Ankit Mehta - Academia.edu (original) (raw)

Papers by Ankit Mehta

Journal of Materials Chemistry C, 2021

Journal of Chemical Physics, Mar 11, 2019

Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase pr... more Monolayer-thin WS2 with (0002) texture grows by chemical vapor deposition (CVD) from gas-phase precursors WF6 and H2S at a deposition temperature of 450 °C on 300 mm Si wafers covered with an amorphous Al2O3 starting surface. We investigate the growth and nucleation mechanism during the CVD process by analyzing the morphology of the WS2 crystals. The CVD process consists of two distinct growth regimes. During (i) the initial growth regime, a fast and self-limiting reaction of the CVD precursors with the Al2O3 starting surface forms predominantly monolayer-thin WS2 crystals and AlF3 crystals that completely cover the starting surface. During (ii) the steady-state growth regime, a much slower, anisotropic reaction on the bottom, first WS2 layer proceeds with the next WS2 layer growing preferentially in the lateral dimensions. We propose that the precursor adsorption reaction rate strongly diminishes when the precursors have no more access to the Al2O3 surface as soon as the WS2 layer completely covers the Al2O3 surface, and that the WS2 crystal basal planes and AlF3 crystals have a low reactivity for WF6 adsorption at 450 °C. Nonetheless, a second layer of WS2 starts to form before the first WS2 layer completely covers the starting surface, albeit the surface coverage of the second layer is low (< 20%, after 25 minutes of CVD reaction). During the steady-state growth regime, predominantly the WS2 crystals in the second monolayer continue to grow in lateral dimensions up to ~40 nm. These crystals reach larger lateral dimensions compared to the crystals in the bottom, first layer due to low reactivity for WF6 adsorption on the WS2 basal plane compared to Al2O3. Presumably, they grow laterally by precursor species that adsorb on and diffuse across the WS2 surface, before being incorporated at the more reactive edges of the WS2 crystals in the second layer. Such process proceeds slowly with only up to 40 % surface coverage of the second WS2 layer after 150 minutes of CVD reaction. The CVD reaction is mediated by the starting surface: WF6 precursor preferentially adsorbs on Al2O3, whereas adsorption is not observed on SiO2. Nevertheless, WS2 grows on SiO2 in close proximity with Al2O3 in 90 nm pitch Al2O3/SiO2 line patterns. Hence, functionalization of the starting surface (e.g., SiO2 with Al2O3) can provide opportunities to grow monolayer-thin WS2 crystals at predetermined locations by selective, lateral growth with tunable crystal size, even at low deposition temperatures.

npj 2D Materials and Applications, 2021

The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future ... more The cleaning of two-dimensional (2D) materials is an essential step in the fabrication of future devices, leveraging their unique physical, optical, and chemical properties. Part of these emerging 2D materials are transition metal dichalcogenides (TMDs). So far there is limited understanding of the cleaning of “monolayer” TMD materials. In this study, we report on the use of downstream H2 plasma to clean the surface of monolayer WS2 grown by MOCVD. We demonstrate that high-temperature processing is essential, allowing to maximize the removal rate of polymers and to mitigate damage caused to the WS2 in the form of sulfur vacancies. We show that low temperature in situ carbonyl sulfide (OCS) soak is an efficient way to resulfurize the material, besides high-temperature H2S annealing. The cleaning processes and mechanisms elucidated in this work are tested on back-gated field-effect transistors, confirming that transport properties of WS2 devices can be maintained by the combination of...

