Typical Chemical Vapor Deposition Research Articles

Stoichiometry-Tunable Synthesis and Magnetic Property Exploration of Two-Dimensional Chromium Selenides.

Emerging 2D chromium-based dichalcogenides (CrXn (X = S, Se, Te; 0 < n ≤ 2)) have provoked enormous interests due to their abundant structures, intriguing electronic and magnetic properties, excellent environmental stability, and great application potentials in next generation electronics and spintronics devices. Achieving stoichiometry-controlled synthesis of 2D CrXn is of paramount significance for such envisioned investigations. Herein, we report the stoichiometry-controlled syntheses of 2D chromium selenide (CrxSey) materials (rhombohedral Cr2Se3 and monoclinic Cr3Se4) via a Cr-self-intercalation route by designing two typical chemical vapor deposition (CVD) strategies. We have also clarified the different growth mechanisms, distinct chemical compositions, and crystal structures of the two type materials. Intriguingly, we reveal that the ultrathin Cr2Se3 nanosheets exhibit a metallic feature, while the Cr3Se4 nanosheets present a transition from p-type semiconductor to metal upon increasing the flake thickness. Moreover, we have also uncovered the ferromagnetic properties of 2D Cr2Se3 and Cr3Se4 below ∼70 K and ∼270 K, respectively. Briefly, this research should promote the stoichiometric-ratio controllable syntheses of 2D magnetic materials, and the property explorations toward next generation spintronics and magneto-optoelectronics related applications.

High yield and wide lateral size growth of α-Mo2C: exploring the boundaries of CVD growth of bare MXene analogues

Synthesis of Mo2C bare MXenes, without surface terminations groups, via chemical vapor deposition (CVD) on metal foils is scientifically a very intriguing crystal growth process, and there are still challenges and limited fundamental understanding to overcome to obtain high yield and wide crystal size lateral growth. Achieving large area coverage via direct growth is scientifically vital to utilize the full potential of their unique properties in different applications. In this study, we sought to expand the boundaries of the current CVD growth approach for Mo2C MXenes and gain insights into the possibilities and limitations of large area growth, with a particular focus on controlling Mo concentration. We report a facile modification of their typical CVD growth protocol and show its influence on the Mo2C synthesis, with growth times spanning up to 3 h. Specifically, prior to initiating the CVD growth process, we introduced a holding step in temperature at 1095 °C. This proved to be beneficial in increasing the Mo concentration on the liquid Cu growth surface. We achieved an average Mo2C crystals coverage of approximately 50% of the growth substrate area, increased tendency of coalescence and merging of individual flakes, and lateral flake sizes up to 170 μm wide. To gain deeper understanding into their CVD growth behavior, we conducted a systematic investigation of the effect of several factors, including (i) a holding step time on Mo diffusion rate through molten Cu, (ii) the Cu foil thickness over the Mo foil, and (iii) the CVD growth time. Phase, chemical and microstructural characterization by x-ray diffraction, x-ray photon spectroscopy, SEM and scanning/transmission electron microscopy revealed that the grown crystals are single phase α-Mo2C. Furthermore, insights gained from this study sheds light on crucial factors and inherent limitations that are essential to consider and may help guide future research progress in CVD growth of bare MXenes.

A Wrinkling and Etching-Assisted Regrowth Strategy for Large-Area Bilayer Graphene Preparation on Cu.

Bilayer graphene is a contender of interest for functional electronic applications because of its variable band gap due to interlayer interactions. Graphene growth on Cu is self-limiting, thus despite the fact that chemical vapor deposition (CVD) has made substantial strides in the production of monolayer and single-crystal graphene on Cu substrates, the direct synthesizing of high-quality, large-area bilayer graphene remains an enormous challenge. In order to tackle this issue, we present a simple technique using typical CVD graphene growth followed by a repetitive wrinkling-etching-regrowth procedure. The key element of our approach is the rapid cooling process that causes high-density wrinkles to form in the monolayer area rather than the bilayer area. Next, wrinkled sites are selectively etched with hydrogen, exposing a significant portion of the active Cu surface, and leaving the remaining bilayer areas, which enhance the nucleation and growth of the second graphene layer. A fully covered graphene with 78 ± 2.8% bilayer coverage and a bilayer transmittance of 95.6% at room temperature can be achieved by modifying the process settings. Bilayer graphene samples are examined using optical microscopy (OM), scanning electron microscopy (SEM), Raman spectroscopy, and an atomic force microscope (AFM) during this process. The outcomes of our research are beneficial in clarifying the growth processes and future commercial applications of bilayer graphene.

