Plasma and Gas‐based Semiconductor Technologies for 2D Materials with Computational Simulation & Electronic Applications

The technique of plasma processing is beneficial for wafer cleaning and precision etching of integrated circuits and essential in manufacture of advanced semiconductor devices with unmatched perfection. Research on two‐dimensional (2D) materials, such as transition metal dichalcogenides(TMDs), offers a promising solution to the challenges in semiconductor miniaturization. TMDs, with their atomic layer thicknesses and silicon‐like bandgaps, can be integrated using existing plasma systems. Different 2D crystal structures, such as 1T and 2H configurations, exhibit distinctive properties. Computational approaches are also developed to provide guidelines for controlled synthesis and etching of large‐scale and high‐quality 2D materials. Plasma/gas‐surface interactions during the synthesis, etching, and phase transformation of 2D materials are explored using atomistic simulations such as density functional theory and molecular dynamics. The reaction energetics, chemical species, and associated kinetics are discovered in the simulation study. These results decipher various mechanisms of 2D materials processing at the microscopic scale and predict certain optimal process parameters. Plasma/gas‐based semiconductor technologies are crucial in electronics because they enable production of advanced semiconductors. Plasma/gas etching allows precise and selective removal of material and plasma‐enhanced chemical vapor deposition enhances chemical reactions for efficient film deposition; therefore, these processes are majorly important for harnessing 2D materials in electronic applications.


Introduction
The plasma processing technique is very useful in the manufacture of advanced semiconductor devices as it alters the properties of the material and purifies surfaces, [1] and ensures a precise control of integrated circuit fabrication in the wafer cleaning and etching stages.However, challenges persist as industry demands are to reduce semiconductor linewidth and miniaturize devices further. [2,3]Although reduction in linewidth escalates the manufacturing cost and encounters physical limitations at the atomic level, innovative approaches such as 2D and 3D integration are being explored to improve manufacturing efficiency and produce high-density circuits.6][7] To address these issues, extensive research on 2D materials such as transition metal dichalcogenides (TMDs) is being actively pursued. [8,9]TMDs have a band gap similar to that of conventional semiconductor materials such as silicon [10] and their thickness is comparable to that of atomic layers, making them suitable for applications in thin devices and streamlined processing. [10]Moreover, the band gap of TMDs can be controlled by manipulating the number of layers. [11]Research is also being conducted on 3D integration technology and the effective application of band gap control in various devices. [12,13]As plasma techniques are already being used in semiconductor manufacturing processes, modifying these systems for 2D materials is a practical option.By adapting systems such as gas and scrubbers to meet the requirements of 2D materials, it is possible to fabricate modern semiconductors. [14]D materials have 1T and 2H structures, depending on their crystallinity. [15]The 1T structure, with a distorted octahedral configuration, exhibits metallic properties whereas the 2H structure, characterized by a triangular prism shape, exhibits semiconductor properties.Methods for synthesizing 2H-molybdenum disulfide (MoS 2 ), [16] 1T-MoS 2 , [17] and 1T-tungsten disulfide (WS 2 ) [18] using plasma-enhanced chemical vapor deposition (PECVD) were reported, [19] as also research on uniformly etching them on a 4-inch wafer scale using reactive ion etching (RIE). [20,21]Various methods of synthesis and etching were investigated, and future research on utilizing plasma is expected to gain further recognition.
Computational approaches were developed in semiconductor technologies to provide guidance for controlled synthesis and etching of high-quality TMDs.Owing to the complexity of plasma/gas-surface interactions, the detailed chemistry underlying TMD processes remains largely unknown, thus hindering process optimization.Comprehensive studies on density functional theory (DFT) and molecular dynamics (MD) simulations were conducted to elucidate the reaction mechanism in the chemical vapor deposition (CVD) growth, [22][23][24][25] plasma/gasbased etching, [21,26] and ion beam-induced phase engineering [27] of TMDs.Sulfidation of metal oxide surfaces, [22] nucleation of embryos, [23] salt-free, [24] and salt-assisted monolayer growth [25] of TMDs were explored from an atomic perspective, especially for the process of synthesis.For TMD etching, halogen gasinduced [26] and plasma radical-induced reactions [21] were examined, and the associated adsorption, diffusion, substitution, and desorption were reproduced.The physical collision of ions and subsequent phase change of the TMD surface were also focused upon. [27]These computational studies identified essential chemical species, reaction energetics, and related kinetics during the process, facilitating the fundamental understanding and rational design of TMD processes such as optimal gas/salt screening.
Plasma/gas-based semiconductor technologies have significant potential in the field of electronics, particularly in the fabrication of advanced semiconductor devices with enhanced capabilities.These techniques are commonly used to create thin films of semiconducting materials that are essential to produce various electronic devices, such as transistors, solar cells, and lightemitting diodes.The high-energy state of matter, consisting of ionized gas in plasma, enables the effective and controlled removal of materials from semiconductor surfaces through etching, enabling the production of intricate micro-/nano-electronic devices.
Gas-based semiconductor technology utilizes CVD to deposit thin films of semiconducting materials.In this process, a gas-containing precursor material is passed over a substrate and they react to form a thin film of various semiconductors, such as silicon, gallium arsenide, and diamond among others.The process of PECVD accelerates the chemical reactions and promotes efficient, uniform, and pure deposition.Plasma/gas-based technologies are integral to contemporary electronic devices and are undergoing continuous refinement for improved efficiency, reliability, and adaptability.We outline various electronic applications of 2D materials fabricated by using plasma/gas-based etching techniques.
In summary, plasma-/gas-based methodologies are indispensable to the manufacture of semiconductors and are promising avenues for future technological breakthroughs.At present, there is active ongoing research on the synthesis and etching of 2D materials, which are the next-generation semiconductor materials.In this paper, we aim to introduce studies that apply plasma-/gas-based synthesis and etching methods to TMDs and utilize computational techniques, such as DFT and MD simulations, to identify the reaction mechanisms.

Plasma Technology-based Synthesis/Etching Methods for 2D Materials
In the semiconductor industry, a range of etching and synthesis techniques are employed to fabricate devices.Common methods for synthesizing semiconductor materials include PECVD, CVD, and sputtering.PECVD deposits thin films of various materials onto a substrate such as a silicon wafer via plasma and chemical reactions.This method is typically used to deposit thin films of silicon dioxide (SiO 2 ) and silicon nitride (Si 3 N 4 ) for applications such as insulation layers, passivation layers, and thin-film transistors.CVD is also used to deposit thin films via chemical reactions.Sputtering, another vacuum deposition method, accelerates the plasma of ionized gases such as argon (Ar) at low vacuum pressure to collide with a target, creating a film on substrates such as wafers or glass.The sputtering equipment consists of a cathode and an anode, with the target as the cathode and substrate as the anode.][30] The formation of the 1T-phase MoS 2 film was deposited on SiO 2 /Si wafer by H 2 S+Ar plasma, as shown Figure 1a. [17]Kim et al. developed the method of fabrication of a MoS 2 -graphene heterostructure (MGH) on a 4-inch wafer at 300 °C by depositing a thin Mo film as seed layer on transferred graphene followed by sulfurization using H 2 S plasma, as shown in Figure 1b. [28]Nearly 5-6 MoS 2 layers with a high density of sulfur vacancies were grown uniformly on the entire substrate and the existence of the layers was confirmed using Raman spectroscopy and high-resolution transmission electron microscope (HR-TEM).PECVD was also used to synthesize MoS 2 -WS 2 vertical heterostructures through penetrative, single-step sulfurization, as determined by time-dependent analysis, as shown in Figure 1c. [29]he sputtering process involves an ionizing gas such as argon at relatively low vacuum levels, accelerated to collide with a target causing atoms to be ejected to create a film on a substrate, such as a wafer or glass.Ohta et al. grew a hexagonal boron nitride (hBN) film on an AIN template substrate using an Fe catalyst layer and plasma-enhanced sputtering deposition (PSD), as shown in Figure 1d. [30]The results of Raman analysis showed that a highly crystalline hBN film was formed at the heterointerface and the possibility of wafer-scale production of the hBN film was perceived in this study.Gupta et al. investigated the sputtering method of thin films of MoS 2 and analyzed the effects of various parameters on the thin film form of MoS 2 , [31] shown in Figure 1e.Tao et al. demonstrated a proof-of-concept, one-step, large-area synthesis of uniform MoS 2 films with a readily controllable number of layers using a magnetron sputtering method, as shown in Figure 1f. [32]This method produced a uniform MoS 2 film over a large area and characteristics of p-type semiconductors were observed in the electrical measurement analysis.It controls the doping effect and can be applied to other 2D materials.
