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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Amorphous silicon enhanced metal-insulator-semiconductor contacts for silicon solar cells

Carrier recombination at the metal-semiconductor contacts has become a significant obstacle to the further advancement of high-efficiency diffused-junction silicon solar cells. This paper provides the proof-of-concept of a procedure to reduce contact recombination by means of enhanced metal-insulator-semiconductor (MIS) structures. Lightly diffused n{sup +} and p{sup +} surfaces are passivated with SiO{sub 2}/a-Si:H and Al{sub 2}O{sub 3}/a-Si:H stacks, respectively, before the MIS contacts are formed by a thermally activated alloying process between the a-Si:H layer and an overlying aluminum film. Transmission/scanning transmission electron microscopy (TEM/STEM) and energy dispersive x-ray spectroscopy are used to ascertain the nature of the alloy. Idealized solar cell simulations reveal that MIS(n{sup +}) contacts, with SiO{sub 2} thicknesses of ∼1.55 nm, achieve the best carrier-selectivity producing a contact resistivity ρ{sub c} of ∼3 mΩ cm{sup 2} and a recombination current density J{sub 0c} of ∼40 fA/cm{sup 2}. These characteristics are shown to be stable at temperatures up to 350 °C. The MIS(p{sup +}) contacts fail to achieve equivalent results both in terms of thermal stability and contact characteristics but may still offer advantages over directly metallized contacts in terms of manufacturing simplicity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Molybdenum oxide MoO{sub x}: A versatile hole contact for silicon solar cells

This letter examines the application of transparent MoO{sub x} (x < 3) films deposited by thermal evaporation directly onto crystalline silicon (c-Si) to create hole-conducting contacts for silicon solar cells. The carrier-selectivity of MoO{sub x} based contacts on both n- and p-type surfaces is evaluated via simultaneous consideration of the contact recombination parameter J{sub 0c} and the contact resistivity ρ{sub c}. Contacts made to p-type wafers and p{sup +} diffused regions achieve optimum ρ{sub c} values of 1 and 0.2 mΩ·cm{sup 2}, respectively, and both result in a J{sub 0c} of ∼200 fA/cm{sup 2}. These values suggest that significant gains can be made over conventional hole contacts to p-type material. Similar MoO{sub x} contacts made to n-type silicon result in higher J{sub 0c} and ρ{sub c} with optimum values of ∼300 fA/cm{sup 2} and 30 mΩ·cm{sup 2} but still offer significant advantages over conventional approaches in terms of contact passivation, optical properties, and device fabrication.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Legendre submanifolds in contact manifolds as attractors and geometric nonequilibrium thermodynamics

It has been proposed that equilibrium thermodynamics is described on Legendre submanifolds in contact geometry. It is shown in this paper that Legendre submanifolds embedded in a contact manifold can be expressed as attractors in phase space for a certain class of contact Hamiltonian vector fields. By giving a physical interpretation that points outside the Legendre submanifold can represent nonequilibrium states of thermodynamic variables, in addition to that points of a given Legendre submanifold can represent equilibrium states of the variables, this class of contact Hamiltonian vector fields is physically interpreted as a class of relaxation processes, in which thermodynamic variables achieve an equilibrium state from a nonequilibrium state through a time evolution, a typical nonequilibrium phenomenon. Geometric properties of such vector fields on contact manifolds are characterized after introducing a metric tensor field on a contact manifold. It is also shown that a contact manifold and a strictly convex function induce a lower dimensional dually flat space used in information geometry where a geometrization of equilibrium statistical mechanics is constructed. Legendre duality on contact manifolds is explicitly stated throughout.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Role of vacancies in tuning the electronic properties of Au-MoS{sub 2} contact

