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 of wells. Generation of high-density threading dislocations is minimized by using slowly composition graded InGaAs buffers (10 %/μm), which show full strain relaxation and a pile-up of lateral dislocations deposited parallel to the interface. High resolution cross-section transmission electron microscopy, photoluminescence and high resolution X-ray diffractometry show very smooth and defect-free multiple wells. We suggest a strain relaxation model for linearly composition-graded epitaxial layers and define local critical thickness as a function of the film thickness. The calculated critical thickness 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 the minimized threading dislocations in the composition graded layers. The analysis with calculated results coincides with the dislocation structures observed using cross-section transmission electron microscopy.