Photoreflectance study of growth mode in InAs-GaAs quasimonolayer single quantum wells

Photoreflectance measurements have been performed in a number of InAs/GaAs single-quantum wells with nominal thickness L ranging from 0.6 to 2.0 ML. The InAs growth mode was investigated by analyzing the evolution, with increasing coverage, of the optical response associated with the InAs layer. For L less than or equal to 1.6 ML, the experimentally derived energies for the optical transition originating in the InAs are consistent with those evaluated in a simple square-well envelope-function scheme. The dependence of the photoreflectance line shape broadening on L is well described up to L=1.4 ML in terms of a disordered InAs/GaAs interface made by interconnected InAs and GaAs islands with a typical size of order 2 nm. For L=1.6 Mi,, the quantum well spectral features broaden abruptly and vanish for L=2 ML, suggesting the disappearance of the InAs 2D layer in favor of a predominant nucleation of large quantum dots.