CdTe:Cl crystals were successfully grown in a closed vapor transport system under terrestrial and microgravity conditions within the framework of the EURECA-1 mission. The axial as well as radial resistivity distribution differs strongly in 1 g and (mu) g crystals. This is assumed to be due to different vapor transport of major components and dopants and different dopant segregation. For CdTe crystal growth in closed systems, the partial pressures depend sensitively on the composition of the feed material and its pretreatment. This affects the reproducibility of vapor transport: Smallest deviations from the congruently subliming composition yield significant changes in the p_Cd/p_Te(2) ratio and transport-limiting diffusion barriers. Therefore, vapor pressure control by means of a semi-closed growth system is proposed for CdTe vapor transport experiments in microgravity. In ground-based experiments, CdTe crystals were grown in the semi-closed system applying Cl, Ga and V as dopants. The conditions for reproducible transport are discussed. Results of electrical material characterization are presented.

The mechanism of physical vapor transport of II-VI semiconducting compounds was studied both theoretically, using a one-dimensional diffusion model, as well as experimentally. It was found that the vapor phase stoichiometry is critical in determining the vapor transport rate. The experimental heat treatment methods to control the vapor composition over the starting materials were investigated and the effectiveness of the heat treatments was confirmed by partial pressure measurements using an optical absorption technique. The effect of residual (foreign) gas on the transport rate was also studied theoretically by the diffusion model and confirmed experimentally by the measurements of total pressure and compositions of the residual gas. An in-situ dynamic technique for the transport rate measurements and a further extension of the technique that simultaneously measured the partial pressures and transport rates were performed and, for the first time, the experimentally determined mass fluxes were compared with those calculated, without any adjustable parameters, from the diffusion model. Using the information obtained from the experimental transport rate measurements as guideline high quality bulk crystal of wide band gap II-VI semiconductor were grown from the source materials which undergo the same heat treatment methods. The grown crystals were then extensively characterized with emphasis on the analysis of the crystalline structural defects.

Mercurous chloride is an acoustical optical material with an unusually low acoustic velocity and high acousto-optical figure of merit, which makes it an interesting candidate for optical delay lines and Bragg cells for optical signal processors. It also has a broad range of spectral transmissivity which makes it an ideal candidate for wide band acoustically tuned optical filter (ATOF) applications. Single crystals of this material can be readily grown in normal gravity by closed-tube physical vapor transport, but the crystals appear to contain structural inhomogeneities which degrade the optical performance. The nature of these defects is not known, but their degree appears to correlate with the Rayleigh number that characterizes their growth; hence, it is suspected that uncontrolled convection may play a role in the defect structure. This prompted a space flight experiment to determine if these defects could be further reduced by virtually eliminating the buoyancy-driven convective flows which are always present to a degree in normal gravity. Single crystals of mercurous chloride (Hg₂Cl₂) were grown in the Space Experiment Facility (SEF) transparent furnace developed by the University of Alabama in Huntsville, Consortium for Materials Development in Space. The Northrop- Grumman Science and Technology Center provided the highly purified starting material and analyzed the crystals that were grown. This experiment was flown on Spacehab 4 (STS-77) in May 1996. The SEF is a transparent furnace which allowed the progress of the growth to be recorded by video. Extensive furnace profiling and modeling has been carried out to relate the growth front location to the thermal environment and to the crystal quality. The results of the flight experiment as well as the ground control experiments are presented.

The effect of desorption from, and diffusion through the wall on inert gas pressure in sealed fused silica ampoules was investigated. It is shown, that desorption from the surface and the bulk of silica may lead to an accumulation of residual gas on the order of a few Torr or more upon annealing. A prior outgassing of the ampoules under vacuum at high temperature reduces the amount of gas released from the glass by at least one order of magnitude. Presence of oxide and other impurities in the source material was found to increase the residual gas pressure, affect its composition, and reduce the vapor transport rate in PVT systems. It is shown, that light gases (hydrogen, helium, and neon) diffuse through silica wall and may change the pressure inside the sealed ampoule considerably even at moderate temperatures.

