HIGH-PRESSURE PHASE-TRANSITIONS IN ZN1-XMNXSE - A RAMAN-SCATTERING AND PHOTOLUMINESCENCE STUDY

High-pressure phase transitions in semiconductor mixed crystals Zn1-xMnxSe (x less than or equal to 0.29) are investigated using Raman scattering and photoluminescence (PL) up to a pressure of 150 kbar. The samples are characterized for composition and crystal structure. A new Raman mode is observed between TO and LO frequencies for samples with x less than or equal to 0.097 and is assigned to a disorder activated zone boundary phonon. Apart from the PL band at 2.1 eV due to the T-4(1) --> (6)A(1) transition between the crystal field split levels of Mn2+, a PL band is observed at 1.94 eV in the samples with x = 0.063 which is assigned to a native defect forming a complex with Mn2+. The intensity of the PL band reduces drastically with pressure and completely disappears at about 15 kbar. A possible model for this behavior is presented. The pressure dependence of the phonon frequencies and the behavior of PL suggest three transitions P-1, P-2, and P-3 for the mixed crystals with re = 0.29, whereas those with x = 0.063 and 0.097 show only P-2 and P-3 transitions. In the sample with x = 0.29. TO mode splits into two components across P-1 and the low-frequency component exhibits softening. The crystals turn opaque across P-2 and the PL signal disappears suggesting that the band gap changes from direct to indirect. Intensities of Raman lines decrease and completely disappear across P-3. The difference in the behavior of the phase transitions for the low and the high values of a is understood on the basis of their different crystal structures.