Chemical Vapor Deposition Conditions Research Articles

Effect of gadolinia–stabilized zirconia nanoparticles manufactured from Mist CVD on the mechanical properties of ceramics sintered by SPS

Zirconia is an engineering ceramic material with excellent comprehensive properties. However, it suffers from the inherent disadvantage of low fracture toughness. To improve its fracture toughness, Gd2O3 is an effective stabilizer. In the present study, Gd2O3–ZrO2 composite powders were synthesized using the Mist CVD method. Following synthesis, these powders were pressed through spark plasma sintering. The crystallinity of the Gd2O3–ZrO2 powders improved as the deposition temperature increased from 600°C to 900°C. The tetragonality (c/2a) of the Gd–TZP composites increased from 0.99941 to 1.01571 as the Gd2O3 content increased from 2 mol% to 4 mol%, but it decreased when the content reached 5 mol%. The Gd2O3–ZrO2 nanoparticles produced via the Mist CVD approach presented a unique hollow spherical structure. Under moist chemical vapor deposition (CVD) conditions with 4 mol% Gd2O3 at 1400°C, the Gd–TZP composites exhibited fracture toughness and hardness values of approximately 12.03 ± 0.15 MPa·m1/2 and 12.16 ± 0.17 GPa, respectively.

Air-Stable Wafer-Scale Ferromagnetic Metallo-Carbon Nitride Monolayer.

The pursuit of robust, long-range magnetic ordering in two-dimensional (2D) materials holds immense promise for driving technological advances. However, achieving this goal remains a grand challenge due to enhanced quantum and thermal fluctuations as well as chemical instability in the 2D limit. While magnetic ordering has been realized in atomically thin flakes of transition metal chalcogenides and metal halides, these materials often suffer from air instability. In contrast, 2D carbon-based materials are stable enough, yet the challenge lies in creating a high density of local magnetic moments and controlling their long-range magnetic ordering. Here, we report a novel wafer-scale synthesis of an air-stable metallo-carbon nitride monolayer (MCN, denoted as MN4/CNx), featuring ultradense single magnetic atoms and exhibiting robust room-temperature ferromagnetism. Under low-pressure chemical vapor deposition conditions, thermal dehydrogenation and polymerization of metal phthalocyanine (MPc) on copper foil at elevated temperature generate a substantial number of nitrogen coordination sites for anchoring magnetic single atoms in monolayer MN4/CNx (where M = Fe, Co, and Ni). The incorporation of densely populating MN4 sites into monolayer MCN networks leads to robust ferromagnetism up to room temperature, enabling the observation of anomalous Hall effects with excellent chemical stability. Detailed electronic structure calculations indicate that the presence of high-density metal sites results in the emergence of spin-split d-bands near the Fermi level, causing a favorable long-range ferromagnetic exchange coupling through direct exchange interactions. Our work demonstrates a novel synthesis approach for wafer-scale MCN monolayers with robust room-temperature ferromagnetism and may shed light on practical electronic and spintronic applications.

Theoretical study on gas‐phase reactions during chemical vapor deposition of TixAl1–xN from TiCl4, AlCl3, and NH3

AbstractFcc‐TixAl1–xN (TiAlN) coatings synthesized via chemical vapor deposition (CVD) reduce cutting tool wear. Although CVD conditions reportedly influence coating quality, no quantitative guidelines are yet available. To quantitatively study the film‐forming mechanism of TiAlN CVD, the gas composition over the surface must be known. Therefore, we developed a gas‐phase elementary reaction model for TiAlN CVD derived from TiCl4/AlCl3/NH3. First, we constructed a novel thermodynamic dataset including molecules that contained both Ti and Al, and calculated the equilibrium composition. Thermal equilibrium calculations in the gas phase showed that the most stable species were AlCl3 and TiCl3 rather than TiCl4. An elementary reaction model was constructed based on the kinetics of the gas‐phase species that were generated. Kinetic analysis revealed that gas‐phase reactions were largely absent under our reactor conditions. The thermal equilibrium calculations indicated that TiCl4 may have given rise to TiCl3. Thus, other reaction pathways of TiCl4 to TiCl3 were explored. We calculated the reaction rate constants of 12 reactions of Ti species and added them to the model, which revealed that TiCl4 decomposed to TiCl3 via TiCl3NH2. Under our conditions, TiCl4 and TiCl3NH2 are the major Ti species and AlCl3 and AlCl2NH2 are the major Al species, which suggests that some of these species may form films. Unlike in the earlier reaction model, NH2 and H radicals were produced, which may have contributed to the surface reactions. For reactors with large Surface/Volume ratio of reactor, the effects of gas‐phase reactions should be considered. Our reaction model enables estimation of the partial pressures of reactor gas species and will therefore aid study of the TiAlN deposition mechanism.

