Bound excitons and resonant Raman scattering inCdxZn1−xTe(0.9≤x≤1)

Abstract

Luminescence and resonant LO-phonon Raman scattering spectra of CdTe, ${\mathrm{Cd}}_{0.97}$ ${\mathrm{Zn}}_{0.03}$Te, and ${\mathrm{Cd}}_{0.9}$ ${\mathrm{Zn}}_{0.1}$Te have been studied using selective, cw excitation in the temperature range of 1.7-100 K. The main spectral features are due to free excitons (polaritons), excitons bound to neutral donors ($D^0$,$X$), excitons bound to neutral acceptors ($A^0$,$X$) and a shallow bound-exciton band of unidentified origin ($S$ band). In CdTe, the fine-structure components of the ($A^0$,$X$) $1S$ exciton have strongly temperature-dependent Lorentzian line shapes which are shown to be due to the interaction with acoustic phonons. The fine-structure components of the ($D^0$,$X$) $1S$ exciton have Gaussian line shapes, reflecting random electric fields acting on the bound exciton. In the mixed semiconductors, all lines are Gaussian because of the random potential fluctuations. Two thermally activated processes are found to quench exciton luminescence: (a) thermalization of the ($A^0$,$X$) exciton into the free-exciton band; (b) dissociation of the exciton into free carriers. The latter is found to be operative in ($A^0$,$X$), free-exciton, and $S$-band luminescence quenching. Strong resonance enhancement of LO-phonon Raman scattering is observed mainly in the ($A^0$,$X$) band. The cross-section spectrum is well explained by an homogeneous line broadening in CdTe and by an inhomogeneous one in ${\mathrm{Cd}}_2{\mathrm{Zn}}_{1-x}\mathrm{Te}$. The intensity of the Raman lines is strongly temperature dependent, reflecting exciton-damping processes. Spectral diffusion is observed under selective excitation within the ($A^0$,$X$) band in the mixed crystals. However, it is attributed to exciton transfer within the states of the $S$ band [which overlaps the ($A^0$,$X$) band] and not within the ($A^0$,$X$) band itself.