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