Recently, the processing of fine patterns in units of several nanometers has become an essential technology.Consequently, active research was conducted on atomic layer etching using plasma reactions.Plasma-enhanced atomic layer etching (PEALE) and reactive ion etching (RIE) are widely used to etch fine patterns.PEALE is an advanced form of atomic layer etching (ALE) that uses plasma to enhance chemical reactions and improve the removal rate of materials with high precision and control.On the other hand, RIE is a type of dry etching process used to pattern thin films on semiconductor wafers or other materials.They are plasma-based processes that use reactive gas ions to selectively etch away the material from the surface of a substrate, as shown in Figure 1g-l.Hirata et al. investigated the etch-stop mechanism in the SiN PEALE process, as shown in Figure 1g. [39]he MD simulations confirmed the presence of SI-C bonds.The etching precision of the SiN films was in the range of Ångström units.The etched amount per cycle (EPC) was analyzed to confirm ALE.Zhu et al. attempted to control the atomic layer of MoS 2 using remote plasma etching, as shown in Figure 1h. [34]XPS, low-energy electron diffraction D), and atomic force microscopy (AFM) were used to characterize the surface chemistry, structure, and topography of MoS 2 .To form the surface layer of MoO x , remote oxygen plasma and surface oxidation of MoS 2 were combined and a subsequent annealing process was used to etch the MoOx surface layer, which carried out the functionalization of MoS 2 and etching of layers.The present study focuses on analyzing the oxidation and etching of a uniform monolayer on multilayer MoS 2 flakes using Raman and AFM techniques.It was demonstrated that O 2 plasma treatment enables soft etching of MoS 2 without physical etching.Lin et al. investigated the po-tential of MoS 2 for manufacturing complementary metal-oxidesemiconductor (CMOS) devices, [35] where the thickness of the multilayer MoS 2 was controlled layer by layer using Cl radical adsorption and Ar + ion desorption.Raman spectroscopy results confirmed that a monolayer of MoS 2 was etched through a onecycle process, as shown in Figure 1i.
This study also demonstrated that the same atomic group etching method could be applied to various 2D materials, such as Molydbenum diselenide (MoSe 2 ) and WS 2 .In a related study, Jeon et al. explored a novel approach to control the MoS 2 layer by using CF 4 plasma etching on a six-layer MoS 2 sample, followed by Raman spectroscopy and XPS analysis.The results of XPS indicated the presence of a partial defect on the MoS 2 surface after the etching process, which was attributed to residual CF 4 gas, as shown in Figure 1j. [36]Jiang et al. investigated the inductively coupled Cl 2 /Ar plasma etching of 4H-SiC and found that the ratio of Cl + ions to Ar + ions affected the etching rate of SiC using Cl 2 /Ar plasma, as shown in Figure 1k. [37]Additionally, the study found that the etching rate increased as the substrate temperature decreased.The XPS analysis confirmed the effective removal of chlorine-related etching residues from the SiC surface after etching.Choi et al. investigated the etching rates of titanium dioxide (TiO 2 ) thin films by O 2 with CF 4 /Ar plasma etching and analyzed the chemical reaction using XPS, and surface roughness of the TiO 2 film using AFM, as shown in Figure 1l. [38]The results indicated that the value of RMS decreased and the surface morphology of the TiO 2 thin film was smoothened after etching.Various synthesis and etching methods were reported as shown in Table 1.

Classification of Physical/Chemical Plasma Etching
Dry etching is a process of removal of surface material that uses plasma generated by a combination of electric and magnetic fields.Several types of dry etching techniques exist such as RIE, ion-beam etching (IBE), and deep reactive ion etching (DRIE).RIE is a chemically reactive ion-based dry etching technique commonly used to create fine patterns whereas IBE uses highenergy ion beams to remove materials from a surface.Ions are generated by an ion source and directed to the surface to be etched.DRIE is a type of RIE that creates deep, high-aspect-ratio patterns on a surface by alternating between the etching and Figure 1.Plasma-based synthesis and etching methods for 2D materials.a) Synthesis of 1T-MoS 2 by using PECVD at 150 °C; [17] b) Photograph of wafer-scale MGH on a 4-inch SiO 2 /Si substrate, cross-sectional HR-TEM images of MGH and electron energy loss spectroscopy elemental mapping images of MGH; [28] c) Synthesis of MoS 2 -WS 2 vertical heterostructures (MWVHs) via single-step penetrative sulfurization by PECVD at 300 °C; [29] d) Fabrication of the wafer-scale sp 2 -bonded BN film on the AIN template by interface segregation; [30] e) Equipment of fabrication for synthesis of MoS 2 by reaction between Mo foil and S, space-confined CVD growth of the samples MoS 2 , and equipment of fabrication for the CVD process for synthesis of MoS 2 by reaction between MoO 3 and S; [31] f) MoS 2 film on sapphire substrate, AFM image of the as-grown MoS 2 film, and height profile by AFM; [32] g) Process sequence for five-step SiN PEALE and depth profiles of bond densities and side view of atomic compositions of a SiN surface; [33] h) Thermal stability of the MoO x layer on MoS 2 by annealing in vacuum system; [34] i) ALET cycle of MoS 2 for layer-by-layer etching, and Raman spectra of one monolayer of MoS 2 before and after exposure to the CL plasma; [35] j) XPS narrow scan data of Mo 3d, S 2p, and F 1s before/after treatment using a H 2 S plasma; [36] k) Etch rate, sputter yield, and Ar concentration in Cl 2 /Ar gas mixture; [37] l) Etch rate of TiO 2 thin film and selectivity of TiO 2 to SiO 2 as a function of the DC-bias voltage and process pressure. [38]as shown in Figure 2a.They compared the chemical and surface morphology changes in the film after each treatment.AFM analysis revealed significant topographical and structural changes in the film after O 2 plasma treatment whereas Ar plasma treatment showed minimal changes.The combination of surface chemical changes and sample surface morphology caused a rapid change in the contact angle, ren-dering the Ar-treated film hydrophilic and the O 2 -treated film superhydrophilic, as shown in Figure 2a.Kim et al. controlled the thickness and band structure of MoS 2 films using a low-intensity ICP source, and the fabricated MoS 2 field effect transistors (FETs) exhibited high n-type doping owing to the formation of S vacancies.The plasma-treated MoS 2 FETs induced effective n-type doping and thinning effects, as shown in Figure 2b. [71]aser damage threshold is the limit at which the laser fluence, intensity, and wavelength cause damage to an optical device or material.In a study by K et al., employed laser induced damage  [70] b) AFM results and frequency differences showing a stepwise decrease as the number of layers decreased; [71] c) Comparison of transmittance spectra of individual FS substrates before and after ion etching and comparison of FS surface roughness; [72] d) Results of etch rate and Ar concentration after Cl 2 /Ar gas mixture plasma etching; [37] e) Schematic of MoS 2 device, transfer characteristics, and variation of field-effect mobilities of devices based on tri-, bi-, and mono-layer MoS 2 obtained from plasma etching system; [73] f) Raman spectra and XPS narrow scan data of Mo 3d, S 2p, and F 1s before/after treatment using a H 2 S plasma; [36] g) Plan-view HIM images of lines spaced with pitches of 22, 20, 18, and 16 nm on silicon substrate; [74] h) Analysis of Y 2 O 3 films and SF 6 plasma-treated films before and after fluorocarbon plasma etching by XPS. [75]Figures adapted with permission from: a) ref. [70], Copyright 2015, Wiley Publishing Group; b) ref. [71], Copyright 2016, IOP Publishing Group; c) ref. [72], Copyright 2017, Optica Publishing Group; d) ref. [37], Copyright 2004, IOP Publishing Group; e) ref. [73], Copyright 2020, Wiley Publishing Group; f) ref. [36], Copyright 2015, IOP Publishing Group; g) ref. [74], Copyright 2019, ACS Publishing Group; h) ref. [75], Copyright 2020, MDPI Publishing Group.
