Spectral distribution of photoionization cross sections by photoconductivity measurements

Abstract

A new technique for measuring the spectral distribution of photoionization cross sections in photoconductors is presented. The method makes use of the fact that the occupancy of an impurity level is not changed during illumination with photons of different energy if the photocurrent is kept constant. A constant photocurrent is achieved by adjusting the light intensity. The spectral distribution of the photoionization cross section is then given by the inverse of the photon flux as a function of the photon energy. To demonstrate the applicability of this measuring technique, measurements were performed on oxygen-doped GaAs samples. Three deep energy levels (two 0.46 and 0.79 eV below the conduction band and one 0.48 eV above the valence band) were investigated and the spectral distribution of five photoionization cross sections were measured. All of them are in good agreement with calculated values, and thus an accurate determination of the threshold energies is achieved. For comparison, similar investigations were performed with p+n junctions giving results which are in good agreement with the photoconductivity measurements.