Raman Spectroscopy of Graphene Applications in Epitaxial Graphene/SiC MVS Chandrashekhar Department of Electrical Engineering University of South Carolina Columbia, SC 29208

Outline Introduction The Raman Effect Phonons in Crystals Raman Spectra of Carbon Materials – Double Resonant D-peak Raman Spectrum of Graphene Raman Spectrum of Epitaxial Graphene – Applications Conclusion

Raman Systems Macro & Micro Confocal available Near field Raman is being developed- – scanning probe technique

Graphene Graphene is a durable ambipolar material – Honeycomb C-Lattice-single graphite sheet – Even in multiple layers, weak interlayer coupling Field effect shown,e-h µ>10000cm 2 /Vs Linear electronic band structure vs. traditional quadratic – Dispersionless/Massless Dirac Fermions-crazy physics

Graphene Phonon Modes Maultzsch et al, Phys. Rev. Lett., (2004) Assumes perfect hexagonal honeycomb

Raman Spectra of Carbon materials 3 Major Peaks – -1 “ sp2-hybridization” At Γ-point – D-disorder or -1 “ sp3 hybridization” at K-point – 2D -1

Double Resonant D-peak Normally, Raman shift is independent of λ pump – q~0 regardless of λ pump In Carbon, NOT TRUE…why?

Double Resonant D-peak If ω ph increases, q is also increasing..how? – How can momentum & energy be conserved? Scattering from defects gives required q – Breaks periodicity of the lattice

Identifying Carbon Allotropes Many different allotropes of carbon

Identifying Carbon Allotropes 3 stage disorder formation Stage 1: From graphite to nanocrystalline graphite -G-peak blueshifts because of scattering from small grains- q increases Stage 2: From nanocrystalline graphite to a-C Stage 3: From a-C to ta-C C~4nm

Raman Spectrum of Graphene D-peak only at edges –”disorder” 2D peak intensity changes with thickness Graf et al. arxiv (2006)

Raman Spectrum of Graphene 2D peak influenced by stacking despite weak coupling Bernal AB vs. Turbostratic “AA”

Raman Spectrum of Graphene Violation of Adiabatic Born-Oppenheimer – Strange Electron phonon coupling in graphene G-peak blueshifts & sharpens/stiffens Pisana et al, Nature Mat., 6, 198 (2007)

Epitaxial Graphene on SiC Till recently, impossible to produce large areas of graphite/graphene SiC annealed at high T – Si evaporates, and C rearranges into graphene Evidence of quantum coherence – Influence of SiC substrate? De Heer et al. Sol. State Comm., 143, 92 (2007)

Epitaxial Graphene Growth 1300C-1600C, 15mins-60mins C-face and Si-face Vacuum in cold wall & Hot wall BP ~10 -7 Torr We will address general trends common in both Cold Wall Hot Wall

Basic Material Properties Typical Si-face RT mobility ~1000cm 2 /Vs Highest Si-face mobility > 5000cm 2 /Vs at 120K

C-face vs. Si-face 2D peak is very different for the two – C-face turbostratic-single peak-looks like single layer as c- axis symmetry is broken-Symmetric peak – Si-face Bernal stack-splits-Asymmetric peak

Raman for Thickness Substrate signal is attenuated ~2-3%/ML Reasonable precision in thickness ~2-5ML res.

Raman for Stress & Disorder G-peak blue-shifts under compressive stress D-peak intensity increase-greater disorder – 0<I D /I G <~2 is range for sp2 carbons G-peak blue-shifts at high disorder 0.5<I D /I G <~2 Ferrari and Robertson, PRB (2000)-disorder Hanfland et al, PRB (1989)-strain

Stress & Disorder Cont. Two Major “Buckets” I.Low disorder I D /I G <0.2, slow growth  stress branch III.High disorder, high growth I D /I G >0.5  disorder branch – Stress relief believed to be through loss of registry

Stress & Disorder Branches Stress Branch Disorder branch

Stress and SiC Off-Cut in Stress Branch C-face is less strained than Si-face Si-face stress increase with offcut-registry Higher stress  higher disorder in stress branch  Disorder increases with offcut

Offcut Cont. Increasing stress suggests some degree epitaxial registry of 6 root3 structure for Si-face Slip/disorder during cooling may explain results? a0a0 a1a1 a 1 =a 0 cos (offcut angle)<a 0  greater stress Offcut (Deg) Calculated Mismatch at RT (%) Measured strain at RT (%) I D /I G < ~ ~0.2 SiC Lattice-Fissel, Physics Reports, 379, pp (2003) Graphite Lattice- Kellett, Richards, J. Nucl Mat., 12, pp (1964)

Released Graphene Membranes Release gets rid of stress-no substrate D-peak emerges-etch induced disorder?

Epitaxial Graphene Mobility Correlation 2D peak width and Hall mobility Robinson et al, Nano Lett., 9, 2873 (2009)

Conclusions Raman discovered by CV Raman In crystals, shift is caused by lattice vibrations Vibrations couple with electrons and EM fields Large area graphene can be produced on SiC Raman can completely characterize epitaxial graphene – Thickness, stress, grain size, mobility, stacking

Questions?