“…The p-type conduction was repeatedly obtained for the films with PO 2 around 40 Pa which could be electrically stable over several months. These results indicate that oxygen-rich condition is favorable for achieving p-type conversion and can be explained by that intrinsic oxygen defects are compensated [23,24]. However, it is worth to point out that further increasing PO 2 beyond a certain value ( $60 Pa) will lead to vanishing of p-type conduction.…”