Temperature-dependent transport properties of hydrogen-induced diamond surface conductive channels

Hall effect experiments have been carried out in diamond surface conductive channels induced by hydrogen termination. The conductivity and the Hall mobility have been studied as a function of temperature and carrier concentration. The conductivity and Hall mobility are exponentially activated with temperature, with almost identical activation energies, whereas the carrier concentration shows no or only a very weak temperature dependence, as expected for metallic conduction. The activation energy is found to decrease with increasing carrier concentration, until it almost vanishes. The experimental results are discussed in the frame of a model based on large-range potential fluctuations which give rise to metallic and insulating regions coexisting at the diamond surface. The conductivity and Hall mobility are analyzed using percolation theory. Below the percolation threshold, transport occurs by thermal excitation over the potential barriers separating the metallic regions. Above the percolation threshold, activated transport persists as the thermal emission over remaining potential barriers contributes to reduce the resistance of long percolating paths. The experimental results have been fitted using a model with three free parameters: metallic conductivity and mobility, averaged potential barrier, and surface fraction occupied by insulating regions. The Hall mobility in the metallic regions has been found to decrease with increasing carrier concentration, indicating a strong influence of surface scattering. The contribution of variable-range hopping at very low temperatures is also discussed.