Impact of an In_xGa_1–xAs Capping Layer in Impeding Indium Desorption from Vertically Coupled InAs/GaAs Quantum Dot Interfaces

This study describes the effect of a thin GaAs spacer of 4.5 nm thickness in a bilayer-coupled InAs quantum dot (QD) heterostructure. Here, we report the first demonstration of InAs/GaAs QDs capped by self-assembled In x Ga 1– x As layers. Self-assembled In x Ga 1– x As layers were introduced into each intermediate layer across the interface of InAs QDs and the GaAs layer in a vertical-coupled bilayer QD (VCBQD) heterostructure to prevent indium desorption from the QDs. A change in the indium content in the seed-layer InAs QDs changes the self-assembly position and modifies the In x Ga 1– x As layer thickness. A theoretical approach was presented to study the formation of self-assembled In x Ga 1– x As layers at each strain-free layer. We showed that the strain energy at the second intermediate (ε zz 2 ) layer is greater than that at the first intermediate (ε zz 1 ) layer; ε zz 2 depends on the vertical strain channel length. The impact of the In x Ga 1– x As layer thickness on the strain energy was studied using high-resolution transmission electron microscopy; a shorter strain channel length was found to facilitate the formation of a more relaxed and larger-sized self-assembled In x Ga 1– x As layer in the active layer. This In x Ga 1– x As layer formed at the intermediate layer acts as a capping layer or a protective shield for the indium adatoms, preventing their desorption from the InAs QDs. Furthermore, we studied the thermal stability of the self-assembled In x Ga 1– x As layer by annealing the VCBQD samples at 700 and 800 °C. This aspect has been investigated for the first time ever in a study of the coupling efficiency between the InAs QDs and In x Ga 1– x As capping layer. A high-resolution in-plane (2θχ/ϕ) reciprocal space mapping (RSM) technique provided the connection between the in-plane reciprocal lattice point of the InAs QDs and In x Ga 1– x As layers and revealed the strain and coupling between them. InAs QDs fully covered with the self-assembled In x Ga 1– x As layer enhanced the photoluminescence intensity by 77% and had an activation energy of 467 meV.