Spin/Carrier Dynamics in Narrow Gap Semiconductors
Transcript of Spin/Carrier Dynamics in Narrow Gap Semiconductors
Giti A. KhodaparastDepartment of Physics, Virginia Tech
Supported by NSF and AFOSR
Graduate Students : Kanokwan Nontapot, Aliya Gifford, Matt Frazier
Post doc.: Rajeev Kini
Spin/Carrier Dynamics in Narrow Gap SemiconductorsSpin/Carrier Dynamics in Narrow Gap Semiconductors
InSbInSb Based Structures:Based Structures:
Prof. J. K. Furdyna, InMnSbDepartment of Physics, University of Notre Dame
Prof. M. B. Santos , InSb QWsDepartment of Physics & Astronomy, University of Oklahoma
Intel and Qinetiq researchers have recently demonstrated prototype InSb quantum well transistors
InSb QW has the lowest energy dissipation and gate delay which is an important metric for logic microprocessors
IIIIII--V SemiconductorsV Semiconductors
InSbInSb Based Based HeterostructuresHeterostructures
Small effective mass,large g-factor
Large spin-orbit coupling
Small e-e interaction
An ideal model of a narrow-gap semiconductor
Narrow Gap : Revisited Narrow Gap : Revisited Spin Orbit Interaction
Rashba Effect
Electronic systems with finite spin splitting in the absence of external magnetic field!!!
Z
Ez
X
Y
V
Beff =E×V/c
Symmetric quantum well
InSb
AlxIn1-xSb AlxIn1-xSb
Asymmetric quantum well
InSb
AlxIn1-xSb
AlxIn1-xSb
InSbInSb Quantum Well StructuresQuantum Well StructuresProf. M. B. Santos Prof. M. B. Santos
Department of Physics & Astronomy, Department of Physics & Astronomy, University of OklahomaUniversity of Oklahoma
Density: 1-4x1011 cm-2
Mobility: 100,000-200,000 cm2/VsAlloy concentration: 9%, 15%
RashbaRashba EffectEffectAsymmetry of the confining potential in a QW remove the degeneracy of band structure Rashba effect:
ttsub
z kmkEE α±+= *
22
2h
E
K
Electronic systems with finite spin splitting in the absence of external magnetic field!!!
Y.A. Bychkov and E.I. Rashba, J. Phys. C 17, 6039 (1984);Y.A. Bychkov and E.E. Rashba, [JETP Lett. 39, 78 (1984)]
RashbaRashba Splitting at B>0Splitting at B>0
E(k)=ħ 2k2/2m ±αk
In addition to:J. Luo, et al. PRB, 41,7685 (1990)
Change in g* at low magnetic fieldG. A. Khodaparast, et al. Phys. Rev. B 70, 155322 (2004)
Spin RelaxationSpin Relaxation
Equilibrium Excitation Recombination Non-equilibrium
Why Why SpintronicSpintronic??
ISD
VG
Normal Field Effective Transistor (FET) B
s
Spin Polarized FET
<Sx>∝cosωtω ∝E - E
Efficiency!!Efficiency!!
Spin Field Effective Transistor: Datta and Das, 1990, APL. Phys. Lett., 56, 665
Our MotivationOur Motivation
Understand charge/spin dynamics in narrow gap structures
Study phenomena such as interband and intraband photo-galvanic effects, in order to generate spin polarized current
We will need FEL!!
Probe the effect of magnetic impurities on the spin/charge dynamics: Poster by Matt Frazier today
Carrier dynamics:Carrier dynamics:PumpPump--Probe SpectroscopyProbe Spectroscopy
TiTi--Sapphire laser Sapphire laser –– 800nm 800nm
Carrier density ~10Carrier density ~101717 cmcm--33
Optical Parametric Optical Parametric Amplifier Amplifier –– 1.11.1μμm m –– 2.4 2.4 μμm, Carrier density ~10m, Carrier density ~101919
cmcm--33
Optical Induced Magneto Optical Optical Induced Magneto Optical Kerr Effect (MOKE)Kerr Effect (MOKE)
•• Selection rules for inter-band transitions, Spin-polarized carriers using circularly polarized beams
Spin Dynamics:Spin Dynamics:Magneto-Optical Kerr (reflectivity)Magneto-Optical Faraday (Transmission)
• Optical MOKE signal arises from the difference between the optical coefficients of a material for left and right circularly polarized light.
