Imaging and Modeling C2 Radical Emissions from Microwave Plasma-Activated Methane/Hydrogen Gas Mixtures: Contributions from Chemiluminescent Reactions and Investigations of Higher-Pressure Effects and Plasma Constriction.

Wavelength and spatially resolved imaging and 2D plasma chemical modeling methods have been used to study the emission from electronically excited C2 radicals in microwave-activated dilute methane/hydrogen gas mixtures under processing conditions relevant to the chemical vapor deposition (CVD) of diamond. Obvious differences in the spatial distributions of the much-studied C2(d3Πg-a3Πu) Swan band emission and the little-studied, higher-energy C2(C1Πg-A1Πu) emission are rationalized by invoking a chemiluminescent (CL) reactive source, most probably involving collisions between H atoms and C2H radicals, that acts in tandem with the widely recognized electron impact excitation source term. The CL source is relatively much more important for forming C2(d) state radicals and is deduced to account for >40% of C2(d) production in the hot plasma core under base operating conditions, which should encourage caution when estimating electron or gas temperatures from C2 Swan band emission measurements. Studies at higher pressures (p ≈ 400 Torr) offer new insights into the plasma constriction that hampers efforts to achieve higher diamond CVD rates by using higher processing pressures. Plasma constriction is proposed as being inevitable in regions where the local electron density (ne) exceeds some critical value (nec) and electron-electron collisions enhance the rates of H2 dissociation, H-atom excitation, and related associative ionization processes relative to those prevailing in the neighboring nonconstricted plasma region. The 2D modeling identifies a further challenge to high-p operation. The radial uniformities of the CH3 radical and H-atom concentrations above the growing diamond surface both decline with increasing p, which are likely to manifest as less spatially uniform diamond growth (in terms of both rate and quality).

Mechanisms of hydrogen atom interactions with MoS2 monolayer.

The mechanisms of H atoms interactions with single-layer MoS2, a two-dimensional transition metal dichalcogenide, are studied by static and dynamic DFT (density functional theory) modeling. Adsorption energies for H atoms on MoS2, barriers for H atoms migration and recombination on hydrogenated MoS2 surface and effects of H atoms adsorptions on MoS2 electronic properties and sulfur vacancy production were obtained by the static DFT calculations. The dynamic DFT calculations give insight into the dynamics of reactive interactions of incident H atoms with hydrogenated MoS2 at H atoms energies in the range of 0.05-1 eV and elucidate the competitive mechanism of hydrogen adsorption and recombination that limits hydrogen surface coverage at the level of 30%. Various pathways of S-vacancies production and H atoms losses on MoS2 are calculated and the effects of MoS2 temperature on these processes are estimated and discussed.

Optical Emission Imaging and Modeling Investigations of Microwave-Activated SiH4/H2 and SiH4/CH4/H2 Plasmas.

Silicon is a known trace contaminant in diamond grown by chemical vapor deposition (CVD) methods. Deliberately Si-doped diamond is currently attracting great interest because of the attractive optical properties of the negatively charged silicon-vacancy (SiV-) defect. This work reports in-depth studies of microwave-activated H2 plasmas containing trace (10-100 ppm) amounts of SiH4, with and without a few % of CH4, operating at pressures and powers relevant for contemporary diamond CVD, using a combination of experiment (spatially resolved optical emission (OE) imaging) and two-dimensional plasma chemical modeling. Key features identified from analysis and modeling of the OE from electronically excited H, H2, Si, and SiH species in the dilute Si/H plasmas include the following: (i) fast H-shifting reactions ensure that Si atoms are the most abundant silicon-containing species throughout the entire reactor volume, (ii) the low ionization potentials of all SiHx (x ≤ 4) species and efficient ion conversion reactions ensure that even trace SiH4 additions cause a change in the dominant ions in the plasma volume (from H3+ to SiHx+), with consequences for electron-ion recombination rates and ambipolar diffusion coefficients, and (iii) the total silicon content in the reactor volume can be substantially perturbed by silicon deposition and H atom etching reactions at the reactor walls. The effects of adding trace amounts of SiH4 to a pre-existing C/H plasma are shown to be much less dramatic but include the following: (i) a Si substrate or fused silica components within the reactor are a ready (unintended) source of gas-phase Si-containing species, (ii) OE from electronically excited Si atoms should provide a reliable measure of the Si content in the hot plasma region, and (iii) Si atoms and/or SiC2 species are the most abundant gas-phase Si-containing species just above the growing diamond surface and thus the most likely carriers of the silicon incorporated into CVD diamond.