Experiments were performed in controlled conditions to understand both morphological and convective instabilities and results were compared with the theoretical calculations. The sharp contrast between the solid (yellow) and the liquid (bright red) phases makes this transparent lead bromide a very suitable material for such an investigation. Crystals doped with 5000 ppm silver bromide were grown. The experimental observations agree with the preliminary results of the numerical prediction. Further, toroidal instabilities resulting from double diffusive convection were observed during crystal growth. Crystals for these interfacial observations were grown in a two zone vertical Bridgman furnace. The acoustic properties were better in doped rather than undoped crystals. Crystals were characterized by x-ray rocking curves and contour scans. Crystals grown at lower Rayleigh numbers showed better quality than crystals grown at higher Rayleigh numbers.

Undoped and doped CdS_0.8 Se_0.2 crystals were grown by physical vapor transport (PVT). The selected dopant was vanadium at a nominal concentration of 150 ppm creating for photorefractive effect. The as-grown crystal has a large crystal size, 1.1 cm in diameter and 6 cm in length, with a medium resistivity of 10⁴ - 10⁷ (Omega) -cm. The results from low temperature photoluminescence (PL) show that the undoped crystal has only one emission band at 2.31 eV and its phonon replicas. The vanadium doped crystal not only show the similar emission band but also has an additional broad band center at 1.95 eV due to the effect of doping. Low temperature (16 K) and room temperature IR transmittance spectra of vanadium doped crystal revealed a broad absorption band between 0.8 and 1.3 eV which may be due to vanadium dopant. Etch pitch density (EPD) measurements were performed, and the results showed EPD in the range of 10⁴/cm² for both types of crystals. Precipitate/inclusion were also found in both crystal, and their distribution patterns may be related to gravity-induced convection during growth process.

Based on design criteria established in preceding work for conditions of moderately elevated pressure (less than or equal to 8 bar) we have designed and evaluated a reactor for chemical vapor deposition (CVD) at high pressure (less than or equal to 100 bar). While at moderate pressure non-turbulent unidirectional forced channel flow past a heated substrate wafer can be realized, at high pressure, the flow is expected to become turbulent. Due to phase front distortions and variations in angle of incidence associated with density fluctuations and density gradients in the high pressure vapor phase in the vicinity of the hot substrate -- the precision of methods of real-time optical process monitoring that employ polarized light, such as, p-polarized reflectance spectroscopy (PRS), is degraded. Above a critical pressure, features in the optical signals related to chemical kinetics and to the kinetics of heteroepitaxy, thus are no longer resolved. Therefore, experimentation at reduced gravity, which extends the pressure range of non-turbulent flow, and alternative robust methods of real-time optical process monitoring are considered. At very high pressures, where real-time process monitoring is severely curtailed, CVD processing must rely on predictions of numerical models -- validated by experimentation at lower pressure/low gravity. Experimentation at high pressure is needed to access materials, properties and/or structures that otherwise cannot be realized.

Homogeneous II-VI narrow gap semiconducting alloys are of interest because of their use in infrared detectors. These compound semiconductor materials are inherently difficult to grow in bulk due to gravity induced convective flows. A microgravity growth environment has therefore been considered. In order to evaluate the quality of crystals grown in space compared to those grown on the ground, it is necessary to characterize both. One important aspect of this characterization is the study of stoichiometry, x. A characterization scheme using scanning tunneling optical spectroscopy (STOS) involves determining the spectral response of the photoexcited tunneling current for a semiconductor. By measuring the photoenhanced tunneling current versus photon energy, the band gap E_g of the semiconductor material can be determined. Such measurements determine E_g equals E_g(x) locally, and thus x can be determined as a function of position provided E_g(x) is known. We consider a one dimensional model, involving a simple analysis of absorption of photons, production of photoelectrons, diffusion of photoelectrons to the surface, and tunneling of these electrons to the STM probe. Our results of photoenhanced tunneling current versus photon energy are qualitatively similar to experiment. After our results are presented, we list questions that need to be considered for an improved version of our analysis which is planned.

