Direct growth of ${\mathrm{MoS}}_2$ on electrolytic substrate and realization of high-mobility transistors

Although electrostatic gating with liquid electrolytes has been thoroughly investigated to enhance electrical transport in two-dimensional (2D) materials, solid electrolyte alternatives are now actively being researched to overcome the limitations of liquid dielectrics. Here, we report direct growth of few-layer ($3-4L$) molybdenum disulfide (${\mathrm{MoS}}_2$), a prototypical 2D transition metal dichalcogenide (TMD), on lithium-ion solid electrolyte substrate by chemical vapor deposition (CVD), and demonstrate a transfer-free device fabrication method. The growth resulted in 5–10 μm sized triangular ${\mathrm{MoS}}_2$ single crystals as confirmed by Raman spectroscopy, x-ray photoelectron spectroscopy, and scanning electron microscopy. Field-effect transistors (FETs) fabricated on the as-grown few-layer crystals show near-ideal gating performance with room temperature subthreshold swings around 65 mV/decade while maintaining an ON/OFF ratio around ${10}^5$. Field-effect mobility in the range of $42–49\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{–1}{\mathrm{s}}^{–1}$ and current densities as high as 120 μA/μm with 0.5 μm channel length has been achieved, back-gated by the solid electrolyte. This is the highest reported mobility among comparable FETs on as-grown single/few-layer CVD ${\mathrm{MoS}}_2$. This growth and transfer-free device fabrication method on solid electrolyte substrates can be applied to other 2D TMDs for studying advanced thin-film transistors and interesting physics, and is amenable to diverse surface science experiments, otherwise difficult to realize with liquid electrolytes.

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Figure 1

Experimental setup and material characterization of few-layer CVD ${\mathrm{MoS}}_2$ directly grown on Li-ion glass. (a) Schematic illustration of the experimental setup used for APCVD growth. (b) False-color SEM image of a typical ${\mathrm{MoS}}_2$ single-crystal triangle. The scale bar is 1 μm. (c) Raman spectrum of as-grown CVD ${\mathrm{MoS}}_2$ on Li-ion glass. $E_{2\mathrm{g}}^1$ and $A_{1\mathrm{g}}$ peaks are located at 384.8 and $408.3\mathrm{c}{\mathrm{m}}^{–1}$, respectively. Peak separation $\sim 23.5\mathrm{c}{\mathrm{m}}^{–1}$ suggests a $3L$ thick ${\mathrm{MoS}}_2$.

Figure 2

High-resolution XPS spectra of ${\mathrm{MoS}}_2$ thin films: (a) Mo-$3d$ and (b) S-$2p$ core-level peaks of ${\mathrm{MoS}}_2$, respectively. A Mo/S ratio of 1:1.97 is extracted from the area fit (solid lines) to the experimental data (○). A close-to-the-ideal ratio of 1:1.97 indicates a fairly stoichiometric ${\mathrm{MoS}}_2$.

Figure 3

Schematics showing the energy band diagram of a back-gated CVD ${\mathrm{MoS}}_2$ FET. The band diagrams were obtained by considering the chemical potential of Ag [37] and channel formation at the ${\mathrm{MoS}}_2$/Li-ion glass interface. The dashed and solid black lines represent the vacuum levels and chemical potential, respectively. (a) In the OFF state, the alignment of the Fermi levels (dashed yellow line) is made spontaneously by the movement of the Li ions in the electrolyte; the Li ions diffuse to the center of the Li-ion glass substrate, leaving negatively charged vacancies behind and thereby constituting EDLs at both interfaces to allow alignment of the Fermi levels. The adjustment in vacuum level in the solid electrolyte is shown with the green dashed line. (b) ${\mathrm{MoS}}_2$ FET is in the ON state. The channel is formed when the ${\mathrm{MoS}}_2$ aligns its chemical potential with that of the Li-ion glass (no EDL forms at the ${\mathrm{MoS}}_2$/Li-glass interface immediately before channel formation).

Figure 4

Electrical characteristics of as-grown 3L CVD ${\mathrm{MoS}}_2$ on Li-ion glass. (a) Schematic diagram of ${\mathrm{MoS}}_2$ FET on Li-ion glass. Electrical connections are shown with orange lines. (b) Capacitance (phase angle) vs frequency characteristics of a Li-ion glass substrate with top and bottom Ni (20 nm) electrode. The frequency spectrum can be divided into three distinct regions: (I) where EDL is formed, (II) where ion migration dominates, and (III) where the bulk substrate works as dielectric. We note that $C_{\mathrm{EDL}}\approx 2.0\mu \mathrm{F}\mathrm{c}{\mathrm{m}}^{–2}$ at low frequency $\left(f=4\mathrm{Hz}\right)$. (c) Quasistatic capacitance-voltage characteristics of the same structure of (b). We note an average value of $C_{\mathrm{EDL}}\approx 2.15\mu \mathrm{F}\mathrm{c}{\mathrm{m}}^{–2}$ from the quasistatic measurement. Transfer characteristics of a $3L$ ${\mathrm{MoS}}_2$ FET in (d) log scale, with $V_{\mathrm{TH}}$ approximately 0.40 V and hysteresis voltage $<60\mathrm{mV}$, and (e) linear scale, where the field-effect mobility of $\sim 47.6\pm 1.7\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{–1}{\mathrm{s}}^{–1}$ is extracted from the slope of the linear $I_{\mathrm{D}}\text{−}{V}_{\mathrm{BG}}$. (f) The output characteristics of the same FET. Linear $I_{\mathrm{DS}}-V_{\mathrm{DS}}$ is observed for small $V_{\mathrm{DS}}$ suggesting an Ohmic-like contact. Current saturation is observed for higher drain voltages. A DC sweep rate of 9 mV/sec was used for electrical transport measurement.

Figure 5

Electrical characteristics of as-grown $4L$ CVD ${\mathrm{MoS}}_2$ on Li-ion glass. (a) Transfer characteristics of a $4L$ ${\mathrm{MoS}}_2$ FET on a log scale. $V_{\mathrm{TH}}$ is approximately 0.40 V. Hysteresis voltage is less than 150 mV, and the ON/OFF ratio is around ${10}^5$. (b) Output characteristics of the same FET. Linear $I_{\mathrm{DS}}\text{−}{V}_{\mathrm{DS}}$ is observed for small $V_{\mathrm{DS}}$ suggesting an Ohmic-like contact. Current saturation is observed for higher drain voltages. A DC sweep rate of 9 mV/sec was used for electrical transport measurement.

Figure 6

A plot of the ON/OFF ratio vs field-effect mobility for single/few-layer as-grown CVD ${\mathrm{MoS}}_2$ FETs. Data from this work (violet and green star) are compared with reported data [7, 8, 11, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54] for as-grown single (not labeled) and few-layer (labeled) CVD ${\mathrm{MoS}}_2$ FETs. Here we considered materials with thicknesses from a single layer up to $5L$ for comparison. The mean value of the mobility data from this work is plotted (without the error bar).