Microwave Plasma Chemical Vapor Deposition Research Articles

Edge effect during microwave plasma chemical vapor deposition diamond-film: Multiphysics simulation and experimental verification

Edge effect during microwave plasma chemical vapor deposition diamond-film: Multiphysics simulation and experimental verification

(Invited) Preparation of N-Type and P-Type Doped Single Crystal Diamonds with Laser-Induced Doping and Microwave Plasma Chemical Vapor Deposition

In this study, n-type doped single crystal diamonds were successfully prepared under normal temperature and pressure conditions using laser-induced doping. 248nm pulsed laser beams with nanosecond duration were irradiated on the single crystal diamond substrate immersing in an 85% phosphoric acid solution and it introduced phosphorus doping to form an n-type doped thin layer. The resistivity of the doped region significantly decreased compared to that of the single crystal diamond. After depositing a titanium electrode, the resistivity of the doped film obtained by Van der Pauw Technique was determined to be 1.3×10-6 Ω·m. Raman spectroscopy confirmed the absence of carbon or graphite phases in the laser-induced doped diamond, indicating that the enhanced conductivity was due to phosphorus incorporation.Furthermore, p-type doped single crystal diamonds were successfully prepared by introducing boron during the growth process using Microwave Plasma Chemical Vapor Deposition（MPCVD）. It was demonstrated that the proposed technique can introduce impurities into single crystal diamonds to form doped conductive thin layers, which has potential applications in the field of microdevices and integrated circuits.

Nanocrystalline Diamond Lateral Overgrowth for High Thermal Conductivity Contact to Unseeded Diamond Surface

We demonstrate diamond lateral overgrowth as a way of increasing the thermal conductivity of thin layers of diamond. The technique can be used as a way of growing diamond on top of semiconductors, creating a thin layer of high thermal conductivity diamond in direct contact with semiconductors and allowing for the encasement of GaN in high thermal conductivity diamond.As we move to higher and higher power densities, the requirements for heat removal become extreme. The best passive way to remove heat from a semiconductor is to have a high thermal conductivity heat spreader near the heat source. Having diamond under the substrate is good. Having diamond under the epi is better and having diamond on top and bottom of the epi is better still.As-grown thin layers of diamond on non-diamond substrates have nano-diamond seeds as the starting material. The diamond thermal conductivity is a function of the grain size (Especially for small grain material) [1]. If we can increase the grain of the diamond near the junction, we can increase the thermal conductivity near the junction and more efficiently spread the heat away from the source before rejecting it into the surrounding material.In the experiment, we start with a bare silicon wafer seeded with nano-diamond seeds then grow about one micron of diamond. We coat it with a few nanometers of SiN. Then pattern the SiN layer with 2μm wide features to facilitate the coalescence of LOG-NCD over the SiN. Then we grow a 2μm thick microwave-plasma CVD NCD film (800W, 15Torr, 750°C, 0.3% CH4/H2) over the patterned SiN film initiating directly from the exposed areas of the host NCD. The diamond growth does not initiate on the patterned silicon nitride without seeds. The exposed diamond acts as a nucleating center initiating the diamond regrowth. The regrown diamond spreads from the diamond crystals exposed from the first growth. The edge crystals closest to the SiN patterned area expand over the silicon nitride forming diamond conformal to the SiN surface. This creates large crystals over the unseeded area. Figure 1 is a transmission electron microscope (TEM) image of the sample. The TEM, the image can be split into two regions. The left has no patterning, the called-out grain is 100nm wide. The right is patterned, and the diamond grains are between 1,000 to 2,000nm wide.We measured the thermal conductivity of the diamond on this samples using the TDTR method. This method uses a laser many times larger in diameter than the thickness of the film. As such, we probe the out-of-plane thermal conductivity. The initial diamond growth near the silicon had an average thermal conductivity of 100W/mK. We measured the second micron of growth without patterns to have Tc of 130W/mK. The diamond directly above the patterns had Tc of 260W/mK with grains 10X larger than the un-patterned diamond.Since the diamond grains are columnar, the difference in thermal conductivity between the patterned and un-patterned diamonds was 2X. Lateral measurements would likely show even larger changes in thermal conductivity. With grain sizes of 100 nm to 250 nm (as is the case with seeded diamond), the in-plane thermal conductivity will be dominated by the quality of the grain/grain interfaces and the lateral grain size dimensions rather than for the quality of the lattice. [1] We used lateral overgrowth to increase the grains to microns rather than 100s of nm. This means that we move into a regime where thermal conductivity is dominated by the quality of the diamond in the grains rather than the grain boundaries. As such the thermal conductivity is increased and can be further improved by improving the crystal quality - a parameter which is controlled by the diamond growth conditions. Typically, samples grown with more than 1% methane / hydrogen ratio show lower thermal conductivity [2]. However higher concentrations are important to establish the initial diamond film without damage to the underlying semiconductor. Here, the lateral overgrowth and methane concentration can be balanced to achieve both high thermal conductivity and low damage to the underlying substrate.

