Spintronics The Search for Effective Spin Polarized Current Injection Into Semiconductors Presented by Alan Gabel Boston University Introduction to Solid.

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Presentation transcript:

Spintronics The Search for Effective Spin Polarized Current Injection Into Semiconductors Presented by Alan Gabel Boston University Introduction to Solid State

Spintronics: Spin-based-electronics Using Spin as well as charge to control electrons and holes This may make possible: – Decreased volatility – Increased Processing Speeds – Decreased Power Consumption – Increased integrated circuit density

What is a Transistor? Basic logic component of an integrated circuit Device where a small applied voltage can control a large current C. Woodford Transistors. Explain That Stuff. [Online] 9/10/2008. [Cited: April 25, 2009.]

Field Effect Transistor (FET) Gate Source Electrode Drain Electrode n-type P-type Φ(x) x EE

Field Effect Transistor (FET) Gate Source Electrode Drain Electrode n-type P-type Φ(x) x EE

Properties of Ferromagnets Ferromagnets have asymmetric density of states with respect to electron spin Electrons see an effective magnetic field from magnetization of ferromagnet Leads to a ‘Zeeman Splitting’ effect M

Properties of Ferromagnets Conduction electrons form a polarized current M

A Spin Based Transistor If current is polarized in same direction as Drain electrode: low resistance If current is polarized opposite to drain electrode: high resistance Electronic Analog of Electro-optic Modulator. S. Datta, B. Das Applied Physics Letters (1990) Gate Source Electrode Drain Electrode Ferromagnet Substrate 2-D Semiconductor

A Spin Based Transistor Voltage on gate creates an electric field, which induces an effective magnetic field – Rashba Effect Magnetic field causes the spins to precess so polarization is anti-parallel to drain electrode Gate Source Electrode Drain Electrode Ferromagnet 2-D Semiconductor Substrate Electronic Analog of Electro-optic Modulator. S. Datta, B. Das Applied Physics Letters (1990)

Key Ingredients Injection of Spin polarized current into semiconductor from source electrode Propagation through the semiconductor Induced spin precession Spin-selective collection of current by drain electrode Spintronics: A Spin-Based Electronics. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molna. Science, 294 (2001), p

Key Ingredients Injection of Spin polarized current into semiconductor from source electrode Propagation through the semiconductor Induced spin precession Spin-selective collection of current by drain electrode Spintronics: A Spin-Based Electronics. S. A. Wolf, D. D. Awschalom, R. A. Buhrman, J. M. Daughton, S. von Molna. Science, 294 (2001), p

What’s the problem anyway? Direct current from ferromagnet to semiconductor produces very low polarization, <1% ‘Conductivity Mismatch’ P 0 =polarization far inside the ferromagnet σ F, σ SC = conductivity of the ferromagnet, semiconductor λ F, λ SC = mean distance travelled by spin carriers before a spin flipping scattering occurs. Fundamental obstacle for electrical spin injection from a ferromagnetic metal into a diffusive semiconductor. G. Schmidt, D. Ferrand, L. W. Molenkamp. Physical Review B (2000)

What if Ferromagnet was a Semiconductor? Use a ferromagneticaly doped semiconductor It Works! P measured between % BUT… Electrical spin injectin in a ferromagnetic semiconductor heterostructure. Y. Ohno, D. Young, B. Beschoten, F. Matsukura, H. Ohno, D. Awschalom. Nature (1999) Injection and detection of a spin-polarized current in a light-emitting diode. R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L. Molenkamp. Nature (1999)

What if Ferromagnet was a Semiconductor? Use a ferromagneticaly doped semiconductor It Works! P measured between % BUT… – Need high magnetic fields (~1.5T) – Need super-low temperatures (<40K) – Not viable for commercial application Electrical spin injectin in a ferromagnetic semiconductor heterostructure. Y. Ohno, D. Young, B. Beschoten, F. Matsukura, H. Ohno, D. Awschalom. Nature (1999) Injection and detection of a spin-polarized current in a light-emitting diode. R. Fiederling, M. Keim, G. Reuscher, W. Ossau, G. Schmidt, A. Waag, L. Molenkamp. Nature (1999)

Tunneling Junction Tunneling current remains polarized Measured P=2% at room temperature Trade-off to insulating layer: – Increases injection efficiency – Decreases overall current Ferromagnet Insulator Semiconductor Current Flow

Conclusions Spintronics promises great, if vague, improvements – but is yet to be realized Obstacles to a working spin transistor are substantial – Device was proposed 20 years ago, and no working model has ever been made Will take hard work and possibly a major breakthrough to succeed.