Pressure-tuned resonance Raman scattering in AgGaSe2.

Abstract

Pressure-tuned resonance Raman scattering is investigated in the ambient and subsequent high-pressure phases of ${\mathrm{AgGaSe}}_{2}$ up to a pressure of 165 kbar using several fixed wavelengths of argon and krypton ion lasers. The nonpolar Raman active mode of ${\mathit{A}}_{1}$ symmetry in the chalcopyrite phase exhibits resonances at 18 and 30 kbar for the incident wavelengths of 676.4 and 647.1 nm, respectively. From the observed resonances, the pressure coefficient of a direct interband transition involving a defect level is estimated to be 6.9 meV/kbar in the chalcopyrite phase. The observed resonances are fitted to those calculated incorporating the corrections for absorption and reflection. In the high-pressure orthorhombic phase above 51 kbar, ${\mathrm{AgGaSe}}_{2}$ has an indirect gap that is lower than the energy of the visible photon; however, enhancement of the intensities of Raman modes is observed with yellow-green wavelengths due to resonance with another direct interband transition. The pressure coefficient of this transition in the opaque phases is found to be smaller than that in the chalcopyrite phase. \textcopyright{} 1996 The American Physical Society.