Relaxation study of In_xGa_(1-x)As/GaAs quantum-well structures grown by MBE on (0 0 1) and (1 1 1)B GaAs for long wavelength applications

摘要

A comparative study of the relaxation mechanisms of thin. single In_xGa_(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-rc1ieving 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_xGa_(1-x)As QWs are able to reach wavelengths as long as 1.1 μm at room temperature.