Effects of post‐annealing on (110) Cu2O epitaxial films and origin of low mobility in Cu2O thin‐film transistor

We grew epitaxial (110) Cu2O films on (110) MgO substrates toward high‐mobility p‐channel oxide thin‐film transistors (TFTs). The (110) Cu2O films exhibited high Hall mobilities ∼90 cm2(Vs)−1 comparable to those of high‐quality single‐crystals, which were obtained in a narrow growth condition for 650 nm‐thick films. TFTs using the epitaxial (110) Cu2O channels exhibited p‐channel operation, but the field‐effect mobilities and the on‐to‐off drain current ratio were far from satisfaction (∼0.04 cm2(Vs)−1 and ∼2, respectively). In order to investigate the origin of the poor mobility, the films were subjected to post‐deposition annealing under various oxygen partial pressures (PO2−A = 0.65–10−3 Pa). Optical measurements revealed that subgap states exist in all the films and their amounts were increased by post‐deposition annealing irrespective of PO2−A. The subgap density of states estimated by the optical analyses are consistent roughly with that estimated from the TFT mobility.