VIBRATIONAL-MODES IN GERMANIUM-RICH SIXGE1-X ALLOYS

We report infrared absorption spectra of single crystal Si¢Ge1-¢ alloys with the low silicon content of 0 ≤ ¢ ≤ 0.01 at room temperature. We have observed local vibration of Si at (384 ± 1) cm-1 in transmittance taken differentially against pure Ge. Its strength has been calibrated using precise values of ¢ obtained from electroreflectance measurements: the peak absorption coefficient is (3.9 ± 0.2) × 10-21 cm2 times the Si concentration in cm-3. The transverse effective charge |eT*| = 0.08 ± 0.02 results from the measured strength and width of the local band. The differential spectra reveal modifications of two-phonon bands of the Ge host lattice below 400 cm-1. A series of weak absorption bands is observed in the region from 440 to 700 cm-1. We assign them tentatively to second-order processes involving the local Si mode and impurity-induced vibrations of Ge. © 1990.