Research ArticlePHYSICAL SCIENCE

Quasi-ballistic carbon nanotube array transistors with current density exceeding Si and GaAs

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Science Advances  02 Sep 2016:
Vol. 2, no. 9, e1601240
DOI: 10.1126/sciadv.1601240
  • Fig. 1 FETs constructed from densely packed semiconducting CNT arrays.

    (A) Schematic of CNT array sitting on a SiO2/Si back gate (G) with top Pd source (S) and drain (D) electrodes. (B) False-colored SEM image of a representative FET highlighting where the contacts overlap the CNT array (light orange) and the CNT array channel (dark orange), with Wch = 4.1 μm and Lch = 150 nm. Inset SEM image (scale bar, 200 nm) shows the CNT array, with 47 CNTs μm−1 and a high degree of alignment. (C) TEM cross-sectional image of Pd/CNT/SiO2 electrode stack where the “humps” in the Pd correspond to CNTs in the array. High-resolution TEM images reveal individual CNTs underneath the Pd “humps” with a diameter of 1.3 to 1.9 nm. (D) Atomic force miscroscopy image of 30 nm of Pd overcoating a CNT array, evidencing the conformity of the Pd to the individual CNTs.

  • Fig. 2 Electrical properties of CNT array FETs and influence of postdeposition treatment on conductance.

    (A) IDS-VGS curves forward sweep for a representative “rinsed + annealed” FET with Lch = 100 nm where the open and filled symbols are plotted on linear and logarithmic scales, respectively. (B) IDS-VDS output curves for the same device as in (A). (C) Gon versus Lch obtained from CNT arrays that have undergone the three different postdeposition treatment conditions, fit to Eq. 1, with a sevenfold total increase in Gon resulting from the “rinsed + annealed” treatment. (D) gm versus Lch for each surface treatment fit to Embedded Image as a guide to the eye, with a threefold total increase in gm resulting from the “rinsed + annealed” treatment.

  • Fig. 3 Spectroscopic characterization of postdeposition treatments.

    (A) XPS of CNT arrays showing C 1s, N 1s, and Cl 2p peaks following solvent rinsing and vacuum annealing. A.U., arbitrary units. (B) Optical absorbance spectra of CNT array films deposited on quartz with the approximate spectral windows corresponding to the first three CNT Sii transitions marked for clarity. Black open circles correspond to “as-deposited,” red open triangles correspond to “rinsed,” and blue open squares correspond to “rinsed + annealed” treatments. (C) FTIR spectroscopy of 10-nm-thick PFO-BPy films. Vacuum annealing reduces the alkyl C–H stretching modes centered at 2855 and 2926 cm−1, corresponding to loss of the polymer side chains.

  • Fig. 4 CNT array FETs with conductance per tube approaching the fundamental quantum conductance limit 2G0 = 4e2/h.

    Three champion devices with conductance reaching 0.46 G0. All three devices also exhibit high Ion/Ioff > 104.

  • Fig. 5 Benchmarking CNT array FET performance against Si MOSFETs.

    Isat of a champion CNT array FET (VDS = −1 V; Lch = 140 nm) and 90-nm node Si MOSFET (43) (VDS = −1.2 V) versus VGSVoff, normalized by the dielectric constant of the gate dielectric (εrεo) and the gate oxide thickness (tox), where VGSVoff is 0 V at an Ioff of 0.1 μA μm−1. Oxide parameters for the CNT array FET are tox = 15 nm and εr = (εair + εSiO2)/2 = 2.45 for the SiO2/CNT/air dielectric stack (31, 62). Oxide parameters for the Si MOSFET are tox = teff = 2.4 nm [effective oxide thickness accounting for inversion capacitance (63)] and εr = εSiO2 = 3.9. The champion CNT array FET exhibits an Isat that is 1.9-fold higher when measured at an equivalent charge density (VGSVoff) εrεotox−1 of −0.85 μC cm−2.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1601240/DC1

    Supplementary Text

    fig. S1. Full sweep transfer characteristics.

    fig. S2. Lead resistance subtraction.

    fig. S3. Optical spectroscopy evidence of high semiconducting purity.

    fig. S4. Effect of annealing on Ion/Ioff.

    fig. S5. Ioff variability.

    fig. S6. Postdeposition treatment effect on output characteristics.

    fig. S7. Optical absorbance spectra used to verify XPS atomic concentrations and CNT diameter distribution.

    fig. S8. Atomic force microscopy height measurement used to quantify packing density for XPS calibration.

    fig. S9. Temperature-dependent FTIR measurement of PFO-BPy side chains and backbone.

    fig. S10. On-conductance comparison of “annealed” versus “rinsed + annealed” treatments.

    fig. S11. Conductance per tube measured for FETs with Lch < 150 nm.

    fig. S12. Contact resistance and mean free path extraction.

    table S1. Surface treatment XPS summary.

    table S2. CNT FET performance comparisons.

    References (6474)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. Full sweep transfer characteristics.
    • fig. S2. Lead resistance subtraction.
    • fig. S3. Optical spectroscopy evidence of high semiconducting purity.
    • fig. S4. Effect of annealing on Ion/Ioff.
    • fig. S5. Ioff variability.
    • fig. S6. Postdeposition treatment effect on output characteristics.
    • fig. S7. Optical absorbance spectra used to verify XPS atomic concentrations and CNT diameter distribution.
    • fig. S8. Atomic force microscopy height measurement used to quantify packing density for XPS calibration.
    • fig. S9. Temperature-dependent FTIR measurement of PFO-BPy side chains and backbone.
    • fig. S10. On-conductance comparison of “annealed” versus “rinsed + annealed” treatments.
    • fig. S11. Conductance per tube measured for FETs with Lch < 150 nm.
    • fig. S12. Contact resistance and mean free path extraction.
    • table S1. Surface treatment XPS summary.
    • table S2. CNT FET performance comparisons.
    • References (6474)

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