Stagnation Flow Reactor Research Articles

Metalorganic vapor phase epitaxy and characterization of Zn1−xFexSe films

The epitaxial growth of the diluted magnetic semiconductor (DMS) Zn1−xFexSe (0<x≤0.22) by metalorganic vapor phase epitaxy (MOVPE) is reported. The films were grown on (100) GaAs substrates in a vertical stagnation flow reactor with a specially designed inlet to minimize prereactions between the groups II and VI precursors. The precursors used in this study were (CH3)2Zn:N(CH2H5)3, Fe(CO)5, and H2Se diluted in H2 carrier gas. The epilayers were characterized by x-ray diffraction (XRD), Raman, absorption, and x-ray photoelectron spectroscopies (XPS). Typical growth rates were from 3–4 μm/h, which are significantly higher than those obtained by molecular beam epitaxy. Thus, in addition to the growth of DMS multilayer structures, MOVPE appears to be very promising for efficient growth of thick DMS films for Faraday magneto-optical applications.

Large-area diamond deposition in an atmospheric pressure stagnation-flow reactor

We present the results of large-area (20 cm2) diamond deposition from a scaled-up stagnation-flow reactor. The reactor uses a unique nozzle geometry that optimizes reagent gas usage. The premixed acetylene–oxygen–hydrogen flames were operated in a highly strained configuration, allowing uniform deposition of diamond with growth rates exceeding 25 μm/h. Substrate temperature control and flame stability of the chemical vapor deposition reactor are described. Diamond films were deposited on a molybdenum substrate with a surface temperature of approximately 1200 K and C/O ratio of 1.03. Diamond film growth results are presented, and film uniformity is assessed using micro-Raman spectroscopy.

Scaleable stagnation-flow reactors for uniform materials deposition: Application to combustion synthesis of diamond

We describe two inherently scaleable geometries for chemical-vapor-deposition and heat-transfer processes that are based on stagnation flows. The ‘‘coflow’’ and ‘‘trumpet-bell’’ designs result in radially uniform fluxes to surfaces and they optimize the use of reagents. Using a trumpet-bell burner, we have grown uniform films of diamond from a substrate-stabilized flat flame of C2H2/H2/O2.

Computational simulation of diamond chemical vapor deposition in premixed C 2H 2/O 2/H 2 and [formula omitted] flames

We have modeled combustion synthesis of CVD diamond in a stagnation-flow reactor under atmospheric conditions. In this configuration a premixed flat flame flows over a flat deposition substrate that lies perpendicular to the flow and parallel to the burner face. Optimal growth conditions occur when the flame is lifted from the burner surface and stabilized at the deposition surface. A similarity transformation for the stagnation flow field reduces the governing equations to a one-dimensional boundary value problem, significantly simplifying the computational task. The simulations include elementary gas-phase and surface chemistry as well as multicomponent molecular transport in the flame gas. Our model shows good qualitative agreement with observed growth parameters for the experimental conditions of Murayama et al. [1], who employed a premixed C 2H 2/H 2/O 2 gas mixture. Modeling CH 4 O 2 flame synthesis demonstrates that methane is less effective for diamond growth due to the decreased flame temperature and stability compared with C 2H 2 combustion.

A mathematical representation of a modified stagnation flow reactor for MOCVD applications

Computed results are presented describing the behavior of a modified stagnation point reactor for an MOCVD system, employing a showerhead type gas distributor. The principal findings of the work are the following: (a) By this arrangement, it is possible to obtain a very high spatial uniformity in the deposition rate, in cases better than 0.35% for a five inch diameter wafer. (b) Both the absolute values of the gas velocity and the standoff distance were found to play a critical role in affecting the uniformity of the deposition rate. Indeed a small standoff distance was found to be an essential ingredient in obtaining a good spatial uniformity of the deposit. (c) “An upside down” orientation was found to be helpful in minimizing thermal natural convection and a further refinement was found to be possible by imposing a desired radial distribution on the gas inlet velocity profile.

Computer modeling of chemical vapor deposition processes

A general purpose computer model for describing the transport phenomena and resulting rate of deposition has been described. Partial differential equations describing the conservation of mass, momentum, energy, and chemical species are solved by a computer program employing the finite difference method. The system considered in this paper is deposition of silicon in a vertical stagnation flow reactor by the reaction of silicon tetrachloride and hydrogen. The program allows for multiple chemical species and natural convection effects. Predicted silicon deposition rates along the substrate are in reasonable agreement with experimental values available in the literature. The effect of gas inlet configuration on the uniformity of deposition has been studied. The model can be used as a tool for design optimization of such reactors.

Thermal and plasma enhanced CVD OF HTc-superconductors

Metallorganic Chemical Vapor Deposition should be possible at high oxygen potentials with high deposition rates and with a high throwing power. Therefore, layers can be deposited on tapes and fiber bundles. Two types of reactors: a parallel plate reactor and a stagnation flow reactor were used. The deposition process is activated by a rf-plasma (13.56 MHz) and/or by heating the deposition surface. The gas flow phenomena in the reactor were studied by TiO 2 -fume experiments. The deposition of the single oxides BaO, CuO and Y 2 O 3 is described.

Kinetics of the Oxidation of Tungsten by CO2 at High Temperatures

The kinetics of oxidation of tungsten by CO2 was studied in a stagnation-flow reactor at CO2 partial pressures from 20 to 337 torr. The study covered the temperature range 2200°—3200°K and included measurements of the effect of CO on the reaction kinetics at CO/CO2 ratios up to 5:1. The rates observed below about 2650°K were essentially independent of the velocity of the impinging gas stream, indicating that the surface kinetics primarily determined the rates up to 2650°K at the lower gas-flow rates used and to higher temperatures at higher flow rates. Where surface kinetics were rate determining, the data could be explained simply by incorporating a temperature-dependent ``sticking coefficient'' for CO2 into the two-layer adatom model previously proposed for the W–O2 reaction. The effect of CO was small and was readily incorporated into the model. Velocity-dependent deviations from the rates predicted by the model were observed at temperatures above 2650°K. These were largely explained by simple mass-transfer theory. At low flow rates and high temperatures it appears that the reactions of desorbed oxygen became important.