Bacteriorhodopsin-based interferometry is a new technique with which high resolution interferograms can be recorded at one or more wavelengths in real time. The erasable nature of bacteriorhodopsin, its panchromatic response to light, and the fact that recording is done on the molecular level, allow for the formation of fringe patterns combining the high resolution of conventional silver halide recording materials with the real time properties of CCD cameras. In this context, we have used bacteriorhodopsin to image changes in the crystal environment at single and dual wavelengths, using a real time sequencing architecture whereby successive exposures of a bR thin film are overlapped to produce a continuous stream of interferometric images. This allows real-time data buffering and immediate, on-line observation of results. In this paper, we present real time single and dual wavelength interferograms of growing KAl(SO₄)₂ and melting sugar crystals recorded on bacteriorhodopsin thin films. The interferograms were produced with a shuttered cw argon or a pulsed Nd:YAG write laser, and a helium neon read laser, and show enhanced contrast and resolution. The results of this effort clearly demonstrate the superior, real time recording capabilities of bacteriorhodopsin thin films.

The dependence of supercooling level ((Delta) T^-) of InSb melt preceding on its melt overheating ((Delta) T⁺) above the equilibrium melting temperature, is investigated for the first time on a III-V semiconductor compound. The dependence of (Delta) T^- on (Delta) T⁺ in the InSb melt is shown to be abrupt-and-discontinuous. The observation can be linked to the semiconductor-metal transition upon melting, and is probably general, occurring also for other III-V and group IV semiconductors.

A method which combines laser vaporization of metal targets with controlled condensation from the vapor phase is used to synthesize nanoscale metal oxide, carbide and nitride particles (10 - 20 nm) of well-defined composition. The metal vapor is generated by pulsed laser vaporization using the second harmonic (532 nm) of a Nd-YAG laser. Following the laser pulse, the ejected atoms react with the reactive gas within the ambient atmosphere and condense to form nanoparticles. The role of convection in the experiments is to remove the small particles away from the nucleation zone (once condensed out of the vapor phase) before they can grow into larger particles. Surface-oxidized silicon nanocrystals, produced by this method, aggregate into a novel weblike microstructure. These aggregates are very porous and have a large surface area. The nanoparticles show a short-lived (less than 20 ns) blue emission at 450 nm characteristic of the SiO₂ coating and a biexponential longer-lived red emission characteristic of the Si core. Other examples of nanoparticles discussed include ZnO and magnetic FeO. Examples of the effects of solvents and temperature on the morphology of the nanoparticles are presented. The advantages of microgravity in the synthesis of multilayer nanoparticles of engineered compositions and morphology are outlined.

Thin films of an organic nonlinear optical (NLO) material, N,N-dimethyl-p-(2,2-dicyanovinyl) aniline (DCVA), have been grown in space aboard the USA Space Shuttle Endeavour on STS (space transportation system) -59 and STS-69. Similar experiments have been conducted in the laboratory as ground controls and have produced single crystals only. In this paper, preliminary results of the space grown film characterization using the differential scanning calorimetry, differential interference contrast microscopy, Fourier transform infrared spectrometry, visible reflection spectroscopy, x-ray diffraction, second harmonic generation, and stylus profilometry are presented. These techniques have implied that ordered, 3.7 (mu) thick, DCVA films, have been grown on disordered substrates.

Polymer dispersed liquid crystal (PDLC) materials are produced in a microgravity environment to investigate the gravitational influence on the phase separated microstructure. PDLC materials contain a dispersion of micron-sized liquid crystal droplets dispersed in a solid polymer. The dispersion is achieved by one of several phase separation methods. The phase separated microstructure determines the operating parameters of PDLCs; therefore, a fundamental understanding of phase separation processes is critically important. Preparation of PDLC materials in a microgravity environment is advantageous for studying the underlying processes of phase separation that are masked in our terrestrial environment. We investigate the effect of gravity on the microstructure of PDLC materials using photo-polymerizable materials aboard NASA's KC-135 aircraft.

