(395c) Surface Conductivity of Undoped, Hydrogen-Terminated Diamond

A unique feature of hydrogen-terminated diamond is its surface conductivity. Although its origin is not fully understood, the proposed electrochemical transfer-doping model is gaining acceptance. A p-type accumulation layer forms in the diamond when the Fermi level of the diamond is higher than the chemical potential of electrons in an adsorbed water layer on the diamond surface. The chemical potential of electrons in the water film is most likely fixed by the oxygen redox couple. The conductivity increases upon exposure to acidic vapors and decreases when exposed to basic vapors, which lower and raise the chemical potential of electons respectively. Addition of hydrogen peroxide, an intermediate in the electrochemical reduction of oxygen, increases the effect. Increasing the surface work function by fluorinating or oxidizing the diamond eliminates the surface conductivity. Measurements of conductivity in air and in vacuum were made as a function of temperature and are consistent with the electrochemical transfer-doping model. This effect may not be unique to diamond, but may also be responsible for observed conductivity in semiconducting carbon nanotubes. The results, in general, support the electrochemical transfer-doping model; however, some uncertainty remains in the nature of the electrochemical couple. Full understanding of the effect may enable a new class of sensors.