Challenges in Chemical Vapour Deposition of Graphene on Metallurgical Alloys Exemplified for NiTi Shape Memory Alloys

Atomically-thin two-dimensional (2D) materials like graphene have been suggested as ultimately thin corrosion barriers and functional coatings for modern metallurgical alloys. The challenges of chemical vapour deposition (CVD) of such 2D materials, particularly graphene, on modern metallurgical alloys are discussed and reviewed here, focusing on the key problems with the metallurgical alloys’ often limited catalytic activity towards 2D materials growth and the key need to preserve the metallurgical alloys’ bulk properties during the high temperature 2D materials CVD processes. Using graphene CVD on NiTi (Nitinol) shape memory alloys as a case study, we illustrate the constraints arising from low catalytic activity and tendency to form oxides due the Ti in the NiTi alloy in terms of graphene growth results. We show that, by using a scalable low-temperature CVD process at 650 to 750 °C, we can deposit fully covering carbon films on the NiTi, albeit at limited structural quality. Notably, we also demonstrate that our CVD process does not degrade the bulk microstructure of the NiTi during carbon deposition and, importantly, leaves the crystallographic shape memory effect evolution intact. This underscores the potential of CVD for depositing graphene films on NiTi alloys while emphasizing the necessity for further exploration of CVD conditions to achieve high-quality graphene deposits akin to those on prior widely investigated dedicated (often sacrificial) high-purity metal substrates such as Ni.

Cluster-Assisted Mesoplasma Chemical Vapor Deposition for Fast Epitaxial Growth of SiGe/Si Heterostructures: A Molecular Dynamics Simulation Study.

Co-condensation of mixed SiGe nanoclusters and impingement of SiGe nanoclusters on a Si substrate were applied using molecular dynamics (MD) simulation in this study to mimic the fast epitaxial growth of SiGe/Si heterostructures under mesoplasma chemical vapor deposition (CVD) conditions. The condensation dynamics and properties of the SiGe nanoclusters during the simulations were investigated first, and then the impingement of transient SiGe nanoclusters on both Si smooth and trench substrate surfaces under varying conditions was studied theoretically. The results show that the mixed nanoclusters as precursors demonstrate potential for enhancing epitaxial SiGe film growth at a high growth rate, owing to their loosely bound atomic structures and high mobility on the substrate surface. By varying cluster sizes and substrate temperatures, this study also reveals that smaller clusters and higher substrate temperatures contribute to faster structural ordering and smoother surface morphologies. Furthermore, the formed layers display a consistent SiGe composition, closely aligning with nominal values, and the cluster-assisted deposition method achieves the epitaxial bridging of heterostructures during cluster impingement, highlighting its additional distinctive characteristics. The implications of this work make it clear that the mechanism of fast alloyed epitaxial film growth by cluster-assisted mesoplasma CVD is critical for extending it as a versatile platform for synthesizing various epitaxial films.

First principles investigation of ammonia borane decomposition for understanding h-BN growth on the copper surface

Ammonia borane (AB) is a highly promising precursor for the large-scale growth hexagonal boron nitride (h-BN) via chemical vapor deposition (CVD) methods. However, the unclear decomposition mechanism of precursor molecules on the Cu surface poses a great challenge for understanding the CVD growth process of h-BN. In this work, we addressed this issue by conducting first-principles calculations to investigate the decomposition mechanism of AB on the Cu surface. Our calculations revealed that AB preferentially decomposed into ammonia and borane. Further analysis demonstrates that borane exhibits a higher dehydrogenation rate than ammonia, suggesting a single atom self-assembly growth mode of h-BN under B-rich conditions. Our calculations revealed the formation of BNHx-related byproducts during h-BN growth using AB as a precursor, resulting from the intricate decomposition of AB on the Cu surface under CVD conditions. Our findings have significant implications for comprehending and controlling the growth of h-BN in CVD process.

