For every semiconducting material, the long-term stability of thin film characteristics is a crucial necessity for device applications. This is particularly true for the p-type semiconductor CuI, where the thin film properties are especially sensitive to environmental influences and motivate the application of capping materials. Utilizing pulsed laser deposition (PLD) and Al2O3 cappings, we performed systematic studies on the N2/O2 partial pressure during growth and the effect of layer thickness. Our results suggest that oxygen, acting as an acceptor, and its diffusion through Al2O3 and CuI dominate the conductivity of PLD grown CuI thin films. The diffusion process of atmospheric oxygen into CuI was traced with 18O-isotopes. Additionally, the transparency and morphology of CuI films are also affected by the oxygen supply during capping growth. These results challenge the currently accepted idea that intrinsic, and not extrinsic, effects determine the conductivity of CuI thin films.
