PhD defense – Hao Wei

Hao WEI will defend his thesis entitled ‘Spintronics with 2D Semiconductors: From Transition Metal Dichalcogenides to Black Phosphorous’ on May 26th 2025, at 2pm in the TRT auditorium.

Abstract:
Spintronics presents a promising alternative to CMOS technology, yet industrial applications remain reliant on only one material system: CoFeB/MgO/CoFeB. The rise of 2D materials introduces an exciting platform due to their unique spin properties: atomic-scale thickness control, tunable barriers, spin filtering, and spin-orbit torque modulation.

However, challenges such as stability, interface engineering, and efficient spin injection hinder their integration into functional spintronic devices. Overcoming these limitations is essential for future spin-based technologies.

This thesis work has focused first on the definition and optimization of interfaces by employing in particular pulsed laser deposition (PLD) and atomic layer deposition (ALD), and developing in situ protocols with controlled atmospheres to combine 2D materials and more usual spintronics materials. This material science work has been key to unlock high quality 2D/FM interfaces towards achieving spin transport results in 2D-based systems that go beyond the current state of the art.

In a first set of studies, we have shown that PLD proves to be an effective technique for growing 2D materials and complex heterostructures. In particular, we have studied a novel complex van der Waals system to illustrate the performance of our PLD approach, namely van der Waals superlattices. These heterostructures of 2D semiconductors have been discussed to provide a novel family of artificial semiconducting lamellar materials, with highly tailorable properties thanks to hetero-2D interfaces hybridization effects. This work has been a starting point to go towards high quality 2D-based interfaces for spintronics.

Taking advantage of this development, we started to use PLD to define new 2D-based spin-polarized interfaces. The use of large-scale and in situ processes helped unlock high-quality 2D/FM spin sources, a pre-requisite to investigate their unique spin properties. We successfully fabricated MoS₂-based magnetic tunnel junctions (MTJs), achieving highly specific spin transport characteristics and strong tunnel magnetoresistance (TMR) signals.

The unusual spin response has been analyzed through ab initio calculations within a collaboration with UC Louvain, revealing the crucial role of 2D/FM hybridizations in these systems. Our results highlight the promising potential of 2D semiconductors to finely tailor the response of spin valves.

Finally, we explored the potential of another 2D semiconductor for spintronics: black phosphorus (BP). While predictions were very promising for this 2D semiconductor in regard to its spintronics properties (high mobilities, low spin-orbit coupling, large spin diffusion lengths…), material science issues such as interface definitions and BP rapid degradation have strongly hampered its experimental exploration.

Here, by exploiting passivation protocols of BP, and working on fabrication approaches in controlled atmospheres, we were able to integrate it in high-quality spin-valve devices. We in turn uncovered its unique spin-filtering properties leading already to high TMR spin signals in excess of 500%. Initial analysis of lateral devices provided some evidence of its long spin diffusion length, paving the way for its use in advanced spintronic architectures.

Overall, these findings underscore the importance of developing robust integration techniques for 2D materials in spintronic devices. Looking forward, the insights gained from this research open exciting prospects for scalable 2D spintronic devices, with potential applications in low-power logic, memory, and quantum computing.

Continued advancements in material engineering, interface control, and device fabrication will be crucial in translating these findings into real-world technologies, bridging the gap between fundamental research and industrial applications.