Theoretical Study On Spin Injection In Silicon-based Semiconductor

The spin degree of freedom has caught the eyes of researchers due to it sheds lights on the next-generation devices which beyond the physical limits of More's Law,with smaller unit sizes,lower power consumptions and novel charge-spin integrated functionalities.Among all the semiconductors,realizing the spin-based electronics(spintronics)on silicon,i.e.,the most prevailing material in semiconductor industry,has special significance because the established mature Si-technology could greatly facilitate the productions and massive applications of spintronic devices.Fortunately,silicon is also considered as an ideal host for spintronics,due to its long spin lifetime and diffusion length,which originates from the absence of crystal inversion asymmetry,weak spin orbit and nuclear hyperfine interactions.In the past decades,milestone progresses have been achieved in Si-based spintronics.Electrical spin injection into silicon across a ferromagnet/insulator/Si(FM/I/Si)tunnel contact were claimed to be observed at low temperature in 2007 and at room temperature in 2009.After that,a number of Si-based tunnel contacts with different tunnel barriers such as Al2O3,SiO2 and crystal-MgO layers were fabricated.Nevertheless,there remains challenges on obtaining reliable and clear spin signals,as well as understanding the spin transport process of the FM/I/Si tunnel contacts.In the second chapter,we give a brief introduction to the non-equilibrium Green's function(NEGF)formalism,including the Self-consistent lattice non-equilibrium Green's function method and Green's function as a special scattering state。In this thesis,we use the non-equilibrium Green's function(NEGF)method to investigate the spin injection across the FM/I/n-Si tunnel contacts by.The transmission coefficient of an arbitrary energy band profile consisting of tunnel layer and Schottky barrier are calculated by the lattice Green's function.And the thermionic emission process is taken into account by the temperature-dependent Fermi energy of n-Si and the Fermi-Dirac distributions.Models for describing the spin transport based on the FM/I/Si structure are also given,including the two-current model,F/N/F junction,and the Fert's model.As pointed out by Fert et al.,a noticeable spin signal by the spin injection from a ferromagnet into semiconductor can be observed only if the contact resistance is engineered into an optimum window:the contact resistance cannot be too low to overcome the conductivity mismatch,nor be too high to keep the electron dwell time shorter than the spin lifetime.In the third chapter,by using the lattice NEGF method,we investigate theoretically the effective RA product and the spin polarization for FM/I/n-Si tunnel contacts,as well as the MR ratio of a vertical spin metal-oxide-semiconductor field-effect transistor(spin MOSFET)with different tunnel barriers.We find that rb of tunnel contacts with low barrier materials,such as TiO2 and Ta2O5,are orders of magnitude smaller than that of the conventional tunnel contacts.Therefore,the maximum MR signal and optimum parameters window for TiO2 and Ta2O5 barrier contacts is larger than the conventional tunnel contacts.Interestingly,we also demonstrate the spin asymmetry coefficient γ of TiO2 barrier contact has a negative value,and γ of Ta2O5 barrier contact can be tuned from negative to positive by changing the thickness of tunnel barrier and temperature.The optimized spin signals and unique spin asymmetry property of low barrier tunnel contact can be used for the designing of efficient spintronic devices.In the final chapter,we give a summary of this thesis and a plan for the future work.