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TRANSCRIPT
Silicon spintronics
Daniel Wolseop Lee
Introduction
• Ferromagnets and semiconductors
• Scaling faces extraordinary challenges
• Spintronics as an alternative
• Spin transistors with different operating principles proposed, but..
Background
• In 1968, optical orientation of spin in Si conducted by Lampel
• Well-defined selection rules for optical transitions in III-V materials
• Electrical spin injection, optical methods using luminescence in a III-V LED proposed
• First demonstration of electrical spin injection made in GaAs using STM
• Electrical spin injection in III-V SC devices at RT (no spin detection)
Comeback of silicon
• Initial attempts to observe MR in two-terminal FM/Si/FM not convincing due to impedance mismatch between FM and SC
• No direct spin injection from high-conductive FM into resistive, non-magnetic SC
• The introduction of a spin-dependent interface resistance between FM and SC provided a solution (Tunnel barrier)
Hot electrons and undoped Si
• Hot electrons do not obey the rules that apply to Fermi electrons
• Spin-polarized hot electrons can thus be injected into a SC without suffering from the impedance mismatch problem
• In 2007, “electronic measurement and control of spin transport in silicon” by Appelbaum et al.
Hot electrons and undoped Si
• Provided informations about spin transport in undoped Si at low T (60-150 K)
• Disadvantages
– Complicated device geometry
– Too small signal magnitude: The transmitted current is only a small fraction (<10−4) of the current in the injector circuit
Spin tunnelling into Si
Room temperature
• The electrical creation, detection and manipulation of spin polarization in Si at RT by Dash et al.
Contact engineering
• The early failures to observe spin injection into SC through a direct contact with FM due to impedance mismatch
• A current across a FM/SC interface induces a non-equilibrium situation to accommodate the depolarization of the current
• This occurs in the FM owing to its much higher rate of spin relaxation, and the current injected in to SC is nearly unpolarized
Contact engineering
• Interface with a large resistance that is spin-dependent
• Establishment of a high-quality tunnel interfaces by a direct contact of FM and SC is crucial
• Primary role of an oxide:
– NOT to overcome the impedance mismatch
– To prevent silicide formation
– Ensure a large and reliable tunnel spin polarization at RT
Contact engineering
• Introducing a tunnel oxide is not the complete solution
• For Si, the contact resistance is determined not by the tunnel oxide, but by the Schottky barrier
– The average time an 𝑒− dwells in the channel exceeds the spin-relaxation time; spin accumulation vanishes
– No high-frequency operation
– Transport not by tunneling but by thermionic emission
Contact engineering
Spin accumulation
• Δ𝑅 𝐴 = 𝑃2𝜌𝐿𝑆𝐷
• Predicted value: 0.001 kΩ µm2 for n-type Si/Al2O3/Ni80Fe20 devices
Spin lifetime
• In Si, the spin of conduction electrons is only weakly coupled to other degrees of freedom
– Inversion symmetry, isotopes..
• Spin relaxation in bulk Si is dominated by the Elliott-Yafet mechanism
• Combined action of momentum scattering and spin-orbit interaction, where the latter is small for Si
Electric-field control of spins
• Owing to spin-orbit interaction, an electric field transverse to the carrier’s direction of motion transforms into an effective magnetic field that acts on the spin
• Spin precession is proportional to the momentum and requires motion of the carriers
• With practical E-field, the distance needed for precessional spin reversal is relatively large even with strong spin-orbit interaction. Scaling is not straightforward
Electric-field control of spins
• Application in sub-100-nm spintronic devices seems unlikely
• Spin polarization manipulation through its magnitude rather than its orientation
What lies ahead?
• Ferromagnetic tunnel contacts have become the standard in Si spintronics
• Better understanding of the magnitude of the induced spin accumulation and the spin lifetime
• Better control and engineering of the contact properties and spin manipulation by E-field
Thank you for your attention!
References
http://i.kinja-img.com/gawker-media/image/upload/s--p_wIwj55--/17vu1nvh407s4jpg.jpg
http://www.jst.go.jp/EN/research/bt14_en.html
Jansen, R. (2012). "Silicon spintronics." Nat Mater 11(5): 400-408.
Min, B. C., et al. (2006). "Tunable spin-tunnel contacts to silicon using low-work-function ferromagnets." Nat Mater 5(10): 817-822.