Large area deposition of boron doped nano-crystalline diamond films at low temperatures using microwave plasma enhanced chemical vapour deposition with linear antenna delivery

We report on the preparation and characterisation of boron (B) doped nano-crystalline diamond (B-NCD) layers grown over large areas (up to 50 cm × 30 cm) and at low substrate temperatures (< 650 °C) using microwave plasma enhanced linear antenna chemical vapour deposition apparatus (MW-LA-PECVD). B-NCD layers were grown in H₂/CH₄/CO₂ and H₂/CH₄ gas mixtures with added trimethylboron (TMB). Layers with thicknesses of 150 nm to 1 μm have been prepared with B/C ratios up to 15000 ppm over a range of CO₂/CH₄ ratios to study the effect of oxygen (O) on the incorporation rate of B into the solid phase and the effect on the quality of the B-NCD with respect to sp³/sp² ratio. Experimental results show the reduction of boron acceptor concentration with increasing CO₂ concentration. Higher sp³/sp² ratios were measured by Raman spectroscopy with increasing TMB concentration in the gas phase without CO₂. Incorporation of high concentrations of B (up to 1.75 × 10²¹ cm³) in the solid is demonstrated as measured by neutron depth profiling, Hall effect and spectroscopic ellipsometry.     

The effect of the CH4 level on the morphology,microstructure, phase purity and electrochemical properties of carbon films deposited by microwave-assisted CVD from Ar-rich source gas mixtures

Carbon thin films were deposited on Si substrates by microwave-assisted chemical vapor deposition (CVD) using variable CH₄ levels in an Ar/H₂ (Ar-rich) source gas mixture. The relationship between the CH₄ concentration (0.5 to 3 vol.%) in the source gas and the resulting film morphology, microstructure, phase purity and electrochemical behavior was investigated. The H₂ level was maintained constant at 5% while the Ar level ranged from 92 to 94.5%. The films used in the electrochemical measurements were boron-doped with 2 ppm B₂H₆ while those used in the structural studies were undoped. Boron doping at this level had no detectable effect on the film morphology or microstructure. Relatively smooth ultrananocrystalline diamond (UNCD) thin films, with a nominal grain size of ca. 15 nm, were only formed at a CH₄ concentration of 1%. At the lower CH₄ concentration (0.5%), faceted microcrystalline diamond was the predominant phase formed with a grain size of ca. 0.5 µm. At the higher CH₄ concentration (2%), a diamond-like carbon film was produced with mixture of sp²-bonded carbon and UNCD. Finally, the film grown with 3% CH₄ was essentially nanocrystalline graphite. The characteristic voltammetric features of high quality diamond (low and featureless voltammetric background current, wide potential window, and weak molecular adsorption) were observed for the film grown with 1% CH_4, not the films' grown with higher CH₄ levels. The C₂ dimer level in the source gas was monitored using the Swan band optical emission intensity at 516 nm. The emission intensity and the film growth rate both increased with the CH₄ concentration in the source gas, consistent with the dimer being involved in the film growth. Importantly, C₂ appears to be involved in the growth of the different carbon microstructures including microcrystalline and ultrananocrystalline diamond, amorphous or diamond-like carbon, and nanocrystalline graphite. In summary, the morphology, microstructure, phase purity and electrochemical properties of the carbon films formed varied significantly over a narrow range of CH₄ concentrations in the Ar-rich source gas. The results have important implications for the formation of UNCD from Ar-rich source gas mixtures, and its application in electrochemistry. Characterization data by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), visible-Raman spectroscopy and electrochemical methods are presented.     

