N. V. Suetin

On the mechanism of CH3 radical formation in hot filament activated CH4/H2 and C2H2/H2 gas mixtures

Abstract Resonance enhanced multiphoton ionization spectroscopy has been used to determine relative number densities of CH 3 radicals in a hot filament chemical vapour deposition (HF-CVD) reactor designed for diamond growth, as a function of process gas (i.e. both CH 4 /H 2 and C 2 H 2 /H 2 gas mixtures), position ( d ), filament temperature ( T f ) and local gas temperature ( T g ). The similar CH 3 radical number density profiles observed upon activation of the two feedstock gas mixtures suggest that CH 3 radical formation in both cases is dominated by gas phase chemistry, in contradiction of the current consensus which invokes surface catalysed hydrogenation as the means of inducing the necessary CC bond fission in the case of C 2 H 2 /H 2 gas mixtures. Three body addition reactions involving C 2 H 2 (and C 2 H 4 ), together with H atoms and H 2 molecules, are identified as probable reactions requiring further study in order to provide a proper description of diamond CVD using a C 2 H 2 /H 2 gas feed.

Investigations of the gas phase chemistry in a hot filament CVD reactor operating with CH4/N2/H2 and CH4/NH3/H2 gas mixtures

Abstract We report a combination of experimental and theoretical studies of hot filament (HF) surface effects and the gas-phase chemistry prevailing in HF activated CH 4 /N 2 /H 2 and CH 4 /NH 3 /H 2 gas mixtures which provide some rationale for the observed low nitrogen doping levels in diamond films grown from such gas mixtures. The experimental studies involve use of resonance enhanced multiphoton ionisation (REMPI) to monitor relative H atom and CH 3 radical number densities in a HFCVD reactor as a function of filament temperature and N 2 /CH 4 and NH 3 /CH 4 gas mixing ratios. With NH 3 , contrary to N 2 , clear depletion of both H atom and CH 3 radical number densities are observed. The experimental observations are successfully reproduced using a previously developed 3-D model of HFCVD reactors. Three-dimensional (3-D) model calculations for C/H/N gas mixtures show significant N atom production due to successive ‘H-shifting’ reactions NH x +H⇔NH x −1 +H 2 ( x =1–3). N atom densities reach 5×10 13 cm −3 ; their reaction with CH 3 radicals accounts for the observed depletion of the latter and results in eventual production of HCN.

DiAmond growth enhancement in d.c. discharge CVD reactors. Effects of noble gas addition and pulsed mode application

Abstract Using the developed two-dimensional model of a d.c. discharge CVD reactor we have examined two possibilities of diamond growth rate enhancement. The first deals with the noble gas addition to inlet hydrogen-methane reactive mixture of a d.c. discharge. The second deals with application of a pulsed d.c. discharge. In the model, heat and mass transfer, electrodynamics phenomena, plasma-chemical and surface kinetics have been taken into account in a self-consistent manner. The electron-molecule reaction rate coefficients in methane/hydrogen/noble gas mixtures are calculated by solving the Boltzmann equation. Our calculations show that the dilution of the CH 4 H 2 mixture with argon allows the input power to reduce substantially and to increase the tolerable CH 4 H 2 inlet ratio. Diamond growth rate in a pulsed discharge may be several times higher than in a continuous d.c. discharge with the same power input. The overall enhancement depends on the relationship between the pulsed width and pause time.

Two-dimensional model of reactive gas flow in a diamond film CVD reactor

Abstract A two-dimensional model of the reactive gas flow, heat transfer and electrodynamic phenomena in a d.c. discharge reactor for diamond film deposition has been developed. Contrary to earlier proposed models we calculate the electron-molecule reaction rate coefficients by solving the Boltzmann equation for the electron energy distribution function. These coefficients are used for chemical gas composition calculations. An effective numerical method for solving the chemical kinetics equations has been developed. The electric field strength was determined from the total current conservation condition and the electron balance equation. The spatial distributions and fluxes of the chemical species and the gas flow parameters have been obtained as a result of the self-consistent calculations.

Emission properties of carbon nanowalls on porous silicon

For the past two decades various methods of carbon nanostructures growth have been proposed. Special substrate pretreatment methods are generally used to grow carbon nanowalls on silicon substrates and among them are mechanical and catalytic methods and ion bombardment in an rf discharge with bias. This work describes the possibility of growing carbon structures on porous silicon in a dc discharge without any additional pretreatment of the substrate surface. Carbon structures were grown on n- and p-type (100) porous silicon substrates produced by using standard photoelectrochemical etching. The analysis of these carbon structures revealed nanocrystalline carbon with multilayer carbon nanotubes and fibers. All samples demonstrated low field emission thresholds (Etr < 3 V/μm) and high current densities, showing an achieved current density of more than 6 A/cm2 for an electric field of E ∼ 15 V/μm. The authors investigated various modifications of porous silicon samples and carbon structures and demonstrated ...

Thermal diffusivity measurements by “instantaneous phase portrai” method☆

Abstract A new method of “instantaneous phase portrait” was developed for measurements of thermal diffusivity χ of thin films with high values of χ, such as diamond films. This method is based on the registration of the air layer refractive index spatial distribution by means of double exposure holographic interferometry and the derivation of the thermal diffusivity value χ from the spatial distribution. The values of thermal diffusivity for copper and steel foils measured by this technique coincide with the tabular ones. The method was also applied to measure thermal diffusivity of thin diamond films. The measured values were in 2–3 cm2 s−1 range.