Defect Density of States of Tin Oxide and Copper Oxide p-type Thin-film Transistors

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, Cu2O. The resulting TFT photoconduction spectra clearly resolve bandgaps that 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. To achieve desirable unipolar p-mode TFTs, the conduction band-edge defect density of oxygen interstitials must be sufficiently large to suppress n-mode conduction. In both channel materials, the metal vacancy peak densities near the valence band edge determine the hole concentrations, which then predict the TFT Fermi level energy, observed on-off ratios, and threshold voltages.