Carbon Diffusion Barrier Research Articles

Site-Selective Synthesis of Bilayer Graphene on Cu Substrates Using Titanium as a Carbon Diffusion Barrier.

Chemical vapor deposition (CVD) is a widely used method for graphene synthesis, but it struggles to produce large-area uniform bilayer graphene (BLG). This study introduces a novel approach to meet the demands of large-scale integrated circuit applications, challenging the conventional reliance on uniform BLG over extensive areas. We developed a unique method involving the direct growth of bilayer graphene arrays (BLGA) on Cu foil substrates using patterned titanium (Ti) as a diffusion barrier. The use of the Ti layer can effectively control carbon atom diffusion through the Cu foil without altering the growth conditions or compromising the graphene quality, thereby showcasing its versatility. The approach allows for targeted BLG growth and achieved a yield of 100% for a 10 × 10 BLG units array. Then a 10 × 10 BLG memristor array was fabricated, and a yield of 96% was achieved. The performances of these devices show good uniformity, evidenced by the set voltages concentrated around 4 V, and a high resistance state (HRS) to low resistance state (LRS) ratio predominantly around 107, reflecting the spatial uniformity of the prepared BLGA. This study provides insight into the BLG growth mechanism and opens new possibilities for BLG-based electronics.

Growth of graphene from solid amorphous carbon: A new geometry for control carbon diffusion barrier

As an alternative strategy to fabricate high-quality graphene over a large area, metal catalysis has been attempted at elevated temperatures with solid carbon sources. However, graphene was generally fabricated on the surface of metal catalysis layer using amorphous carbon as solid carbon sources. In this study, a thin Al2O3 barrier (1 nm) deposited on an amorphous-C/Ni bilayer stack is demonstrated to enable direct growth of few layer graphene (FLG) identified as 3–4 layers on substrate surface at 700 °C and 800 °C. Moreover, the obtained graphene shows a good transmittance (93%) under the light source of 550 nm. The findings provide a way to directly synthesis FLG on the required substrate at low temperature, which may dramatically broaden the applications range of graphene.

Cooperatively enhanced coking resistance via boron nitride coating over Ni-based catalysts for dry reforming of methane

Dry reforming of methane provides a feasible solution to reduce the greenhouse gas via simultaneously converting CH4 and CO2 into useful syngas. However, conventional Ni-based catalysts suffer from deactivation due to serious carbon coking during dry reforming of methane process. Herein, we propose a concept by reducing crystallinity of formed carbon and accelerating carbon oxidization to enhance the coking resistance of Ni-based catalysts. This concept was conducted through a boron nitride coating strategy over reducible oxides supported Ni catalysts, in which the increased carbon diffusion barrier, induced by the boron nitride coating, would lead to more amorphous carbon. While the amorphous carbon was efficiently gasified by active oxygen species derived from reducible oxides. Our findings demonstrate that the coke issues can be solved by building a dynamic carbon removal route.

Demonstration of Solar Cell on a Graphite Sheet with Carbon Diffusion Barrier Evaluation.

An amorphous Si (a-Si) solar cell with a back reflector composed of zinc oxide (ZnO) and silver (Ag) is potentially the most plausible and flexible solar cell if a graphite sheet is used as the substrate. Graphite supplies lightness, conductivity and flexibility to devices. When a graphite sheet is used as the substrate, carbon can diffuse into the Ag layer in the subsequent p-i-n process at 200–400 °C. To prevent this, we added an oxide layer as a carbon diffusion barrier between the carbon substrate and the back reflector. For the carbon diffusion barrier, silicon oxide (SiO2) or tin oxide (SnOx) was used. We evaluated the thermal stability of the back reflector of a carbon substrate using secondary-ion mass spectrometry (SIMS) to analyze the carbon diffusion barrier material. We confirmed the deposition characteristics, reflectance and prevention of carbon diffusion with and without the barrier. Finally, the structures were incorporated into the solar cell and their performances compared. The results showed that the back reflectors that were connected to a carbon diffusion barrier presented better performance, and the reflector with an SnOx layer presented the best performance.