threshold (LIDT) measurements using the 1-on-1 technique, which involves progressively increasing the laser light's intensity on the target until damage is observed at a wavelength of 355 nm showed a threshold for bare FS substrates with etch depths of (50 ± 3.5) and (100 ± 7) nm that was 8.4 times higher as compared to unetched samples, as shown in Figure 2c. [72]The results at depths ranging from ≈50 to 200 nm improved the performance of the FS surface material in terms of RMS, flatness, and LIDT.Jiang et al. [37] studied the inductively coupled Cl 2 /Ar plasma etching of 4H-SiC and found that the ratio of Cl to Ar ions had the greatest impact on SiC etching.The etching rate increased as the substrate temperature decreased.XPS analysis confirmed the effective removal of chlorine-related etching residues from the SiC surface after etching, as shown in Figure 2d.Chee et al. developed a layer-by-layer chemical etching method of MoS 2 using mild sulfur hexafluoride (SF 6 ) plasma treatment, as shown in Figure 2e. [73]Defects were observed only from the 3 rd layer to the monolayer in the MoS 2 layers, and the damage was well-controlled during the plasma process.The soft plasma etching system can only be induced by chemical etching and results in high mobility for each thickness.The thickness of the etched MoS 2 film was measured using various methods such as OM imaging, Raman spectroscopy, PL spectroscopy, and AFM.Jeon et al. developed a method for controlling the layer of MoS 2 using CF 4 plasma etching, [36] as shown in Figure 2f.The six-layer MoS 2 was etched using the CF 4 plasma etching technique.The number of layers and chemical structure of MoS 2 were confirmed using Raman spectroscopy and XPS.However, the XPS results indicated a partial defect on the MoS 2 surface after etching owing to residual CF 4 gas that was later removed by H 2 S treatment.Lewis et al. demonstrated that the use of Cr 8 F 8 (private) 16 molecules as resisted in the HIBL lithography method can generate structures smaller than 10 nm in Si and tungsten (W) during ICP-RIE etching, as shown in Figure 2g. [74]This is because of the ability of the resist material to rotate into a film with a thickness of less than 5 nm, which reduces side scattering when the beam passes through the resist, resulting in high resolution.The high molecular weight and low density of the material also limit the number of scattering sites where the beam meets, thereby improving the resolution.A higher yield was also achieved using a He + ion beam.Wang et al. [75] investigated the etching behavior of yttrium oxyfluoride and compared a Y 2 O 3 film deposited by sputtering after exposure to CF 4 /O 2 plasma with an SF 6 plasma-treated Y 2 O 3 film.After etching, the cross-sectional thickness of the material was measured by TEM.b) Fabrication of 2D heterostructures using atomic layer etching method; [76] c) Atomic layer etching method for MoS 2 ; [77] d) Exploration of intrinsic mechanism of oxygen-assisted etching; [78] e) Patterning of few-layered MoS 2 and MoSe 2 by oxidation lithography and etching; [79] f) Reactive plasma etching for layer-by-layer thinning of MoSe 2 ; [80] g) Comparison of roughness-factor corrected HER peak current densities; [81] h) Schematic of the fabrication process of Ag nanoprism array and corresponding SEM image; [82] i) VLM image of monolayered WS 2 flakes and similar flakes analyzed for thickness using AFM height measurement; [83] j) Comparison of pristine and etched-to 1L WSe 2 flakes; [84] k) Cross-sectional HRTEM images and schematic of device processing occurring at the edge-contacted metal-WSe 2 interface; [85] l) Schematic of device fabrication process and optical microscope images of the device after plasma process. [86]Figures adapted with permission from: a) ref. [21], Copyright 2023, ACS Publishing Group; b) ref. [76], Copyright 2019, IOP Publishing Group; c) ref. [77], Copyright 2016, Nature Publishing Group; d) ref. [78], Copyright 2022, ACS Publishing Group; e) ref. [79], Copyright 2020, Elsevier Publishing Group, f) ref. [80], Copyright 2017, Elsevier Publishing Group; g) ref. [81], Copyright 2017, Elsevier Publishing Group; h) ref. [82], Copyright 2019, Elsevier Publishing Group; i) ref. [83], Copyright 2020 ACS Publishing Group; j) ref. [84], Copyright 2020, ACS Publishing Group; k) ref. [85], Copyright 2022, Royal Society of Chemistry Publishing Group; (l) ref. [86], Copyright 2021, Wiley Publishing Group.

Types of 2D-TMD Materials Suitable for Plasma Etching
The increasing interest in 2D materials, which are the nextgeneration semiconductor materials, has led to active research in this field.Therefore, it is necessary to develop various processes and techniques for utilizing these materials as semiconductor components.As the semiconductor linewidth decreases, precise etching with a high aspect ratio becomes a critical requirement.The unique layered structure and varying band gap of 2D materials with respect to the number of layers necessitate the development of atomic etching technologies capable of precisely removing individual layers.Consequently, research on such technologies is currently receiving significant attention and efforts are underway to apply them to various 2D materials, as shown in Figure 3a-l.In the case of MoS 2 etching, Kim et al. [21] presented a novel technology for the mass production of 2D materials using large-scale synthesis and etching by PECVD and RIE.The synthesized and etched materials were comprehensively analyzed using various advanced techniques, such as TEM, Raman spectroscopy, AFM, OM images, and ellipsometer measurements, as shown in Figure 3a.To understand the plasma state during the etching process, they employed optical emission spectroscopy (OES) to diagnose the plasma state in real time; in addition, to understand the etching mechanism of MoS 2 , they utilized DFT to predict and simulate the chemical reaction during the etching process, as shown in Figure 3a.Heyne et al. [76] studied the heterostructures of low-dimensional semiconducting materials such as 2D-TMDc.The present study investigates the optimization of a cyclic Ar/Cl 2 atomic layer etching process that enables the etching of Si on MoS 2 with minimal damage.This is followed by the selective conversion of patterned Si into WS 2 and the impact of Si atomic layer etching on MoS 2 is evaluated.The results showed that the removal of MoS 2 within the ion energy range used in this study occurred in one or two over-etching steps, leading to the formation of MoO x whereas no significant lattice distortion was observed in the remaining layers.Moreover, the combination of Si atomic layer etching on top of MoS 2 with subsequent Si-to-WS 2 selective conversion created WS 2 /MoS 2 heterostructures with clear Raman signals and a horizontal lattice alignment, as shown in Figure 3b.Xiao et al. [77] investigated an etching method for 2D MoS 2 because the band gap of MoS 2 exhibits layer-dependent behavior, that is, the band gap changes as MoS 2 transitions from a multi-layered to single-layered structure.To selectively remove MoS 2 without damaging the substrate or other materials, atomic layer etching was conducted using SF 6 and N 2 plasma and the etching results were verified AFM, as shown in Figure 3c.
Zhang et al. [78] achieved the precise production of the etching pattern of MoSe 2 by adjusting the oxygen plasma pretreatment and etching times.The resulting atom-characterized etched pattern exhibited a distinctive zigzag shape, which was in excellent agreement with the preferred etching orientation.This observation confirmed the effectiveness of the anisotropic etching process, as validated by both experimental and theoretical calculations.The results of etching were systematically analyzed using advanced microscopy techniques, such as OM, SEM, and TEM imaging, which provided insights into the morphology and structural features of the etched material, as shown in Figure 3d.Ryu et al. [79] investigated MoS 2 and MoSe 2 nanoelectronic devices.These 2D materials are highly sensitive to physical and chemical interactions that occur during processing.To fabricate devices on the nanometer scale, a low-intrusion patterning method is required.In this study, the oxygen scanning probe lithography (O-SPL) and oxygen plasma were used to create field-effect transistors and nanoconstrictions on multilayers of MoS 2 and MoSe 2 , as shown in Figure 3e.In addition, Ryu et al. [79] developed a scanning probe lithography method to reduce the size of the MoX 2 films; this involved pretreating the surface of the material with O-SPL to generate a uniform oxide layer of 3-4 nm on the MoX 2 flake surface.They compared the output characteristics of nanoribbon-based transistors with those of microchannel transistors made of MoS 2 and MoSe 2 and confirmed an improvement in the performance of the transistor, as shown in Figure 3e.Sha et al. [80] attempted to control the layer of MoSe 2 by soft and reactive plasma etching.2D TMDs such as molybdenum diselenide (MoSe 2 ) have recently attracted significant attention owing to their complementary characteristics to graphene.However, unlike graphene that has no band gap, the band structure of MoSe 2 can transition from an indirect to a direct band gap when changing from bulk to a single-layered material.Such a change in the number of layers requires atomic layer synthesis and etching.They presented a method of depositing MoSe 2 monolayers by gentle etching using SF 6 and N 2 plasma.The results were confirmed using Raman spectroscopy, TEM, and AFM.By controlling the etching speed, MoSe 2 could be completely removed or the desired number of MoSe 2 layers, even a single layer, could be obtained, as shown in Figure 3f.
In WS 2 etching carried out by Daniel et al., [81] the activity of TMDs in the hydrogen evolution reaction (HER) was enhanced by increasing the concentration of sulfur in the active metalsulfide sites.In this study, the electrochemical sulfidation of WS 2 nanoarrays exposed to air and its effect on the HER activity were investigated.Electrochemical and XPS experiments revealed that inert tungsten oxide contributes to the change in HER and electron transfer properties, and this result demonstrates a strong dependence on the TMD composition along with sulfur doping and subsequent HER activity, as shown in Figure 3g.In contrast to MoS 2 , they obtained a WS x structure with a high surface content of WO 2 and low content of S, rather than a sulfurrich WS x structure, by applying the electrochemical sulfidation method in solution at room temperature to obtain a WS x structure.The poorly integrated sulfur in the WS x structure was presumed to be due to sulfidation and the formed WO x compounds were insoluble in acidic solutions.These results explained the key role of TMD characteristics in successfully integrating sulfur into the TMD structure using electrochemical methods, as shown in Figure 3g.Owing to their atomic thinness, TMDs generally exhibit weak light emission because of poor light absorption.The enhancement of their optical properties through surface plasmon resonance is suitable for the application of atomically thin TMDs in optoelectronics and photonics, as shown in Figure 3h.In their study on TMDs, Yang et al. [82] fabricated a monolayer of WS 2 grown by CVD and transferred it onto an Ag/WS 2 hybrid structure, achieving significantly improved photoluminescence emission and Raman scattering.According to calculations, the plasmonic enhancement mainly arises from the highly enhanced local electric fields and charge transfer.They prepared Ag nanoprism arrays using nanosphere lithography by adjusting the nanosphere spacing to maximize the photoluminescence emission of monolayered WS 2 .The PL and Raman spectra of the Ag/WS 2 plasmonic hybrid structure showed a uniform enhancement effect, with the integrated intensity of the neutral excitons of the monolayered WS 2 increasing over sixfold.Using FDTD simulations, the mechanism of the surface-plasmon-enhanced optical properties was elucidated, and it was observed that local surface plasmons were formed through the action of an external electric field, as shown in Figure 3h.Kumar et al. [83] studied the polymorphic in-plane heterostructures of monolayered WS 2 .To implement nanoscale devices based on TMDs, uniform control of atomically thin-film layers with excellent properties is required.In this study, a WS 2 heterostructure was grown on a substrate by CVD at atmospheric pressure.The structures formed in the 1H and 1T phases were extensively analyzed using various spectroscopic and microscopic techniques, as shown in Figure 3i.
