Influence of the surface morphology on the relaxation of low-strained InxGa1-xAs linear buffer structures

The relaxation of low-strained InGaAs linear-graded buffer layers up to 30 and 60% In content grown on GaAs substrates is studied. Strain-limited designs allow to preserve bidimensional (2D) growth mode with a relaxation driven by dislocation multiplication for large thickness, as in the case of single layers. However, it is shown that low-strained linear buffer layers are not able to inhibit tridimensional (3D) growth mode. Under standard growth conditions (500 degrees C), the surface morphology evolves from a near-flat one for samples up to x = 0.3, to a rough one at higher In content, characterised by saw-tooth morphology with well-defined facets. The development of the surface roughness changes the strain relaxation mechanism from dislocation multiplication inside the layer to surface elastic strain relaxation plus surface nucleation and a high density of threading dislocations. A constant surface strain in linear grades during all the dislocation-driven relaxation is proposed to allow the observed 2D to 3D transition. The influence of the grading rate, composition steps, and growth temperature on the surface morphology and the relaxation are analysed. Appropriate growth conditions and buffer design up to high Indium content allow to produce structures with improved structural and optical quality.