In-plane core-shell nanowires with strong spin-orbit coupling for scalable mesoscopic devices – Inspiring

In semiconductors with strong spin-orbit (SO) coupling and broken inversion symmetry, electron momentum is locked to its spin via the Rashba effect. This property had important consequences in the design of spintronic devices such as the electrically gated spinFET, in the charge-spin inter-conversion or its counterpart, in the topological protection of electron spin current with the quantum spin Hall effect and, more recently, in the quest of Majorana zero modes, candidates to achieve robust Qubits. InSb and HgTe which exhibit the largest SO interaction among III-V and II-VI materials respectively are particularly appealing for these applications but require a reliable nanoscale technology to fabricate complex architectures working in the coherent regime.
The INSPIRING project gathers three academic partners (IEMN, LETI and Néel) which will strive to develop such a technology by combining molecular beam epitaxy and nanoscale lithography in a dielectric mask for the selective growth of in-plane nanostructures at the substrate surface in ultrahigh vacuum. This nanoscale selective area growth, already experimented at IEMN with different materials such as GaSb, InP or InAs for ultimate MOSFET fabrication, could alleviate technological issues related to the mismatch accommodation or the semiconductor etching which are encountered for InSb and HgTe.
Concerning InSb, preliminary investigation reported by IEMN and Microsoft StationQ in Delft, are promising but the electron properties in the nanostructures are still impacted by the large lattice mismatch with semi-insulating III-V substrates and defects from the surface. The quasi-lattice matching between InSb and CdTe should provide a solution both for the selective area growth of InSb on an insulating CdTe substrate and its subsequent embedding with a protective CdTe shell. A similar structure will also be considered between HgTe and CdTe II-VI materials, allowing the development of a common patterned substrate for both studies. For HgTe, for which progress has been hindered by the extreme sensitivity of the material to external technological perturbation, selective area growth of in-plane nanostructures would be a real technological breakthrough given its very interesting topological properties.
Beyond the development of selective area growth, the second objective of the INSPIRING project is to characterize the electron transport properties within the nanostructures and achieve ballistic transport over µm-scale distances. As these properties can be profoundly impacted by technological processing, we anticipate the use of multi-probe scanning tunneling microscopy so that intact nanostructures are directly contacted in ultrahigh vacuum. Such a study will require the development of an efficient high vacuum capping/uncapping process of the epitaxial layers for their clean transfer between experiments. A second step will consist in processing elementary nanodevices to evidence electronic properties through low temperature magneto-transport experiments.
In this context, IEMN, LETI and Néel have a strong and complimentary experience in the fields of molecular beam epitaxy, near field characterization and magneto-transport measurements as well as the nanofabrication facilities to reach the objectives of the INSPIRING project and offer new horizons for spin-orbitronics.