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

Doping wide-gap materials p type is highly desirable but often difficult. This makes the recent discovery of p-type delafossite oxides, CuM^IIIO₂, very attractive. The CuM^IIIO₂ also show unique and unexplained physical properties: Increasing band gap from M^III0ex{0ex}=0ex{0ex}Al, Ga, to In, not seen in conventional semiconductors. The largest gap CuInO₂ can be mysteriously doped both n and p type but not the smaller gaps CuAlO₂ and CuGaO₂. 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.