Slide 1

Philip Kim Department of Physics Columbia University Toward Carbon Based Electronics Slide 2 Outline: Carbon Based Electronics 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 Slide 3 SP 2 Carbon: 0-Dimension to 3-Dimension Fullerenes (C 60 ) Carbon Nanotubes Atomic orbital sp 2 GraphiteGraphene 0D1D2D3D Slide 4 Graphene : Dirac Particles in 2-dimension Band structure of graphene (Wallace 1947) kxkx kyky Energy kx'kx' ky'ky' E Zero effective mass particles moving with a constant speed v F hole electron Slide 5 Single Wall Carbon Nanotube kyky kxkx kxkx kyky Allowed states Metallic nanotube E k 1D E Semiconducting nanotube E g ~ 0.8 ev / d (nm) Slide 6 Length ( m) Resistance (k ) T = 250 K = 8 k / m Extremely Long Mean Free Path in Nanotubes Multi-terminal Device with Pd contact * Scaling behavior of resistance: R(L) L ( m) R (k ) T = 250 K R (k ) L ( m) R ~ R Q R ~ L l e ~ 1 m M. Purewall, B. Hong, A. Ravi, B. Chnadra, J. Hone and P. Kim, PRL (2007) Room temperature mean free path > 0.5 m Slide 7 Nanotube FET Band gap: 0.5 1 eV On-off ratio: ~ 10 6 Mobility: ~ 100,000 cm 2 /Vsec @RT Ballistic @RT ~ 300-500 nm Fermi velocity: 10 6 m/sec Max current density > 10 9 A/cm 2 V sd (V) 0-0.4-0.8-1.2 I sd ( A) Ph. Avouris et al, Nature Nanotechnology 2, 605 (2007) Schottky barrier switching Slide 8 Advantages of CNTFET Novel architecture -> Band-to-band tunneling FET: subthreshold slop ~ 40 meV/dB @RT No-dangling bond at surface -> high k-dielectric compatible C g ~ C Q can be attainable; small RC, low energy Thin body (1-2 nm) -> suppressed short channel effect channel length ~ 10 nm has been demonstrated Javey et al. PRL (2004). Appenzeller et al., PRL (2002) Slide 9 Rodgers, UIUC Aligned growth of Nanotubes 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 Artistic dream (DELFT) IBM, Avouris group Nanotube Ring Oscillators graphene Slide 10 Discovery of Graphene Large scale growth efforts: CVD, MBE, chemical synthesis Slide 11 Jun 07 Dec 06 Mar 07 Sep 06 Jun 06 Mar 06 Dec 05 Sep 05 Jun 05Mar 05 Dec 04 Sep 04Sep 07 Growth of Graphene Papers Scotch tape method Discovery of QHE in graphen Jun 07 Dec 06 Mar 07 Sep 06 Jun 06 Mar 06 Dec 05 Sep 05 Jun 05Mar 05 Dec 04 Sep 04Sep 07 factor 4.5 / year Slide 12 Graphene Mobility n (10 12 cm -2 ) Mobility (cm 2 /V sec) TC17 TC12 TC145 TC130 Mechanically exfoliated graphene Tan et al. PLR (2007) Scattering Mechanism? Ripples Substrate (charge trap) Absorption Structural defects Modulate Doped GaAs: Pfeiffer et al. GaAs HEMT Slide 13 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) Graphene mobility: > 100,000 cm 2 /Vsec @ room temperature unsuspended best before annealing after annealing Density ( 10 12 cm -2 ) Mobility (cm 2 /V sec) T (K) R ( ) |n|=2 X 10 11 cm -2 Resistance at High Density 0.24 /K 0.13 /K Strong density dependence! Enhanced Room Temperature Mobility of Graphene Slide 14 Low temperature direct atomic layer deposition (ALD) of HfO 2 as high- 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 g m = 328S (150S/m) Poor on-off ratio: ~ 5-10 due to zero gap in bulk Graphene FET characteristics Meric, Han, Young, Kim, and Shepard (2008) Slide 15 Graphene FET: High Saturation Velocity GaAs: 0.7x10 7 cm/s v Fermi = 1x10 8 cm/s For comparison: Silicon: 1x10 7 cm/s Operation current density > 1 mA/ m V top = 0 V V top = -1.5 V V top = -2 V V top = -3 V V top Dirac = 2 V @ V g = -40 V E F (eV) v sat (10 8 cm/s) Saturation velocity Meric, Han, Young, Kim, and Shepard (2008) Slide 16 Graphene Device Fabrication Developing Graphene Nanostructure Fabrication Process Contacts: PMMA EBL Evaporation Graphene patterning: HSQ EBL Development Graphene etching: Oxygen plasma Local gates: ALD HfO 2 EBL Evaporation graphene Graphene