Presentation on theme: "皮克宇* Department of Physics and Astronomy UC Riverside 4月26日, 2011 NTNU"— Presentation transcript:
1Spintronic and electronic transport properties in graphene – The cornerstone for spin logic devices. 皮克宇*Department of Physics and AstronomyUC Riverside4月26日, 2011NTNU*Current location: Hitachi Global Storage Technologies
2Outline I. Introduction. Gate tunable spin transport in signal layer graphene at room temperature.III. Enhanced spin injection efficiency: Tunnel barrier study.Spin relaxation mechanism in graphene:--- Charged impurities scattering.--- Chemical doping on graphene spin valves.
3Motivation for Spintronics Silicon electronics and the “end-of-the-roadmap”….How to improve computers beyond the physics limits of existing technology?Spintronics: Utilize electron spin in addition to charge for information storage and processing.Spin up“1”Spin down“0”Spins fordigitalinformationOROutline here
4Technological Approach Storage:Magnetic Hard Drives and Magnetic RAM use metal-based spintronics technologies.Logic:Silicon-based electronics are the dominant technology for microprocessors.Ferromagnetic Materials:Non-volatileRadiation hardFast switchingSemiconducting Materials:Tunable carrier concentrationBipolar (electrons & holes)Large on-off ratios for switchesOutline hereSpintronics may enable the integration of storage and logic for new, more powerful computing architectures.Hanan Dery et al., arXiv (2011).
5Material 1D 2D 3D Discover in 2004 !! Good electrical properties and potential good spintronic properties.Carbon Family (Z=6) ~ One of the candidates for the cornerstone of this bridge.Carbon Nanotube1DK. Tsukagoshi, B. W. Alphenaar, and H. Ago, Nature 401, 572 (1999).Graphene2DDiscover in 2004 !!K. S. Novoselov et al., Science 306, 666 (2004).Graphite3DM. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (2007).
6Properties of Graphene Physical StructureAtomicsheetof carbonElectronic Band StructureHigh 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.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
7Graphene Spin transport E. W. Hill et al., IEEE Trans. Magn. 42, 2694 (2006). (Prof. Geim’s group at Manchester )M. Ohishi et al., Jpn. J. Appl. Phys 46, L605 (2007). (Prof. Suzuki’s group at Osaka)S. Cho et al., Appl. Phys. Lett. 91, (2007). (Prof. Fuhrer’s group at Maryland)M. Nishioka, and A. M. Goldman, Appl. Phys. Lett. 90, (2007). (Prof. Goldman’s group at Minnesota)N. Tombros et al., Nature, 571 (2007). (Prof. van Wees’ group at University of Groningen)W. H. Wang et al., Phys. Rev. B (Rapid Comm.) 77, (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.
8Hybrid Spintronic Devices Spin InjectorSpin DetectorLateralSpin ValveFerromagnetic ElectrodesMM_+Spin Transport LayerRoom temperature operationDesired CharacteristicsGraphene (beginning in 2007)YesOK, 5 microns. Small graphene flakes.Theory: yes, Experiment: noYes (With tunnel barrier)Good potentialOutline hereHigh spin injection efficiencyGate-tunable spin transportSpin transport over long distancesLong spin lifetimesAllows spin manipulation
9Outline I. Introduction. Gate tunable spin transport in signal layer graphene at room temperature.III. Enhanced spin injection efficiency: Tunnel barrier study.Spin relaxation mechanism in graphene:--- Charged impurities scattering.--- Chemical doping on graphene spin valves.
10Sample preparationRamanIdentify single layer graphene with optical microscope and confirm with Raman spectrum.
