Dual Role of Adsorbent and Non-monotonic Transfer p-Doping of Diamond

Exposure to oxygen is usually detrimental for materials and devices as it leads to undesirable surface oxidation or even deeper corrosion. However, experiments with hydrogen-terminated H-diamond show that oxygen adsorption plays an instrumental role in inducing the p-type surface conductivity. Using first-principles calculations, we explore how the surface-physisorbed molecular O₂ serves as an electron acceptor in the transfer doping of diamond. On the other hand, calculations reveal that in a chemisorbed state, oxygen groups substitute H, which lowers the bands in diamond and inhibits the transfer doping. This explains the non-monotonic carrier density dependence on the exposure to oxygen (or, similarly, other adsorbent-acceptor). We further find that ozone can be more efficient for p-type doping of H-diamond due to O₃ having lower LUMO energy levels than in molecular O₂.