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

2. 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

3. 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.] http://www.explainthatstuff.com/howtransistorswork.html.

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

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

6. 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

7. Properties of Ferromagnets Conduction electrons form a polarized current M

8. 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. 7 56 Applied Physics Letters (1990) Gate Source Electrode Drain Electrode Ferromagnet Substrate 2-D Semiconductor

9. 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. 7 56 Applied Physics Letters (1990)

10. 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. 1488.

11. 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. 1488.

12. 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. 8 62 (2000)

13. What if Ferromagnet was a Semiconductor? Use a ferromagneticaly doped semiconductor It Works! P measured between 90-100% BUT… Electrical spin injectin in a ferromagnetic semiconductor heterostructure. Y. Ohno, D. Young, B. Beschoten, F. Matsukura, H. Ohno, D. Awschalom. Nature 402 790 (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 402 787 (1999)

14. What if Ferromagnet was a Semiconductor? Use a ferromagneticaly doped semiconductor It Works! P measured between 90-100% 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 402 790 (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 402 787 (1999)

15. 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

16. 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.