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Toward Carbon Based Electronics

Toward Carbon Based Electronics. Philip Kim Department of Physics Columbia University. Material Platform: Low dimensional graphitic systems. 1-D: Carbon Nanotubes (since 1991) 2-D: Graphene (since 2004). Device Concepts. Conventional: (extended) CMOS, SET. Non-Conventional:

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Toward Carbon Based Electronics

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  1. Toward Carbon Based Electronics Philip Kim Department of Physics Columbia University

  2. Material Platform: Low dimensional graphitic systems • 1-D: Carbon Nanotubes (since 1991) • 2-D: Graphene (since 2004) Device Concepts Conventional: (extended) CMOS, SET Non-Conventional: Quantum Interference, Spintronics, valleytronics Outline: Carbon Based Electronics

  3. Atomic orbital sp2 Graphene 0D 1D 2D 3D Carbon Nanotubes Fullerenes (C60) Graphite p s SP2 Carbon: 0-Dimension to 3-Dimension

  4. E hole Energy kx' ky' electron ky kx Graphene : Dirac Particles in 2-dimension Band structure of graphene (Wallace 1947) Zero effective mass particles moving with a constant speed vF

  5. Allowed states Semiconducting nanotube Metallic nanotube E E ky ky k1D k1D kx kx Single Wall Carbon Nanotube Eg ~ 0.8 ev / d (nm)

  6. T = 250 K T = 250 K Resistance (kW) R (kW) R (kW) L (mm) le ~ 1 mm Length (mm) L (mm) Extremely Long Mean Free Path in Nanotubes Multi-terminal Device with Pd contact r = 8 kW/mm R ~ L R ~ RQ * Scaling behavior of resistance: R(L) Room temperature mean free path > 0.5 mm M. Purewall, B. Hong, A. Ravi, B. Chnadra, J. Hone and P. Kim, PRL (2007)

  7. Isd (mA) -1.2 -0.8 -0.4 0 Vsd (V) Schottky barrier switching Ph. Avouris et al, Nature Nanotechnology 2, 605 (2007) Nanotube FET Band gap: 0.5 – 1 eV On-off ratio: ~ 106 Mobility: ~ 100,000 cm2/Vsec @RT Ballistic @RT ~ 300-500 nm Fermi velocity: 106 m/sec Max current density > 109 A/cm2

  8. Thin body (1-2 nm) -> suppressed short channel effect • channel length ~ 10 nm has been demonstrated Javey et al. PRL (2004). • No-dangling bond at surface -> high k-dielectric compatible Cg ~ CQ can be attainable; small RC, low energy Appenzeller et al., PRL (2002) Advantages of CNTFET • Novel architecture -> • Band-to-band tunneling FET: • subthreshold slop ~ 40 meV/dB @RT

  9. IBM, Avouris group graphene Aligned growth of Nanotubes Rodgers, UIUC Nanotube Ring Oscillators Artistic dream (DELFT) Nanotube Electronics: Challenges Pros: High mobility High on-off ratio High critical current density Small channel length Small gate capacitance Large Fermi velocity Con: Controlled growth

  10. Discovery of Graphene Large scale growth efforts: CVD, MBE, chemical synthesis

  11. factor 4.5 / year Discovery of QHE in graphen Scotch tape method Jun 06 Mar 06 Jun 07 Mar 05 Jun 05 Mar 07 Dec 04 Dec 05 Dec 06 Sep 04 Sep 05 Sep 06 Sep 07 Jun 06 Mar 06 Jun 07 Mar 05 Jun 05 Mar 07 Dec 04 Dec 05 Dec 06 Sep 04 Sep 05 Sep 06 Sep 07 Growth of Graphene Papers

  12. GaAs HEMT Mechanically exfoliated graphene Modulate Doped GaAs: Pfeiffer et al. TC17 Mobility (cm2/V sec) TC12 Scattering Mechanism? TC145 • Ripples • Substrate (charge trap) • Absorption • Structural defects TC130 n (1012 cm-2) Graphene Mobility Tan et al. PLR (2007)

