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NASA NTRS1973

Correlation of experimental performance data for a CVD tungsten-niobium, planar thermionic converter

Approximate expressions are presented which correlate experimental performance data from a CVD tungsten-niobium, planar thermionic converter. The current voltage characteristics are given as functions of emitter and collector temperatures and cesium pressure for currents below the knee in the ignited mode. The correlation covers the temperature ranges of 1700 to 1950 K for the emitter, 900 to 1050 K for the collector, and 580 to 645 K for the cesium reservoir.

Sockol, P. M.↗

NASA NTRS1974

Characteristics of epitaxial garnets grown by CVD using single metal alloy sources

Single metal alloys have been explored as the cation source in the chemical vapor deposition (CVD) of iron garnets. Growth of good quality single crystal garnet films containing as many as five different cations has been achieved over a wide range of deposition conditions. The relationship of film composition to alloy compositions and deposition conditions has been determined for several materials. By proper choice of the alloy composition and the deposition conditions, uncrazed deposits were grown on (111) gadolinium gallium garnet (GGG) substrates. Data on physical, magnetic and optical properties of representative films is presented and discussed.

Besser, P. J.↗

NASA NTRS1975

Preparation of tantalum-based alloys by a unique CVD process

The paper describes a sequential pulsing technique for deposition of refractory alloys and evaluates the technique for the deposition of the tantalum-base alloys Ta-10W (Ta-10 st% W) and T-111 (Ta-8 wt% W-2 wt% Hf). The deposition cycle for Ta-10W was chosen as alternate injections of TaCl5 plus hydrogen and WCl6 plus hydrogen. The cycle for T-111 was chosen as injections of TaCl5 plus hydrogen interspersed with injections of WCl6 plus hydrogen. A temperature range of 900-1300 C was chosen for both alloys. The ability of the pulse process to blanket a uniformly heated section of substrate with a mixture of gases, whose composition varies not with position on the substrate but instead with time of residence in the reactor, allows metal of uniform thickness to be deposited. It is shown that Ta and W can be deposited at high temperature with the formation of a dense columnar grain structure, so that the feasibility of preparing uniformly thick deposits of these elements by a 'pulsing' modification of CVD is demonstrated. A similar attempt to deposit T-111 was unsuccessful due to the difficulty in reducing HfCl4.

Bryant, W. A.↗

NASA NTRS1980

Calculation of residual principal stresses in CVD boron on carbon filaments

A three-dimensional finite element model of the chemical vapor deposition (CVD) of boron on a carbon substrate (B/C) is developed. The model includes an expansion of the boron after deposition due to atomic rearrangement and includes creep of the boron and carbon. Curves are presented to show how the principal residual stresses and the filament elongation vary as the parameters defining deposition strain and creep are varied. The calculated results are compared with experimental axial residual stress and elongation measurements made on B/C filaments. This comparison requires that for good agreement between calculated and experimental results, the deposited boron must continue to expand after deposition, and that the build-up of residual stresses is limited by significant boron and carbon creep rates.

Behrendt, D. R.↗

NASA NTRS1980

Epitaxial and polycrystalline GaAs solar cells using OM-CVD techniques

GaAs epitaxial films were grown by chemical vapor deposition using organo-metallic sources (OM-CVD) on single crystal and polycrystalline bulk GaAs, as well as on bulk polycrystalline and recrystallized thin-film Ge substrates. Details of Antireflecting Metal-Oxide-Semiconductor (AMOS) solar cells fabricated on GaAs films grown on bulk polycrystalline Ge and recrystallized Ge thin-film substrates will be discussed, as well as preliminary photovoltaic results obtained for n(+)/p homojunction structures.

