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We address the issue of the composition and strain dependence of the piezoelectric effect in semiconductor materials, which is manifested by the appearance of an electric field in response to shear crystal deformation. We propose a model based on expressing the direct and dipole contributions to the polarization in terms of microscopic quantities that can be calculated by density functional theory. We show that when applied to the study of In x Ga 1−x As alloys, the model is able to explain and accurately predict the often-observed discrepancies between the experimentally deduced values of e 14 and those linearly interpolated between the values of InAs and GaAs. The values of the piezoelectric coefficient predicted by our approach compare very well with values deduced from photocurrent measurements of quantum well samples grown on ͑111͒ GaAs substrates by molecular beam epitaxy.

We address fundamental issues relating to the symmetry of the shape and the nonuniform composition of InGaAs quantum dot islands. Using atomistic simulations in the framework of the Tersoff empirical potential, we study the effect of compositional gradients in the In distribution on the piezoelectric effect in quantum dots. We demonstrate that the internal piezoelectric fields contribute strongly to the experimentally observed optical anisotropies. This is confirmed by accurate high-resolution transmission electron microscopy analysis over hundreds of islands grown in different conditions that reveals the absence of structural anisotropy under our growth conditions.

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