Microwave Plasma-enhanced Chemical Vapor Deposition Research Articles

The effects of carbon incorporation on the refractive index of PECVD silicon oxide layers

Silicon oxide (SiOx) has many applications, including as a low-refractive index material. Plasma enhanced chemical vapor deposition (PECVD) processes are facile, low temperature routes to produce thin SiOx layers. A route to decrease the refractive index of SiOx films is to increase the layer porosity although maintaining structural and optical stability remains challenging. Organic carbon-containing sacrificial layers have been shown to modify the growth and resulting structure of PECVD SiOx layers. In this work, we study the effect of adding methane (CH4) to the standard SiOx process gas mixture (silane and nitrous oxide) and varying deposition temperatures and microwave power in an industrial-scale, microwave PECVD reactor. Spectral ellipsometry was used to measure the optical properties of deposited layers, Fourier-transformed infrared (FTIR) spectroscopy to determine bonding and the layer porosity, and optical emission spectroscopy to characterize the plasma. We propose two regimes characterized by whether adding CH4 increases or decreases the refractive index and porosity of deposited layers compared to SiOx layers grown under standard conditions. However, the magnitude of the effect of adding CH4 was not large and would not find industrial application. Furthermore, the deposited layers’ refractive indices increased over time, indicating that the effects of adding CH4 to the process gas mixture were not stable. To help explain our results and to provide guidance for future efforts to better control the refractive index of PECVD SiOx layers via carbon incorporation while maintaining layer stability, we propose possible growth pathways for our layers considering both plasma reactions and surface processes.

Enhanced Electromagnetic Wave Shielding Effectiveness of Carbon-Based Nonwoven Fabrics by H<sub>2</sub> Plasma Treatment

Electromagnetic wave shielding effectiveness of the nonwoven fabrics was measured in the wide operating frequency range, namely 0.4GHz to 20GHz. The shielding effectiveness of the nonwoven fabric was below 45dB in the range of 0.04GHz to 15GHz and then it increased to above 45dB in the range of 15GHz to 20GHz. To enhance the electromagnetic wave shielding effectiveness of the nonwoven fabrics, 3 minutes H2 plasma treatment of the nonwoven fabrics was carried out under the microwave plasma-enhanced chemical vapor deposition system. By H2 plasma treatment, the shielding effectiveness of the nonwoven fabrics was greatly enhanced in the whole operating frequency range. The surface electron conductivity of the nonwoven fabrics was also enhanced from 2.11×103 S/m to 3.02×103S/m by H2 plasma treatment. The surface and cross sectional morphologies of the nonwoven fabrics with or without H2 plasma treatment were investigated and compared with each other. Crystal structure variation of the nonwoven fabrics by H2 plasma treatment was also investigated. Based on these results, the cause for the enhancement of the shielding effectiveness of the nonwoven fabrics by H2 plasma treatment was suggested and discussed.

Microstructures Manufactured in Diamond by Use of Laser Micromachining.

Different microstructures were created on the surface of a polycrystalline diamond plate (obtained by microwave plasma-enhanced chemical vapor deposition—MW PECVD process) by use of a nanosecond pulsed DPSS (diode pumped solid state) laser with a 355 nm wavelength and a galvanometer scanning system. Different average powers (5 to 11 W), scanning speeds (50 to 400 mm/s) and scan line spacings (“hatch spacing”) (5 to 20 µm) were applied. The microstructures were then examined using scanning electron microscopy, confocal microscopy and Raman spectroscopy techniques. Microstructures exhibiting excellent geometry were obtained. The precise geometries of the microstructures, exhibiting good perpendicularity, deep channels and smooth surfaces show that the laser microprocessing can be applied in manufacturing diamond microfluidic devices. Raman spectra show small differences depending on the process parameters used. In some cases, the diamond band (at 1332 cm−1) after laser modification of material is only slightly wider and shifted, but with no additional peaks, indicating that the diamond is almost not changed after laser interaction. Some parameters did show that the modification of material had occurred and additional peaks in Raman spectra (typical for low-quality chemical vapor deposition CVD diamond) appeared, indicating the growing disorder of material or manufacturing of the new carbon phase.