Understanding the electronic properties between molybdenum disulfide (MoS{sub 2}) and metal electrodes is vital for the designing and realization of nanoelectronic devices. In this work, influence of intrinsic vacancies in monolayer MoS{sub 2} on the electronic structure and electron properties of Au-MoS{sub 2} contacts is investigated using first-principles calculations. Upon formation of vacancies in monolayer MoS{sub 2}, both tunnel barriers and Schottky Barriers between metal Au and monolayer MoS{sub 2} are decreased. Perfect Au-MoS{sub 2} top contact exhibits physisorption interface with rectifying character, whereas Au-MoS{sub 2} contact with Mo-vacancy shows chemisorption interface with Ohmic character. Partial density of states and electron density of defective Au-MoS{sub 2} top contacts are much higher than those of perfect one, indicating the lower contact resistance and higher electron injection efficiency of defective Au-MoS{sub 2} top contacts. Notably, Mo-vacancy in monolayer MoS{sub 2} is beneficial to get high quality p-type Au-MoS{sub 2} top contact, whereas S-vacancy in monolayer MoS{sub 2} is favorable to achieve high quality n-type Au-MoS{sub 2} top contact. Our results provide guidelines for designing and fabrication of novel 2D nanoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Mechanism of Ti/Al/Ti/W Au-free ohmic contacts to AlGaN/GaN heterostructures via pre-ohmic recess etching and low temperature annealing

The physical mechanism of low-thermal-budget Au-free ohmic contacts to AlGaN/GaN heterostructures is systematically investigated with current-voltage, high-resolution transmission electron microscopy, and temperature-dependent contact resistivity characterizations. With a low annealing temperature of 600 °C, pre-ohmic recess etching of the AlGaN barrier down to several nanometers is demonstrated to be an effective method to reduce the contact resistance between Ti/Al/Ti/W ohmic metals and AlGaN/GaN heterostructures. However, further over recess of the AlGaN barrier leads to only sidewall contact to 2D electron gas channel and thus degraded contact performance. It is verified by temperature-dependent contact resistivity measurements that field emission (tunneling) dominates the current transport mechanism in Au-free ohmic contacts with AlGaN barrier partially and over recessed, while both field emission and thermionic emission contribute to traditional Ti/Al/Ni/Au ohmic contacts to AlGaN/GaN heterostructures that annealed at high temperature (850 °C)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Theoretical and experimental investigations of nano-Schottky contacts

Formation of metal-semiconductor (M-S) contacts at sub-20 nanometer range is a key requirement for down-scaling of semiconductor devices. However, electrical measurements of M-S contacts at this scale have exhibited dramatic change in the current-voltage (I-V) characteristics compared to that of conventional (or planar) Schottky contacts. This change is actually attributed to the limited metal contact region where the transferred charge from the semiconductor into the metal is confined to a small surface area, which in turn results in an enhanced electric field at the nano-M-S interface. We here present detailed theoretical models to analyze the nano-M-S junctions at 10 nm contact range and then implement this analysis on the experimental data we conducted under these conditions. Both theoretical and experimental results demonstrate a significant effect of the contact size on the electronic structure of the M-S junctions and thus on the I-V characteristics. This effect is rather prominent when the size of the metal contact is substantially smaller than the width of conventional depletion region of the relevant planar M-S contacts.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Nitride Layer Screening as Carrier-Selective Contacts for Silicon Heterojunction Solar Cells

A three-architecture method for screening new materials' viability as carrier-selective contacts for silicon solar cells is presented. Test-structure solar cells were fabricated with a standard silicon heterojunction contact for the front side, and one of three treatments on the back: bare silicon, intrinsic amorphous silicon (i-aSi:H), or n-type amorphous silicon on an i-aSi:H passivation layer. Then, the candidate contact material of interest was deposited on the back of each test structure and the cells were finished with evaporated Al. By analysing the current-voltage characteristics of each type of architecture, the carrier selectivity and surface passivation quality of novel contact materials or material stacks can be rapidly screened. To demonstrate the utility of this method, we present a preliminary investigation of nitride compounds as electron-selective contacts in silicon solar cells, in particular zinc tin nitride (ZnSnN2), which is naturally n-type and has favourable band alignments with c-Si. ZnSnN2 was deposited by reactive sputtering as an electron contact on each test structure. No passivation was observed, but decent electron-selectivity was observed when in direct contact with the silicon wafer. When combined with intrinsic amorphous silicon, poor performances were obtained with poor selectivity and the occurrence of an S-shape, likely due to insufficient selectivity of the ZTN. Similar results were obtained for 2 nm and 20 nm thick layers, indicating selectivity did not result from the Al over-layer, but was in fact due to the ZTN itself. Overall, this work shows the three-architecture screening method is a useful tool for assessing novel contact materials for silicon heterojunction solar cells.

