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Quantum Capacitance Effects In Carbon Nanotube Field-Effect Devices

Quantum Capacitance Effects In Carbon Nanotube Field-Effect Devices. L. Latessa, A. Pecchia, A. Di Carlo. University of Rome ‘Tor Vergata’. P. Lugli. Lehrstuhl fur Nanoelectronic, Munchen. Devices based on field effect (CNTFET). Planar geometry top/bottom gate (IBM). ULSI Devices

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Quantum Capacitance Effects In Carbon Nanotube Field-Effect Devices

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  1. Quantum Capacitance Effects In Carbon Nanotube Field-Effect Devices L. Latessa, A. Pecchia, A. Di Carlo University of Rome ‘Tor Vergata’ P. Lugli Lehrstuhl fur Nanoelectronic, Munchen

  2. Devices based on field effect (CNTFET) Planar geometry top/bottom gate (IBM) • ULSI Devices • Quasi-1D coherent transport over • long distances • Si integrable technology Vertical Geometry coaxial gate (Infineon)

  3. gDFTB Green’s functions Density Functional Tight-Binding (A.Pecchia, L.Latessa, A.Di Carlo) Quantum treatment Atomistic simulations: Approximated DFT:

  4. L R NEGF Extention Density Matrix Mullikan Charges SCC loop A. Pecchia, A. Di Carlo, Rep Prog Phys 67, 1-65 (2004)

  5. 3D Poisson Multigrid The cylindrical gate is added as an appropiate Dirichlet boundary condition Metal Gate Insulating dielectric and CNT

  6. Coaxially gated CNTFET • Working mechanisms: • Schottky barrier modulation • at the contacts • local modulation of • channelconductance • Model: • semiconducting CNT(10,0) • neglect Schottky barrier • Charge injection from p-doped • CNT contacts

  7. Vg Cox VCNT CQ CNTFET control capacities CS CG CG CD (S.Luryi, Appl.Phys.Lett. 52, 501 (1988), sistemi 2DES)

  8. Gate capacitance Gate is a Macroscopic metal Oxide CNT In a classic MOS CQ >> Cox => modulation depends on Cox In a well-tempered MOS CG >> CS,CD In 1D systems CQ is small Vds can influence the channel charge and barrier

  9. Drain Source Drain Source Caratteristiche di uscita IDS/VDS

  10. Calculation of CQ Bare Potential Partial Screening

  11. rs=5.03 rs=6.71 rs=10.06 rs=20.12 Over-screening in CNT rs=6.71 rs=5.03 Negative CQ Ctot > Cins The can CNT better than a metal!

  12. Negative compressibility Experimental measurements in GaAs quantum wells J.P.Eisenstein et al., Phys.Rev.Lett. 68, 647 (1992)  Analytic results for quasi-1D electron systems L.Calmels, A.Gold, Phys. Rev.B. 53, 10846 (1996) K0 : free electrons compressibility KHFA: Hartree-Fock exchange compress.

  13. Many-body exchange effect Calcolo DFT Charge carrier density nC: TOTAL SCREENING CNT ~ macroscopico CQ= e2(EF) High density: PARTIAL SCREENING Dominated by DOS Low density: OVER-SCREENING Exchange interaction preveal

  14. rs=5.03 rs=6.71 Substituting UH with Uee accounts only for Hartree rs=10.06 rs=20.12 XC in gDFTB In DFTB the XC term is treated with a Hubbard on-site energy The capacity becomes positive again

  15. 1/DOS Quantum Capacitance vs DOS

  16. Over-screening and modulation VGS Unconventional Trans-characteristics Possible applications ? VGS

  17. Conclusions Comprehensive calculation of Quantum Capacity in a CNT(10,0) using NEGF XC effects can give deviations from “classical” CQ Negative CQ below a crytical carrier density which compares well with analytic results in quasi 1D systems More complicated behaviour when more subbands are occupied

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