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Carbon Nanotube Field-Effect Transistors: An Evaluation

Carbon Nanotube Field-Effect Transistors: An Evaluation. D.L. Pulfrey, L.C. Castro, D.L. John. Department of Electrical and Computer Engineering University of British Columbia Vancouver, B.C. V6T1Z4, Canada pulfrey@ece.ubc.ca. S.Iijima, Nature 354 (1991) 56.

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Carbon Nanotube Field-Effect Transistors: An Evaluation

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  1. Carbon Nanotube Field-Effect Transistors: An Evaluation D.L. Pulfrey, L.C. Castro, D.L. John Department of Electrical and Computer Engineering University of British Columbia Vancouver, B.C. V6T1Z4, Canada pulfrey@ece.ubc.ca

  2. S.Iijima, Nature 354 (1991) 56 Single-wall and multi-wall NANOTUBES Compare: flaxen hair - 20,000 nm

  3. CNT formation by catalytic CVD 2000nm 5m islands in PMMA patterned by EBL LPD of Fe/Mo/Al catalyst Lift-off PMMA No field CVD from methane at 1000C J.Kong et al., Nature, 395, 878, 1998 A. Ural et al., Appl. Phys. Lett., 81, 3464, 2002 Growth in field (1V/micron)

  4. 2p orbital, 1e-(-bonds) Single-Walled Carbon Nanotube Hybridized carbon atom  graphene monolayer  carbon nanotube

  5. Structure (n,m): (5,2) Tube VECTOR NOTATION FOR NANOTUBES Chiral tube Adapted from Richard Martel

  6. E-EF (eV) vs. k|| (1/nm) Eg/2 (5,0) semiconducting (5,5) metallic

  7. Doping • Substitutional unlikely • Adsorbed possible • e.g., K, O Tubes are naturally intrinsic • Interior possible

  8. Phonons • Acoustic phonons (twistons) mfp  300 nm • Optical phonons • mfp  15 nm Ballistic transport possible

  9. Nanotube Fabricated Carbon Nanotube FETs • Few prototypes • [Tans98]: 1st published device • [Wind02]: Top-gated CNFET • [Rosenblatt02]: Electrolyte-gated

  10. CLOSED COAXIAL NANOTUBE FET STRUCTURE chirality: (16,0) radius: 0.62 nm bandgap: 0.63 eV length: 15 - 100 nm oxide thickness: (RG-RT): 2 - 6 nm

  11. E kz kx kx MODE CONSTRICTION and TRANSMISSION Doubly degenerate lowest mode T CNT (few modes) METAL (many modes)

  12. Eb Quantum Capacitance Limit gate Cins insulator CQ nanotube source

  13. Quantum Capacitance and Sub-threshold Slope High k dielectrics: zirconia - 25 water - 80 70 mV/decade ! - Javey et al., Nature Materials, 1, 241, 2002

  14. AMBIPOLAR CONDUCTION Experimental data: M. Radosavljevic et al., arXiv: cond-mat/0305570 v1 Vds= - 0.4V Vgs= -0.15 +0.05 +0.30

  15. Minimize the OFF Current S,D = 3.9 eV Increasing G  3.0, 4.37 eV G = 4.2 eV Increasing S,D  3.9, 4.2, 4.5 eV ON/OFF 103

  16. E E 1D DOS E EFS g(E) EFD 0.5 f(E) f(E) 0.5 General non-equilibrium case Non-equilib f(E) Q(z,E)=qf(E)g(E) Solve Poisson iteratively

  17. CURRENT in 1-D SYSTEMS

  18. Quantized Conductance In the low-temperature limit: Interfacial G: even when transport is ballistic in CNT 155 S for M=2

  19. Measured Conductance G  0.4 Gmax at 280K !! A. Javey et al., Nature, 424, 654, 2003 • No tunneling barriers • Low R contacts (Pd)

  20. Drain Saturation Current VGS Eb EF If T=1 Get BJT behaviour! Zero-height Schottky barrier

  21. ON Current: Measured and Possible CQ limit S,D= 3.9eV G = 4.37eV 80% of QC limit! Present world record Javey et al., Nature, 424, 654, 2003