14 SOLAR ENERGY↗
DOE OSTI2019

Exchange current density model for the contact-determined current-voltage behavior of solar cells

An analytic expression for the current–voltage [J(V)] behavior of a solar cell as limited by equilibrium exchange current densities of both carriers at both contacts is derived. The partial currents at both contacts to a generic semiconductor absorber are assumed to be linearly proportional to the excess carrier concentration at the interface with the contacts (e.g., as with Schottky-like contacts). The assumption that the quasi-Fermi levels in the absorber are approximately flat leads to an algebraic solution for the applied voltage as a function of current, which is inverted to obtain the analytic J(V) curve. The J(V) curve reveals distinct behavior associated with electrons and holes, separately, and allows for the determination of all critical performance parameters. In particular, it demonstrates how the characteristic features of the J(V) curve depend on the relative rate at which a particular carrier (electron or hole) is collected at one contact vs the other, rather than the relative rate of electron vs hole collection at a single contact. Furthermore, the model provides a unified explanation of how majority carrier extraction limitations cause nonideal J(V) behaviors such as S-shaped curves and dark/light crossover (i.e., failure of superposition). The efficacy and limitations of the model when applied to Schottky-type and doped semiconductor contacts are discussed. The work serves as a theoretical guide to scientists studying solar cells that are thought to be primarily limited by their contacts.

Solar cells↗
DOE OSTI2025

Identifying impacts of contact tracing on HIV epidemiological inference from phylogenetic data

Abstract Robust sampling methods are foundational to inferences using phylogenies. Yet the impact of using contact tracing, a type of non-uniform sampling used in public health applications such as infectious disease outbreak investigations, has not been investigated in the molecular epidemiology field. To understand how contact tracing influences a recovered phylogeny, we developed a new simulation tool called SEEPS (Sequence Evolution and Epidemiological Process Simulator) that allows for the simulation of contact tracing and the resulting transmission tree, pathogen phylogeny, and corresponding virus genetic sequences. Importantly, SEEPS takes within-host evolution into account when generating pathogen phylogenies and sequences from transmission histories. Using SEEPS, we demonstrate that contact tracing can significantly impact the structure of the resulting tree, as described by popular tree statistics. Contact tracing generates phylogenies that are less balanced than the underlying transmission process, less representative of the larger epidemiological process, and affects the internal/external branch length ratios that characterize specific epidemiological scenarios. We also examined real data from a 2007–2008 Swedish HIV-1 outbreak and the broader 1998–2010 European HIV-1 epidemic to highlight the differences in contact tracing and expected phylogenies. Aided by SEEPS, we show that the data collection of the Swedish outbreak was strongly influenced by contact tracing even after downsampling, while the broader European Union epidemic showed little evidence of universal contact tracing, agreeing with the known epidemiological information about sampling and spread. Overall, our results highlight the importance of including possible non-uniform sampling schemes when examining phylogenetic trees. For that, SEEPS serves as a useful tool to evaluate such impacts, thereby facilitating better phylogenetic inferences of the characteristics of a disease outbreak. SEEPS is available at https://github.com/MolEvolEpid/SEEPS.

Virology↗
DOE OSTI2015

Experimental Investigation of Electric Contact Resistance in Spark Plasma Sintering Tooling Setup

An experimental investigation on electric contact resistance in the tooling utilized in spark plasma sintering (SPS) is conducted. Two different SPS regimes are operated without the presence of processed powder to obtain the punch–punch (horizontal) and punch–die (vertical) contact resistance assessments separately. The processing temperature and applied pressure are considered to be the main factors that affect the contact resistance. These effects are analyzed by performing experiments at a maximum temperature of 1600°C with constant heating rate of 100°C/min. The applied pressure is ranging from 20 to 60 MPa and a 1 min holding time‐step is used to ensure stable measurements. It is shown that the contact resistance goes down as the temperature ramps up. An increase in the pressure leads to a decrease in the contact resistance. The mechanisms that cause these variations in the contact resistance are discussed. The dependence of assessed contact resistance of the utilized SPS system on the processing temperature and applied pressure is analytically described based on the regression analysis of the collected experimental data. A finite‐element simulation based on the obtained experimental results evidences that the electric contact resistance dominates the temperature evolution. The obtained results are of significant importance for modeling and optimization of SPS processes.

