Cold-wall Chemical Vapor Deposition Research Articles

Large-area and few-layered 1T′-MoTe2 thin films grown by cold-wall chemical vapor deposition

This study employs cold-wall chemical vapor deposition to achieve the growth of MoTe2 thin films on 4-inch sapphire substrates. A two-step growth process is utilized, incorporating MoO3 and Te powder sources under low-pressure conditions to synthesize MoTe2. The resultant MoTe2 thin films exhibit a dominant 1T′ phase, as evidenced by a prominent Raman peak at 161 cm−1. This preferential 1T′ phase formation is attributed to controlled manipulation of the second-step growth temperature, essentially the reaction stage between Te vapor and the pre-deposited MoO x layer. Under these optimized growth conditions, the thickness of the continuous 1T′-MoTe2 films can be precisely tailored within the range of 3.5–5.7 nm (equivalent to 5–8 layers), as determined by atomic force microscopy depth profiling. Hall-effect measurements unveil a typical hole concentration and mobility of 0.2 cm2 Vs−1 and 7.9 × 1021 cm−3, respectively, for the synthesized few-layered 1T′-MoTe2 films. Furthermore, Ti/Al bilayer metal contacts deposited on the few-layered 1T′-MoTe2 films exhibit low specific contact resistances of approximately 1.0 × 10−4 Ω cm2 estimated by the transfer length model. This finding suggests a viable approach for achieving low ohmic contact resistance using the 1T′-MoTe2 intermediate layer between metallic electrodes and two-dimensional semiconductors.

MOCVD growth of Si-doped α-(AlGa)2O3 on m-plane α-Al2O3 substrates

We reported the growth of (AlGa)2O3 layers on (10 10̅ ) α-Al2O3 substrates using cold-wall metalorganic chemical vapor deposition, and the electrical characterization of Si-doped (AlGa)2O3 layers. In the Ga2O3 growth, the α phase was dominant at low growth temperature, achieving the growth rate of 2.4 μm h−1 at 650 °C. Sheet resistance and electrical conductivity of the Ga2O3 layers with a Si concentration of 3 × 1020 cm−3 were 1 × 104 Ω/square and 8.3 S cm−1, respectively, at the measurement temperature of 500 °C. The Al composition in the (AlGa)2O3 layers was controlled from 0% to 74%. In our initial attempts, we obtained electrically conductive α-(AlGa)2O3 layers by Si doping (2 × 10−9 S cm−1 in the sample with an Al composition of 56%). Hybrid functional calculations suggest the conductivities are limited by compensation of Si through cation vacancy complexes, and not by the significant amounts of co-incorporated C and N that are predicted to be electrically passivated by hydrogen.

Oxidation Resistance of Ir/HfO2 Composite Coating Prepared by Chemical Vapor Deposition: Microstructure and Elemental Migration

In this study, a HfO2 coating was developed on an Ir matrix using a customized open-tube airflow, cold-wall chemical vapor deposition instrument. The preparation process and structure of the as-prepared coating were investigated to gain insights into its characteristics. The HfO2 coating effectively prevents direct contact between Ir and O, leading to a reduction in the oxidation rate of Ir. Furthermore, defects such as micropores and cracks generated during sealed oxidation erosion contribute to Ir’s decelerated oxidation failure. The as-prepared HfO2 coating exhibits low thermal conductivity and a high heat radiation rate, reducing the coating’s surface temperature. These characteristics significantly enhance adversity tolerance and increase the working temperature of the coating. Moreover, the as-prepared HfO2 coating can serve as a diffusion barrier, blocking both the direct contact of O with the Ir coating and the diffusion of other elements to the Ir coating. As a result, the rates of diffusion of other elements to the Ir coating are reduced.

Low-resistivity Ohmic contacts of Ti/Al on few-layered 1T'-MoTe2/2H-MoTe2 heterojunctions grown by chemical vapor deposition.