Imaging and Modeling the Optical Emission from CH Radicals in Microwave Activated C/H Plasmas.

We report a combined experimental/modeling study of optical emission from the A2Δ, B2Σ-, and C2Σ+ states of the CH radical in microwave (MW) activated CH4/H2 gas mixtures operating under a range of conditions relevant to the chemical vapor deposition of diamond. The experiment involves spatially and wavelength resolved imaging of the CH(C → X), CH(B → X), and CH(A → X) emissions at different total pressures, MW powers, C/H ratios in the source gas, and substrate diameters. The results are interpreted by extending an existing 2D (r, z) plasma model to include not just electron impact excitation but also chemiluminescent (CL) bimolecular reactions as sources of the observed CH emissions. Three possible CL reactions (of H atoms with CH2(a1A1) and CH2(X3B1) radicals and of C(1D) atoms with H2) are identified as plausible sources of electronically excited CH radicals (particularly of the lowest energy CH(A) state radicals). Each or all of these could contribute to the observed emissions and, collectively, are deduced to be the major source of the CH(A) emissions observed at the high temperatures (Tgas ∼ 3000 K) and pressures (75 ≤ p ≤ 275 Torr) explored in the present study. We suggest that such CL contributions are likely to be commonplace in such high pressure, high temperature plasma environments and highlight some of the risks associated with using relative emission intensities as an indicator of the electron characteristics in such plasmas.

N-Doped Carbon NanoWalls for Power Sources.

Cycling stability and specific capacitance are the most critical features of energy sources. Nitrogen incorporation in crystalline carbon lattice allows to increase the capacitance without increasing the mass of electrodes. Despite the fact that many studies demonstrate the increase in the capacitance of energy sources after nitrogen incorporation, the mechanism capacitance increase is still unclear. Herein, we demonstrate the simple approach of plasma treatment of carbon structures, which leads to incorporation of 3 at.% nitrogen into Carbon NanoWalls. These structures have huge specific surface area and can be used for supercapacitor fabrication. After plasma treatment, the specific capacitance of Carbon NanoWalls increased and reached 600 F g-1. Moreover, we made a novel DFT simulation which explains the mechanism of nitrogen incorporation into the carbon lattice. This work paves the way to develop flexible thin film supercapacitors based on carbon nanowalls.

Combined Spatially Resolved Optical Emission Imaging and Modeling Studies of Microwave-Activated H2/Ar and H2/Kr Plasmas Operating at Powers and Pressures Relevant for Diamond Chemical Vapor Deposition.

Microwave (MW) activated H2/Ar (and H2/Kr) plasmas operating under powers and pressures relevant to diamond chemical vapor deposition have been investigated experimentally and by 2-D modeling. The experiments return spatially and wavelength resolved optical emission spectra of electronically excited H2 molecules and H and Ar(/Kr) atoms for a range of H2/noble gas mixing ratios. The self-consistent 2-D( r, z) modeling of different H2/Ar gas mixtures includes calculations of the MW electromagnetic fields, the plasma chemistry and electron kinetics, heat and species transfer and gas-surface interactions. Comparison with the trends revealed by the spatially resolved optical emission measurements and their variations with changes in process conditions help guide identification and refinement of the dominant plasma (and plasma emission) generation mechanisms and the more important Ar-H, Ar-H2, and H-H2 coupling reactions. Noble gas addition is shown to encourage radial expansion of the plasma, and thus to improve the uniformity of the H atom concentration and the gas temperature just above the substrate. Noble gas addition in the current experiments is also found to enhance (unwanted) sputtering of the copper base plate of the reactor; the experimentally observed increase in gas phase Cu* emission is shown to correlate with the near substrate ArH+ (and KrH+) ion concentrations returned by the modeling, rather than with the relatively more abundant H3+ (and H3O+) ions.

