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A main source of high carbon levels in silicon crystals grown from melt under reduced pressures and contained in silica crucibles supported by graphite retainer/susceptor has been identified by thermodynamic analysis. The calculations have been verified by experimental results and the carbon level can be reduced by approximately 50% with the use of molybdenum retainers.

In this paper we describe an experimental and theoretical study of the effects of susceptor slope and natural convection on the growth rate in horizontal epitaxial reactors. A mathematical model for mass, momentum and energy transfer was developed for this purpose. The growth of gallium arsenide by the reaction of trimethylgallium and arsine gas in hydrogen was used as the experimental vehicle for this study. The growth rates predicted by this model were compared with experimental data for reactor pressures from 76–760 Torr and for susceptor slopes spanning the range from conventional low angle (7°) to near stagnation flow geometries (81°). Excellent correlation between modeled and experimental results was demonstrated over this range of susceptor slopes, with no adjustable parameters. Natural convection effects were found to dominate at higher reactor pressure and steeper susceptor slopes. Growth rate uniformity and growth efficiency issues are also discussed in this paper. It is shown that, for the reactor geometry considered here, the usable area growth efficiency is maximized for susceptor slopes around 5°. However, optimization of this parameter is not of itself sufficient, since material quality considerations necessitate operation at pressures where recirculation effects are absent.

The epitaxial growth and fluid dynamics characteristics of pedestal Chemical Vapor Deposition (CVD) reactors were studied analytically, with emphasis on the effects of susceptor rotation and thermal diffusion on the epitaxial growth rate. The coupled transport equations were solved by numerical integration and a multiple shooting technique. The pressure, velocity, temperature and concentration distributions were calculated and studied for various rotation speeds and operating conditions. The ranges of the process parameters in actual CVD reactors were estimated. It is found that different combinations of the external forced flow and the susceptor rotation have similar temperature and concentration distributions for the Schmidt number around unity when the results are presented in terms of a properly normalized vertical coordinate. The changes in susceptor rotation and external forced flow have nearly identical effects on the epitaxial growth rate for the Schmidt numbers greater than unity. At small Schmidt numbers, the epitaxial growth rate can be enhanced by increasing the forced flow velocity.

A method for melting and sustaining large volumes of UO2 has been developed. The method, known as the Inductive Ring Susceptor Technique, melts UO2 powder via inductively heated refractory metal rings. The technique has been fully demonstrated by a series of experiments which melted 5-5 kg charges of powdered SiO2, AI2O3,and UO2

Design of CVD reactors is considered with the objective of maintaining stable boundary layer and achieving uniform deposition based on the theoretical relationships obtained in Part I for the mass transfer coefficient. In addition, a condition for the stability of boundary layer is utilized. A condition for uniform deposition for typical CVD reactors is derived. These considerations lead to two new design alternatives. The first alternative, which ensures uniform deposition over the entire susceptor length, involves making the contour of the reactor top wall follow a certain prescribed shape with a horizontally placed susceptor. The second design alternative. which could conceivably ensure uniform deposition for any length of the susceptor and stable boundary layer for any velocity, involves applying slight uniform suction through the holes/slits between substrates (susceptor portions supporting the substrates). The only limitation on the susceptor length (or the number of substrates) is the depletion of reactants, which can be appropriately accounted for. Detailed design and operating considerations for the design alternative are given.

We have designed and built a novel, computer-controlled VPE reactor for the growth of thin layer structures with abrupt composition changes. In this vertical “chimney” reactor, the carrier gas together with components are introduced at the bottom of the reactor tube and passed through the inside of a hollow susceptor. It is used for MOVPE growth in the system GaAs to AlAs. We have grown GaAs layers of thicknesses down to 7 A in between 50 A thick layers of Al0.55Ga0.45As. Also, we have grown a superlattice type structure containing GaAs and AlAs layers of 35 A thick. Direct observation of the thicknesses of the layers and of the transition width was obtained by means of Transmission Electron Microscopy. By using lattice imaging, it is demonstrated that the composition change from GaAs to AlAs is achieved in approximately one atomic layer.

