Suppressing Nucleation in Metal–Organic Chemical Vapor Deposition of MoS₂ Monolayers by Alkali Metal Halides

Abstract

Toward the large-area deposition of MoS 2 layers, we employ metal−organic precursors of Mo and S for a facile and reproducible van der Waals epitaxy on c-plane sapphire. Exposing c-sapphire substrates to alkali metal halide salts such as KI or NaCl together with the Mo precursor prior to the start of the growth process results in increasing the lateral dimensions of single crystalline domains by more than 2 orders of magnitude. The MoS 2 grown this way exhibits high crystallinity and optoelectronic quality comparable to singlecrystal MoS 2 produced by conventional chemical vapor deposition methods. The presence of alkali metal halides suppresses the nucleation and enhances enlargement of domains while resulting in chemically pure MoS 2 after transfer. Field-effect measurements in polymer electrolyte-gated devices result in promising electron mobility values close to 100 cm 2 V −1 s −1 at cryogenic temperatures. KEYWORDS: Chemical vapor deposition, two-dimensional transition metal dichalcogenides, nucleation and growth, microstructure engineering, FET devices T he chemical vapor deposition of two-dimensional materials is a highly promising method to produce atomically thin layers at a large scale for harnessing their attractive properties. Monolayer MoS 2 is a model 2D semiconductor that can be used to realize field-effect transistors with high current on/off ratios. 1 It is a naturally occurring material with a good chemical stability that exhibits a wide range of attractive properties such as a spin−orbit couplinginduced band splitting, 2,3 a mechanically tunable bandgap, 4−8 and a low temperature superconductivity. 9−13 Toward the large-scale synthesis of MoS 2 thin films, a conventional chemical vapor deposition method of producing MoS 2 monolayers typically involves low vapor pressure solid powder precursors such as MoO 3 and sulfur. It has been investigated for centimeter-scale deposition of polycrystalline monolayer MoS 2 with grain sizes of nanometer to micrometer and with controllable coverage. 14,15 However, low vapor pressures of the solid precursors require them to be loaded inside a heated zone of the reactor chamber leading to a limited control over the vapor phase composition and deposition rate. Thus, this synthesis approach heavily undermines the ability to control the nucleation density, thickness, and coverage.Here, we aim to address this issue by employing well-known metal−organic precursors of molybdenum, Mo(CO) 6 , which is a high vapor pressure solid, and of sulfur, H 2 S in gas phase. 16−19 This metal-organic chemical vapor deposition (MOCVD) approach allows reliably setting the concentration of precursor gases within the gaseous mixture that is transported to the substrate by controlling the evaporation rates of the solid precursor and mass flow rates. An extensive vapor phase thermodynamics study performed by Kumar et al. 19 has shown that growth temperatures above 850°C at atmospheric pressure lead to layer-by-layer growth of MoS 2 without extraneous deposition of carbon ...

Considerations for Utilizing Sodium Chloride in Epitaxial Molybdenum Disulfide

“…When NaCl is kept at 500 o C or below, there is no significant increase in domain size. Once NaCl is heated to 600 o C, similar to previous reports, 13 there is a distinct >20× increase in domain size compared to the growth without NaCl ( Fig. 2a and Fig.…”

“…These mobility values fall in the range of reported room-temperature measurements (0.02~30 cm 2 /V.s) on single or poly crystalline MoS2. [11][12][13]17,18,20,31,37,40,41 The most significant impact may be a >100× reduction of on/off ratio (on/offNaCl = 10 4~1 0 5 ; on/off =10 6~1 0 7 ) ( Fig. 4b,d), a ~20V shift in Vth, and dramatic increase in Vth variability ( Fig.…”

Molecular Nanowire Bonding to Epitaxial Single‐Layer MoS2 by an On‐Surface Ullmann Coupling Reaction

“…The case of connecting both ends of a poly‐DBT wire to MoS 2 islands is much rarer than connecting one end; however, we believe a more suitable organic molecule might increase the success rate in connecting multiple MoS 2 islands. Obviously, device production requires the use of insulating substrates, and it will be an interesting topic to study if well‐defined MoS 2 grown by CVD methods on SiO 2 , Al 2 O 3 or other oxides can be used for this purpose 48–50. We note that the role of Au surface may be an important factor here, as metal atoms are needed to initiate the Ullmann coupling during the chain growth, but we speculate that addition of trace amounts of metals species (as in solution phase Ullmann coupling) can help overcome this problem.…”

Vapor‐phase growth of high‐quality wafer‐scale two‐dimensional materials

“…However, the nucleation and growth of the second layer on the monolayer were difficult because of the limited catalysts on the surface and the surface self‐limited growth mechanism . Recently, alkali metal halides have been demonstrated to induce a surface poisoning effect that would limit nucleation and to be used as effective promoters in the fast growth of TMDs . Based on these works, by using NaCl‐coated Mo foils as metal precursors and the face‐to‐face metal‐precursor feeding route, their group further achieved the large‐scale, thickness‐tunable growth of MoS 2 on a 6‐in.…”

“…62,70 Recently, alkali metal halides have been demonstrated to induce a surface poisoning effect that would limit nucleation and to be used as effective promoters in the fast growth of TMDs. [71][72][73] Based on these works, by using NaCl-coated Mo foils as metal precursors and the face-to-face metal-precursor feeding route, their group further achieved the large-scale, thickness-tunable growth of MoS 2 on a 6-in. soda-lime glass ( Figure 2E).…”