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

Abstract

Photoluminescence, reflectivity, and thermally detected optical-absorption (TDOA) experiments have been performed at liquid-helium temperatures on two strained ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As/GaAs quantum-well (QW) structures grown by metalorganic molecular-beam epitaxy (MOMBE). The QW thicknesses vary from 3--15 ML and 4--16 ML by two monolayer (ML) steps. It is demonstrated that the MOMBE technique allows the thickness to be controlled with an accuracy of 1 ML. The electron--heavy-hole excitonic transitions (${\mathit{e}}_{1}$${\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\ifmmode\pm\else\textpm\fi{}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.