Towards GaAs/AlxGa1-xAs 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. Although, as the physical dimensions of such devices approach the atomic scale, managing the compositional grading effect of the constituent materials at the heterointerface becomes critical and can be particularly challenging in nanowires grown in vapor-liquid-solid mode.
We grow self-catalyzed GaAs nanowires with AlxGa1-xAs axial insertions on Si substrates using our previously developed nanowire growth technique called droplet-confined alternate pulsed-epitaxy (DCAPE) [1]. Here, nanowires are grown using alternating short beam pulses as opposed to the conventional molecular beam epitaxy analog where material is continuously supplied. With this approach, precise control over the axial growth rate, droplet composition and contact angle is granted along with the unique opportunity to grow nanowire heterostructures at CMOS compatible temperatures. To examine the compositional grading mechanism of Al in our AlxGa1-xAs axial insertions we employ high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Quantitative AlxGa1-xAs profiles are extracted from atomically resolved images to study in detail the profile’s interface sharpness and symmetry in relation to the profiles total Al content, heterostructure growth temperature and nanowire diameter. Additionally, building on a previously existing heterostructure growth model that describes the AlxGa1-xAs-to-GaAs interface while considering the solid-liquid thermodynamics and the so-called reservoir effect [2], we derive our own semi-empirical model that describes remarkably well the AlxGa1-xAs profile as a whole and allows for a rigorous analysis of both the profile’s leading and trailing interfaces.
We show the AlxGa1-xAs profile’s leading interface sharpness can be maximized in thin nanowires, largely due to the Al prefilling possibilities provided by DCAPE [3]. Furthermore, we demonstrate increases in trailing interface sharpness by decreasing the nanowire radius and, considerably more so, by decreasing the heterostructure growth temperature [3]. In the best case, practically symmetrical insertions with interface lengths of only 1 – 3 monolayers are achieved for a wide range of peak Al contents, accordingly approaching the absolute limit of atomically sharp interfaces [3].

References

[1] Balaghi et al., Nano Lett. 16, 4032 (2016)
[2] Priante et al., Nano Lett. 16, 1917 (2016)
[3] Hilliard et al., unpublished results