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.