This study explores the phase-controlled growth of few-layered 2H-MoTe2, 1T'-MoTe2, and 2H-/1T'-MoTe2 heterostructures and their impacts on metal contact properties. Cold-wall chemical vapor deposition (CW-CVD) with varying growth rates of MoOx and reaction temperatures with Te vapors enabled the growth of continuous thin films of either 1T'-MoTe2 or 2H-MoTe2 phases on two-inch sapphire substrates. This methodology facilitates the meticulous optimization of chemical vapor deposition (CVD) parameters, enabling the realization of phase-controlled growth of few-layered MoTe2 thin films and their subsequent heterostructures. The study further investigates the influence of a 1T'-MoTe2 intermediate layer on the electrical properties of metal contacts on few-layered 2H-MoTe2. Bi-layer Ti/Al contacts directly deposited on 2H-MoTe2 exhibited Schottky behavior, indicating inefficient carrier transport. However, introducing a few-layered 1T'-MoTe2 intermediate layer between the metal and 2H-MoTe2 layers improved the contact characteristics significantly. The resulting Al/Ti/1T'-MoTe2/2H-MoTe2 contact scheme demonstrates Ohmic behavior with a specific contact resistance of around 1.7 × 10-4 Ω cm2. This substantial improvement is attributed to the high carrier concentration of the 1T'-MoTe2 intermediate layer which could be attributed tentatively to the increased tunneling events across the van der Waals gap and enhancing carrier transport between the metal and 2H-MoTe2.

Fabrication of SiC-on-Insulator (SiCOI) Layers by Chemical Vapor Deposition of 3C-SiC on Si-in-Insulator Substrates at Low Deposition Temperatures of 1120 °C

Compared to bulk silicon carbide (SiC) wafers, SiC-on-insulator (SiCOI) substrates enable new device designs of electronic switches as well as novel photonic applications. One application is a micro-resonator for the usage in a Kerr frequency comb. For SiCOI substrates, a deposition temperature below 1200 °C is advisable due to stability reasons of the buried oxide layer during chemical vapor deposition (CVD) process conditions. To create 3C-SiC-on-insulator layers, a cold-wall CVD reactor was utilized, with propane and silane as the sources for carbon and silicon, respectively. To improve the cracking of the carbon source gas at low temperatures, the inner setup of the utilized cold-wall CVD reactor was changed to a non-water-cooled system. The change of the inner reactor setup was investigated numerically, and the grown epitaxial layers were characterized by Raman, EDX, SEM-imaging and XRD spectroscopy. We demonstrate successful deposition of 3C-SiC epitaxial layer substrates at temperatures below 1200 °C without delamination on SOI.

Scalable growth of atomically thin MoS2 layers in a conventional MOCVD system using molybdenum dichloride dioxide as the molybdenum source

To bring atomically thin transition metal dichalcogenides (TMDs) to practical application, a highly reproducible process to grow them over a large scale is indispensable. Here, we develop a carbon-free reproducible route for the scalable growth of high-quality monolayer MoS2 by selecting molybdenum dichloride dioxide (MoO2Cl2) as the Mo source and integrating the precursor with a standard low-pressure cold-wall metalorganic chemical vapor deposition (MOCVD) system. Specifically, combined with H2S as the sulfur source and using catalytic Dragontrail glass (DT-glass) as the main substrate, we investigate the effect of MoO2Cl2 flux, temperature, and deposition time on the growth, confirming MoO2Cl2 with reasonably high vapor pressure is well compatible with the MOCVD system that enables precise control of sources for uniform nucleation and growth of MoS2 over a large area. We successfully demonstrate the growth of carbon-free MoS2 monolayers on DT-glass with decent crystalline, optical, and electrical properties. Additionally, the initial attempts also manifest that the proposed strategy could be generalized for growing ultrathin MoS2 on other technologically important substrates, such as SiO2/Si and quartz, exhibiting superior optical quality and wafer-scale uniformity. This work provides a new avenue for the large-scale production of high-quality MoS2 monolayers and facilitates their use in practical applications.

Maximizing the thermal hotspot reduction by optimizing the thickness of multilayer hBN heat spreader

This article investigates the thickness-dependent heat spreading performance of multilayer hexagonal boron nitride (hBN). The growth by a cold-wall chemical vapor deposition system results in large-area and high crystal quality of multilayer hBN. The full width at half maximum of Raman E2g peak is approximately 25 cm−1 and the bandgap is larger than 5.8 eV. Varying the growth duration from 5 to 15 min on Ni foil results in hBN thicknesses of 5–12 nm with comparable crystal quality. The electrothermal analysis using a sensitive Pt/Cu/Ti micro-coil introduces the figure of merit (FoM) of hBN as an insulator heat spreader. Fittings on FoM plots suggest that multilayer hBN with a thickness of ∼2 nm is optimum for a 25% reduction in the thermal hotspot. The initial drop in lateral thermal resistance is significant for a few nanometers-thick hBN, where a 22.5% reduction is measured from the 5.6 nm-thick hBN heat spreader. Further thickening of hBN reduces the lateral thermal resistance by approximately 0.37% nm−1. Findings from this work provide a significant contribution to the implementation of hBN as a direct contact heat spreader on the actual power devices.

