Process study of thermal plasma chemical vapor deposition of diamond,part I: Substrate material,temperature, and methane concentration

Effects of process parameters on diamond film synthesis in DC thermal plasma jet reactors are discussed including substrate material, methane concentration and substrate temperature. Diamond has been deposited on silicon, molybdenum, tungsten, tantalum, copper, nickel, titanium, and stainless steel. The adhesion of diamond film to the substrate is greatly affected by the type of substrate used. It has been found that the methane concentration strongly affects the grain size of the diamond films. Increased methane concentrations result in smaller grain sizes due to the increased number of secondary nucleations on the existing facets of diamond crystals. Substrate temperature has a strong effect on the morphology of diamond films. With increasing substrate temperature, the predominant orientation of the crystal growth planes changes from the (111) to the (100) planes. Studies of the variation of the film quality across the substrate due to the nonuniformity of thermal plasma jets indicate that microcrystalline graphite formation starts at the corners and edges of diamond crystals when the conditions become unfavorable for diamond deposition.     

Diamond Deposition on Substrates with Different Geometries in a Thermal Plasma Reactor

The effects of process parameters on diamond film deposition have been considered in an atmospheric-pressure dc thermal plasma jet reactor. Two different precursor injection systems have been evaluated, counterflow and side injection. The precursor flow rate using ethanol has been found to strongly affect crystal size as well as orientation of crystal growth planes. Further, crystal size on sharp edges has been found to be up to five times larger than on planar surfaces. The effects of substrate geometry on the morphology and area of deposited diamond have been investigated as well. The results of this study show that dc thermal plasma jets can provide high diamond deposition rates, for example on wires and drills, although crystal size and film thickness show substantial variation.     

Effect of pressure on the growth of boron and nitrogen doped HFCVD diamond films on WC–Co substrate

Boron and nitrogen compounds are added in the acetone/hydrogen gas mixture to deposit hot filament chemical vapor deposition (HFCVD) diamond films on the cobalt cemented tungsten carbide (WC–Co) substrate under the pressure of 1–4 kPa. The as‐deposited diamond films are characterized by field emission scanning electron microscope (FESEM), atomic force microscopy (AFM), X‐ray diffraction (XRD) spectroscopy and Raman spectroscopy. The results reveal that the surface morphology, growth rate, structure and quality of the diamond films vary with the pressure and the type of the impurity addition. The diamond grains tend to develop into the nanometer scale with the decrease of the pressure. However, adding of boron or nitrogen impurities in the gas mixture will weaken the nanocrystallization effect by reducing the carbon supersaturation. Density functional theory (DFT) calculations indicate that co‐adsorption of B and N containing radicals can favor the adsorption of CH₃ on diamond (100) surface. Thus, at low pressure of 1 kPa, large grained cubic (100) facet diamond rather than typical nanometer diamond is produced for B–N co‐addition gas mixture. The present results appear to be useful to efficiently synthesize high quality doped diamonds with desirable properties for mechanical application. Copyright © 2015 John Wiley & Sons, Ltd.     

Scanning Probe Microscopy and Spectroscopy of CVD Diamond Films

 The surface morphology and electronic properties of as-deposited CVD diamond films and the diamond films which have been subjected to boron ion implantation or hydrogen plasma etching have been systematically studied by high resolution scanning probe microscopy and spectroscopy techniques. AFM and STM image observations have shown that (a) both the as-deposited CVD diamond films and the boron ion implanted films exhibit similar hillock morphologies on (100) crystal faces and these surface features are formed during the deposition process; (b) boron ion implantation does not cause a discernible increase in surface roughness; (c) atomic flatness can be achieved on crystal faces by hydrogen plasma etching of the film surface. Scanning tunnelling spectroscopy analysis has indicated that (a) the as-deposited diamond films and the hydrogen plasma etched diamond films possess typical p-type semiconductor surface electronic properties; (b) the as-deposited diamond films subjected to boron implantation exhibit surface electronic properties which change from p-type semiconducting behaviour to metallic behaviour; (c) the damage in the boron implanted diamond films is restricted to the surface layers since the electronic properties revert to p-type on depth profiling.     

