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

Shrt abstract:

Using our nanowire growth technique called droplet-confined alternate pulsed-epitaxy, which provides precise control over the axial growth rate and droplet composition, the growth mechanism of self-catalyzed GaAs epitaxial nanowires containing AlxGa1-xAs/AlyGa1-yAs axial short-period superlattices, was systematically examined. Significant increases in interfacial abruptness were confirmed by reducing the nanowire diameter and superlattice growth temperature. Additionally, we found that an unstable droplet contact angle impacts the superlattice growth rate considerably. The unique versatility of our short-period superlattices was tested by using them as axial barriers for GaAs quantum dot heterostructures embedded in nanowires. Furthermore, encapsulating the nanowire in lattice-mismatched shells operating as radial barriers, we demonstrated the tuneabilty of the quantum dot emission energy via strain engineering.

Long abstract
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 self-catalyzed 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 20 nm. High-angle annular dark-field scanning transmission electron microscopy (figure 1(a)), 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. We confirmed with monolayer resolution, controlled Al contents in a wide compositional range and superlattice period widths of just a few monolayers (figure 1 (b)).
The quality of our short-period superlattices was successfully tested via their employment as axial barriers in quantum dot nanowire heterostructures (figure 1 (c)). Additionally, by introducing a lattice-mismatched ternary alloy shell as the radial barrier (In(x)Al(1-x)As in this case), we demonstrated controlled strain-induced redshifts of the quantum dot emission energy by adjusting the In content of the shell. For 39% In, we measured a redshift in the quantum dot emission of 180 meV as shown by the photoluminescence measurements in figure 1 (d). Notwithstanding, limitations in what can be accomplished are present and possible strategies to overcome them will be presented.

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