11Sample preparation Co SiO2 Si SLG Co Co (7°) MgO (0°) Back Gate SLG 2nmSLGSiO2Back GateSLGSiSiO2OpticalStandard ebeam lithographyCoSLGSEM500 nmSLG
13Spin Injection and Chemical Potential FMgraphenee-Spin-dependentChemical potentialChemicalPotential(Fermi level)Outline hereDensity of statesDensity of states
14Local and Nonlocal Magnetoresistance Local spin transport measurement:Spin InjectorSpin Detectorcharge currentIVspin currentNon-local spin transport measurement:Outline hereSpin InjectorSpin Detectorcharge currentspin currentIINJVNL+-Using lock-in detectionM. Johnson, and R. H. Silsbee,PRL, 55, 1790 (1985)
20ChallengesCreate spin polarized current in graphene.How to increase the spin injection efficiency?Keep spin current polarized in graphene.What is the spin relaxation mechanism in graphene?
21Outline I. Introduction. Gate tunable spin transport in signal layer graphene at room temperature.III. Enhanced spin injection efficiency: Tunnel barrier study.Spin relaxation mechanism in graphene:--- Charged impurities scattering.--- Chemical doping on graphene spin valves.
22Interface resistance (R1, R2 )(Ω) Theoretical analysisHow to achieve efficient spin injection?Takahashi, et al, PRB 67, (2003)CoL=λG=W=2 μmPF=0.5, PJ=0.4ρG=2 kΩ2000040000Interface resistance (R1, R2 )(Ω)RNL(Ω)60120TunnelingcontactsTransparentMgOSLGInsert a thin tunnel barrierto make R1, R2 >> RGOutline hereHow to fabricate pin-hole free tunnel barrier.
23MgO Barrier with Ti adhesion layer 1 nm MgO on graphite (AFM)TiOutline hereMgONo TigraphiteRMS roughness: 0.766nmRMS roughness: 0.229nmW. H. Wang, W. Han et. al. ,Appl. Phys. Lett. 93, (2008).
24Tunneling spin injection into SLG Fabrication and Electrical characterizationCo (7°)IV-+Ti/MgO(0°)Ti/MgO(9°)CoMgOTiO2ISLGSLGSiO2SiO2-0.6-0.40.30.6-8VDC(V)-448IDC (μA)2-probe300 K3-probe300 K-1010IDC (mA)50100150200dV/dI (kW)Outline here
25Tunneling spin injection into SLG Large Non-local MR with high spin injection efficiencyJohnson & Silsbee, PRL, 1985.Jedema, et al, Nature,Outline hereDRNL=130 W , PJ=31 %Wei Han, K. Pi et. al., PRL 105, (2010).
26Comparison of Co/SLG and Co/MgO/SLG 2nmMgO1nmMgO3nmSLGSLGSiO2SiO2L=1 mmL=2.1 mmOutline hereVg=0 VVg=0 VDRNL= 0.02 W P ~ 1%DRNL=130 W P ~ 31%Tunnel barrier increases spin signal by factor of ~1,000
27Theoretical analysis For Ohmic spin injection with Co/SLG For Tunneling spin injection with Co/MgO/SLGOutline here
28Gate Tuning of Spin Signal Drift-Diffusion Theory for Different Types of ContactsProportional tographene conductivityOutline hereInversely proportionalto graphene conductivity
29Gate Tuning of Spin Signal Outline hereTransparent contactPin-hole contact
30Gate Tuning of Spin Signal Outline hereTunneling contactCharacteristic gate dependence of tunneling spin injection is realized.
31Outline I. Introduction. Gate tunable spin transport in signal layer graphene at room temperature.III. Enhanced spin injection efficiency: Tunnel barrier study.Spin relaxation mechanism in graphene:--- Charged impurities scattering.--- Chemical doping on graphene spin valves.
32Spin relaxation in graphene Experiment:Spin lifetime ~ 500 ps(for single layer graphene)Theory:Spin lifetime ~ 100 ns – 1 msTwo types of spin relaxation mechanisms:Elliot-Yafet mechanismD’yakonov-Perel mechanismdefectsOutline hereSpin flip during momentum scattering events.spins precess in internal spin-orbit fields.Charged impurities (Coulomb) are the most important type of momentum scattering.Are charged impurities important for spin relaxation?C. Jozsa, et al., Phys. Rev. B, 80, (R) (2009).N. Tombros, et al., Phys. Rev. Lett. 101, (2008).