  13. Graphene mobility: > 100,000 cm2/Vsec @ room temperature after annealing Strong density dependence! 0.24 W/K Resistance at High Density Mobility (cm2/V sec) before annealing |n|=2X1011 cm-2 0.13 W/K R (W) unsuspended best Density ( 1012 cm-2) T (K) Enhanced Room Temperature Mobility of Graphene High mobility materials have been under intensive research as an alternative to Silicon for higher performance mobility: Si (1,400 cm2/Vsec), InSb (77,000 cm2/Vsec)

  14. Graphene FET characteristics • Low temperature direct atomic layer deposition (ALD) of HfO2 as high-κ gate dielectric • Top-gate electrode is defined with a final lithography step. • I-V measurements at two different back gate voltages show a distinct “kink” for different top-gate voltages • Transconductance can be as high as gm = 328μS (150μS/μm) • Poor on-off ratio: ~ 5-10 due to zero gap in bulk Meric, Han, Young, Kim, and Shepard (2008)

  15. Saturation velocity vsat (108 cm/s) Vtop = -3 V Vtop = -2 V EF (eV) For comparison: Vtop = -1.5 V vFermi= 1x108 cm/s Silicon: 1x107 cm/s Vtop = 0 V GaAs: 0.7x107 cm/s VtopDirac = 2 V @ Vg = -40 V Operation current density > 1 mA/mm Graphene FET: High Saturation Velocity Meric, Han, Young, Kim, and Shepard (2008)

  16. Graphene Device Fabrication • Developing Graphene Nanostructure Fabrication Process graphene Contacts: PMMA EBL Evaporation Graphene patterning: HSQ EBL Development Graphene etching: Oxygen plasma Local gates: ALD HfO2 EBL Evaporation • Graphene device structure with local gate control Oezyilmaz, Jarrilo-Herrero and Kim APL (2007)

  17. Graphene Nanostructures AB Ring Quantum Dot Graphene with local barrier Goldhaber-Gordon (Stanford) Geim (Manchester) Morpurgo (DELFT) Graphene nanoribbons & nanoconstrictions Graphene PN junctions Graphene Side Gates Ensslin (ETH) Marcus (Harvard) Kim (Columbia)

  18. Dirac Particle Confinement Graphene Gold electrode W W 1 mm 10 nm < W < 100 nm x y Graphene nanoribbon theory partial list W Zigzag ribbons Egap~ hvF Dk ~ hvF/W Graphene Nanoribbons: Confined Dirac Particles

  19. 100 Eg (meV) Eg = E0 /(W-W0) 10 P1 P2 P3 P4 D1 D2 1 0 30 60 90 W (nm) Scaling of Energy Gaps in Graphene Nanoribbons Han, Oezyilmaz, Zhang and Kim PRL (2007)

  20. G (e2/h) 10-7 10-5 10-3 10-1 SEM image of device Top gate source drain top gate Hf-oxide graphene 1 mm 30 nm wide x 100 nm long 75 OFF 10-1 50 10-2 25 G (e2/h) VBG (V) 10-3 0 10-4 -25 10-5 -50 10-6 -75 -8 -4 0 4 8 -8 -4 0 4 8 VLG (V) VLG (V) Top Gated Graphene Nano Constriction source drain graphene SiO2 Back gate

  21. Son, et al, PRL. 97, 216803 (2006) Eg (meV) 2mm q (degree) 40 20 0 60 0 30 90 Graphene Nanoribbons Edge Effect Crystallographic Directional Dependence Rough Graphene Edge Structures

