Channelizing the Exemplary ‘Lane Discipline’ of Topological Insulators for Next-Generation Electronics

Two-dimensional topological insulators (TIs), like monolayer group-IV and V Xenes with large intrinsic spin-orbit coupling, offer electrically tunable phase transitions due to their buckled lattice structure and present exciting prospects for futuristic low-power electronics. These materials possess robust conducting modes with enforced ‘lane discipline’ at their edges, which provide dissipationless channels for carrier transport and are unaffected even in the presence of scattering centers-–the primary source of energy dissipation in modern semiconductor devices. These salient features make 2D topological insulators an excellent material choice for several interesting applications in low-power electronics. We discuss a couple of avenues, namely valleytronics and steep-subthreshold transistors, where 2D TIs have an immense potential to make significant contributions.<br/>Firstly, we demonstrate an all-electrical valley filtering device that exploits the topological robustness of 2D TIs. Our device design draws inspiration from the previous proposals based on bilayer graphene in using the valley-polarized interface states appearing at the domain wall between topologically distinct phases. Spatially separating these interface states by the introduction of a large gap quantum spin Hall region in between the states added to their topological robustness and significantly boosted the valley filtering performance of our device. By adopting the scattering matrix formalism on a suitably designed device structure, valley-resolved transport calculations in the presence of non-magnetic short-range disorder have been done to gauge the valley filter performance in practical scenarios. Based on our numerical simulations, we also outline the role of SO coupling strength, device geometry, and other factors while designing an optimized valleytronic filter device with superior efficiency.<br/>Next, we explore the emerging realm of topological transistors where one can achieve a subthreshold slope steeper than the thermionic Boltzmann’s limit, which is indispensable for low-power device operation. Rashba spin-orbit interaction in 2D TIs enables a faster-than-linear response of the band movement to the applied field via the topological quantum field effect that can have encouraging implications in achieving sub-thermionic performance. Combining this with the prevailing advantage of dissipationless ON state conduction via robust, helical edge modes of the TI channel, makes topological quantum field-effect transistors an excellent device choice for ultra-low power electronics. However, realizing a working device design with the above merits requires a deep exploration of the topological quantum field-effect transition physics. Our numerical and analytical calculations highlighted the drawbacks in the subthreshold performance of a primitive design of the topological transistor. This led us to a partial solution to beat the thermionic limit by adopting a modified gating strategy. However, this strategy fails to preserve the dissipationless ON state performance due to bulk modes conduction. We then show that an out-of-plane antiferromagnetic exchange induced in the material via proximity coupling, effectuating transitions between the quantum spin-valley Hall and the spin quantum anomalous Hall phases, allows the ON state to remain topologically robust while surpassing the thermionic limit on the subthreshold performance. Further dephasing studies have been done on all the transistor designs mentioned above to gauge their performance in experimentally-relevant regimes. They reveal the robustness of the topological edge states under momentum relaxation scattering and the effect of varying degrees of momentum relaxation strengths on the subthreshold performances.<br/>Our work thus provides a strong foundation for future investigations on harnessing topological phase transitions in two-dimensional topological insulators for next-generation low-power electronics.