Enhanced growth rates of N-type phosphorus-doped polycrystalline diamond via in-liquid microwave plasma CVD

Phosphorus-doped diamond (PDD) exhibits excellent properties, making it suitable for a wide range of applications, such as electronic devices and electrodes. Here, we report the first synthesis of PDD by in-liquid microwave plasma CVD (IL-MPCVD) under high-pressure and low-power conditions. A mixture of methanol (MeOH) and ethanol (EtOH) with triethyl phosphate ((C2H5)3PO4) and (P/C = 1000 ppm) was used for the PDD deposition. Samples were characterized by laser microscopy, Raman spectroscopy, and glow discharge optical emission spectroscopy. Notably, PDD was successfully produced at a growth rate of 280 μm/h, which is two orders of magnitude higher than conventional CVD methods. Additionally, cyclic voltammetry (CV) and impedance spectroscopy (EIS) were used to evaluate the electrochemical properties of PDD. As a result, we confirmed the wide potential window characteristic of conductive diamond and determined that the donor density was [P] = 3.8 × 1017cm⁻³. Therefore, it is clear that IL-MPCVD is applicable for very high growth rates in the CVD process for PDD synthesis.

Spectroscopic analysis of pulsed-mode plasma with argon addition for diamond growth

The advancement of high-speed growth technology for large-scale single-crystal diamonds is desired. In the widely used microwave plasma chemical vapor deposition method, the gas temperature in the plasma atmosphere significantly contributes to the generation of reactive radicals and enhancement of crystal growth. However, the impact of electron-dominated reactions in the plasma on the crystal growth remains unclear. In this study, we actively controlled the plasma environment by adding argon gas and adopting microwave pulse modulation to generate the plasma. We estimated the gas temperature and electron density of the plasma using optical emission spectroscopy. Our results implied that an increase in gas temperature alone hardly explained the enhancement of the growth rate by the argon addition or pulse modulation. In addition to the increase in the electron density due to the argon addition and pulse modulation, gas-phase chemical reaction calculations showed that the radical production enhancement became remarkable under an electron density higher than a threshold (1017 m−3). Therefore, the enhancement of the growth rate mentioned above may be attributed to electron-dominated reactions in the discharge region. These findings suggest that increasing the electron density can further improve the growth rate and potentially enable diamond synthesis at lower temperatures than traditional methods.

Microstructure Control and Mechanical Properties of Ultra-nanocrystalline Diamond Films

Ultra-nanocrystalline diamond (UNCD) films exhibit excellent mechanical properties because of their unique structure, such as small grain size and high grain boundary ratio. The effect of nitrogen atoms in nitrogen-doped UNCD films on their structure and mechanical properties deserves in-depth study. In this study, we focus on the microstructure control of UNCD films and its effect on the mechanical properties. Ultra-nanocrystalline diamond films were prepared using microwave plasma chemical vapor deposition under varying nitrogen flow rates. It was observed that the hardness and elastic modulus of UNCD films initially increased and then decreased with the increase of nitrogen flow rate. The structure of UNCD films was characterized using transmission electron microscopy and the crystal quality was analyzed through Raman spectroscopy. Nitrogen-interstitial atom defects were identified in the photoluminescence spectra. These defects contribute to shorter interatomic bonds and enhance the covalent bonds in the crystalline structure, resulting in an increase in hardness. The sp2/sp3 ratio and effective nitrogen atom concentration in the UNCD film were analyzed by X-ray photoelectron spectroscopy. The results indicate that an increase in the effective nitrogen atom concentration corresponds with a notable reduction in grain size, which is simultaneously accompanied by a rise in the number of grain boundaries. Meanwhile, the increase of sp2 content in the UNCD leads to the increase of grain boundary width and density, resulting in the increase of hardness and elastic modulus. This work provides theoretical basis and experimental guidance for the development of high performance UNCD films through precisely regulating the microstructure.

