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 ${\mathrm{M}}^{\mathrm{III}}=\mathrm{Al},\mathrm{Ga}$, 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.

• Received 25 September 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.066405