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NASA NTRS1991

Crystal regularity with high-resolution synchrotron X-radiation diffraction imaging

New, high-resolution sources of X-radiation such as monochromatic synchrotron radiation beams with subarcsec divergence allow observation of regularities in a range of crystals with sufficient clarity for comprehensive analyses, whose results can deepen understanding of the nature of various crystal irregularities, their sources, and their effects on device performance. An account is presented of the results thus achievable with irregularities encountered in lattice orientation and strain, grain and subgrain boundaries, dislocations, domain boundaries, additional phases, and surface scratches. Significant achievements to date encompass the observation of critical anomalies in lead tin telluride, the reconciliation of disparate observations of GaAs, the determination of the performance effects of irregularities in mercuric iodide, and the characterization of the origins of crystal growth in bismuth silicon oxide.

Steiner, Bruce↗

NASA NTRS1991

High resolution synchrotron X-radiation diffraction imaging of crystals grown in microgravity and closely related terrestrial crystals

Irregularities in three crystals grown in space and in four terrestrial crystals grown under otherwise comparable conditions have been observed in high resolution diffraction imaging. The images provide important new clues to the nature and origins of irregularities in each crystal. For two of the materials, mercuric iodide and lead tin telluride, more than one phase (an array of non-diffracting inclusions) was observed in terrestrial samples; but the formation of these multiple phases appears to have been suppressed in directly comparable crystals grown in microgravity. The terrestrial seed crystal of triglycine sulfate displayed an unexpected layered structure, which propagated during directly comparable space growth. Terrestrial Bridgman regrowth of gallium arsenide revealed a mesoscopic structure substantially different from that of the original Czochralski material. A directly comparable crystal is to be grown shortly in space.

Steiner, Bruce↗

NASA NTRS1991

High resolution diffraction imaging of crystals grown in microgravity and closely related terrestrial crystals

Irregularities found in three crystals grown in space, in four crystals grown entirely on the ground were examined and compared. Irregularities were observed in mercuric iodide, lead tin telluride, triglycine sulfate, and gallium arsenide by high resolution synchrotron x radiation diffraction imaging. Radiation detectors made from mercuric iodide crystals grown in microgravity were reported to perform far better than conventional detectors grown from the same material under full gravity. Effort is now underway to reproduce these 'space' crystals, optimize their properties, and extend comparable superiority to other types of materials.

Steiner, B.↗

NASA NTRS1988

Data acquisition from a crystal growth furnace

Digital acquisition from a furnace for growing the lead tin telluride crystal based on Asyst software is described. It is shown that the use of Asyst for the acquisition of experimental data makes it possible to handle large amounts of data and to drastically reduce the time required for displaying the data from several hours to less than 5 minutes.

Woodell, Glenn A.↗

NASA NTRS1993

Growth of PbSnTe single crystal by traveling-zone method in low gravity (M-2)

The single-crystal lead tin telluride (PbSnTe) semiconductor is most promising as a laser radiation element and infrared detecting element in the far infrared region. However, it is very difficult to grow a large single crystal with a homogeneous composition on Earth because the elements have a very strong tendency to separate from each other in the molten phase due to differences in their specific gravities and melting points. The purpose of the experiment is to grow a single crystal of PbSnTe by a traveling zone method in microgravity, and to study the spatial fluctuation of the composition and the electrical properties of the crystal.

Segawa, Y.↗

NASA NTRS1998

The Effect of Microgravity Direction on the Growth of PbSnTe

The Space Shuttle Columbia was launched as STS-75 at 2018 GMT (Greenwich Mean Time) on February 22, 1996. One of the two major experiment packages was the Third United States Microgravity Payload (USMP-3), and one of the principal instruments on the USMP was the Advanced Automated Directional Solidification Furnace (AADSF). The AADSF is a multizone directional solidification furnace, and at the time of the USMP-3 flight was capable of processing only one sample per Shuttle mission which, for that flight, was a lead tin telluride (PbSnTe) crystal growth experiment. In the one year since the flight experiment the sample has been retrieved from the spacecraft and analysis has begun. After presenting introductory material on why PbSnTe was chosen as a test material, why microgravity processing was expected to produce desired results, and what we expected to find in conducting these tests, this report discusses the results to date which are far from complete.