Wei, Xialu↗
DOE OSTI2015

Low cost back contact heterojunction solar cells on thin c-Si wafers. integrating laser and thin film processing for improved manufacturability

An interdigitated back contact (IBC) Si wafer solar cell with deposited a-Si heterojunction (HJ) emitter and contacts is considered the ultimate single junction Si solar cell design. This was confirmed in 2014 by both Panasonic and Sharp Solar producing IBC-HJ cells breaking the previous record Si solar cell efficiency of 25%. But manufacturability at low cost is a concern for the complex IBC-HJ device structure. In this research program, our goals were to addressed the broad industry need for a high-efficiency c-Si cell that overcomes the dominant module cost barriers by 1) developing thin Si wafers synthesized by innovative, kerfless techniques; 2) integrating laser-based processing into most aspects of solar cell fabrication, ensuring high speed and low thermal budgets ; 3) developing an all back contact cell structure compatible with thin wafers using a simplified, low-temperature fabrication process; and 4) designing the contact patterning to enable simplified module assembly. There were a number of significant achievements from this 3 year program. Regarding the front surface, we developed and applied new method to characterize critical interface recombination parameters including interface defect density Dit and hole and electron capture cross-section for use as input for 2D simulation of the IBC cell to guide design and loss analysis. We optimized the antireflection and passivation properties of the front surface texture and a-Si/a-SiN/a-SiC stack depositions to obtain a very low (< 6 mA/cm2) front surface optical losses (reflection and absorption) while maintaining excellent surface passivation (SRV<5 cm/s). We worked with kerfless wafer manufacturers to apply defect-engineering techniques to improve bulk minority-carrier lifetime of thin kerfless wafers by both reducing initial impurities during growth and developing post-growth gettering techniques. This led insights about the kinetics of nickel, chromium, and dislocations in PV-grade silicon and to achieving millisecond lifetimes in kerfless silicon materials. Laser fired contacts to n-Si were developed for the first time using a Al/Sb/Ti metal stack giving contact resistances < 5 mOhm-cm2 when fired through several different dielectric layers. A new 2 step laser+chemical etch isolation technique was developed using a sacrificial top coating which avoids laser damage to Si passivation. Regarding the heterojunction emitter, analysis of front FHJ (1D) and IBC (2D) cells with range of p-layer conditions found that a 2-stage high/low doped p-layer was optimum: the low doped region has lower defects giving higher Voc and the high doped region gave a better contact to the metal. A significant effort was spent studying the patterning process and its contribution to degradation of passivation and reproducibility. Several promising new cleaning, contact and deposition patterning and processing approaches were implemented leading to fabrication of several runs with cells having 19-20% efficiency which were stable over several months. This program resulted in the training and support of 12 graduate students, publication of 21 journal papers and 14 conference papers.

14 SOLAR ENERGY↗
DOE OSTI2021

Photometric Study and Absolute Parameter Estimation of Six Totally Eclipsing Contact Binaries

High precision CCD observations of six totally eclipsing contact binaries were presented and analyzed. It is found that only one target is an A-type contact binary (V429 Cam), while the others are W-type contact ones. By analyzing the times of light minima, we discovered that two of them exhibit secular period increase while three manifest long-term period decrease. For V1033 Her, a cyclic variation superimposed on the long-term increase was discovered. By comparing the Gaia distances with those calculated by the absolute parameters of 173 contact binaries, we found that the Gaia distance can be applied to estimate the absolute parameters for most contact binaries. The absolute parameters of our six targets were estimated using their Gaia distances. The evolutionary status of contact binaries was studied, and we found that the A- and W-subtype contact binaries may have different formation channels. The relationship between the spectroscopic and photometric mass ratios for 101 contact binaries was presented. It was discovered that the photometric mass ratios are in good agreement with the spectroscopic ones for almost all of the totally eclipsing systems, which is corresponding to the results derived by Pribulla et al. and Terrell & Wilson.