Oxygen Driven Defect Engineering of Monolayer MoS2 for Tunable Electronic, Optoelectronic, and Electrochemical Devices

AbstractMolybdenum disulfide (MoS2), a two‐dimensional (2D) semiconducting material harbors intrinsic defects that can be harnessed to achieve tuneable electronic, optoelectronic, and electrochemical devices. However, achieving precise control over defect formation within monolayer MoS2, remains a notable challenge. Here, an in‐situ defect engineering approach for monolayer MoS2 using a pressure‐dependent chemical vapor deposition (CVD) process is presented. Monolayer MoS2 grown in a low pressure CVD conditions (LP‐MoS2) produces sulfur vacancy (Vs) induced defect‐rich crystals primarily attributed to the oxygen‐deficient growth conditions. Conversely, atmospheric pressure CVD‐grown MoS2 (AP‐MoS2) passivates these defects with oxygen from ambient conditions. This disparity in defect profiles profoundly impacts crucial functional properties and device performance. AP‐MoS2 shows a drastically enhanced photoluminescence, which is significantly quenched in LP‐MoS2 attributed to in‐gap electron donor states induced by the Vs defects. However, the n‐doping induced in LP‐MoS2 generates enhanced photoresponsivity and detectivity in fabricated photodetectors compared to the AP‐MoS2‐based devices. Defect‐rich LP‐MoS2 outperforms AP‐MoS2 as channel layers of field‐effect transistors (FETs), as well as electrocatalytic material for hydrogen evolution reaction (HER). This work presents a single‐step CVD approach for in situ defect engineering in monolayer MoS2 and presents a pathway to control defects in other monolayer 2D materials.

Toward Controlled Synthesis of 2D Crystals by CVD: Learning from the Real-Time Crystal Morphology Evolutions.

The rich morphology of 2D materials grown through chemical vapor deposition (CVD), is a distinctive feature. However, understanding the complex growth of 2D crystals under practical CVD conditions remains a challenge due to various intertwined factors. Real-time monitoring is crucial to providing essential data and enabling the use of advanced tools like machine learning for unraveling these complexities. In this study, we present a custom-built miniaturized CVD system capable of observing and recording 2D MoS2 crystal growth in real time. Image processing converts the real-time footage into digital data, and machine learning algorithms (ML) unveil the significant factors influencing growth. The machine learning model successfully predicts CVD growth parameters for synthesizing ultralarge monolayer MoS2 crystals. It also demonstrates the potential to reverse engineer CVD growth parameters by analyzing the as-grown 2D crystal morphology. This interdisciplinary approach can be integrated to enhance our understanding of controlled 2D crystal synthesis through CVD.

Selectivity and Growth Rate Modulations for Ruthenium Area‐selective Deposition by Co‐Reagent and Nanopattern Design

AbstractArea‐selective deposition (ASD) is a bottom‐up technique that provides numerous opportunities for nanoelectronic device fabrication. For example, advanced nano‐interconnect structures with barrierless metals like Ru in the contact holes can be created by Ru ASD with the bottom metals as growth surfaces and dielectrics as non‐growth surfaces. This work investigates Ru ASD by chemical vapor deposition (CVD) on industrially relevant substrates and nanopatterns and reveals how selectivity and growth rate are modulated by the CVD conditions and type of nanopattern. For low‐k dielectric/Cu substrate combinations, the selectivity reverses from metal‐on‐metal to metal‐on‐dielectric upon only changing the CVD co‐reagent from H2 to NH3. In contrast, NH3 is the preferred co‐reagent for SiO2‐TiN line patterns with critical dimension (CD) of 40 nm due to the more favorable adsorption and diffusion kinetics that cause growth rate and selectivity enhancement. Consistent with a diffusion‐mediated mechanism, the growth rate enhances even more for Ru CVD on nanoscale contact holes with CD of 10.5 nm, becoming 2.4 times higher as compared to unpatterned substrates. Thus, the ASD process changes drastically when pattern dimensions reach the nanoscale. The reported insights facilitate rational design of metal ASD processes for multiple applications in nanofabrication.