Fripp, A. L.↗

NASA NTRS1999

Growth of Compound Semiconductors in a Low Gravity Environment: Microgravity Growth of PbSnTe

The growth of the alloy compound semiconductor lead tin telluride (PbSnTe) was chosen for a microgravity flight experiment in the Advanced Automated Directional Solidification Furnace (AADSF), on the United States Microgravity Payload-3 (USNP-3) in February, 1996 and on USNW- 4 in November, 1997. The objective of these experiments was to determine the effect of the reduction in convection, during the growth process, brought about by the microgravity environment. The properties of devices made from PbSnTe, an alloy of PbTe and SnTe, are dependent on the ratio of the elemental components in the starting crystal. Compositional uniformity in the crystal is only obtained if there is no significant mixing in the liquid during growth. The technological importance of PbSnTe lies in its band gap versus composition diagram which has a zero energy crossing at approximately 40% SnTe. This facilitates the construction of long wavelength (greater than 6 gm) infrared detectors and lasers. The properties and utilization of PbSnTe are the subject of other papers. 1,2 PbSnTe is also interesting from a purely scientific point of view. It is, potentially, both solutally and thermally unstable due to the temperature and density gradients present during growth. Density gradients, through thermal expansion, are imposed in directional solidification because temperature gradients are required to extract heat. Solutal gradients occur in directional solidification of alloys due to segregation at the interface. Usually the gradients vary with both experiment design and inherent materials properties. In a simplified one dimensional analysis with the growth axis parallel to the gravity vector, only one of the two instabilities work at a time. During growth, the temperature in the liquid increases ahead of the interface. Therefore the density, due to thermal expansion, is decreasing in that direction. However, the phase diagram shows that the lighter SnTe is preferentially rejected at the interface. This causes the liquid density to increase with distance away from the interface.

Fripp, A. L.↗

NASA NTRS2001

Effect of Marangoni Convection Generated by Voids on Segregation During Low-G and 1-G Solidification

Solidification experiments, especially microgravity solidification experiments, are often compromised by the evolution of unwanted voids or bubbles in the melt. Although these voids and/or bubbles are highly undesirable, there is currently no effective means of preventing their formation or of eliminating their adverse effects, particularly during microgravity experiments. Marangoni convection caused by these voids can drastically change the transport processes in the melt. Recent microgravity experiments by Matthiesen (1) Andrews (2) and Fripp (3) are perfect examples of how voids and bubbles can affect the outcome of costly space experiments and significantly increase the level of difficulty in interpreting their results. Formation of bubbles have caused problems in microgravity experiments for a long time. Even in the early Skylab mission an unexpectedly large number of bubbles were detected in the four materials processing experiments reported by Papazian and Wilcox (4). They demonstrated that while during ground-based tests bubbles were seen to detach from the interface easily and float to the top of the melt, in low-gravity tests no detachment from the interface occurred and large voids were grown in the crystal. More recently, the lead-tin-telluride crystal growth experiment of Fripp et al.(3) flown aboard the USMP-3 mission has provided very interesting results. The purpose of the study was to investigate the effect of natural convection on the solidification process by growing the samples at different orientations with respect to the gravitational field. Large pores and voids were found in the three solid crystal samples processed in space. Post-growth characterization of the compositional profiles of the cells indicated considerable levels of mixing even in the sample grown in the hot-on-top stable configuration. The mixing was attributed to thermocapillary convection caused by the voids and bubbles which evolved during growth. Since the thermocapillary convection is orientation-independent, diffusion-controlled growth was not possible in any of the samples, even the top-heated one. These results are consistent with recent studies of thermocapillary convection generated by a bubble on a heated surface undertaken by Kassemi and Rashidnia (5-7) where it is numerically and experimentally shown that the thermocapillary flow generated by a bubble in a model fluid (silicone oil) can drastically modify the temperature field through vigorous mixing of the fluid around it, especially under microgravity conditions.