Superconformal silicon carbide coatings via precursor pulsed chemical vapor deposition

In this work, silicon carbide (SiC) coatings were successfully grown by pulsed chemical vapor deposition (CVD). The precursors silicon tetrachloride (SiCl4) and ethylene (C2H4) were not supplied in a continuous flow but were pulsed alternately into the growth chamber with H2 as a carrier and a purge gas. A typical pulsed CVD cycle was SiCl4 pulse—H2 purge—C2H4 pulse—H2 purge. This led to growth of superconformal SiC coatings, which could not be obtained under similar process conditions using a constant flow CVD process. We propose a two-step framework for SiC growth via pulsed CVD. During the SiCl4 pulse, a layer of Si is deposited. In the following C2H4 pulse, this Si layer is carburized, and SiC is formed. The high chlorine surface coverage after the SiCl4 pulse is believed to enable superconformal growth via a growth inhibition effect.

Strongly suppressed diffuse scattering in periodic graphene metamaterials.

As an emerging two-dimensional material, graphene offers an alternative material platform for exploring new metamaterial phenomena and device functionalities. In this work, we examine diffuse scattering properties in graphene metamaterials. We take periodic graphene nanoribbons as a representative example and show that diffuse reflection in graphene metamaterials as dominated by diffraction orders is restricted to wavelengths less than that of first-order Rayleigh anomaly, and is enhanced by plasmonic resonances in graphene nanoribbons, as similar to metamaterials made of noble metals. However, the overall magnitude of diffuse reflection in graphene metamaterial is less than 10-2 due to the large period to nanoribbon size ratio and ultra-thin thickness of the graphene sheet, which suppress the grating effect from the structural periodicity. Our numerical results indicate that, in contrast to the cases of metallic metamaterials, diffuse scattering plays a negligible role in spectral characterization of graphene metamaterials in cases with large resonance wavelength to graphene feature size ratio, which corresponds to typical chemical vapor deposition (CVD)-grown graphene with relatively small Fermi energy. These results shed light on fundamental properties of graphene nanostructures and are helpful in designing graphene metamaterials for applications in infrared sensing, camouflaging, and photodetection, etc.

The Importance of Decarbonylation Mechanisms in the Atomic Layer Deposition of High-Quality Ru Films by Zero-Oxidation State Ru(DMBD)(CO)3.

Achieving facile nucleation of noble metal films through atomic layer deposition (ALD) is extremely challenging. To this end, η4 -2,3-dimethylbutadiene ruthenium(0) tricarbonyl (Ru(DMBD)(CO)3 ), a zero-valent complex, has recently been reported to achieve good nucleation by ALD at relatively low temperatures and mild reaction conditions. The authors study the growth mechanism of this precursor by in situ quartz-crystal microbalance and quadrupole mass spectrometry during Ru ALD, complemented by ex situ film characterization and kinetic modeling. These studies reveal that Ru(DMBD)(CO)3 produces high-quality Ru films with excellent nucleation properties. This results in smooth, coalesced films even at low film thicknesses, all important traits for device applications. However, Ru deposition follows a kinetically limited decarbonylation reaction scheme, akin to typical chemical vapor deposition processes, with a strong dependence on both temperature and reaction timescale. The non-self-limiting nature of the kinetically driven mechanism presents both challenges for ALD implementation and opportunities for process tuning. By surveying reports of similar precursors, it is suggested that the findings can be generalized to the broader class of zero-oxidation state carbonyl-based precursors used in thermal ALD, with insight into the design of effective saturation studies.

