Combined photoacoustic and photoconductive spectroscopic investigation of nonradiative recombination and electronic transport phenomena in crystalline n-type CdS. I. Experiment.

Abstract

Microphone gas-coupled photoacoustic spectroscopy (PAS) and photocurrent spectroscopy (PCS) have been applied simultaneously to high-resistivity, (0001)-oriented, single crystals of pure n-type CdS in order to obtain detailed information about the importance of the nonradiative capture or recombination channels at defect states below the band-gap energy of these materials. The primary spectral responses of well-characterized samples at open circuit and in the presence of perturbing transverse ac or dc electric fields, as well as secondary PA and PC spectra were found to be largely consistent with a substantial enhancement of the nonradiative recombination quantum efficiency at sub-band-gap wavelengths, concomitant with free-carrier trapping at defect centers (presumably sulphur vacancies) responsible for the observed red quenching of the photoconductivity. Our combined spectroscopic approach, besides the new direct information it yields concerning intrinsic band-to-band transition contributions to the PA and PC spectra as well as band-gap defect structure information, is also shown to raise the level of confidence regarding data interpretation through consistency arguments between the PAS and PCS results, an insight not easily obtained solely through conventional PC or optical-absorption measurements.