Brillouin spectra of the longitudinal acoustic (LA) mode, traveling along the [001] direction, in silicon and germanium have been recorded in $180°$ backscattering using 457.9–514.5-nm laser lines. The wave velocity of the LA phonon propagating in the [001] direction was determined at hypersonic frequencies, from the measured acoustic phonon dispersion in silicon and germanium. The elastic modulus $c_{11}$ of the two semiconductors has been calculated from the respective measured hypersonic wave velocities and the results are compared with values determined from lower-frequency ultrasonic and other measurements. Interestingly, the hypersonic velocities are consistently lower by $\approx 1–2\%$ than the ultrasonic ones, but they generally agree within the present experimental accuracy.

• Received 22 December 1999

DOI:https://doi.org/10.1103/PhysRevB.62.12902