ALD-Based Synthesis of Wafer-Scale Transition Metal Dichalcogenides Towards Nanoelectronics and Optoelectronics Applications

Two-dimensional (2D) transition metal dichalcogenides (TMDs), such as MoS2 and WS2 have been widely studied due to their unique physical and electronic properties even at one atomic layer thickness. Since the first report on the MoS2 field-effect transistor (FET) device [1], great efforts have been made in the past decade to extend the intriguing features of TMDs to practical applications. However, such integration has been severely bottlenecked by the lack of effective approaches to achieve wafer-scale, uniform, crystalline and stoichiometric TMD films.Till now, it is still prevalent to fabricate TMD-based electronic devices using mechanical exfoliation methods. Although single-crystal flakes with least defects can be obtained, the size, thickness and location of the exfoliated TMD flakes are largely uncontrollable, making it unsuitable for large-scale device integration and massive production. So far, various synthetic approaches have been proposed to grow large-area TMD thin films. Chemical vapor deposition (CVD)-based method is one of the major routines to synthesize high-quality monolayer and few-layer TMD flakes [2-4]. For example, in typical CVD processes to grow MoS2, the films can be achieved either by the reduction reaction between the S and MoO3 source powders or by sulfurizing the Mo (or MoOx) films pre-deposited by using physical vapor deposition. However, the nucleation sites in the CVD process are uncontrollable which may lead to the vertical stacking of the MoS2 flakes in triangle shape deteriorating the crystalline properties of the film. On the other hand, the pre-deposited Mo or MoOx film by using e-beam evaporation or sputtering usually suffer from rough surface morphology and unsatisfactory film uniformity which further limit the homogeneity of the large-scale film and device integration.Atomic layer deposition (ALD) is a surface-controlled film fabrication and can provide a possible route towards the synthesis of TMD thin films since the ALD approach follows the layer-by-layer deposition mechanism [5,6]. As compared to the conventional CVD method, ALD can enable precise thickness control on atomic scale and excellent film uniformity as well as good stoichiometry and crystallinity with proper annealing steps. In this work, we employed the ALD-based synthesis method to grow wafer-scale MoS2 and WS2 thin films. Non-toxic MoCl5, WCl5 and hexamethyldisilathiane (HMDST) precursors have been used which are different and superior to those used in previous work. Film characterizations have confirmed the wafer-level uniformity, crystallinity and stoichiometry of the synthesized films. Homogeneous electrical behaviors have also been obtained from the fabricated FET device arrays. In addition, optoelectronic devices such as photodetectors and simple logic gates such as n-type inverter arrays have been demonstrated using the wafer-scale MoS2 FET arrays. Furthermore, we have used the substitutional doping in the ALD process to achieve controllable p-type doping of MoS2 films, which can provide solid basis for the complimentary metal-oxide-semiconductor (CMOS) integrated circuit applications. This work shows that the ALD-based synthesis of the 2D TMD thin films is an attractive approach and has provided a promising platform to further push forward the circuit and system level applications of the TMD materials.

Nanoscale water film at a super-wetting interface supports 2D material transfer

We observed a super-wetting behavior at typical chemical vapor deposition grown transition metal di-chalcogenide (TMDC) 2D material–substrate interface. Such an interface was found to keep a flat, nanoscale water film when interacting with liquid state water. Detailed in situ atomic force microscope studies found the variable thickness of the water layer. Both surfaces at the two sides of the interface, viz. the TMDC (MoS2 or WS2 as demonstrated) bottom surface and the exposed substrate (typically, the as-grown SiO2) surface, were found to be super-wetting. This insured effectively their stable super-wetting behaviors. We further verified a prompt relationship of the nanoscale water layer with the water-mediated transfer of the TMDC. The super-wetting layer was found essential and critical for the effective transfer of TMDCs to other surfaces.

Ultrafast growth of large single crystals of monolayer WS2 and WSe2.

Monolayer transition metal dichalcogenides (TMDs) have attracted considerable attention as atomically thin semiconductors for the ultimate transistor scaling. For practical applications in integrated electronics, large monolayer single crystals are essential for ensuring consistent electronic properties and high device yield. The TMDs available today are generally obtained by mechanical exfoliation or chemical vapor deposition (CVD) growth, but are often of mixed layer thickness, limited single crystal domain size or have very slow growth rate. Scalable and rapid growth of large single crystals of monolayer TMDs requires maximization of lateral growth rate while completely suppressing the vertical growth, which represents a fundamental synthetic challenge and has motivated considerable efforts. Herein we report a modified CVD approach with controllable reverse flow for rapid growth of large domain single crystals of monolayer TMDs. With the use of reverse flow to precisely control the chemical vapor supply in the thermal CVD process, we can effectively prevent undesired nucleation before reaching optimum growth temperature and enable rapid nucleation and growth of monolayer TMD single crystals at a high temperature that is difficult to attain with use of a typical thermal CVD process. We show that monolayer single crystals of 450 μm lateral size can be prepared in 10 s, with the highest lateral growth rate up to 45 μm/s. Electronic characterization shows that the resulting monolayer WSe2 material exhibits excellent electronic properties with carrier mobility up to 90 cm2 V−1 s−1, comparable to that of the best exfoliated monolayers. Our study provides a robust pathway for rapid growth of high-quality TMD single crystals.

