Sound velocity in amorphous films of germanium and silicon

Sound velocities in evaporated amorphous Ge and Si films have been determined to be 81.6±1 and 76.8±3% of the corresponding directionally averaged crystalline speed, respectively. These results, combined with previous work, indicate a decrease of sound velocity through the preparation sequence of glow discharge, ion implantation, evaporation, and sputtering. The reason for this dependence is not known, but may be due to an increase in voids through the preparation sequence. The experimental technique was verified by also measuring sound velocities for crystalline films of Ge and Si. These velocities were 100.4±0.4 and 95.8±5% of the known corresponding directionally averaged crystalline speed for Ge and Si, respectively. Reasons for departure from 100% are discussed and the technique is concluded to yield sound velocities with an accuracy of a few percent.