Raman studies of isotope effects in Si and GaAs

We have measured by Raman scattering changes of the optic phonon energy and line width in Si and GaAs with isotopic composition. The phonon energies of isotopically pure samples show the expected dependence on the average atomic mass in Si and the reduced mass in GaAs, respectively, as well as small anharmonic contributions. In isotopically disordered samples we find frequency shifts of 1.15(20)cm(-1) for (Si0.5Si0.5)-Si-28-Si-30 and 0.31(20)cm(-1) for the TO phonon of (GaAs)-Ga-nat, induced by mass disorder which also contributes to the line broadening. We give theoretical estimates of these effects, (C) 1999 Elsevier Science B.V. All rights reserved.