Dependence of structural and optical properties of In0.23Ga0.77As/GaAs quantum wells on misfit dislocations: Different critical thickness for dislocation generation and degradation of optical properties

The critical layer thickness Lc for dislocation generation and the characteristic thickness for consequent modification of the optical properties of strained quantum wells are shown to be remarkably different. We visualize the misfit dislocations directly using scanning cathodoluminescence and transmission electron microscopy. The dislocations are found to be asymmetrically distributed within the (001) interface plane. The critical thickness for In0.23Ga0.77 As on GaAs is determined to be Lc ≊15 nm in agreement with the theory of Matthews and Blakeslee [J. Cryst. Growth 27, 118 (1974)]. The deterioration of quantum efficiency, a change of recombination dynamics, the increase of spectral broadening and a shift of the band gap occur at much larger thickness than Lc since these quantities are barely affected by low dislocation densities. The relaxation is heterogeneous; strained and fully relaxed domains coexist. Using double crystal x-ray diffraction we find that partially relaxed layers with thickness larger Lc have no longer tetragonal but monoclinic crystal symmetry in quantitative agreement with the asymmetry of the dislocation density. Along the dislocation lines the lattice planes show an anisotropic tilting. The investigation of recombination dynamics and excitonic lifetime in this material system shows a change of the character of recombination dynamics from almost purely radiative to increasingly nonradiative transitions upon an increase of dislocation density. The delayed onset of degradation of the quantum efficiency and the drop of recombination lifetime is quantitatively explained by a diffusion model for charge carriers.