Force-Constant Changes due to Impurities in GaAs and GaP

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Abstract
The theory of localized modes due to Dawber and Elliott has been extended to include force-constant changes near the impurity site. This has been used to evaluate the force-constant changes due to impurities in GaAs and GaP. Calculations have been made by assuming a band model for the density of phonon states of the host crystals. The results are compared to a calculation of force-constant changes of these impurities in a molecular model. It is observed that the band model is more suitable than the molecular model for calculating the force-constant changes. It is pointed out that the present theory is inadequate for treating very light impurities.