HOST AND IMPURITY ISOTOPE EFFECTS ON LOCAL VIBRATIONAL-MODES OF GAASC(AS) AND GAASB(AS)

New high-resolution FTIR transmission spectra are presented for the fine structure of local vibrational modes associated with B-10, B-11 and C-12 impurities in GaAs. Attempts are made at reproducing these results from cluster calculations based on a range of harmonic force constant models. For the boron impurities good agreement with the fine structure arising from host crystal isotope effects is readily achieved, but the impurity isotope shift cannot be satisfactorily reproduced by harmonic models which can be deemed physically reasonable. The same conclusion applies to the carbon results and, in this case, even the host isotope effects are not so well reproduced. We derive a general relation between impurity isotope shifts and the width of the fine structure from host crystal isotope effects. Both the detailed model calculations and this more general relationship point to the conclusion that, for any model to be satisfactory, anharmonicity must be included.