Ivan O. Clark

Effects of supercooling in the initial solidification of PbTe-SnTe solid solutions

Abstract Deviations from compositions anticipated by the thermal equilibrium phase diagram have been observed in Bridgman-grown crystals of Pb 1− x Sn x Te, in the first to freeze region of the boule. A set of experiments were conducted to determine the extent of thermal supercooling of Pb 1− x Sn x Te in a Bridgman-like configuration. The results of the compositional profiles and the supercooling measurements are consistent with a diffusionless transformation occurring at the onset of solidification and the length of uncontrolled growth is inversely related to the temperature gradient of the furnace.

MOCVD of GaAs in a horizontal reactor: modeling and growth

Metalorganic chemical vapor deposition (MOCVD) of GaAs in a horizontal reactor was performed and the process mathematically modeled in two dimensions. Growth was performed at pressures of 1.0 and 0.1 atm. The mas flow rates of the reactants, trimethylgallium (TMG) and arsine, were held constant in all runs with the hydrogen carrier gas comprising the remainder of the flow. An advanced computational fluid dynamics code enhanced for chemically reacting non-isothermal flows was used to model the deposition process. Parameters used in the model included: reactor geometry and operating pressure; thermal boundary conditions; ratio of reactants; chemical reactions; total inlet gas flow rate; and molecular weights, thermal conductivities, heat capacities, viscosities, and binary diffusion coefficients of the gas-phase species. Densities were calculated from the ideal gas law. Heats of reaction were obtained from the enthalpies of formation of the individual species. A single surface reaction Ga(CH3)3 + AsH3 → GaAs(s)+3CH4 was assumed. Film thickness profiles predicted by the model were compared with those of the GaAs thin films grown in the modeled reactor. The predictions and data are in good agreement over the entire length of the deposition region for the low pressure and high flow rate run. At the lower flow rate and at atmospheric pressure, the model predicted much more uniform deposition profiles than were obtained experimentally. These results and possible explanations for the discrepancies between the model and the data will be presented. Work is in progress to refine the model by including additional chemical reactions, Soret diffusion, and three-dimensional simulations.

Characterization of MOCVD fluid dynamics by laser velocimetry

Abstract The process of metalorganic chemical vapor deposition (MOCVD) is critically dependent on the fluid dynamics of the reacting and carrier gases. Quantitative understanding of the basic fluid dynamics associated with a reactor design is central to the application of engineering techniques to achieve design improvements in efficiency, bulk production and growth uniformity. A laser velocimetry (LV) system has been adapted specifically to acquire such quantitative information for the low Reynolds number mixed convective flows commonly encountered in MOCVD reactors. The instrument uses three color-separated wavelengths of an argon-ion laser to make three simultaneous independent orthogonal measurements of the flow field at specified interior locations. The technique is noninvasive, provides high spatial resolution, and is compatible with reactors operating at growth temperatures. The fluid dynamics of a horizontal MOCVD reactor used for GaAs growth were characterized by laser velocimetry measurements. The full three-component LV system was employed to determine a three-dimensional map of the flow field inside the quartz reactor vessel. Data have been obtained from the vessel both at room temperature and heated to growth temperature by RF induction. The two cases were compared to evaluate the effects of gravity-induced convection and buoyancy. Significant convective effects were found in the heated reactor, while fluidic switching effects were observed during room temperature operation. Instrumentation details and flow field measurements are presented.

Experimental investigation of the effects of gravity on thermosolutal convection and compositional homogeneity in bridgman grown, compound semiconductors☆

Lead-tin-telluride has been grown in a thermally stable mode (solutally unstable) and in a solutally stable (thermally unstable) mode in a Bridgman configuration. Significant differences in the crystal morphology and the compositional homogeneity have been found between the two configurations. In addition, for the solutally stable configuration, evidence has been found that the flow characteristics in the melt change drastically during the course of the run.

MOCVD manifold switching effects on growth and characterization

Abstract The components of metalorganic chemical vapor deposition (MOCVD) supply manifolds have significant effects on the ability to rapidly introduce and exhaust growth materials. This in turn has direct bearing on the achievable heterojunction interface abruptness for a given manifold design. Research to quantify the effects of various manifold components on the switching speed has been performed with a combined modeling and experimental approach. An overview of the experimental investigation is reported. Manifold switching sections were incorporated in a manifold line instrumented with an atmospheric pressure sampling mass spectrometer. A constant flow rate of nitrogen was established in the manifold line to serve as the carrier gas. Into this flow, controlled concentration pulses of argon were introduced from one switching section at a time to model the transport of source gases. A square-wave generator was used to control the on-time of the source for both single and multiple pulse experiments. The results of the single pulse experiments are reported here. The sudden displacement of the valve seats in the air-actuated bellows valves was found to introduce significant pressure and concentration spikes. These spikes demonstrate the need to carefully resolve the effects of pressure and concentration on the measurement technique. For the mass spectrometer system, measurement artifacts occur due to the pressure pulses increasing the rate at which gas enters the sampling orifice. Similar measurement artifacts would be expected in any concentration measurement system which measures number density or which employs a sampling orifice.

