Interface Defect Engineering of a Large‐Scale CVD‐Grown MoS₂ Monolayer via Residual Sodium at the SiO₂/Si Substrate

Abstract

In particular, extensive efforts regarding chemical vapor deposition (CVD) methods resulted in a large-scale MoS 2 monolayer of high quality for practical applications. [7][8][9][10][11][12] However, despite the large on-off ratio and high mobility, [3,9] 2D field-effect transistors (FET) composed of monolayer MoS 2 have become burdened with electrical contact issues such as Fermi level pinning (FLP) and high contact resistances. [13][14][15][16][17] The simplest and most abundant mono-sulfur vacancy (V S ) defects in the monolayer MoS 2 cause strong FLP at metal electrodes, which generally induce n-type semiconductor behavior. [18][19][20][21][22] Thus, many methods, which include substitution, charge transfer, intercalation, and electrostatic doping, have been developed to improve the contact properties or modulate the polarity of transition metal dichalcogenides (TMDs). [23][24][25] Meanwhile, the alkali metal halide-assisted CVD methods enable the rapid fabrication of the continuous and waferscale uniform monolayer TMDs directly on several oxide substrates. [26][27][28][29][30][31][32][33][34] Alkali metal cations promote a rapid increase of the single domain size of monolayer TMDs due to dehydration and nucleation suppression, regardless of the halide anions. [28,31,34] However, excess amounts of alkali metals destroy the crystallinity of monolayer TMDs. [29] On the other hand, back-gated FETs based on the MoS 2 monolayers directly grown this way or transferred onto the SiO 2 /Si substrates, used in current Si processing techniques to manufacture electronic devices, [19] showed comparable electrical performances to those fabricated without using alkali metal halides. [8,9,27,34] Nevertheless, such FETs also exhibited n-type behavior. [27,34] Na impurities are notoriously prone to contaminating the SiO 2 surface. [36] Alkali Na metals readily absorb at the oxide substrates during the CVD growth process of monolayer TMDs. [28,29] Furthermore, Na's incorporation at the SiO 2 surface of siloxane or silanol releases electrons into the monolayer MoS 2 conduction band; [37] whereas, in graphene deposited on SiO 2 , Na produces holes. [38] To understand the effect of trapped Na cations, we synthesized the large-scale uniform MoS 2 monolayers on the SiO 2 / Si substrates using NaCl catalysts [27] and then performed two-probe I-V measurements by the forward and reverse bias sweeping between ±30 V in the ambient air condition at room temperature (see Figures S1-S3, Supporting Information). Notably, initial low or negligible source/drain I-V signals in the µA range enhanced to the mA range after the Alkali metal halide-assisted chemical vapor deposition (CVD) methods can produce wafer-scale uniform monolayer transition metal dichalcogenides (TMDs). Further defect engineering is necessary to obtain high-performance functional devices. While defect engineering has focused on the surface of the monolayer TMDs or the contact property, interface defect engineering is rare and non-trivial. Based on a NaCl-assisted CVD-grown l...

Engineering of Oxidized Line Defects on CVD-Grown MoS2 Flakes

“…22 NaClassisted CVD-grown MoS 2 was reported to show n-type doping and passivated interfacial defects due to the existence of residual Na cations. 27 Therefore, it is critical to understand the formation mechanism and establish controllabe fabrication means of defects on MoS 2 flake to explore its potential applications utilizing defects to realize effective applications are critical in defect engineering. 28,29 In this work, we systematically analyzed the line defects formed on monolayer MoS 2 by introducing oxygen during the chemical vapor deposition (CVD) growing process.…”

“…Holes or cracks were introduced in monolayer MoS 2 by oxygen plasma exposure or hydrogen annealing treatment to enhance the hydrogen evolution reaction (HER) performance . NaCl-assisted CVD-grown MoS 2 was reported to show n-type doping and passivated interfacial defects due to the existence of residual Na cations . Therefore, it is critical to understand the formation mechanism and establish controllabe fabrication means of defects on MoS 2 flake to explore its potential applications utilizing defects to realize effective applications are critical in defect engineering. , …”

Plasma Processing and Treatment of 2D Transition Metal Dichalcogenides: Tuning Properties and Defect Engineering

“…For example, it has been shown that O 2 plasma treatment of the h-BN substrate in a 2D SnS 2 /h-BN heterostructure produces defects in h-BN which act as charge trapping centers and enables synaptic characteristics to be used as artificial synaptic transistors . On the other hand, such a hysteresis causes reliability issues for the devices in electronics and optoelectronics. , Studies have shown that the passivation of surface defects in SiO 2 substrates by Na + or various organosilane self-assembled monolayers with different end groups such as −SH and −NH 2 can enhance the electron transfer in MoS 2 -based FETs. , Therefore, not only defect engineering of substrates, but also passivation of their surface defects by plasma functionalization or doping can be considered as interesting tools to further tailor the performance of TMD-based electronic devices.…”

“…725,727 Studies have shown that the passivation of surface defects in SiO 2 substrates by Na + or various organosilane self-assembled monolayers with different end groups such as −SH and −NH 2 can enhance the electron transfer in MoS 2 -based FETs. 728,729 Therefore, not only defect engineering of substrates, but also passivation of their surface defects by plasma functionalization or doping can be considered as interesting tools to further tailor the performance of TMD-based electronic devices.…”

Effective Optical Properties of Laterally Coalescing Monolayer MoS2

“…However, experimental optical dispersion data analysis is lacking for 2D TMDC nanoscale nucleation grain dimensions relevant to prominent epitaxial growth methodologies (such as solid-source chemical vapor deposition or metalorganic chemical vapor deposition). , In this work, we define grain dimension as the growth of monolayer nucleation sites that coalesce in the lateral dimension at constant thickness. The ability to accurately and precisely monitor such nanoscale monolayer MoS 2 growth conditions via optical characteristics (at technique-relevant nucleation grain size dimensions) is expected to offer greater control of TMDC defect engineering, , selective chemical doping or lattice substitution, , grain-size-controlled mechanics, , and kinetic interfacial grain boundary phenomena. − Comprehensive optical characterization is also expected to provide accessibility toward engineering predicted transdimensional TMDC optical phenomena such as quantum excitonics, plasmonics, and plexitonics by guiding in-line growth dynamics. − …”