Analysis of carbon nitride growth in pedestal reactors by chemical vapor deposition

The chemical vapor deposition of polycrystalline carbon nitride in stagnation flow reactors is simulated. A model is used to predict the gas phase chemistry, temperature and velocity profiles, potential gaseous film growth precursors, and to evaluate the likelihood of bond rearrangement occurring in the bulk phase or on the deposition surface once the gaseous precursors are adsorbed. Numerical studies are carried out to predict the effects of inlet and substrate temperatures, reactor pressure, and inlet gas composition on the gas phase chemistry. Potential gaseous film growth precursors of carbon nitride are determined by quantitatively comparing the calculated results against existing experimental data. Results of the model indicate that the gas phase chemistry, including the gas composition at the deposition surface, is strongly affected by reactor pressure and inlet gas composition. However, the gas composition at the deposition surface does not depend strongly on the inlet temperature, while it is found to be strongly dependent on the substrate temperature. Since no correlation is found between the predicted near-surface concentrations of potential film growth precursors and experimentally measured bond types in the carbon nitride films, the experimentally measured bond types in the films must therefore result from chemical bond rearrangement occurring on the deposition surface or in the bulk phase once the gaseous precursors are adsorbed. Comparison between the calculated film growth rate using potential growth precursors and experimental data indicates that the CH_x (x = 0,2,3), C₂H₂, NH_x (x = 0,1,2), and H_xCN (x = 1,2) species are the most probable crystalline carbon nitride growth species. Among these, C and CH₃ dominate the carbon contribution to the film growth, and N is the primary nitrogen bearing species responsible for the film growth. The sum of predicted film growth rates for carbon bearing species is comparable to the experimentally determined film growth rate.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

Quality of diamond wafers grown by microwave plasma CVD: effects of gas flow rate

We found a strong impact of gas flow rate on diamond growth process in a 5 kW microwave plasma chemical vapour deposition reactor operated on CH₄-H₂ gas mixtures. Diamond films of 0.1–1.2 mm thickness and 2.25 in. in diameter were produced at H₂ flow rates varied systematically from 60 sccm to 1000 sccm at 2.5% CH₄. The highest growth rate, 5 μm h⁻¹, was observed at intermediate F values (≈300 sccm). Carbon conversion coefficient (the number of C atoms going from gas to diamond) increases monotonically up to 57% with flow rate decrease, however, this is accompanied with a degradation of diamond quality revealed from Raman spectra, thermal properties and surface morphology. High flow rates were necessary to produce uniform films with thermal conductivity >18 W cm⁻¹ K⁻¹. Diamond disks with very low optical absorption (loss tangent tgδ<10⁻⁵) in millimetre wave range (170 GHz) have been grown at optimized deposition conditions for use as windows for high-power gyrotrons.     

Investigation of the combined effect of argon addition and substrate bias on the growth of ultrananocrystalline diamond layers

In our recent project the combined effect of argon addition and substrate bias was investigated in the microwave plasma assisted chemical vapor deposition of diamond, focused on the ultrananocrystalline phase. Over the conventional qualifying techniques, i.e., Raman and SEM studies, we have led a special in-situ mass spectrometry investigation to explore the growth mechanism of UNCD, analysing the gas composition close to the surface. To achieve this aim, ion beam mass spectrometry (IBMS) was used for in-situ, real time, mass-selective analysis of the incoming species playing an important role in the MWPECVD (Microwave Plasma Enhanced Chemical Vapor Deposition) of the ultrananocrystalline diamond. In our experiments Ar, CH₄, and H₂ gases were used as source gases in a wide range of concentrations applying different values of substrate bias to deposit different phases of diamond. By the IBMS technique we can measure the fluxes of different species: C_xH_y (x = 1–2, y = 0–2) during the phases of deposition, either under the conditions of microcrystalline diamond (MCD), nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) synthesis. As a result of it, we can compare the different mechanisms of layer formation: i.e.: whether C₁ species or C₂ mediated growth method takes place, or probably both C₁ and C₂ species propagate the diamond lattice. Based on the given tendency by comparing the IBMS data (i.e.: fluxes of surface species) with the growth rate, morphology, and Raman spectra of the layers we propose, that in the case of UNCD a similar (but not exactly the same) growth mechanism can be found as in the case of MCD i.e.: C₁ species are the most likely precursors.     

