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 pho tocurrent showed characteristic behavior that can be quantitatively re lated to a fast recombination of electrons and effective trapping of h oles, 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 dens ity and location of two distinctive trap levels within the energy band gap 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 vale nce band edge, an electron recombination center in the middle of the b andgap, and a shallow hole-trap state having an extremely high density of similar to 10(19) cm(-3). Based on these data, we suggest a bandga p and trap-state model for intrinsic CVD diamond. The details of this model and the characteristic properties of the defects/traps are consi stent with experimental results and theoretical findings made by other authors covering a large diversity of areas, such as photoluminescenc e and cathodoluminescence, photoabsorption, carrier lifetimes and mobi lities, electron paramagnetic resonance, cold-cathode electron emissio n, and photoconductive current switching. (C) 1997 Elsevier Science S. A.