The chemical vapor deposition of epitaxial layers of silicon is a widely used process in the electronic industry. It is a batch process and the relatively small capacity (i.e., 20–30 wafers) of epitaxial reactors significantly contributes to the expense of the process. We thus embarked on a research project aimed at a significant expansion of the reactor capacity. The first step was to conduct a complete characterization of the presently used reactors by means of flow visualization, temperature measurements and mass spectrometric studies; results of these studies will be briefly presented and discussed. The main conclusion from these studies was that up-scaling of present reactors is not economical. We thus designed and constructed a novel epitaxial reactor, radically different from current types. In this reactor the susceptor structure consists of parallel graphite discs. Wafers are fastened to one or both sides of these discs. The nutrient gaseous mixture is injected into spaces between discs by a specially designed gas distributor, which delivers the same amount of the mixture to all interdisc spacings, thus insuring the wafer-to-wafer thickness uniformity. A combination of the rotation of the susceptor discs with the gas distributor motion insures the on-the-wafer thickness uniformity. The above described parallel packing allows much higher reactor capacities (e.g., 50–100 wafers). It also results in a more economical reactor in terms of consumption of energy and chemicals. We shall illustrate the application of the novel reactor (known as the “RCA Rotary Disc Reactor”) to epitaxial deposition of silicon from SiCl2H2.

The thickness of the stagnant layer observed by smoke gas simulator is correlated to that of the epitaxial layer of GaAs. With this knowledge we developed an atmospheric pressure MOCVD system with a barrel reactor for large area epitaxial growth of GaAs and GaAlAs. The susceptor is a hexagonal frustum in shape, on which six 2 inch diameter wafers are set. It was known that the tapering angle of the susceptor and the number of its revolutions affect uniformity in the thickness of epitaxial layer. Under the optimized growth conditions, variation of layer thickness in a wafer was less than ± 5%. Variation of average layer thickness of a wafer was less than ± 2% in a run and that of six wafers in a run was less than ±2% from run to run. In S-doping by using H 2 S, the uniformity of carrier concentration depended on growth temperature and [AsH 3 ]/[TMGa] ratio. Under the optimized growth conditions, variation of the carrier concentration in a wafer, the average carrier concentration of a wafer in a run and that of six wafers in a run from run to run were less than ±5%.

Materials selection and performance for thermionic diode application in fusion reactor systems is examined. The volumetric heating, atom displacement damage and He production due to neutron attenuation in four diode designs are calculated with ANISN-W and relevant kerma factors. The preferred design includes a graphite or silicon carbide first wall/susceptor, a refractory metal emitter and refractory metal collector.

Lateral selective growth has been used to integrate multiple color light emitting diodes and detectors with different cutoff wavelengths on the same substrate. This was achieved by using a specially designed growth chamber and susceptor that allows the substrate to move with respect to a stationary GaAs mask in a metalorganic chemical vapor deposition system. The variation of thickness of the deposited film near the edge of the mechanical mask has been studied.

In deriving theoretical transport controlled growth rates for CVD processes the effect of the temperature gradient close to the reaction surface is usually neglected in order to obtain a simple analytical treatment, and values of gas properties corresponding to some average temperature are taken. A simple consideration of the differential equations for fluid flow and heat and mass transfer shows that one might expect the steep temperature profile commonly encountered at a susceptor surface to lead to a considerable distortion of the diffusion boundary layer and hence of the growth rate. Nevertheless, the agreement between theory based on constant gas property conditions and experiment for various reactor geometries is usually reasonable. In this paper we present an analysis to show why this is so.

s-SiC was formed on Si substrates in an horizontal, rf powered reactor. The reaction of Si with CH4} was either directly at temperatures of 1200-1350‡C or by a two step reaction in which CH4} was first cracked at 900‡C and then the deposited C reacted with Si at temperatures of 1200-1350‡C. Important features of the study were the minimization of contam-inants from susceptor and substrate support materials, removal of native oxides and a novel susceptor design. A rectangular, TaC coated Ta susceptor was used in conjunction with sapphire supports. Preheated He was used as a carrier gas. Growth characteristics and film structure are presented as a function of CH4} concentration and substrate temperature.