Interface studies of Mo/Si multilayers with carbon diffusion barrier by grazing incidence extended X-ray absorption fine structure

Single layer Mo and Si thin films deposited for different deposition times on c-Si substrates by Ion beam sputtering technique have been characterized by Grazing Incidence X-ray Reflectivity (GIXRR) measurements. Subsequently, few Mo/Si multilayers having 5 bi-layers with different Mo to bi-layer thickness ratio (Γ) at a fixed bi-layer thickness have been deposited and characterized by GIXRR measurements. Investigation has been carried out on the different behaviors of the interfaces and also variation of the interface width with Γ has been studied. Finally, 25 bi-layer Mo/Si multilayer samples having the lowest Γ value and with and without carbon diffusion barrier layer at both the Mo/Si interfaces are deposited and the interfaces have been characterized by hard X-ray and soft X-ray reflectivity measurements. The study shows improvement in reflectivities of the multilayer with carbon diffusion barrier layer from 38% to 54% at 130 Å wavelength. To have further insight into the modification of the interface due to introduction of the carbon barrier layer, depth selective study of both the multilayers have been carried out by Grazing Incidence Extended X-ray Absorption Fine Structure measurements using synchrotron radiation and thereby investigating the local structures around Mo atoms in Si bulk region and Si-on-Mo interface. This study confirms the reduction of diffusion of Mo and Si layer into each other when the carbon buffer layers are present at the interfaces, thus reducing the chance of formation of MoSi2.

Controlling the carbide formation and chromium depletion in W-Cr alloy during field assisted sintering

Tungsten is a refractory metal, suitable for a broad range of high-temperature applications, due to its high melting point and high-temperature strength. Recently, tungsten alloys became of great interest in thermonuclear fusion research. The alloys maintain low sputtering yield and high melting point, but greatly improve some drawback properties comparing to pure tungsten. These alloys are commonly prepared by mechanical alloying and compacted using Spark Plasma Sintering/Field Assisted Sintering. Powder in a graphite die is consolidated by applying pulsed electric current and pressure. Graphite foil is commonly placed between the die and the powder, to prevent adhesion. However, for number of applications it is necessary to avoid carbon contamination. In the present study we have examined carbon contamination mechanism of W - 10 wt.%Cr - 1 wt.%Hf alloy during Field Assisted Sintering along with the alloy's properties. The experiments have shown that the graphite foil leads to significant dealloying via mechanism of liquid phase sintering. As a result, tailored properties of the material are deteriorated after several minutes of the consolidation process. Therefore, application of carbon diffusion barrier was proposed, examined and compared with the alloy sintered with graphite foil.

Grain Boundary Plays a Key Role in Carbon Diffusion in Carbon Irons Revealed by a ReaxFF Study

Carbon diffusion is a critical process to the manufacture of many industry products, such as iron carbides, stainless steels, and carbon materials. Here we investigate carbon diffusion induced by tensile, screw dislocation, edge dislocation, and polycrystal boundary through reactive molecular dynamics simulations with ReaxFF potentials. The temperature enhances the dynamics and therefore the carbon diffusion. The pre-existing defects promote the carbon diffusion with a linear relationship between carbon diffusion barrier and strain as well as line defect concentrations. Furthermore, we also observed a linear relationship between the carbon diffusion barrier and the volume fractions of the polycrystalline boundary, indicating that the grain boundary mechanism is prominent in carbon diffusion in the carbon iron.

Direct formation of graphene on dielectric substrate: Controlling the location of graphene formation adopting carbon diffusion barrier

The authors report a method to form multilayered graphene directly on top of a dielectric SiO2/Si substrate by the solid-phase crystallization of amorphous carbon (a-C) using Ni as a catalyst and Ti as a carbon diffusion barrier layer; the layer sequence is Ti/Ni/a-C/SiO2/Si. During annealing, carbon diffuses through Ni and forms TiC at the Ti/Ni interface, blocking further carbon diffusion to the Ti layer. During cooling, the remnant carbon in the Ni layer precipitates out at the Ni/SiO2 interface, forming multilayers of graphene. Then, both Ti and Ni are etched away using Radio Corporation of America standard cleaning (SC-I) and FeCl3-based wet etching. The graphene layers formed on top of the dielectric substrate can be utilized without further transfer methods. The best-quality of graphene is formed at 600 °C with the Raman signal D- to G-peak intensity ratio of 0.29. Auger electron spectroscopy depth profiles and sequential etching tests with SC-I and FeCl3-based etchant confirms that the Ti layer is transformed to TiOxCy or TiC layer, which may reduce the carbon diffusion flux through this layer, as expected.

