RAMAN-SCATTERING IN GERMANIUM-SILICON ALLOYS UNDER HYDROSTATIC-PRESSURE

The pressure dependence of vibrational Raman scattering in polycrystalline Ge1-xSix alloys is studied up to approximately 100 kbar across the compositional range, and the mode Gruneisen parameters gamma for the Si-Si, Ge-Ge, and Ge-Si optical phonons are determined from the Raman shifts. Gamma(Si-Si) increases with the Ge fraction of the alloy and gamma(Ge-Ge) increases with the Si fraction. The dependence of the Raman shift on x and pressure is described by a modified cellular isodisplacement model of optical phonons. In particular, the predicted dependence of the Ge-Si mode frequency on alloy composition at ambient pressure is found to agree much better with experiment when the variation of the average Ge-Si bond length with alloy composition is included in this model.