Towards axial heterostructures with atomically-sharp interfaces in self-catalyzed nanowires

Axial heterostructures have diverse functionality in electronic and optoelectronic devices. Implementing such systems in freestanding semiconducting nanowires further broadens the scope of potential applications, for example: distributed Bragg reflectors, high-efficiency light-emitting diodes, and quantum dot heterostructures. The challenge, however, lies in reducing the compositional grading effect of the constituent heterostructure materials across the interfaces in nanowires grown in vapor-liquid-solid mode. Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow Al(x)Ga(1-x)As axial insertions in self-catalyzed GaAs nanowires. A full nanowire with six insertions is shown in Figure 1 (top). High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and nanowire growth models are utilized to gain an understanding of the compositional grading mechanism and its dependence on the growth temperature (TG), the nanowire radius (RNW), and the amount of supplied Al. Figure 1 (bottom) is an example HAADF-STEM image showing two atomically resolved Al(x)Ga(1-x)As insertions embedded in a zincblende GaAs nanowire, whereas Figure 2 shows the extracted Al-content (x) axial profiles for three selected insertions with different TG and RNW (the same amount of supplied Al). We show that lower TG and/or smaller RNW result in sharper interfaces, with a more profound improvement for the AlxGa1-xAs-to-GaAs interface. In the best case, almost symmetric insertions with interface thicknesses of only 2 – 3 monolayers are achieved, approaching the absolute limit of atomically sharp interfaces. Thermodynamics, kinetics, and nanowire geometry are all factors considered during the formation of our Al(x)Ga(1-x)As insertions. Using two existing heterostructure growth models [2, 3] to fit our experimental data we can extract valuable quantitative information regarding the interface characteristics. Our results show for the first time that not only TG, but also size effects, i.e. a decreasing RNW, play a large role in the thermodynamic stability at the liquid-solid interface, setting previously unknown limitations on the maximum attainable interface sharpness. These findings and their implications will be discussed in detail. The functionality of our insertions is successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures. By growing the quantum dot heterostructure at 350 °C, consequently sharpening the quantum dot interfaces, we observe a one order of magnitude decrease in quantum dot emission linewidth (Figure 3) in comparison to a similar system grown at 550 °C. 1. Balaghi et al., Nano Lett. 16, 4032 (2016) 2. Luna et al., Phys. Rev. Lett. 109, 126101 (2012) 3. Priante et al., Nano Lett. 16, 1917 (2016)