Spatially Resolved Optical Emission and Modeling Studies of Microwave-Activated Hydrogen Plasmas Operating under Conditions Relevant for Diamond Chemical Vapor Deposition.

A microwave (MW) activated hydrogen plasma operating under conditions relevant to contemporary diamond chemical vapor deposition reactors has been investigated using a combination of experiment and self-consistent 2-D modeling. The experimental study returns spatially and wavelength resolved optical emission spectra of the d → a (Fulcher), G → B, and e → a emissions of molecular hydrogen and of the Balmer-α emission of atomic hydrogen as functions of pressure, applied MW power, and substrate diameter. The modeling contains specific blocks devoted to calculating (i) the MW electromagnetic fields (using Maxwell's equations) self-consistently with (ii) the plasma chemistry and electron kinetics, (iii) heat and species transfer, and (iv) gas-surface interactions. Comparing the experimental and model outputs allows characterization of the dominant plasma (and plasma emission) generation mechanisms, identifies important coupling reactions between hydrogen atoms and molecules (e.g., the quenching of H( n > 2) atoms and electronically excited H2 molecules (H2*) by the alternate ground-state species and H3+ ion formation by the associative ionization reaction of H( n = 2) atoms with H2), and illustrates how spatially resolved H2* (and Hα) emission measurements offer a detailed and sensitive probe of the hyperthermal component of the electron energy distribution function.

What [plasma used for growing] diamond can shine like flame?

Diamond synthesis by chemical vapour deposition (CVD) from carbon-containing gas mixtures has by now long been an industrial reality, but commercial interest and investment into the technology has grown dramatically in the last several years. This Feature Article surveys recent advances in our understanding of the gas-phase chemistry of microwave-activated methane/hydrogen plasmas used for diamond CVD, including that of added boron-, nitrogen- and oxygen-containing dopant species. We conclude by considering some of the remaining challenges in this important area of contemporary materials science.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. II: CH4/N2/H2 Plasmas.

We report a combined experimental and modeling study of microwave-activated dilute CH4/N2/H2 plasmas, as used for chemical vapor deposition (CVD) of diamond, under very similar conditions to previous studies of CH4/H2, CH4/H2/Ar, and N2/H2 gas mixtures. Using cavity ring-down spectroscopy, absolute column densities of CH(X, v = 0), CN(X, v = 0), and NH(X, v = 0) radicals in the hot plasma have been determined as functions of height, z, source gas mixing ratio, total gas pressure, p, and input power, P. Optical emission spectroscopy has been used to investigate, with respect to the same variables, the relative number densities of electronically excited species, namely, H atoms, CH, C2, CN, and NH radicals and triplet N2 molecules. The measurements have been reproduced and rationalized from first-principles by 2-D (r, z) coupled kinetic and transport modeling, and comparison between experiment and simulation has afforded a detailed understanding of C/N/H plasma-chemical reactivity and variations with process conditions and with location within the reactor. The experimentally validated simulations have been extended to much lower N2 input fractions and higher microwave powers than were probed experimentally, providing predictions for the gas-phase chemistry adjacent to the diamond surface and its variation across a wide range of conditions employed in practical diamond-growing CVD processes. The strongly bound N2 molecule is very resistant to dissociation at the input MW powers and pressures prevailing in typical diamond CVD reactors, but its chemical reactivity is boosted through energy pooling in its lowest-lying (metastable) triplet state and subsequent reactions with H atoms. For a CH4 input mole fraction of 4%, with N2 present at 1-6000 ppm, at pressure p = 150 Torr, and with applied microwave power P = 1.5 kW, the near-substrate gas-phase N atom concentration, [N]ns, scales linearly with the N2 input mole fraction and exceeds the concentrations [NH]ns, [NH2]ns, and [CN]ns of other reactive nitrogen-containing species by up to an order of magnitude. The ratio [N]ns/[CH3]ns scales proportionally with (but is 102-103 times smaller than) the ratio of the N2 to CH4 input mole fractions for the given values of p and P, but [N]ns/[CN]ns decreases (and thus the potential importance of CN in contributing to N-doped diamond growth increases) as p and P increase. Possible insights regarding the well-documented effects of trace N2 additions on the growth rates and morphologies of diamond films formed by CVD using MW-activated CH4/H2 gas mixtures are briefly considered.