氢终止金刚石表面的形成机理

为考察金刚石形成氢终止表面的反应机制,采用微波氢等离子体处理以及电阻丝氢气气氛加热处理进行对比研究.利用光发射谱(OES)和漫反射傅里叶变换红外光谱(DRIFTS)分别表征了微波氢等离子体中的活性基团和金刚石表面氢终止浓度.结果表明,微波氢等离子体环境下,随着衬底温度、等离子体密度和能量的增加,温度至700 ℃ (800 W/3 kPa)时,等离子体中出现了明显的CH基团;相应地,金刚石表面氢终止浓度随温度、等离子体密度和能量的增加而增加.采用氢气气氛下电阻丝加热的方法同样形成了氢终止金刚石表面,表明微波等离子体处理金刚石表面形成氢终止主要源于由温度控制的表面化学反应,而非等离子体的物理刻蚀作用.氧终止金刚石表面形成氢终止的机制是表面C=O键在高于500 ℃时分解为CO,相应的悬挂键由氢原子或氢分子占据.     

Hydrogen bonding configuration and thermal stability of ambient exposed and in situ hydrogenated polycrystalline diamond surfaces studied by high resolution electron energy loss spectroscopy

In this work we report on an investigation of hydrogen bonding and thermal stability on the surface of poly-crystalline diamond by high resolution electron energy loss spectroscopy (HR-EELS). Diamond films were grown on silicon substrates from CH(4)/H(2) as well as from CD(4)/D(2) gas mixtures by hot filament chemical vapor deposition (HF-CVD). The impact of ex situ ambient exposure on hydrogen bonding and its thermal stability was examined for: (i) as deposited films from a CH(4)/H(2) gas mixture; (ii) the same sample treated ex situ in micro-wave activated hydrogen plasma; and (iii) as deposited films from a CD(4)/D(2) gas mixture. In order to clarify the changes in the hydrogen bonding configuration detected on the different surfaces as a function of thermal annealing in situ hydrogenation by thermally activated atomic hydrogen was performed and examined. This study provides direct evidence that the exposure to ambient conditions and low temperature vacuum annealing have a pronounced effect on the hydrogen-carbon bonding configuration onto the poly-crystalline diamond surfaces.     

金刚石附氢(100)面脱氢势垒的量子化学研究

采用MNDO（UHF）方法计算了金刚石（100）－（1×1）：2H双氢化表面和（100）－（2×1）：H单氢化表面的脱氢势垒，论证了决定金刚石附氢表面脱氢势垒大小的主要因素是气相－表面吸附氢原子间的相互排斥大小，得出（100）面两种表面结构脱氢势垒的理论预言值分别为71和59kJ·mol-1，均大于（111）面脱氢势垒的理论预言值42kJ·mol-1．揭示了在同等生长条件下金刚石（111）面可供成核和生长的反应基多于（100）面，与实验上得到的同等生长条件下（111）面的相对生长率大于（100）面的结论是一致的。     

Determination of atomic hydrogen in non-thermal hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry

Atomic hydrogen plays important roles in chemical vapor deposition of functional materials, plasma etching and new approaches to chemical synthesis of hydrogen-containing compounds. The present work reports experimental determinations of atomic hydrogen near the grounded electrode in medium-pressure dielectric barrier discharge hydrogen plasmas by means of molecular beam threshold ionization mass spectrometry (MB-TIMS). At certain discharge conditions (a.c. frequency of 24 kHz, 28 kV of peak-to-peak voltage), the measured hydrogen dissociation fraction is decreased from approximately 0.83% to approximately 0.14% as the hydrogen pressure increases from 2.0 to 14.0 Torr. A simulation method for extraction of the approximate electron beam energy distribution function in the mass spectrometer ionizer and a semi-quantitative approach to calibrate the mass discrimination effect caused by the supersonic beam formation and the mass spectrometer measurement are reported.     