A rigorous computer model was developed to study growth rate uniformity in MOVPE. Two different horizontal reactors were modeled in this study as a function of carrier gas composition, flow rates, and pressure. The accuracy of the simulation was verified by comparison to experimental growth rates and published gas temperature measurements. Large recirculation patterns caused by buoyancy were found in the gas for several operating conditions. These recirculation patterns affected the gas temperature, growth rate, and deposition uniformity. Furthermore, time-dependent simulations showed that recirculation reduced interface abruptness. A simple analytical formula is derived for predicting buoyant recirculation in horizontal reactors. Reduced height over the susceptor and reduced pressure are very effective in eliminating this problem.

Gas flow control tubes were designed to obtain uniform epitaxial layers grown by metalorganic vapor phase epitaxy (MOVPE). Four tube-shape parameters for the reactor, the angle of inclination of the susceptor, the angle of inclination of the top wall of the tube, and the curvature and height of the top wall were varied and the uniformity of the InP epitaxial layers were studied. It was clear that the inclined susceptor and inclined top wall improved the thickness uniformity in the direction of the gas flow. It was also found that the curvature of the top wall of the tube was effective in the control of the epitaxial layer thickness in the direction normal to the gas flow. Uniform epitaxial layers over entire 5 cm diameter wafers could be obtained by optimizing the design of the tube.

A slightly modified MO-CVD process was used to grow epitaxial layers of GaP and GaAlP on GaP and Si substrates. PH3 is cracked before reaching the substrate by leading it through the hot susceptor. The morphology of epitaxial GaP layers is strongly dependent on substrate orientation and growth temperature. Layers with mirror-like surfaces are achieved on 3° off (100)GaP substrates at 730°C. The background doping level is n ≈ 3 × 1016 cm-3. Nitrogen doping was achieved by addition of NH3. This lowers the growth rate similar as in the halide process. CL spectra show the successful incorporation of N as isoelectronic impurity. First, non-optimized LED structures with external quantum efficiencies of 3 × 10-5 are realized.

Gas phase transport in Si epitaxial growth has been investigated by means of in situ mass spectrometry measurements of reaction gas concentration profiles. Five species, , , , , and are observed in the reaction gas. Not only (source gas) but the other Si chlorides as well contribute to the surface growth reaction. The primary reaction species shifts from to the other Si chlorides with decreasing gas velocity and with increasing temperature and position along the susceptor.

A two-dimensional numerical model employing the method of finite elements is described for mass transfer-controlled chemical vapor deposition in channel reactors. This model includes all relevant transport processes occurring in the two primary dimensions, including thermal diffusion, buoyancy forces, and the diffusion of energy, mass, and momentum in the axial (flow) direction. The model represents conditions occurring in reactors with large aspect ratios (channel width/height) in the absence of longitudinal roll waves, conditions conducive to the best growth uniformity. The model was used to calculate growth rate uniformity for metalorganic chemical vapor deposition (MOCVD) growth of InP from trimethyl indium. Thermal diffusion was found to be a significant term in the mass transfer equation, decreasing growth rate by about 30% near the front of the susceptor and by lesser amounts further downstream. Growth rate was found to be insensitive to susceptor temperature but dependent on the binary diffusion coefficient and thermal diffusion factor of the reactant in the carrier gas. Use of a tilted susceptor allows for conditions to be established for which there is a region of relatively uniform growth rate. Tilt angle must be chosen to balance between high growth rate and uniformity over large area.

Single crystals of CdTe with volumes up to 10 cm3 have been grown by an improved sealed-ingot zone refining technique, in which large vertical oscillations (e.g., 4 mm at 100 cpm) are imposed on the ingot during the last zone pass. The conductivity and carrier type are correlated with susceptor temperature. The effectiveness of zone refining has been studied by measurements on ingots doped with radiotracers of Fe, Co, Sn, W and Bi. The following distribution coefficients were determined: Fe, 0.53; Co, 0.27; Sn, 0.025.

