Transcript Spintronics - Boston University Physics

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.]
http://www.explainthatstuff.com/howtransistorswork.html.
Field Effect Transistor (FET)
Source
Electrode
Drain
Electrode
Gate
n-type
Φ(x)
P-type
E
n-type
E
x
Field Effect Transistor (FET)
Source
Electrode
Drain
Electrode
Gate
++++++++++
n-type
Φ(x)
P-type
E
n-type
E
x
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
Source
Electrode
Drain
Electrode
Gate
Ferromagnet
2-D Semiconductor
Ferromagnet
Substrate
• 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)
A Spin Based Transistor
Source
Electrode
Drain
Electrode
Gate
++++++++++
Ferromagnet
2-D Semiconductor
Ferromagnet
Substrate
• 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
Electronic Analog of Electro-optic Modulator. S. Datta, B. Das. 7 56 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. 1488.
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.
What’s the problem anyway?
• Direct current from ferromagnet to semiconductor
produces very low polarization, <1%
• ‘Conductivity Mismatch’
P0 =polarization far inside the ferromagnet
σF , σSC = conductivity of the ferromagnet , semiconductor
λF , λSC = 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)
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)
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)
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.