In the case of WSe 2 , Nipane et al. [84] developed a controllable and reproducible etching process with high selectivity for the thickness control and patterning of 2D materials.However, owing to the similarity in thickness and structural properties between the monolayers, it is difficult to apply conventional thinfilm etching processes.They investigated the ALE of monolayered WSe 2 , while maintaining its physical, optical, and electronic properties, as shown in Figure 3j.Top-down etching was performed, and each layer was selectively removed from the top.The ALE-treated WSe 2 layers exhibited bright photoluminescence and a high room-temperature hole mobility of 515 cm 2 V −1 s −1 , which is essential for high-performance 2D device fabrication.In addition, they used sacrificial monolayered WSe 2 to protect the channel during the etching process and fabricated highly pure 2D  [21] b) Measured (lines + symbols) and model-predicted (lines) plasma parameters as functions of O 2 content in 50% Cl 2 + HBr + O 2 gas mixture; [87] c) Time averaged fluxes of radicals and ions to the wafer in a TF-CCP sustained in an Ar/C 4 F 8 /O 2 mixture; [88] d) Steady-state densities of neutral species versus fraction of second fluorocarbon component in CF 4 + C 4 F 8 + Ar and CF 4 + CHF 3 + Ar plasmas; [89] Figures adapted with permission from: a) ref. [21], Copyright 2023, ACS Publishing Group; b) ref. [87], Copyright 2019, Elsevier Publishing Group; c) ref. [88], Copyright 2019 AIP Publishing Group; d) ref. [89], Copyright 2022 Elsevier Publishing Group.devices using graphene as the test bed.Ngo et al. [85] investigated the techniques for forming edge contacts in 2D FETs with the aim of achieving high mobility and overcoming Fermi-level pinning.However, most studies on the effects of edge contacts focused on graphene and MoS 2 .This paper presents unusual electrical transport results for edge-contacted WSe 2 FETs.These WSe 2 FETs exhibited p-type behavior with a small, fixed coefficient of 0.04, independent of contact metals, such as chromium and indium with low work functions, and palladium with a high work function.HR-TEM and energy-dispersive X-ray spectroscopy analyses showed the formation of oxides near the metal contact interface owing to plasma etching, which was the cause of strong Fermi-level pinning, as shown in Figure 3k.Kim et al. [86] studied the doping of van der Waals layered semiconducting materials as an essential technique for realizing the full potential of nanoelectronics, as shown in Figure 3l.In this study, defect engineering and area-selective n-doping of bipolar multilayer WSe 2 were performed using Ar plasma treatment.The contact area of WSe 2 was exposed to weak Ar plasma treatment to induce S vacancies whereas the channel area was protected with hexagonal boron nitride.The results were systematically analyzed using structural and optical characterization techniques, and the origin of n-type characteristics of the plasma-treated WSe 2 was proposed through plane-wave DFT calculations.Defect-engineered n-doping of multilayered WSe 2 was achieved through Ar-ion plasma treatment, as shown in Figure 3l.

Plasma Etching with Various Mixed Gases for 2D Materials
As previously mentioned, plasma possesses the capability for both physical and chemical etching, which varies depending on the type of gas utilized.Therefore, for materials such as SiO 2 or SiN x , a combination of two or three types of gases has commonly been employed in plasma etching processes.While mixed gases have been used for plasma etching, their application in the context of 2D materials has been relatively limited.The anticipated effects from such applications include chemical plasma etching characterized by F-series selectivity and the use of Ar and O 2 , which, though non-selective, leave no residual material.The research into mixed gases was initially conducted to leverage the full spectrum of benefits associated with physical etching.Gas mixtures are used in plasma etching to enhance the etching outcome, such as the etching speed and aspect ratio, and execute the physical and chemical etching processes simultaneously, as shown in Figure 4a-d.Kim et al. developed a method of layer-by-layer control of MoS 2 . [21]The objective of this study was to achieve precision and purity in the large-scale (4-inch) layer control of MoS 2 using two sequential plasma processes, namely PECVD and RIE.MoS 2 was synthesized on a 4-inch wafer by PECVD, and the grown bulk layer was then etched upon using a gas mixture that was computationally screened during the cycling phase using RIE, as shown in Figure 4a. Lee et al. investigated the etching mechanism and characteristics of plasma chemistry. [87]Plasma diagnostics, such as Langmuir probes and 0D (global) plasma modeling, provide information on gas-state plasma parameters, formation and collapse dynamics of plasma active species, and steady-state plasma composition.Substituting O 2 with HBr at a fixed Cl 2 ratio in the feed gas leads to 1) a slight increase in electron density, ion density, and ion energy flux; 2) an impact on the neutral species dynamics due to reactions involving O atoms and their reaction products; and 3) an increase in the total halogen atom density.In addition, the change in the HBr/O 2 mixing ratio induces contrasting kind of behavior in the Si and SiO 2 etching rates and increases the SiO 2 /Si etching selectivity by a factor of almost 10 in highly oxygenated plasmas.The kinetic analysis of Si and SiO 2 etching using the predicted fluxes of the plasma active species revealed different etching mechanisms for these materials, as shown in Figure 4b.Huang et al. studied the plasma etching of high-aspectratio (HAR) features, which was a critical step in the production of high-capacity memory. [88]It was a challenge to maintain the critical dimensions (CDs) while removing or reducing the aspect ratio-dependent etching (ARDE), distortion, contact roughness, and AR exceeding 50 (approaching 100), as shown in Figure 4c.They employed an integrated reactor and feature-scale modeling to investigate the HAR feature etching in SiO 2 with up to 80 AR using sustained triple-frequency capacitively coupled plasma in an Ar/C 4 F 8 /O 2 mixture, as shown in Figure 4c.Efremov et al. investigated the plasma parameters, steady-state gas composition, and heteroprocess dynamics in CF 4 + C 4 F 8 + Ar and CF 4 + CHF 3 + Ar gas mixtures under 13.56 MHz inductive coupling conditions, as shown in Figure 4d. [89]They used consistent input parameters, such as pressure (6 mTorr) and input power (700 W), as constants, and the variables were ratios of the C-F components and bias power.When the second fluorocarbon gas is replaced with CF 4 , it exerts a somewhat weak effect on the electron-and ion-related plasma characteristics and induces sufficient changes in the neutral species dynamics that accelerate the formation of polymer radicals and suppress the F-atom density.This is due to the effective collapse of F-atoms in the reaction sequence CHF x + F → CF x + HF.By contrast, changes in the bias power only alter the strength of ion collisions through a negative DC bias voltage without affecting the plasma-active species dynamics, as shown in Figure 4d.

Computational Approaches for 2D Materials Processing
For high-quality, and large-scale fabrication of 2D materials, a complete understanding of the surface chemistry underlying the semiconductor processes is essential.Computational studies were conducted to elucidate the reaction mechanisms during the process involving synthesis, etching, and phase transformation of TMDs.This section focuses on plasma/gas-surface interactions using atomistic simulations, and the detailed energetics and reaction kinetics are presented.
Computational synthesis of MoS 2 layers in the CVD process has received significant attention.The CVD method exhibits good scalability, controllability, and low cost; however, further refinement is inadequate owing to a lack of understanding of the reaction chemistry.Hong et al. [22] reported a mechanistic analysis of sulfidation of the MoO 3 surface for CVD growth of MoS 2 .An MD simulation with a reactive force field (ReaxFF) interatomic potential was used to identify the reaction pathway, as shown in Figure 5a-d.A MoO 3 /Al 2 O 3 stack was prepared in a simulation model that mimicked pre-deposited molybdenum oxide on an alumina substrate and was heated from 100 to 2000 K.During heating, evolution of O 2 occurred from the MoO 3 surface leaving undersaturated 3-5 fold coordinated Mo atoms that acted as reactive sites for subsequent sulfidation (Figure 5a).Subsequently, the self-reduced MoO 2.6 /Al 2 O 3 surface was exposed to gaseous S 2 molecules, leading to further reduction and sulfidation (MoO 1.99 S 0.24 ) as depicted in Figure 5b.SO, and SO 2 gas molecules were also observed during sulfidation.The continuous supply of S 2 gas substituted the remaining O atoms on the MoO x S y surface with S atoms, resulting in the MoO 1.70 S 0.56 structure.In addition, several voids were identified in the substrate that originated from Mo migration and reorganization of the Al 2 O 3 underlayer (Figure 5c); this was consistent with the experimental observations. [91]The final snapshot also confirms several types of Mo-S bonds, such as S terminations, MoS 2 edges, and Mo-S-Mo bridge structures (Figure 5d).
