Influence of Indium Composition on InAlAs QCLs

In this work, we explored the impact of indium composition (x) on the structural and optical characteristics of In _x Al _1-x As layers within the context of quantum cascade laser (QCL) structures grown on InP (100) substrates using the Metal Organic Vapor Phase Epitaxy (MOVPE) method. The quality of the In _x Al _1-x As QCL is notably influenced by the growth with low indium composition, evident in terms of crystallinity, interface sharpness, and optical properties. The properties of the InAsP layer at the InP/ In _x Al _1-x As junction are particularly sensitive to the indium composition. A drop below 0.52 in indium composition leads to a substantial lattice mismatch between the In _x Al _1-x As layer and the InP substrate, typically exceeding [3 8]%. This mismatch induces defects or traps within the bandgap, significantly impacting carrier localization in this system. Our study demonstrates that cultivating In _x Al _1-x As with a low indium concentration results in a strained (lattice-mismatched) In _x Al _1-x As layer. This finding is significant as it can be leveraged to balance strain in high indium content InGaAs layers, particularly in the context of applications involving quantum cascade lasers.