device structure with local gate control Oezyilmaz, Jarrilo-Herrero and Kim APL (2007) Slide 17 Graphene Nanostructures Geim (Manchester) Morpurgo (DELFT) Goldhaber-Gordon (Stanford) Kim (Columbia) Ensslin (ETH) Marcus (Harvard) Quantum Dot AB Ring Graphene with local barrier Graphene PN junctionsGraphene nanoribbons & nanoconstrictions Graphene Side Gates Slide 18 1 m Gold electrode Graphene 10 nm < W < 100 nm W Zigzag ribbons Graphene nanoribbon theory partial list Graphene Nanoribbons: Confined Dirac Particles W Dirac Particle Confinement x y E gap ~ hv F k ~ hv F /W W Slide 19 Scaling of Energy Gaps in Graphene Nanoribbons E g = E 0 /(W-W 0 ) Han, Oezyilmaz, Zhang and Kim PRL (2007) Slide 20 -8-4048 75 50 25 0 -25 -50 -75 V LG (V) V BG (V) 10 -7 10 -5 10 -3 10 -1 G (e 2 /h) Top Gated Graphene Nano Constriction source Back gate SiO 2 drain graphene -8-4048 10 -6 10 -5 10 -4 10 -3 10 -2 10 -1 V LG (V) G (e 2 /h) OFF SEM image of device source drain top gate graphene 1 m 30 nm wide x 100 nm long Hf-oxide Top gate Slide 21 Crystallographic Directional Dependence Son, et al, PRL. 97, 216803 (2006) 2 m 030 60 90 0 20 40 E g (meV) (degree) Graphene Nanoribbons Edge Effect Rough Graphene Edge Structures Slide 22 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) Transport gap Slide 23 T -1 ln(R) 3 2 1 0 -2 0.20.10.0 Arrhenius plot Variable Range Hopping in Graphene Nanoribbons Conductance ( S) V g (V) W = 37 nm 0.1 1 10 100 6040200 4K 15K 100K 200K 300K E x EFEF d: dimensionality 70 nm 48 nm 37 nm 22 nm 15 nm 31 nm ln(R) T -1/3 70 nm 48 nm 37 nm 22 nm 15 nm 31 nm 2D VRH ln(R) T -1/2 70 nm 48 nm 37 nm 22 nm 15 nm 31 nm 1D VRH T Slide 24 Rodgers, UIUC Aligned growth of Nanotubes 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 Artistic dream (DELFT) tunability of band gaps Edge control This can be turned into advantage: doping site, functionality, and etc Slide 25 Graphene Electronics: Conventional & Non-conventional Conventional Devices Cheianov et al. Science (07) Graphene Veselago lense FET Band gap engineered Graphene nanoribbons Nonconventional Devices Trauzettel et al. Nature Phys. (07) Graphene psedospintronics Son et al. Nature (07) Graphene Spintronics Graphene quantum dot (Manchester group) Slide 26 Pd (under HfO 2 ) Pd (over HfO2) SWCNT (under HfO 2 ) HfO 2 on SiO 2 /Si+ Carbon Nanotube Superlattice 20 nm 60 nm 1 m Purewal, Takekosh, Jarillo-Herrero, Kim (2008) Kouwenhoven PRL (1992) Conductance ( S) 0 1 Slide 27 Klein Tunneling Transmission coef Novoselov et al (2006) Ballistic Quantum Transport in Graphene Heterojunction n np x potential Ballistic transport in the barrier Graphene NPN junctions Realistic smooth potential distribution Total Internal Reflection Cheianov and Falko (2006) Zhang and Fogler (2008) Tunneling through smooth pn junction Requirements for Experimental Observation: Long Mean free path -> Ballistic conduction Small d -> better collimination Top gate width: 50 nm < L m graphene electrode 1 m SEM image of device Slide 28 Transport Ballistic Graphene Heterojunction V BG = 90 V V BG = -90 V ppp pnp npn nnn graphene electrode 1 m Young and Kim (2008) V TG (V) -100-2-4-6-80108642 Conductance (mS) 6 4 10 8 12 PN junction resistance Zhang and Fogler (2008) 18 V -18 V L n 1,, k 1, T T T R R* n 1,, k 1, n 2,, k 2 Conductance Oscillation: Fabry-Perot k 1 /k 2 = sin / sin = 2L /cos Mean free path ~ 200 nm Junction length < 100 nm See also Shavchenko et al and Goldhaber-Gordons recent preprint Slide 29 Quantum Oscillations in Ballistic Graphene Heterojunction n top (10 12 cm 2 ) n back (10 12 cm 2 ) 5 -5 0 0 5 0 1 dR/dn top ( h/e 2 10 -15 cm -2 ) Resistance Oscillations Oscillation persist high temperature! Slide 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