Figure 5e,f presents a systematic study of the initial stage of MoS 2 nucleation on a gold substrate during the CVD process, as reported by Shao et al. [23] The DFT and ab initio molecular dynamics (AIMD) simulations provide a step-by-step mechanism for the growth of Mo x S y clusters.Initially, at high temperatures ranging from 1000 to 1400 K, the MoO 3 molecules were readily deoxidized by S 2 gas, leaving molybdenum sulfide on the Au (1 1 1) surface, as shown in Figure 5e.Along with the overfeeding of S 2 gas, sulfur passivation was energetically favorable at the fcc hollow sites of the Au (1 1 1) substrate.On the S-passivated gold surface, the Mo and S atoms quickly aggregated to form Mo 3 S 7 clusters, as shown in Figure 5f.After the addition of Mo and S atoms with an appropriate stoichiometry, some S atoms diffused into the interface between the Mo x S y cluster and Au surface and lifted the Mo atoms to form a three-atom-thick (S-Mo-S) Mo 6 S 15 embryo with the T phase; this was experimentally reported by Xu et al. [92] By calculating the formation energy, the small T-phase Mo x S y seed was found to be transformed into the H phase when the cluster size reached Mo 10 S y , and its stable H phase was maintained during subsequent growth.
A combination of DFT and AIMD simulations was performed by Lei et al. [24] to examine the entire process of MoS 2 growth by CVD, starting from the sublimation of solid MoO 3 , gas-phase sulfurization, and finally the fusion of building molecules to form the MoS 2 crystal lattice.Figure 5g [22] e) Snapshot of MoO 3 molecule with the appearance of S 2 molecules; f) Entire simulation process from separate atoms to cluster for MoS 2 nucleation on Au substrate; [23] .g) Snapshot of solid MoO 3 sublimation; h) Close-ups of intermediates showing Mo 3 O 9 cleavage into MoO x S y molecule; i) Energy diagram from MoO 3 precursor to MoS 2 monolayer; j) Incorporation of MoS 6 into MoS 2 crystal lattice during CVD growth; [24] k) Reaction map of MoO 2 Cl 2 sulfurization toward MoS 6 ; l) Comparison of sulfurization energetics between salt-assisted (red) and conventional CVD growth (black) of MoS 2 monolayer; m) Halogen-dependent MoO 2 X 2 (X = F, Cl, Br, and I) sulfurization barrier in rate-limiting step for salt-assisted CVD synthesis. [25]; n) Schematic of the confined WSe 2 synthesis strategy; o) the binding energy of WO (W 3 O 9 ), Se (Se 2 and WSe 2 clusters (W 3 Se 6 ) on various substrates during the confined WSe 2 growth. [90]Figures adapted with permission from: a-d) ref. [22], Copyright 2017, ACS Publishing Group, e,f) ref. [23], Copyright 2021, ACS Publishing Group, g-j) ref. [24], Copyright 2021, ACS Publishing Group, k-m) ref. [25], Copyright 2022, ACS Publishing Group.n,o) ref. [90], Copyright 2023, Nature Publishing Group.
To promote the growth rate of conventional CVD, alkali metal halides were introduced in the synthesis and the salt-assisted growth mechanism of MoS 2 was analyzed by the same group. [25]sing sodium chloride (NaCl), the MoO 3 precursor was converted into molybdenum oxyhalide (MoO 2 Cl 2 ), which is the major Mo feedstock gas in the salt-assisted CVD.At a high tem-perature of 2500 K, MoO 2 Cl 2 was sulfurized by S 2 gas to form intermediates, such as MoOCl 2 S, MoOClS 2 , and fully sulfurized MoS n (n = 4-6) molecules.After identifying the major intermediates, three competing pathways for the MoO 2 Cl 2 sulfurization were formulated, as shown in Figure 5k.The O and Cl atoms in MoO 2 Cl 2 were sequentially substituted with S atoms, and according to the replacement order, the reaction route varied from I to III.Each sulfurization path had a rate-determining barrier of 0.55 (I), 0.63 (II) and 0.66 (III) eV, suggesting that route I was the most preferable.The energy barrier of salt-assisted sulfurization (0.55 eV in route I) was much lower than that of Mo 3 O 9 sulfurization (0.95 eV) in saltless CVD, as shown in Figure 5l; therefore, the alkali halide promoted the growth rate of the MoS 2 monolayer.The rate-limiting sulfurization barrier was linearly correlated with the electronegativity of the halogen atom, as shown in Figure 5m.The lower the electronegativity of a halogen element, the easier it is to substitute the Mo-O and Mo-halogen bonds with Mo-S in the oxyhalide molecules; in other words, oxyiodide had the lowest barrier among the four halogen elements (I, Br, Cl, and F).
For the high-quality single-crystalline TMD growth in wafer, Kim et al. [90] reported the confined CVD method of tungsten diselenide (WSe 2 ).A key strategy was the patterning of amorphous silicon oxide (a-SiO 2 ) on the c-plane sapphire (c-Al 2 O 3 ) or amorphous hafnium oxide (a-HfO 2 ) substrates for selectively controlling the growth rate of each area, as depicted in Figure 5n.A single domain of WSe2 nuclei was formed inside the micrometersized pattern and filled the entire trench, yielding a single crystalline array across the 2-inch wafer.To examine the selective growth mechanism in the trench structure, the binding energies of precursor and product molecules on a-SiO 2 , c-Al 2 O 3 , a-HfO 2 substrates were calculated using DFT simulation, as displayed in Figure 5o.The WO 3 (W 3 O 9 ), Se (Se 2 ), and WSe 2 (W 3 Se 9 ) clusters exhibited stronger binding energies on c-Al 2 O 3 and a-HfO 2 substrates, compared to the a-SiO 2 surface.It implies that the precursors and products preferentially adsorb on the c-Al 2 O 3 and a-HfO 2 substrates (inside the trench) instead of the a-SiO 2 surface, leading to the selective growth of WSe 2 .This scenario was experimentally confirmed by the growth selectivity of WSe 2 on these substrates.
Another target process of computational simulations is the etching of 2D materials.Amongst the extraordinary properties of TMDs, the layer-dependent electronic feature [93] is crucial for the on-demand fabrication of nanoelectronics where the band gap changes from indirect (1.29 eV) to direct (1.86 eV) as the TMD material varies from bulk to monolayer, respectively.The direct band gap of the monolayer is also beneficial for strong luminescence and high quantum yield in optoelectronics. [94]In this respect, the elaborate layer control of TMDs is essential for nanodevice applications; however, the surface chemistry during etching is largely unknown.Farigliano et al. performed DFT and AIMD simulations to investigate the mechanism of F 2 gasinduced MoS 2 etching, as shown in Figure 6a-c. [26]Upon exposure to fluorine gas, the F 2 molecules showed dissociative adsorption on the MoS 2 substrate, forming SF and SF 2 groups at the surface, where the altitude of the F 2 gases above the surface decreased rapidly within a few picoseconds, as shown in Figure 6a.At temperatures ranging from 100 to 500 K, the adsorbed F atoms diffused to the adjacent S-top or hollow sites, resulting in SF 3 groups (Figure 6b).In the SF x group-incorporated MoS 2 surface heated at 500 to 1500 K, the SF 3 molecule desorbed and left the S vacancy that was filled by the F atom, releasing the MoF 4 moiety.The desorbed SF 3 and MoF 4 molecules then attracted neighboring F atoms, producing SF 4 , SF 5 , and MoF 6 gases, as shown in Figure 6c.
In the plasma etching of MoS 2, owing to the high kinetic energy and reactivity of the plasma species, wafer-scale manufacturing with a short process time at room temperature is possible, but a highly sophisticated design of the plasma process is essential for atomic-level precision and high purity.In particular, the type of process gas is one of the major factors that determine the elementary reaction and resultant layer quality.Kim et al. [21] utilized DFT-based energetics on plasma-surface interactions for the computational screening of an optimal gas mixture, as shown in Figure 6d-f.For the candidate elements of F, O, Cl, and H in the gas mixture, the etching capability of each species was tested by comparing the formation energies of the plasma radical-induced defective MoS 2 surfaces, as depicted in Figure 6d.Substitution, interstitials, and adsorption are surface defects; and the F, O, and Cl radicals that replaced the S atom of the MoS 2 substrate (F S , O S , and Cl S ) were the most favorable among the other types of defects.Therefore, these elements demonstrated the etching capability for the removal of S with high selectivity.
