皮克宇 *

皮克宇 *

Department of Physics and Astronomy UC Riverside

4月 26日 , 2011

NTNU

*Current location: Hitachi Global Storage Technologies

Spintronic and electronic transport properties in graphene – The cornerstone for spin logic

devices.

I. Introduction.

Outline

III. Enhanced spin injection efficiency: Tunnel barrier study.

IV. Spin relaxation mechanism in graphene: --- Charged impurities scattering. --- Chemical doping on graphene spin valves.

II. Gate tunable spin transport in signal layer graphene at room temperature.

Silicon electronics and the “end-of-the-roadmap”…. How to improve computers beyond the physics limits of

existing technology?

Motivation for Spintronics

Spintronics: Utilize electron spin in addition to charge for information storage and processing.

Spin up“1”

Spin down“0”

Spins fordigital information

OR

Logic:Silicon-based electronics are the

dominant technology for microprocessors.

Technological Approach

Storage:Magnetic Hard Drives and

Magnetic RAM use metal-based spintronics technologies.

Ferromagnetic Materials:• Non-volatile• Radiation hard• Fast switching

Semiconducting Materials:• Tunable carrier concentration• Bipolar (electrons & holes)• Large on-off ratios for switches

Spintronics may enable the integration of storage and logic for new, more powerful computing architectures.

Hanan Dery et al., arXiv 1101.1497 (2011).

Material

Good electrical properties and potential good spintronic properties.

Carbon Family (Z=6) ~ One of the candidates for the cornerstone of this bridge.

Carbon Nanotube

1D

K. Tsukagoshi, B. W. Alphenaar, and H. Ago, Nature 401, 572 (1999).

Graphite

3D

M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, 252505 (2007).

Graphene

2D

Discover in 2004 !!K. S. Novoselov et al., Science 306, 666 (2004).

Properties of Graphene

High mobility -- up to 200,000 cm2/Vs (typically 1,000 – 10,000 cm2/Vs).

Zero gap semiconductor with linear dispersion: “massless Dirac fermions”.

Tunable hole/electron carrier density by gate voltage.

Possible for large scale device fabrication.

Electronic Band StructurePhysical Structure

Atomicsheetof carbon

C. Berger et al., Science 312, 1191 (2006).

K. S. Kim et al., Nature 457, 706 (2009).

Possibility for long spin lifetime at RTLow intrinsic spin-orbit coupling

Graphene Spin transport1. E. W. Hill et al., IEEE Trans. Magn. 42, 2694 (2006). (Prof. Geim’s group at Manchester )

2. M. Ohishi et al., Jpn. J. Appl. Phys 46, L605 (2007). (Prof. Suzuki’s group at Osaka)

3. S. Cho et al., Appl. Phys. Lett. 91, 123105 (2007). (Prof. Fuhrer’s group at Maryland)

4. M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, 252505 (2007). (Prof. Goldman’s group at Minnesota)

5. N. Tombros et al., Nature, 571 (2007). (Prof. van Wees’ group at University of Groningen)

6. W. H. Wang et al., Phys. Rev. B (Rapid Comm.) 77, 020402 (2008). (Prof. Kawakami’s group at Riverside)

Figure 2 in ref. 5.

Observed Local and non-local magnetoresistance.

Figure 3 in ref. 5.

Gate dependent non-local magnetoresistance.

Figure 4 in ref. 5.

Hanle spin precession.

• Demonstrated the first gate tunable spin transport in graphene spin valve at room temperature.

Hybrid Spintronic Devices

Spin transport over long distances

Long spin lifetimes

Allows spin manipulation

Gate-tunable spin transport

High spin injection efficiency

Room temperature operation

Desired Characteristics Graphene (beginning in 2007)

Yes

OK, 5 microns. Small graphene flakes.