It is shown that in the induction heating of a long cylinder of sufficient wall thickness, the temperature difference between the inside and outside wall, for a given power input, is inversely proportional to the square root of the frequency. This can be most easily observed in materials of low thermal conductivity, and is here applied to pyrolytic graphite, which is a good thermal insulator normal to the surface on which it is deposited. At outer temperatures of 1000–1500°C, the temperature difference across a pyrolytic graphite wall is negligible at 500 kc/s, but is remarkably large at frequencies below 10 kc/s. For example, using about 25 kW of power at 3·6 kc/s, a pyrolytic graphite crucible 105 mm dia. and 160 mm long, with about 10 mm total wall thickness, was heated to a surface temperature of 1500°C, while at the same time the interior temperature rose to 2700°C in equilibrium. Using a pyrolytic graphite susceptor in this way, sizable systems can be heated to high temperatures without any additional thermal insulation. The larger the diameter of the susceptor, the lower the frequency which can be used for efficient coupling, and thus the larger the temperature difference which can be established across the susceptor wall. Furthermore, when used as radiation screening, thermal shields of slotted pyrolytic graphite are up to 100 per cent more effective at frequencies below 10 kc/s than at radio frequencies.

Liquid encapsulated growth of GaP is complicated by two features not present in ordinary Czochralski growth: molten B 2 O 3 encapsulates the melt, and high pressure gas (55 atm) fills the growth chamber. The growth of large ingots of GaP (200 g, 35 mm diameter) under these conditions is more difficult than the previously described growth of smaller ingots. Diameter control is more difficult, and the large ingots often crack spontaneously. In the liquid encapsulated Czochralski process, the growth conditions are dominated first by radiative heat transfer and later by convective heat transfer. Evidence to support this conclusion includes the shapes of growing crystals and the configurations of the growth interface. By using a knowledge of the heat transfer mechanisms, a reliable technique has been developed for adjusting rf power input to obtain uniform diameter single crystals. Unless precautions are taken, however, such ingots will crack spontaneously. The vertical temperature gradient in the growth chamber can be as high as 500 °C/cm. The resulting thermal stress causes plastic deformation in the growing ingots, leading to frozen-in stresses which may exceed the yield stress of GaP. These stresses have been reduced sufficiently to avoid cracking by using an afterheater which is an integral part of the rf susceptor. The use of an afterheater can lead to the occurrence of twin lamellae, but these have been eliminated by using a suitably shaped crucible (e.g., flat bottomed).

Thermal etching of α-SiC crystals in argon atmosphere has been studied at temperatures ranging from 1800 °C to 2650 °C, on tantalum, tantalum carbide and graphite susceptors. The etching was done with and without the access of SiC vapors. The etching rate was determined as a function of the temperature on various susceptor materials. Microscopic etching patterns obtained on surfaces of α-SiC crystals etched on different susceptors are presented.

An apparatus for tensile testing in vacuum to 5400°F is described. A load‐elongation curve is plotted during testing of a standard specimen at a selected nominal strain rate between 0.005 and 5.0 in./in./min. The load train design provides that the grips, as well as the specimen, are in the furnace hot zone. The specimen is heated by radiation from an induction‐heated susceptor, and its temperature is uniform within the limits of detection of an optical pyrometer.

An induction furnace operating at 500 kc/s is described which is capable of providing temperatures in excess of 3000° C at high vacuum or in any suitable gas mixture at pressures up to 8 atmospheres. The work coil is mounted inside the furnace chamber, which can then be designed as a pressure vessel and constructed of electrically conducting steel. By this technique close coupling is provided between the work coil and the susceptor crucible: if desired, a heat treatment cycle to 3000° C can be completed in less than one minute. To avoid any contamination, the furnace is thermally uninsulated, so that all the heat dissipated in the crucible is lost by radiation, limiting its total surface area to about 20 cm2 for temperatures of 3500° C with a 25 kw generator. Application of the theory of induction heating shows that the effective resistance of the susceptor crucible can be increased by several methods, thus improving the heating efficiency of the furnace. The theory may also be applied for determining the resistivities of various susceptor materials at very high temperatures.

An r.f. furnace and control equipment that are suitable for growing single crystals of refractory oxides, such as Al2O3, by the Czochralski technique is described and examples of sapphire and ruby boules grown using this equipment are shown. Thoria is employed throughout the furnace. Thoria has a lower heat capacity and a higher resistivity at the working temperature than any other refractory oxide. This enables iridium susceptor crucibles to be used at temperatures close to their melting point.

