Activating p-type conduction and visible transparency in delafossite CuAlO2 films: The critical role of the copper valence state transition

P-type oxide semiconductors with high p-type conductivity and visible transparency are highly desired for various electronic devices. However, achieving these two demands is challenging due to the localized oxygen 2p orbitals and compensating donors in oxides. This is seen in the representative delafossite CuAlO2, despite tremendous theoretical and experimental efforts in perfecting the lattice quality and physical properties. Here, we reveal the critical role of the copper valence state transition in activating high p-type conduction and visible transparency in CuAlO2. A two-step synthesis strategy is developed to fabricate c-axis-oriented CuAlO2 films with similar crystallinity but with very different Cu2+/Cu+ proportions on sapphire substrates. In combination with structural and transport characterizations, this study distinguishes the key impact of the cooper valence state from the more commonly noticed lattice crystallization, for simultaneously achieving a room-temperature p-type conductivity of >1 S/cm and visible transmittance of >75 %. We further demonstrate that the strategy of obtaining high-performance CuAlO2 films is applicable to n-type SiC substrates for fabricating pn heterojunction diodes with a rectifying ratio of 106. This study provides critical insights into designing and synthesizing high-performance p-type oxide semiconductors.