Theory: yes, Experiment: no

Yes (With tunnel barrier)

Good potential

Yes

Spin Injector Spin Detector

0 +_

LateralSpin Valve

Ferromagnetic Electrodes

Spin Transport Layer

M M

I. Introduction.

Outline




Sample preparation

Raman

Identify single layer graphene with optical microscope and confirm with Raman spectrum.

Optical

Standard ebeam lithography

Co

SLG

Sample preparation

SLGSiO2

MgO (0°)

Co (7°)Co

SiO2

MgOSLG

2nm

500 nm

SLG

SEM

SiBack Gate

Device characterization

I (μA)

dV/d

I (kΩ

)

R4pt

R3pt

R3pt – R4pt

Vg = 0 V E1 E2 E3 E4

IV

Relectrode + Rcontact

< 300 ohms

Transparent contact of Co/SLG

Contact resistance

Gate dependent resistance

0

0.5

1.0

1.5

0 200-200

E1 E2 E3 E4

I

V

MgOCo

SLG

E1 E2 E3 E4

IV

-60 -40 -20 0 20 400.0

0.5

1.0

1.5

Cond

ucta

nce

(mG)

Gate Voltage (V)

m ~ 2500 cm2/Vs

Spin Injection and Chemical Potential

FM graphene

Chemical Potential(Fermi level)

m

e-

m

m

Spin-dependentChemical potential

Density of states Density of states

Local and Nonlocal Magnetoresistance

Local spin transport measurement:

Spin Injector Spin Detector

charge current

IV

spin current

Non-local spin transport measurement:

Spin Injector Spin Detectorcharge current

spin current

IINJ VNL

+ -

M. Johnson, and R. H. Silsbee, PRL, 55, 1790 (1985)

Using lock-in detection

Nonlocal Magnetoresistance

IINJ VNL

L

H

Injector Detectors

Vp>0

IINJ VNL

L

H

Injector Detectors

VAP<0

Parallel Anti-Parallel

Spin down Spin down

Spin up Spin up

Nonlocal MR = (VP - VAP)/IINJ

m

m

m

m

Spi

n de

pend

ent

chem

ical

pot

entia

l

Spi

n de

pend

ent

chem

ical

pot

entia

l

Spin Signal

Nonlocal MR = ΔRNL = ΔVNL/Iinj

-100 0 100-80

-40

0

40

80

RNL

(m)

H (mT)

ΔRNL

RT

0 100 200 3000

100

200

RNL

(m

)

Temperature(K)

Nonlocal MR--- Temperature dependent

Room temperature spin transport

L = 1 μm

RN

l (m

Ω)

H (mT)

Nonlocal MR—Spacing dependence

RN

L (m

Ω)

H (mT)

L = 3 μm

E1

SLG

E2

E3

E4

E5

E6 E7

1 um

2μm 1 3μm

L (mm)

ΔR

(m

)

λS ~1.6 μm

RN

L (m

Ω)

H (mT)

L = 2 μm

Wei Han, K. Pi et al., APL. 94, 222109 (2009)

-600 -300 0 300 600-40

-20

0

20

40

Non

-loca

l sig

nal (

m)

H (Oe)

-600 -300 0 300 600-40

-20

0

20

40

Non

-loca

l sig

nal (

m)

H (Oe)

-600 -300 0 300 600-40

-20

0

20

40

Non

-loca

l sig

nal (

m)

H (Oe)

Graphene spin valve

Gate tunable non-local spin signal

spin injection efficiency is low.

P~ 1%.

L = 3 μm

-160 -80 0 80 160

-1.0

-0.5

0

0.5

1.0

H (mT)

RN

L (m

Ω)

Hanle spin precession – spin lifetime measurement

2

0

1 exp cos( )exp( / )44NL L sLR t t dtDtDt

IINJ VNL

H

L

D = 0.025 m2/ss = 84 psλs = 1.5 μm

Diffusioncoefficient

SpinLifetime

spin lifetime is “short”.

