Raman and photoluminescence studies on intrinsic and Cr-doped ZnSe single crystals

Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal-optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse-optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LQ phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of Cr-Zn-Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons with the electronic continuum of the conduction band in ZnSe is attributed co the renormalization of Raman peaks. The large electron capture cross section of deep-level Cr2+ and Cr1+ impurities prevents the observation of band-to-band photoluminescence transitions at similar to 2.67 eV in ZnSe:Cr; the charge transfer process Cr2+ <-> Cr1+ results in two emission bands at 2.25 and 1.40 eV. (C) 1998 American Institute of Physics. [S0021-8979(98)02411-6].