Nitride film deposition using ammonia plasma pre-treatment to promote good adhesion of nitride film to copper

Over the last decade, there is tremendous amount of technological advances in micro fabrication technology to manufacture devices of high complexities and compact devices to capture global market in semiconductor industry. As the device size shrinks, process integration gets more complicated and interaction between the dielectric layer and metallic layers becomes extremely important. This work presents an integration of a high quality Nitride film to copper. Native copper oxide layer was removed by ammonia plasma pre-treatment using a typical PE CVD (Plasma Enhanced Chemical Vapor Deposition) chamber and a high quality Nitride film deposition was followed on the surface of copper. The copper films exhibited various degrees of haze and discoloration such as dark greenish or dark gold, caused by the copper silicide layer formation at the interface between copper and Nitride. The silicide formation was able to be prevented by maintaining a stable Nitrogen plasma environment prior to introducing SiH4 gas flow into a PE CVD chamber. Results show that Nitride film combined with an in-stiu pre-treatment has excellent film properties in its integration with Copper.

Uniform growth of high-concentration MoS2 nanoflakes on plasma treated carbon nanotube sheet

Abstract In this report, we demonstrate a novel two-step approach that is capable of growing uniform high-concentration MoS2 nanoflakes on the carbon nanotube (CNT) sheet substrate. The CNT sheet is simply activated by the oxygen plasma to separate large CNT bundles and to create more active surface sites. A typical chemical vapor deposition (CVD) process is employed, enabling the direct growth of MoS2 nanoflakes on the activated CNT substrate. Morphologies and structures of MoS2/CNT nanohybrid are characterized by SEM/EDS, high-resolution TEM (HRTEM) and Raman spectroscopy. Our results suggest that the plasma treatment of CNT sheet is critical to obtain well-dispersed large quantity of MoS2 nanoflakes on CNT assemblies.

Transfer-free CVD graphene for highly sensitive glucose sensors

Chemical vapor deposition (CVD) graphene film is a promising electrode-modifying material for fabricating high-performance glucose sensor due to its high electrical conductivity and two-dimensional structure over large area. However, the use of typical metal-based CVD graphene suffers from the residue contamination of polymer transfer-support and heavy metal ions. In this work, we directly grew few-layer graphene on the SiO2/Si substrate without transfer process and then fabricated graphene-based glucose sensors by sequentially immobilizing glucose oxidase and depositing Nafion layer on its surface that was functionalized by oxygen-plasma treatment. Our transfer- and metal-free process shows distinct advantage over the common metal-CVD method in improving the electrochemical performance by eliminating the contamination of transfer-residue. Thus-obtained glucose sensor shows a high sensitivity (16.16 μA mM−1 cm-2) with a detection limit of 124.19 μM. This method is simple and promising for the development of highly sensitive glucose sensors.

Revealing the microscopic CVD growth mechanism of MoSe2 and the role of hydrogen gas during the growth procedure

Understanding the microscopic mechanisms for the nucleation and growth of two-dimensional molybdenum diselenide (2D MoSe2) via chemical vapor deposition (CVD) is crucial towards the precisely controlled growth of the 2D material. In this work, we employed a joint use of transmission electron microscopy and CVD, in which the 2D MoSe2 were directly grown on a graphene membrane based on grids, that enables the microstructural characterization of as-grown MoSe2 flakes. We further explore the role of hydrogen gas and find: in an argon ambient, the primary products are few-layer MoSe2 flakes, along with MoOx nanoparticles; while with the introduction of H2, single-layer MoSe2 became the dominant product during the CVD growth. Quantitative analysis of the effects of H2 flow rate on the flake sizes, and areal coverage was also given. Nevertheless, we further illuminated the evolution of shape morphology and edge structures of single-layer MoSe2, and proposed the associated growth routes during a typical CVD process.

