Role of elastic exciton-defect scattering in resonant Raman and resonant Brillouin scattering in CdSe

Abstract

We report a detailed resonant Brillouin and Raman study in the wurtzite semiconductor CdSe in the vicinity of its lowest-energy $A$ exciton. By using a spectrometer and laser system with a combined spectral resolution of 0.16 ${\mathrm{cm}}^{-1\mathrm{}}$ we were able to measure the widths and line shapes of Raman and Brillouin modes. In a nominally undoped CdSe sample we found that wave vector is conserved in the light-scattering process only for incident photon frequencies below the longitudinal exciton frequency. For incident laser frequencies above the longitudinal exciton frequency, scattering processes which do not conserve wave vector became dominant. We propose that this breakdown in wave-vector conservation is due to elastic exciton-defect scattering, and our results are in qualitative agreement with the predictions of the elastic exciton-defect scattering theory of Gogolin and Rashba. In another CdSe sample doped with Li, we found that its Raman and Brillouin spectra are always dominated by wave-vector-nonconserving modes even for incident photon frequencies below the $A$-exciton frequency. From the Brillouin spectra we deduced the following parameters for the $A$ exciton in CdSe: $M_{\perp } \left(\mathrm{exciton}\mathrm{mass}\mathrm{perpendicular}\mathrm{to}\mathrm{the} c \mathrm{axis}\right)=0.40\mathrm{}{m}_0$ (free electron mass), $M_{\parallel }\sim 1.3m_0$, ${\omega }_T \left(\mathrm{transverse}\mathrm{exciton}\mathrm{frequency}\right)=14713\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$ (at ∼10 K), and the splitting between longitudinal and transverse exciton frequencies is 4 ${\mathrm{cm}}^{-1\mathrm{}}$.