Effect of substrate roughness on the nucleation and growth behaviour of microwave plasma enhanced CVD diamond films – a case study

The influence of substrate surface roughness on the nucleation and growth of diamond films by chemical vapour deposition (CVD) is investigated. Silicon substrates were grinded with six different grit sizes of abrasive papers with a rotating wheel. Si was also etched by Ar+ ions to produce average surface roughness Ra = 11.29 nm on the mirror polished side (Ra = 1.17 nm). A comparison of the results of the effect of substrate roughness, on the growth behaviour of nanocrystalline diamond (NCD) films, by using both the resonant cavity and the linear antenna CVD systems, are presented here. Scanning electron microscopy (SEM) images and Raman spectroscopy reveal that under both the linear antenna and the resonant cavity microwave plasma CVD conditions, grown films are NCD. The diamond nanocrystals sizes vary from 80 to 180 nm, grown by both the reactors after few hours of deposition, irrespective of the substrate roughness, whereas their quality (defined by the relative percentage ratios of the Raman sp3 peak intensity to the non-sp3 peak intensity) varies from 33% to 45%, depending on the substrate surface roughness. Such nanocrystals grew into plate-like flat 1–6 μm size diamond grains after prolonged hours (64–69 h) of CVD growth. It is found specifically that the roughness created by the argon plasma treatment of the silicon substrate surfaces effectively enhances the nucleation and growth behaviour of the diamond films.

Epitaxial growth of high-quality Ge layers on Si with Ge2H6 under UHV-CVD conditions

Epitaxial growth of Ge films on Si(100) substrates has been studied under ultra-high vacuum chemical vapor deposition (CVD) conditions by using digermane (Ge2H6) as the precursor. It was found out that high quality layers with thicknesses beyond 500 nm could be produced at complementary metal–oxide–semiconductor compatible conditions, demonstrating low defect density, sharp and narrow x-ray diffraction peaks, as well as room temperature photoluminescence around 1550 nm. The surface roughness values are comparable to prior reduced pressure CVD results at similar growth temperatures. By employing higher growth temperatures, growth rates are significantly enhanced, resulting in much thicker layers beyond 2000 nm. Smoother sample surface could also be obtained, yielding a state-of-the-art surface root-mean-square roughness value of 0.34 nm for the as-grown sample. At the same time, after being annealed at 750 °C for 20 min, the full width at half maximum (FWHM) of x-ray diffraction 004 rocking curve spectrum of the Ge layer is as low as 88 arcseconds, which stands the best among all Ge/Si samples. The current work has provided important reference for Ge/Si growth with Ge2H6 in low pressure regime and solidified material grounding for Ge-based optoelectronics and Si photonics.

Effect of different oxygen precursors on alumina deposited using a spatial atomic layer deposition system for thin-film encapsulation of perovskite solar cells

An atmospheric-pressure spatial atomic layer deposition system operated in atmospheric-pressure spatial chemical vapor deposition conditions is employed to deposit alumina (AlO x ) thin films using trimethylaluminum and different oxidants, including water (H2O), hydrogen peroxide (H2O2), and ozone (O3). The impact of the oxygen precursor on the structural properties of the films and their moisture-barrier performance is investigated. The O3-AlO x films, followed by H2O2-AlO x , exhibit higher refractive indexes, lower concentrations of OH− groups, and lower water-vapor-transmission rates compared to the films deposited using water (H2O-AlO x ). The AlO x films are then rapidly deposited as thin-film-encapsulation layers on perovskite solar cells at 130 °C without damaging the temperature-sensitive perovskite and organic materials. The stability of the p–i–n formamidinium methylammonium lead iodide solar cells under standard ISOS-D-3 testing conditions (65 °C and 85% relative humidity) is significantly enhanced by the encapsulation layers. Specifically, the O3-AlO x and H2O2-AlO x layers result in a six-fold increase in the time required for the cells to degrade to 80% of their original efficiency compared to un-encapsulated cells.

