Acoustic phonon dispersion at hypersonic frequencies in Si and Ge

Brillouin spectra of the longitudinal acoustic (LA) mode, traveling along t he [001] direction, in silicon and germanium have been recorded in 180 degr ees backscattering using 457.9-514.5-nm laser lines. The wave velocity of t he LA phonon propagating in the [001] direction was determined at hypersoni c frequencies, from the measured acoustic phonon dispersion in silicon and germanium. The elastic modulus c(11) of the two semiconductors has been cal culated from the respective measured hypersonic wave velocities and the res ults are compared with values determined from lower-frequency ultrasonic an d other measurements. Interestingly, the hypersonic velocities are consiste ntly lower by approximate to1-2 % than the ultrasonic ones, but they genera lly agree within the present experimental accuracy.