Optical investigations in (In,Ga)As/GaAs quantum wells grown by metalorganic molecular-beam epitaxy

Photoluminescence, reflectivity, and thermally detected optical-absorption (TDOA) experiments have been performed at liquid-helium temperatures on two strained ${\mathrm{In}}_{\mathit{x}}$ ${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As/GaAs quantum-well (QW) structures grown by metalorganic molecular-beam epitaxy (MOMBE). The QW thicknesses vary from 3–15 ML and 4–16 ML by two monolayer (ML) steps. It is demonstrated that the MOMBE technique allows the thickness to be controlled with an accuracy of 1 ML. The electron–heavy-hole excitonic transitions (${\mathit{e}}_1$ ${\mathrm{hh}}_1$) are detected for all the QW’s. TDOA enables the light-hole excitonic transition to be observed for the thinnest well in the second sample. The QW excitonic absorption energies are compared with calculations within the framework of the envelope-function approximation by taking into account the strain effects and the indium segregation phenomenon. An accurate determination of the strain conduction-band offset is derived (${\mathit{Q}}_{\mathit{c}}$=0.64±0.01) and it is found that the indium segregation is very weak at the QW interfaces compared to structures grown by MBE. The light-hole band configuration is type I for the evaluated alloy composition (x=0.21–0.22). Under low excitation intensity, intermediate photoluminescence emissions appear between the ${\mathit{e}}_1$ ${\mathrm{hh}}_1$ lines corresponding to nominal thicknesses; they can be associated with fluctuations of 1 ML thickness, which are reported, to our knowledge, for the first time, in this type of QW system. Efficient thermally activated interwell transfer of excitons is also in evidence from photoluminescence experiments. A simple model is used to account for the transfer from narrower to broader wells when the temperature is increased.