Electrochemical performance of thin free-standing boron-doped diamond nanosheet electrodes

In the following work we describe preparation and the electrochemical performance of thin and free-standing heavy boron-doped diamond (BDD) nanosheets. The investigated foils were deposited on Ta substrate using microwave plasma-enhanced chemical vapor deposition technique (MPECVD). Foils of two B-dopant densities were investigated, obtained on the base of 10 k and 20 k ppm [B]/[C] ratio in the gas admixture. The obtained foils can be easily peeled from substrate in deionized water to be then attached to other material, in this case polydimethylsiloxane (PDMS). We have shown that the top surface and the bottom side of investigated boron-doped diamond nanosheet possess significantly altered morphology and physico-chemical properties, revealed by electron microscopy, Raman spectroscopy and electrochemistry. The voltammetric response of investigated BDD foils as working electrodes indicates the highest activity for the nanosheet with higher dopant concentration, in particular on its top surface. Furthermore, electrodes are characterized with altered kinetics, characteristic for partially blocked electrodes with quasi-reversible charge transfer.

Phase Transformation of Nanocrystalline Diamond Films: Effect of Methane Concentration

Ultra-nanocrystalline diamond films were prepared by a microwave plasma-enhanced chemical vapor deposition reactor using CH4/H2 gas mixture with a power as low as 650 W. The effects of CH4 concentration on nanostructure of the films and gas-phase species in plasma were investigated. The CH4 concentrations of 1.5%, 3.0%, 3.5%, and 4.0% were used and balanced with H2 to a total flow rate of 200 sccm. Morphology and composition of the films were characterized by SEM, Raman spectroscopy and Auger spectroscopy. The gas-phase species and electron density in the plasma were explored by optical emission spectroscopy and plasma-impedance measurement. The increasing CH4 concentration from 1.5% to 4.0% increased C2Hx species and decreased electron density. Phase of the film transform from nano- into ultranano- diamond phase but the growth rate revealingly decreased from 300 to 210 nm/h. Raman spectra indicate the higher CH4 concentration promted phase of the film transiton from NCD to UNCD. While Auger spectra revealed that UNCD film deposited with 4.0%CH4 was composed of 90.52% diamond phase but only 9.48% of graphite phase. The relation between phase transformation of the films and growth mechnism controlled by gas-phase species in the plasma will be dissused.

Graphene In Situ Coated High-Oxygen Vacancy Co3O4−x Sphere Composites for High-Stability Supercapacitors

In this paper, the graphene in situ coated Co3O4 core–shell heterogeneous composites have been facilely fabricated via microwave plasma-enhanced chemical vapor deposition method. The graphene thin-shell-layer-covered Co3O4−x particles were revealed by FE-SEM, XRD, XPS and Raman spectra. And the ratio of Co2+/Co3+ is adjusted, and abundant surface oxygen vacancies are created by the microwave plasma etching, which can contribute to the improvement of electrochemical performance for the Co3O4/graphene core–shell composites. Results present that the graphene in situ coated Co3O4−x has a specific capacitance of 192.8 F g−1 under the current density of 0.5 A g−1, which is 4.5 times than that of the original Co3O4 sphere. Meanwhile, the core–shell heterogeneous composite displays excellent cyclic stability with ~ 98.5% specific capacitance retained after 20,000 cycle tests.

Enabling highly effective underwater oxygen-consuming reaction at solid-liquid-air triphasic interface

