Structural studies have been performed using transmission electron microscopy and diffraction on Ill-V ternary and quaternary alloy epitaxial layers. The results revealed that 'natural superlattice' structures had spontaneously formed in some of the layers by phase separation and/or atomic ordering which occurred at the surface during growth. The structure of these natural superlattices and the growth conditions for their occurrence are described and their effect on the electrical and optical properties of the layers, e.g. bandgap narrowing, and possible mechanisms of formation are briefly discussed.

Molecular beam epitaxial growth of a normally homogeneous InAs0.5Sb0.5 alloy below 430 °C results in its coherent phase separation into platelets of two different alloy compositions with tetragonally distorted crystal lattices. This produces a ‘‘natural’’ strained layer superlattice (n-SLS) with clearly defined interfaces modulated in the [001] growth direction. A description of the n-SLS growth mode in InAsSb is outlined, and the optical response of a n-SLS structure, which extends to 12.5 μm−considerably further than that of a homogeneous InAs0.5Sb0.5 layer (8.9 μm)−is reported.

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