Linewidths and Two-Electron Processes in Spin-Flip Raman Scattering from CdS and ZnSe

We present inelastic-light-scattering data and analyses for spin-flip scattering from conduction electrons in CdS and ZnSe. Cross sections, linewidths, and line shapes are studied as functions of magnetic field, temperature, scattering angle, and donor concentration. Both free-conduction-electron spin-flip processes and spin-flip processes involving conduction electrons bound to shallow donors are observed. These processes exhibit different selection rules and temperature dependences; the free-electron spin-flip processes exhibit only ${\alpha }_{\mathrm{ij}}$ scattering in which $i\ne j$ and $i$ or $j\parallel \overrightarrow{\mathrm{H}}$ as expected, while the bound-electron spin-flip processes also exhibit strong ${\alpha }_{\mathrm{xx}}$ and ${\alpha }_{\mathrm{yy}}$ scattering ($z$ is the [0001] optic axis), in agreement with the selection rules calculated for shallow donors at $C_{3V}$ Cd sites by Thomas and Hopfield. For right-angle scattering, the free-electron linewidth increases from 0.05 ${\mathrm{cm}}^{-1\mathrm{}}$ (half-width at half-height) at 2 °K to about 4 ${\mathrm{cm}}^{-1\mathrm{}}$ at ∼ 150 °K in both ZnSe and CdS. This broadening is not due to a decrease in spin lifetime, but rather to a spin diffusion, as directly confirmed by the angular dependence of the spin-flip linewidth. The linewidth is observed to vary as $q^2$, where $q$ is the momentum transfer in the light-scattering process. Bound-electron scattering exhibits a linewidth which is independent of scattering angle and nearly independent of temperature over the 2-150 °K range. The spin-diffusion model is thus not applicable to bound-electron scattering. The double spin-flip process observed involves two interacting electrons with an apparent attractive energy of 0.25 ± 0.05 ${\mathrm{cm}}^{-1\mathrm{}}$. Selection rules, relative cross sections, field dependence, and binding energy of the double spin-flip transition are discussed. At sufficiently high input powers (≥3 MW/${\mathrm{cm}}^2$) the CdS single spin-flip scattering becomes stimulated, resulting in a tunable, visible, spin-flip laser.