A method of measuring the thermal conductivity of refractory oxides at high temperatures is described. Heat is generated inductively in a metal susceptor inside a hollow specimen in the shape of a prolate spheroid, the temperature of which is measured at two points by means of an optical pyrometer. The conductivities of alumina, magnesia, and beryllia were measured between 1000° and 1800°C.

The paper deals with the design and lay-out of a horizontal epitaxial system capable of holding 4–6 slices simultaneously. Epitaxial films prepared by hydrogen reduction of silicontetrachloride are of good quality for fabricating planar devices. The effects of various factors such as growth rate, wefer preparation, susceptor design, etc. influencing the quality of the films in terms of resistivity, thickness and defect density have been investigated.

High-quality recrystallized silicon films on fused silica substrates have been produced with a new micro-zone-melting method using an RF-heated carbon susceptor. In this method, the fused silica substrate, on which a 0.5-1.0-µm-thick polycrystalline silicon film encapsulated with a 1.2-µm-thick CVD-SiO 2 layer has been deposited, is moved across the carbon susceptor surface, which has a narrow-strip high-temperature zone. Recrystallized silicon films with

Atomic layer epitaxy (ALE) of III‐V semiconductors is reported for the first time using metalorganic and hydride sources. This is achieved by using a new growth chamber and susceptor design which incorporates a shuttering mechanism to allow successive exposure to streams of gases from the two sources. Also, most of the gaseous boundary layer is sheared off after exposure to the gas streams. GaAs and AlAs deposited by ALE are single crystal and show good optical properties.

A dilatometer specially designed for a study of the sintering of silicon nitride is described. Densification kinetics could be measured at temperatures up to 2300K. Thermodynamic considerations dictated that the specimens be maintained in a N2 gas pressure of up to 2 MPa. Hot-pressed boron nitride was the only material found suitable for the high-temperature parts of the dilatometer. The pressure vessels contained an RF furnace with graphite susceptor thermally insulated with zirconia granules and housed in a vitreous silica tube. This simple arrangement allowed high heating rates and a minimum of maintenance. The dilatometer is readily adaptable to high-temperature properties measurements where inert gas overpressures are desirable.

Laterally selective growth of III‐V compounds has been successfully demonstrated by metalorganic chemical vapor deposition. This was achieved by using a specially designed growth chamber and susceptor that allows the substrate to move with respect to a stationary GaAs or Si mask. We have used this technique to selectively deposit GaAs1−xPx with different values of x and a GaAs‐GaAsP superlattice on a single GaAs substrate. We have also selectively grown multiple color light‐emitting diodes on a GaAs substrate.

The dependence of plasma silicon dioxide deposition rates on the thickness of underlying electrically insulating thermal oxide grown on sample wafers prior to plasma deposition has been studied. Insulating films can reduce the deposition rates by approximately 25% if both wafers facing each other in the susceptor assembly are insulated from it, and by approximately 15% if only one is insulated. A semiquantitative description of the effect is given, based on variations of RF plasma excitation power due to insulating target wafers from the susceptor.

Rapid thermal annealing of GaAs within an enclosed, SiC‐coated graphite susceptor is shown to eliminate slip formation during implant activation treatments (900 °C, 10 s) and to provide much better protection against surface degradation at the edges of wafers compared to the more conventional proximity method. The peak carrier concentration obtained in Si‐implanted (3×1012 cm−2, 60 keV) GaAs by both methods is comparable, but the wafers annealed in the susceptor have tighter carrier profile width distributions measured over the whole wafer area.

Crystal growth of 2H silicon carbide showing temperature profile along susceptor as affecting factor

The far infrared reflectance technique has been used to characterize ZnTe and CdTe films grown using Metal Organic Chemical Vapor Deposition (MOCVD) on GaAs substrates. Carrier concentrations, derived from the reflectance measurements, decreased with increasing ZnTe film thickness as a result of the large lattice mismatch between ZnTe and GaAs. This mismatch produces a high strain, high dislocation region near the interface. The uniformity of CdTe films grown on GaAs substrates in two different types of reactors are compared. The thickness and carrier concentration is found to be far more uniform for films grown in a vertical flow, rotating susceptor reactor than for films grown in a conventional linear flow reactor.