Boron-Silicon Thin Film Formation Using a Slim Vertical Chemical Vapor Deposition Reactor

A boron-silicon film was formed from boron trichloride gas and dichlorosilane gas at about 900℃ in ambient hydrogen at atmospheric pressure utilizing a slim vertical cold wall chemical vapor deposition reactor designed for the Minimal Fab system. The gas flow rates were 80, 20 and 0.1 - 20 sccm for the hydrogen, dichlorosilane and boron trichloride gases, respectively. The gas transport condition in the reactor was shown to quickly become stable when evaluated by quartz crystal microbalances at the inlet and outlet. The boron-silicon thin film was formed by achieving the various boron concentrations of 0.16% - 80%, the depth profile of which was flat. By observing the cross-sectional TEM image, the obtained film was dense. The boron trichloride gas is expected to be useful for the quick fabrication of various materials containing boron at significantly low and high concentrations.

Solar-thermal cold-wall chemical vapor deposition reactor design and characterization for graphene synthesis

Manufacturing processes are often highly energy-intensive, even when the energy is primarily used for direct heating processes. The required energy tends to derive from local utilities, which currently employ a blend of sources ranging from fossil fuels to renewable wind and solar photovoltaics, among others, when the end manufacturing need is thermal energy. Direct solar-thermal capture provides a compelling alternative that utilizes renewable energy to reduce greenhouse gas emissions from industrial processes, but one that has rarely been employed to date. In this study, a 10 kWe custom-built high flux solar simulator (HFSS) that closely approximates the solar spectrum produces a heat flux distribution with an adjustable peak between 1.5 and 4.5 MW/m2. The HFSS system is coupled to a cold-wall chemical vapor deposition (CVD) system that is equipped to automate graphene synthesis while providing safe operation, precise control, and real-time monitoring of process parameters. A numerical heat transfer model of a thin copper substrate is derived and validated to compute the substrate’s temperature profile prior to the synthesis process. The peak substrate temperature is correlated to the HFSS supply current and vacuum pressure, as it serves as a critical design parameter during graphene synthesis. We report the synthesis of high-quality graphene films on copper substrates with an average Raman peak intensity ratio ID/IG of 0.17. Backscattered electron microscopy reveals a characteristic grain size of 120 μm, with an area ratio of 16 when compared to that of low-quality graphene on copper. The reported solar-thermal CVD system demonstrates the ability to produce a high-value product, namely, graphene on copper, directly from a renewable energy resource with process control and automation that enables synthesis under a variety of conditions.

Anti-dewetting of Cu thin film on nanostructured black Si template for continuous CVD growth of monolayer graphene

The growth of high-quality continuous film of graphene on less than [Formula: see text]m-thick Cu film is proven to be a challenging task due to the solid-state dewetting of Cu during the high-temperature chemical vapor deposition (CVD) process. In this paper, we introduce the use of nanostructured black Si (b-Si) as a template for Cu evaporation to mitigate the dewetting of Cu thin film. Using a cold-wall CVD system at a process temperature of 825[Formula: see text]C, even Cu thickness, [Formula: see text] nm on polished SiO2/Si substrate is poor for maintaining Cu as a continuous film. If the polished SiO2/Si is replaced with SiO2/b-Si, the minimum [Formula: see text] nm is sufficient. According to the Cu trapping mechanism, moving Cu particle is trapped in the nanostructured trenches of SiO2/b-Si during annealing and CVD growth processes. Continuous monolayer graphene with a grain size of [Formula: see text]m without defect is obtained on Cu/SiO2/b-Si substrate. The improved adhesion of Cu to the SiO2/b-Si enables dry-transfer of graphene by mechanical peeling using a polyvinyl alcohol (PVA) film. Our solution is promising for obtaining flat graphene and a recyclable Cu/SiO2/b-Si substrate.

Toward batch synthesis of high-quality graphene by cold-wall chemical vapor deposition approach

Toward batch synthesis of high-quality graphene by cold-wall chemical vapor deposition approach

Iridium‐Catalyzed Single‐Walled Carbon Nanotube Synthesis by Alcohol‐Gas‐Source Method Under Low Ethanol Pressure: Growth Temperature Dependence

AbstractSingle‐walled carbon nanotubes (SWCNTs) are grown via an Ir‐catalyzed gas‐source‐type cold‐wall chemical vapor deposition (CVD) method under high vacuum. A nozzle injector is used to supply ethanol gas. The grown SWCNT surface shows a colored concentric morphology. This corresponds to the in‐plane distribution of SWCNT lengths, which is induced by in‐plane variations in the ethanol pressure. Raman spectroscopy and scanning electron microscopy are used to investigate the dependence of the SWCNT yield on the growth temperature and ethanol pressure. The ethanol pressure that gives the highest SWCNT yield decreases with decreasing growth temperature. Optimization of the ethanol pressure enables SWCNT growth between 500 °C and 800 °C, and vertically aligned SWCNTs are grown at 700 °C and 800 °C. The SWCNT diameters are small at all growth temperatures, namely less than 0.9 nm for growth at 500 °C and 600 °C, and less than 1.0 nm for growth at 700 °C and 800 °C. The estimated activation energy for SWCNT growth with an Ir catalyst is 0.97 eV, which is much smaller than that with a Co catalyst.