Kassemi, M.↗

NASA NTRS1976

Electrochemical deposition of silver crystals aboard Skylab 4

Silver crystals were grown aboard Skylab 4 by an electro-chemical reaction and subsequently returned to earth for comparison with crystals grown at 1- and 5-g. Both the Skylab and earth-grown crystals show a variety of structures. Certain tendencies in structure dependency on gravity level, however, can be discerned. In addition, downward growing dendrite streamers; upward growing chunky crystal streamers; growth along an air/liquid interface; and ribbon, film, and fiber crystal habits were observed in experiments conducted on the ground with solutions of varying concentrations. It was also observed that the crystal structures of space and ground electro-deposited silver crystals were very similar to the structures of germanium selenide and germanium telluride crystals grown in space and on the ground by a vapor transport technique. Consideration of the data leads to the conclusions that: (1) the rate of electrochemical displacement of silver ions from a 5 percent aqueous solution by copper is predominantly diffussion controlled in space and kinetically controlled in 1- and higher-g because of augmentation of mass transport by convection; (2) downward and upward crystal streamers are the result of gravity-driven convection, the flow patterns of which can be delineated. Lateral growths along an air/liquid interface are the result of surface-tension-driven convection, the pattern of which also can be delineated; (3) electrolysis in space or low-g environments can produce either dendritic crystals with more perfect microcrystalline structures or massive, single crystals with fewer defects than those grown on ground or at higher g-levels. Ribbons or films of space-grown silicon crystals would find a ready market for electronic substrate and photocell applications. Space-grown dendritic, metal crystals present the possibility of unique catalysts. Large perfect crystals of various materials are desired for a number of electronic and optical applications; and (4) vapor transport growth of germanium selenide and germanium telluride is affected by convection mechanisms similar to the mechanisms hypothesized for the electrochemical deposition of silver crystals. Evidence and considerations leading to the preceding summaries and conclusions are presented. The implications of the findings and conclusions for technological applications are discussed, and recommendations for further experiments are presented.

Grodzka, P. G.↗

DOE OSTI2024

Enhanced Thermal Stability of Conductive Mercury Telluride Colloidal Quantum Dot Thin Films Using Atomic Layer Deposition

Colloidal quantum dots (CQDs) are valuable for their potential applications in optoelectronic devices. However, they are susceptible to thermal degradation during processing and while in use. Mitigating thermally induced sintering, which leads to absorption spectrum broadening and undesirable changes to thin film electrical properties, is necessary for the reliable design and manufacture of CQD-based optoelectronics. Here, low-temperature metal–oxide atomic layer deposition (ALD) was investigated as a method for mitigating sintering while preserving the optoelectronic properties of mercury telluride (HgTe) CQD films. ALD-coated films are subjected to temperatures up to 160 °C for up to 5 h and alumina (Al 2 O 3 ) is found to be most effective at preserving the optical properties, demonstrating the feasibility of metal–oxide in-filling to protect against sintering. HgTe CQD film electrical properties were investigated before and after alumina ALD in-filling, which was found to increase the p-type doping and hole mobility of the films. The magnitude of these effects depended on the conditions used to prepare the HgTe CQDs. With further investigation into the interaction effects of CQD and ALD process factors, these results may be used to guide the design of CQD–ALD materials for their practical integration into useful optoelectronic devices.

36 MATERIALS SCIENCE↗

NASA NTRS2009

The 55Fe X-ray Energy Response of Mercury Cadmium Telluride Near-Infrared Detector Arrays

A technique involving Fe-55 X-rays provides a straightforward method to measure the response of a detector. The detector's response can lead directly to a calculation of the conversion gain (e(-) ADU(-1) ), as well as aid detector design and performance studies. We calibrate the Fe-15 X-ray energy response and pair production energy of HgCdTe using 8 HST WFC3 1.7 micron flight grade detectors. The results show that each Ka X-ray generates 2273 +/- 137 electrons, which corresponds to a pair-production energy of 2.61 +/- 0.16 eV. The uncertainties are dominated by our knowledge of the conversion gain. In future studies, we plan to eliminate this uncertainty by directly measuring conversion gain at very low light levels.

Fox, Ori↗

NASA NTRS1988

MCT crystal growth

Convection and segregation in directional solidification and crystal growth by the Bridgman-Stockbarger technique are traditionally treated by assuming axisymmetric thermal condition on the ampoule wall. It is, however, difficult to achieve such a condition in an experimental setup. Any deviation from an axisymmetric temperature field on the wall of a vertical ampoule represents a horizontal temperature gradient. The horizontal density gradient that results from thermal expansion in the melt under this condition must lead on earth to some buoyance-driven convection, no matter what the axial (vertical) temperature distribution that is imposed on the melt. The magnitude of such convective flows for conditions representative of the MSFC mercury-cadmium-telluride (MCT) Bridgman setup is studied.

