The formation enthalpy of the InGa defect in GaAs was calculated ab initio by a hybrid method consisting of an atomistic- and a continuum calculation. The atomic relaxation of the lattice near the defect was treated with an atomistic calculation, whereas the long-ranged elastic strain energy was obtained from a linear elastic continuum calculation. The solubility of InAs in GaAs at room temperature and the maximum temperature of the miscibility gap in the solid solution phase were determined from the defect formation enthalpy. The results are in good agreement with a thermodynamic assessment. The effect of lattice coherency on the c/a ratio of epitaxial InAs monolayers grown on (100) GaAs substrates is determined. The stability of such InAs layers with respect to the Ga-rich InxGa1-xAs solid solution is predicted.
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