Inkjet Printing of PrBa0.5Sr0.5Co1.5Fe0.5O5+ δ Cathode for High-Performance Proton Ceramic Fuel Cells

Ceramic fuel cells have been actively studied and are in the commercialization stage because of their economic and efficiency advantages, including the lack of precious metal catalysts, including Pt, fuel flexibility, and high efficiency. Solid oxide fuel cells (SOFCs) are the most typical type of ceramic fuel cell that conduct oxide ions. However, the operating temperature of SOFCs is high (>700 °C) owing to the high activation energy required for ion conduction. Prolonged operation at high temperatures can cause chemical and mechanical degradation in materials and peripheral parts, discouraging their use as long-time stationary generators, which is the main target market for SOFCs. For these reasons, expectations and interest in proton ceramic fuel cells (PCFCs) have recently increased owing to excellent power and durability exhibited at intermediate temperatures (<600 °C). According to recent reports, the durability of PCFC is superior to that of SOFC, even when methane fuel or city gas is used. The cathode reaction accounts for the highest resistance in PCFC operation. Therefore, the development of a high-performance cathode is the biggest factor for the success of PCFCs. To develop a high-performance cathode, the following factors must be optimized: 1) surface and interface shapes and components that can maximize the charge transfer reaction, and 2) porous microstructure that can effectively deliver oxygen gas to the reaction sites, and 3) electrolyte-electrode adhesion at the interface. The potential of inkjet printing (IJP) as a fabrication technology capable of optimizing these structures and components was recently confirmed by our group. IJP has attracted attention as a thin-film deposition technology that can produce complex and sophisticated microstructures, such as various multi-component ceramic materials, while controlling them at the nano-level. Recently, solution processes such as spin coating, tape casting, and screen printing have been frequently adopted in device manufacturing owing to their advantages such as simple accessibility and convenient management. However, fabricating films with nanoscale thickness is very difficult. Moreover, the precision with which a microstructure with the desired shape can be fabricated is limited. Conventional thin-film processes, such as physical vapor deposition and chemical vapor deposition, require complex and difficult deposition conditions, such as high-temperature or high-vacuum maintenance. In addition, equipment management is difficult, a large number of parts and a large system are required, and large-area deposition is highly challenging. Meanwhile, in the case of IJP technology, it is feasible to deposit high-quality nanoscale thin films relatively conveniently without requiring vacuum or temperature maintenance. Furthermore, they can be manufactured over large areas. IJP can be used to conveniently fabricate various patterns in the desired shape without masking or lithography. Moreover, it can precisely control the concentration and combination of printing inks to produce thin films with desired material compositions and microstructures. We successfully fabricated perovskite-type PBSCF cathodes for PCFCs using thermal inkjet printing (T-IJP) on a BaZrxCe0.8-xY0.1Yb0.1O3-δ electrolyte using an inexpensive desktop printer. Ceramic ink solutions were formulated using PBSCF nanopowder as a solid material and dispersed in anhydrous ethanol/propylene glycol methyl ether (PGME)-based solvents. The ink properties were improved by adding various polymers such as surfactants and dispersants. Printability was verified by analyzing the viscosity, surface tension, and particle size of the pigments. The shape and microstructure of the PBSCF cathode layer were successfully adjusted by controlling the number of printing cycles and grayscale value (color level) using commercial printing software. We verified the tendency of these specimens by observing the images using a scanning electron microscope (SEM) equipped with energy dispersive spectra. We also conducted performance evaluations and electrochemical impedance spectroscopy (EIS). Finally, we obtained an optimized PBSCF cathode (G-80) with adequate thickness and a favorable number of pores and cracks. The samples had identical thicknesses of 6–7 μm. This enabled a more reliable and accurate comparison and examination of the tendencies of the specimen microstructures by using grayscale values. We enhanced the performance compared to that of our previous T-IJP work. Among the four specimens measured in this study, G-80 attained a high power output of 728 mWcm-2 in the sub-IT regime (< 600 °C). This performance is higher than that achieved in previous similar studies.

(Invited) Optical Properties and Device Application of One-Dimensional Vdw Heterostructures Based on Single-Walled Carbon Nanotubes

