Quantification of segregation and mass transport in In_xGa_1-xAs/GaAs Stranski-Krastanow layers

We report on transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy measurement of mass transport and segregation in InAs Stranski-Krastanow layers grown on GaAs(001) by molecular beam epitaxy at growth temperatures of 480 and 530 °C. Plan-view TEM reveals regularly shaped islands with a density of 7.8×10¹⁰ cm^-2 (480 °C) and 1.5×10¹⁰ cm^-2 (530 °C), respectively. Uncapped islands were investigated by strain state analysis of electron wave functions reconstructed from high-resolution TEM images. In-concentration profiles of the islands were obtained by the measurement of lattice-parameter profiles of the islands and the application of finite-element calculations. We find that the islands contain Ga-atoms with a percentage of 50% (480 °C) and 67% (530 °C). The capped InAs-layers were investigated with PL and TEM. In agreement with TEM, PL indicates a smaller and deeper potential well of the islands grown at 480 °C. Concentration profiles of the wetting layers were measured with TEM using the composition evaluation of lattice fringe images method, clearly revealing segregation profiles. The obtained segregation efficiency of In-atoms is 0.77±0.02 (480 °C) and 0.82±0.02 (530 °C). As an explanation for the strong mass transport of Ga from the substrate to the islands we show that the segregation of In atoms during the growth of the binary InAs can lead to the generation of vacancies in the metal sublattice. The vacancies are filled by Ga-atoms migrating along the surface or by a diffusion of the vacancies from the wetting layer and the islands into the GaAs buffer, leading to a unidirectional diffusion of Ga atoms from the buffer into the Stranski-Krastanow layer.