Surface conductivity of air-exposed hydrogenated diamond: A survey into the subsurface electronic structure and the role of oxygen-related adsorbates

The surface conductive and insulating states of microwave plasma hydrogenated diamond (HD) single crystal (1 0 0) surfaces have been investigated by using electron diffraction, electron energy loss and photoemission spectroscopies. Before each experiment, the surface was exposed to air and left to reach the conductive state; then, after measurements, the sample was annealed in ultra-high vacuum, its surface reached the insulating state, and the measurements were repeated. We have determined coherently scattering domains average dimension of (2 × 1) reconstructed HD surfaces both in insulating and conductive state, resulting in a smaller dimension in the latter case, due to adsorbates inducing surface conductivity. Airborne adsorbates containing oxygen, which actively contribute to enhancing surface conductivity, are randomly distributed and cover about 23 % of the surface. Monte Carlo simulations show that the hydrogenation process produces hydrogen penetration beyond the topmost surface layer; this result supports X-ray core-level photoemission spectroscopy data interpretation. The role of oxygen and oxygen-related adsorbates in the surface conductivity of hydrogenated diamond has also been studied by using controlled exposure to atomic oxygen. We have found that oxygen adsorbs on the surface without bonding to carbon. This behavior contrasts with that of airborne adsorbates, which are the primary contributors to surface conductivity.