Energy efficient two-dimensional process-in-memory devices

The motion of negatively charged electrons generates charge current in solid state conductors and nowadays electronic elements and devices such as diodes, transistors and even computer processors function due to the manipulation of the charge current. Electrons, beside orbiting the positive nucleus in an atom, also spin about their own axis, giving rise to a quantum mechanical degree of freedom known as spin. Depending on the sense of rotation clock-wise or counter-clock-wise the spin of an electron is visualized as pointing up or down. The magnetic devices used extensively in memory applications like hard disk drives or magnetic random access memories exploit the spin of electrons by manipulating a spin polarized charge current, where the current has a majority of the up or down spin electrons, unlike the pure charge current. Spin based memories are attractive for their non-volatility and high durability but offer low resistance changes, whereas semiconductor logic transistors, such as the field effect transistors, provide large resistance tunability but do not provide memory function. In both approaches, non-volatility of data and energy efficiency remain major issues to resolve. However, a tie-up of the spin and orbital motion of electrons, known as spin-orbit coupling in solids has the capability of generating, in non-magnetic materials like metals and semiconductors, dissipationless pure spin currents, where only the spins move and not the charges. This approach of utilizing the spin -orbit coupling is expected to lead to the development of a new technology addressed as spin-orbitronics by using spin-orbit torque at interfaces of magnetic and non-magnetic layers through which non-volatile, energy efficient electronic devices will be realized. In particular, the emergence of atomically thin two-dimensional (2D) materials, has led to the emergence of quantum devices that are not only energy efficient, but also complement the existing silicon technology. In this project, a spin-orbit torque driven field-effect transistor (SOTdFET) fabricated from 2D van der Waals heterostructure is designed and realized, in the perspective of practical process- in-memory (PiM) devices with low power consumption, sustainability, endurance and integrability with the existing complementary metal-oxide-semiconductor (CMOS) architecture. The outcome of the project will provide alternative approaches to Boolean logic operations, by exploiting hybrid 2D materials in building-blocks for prospective quantum processors and quantum spintronic devices.

Our project was aimed at the exploration of a new route to energy-efficient, scalable computing by developing a spin-orbit torque driven field-effect transistor (SOTdFET) using ultra-thin, layered materials known as van der Waals heterostructures. This approach is predicted to enable "process-in-memory" (PiM) devices-components that can store and compute data in the same place-reducing energy use, improving endurance, and integrating with today's CMOS technology. The proposed device had three basic components: a ferromagnet, a semimetal, a multiferroic and a semiconductor. Substantial challenges were encountered to realize a stable crystalline phase of a van der Waals multiferroic which redirected our efforts toward other promising quantum materials initially included in the project: Dirac semimetals and van der Waals ferromagnets: PtSe2 (platinum diselenide), a layered Dirac semimetal. ZrTe5 (zirconium pentatelluride), a Dirac semimetal at the boundary between different topological phases. CrTe2 (chromium ditelluride), a van der Waals ferromagnet. Our research on the ultra-thin flakes of PtSe2 revealed an unusual coexistence of spin-orbit coupling, Kondo physics (where conduction electrons interact with magnetic impurities), and topological electronic behavior. This combination points to new ways to control electron "orbit" and spin-an emerging field known as orbitronics-which could be harnessed in future low-power quantum technologies. In CrTe2, by using chemical, structural and electronic diagnostic tools, we discovered that spontaneous self-intercalation-where extra chromium atoms insert themselves between the layers-induces a strong in-plane magnetic anisotropy where the material's magnetization prefers to lie within the layers, which is ideal for devices that use spin-orbit torques. This positions CrTe2 as a key ferromagnetic partner for PtSe2 in layered heterostructures tailored for efficient spin-based switching. For the ZrTe5, the presence of a topological phase transition and a non-trivial Berry curvature-signatures of robust, geometry-driven electron behavior is established. By combining ZrTe5 with the superconductor NbSe2, we obtained the first proof-of-concept for van der Waals topological quantum device for future quantum technology. Together, these findings advance the material foundations needed for practical PiM devices and compatible integration with existing semiconductor platforms. While the path to a full SOTdFET is still unfolding, our results chart a promising route toward sustainable, high-performance computing powered by quantum materials.