We will demonstrate that atomic species in mixed III-V and group III nitride layers, differing in their covalent tetrahedral radii, are not distributed at random on their respective sub-lattices. Two types of deviations from randomness are observed: phase separation and atomic ordering. In both systems, phase separation is two-dimensional in nature, evolves while the layer is growing and occurs along soft directions lying in the growth plane. This microstructural feature develops to reduce the strain energy of the system.

Mixed III-V layers exhibit CuPt type ordering where lattice periodicity along two of the four <111> directions is doubled. Ordering is caused by the (2 x 4) surface reconstruction induced sub-surface stresses which bias the occupation of sites by species differing in their tetrahedral radii. On the other hand, 1:1 atomic ordering in mixed group III nitride layers having the wurtzitic structure appears to be driven by the preference of atomic species, differing in their tetrahedral, for different sites on facets.

Influence of the above microstructural features on electronic and optical properties will also be discussed.

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