STRAIN-SHIFT COEFFICIENTS FOR PHONONS IN SI1-XGEX EPILAYERS ON SILICON

The measurement of the strain coefficients for the three longitudinal-optical phonons in strained Si1-xGex (0 < x < 0.35) molecular-beam-epitaxy-grown epilayers on (100) Si is reported. Strain in the epilayers was varied by annealing metastable pseudomorphically grown epilayers at various temperatures and was measured by double-crystal x-ray diffractometry. The strain-shift coefficients for the phonons were obtained from Raman-scattering measurements of the annealed specimens. For all compositions it was found that the Si-Si phonon frequencies vary linearly with strain. The strain-shift coefficient, however, showed a small composition dependence, varying from about - 750 cm-1 at x = 0.08 to about - 950 cm-1 at x = 0.35, corresponding to a stress factor-tau almost-equal-to 0.4 + 0.57x + 0.13x2 cm-1/kbar for this vibration. The magnitude of the corresponding coefficients for the Si-Ge and Ge-Ge lines are slightly less than that of the Si-Si line, and vary in a similar way with x.