High Temperature Surface Conductivity of Hydrogenated Diamond Films Exposed to Humid Air

Surface conductivity of thin diamond films was measured as a function of temperature up to 450°C. Hydrogenated diamond was synthesized by chemical vapor deposition in hydrogen/carbon plasma. Low values of charge carrier activation energy ( ≈ 10 meV) were observed, when hydrogenated diamond films were exposed to the ambient humid air. However, the activation energy increased by two orders of magnitude as film temperature exceeded 300°C. We have attributed this behavior to the desorption of the $H_2O$ adlayer. The jump of the activation energy did not occur, when experiment was performed in vacuum. We have also shown that donor doping leads to the up-shift of the Fermi level much above the acceptor-like band gap levels induced by surface C-H bonds, which cannot be compensated by transfer of electrons from diamond to the double $H-H_2O$ layer.