Yeh, Y. C. M.↗

NASA NTRS1981

XPS study of interface formation of CVD SiO2 on InSb

The interfacial chemistry of CVD SiO2 films deposited on thin native oxides grown on InSb substrates is examined using X-ray photoemission spectroscopy (XPS) and a relatively benign chemical etching technique for depth profiling. An intensity analysis of XPS spectra is used to derive the compositional structure of the interfaces obtained in the SiO2/native oxide/InSb system. Peak positions in these spectra are used to follow the change in substrate surface potential during the etch sequence, and to establish the chemical nature of the species formed during deposition and subsequent processing. Reaction of the substrate with oxygen resulted in an In-rich native oxide and 1-2 monolayers of excess elemental Sb at the native-oxide/substrate interface, incompletely oxidized silane reduced the native oxide, leaving less than 1 monolayer of elemental In at the SiO2/native oxide interface. Etch removal of this thin In-rich layer leads to a change in the substrate surface potential of 0.06 eV, corresponding to a net increase in positive charge. The results are consistent with simple thermodynamic considerations; they are also compared to previously reported studies of deposited dielectrics on III-V compound semiconductors.

Vasquez, R. P.↗

NASA NTRS1984

Growth of InSb and InAs(1-x)Sb(x) by OM-CVD

Organometallic chemical vapor deposition (OM-CVD) growth of InSb and InAs(1-x)Sb(x) has been obtained using triethylindium (TEI), trimethylantimony (TMS), and arsine (AsH3) on (100) GaAs, (100) InSb, and (111)-B InSb substrates. InSb with excellent morphology was achieved on both (100) InSb and (111)-B InSb substrates. The measured electron mobility at 300 K of undoped InSb grown on (100) GaAs semi-insulating substrates was 40,000 sq cm/V-s at a carrier concentration of ND-NA = 2.0 x 10 to the 16th per cu cm. Carrier concentration of ND-NA = 1.2 x 10 to the 15th per cu cm has been measured at 77 K. InAs(1-x)Sb(x) (x = 0.07-0.75) with mirror-like surfaces have been grown on (100) InSb and InAs substrates. This composition range of x between 0.55 and 0.75 (Eg = 0.1 eV) has been successfully achieved for the first time. Solid composition variations as a function of growth temperature and InSb substrate orientations are also discussed.

Chiang, P. K.↗

NASA NTRS1988

Kinetics of low pressure CVD growth of SiO2 on InP and Si

The kinetics of low pressure CVD growth of SiO2 from SiH4 and O2 has been investigated for the case of an indirect (remote) plasma process. Homogeneous (gas phase) and heterogeneous operating ranges have been experimentally identified. The process was shown to be consistent within the heterogeneous surface-reaction dominated range of operation. A kinetic rate equation is given for growth at 14 W RF power input and 400 mtorr total pressure on both InP and Si substrates. The process exhibits an activation energy of 8.4 + or - 0.6 kcal/mol.

Iyer, R.↗

NASA NTRS1988

Numerical modeling of chemical vapor deposition (CVD) in a horizontal reactor

In the present numerical prediction of the deposition rate of silicon from silane in a CVD process, the conservation equations for mass, momentum, energy, and chemical species are solved on a staggered grid using the SIMPLE algorithm, while the rate of chemical reactions in the gas phase and on the susceptor surface is obtained from an Arrhenius rate equation. Predicted deposition rates as a function of position along the susceptor with and without the gas phase chemical reaction are compared with the available experimental and numerical data; agreement is excellent except at the leading edge of the susceptor, where the deposition rate is overpredicted.

Sheikholeslami, M. Z.↗

NASA NTRS1992

Gravitational and thermal effects in CVD flow and deposition

A three-phased approach is used to investigate the transport phenomena occurring in horizontal CVD reactors. The three phases are made up of laser velocimetry experimental investigations, numerical CFD modeling, and metal-organic chemical vapor deposition experimental studies. Laser velocimetry measurements of flow in a horizontal reactor with a cylindrical flow channel and tilted susceptor demonstrate velocity components arising from thermal expansion and buoyancy effects that significantly exceed the velocity components due to the external applied flow. MOCVD of InP and GaAs thin films in a horizontal reactor channel with a rectangular cross section and a tilted susceptor has also been examined.