Low-Temperature Nitrogen Doping of Nanocrystalline Graphene Films with Tunable Pyridinic-N and Pyrrolic-N by Cold-Wall Plasma-Assisted Chemical Vapor Deposition.

We report a viable method to produce nanocrystalline graphene films on polycrystalline nickel (Ni) with enhanced N doping at low temperatures by a cold-wall plasma-assisted chemical vapor deposition (CVD) method. The growth of nanocrystalline graphene films was carried out in a benzene/ammonia/argon (C6H6/NH3/Ar) system, in which the temperature of the substrate heated by Joule heating can be further lowered to 100 °C to achieve a low sheet resistance of 3.3 kΩ sq–1 at a high optical transmittance of 97.2%. The morphological, structural, and electrical properties and the chemical compositions of the obtained N-doped nanocrystalline graphene films can be tailored by controlling the growth parameters. An increase in the concentration of atomic N from 1.42 to 11.28 atomic percent (at.%) is expected due to the synergetic effects of a high NH3/Ar ratio and plasma power. The possible growth mechanism of nanocrystalline graphene films is also discussed to understand the basic chemical reactions that occur at such low temperatures with the presence of plasma as well as the formation of pyridinic-N- and pyrrolic-N-dominated nanocrystalline graphene. The realization of nanocrystalline graphene films with enhanced N doping at 100 °C may open great potential in developing future transparent nanodevices.

Practical Route for the Low-Temperature Growth of Large-Area Bilayer Graphene on Polycrystalline Nickel by Cold-Wall Chemical Vapor Deposition.

We report a practical chemical vapor deposition (CVD) route to produce bilayer graphene on a polycrystalline Ni film from liquid benzene (C6H6) source at a temperature as low as 400 °C in a vertical cold-wall reaction chamber. The low activation energy of C6H6 and the low solubility of carbon in Ni at such a low temperature play a key role in enabling the growth of large-area bilayer graphene in a controlled manner by a Ni surface-mediated reaction. All experiments performed using this method are reproducible with growth capabilities up to an 8 in. wafer-scale substrate. Raman spectra analysis, high-resolution transmission electron microscopy, and selective area electron diffraction studies confirm the growth of Bernal-stacked bilayer graphene with good uniformity over large areas. Electrical characterization studies indicate that the bilayer graphene behaves much like a semiconductor with predominant p-type doping. These findings provide important insights into the wafer-scale fabrication of low-temperature CVD bilayer graphene for next-generation nanoelectronics.

Graphene Directly Growth on Non-Metal Substrate from Amorphous Carbon Nano Films Without Transfer and Its Application in Photodetector

A layer of nano amorphous carbon was fabricated on the target substrate by precisely controlled magnetron sputtering, and then a layer of copper film was fabricated on the amorphous carbon. By using a vertical cold wall chemical vapor deposition system under protective atmosphere, the carbon atoms at high temperature was catalyzed by copper to form graphene films. The amorphous carbon nano thin film was converted into monolayer graphene on a SiO2 substrate directly. The experimental results show that the graphene film has high crystal quality and conductivity. Compared with other methods, the process is simple and the process window is wider. By virtue of this technique, a graphene-Si photodetector was also demonstrated. The photoelectric response and frequency characteristics have been studied which shows good device characteristics.

Graphene Coating Obtained in a Cold-Wall CVD Process on the Co-Cr Alloy (L-605) for Medical Applications.

Graphene coating on the cobalt-chromium alloy was optimized and successfully carried out by a cold-wall chemical vapor deposition (CW-CVD) method. A uniform layer of graphene for a large area of the Co-Cr alloy (discs of 10 mm diameter) was confirmed by Raman mapping coated area and analyzing specific G and 2D bands; in particular, the intensity ratio and the number of layers were calculated. The effect of the CW-CVD process on the microstructure and the morphology of the Co-Cr surface was investigated by scanning X-ray photoelectron microscope (SPEM), atomic force microscopy (AFM), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Nanoindentation and scratch tests were performed to determine mechanical properties of Co-Cr disks. The results of microbiological tests indicate that the studied Co-Cr alloys covered with a graphene layer did not show a pro-coagulant effect. The obtained results confirm the possibility of using the developed coating method in medical applications, in particular in the field of cardiovascular diseases.

