Spectral Dependence of the Photoconductance in CdCr2Se4

Abstract

The spectral response of the photoconductance in In-doped single crystals of the ferromagnetic semiconductor CdCr2Se4 was measured in the range of photon energies from 1 to 3 eV, between 80° and 340°K. The photoresponse is attributable to three distinct transitions: I varies between 1.2 and 1.35 eV, II and III remain constant near 1.45 and 1.8 eV, respectively. Transition I is identified with that which had been previously observed in absorption measurements, and shows the same characteristic ``red shift'' associated with magnetic ordering as the temperature is lowered. Transitions I and II are superimposed near room temperature, and become separated only below 200°K, after I had shifted to lower energies. Transitions II and III are relatively temperature insensitive. The relative photoresponses associated with transitions I and II remain nearly unchanged over a temperature range in which the total photoresponse changes by orders-of-magnitude. This points to the likelihood that the carriers excited by these two transitions move in the same final state. Transition III corresponds to a high-lying one which had been previously inferred from magneto-optical measurements.