We report on a novel computer-controlled experiment set-up for growing organic nonlinear optical (NLO) crystals by effusive- ampoule physical vapor transport (EAPVT). In this approach, incongruent or impurity vapor components are continuously removed from the vicinity of the growing crystal to vacuum. This results in considerably higher transport rates than are obtained in closed ampoule arrangements. As a consequence, crystal growth can be conducted with reduced temperature gradients, which is important for the growth of structurally perfect crystals. We present design considerations for an EAPVT apparatus, its construction, and its application to the growth of single crystals of 4-(N,N-dimethylamino)-3- acetamidonitrobenzene (DAN), an organic NLO material. The insight gained from this ground-based experimental work was used for the design of the flight hardware used aboard the U.S. Space Shuttle.

Single crystals of pure and binary alloy of m.dinitrobenzene and m.nitroaniline were grown by Bridgman method in a two zone transparent furnace. Effect of doping and growth velocity on the solid-liquid interface morphology and quality of crystal was determined by studying the optical transparency, birefringence and nonlinear optical characteristics.

Techniques to overcome the limits of electronic data transmission have centered around the development of second and third order nonlinear optical devices. However, development of materials to make these devices has been slow. In order to maximize nonlinear optical response, it is necessary for the materials to be aligned in one direction. Various techniques that require extra energy have been used to accomplish this task. It would therefore be advantageous to develop a process which would take advantage of order enhancing groups to increase the alignment naturally. Polydiacetylenes are a class of conjugated polymers that possess large third order nonlinear optical responses. Their wide-ranging chemical and electronic properties and morphology make them very attractive materials for device applications. Paley, et al. developed a process called photodeposition in which a thin polymer film is deposited directly from monomer solution on to a variety of substrates. Utilization of this process to deposit ordered thin films will facilitate easy device preparation. We present synthetically prepared derivatives of a polydiacetylene, PDAMNA, which possess order encouraging side groups. Photodeposition of these compounds yield good optical quality thin films which display orientation normal to the substrate as determined by polarized UV-Vis spectroscopy. Furthermore, SEM micrographs show the chains normal to the surface and the subsequent decrease in order as the thickness increases. Energy calculations of the different conformers of the dimers shows a 24 kcal/mole difference between the alternating and non-alternating structures. These data show that partially ordered polydiacetylene thin films can be obtained through photodeposition.

A program for crystal shape calculation on the basis of experimental data on molecular crystal structure was elaborated. The atom-atom potential method was used to calculate intermolecular energy, energy of molecular layers, attachment energy, and relative rates of crystal faces growth. Using this program the crystal shape of the well known NLO compound DIVA was calculated. The prospects for further development of prediction of NLO properties on the basis of known crystal structure by combination crystal hyperpolarizability and crystal shape calculations are discussed.

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.

A numerical model of the thermal field in the furnace-ampoule system developed for the 'contactless' physical vapor transport growth geometry is presented. The model is used to assess the effect of the system geometry and growth parameters on the conditions of 'contactless' growth of cadmium telluride crystals.

The dynamics of the flow field during physical vapor transport of mercurous chloride is analyzed computationally for practical crystal growth conditions to distinguish between unsteady flow and thermal stress as the cause of crystal inhomogeneity. We analyze the flow field over the parametric range of 1.56 multiplied by 10² less than or equal to Ra less than or equal to 2.08 multiplied by 10⁵, .011 less than or equal to Ar less than or equal to .112, and show that the effect of thermal stress as the cause of crystal inhomogeneity can be investigated by performing experiments with low aspect ratio enclosures for Ar less than .028 and Ra less than 2.4 multiplied by 10³ for which the flow becomes diffusive-advective. Dynamical characteristics of the flow field indicate a transition from chaotic flow Ra equals 2.08 multiplied by 10⁵ to steady flow Ra less than 1.4 multiplied by 10⁴ occurs through period doubling. A microgravity environment can be used effectively to grow high quality crystals, if thermal stresses are not the cause of crystal inhomogeneity, since unsteady flows are damped regardless of the aspect ratio of the enclosure.

