Acoustic phonon dispersion at hypersonic frequencies in Si and Ge

Brillouin spectra of the longitudinal acoustic (LA) mode, traveling along the [001] direction, in silicon and germanium have been recorded in 180 degrees 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 approximate to1-2 % than the ultrasonic ones, but they generally agree within the present experimental accuracy.