Towards Low-Temperature CVD Synthesis and Characterization of Mono- or Few-Layer Molybdenum Disulfide

Abstract

Molybdenum disulfide (MoS₂) transistors are a promising alternative for the semiconductor industry due to their large on/off current ratio (>10¹⁰), immunity to short-channel effects, and unique switching characteristics. MoS₂ has drawn considerable interest due to its intriguing electrical, optical, sensing, and catalytic properties. Monolayer MoS₂ is a semiconducting material with a direct band gap of ~1.9 eV, which can be tuned. Commercially, the aim of synthesizing a novel material is to grow high-quality samples over a large area and at a low cost. Although chemical vapor deposition (CVD) growth techniques are associated with a low-cost pathway and large-area material growth, a drawback concerns meeting the high crystalline quality required for nanoelectronic and optoelectronic applications. This research presents a lower-temperature CVD for the repeatable synthesis of large-size mono- or few-layer MoS₂ using the direct vapor phase sulfurization of MoO₃. The samples grown on Si/SiO₂ substrates demonstrate a uniform single-crystalline quality in Raman spectroscopy, photoluminescence (PL), scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy. These characterization techniques were targeted to confirm the uniform thickness, stoichiometry, and lattice spacing of the MoS₂ layers. The MoS₂ crystals were deposited over the entire surface of the sample substrate. With a detailed discussion of the CVD setup and an explanation of the process parameters that influence nucleation and growth, this work opens a new platform for the repeatable synthesis of highly crystalline mono- or few-layer MoS₂ suitable for optoelectronic application.

Keywords: MoS2; Raman spectroscopy; X-ray photoelectron spectroscopy; atomic force microscopy; chemical vapor deposition; monolayer; photoluminescence; scanning electron microscopy; scanning tunneling electron microscopy.

Figure 1

Schematic of CVD Setup.

Figure 2

Ceramic boats with sulfur (85 mg) and MoO₃ (15 mg).

Figure 3

Temperature cycle demonstrating the importance of cooling rate.

Figure 4

SEM micrographs of MoS₂, on Si/SiO₂ substrates (Carl Zeiss Auriga—BU FIB FESEM). (a) SEM monolayer MoS2, (b) shapes and bi-layer MoS2, (c) coalescence of MoS₂ crystals, (d) few-layer MoS₂ crystals.

Figure 5

Structural characterization of MoS₂ on silicon/silicon dioxide. Raman mapping: (a) area under E_2g 382 cm⁻¹, and (b) area under the A_1g 403 cm⁻¹ peak; (c) Raman spectra in the red and blue star areas, respectively (monolayer peaks in red: 19 (1/cm), multilayer peaks in blue: 23 (1/cm)).

Figure 6

Photoluminescence of monolayer MoS₂. (a) Raman map for MoS₂ sample area; (b) PL spectrum for ML, multilayer, and bulk MoS₂.

Figure 7

MoS₂ synthesized on Si/SiO₂ substrate. (a) AFM images of monolayer MoS₂ flakes; (b) AFM images of individual monolayer MoS₂ flake with AFM tip trace path (pink line); (c) thickness measurements of monolayer MoS₂ along the blue line.

Figure 8

X-ray photoelectron spectroscopy of monolayer MoS₂. (a) Mo3d deconvoluted peaks. (b) Sulfur deconvoluted peaks.

Figure 9

(a) ADF-STEM image of monolayer MoS₂. (b) Fourier transform image of MoS₂ lattice showing high crystallinity. (c) Computed lattice spacing.