Oxygen at the interface of CVD diamond films on silicon

The oxygen incorporation at the interface between the silicon substrate and chemical vapour deposited (CVD) diamond films nucleated by the bias-enhanced nucleation (BEN) procedure has been studied by heavy-ion elastic recoil detection (ERD). Using standard process conditions for the realisation of heteroepitaxial films, oxygen with a concentration equivalent to about 1 nm SiO₂ has been found, which was mainly incorporated during textured growth with a certain CO₂ admixture to the process gas. By completely omitting CO₂ during nucleation and growth, the oxygen at the interface can be reduced by nearly one order of magnitude to 6.3×10¹⁵ at cm⁻², corresponding to 0.14 nm SiO₂. Intentional addition of highly enriched C¹⁸O₂ to the gas phase shows that the oxygen incorporation is strongly enhanced during BEN with hydrocarbon in the gas phase. The results indicate that roughening of the surface, the deposition of Si_xO_yC_z phases and strong lateral inhomogeneities at the silicon interface may explain the coexistence of epitaxial crystallites and amorphous phases. It is suggested that a further reduction of the oxygen concentration at the interface may have consequences for an improved heteroepitaxy of diamond on silicon.     

Electrical Properties of B-doped homoepitaxial diamond films grown from UHP gas sources

It is confirmed that a small amount of nitrogen incorporated into chemical vapor deposited diamond films dramatically affects their electrical properties. Nitrogen can be incorporated into diamond films through the leak of vacuum system and/or from the impurity in source gases. Because a nitrogen atom can be a deep donor in diamond crystal, the p-type semiconducting properties of boron doped diamond films can be degraded even by the small amount of nitrogen. The small amount of nitrogen in chemical vapor deposited diamond films was measured by cathodoluminescence spectroscopy. For the detection of nitrogen, the N–V center was intentionally induced by defect formation through ion beam irradiation and subsequent annealing. The luminescence intensity of the N–V center was decreased by reducing the leak of the vacuum system and by upgrading the purity of the source gases. Both the carrier density and the Hall mobility of the boron doped diamond films were successfully improved by the control of nitrogen contamination. Using extremely high pure CH₄, H₂ and B₂H₆ in a tightly sealed vacuum system, the total amount of nitrogen impurity in the source gas was controlled to <80 ppm in the N/C atomic ratio resulting in a Hall mobility of 1600 cm²/Vs with a hole concentration of >10¹⁴ cm⁻³ at the room temperature in a 10-ppm-boron doped homoepitaxial diamond film.     

Calculated energies of adsorption of non-hydrocarbon species on diamond H/C(1 1 1) surface and the abstraction energies of these species abstracted by hydrogen atoms using ab initio calculation

The energetics of adsorption of non-hydrocarbon radical species on H/C(1 1 1) diamond surface and the abstraction energies of these species abstracted by hydrogen atoms, which are in excess in gas phase in the diamond thin film growth using the chemical vapor deposition (CVD) method, were examined using ab initio calculation method. Based on the calculated results for the examined species, which include H, F, OH, NH₂, Cl, CH_mX_n (X=F or Cl) radicals, the tendency of incorporation of F, O, N, H and Cl atoms in the diamond thin film is discussed. The high adsorption energy and the high abstraction energy abstracted by excess gas-phase H atoms for F radicals suggest that F atom has the highest tendency to stay in the diamond thin film among the examined non-carbon atoms. In contrast, the comparable adsorption energy of Cl atom with other examined radicals except F radical, and its low abstraction energy, indicate that Cl atom possesses the least tendency to be incorporated in the diamond thin film. For O, N and H atoms, their calculated abstraction energy values suggest that the overall order of tendency of incorporation in diamond thin film is F>O>N>H>Cl. In addition, the energetically comparable adsorption energy for the CH₂Cl radical, compared with the other examined CH_mX_n species, and the low abstraction energy of Cl atom support that CH₂Cl is a good growth species in diamond CVD thin film growth.     