Preliminary study of the influence of Grashof and Reynolds numbers on the flow and heat transfer in an MOCVD reactor

The combined effect of Grashof and Reynolds numbers on the flow and heat transfer in a metal organic chemical vapor deposition (MOCVD) reactor is investigated both experimentally and numerically. Experimental data for pure hydrogen, helium, and nitrogen with induction heating are obtained at the Chemical Vapor Deposition Facility for Reactor Characterization at NASA Langley Research Center (LaRC). The test facility measures the velocity field inside the reactor using a three dimensional laser velocimeter. Temperatures of the fused silica walls are recorded using an infrared camera. Each gas is tested over a range of flow rates. These experimental runs are repeated using a three-dimensional computational fluid dynamics code which models the flow and heat transport throughout the reactor. The model accounts for the mechanisms of conjugate heat conduction, convection, and radiation. The analytical results are compared with the experimental data and used to assess the heat and mass transfer in the system as a function of the Richardson number, Ri equals Gr/Re2.

Comparison of Numerical Model Results with Diffusion Flames in Microgravity

The effects of gravity on methane diffusion flames were studied using a finite difference numerical model. Variables under consideration included velocity, pressure, temperature, enthalpy, reactive species, and inert carrier gas. Results obtained showed that flames in zero gravity were wider and taller than those in normal gravity. The use of a six reaction scheme produced similar predictions of flame width and height compared to those of the single reaction case. The numerical models predicted a considerable difference in the flame base region between normal gravity and low gravity. This region is considered to be beyond the boundaries of current analytical models.

Modeling of InP metal organic chemical vapor deposition

The growth of InP by metalorganic chemical vapor deposition (MOCVD) in a horizontal reactor is being modeled with a commercially available computational fluid dynamics modeling code. The mathematical treatment of the MOCVD process has four primary areas of concern: (1) transport phenomena, (2) chemistry, (3) boundary conditions, and (4) numerical solution methods. The transport processes involved in CVD are described by conservation of total mass, momentum, energy, and atomic species. Momentum conservation is described by a generalized form of the Navier-Stokes equation for a Newtonian fluid and laminar flow. The effect of Soret diffusion on the transport of particular chemical species and on the predicted deposition rate is examined. Both gas-phase and surface chemical reactions are employed in the model. Boundary conditions are specified at the inlet and walls of the reactor for temperature, fluid flow, and chemical species. The coupled set of equations described above is solved by a finite difference method over a nonuniform rectilinear grid in both two and three dimensions. The results of the 2-D computational model is presented for gravity levels of zero- and one-g. The predicted growth rates at one-g are compared to measured growth rates on fused silica substrates.

Flow-field velocity measurements for nonisothermal systems

An effort to characterize the fluid dynamics of nonisothermal, chemically reactive flows of gaseous mixtures inside fused silica chemical vapor deposition (CVD) reactor vessels is underway at the NASA Langley Research Center. This effort is in support of microgravity investigations of fluid dynamics and multiphase flows. A quantitative understanding of the basic fluid dynamics associated with CVD is necessary to achieve improvements in layer thickness and compositional uniformity, in abruptness of alloy interfaces, and in growth efficiency. To perform this research, a three-component laser velocimetry (LV) system has been adapted specifically for quantitative determination of the mixed convective flows found in chambers used for crystal growth and film formation by CVD. A discussion of the advantages and disadvantages of this instrument compared to flow visualization and particle image velocimetry (PIV) techniques is presented. A fundamental limitation on the application of all particle-based velocimetry techniques in nonisothermal systems is addressed which involves a measurement bias due to the presence of thermal gradients. This bias arises from thermophoretic effects which cause seed particle trajectories to deviate from gas streamlines. Data from a horizontal research CVD reactor are presented which indicate that current models for the effects of this thermophoretic force are not adequate to predict the thermophoretic bias in arbitrary flow configurations. Thermal effects on the flow field inside the research reactor were investigated by comparing data obtained from the reactor both at room temperature and heated to growth temperature by radio frequency (rf) induction. Heating of the susceptor was found to increase the gas velocities parallel to the face of a slanted susceptor by up to a factor of five and to result in as much as a factor of eight increase in velocity components directed toward the hot surface.