Se-precipitation in ZnSe under moderate-power laser-irradiation at high-pressure

We report evidence for the formation of Se inclusions in ZnSe under laser-irradiation during pressure-Raman experiments. Spectra of high-quality $^{68}$Zn$^{76}$Se crystals are recorded at 300K for pressures of 0-13GPa using 647nm excitation at powers of 10 and 100 mW (focal spot $\sim $ 50$\mu $m.) For runs at the higher power a new Raman peak appears at 1.8 GPa, and shifts to lower energy at the rate --3.5 cm$^{-1}$/GPa with further increase of pressure. Its frequency, 228cm$^{-1}$ at 1.8GPa, is within 7 cm$^{-1}$ of the A1 and E$^{\mbox{'}\mbox{'}}$ Raman peaks in trigonal Se, which both exhibit negative, strongly non-linear, pressure shifts.\footnote{W. Richter, \textit{et. al.}, phys. stat. sol. (b)\underline {56}, 223(1993); K. Aoki, \textit{et. al.}, J. Phys. Soc. Japan \underline {48}, 906 (1980).} In particular, the pressure-shift of the new ZnSe peak gives a reasonable fit to the average dependence of the Se A1 peak over the range 2-8 GPa. No assignment to any of the ZnSe acoustic modes (one- or two- phonon) that also soften with pressure is feasible for the new peak. It is most likely related to the Se A1-mode in Se-inclusions, whose tendency to precipitate appears to increase with pressure.