Piezoresistivity of n-type conductive ultrananocrystalline diamond

Recent developments of a piezoresistive sensor prototype based on n-type conductive ultrananocrystalline diamond (UNCD) are presented. Samples were deposited using hot filament chemical vapor deposition (HFCVD) technique, with a gas mixture of H₂, CH₄ and NH₃, and were structured using multiple photolithographic and etching processes. Under controlled deposition parameters, UNCD thin films with n-type electrical conductivity at room temperature (5 × 10^− 3 – 5 × 10¹ S/cm) could be grown. Respective piezoresistive response of such films was analyzed and the gauge factor was evaluated in both transverse and longitudinal arrangements, also as a function of temperature from 25 °C up to 300 °C. Moreover, the gauge factor of piezoresistors with various sheet resistance values and test structure geometries was evaluated. The highest measured gauge factor was 9.54 ± 0.32 at room temperature for a longitudinally arranged piezoresistor with a sheet resistance of about 30 kΩ/square. This gauge factor is well comparable to that of p-type boron doped diamond; however, with a much better temperature independency at elevated temperatures compared to the boron-doped diamond and silicon. To our best knowledge, this is the first report on piezoresistive characteristics of n-type UNCD films.     

Growth of atomically step-free surface on diamond {111} mesas

Diamond homoepitaxy by microwave plasma-enhanced chemical vapor deposition was investigated on {111} substrate. Growth at a low CH₄/H₂ ratio of 0.025% in a gas phase results in the formation of an atomically step-free surface over 10 × 10-µm² mesas of diamond {111} substrate, when there are no screw dislocations in the mesas. This was achieved through ideal lateral growth, in which two-dimensional terrace nucleation was completely suppressed. The application of the selective formation of the step-free surface and the lateral growth of diamond films will open the way for the realization of high-quality electronic devices using diamond.     

High-quality diamond film deposition on a titanium substrate using the hot-filament chemical vapor deposition method

Diamond film on titanium substrate has become extremely attractive because of the combined properties of these two unique materials. Diamond film can effectively improve the properties of Ti for applications as aerospace and biomedical materials, as well as electrodes. This study focuses on the effects of process parameters, including gas composition, substrate temperature, gas flow rate and reactor pressure on diamond growth on Ti substrates using the hot-filament chemical vapor deposition (HFCVD) method. The nucleation density, nuclei size as well as the diamond purity and growth tendency indices were used to quantify these effects. The crystal morphology of the material was examined with scanning electron microscopy (SEM). Micro-Raman spectroscopy provided information on the quality of the diamond films. The growth tendency of TiC and diamond film was determined by X-ray diffraction analysis. The optimal conditions were found to be: CH₄:H₂ = 1%, gas flow rate = 300 sccm, substrate temperature T_sub = 750 °C, reaction pressure = 40 mbar. Under these conditions, high-quality diamond film was deposited on Ti with a growth rate of 0.4 μm/h and sp² carbon impurity content of 1.6%.     

Grain size dependent physical and chemical properties of thick CVD diamond films for high energy density physics experiments

We report on the grain size dependent morphological, physical and chemical properties of thick microwave-plasma assisted chemical vapor deposited (MPCVD) diamond films that are used as target materials for high energy density physics experiments at the Lawrence Livermore National Laboratory. Control over the grain size, ranging from several μm to a few nm, was achieved by adjusting the CH₄ content of the CH₄/H₂ feed gas. The effect of grain size on surface roughness, morphology, texture, density, hydrogen and graphitic carbon content was systematically studied by a variety of techniques. For depositions performed at 35 to 45 mbar and 3000 W microwave power (power density ~ 10 W cm^− 3), an abrupt transition from micro-crystalline diamond to nanocrystalline diamond was observed at 3% CH₄. This transition is accompanied by a dramatic decrease in surface roughness, a six percent drop in density and an increasing content in hydrogen and graphitic carbon impurities. Guided by these results, layered nano-microhybrid diamond samples were prepared by periodically changing the growth conditions from nano- to microcrystalline.     