Microwave Plasma-Activated Chemical Vapor Deposition of Nitrogen-Doped Diamond. I. N2/H2 and NH3/H2 Plasmas.

We report a combined experimental/modeling study of microwave activated dilute N2/H2 and NH3/H2 plasmas as a precursor to diagnosis of the CH4/N2/H2 plasmas used for the chemical vapor deposition (CVD) of N-doped diamond. Absolute column densities of H(n = 2) atoms and NH(X(3)Σ(-), v = 0) radicals have been determined by cavity ring down spectroscopy, as a function of height (z) above a molybdenum substrate and of the plasma process conditions, i.e., total gas pressure p, input power P, and the nitrogen/hydrogen atom ratio in the source gas. Optical emission spectroscopy has been used to investigate variations in the relative number densities of H(n = 3) atoms, NH(A(3)Π) radicals, and N2(C(3)Πu) molecules as functions of the same process conditions. These experimental data are complemented by 2-D (r, z) coupled kinetic and transport modeling for the same process conditions, which consider variations in both the overall chemistry and plasma parameters, including the electron (Te) and gas (T) temperatures, the electron density (ne), and the plasma power density (Q). Comparisons between experiment and theory allow refinement of prior understanding of N/H plasma-chemical reactivity, and its variation with process conditions and with location within the CVD reactor, and serve to highlight the essential role of metastable N2(A(3)Σ(+)u) molecules (formed by electron impact excitation) and their hitherto underappreciated reactivity with H atoms, in converting N2 process gas into reactive NHx (x = 0-3) radical species.

Tailoring of the carbon nanowall microstructure by sharp variation of plasma radical composition.

In this paper we propose a new and simple method to tune the carbon nanowall microstructure by sharp variation of CH4/H2 plasma conditions. Using theoretical calculations we demonstrated that the sharp variation of gas pressure and discharge current leads to significant variation of plasma radical composition. In some cases such perturbation creates the necessary conditions for the nucleation of smaller secondary nanowalls on the surface of primary ones.

Exploring the plasma chemistry in microwave chemical vapor deposition of diamond from C/H/O gas mixtures.

