Structural and optical properties of (100) InAs single-monolayer quantum wells in bulklike GaAs grown by molecular-beam epitaxy

A single (100) monolayer InAs in GaAs is grown by conventional molecular-beam epitaxy. The structural properties of these highly strained single-monolayer quantum-well samples are investigated with high-resolution double-crystal x-ray diffractometry, double-crystal x-ray reflection topography, and transmission electron microscopy. The calculation of the x-ray-diffraction patterns within the framework of the dynamical diffraction theory provides the precise determination of the strain state and the thickness of the layer even in the submonolayer region. This approach allows us to prove the superior crystalline quality of the samples. In particular, no misfit dislocations are generated at the highly strained InAs/GaAs heterointerface as observed by x-ray topography. These results are confirmed by the high-resolution lattice image of the heterointerface, indicating the excellent homogeneity and flatness of the InAs lattice plane.

In addition, we present evidence for a strong impact of the strain on the incorporation of cations in the crystal lattice during epitaxial growth. This structural information on an atomic scale is necessary for a correct interpretation of photoluminescence (PL) and photoluminescence excitation (PLE) spectra. We correlate the results of our structural investigation with a simple analysis of the PL linewidth and the blue shift (Stokes shift) of the heavy-hole exciton resonance in the PLE spectrum with respect to the electron heavy-hole transition in the PL spectrum. Both the PL linewidth and the Stokes shift are explained in terms of a fluctuation of the well width of 1 monolayer and a lateral island size of about 10 nm, in excellent agreement with the lattice image of the structure. The important result of our investigation is that a single monolayer of InAs yields a pronounced redshift of the quantum-well luminescence as compared with that of bulklike GaAs. Additionally, the PLE spectrum provides direct experimental evidence for a weak confinement of the light-hole excitons in the InAs plane. This result clearly indicates a type-I band alignment for both heavy holes and light holes, as predicted by a simple envelope-function calculation. Finally, the samples exhibit a surprisingly high luminescence efficiency, resulting from a very efficient trapping of the photoexcited carriers into the InAs-monolayer quantum well with trapping times of the order of only 20 ps.