Clark, I. O.↗

NASA NTRS1992

Oxidation kinetics of CVD silicon carbide and silicon nitride

The long-term oxidation behavior of pure, monolithic CVD SiC and Si3N4 is studied, and the isothermal oxidation kinetics of these two materials are obtained for the case of 100 hrs at 1200-1500 C in flowing oxygen. Estimates are made of lifetimes at the various temperatures investigated. Parabolic rate constants for SiC are within an order of magnitude of shorter exposure time values reported in the literature. The resulting silica scales are in the form of cristobalite, with cracks visible after exposure. The oxidation protection afforded by silica for these materials is adequate for long service times under isothermal conditions in 1-atm dry oxygen.

Fox, Dennis S.↗

NASA NTRS1992

Facility for continuous CVD coating of ceramic fibers

An inductively heated CVD furnace of pilot-plant scale, whose hot zone is 150 mm in diameter x 300 mm in length, has been adapted for continuous coating of ceramic yarns. Coatings at very low pressures are possible in this facility due to the fact that the entire apparatus, including yarn feeding and collecting equipment, is under vacuum. SiC yarn has been coated with 0.1-0.2 microns of BN at yarn speeds of 60 cm/min; a 500-m spool; was coated in about 14 hrs. Coating capacity was tripled by adding pulleys to allow three yarn passes through the furnace.

Moore, Arthur W.↗

NASA NTRS1994

Thin CVD Coating Protects Titanium Aluminide Alloys

Feasibility of using very thin CVD coatings to provide both protection against oxidation and surfaces of low catalytic activity for thin metallic heat-shield materials demonstrated. Use of aluminum in compositions increases emittances of coatings and reduces transport of oxygen through coatings to substrates. Coatings light in weight and applied to foil-gauge materials with minimum weight penalties.

Clark, Ronald↗

NASA NTRS1992

Design of A CVD Reactor for the Deposition of Diamond in A Microgravity Environment

There is a growing body of theoretical and experimental evidence suggesting that the uniformity, rate, adhesion, quality, and other key properties of Chemical Vapor Deposition (CVD) diamond coatings are influenced by the gas mixing kinetics in the thermal plasma environment of the reaction chamber. The implementation of, for example, Microwave Plasma Enhanced Chemical Vapor Deposition (MPECVD) process in microgravity has, thus, been suggested. Such a diamond deposition system, which overcomes the limitations of present systems and which is distiguishable from them by the use of high pressure MPECVD and recirculation of the would be effluent hydrogen and carbon, is described. Given the key fact that there is nothing in the effluent of the MPECVD process that is truly a byproduct or 'waste', the system can, at least in principle, lend itself to being closed loop yet dynamic. The exhaust contains hydrogen and carbon species which can be recirculated to the plasma reactor, that is, since there are no unusable reaction byproducts, the effluent can be fed back to the reaction chamber with no detriment, thus allowing deployment in a microgravity environment.

M L Languell↗

NASA NTRS1994

Numerical Analysis of an Impinging Jet Reactor for the CVD and Gas-Phase Nucleation of Titania

We model a cold-wall atmospheric pressure impinging jet reactor to study the CVD and gas-phase nucleation of TiO2 from a titanium tetra-iso-propoxide (TTIP)/oxygen dilute source gas mixture in nitrogen. The mathematical model uses the computational code FIDAP and complements our recent asymptotic theory for high activation energy gas-phase reactions in thin chemically reacting sublayers. The numerical predictions highlight deviations from ideality in various regions inside the experimental reactor. Model predictions of deposition rates and the onset of gas-phase nucleation compare favorably with experiments. Although variable property effects on deposition rates are not significant (approximately 11 percent at 1000 K), the reduction rates due to Soret transport is substantial (approximately 75 percent at 1000 K).