A typical one-dimensional (1D) van der Waals (vdW) heterostructure consists of SWCNT, boron nitride nanotube (BNNT), and molybdenum disulfide nanotube (MoS2NT), grown coaxially by successive chemical vapor deposition (CVD) steps [1, 2]. The coaxially nested structure based on SWCNTs, which has diverse electronic properties (metallic or semiconducting), can expand the board application possibilities of 1D vdW heterostructures [3]. By comparing the optical properties of films of BNNT@MoS2NT and SWCNT@BNNT@MoS2NT, we found strong photoluminescence (PL) from monolayer MoS2NT and quenching of PL by coupling to SWCNT through thin BNNT [4]. The inter-tube excitons are demonstrated by ultrafast optical spectroscopy [5]. The inter-tube excitons, the counterpart of inter-layer excitons for 2D heterostructures, suggest a possibility of efficient photovoltaic applications of the 1D heterostructure [6]. In addition to MoS2 nanotubes, we will discuss the growth control of WS2 and NbS2 nanotubes. A general strategy we can tune the CVD condition from 2D flake to 1D tube is proposed. The junctions of MoS2 and WS2 nanotube are also realized as the outermost layer. More specific optical properties are studied for semiconductor SWCNTs and chirality-sorted SWCNTs [7,8]. Semiconductor SWCNT or chirality-sorted SWCNT wrapped with BNNT can be regarded as the ideal building blocks of field-effect transistors (FET) [9]. We have demonstrated the radial semiconductor–insulator–semiconductor (S-I-S) tunneling heterojunction diode by using a micrometer long 1D vdW heterostructure SWCNT@BNNT@MoS2NT [10]. We will discuss a more recent array of 1D heterostructure FET devices.Part of this work was supported by JSPS KAKENHI Grant Number JP20H00220, and by JST, CREST Grant Number JPMJCR20B5, Japan.

Single‐Source Precursors for the Chemical Vapor Deposition of Iron Germanides

AbstractBinary iron‐germanium phases are promising materials in magnetoelectric, spintronic or data storage applications due to their unique magnetic properties. Previous protocols for preparation of FexGey thin films and nanostructures typically involve harsh conditions and are challenging in terms of phase composition and homogeneity. Herein, we report the first example of single source chemical vapor deposition (CVD) of FexGey films. The appreciable volatility of [Ge[Fe2(CO)8]2], [Cl2GeFe(CO)4]2 and Me₂iPr₂NHC ⋅ GeCl2 ⋅ Fe(CO)4 allowed for their application as precursors under standard CVD conditions (Me₂iPr₂NHC=1,3‐diisopropoyl‐4,5‐dimethylimidazol‐2‐ylidene). The thermal decomposition products of the precursors were characterized by TGA and powder XRD. Deposition experiments in a cold‐wall CVD reactor resulted in dense films of FexGey. During the optimization of synthetic conditions for precursor preparation the new iron‐germanium cluster Cl2Ge[Fe2(CO)8]Ge[Fe2(CO)8] was obtained in experiments with a higher stoichiometric ratio of GeCl2 ⋅ 1,4‐dioxane vs. Fe2(CO)9.

Etching Kinetics of Nanodiamond Seeds in the Early Stages of CVD Diamond Growth.

We present an experimental study on the etching of detonation nanodiamond (DND) seeds during typical microwave chemical vapor deposition (MWCVD) conditions leading to ultra-thin diamond film formation, which is fundamental for many technological applications. The temporal evolution of the surface density of seeds on the Si(100) substrate has been assessed by scanning electron microscopy (SEM). The resulting kinetics have been explained in the framework of a model based on the effect of the particle size, according to the Young-Laplace equation, on both chemical potential of carbon atoms in DND and activation energy of the reaction with atomic hydrogen. The model describes the experimental kinetics of seeds' disappearance by assuming that nanodiamond particles with a size smaller than a "critical radius," r*, are etched away while those greater than r* can grow. Finally, the model allows to estimate the rate coefficients for growth and etching from the experimental kinetics.

Machine learning method for determining chemical vapor deposition conditions for large‐area graphene growth

AbstractThe synthesis of wafer‐scale single‐crystal graphene sheets has become essential to realize future electronic device applications. To synthesize large‐area boundary‐free graphene, it is effective to use chemical vapor deposition (CVD) on copper (Cu) surfaces that possess a thin oxide layer. In this study, we constructed machine learning (ML) modeling to design experimental CVD conditions for the formation of large‐area graphene. The constructed ML model predicted the graphene domain size from the experimental CVD growth conditions and the spectral information of the Cu surface. Furthermore, we demonstrated the formation of large‐area graphene domain on the Cu surface using the CVD conditions determined by the constructed ML model, which provided faster graphene growth compared to previously reported strategies.