Excitation Temperature and Constituent Concentration Profiles of the Plasma Jet Under Plasma Spray-PVD Conditions

Plasma spray-physical vapor deposition (PS-PVD) is a promising technology to produce columnar structured thermal barrier coatings with excellent cyclic lifetime. The characteristics of plasma jets generated by standard plasma gases in the PS-PVD process, argon and helium, have been studied by optical emission spectroscopy. Abel inversion was introduced to reconstruct the spatial characteristics. In the central area of the plasma jet, the ionization of argon was found to be one of the reasons for low emission of atomic argon. Another reason could be the demixing so that helium prevails around the central axis of the plasma jet. The excitation temperature of argon was calculated by the Boltzmann plot method. Its values decreased from the center to the edge of the plasma jet. Applying the same method, a spurious high excitation temperature of helium was obtained, which could be caused by the strong deviation from local thermal equilibrium of helium. The addition of hydrogen into plasma gases leads to a lower excitation temperature, however a higher substrate temperature due to the high thermal conductivity induced by the dissociation of hydrogen. A loading effect is exerted by the feedstock powder on the plasma jet, which was found to reduce the average excitation temperature considerably by more than 700 K in the Ar/He jet.     

Control of substrate temperature during diamond deposition

A method is reported for controlling substrate temperature during chemical vapor deposition of diamond in high heat flux environments such as thermal plasmas or flames. The method is based on flowing a variable-composition argon-helium mixture through channels between the substrate and a water-cooled base. A fiberoptic pyrometer temperature reading from the back of the substrate provides feedback for active substrate temperature control. Experiments in an atmospheric-pressure RF reactor under diamond growth conditions indicated that substrate temperature could be sensitively varied by over 600 K by varying the composition of the argon-helium mixture. The effect is explained in terms of the variable thermal contact resistance at the interface between the substrate and the base.     

Catalytic activity of platinum nanoparticles on highly boron-doped and 100-oriented epitaxial diamond towards HER and HOR

Platinum nanoparticles supported on boron-doped single-crystalline diamond surfaces were used as a model system to investigate the catalytic activity with respect to the influence of particle morphology, particle density and surface preparation of the diamond substrates. We report on the preparation, characterization and activity regarding hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) of these Pt/diamond electrodes. Two kinds of diamond layers with boron doping above 10(20) cm(-3) were grown epitaxially on (100)-oriented diamond substrates; post-treatments of wet chemical oxidation and radio frequency (rf) oxygen plasma treatments were applied. Electrochemical deposition of Pt was performed using a potentiostatic double-pulse technique, which allowed variation of the particle size in the range between 1 nm and 15 nm in height and 5 nm and 50 nm in apparent radius, while keeping the particle density constant. Higher nucleation densities on the plasma processed surface at equal deposition parameters could be related to the plasma-induced surface defects. Electrochemical characterization shows that the platinum particles act as nanoelectrodes and form an ohmic contact with the diamond substrate. The catalytic activity regarding HER and HOR of the platinum nanoparticles exhibits no dependence on the particle size down to particle heights of ～1 nm. The prepared Pt on diamond(100) samples show a similar platinum-specific activity as bulk platinum. Therefore, while keeping the activity constant, the well-dispersed particles on diamond offer an optimized surface-to-material ratio.     

Reactions of hydrocarbons in a supersonic vacuum plasma jet

The plasma plume of a hydrogen plasma jet used for diamond synthesis is analyzed by a Pitot tube and by mass spectrometry. In the investigated pressure range of 2–10 mbar, supersonic gas velocities with Mach numbers of up to 2 were observed, which decreased with increasing pressure and increasing distance from the nozzle. The injection of the carbon-containing species either at the exit of the jet nozzle or simply into the background gas of the reaction chamber confirmed the importance of recirculation of background gas into the plasma plume. In the case of background injection the rise of the total carbon content in the plume with increasing distance from the nozzle is much slower than in the case of nozzle injection. The results of a numerical model of the hydrocarbon gas-phase reactions in the jet are presented. The model considers the entrainment of background gas into the plasma plume. Two domains along the jet axis can be distinguished. The first one in the vicinity of the nozzle is dominated by methyl radicals, the second one by atomic carbon. Increase of the hydrogen dissociation level results in the broadening of the atomic carbon domain and the rise of C₂ far from the nozzle. Background injection of CH₄ leads to lower total carbon content in the plume but has little effect on the species distribution along the jet axis.     

