Microstructural characterization of heteroepitaxial layers of III-V compound semiconductors

This work describes results obtained from TEM, TED and HREM studies of MBE and MOCVD InASySb1-y, MOCVD InxGa1-xAs, MOCVD InPySb1-y and MOCVD GaPySb1-y layers which were grown over a wide range of conditions. These semiconductor layers are of considerable importance for a variety of applications in optoelectronic and high-speed devices. TEM/TED investigations showed that phase separation occurs in MBE InAsSb layers, resulting in two phases with platelet structures ~5 to ~200nm thick approximately parallel to the layer surface. Phase separation was dependent on growth temperature and layer composition. Anisotropic geometry of the platelets was observed when viewed in the [110] and [110] directions. The compositions of the two phases were derived by TED and EDX analyses. A model for the phase-separated layers was proposed based on the presence of a miscibility gap and using the lateral and island growth mechanisms. TEM results of InGaAs, InPSb and GaPSb layers showed a fine scale modulated contrast (8-20nm in scale) which is a characteristic of alloy clustering occurring by spinodal decomposition, and a fine scale speckle contrast (4-5nm in scale). TEM/TED studies showed that [110] diffuse intensity lines in [001] TED patterns of InGaAs are not related to the fine scale modulated contrast but to the fine scale speckle contrast. It was concluded that a fine scale modulated contrast due to alloy clustering coexists with a fine scale speckle contrast associated with static atomic displacements from the average lattice in InGaAs. For InPSb and GaPSb, a fine needle-like contrast was also observed, which corresponds to diffuse streaks in the [110] patterns. This fine needle-like contrast was attributed to segregation of atoms at missing rows of atoms in the reconstructed growing surface. TED investigations revealed CuPt-type ordering in some of the InGaAs, InAsSb and InPSb layers. Regardless of alloy systems and growth conditions, the ordering occurred on only two of the four possible {111} variants. The degree of ordering was strongly dependent on growth conditions. Two variants of the ordered regions in InGaAs nucleated separately. TED/HREM studies of the ordered structure in InGaAs revealed a direct relationship between the inclination and elongation of superlattice spots and the morphology of anti-phase boundaries present within the domains. Two competing processes of surface-induced ordering, and bulk-induced disordering within a transition region, were considered to interpret the growth condition dependence of the ordering in InGaAs. A model for the ordering observed was proposed based on the surface reconstruction mechanism. MBE InAsSb strained layer superlattices (SLSs) were examined by TEM and HREM techniques. Defect configuration and the atomic structure of tetragonal distortion of the SLSs were directly imaged. Defect behaviour was dependent on the geometry of the SLSs. Possible relaxation mechanisms for the SLSs were proposed.