Local force variations caused by isoelectric impurities: Method of determination from first principles

Abstract It is shown how the variation of lattice dynamical force constants caused by substitutional isoelectronic impurities can be evaluated ab initio . The approach, illustrated on the example of Al in GaAs, is based on local density functional and uses ionic pseudopotentials of Al, Ga, As as the only input; Hellmann-Feynman theorem is applied in order to extract from self-consistent electronic charge densities the forces acting on atoms in periodic patterns in which entire planes of impurities are displaced. The defect-induced variations of inter planar force constants are converted into the inter atomic ones, which can be compared with those determined by phenomenological models from the measured local mode frequencies. A method is presented which allows to account for the effect of relaxation without requiring an explicit determination of the latter. Particular problems resulting from dealing with entire plane of defects are discussed and an estimate for relaxation is given.