Evaluation of the effect of oxygen addition on charge carrier transport properties in single-crystal diamond growth

The effects of oxygen addition during single-crystal diamond growth by microwave plasma CVD method on crystal surface morphology and charge carrier transport properties were evaluated. Diamond single-crystals were homoepitaxially grown on (001) surface of high-pressure and high-temperature (HPHT) synthesized type IIa single-crystal diamond substrates with a fixed methane concentration of 1 % and oxygen concentrations of 0–0.8 %. Inverted pyramid-shaped pits were generated as the oxygen concentration increased. The free exciton recombination emission intensity in the cathodoluminescence spectrum reached a maximum at oxygen concentration of 0.5 %. The samples with oxygen concentrations of 0.5 % and 0.8 % showed excellent charge collection efficiency and energy resolution. On the other hand, the mobility-lifetime (μτ) product was only (7.1 ± 1.7) × 10−5 cm2/V, which is approximately 1/4 of that of the sample with 0.2 % methane and no oxygen addition.

Preparation of CNT/diamond composite via MPCVD: The interface behavior

Carbon nanotubes (CNTs) and diamonds are allotropes of carbon, exhibiting excellent thermal, mechanical, optical, and electrical properties. Integrating CNTs with diamonds could advance the thermal management of high-heat-flux devices, light absorption, and novel semiconductor devices. This study reports the synthesis of vertical CNTs on a diamond substrate using microwave plasma chemical vapor deposition (MPCVD). Enhanced interfacial adhesion between the CNTs and diamond was confirmed by subjecting the CNT/diamond composite to ultrasonic vibrations and analyzing the morphology via scanning electron microscopy. The growth mechanism of CNTs on the diamond and the CNT/diamond interface bonding behavior were investigated using scanning transmission electron microscopy, electron energy loss spectroscopy, energy-dispersive X-ray spectroscopy, and optical emission spectroscopy. The results indicate that the carbon supplied from two different sources drove the growth of CNTs primarily through the root-growth mechanism and, to a lesser extent, through the tip-growth mechanism. The tip-growth mechanism may involve the formation of a covalent connection at the CNT/diamond interface. For the root-growth mechanism, two interfaces emerged in the CNT/diamond composite, namely Fe/diamond and CNT/Fe. Additionally, mixed cementite phases connected the CNTs to the diamond substrate. Elucidating the bonding mechanism between the CNTs and diamond can provide insights into the dynamics of the carbon atoms in the catalyst and offer a deeper understanding of the growth mechanism of CNTs on the diamond substrate.

Directional growth of graphene by microwave plasma CVD constructed TiO2@Ag@graphene ternary junction for efficient hydrogen production

This work proposes a novel strategy which the microwave plasma chemical vapor deposition (MPCVD) technology was used as one important preparation process for synthesizing TiO2@Ag@graphene ternary junction. The ternary junction exhibits an enhanced photocatalytic hydrogen production rate of 4.44 mmol·g−1·h−1, which is approximately 9.25 times higher than that of pure TiO2 (0.48 mmol·g−1·h−1). The remarkable photocatalytic activity can be attributed to the strong synergistic effect between TiO2@Ag and graphene. The loaded Ag and graphene can not only act as electronic conductors to effectively promote the separation of charges, but also obviously improve the response of visible light of TiO2. The photogenerated electrons are transferred from TiO2 to graphene via the Ag medium was further verified by DFT calculations. Constructing an Ag-graphene interface can activate carbon atoms, which can quickly capture electron to fill the π* orbital of graphene. TiO2@Ag@graphene ternary junction exhibits a lower ΔGH* value of 0.51 eV than that of TiO2@Ag (1.49 eV), which indicates that TiO2@Ag@graphene ternary junction with optimal absorption energy for the hydrogen intermediate.

