MNSE - ROCK-SALT VERSUS ZINCBLENDE STRUCTURE

MnSe epilayers are grown on (001) GaAs substrates by molecular beam ep itaxy (MBE), which exhibit NaCl structure as shown by x-ray diffractio n. Their zone-center transverse optic (TO) and longitudinal optic (LO) phonons have been observed as minima in infrared transmission with ob lique incidence, i.e., in the Berreman geometry, using Fourier transfo rm infrared spectroscopy. In contrast, they are absent in the Raman sp ectrum, consistent with the rule of mutual exclusion. The TO and LO fr equencies thus deduced, 139 and 239 cm(-1) respectively, differ signif icantly from the corresponding values of 219.5 and 257 cm(-1) obtained for zinc-blende MnSe. The latter were deduced from the x-->1 values o f the TO and LO frequencies from a Raman study of MBE-grown Cd1-xMnxSe epilayers. A similar study of ZnTe/MnSe multiple quantum well structu re also yielded the LO at 249 cm(-1), compatible with the zinc-blende character of MnSe in such structures, combined with the strain experie nced by them.