Fine Structure and Magneto-Optic Effects in the Exciton Spectrum of Cadmium Sulfide

The valence band of cadmium sulfide is split by spin-orbit and crystal field effects into three nearly degenerate bands at k=0. The magneto-optic absorption spectrum of direct excitons formed from the top valence band and the conduction band has been studied in detail. Most of the experiments reported have been performed in light polarized parallel to the hexagonal axis. In this geometry, the exciton series consists of weak lines amenable to magneto-optic experiments. When the magnetic field and the wave vector of the light are perpendicular to each other and to the hexagonal axis, the reversal of the magnetic field produces large changes in the absorption spectrum. This effect can be quantitatively understood as an interference effect between allowed exciton transitions (optical matrix elements independent of the wave vector of the light) and forbidden exciton transitions (optical matrix elements proportional to the wave vector of the light). It is shown that in CdS the forbidden processes having a principal quantum number 2 are somewhat stronger than allowed processes of the same quantum number. By using group theory and the effective-mass approximation, the electron and hole anisotropic $g$ values and masses are determined from an analysis of the exciton spectrum. The electron mass, ${0.20}_5$ $m$ (almost isotropic), determined in this analysis is compatible with the assumption that the k=0 conduction band valley is the lowest conduction band valley. The hole masses for the top valence band are $m_{h\perp }=0.7\mathrm{} m$ and $m_{h\mathrm{II}}\approx 5m$. An experimental upper limit on the slope of the conduction band at k=0 is obtained.