HRTEM Study of Size-Controlled Bi Quantum Dots in Annealed GaAsBi/AlAs Multiple Quantum Well Structure

High-resolution transmission electron microscopy (HRTEM) study of statistically large number of Bi quantum dots (QD) in annealed GaAsBi/AlAs multiple quantum well (MQW) structures is presented in this work. Superlattices containing 20 alternating periods of 10nm thick GaAsBi quantum well and AlAs 20nm thick barrier layers are grown on semi-insulating GaAs at 330 degrees C temperature. Dispersed Bi quantum dots are formed within the bismide layers during a post-growth annealing at 750 degrees C. Energy dispersive X-ray (EDX) mapping and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis reveal that GaAsBi phase is altered during the annealing treatment forming a partially Bi-depleted GaAsBi phase and semiconducting Bi nanoparticles. AlAs layers are found to act as barriers for Bi out-diffusion, thus controlling the size of the QDs. The analysis of HRTEM micrographs confirms that Bi quantum dots consist of a rhombohedral Bi phase randomly distributed within zinc blende GaAsBi layers. Geometric phase analysis (GPA) also shows that GaAsBi layers are strained with respect to AlAs layers, as supported by the high-resolution X-ray diffraction (HRXRD) data. These findings demonstrate a highly controlled synthesis of Bi-based quantum dots and pave the way for future applications in the field of infrared devices.