Phonon spectroscopy of chromium-doped GaAs using superconducting tunnel junctions

Abstract

The phonon absorption spectra of samples of n-type, semi-insulating (SI) and p-type GaAs containing chromium from a variety of sources have been measured at frequencies between 80 and 720 GHz. The measurements have been made at 1K using a quasi-monochromatic phonon spectrometer with superconducting tunnel junctions as generators and detectors. In semi-insulating and lightly p-type samples a number of resonant absorptions due to chromium are found including a group of three at frequencies of 125, 147 and 167 GHz and a weaker and broader resonance at 320 GHz. In n-type samples these resonances do not occur under dark conditions, but can be induced by the application of sub-band-gap illumination. It is shown that there is strong evidence that a number of resonances including these four are due to the centre that causes the zero-phonon structure at 0.839 eV in the optical absorption. Intense elastic phonon scattering which occurs in semi-insulating and lightly p-type samples but not in n-type material is tentatively attributed to substitutional Cr3+. This scattering appears to be particularly strong in the region of 720 GHz. In two samples a sharp resonant absorption at 444 GHz is noted and attributed to Fe2+. Two other resonant absorptions of unknown origin are reported.