Deposition characteristics of titanium coating deposited on SiC fiber by cold-wall chemical vapor deposition

The deposition characteristics of titanium coating on SiC fiber using TiCl4-H2-Ar gas mixture in a cold-wall chemical vapor deposition were studied by the combination of thermodynamic analysis and experimental studies. The thermodynamic analysis of the reactions in the TiCl4-H2-Ar system indicates that TiCl4 transforms to titanium as the following paths: TiCl4 → TiCl3 → Ti, or TiCl4 → TiCl3 → TiCl2 → Ti. The experimental results show that typical deposited coating contains two distinct layers: a TiC reaction layer close to SiC fiber and titanium coating which has an atomic percentage of titanium more than 70% and that of carbon lower than 30%. The results illustrate that a carbon diffusion barrier coating needs to be deposited if pure titanium is to be prepared. The deposition rate increases with the increase of temperature, but higher temperature has a negative effect on the surface uniformity of titanium coating. In addition, appropriate argon gas flow rate has a positive effect on smoothing the surface morphology of the coating.

The Role of Pre Hydrogen Flow in Nucleation of Graphene on Silicon Nitride

Graphene has been considered as one of most attractive materials in nano-scaled electronic devices. Graphene film has been mainly grown on many metal (eg., Ni , Cu, Fe) by chemical vapor deposition (CVD) process and transferred on dielectric substrate. However, the complicated transfer techniques for metal-catalyzed graphene inevitably have resulted in wrinkles, holes, and metal etching residues. Thus, there is a substantial need to develop a scalable method for reliable production of large-area graphene directly on insulating substrates. We synthesized the nucleation of graphene on silicon nitride (Si3N4) substrate and report the influence of pre hydrogen flow during CVD process on silicon nitride which is the stable dielectric and suitable for carbon diffusion barrier. Test wafer was cleaned in 10:1 HF solution and nucleation of graphene was synthesized by CVD process using CH4, Ar, H2gas at 1000°C. We evaluated the graphene nucleation with and without pre hydrogen flow on silicon nitride film as shown in Fig.1. Although amorphous carbon cluster was formed on silicon nitride without pre hydrogen flow as shown in Fig. 2(a), Raman shift peak related graphene was obtained with pre hydrogen flow condition as shown in Fig. 2(b). It is assumed that hydrogen ions are bonded with silicon atom on silicon nitride surface during pre hydrogen process and then chemisorptions of carbon precursors on the H-terminated surface are easily made. Fig.3 shows the role of hydrogen [1]. To synthesize the graphene film on silicon nitride, it is essential to be terminated with hydrogen at silicon nitride surface. Reference [1] Jae-Hyun Lee et al, “Wafer-Scale Growth of Single-Crystal Monolayer Graphene on Reusable Hydrogen-Terminated Germanium”, Science, Vol. 344, 2014 Figure 1

Synthesis of transfer-free graphene by solid phase reaction process in presence of a carbon diffusion barrier

Synthesis of transfer-free high quality graphene on an arbitrary substrate by metal assisted graphitization with control carbon diffusion process has lot of significance for device applications. Here, we demonstrate the synthesis of monolayer graphene using common triblock co-polymer (EOnPOmEOn) as carbon source by solid phase reaction. In the synthesis process, we used Ni and NiO thin film as catalyst and carbon diffusion barrier, respectively on the top of polymer deposited SiO2/Si substrate. The long carbon chain of triblock co-polymer pluronic F127(EO106PO70EO106) can be graphitized in presence of Ni layer, where carbon diffusion to the top surface is controlled by NiO thin layer. The effects of carbon diffusion, thickness of polymer layer and annealing process are studied to obtain high quality monolayer graphene. Our findings reveal that the NiO/Ni stacked, thickness of polymer layer and reaction process in H2 atmosphere strongly influence the quality of synthesized graphene.