Microwave (MW)-activated CH(4)/CO(2)/H(2) gas mixtures operating under conditions relevant to diamond chemical vapor deposition (i.e., X(C/Σ) = X(elem)(C)/(X(elem)(C) + X(elem)(O)) ≈ 0.5, H(2) mole fraction = 0.3, pressure, p = 150 Torr, and input power, P = 1 kW) have been explored in detail by a combination of spatially resolved absorption measurements (of CH, C(2)(a), and OH radicals and H(n = 2) atoms) within the hot plasma region and companion 2-dimensional modeling of the plasma. CO and H(2) are identified as the dominant species in the plasma core. The lower thermal conductivity of such a mixture (cf. the H(2)-rich plasmas used in most diamond chemical vapor deposition) accounts for the finding that CH(4)/CO(2)/H(2) plasmas can yield similar maximal gas temperatures and diamond growth rates at lower input powers than traditional CH(4)/H(2) plasmas. The plasma chemistry and composition is seen to switch upon changing from oxygen-rich (X(C/Σ) < 0.5) to carbon-rich (X(C/Σ) > 0.5) source gas mixtures and, by comparing CH(4)/CO(2)/H(2) (X(C/Σ) = 0.5) and CO/H(2) plasmas, to be sensitive to the choice of source gas (by virtue of the different prevailing gas activation mechanisms), in contrast to C/H process gas mixtures. CH(3) radicals are identified as the most abundant C(1)H(x) [x = 0-3] species near the growing diamond surface within the process window for successful diamond growth (X(C/Σ) ≈ 0.5-0.54) identified by Bachmann et al. (Diamond Relat. Mater.1991, 1, 1). This, and the findings of similar maximal gas temperatures (T(gas) ~2800-3000 K) and H atom mole fractions (X(H)~5-10%) to those found in MW-activated C/H plasmas, points to the prevalence of similar CH(3) radical based diamond growth mechanisms in both C/H and C/H/O plasmas.

Optical emission from microwave activated C/H/O gas mixtures for diamond chemical vapor deposition.

The spatial distributions and relative abundances of electronically excited H atoms, OH, CH, C(2) and C(3) radicals, and CO molecules in microwave (MW) activated CH(4)/CO(2)/H(2) and CO/H(2) gas mixtures operating under conditions appropriate for diamond growth by MW plasma enhanced chemical vapor deposition (CVD) have been investigated by optical emission spectroscopy (OES) as a function of process conditions (gas mixing ratio, incident MW power, and pressure) and rationalized by reference to extensive 2-dimensional plasma modeling. The OES measurements clearly reveal the switch in plasma chemistry and composition that occurs upon changing from oxygen-rich to carbon-rich source gas mixtures, complementing spatially resolved absorption measurements under identical plasma conditions (Kelly et al., companion article). Interpretation of OES data typically assumes that electron impact excitation (EIE) is the dominant route to forming the emitting species of interest. The present study identifies a number of factors that complicate the use of OES for monitoring C/H/O plasmas. The OH* emission from EIE of ground state OH(X) radicals can be enhanced by excitation energy transfer from metastable CO(a(3)Π) molecules. The CH* and C(2)* emissions can be boosted by chemiluminescent reactions between, for example, C(2)H radicals and O atoms, or C atoms and CH radicals. Additionally, the EIE efficiency of each of these radical species is sensitively dependent on any spatial mismatch between the regions of maximal radical and electron density, which itself is a sensitive function of elemental C/O ratio in the process gas mixture (particularly when close to 1:1, as required for diamond growth) and the H(2) mole fraction.

Spectroscopic and modeling investigations of the gas-phase chemistry and composition in microwave plasma activated B2H6/Ar/H2 mixtures.

This paper describes a three-pronged study of microwave (MW) activated B(2)H(6)/Ar/H(2) plasmas as a precursor to diagnosis of the B(2)H(6)/CH(4)/Ar/H(2) plasmas used for the chemical vapor deposition of B-doped diamond. Absolute column densities of B atoms and BH radicals have been determined by cavity ring-down spectroscopy as a function of height (z) above a molybdenum substrate and of the plasma process conditions (B(2)H(6) and Ar partial pressures, total pressure, and supplied MW power). Optical emission spectroscopy has been used to explore variations in the relative densities of electronically excited BH, H, and H(2) species as a function of the same process conditions and of time after introducing B(2)H(6) into a pre-existing Ar/H(2) plasma. The experimental measurements are complemented by extensive 2-D(r, z) modeling of the plasma chemistry, which results in refinements to the existing B/H chemistry and thermochemistry and demonstrates the potentially substantial loss of gas-phase BH(x) species through reaction with trace quantities of air/O(2) in the process gas mixture and heterogeneous processes occurring at the reactor wall.

On the mechanism of H atom production in hot filament activated H2 and CH4/H2 gas mixtures.

