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 degrees 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 degrees C. We have attributed this behavior to the desorption of the H2O 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-H2O layer.