Al(x)Ga(1-x)As /Al(y)Ga(1-y)As axial short-period superlattices in self-catalyzed nanowires

Short-period superlattices have diverse functionality in electronic and optoelectronic devices. Implementing such systems as axial heterostructures 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 superlattice 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, was employed to grow Al(x)Ga(1-x)As/Al(y)Ga(1-y)As axial superlattices in self-catalyzed GaAs nanowires with diameters as thin as 25 nm. High-angle annular dark-field scanning transmission electron microscopy, energy-dispersive X-ray spectroscopy, and growth models were utilized to gain an understanding of the compositional grading mechanism. By varying several growth parameters involving growth temperature, nanowire diameter, and droplet contact angle, the link between them and the superlattice characteristics was explored. We found that interfacial abruptness increases significantly by reducing the superlattice growth temperature and nanowire radius. Moreover, we studied the impact of an unstable contact angle on the superlattice growth rate, showing good agreement with analytical growth models and demonstrating the importance of growth rate stability in obtaining reproducible Al contents across successive superlattice periods.
Finally, we confirmed with monolayer resolution, controlled Al contents in the whole compositional range and superlattice period widths of just a few monolayers. Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome them will be presented. The quality of our short-period superlattices was successfully tested via their employment as barriers in quantum dot nanowire heterostructures.

[1] Balaghi et al., Nano Lett. 16, 4032 (2016)