Gokoglu, Suleyman A.↗

NASA NTRS1995

CVD silicon carbide monofilament reinforced SrO-Al2O3-2SiO2 (SAS) glass-ceramic composites

Unidirectional CVD SiC fiber-reinforced SrO.Al2O3.2SiO2 (SAS) glass-ceramic matrix composites have been fabricated by hot pressing at various combinations of temperature, pressure and time. Both carbon-rich surface coated SCS-6 and uncoated SCS-0 fibers were used as reinforcements. Almost fully dense composites have been obtained. Monoclinic celsian, SrAl2Si2O8, was the only crystalline phase observed in the matrix from x-ray diffraction. During three point flexure testing of composites, a test span to thickness ratio of approximately 25 or greater was necessary to avoid sample delamination. Strong and tough SCS-6/SAS composites having a first matrix crack stress of approximately 300 MPa and an ultimate bend strength of approximately 825 MPa were fabricated. No chemical reaction between the SCS-6 fibers and the SAS matrix was observed after high temperature processing. The uncoated SCS-0 fiber-reinforced SAS composites showed only limited improvement in strength over SAS monolithic. The SCS-0/SAS composite having a fiber volume fraction of 0.24 and hot pressed at 1400 deg C exhibited a first matrix cracking stress of approximately 231 +/- 20 MPa and ultimate strength of 265 +/- 17 MPa. From fiber push-out tests, the fiber/matrix interfacial debonding strength (tau(sub debond)) and frictional sliding stress (tau(sub friction)) in the SCS-6/SAS system were evaluated to be approximately 6.7 +/- 2.3 MPa and 4.3 +/- 0.6 MPa, respectively, indicating a weak interface. However, for the SCS-0/SAS composite, much higher values of approximately 17.5 +/- 2.7 MPa for tau(sub debond) and 11.3 +/- 1.6 MPa for tau(sub friction) respectively, were observed; some of the fibers were so strongly bonded to the matrix that they could not be pushed out. Examination of fracture surfaces revealed limited short pull-out length of SCS-0 fibers. The applicability of various micromechanical models for predicting the values of first matrix cracking stress and ultimate strength of these composites were examined.

Bansal, Narottam P.↗

NASA NTRS1995

Microstructural and strength stability of a developmental CVD SiC fiber

The effects of thermal exposure on the room temperature tensile strength and microstructure of a developmental 50 micron CVD SiC fiber have been studied. The fibers were heat treated between 600 and 2000 C in 0.1 MPa argon and air environments for up to 100 hr. In the as-fabricated condition, the fibers showed approximately 6 GPa tensile strength. After argon treatment, the fibers showed strength degradation after 1 hr exposure beyond 1000 C, but those exposed between 1600 and 2000 C retained approximately 2 GPa strength. TEM results showed microstructural changes both in the surface coating and SiC sheath. Flaws created by the rearrangement of carbon in the surface coating and growth of equiaxed SiC grain zone in the SiC sheath are the suggested mechanisms of strength degradation. After air treatment, fibers showed strength degradation after only 2 min exposure at 600 C. Strength retention after 2 min at 1500 C was approximately 2 GPa. Oxidation of the surface coating is the primary reason for strength degradation.

Bhatt, Ramakrishna T.↗

NASA NTRS1996

CVD Of Thin Films From Single Organometallic Precursors

Method of forming thin inorganic films involves chemical vapor deposition (CVD) from single organometallic precursors. No toxic constituents, minimizes impurities, and yields films having substantially uniform crystal structure and composition. Especially suitable for depositing high-quality passivating or buffer layers of GaS on GaAs semiconductor substrates. Also applicable to formation of high-quality films for purposes other than buffering or passivation, and to different materials in which another element from same group in periodic table of elements substituted for all or portion of each element in GaS/GaAs system.

Hepp, Aloysius F.↗