gaps ${\mathrm{CuAlO}}_{2}$ and ${\mathrm{CuGaO}}_{2}$. Here, we show that both properties are results of a large disparity between the fundamental gap and the apparent optical gap, a finding that could lead to a breakthrough in the study of bipolarly dopable wide-gap semiconductor oxides.