Synthesis of Polycrystalline Diamond Films in Microwave Plasma at Ultrahigh Concentrations of Methane

Polycrystalline diamond (PCD) films are usually grown by chemical vapor deposition (CVD) in hydrogen–methane mixtures. The synthesis conditions determine the structure and quality of the grown material. Here, we report the complex effect of the microwave plasma CVD conditions on the morphology, growth rate and phase composition of the resulting PCD films. Specifically, we focus on the factors of (i) increased methane concentrations (νc) that are varied over a wide range of 4%–100% (i.e., pure methane gas) and (ii) substrate temperatures (Ts) varied between 700–1050 °C. Using scanning electron microscopy, X-ray diffraction and Raman spectroscopy, we show that diamond growth is possible even at ultrahigh methane concentrations, including νc = 100%, which requires relatively low synthesis temperatures of Ts &lt; 800 °C. In general, lower substrate temperatures tend to facilitate the formation of higher-quality PCD films; however, this comes at the cost of lower growth rates. The growth rate of PCD coatings has a non-linear trend: for samples grown at Ts = 800 °C, the growth rate increases from 0.6 µm/h at νc = 4% to 3.4 µm/h at νc = 20% and then falls to 0.6 µm/h at νc = 100%. This research is a step toward control over the nature of the CVD-grown PCD material, which is essential for the precise and flexible production of diamond for various applications.

Kinetic study on heterogeneous nucleation and incubation period during chemical vapor deposition.

The kinetics of heterogeneous nucleation during chemical vapor deposition (CVD) is still unclear despite its importance. Nucleation delay is often observed in many CVD processes, which is known as the incubation period (τi). In this study, the effects of concentration (C) and sticking probability (η) of film-forming species on τi were formulated based on our kinetic model. To discuss the kinetics, τi -1 with the rate dimension was used and formulated using C and η. Because η onto heterogeneous surfaces (ηhetero) is difficult to evaluate, the study was initiated with η onto homogeneous surfaces (ηhomo), followed by a discussion on its reasonability. The formulation was validated using the experimental dataset for SiC-CVD from CH3SiCl3/H2 onto BN underlayers because CVD involves multiple film-forming species with different ηhomo ranging from 10-6 to 10-2 and thus is a suitable system for studying the effect of ηhomo. High-aspect-ratio (1000:1) parallel-plate microchannels consisting of τi-involving BN and a τi-free Si surface were utilized to separate these film-forming species along the microchannel depth. τi was exceptionally long, up to several hours, depending on the CVD conditions. τi -1 was found to be proportional to Cn, where n is the reaction order. n was quantified as ≈1.6, suggesting the initial nucleation was triggered by the impingement of two adspecies in the second order and lowered possibly by the discrepancy between C in the gas-phase and that actually producing adspecies on the surface. τi -1 was also found to be proportional to ηhomo. The exceptionally long τi was likely originated from the significantly lower ηhetero than ηhomo and the higher activation energy for ηhetero than that for ηhomo.

N-type diamond synthesized with tert-butylphosphine for long spin coherence times of perfectly aligned NV centers

The longest spin coherence times for nitrogen-vacancy (NV) centers at room temperature have been achieved in phosphorus-doped n-type diamond. However, difficulty controlling impurity incorporation and the utilization of highly toxic phosphine gas in the chemical vapor deposition (CVD) technique pose problems for the growth of n-type diamond. In the present study, n-type diamond samples were synthesized by CVD using tert-butylphosphine, which is much less toxic than phosphine. The unintentional incorporation of nitrogen was found to be suppressed by incrementally increasing the gas flow rates of H2 and CH4. It was found that the spin coherence time (T2) increased with decreasing the nitrogen concentration, which suggests that the nitrogen concentration limits the length of T2. In the sample with the lowest nitrogen concentration, T2 increased to 1.62 ± 0.10 ms. Optically detected magnetic resonance spectra indicated that all of the measured NV centers were aligned along the [111] direction. Hall measurements confirmed n-type conduction in three measured samples prepared under different growth conditions. The highest measured Hall mobility at room temperature was 422 cm2/(V s). This study provides appropriate CVD conditions for growing phosphorus-doped n-type diamond with perfectly aligned NV centers exhibiting long spin coherence times, which is important for the production of quantum diamond devices.