Challenges

• Create spin polarized current in graphene.

• Keep spin current polarized in graphene.

How to increase the spin injection efficiency?

What is the spin relaxation mechanism in graphene?

I. Introduction.

Outline




Theoretical analysis

How to achieve efficient spin injection?

2 2/ 2 / 1

2 2 2 21 1

2 24 ( ) [ (1 ) ]

1 1 1 1G G

i iF FJ F

L LG G G GNL G

i iJ F J F

R RR RP PR R R R

R R e eP P P P

Insert a thin tunnel barrier to make R1, R2 >> RG

MgOCo

SLG

L=λG=W=2 μmPF=0.5, PJ=0.4ρG=2 kΩ

0 20000 40000Interface resistance

(R1, R2 )(Ω)

RN

L(Ω)

0

60

120

Tunnelingcontacts

Transparentcontacts

How to fabricate pin-hole free tunnel barrier.

Takahashi, et al, PRB 67, 052409 (2003)

MgO Barrier with Ti adhesion layer

1 nm MgO on graphite (AFM)

MgO

graphite TiNo Ti

W. H. Wang, W. Han et. al. ,Appl. Phys. Lett. 93, 183107 (2008).

RMS roughness: 0.229nm

RMS roughness: 0.766nm

Tunneling spin injection into SLG

I V-+

Fabrication and Electrical characterization

SLGSiO2

Ti/MgO (0°)

Ti/MgO (9°)

Co (7°)

MgO

SLGSiO2

TiO2

I

Co

-0.6 -0.4 0 0.3 0.6-8

VDC(V)

-4

0

4

8

I DC (μ

A)

2-probe

300 K

3-probe

300 K

-10 0 100

IDC (mA)

50

100

150

200

dV/d

I (k

)

Tunneling spin injection into SLG

2/ NLJ G

NLG

PR e

W

RNL=130 , PJ=31 %

Large Non-local MR with high spin injection efficiency

Johnson & Silsbee, PRL, 1985. Jedema, et al, Nature, 2002 .

(0 ) 0.35 ,(0 ) 130.4 ,

~ 2.2 ,2.1 ,2 ,

NL

G

V mSR VW mL m

m

mm

m

Wei Han, K. Pi et. al., PRL 105, 167202 (2010).

-600 -300 0 300 600-20

-15

-10

-5

0

5

10

15

20

Non

-loca

l sig

nal (

m)

H (Oe)-800 -400 0 400 800

-100

-50

0

50

100

Non

-loca

l sig

nal ()

H (Oe)

Comparison of Co/SLG and Co/MgO/SLG

RNL= 0.02 P ~ 1%

Co

1nm

SiO2

MgOSLG

3nm

Co

SiO2

MgOSLG2nm

Tunnel barrier increases spin signal by factor of ~1,000

RNL=130 P ~ 31%

Vg=0 V

L=1 mm L=2.1 mm

Vg=0 V

/ /2 2 22

2 / 2 /2 2 2 2

4 4 1( ) [ ] ~(1 ) 1 (1 ) 1

G G

G G

L LF F F F

NL G GL LF G F G

p R p Re eR Rp R e p e R

2 2/ 2 / 1

2 2 2 21 1

2 24 ( ) [ (1 ) ]

1 1 1 1N G

i iF FJ F

L LG G G GNL G

i iJ F J F

R RR RP PR R R R

R R e eP P P P

For Ohmic spin injection with Co/SLG

For Tunneling spin injection with Co/MgO/SLG

2 2/ 2 / 1

2 2 2 21 1

2 24 ( ) [ (1 ) ]

1 1 1 1G G

i iF FJ F

L LG G G GNL G

i iJ F J F

R RR RP PR R R R

R R e eP P P P

/2 1~GLG GNL J

G

RR P e

W

Theoretical analysis

Gate Tuning of Spin Signal

Drift-Diffusion Theory for Different Types of Contacts

Proportional tographene conductivity

Inversely proportional to graphene conductivity


Transparent contact Pin-hole contact


Characteristic gate dependence of tunneling spin injection is realized.