Simultaneous growth of diamond and nanostructured graphite thin films by hot-filament chemical vapor deposition

Diamond and graphite films on silicon wafer were simultaneously synthesized at 850 °C without any additional catalyst. The synthesis was achieved in hot-filament chemical vapor deposition reactor by changing distance among filaments in traditional gas mixture. The inter-wire distance for diamond and graphite deposition was kept 5 and 15 mm, whereas kept constant from the substrate. The Raman spectroscopic analyses show that film deposited at 5 mm is good quality diamond and at 15 mm is nanostructured graphite and respective growths confirm by scanning auger electron microscopy. The scanning electron microscope results exhibit that black soot graphite is composed of needle-like nanostructures, whereas diamond with pyramidal featured structure. Transformation of diamond into graphite mainly attributes lacking in atomic hydrogen. The present study develops new trend in the field of carbon based coatings, where single substrate incorporate dual application can be utilized.     

Hydrogen plasma interaction with (100) diamond surfaces

Polycrystalline diamond films exhibiting (100) oriented surfaces have been subject to a low pressure hydrogen plasma for durations up to 20 h. The topography of spatially defined 20 × 20 μm areas of the samples were imaged by atomic force microscopy at intervals during the plasma exposure. The mean surface roughness of individual (100) crystallites decreased from ca. 2.4 nm to <1 nm over the period and was independent of the twist and tilt angles of the crystallite. Whilst small hillock growth features were etched completely by the plasma treatment, there was no evidence for etch pits evident in similar experiments carried out with (100) natural diamond. Very low lateral etch rates of the (100) crystallites of 28 ± 4 nm/h were measured for crystallites bounded by (111) planes. High resolution XPS analysis of the C(1s) and O(1s) transitions of the same samples showed that the surface graphitic phase, present in the as-prepared samples, was removed to below detectable limits. The surface oxygen content was reduced from around 9-10% to ca. 3% after prolonged plasma exposure. The C(1s) and O(1s) band contours revealed the presence of oxygen in the form of ether and carbonyl functional groups. The ether:carbonyl: areal density ratio on (100) crystallites decreased only slightly from 83:17 to 64:37 after 20 h of plasma treatment based on fitting of the O(1s) band envelope. Etching products arising from the plasma interaction with the diamond surface were not detected by either optical emission spectroscopy or mass spectrometry.     

Structural evolution of perfluoro-pentacene films on Ag(111): transition from 2D to 3D growth

The structural evolution and thermal stability of perfluoro-pentacene (PF-PEN) thin films on Ag(111) have been studied by means of low-temperature scanning tunnelling microscopy (STM), low-energy electron diffraction (LEED), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS). Well-defined monolayer films can be prepared by utilizing the different adsorption energy of mono- and multilayer films and selectively desorbing multilayers upon careful heating at 380 K, whereas at temperatures above 400 K, a dissociation occurs. In the first monolayer, the molecules adopt a planar adsorption geometry and form a well-ordered commensurate (6 × 3) superstructure where molecules are uniformly oriented with their long axis along the <110> azimuth. This molecular orientation is also maintained in the second layer, where molecules exhibit a staggered packing motif, whereas further deposition leads to the formation of isolated, tall islands. Moreover, on smooth silver surfaces with extended terraces, growth of PF-PEN onto beforehand prepared long-range ordered monolayer films at elevated temperature leads to needle-like islands that are uniformly aligned at substrate steps along <110> azimuth directions.     

The role of radicals and clusters in thermal plasma flash evaporation processing

The prominent features of plasma flash evaporation may be caused by cluster deposition under high net flux of radicals. In order to clarify such features quantitatively, the atomic oxygen flux (Γ _o ) and cluster size (d) in 200 Torr RF thermal Ar-O₂ plasma were measured on the substrate. It was found that Γ _o higher than 10¹⁹ atoms/cm²·s can be easily acheived in this processing and this value corresponds to frozen atomic oxygen fraction 7 %. The cluster size of YBCO was estimated to be about 0.3 to 10 nm. 0.8 nm cluster deposition under Γ _o higher than 6 × 10¹⁸ atoms/cm²·s made it possible to deposit fine epitaxial YBCO films with Tc = 90K on SrTiO₃ (100) with extremely high deposition rate around 2.2 μm/min.     

