Considerations for Utilizing Sodium Chloride in Epitaxial Molybdenum Disulfide

Abstract

The utilization of alkali salts, such as NaCl and KI, have enabled the successful growth of large single domain and fully coalesced polycrystalline two-dimensional (2D) transition metal dichalcogenide layers. However, the impact of alkali salts on photonic and electronic properties are not fully established. In this work, we report alkali-free epitaxy of MoS2 on sapphire and benchmark the properties against alkaliassisted growth of MoS2. This study demonstrates that although NaCl can dramatically increase the domain size of monolayer MoS2 by 20 times, it can also induce strong optical and electronic heterogeneities in as-grown large-scale films. This work elucidates that utilization of NaCl can lead to variation in growth rates, loss of epitaxy, and a high density of nanoscale MoS2 particles (4  0.7/μm 2 ). Such phenomena suggest that alkali atoms play an important role in Mo and S adatom mobility and strongly influence the 2D/sapphire interface during growth. Compared to alkali-free synthesis under the same growth conditions, MoS2 growth assisted by NaCl results in >1% tensile strain in as-grown domains, which reduces photoluminescence by ~20× and degrades transistor performance.

Controlled Growth of Large-Area Bilayer Tungsten Diselenides with Lateral P–N Junctions

“…20,40,42 The addition of NaCl transforms some WOx into oxyhalide (WOxCl), which is more easily evaporated at high temperature, leading to increased W adatom flux from the vapor and enhanced growth of the bilayer (Figure 1b). 20,43,44 When pure NaCl is used as the growth promoter, a large amount of WOxCl forms, leading to reduced surface adsorption and further increased W vapor adatom flux, so that multilayer (ML) WSe2 dominates the final structures (Figure 1c). Figure 2 shows optical micrographs of WSe2 flakes grown using these three different promoter mixtures.…”

Caveats in obtaining high-quality 2D materials and property characterization

“…Such variations in coupling can also affect the quality of the grown material significantly, resulting in a tensile strain in the layers and a reduction of the PL intensity of MoS 2 layers by roughly 100Â. Furthermore, these studies indicate that while NaCl may serve to control nucleation and promote large area growth of MoS 2 , NaCl can also degrade the electrical performance of these grown semiconducting layers even when the films are transferred from the growth substrate [27]. More importantly, however, is the 501 years of research in the semiconductor industry that provides overwhelming evidence that the presence of alkali ions leads to higher oxide leakage currents and reliability problems in semiconductor devices when operated at high temperatures and voltages [28,29,30,31,32,33].…”

“…While salt-assisted growth does result in large-area TMD films with fewer grain boundaries, recent studies show Na atoms present at the TMD/substrate interface, which can lead to variation in coupling between the growth substrate and TMD layers. This causes spatial variation in electronic properties of the grown TMD [27]. Such variations in coupling can also affect the quality of the grown material significantly, resulting in a tensile strain in the layers and a reduction of the PL intensity of MoS 2 layers by roughly 100Â.…”

Wafer‐Scale Fabrication of Nanopore Devices for Single‐Molecule DNA Biosensing using MoS2

“…[33] However, Na-based compounds assisting the MoS 2 growth were reported to diminish such an epitaxial interface due to Na intercalation under the grown film large-area epitaxial monolayer. [46] Indeed, optical images ( Figure S5, Supporting Information) reveal rare spots of extruded grain boundaries, which suggests imperfect grain stitching. Nonetheless, for our application, we have not observed any odd grain boundaries nor low-quality suspended areas on fabricated membrane devices.…”