Another concern in gas screening is the prevention of chemical contamination in which the F residue disturbs the electrical conductivity of MoS 2 from n-to p-type.To characterize F contamination, the adsorption energies of F atoms on the pristine and plasma-treated MoS 2 surfaces were calculated, as shown in Figure 6e.Surfaces where sulfur is substituted with F, O, and Cl radicals (F S , O S and Cl S ) were employed as plasma-induced configurations.It is interesting to note that the O atom showed positive adsorption energy of F (0.04 eV at ⑤ O-top site) suppressing the F residue whereas the other F and Cl elements strongly attracted the F adsorbate at all sites with large negative adsorption energies.The physical background of the surface configurationdependent F adsorption was interpreted in terms of the orbital interaction and associated charge transfer, as shown in Figure 6f.The orbital overlap between the adsorbate and surface and related charge transfer was reduced for the O-replacing surface compared to the F/Cl-substituting case, which led to the blockage of F adsorption.From these data, the F and O species were paired as the process gas elements, having synergetic effects on removing the host atoms from the MoS 2 substrate, together with mitigating the F-impurity.Combined with the physical etchant of Ar, the resulting Ar + CF 4 + O 2 mixture showed uniform layer-by-layer thinning of 4-inch MoS 2 without damaging the atomic structure or chemical impurities in subsequent experiments. [21]inally, the computational study of 2D material processing was extended to ion beam-induced phase transformations.Upon irradiation with ions, the phase of the TMD layers changed from crystalline to amorphous, shifting the electrical conductivity from semiconductor to metal, which lowered the Schottky barrier [95] between the metal contact and TMD channel.Valerius et al. [27] conducted an MD study of the Xe + ion-induced phase change in MoS 2 to optimize the conditions of incidence, as shown in Figure 6g.By irradiating the MoS 2 /graphene/iridium stack with Xe + ions of kinetic energy of 500 eV at a polar (azimuthal) angle of 75°(27.5°),S atoms were sputtered from the top S layer, as shown in Figure 6h.The Mo and bottom S layers were not sputtered, and the graphene and iridium substrates remained intact.The damage due to the bombardment resulted in an amorphized area of 8 Å radius, consisting of the undercoordinated and disordered Mo and S atoms (Figure 6i).Further investigation on the effect of polar angle on sputtering is shown in Figure 6j.The etching; [26] d) Formation energy of the defective MoS 2 surface; e) Adsorption energy of F atom on pristine (black), F-(blue), O-(red), and Cl-substituting surface (green); f) Charge density difference and partial density of states (PDOS) of F-adsorbed systems for gas screening of MoS 2 plasma etch; [21] g) Perspective view of Xe + ion incidence on MoS 2 /graphene/iridium; h) Side view of the same event; i) Top view of the amorphized area by the same event; j) Dependence of sputtering yield of Mo and S atoms according to incident polar angle; k) Dependence of sputtering yield of top-and bottom-S atoms according to incident azimuthal angle for MoS 2 phase change by ion beam. [27]Figures adapted with permission from: a-c) ref. [26], Copyright 2023, Elsevier Publishing Group, d-f) ref. [21], Copyright 2023, ACS Publishing Group, g-k) ref. [27], Copyright 2020, IOP Publishing Group.
sputtering yield had maximum value at 50-65°and 40-60°for the S and Mo atoms, respectively, and the S atom had a higher probability of being sputtered than the Mo atoms by a factor of at least 20 at all polar angles.In addition, the S atoms in the top layer had a larger sputtering yield than those in the bottom layer by a factor of 50 at all azimuthal angles, as shown in Figure 6k.Therefore, the grazing Xe + ion incidence at a polar angle of 75°a morphized the MoS 2 surface effectively, and there were no undesirable occurrences such as underlayer damage, ion implantation, or intermixing in the following experiments. [27]n future computational studies, the key issues that need to be addressed are (i) the development of highly sophisticated inter-atomic potential in MD simulation, and (ii) the coupling between the surface chemistry and the plasma/gas dynamics in the reactor.First, the inter-atomic potential is a mathematical function to compute the potential energy of the atomic system in MD simulation.The reactive force-field (ReaxFF) [96] is one of the widely used potentials in the MD studies, based on the bond order scheme.This bond order formalism consists of empirical parameters fitted from first-principles calculation and accounts for the transition among , , and  bond characters, which enables the description of the bond formation and -dissociation during the reaction events.However, the current ReaxFF parameters partially cover the periodic table, where almost half of transition metal elements cannot be described using the ReaxFF parameter sets.Although the ReaxFF parameters are reported for several atomic elements, one should not directly combine these sets.For the same atomic element in the different chemical environment (i.e., process condition), the empirical parameters should be refitted extensively, indicating its poor transferability.
Alternatively, the AIMD has received great attention for overcoming the drawbacks of ReaxFF MD.The AIMD is independent of empirical parameters and directly solves the Schrödinger equation to calculate the potential energy of the system. [97,98]This quantum mechanically-derived potential energy is used to evolve the atomic trajectory based on the Newtonian equation of motion.Thus, the AIMD deal with full-periodic-table-atoms precisely and ensure transferability across the various phases and environments.However, the AIMD requires huge computational cost limiting the length-and time scale of simulation model (≈1 nm and ≈10 ps) which are much shorter than those of the ReaxFF MD (≈10 nm and ≈1 ns).
The machine learning potential (MLP) [98,99] is a promising strategy to meet the high level of accuracy, cost-effectivity, and transferability simultaneously.The MLP is constructed in a stepby-step procedure: 1) data collection of reference structures with the associated energy and force from first-principles calculation; 2) descriptor setup representing the local atomic environments; 3) regression of the descriptor function to fit the MLP.Several regression methods such as artificial neural network, and kernel-based scheme are employed to predict the potential energy of the input atomic structure, exhibiting high accuracy at DFT level while lowering their computational cost.In this regard, the development-and application of MLP will be a key issue to be considered in future process simulations.
In addition, most of the simulation works in TMD processing are limited to surface chemistry, while there is a lack of understanding in plasma-and gas dynamics in the reactor.Although changes in composition, flux, and kinetic energy of reactive species have significant influence on the surface reaction and the subsequent process results [100] , computational approach for reflecting the plasma/gas dynamics in surface chemistry model has not been reported in the TMD fields, to the best of our knowledge.Therefore, multi-physics simulation combining the electromagnetics (plasma), fluid dynamics, heat transfer, gas chemistry, and surface reaction will be of primary importance to be addressed in the future.

Applications of 2D FETs by Plasma/Gas Etching and Doping Techniques
This section outlines the use of plasma and gas etching techniques for fabricating electronic devices using 2D materials.[107] We have described below a range of electronic applications, starting with 2D FETs as representative devices, and diodes, etch-stop layers, cleaning processes, sensors, and phototransistors.
A 2D FET is an electronic device made from 2D materials such as graphene or TMDs that are ultra-thin and have high carrier mobility enabling effective gate control modulation of carrier densities for fast-switching devices.The device operates by creating an electric field between the gate electrode and a semiconducting channel made of a 2D material, which modifies the conductivity of the channel and contact resistance. [108]This modification can switch a transistor on and off, creating binary digital signals that underpin modern computing.[111][112] Figure 7 depicts the applications of 2D FETs.A self-aligned fabrication technique for recessed-channel WSe 2 FETs was demonstrated using UV-ozone oxidation and chemical etching, as shown in Figure 7a. [101]The self-limiting nature of oxidation resulted in an ALE that thinned out the layers in a layer-bylayer manner, as confirmed by the cross-sectional TEM images presented in Figure 7b.It is to be noted that ALE can also be achieved by oxidation and re-sulfurization that selectively etch 2D heterostructures. [101]The WSe 2 FET with a top oxidized layer exhibited a degenerate p-doped state owing to the strong doping effect of the oxide layer.Following KOH etching, the off-state current was considerably reduced, achieving a high on/off ratio of nearly 10 8 .This process can be repeated to obtain a recessed channel with a high on/off ratio.The WSe 2 FETs with the recessed channel exhibited a high field effect mobility of 184-190 cm 2 V −1 s −1 and low contact resistance of 0.9-6.1 kΩ•μm.
