Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - Valleys (Valley and spin devices based on two-dimensional semiconductors)

Teaser

The main objective of this research proposal is to realize new types of electronic devices based on the valley/spin degree of freedom in two-dimensional semiconductors from the transition metal dichalcogenide family. These materials are analogous to graphene but have a direct...

Summary

The main objective of this research proposal is to realize new types of electronic devices based on the valley/spin degree of freedom in two-dimensional semiconductors from the transition metal dichalcogenide family. These materials are analogous to graphene but have a direct band gap. Together with the unique band structure, this allows manipulating the spin and valley degrees of freedom interchangeably. In addition, it can give rise to a mechanism for protecting the spin which could in future result in very high spin relaxation lengths. This proposal will explore various spin/valley injection mechanisms as well as detection mechanisms with the goal of realizing an all-electric valleytronic device. Various new device architectures will be realized in the central part of the proposal.
The research we propose here will address practical applications and fundamental questions related to the main feature that distinguishes 2D TMDC materials from other semiconductors: the valley/spin degree of freedom. The lack of inversion symmetry could lead to interesting new physics due to strong spin-orbit and spin-valley couplings that could be exploited for the construction of an entirely new type of electronics, called valleytronics. Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.

Work performed

We have successfully integrated 2D materials and heterostructures with ferromagnetic electrodes. Toward this goal, we have fabricated light emitting diodes based on naturally n-doped MoS2 and intrinsic WSe2, in which doping level could be electrical modulated by back gate and deposited a magnetized permalloy contact for spin injection. Valley-polarized holes are then injected into WSe2 through the magnetized permalloy contact. The circular polarization of light emitted from the heterojunction, proves that the holes have indeed retained their polarization as they traversed the µm range distance between the injecting ferromagnetic electrode and the heterojunction. This result represents the first instance of spin injection into a 2D material using a local electrode.

We proceeded by demonstrating the inverse action in which polarized light excites spin-polarized charge carriers in WSe2 which then diffuse into graphene and are detected electrically using the non-local geometry. This paper represents the first instance of optical spin injection and transport in a 2D material.
In the direction of implementing valley transport in pure electrical devices, we have realised the first quantum point contact to monolayer MoS2 encapsulated in hBN. The achieved high mobility exceeding 1000 cm2V−1s−1 at 1.5 K allowed us to observe conductance quantisation. Surprisingly, the achieved conductance quantisation occurs in steps of e2/h, which is a signature of complete lifting of spin degeneracy and that the resulting conduction channels are spin and valley polarized.
During preparation of heterostructures composed of MoS2 and WSe2 as part of the work on spin device, we have perfected the heterostructure fabrication and realized a very clean interface between the two materials which resulted in the near-complete suppression of intralayer excitons and the generation of interlayer excitons. This inspired us to investigate if such interlayer excitons could be used for the realisation of excitonic transistors, switches in which excitons currents can be electrically controlled using external voltages, analogously to the way electrical currents are controlled by external field via gate electrodes. Not only did this work, but the resulting device actually operated at room temperature and was the first instance of demonstrated exciton switching in a 2D material and at room temperature in general and was published in Nature. In addition to exciton flux switching, we could also enhance the exciton diffusion length to cca 5 um and achieve trapping and anti-trapping of excitons.

Polarisation-dependent optical measurements however revealed an absence of valley polarisation due to the indirect nature of the optical transition. We have therefore improved the design by exchanging MoS2 for MoSe2. Due to a 10x factor reduction in lattice mismatch, such structures now show valley polarisation. This polarisation could also be switched using external electric field and could lead to the development of integrated valley logic devices.

Final results

The work until the end of the project will focus on realizing practical devices based on the valley degree of freedom, including the newly opened avenue of excitonic devices with offer new opportunities for realizing the main goals of the project. Materials-related aspects will also be explored with the aim of identifying and addressing limits to these devices that are imposed by the material quality.

Results of this research will enrich the applications of 2D materials and possibly result in a new paradigm for computing.