Interfacial improvement of diamond through epitaxial lateral overgrowth with periodic Ir/SiO2/Ir stripe mask

Epitaxial lateral overgrowth (ELO) of diamond film with SiO2 stripe mask has been carried out through microwave plasma chemical vapor deposition systems. To solve the peel-off of SiO2, this study evaluates different kinds of metal buffer layers and chooses Ir as the optimal metal. The effects of the interface feature of two types of stripe masks (Ir/SiO2 and Ir/SiO2/Ir stripes) on the internal stress status and the fluorescence properties are characterized. The optical image confirms that the diamond emerges in the interval of stripes at the initial growth stage and then forms a continuous film with smooth surface and step-flow mode after 37 h lateral overgrowth. Photoluminescence (PL) mapping demonstrates that the SiO2 mask is etched by H2 plasma and results in a high SiV− intensity, especially above the mask. The Ir cap layer on the Ir/SiO2 mask surface is effective in preventing the SiO2 from being etched in the plasma atmosphere, greatly alleviating the PL intensity of the SiV− color center compared with that of the Ir/SiO2 mask. In addition, the Ir cap layer can suppress the stress in the epitaxial layer. The findings of this study can advance the existing research on ELO technology and be potentially applied to vacancy-related color centers.

Oriented growth of 5-inch optical polycrystalline diamond films by suppressing dark features

Optical diamond materials are essential for infrared optical windows, microwave windows, and laser windows due to their exceptional properties. However, dark features inside the diamond are significant factors limiting optical properties, particularly at high deposition rates. This work focuses on adjusting crystallographic parameters to prepare (110) oriented optical-grade polycrystalline diamond films, aiming to mitigate dark features. Diamond films deposited via microwave plasma chemical vapor deposition (MPCVD) exhibit discernible quality stratification. Characterization analysis of diamond film samples with varying levels of black defects reveals a correlation: films with higher dark feature content tend to exhibit a preference for the (111) orientation, accompanied by a significant presence of penetration twins and interstitial voids. Conversely, transparent diamond films demonstrate a prominent (110) orientation, featuring numerous contact twins that alleviate competition from penetration twins and original grains during growth. Electron backscatter diffraction (EBSD) and scanning electron microscope (SEM) results reveal that transparent diamond films cultivated with a pronounced preference for the (110) orientation manifest consistent and seamless contact twins, leading to fewer dark features and improved optical transmittance. In contrast, (111) oriented diamond film exhibits numerous penetration twins, and holes form between them. Based on the analysis, 5-inch optical polycrystalline diamond films with (110) orientation were achieved successfully by suppressing dark features.

Novel superhard BC10N synthesized by microwave plasma CVD

Microwave Plasma Chemical Vapor Deposition (MPCVD) was used to synthesize novel superhard BC10N on silicon substrates. Feedgas mixtures of H2, CH4, N2, and B2H6 were used for a range of systematically varied microwave power, chamber pressure, and flow rate conditions. Optical Emission Spectroscopy (OES) was used to guide the synthesis of BC10N. Plasma optical emission from the C2 peak increases with pressure up to 80 Torr and saturates in the 80–100 Torr range. The C2 emission also follows the same trend, growing non-linearly with increasing microwave power (for fixed chamber pressure of 30 Torr). The presence of abundant C2 in the plasma is advantageous under specific growth conditions to produce superhard carbon-rich BC10N, a material that has been theoretically predicted but not yet synthesized. Findings obtained from X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FTIR) elucidate the existence of BC, CN, BN, and B-N-C bonding in the synthesized coating. Nanoindentation hardness measurements are within the superhard regime, showing an average hardness of 63 GPa.

Fast, Efficient Tailoring Growth of Nanocrystalline Diamond Films by Fine-Tuning of Gas-Phase Composition Using Microwave Plasma Chemical Vapor Deposition.

