Structural characterization of InGaAs/InP heterostructures grown under compressive and tensile stress

Abstract

A systematic study of strain relaxation mechanisms by TEM, XRD, RBS-channelling, SEM-CL and AFM in In x Ga 1− x As/InP heterostructures grown by MOCVD under tensile and compressive initial misfit is reported. It is found that the layers under compression (0.61< x<0.74) relax nearly symmetrically along the two 〈1 1 0〉 directions. The residual strain vs. the layer thickness follows the same law experimentally determined for compressive MBE-grown In x Ga 1−x As/GaAs (x<0.2) specimens showing no composition, growth technique or temperature effect. The tensile layers (0.2< x<0.36) relax asymmetrically with a critical thickness larger than for the compressive case. The relaxation is faster along the [1 1 0]-direction with the asymmetry that increases by increasing the tensile misfit. Cracks, formed after the growth, are found to present a different density along [1 1 0] and [1 −1 0] depending on the residual strain. Grooves also develop along the two 〈1 1 0〉 directions as soon as a measurable strain relaxation appears. A possible correlation between grooves, planar defects and the mechanism of strain release is discussed. Strain-balanced In x Ga 1− x As/In y Ga 1− y As multi-quantum wells (MQWs) grown on a compositionally graded In x Ga 1− x As/GaAs buffer to extend the absorption edge of photovoltaic devices to 1 eV are finally investigated. The MD network results in a marked cross-hatched morphology that affects the lateral strain distribution in the whole structure. Lateral composition fluctuations in the MQW are discussed in terms of step bunching mechanism occurring at the valleys of the cross-hatched surface.