Crystal growth kinetics from the vapor phase or from solution can be described by the diffusion of growth species to the echelon of equidistant steps present at vicinal surfaces. Diffusion takes place in a thin boundary layer adjacent to the interface. Present theories of this process neglect a convective transport mechanism in the boundary layer. In this work, we reexamine this zero-flow assumption. We consider the difference in the densities between the mother phase and the growing crystalline phase as the driving force for the flow. This force is localized at the step positions when only lateral growth of the steps is permitted. In such a case a highly nonuniform flow pattern is obtained. It consists of two vortices with the line between these vortices corresponding to a flow directed towards the step. This nonuniform part of the flow is found to extend into the mother phase up to an inter- step distance. This is the region where diffusion in the horizontal direction takes place. Consequently, the results suggest the importance of convective transport in the boundary layer. Finally, a constant horizontal flow, far from the surface, is predicted.

The Microgravity Research Division of NASA funds materials science research through biannual research programs known as NASA Research Announcements (NRA). Selection is via external peer review with proposals being categorized for ground based research or flight definition status. Topics of special interest to NASA are described in the NRAs and guidelines for successful proposals are outlined. The procedure for progressing from selection to a manifested flight experiment will involve further reviews of the science and also of the engineering needed to complete the experiment successfully. The topics of interest to NASA within the NRAs cover a comprehensive range of subjects, but with the common denominator that the proposed work must necessitate access to the microgravity environment for successful completion. Understanding of the fundamental nature of microstructure and its effects on properties is a major part of the program because it applies to almost all fields of materials science. Other important aspects of the program include non-linear optical materials, glasses and ceramics, metal and alloys and the need to develop materials science specifically to support NASA's Human Exploration and Development of Space (HEDS) enterprise. The transition to the International Space Station (ISS) represents the next stage of the Materials Science program.

A knowledge of the quasi-steady acceleration environment on the NASA Space Shuttle Orbiter is of particular importance for materials processing experiments which are limited by slow diffusive processes. The quasi-steady (less than 1 HZ) acceleration environment on STS-73 (USML-2) was measured using the orbital acceleration research experiment (OARE) accelerometer. One of the facilities flown on USML-2 was the crystal growth furnace (CGF), which was used by several principal investigators (PIs) to grow crystals. In this paper the OARE data mapped to the sample melt location within this furnace is presented. The ratio of the axial to radial components of the quasi-steady acceleration at the melt site is presented. Effects of orbiter attitude on the acceleration data is discussed.

Successful ground based research of certain classes of semiconducting alloys is oftentimes difficult to accomplish due to the ever present effects of gravity acting on the sample during processing. Buoyancy-driven convection in the melt can prevent the formation of a stable boundary layer and hence diffusion-controlled growth conditions. In theory, if such experiments are conducted in the microgravity environment of space, buoyancy-driven convection effects due to gravity are essentially eliminated. Even under weightless conditions, however, some classes of semiconducting alloys remain sensitive to the direction of the residual acceleration vector. The residual acceleration environment is comprised of drag, gravity-gradient, and rotational acceleration contributions that are always present on orbit. These contributors are a function of shuttle attitude, altitude, atmospheric conditions, and the distance away from the shuttle center of gravity. It is possible, through orbital dynamics studies, to provide a shuttle attitude that aligns the residual acceleration vector in an ideal direction relative to the crystal growth axis. Due to shuttle hardware limitations it is often not possible to maintain these desired shuttle attitudes over extended periods of time while critical sample processing is being conducted. The development of a rotating mechanism, integrated with a crystal growth furnace, will allow experimenters the opportunity to process their samples under ideal residual acceleration conditions with no deviations from the desired shuttle attitude, while meeting all the on-orbit shuttle hardware requirements. Such a mechanism can be incorporated into existing hardware and can be flown on multiple shuttle flights to satisfy various science team sample processing requirements, independent of the varying on-orbit conditions that exist from flight to flight.

A numerical model of HgCdTe solidification was implemented using finite the element code FIDAP. Model verification was done using both experimental data and numerical test problems. The model was used to eluate possible effects of double- diffusion convection in molten material, and microgravity level on concentration distribution in the solidified HgCdTe. Particular attention was paid to incorporation of HgCdTe phase diagram. It was found, that below a critical microgravity amplitude, the maximum convective velocity in the melt appears virtually independent on the microgravity vector orientation. Good agreement between predicted interface shape and an interface obtained experimentally by quenching was achieved. The results of numerical modeling are presented in the form of video film.