Nano-Diamond Synthesis in Strong Magnetic Field

Diamond is one of the five known allotropes of carbon. Diamonds are important due to their esthetic beauty and excellently remarkable properties. Single crystalline diamond has been synthesized by catalytic processes under high temperature and high pressure. Polycrystalline diamond has been prepared by various activated hot filament and plasma chemical vapor deposition methods. Industrial synthesis of diamonds has been limited by the extreme synthetic conditions, purity, crystallinity, size and high cost. Synthetic advancement for future industrial production will require better understanding and exploitation of the mechanism of formation. Current mechanistic uncertainty arises out of limited consideration of atomic scale dynamics. A new comprehensive model by Little addresses atomic scale electronic processes with nonclassical significance of the intermediary states. This new nonclassical, electronic mechanistic perspective identifies the creation, stabilization and condensation of carbon and metal intermediates for pathways from carbon precursors thru solvated carbon and carbides to sp³ carbon condensation as diamond. In order to test these mechanistic ideas and exploit the impact for better diamond synthesis, catalytic carbon condensation has been explored in strong magnetic field in an effort to influence various radicals and high spin metal and carbon intermediates along the pathway for more efficient diamond condensation. In this effort, static magnetic fields in excess of 15 T are observed to direct these carbon and metal intermediary microstates to promote nano-diamond nucleation and growth at atmospheric pressure and temperature of 900°C. The observed nano-diamond formation is consistent with the predicted faster kinetics due to lower potential energies of intermediates along pathways to diamond. The predicted stability of nano-diamond relative to nano-graphite also accounts for the observed nano-diamond in this work. This novel use of static magnetic field for diamond synthesis is compared with advancements since the first synthetics bulk diamond formation by scientists at both ASEA (Sweden) and GE Research Laboratory, Schenectady, NY, 50 years ago. On the basis of this predicted, invented, and intrinsic magnetic influence on the dynamics of carbon condensation and the promise for faster, feasible single crystal diamond formation, this discovery of the use of strong magnetic field for diamond production is asserted a major advancement during this 50-year period since the first successful synthesis.     

Study of the Interface Microstructures of CVD Diamond Films by TEM

 The characteristics of the interface microstructures between a CVD diamond film and the silicon substrate have been studied by transmission electron microscopy and electron energy loss spectroscopy. The investigations are performed on plan-view TEM specimens which were intentionally thinned only from the film surface side allowing the overall microstructural features of the interface to be studied. A prominent interfacial layer with amorphous-like features has been directly observed for CVD diamond films that shows a highly twinned defective diamond surface morphology. Similar interfacial layers have also been observed on films with a <100> growth texture but having the {100} crystal faces randomly oriented on the silicon substrate. These interfacial layers have been unambiguously identified as diamond phase carbon by both electron diffraction and electron energy loss spectroscopy. For the CVD diamond films that exhibit heteroepitaxial growth features, with the {100} crystal faces aligned crystallographically on the silicon substrate, such an interfacial layer was not observed. This is consistent with the expectation that the epitaxial growth of CVD diamond films requires diamond crystals to directly nucleate and grow on the substrate surface or on an epitaxial interface layer that has a small lattice misfit to both the substrate and the thin film material.     

Two-dimensional model for thermal plasma chemical vapor deposition

A formulation of a global mathematical two-dimensional model for Thermal Plasma Chemical Vapor Deposition (TPCVD) is reported. Both gas-phase and surface chemical kinetics as well as ordinary and thermal diffusion are incorporated. Flow is assumed to be steady, laminar and swirlless at this stage. The results include velocity, pressure, density, temperature and chemical species distributions in the reactor, and the heat flux and the film growth characteristics at the substrate.The model has been applied to a low pressure diamond TPCVD. Two basic cases have been investigated: (1) supersonic jet regime, and (2) high speed subsonic jet regime. The results for both cases are presented and compared. In both cases, the hydrocarbon species needed for the diamond formation are assumed to be premixed in the plasma jet.The main conclusions are: (1) The low pressure high speed jets are very narrow and slow down only at the substrate through a bow shock, (2) the faster the jet, the bigger the total deposited amount of diamond but also the higher the heat flux and diamond growth rate nonuniformities.     

镍基板上低温合成定向纳米碳管

纳米碳管具有非常优异的场发射效应 ,亮度高、均匀且稳定的纳米碳管场效应发射器 ,例如平板显示器、阴极射线管以及信号灯等有着非常广阔的应用前景 [1] .由于纳米碳管的场发射效应与纳米碳管的方向性有关[2 ] ,因此定向纳米碳管的制备及其场发射性能研究是当前的一个研究热点