Thermal chemical vapor deposition of tin oxide nanowires in a hydrogen reduction atmosphere

We fabricated tin oxide nanowires (NWs) on Au/Si3N4/Si(001) substrates using thermal chemical vapor deposition (CVD) in a hydrogen reduction atmosphere. Unlike typical thermal CVD conditions, a mixture of H2 and Ar gas was used as the carrier gas, and a catalyst powder such as graphite was not used. The hydrogen gas maintained the reducing atmosphere and easily vaporized the SnO2 powder without a catalyst, to supply vapors of Sn and O. This process enables vapor-liquid-solid (VLS) growth of SnO2 NWs at temperatures below 600 °C. The formation of a eutectic AuSn alloy that acts as a liquid seed during the VLS process contributes to lowering the growth temperature. X-ray diffraction and transmission electron microscopy analysis show that SnO2 NWs and SnO NWs coexist in the as-prepared sample. The oxygen-deficient atmosphere results in nucleation and growth of SnO NWs.

Enhanced Carbon Nanotubes Growth Using Nickel/Ferrocene-Hybridized Catalyst.

Tall, crystalline carbon nanotubes (CNTs) are desired to successfully integrate them in various applications. As the crystallinity of CNTs improves with increasing growth temperatures, higher growth temperatures are required to obtain crystalline CNTs. However, in a typical chemical vapor deposition (CVD) process, CNT growth rate reduces when the growth temperature exceeds a specific level due to the degradation of the catalyst particles. In this study, we have demonstrated the improved catalytic activity of nickel/ferrocene-hybridized catalyst as compared to sole ferrocene catalyst. To demonstrate this, CNTs are grown on bare silicon (Si) as well as nickel (Ni) catalyst-deposited substrates using volatile catalyst source (ferrocene/xylene) CVD at the growth temperatures ranging from 790 to 880 °C. It was found that CNTs grown on bare Si substrate experience a reduction in height at growth temperature above 860 °C, whereas the CNTs grown on 10 nm Ni catalyst-deposited substrates experience continuous increase in height as the temperature increases from 790 to 880 °C. The enhancement in the height of CNTs by the addition of Ni catalyst is also demonstrated on 5, 20, and 30 nm Ni layers. The examination of CNTs using electron microscopy and Raman spectra shows that the additional Ni catalyst source improves the CNT growth rates and crystallinity, yielding taller CNTs with a high degree of structural crystallinity.

Graphene growth with ‘no’ feedstock

Synthesis of graphene by chemical vapor deposition (CVD) from hydrocarbons on Cu foil substrates can yield high quality and large area graphene films. In a typical CVD process, a hydrocarbon in the gas phase is introduced for graphene growth and hydrogen is usually required to achieve high quality graphene. We have found that in a low pressure CVD system equipped with an oil mechanical vacuum pump located downstream, graphene can be grown without deliberate introduction of a carbon feedstock but with only trace amounts of C present in the system, the origin of which we attribute to the vapor of the pump oil. This finding may help to rationalize the differences in graphene growth reported by different research groups. It should also help to gain an in-depth understanding of graphene growth mechanisms with the aim to improve the reproducibility and structure control in graphene synthesis, e.g. the formation of large area single crystal graphene and uniform bilayer graphene.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. II: CH4/N2/H2 Plasmas.

