The optical properties of single crystals of cadmium selenide

Abstract

Cadmium selenide is a semi-conductor with interesting optical and photoconductive pro­perties in the red part of the spectrum. This paper is concerned mainly with the measure­ment of a number of the optical properties of cadmium selenide in which pure single crystals grown by sublimation in atmospheres of inert gas were used. Thin plates of CdSe less than 1 μ thick were obtained, and these are transparent right down to short wavelengths in the region of 4500Å. At liquid-helium temperatures, sharp absorption bands may be observed in the visible part of the spectrum, both in the red part near the absorption edge, and in the green part. From the position of these bands and the shape of the absorption curves, in­ formation has been obtained concerning the band structure. Two levels occur near the top of the valence band with a small energy separation ( ca . 0.026eV) and these are connected with crystal field splitting. The energy separation of the deeper levels is ca . 0.41 eV and these levels are thought to arise from spin-orbit interaction. Line spectra have also been observed, and these are connected with levels (possibly exciton levels) close to the conduction band. The line spectra appear to be less complicated than those previously observed by Gross & Sobolev (1960). The bands and lines in CdSe appear near the optical absorption edge. Measurements have now been made for wavelengths near the absorption edge of the refractive indices and birefringence at temperatures down to liquid-helium temperature. The birefringence changes in an unusual manner in the wavelength region between the two bands in the red part of the spectrum. These results should be of use in a complete discussion of the optical properties of CdSe.