Relaxation study of In xGa 1− xAs/GaAs quantum-well structures grown by MBE on (0 0 1) and (1 1 1)B GaAs for long wavelength applications

Abstract

A comparative study of the relaxation mechanisms of thin, single In x Ga 1− x As/GaAs quantum wells (QWs) grown simultaneously on (0 0 1) and (1 1 1)B GaAs substrates is presented. Transmission electron microscopy (TEM) studies indicate that the primary relaxation mechanism for high-indium content (0 0 1) samples is the formation of 3D islands. For the growth conditions used, this occurs at an indium content above 0.24 in our samples. Although initially coherent with the substrate, above a certain critical thickness catastrophic relaxation occurs through a very high density of edge dislocations (>10 12 cm −2) as the islands coalesce. A similar relaxation mechanism is not observed in the (1 1 1) orientation. Here, 3D islanding is suppressed and instead a novel configuration of misfit dislocations (MD) appears at x>0.24. This new MD configuration presents an additional misfit-relieving component not taken into account in the previous theoretical analysis of the critical layer thickness (CLT) for (1 1 1)B. However, the mechanism is relatively inefficient when compared to the catastrophic relaxation in the (0 0 1) case, and therefore, the (1 1 1)B substrate still offers considerable advantages. This is illustrated by photoluminescence results, which show that (1 1 1)B In x Ga 1− x As QWs are able to reach wavelengths as long as 1.1 μm at room temperature.