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Enhanced Spin-Orbit Coupling in Silicon

Creative Commons 'BY-SA' version 4.0 license

Silicon is the standard electronics semiconductor for its abundance and versatility. However, its insignificant intrinsic spin-orbit coupling leads to weak spin-charge conversion and hinders its application in spin-transport electronics. Spin-orbit coupling can be enhanced by lifting the degenerate spin bands through strain gradient and Rashba effect, and further increased with proximity effect. By creating strain gradient in Si, we broke its centrosymmetric crystal structure and created flexoelectricity and charge separation. The combination of Si inversion asymmetry and electric polarization arose Rashba spin-orbit coupling with estimated spin-Hall angle of 0.1132, same order as Pt. The intrinsic spin-Hall effect in p-Si and n-Si is proven from observation of spin-Hall magnetoresistance, emergent phase transitions and dissipationless spin current. The methods we employed to enhance, characterize and utilize spin-orbit coupling are documented in this thesis.

This thesis begins with reviewing the mechanisms for spin transport manipulation and the current understandings of spin transport in Si. We introduce the effects of strain gradient, Rashba and proximity that can lift the spin degeneracy, and the methods for incorporating the effects into Si with material structure of p-Si/MgO/NiFe/Pd using standard fabrication procedures. We validated our device setup through spin-Hall magnetoresistance measurements, and characterized the type and strength of spin-orbit coupling. Using the enhanced spin-orbit coupling, we measured its influence with magnetoresistance from NiFe, since spin current should influence the magnetization of a magnet in contact. As thermal transport was primarily carried by Si, we measured thermal properties of the device using three-omega method to study the changes in Si with spin polarization. In Si, spins relax through Elliot-Yafet mechanism, and our measured thermal properties reflect the interaction between spins and phonons. Further investigation showed signatures of phase transitions in thermal transport properties. To enhance the spin polarization, we constricted the thickness of Si to its spin diffusion length, and consequently arose metal-to-insulator transition and additional phase transitions. In a device composed of MgO and Si bilayer, we generated and detected dissipationless spin current solely through Si without a ferromagnet. In this study, we demonstrate the functionality of Si as a spin transport material with tunable spin-orbit coupling, advocating the feasibility of Si spintronics.

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