Surface stoichiometry, epitaxial morphology and strain relaxation during molecular beam epitaxy of highly strained InAs/Ga 0.47In 0.53As heterostructures

Abstract

We have investigated the influence of the surface stoichiometry (i.e. cation- or anion-stable surface) of the InAs film on the growth mechanisms at work during molecular beam epitaxy of highly mismatched (3.2%) InAs films buried in a Ga 0.47In 0.53As matrix. In situ reflection high-energy electron diffraction (RHEED) studies reveal that under As-stable conditions InAs grows on Ga 0.47In 0.53As in the Stranski-Krastanov mode and that the strain relaxation occurs via three different successive paths. After an initial stage with the InAs film fully strained on the Ga 0.47In 0.53As buffer, a slow relaxation process ascribed to relaxation through the formation of small coherent islands takes place. Then, above 1.5% relaxation, misfit dislocations are generated and complete very efficiently the strain relief. This fast relaxation path is closely connected to the detection of large islands in the RHEED pattern. In contrast, when InAs is grown under In-stable conditions islanding is totally inhibited and relaxation is strongly delayed. The results of ex situ structural and optical investigations demonstrate that samples of superior quality are obtained under these growth conditions. This finding is interpreted in terms of a reduced surface-diffusion imposed to the As adatoms, which are in this case the growth-limiting species, by the highly reactive In-stable surface which acts as a virtual surfactant. In addition to opening a new route for the synthesis of high quality strained-layer heterostructures, our results call for new heteroepitaxial growth models to be developed. In particular, to be realistic any new model should now account for the strong influence of the surface stoichiometry of a strained film on its epitaxial morphology and relaxation state.