A study of the electronic and thermal strains in direct and indirect band-gap semiconductors by the use of photoacoustic spectroscopy

When a modulated beam of 1ight impinges on a solid, elastic waves are produced as a result o-f the surface motion due to thermal expansion. In semiconductors, in addition to this thermal expansion effect, there is also mechanical strain induced in the material due to the photogeneration of excess free carriers. Theory predicts the dependence of this strain on the pressure dependence of the energy gap, dEg/dp, thereby giving a theoretical foundation for the study of the difference of this effect in a direct band—gap semiconductor and an indirect band-gap semiconductor in view of the difference in sign of their pressure coefficients of the band gap.An experiment has been performed to investigate this difference by carrying out photoacoustic measurements on the magnitude and phase of the acoustic signals generated as a result of these effects, in GaAs (direct) and silicon (indirect). In GaAs, the results have been found to be very unsatisfactory due to the experimental factors cited. As for silicon, no results have been obtained because it required modulation -frequencies of over 1MHz in order to sustain a detectable strain. However from the information obtained, we learn that the electronic strain is the dominant contributor to the total acoustic signal, in both cases. Measures to take in order to improve the situation have been explored in some detail. The importance of the beam spot size has been highlighted. Generally, several important points which need more attention have been revealed.