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Giti KhodaparastDepartment of PhysicsDepartment of Physics
Virginia TechVirginia Tech
NarrowNarrow--Gap Semiconductors, Spin Splitting Gap Semiconductors, Spin Splitting With no Magnetic Field and moreWith no Magnetic Field and more……....
University of Virginia , Oct 11th 2007
Supported by:NFS-DMR-0507866
AFOSR Young Investigator Award
InSbInSb Based SamplesBased Samples
J. K. Furdyna, InMnSbDepartment of Physics, University of Notre Dame,
M. B. Santos , InSb QWsDepartment of Physics & Astronomy, University of Oklahoma
IIIIII--V SemiconductorsV Semiconductors
δ-doped Si
InSb cap 100Å
AlxIn1-xSb Top Layer 100Å
AlxIn1-xSb Spacer 1000Åδ-doped Si
AlxIn(1-x)Sb Barrier 300Å
InSb Well 11.5 nm to 30nm
AlxIn(1-x).Sb Barrier 300Åδ -doped Si
AlxIn1-xSb Layer 600Å
AlxIn1-xSb Buffer 4μm
AlSb Buffer 2150Å
GaAs (001) substrate
InSbInSb Quantum WellsQuantum Wells
Density: 1-4x1011 cm-2
Mobility: 100,000-200,000 cm2/VsAlloy concentration: 9%, 15%
S. J. Chung, N. Goel, M. B. Santos, University of Oklahoma
Intel and Qinetiq researchers have recently demonstrated prototype InSbquantum well transistors.
InSb QW has the lowest energy dissipation and gate delay which is an important metric for logic microprocessors.
Single electron charging effect in a surface-gated InSb/AlInSb has been reported.Tim Ashley’s group , New Journal of Physics, 9, (2007)
Basic CharacteristicsBasic Characteristics
Most nonMost non--parabolicparabolic--51510.0140.014InSbInSb
More nonMore non--parabolicparabolic--15150.0230.023InAsInAs
Least nonLeast non--parabolicparabolic--0.50.50.0670.067GaAsGaAs
E(k)E(k)gg--factorfactorm*/mm*/moo
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
Quantum Hall Effect in Quantum Hall Effect in InSbInSbR
xy (h
/e2 )
0.0
0.5
1.0
1.5
1.4K33mK
2/3
2
1
B (T)0 5 10 15
Rxx
(kΩ
)
0
1
2
12
0 20.00
0.05 2/3
1/2
9 5
Shubnikov de Haas oscillations down to low B (0.4T)
Spin splitting resolved at starting at low B (>0.8T)
Integer Quantum Hall Effect
No evidence of Fractional Quantum Hall Effect, but not insulating at ν<1
Ballistic transport in InSb mesoscopic structuresHong Chen, J.J. Heremans, J.A. Peters, N. Goel, S.J. Chung, and M.B. Santos, Applied Physics Letters 84, 5380 (2004)
S. J. Chung et al. Physica E7, 809 (2000)
Intensity Anomaly in SpinIntensity Anomaly in Spin--Split CR, Split CR, InSb/AlInSbInSb/AlInSb QWQW
Landau level calculation predicts that blue transition is stronger…
… but experiment determines that red transition is stronger!
Spin Effects in InSb Quantum Wells,” G.A. Khodaparast, et al.,Physica E20, 386 (2004).
ISD
VG
Normal Field Effective Transistor (FET)
Spin Polarized FET
Spin Field Effective Transistor: Datta and Das, 1990, APL. Phys. Lett., 56, 665
Narrow Gap : RevisitedNarrow Gap : Revisited
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 Structures
Zero Field Spin SplittingZero Field Spin Splitting
Asymmetry of the confining potential in a QW remove the degeneracy of band structure Rashba effect
Bulk Inversion asymmetry is known as Dresselhauseffect
ttsub
z kmkEE α±+= *
22
2h
E
K
Zero Field Spin Splitting, Rashba (J. Phys. C 17, 6039 1983) or Dresselhaus effects (Phys Rev 100, 580, 1995)
Spin Polarized CurrentSpin Polarized Current
CB
HH
|+1/2>|-1/2>
|-3/2>|+3/2>CB
|+1/2>|-1/2>
K+x
KxK-x0
σ+
Jx
Jx
Kx0
E
E(a) (b)
S. D. Ganichev and W. Prettl, J. Phys: Condens. Matter 15 (2003) R935-R983
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)
Electron Spin Resonance in Electron Spin Resonance in InSbInSb QWsQWs
B(T)0 1 2
Δ Rxx
(arb
. uni
t)
CR
ESR
n = 1.3x1011cm-2
λ = 432μm45o tilt ν=3
ν=4
ν=5
B (T)0 1 2 3
ΔR
xx (a
rb. u
nits
)
ESR
3456ν = 7
CR
S644, 4.2K30o tilt
λ = 184μm
G. A. Khodaparast et al., “Spectroscopy of Rashba spin splitting in InSb quantum wells”,
Phys. Rev. B 70, 155322 (2004),
Our MotivationOur Motivation
To understand charge/spin dynamics in narrow gap structures
To study phenomena such as interband and intraband photo-galvanic effects, in order to generate spin polarized current
To probe the effect of magnetic impurities on the spin/charge dynamics
Spin Relaxation ProcessSpin Relaxation Process
D'yakonovD'yakonov--Perel MechanismPerel Mechanism
System acts as if in a magnetic field dependent on wavevector k
Spins precess in the field, and relax as k varies due to scattering
B(k’)B(k)
ElliotElliot--YafetYafet MechanismMechanismWave function of the electron is a mixture of spin up and spin down states, with a finite probability of a flip of the spin during scattering events even for spin-conserving scattering process.
