Y.A. Mankelevich

Reevaluation of the mechanism for ultrananocrystalline diamond deposition from Ar∕CH4∕H2 gas mixtures

Various mechanisms for the growth and renucleation of ultrananocrystalline diamond (UNCD) films are discussed and evaluated in the light of experimental and theoretical evidences in recent publications. We propose that the most likely model for UNCD growth is that where most of the diamond is formed via a similar mechanism to that of microcrystalline diamond films, i.e., gas phase H atoms abstracting surface hydrogens, followed by a CHx, x=0–3, addition. Calculations of the gas composition close to the substrate surface in the microwave plasma reactor for both the microcrystalline diamond and the UNCD growth, at substrate temperatures of 1073 and 673K, suggest that CH3 and C atoms are the most likely precursors for the growth of UNCD. However, the deposition is interrupted by an event which prevents the smooth growth of a continuous layer, and instead creates a surface defect which changes the growth direction and acts as a renucleation site. The possible nature of this event is discussed in detail. Using...

On the possibility of O2(a 1Δg) production by a non-self-sustained discharge for oxygen–iodine laser pumping

O2(a 1Δg) production in a non-self-sustained discharge (ND) in pure oxygen and oxygen mixtures with inert gases (Ar and He) has been studied. A self-consistent model of ND in pure oxygen is developed, allowing us to simulate all the obtained experimental data. Agreement between the experimental and simulated results for pure oxygen over a wide range of reduced electric fields was reached only after taking into account the ion component of the discharge current. It is shown that the correct estimation of the energetic efficiency of O2(a 1Δg) excitation by discharge using the EEDF calculation is possible only with the correct description of the energy deposit into the plasma on the basis of an adequate discharge model. The testing of an O2(a 1Δg) excitation cross-section by direct electron impact, as well as a kinetic scheme of processes involving singlet oxygen, has been carried out by the comparison of experimental and simulated data. The tested model was then used for simulating O2(a 1Δg) production in ND in oxygen mixtures with inert gases. The study of O2(a 1Δg) production in Ar : O2 mixtures with small oxygen content has shown that the ND in these mixtures is spatially non-uniform, which essentially decreases the energetic efficiency of singlet oxygen generation. While simulating the singlet oxygen density dynamics, the process of three-body deactivation of O2(a 1Δg) by O(3P) atoms was for the first time taken into account. The maximal achievable concentration of singlet oxygen in ND can be limited by this quenching. On the basis of the results obtained and the model developed, the influence of hydrogen additives on singlet oxygen kinetics in argon–oxygen–hydrogen mixtures has been analysed. The simulation has shown that fast quenching of O2(a 1Δg) by atomic hydrogen is possible due to significant gas heating in the discharge that can significantly limit the yield of singlet oxygen in hydrogen-containing mixtures.

Experiment and modeling of the deposition of ultrananocrystalline diamond films using hot filament chemical vapor deposition and Ar/CH4/H2 gas mixtures : A generalized mechanism for ultrananocrystalline diamond growth

Ar∕CH4∕H2 gas mixtures have been used to deposit nanocrystalline diamond (NCD) and ultrananocrystalline diamond (UNCD) films using hot filament (HF) chemical vapor deposition. The Ar:H2 concentration was maintained at Ar∕(H2+Ar)=80% while the CH4 concentration was varied over the range CH4∕(H2+CH4)=0.3–6.0. For higher methane concentrations, the filament became coated in a graphitic layer which prevented film growth. For lower CH4 additions, the film morphology depended upon the CH4 concentration, with different gas mixing ratios producing microcrystalline diamond (MCD), NCD, or UNCD films. A two-dimensional computer model was used to calculate the gas phase composition for all these conditions at all positions within the reactor. Using the experimental and calculated data, we show that the observed film morphology can be rationalized using a model based on competition between H atoms, CH3 radicals, and other C1 species reacting with dangling bonds on the surface. The relative concentrations of each of th...

Two-dimensional simulation of a hot-filament chemical vapor deposition reactor

Abstract A two-dimensional model of a hot-filament (HF) CVD reactor has been developed to study the gas-phase and surface processes of diamond growth. Full transport equations were solved numerically to calculate the gas temperature, fluid flow, and species concentration fields. Catalytic chemistry at the filament surface was considered. The distribution of the hydrogen atom concentration and the gas temperature in an HFCVD reactor were obtained analytically. The expressions for diamond growth rate and the hydrogen atom destruction coefficient at the substrate were derived from the surface kinetics. The gas-phase reaction mechanism was obtained using a brute force sensitivity analysis. The calculated results were compared with existing experimental data.

