Structure has been observed in the photoconductivity spectrum of semiconducting diamond at energies below the ionization threshold. Photoconductivity maxima in this spectral region have been found to coincide exactly with maxima in the absorption spectrum associated with transitions to excited states of the aluminum acceptor center (ionization energy=0.373 eV). In general, the acceptor spectrum is better resolved in these photoconductivity measurements than in absorption measurements on the same specimen, and this is particularly true for synthetic semiconducting diamonds in which the acceptor concentration is very high. Measurements in the temperature range 4 to 150°K have shown that a two-stage process is predominantly responsible for the appearance of the excited-states spectrum in photoconductivity: optical excitation of holes from the ground state to these excited states, followed by thermal excitation into the valence band. This process is referred to as "photothermal ionization." An investigation of the electric field dependence of this process has shown that the only effect is a broadening of the spectrum at high fields which are comparable in magnitude with the binding field experienced by a hole in an excited state. At temperatures close to 4°K, where the thermal contribution is negligible, residual structure can still be observed which is specimen-dependent, and the features may appear as either minima or maxima superimposed on the low-energy tail of the photo-ionization continuum. This low-energy tail is presumably due to direct photoexcitation of slightly perturbed acceptor centers, and minima can appear on this tail because of the strong competitive absorption to the excited states. However, in many specimens, tunneling from the excited states appears to be possible, and the features are then observed as maxima. This picture is further substantiated by the fact that in those diamonds for which tunneling does not normally occur, it can be induced by simultaneously illuminating the crystal with radiation of energy lying within the photoconductivity continuum. This increases the number of ionized centers present, and so increases the probability of tunneling.

• Received 21 February 1968

DOI:https://doi.org/10.1103/PhysRev.171.843