Epitaxial Si/InAs/Si heterostructure grown on ͑001͒ Si substrate by molecular beam epitaxy and annealed at 800°C was investigated by high resolution transmission electron microscopy. Extensive interdiffusion leads to the formation of an InAs solid solution in the Si cap layer. Additionally, InAs-enriched regions with extensions of ϳ6 nm, which exhibit two kinds of ordering are observed. The ordering of InAs molecules has occurred, respectively, in ͑101͒ and (101) planes inclined and ͑110͒ and (11 0) planes parallel to the ͓001͔ growth direction. It is attributed to the energy gain from the reduced number of mixed Si-As and Si-In bonds. The sample show photoluminescence in the 1.3 m region, which is tentatively attributed to the recombination of excitons localized in the ordered regions.

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