Role of Phonons in the Oscillatory Photoconductivity Spectrum of Semiconducting Diamond

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Abstract
The oscillatory photoconductivity spectrum associated with the acceptor center in semiconducting diamond has been measured over the spectral energy range 0.35-2.6 eV (3.5-0.47 μ). The energy separation between the acceptor ground state and the top of the valence band is more than twice the longitudinal optical (LO) phonon energy, and the spectrum is interpreted in terms of hole capture from the valence band to excited states of the acceptor center together with the cascade emission of one or more optical phonons. Transitions involving the emission of up to 14 phonons have been detected. The mean phonon energy in the cascade process is not constant, as has been generally assumed, but decreases gradually as the number of phonons involved increases. This is shown to be consistent with the curvature of the valence band and the dispersion curves for LO phonons. In addition, features have been observed in the photoconductivity spectrum which are considered to be due to the phonon-assisted hole capture to excited states of the acceptor center for which transitions from the ground state are forbidden, and which therefore have not previously been observed in absorption measurements on natural semiconducting diamond.