For a typical Bridgman semiconductor crystal-growth process in space with a 0.2 Tesla axial magnetic field, the velocity in the melt motion driven by the residual accelerations is so small that nonlinear inertial effects and convective heat transfer are negligible, and the governing equations are linear. Therefore the melt motions driven by (1) the time- averaged or steady residual acceleration, (2) the continuous, random fluctuations of acceleration or g-jitters, and (3) the isolated spikes of much larger acceleration due to thruster firings, etc., are decoupled and can be treated independently. In addition, the solution for any instantaneous orientation of the acceleration vector is given by a time-dependent superposition of two solutions for an axial acceleration and a transverse acceleration. Solutions are presented for the magnetically damped buoyant convections driven by the axial and transverse components of the steady acceleration, the g- jitters and the spikes of larger acceleration. The axial magnetic field provides much stronger damping of the transverse vorticity than of the axial vorticity, so that the melt motion may be quite different from that without magnetic damping.

The body force generated by a rotating magnetic field applied to a cylindrical column of liquid metal of finite height is investigated theoretically. Although an exact analytical formula has not been found, the proposed approach leads to a good approximate solution. It is demonstrated that the force field is significantly affected by the angular frequency of the rotating magnetic field. In the low-frequency limit, only the azimuthal component is present, while in the high frequency regime, a complex force field is induced which is composed of both azimuthal and meridional components. The resulting azimuthal flow in the Stokesian regime has been numerically obtained and a counter-rotating profile has been demonstrated. The calculated results can be used to determine the fluid flow behavior during crystal growth in a weak rotating magnetic field.

Flow and heat and mass transfer in a two-layer liquid system is investigated when the layers are subjected to a constant magnetic field. The physical system consists of two immiscible liquids in a layer configuration with a free surface and a boundary between layers simulating two crystal growth methods, namely, floating zone and horizontal zone melting. Mass transfer effects due to crystallization of one of the layers is also considered. The diversity of flows is reduced to three types depending on (1) the dominance of the free surface effects of the top layer, (2) a balance between the thermocapillary effect on the free surface and the interface effect between the two layers, or (3) the dominance of interfacial effects, in determining the system thermo-fluid characteristics. The use of an additional liquid layer atop the basic crystallizing layer leads to a reduction of the radial dopant inhomogeneity in comparison to the one-layer case. The effect an externally imposed magnetic field on dopant distribution in this two-layer configuration is ambiguous and depends on its direction and intensity resulting in the necessity of optimizing the applied magnetic field for a specific situation.

A solid state electrochemical method for flow visualization was developed to investigate the orientation of flow due to natural convection in semiconductor melts, contained in a simulated vertical Bridgman configuration. The Bridgman ampoule was constructed from recrystallized alumina and multiple solid-state electrochemical cells/sensors were incorporated along the periphery of the ampoule, electrically insulated from one another. Liquid tin was used as the model fluid to represent a high temperature, low Prandtl number, opaque Bridgman melt. Atomic oxygen was used as a tracer species, which could be potenstiostatically injected or extracted locally at one of the sensors and the oxygen concentration changes were monitored at the other cell locations on the melt/electrolyte boundaries in the potentiometric mode as a function of time. Various convective flow patterns were inferred from these results for different aspect ratios of the melt and as a function of the imposed temperature gradient. The experimental results were shown to agree well with theoretical predictions. The technique was also able to discern transcritical points in the dynamic state of the melt.

Both quantum and electromagnetic theories of light predict the existence of angular momentum of the light (or photon spin) S and the possibility of its transfer to the matter /1/. Complex electromagnetic fields (e.g., higher order Laguerre-Gaussian beams) may carry, in addition, an "orbital" angular momentum L, associated with their spatial non uniformity /2/. These momentums may be transferred to matter when the light is absorbed by the matter. In general case, the torque r exerted by absorbed light has contributions from both momentums S and L, and may be presented as T(Pa/O))(1+cY), where a is the absorbed power, Co - light frequency, 1- is the azimuthal index (or charge) of the beam, and a=O for linear and = for circular polarizations /2, 3/.