The Journal of Physical Chemistry C, 2020

Grain boundaries between 60° rotated and twinned crystals constitute the dominant type of extende... more Grain boundaries between 60° rotated and twinned crystals constitute the dominant type of extended line defects in two-dimensional transition metal dichalcogenides (2D MX2) when grown 2 on a single crystalline template through van der Waals epitaxy. The two most common 60°grain boundaries in MX2 layers, i.e., -and -boundaries, introduce distinct distortion and strain into the 2D lattice. They impart a localized tensile or compressive strain on the subsequent layer, respectively, due to van der Waals coupling in bilayer MX2 as determined by combining atomic resolution electron microscopy, geometric phase analysis and density functional theory. Based on these observations, an alternate route to strain engineering through controlling intrinsic van der Waals forces in homo-bilayer MX2 is proposed. In contrast to commonly used external means, this approach enables localized application of strain to tune electronic properties of the 2D semiconducting channel in ultra-scaled nanoelectronic applications. Owing to their unique optical and electronic properties, two-dimensional (2D) transition metal dichalcogenides (MX2, with M a transition metal, and X a chalcogen) have garnered significant interest in the past decade for applications in next-generation nanoelectronics. Molybdenum disulfide (MoS2) is the most investigated member of the MX2 family. In its bulk form, MoS2 has a layered structure in which individual Mo atoms are covalently bound to S atoms within a single layer; while the individual layers are coupled by a weak van der Waals interaction. MoS2 is an ntype semiconductor with a layer-dependent bandgap ranging from 1.2 to 1.9 eV. The band gap decreases with the number of layers and evolves to a direct band-gap in a single layer due to 2D-confinement1-3. Apart from this layer thickness dependence, 2D MX2 optoelectronic properties are strongly influenced by changes to the atomic arrangement, such as polymorphism,4,5 stacking differences,6-9 presence of grain-boundaries10-14 and strain.15-21 Introducing lattice strain in atomically thin materials emerges as a promising route to modify a wide range of their properties including electrical,15,16 optical,17-19 magnetic20 and catalytic.21

2D Materials, 2020

As interest in layered van der Waals (vdW) materials keeps increasing, fundamental knowledge abou... more As interest in layered van der Waals (vdW) materials keeps increasing, fundamental knowledge about their synthesis is gaining more and more value. The defect-free heteroepitaxial integration of vdW materials on large-area substrates is currently thoroughly being researched since it might encompass a successful transition of these materials to industrial applications. To date, Transition Metal Dichalcogenides (TMDs) are considered as one of the most promising vdW materials within the field of nanoelectronics. Nevertheless, the electrical characterization of heteroepitaxially grown TMDs still shows inferior performance as compared to exfoliated TMD flakes. This is mainly attributed to the high density of defects resulting from their challenging vdW heteroepitaxial synthesis. In this work, we have investigated in depth the vdW homoepitaxial synthesis of the WSe2 TMD compound. We have demonstrated that even for homoepitaxy, the simplest type of crystal growth, challenges such as the formation of 60 o twins need to be addressed. We evidenced the presence of 60 o twins during vdW homoepitaxy which is assigned to stacking faults. The formation of these stacking faults is associated with their very similar binding energy as revealed by Density Functional Theory (DFT) calculations. Therefore, stacking faults are identified in this work to be the fundamental limitation of lowly-defective TMD vdW epitaxy. Furthermore, a generalized model is developed that determines the lower limit of the defect density based on the degree of control on the bilayer stacking phase and the nucleation density of the TMD compound. This model therefore assesses and quantifies for the first time the ultimate defect density level that can be achieved with vdW epitaxially grown 2D materials.

2D Materials, 2018

Tin disulfide (SnS2) is a n-type semiconductor with a hexagonally layered crystal structure and h... more Tin disulfide (SnS2) is a n-type semiconductor with a hexagonally layered crystal structure and has promising applications in nanoelectronics, optoelectronics and sensors. Such applications require the deposition of SnS2 with controlled crystallinity and thickness control at monolayer level on large area substrate. Here, we investigate the nucleation and growth mechanism of two-dimensional (2D) SnS2 by chemical vapor deposition (CVD) using SnCl4 and H2S as precursors. We find that the growth mechanism of 2D SnS2 is different from the classical layerby-layer growth mode, by which monolayer-thin 2D transition metal dichalcogenides can be formed. In the initial nucleation stage, isolated 2D SnS2 domains of several monolayers high are formed. Next, 2D SnS2 crystals grow laterally while keeping a nearly constant height until layer closure is achieved, due to the higher reactivity of SnS2 crystal edges than basal planes. We infer that the thickness of the 2D SnS2 crystals is determined by the height of initial SnS2 islands. After layer closure, SnS2 grows on grain boundaries and results in 3D growth mode, accompanied by spiral growth. Our findings suggest an approach to prepare 2D SnS2 with a controlled thickness of several monolayers and add more knowledge on the nucleation and growth mechanism of 2D materials.

