Are protons involved in the hydrogen-induced surface conductivity of diamond(001)?

We have investigated the mechanism underlying the surface conductivity obse rved for synthetic diamond(001) samples. It is found that (I) the surface c onductivity depends on the pressure of the surrounding gas atmosphere, (2) the temperature dependence can be described as being due to a thermally act ivated process and (3) a chromogeneous WO3 cathode turns color from transpa rent to blue and gas bubbles develop underneath the cathode if an electrica l de current is passed through such an arrangement. In contrast, the comple mentary WO3 anode neither changed color nor were gas bubbles observed. One concludes that hydrogen develops at the negatively biased WO3-diamond inter face, recombining to gaseous H-2. However, most of the hydrogen invades the WO3 cathode coloring it blue, thereby clearly indicating the involvement o f protons. Though the conductivity crucially depends on temperature, time a nd history of a sample, the resistances of various and even differently pre pared diamond specimens covering eleven orders of magnitude can be scaled o nto one master curve reflecting a thermally activated behavior. Taken toget her, we propose that the mechanism of the diamond(001) surface conductivity as observed under ambient conditions is at least partly due to proton cond uction.