Photoluminescence excitation by band-gap optical absorption in chemical vapor deposition diamond films

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Abstract
Photoluminescence excitation (PLE) spectra at photon energies near the indirect band gap of diamond have been obtained for diamond films grown by the filament-assisted chemical vapor deposition (CVD) method. The PLE intensity was observed to increase abruptly with photon energy above 5.5 eV. This increase coincides with the onset of phonon-emission-assisted interband absorption, which was observed independently by diffuse transmittance measurements. A lower-energy PLE threshold at ∼5.25 eV, which coincides approximately with the onset of phonon-absorption-assisted interband absorption, was observed in the spectrum of a gem-quality natural diamond, but not in the spectra of the CVD-grown films. Emission spectra of the luminescence excited by above-band-gap photons have features similar to luminescence spectra of the same specimens excited by 20-keV electrons. The spectrally integrated intensities of the luminescence excited by above-band-gap photons and by electrons were found to vary from specimen to specimen in a linearly related manner, suggesting that similar recombination processes occur in both cases.