Analysis of Volatiles Produced in Foods and Packages during Microwave Cooking Migration into Food during Microwave- and Conventional-Oven Heating Food and Drug Administration Studies of High-Temperature Food Packaging Interactions of Food, Drug, and Cosmetic Dyes with Nylon and Other Polyamides Application of a Poly(tetrafluoroethylene) Single-Sided Migration Cell for Measuring Migration through Microwave Susceptor Films Determining Volatile Extractives from Microwave Susceptor Food Packaging Toward a Threshold of Regulation for Packaging Components: A Feasibility Study Packaging Industries and the Food-Additives Amendment: Reprise Food Contact Materials and Articles: Standards for Europe Food-Package Interaction Safety: European Views Influence of Microwave Heating on the Formation of N-Nitrosamines in Bacon Thermodynamics of Permeation of Flavors in Polymers: Prediction of Solubility Coefficients Determination of Flavor-Polymer Interactions by Vacuum-Microgravimetric Method Determination of Food-Packaging Interactions by High-Performance Liquid Chromatography Sensory-Directed Analytical Concentration Techniques for Aroma-Flavor Characterization and Quantitation Interaction of Orange Juice and Inner Packaging Material of Aseptic Product Sorption of Flavor Compounds by Polypropylene Sorption Behavior of Citrus-Flavor Compounds in Polyethylenes and Polypropylenes: Effects of Permeant Functional Groups and Polymer Structure Scalping from a Paste into a Series of Polyolefins: Absence of Correlation between Quantity Sorbed and Polymer Crystallinity

The seeded growth furnace with a slotted molybdenum susceptor has been operated during the past quarter. The susceptor design permits flat sheets of refractory materials to be used in making up the tray. Fused quartz and sapphire trays were used. Local surface cooling was obtained by means of a water-cooled cold finger. Seeded surface growth has been obtained in this furnace using a seed with (111) axis and (112) principal faces. The silicon grown out of solution has a region approximately 3 mm x 5 mm, which is substantially single-crystal with the same crystallographic orientation as the seed. Sessile drop experiments have been initiated as a rapid method of determining the suitability of various refractory materials as trays for tin melts. These experiments indicate the presence of some impurity in the starting tin, which forms a surface film that prevents the tin from wetting silicon. This film can be removed by heating the tin to 900/sup 0/C in hydrogen.

An experimental and theoretical study has been undertaken of the effects of natural and forced convection in horizontal epitaxial reactors. The epitaxial growth of GaAs was used as the experimental vehicle for this study. A mathematical model for mass, momentum and energy transfer in the reactor was developed. Excellent correlation between modeled and experimental results was demonstrated over a wide range of reactor pressures and susceptor geometries. Recirculation of hot gases, caused by natural convection, was found to result in a strong pressure dependence of growth rate at higher susceptor slopes. Low reactor pressures have been shown to be a more effective way to eliminate recirculation than high gas flow rates.

The feasibility of growing silicon ribbons by the inverted ribbon growth process has been established using both nonwetting boron nitride (BN) dies as well as wetting composite dies coated with chemical vapor deposited silicon nitride (CVD Si/sub 3/N/sub 4/). The former can be termed as the inverted Stepanov process, the latter as the inverted edge-defined film-fed growth (EFG) process. Flat die and V-shaped crucible-die configurations have been employed. The shift of the die material from nonwetting BN to nonwetting SiO/sub 2/ was more difficult than anticipated. The V-shaped susceptor has been modified to decrease the vertical gradient inside the susceptor in order to minimize the SiO formation and the related hysteresis. Preliminary evaluation of the reactivity of liquid silicon with CVD Si/sub 3/N/sub 4/ and CVD Si/sub 3/O/sub x/N/sub y/ indicaped that these materials are considerably more resistant to reaction with and/or dissolution in silicon than other materials examined to date. Silicon ribbon growth was carried out using CVD Si/sub 3/N/sub 4/ coated composite dies in the V-shaped crucible-die configuration. A one-dimensional heat flow model has been developed to simulate numerically the major thermal aspects of the inverted Stepanov growth process.