Tunneling contact

I. Introduction.

Outline




Spin relaxation in graphene

Experiment:Spin lifetime ~ 500 ps(for single layer graphene)

Theory:Spin lifetime ~ 100 ns – 1 ms

C. Jozsa, et al., Phys. Rev. B, 80, 241403(R) (2009).N. Tombros, et al., Phys. Rev. Lett. 101, 046601 (2008).

Charged impurities (Coulomb) are the most important type of momentum scattering.

Are charged impurities important for spin relaxation?

Elliot-Yafet mechanism

defects

Spin flip during momentum scattering events.

D’yakonov-Perel mechanism

spins precess in internal spin-orbit fields.

Two types of spin relaxation mechanisms:

Single-Layer Graphene (SLG)

SiSiO2

(backgate)

Co electrode

Graphene spin valve device

I V+ -

MBE cell

Charged impurities(we use Au in this study)

We add charged impurities onto a graphene spin valve to study its effect on spin lifetime.

Experiment

K. Pi, Wei Han et.al., Phys. Rev. Lett. 104, 187201 (2010).

How to perform the experiment????

• With small amounts of adatom coverage, metal impurties

will oxidize.

• Clean environment and fine control of deposition rate.

In-situ Measurement.

Molecular beam epitaxy Growth.

Challenges

500 nm

SLG

The UHV System

Small MBE Chamber• Measure Transport Properties• Vary Temperature from 18K to

300K• Ports for 4 different materials• Apply a magnetic field

SEM image Magnet

In situ measurement

Au 2 sAu 4 sAu 6 sAu 8 s

No Au

Gate Voltage (V)

Con

duct

ivity

(mS

)

Au is selected for this study because Au behaves as a point-like charged impurity on graphene.

Gate dependent conductivity vs.

Au deposition time

Au deposition (Sec)

m (c

m2 /V

s)

Coulomb scattering is the dominant charge scattering mechanism.

T=18 K

Deposition rate ~ 0.04 Å/min (5x1011 atom/cm2s)

K. M. McCreary, K. Pi et al., Phys. Rev. B 81, 115453 (2010).

Effect of Au doping on non-local signal

Au doping does not introduce extra spin scattering.

Introducing extra spin scattering.

Gate (V)

Rnl (

)

Simulation

Without introducing extra spin scattering.

Gate (V)

Rnl (

)

Simulation

Au 2 sAu 4 sAu 6 sAu 8 s

No Au

Gate Voltage (V)C

ondu

ctiv

ity (m

S)

datafit

Au = 0 sDirac Pt.Δ

RN

L (Ω

)datafit

Au = 8 sDirac Pt.Δ

RN

L (Ω

)

-0.01 0.010H (T)

-0.01 0.010H (T)

datafit

Au = 0 sElectronsΔ

RN

L (Ω

)

-0.01 0.010H (T)

datafit

Au = 8 sElectronsΔ

RN

L (Ω

)

-0.01 0.010H (T)

datafit

Au = 0 sHoles

H (T)

ΔR

NL (

Ω)

-0.01 0.010

datafit

Au = 8 sHoles

H (T)

ΔR

NL (

Ω)

-0.01 0.010

Hanle precession

Directly compare spin lifetime between different amounts of Au doping.

Spin lifetime and the diffusion coefficient are determined from Hanle spin precession data

Effect of charged impurities on spin lifetime

Au deposition (s)

Spin

life

time

(ps)

(2.9x1012 cm-2)

Spin relaxation

Charged impurities are not the dominant spin relaxation mechanism.

Momentum scattering

0 2 4 6 8Au deposition (sec)

0.00

0.02

0.04

0.06

D (m

2 /s)

Dirac Pt.ElectronsHoles

• Spin relaxation mechanisms are correlated.