Effect of Cu thickness and temperature on growth of graphene on 8-inch Cu/SiO2/Si wafer using cold-wall CVD reactor

The growth of graphene by chemical vapor deposition (CVD) method on Cu thin film using a cold-wall CVD reactor is desirable for developing a compatible graphene transfer process on large SiO 2 /Si wafer. We reported the growth of graphene on the evaporated Cu on an 8-inch SiO 2 /Si wafer for the varied thickness of 100–600 nm. We found the temperature for the cold-wall CVD reactor needs to be 825 °C or higher for methane decomposition and graphene growth. The high-temperature process results in the formation of isolated and interconnected Cu islands due to the dewetting mechanism. The evaporated Cu film on SiO 2 /Si wafer needs to be sufficiently thick minimum of 600 nm to mitigate the dewetting during the 825 °C process. Successfully grown graphene on 8-inch Cu/SiO 2 /Si wafer using a cold-wall reactor was monolayer without crystal defect. The grain size of graphene was at an average of 2.5 µm, correlated to the grain size of Cu film which is slightly smaller at an average of 2 µm. We anticipate further enlargement of Cu grain size along with graphene grain size by the thickening of Cu film and by using higher process temperature.

Integrated Wafer Scale Growth of Single Crystal Metal Films and High Quality Graphene.

We report on an approach to bring together single crystal metal catalyst preparation and graphene growth in a combined process flow using a standard cold-wall chemical vapor deposition (CVD) reactor. We employ a sandwich arrangement between a commercial polycrystalline Cu foil and c-plane sapphire wafer and show that close-spaced vacuum sublimation across the confined gap can result in an epitaxial, single-crystal Cu(111) film at high growth rate. The arrangement is scalable (we demonstrate 2″ wafer scale) and suppresses reactor contamination with Cu. While starting with an impure Cu foil, the freshly prepared Cu film is of high purity as measured by time-of-flight secondary ion mass spectrometry. We seamlessly connect the initial metallization with subsequent graphene growth via the introduction of hydrogen and gaseous carbon precursors, thereby eliminating contamination due to substrate transfer and common lengthy catalyst pretreatments. We show that the sandwich approach also enables for a Cu surface with nanometer scale roughness during graphene growth and thus results in high quality graphene similar to previously demonstrated Cu enclosure approaches. We systematically explore the parameter space and discuss the opportunities, including subsequent dry transfer, generality, and versatility of our approach particularly regarding the cost-efficient preparation of different single crystal film orientations and expansion to other material systems.

Chemical Vapour Deposition of Graphene-Synthesis, Characterisation, and Applications: A Review.

Graphene as the 2D material with extraordinary properties has attracted the interest of research communities to master the synthesis of this remarkable material at a large scale without sacrificing the quality. Although Top-Down and Bottom-Up approaches produce graphene of different quality, chemical vapour deposition (CVD) stands as the most promising technique. This review details the leading CVD methods for graphene growth, including hot-wall, cold-wall and plasma-enhanced CVD. The role of process conditions and growth substrates on the nucleation and growth of graphene film are thoroughly discussed. The essential characterisation techniques in the study of CVD-grown graphene are reported, highlighting the characteristics of a sample which can be extracted from those techniques. This review also offers a brief overview of the applications to which CVD-grown graphene is well-suited, drawing particular attention to its potential in the sectors of energy and electronic devices.

Superclean Growth of Graphene Using a Cold-Wall Chemical Vapor Deposition Approach.

Chemical vapor deposition (CVD) has become a promising approach for the industrial production of graphene films with appealing controllability and uniformity. However, in the conventional hot-wall CVD system, CVD-derived graphene films suffer from surface contamination originating from the gas-phase reaction during the high-temperature growth. Shown here is that the cold-wall CVD system is capable of suppressing the gas-phase reaction, and achieves the superclean growth of graphene films in a controllable manner. The as-received superclean graphene film, exhibiting improved optical and electrical properties, was proven to be an ideal candidate material used as transparent electrodes and substrate for epitaxial growth. This study provides a new promising choice for industrial production of high-quality graphene films, and the finding about the engineering of the gas-phase reaction, which is usually overlooked, will be instructive for future research on CVD growth of graphene.