Optical Phonon Effects in the Infra-red Spectrum of Acceptor Centres in Semiconducting Diamond

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Abstract
The infra-red absorption spectrum of two natural Type IIb semi-conducting diamonds has been measured by high resolution grating spectroscopy in the range 0.1 to 0.7 eV and at temperatures from 70° to 293 °K. The various absorption features (observed in much greater detail at low temperature than hitherto) were as follows: (i) a series of three lines due to excited states of bound holes at 0.305, 0.347 and 0.363 eV together with many smaller peaks, (ii) secondary peaks at 0.462, 0.508, 0.625 and 0.670 eV in the continuum and (iii) a low-level absorption near 0.16 eV. The observed widths of the excited states are inconsistent with the broadening theory of Lax and Burstein. However Kane's recent theory of lifetime broadening predicts the observed width of the 0.305 eV line to the correct order of magnitude. Hardy has shown that both (a) optical-phonon-assisted transitions and (b) impurity-induced single-phonon absorption are likely to occur in diamond. These conclusions are confirmed by (a) the observation of peaks in the continuum at energies separated from the excited states by 0.159 eV, the TO phonon energy and (b) the presence of absorption just below the Raman energy at 0.165 eV. The cross section for this single-phonon absorption is much higher than for the Type I (nitrogen) or irradiation-induced lattice absorption.