The rotation of the polarization plane is proportional to the magnetization
Carrier RelaxationCarrier Relaxation
• Pump – 800 nm
• Carrier density ~1017 cm-3
• Sharp decrease, followed by an exponential recovery
• Carrier lifetime ~ 40psΔR
/R (2
% P
er D
iv.)
-200 -150 -100 -50 0 50 100 150 200Time Delay (ps)
77K
b)Asymmetric QWSymmetric QW
Spin relaxationSpin relaxationPh
oto
Indu
ced
MO
KE
(1m
V pe
r div
.)
-200 -100 0 100 200Time Delay (ps)
77 K
Fluence50 μJ/cm2
σ+
σ−
• Spin relaxation for left and right circularly polarized pump.
• Relaxation time ~ 40ps
Carrier relaxation at 2Carrier relaxation at 2μμmm
Asymmetric Quantum Well
• Pump – 2 μm, Probe 800 nm
• Carrier density ~1019 cm-3
• Sharp decrease, followed by an exponential recovery.
• Carrier lifetime ~ 8ps
RT
-1.0
-0.5
0
0.5
1.0
-10 0 10 20
Delay (ps)
Δ R/R
(%)
RT
Spin relaxation at 2Spin relaxation at 2μμmmAsymmetric Quantum WellAsymmetric Quantum Well
• Spin polarized carriers excited using circular polarized light
• Spin lifetime ~ 4 ps
-1.5
0
1.5
3.0
0 5 10
Delay (ps)
MO
KE( μ
V) σ-
RT
Carrier relaxation at 2Carrier relaxation at 2μμmmSymmetric Quantum WellSymmetric Quantum Well
• Pump – 2 μm, Probe 800 nm
• Carrier density - ~1019 cm-3
• Sharp decrease, followed by an exponential recovery.
• Carrier lifetime ~ 4 ps
-3
-2
-1
0
1
2
-2.5 0 2.5 5.0 7.5 10.0
Delay (ps)
ΔR/R
(%)
RT
Spin relaxation at 2Spin relaxation at 2μμmmSymmetric Quantum Well
• Spin polarized carriers excited using circular polarized light
• Spin lifetime ~ 2 ps
-10
0
10
-3 0 3 6
Delay (ps)
MO
KE
(μV
)
σ+
RT
Time Resolved Cyclotron Resonance Measurements, Time Resolved Cyclotron Resonance Measurements, Stanford FELStanford FEL
delay
Pelliclebeamcombiner
FELprobe
Ti:Spump
synchronization
Ga:Ge detectorBSample
0-8 T
λ = 800 nmpulse ~ 200 fsmax 1 mJ/pulse
λ = 3-60 μmpulse 600 fs – 2 psmax 1 μJ/pulse
Prof. Kono’s Group at Rice University
Time Resolved Cyclotron MeasurementsTime Resolved Cyclotron Measurements
(T0 -T)/T
0
Delay (
ps)
B (T)
Interband pump + Intraband probe
Monitor dynamics of relaxing carriers in conduction band directly in time:
Effective mass m*(t)Density n(t)Scattering time τ(t)
G. A. Khodaparast et al, PRB 67 035307 (2003)
Carrier Dynamics in Carrier Dynamics in InSbInSb QWsQWs
0
0.1
0.2
0.3
0.4
0.5
0 100 200 300 400
ΔT/
T
Time Delay (ps)
Differential Transmission
Interband Pumping (800 nm)
Probe Beam (42 μm)
Undoped InSb MQW containing 25 periods of 35 nm InSb wells
1.5K
Spin Polarized CurrentSpin Polarized Current
S. D. Ganichev and W. Prettl, J. Phys: Condens. Matter 15 (2003) R935-R983
Spin Polarized CurrentSpin Polarized CurrentUsing FELUsing FEL
•Spin Current as a result of spin flip scattering
•Both inter-band and inter-subbandhas been observed in GaAs based QWs
S. D. Ganichev and W. Prettl, J. Phys: Condens. Matter 15 (2003) R935-R983 C. L. Yang et al. , PRL, 96, 186605 (2006)
Summary/ Future WorkSummary/ Future Work
- InSb based structures important: fundamental physics and applications
High carrier mobility, large Rashba effect,….
-We have some understanding of carrier/spin relaxations in this system
- We can take advantage of FEL to probe spin polarized current in this structure
Thank you for your attention