33ExperimentMBE cellCharged impurities(we use Au in this study)We add charged impurities onto a graphene spin valve to study its effect on spin lifetime.IVCo electrode+-Single-Layer Graphene (SLG)SiO2Si(backgate)Graphene spin valve deviceK. Pi, Wei Han et.al., Phys. Rev. Lett. 104, (2010).
34How to perform the experiment???? ChallengesHow to perform the experiment????With small amounts of adatom coverage, metal impurtieswill oxidize.Clean environment and fine control of deposition rate.In-situ Measurement.Molecular beam epitaxy Growth.
35The UHV System SEM image Small MBE Chamber Measure Transport PropertiesVary Temperature from 18K to 300KPorts for 4 different materialsApply a magnetic field500 nmSLGMagnetSEM image
36Gate dependent conductivity In situ measurementAu is selected for this study because Au behaves as a point-like charged impurity on graphene.Gate dependent conductivityvs.Au deposition timeNo AuGate Voltage (V)Conductivity (mS)Au 6 sAu 2 sAu 4 sAu 8 sAu deposition (Sec)m (cm2/Vs)T=18 KCoulomb scattering is the dominant charge scattering mechanism.Deposition rate ~ 0.04 Å/min (5x1011 atom/cm2s)K. M. McCreary, K. Pi et al., Phys. Rev. B 81, (2010).
37Introducing extra spin scattering. Effect of Au doping on non-local signalNo AuGate Voltage (V)Conductivity (mS)Au 6 sAu 8 sAu 4 sAu 2 sWithout introducing extra spin scattering.Gate (V)DRnl (W)SimulationIntroducing extra spin scattering.Gate (V)DRnl (W)SimulationAu doping does not introduce extra spin scattering.
38Hanle precessionDirectly compare spin lifetime between different amounts of Au doping.datafitAu = 0 sHolesH (T)ΔRNL (Ω)-0.010.01Au = 8 sdatafitAu = 0 sElectronsΔRNL (Ω)-0.010.01H (T)Au = 8 sdatafitAu = 0 sDirac Pt.ΔRNL (Ω)Au = 8 s-0.010.01H (T)
39Effect of charged impurities on spin lifetime Spin lifetime and the diffusion coefficient are determined from Hanle spin precession dataAu deposition (s)Spin lifetime (ps)(2.9x1012 cm-2)Spin relaxationMomentum scattering2468Au deposition (sec)0.000.020.040.06D (m2/s)Dirac Pt.ElectronsHolesCharged impurities are not the dominant spin relaxation mechanism.
40Slight enhancement of spin lifetime Spin relaxation mechanisms are correlated.tc : Spin relaxation by Coulomb scattering.tj : Spin relaxation by other defects (latticedefects, sp3 bound etc.).Y. Gan et al., Small 4, 587 (2008).S. Molola et al., Appl. Phys. Lett. 94, (2009).Wei Han et al., arXiv (2011).Recent study shows that Co contact plays an important role.Effect of D’yakonov-Perel mechanism.E-Y mechanism: ts ~ tmD-P mechanism: ts ~ tm-1F. Guinea et al., Solid State comm. 149, 1140 (2009).Further study is needed.
41Enhancement of spin signal by chemical doping At fixed gate voltage, Au doping can enhance conductivity.No significant spin relaxation from charged impurities.By Au doping we are able to enhance spin life time from 50 ps to 150 ps.2.01.51.00.50.0Conductivity (mS)Possible to tune spin properties by chemical doping instead of applying high electric field (gate voltage).
42Conclusion Achieved tunneling contact on graphene spin valves. Au deposition (s)Spin lifetime (ps)Demonstrated charged impurities are not the dominant spin relaxation mechanism.Manipulation of spin transport in graphene by surface chemical doping.
43Thank you. Acknowledgements Roland Kawakami Wei Han Kathy McCreary Postdoc: Wei-Hua Wang(Academia Sinica in Taiwan)Yan LiAdrian SwartzJared WongRichard ChiangCollaboratorsWenzhong BaoFeng MiaoJeanie Lau (PI)Peng WeiJing Shi (PI)Shan-Wen Tsai (PI)Francisco Guinea (PI)Mikhail Katsnelson (PI)Thank you.