• Effect of D’yakonov-Perel mechanism.

1/ s 1/C 1/ jj

c : Spin relaxation by Coulomb scattering.

j : Spin relaxation by other defects (lattice

defects, sp3 bound etc.).

Slight enhancement of spin lifetime

Y. Gan et al., Small 4, 587 (2008).S. Molola et al., Appl. Phys. Lett. 94, 043106 (2009). Wei Han et al., arXiv 1012.3435 (2011).

E-Y mechanism: s ~ m

D-P mechanism: s ~ m-1

F. Guinea et al., Solid State comm. 149, 1140 (2009).

Further study is needed.

Recent study shows that Co contact plays an important role.

Enhancement of spin signal by chemical doping

• At fixed gate voltage, Au doping can enhance conductivity. • No significant spin relaxation from charged impurities.

Possible to tune spin properties by chemical doping instead of applying high electric field (gate voltage).

2.0

1.5

1.0

0.5

0.0

Con

duct

ivity

(mS)

By Au doping we are able to enhance spin life time from 50 ps to 150 ps.

Conclusion

Achieved tunneling contact on graphene spin valves.

Au deposition (s)

Spi

n lif

etim

e (p

s)

Demonstrated charged impurities are not the dominant spin relaxation mechanism.

Manipulation of spin transport in graphene by surface chemical doping.

Roland Kawakami

Wei HanKathy McCrearyPostdoc: Wei-Hua Wang (Academia Sinica in Taiwan)Yan LiAdrian SwartzJared WongRichard Chiang

Collaborators

Wenzhong BaoFeng MiaoJeanie Lau (PI)Peng WeiJing Shi (PI)Shan-Wen Tsai (PI)Francisco Guinea (PI)Mikhail Katsnelson (PI)

Acknowledgements

Thank you.

• Hydrogen storage.

--- AI doped graphene as hydrogen storage at room temperature.

Z. M. Ao et al., J. Appl. Phys. 105, 074307 (2009).

• Adatoms on Graphene; Wave function hybridization between TM and graphene may lead us to the new physics.

--- Fe on graphene is predicted to result in 100% spin polarization.

--- Pt may induce localized magnetic states in Graphene. Y. Mao et al., Journal of Physics: Condensed Matter 20, 2008 (2008).

B. Uchoa et al., Phys. Rev. Lett. 101, 026805 (2008).

New physics in TM doped graphene system

The UHV System

We use same system to study the charge transfer and charge scattering mechanism of transition metals doped graphene.

5 mm

SEM image Magnet

Dirac point shift vs. Ti and Fe coverageC

ondu

ctiv

ity (m

S)

Con

duct

ivity

(mS)

Gate Voltage (V) Gate Voltage (V)

ØTi = 4.3 eV ØFe = 4.7 eV Øgraphene = 4.5 eV

Both Ti and Fe coverage show n-type doping

No Ti (0 ML)

0.0038 ML

0.0077 ML

0.015 ML

No Fe (0 ML)

0.041 ML

0.123 ML

0.205 ML

Dira

c Po

int (

V)

Ti coverage (ML)

0

-40

-80

0.00 0.01 0.02

Dira

c Po

int (

V)

Fe coverage (ML)

0

-30

-60

0.0 0.1 0.2

Keyu Pi et al., PRB 80, 075406 (2009).

Dirac point shift vs. Pt coverage

TM coverage (ML)D

irac

poin

t shi

ft (V

)

Pt-1Pt-2Fe-1Fe-2Fe-3Ti-1Ti-2Ti-3

Con

duct

ivity

(mS)

Gate Voltage (V)

ØPt = 5.9 eV

• The trend of Dirac point shift follows the work function.

• All the Pt and Fe samples show the n-type doping behavior.

Regardless of the metal work function, all TMs we have studied result in n-type doping when making contact with graphene.

