Surface transfer doping of diamond

Hydrogen-terminated diamond can exhibit a pronounced two-dimensional hole conductivity when it is exposed to appropriate adsorbates. This phenomenon was first encountered in the form of the so-called hydrogen-induced surface conductivity but is meanwhile understood as a novel kind of doping mechanism by which the surface adsorbates act like acceptors. Such surface acceptors can be solvated ionic species that undergo electrochemical reactions after accepting an electron from the diamond's valence band. But they can just as well be neutral molecular species with a sufficiently high electron affinity such as fullerenes or fluoro-fullerenes. This review summarizes the physical principles behind this unusual type of doping.