Axial heterostructures in vapor-liquid-solid grown nanowires: at the limit of interface sharpness

When lowering the dimensions of semiconductor heterostructures, controlled compositional changes become paramount owing to the increasing role of the heterointerfaces in the electronic properties of the heterostructures. A common issue faced in fabricating axial heterostructures in vapor-liquid-solid (VLS)-grown nanowires is the compositional grading, which is caused by the involved growth mechanisms and broadens the interfaces.
Here, our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1] (an adaptation of conventional molecular beam epitaxy), which grants precise control over the axial growth rate and droplet composition, is employed to grow AlxGa1-xAs axial insertions in self-catalyzed GaAs nanowires. Some examples showcasing the capability of DCAPE in growing systems of complex, abrupt heterostructures are shown in Fig. 1a, where AlxGa1-xAs is revealed as horizontal dark grey lines. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and a semi-empirical growth model [2] 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 total Al content [2]. We show that pre-filling the Ga droplet with Al before the growth of an insertion is enough to sharpen the GaAs-to-AlxGa1-xAs interface at any TG. On the other hand, though, sharper AlxGa1-xAs-to-GaAs interfaces are obtained only at lower TG and/or smaller RNW, where the so-called reservoir effect is markedly reduced. In the best case, symmetric AlxGa1-xAs insertions with interface widths of only 2– 3 monolayers (comparable to, or better than, state-of-the-art thin film heterostructures) are achieved for the full range of x (Fig. 1b), proving the strength of VLS in the fabrication of sharp axial heterostructures.

Figure 1: (a) HAADF-STEM images with 5 examples of GaAs/AlxGa1-xAs heterostructures grown in self-catalyzed GaAs nanowires. Dark contrast denotes Al-containing regions. (b) Symmetric AlxGa1-xAs profiles obtained after optimization of all growth parameters.

[1] L. Balaghi et al., Nano Lett. 16, 4032 (2016).
[2] D. Hilliard et al., in preparation.