皮克宇 *
description
Transcript of 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
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.
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
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.
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
Gate Tuning of Spin Signal
Transparent contact Pin-hole contact
Gate Tuning of Spin Signal
Characteristic gate dependence of tunneling spin injection is realized.
Tunneling contact
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.
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).