47 OTHER INSTRUMENTATION↗
NASA NTRS1981

Dynamics of solid dispersions in oil during the lubrication of point contacts. Part 1: Graphite

A Hertzian contact was lubricated with dispersed graphite in mineral oils under boundary lubrication conditions. The contact was optically observed under pure rolling, combined rolling and sliding, and pure sliding conditions. The contact was formed with a steel ball on the flat surface of a glass disk. Photomicrographs are presented which show the distribution of the graphite in and around the contact. Friction and surface damage are also shown for conditions when the base oils are used alone and when graphite is added to the base oils. Under pure rolling and combined rolling and sliding conditions, it is found that, for low speeds, a graphite film can form which will separate the contacting surfaces. Under pure sliding conditions, graphite accumulates at the inlet and sweeps around the contact, but very little of the graphite passes through the contact. The accumulated graphite appears to act as a barrier which reduces the supply of oil available to the contact for boundary lubrication. Friction data show no clear short term beneficial or detrimental effect caused by addition of graphite to the base oil. However, during pure sliding, more abrasion occurs on the polished balls lubricated with the dispersion than on those lubricated with the base oil alone. All observations were for the special case of a highly-polished ball on a glass surface and may not be applicable to other geometries and materials, or to rougher surfaces.

Cusano, C.↗
NASA NTRS1990

Influence of linear profile modification and loading conditions on the dynamic tooth load and stress of high contact ratio gears

A computer simulation for the dynamic response of high-contact-ratio spur gear transmissions is presented. High contact ratio gears have the potential to produce lower dynamic tooth loads and minimum root stress but they can be sensitive to tooth profile errors. The analysis presented examines various profile modifications under realistic loading conditions. The effect of these modifications on the dynamic load (force) between mating gear teeth and the dynamic root stress is presented. Since the contact stress is dependent on the dynamic load, minimizing dynamic loads will also minimize contact stresses. It is shown that the combination of profile modification and the applied load (torque) carried by a gear system has a significant influence on gear dynamics. The ideal modification at one value of applied load will not be the best solution for a different load. High-contact-ratio gears were found to require less modification than standard low-contact-ratio gears. High-contact-ratio gears are more adversely affected by excess modification than by under modification. In addition, the optimal profile modification required to minimize the dynamic load (hence the contact stress) on a gear tooth differs from the optimal modification required to minimize the dynamic root (bending) stress. Computer simulation can help find the design tradeoffs to determine the best profile modification to satisfy the conflicting constraints of minimizing both the load and root stress in gears which must operate over a range of applied loads.

Lee, Chinwai↗
NASA NTRS1995

Sensitivity analysis of frictional contact response of axisymmetric composite structures

A computational procedure is presented for evaluating the sensitivity coefficients of the static frictional contact response of axisymmetric composite structures. The structures are assumed to consist of an arbitrary number of perfectly bonded homogeneous anisotropic layers. The material of each layer is assumed to be hyperelastic, and the effect of geometric nonlinearity is included. The sensitivity coefficients measure the sensitivity of the response variations in different material, lamination and geometric parameters of the structure. A displacement finite element model is used for the discretization. The normal contact conditions are incorporated into the formulation by using a perturbed Lagrangian approach with the fundamental unknowns consisting of nodal displacements, and Lagrange multipliers associated with the contact conditions. The Lagrange multipliers are allowed to be discontinuous at interelement boundaries. Tangential contact conditions are incorporated by using a penalty method in conjunction with the classical Coulomb's friction model. The Newton-Raphson iterative scheme is used for the solution of the resulting nonlinear algebraic equations, and for the determination of the contact region, contact conditions (sliding or sticking), and the contact pressures. The sensitivity coefficients are evaluated by using a direct differentiation approach. Numerical results are presented for the frictional contact of a composite spherical cap pressed against a rigid plate.