A Comprehensive Study of the Carbon Contamination in Tellurite Glasses and Glass‐Ceramics Sintered by Spark Plasma Sintering (SPS)

Highly transparent tellurite glasses and glass‐ceramics based on the 85TeO2–15WO3 composition (mol%) were produced by spark plasma sintering (SPS) from powders previously prepared by the conventional melt‐quenching technique and then grinded. We report a study based on the understanding of carbon contamination that is commonly observed by this nonconventional sintering technique and which constitutes a drawback for optical applications. First, the influence of the particle size of the initial amorphous powders as well as an additional pressureless sintering step prior to SPS experiments and the use of a physical carbon diffusion barrier have been investigated to reduce the carbon contamination in glass bulks. Second, once reducing the carbon contamination, glass‐ceramics were obtained by varying the SPS conditions. The noncentrosymmetric γ‐TeO2 phase crystallized within the bulk volume while maintaining good optical transparency and led to the generation of second harmonic. This approach paves the way to further applications in the domain of linear and nonlinear optics.

Fabrication of a Schottky junction diode with direct growth graphene on silicon by a solid phase reaction

We demonstrate fabrication of a Schottky junction diode with direct growth graphene on n-Si by the solid phase reaction approach. Metal-assisted crystallization of a-C thin film was performed to synthesize transfer-free graphene directly on a SiO2 patterned n-Si substrate. Graphene formation at the substrate and catalyst layer interface is achieved in presence of a Co catalytic and CoO carbon diffusion barrier layer. The as-synthesized material shows a linear current–voltage characteristic confirming the metallic behaviour of the graphene structure. The direct grown graphene on n-Si substrate creates a Schottky junction with a potential barrier of 0.44 eV and rectification diode characteristic. Our finding shows that the directly synthesized graphene on Si substrate by a solid phase reaction process can be a promising technique to fabricate an efficient Schottky junction device.

Study of diamond nucleation and growth on Ti6Al4V with tungsten interlayer

Diamond nucleation and growth on Ti6Al4V substrates with tungsten (W) interlayers were investigated using hot filament chemical vapor deposition (HFCVD). W interlayers were firstly deposited on Ti6Al4V substrates under different conditions by sputtering and diamond was then deposited on W-coated Ti6Al4V using the same HFCVD reactor. X-ray diffraction, scanning electron microscopy, energy-dispersive spectroscopy, atomic force microscopy, Raman spectroscopy, synchrotron near-edge X-ray absorption fine structure spectroscopy and Rockwell C indentation tests were used for characterization. The effect of thickness and surface morphology of W interlayer on the nucleation, growth, and adhesion of diamond on the Ti alloy was investigated. The results show that the pre-deposited W interlayers act as a diffusion barrier for carbon, and thus significantly enhancing the nucleation of diamond. In addition, the W interlayer reacts with carbon to form WC and W2C during the diamond deposition, thus enhancing the adhesion of diamond on Ti6Al4V.

Effect of Oxidation with Lanthanum Sol–Gel Coating on the 330Cb Carburization at 900 °C

Corrosion and carburization are the main reasons for the degradation of mechanical properties of conveyor units. In order to improve alloy carburization resistance, an adherent oxide scale can act as a carbon diffusion barrier. Oxidation of Alloy 330Cb in air at 900 °C was performed in order to grow a protective chromia scale. Nevertheless, after oxidation some scale spallation occurred during cooling. Then, argon-annealed lanthanum sol–gel coatings were applied in order to improve the oxide scale adherence. This led to a thinner chromia scale. The convoluted scale structure suggested that a mixed diffusion process was induced by lanthanum. After carburization during 22 h at 900 °C under Ar/10 % CH4, the Cr2O3 scale was converted into carbides and it appears that is necessary to have a thick oxide scale to get a good protection.

α-iron facet with enhanced carbon mobility

Several in situ studies of carbon nanotube and carbon nanofiber growth on transition-metal nanoparticles have observed the growth of graphitic material from specific nanoparticle facets. In many of the studies these facets have been associated with rough nanoparticle surfaces with atomic size steps. Taking as a reference case a recent in situ study [G. E. Begtrup et al., Phys. Rev. B 79 205409 (2009)] where the facet producing graphitic material in situ was associated with the $\ensuremath{\alpha}$-iron (111) surface, we demonstrate, using density-functional theory calculations, that the diffusion barrier for carbon entering and leaving the nanoparticle and migrating in the vicinity of the nanoparticle surface is smaller on the undercoordinated (111) facet than on the more closely packed (110) and (100) facets. We explain this behavior by the flexibility of the atomic steps on the (111) surface. This idea is general and can be used to explain similar behavior in other transition-metal nanoparticles as observed in the in situ studies.