44New physics in TM doped graphene system 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.Y. Mao et al., Journal of Physics: Condensed Matter 20, 2008 (2008).--- Pt may induce localized magnetic states in Graphene.B. Uchoa et al., Phys. Rev. Lett. 101, (2008).Hydrogen storage.--- AI doped graphene as hydrogen storage at room temperature.Z. M. Ao et al., J. Appl. Phys. 105, (2009).
45The UHV System 5 mm SEM image We use same system to study the charge transfer and charge scattering mechanism of transition metals doped graphene.5 mmSEM imageMagnet
46Dirac point shift vs. Ti and Fe coverage ØTi = 4.3 eVØgraphene = 4.5 eVØFe = 4.7 eVNo Ti (0 ML)No Fe (0 ML)ML0.041 MLMLConductivity (mS)Conductivity (mS)0.123 ML0.015 ML0.205 MLGate Voltage (V)Gate Voltage (V)Dirac Point (V)Ti coverage (ML)-40-800.000.010.02Dirac Point (V)Fe coverage (ML)-30-600.00.10.2Both Ti and Fe coverage show n-type dopingKeyu Pi et al., PRB 80, (2009).
47Dirac point shift vs. Pt coverage ØPt = 5.9 eVTM coverage (ML)Dirac point shift (V)Pt-1Pt-2Fe-1Fe-2Fe-3Ti-1Ti-2Ti-3No Pt (0 ML)0.025 MLConductivity (mS)0.071 ML0.127 MLGate Voltage (V)Dirac Point (V)-20-40Pt coverage (ML)0.000.050.100.15The 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.
48Interfacial dipole DV(d) = Dtr(d) + Dc(d) Become n-type doping WGDEFDVWEFGraphene+q-qdWG-DEFDVWEFGraphene+q-qdWGDEFDVWEFGraphene+q-qDtr(d) : The charge transfer between graphene and the metal (difference in work functions).WMDc(d) : the overlap of the metal and graphene wave functionsMetalDc(d) = e−gd (a0 + a1d + a2d2)Highly depends on d.G. Giovannetti et al., Physical Review Letters 101, (2008).
49Possible reason for anomalous n-type doping Graphenep-typedn-typeTransition metal--- 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.G. Giovannetti et al., Physical Review Letters 101, (2008).M. A. Pushkin et al, Bulletin of the Russian Academy of Science: Physics 72, 878 (2008).
50Experimental evidence of interfacial dipole. 24680.871.752.623.50Pt Coverage (ML)Pt Coverage (Å)Dirac Point (V)AFM 1AFM 20 nm10 nm3.19 ML0.62 MLBy Theoretical calculation, d increase as material coverage went from adatoms to continuous film.dddGrapheneExperimental evidence of interfacial dipole.K. T. Chan, J. B. Neaton, and M. L. Cohen, Phys. Rev. B 77,
51Scattering 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).
52Mobility change vs. TM coverage Mobility, m (103 cm2/Vs)321Fe-2Conductivity (mS)Pt-2Ti-12.01.00.01.50.5-4-240.0000.0150.030n (1012 cm-2)Coverage (ML)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.Dirac point shift with TM coverage:Ti >Fe >PtMobility drop with TM coverage:Ti >Fe >Pt?Dirac point shift vs. Mobility change
53Mobility change vs. Dirac point shift Fitting equation:0.1 MLμ/μ0 = (Γ0 + ΓTM)-1/Γ0-1= (1 + ΓTM/Γ0)-1Normalized mobility, μ/μ0ΓTM/Γ0 = (AVD,shift)βPt-1Pt-2Ti-1Ti-2Ti and Pt fall on the universal curve.Coulomb scattering is the dominant effect.0.008 MLDirac Point Shift (V)Fe-2Electron data follows the universal curve.Hole data is significantly different.This implies some wave function hybridization in the Fe system.ElectronHoleμ/μ0Dirac Point Shift (V)Keyu Pi, K. M. McCreary et al., PRB 80, (2009).