III-V semiconductor nanowires with unique heterostructure possibilities

III-V semiconductor heterostructures have contributed to a wealth of studies in solid state physics, as well as to applied research and technology in electronics and photonics. More recently, the nanowire geometry introduced new possibilities, such as one-dimensional quantum transport, formation of Majorana modes, enhanced light-matter coupling, photon entanglement, as well as monolithic integration in Si-CMOS platforms for the realization of more-than-Moore hybrid systems. This talk will be focusing on the bottom-up fabrication and the structural and electronic properties of III-As nanowire heterostructures.

The first type of heterostructures will be radial ones, comprising a GaAs core and a lattice-mismatched InxAl1-xAs or InxGa1-xAs shell. Molecular beam epitaxy and a combination of vapor-liquid-solid and vapor-solid growth modes are employed to grow free-standing nanowires on Si substrates [1, 2]. Owing to its high surface-to-volume ratio and the peculiar geometry, the thin core can be hydrostatically tensile strained to extremely high levels, depending on the In content x and the thickness of the shell. As a welcome effect, the bandgap of GaAs can be tuned to be anywhere between the strain-free value of 1.4 and 0.8 eV, allowing for potential applications in telecom photonics [3]. Furthermore, the electron mobility in the GaAs core is increased with increasing the tensile strain, as a result of a corresponding decrease in electron effective mass [4]. This is of major importance for the realization of transistors with high speed and low-power consumption.

The second type of heterostructures will be axial ones, where the composition is modulated from GaAs to AlxGa1-x¬As and back to GaAs along the nanowire axis. Here, we develop a pulsed-growth technique [5], which grants precise control over the axial growth rate and droplet composition. Using advanced transmission electron microscopy methods and a thermodynamic equilibrium model, it becomes possible to quantitatively describe the compositional grading of Al across the heterostructure interfaces and to identify the control parameters. In the end, symmetric GaAs to AlxGa1-x¬As and AlxGa1-x-As to GaAs interfaces with 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 [6]. This is particularly important for quantum heterostructures and nano-devices, where the compositional grading at interfaces can critically affect the electronic properties and the device characteristics.

Acknowledgements
Part of this work was supported by the program for the promotion of the exchange and scientific cooperation between Greece and Germany (IKYDA 2020), project title: Strain tuning of III-V semiconductor nanowires (TUNE).

References
1. T. Tauchnitz et al., Cryst. Growth Des. 17 (2017) 5276.
2. T. Tauchnitz et al., Nanotechnology 29 (2018) 504004.
3. L. Balaghi et al., Nat. Commun. 10 (2019) 2793.
4. L. Balaghi et al., Nat. Commun. 12 (2021) 6642.
5. L. Balaghi et al., Nano Lett. 16 (2016) 4032.
6. D. Hilliard et al., in preparation (2023).