The oxidation technique was employed as a contact doping method to achieve high-performance 2D FETs with low contact resistance.[115] This process protects the underlying layers during oxidation, enabling doping of the underlying channel materials owing to the strong electron-withdrawing behavior of the oxide layer.The utilization of UV-ozone treatments operated at room temperature has been observed to facilitate the self-limited oxidation of monolayer TMDs with notable reproducibility and stability.In contrast, plasma treatments utilize highly reactive species preventing precise control of oxide thickness.Nevertheless, O 2 plasma treatments exhibit a greater time efficiency in generating a self-limiting oxide layer when compared to UV-ozone treatments.Yamamoto et al. successfully utilized self-limiting oxidation via UV-ozone treatment followed by air exposure to fabricate WSe 2 p-FETs with low contact resistance. [116]ir exposure is necessary to mitigate the strong hole-doping effects of the oxide layer, otherwise doped WSe 2 exhibits metallic transport behavior.With air exposure, the WSe 2 FETs demonstrated clear on/off switching behavior and exhibited a contact resistance as low as 66 kΩ•μm.Alternatively, multilayer oxidation was achieved by subjecting TMDs to an oxygen plasma treatment at elevated temperatures.For instance, at 250 °C, a three-layered WO 3-x was formed, leading to increased hole doping density in the underlying WSe 2 and reduction in contact resistance to 0.5 kΩ•μm. [117]As air exposure can significantly degrade the performance of a device, other research groups employed an hBN mask to protect the channel layer from the oxidation process while oxidizing the non-covered contact regions to enhance the contact properties. [118,119]By combining this approach with e-beam irradiation, selective patterning of the doping profile for oxygenplasma-doped WSe 2 was achieved, enabling a narrow p-channel length as short as sub-100 nm.The patterned short channel resulted in a very high saturation current of ≈280 μA μm −1 and Figure 7. Applications of 2D FETs using plasma and gas etching techniques.a) Fabrication process for recessed-channel WSe 2 FETs by UV-ozone oxidation and KOH etching; [101] b) Cross-sectional TEM images of before and after etching process for oxidized WSe 2 ; [101] c) Schematic for short-channel WSe 2 FET formed by e-beam irradiation and its transfer characteristics; [102] d) Schematic and optical images of graphene-passivated MoS 2 (G-MoS 2 ) FET before and after XeF 2 gas exposure; [103] e) Transfer characteristics of G-MoS 2 FETs before and after fluorination process; [103] f) Band diagrams and transfer characteristics of untreated and Ar plasma-treated WSe 2 at the contact; [86] g) Thickness control of BP FET by Ar plasma treatment; [104] h) Etched BP thickness as a function of duration of plasma treatment; [104] Transfer characteristics, mobility, and on/off ratio of BP FET before and after plasma treatment. [104]Figures adapted with permission from: a,b) ref. [101], Copyright 2022, ACS Publishing Group, c) ref. [102], Copyright 2022, Wiley Publishing Group, d,e) ref. [103], Copyright 2022, Wiley Publishing Group, f) ref. [86], Copyright 2021, Wiley Publishing Group, g-i) ref. [104], Copyright 2015, ACS Publishing Group.
an excellent on/off ratio of 10 9 , as shown in Figure 7c. [102]The same research group has recently introduced a novel approach for fabricating lateral p + -p-p + junction FETs by combining van der Waals (vdW) integration and contact spacer doping techniques. [120]Similarly, they employed oxygen plasma doping on WSe 2 and formation of vdW top-gate stack using dry transfer.This process ensures efficient top-gate controllability without the generation of undesired fringing fields.The application of contact spacer doping techniques through plasma treatments demonstrates its capability to yield high-performance 2D p-FETs thanks to the high work-function of the oxide layer. [121,122]elective etching is an important technique for precise patterning and shaping of 2D materials, as it is critical for designing electronic and optoelectronic devices.Ryu et al. developed a unique etching technique that selectively etched hBN in graphene-hBN heterostructures using XeF 2 gas exposure. [103]Graphene was fluorinated by gas exposure and served as an etch stop layer against further etching.Such strong etch selectivity between hBN and graphene enabled the formation of a fluorinated graphene (FG) interlayer on the MoS 2 FETs, as illustrated in Figure 7d.Electrical characterization of Gr-MoS 2 heterostructures with and without XeF 2 exposure clearly demonstrated that effective on/off switching was achieved when FG contacts were formed; otherwise, graphene dominates the carrier transport without fluorination, as shown in Figure 7e.As the FG layer acts as an efficient charge injection layer, the MoS 2 FET exhibited linear output characteristics and a high mobility of 64 cm 2 V −1 s −1 at room temperature.
Kim et al. demonstrated that defect engineering could enhance the contact properties of 2D FETs.They accomplished this by utilizing a mild argon-plasma treatment to induce Se vacancies in WSe 2 . [86]By protecting the channel region with hBN, only the contact regions were exposed to the plasma treatment.This technique not only reduced the thickness of the WSe 2 layers but also produced an n-doping effect, thereby improving carrier injection at the contact, as depicted in the band diagrams presented in Figure 7f.This significant improvement led to an increase in the on/off ratio of the WSe 2 FETs from 12.8 to 10 7 , as shown in Figure 7f.
Black phosphorus (BP) is a layered 2D semiconductor that has gained attention in recent years owing to its p-type polarity, Figure 8.Other electronic applications using plasma and gas etching techniques.a) Schematic of cycle of thinning process for BP by cyclic oxidation/annealing; [126] b) I D -V DS characteristics and band diagrams of thick/thin BP heterostructures, showing rectifying behavior; [126] c) Schematic and optical images for SnS/SnS 2 heterostructure diode; [127] d) Rectifying behavior and photoresponse of diode; [127] e) Schematic, optical, and crosssectional TEM images of XeF 2 etching for Gr-heterostructure; [128] f) ALE of WSe 2 by UV-ozone oxidation and KOH treatments.The ALE technique enables clean surface of graphene devices with high field effect mobility; [84] g) Illustration of oxygen-doped MoSe 2 nanosheets and h) its real-time response for TMA at various concentrations; [129] i) Schematic of a WSe 2 phototransistor with asymmetric split gates configuration to enhance the photocurrent. [130]igures adapted with permission from: a,b) ref. [126], Copyright 2017, ACS Publishing Group, c,d) ref. [127], Copyright 2018, Nature Publishing Group, e) ref. [128], Copyright 2018, Nature Publishing Group, f) ref. [84], Copyright 2021, ACS Publishing Group, g,h) ref. [129], Copyright 2020, Springer Publishing Group, i) ref. [130], Copyright 2022, Wiley Publishing Group.
anisotropic transport, and high carrier mobility that approaches the ballistic limit. [123,124]However, BP is known to be highly airsensitive and can rapidly degrade upon exposure to ambient air because its surface is easily oxidized in the presence of oxygen and moisture, leading to the formation of various oxidation products such as phosphorus oxides. [125]The formation of an oxide layer significantly degrades the electrical properties of BP FETs owing to the introduction of impurities.Jia et al. developed a modulated plasma treatment that simultaneously reduced the thickness of the BP and eliminated chemical degradation. [104]igure 7g illustrates their approach, which involved using Ar plasma treatment to thin the layer and remove impurities from the BP surface.The treated BP was subsequently passivated using PMMA to prevent further oxidation by air.The thickness of the etched BP increased linearly with increasing duration of the plasma treatment (Figure 7h).By removing the charged impurities, the on/off ratio and field effect mobility increased to 10 5 and 415 cm 2 V −1 s −1 , respectively, as shown in Figure 7i.
Figure 8 outlines other electronic applications of 2D materials using plasma/gas-etching techniques.A diode is an electronic component that allows an electric current to flow in one direction only.It is made up of semiconductor materials and typically has a junction between two regions with different types of doping or band structures.Robbins et al. accomplished cyclical thin-ning of BP via surface oxidation and vacuum annealing leading to a change in its band structure. [126]To achieve the desired BP thickness, a thin Al layer was deposited on the patterned BP and oxidized in air to create Al 2 O 3 , which is a protective layer that prevents further air degradation.Cyclical thinning was then carried out by repeated oxidation and annealing until the required thickness was obtained, as shown in Figure 8a.ALD was conducted in situ after the last annealing step to deposit a thick protective Al 2 O 3 layer that covered the entire sample.As the band gap of BP is significantly dependent on its thickness, the formation of a thin/thick heterojunction results in an asymmetric band structure with rectifying behavior, as shown in Figure 8b.
Other research groups explored the use of plasma treatment to create lateral 2D p-n junctions to achieve rectifying behavior in diodes.A lateral homogeneous p-n junction diode was fabricated by combining edge and surface contacts with WSe 2 . [131]During the SF 6 /O 2 plasma etching process, amorphous tungsten oxide forms at the etched edge of WSe 2 , inducing p-type semiconducting behavior.As WSe 2 with surface contacts is an n-type semiconductor, the asymmetric contact strategy yielded rectifying behavior with a good ideality factor owing to hBN encapsulation and the homogeneous junction.Another approach to creating a p-n junction in a diode is through the plasma-etching-induced phase transformation of SnS 2 , as illustrated in Figure 8c. [127]Ar-plasma treatment was used to remove the sulfur atoms from the surface of SnS 2 resulting in the transformation of SnS 2 to SnS.As SnS 2 and SnS are n-type and p-type semiconductors, respectively, a vertical p-n diode was naturally constructed through Ar plasma treatment.The diode exhibited typical rectifying behavior and photocurrent under white-light exposure, as shown in Figure 8d.
Selective etching is a crucial process in the design of electronic and optoelectronic devices, particularly when using 2D materials that are highly susceptible to surface-reactive processes.Son et al. developed an atomically precise etch stop for 3D integrated systems by using FG. [128]The strong etch selectivity between graphene and hBN to XeF 2 gas exposure was employed to shape the 2D van der Waals heterostructures, as illustrated in Figure 8e.As seen in the cross-sectional TEM images, the hBN without the top graphene layer was fully etched whereas the hBN underneath the graphene layer was protected by the FG.By utilizing FG as a protective layer and electrodes, graphene FETs exhibited a room temperature mobility of 40 000 cm 2 V −1 s −1 at a carrier density of 4 × 10 12 cm −2 , owing to hBN encapsulation and protection from XeF gas exposure.Graphene etching not only simplifies device fabrication by a one-step etching process, but also enables the creation of complex 3D integrated systems such as connecting multiple active layers using interlayer vias.
