Recovery of surface conductivity of H-terminated diamond after thermal annealing in vacuum

A unique feature of diamond among other semiconductors is the formation of a high conductive p-type layer which is usually obtained after hydrogen-termination of the surface. It is generally accepted that the appearance of surface conductivity (SC) requires the presence of atmospheric adsorbates. We present a combination of conductivity and spectroscopic measurements dealing with the loss and the formation of SC as a function of annealing in vacuum (temperatures $60--900\ifmmode^\circ\else\textdegree\fi{}\mathrm{C})$ and exposure to different atmospheres. For temperatures below $190\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in vacuum the SC decreases by more than five orders of magnitude and comes back to its initial value when the sample is exposed to air. After annealing between 250 and $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ exposure to normal atmospheric conditions is no longer sufficient to recover SC, although the H termination is preserved. In this state the SC is fully restored upon air exposure after the surface has been exposed to ozone or oxygen radicals. We propose a model where oxygen-related sites are catalytically involved in the transfer-doping mechanism such that the rate of electron transfer from the diamond into solvated adsorbates is enhanced.