Electrode surfaces with superhydrophobic and conductive properties exhibit a unique energy-mass transfer behavior in electrolytes. Therefore, the rational design of electrode surface structures, conductivity, and infiltration performance is expected to yield more efficient electrochemical reactions and solve the gas-deficit problem that hinders many underwater gas-consuming reaction systems. In this paper, hydrogenated carbon nano-onions displaying superhydrophobicity and conductivity were prepared by microwave plasma-enhanced chemical vapor deposition, which exhibited remarkable transferability and designability. The fabricated carbon nano-onions were used to modify the surfaces of various materials and structures toward their application to self-cleaning, oil-water separation, and enabling highly effective underwater oxygen-consuming reactions. Systematic analyses of the different wetting states of the superhydrophobic electrodes coated with the fabricated carbon nano-onions indicated that the different wetting states have important effects on their behaviors for underwater nucleation reactions, thus changing the efficiency of underwater oxygen-consuming reaction systems. The results reveal that designing a superhydrophobic electrode with a Wenzel-Cassie coexistent underwater wetting state, rather than a Cassie state or Wenzel state, is the key to enabling highly effective underwater oxygen-consuming reactions.

Fabrication of electrolyte-gate nanocrystalline diamond- based field effect transistor (NCD-EGFET) for HIV-1 Tat protein detection

In this paper, we reported on the fabrication process of electrolyte-gate field effect transistor using nanocrystalline diamond as a sensing transducer. The fabrication procedure was begin with the growth of nanocrystalline diamond thin film on silicon/silicon dioxide (Si/SiO2) substrate using microwave plasma-enhanced chemical vapour deposition (CVD). Then the photolithography process was performed in order to design and pattern the field effect transistor device with the active gate channel of 60 µm length and 20 µm width. Each device consists of three active gate channel which connecting to three different pairs of source and drain contact. The surface morphology of fabricated NCD-EGFET was characterized using Scanning Electron Microscope to clarify the active gate channel of the device and the grain size of nanocrystaline diamond. The current-voltage (I-V) measurement of the device were carried out to study the electrical behaviour for HIV-1 Tat protein detection via RNA aptamer as sensing probe.

Adhesion-Increased Carbon Nanowalls for the Electrodes of Energy Storage Systems

Carbon nanowalls (CNWs), which are used as electrodes for secondary batteries in energy storage systems (ESSs), have the widest reaction surface area among the carbon-based nanomaterials, but their application is rare due to their low adhesion with substrates. Indium tin oxide (ITO), a representative transparent conducting oxide (TCO) material, is widely used as the electrode for displays, solar cells, etc. Titanium nitride (TiN) is a well-used material as an interlayer for improving the adhesion between two materials. In this study, ITO or TiN thin films were used as an interlayer to improve the adhesion between a CNW and a substrate. The interlayer was deposited on the substrate using a radio frequency (RF) magnetron sputtering system with a four-inch TiN or ITO target. CNWs were grown on the interlayer-coated substrate using a microwave-plasma-enhanced chemical vapor deposition (MPECVD) system with a mixture of methane (CH4) and hydrogen (H2) gases. The adhesion of the CNW/interlayer/substrate structure was observed through ultrasonic cleaning.

Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source.

Electron emission signifies an important mechanism facilitating the enlargement of devices that have modernized large parts of science and technology. Today, the search for innovative electron emission devices for imaging, sensing, electronics, and high-energy physics continues. Integrating two materials with dissimilar electronic properties into a hybrid material is an extremely sought-after synergistic approach, envisioning a superior field electron emission (FEE) material. An innovation is described regarding the fabrication of a nanostructured carbon hybrid, resulting from the one-step growth of boron-doped nanocrystalline diamond (BNCD) and carbon nanospikes (CNSs) by a microwave plasma-enhanced chemical vapor deposition technique. Spectroscopic and microscopic tools are used to investigate the morphological, bonding, and microstructural characteristics related to the growth mechanism of these hybrids. Utilizing the benefits of both the sharp edges of the CNSs and the high stability of BNCD, promising FEE performance with a lower turn-on field of 1.3 V/μm, a higher field enhancement factor of 6780, and a stable FEE current stability lasting for 780 min is obtained. The microplasma devices utilizing these hybrids as a cathode illustrate a superior plasma illumination behavior. Such hybrid carbon nanostructures, with superb electron emission characteristics, can encourage the enlargement of several electron emission device technologies.

