Our work centers on understanding the morphological and compositional evolution, as well as the strain relaxation and defect formation in lattice mismatched compound semiconductor thin films. 3D roughening will occur in lattice mismatched films above some critical thickness. 3D islands and surface ripples have been shown to relieve strain to some degree. Theory shows that pits are even slightly more efficient at relieving strain than islands, but experimental evidence of pit nucleation has only appeared recently. This work presents a detailed study of the morphological evolution of In0.25Ga0.75As alloy layers on (001) GaAs substrates (lattice mismatch » 1.8%) as a function of thickness and growth conditions and shows that pit nucleation is an additional mechanism for ripple formation.
Another area of interest is in lateral composition modulation, i.e.; controlled phase separation in multilayer structures. By and large, phase separation is undesirable and has been avoided in device structures. However, reproducibly obtaining regular and robust arrays of phase-separated material is a promising way to acquire low dimensional structures such as quantum dots or wires. We have achieved such arrays by the deposition of short period superlattices, where each layer is on the order of one or two monolayers thick. We have demonstrated lateral composition modulation in several different materials systems, such as GaAs/InAs, AlAs/InAs, and GaSb/GaAs. We have shown that the appearance of lateral composition modulation is correlated to roughening of the surface front. Also, the microstructure is largely dictated by the relative lattice mismatch between the individual superlattice layers. These results are consistent with continuum perturbation models that predict the coupling of morphological and compositional instabilities under the appropriate circumstances.