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Influence of gate capacitance on CNTFET performance using Monte Carlo simulation H. Cazin d'Honincthun, S. Retailleau, A. Bournel, P. Dollfus, J.P. Bourgoin* UPS Institut d'Electronique Fondamentale (IEF) UMR 8622 – CNRS-Université Paris Sud 11, Orsay, France *Laboratoire d’électronique moléculaire, CEA/DSM/DRECAM/SPEC, CEA Saclay, France

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Plan Carbon nanotubes: high potential material Carbon nanotubes: Model for calculations (Monte Carlo) Evaluation of Mean Free Path Carbon nanotube field effect transistor simulation

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Potentialities of Carbon nanotubes Charge transport and confinement Charge transport is quasi-ballistic: few interactions Charge confinement (1D): good control of electrostatics Band structure « quasi » symmetric Transport properties identical for electron and hole (same m*) Carbon nanotubes can be either metallic as well as semi-conducting All-nanotube-based electronic is possible No dangling bonds Possibility to use High K material for oxide

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Plan Carbon nanotubes: high potential material Carbon nanotubes: Model for calculations (Monte Carlo) Evaluation of Mean Free Path Carbon nanotube field effect transistor simulation

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Particle Monte-Carlo method Carrier trajectories: Succession of free flight and scattering events Study of carrier transport in this work: solving the Boltzmann equation by the Monte Carlo method electron Statistical solution :by Monte Carlo method assembly of individual particles 1 particle N particles allow us to reconstruct scattering rates - time of free flights t f - type of scattering i - effect of scattering ( E, ) (Monte Carlo algorithm) random selection of

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Band structure and phonons E 12 E G /2 subbands 1 subbands 2 subbands 3 Energy dispersion calculated by: Tight-binding Zone-folding Phonon dispersion curves calculated by zone folding method Analytical approximation of dispersion curves [Pennington, Phys. Rev. B 68, (2003)] Longitudinal acoustic and optical modes are considered as dominant like in graphene Analytical approximation of the three first subbands Analytical approximation of the three first subbands Approximation + RBM phonon (E RBM ≈ Cst./d t ) [M. Machon et al., Phys. Rev. B 71, , (2005)] Ex: Tube index n=19, E RBM =19 meV

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Scattering Events Calculation Scattering rates : 1st order Perturbation Theory by deformation potential model with: D aco = 9 eV [L. Yang, M.P. Anantram et al. Phys. Rev. B 60, (1999) ] D RBM = 0.65eV/cm (for n = 19) [M. Machon et al., PRB 71, (2005)] Intrasubband acoustic scattering Elastic process Intersubband transition Inelastic process intravalley scattering from subband 1intervalley scattering from subband 1 n = 19

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Plan Carbon nanotubes: high potential material Carbon nanotubes: Model for calculations (Monte Carlo) Evaluation of Mean Free Path Carbon nanotube field effect transistor simulation

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Mean Free Path The mean free path depends on diameter and electric field : l MFP = nm for elastic scattering l MFP = nm for inelastic scattering Agreement with experimental works (L. field: nm H. Field: nm) Potential of quasi-ballistic transport in the low-field regime Simulation in steady-state regime

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Plan Carbon nanotubes: high potential material Carbon nanotubes: Model for calculations (Monte Carlo) Evaluation of Mean Free Path Carbon nanotube field effect transistor simulation

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS The modelled transistor Coaxially gated carbon nanotube transistor with heavily-doped source drain extensions CNTFET Characteristics: Zigzag tube (19,0) - d t = 1.5nm – E G = 0.55eV Cylindrically gated Ohmics Contacts (heavily n-doped) Intrinsic channel High K gate insulator HfO 2 ( EOT = 16, 1.4, 0.4, 0.1 nm - C OX = 72, 210, 560,1060 pF/m ) L c = 100 nm L = 20 nm e t = 0.18 nm d t = 1.5 nm EOT (nm) Oxide : HfO 2 SD N D = /m

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Conduction band First subband energy profile Weak field in the channel due to strong electrostatic gate control Weak field in the channel quasi ballistic transport

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Capacitive effects Determination of C G for low V DS : ≈ C G tends towards 4pF/cm Evolution of C ox and C G as a function of 1/EOT Reminder : C G leads towards C Q for C ox >> C Q For C ox >> C Q quantum capacitance limit Weak Field 1D low density of states Channel potential is fixed

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS I D -V GS Characteristics Expected result : I D increases with oxide capacitance C ox but limited enhancement for high C OX values Expected result : I D increases with oxide capacitance C ox but limited enhancement for high C OX values

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Evaluation of Ballistic Transport Unstrained Si. DG- MOSFET vs c-CNTFET CNTFET : 70% of ballistic electrons at the drain end for L ch = 100nm Unstrained Si DG : 50% of ballistic electrons pour L ch = 15 nm CNTFET : 70% of ballistic electrons at the drain end for L ch = 100nm Unstrained Si DG : 50% of ballistic electrons pour L ch = 15 nm

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS I D -V DS characteristics I ON ≈ 10 – 30 µA. I ON / I OFF ≈ – C OX (pF/m) /EOT(nm) 210/ / / 0.1 Expe.* 1.5 S (mV/dec) I ON (µA) g m (µS) High values of S and g m are obtained Acceptable agreement with experiments High values of S and g m are obtained Acceptable agreement with experiments Device parameters extracted from simulation (I off =0.1nA) * H. Dai, Nano Letter, vol.5, 345, (2005) : L=80nm, EOT=1.5nm V DD = 0.4V

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS I ON /I OFF ratio (V DD ) * H. Dai, Nano Letter, vol.5, 345, (2005) : L=80nm, EOT=1.5nm Performances CNT for L C =100nm and V DD <0.4V Performances Si for L C =15nm and V DD >0.7V Performances CNT for L C =100nm and V DD <0.4V Performances Si for L C =15nm and V DD >0.7V

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Hugues Cazin d’Honincthun – URSI - 20/03/2007 UPS Conclusion This Monte Carlo simulation of a CNTFET shows excellent performances: Excellent electrostatic gate control (quantum capacitance regime) High fraction of ballistic electrons in the transistor channel High performances at relatively low power supply

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