Transmission Electron Microscopy of MBE-Grown Self-Assembled InAs Quantum Dots Capped with GaAsSb Layer(s)

Single-layer/multi-layer GaAs(001)-based heterostructures consisting of self-assembled Stranski-Krastanov InAs quantum dots capped with GaAsSb layer(s) are attractive materials for long-wavelength (up to 1550 nm) telecommunication applications. The GaAsSb layer acts as strain reducing layer, however the effect of GaAsSb capping on the size and shape of InAs quantum dots is not well-understood. In this work, the single-layer/multi-layer InAs quantum dots capped with 12-nm-thick GaAs_0.86Sb_0.14 layer(s) were grown using molecular beam epitaxy (MBE), which were characterized using various transmission electron microscopy (TEM)-based techniques such as diffraction-contrast two-beam imaging, high-resolution phase contrast TEM imaging and high-resolution scanning transmission electron microscopy (STEM) in conjunction with energy dispersive X-ray (EDX) spectroscopy. Samples for (S)TEM were prepared using focused ion beam (FIB)-enabled in-situ lift-out method using a Helios 5 UC machine which was operated at 2 KV during final thinning to reduce Ga-ion-induced damage in the thin electron-transparent (S)TEM samples. A ThermoFisher Scientific Themis G3 TEM equipped with four quadrant silicon drift detectors was operated at 300 KV for structural and chemical characterization of the epitaxially-grown quantum dot heterostructures. Cross-section (S)TEM images revealed that although the dots were strain-coupled and vertically well-aligned in both bi- and hepta-layer samples, the aspect ratio (height to base) of quantum dots typically reduced with increasing layer number in the hepta-layer sample. The wetting layer contrast was clearly distinguishable in the STEM images, which confirmed that the growth mode was Stranski-Krastanov. More detailed investigation of intermixing in the buried quantum dots is being undertaken using STEM-EDX spectrum imaging with a judicious choice of electron dose, probe size and pixel size as well as dwell time for X-ray collection. The EDX quantification result for the bi-layer sample obtained using Velox software with optimum prefiltering yielded a reliable profile for Indium distribution across quantum dot and capping layer regions. Current efforts to deduce Sb concentration profile at sufficient spatial resolution and analytical sensitivity will be described in detail.