  22. Transport ‘gap’ Localization of Edge Disordered Graphene Nanoribbons See also: Gunlycke et al, Appl. Phys. Lett. 90 (14), 142104 (2007). Areshkin et al, Nano Lett. 7 (1), 204 (2007) Lherbier et al, PRL 100 036803 (2008) Querlioz et al., Appl. Phys. Lett. 92, 042108 (2008)

  23. 100 W = 37 nm 10 Conductance (mS) d: dimensionality 4K 15K 1 100K 200K 300K Arrhenius plot 2D VRH 1D VRH 0.1 15 nm 15 nm 15 nm 22 nm 22 nm 22 nm 0 20 40 60 3 Vg (V) 31 nm 31 nm 31 nm 2 37 nm 37 nm 37 nm ln(R) ln(R) 1 ln(R) 48 nm 48 nm 48 nm 0 -1 70 nm 70 nm 70 nm -2 0.0 0.1 0.2 T-1/2 T-1/3 T-1 Variable Range Hopping in Graphene Nanoribbons E EF T x

  24. Aligned growth of Nanotubes Rodgers, UIUC Artistic dream (DELFT) Graphene Electronics: Challenges Pros: High mobility High on-off ratio High critical current density Small channel length Small gate capacitance Large Fermi velocity Con: Controlled growth tunability of band gaps Edge control This can be turned into advantage: doping site, functionality, and etc…

  25. Graphene quantum dot Band gap engineered Graphene nanoribbons FET (Manchester group) Nonconventional Devices Graphene Veselago lense Graphene psedospintronics Graphene Spintronics Son et al.Nature (07) Cheianov et al.Science (07) Trauzettel et al.Nature Phys. (07) Graphene Electronics: Conventional & Non-conventional Conventional Devices

  26. 1 Pd (under HfO2) Conductance (mS) SWCNT (under HfO2) 0 Pd (over HfO2) 1 mm HfO2 on SiO2/Si+ Pd (under HfO2) 20 nm 60 nm Carbon Nanotube Superlattice Purewal, Takekosh, Jarillo-Herrero, Kim (2008) Kouwenhoven PRL (1992)

  27. electrode graphene potential x Top gate width: 50 nm < Lm Tunneling through smooth pn junction 1 mm SEM image of device Cheianov and Fal’ko (2006) Zhang and Fogler (2008) n p n Klein Tunneling Total Internal Reflection Novoselov et al (2006) Transmission coef Ballistic Quantum Transport in Graphene Heterojunction Realistic smooth potential distribution Requirements for Experimental Observation: • Small d -> • better collimination Graphene NPN junctions • Long Mean free path • -> Ballistic conduction Ballistic transport in the barrier

  28. Transport Ballistic Graphene Heterojunction 12 electrode nnn ppp graphene 10 8 PN junction resistance npn Conductance (mS) pnp Zhang and Fogler (2008) 1 mm 6 Conductance Oscillation: Fabry-Perot n1,, k1, T T 4 R 18 V n2,, k2 L q´ R* T -18 V n1,, k1, -10 -8 -6 -4 -2 0 0 2 4 6 8 10 q VTG (V) k1 /k2= sinq’/ sinq Df= 2L /cosq’ Young and Kim (2008) Mean free path ~ 200 nm VBG = 90 V Junction length < 100 nm VBG =-90 V See also Shavchenko et al and Goldhaber-Gordon’s recent preprint

  29. Quantum Oscillations in Ballistic Graphene Heterojunction Oscillation persist high temperature! dR/dntop ( h/e2 10-15 cm-2) 5 nback (1012 cm2) 0 1 0 -1 -5 ntop (1012 cm2) -5 0 5 Resistance Oscillations

  30. Conclusions • Carbon nanotube FET is mature technology demonstrating substantial improvement over Si CMOS • Controlled growth and scaling up of CNTFET remains as a challenge • Graphene provides scaling up solution of carbon electronics with high mobility • Controlled growth of graphene and edge contol remains as a challenge • Novel quantum device concepts have been demonstrated on graphene and nanontubes

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