Atomically precise etching was also used for cleaning surfaces.Nipane et al. demonstrated the damage-free ALE of WSe 2 through self-limiting UV-ozone oxidation and subsequent KOH treatment, as shown in Figure 8f. [84]A UV-ozone treatment system was developed to eliminate polymer residues induced during fabrication of a device.Thus, the UV-ozone treatment not only oxidized the top layer of WSe 2 but also cleaned its surface.Following KOH treatment to etch the top layer of oxidized WSe 2 , the underlying layer (graphene), remained intrinsic and exhibited approximately three times higher mobility than conventionally processed devices.
Figure 8g-i illustrates sensors and photonic devices as possible applications of plasma/gas etching techniques for 2D materials.The efficiency of such devices is expected to improve by increasing the specific surface and light absorption areas through etching.As shown in Figure 8g,h, oxygen-doped MoSe 2 hierarchical nanostructures were formed through a mild calcination treatment at elevated temperatures in air. [129]The MoSe 2 nanosheets treated at 200 °C exhibited a chemical sensing response with a detection limit of 8 ppb owing to their large specific surface areas.This oxygen-doping process could possibly be replicated by a mild O 2 plasma treatment, creating defects, and introducing oxygen atoms into the crystal structure to develop high-performance sensors.Another potential application is the enhancement of photodetection in photonic devices with formation of repeated junctions using plasma/gas etching techniques.Ra et al. achieved enhanced photocurrent generation using an asymmetric split-gate configuration, as shown in Figure 8i. [130]his structure enabled the effective modulation of the potential profile of the WSe 2 channel resulting in the formation of multiple p-n junctions.The increased number of junctions led to significantly enhanced photocurrent generation by suppressing the e-h pair recombination.The formation of multiple junctions can potentially be replicated by ALE combined with oxidation doping.

Outlook and Conclusions
Semiconductor manufacturing is at the forefront of technological innovation, driven by the adoption of plasma/gas-based processes.As the physical limits of semiconductor line width reduction have been reached, processes utilizing plasma have become essential for innovation in manufacturing process technology.In conventional methods like CVD synthesis, high-temperature process conditions cause damage and defects in semiconductor materials, and wet etching have proven inadequate for fine patterning.Tailoring plasma processes to specific materials is paramount, emphasizing the need for optimization, precise analysis, and a comprehensive grasp of etching mechanisms.Particularly, the plasma process such as PECVD and RIE is highly versatile to synthesis and etching of 2D material, which is one of the most promising next-generation channel materials due to its unique physical properties.Among the families of 2D materials, TMDs possesses an atomic-level thickness, high current on-off ratio and a bandgap similar to silicon.It can be directly employed without fine patterning, reducing semiconductor manufacturing costs.
For the high-quality 2D materials processing, computational approaches have emerged as indispensable tools in semiconductor manufacturing.Computational simulation explores plasma/gas-surface interactions during the process of synthesis, etching, and phase change of TMDs and reveals the underlying mechanism.Comprehensive studies of DFT and MD simulation identified essential chemical species, reaction energetics, and associated kinetics, covering a broad range of surface chemistry from physical collision, adsorption, diffusion, and desorption.Based on the understanding of the detailed chemistries, the computational model predicts the process quality and facilitates the rational design of TMD process such as optimal gas and salt screening.
The plasma/gas-based semiconductor technologies are also commonly employed to produce various electronic components.Plasma enables the efficient and controlled removal of materials from semiconductor surfaces through etching, facilitating the production of complicated micro-/nano-electronic devices.Gasbased semiconductor technology also employs CVD to deposit 2D semiconductor materials.PECVD expedites the chemical reactions and encourages efficient, uniform, and pure deposition.Plasma and gas-based technologies play a pivotal role in modern electronic devices and are continuously refined to enhance efficiency, reliability, and adaptability.In this regard, we elucidated multiple electronic applications and processing for 2D materials, including FETs, diodes, as well as etch-stop and cleaning processes, achieved through the utilization of plasma/gas-based etching methods.Moreover, we proposed potential applications in the realm of sensors and photonic devices, making use of plasma/gas etching techniques for 2D materials.
In conclusion, the semiconductor industry's relentless pursuit of advancement necessitates the continued refinement of plasma/gas-based techniques, the application of emerging simulation approaches, the development of diagnostic capabilities, and the exploration of alternative materials to sustain progress in semiconductor manufacturing for further technological breakthroughs.

Figure 2 .
Figure 2. Classification of physical and chemical plasma etching of 2D materials.a) Relative atomic percentages for C, O, and Cl derived from XPS spectra, photoemission spectra of Parylene C after 20 min O 2 plasma and 30 min Ar plasma treatment;[70] b) AFM results and frequency differences showing a stepwise decrease as the number of layers decreased;[71] c) Comparison of transmittance spectra of individual FS substrates before and after ion etching and comparison of FS surface roughness;[72] d) Results of etch rate and Ar concentration after Cl 2 /Ar gas mixture plasma etching;[37] e) Schematic of MoS 2 device, transfer characteristics, and variation of field-effect mobilities of devices based on tri-, bi-, and mono-layer MoS 2 obtained from plasma etching system;[73] f) Raman spectra and XPS narrow scan data of Mo 3d, S 2p, and F 1s before/after treatment using a H 2 S plasma;[36] g) Plan-view HIM images of lines spaced with pitches of 22, 20, 18, and 16 nm on silicon substrate;[74] h) Analysis of Y 2 O 3 films and SF 6 plasma-treated films before and after fluorocarbon plasma etching by XPS.[75] Figures adapted with permission from: a) ref.[70],Copyright 2015, Wiley Publishing Group; b) ref.[71],Copyright 2016, IOP Publishing Group; c) ref.[72],Copyright 2017, Optica Publishing Group; d) ref.[37],Copyright 2004, IOP Publishing Group; e) ref.[73],Copyright 2020, Wiley Publishing Group; f) ref.[36],Copyright 2015, IOP Publishing Group; g) ref.[74],Copyright 2019, ACS Publishing Group; h) ref.[75],Copyright 2020, MDPI Publishing Group.
shows the sublimation of the MoO 3 surface, forming ring-like trimolybdenum nonaoxide (Mo 3 O 9 ) at 1700 K.In S 2 atmosphere at a high temperature of 2500 K, the bridge O atoms in Mo 3 O 9 interacted with S 2 gases to open the ring structure and convert it into a linear Mo 3 -containing chain, as shown in Figure 5h.Further sulfurization broke the Mo 3 -chain, leading to a shorter Mo 2 -containing chain and fully separated MoO x S y molecules.Based on the intermediates and products in the dynamic simulation, the detailed energetics of Mo 3 O 9 sulfurization were investigated using DFT, as illustrated in Figure 5i.The ring opening of Mo 3 O 9 by S 2 gas showed moderate activation energy (≈0.8 eV) and exothermic energy release (0.52 eV).The breakage of the Mo 3 -containing chain into the Mo 2 -chain and MoO 3 S 2 monomers resulted in a relatively high energy barrier (0.95 eV) and an endothermic energy increase, suggesting a rate-limiting step of sulfurization.The O atoms in MoO 3 S 2 were gradually replaced by S atoms, resulting in fully sulfurized MoS 6 with an activation energy of 0.7 eV.The MoS 6 acted as a building molecule attaching to the bare S-terminated Mo-zigzag edge and closest S near the kink site (Figure 5j), having energy barriers of 1.38 and 0.93 eV, respectively.After the fusion of one MoS 2 -unit with the crystal, an S 4 molecule was released, and the kink site of the lattice propagated during the synthesis.

Figure 6 .
Figure 6.Computational approaches for etching and phase engineering of 2D materials.a) Variation of z-coordinate of F 2 molecules from the MoS 2 surface; b) Diffusion trajectories of F atoms on the surface plane; c) Atomistic snapshots of SF x group-incorporated surface for F 2 gas-induced MoS 2etching;[26] d) Formation energy of the defective MoS 2 surface; e) Adsorption energy of F atom on pristine (black), F-(blue), O-(red), and Cl-substituting surface (green); f) Charge density difference and partial density of states (PDOS) of F-adsorbed systems for gas screening of MoS 2 plasma etch;[21] g) Perspective view of Xe + ion incidence on MoS 2 /graphene/iridium; h) Side view of the same event; i) Top view of the amorphized area by the same event; j) Dependence of sputtering yield of Mo and S atoms according to incident polar angle; k) Dependence of sputtering yield of top-and bottom-S atoms according to incident azimuthal angle for MoS 2 phase change by ion beam.[27]Figures adapted with permission from: a-c) ref.[26],Copyright 2023, Elsevier Publishing Group, d-f) ref.[21],Copyright 2023, ACS Publishing Group, g-k) ref.[27],Copyright 2020, IOP Publishing Group.

Table 1 .
Different synthesis/etching methods for 2D materials with various parameters.