Energy levels and exciton oscillator strength in submonolayer InAs-GaAs heterostructures

We have studied monolayer (ML) and submonolayer InAs insertions (1--0.08 ML) in a GaAs matrix by photoluminescence, photoluminescence excitation, and optical-reflectance spectroscopy. Linewidths of heavy-hole and light-hole exciton peaks as narrow as 0.15 meV are observed. A surprisingly high exciton oscillator strength, its weak dependence on the average thickness of the InAs layer, and the pronounced anisotropy and splitting of heavy- and light-hole exciton peaks are all revealed in the optical studies and are attributed to the formation of ordered arrays of InAs wirelike islands. Furthermore, from photoluminescence and reflectance-anisotropy measurements, we confirm that the wire arrays are elongated along the [011\ifmmode\bar\else\textasciimacron\fi{}] direction.

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