Vibrational Modes in Si$\text{}_{x}$Ge$\text{}_{1-x}$ Alloys: Temperature and Compositional Dependence

We report infrared absorption spectra of crystalline Si$\text{}_{x}$Ge$\text{}_{1-x}$ alloys with silicon content 0 ≤ x ≤ 1 at room and liquid nitrogen temperature. We covered the spectral range from 375 to 1200 cm$\text{}^{-1}$ that includes the "Si-Ge" and "Si-Si" single-phonon transitions, the continuum of two-phonon processes, and the localized mode of interstitial oxygen. We study the change of vibrational structure and correlation between reference (pure Si and Ge) and alloy spectra. We observed shifts to lower wave numbers by about 1 cm$\text{}^{-1}$ of the two-phonon absorption bands per 1% increase in germanium concentration. Pronounced changes of the vibrational spectra upon lowering the temperature were detected.