Ultraviolet Intrinsic and Extrinsic Photoconductivity of Natural Diamond

A detailed investigation has been made of the photoconductivity response of several types of natural diamond in the spectral energy range from 3.0 to 5.9 eV. Oscillatory structure in the photoconductivity beyond the fundamental absorption edge at 5.49 eV has been shown to be closely related to similar oscillatory structure in the luminescence-excitation spectra, but does not appear in the absorption spectrum. The oscillations appear only together with the N9 absorption/luminescence system with no-phonon lines at 5.25 and 5.26 eV, which has recently been attributed to the creation and decay of indirect excitons bound to nearest-neighbor nitrogen-aluminum donor-acceptor pairs ($5DA$ complex). The photoconductivity minima and the luminescence-excitation maxima are attributed to the enhanced probability of the creation of bound-exciton N9 states if the residual kinetic energy of free excitons is very low after the dissipation of the majority of their excess energy through the cascade emission of optical phonons. Photoconductivity quenching has also been observed in the region of the no-phonon lines of the N9 system, and the temperature dependence of this effect has been found to be consistent with one of the proposed decay modes of the $5DA$ complex. A photoconductivity threshold has been observed at 4.05 eV in all insulating diamonds, which is considered to be the photoionization limit for the isolated substitutional nitrogen donor. Photo-Hall-effect measurements have been made which support this explanation.