Direct deposition of diamond films on steel using a three-step process

Diamond coatings were successfully deposited on tool steel substrates without using any external diffusion barrier layers. The diamond film deposition was performed in a hot filament chemical vapour deposition (HFCVD) reactor. In the first step, a high substrate temperature and a high methane percentage were used to achieve a faster critical carbon concentration and hence a shorter incubation time for diamond nucleation. Subsequently, the substrates were taken out of the reactor and subjected to ultrasonic scratching in diamond slurry in order to increase the diamond nucleation density. Then the final deposition was performed in the reactor under typical diamond growth conditions. X-ray diffraction analyses showed that Fe 3C was the dominant carbide phase formed on the steel surface after the first step. The carbides produced in the first step act as a diffusion barrier for both iron and carbon. Thus, they accelerate the diamond nucleation and help further diamond growth. Scotch tape adhesion tests showed that the diamond film adheres well to the substrate.

High-temperature properties of joint interface of VPS-tungsten coated CFC

Tungsten coated carbon fiber composite (CFC) is a candidate material for the high heat flux components in fusion reactors. In order to investigate the high-temperature properties at the joint interface of coating, heat load experiments by using electron beam were performed on VPS-tungsten coated CX-2002U samples. After the heat load test for 3.6 ks at 1400 °C, tungsten–rhenium multilayer (diffusion barrier for carbon) at the joint interface of coating was observed clearly. But, at the temperatures above 1600 °C, the multilayer was disappeared and a tungsten carbide layer was formed in the VPS-tungsten coating. At the temperatures below 1800 °C, the thickness of this layer logarithmically increased with increasing its loading time. At 2000 °C, the growth of the tungsten carbide layer was proportional to the square root of loading time. These results indicate that the diffusion barrier for carbon is not expected to suppress the carbide formation at the joint interface of the VPS-tungsten coating above 1600 °C.

Changes of composition and microstructure of joint interface of tungsten coated carbon by high heat flux

Tungsten coatings of 0.5 and 1 mm thickness were successfully deposited by the vacuum plasma spraying (VPS) technique on carbon/carbon fiber composite (CFC), CX-2002U and isotropic fine grained graphite, IG-430U. High heat flux experiments by irradiation of electron beam with uniform profile were performed on the coated samples in order to prove the suitability and load limit of such coating materials. The cross-sectional composition and structure of the interface of VPS–W and carbon material samples were investigated. Compositional analyses showed that the Re/W multi-layer acts as diffusion barrier for carbon and suppresses tungsten carbide formation in the VPS–W layer at high temperature about 1300°C. Microstructure of the joint interface of the sample changed in the case of a peak temperature of about 2800°C. The multi-layer structure completely disappeared and compositional distribution was almost uniform in the interface of the sample after melting and resolidification. The diffusion barrier for carbon is not expected to act in this stage.

Development of a diffusion barrier layer for silicon and carbon in molybdenum—a physical vapor deposition approach

During the last 2 decades, research on high-temperature, oxidation-resistant coating systems for refractory metals has focused on a variety of silicides (e.g., Mo and Ta silicides), due to their excellent resistance to oxidation. However, commercialization efforts have been thwarted in large measure due to the diffusion of silicon from the coating to the substrate, resulting not only in the depletion of silicon from the coating, but also the formation of less oxidation-resistant subsilicides. Consequently, the development of a high-temperature, diffusion barrier layer for silicon has assumed considerable importance. Furthermore, introduction of carbon in the system, e.g., during the synthesis of MoSi2-SiC composite thin films on molybdenum substrates, results in the diffusion of both silicon and carbon into the substrate, necessitating the development of a barrier layer for both elements. This article examines the possibility of using a novel approach—that of reactive radio frequency (RF) sputtering—for synthesizing a diffusion barrier (based on the Mo-Si-C-N quaternary system) for both silicon and carbon. It is shown that reactive rf magnetron sputtering of a composite target (MoSi2 + 1.96 moles SiC) in an argon-nitrogen atmosphere results in the formation of an amorphous layer, of an as-yet undetermined stoichiometry, preventing the diffusion of both silicon and carbon into the molybdenum substrate. This layer is thermally and chemically stable up to at least 1260 °C.