Strong oscillations detected by picosecond ultrasonics in silicon: Evidence for an electronic-structure effect

We present the results of picosecond ultrasonic experiments performed at different laser wavelengths in different samples all grown on silicon (100) substrates. In the blue range we report on strong oscillations in the reflectivity curves. We present experimental and numerical results that identify the origin of these oscillations in a strong acousto-optic interaction in the silicon substrate. Then we propose to explain the high amplitude of these oscillations by a relation with direct interband transitions in silicon which fall in the same range. This explanation is supported by two other experimental evidences: First, sudden changes in the phase of the detected signal are observed as the laser wavelength is tuned around the interband transitions. Second, we report on a shift of this phase change as the substrate is heated. This study shows that the effect of interband transitions on picosecond ultrasonic studies we had first demonstrated in thin metallic films can be extended to semiconductors.