Optical phonons in isotope superlattices of GaAs, GaP, and GaSb studied by Raman scattering

We have investigated the LO-phonon Raman spectra of [100] oriented gallium isotope superlattices ((GaX)-Ga-69)(n)((GaX)-Ga-71)(n) [X=P,As; both elements have a single stable isotope] at low temperature, When the number of monolayers 2n within one superlattice (SL) unit cell is varied, anticrossings between phonons confined in the (GaX)-Ga-69 and (GaX)-Ga-71 layers an observed. We have used a planar bond-charge model to calculate the frequencies and intensities of the modes as a Function of layer thickness. For the Gap isotope SL's, we find that a simulation of isotopically mixed interface layers is in good agreement with the experiment, while the assumption of ideal interfaces does not reproduce the data well. Spectra from the GaAs isotope SL's an substantially broadened compared to the LO phonon width in bulk samples, thus allowing only a qualitative discussion of phonon-confinement effects. Predictions for GaSb isotope SL's, in which both Ga and Sb isotopes can be substituted, are given. Raman spectra of bulk GaAs with varying gallium-isotope ratio are also discussed. [S0163-1829(99)04019-9].