CLUSTER CALCULATIONS OF LOCAL VIBRATIONAL-MODE FREQUENCIES OF IMPURITIES IN III-V SEMICONDUCTORS - APPLICATIONS TO DEFECT COMPLEXES INVOLVING CAS IN GAAS

A direct method for calculating local vibrational mode (LVM) frequencies from defect complexes in zinc blende structure semiconductors is discussed. Interatomic interactions are described by an extension of the Keating model. Sets of force constants for eleven pure host crystals, including homopolar and III-V semiconductors, are found by fitting to available phonon dispersion relations. Simulations of isolated impurities in III-V hosts are reported and, from these, some trends are deduced for the changes in force constants needed for reproduction of experimental measurements. The use of the method in the interpretation of experimental results from defect complexes is illustrated by detailed calculations of (i) isotopic fine structures of the C(As) line and C(As) - (As)interstitial lines in GaAs and (ii) line patterns associated with carbon in AlxGa1-xAs.