Transition from Self-Organized Criticality into Self-Organization during Sliding Si3N4 Balls against Nanocrystalline Diamond Films

The paper investigates the variation of friction force (Fx) during reciprocating sliding tests on nanocrystalline diamond (NCD) films. The analysis of the friction behavior during the run-in period is the focus of the study. The NCD films were grown using microwave plasma-enhanced chemical vapor deposition (MW-PECVD) on single-crystalline diamond SCD(110) substrates. Reciprocating sliding tests were conducted under 500 and 2000 g of normal load using Si3N4 balls as a counter body. The friction force permanently varies during the test, namely Fx value can locally increase or decrease in each cycle of sliding. The distribution of friction force drops (dFx) was extracted from the experimental data using a specially developed program. The analysis revealed a power-law distribution f−µ of dFx for the early stage of the run-in with the exponent value (µ) in the range from 0.6 to 2.9. In addition, the frequency power spectrum of Fx time series follows power-law distribution f−α with α value in the range of 1.0–2.0, with the highest values (1.6–2.0) for the initial stage of the run-in. No power-law distribution of dFx was found for the later stage of the run-in and the steady-state periods of sliding with the exception for periods where a relatively extended decrease of coefficient of friction (COF) was observed. The asperity interlocking leads to the stick-slip like sliding at the early stage of the run-in. This tribological behavior can be related to the self-organized criticality (SOC). The emergence of dissipative structures at the later stages of the run-in, namely the formation of ripples, carbonaceous tribolayer, etc., can be associated with the self-organization (SO).

Single-crystalline boron-doped diamond superconducting quantum interference devices with regrowth-induced step edge structure

Superconducting quantum interference devices (SQUIDs) are currently used as magnetic flux detectors with ultra-high sensitivity for various applications such as medical diagnostics and magnetic material microstructure analysis. Single-crystalline superconducting boron-doped diamond is an excellent candidate for fabricating high-performance SQUIDs because of its robustness and high transition temperature, critical current density, and critical field. Here, we propose a fabrication process for a single-crystalline boron-doped diamond Josephson junction with regrowth-induced step edge structure and demonstrate the first operation of a single-crystalline boron-doped diamond SQUID above 2 K. We demonstrate that the step angle is a significant parameter for forming the Josephson junction and that the step angle can be controlled by adjusting the microwave plasma-enhanced chemical vapour deposition conditions of the regrowth layer. The fabricated junction exhibits superconductor–weak superconductor–superconductor-type behaviour without hysteresis and a high critical current density of 5800 A/cm2.

Selective Growth and Contact Gap-Fill of Low Resistivity Si via Microwave Plasma-Enhanced CVD.

Low resistivity polycrystalline Si could be selectively grown in the deep (~200 nm) and narrow patterns (~20 nm) of 20 nm pitch design rule DRAM (Dynamic Random Access Memory) by microwave plasma-enhanced chemical vapor deposition (MW-CVD). We were able to achieve the high phosphorus (CVD gap-fill in a large electrical contact area which does is affected by line pitch size) doping concentration (>2.5 × 1021 cm−3) and, thus, a low resistivity by adjusting source gas (SiH4, H2, PH3) decomposition through MW-CVD with a showerhead controlling the decomposition of source gases by using two different gas injection paths. In this study, a selective growth mechanism was applied by using the deposition/etch cyclic process to achieve the bottom–up process in the L-shaped contact, using H2 plasma that simultaneously promoted the deposition and the etch processes. Additionally, the cyclic selective growth technique was set up by controlling the SiH4 flow rate. The bottom-up process resulted in a uniform doping distribution, as well as an excellent filling capacity without seam and center void formation. Thus, low contact resistivity and higher transistor on-current could be achieved at a high and uniform phosphorus (P)-concentration. Compared to the conventional thermal, this method is expected to be a strong candidate for the complicated deep and narrow contact process.

Effect of oxygen vacancies on the surface morphology and structural properties of WO3– x nanoparticles

WO3 is an essential material for energy storage and catalytical technology, the oxygen vacancies level will play an essential role in its potential application. In this paper, porous tungsten oxide (WO3–x) with various oxygen contents was quickly fabricated by a microwave plasma-enhanced chemical vapor deposition method. A detailed characterization of structure and morphology features with the plasma handling process was recorded by X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy, respectively. The etching mechanism of porous WO3–x under H2 plasma was discussed based on the systemically characterization. These results will help us to design the active sites or structure in the metal oxide materials.