Baird, James K.↗

NASA NTRS2009

A Model for Predicting Thermoelectric Properties of Bi2Te3

A parameterized orthogonal tight-binding mathematical model of the quantum electronic structure of the bismuth telluride molecule has been devised for use in conjunction with a semiclassical transport model in predicting the thermoelectric properties of doped bismuth telluride. This model is expected to be useful in designing and analyzing Bi2Te3 thermoelectric devices, including ones that contain such nano - structures as quantum wells and wires. In addition, the understanding gained in the use of this model can be expected to lead to the development of better models that could be useful for developing other thermoelectric materials and devices having enhanced thermoelectric properties. Bi2Te3 is one of the best bulk thermoelectric materials and is widely used in commercial thermoelectric devices. Most prior theoretical studies of the thermoelectric properties of Bi2Te3 have involved either continuum models or ab-initio models. Continuum models are computationally very efficient, but do not account for atomic-level effects. Ab-initio models are atomistic by definition, but do not scale well in that computation times increase excessively with increasing numbers of atoms. The present tight-binding model bridges the gap between the well-scalable but non-atomistic continuum models and the atomistic but poorly scalable ab-initio models: The present tight-binding model is atomistic, yet also computationally efficient because of the reduced (relative to an ab-initio model) number of basis orbitals and flexible parameterization of the Hamiltonian.

Lee, Seungwon↗

DOE OSTI2024

Spin-dependent electron transfer in electrochemically transparent van der Waals heterostructures for oxygen evolution reaction

Spin selective catalysis is an emerging approach for improving the thermodynamics and kinetics of reactions. The role of electron spins has been scarcely studied in catalytic reactions. One exception is the oxygen evolution reaction (OER) where strongly correlated metals and oxides are used as catalysts. In OER, spin alignment facilitates the transition of singlet state of the reactant to the triplet state of O 2 . However, the influence of strong correlations on spin exchange mechanism and spin selective thermodynamics of most catalytic reactions remain unclear. Here we decouple the strongly correlated catalyst from the electrolyte to study spin exchange in two-dimensional (2D) magnetic iron germanium telluride (FGT) heterostructure. We demonstrate that transmission of spin and electrochemical information between the catalyst and the reactant can occur through quantum exchange interaction despite the catalyst of FGT being completely encapsulated by graphene or hexagonal boron nitride (hBN). The strong correlations in FGT that lead to enhanced spin exchange in OER are observed in graphene or hBN layers with thicknesses of up to 6 nm. We demonstrate that spin alignment in FGT leads to a lowering of thermodynamic barrier for adsorption of hydroxide ion and electron transfer to the catalyst. This results in up to fivefold enhancement in OER performance and improved kinetics. Our results provide clear evidence that transmission of both quantum mechanical and electrochemical information through quantum spin exchange interaction in FGT leads to an enhancement in catalytic performance.

2D materials↗

NASA NTRS2002

Building A New Kind of Graded-Z Shield for Swift's Burst Alert Telescope

The Burst Alert Telescope (BAT) on Swift has a graded-Z Shield that closes out the volume between the coded aperture mask and the Cadmium-Zinc-Telluride (CZT) detector array. The purpose of the 37 kilogram shield is to attenuate gamma rays that have not penetrated the coded aperture mask of the BAT instrument and are therefore a major source of noise on the detector array. Unlike previous shields made from plates and panels, this shield consists of multiple layers of thin metal foils (lead, tantalum, tin, and copper) that are stitched together much like standard multi-layer insulation blankets. The shield sections are fastened around BAT, forming a curtain around the instrument aperture. Strength tests were performed to validate and improve the design, and the shield will be vibration tested along with BAT in late 2002. Practical aspects such as the layup design, methods of manufacture, and testing of this new kind of graded-Z Shield are presented.

Robinson, David W.↗

NASA NTRS2010

Fabrication of Lanthanum Telluride 14-1-11 Zintl High-Temperature Thermoelectric Couple

