Effect of Uniaxial Stress on the Reststrahlen Spectrum of GaAs

We have measured the effect of uniaxial stress on the reststrahlen spectrum of GaAs. Measurements were performed on pure and heavily doped samples. The signs of the splittings and shifts observed for [111] and [001] stress agree with previous Raman-scattering work. We are able to obtain more complete data for the LO phonons because of differences in the Raman and infrared (ir) selection rules. The magnitudes of the shifts and splittings obtained in the ir measurements are somewhat larger than in the uniaxial Raman work. We interpret this fact as evidence for relaxation of the stress near the surface: The penetration length of the light in the Raman experiment is considerably less than in the reststrahlen region.