Uniform Vapor-Pressure-Based Chemical Vapor Deposition Growth of MoS2 Using MoO3 Thin Film as a Precursor for Coevaporation

Abstract

Chemical vapor deposition (CVD) is a powerful method employed for high-quality monolayer crystal growth of 2D transition metal dichalcogenides with much effort invested toward improving the growth process. Here, we report a novel method for CVD-based growth of monolayer molybdenum disulfide (MoS₂) by using thermally evaporated thin films of molybdenum trioxide (MoO₃) as the molybdenum (Mo) source for coevaporation. Uniform evaporation rate of MoO₃ thin films provides uniform Mo vapors which promote highly reproducible single-crystal growth of MoS₂ throughout the substrate. These high-quality crystals are as large as 95 μm and are characterized by scanning electron microscopy, Raman spectroscopy, photoluminescence spectroscopy, atomic force microscopy, and transmission electron microscopy. The bottom-gated field-effect transistors fabricated using the as-grown single crystals show n-type transistor behavior with a good on/off ratio of 10⁶ under ambient conditions. Our results presented here address the precursor vapor control during the CVD process and is a major step forward toward reproducible growth of MoS₂ for future semiconductor device applications.

Figure 1

Experimental setup. (a) Schematic representation of the atmospheric pressure CVD setup and the relative S, MoO₃, and substrate positioning. (b) Cross-sectional view of the substrate boat: target substrate was placed face down toward the film at a small distance d. (c) Temperature profile of MoO₃ and S at a setpoint value of 750 °C.

Figure 2

Growth results. (a) Contrast between bare Si/SiO₂ substrate and substrate after the growth of MoS₂. (b) Sketch of the sample with the corresponding image positions. The blue regions marked here are outside of the boat. (c) Grain-size histogram with the surface coverage. (d) Optical images of the substrate at the positions specified in (b). Scale bar is 100 μm in each image.

Figure 3

Characterization of the MoS₂ single crystals. (a) Optical image of one of the largest monolayer crystals observed. (b) SEM image of a single crystal. (c) AFM topography and height profile (inset) taken at an edge of a monolayer domain. (d) E_2g¹ and A_1g vibrational modes of atoms and Raman single spectra for a single crystal.

Figure 4

PL characterization. (a) PL single spectra of a MoS₂ crystal grown with the optimized growth recipe. (b) PL intensity mapping of A peak for the same crystal. Inset shows the mapping of B peak.

Figure 5

TEM characterization. (a) Low magnification DF-STEM image of the transferred thin film on the copper grid. (b) HRSTEM image of the monolayer MoS₂ showing hexagonal atom arrangement. (c) DF-STEM image for the folded edge of the monolayer.

Figure 6

Electrical characteristics of the as-grown MoS₂ single crystals. (a) Output characteristics of the transistor device by sweeping the gate voltage (V_G) from −10 to 80 V. (b) Transfer characteristics. Plots are provided for the device shown in inset of (b).