Nanocrystalline diamond (NCD) films are attractive for many applications due to their smooth surfaces while holding the properties of diamond. However, their growth rate is generally low using common Ar/CH4 with or without H2 chemistry and strongly dependent on the overall growth conditions using microwave plasma chemical vapor deposition (MPCVD). In this work, incorporating a small amount of N2 and O2 additives into CH4/H2 chemistry offered a much higher growth rate of NCD films, which is promising for some applications. Several novel series of experiments were designed and conducted to tailor the growth features of NCD films by fine-tuning of the gas-phase compositions with different amounts of nitrogen and oxygen addition into CH4/H2 gas mixtures. The influence of growth parameters, such as the absolute amount and their relative ratios of O2 and N2 additives; substrate temperature, which was adjusted by two ways and inferred by simulation; and microwave power on NCD formation, was investigated. Short and long deposition runs were carried out to study surface structural evolution with time under identical growth conditions. The morphology, crystalline and optical quality, orientation, and texture of the NCD samples were characterized and analyzed. A variety of NCD films of high average growth rates ranging from 2.1 μm/h up to 6.7 μm/h were successfully achieved by slightly adjusting the O2/CH4 amounts from 6.25% to 18.75%, while that of N2 was kept constant. The results clearly show that the beneficial use of fine-tuning of gas-phase compositions offers a simple and effective way to tailor the growth characteristics and physical properties of NCD films for optimizing the growth conditions to envisage some specific applications.

Improving Trace Detection of Methylene Blue by Designing Nanowire Array on Boron-Doped Diamond as Electrochemical Electrode

In this study, a boron-doped diamond nanowire array (BDD-NWA)-based electrode is prepared by using a microwave plasma chemical vapor deposition system and treated with inductively coupled plasma reactive ion etching. The BDD-NWA electrode is used for trace detection of methylene blue, which has a wide linear range of 0.04–10 μM and a low detection limit of 0.72 nM. Both the superhydrophilicity (contact angle ~0°) and the dense nanowire array’s structure after the etching process improve the sensitivity of the electrochemical detection compared to the pristine BDD. In addition, the electrode shows great repeatability (peak current fluctuation range of −3.3% to 2.9% for five detection/cleaning cycles) and stability (peak current fluctuation range of −5.3% to 6.3% after boiling) due to the unique properties of diamonds (mechanical and chemical stability). Moreover, the BDD-NWA electrode achieves satisfactory recoveries (93.8%–107.5%) and real-time monitoring in tap water.

Enhancing CO2 Reduction: Insights from In-Liquid Microwave Plasma Chemical Vapor Deposition

Enhancing CO₂ Reduction: Insights from In-Liquid Microwave Plasma Chemical Vapor Deposition

Formation of Diamond Films with Different Grain Sizes on TiO2 Nanotubes and Its Field Emission Properties

Diamond films with different grain sizes are deposited on TiO2 nanotube (TNT) arrays prepared by anodic oxidation using a microwave plasma chemical vapor deposition reactor. The scanning electron microscope, X‐Ray diffractometer, and Raman results indicate that the diamond film is successfully deposited and that the substrate undergoes a phase transition due to the diamond deposition temperature, resulting in the rutile phase becoming dominant with lower energy bandgap and work function. Notably, after 5 h of deposition, a relatively continuous microcrystalline film is formed on the TNTs, and the diamond grains change from spherical to pyramidal and show excellent electron field emission behavior, with a low turn‐on field of 0.5 V μm−1 and a current density of 85.9 μA cm−2. In addition, the deposition environment of the nanocrystalline diamond has a large impact on the substrate morphology, resulting in a blocked electron transport path, which reduces the overall field emission performance. The enhanced performance is attributed to the synergistic effect between the highly efficient 1D electron path of the TNTs, the negative electron affinity of the diamond surface, and the lower work function of rutile TiO2.

Synthesis and Mechanism Study of Carbon Nanowires, Carbon Nanotubes, and Carbon Pompons on Single-Crystal Diamonds

Carbon nanomaterials are in high demand owing to their exceptional physical and chemical properties. This study employed a mixture of CH4, H2, and N2 to create carbon nanostructures on a single-crystal diamond using microwave plasma chemical vapor deposition (MPCVD) under high-power conditions. By controlling the substrate surface and nitrogen flow rate, carbon nanowires, carbon nanotubes, and carbon pompons could be selectively deposited. The results obtained from OES, SEM, TEM, and Raman spectroscopy revealed that the nitrogen flow rate and substrate surface conditions were crucial for the growth of carbon nanostructures. The changes in the plasma shape enhanced the etching effect, promoting the growth of carbon pompons. The CN and C2 groups play vital catalytic roles in the formation of carbon nanotubes and nanowires, guiding the precipitation and composite growth of carbon atoms at the interface between the Mo metal catalysts and diamond. This study demonstrated that heterostructures of diamond–carbon nanomaterials could be produced under high-power conditions, offering a new approach to integrating diamond and carbon nanomaterials.

