Growth of high indium content InGaAs/(Al)GaAs on GaAs and strain relaxation in composition graded buffers

We demonstrate atomically smooth growths of high indium content InGaAs/(Al) GaAs on GaAs by using molecular beam epitaxy with a low growth temperature and an optimized in-situ 1 ML GaAs-plus growth interruption at both sides o f wells. Generation of high-density threading dislocations is minimized by using slowly composition graded InGaAs buffers (10 %/mum), which show full strain relaxation and a pile-up of lateral dislocations deposited parallel to the interface. High resolution cross-section transmission electron micro scopy, photoluminescence and high resolution X-ray diffractometry show very smooth and defect-free multiple wells. We suggest a strain relaxation mode l for linearly composition-graded epitaxial layers and define local critica l thickness as a function of the film thickness. The calculated critical th ickness show a good correspondence with the depths of the dislocation-free regions formed at the tops of the graded layers. From analysis of the model , we explain the origin of the dislocation structures and the reason for th e minimized threading dislocations in the composition graded layers. The an alysis with calculated results coincides with the dislocation structures ob served using cross-section transmission electron microscopy.