Preparation of superhard BxCy fibers by microvortex-flow hyperbaric laser chemical vapor deposition

A novel class of freestanding B_xC_y fibers was prepared by hyperbaric-pressure laser chemical vapor deposition. Utilizing mixtures of diborane and helium with hydrocarbons, such as methane, ethene, and pentane, B_xC_y-alloy fibers were prepared at axial rates of up to 12.2 μm/s. Regions of kinetically-limited and transport-limited growth were identified, and the activation energy for deposition from B₂H₆ + C₅H₂₀ + He mixtures (at relative concentrations of 1:25:10) was found to be 197 ± 27 kJ/mol, while the rate constant was approximately 1810 μm/s. Compositions ranged from B_0.4C_0.6 to B_0.03C_0.97 depending on the gas mixture and laser powers employed; axial and radial fiber compositions/microstructure were explored using Auger spectroscopy and electron microscopy. Glassy B_xC_y fibers with Vickers hardnesses of 42–45 GPa were grown at laser powers below 150 mW. The growth kinetics of pure boron fibers was also investigated from BCl₃ + H₂, BF₃ + H₂, and B₂H₆ + H₂ gas mixtures, producing fine-grained α-boron and large single-crystals of β-boron. Micro-scale vortices in the gas flow emanating from the reaction zone were observed using particle image velocimetry; such vortices enhance axial fiber growth rates through rapid gas mixing. Arrays of fibers were also grown in-parallel using diffractive optical elements.     

Characterization of the near-surface gas-phase chemical environment in atmospheric-pressure plasma chemical vapor deposition of diamond

A numerical model was developed and used to study the near-surface gas-phase chemistry during atmospheric-pressure radio-frequency (RF) plasma diamond chemical vapor deposition (CVD). Model predictions of the mole fractions of CH₄, C₂H₂, C₂H₄ and C₂H₆ agree well with gas chromatograph measurements of those species over a broad range of operating conditions. The numerical model includes a two-dimensional analysis of the sampling disturbance in the thin boundary layer above the substrate, accounts for chemistry in the gas chromatography sampling line, and utilizes a reaction mechanism that is significantly revised from a previously reported version. The model is used to predict the concentrations of H, CH₃, C₂H₂ and C at the diamond growth surface. It is suggested that methyl, acetylene and atomic carbon may all contribute significantly to film deposition during atmospheric-pressure RF plasma diamond CVD. The growth mechanism used in the model is shown to predict growth rates well at moderate substrate temperatures (∼1100 to 1230 K) but less well for lower (∼1000 K) and higher (∼1300 K) temperatures. The near-surface gas-phase chemical environment in atmospheric-pressure RF plasma diamond CVD is compared with several other diamond CVD environments. Compared with these other methods the thermal plasma is predicted to produce substantially higher concentration ratios at the surface of both H/CH₃ and C₂H₂/CH₃.     

Homoepitaxial diamond film with an atomically flat surface over a large area

Homoepitaxial diamond films with atomically flat surface were grown using the microwave plasma chemical vapor deposition method at a low CH₄ concentration of less than 0.05% in a CH₄ and H₂ mixed gas system. In Ib (001) diamond substrates having misorientation angles of 0.5°, atomic force microscope image on the surface of film grown at 0.025% CH₄ concentration showed that the films had atomically flat surface with mean roughness of 0.04 nm in area as large as 4×4 mm² (the whole region of the substrate).     