Simulations of chemical vapor deposition diamond film growth using a kinetic Monte Carlo model

A one-dimensional kinetic Monte Carlo model has been developed to simulate the chemical vapor 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 reaction rates for these processes are re-evaluated in detail and their effect upon the predicted growth rates and morphology are described. We find that for standard CVD diamond conditions, etching of sp3 carbon species from the growing surface is negligible. Surface migration occurs rapidly, but is mostly limited to CH2 species oscillating back and forth between two adjacent radical sites. Despite the average number of migration hops being in the thousands, the average surface diffusion length for a surface species—before it either adds to the diamond lattice or is removed back to the gas phase—is <2 sites. β-scission helps to smooth the surface, but is only a relatively minor process removing <2% of adsorbed species. At low substrate ...

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.

Three-dimensional simulation of a HFCVD reactor

Abstract A three-dimensional (3D) model of a hot-filament CVD reactor is developed to study the gas-phase and surface processes of diamond growth. The gas-phase and surface reaction mechanisms, the molecular diffusion and thermodiffusion, catalytic hydrogen dissociation at the filament and surface kinetics at the substrate are taken into account in transport equations that are numerically solved. The model has been checked using experimental measurements of absolute methyl and atomic hydrogen concentration. An importance of 3D effects for the species concentration and gas temperature distribution is revealed. The filament and substrate temperature dependence of species concentrations are calculated and compared with experimental data.

Chemical kinetics in carbon depositing d.c.-arc jet CVD reactors

Experimental and theoretical studies of the behaviour of hydrocarbon species in d.c.-arc jet chemical vapour deposition reactors are reported, as a function of carbon source gas flow rate. CH(X) and C (a) radical number densities have been measured in 2 absorption (by cavity ring-down spectroscopy) and via their optical emission in an arc jet plume operating with a standard CH y 4 H yAr feedstock gas mixture. The C (a) radical number density is seen to exhibit a linear (or sub-linear) dependence on CH 22 4 flow rate, in accord with previous findings (J. Appl. Phys. 82 (1997) 2072) for both C (a) and C (X) radicals in a lower power 23 d.c.-arc jet. The present findings, together with the comprehensive set of earlier experimental data on gas velocity and gas temperature measurements (Diam. Relat. Mater. 7 (1998) 165; Plasma Sources Sci. Technol. 10 (2001) 595) have been used in developing a model of the plasma plume. The present calculations suggest that the observed high diamond growth rates ()50– 100 mmyh) are most probably related to atomic C, which is present at concentrations as high as 10 cm , though C species 14 y3 2

Experimental data vs. 3-D model calculations of HFCVD processes : correlations and discrepancies

Abstract An existing 3-D model [Mankelevich et al. Diamond Relat. Mater . 7 (1998) 1133] has been used to explain experimentally measured spatially resolved CH 3 radical number densities in hot filament CVD reactors operating with both CH 4 /H 2 and C 2 H 2 /H 2 process gas mixtures and to examine in detail the process of C 2 ↔C 1 inter-conversion in the gas phase. It has been shown that cooler regions distant from the filament need to be modelled in order to obtain the significant C 2 →C 1 conversion observed in HFCVD reactors with C 2 H 2 /H 2 process gas mixtures. The origin and effect of the non-equilibrated H 2 molecule vibrational state population distribution are studied for the first time in the context of HFCVD reactor models.

Simulations of chemical vapor deposition diamond film growth using a kinetic Monte Carlo model and two-dimensional models of microwave plasma and hot filament chemical vapor deposition reactors

A one-dimensional kinetic Monte Carlo (KMC) model has been developed to simulate the chemical vapor deposition of a diamond (100) surface under conditions used to grow single-crystal diamond (SCD), microcrystalline diamond (MCD), nanocrystalline diamond (NCD), and ultrananocrystalline diamond (UNCD) films. The model considers adsorption, etching/desorption, lattice incorporation and surface migration but not defect formation or renucleation processes. Two methods have been devised for estimation of the gas phase concentrations of species at the growing diamond surface, and are used to determine adsorption rates for C1Hx hydrocarbons for the different conditions. The rate of migration of adsorbed carbon species is governed by the availability of neighboring radical sites, which, in turn, depend upon the rates of H abstraction and of surface-radical migration. The KMC model predicts growth rates and surface roughness for each of diamond types consistent with experiment. In the absence of defect formation an...