Chemical modulation of valence band and photoelectric properties of epitaxial p-type infrared transparent conducting CuScO2 thin films

Abstract

Epitaxial p-type infrared transparent conducting oxide (TCO) CuScO2 thin films were grown by polymer-assisted-deposition (PAD) method on a-plane sapphire substrates. Valance band of CuScO2 (largely consisting of localized O-2p and Cu-3d orbitals) was chemically modified by annealing the films under O2 presence with the goal to delocalize carrier holes. Films with c-axis orientation were obtained at 1%-3% O2 pressure. The orientation relationship between CuScO2 and the substrate was [3 R](0001)//a-Al2O3(1̄210). Conduction processes followed the small-polaron hopping mechanism with the hopping activation energies in the 0.032-0.51 eV range (depending on the O2 partial pressure). The films demonstrated p-type conductivity. The conductivity of CuScO2 thin film annealed at 1% O2 partial pressure at room temperature was 22.1 S cm-1. Films prepared with O2 pressure equal to 1 and 3% retained their optical transparency (>90 and >75%, respectively) in the visible and mid-IR regions. To the best of our knowledge, these are the best performing p-type transparent conductive thin films produced by chemical methods. To demonstrate the practical usage of these films, we fabricated p-CuScO2/n-In2O3 heterojunction diodes, which showed ∼2.5 V threshold voltage, which agreed reasonably with CuScO2 bandgap value. High optical transparency and electrical conductivity of these films make them very promising for optoelectronic applications, in which wide wavelength ranges, from visible to IR, are required.