Surface doping: a special feature of diamond

A unique feature of diamond surfaces is a highly conductive p-type layer, which is usually observed when the surfaces are hydrogen terminated. This phenomenon has recently attracted a lot of interest since a number of electronic applications proposed for diamond are based on the effect. Nevertheless, its microscopic origin is still a matter of debate. In this paper we propose an electron transfer from the diamond valence band to adsorbates at the surface to be the effective doping mechanism. These adsorbates act not as isolated species but as components of a mildly acidic aqueous surface layer and thus charge exchange has to be described by electrochemical arguments. The model is supported by experiments, which show that the hydrogenation of the surface is necessary but not sufficient for inducing the hole accumulation layer at the surface.