Karaoglan, I.↗
NASA NTRS2000

Effects of Evaporation/Condensation on Spreading and Contact Angle of a Volatile Liquid Drop

Effects of evaporation/condensation on spreading and contact angle were experimentally studied. A sessile drop of R-113 was tested at different vapor environments to determine the effects of evaporation/condensation on the evolution of contact diameter and contact angle of the drop. Condensation on the drop surface occurs at both the saturated and a nonsaturated vapor environments and promotes the spreading. When the drop is placed in the saturated vapor environment it tends to completely wetting and spreads rapidly. In a nonsaturated vapor environment, the evolution of the sessile drop is divided three stages: condensation-spreading stage, evaporation-retracting stage and rapid contracting stage. In the first stage the drop behaves as in the saturated environment. In the evaporation -retracting stage, the competition between spreading and evaporation of the drop determines the evolution characteristics of the contact diameter and the contact angle. A lower evaporation rate struggles against the spreading power to turn the drop from spreading to retracting with a continuous increase of the contact angle. The drop placed in open air has a much higher evaporation rate. The strong evaporation suppresses the spreading and accelerates the retraction of the drop with a linear decrease of the contact diameter. The contraction of the evaporating drops is gradually accelerated when the contact diameter decreases to 3 min and less till drying up, though the evaporation rate is gradually slowing down.

Zhang, Nengli↗
NASA NTRS2001

Study of Electrical Contacts and Devices in Advanced Semiconductors

Research conducted at Tuskegee University concentrates on electrical contacts to GaN films and their characterization with the objective of understanding contact formation and realizing low-resistance metal contacts. Contact properties are known to be strongly related to surface preparation. It appears that the as-received material had a thin oxide film on the surface of the GaN film. Various cleaning treatments were employed in order to render the surface contamination free and removal of the oxide film. Metal films were then deposited by e-beam evaporation. Electrical characteristics of these contacts indicated that the optimal treatment was an organic solvent cleaning followed by etching in buffered oxide solution. Contacts established with Al were observed to be ohmic in nature, whereas Au, Cr, Ti, and Pt exhibit rectifying contacts. Platinum contacts were almost ideal as shown by an ideality factor of 1.02.

Hall, H. P.↗
NASA NTRS2004

Subsurface Stress Fields in FCC Single Crystal Anisotropic Contacts

Single crystal superalloy turbine blades used in high pressure turbomachinery are subject to conditions of high temperature, triaxial steady and alternating stresses, fretting stresses in the blade attachment and damper contact locations, and exposure to high-pressure hydrogen. The blades are also subjected to extreme variations in temperature during start-up and shutdown transients. The most prevalent high cycle fatigue (HCF) failure modes observed in these blades during operation include crystallographic crack initiation/propagation on octahedral planes, and non-crystallographic initiation with crystallographic growth. Numerous cases of crack initiation and crack propagation at the blade leading edge tip, blade attachment regions, and damper contact locations have been documented. Understanding crack initiation/propagation under mixed-mode loading conditions is critical for establishing a systematic procedure for evaluating HCF life of single crystal turbine blades. This paper presents analytical and numerical techniques for evaluating two and three dimensional subsurface stress fields in anisotropic contacts. The subsurface stress results are required for evaluating contact fatigue life at damper contacts and dovetail attachment regions in single crystal nickel-base superalloy turbine blades. An analytical procedure is presented for evaluating the subsurface stresses in the elastic half-space, based on the adaptation of a stress function method outlined by Lekhnitskii. Numerical results are presented for cylindrical and spherical anisotropic contacts, using finite element analysis (FEA). Effects of crystal orientation on stress response and fatigue life are examined. Obtaining accurate subsurface stress results for anisotropic single crystal contact problems require extremely refined three-dimensional (3-D) finite element grids, especially in the edge of contact region. Obtaining resolved shear stresses (RSS) on the principal slip planes also involves considerable post-processing work. For these reasons it is very advantageous to develop analytical solution schemes for subsurface stresses, whenever possible.

Arakere, Nagaraj K.↗