EE--Y Mechanism:Y Mechanism:
DD--P Mechanism:P Mechanism:
Spin Relaxation Time [Spin Relaxation Time [ττss]]
τp => momentum relaxation time
PRB, Volume 74,075331 2006
Fast or Slow Relaxation?Fast or Slow Relaxation?
Kimberley Hall, Michael FLATTÉ (APL, 88, 162503 (2006)DAVID D. AWSCHALOM AND MICHAEL E. FLATTÉNature physics | VOL 3 | MARCH 2007
Kerr 130 years ago !!Kerr 130 years ago !!
Change in the polarization state when a linearly Change in the polarization state when a linearly polarized light reflected from a soft polished iron polepolarized light reflected from a soft polished iron pole--piece of a strong electromagnet.piece of a strong electromagnet.
M
KK K’
Eis
Eip
ErsErp
Time Resolved Spectroscopy
Time Resolved Faraday/Kerr Effect
Experimental SetupExperimental Setup
Spin RelaxationSpin Relaxation
Equilibrium Excitation Recombination Non-equilibrium
http://www.physics.ucsb.edu/~awschalom/
Test Spin Test Spin DecoherenceDecoherence in in GaAsGaAs
We observed in We observed in GaAsGaAsbut not in our but not in our narrow gap narrow gap semiconductors!!semiconductors!!
Dia et al. , Appl. Phys. Lett. 73, 3132 (1998)
Univ. of Oklahoma
AlAlxxInIn11--xxSb Band GapSb Band Gap77 K, ~530 meVRT, ~470 meV
AlIn
Sb
AlIn
Sb
InSb2.6 μm477 meV
InSbInSb QWsQWs Low Low FluenceFluence and Degenerateand Degenerate
• Pump – 800 nm• Photo-Induced Carrier density ~1017 cm-3
MO
KE
(1 m
v pe
r di
v.)
80400-40Time Delay (ps)
ΔR
/R (2%
per div.)
80400-40
Sample S1
d)
77KFluence 50 μJ/cm2
InSbInSb QWsQWs, Pump 2 , Pump 2 μμm, Probe 800 nmm, Probe 800 nmM
OKE
(0.5
μV
Per D
iv.)
1086420Time Delay (ps)
InSb QW30nmPump 2 μmProbe 800nm
(b)
RT
Fluence 5 mJ/cm2
ΔR
/R (2
% P
er D
iv.)
12840-4Time Delay (ps)
InSb QW30 nm
Pump 2 μmProbe 800nm
InSbInSb, 11.5 nm wide QW, Pump 2.6 , 11.5 nm wide QW, Pump 2.6 μμm, Probe 800 nmm, Probe 800 nmM
OK
E (1
00 μ
V P
er D
iv.)
20151050-5Time Delay (ps)
10 mJ/cm2
5 mJ/cm2
2 mJ/cm2
(c)
Pump 2.6 μmProbe 800 nm
InSb QW11.5 nm
77 KPumping only inside the QW
Boggess et al., Appl. Phys., Lett., 77, 1333 (2000).Murzyn et al., Appl. Phys., Lett., 83, 5220(2003).Hall et al., Phys. Rev. B., 68, 115311(2003).Murdin et al., Phys. Rev. B, 72, 085346 (2005)Litvinenko et al. , Phys. Rev. B, 74, 075331 (2006).
For thicknesses larger than 1 microns , spin relaxations 20-50 psFor thin InAs 0.15 micron, spin relaxation of ~ 1 ps has been reported
Spin Relaxation in Narrow Gap SemiconductorsSpin Relaxation in Narrow Gap Semiconductors
Litvinenko et al., New Journal of Physics 8, 49, (2006)In InSb QWs reported relaxations below 2 ps!!Recent transport measurements on our InSb QW samples suggest spin coherence time of ~ 12 ps. (Prof. Jean Heremans group at VT)
Narrow Gap Ferromagnetic SemiconductorsNarrow Gap Ferromagnetic SemiconductorsMost current understanding of III-Mn-V :
Small lattice constantsLarge gap and Small hole effective masses
Such as GaMnAs
There are observations where the photo-induced spin relaxation is influencedby Mn ions , Wang et al., J. Phys.: Condens. Matter, 18, R501 (2006)
and there are cases which no interaction has been observed, Kimel, et al .PRL, 92, 237203 (2004)
