Defect density of states of tin oxide and copper oxide p-type thin-film transistors

Abstract

The complete subgap defect density of states (DoS) is measured using the ultrabroadband (0.15 to 3.5 eV) photoconduction response from p-type thin-film transistors (TFTs) of tin oxide, SnO, and copper oxide, Cu$_2$O. The TFT photoconduction spectra clearly resolve all bandgaps that further show the presence of interfacial and oxidized minority phases. In tin oxide, the SnO majority phase has a small 0.68 eV bandgap enabling ambipolar or p-mode TFT operation. By contrast, in copper oxide TFTs, an oxidized minority phase with a 1.4 eV bandgap corresponding to CuO greatly reduces the channel hole mobility at the charge accumulation region. Three distinct subgap DoS peaks are resolved for the copper oxide TFT and are best ascribed to copper vacancies, oxygen-on-copper antisites, and oxygen interstitials. For tin oxide TFTs, five subgap DoS peaks are observed and are similarly linked to tin vacancies, oxygen vacancies, and oxygen interstitials. Unipolar p-type TFT is achieved in tin oxide only when the conduction band-edge defect density peak ascribed to oxygen interstitials is large enough to suppress any n-mode conduction. Near the valence band edge in both active channel materials, the metal vacancy peak densities determine the hole concentrations, which further simulate the observed TFT threshold voltages.