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Contenuto archiviato il 2024-05-28

Spin Transport in Silicon Nanodevices

Final Report Summary - SILICONSPIN (Spin Transport in Silicon Nanodevices)

Modern electronics operating by the flow of negatively charged electrons will soon reach their fundamental limitations in their dimensions, performance and power dissipation. To tackle these challenges, one of the alternatives for processing information beyond the charge based technology is to encode digital data in the spin orientation of electrons. Spin-based logic and memory systems have the potential to be non-volatile, faster and with lower energy dissipation. Several schemes considering the spin of electrons have been proposed, for example spin field effect transistor¬ and spin-logic devices. The basic operations of such devices rely on efficient creation of spin polarization, longer spin lifetime and coherence length, and spin manipulation in the channel material. The major challenge in this direction is developing a suitable material system where these basic spin functionalities can be realized at room temperature.

Noteworthy progress has been made recently in spin transport in semiconductors and 2D materials. For example pure spin transport in silicon at room temperature has been demonstrated by non-local methods. Graphene has been shown to be an excellent material for spin transport and proximity induced effects. The 2D hBN layers are found to be very good insulator for tunnel barriers. Latter on topological insulators were discovered and reported to have spin polarized surface states due to spin-momentum locking. However, spintronic potential of these materials we not fully explored yet and basic rules govern their behaviour are not fully understood. My research focus in last four years was on spin transport in semiconductors, graphene, topological insulators and heterostructures with other 2D materials as described below.

We demonstrated large spin polarizations in silicon and germanium at room temperature by electrical, thermal and dynamical methods, in a direct tunneling regime. The magnitude of spin accumulation created and the spin lifetime in silicon (Si) and germanium (Ge) also revealed a scaling with the tunnel resistance. We also reported that the spin accumulation and spin relaxation in Si and Ge to be anisotropic with respect to magnetization direction of the detector ferromagnet. Further in this direction we explored 2D h-BN insulating tunnel barriers for spin injection into Si and in magnetic tunnel junctions showing magnetoresistance up to room temperature. In addition to electrical injection of spin polarization from ferromagnetic contacts, thermal and dynamical methods to create spin polarization in semiconductors are also looking promising. Using thermal heating of silicon we for the first reported the spin accumulation in n-type silicon. Furthermore, employing dynamical methods for spin injection, we for the first time observed spin pumping into n-type Si. In these measurements, we observed a broadening of ferromagnetic resonance linewidth with microwave frequency. These basic experiments have helped in better understanding of the process of spin injection and accumulation in semiconductors and to discover the basic rules govern their behavior.

In the field of graphene spintronics, we demonstrated record long distance spin transport and nanosecond spin lifetimes in large area CVD graphene at room temperature. The obtained spin lifetime and diffusion lengths are highest at room temperature on SiO2/Si substrates. As the use of oxide tunnel barriers gives rise to doping and roughness related issues in graphene, we used 2D h-BN as a tunnel barrier on graphene and observed an enhancement in spin polarization and lifetime. Using multilayer h-BN tunnel barriers, we demonstrated a large tunnel spin polarization and sign inversion of the signal. Finally, we measured strong magnetoresistance signal in topological insulators using ferromagnetic tunnel contacts up to room temperature, due to spin-momentum locking in the surface states.

These excellent spintronic properties of the individual materials and their heterostructures developed during this career integration period promise novel devices with custom-designed spin properties. The project has also generated outputs that constitute big scientific advances in the field of spintronics, 2D materials and their heterostructures. I am very confident that, our research offer a unique route to generate and control spin current in semiconductors and 2D materials, and in future this research field will be even more fascinating than today.

Regarding the career development and integration to my host institute, I got promoted to permanent Associate Professor position in 2015 after a successful tenure track period. I have established my research laboratory and also leading an independent research group in the field of quantum and spintronic devices. I have attracted external funding from Swedish research council, EU ERNET FlagEra, and EU H2020 Graphene Flagship as project leader. During this period I supervised 2 PhD thesis, 2 postdocs, 1 Licentiate thesis, seven Masters, and seven Bachelor thesis on spintronics with semiconductors and 2D materials. I also integrated my research topics in teaching of Semiconductor Physics and Nanoscience course for masters and PhD students. To further enhance my skills, I have attended extensive leadership and pedagogical courses organized at Chalmers. I was also co-organizer of junior faculty club and web development team at Chalmers. Also I was an active member of faculty team for coaching Chalmers management.
I developed a strong collaboration network with international researchers during this period, both in experiments and theory. Also I have obtained experience in outreach activities by giving > 25 invited talks, organizing conferences, chairing conferences, participating in scientific advisory board meetings, and organizing summer schools for graduate students. I have also served as an editor, reviewer for several high impact journals and funding agency. Currently, I am one of the editors of Nature Scientific Reports. I have done seminal works on spintronics with silicon, topological insulators, graphene and 2D material based devices, which is well recognized in the scientific community with over 31 publications in high impact journals (Nature, Nature materials, Nature communications, Nano Lett, ACS Nano) with > 1250 citations, h index > 16. More details can be found in the web page