Photoconductivity of CVD diamond under bandgap and subbandgap irradiations

We systematically studied the photoconductivity of nominally intrinsic diamond films grown by two CVD methods. In the 200-275 nm wavelength range covering the bandgap energy (5.5 eV or 225 nm), the measured photocurrent showed characteristic behavior that can be quantitatively related to a fast recombination of electrons and effective trapping of holes, mainly by trap states near the valence band edge. In addition to photoconductivity, thermoelectric emission spectroscopy and thermally stimulated current measurements were made. From the results, the density and location of two distinctive trap levels within the energy bandgap was estimated. The results are not only self-consistent, but also agree with other authors' findings, such as a hole-dominated current, without or under bandgap illumination, a Fermi level close to the valence band edge, an electron recombination center in the middle of the bandgap, and a shallow hole-trap state having an extremely high density of ∼ 10¹⁹ cm^-3. Based on these data, we suggest a bandgap and trap-state model for intrinsic CVD diamond. The details of this model and the characteristic properties of the defects/traps are consistent with experimental results and theoretical findings made by other authors covering a large diversity of areas, such as photoluminescence and cathodoluminescence, photoabsorption, carrier lifetimes and mobilities, electron paramagnetic resonance, cold-cathode electron emission, and photoconductive current switching.