Exciton Structure in Photoconductivity of CdS, CdSe, and CdS: Se Single Crystals

Abstract

Exciton-like structure has been observed in the photoconductive spectral response curves of CdS, CdSe, and CdS: Se single crystals at 77 and 4.2°K. It is observed that a number of peaks in the photoconductivity spectra of CdS and CdSe correspond to the exciton spectra identified by other optical measurements. In CdS the photoconductivity peaks corresponding to the $n=1, 2, 3$ states and the series limit of the exciton from the first and second valence bands and the ground state of the exciton associated with the third valence band were observed. For CdSe the peaks corresponding to the $n=1, 2, 3$ states of the exciton from the first and second valence bands were identified. Only the ground state was identified in the solid solutions. Assignment of the observed photoconductivity peaks was carried out by observing optical selection rules in polarized light. Within a given series the peaks form nearly hydrogen-like energy spacings, and it is observed that the exciton absorption lines always corresponded to photocurrent maxima. From the position of the photoconductivity maxima corresponding to the ground states of the excitons from the first and second valence bands, the exciton ground-state energy levels of the solid solutions were estimated and found to vary nonlinearly with composition. The coincidence of photoconductivity peaks and exciton absorption lines clearly suggests participation of excitons in photoconductivity. Several possible mechanisms for production of free carriers via exciton states are suggested.