No Pt (0 ML)

0.025 ML

0.071 ML

0.127 ML

Dira

c Po

int (

V) 0

-20

-40

Pt coverage (ML)0.00 0.05 0.10 0.15

Interfacial dipole

WM

Metal

d

WG

EF

V

W

EF

Gra

phen

e

+q-q

V(d) = tr(d) + c(d)

dWG

EF

V

W

EF

Gra

phen

e

+q-q

dWG

EF

V

W

EF

Gra

phen

e

+q-q

Become n-type doping

G. Giovannetti et al., Physical Review Letters 101, 026803 (2008).

tr(d) : The charge transfer between graphene and the metal (difference in work functions).

c(d) : the overlap of the metal and graphene wave functions

c(d) = e−gd (a0 + a1d + a2d2)

Highly depends on d.

G. Giovannetti et al., Physical Review Letters 101, 026803 (2008).

Possible reason for anomalous n-type doping

--- An interfacial dipole having 0.9eV extra barrier for an equilibrium distance ~ 3.3 Å makes the required work function for p-type doping > 5.4eV. ( This explains why Fe with ØFe = 4.7 eV dopes n-type). --- Nano-clusters (smaller than ~ 3nm) have different work function values when compared with bulk material.

M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008).

Transition metal

Graphened

p-type

n-type

Experimental evidence of interfacial dipole.

0 2 4 6 8

0 0.87 1.75 2.62 3.50

Pt Coverage (ML)

Pt Coverage (Å)

Dira

c P

oint

(V)

AFM 1

AFM 2

0 nm

10 nm

3.19 ML0.62 ML

AFM 1 AFM 2

K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77, 235430 2008.

Grapheneddd

By Theoretical calculation, d increase as material coverage went from adatoms to continuous film.

Interfacial dipole

Scattering introduced by TM

• Long range scattering. (Charge impurity)

• Short-range scattering.

(Point defect, wave function hybridization etc.)

• Surface corrugations. (Ripple)

F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson, and K. S. Novoselov, Nature Mater. 6, 652 (2007).

J.-H. Chen, C. Jang, S. Adam, M. S. Fuhrer, E. D. Williams, and M. Ishigami, Nature phys. 4, 377 (2008).

The electron and hole mobilities (μe, μh) are determined by taking a linear fit of the σ vs. n curve just away from the Dirac point (μe,h= |Δσ/Δne| )

Fe data show strong electron hole asymmetry.

Mobility change vs. TM coverage

Dirac point shift with TM coverage: Ti >Fe >Pt

Mobility drop with TM coverage: Ti >Fe >Pt

Dirac point shift vs. Mobility change?

Mobility, m (10

3 cm2/Vs)

3

2

1

0

3

2

1

0

3

2

1

0

Fe-2Con

duct

ivity

(mS)

Pt-2

Ti-1

2.0

1.0

0.0

1.5

1.0

0.5

0.0

2.0

1.0

0.0-4 -2 0 2 4 0.000 0.015 0.030

n (1012 cm-2) Coverage (ML)

HoleElectron

Dirac Point Shift (V)

Nor

mal

ized

mob

ility

, μ/μ

0

Dirac Point Shift (V)

Pt-1Pt-2Ti-1Ti-2

Fe-2

μ/μ0 = (Γ0 + ΓTM)-1/Γ0-1

= (1 + ΓTM/Γ0)-1

Fitting equation:

• Electron data follows the universal curve.

• Hole data is significantly different.

• This implies some wave function hybridization in the Fe system.

Ti and Pt fall on the universal curve.Coulomb scattering is the dominant effect.

Mobility change vs. Dirac point shift

0.1 ML

0.008 ML

ΓTM/Γ0 = (AVD,shift)β

μ/μ 0

Keyu Pi, K. M. McCreary et al., PRB 80, 075406 (2009).

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皮克宇 *