Research ArticleNANOMATERIALS

Selective control of electron and hole tunneling in 2D assembly

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Science Advances  19 Apr 2017:
Vol. 3, no. 4, e1602726
DOI: 10.1126/sciadv.1602726
  • Fig. 1 Illustration of MIS-C and the characterization of the device structure.

    (A) Schematic illustration of an MIS-C under the state of forming an inversion of holes for tunneling. “e+” represents the hole carrier. (B) Schematic representation of the carristor layout and electrical connections for electrical transport measurements. (C) Optical image of a typical MIS-C with Ti/Au top and bottom contacts. Scale bar, 5 μm. (D) High-resolution TEM image of an MGr-hBN-WS2 heterostructure fabricated on a SiO2/Si substrate with a titanium metal capping layer (corresponding to the top lead). The layer thicknesses of the MIS-C are 10.5 and 13.1 nm for WS2 and MGr, respectively. Scale bar, 10 nm. (E) High-magnification TEM image of the ultrathin hBN tunnel barrier with a thickness of ~3 nm between the WS2 and the graphene. (F) Measured histogram of the height distributions for hBN on SiO2, confirming a height profile of <3 nm. Inset: AFM topographic image acquired from the device in (C) showing SiO2 covered by hBN. a.u., arbitrary units. (G) XPS spectra of W 4f and B 1s core levels, located at 30 to 40 eV and 190 eV, respectively.

  • Fig. 2 Room temperature electrical characteristics of an MIS-C and energy band diagram.

    (A) ITB-VTB output curve measured across the heterostructure (between leads T1 and B) showing an ideality factor of 1.2; the silicon back gate was set to ground during the measurements. Inset: Schematic of the energy band diagram of a vertically stacked MIS-C under forward (top) and reverse (bottom) bias conditions. EFG, graphene Fermi level; EFn (EFp), quasi-Fermi level for electrons (holes). (B) Calculated density of generated hole inversion and electron accumulation at the interface as a function of VTB for an n-type WS2. Note that the surface has weak inversion states at zero bias due to φms.

  • Fig. 3 Temperature- and electrostatic gating–dependent I-V characteristics.

    (A) Semilog ITB-VTB output curves of an MIS-C for temperatures between 90 and 290 K. Inset: The corresponding linear scale data. (B) Out-of-plane resistivity versus temperature curve for an MIS-C (blue) and a GW-B (yellow) at VTB = 1 V. (C) Arrhenius plot of ITB at VTB = −1 V (circle) and 1 V (square) for an MIS-C (blue) and a GW-B (yellow). EA of 108 meV for semiconducting and −155 meV for metallic behavior are extracted to yellow circles that are correlated to φB. (D) Semilog scale of ITB-VTB output curves for different back-gate voltages with an active contact area of 50 μm2. (E) Surface potential (left axis) and Schottky barrier height (right axis) as functions of VG modulation for MIS-C and GW-B, respectively. Note that φs has a saturation regime at VG < 0 and becomes negative at VG > 0. Inset: Rectification ratio as a function of VG extracted from the data (D) at VTB = ±1 V. (F) Barrier function as a function of 1000 T−1 in the temperature range from 90 to 290 K.

  • Fig. 4 Transfer characteristics of the carristor.

    (A) Room temperature semilog ITB-VG transfer characteristics of an MIS-C at VTB = −0.5 V. Inset: The corresponding linear scale ITB-VG at various VTB. (B) Fermi level shift of graphene versus VG. These data are extracted from VG-dependent surface potential variations and converted by applying the Schottky-Mott model based on the following expression: φs = WG + ΔEFGWs (where Ws is the WS2 work function). Inset: Energy band diagram under different gate polarities. Effect of p-doping (top; VG < 0); the shifting of EFG to a lower level results in φs forming an inversion layer; thus, ITB is dominated by holes. Effect of n-doping (bottom; VG > 0); the shifting of EFG to an upper level results in φs forming an accumulation of electrons; ITB is dominated by the abundance of electrons. (C) In-plane resistivity as a function of the gate voltage in an MGr channel FET measured at VSD = 10 mV under room temperature. Insets: Output curve of the same device at a back-gate voltage VBG = 20 V (bottom) and an optical image of a 10-nm-thick MGr with source/drain (S/D) contact (top right). (D) On/off ratio versus temperature for carristors and barristors. Inset: Plot of ln(G) versus 1000 T−1 for an MIS-C at on and off states, where conductance G is defined as 1/ρT. The barristor data are extracted from previous studies (5, 6, 9, 11).

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/4/e1602726/DC1

    Supplementary Text

    fig. S1. The 3D schematic structure of the carristor.

    fig. S2. Optical images of MIS-C fabrication steps.

    fig. S3. Optical images of GW-B (or GM-B) fabrication steps.

    fig. S4. Band alignment of MIS-C and XPS spectra.

    fig. S5. Schematic representation and I-V curves.

    fig. S6. ITB-VTB curve for GW-B at different temperatures from 90 to 290 K VG = 0.

    fig. S7. Electrical transport characteristics of GW-B.

    fig. S8. Electrical transport characteristics of GM-B.

    fig. S9. Highly nonlinear dependence of ITB as a function of VTB for experimental (black line) and calculated data (red dots) under zero gate field and T = 300 K.

    fig. S10. Semilog ITB-VG curves for MIS-C at different VTB from −0.1 to −0.5 V at room temperature.

    table S1. List of symbols and descriptions used in the study.

    References (3541)

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Text
    • fig. S1. The 3D schematic structure of the carristor.
    • fig. S2. Optical images of MIS-C fabrication steps.
    • fig. S3. Optical images of GW-B (or GM-B) fabrication steps.
    • fig. S4. Band alignment of MIS-C and XPS spectra.
    • fig. S5. Schematic representation and I-V curves.
    • fig. S6. ITB-VTB curve for GW-B at different temperatures from 90 to 290 K VG = 0.
    • fig. S7. Electrical transport characteristics of GW-B.
    • fig. S8. Electrical transport characteristics of GM-B.
    • fig. S9. Highly nonlinear dependence of ITB as a function of VTB for experimental (black line) and calculated data (red dots) under zero gate field and T = 300 K.
    • fig. S10. Semilog ITB-VG curves for MIS-C at different VTB from −0.1 to −0.5 V at room temperature.
    • table S1. List of symbols and descriptions used in the study.
    • References (35–41)

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