This article reports systematic measurements of the power utilization by Ta (and Re) hot filaments (HFs) operating in a poor vacuum, in pure He, N(2), and H(2), and in CH(4)/H(2) gas mixtures of relevance to diamond growth by HF chemical vapor deposition, as functions of filament temperature T(fil) (in the range of 1800-2700 K) and gas pressure p (in the range of 10(-2)-100 Torr). In the cases of H(2) and the CH(4)/H(2) gas mixtures, the power consumption studies are complemented by in situ measurements of the relative H atom densities [H] near the HF--which are seen to maximize at p approximately 10-20 Torr and thereafter to remain constant or, at the highest T(fil), to decline at higher p. These (and many previous) findings are rationalized by a companion theoretical analysis, which reduces the complex array of chemisorption and desorption processes that must contribute to the HF-surface mediated dissociation of H(2) to a two-step mechanism involving H atom formation by dissociative adsorption at bare (S(*)) sites and by desorption at hydrogenated (SH) sites on the HF surface.

Simplified Monte Carlo simulations of chemical vapour deposition diamond growth.

A simple one-dimensional Monte Carlo model has been developed to simulate the chemical vapour deposition (CVD) of a diamond (100) surface. The model considers adsorption, etching/desorption, lattice incorporation, and surface migration along and across the dimer rows. The top of a step-edge is considered to have an infinite Ehrlich-Schwoebel potential barrier, so that mobile surface species cannot migrate off the edge. The reaction probabilities are taken from experimental or calculated literature values for standard CVD diamond conditions. The criterion used for the critical nucleus needed to form a new layer is considered to be two surface carbon species bonded together, which forms an immobile, unetchable step on the surface. This nucleus can arise from two migrating species meeting, or from direct adsorption of a carbon species next to a migrating species. The analysis includes film growth rate, surface roughness, and the evolving film morphology as a function of varying reaction probabilities. Using standard CVD diamond parameters, the simulations reveal that a smooth film is produced with apparent step-edge growth, with growth rates (∼1 µm h(-1)) consistent with experiment. The ß-scission reaction was incorporated into the model, but was found to have very little effect upon growth rates or film morphology. Renucleation events believed to be due to reactive adsorbates, such as C atoms or CN groups, were modelled by creating random surface defects which form another type of critical nucleus upon which to nucleate a new layer. These were found to increase the growth rate by a factor of ∼10 when the conditions were such that the rate-limiting step for growth was new layer formation. For other conditions these surface defects led to layered 'wedding cake' structures or to rough irregular surfaces resembling those seen experimentally during CVD of nanocrystalline diamond.

Experimental and modeling studies of B atom number density distributions in hot filament activated B2H6/H2 and B2H6/CH4/H2 gas mixtures.

Experimental and modeling studies of the gas-phase chemistry occurring in dilute, hot filament (HF) activated B2H6/H2 and B2H6/CH4/H2 gas mixtures are reported. Spatially resolved relative number densities of B (and H) atoms have been measured by resonance enhanced multiphoton ionization methods, as a function of process conditions (e.g. the HF material and its temperature, the B2H6/H2 mixing ratio, and the presence (or not) of added CH4). Three-dimensional modeling of the H/B chemistry prevailing in such HF activated gas mixtures using a simplified representation of the gas phase chemistry succeeds in reproducing all of the experimentally observed trends, and in illustrating the key role of the "H-shifting" reactions BHx + H <= => BHx-1 + H2 (x = 1-3) in enabling rapid interconversion between the various BHx (x = 0-3) species. CH4 addition, at partial pressures appropriate for growth of boron-doped diamond by chemical vapor deposition methods, leads to approximately 30% reduction in the measured B atom signal near the HF. The modeling suggests that this is mainly due to concomitant H atom depletion near the HF, but it also allows us a first assessment of the possible contributions from B/C coupling reactions upon CH4 addition to HF activated B2H6/H2 gas mixtures.