The flow pattern of process gas at atmospheric pressure in a rectangular cross-section, horizontal MOCVD reactor was determined by pyrolyzing trimethylindium to produce an indium "smoke." Growth tubes with 9cm and 30cm entrance lengths and various internal geometries were tested. Entrance effects dominated the flow behavior for all tubes tested. This behavior is not generally recognized but is predicted from entrance-length theory. Use of a gas diffuser positioned slightly upstream from the susceptor produced no visible backflow of pyrolyzed reagent while greatly improving uniformity of flow over the susceptor. Im-proved control over GaAs film growth was obtained using a diffuser. These results lead to an improved design for a horizontal MOCVD reactor growth tube.© (1987) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

A new material, Tribocor 532N (an alloy of 50 wt % niobium-30 wt % titanium-20 wt % tungsten (50Nb-30Ti-20W) nitrided at >1800/degree/C), has been shown to be an excellent crucible/susceptor for melting and casting uranium and its alloys in a noncarbon environment. Testing has progressed from very small-scale initial tests to intermediate-scale foundry tests with performance better than graphite containment. Lifetime tests of Tribocor 532N indicate an improvement of more than an order of magnitude as compared to graphite. The Tribocor 532N is far superior to oxide-based ceramics and experiences no cracking from thermal cycling, is capable of directly suscepting in an induction field for induction heating, and is stable with graphite---allowing any portion of a graphite stack to be replaced interchangeably with Tribocor 532N. Tribocor 532N can be coated with standard paint, and it performs equally well with the more economical zirconia coating instead of yttria, while also being compatible with erbia coatings that are suitable for enriched uranium. Since Tribocor 532N contains no carbon, a Tribocor crucible should introduce less carbon during melt cycles and enable more recycled metal to be used without increasing carbon levels. The long lifetime projections for Tribocor 532N make the material economical. Amore » large-scale crucible of Tribocor 532N will be tested. 14 refs., 11 figs., 7 tabs.« less

An investigation of the heating behavior of GaAs and InP in a Rapid Thermal Annealing (RTA) system shows a significant variation in the temperature response of these materials. These compound semiconductors are heated with incoherent light in an RTA system where a Si sample is used for temperature feedback. The temperatures of GaAs and InP samples thermally isolated from the Si control sample during heating may overshoot or undershoot the desired temperature by hundreds of degrees. When the samples are thermally connected to the Si control sample by means of a susceptor, the temperatures of the III-V semiconductors follow the programmed temperature closely. Much of the variation in temperature response can be correlated with free carrier concentrations in the materials, but other material properties also play a role. This significant variation in temperature response has practical implications for the design and application of RTA systems.© (1986) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Thirty-five (35) furnace runs were carried out during this quarter, of which 25 produced a total of 120 web crystals. Lengths in excess of two meters and widths over 27 mm were achieved. A variety of lid and susceptor configurations have been used to modify the thermal geometry of the system in an effort to grow wider and longer web crystals of high quality. The two main thermal models for the dendritic growth process have been completed and are being used to assist the design of the thermal geometry of the web growth apparatus. As longer and wider web is being grown, strain in some crystals has been noted. Residual stress measurements based on crystal splitting are being made in some of the crystals being grown and these results will be correlated with predictions of the thermal models. The models will then be used to generate susceptor lid configurations producing smaller thermal stresses in the growing crystal. In addition to the residual stress measurements, both x-ray topographic and etching examinations have been made on web crystals. Dendritic web samples have been fabricated into solar cells using a standard cell configuration and a standard process for a N/sup +/-P-P/sup +/ configuration.more » The data indicates that web material can make very good solar cells with AM1 efficiencies greater than 14 percent. Work is presently in progress to correlate the cell performance with the material quality. Studies are also underway to develop improved processing techniques for dendritic web material. The detailed engineering design was completed for a new dendritic web growth facility of greater width capability than previous facilities. Fabrication of component parts and assembly have begun.« less

Gas phase transport in Si epitaxial growth has been investigated by means of in situ mass spectrometry measurements of reaction gas concentration profiles. Five species, , , , , and are observed in the reaction gas. Not only (source gas) but the other Si chlorides as well contribute to the surface growth reaction. The primary reaction species shifts from to the other Si chlorides with decreasing gas velocity and with increasing temperature and position along the susceptor.