Journal of Materials Chemistry C, 2018

Catalytic decomposition of H2S by SnS2, with generation of H2, plays a critical role in the SnCl4... more Catalytic decomposition of H2S by SnS2, with generation of H2, plays a critical role in the SnCl4/H2S CVD process.

Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2018

The structure, crystallinity, and properties of as-deposited two-dimensional (2D) transition meta... more The structure, crystallinity, and properties of as-deposited two-dimensional (2D) transition metal dichalcogenides are determined by nucleation mechanisms in the deposition process. 2D materials grown by atomic layer deposition (ALD) in the absence of a template are polycrystalline or amorphous. Little is known about their nucleation mechanisms. Therefore, the nucleation behavior of WS2 during plasma enhanced ALD from WF6, H2 plasma, and H2S at 300 °C is investigated on amorphous ALD Al2O3 starting surface and on monocrystalline, bulk sapphire. Preferential interaction of the precursors with the Al2O3 starting surface promotes fast closure of the WS2 layer. The WS2 layers are fully continuous at WS2 content corresponding to only 1.2 WS2 monolayers. On amorphous Al2O3, (0002) textured and polycrystalline WS2 layers form with grain size of 5 to 20 nm due to high nucleation density (∼1014 nuclei/cm2). The WS2 growth mode changes from 2D (layer-by-layer) growth on the initial Al2O3 surf...

Chemistry of Materials, 2017

Two-dimensional (2D) transition metal dichalcogenides are potential low dissipative semiconductor... more Two-dimensional (2D) transition metal dichalcogenides are potential low dissipative semiconductor materials for nanoelectronic devices. Such applications require the deposition of these materials in their crystalline form and with controlled number of monolayers on large area substrates, preferably using growth temperatures compatible with temperature sensitive structures. This paper presents a low temperature Plasma Enhanced Atomic Layer Deposition (PEALD) process for 2D WS2 based on a ternary reaction cycle consisting of consecutive WF6, H2 plasma and H2S reactions. Strongly textured nanocrystalline WS2 is grown at 300 °C. The composition and crystallinity of these layers depends on the PEALD process conditions, as understood by a model for the redox chemistry of this process. The H2 plasma is essential for the deposition of WS2 as it enables the reduction of-W 6+ Fx surface species. Nevertheless, the impact of sub-surface reduction reactions needs to be minimized to obtain WS2 with well-controlled composition (S/W ratio of two).

The data sets and the processing script (jupyter notebooks) are part of the publication "&lt... more The data sets and the processing script (jupyter notebooks) are part of the publication "Wide field of view crystal orientation mapping of layered materials". The programming language used to process the data is written with python 3. The experiments were performed on a JEOL JSM-5110 SEM at 20kV. The direct electron detector used was minipix camera of Advacam. In the notebooks, the data processing of the different 4D-STEM dataset is performed using the pixstem library and some self written packages. At the end of the processing, information on orientation, strain and multilayers of 2D materials (graphene, graphene oxide and MoS2) is retrieved. For the orientation mapping, the grain boundaries and average grain orientation are identified where due to the large field of view a large number of statistics on the grains is retrieved. Dataset 1: 512x512 scan of graphene with a field of view of 3600 um2 &l...

Journal of microscopy, 2017

The crystal and defect structure of SnS crystals grown using chemical vapour deposition for appli... more The crystal and defect structure of SnS crystals grown using chemical vapour deposition for application in electronic devices are investigated. The structural analysis shows the presence of two distinct crystal morphologies, that is thin flakes with lateral sizes up to 50 μm and nanometer scale thickness, and much thicker but smaller crystallites. Both show similar Raman response associated with SnS. The structural analysis with transmission electron microscopy shows that the flakes are single crystals of α-SnS with [010] normal to the substrate. Parallel with the surface of the flakes, lamellae with varying thickness of a new SnS phase are observed. High-resolution transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), first-principles simulations (DFT) and nanobeam diffraction (NBD) techniques are employed to characterise this phase in detail. DFT results suggest that the phase is a strain stabilised β' one grown epitaxially on the α-SnS crys...