  22. Predicted Drain Current -ve 0 +ve Vgs=Vds=0.4V 70mA/m !!

  23. Transconductance Low VDS: modulate for G High VDS: modulate VGS for gm

  24. Transconductance: Measured and Possible CQ limit S,D= 3.9eV G = 4.37eV 80% of QC limit! Highest measured: Rosenblatt et al. Nano. Lett., 2, 869, 2002

  25. CNFET Logic A.Javey et al., Nature Materials, 1, 241, 2002 Gain=60 0,0 1st OR-gate

  26. Recognition-based assembly CNTs Functionalized with DNA Williams, Veenhuizen, de la Torre, Eritja and Dekker Nature,420, 761, 2002.

  27. Self-assembly of DNA-templated CNFETsK.Keren et al., Technion.

  28. Self-assembly of DNA-templated CNFETsK.Keren et al., Technion.

  29. CONCLUSIONS • Schottky barriers play a crucial role in determining the drain current. • Negative barrier devices enable: • control of ambipolarity, • high ON/OFF ratios, • near ultimate-limit S, G, ID, gm. • CNFETs can be self-assembled via biological recognition. • CNs have excellent thermal and mechanical properties. • CNFETs deserve serious study as molecular transistors.

  30. Extra Slides

  31. Compelling Properties of Carbon Nanotubes • Nanoscale • Bandgap tunability • Metals and semiconductors • Ballistic transport • Strong covalent bonding: • -- strength and stability of graphite • -- reduced electromigration (high current operation) • -- no surface states (less scattering, compatibility with many insulators) • High thermal conductivity • -- almost as high as diamond (dense circuits) • Let’s make transistors!

  32. CHIRAL NANOTUBES Armchair Zig-Zag Chiral From: Dresselhaus, Dresselhaus & Eklund. 1996 Science of Fullerenes and Carbon Nanotubes. San Diego, Academic Press. Adapted from Richard Martel.

  33. Carbon Nanotube Properties • Graphene sheet 2D E(k//,k) • Quantization of transverse wavevectors k (along tube circumference)  Nanotube 1D E(k//) • Nanotube 1D density-of-states derived from [E(k//)/k]-1 • Get E(k//)vs. k(k//,k) from Tight-Binding Approximation

  34. Density of States k|| or kz

  35. Tight Binding David John, UBC Wolfe et al., “Physical Properties of Semiconductors”

  36. David John Density of States (5,0) tube E(eV) vs. DOS (100/eV/nm) E(eV) vs. k|| (1/nm)

  37. Tuning the Bandgap T. Odom et al., Nature, 391, 62, 1998 Eg < 0.1 eV for d > 7 nm “zero bandgap” semiconductor

  38. The Ideal Structure nanotube oxide gate Coaxial Planar

  39. CNT formation by catalytic CVD 5m islands in PMMA patterned by EBL 1000nm LPD of Fe/Mo/Al catalyst 300nm Lift-off PMMA CVD from methane at 1000C 2000nm J.Kong et al., Nature, 395, 878, 1998

  40. CNT formation by E-field assisted CVD V applied between Mo electrodes. CVD from catalytic islands. No field 10V applied A. Ural et al., Appl. Phys. Lett., 81, 3464, 2002

  41. Nanotube Bottom-gated Nanotube FETs 1st CNFET S. Tans et al., Nature, 393, 49, 1998 Note very high ID 10mA/m A. Javey et al., Nature, 424, 654, 2003

  42. Phenomenological treatment of metal/nanotube contacts Evidence of work function-dependence of I-V: A. Javey et al., Nature, 424, 654, 2003 Zero holebarrier

  43. Schrödinger-Poisson Model • Need full QM treatment to compute: • -- Q(z) within positive barrier regions • -- Q in evanescent states (MIGS) • -- S  D tunneling • -- resonance, coherence

  44. Schrödinger-Poisson Model L.C. Castro, D.L. John S CNT D Unbounded plane waves

  45. Increasing the Drain Current Vgs=Vds=0.4V 70mA/m !!

  46. Array of vertically grown CNFETs W.B. Choi et al., Appl. Phys. Lett., 79, 3696, 2001. 2x1011 CNTs/cm2 !!

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