Characterization of nitrogen doped graphene bilayers synthesized by fast, low temperature microwave plasma-enhanced chemical vapour deposition

New techniques to manipulate the electronic properties of few layer 2D materials, unveiling new physical phenomena as well as possibilities for new device applications have brought renewed interest to these systems. Therefore, the quest for reproducible methods for the large scale synthesis, as well as the manipulation, characterization and deeper understanding of these structures is a very active field of research. We here report the production of nitrogen doped bilayer graphene in a fast single step (2.5 minutes), at reduced temperatures (760 °C) using microwave plasma-enhanced chemical vapor deposition (MW-PECVD). Raman spectroscopy confirmed that nitrogen-doped bilayer structures were produced by this method. XPS analysis showed that we achieved control of the concentration of nitrogen dopants incorporated into the final samples. We have performed state of the art parameter-free simulations to investigate the cause of an unexpected splitting of the XPS signal as the concentration of nitrogen defects increased. We show that this splitting is due to the formation of interlayer bonds mediated by nitrogen defects on the layers of the material. The occurrence of these bonds may result in very specific electronic and mechanical properties of the bilayer structures.

Facile synthesis of heterogeneous Co3O4-nanowires/vertical-graphene-nanosheets on Ni foam for high performance supercapacitors

The heterogeneous Co3O4-nanowires/vertical-graphene-nanosheets on Ni foam (Co3O4NW/VGN/NF) was synthesized by microwave plasma-enhanced chemical vapor deposition (MPCVD) and hydrothermal process. The morphology of Co3O4NW/VGN/NF shows a uniform porous structure. The Co3O4NW/VGN/NF electrode exhibits a high area specific capacitance of 8.44 F cm−2 at 5 mA cm−2. The excellent capacitive performance of the composite material can be ascribed to the improved charge transfer rate and ion diffusion path.

Interface engineering of ultrananocrystalline diamond/MoS2-ZnO heterostructures and its highly enhanced hydrogen gas sensing properties

High-performance room temperature based hydrogen gas sensor is fabricated using ultra-nanocrystalline diamond/molybdenum disulfide/zinc oxide nanorods (UNCD/MoS2/ZNRs) nanohybrids, through microwave plasma enhanced CVD (MPE-CVD) and hydrothermal techniques. The interfacial effect and structural analysis of UNCD/MoS2/ZNRs layers were comprehensively studied by several analyses. The systematic investigations were revealed that adding MoS2 layer between the ZNRs and UNCD, strongly influence the gas sensing device performance. In addition, the semi-grown UNCD films serve as an excellent substrate for the synthesis of MoS2/ZNRs. Thus the UNCD/MoS2/ZNRs hybrid sensors exhibits the superior sensitivity of 70.6%, which is overwhelmingly superior to that of ZNRs (7.2%). The outstanding improvement is observed due to the efficient material defects and formation of various heterojunction interfaces in the UNCD/MoS2/ZNRs hybrid. It was revealed that the exfoliated MoS2 layer under ZNRs induces more active sites for the adsorption of O2. Moreover, the chemisorbed O2 ions in the surface react with H2, and release a huge number of electrons to the conduction band. Also, the UNCD/MoS2/ZNRs nanohybrid exhibits excellent robustness along with ultra-high gas sensitivity and selectivity due to the addition of semi-continuous UNCD materials. The outstanding features of this UNCD/MoS2/ZNRs nanohybrid potentially extend to nanodiamond and TMD based hybrid materials for future gas sensor/storage applications.