We report a combined experimental and modeling study of microwave-activated dilute CH4/N2/H2 plasmas, as used for chemical vapor deposition (CVD) of diamond, under very similar conditions to previous studies of CH4/H2, CH4/H2/Ar, and N2/H2 gas mixtures. Using cavity ring-down spectroscopy, absolute column densities of CH(X, v = 0), CN(X, v = 0), and NH(X, v = 0) radicals in the hot plasma have been determined as functions of height, z, source gas mixing ratio, total gas pressure, p, and input power, P. Optical emission spectroscopy has been used to investigate, with respect to the same variables, the relative number densities of electronically excited species, namely, H atoms, CH, C2, CN, and NH radicals and triplet N2 molecules. The measurements have been reproduced and rationalized from first-principles by 2-D (r, z) coupled kinetic and transport modeling, and comparison between experiment and simulation has afforded a detailed understanding of C/N/H plasma-chemical reactivity and variations with process conditions and with location within the reactor. The experimentally validated simulations have been extended to much lower N2 input fractions and higher microwave powers than were probed experimentally, providing predictions for the gas-phase chemistry adjacent to the diamond surface and its variation across a wide range of conditions employed in practical diamond-growing CVD processes. The strongly bound N2 molecule is very resistant to dissociation at the input MW powers and pressures prevailing in typical diamond CVD reactors, but its chemical reactivity is boosted through energy pooling in its lowest-lying (metastable) triplet state and subsequent reactions with H atoms. For a CH4 input mole fraction of 4%, with N2 present at 1–6000 ppm, at pressure p = 150 Torr, and with applied microwave power P = 1.5 kW, the near-substrate gas-phase N atom concentration, [N]ns, scales linearly with the N2 input mole fraction and exceeds the concentrations [NH]ns, [NH2]ns, and [CN]ns of other reactive nitrogen-containing species by up to an order of magnitude. The ratio [N]ns/[CH3]ns scales proportionally with (but is 102–103 times smaller than) the ratio of the N2 to CH4 input mole fractions for the given values of p and P, but [N]ns/[CN]ns decreases (and thus the potential importance of CN in contributing to N-doped diamond growth increases) as p and P increase. Possible insights regarding the well-documented effects of trace N2 additions on the growth rates and morphologies of diamond films formed by CVD using MW-activated CH4/H2 gas mixtures are briefly considered.

A first principles study on the CVD graphene growth on copper surfaces: A carbon atom incorporation to graphene edges

Carbon atom reactions in the chemical vapor deposition (CVD) processes for graphene production on copper surfaces have been studied by first principles molecular dynamics (MD) simulations at a typical CVD growth temperature. This study focuses on the processes of a carbon atom incorporation to graphene edges. The energy barriers of these carbon atom incorporation reactions have been calculated as ~1eV, which are comparable or slightly larger than the barriers of carbon atom dimerization. We have also found that the surface copper atoms form step like structures to terminate the carbon dangling bonds at graphene edges, which are markedly different from the graphene-copper interactions observed in static calculations.

Energy consumption and material fluxes in hard coating deposition processes

Abstract Hard coatings grown by physical vapor deposition (PVD) or chemical vapor deposition (CVD) on cutting tools are applied to considerably increase the tools' performance and lifetime. Besides differences in types, thicknesses, structures and properties of the coatings synthesized by PVD and CVD, the deposition processes differ significantly in their throughput of tools as well as their energy and material consumption. Within this work, a methodology to analyze the energy and material fluxes of typical PVD and CVD processes for the deposition of hard coatings on cutting tools is introduced. Three case studies are considered: ( i ) cathodic arc evaporation of TiCN, ( ii ) magnetron sputter deposition of TiN, and ( iii ) CVD of a TiCN/Al 2 O 3 bilayer coating. The material fluxes and energy consumption for each process step of the respective deposition processes were monitored and are illustrated by individual Sankey diagrams. The visualization by Sankey diagrams allows to readily identify the main energy and mass consuming process steps. Finally, a normalization procedure enabling the comparison of different hard coating production routes is presented and discussed.

Quantum Chemical Simulation of Carbon Nanotube Nucleation on Al2O3 Catalysts via CH4 Chemical Vapor Deposition.

We present quantum chemical simulations demonstrating how single-walled carbon nanotubes (SWCNTs) form, or "nucleate", on the surface of Al2O3 nanoparticles during chemical vapor deposition (CVD) using CH4. SWCNT nucleation proceeds via the formation of extended polyyne chains that only interact with the catalyst surface at one or both ends. Consequently, SWCNT nucleation is not a surface-mediated process. We demonstrate that this unusual nucleation sequence is due to two factors. First, the π interaction between graphitic carbon and Al2O3 is extremely weak, such that graphitic carbon is expected to desorb at typical CVD temperatures. Second, hydrogen present at the catalyst surface actively passivates dangling carbon bonds, preventing a surface-mediated nucleation mechanism. The simulations reveal hydrogen's reactive chemical pathways during SWCNT nucleation and that the manner in which SWCNTs form on Al2O3 is fundamentally different from that observed using "traditional" transition metal catalysts.