(Invited) Preparation of N-Type and P-Type Doped Single Crystal Diamonds with Laser-Induced Doping and Microwave Plasma Chemical Vapor Deposition

In this study, n-type doped single-crystal diamonds were successfully prepared under normal temperature and pressure conditions using laser-induced doping. 248nm pulsed laser beams with nanosecond duration were irradiated on the single crystal diamond substrate immersing in an 85% phosphoric acid solution and it introduced phosphorus doping to form an n-type doped thin layer. The resistivity of the doped region significantly decreased compared to that of the single-crystal diamond. After depositing a titanium electrode, the resistivity of the doped film obtained by Van der Pauw Technique was determined to be 1.5×10-4 Ω·cm. Furthermore, p-type doped single-crystal diamonds were successfully prepared by introducing boron during the growth process using Microwave Plasma Chemical Vapor Deposition（MPCVD）. It was demonstrated that the proposed technique can introduce impurities into single crystal diamonds to form doped conductive thin layers.

Nitrogen adsorption induced surface kinetics changes of diamond growth by microwave plasma CVD

The strong temperature dependence of the contribution of nitrogen to diamond growth enhancement by microwave plasma chemical vapor deposition (MPCVD) have been firstly reported and systematically studied. Generally, an enhanced hydrogen desorption from the growing surface induced by the incorporated nitrogen atoms in diamond sublayer has been successfully used to explain the weak temperature depended growth enhancement of diamond by nitrogen addition, but cannot be employed to explain the observed strong temperature dependence of nitrogen contribution to diamond growth enhancement observed in this work. This indicates a significant change of surface kinetics of diamond growth induced by nitrogen addition should be occurred around 700 °C where a maximum value achieved. At this relative low temperature, the growing surface sites should be almost fully covered with hydrogen atoms with relative low nitrogen incorporated in the diamond. The adsorbed nitrogen may then make a contribution to promote hydrogen desorption, which is believed to play a dominant role in affecting the contribution of nitrogen. Experimental and calculation results do show that both the nitrogen adsorbed on the growing surface and incorporated in the sublayer may have different contribution to the growth rate enhancement, resulting in strong and weak temperature dependence, respectively.

Low-Temperature Deposition of Diamond Films by MPCVD with Graphite Paste Additive

Modern integrated circuits (ICs) take advantage of three-dimensional (3D) nanostructures in devices and interconnects to achieve high-speed and ultra-low-power performance. The choice of electrical insulation materials with excellent dielectric strength, electrical resistivity, strong mechanical strength, and high thermal conductivity becomes critical. Diamond possesses these properties and is recently recognized as a promising dielectric material for the fabrication of advanced ICs, which are sensitive to detrimental high-temperature processes. Therefore, a high-rate low-temperature deposition technique for large-grain, high-quality diamond films of the thickness of a few tens to a few hundred nanometers is desirable. The diamond growth rate by microwave plasma chemical vapor deposition (MPCVD) decreases rapidly with lowering substrate temperature. In addition, the thermal conductivity of non-diamond carbon is much lower than that of diamond. Furthermore, a small-grain diamond film suffers from poor thermal conductivity due to frequent phonon scattering at grain boundaries. This paper reports a novel MPCVD process aiming at high growth rate, large grain size, and high sp3/sp2 ratio for diamond films deposited on silicon. Graphite paste containing nanoscale graphite and oxy-hydrocarbon binder and solvent vaporizes and mixes with gas feeds of hydrogen, methane, and carbon dioxide to form plasma. Rapid diamond growth of diamond seeds at 450 °C by the plasma results in large-grained diamond films on silicon at a high deposition rate of 200 nm/h.