S. Koshihara et al., PRL 78, 4617 (1997); A. Oiwa et al., APL 78, 518 (2001).
CW Optical Control of FerromagnetismCW Optical Control of Ferromagnetism
LightLight--induced ferromagnetisminduced ferromagnetism
LightLight--induced coercivity decreaseinduced coercivity decrease
MnMn Content 2.0Content 2.0--2.8%, p~ 2x102.8%, p~ 2x102020 cmcm--33, , μμ ~ 100 cm~ 100 cm22/Vs/Vs
InMnSbInMnSb FilmsFilms
InMnSb (0.23 μm)
InSb buffer layer (0.1 μm)
CdTe substrate (4.5 μm)
GaAs substrate
T.Wojtowicz, X. Liu, J.K Furdyna, University of Notre Dame
Tc= 10K
InMnSbInMnSb Based Ferromagnetic StructuresBased Ferromagnetic Structures
-400 -200 0 200 400
-10
-5
0
5
10 T(K) 1.55 2.1 4 6 8 9 15 100
(a)
MC
D (%
)
Magnetic field (Gs)
(b)
400 200 0 -200 -400
-40
-20
0
20
40
ρ Hal
l(B) (
mΩ
cm)
Magnetic field (Gs)
Large spin-orbit coupling
• Negative RA, not expected for a p-type structure
• RA positive in GaMnAs,InMnAsnegative in GaMnSb, not fully understood
Wojtowicz T, Appl. Phys. Lett. 82, 4310, 2003Wojtowicz T, Physica E20, 325, 2004
FluenceFluence Dependence of Relaxation TimesDependence of Relaxation Times
MO
KE
(20
μV
Per
Div
.)
420-2Time Delay (ps)
Pump 1.3 μmProbe 800 nm
77 K RT
(b)M
OK
E (5
0µV
Per
Div
.)
151050-5-10Time Delay (ps)
Fluence 20 μJ/cm2
InMnSb(B)
ΔR/R
(10% per D
iv.)
(c)
850 nm
Fluence20 μJ/cm2
Photo-induced 1017 cm-2
Fluence5 mJ/cm2
Photo-induced 1019 cm-2
InMnSbInMnSb Samples, Different Samples, Different MnMn Effusion Cell TempEffusion Cell Temp
MO
KE
(1 m
v Pe
r D
iv.)
-100 -50 0 50 100Time Delay (ps)
InMnSb(C)
77 K850 nm
Fluence50 μJ/cm
2
σ+
σ−
ΔR
/R (10%
Per Div.)
(d)
12x10-3
11
10
9
8
MO
KE
(V)
3210-1-2Time Delay (ps)
800 nm, 5 K 800 nm, 5 K 800 nm, 5K (0.7 T)
Fluence 50 μJ/cm2
Temperature Dependence of InMnSbTemperature Dependence of InMnSb
K. Nontapot et al., APL, 90, 143109 (2007)
MO
KE
Sign
al (0
.5 m
V Pe
r. D
iv.)
840-4Time Delay (ps)
Sample A 5K
Sample D 2 K
Pump/Probe800 nm
(a)
MO
KE
Sign
al (0
.5 m
V Pe
r. D
iv.)
86420-2Time Delay (ps)
25 K 77 K 2 K
Sample D (2.8% Mn)
Pump/Probe800 nm
(b)
Dynamics in Dynamics in InMnAsInMnAs
-6
-4
-2
0
Pho
toin
duce
d M
OK
E S
igna
l (10
-5 V
)
3210Time Delay (ps)
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
ΔR/R
(%)
3210Time Delay (ps)
ReflectivityPhotoinducedMOKE
J. Wang J, G. A. Khodaparast, J. Kono, T. Slupinski, A. Oiwa, and H. MunekataJournal of Modern Optics 51, 2771,2004
Summary/Future PlansSummary/Future Plans
•We have explored spin/charge dynamics in a series of Narrow GapSemiconductors
• We can alter the relaxation as a function of photo-induced
• The relaxations in ferromagnetic samples very much depends on the sample properties
Working on spin-polarized current generationFabricating 1-D system to probe the dynamics in lower dimensionsTaking advantages of the FEL to probe the conduction bands
Inter Inter subbandsubband DynamicsDynamics
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
Brett, Emily, Kanokwan, Matt, Rajeev
Jonathan and Ashley
Group Members
Thank you for your attention
Time Resolved Cyclotron MeasurementsTime Resolved Cyclotron Measurements
(T0-T)/T0Dela
y (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)
Relaxations in Relaxations in UndopedUndoped InSbInSb QWQW
•Photo-Induced Carrier density ~1017 cm-3
• Carrier/Spin lifetime longer than 40ps
ΔR/R (7%
per Div.)
-80 -40 0 40 80Time Delay (ps)
MO
KE
(0.5
mV
per D
iv.)
80400-40Time Delay (ps)
Fluence50 μJ/cm2
77 K
Pump/Probe775 nm24 MQW (30nm wells)
Undoped InSb MQW containing 25 periods of 30 nm InSb wells