Transcript Stress and Disorder in Epitaxial Graphene/SiC by Raman

Raman Spectroscopy of Graphene
Applications in Epitaxial Graphene/SiC
MVS Chandrashekhar
Department of Electrical Engineering
University of South Carolina
Columbia, SC 29208
Outline
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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>10000cm2/Vs
• Linear electronic band structure vs. traditional
quadratic
– Dispersionless/Massless Dirac Fermions-crazy physics
Graphene Phonon Modes
• Maultzsch et al, Phys. Rev. Lett., 92 075501
(2004)
• Assumes perfect hexagonal honeycomb
Raman Spectra of Carbon materials
• 3 Major Peaks
– G-graphite @1580cm-1
“sp2-hybridization”
• At Γ-point
– D-disorder or diamond @1350cm-1 “ sp3 hybridization”
• at K-point
– 2D peak @2700cm-1
Double Resonant D-peak
• Normally, Raman shift is independent of λpump
– q~0 regardless of λpump
• In Carbon, NOT TRUE…why?
Double Resonant D-peak
• If ωph 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
C~4nm
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
Raman Spectrum of Graphene
Graf et al. arxiv (2006)
• D-peak only at edges –”disorder”
• 2D peak intensity changes with thickness
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
De Heer et al. Sol. State Comm., 143, 92 (2007)
• 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?
Epitaxial Graphene Growth
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Cold Wall
Hot Wall
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
Basic Material Properties
• Typical Si-face RT mobility ~1000cm2/Vs
• Highest Si-face mobility > 5000cm2 /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 caxis 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
Ferrari and Robertson, PRB (2000)-disorder
Hanfland et al, PRB (1989)-strain
• G-peak blue-shifts under compressive stress
• D-peak intensity increase-greater disorder
– 0<ID/IG<~2 is range for sp2 carbons
• G-peak blue-shifts at high disorder 0.5<ID/IG<~2
Stress & Disorder Cont.
• Two Major “Buckets”
• I.Low disorder ID/IG<0.2, slow growth stress branch
• III.High disorder, high growth ID/IG>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.
a1
a0
a1 =a0 cos (offcut angle)<a0
greater stress
Offcut
(Deg)
Calculated
Mismatch
at RT (%)
Measured
strain at
RT (%)
ID/IG
0
-0.2
-0.2
<0.1
4
-0.4
-0.25
~0.15
8
-1.1
-0.3
~0.2
SiC Lattice-Fissel, Physics Reports, 379, pp149-255 (2003)
Graphite Lattice- Kellett, Richards, J. Nucl Mat., 12, pp184-192 (1964
• Increasing stress suggests some degree epitaxial
registry of 6 root3 structure for Si-face
• Slip/disorder during cooling may explain results?
Released Graphene Membranes
• Release gets rid of stress-no substrate
• D-peak emerges-etch induced disorder?
Epitaxial Graphene Mobility
Robinson et al, Nano Lett., 9, 2873 (2009)
• Correlation 2D peak width and Hall mobility
Conclusions
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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?