The development of more efficient thermoelectric couple technology capable of operating with high-grade heat sources up to 1,275 K is key to improving the performance of radioisotope thermoelectric generators. Lanthanum telluride La3-xTe4 and 14-1-11 Zintls (Yb14MnSb11) have been identified as very promising materials. The fabrication of advanced high-temperature thermoelectric couples requires the joining of several dissimilar materials, typically including a number of diffusion bonding and brazing steps, to achieve a device capable of operating at elevated temperatures across a large temperature differential (up to 900 K). A thermoelectric couple typically comprises a heat collector/ exchanger, metallic interconnects on both hot and cold sides, n-type and ptype conductivity thermoelectric elements, and cold-side hardware to connect to the cold-side heat rejection and provide electrical connections. Differences in the physical, mechanical, and chemical properties of the materials that make up the thermoelectric couple, especially differences in the coefficients of thermal expansion (CTE), result in undesirable interfacial stresses that can lead to mechanical failure of the device. The problem is further complicated by the fact that the thermoelectric materials under consideration have large CTE values, are brittle, and cracks can propagate through them with minimal resistance. The inherent challenge of bonding brittle, high-thermal-expansion thermoelectric materials to a hot shoe material that is thick enough to carry the requisite electrical current was overcome. A critical advantage over prior art is that this device was constructed using all diffusion bonds and a minimum number of assembly steps. The fabrication process and the materials used are described in the following steps: (1) Applying a thin refractory metal foil to both sides of lanthanum telluride. To fabricate the n-type leg of the advanced thermoelectric couple, the pre-synthesized lanthanum telluride coupon was diffusion bonded to the metal foil using a thin adhesion layer. (2) Repeating a similar process for the 14-1-11 Zintl p-type leg of the advanced thermoelectric couple. (3) Bonding thick CTE-matched metal plates on the metallized lanthanum telluride and Yb14MnSb11 to form the hot and cold sides of the thermoelectric couple. The calculated conversion efficiency of such an advanced couple would be about 10.5 percent, about 35 percent better than heritage radioisotope thermoelectric technology that relies on Si-Ge alloys. In addition, unlike Si-Ge alloys, these materials can be combined with many other thermoelectric materials optimized for operation at lower temperatures to achieve conversion efficiency in excess of 15 percent (a factor of 2 increase over heritage technology).

Ravi, Vilupanur A.↗

DOE OSTI2026

Nitrogen p-type doping in polycrystalline zinc selenide telluride films

Nitrogen-doped zinc selenide telluride (N:ZnSe x Te 1−x ) is of interest because it is one of the widest-gap II–VI semiconductors that can still be doped p-type with reasonably high hole concentrations. We sputter deposit N:ZnSe x Te 1−x films, varying Se/(Se + Te), or x, from 0 to 0.7, N 2 flow rate from 0.25 to 0.75 sccm, and substrate temperature from 250 to 370 °C. Increasing x from 0 to 0.39 at the optimal temperature of 370 °C and N 2 flow rate of 0.5 sccm leads to 1.3 atomic % nitrogen incorporation and wurtzite phase stabilization. Such doping and alloying increases hole concentration from 3 × 10 18 cm −3 to 3 × 10 19 cm −3 , although mobility drops from 0.4 to 0.02 cm 2 V −1 s −1 . Our sputtered N:ZnSe 0.38 Te 0.62 has an absorption onset 0.1 eV greater than the ZnTe value of 1.87 eV. Increasing x from 0 to 0.51 enhances transmittance by moving absorption onset from 1.87 to 2.11 eV with diminished band gap bowing likely due to disorder, and increases the work function from 5.12 to 5.42 eV. This combination of tunable properties makes sputtered N:ZnSe x Te 1−x desirable for transparent p-type contacts in polycrystalline Cd(Se,Te) optoelectronic devices.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Pyrolyzer Assisted Vapor Transport Deposition of Antimony-Doped Cadmium Telluride

In this study, we developed a new method for in situ Sb doping of CdTe thin films combining vapor transport deposition with a Group V pyrolyzer to address Sb doping concentration and doping efficiency. The Sb doped CdSeTe (CdSeTe:Sb) films were deposited in solar cell structures under variations of Sb dopant source heater, vapor pyrolyzer temperature, and Cd vapor excess. Results indicate that although these parameters do not affect the CdTe morphology or crystal structure, they critically influence doping efficiency and trap concentration. Capacitance-voltage measurements show that a higher dopant heater (TD) or pyrolyzer (TP) temperature leads to higher net carrier concentration, achieving a net carrier concentration of 1016 cm-3 and 20% doping efficiency with a TD/TP combination of 600 degrees C/1100 degrees C. By tuning the Cd/Sb flux ratio during CdSeTe:Sb deposition, the lowest defect concentration is achieved at Cd/Sb of 1.4:1, which produced the best VOC CdSeTe:Sb cell. This demonstrates a path to produce high net carrier concentration polycrystalline CdTe thin film with a low concentration of dopant-induced defects.

14 SOLAR ENERGY↗