A Comparative Study of GaAs, InGaAs, and AlGaAs Capping Layers on Multilayer InAs Sub-monolayer Quantum Dots Grown by MBE

Abstract

Heterogeneously coupled InAs sub-monolayer quantum dot (SML-QD) structures were grown via molecular beam epitaxy (MBE) in this study, where a set of comparative samples was fabricated by employing GaAs, In_0.13Ga_0.87As, and Al_0.3Ga_0.7As as capping layers, respectively. The effects of different capping layers on the optical properties and crystalline quality of the vertical multilayer InAs SML-QD structures were systematically investigated through atomic force microscopy (AFM), photoluminescence (PL) measurements, and high-resolution X-ray diffraction (HRXRD). AFM measurements revealed that Sample C with an AlGaAs capping layer exhibits the narrowest height distribution and optimal uniformity of InAs SML-QDs compared to Sample A with a GaAs capping layer and Sample B with an InGaAs capping layer. The HRXRD analysis revealed that each sample displayed multiple satellite peaks positioned around the GaAs substrate peak. Temperature-dependent PL fitting revealed that the activation energy for carriers follows the order of Sample B > Sample C > Sample A. The behavior of the InGaAs-capped sample is consistent with enhanced In-related intermixing and structural inhomogeneity, which may lead to broader localized-state distributions in the InAs-containing active region. This dual carrier confinement effect endows Sample B with the highest thermal escape barrier. However, due to the size inhomogeneity of the QDs, Sample B exhibits the lowest emission intensities at both 77 K and 300 K. In contrast, the AlGaAs ultrathin capping layer is likely to suppress excessive intermixing-related inhomogeneity, which is consistent with the improved room-temperature PL response of Sample C. Therefore, the AlGaAs capping layer is more suitable for fabricating high-performance optoelectronic devices operating at room temperature.