The web furnace has been set up, calibrated, and made operational for pulling dendritic-web samples. Considerable work has been completed in the investigation of the effect of changes in the furnace thermal geometry, as accomplished by variations in the number, size, shape, and location of thermal shields, on the growth of dendritic-web. Numerous growth runs were made to grow primitive dendrites for use as the dendritic seed crystals for the web growth. Some preliminary investigations were conducted to try and determine the optimum twin spacing in the dendritic seed crystal for web growth. Models were developed and computer programs applied to ascertain the thermal geometries present in the susceptor, crucible melt, meniscus, and web. A major result of this analysis has been the prediction of an upper limit on the pull rate of approximately 4 cms. per minute with the thermal geometry presented in our furnace. The facilities for obtaining characterization data were set-up and made operational. Data on twin spacings and number of twin planes in the dendritic seed crystals and resulting web samples was obtained. Resistivity and majority charge carrier type determinations were made on a few select web samples. All samples to date have been high resistivity,more » undoped, p-type. (WDM)« less

The seeded growth furnace with the slotted molybdenum susceptor has been operated during the past quarter. Flat sheets of sapphire were used as the tray, and local cooling at the tin-silicon alloy surface was provided by a symmetric water-cooled jet. The silicon surface growth obtained during the past quarter has been consistently polycrystalline. This poor growth is attributed to impurities in the seeded growth furnace, caused by leakage into the system. Two sources have been identified: deteriorating neoprene O-rings and leaks of cooling water from the jet nozzle. The O-rings were replaced with indium gaskets, and the nozzle is being rebuilt. Weight loss measurements of the sapphire tray plates indicate that they suffer negligible attack by tin-silicon alloy melts at 1000/sup 0/ to 1100/sup 0/C. The growth habit of crystals nucleated spontaneously at the tin-silicon alloy melt surface has been studied. The crystal growth observed is consistent with dendritic and web growth from (111) singly twinned triangular nuclei. Examples of planar surface growth of small (less than 0.1 mm), thin crystals from these twinned surface seeds have been observed.

A report of the fifth quarter's work on the web-dendritic ribbon growth at the University of South Carolina is presented. A brief description of the work initiated and carried out during this period to meet the program goals is given along with a copy of the Program Plan covering the entire period of the contract. The web growth portion of this program was spent in the design, installation and testing of a new furnace geometry for the growth of dendritic web ribbon. The new installation was completed and the testing 90% accomplished. Results of the testing to determine the effect of the relative position of the r.f. coil with respect to the susceptor on the thermal profile in the melt are described. The one-dimensional thermal model has been used to determine the relationship between growth rate, web thickness, and mensicus height for stable growth. The results of this analysis are also given. An analysis was completed of the thermal radiation from the meniscus including the effect of the curvature of the meniscus with the results indicating that inclusion of the curvature leads to approximately a 10% increase in the radiation loss.

Efforts were continued to overcome the instabilities experienced when silica dies are used for growth of silicon ribbon in the inverted Stepanov configuration. These instabilities are associated primarily with the formation and evolution of the silicon monoxide; the escape of the gas causes hysteresis of the contact angle and mechanical vibration of the melt. As a result of this, the meniscus during the ribbon growth is not ''pinned'' at the die edge, but is in constant motion. The V-shaped susceptor has been modified to decrease the vertical gradient inside the susceptor in order to minimize the SiO formation and the related hysteresis. A pressure differential has also been applied across the melt in an effort to suppress the hysteresis. Various types of thermal trimmers have been tried to provide suitable thermal gradients at the solid-liquid ribbon growth interface. So far, the instabilities have not been suppressed sufficiently to allow for stable ribbon growth. The thermal stress exerted on the ribbon during growth can be reduced by minimizing the curvature in the ribbon temperature profile. (WDM)