Ultranarrow Semiconductor WS2 Nanoribbon Field-Effect Transistors

Abstract

Semiconducting transition metal dichalcogenides (TMDs) have attracted significant attention for their potential to develop high-performance, energy-efficient, and nanoscale electronic devices. Despite notable advancements in scaling down the gate and channel length of TMD field-effect transistors (FETs), the fabrication of sub-30 nm narrow channels and devices with atomic-scale edge control still poses challenges. Here, we demonstrate a crystallography-controlled nanostructuring technique to fabricate ultranarrow tungsten disulfide (WS₂) nanoribbons as small as sub-10 nm in width. The WS₂ nanoribbon junctions having different widths display diodic current-voltage characteristics, providing a way to create and tune nanoscale device properties by controlling the size of the structures. The transport properties of the nanoribbon FETs are primarily governed by narrow channel effects, where the mobility in the narrow channels is limited by edge scattering. Our findings on nanoribbon devices hold potential for developing future-generation nanometer-scale van der Waals semiconductor-based devices and circuits.

Keywords: 2D semiconductors; TMDs; WS2; crystallographically controlled nanostructuring; diodes; field-effect transistors; nanoribbon; transition metal dichalcogenides; zigzag edges.

Figure 1

Ultranarrow nanoribbon WS₂ devices. (a, b) Device schematic and colored scanning electron microscope (SEM) image of the fabricated multilayer WS₂ nanoribbons of different widths (W = 9–47 nm) on n⁺²Si/SiO₂ substrate. The measurement geometry is also shown in the schematic. (c) High-resolution transmission electron microscopy (HRTEM) image of nanoribbon reveals the crystalline edge of the WS₂ nanoribbon after the wet etching process. The edge region has been marked by the blue arrow to observe the sharp termination of the WS₂ crystal. The adjacent material to the WS₂ edge is PDMS residuals from the TEM membrane transfer process. (d) Key process steps for fabricating WS₂ nanoribbons with crystallography-controlled edges by combining dry reactive ions, wet-chemical etching, and nanopatterning processes. (e) Comparison of channel widths of our WS₂ nanoribbon, fabricated by the wet etching method with the state-of-the-art top-down nanoribbon fabrication processes using dry etching techniques.

Figure 2

Diodic behavior of WS₂ nanoribbon junctions. (a) Device schematic with the contacts positioned on WS₂ nanoribbon with narrow and wider regions to measure the junction properties. (b) Diodic behavior of WS₂ nanoribbon (W = 47 nm) as shown in I_ds vs V_ds plots at different V_g. The same plots on the log scale are shown in Figure S5a. (c) I_ds vs V_ds plots at various V_g for W = 70 nm wide WS₂ junction show less diodic behavior. Figure S5e shows the same plots on a logarithmic scale. (d) Comparison of I_ds vs V_ds properties of devices with 47 and 70 nm channel width in logarithmic scale. (e) I_ds vs V_ds plots, showing diodic behavior of devices with different nanoribbon widths (18–47 nm) at V_g = 80 V. Inset shows the current rectification ratio (I_ds,5V/I_ds,–5V) of the nanoribbon FET with various widths with an exponential fit (solid line) as guides to the eye. (f) Schematic band diagrams (top) of separated wider and narrower nanoribbon sections. The mismatch of Fermi level position with respect to the vacuum level is due to different doping and associated work functions in narrower and wider parts of the etched WS₂ flake. Band diagram (bottom) of the junction with wider and narrower nanoribbon sections, where a charge depletion region emerges due to charge transfer across the junction. (g) Kelvin probe force microscopy (KPFM) profile of etched WS₂ with wider and narrower nanoribbon sections. The respective atomic force microscopy (AFM) image of the KPFM profile section is shown for clear visualization in the inset, and the AFM analysis is provided in Figure S6. (h) The measured work function of the etched WS₂ flake with narrower and wider sections across the white line is shown in the KPFM profile. (i) Estimated Schottky barrier height at the nanoribbons with channel widths of 47 and 70 nm.

Figure 3

Field-effect transistor and narrow channel effect of WS₂ nanoribbons at room temperature. (a,b) Transfer characteristics (I_ds as a function of V_g) at different V_ds for WS₂ FETs with channel widths of 47 and 70 nm, respectively. (c,d) Transfer characteristics of WS₂ nanoribbon FETs with different channel widths W = 18–47 nm at V_ds = 5 V, in linear and semilog scale, respectively. The bottom gray plot in (d) denotes the gate leakage current. (e) A schematic to explain the narrow channel effect, where the fringe depletion enhances carrier depletion and consequently the change in threshold voltage (V_th) in the channels. The bottom basal plane is protected by the substrate (SiO₂); hence, a minute-scale fringing effect is expected on this nanoribbon plane. The top and edge are open to the ambient environment and are expected to have a large fringing effect on these sides. (f) The estimated V_th for WS₂ nanoribbon FETs with different channel widths.

Figure 4

Temperature dependence of WS₂ nanoribbon FET device parameters. (a, b, c) Color contour plots of the transfer characteristics for the WS₂ FETs with 18, 28, and 34 nm channel widths at different temperatures with V_g in the range of 40–80 V at V_ds = 5 V. (d, e, f) Mobility μ as a function of the temperature T of the WS₂ nanoribbon FETs with 18, 28, and 34 nm channel widths, along with the power-law fitting with μ ∝ T^γ (solid line) for different temperature ranges. The exponent γ depends on the scattering mechanisms in the nanoribbons. Error bars are estimated from the error of the fitting parameter in determining the transconductance across the channel.