The role of inert gas in MW-enhanced plasmas for the deposition of nanocrystalline diamond thin films

Nanocrystalline diamond thin films have been deposited using microwave plasma enhanced deposition with gas mixtures of composition H₂/CH₄/X, where X was one of the inert gases He, Ne, Ar and Kr and typically constituted > 90% of the total gas flow. The diamond films obtained with each gas mixture deposited at approximately the same rate (0.15–0.5 µm h^? 1), and all showed similar morphologies and average grain sizes, despite very obvious differences in the appearance and gas temperatures of the respective plasmas. These plasmas were probed by optical emission and cavity ring-down spectroscopy, and results from companion 2D chemical kinetic modelling of the Ar/H₂/CH₄ and He/H₂/CH₄ plasma were used to guide interpretation of the experimental observations. We conclude that the inert gas, though acting primarily as a buffer, also has significant effects on the thermal conduction of the gas mixtures, the electron temperature and electron energy distribution, and thereby changes the main channels of ionization and input power absorption. As a result, inert gas dilution elevates the electron and gas temperatures, enhances the hydrogen dissociation degree and affects the H/C mixture composition and deposition mechanisms.     

Properties of boron-doped epitaxial diamond layers grown on (110) oriented single crystal substrates

Boron doped diamond layers have been grown on (110) single crystal diamond substrates with B/C ratios up to 20 ppm in the gas phase. The surface of the diamond layers observed by scanning electron microscopy consists of (100) and (113) micro-facets. Fourier Transform Photocurrent Spectroscopy indicates substitutional boron incorporation. Electrical properties were measured using Hall effect from 150 to 1000 K. Secondary ion mass spectrometry analyses are consistent with the high incorporation of boron determined by electrical measurements. A maximum mobility of 528 cm² V^− 1 s^− 1 was measured at room temperature for a charge carrier concentration of 1.1 10¹³ cm^− 3. Finally, properties of boron doped (110) diamond layers are compared with layers on (100) and (111) orientated substrates.     

Effect of co-adsorbed dopants on initial diamond growth steps: H abstraction from an adsorbed CH3

The effect induced by a neighbouring co-adsorbed dopant on H abstraction from an adsorbed CH₃ species on diamond has been investigated by using an ultra-soft pseudo-potential density functional theory (DFT) method under periodic boundary conditions. Both the (100) and (111) diamond surface orientations were considered with various types of dopants in two different hydrogenated forms; AH_x (A = N, B, S, or P; X = 0 or 1 for S; X = 1 or 2 for N, B and P, and X = 2 or 3 for C). The H abstraction by gaseous radical H was found to be energetically favoured by the presence of the dopants in all of their different hydrogenated forms. For NH₂, SH, or PH₂, this effect is induced by a destabilisation of the diamond surface by sterical repulsions between the adsorbed growth species CH₃ and the co-adsorbed dopant. For BH₂ and the dopants in their radical form, the abstraction reaction is favoured due to the formation of a new covalent bond between the dopant and the co-adsorbed CH₂ (product of the abstraction reaction), which strongly stabilises the surface after the abstraction process.     

Fast coating of ultramicroelectrodes with boron-doped nanocrystalline diamond

The present work is focused on the deposition of thin boron-doped nanocrystalline diamond (B-NCD) films on electrochemically etched tungsten wires by the hot filament assisted CVD method. The goal is the manufacturing of robust inert ultramicroelectrodes (UMEs) with a superior performance to be used for localised electrochemical analysis. The conductive diamond films can confer high stability of chemical and physical properties as well as low background current.Filament and substrate temperatures were kept constant at 2350 °C and 670 °C, respectively. The total system pressure was equal to 50 mbar and the CH₄/H₂ gas flow ratio was 0.07. Boron was used as the doping agent by solving B₂O₃ in ethanol, with a B/C content of 15000 ppm, and the solution was then dragged with argon gas flowing through a bubbler. The (Ar+B)/H₂ ratio values varied within the range of 0.06–0.21. The film growth rate decreases with the boron content increasing, but larger (Ar+B)/H₂ ratios result in smoother surfaces. UMEs insulation was carried out with epoxy resin in a home built device.The production of very sharp tungsten tips fully coated with B-NCD after just 30 min, for a (Ar+B)/H₂ ratio of 0.21, is one of the main outcomes of this work. The cyclic voltammetry showed a stable behavior with a wide electrochemical window of ~ 2.25 V in a 0.05 M NaCl solution proving the applicability of the developed UME for localized electroanalytical studies in biomedical and corrosion applications.