CARRIER-INDUCED STRAIN EFFECT IN SI AND GAAS NANOCRYSTALS

New experimental results on Raman scattering from porous silicon and s ilicon and gallium arsenide nanocrystals are reported. In all of these systems, almost all vibrational modesl become Raman active and are re markably soft. A carrier-induced strain model is proposed to explain t he optical properties of these nanocrystal systems. According to this carrier-induced strain model, the selection rule of crystal momentum c onservation for Raman scattering is greatly relaxed in Si and GaAs nan ocrystals due to the dilatation strain caused by coupling of excited f ree carriers with the particle lattice and the optical properties of s uch systems are dominated by multiphonon assisted free-electron transi tion processes. (C) 1994 American Institute of Physics.