Graphene synthesis by microwave plasma chemical vapor deposition: analysis of the emission spectra and modeling

In this article, we report on some of the fundamental chemical and physical processes responsible for the deposition of graphene by microwave plasma-enhanced chemical vapor deposition. The graphene is grown by plasma decomposition of a methane and hydrogen mixture (CH4/H2) at moderate pressures over polycrystalline metal catalysts. Different conditions obtained by varying the plasma power (300–400 W), total pressure (10–25 mbar), substrate temperature (700 °C–1000 °C), methane flow rate (1–10 sccm) and catalyst nature (Co–Cu) were experimentally analyzed using the in situ optical emission spectroscopy technique to assess the species rotational temperature of the plasma and the H-atom relative concentration. Then, two modeling approaches were developed to analyze the plasma environment during graphene growth. As a first approximation, the plasma is described by spatially averaged bulk properties, and the species compositions are determined using kinetic rates in the transient zero-dimensional (0D) configuration. The advantage of this approach lies in its small computational demands, which enable rapid evaluation of the effects of reactor conditions and permit the identification of dominant reactions and key species during graphene growth. This approach is useful for identifying the relevant set of species and reactions to consider in a higher-dimensional model. The reduced chemical scheme was then used within the self-consistent two-dimensional model (2D) to determine auto-coherently the electromagnetic field, gas and electron temperatures, heavy species, and electron and ion density distributions in the reactor. The 0D and 2D models are validated by comparison with experimental data obtained from atomic and molecular emission spectra.

Functionalized Carbon Nanowalls as Pro-Angiogenic Scaffolds for Endothelial Cell Activation.

Substrates that can be utilized to assist in expression of the inherent characteristics of cells under in vitro conditions are necessary to mimic the in vivo scenario to the maximum extent possible. We demonstrate the application of functionalized carbon nanowalls (CNW) in facilitating human endothelial cell attachment and proliferation. The CNW films were grown on silicon substrates by surface-wave microwave plasma-enhanced chemical vapor deposition (SWMWPECVD) technique. These CNW were then functionalized using nitrogen (N2) gas plasma to form functionalized N2 doped CNW (CNW-N2). Characterization of CNWs revealed a uniform petal-like morphology with the individual CNW width measured to be 1-5 nm and the film thickness in the range of 10-12 μm. The N2 functionalized CNWs proved to be highly efficient in providing cell-anchorage to the endothelial cells. Profiles of major cytoskeletal proteins revealed a higher degree of expression in the functionalized CNWs depicting the maintenance of structural integrity of cells. Interestingly, the CNW-N2 substrate was found to promote pro-angiogenic factors in the cells, which is observed here for the first time, that could pave way for the utilization of this substrate for detailed studies of angiogenesis processes in vitro and further biomedical applications.

Gradients of microstructure, stresses and mechanical properties in a multi-layered diamond thin film revealed by correlative cross-sectional nano-analytics

Thin diamond films deposited by chemical vapour deposition (CVD) usually feature cross-sectional gradients of microstructure, residual stress and mechanical properties, which decisively influence their functional properties. This work introduces a novel correlative cross-sectional nano-analytics approach, which is applied to a multi-layered CVD diamond film grown using microwave plasma-enhanced CVD and consisting of a ∼8 μm thick nanocrystalline (NCD) base and a ∼14.5 μm thick polycrystalline (PCD) top diamond sublayers. Complementary cross-sectional 30 nm beam synchrotron X-ray diffraction, depth-resolved micro-cantilever and hardness testing and electron microscopy analyses reveal correlations between microstructure, residual stress and mechanical properties. The NCD sublayer exhibits a 1.5 μm thick isotropic nucleation region with the highest stresses of ∼1.3 GPa and defect-rich nanocrystallites. With increasing sublayer thickness, a 110 fibre texture evolves gradually, accompanied by an increase in crystallite size and a decrease in stress. At the NCD/PCD sublayer interface, texture, stresses and crystallite size change abruptly and the PCD sublayer exhibits the presence of Zone T competitive grain growth microstructure. NCD and PCD sublayers differ in fracture stresses of ∼14 and ∼31 GPa, respectively, as well as in elastic moduli and hardness, which are correlated with their particular microstructures. In summary, the introduced nano-analytics approach provides complex correlations between microstructure, stresses, functional properties and deposition conditions.