The vacuum-annealed undoped polycrystalline CVD diamond electrodes: the impedance-spectroscopy and photoelectrochemical studies

The electrochemical impedance, photocurrent, and photopotential are measured in the supporting electrolyte solution (2.5 M H2SO4) for electrodes made of the undoped polycrystalline CVD diamond films annealed in vacuum at 1500-1640 °C. Analysis of the impedance spectra allowed concluding that both the amount of a nondiamond conducting phase in the electrode bulk and its conductivity increased upon annealing. With increasing annealing temperature, the photopotential passed through a maximum at ∼1570 °C. The photosensitivity of “metal-like” samples (annealed at temperatures ≥1630 °C) is negligibly small. Judging from the sign of the photopotential (positive) and photocurrent (cathodic), the studied material formally behaves as a p-type semiconductor. It is suggested that the photoeffects are caused by the structure defects, in particular, dislocations in diamond crystallites, formed close to intercrystalline boundaries during the high-temperature annealing.