Direct Integration of GaSb with GaAs(111)A Using Interfacial Misfit Arrays
Not Available
36 MATERIALS SCIENCE↗
SEARCH · Engineering Papers
Results for “interface defects”
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.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
Nonempirical Range-Separated Hybrid Functional with Spatially Dependent Screened Exchange
Electronic structure calculations based on density functional theory (DFT) have successfully predicted numerous ground-state properties of a variety of molecules and materials. However, exchange and correlation functionals currently used in the literature, including semilocal and hybrid functionals, are often inaccurate to describe the electronic properties of heterogeneous solids, especially systems composed of building blocks with large dielectric mismatch. Here, in this study, we present a dielectric-dependent range-separated hybrid functional, screened-exchange range-separated hybrid (SE-RSH), for the investigation of heterogeneous materials. We define a spatially dependent fraction of exact exchange inspired by the static Coulomb-hole and screened-exchange (COHSEX) approximation used in many-body perturbation theory, and we show that the proposed functional accurately predicts the electronic structure of several nonmetallic interfaces, three- and two-dimensional, pristine, and defective solids and nanoparticles.
37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
BiVO 4 –Liquid Junction Photovoltaic Cell with 0.2% Solar Energy Conversion Efficiency
BiVO 4 is an important photoanode material for water oxidation, but its photoelectrochemistry regarding the triiodide/iodide redox couple is not well understood. Here, we use a combination of open circuit potential measurements, photoelectrochemical scans, and liquid surface photovoltage spectroscopy (SPS) to confirm that BiVO 4 /triiodide/iodide electrolyte contacts produce up to 0.55 V photovoltage under 23 mW/cm –2 illumination from a 470 nm LED. Inspired by these results, we construct FTO/BiVO 4 /KI(I 2 ) aq /Pt sandwich photoelectrochemical cells from electrochemically grown 0.5 × 0.5 cm 2 BiVO 4 and Mo-doped BiVO 4 films. Under AM 1.5 illumination, the devices have up to 0.22% energy conversion efficiency, 0.32 V photovoltage, and 1.8 mA cm –2 photocurrent. Based on SPS, hole transfer to iodide is sufficiently fast to prevent the competing water oxidation reaction. Mo doping increases the incident photon-to-current efficiency to up to 55% (at 425 nm under front illumination) by improving the BiVO 4 conductivity, but this comes at the expense of a lower photovoltage resulting from recombination at the Mo defects and a detrimental Schottky junction at the interface with FTO. Additional photovoltage losses are caused by the offset between the BiVO 4 valence band edge and the triiodide/iodide electrochemical potential and by electron back transfer to iodide at the FTO back contact (shunting). Overall, this work provides the first example of a BiVO 4 –liquid photovoltaic cell and an analysis of its limitations. Finally, even though the larger band gaps of metal oxides constrain their solar energy conversion efficiency, their transparency to visible light and deep valence bands makes them suitable for tandem photovoltaic devices.
14 SOLAR ENERGY↗
Vacancy-induced dislocations within grains for high-performance PbSe thermoelectrics
To minimize the lattice thermal conductivity in thermoelectrics, strategies typically focus on the scattering of low-frequency phonons by interfaces and high-frequency phonons by point defects. In addition, scattering of mid-frequency phonons by dense dislocations, localized at the grain boundaries, has been shown to reduce the lattice thermal conductivity and improve the thermoelectric performance. Here we propose a vacancy engineering strategy to create dense dislocations in the grains. In Pb 1$-$x Sb 2x/3 Se solid solutions, cation vacancies are intentionally introduced, where after thermal annealing the vacancies can annihilate through a number of mechanisms creating the desired dislocations homogeneously distributed within the grains. This leads to a lattice thermal conductivity as low as 0.4Wm -1 K -1 and a high thermoelectric figure of merit, which can be explained by a dislocation scattering model. As a result, the vacancy engineering strategy used here should be equally applicable for solid solution thermoelectrics and provides a strategy for improving zT.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Traps and transport resistance are the next frontiers for stable non-fullerene acceptor solar cells
Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.
14 SOLAR ENERGY↗
Fabricating superconducting interfaces between artificially grown LaAlO{sub 3} and SrTiO{sub 3} thin films
Realization of a fully metallic two-dimensional electron gas (2DEG) at the interface between artificially grown LaAlO{sub 3} and SrTiO{sub 3} thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially grown LaAlO{sub 3} and SrTiO{sub 3} thin films. Our results highlight the importance of two factors—the growth temperature and the SrTiO{sub 3} termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low temperature in the artificially grown LaAlO{sub 3}/SrTiO{sub 3} interface is due to the nanoscale SrO segregation occurring on the SrTiO{sub 3} film surface during the growth and the associated defects in the SrTiO{sub 3} film. By adopting an extremely high SrTiO{sub 3} growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO{sub 3} layers and artificially grown SrTiO{sub 3} thin films. This study paves the way to the realization of functional LaAlO{sub 3}/SrTiO{sub 3} superlattices and/or artificial LaAlO{sub 3}/SrTiO{sub 3} interfaces on other substrates.
36 MATERIALS SCIENCE↗
Effect of rapid thermal processing on copper precipitation in p/p{sup +} silicon epitaxial wafers with heavily boron-doped substrates
The effect of rapid thermal processing (RTP) on the formation of copper precipitation in p/p{sup +} silicon (Si) epitaxial wafers was systematically investigated by defect etching and optical microscopy. After RTP preannealing at high temperature (1250 °C/60 s, with cooling rate 30 K/s) followed by the 750 °C/8 h + 1050 °C/16 h low-high (L-H) two-step annealing, it was revealed that the bulk microdefects were found only inside the p{sup +} substrate, manifesting no defects generated in the epitaxial layer. However, it was found that the width of denude zone (DZ) in samples only subjected to L-H two-step annealing was narrower than that of epitaxial layer, which meant that oxygen precipitation was formed in epitaxial layer. It can be concluded that RTP was beneficial to the formation of DZ. Additionally, it was found that the width of DZ has a sharp dependence on the introducing temperature of copper contamination, that is, the corresponding equilibrium concentration of interstitial copper in the Si influence the thermodynamics and kinetics process of the formation of copper precipitation significantly.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Measurement of bandgap energies in low-k organosilicates
In this work, experimental measurements of the electronic band gap of low-k organosilicate dielectrics will be presented and discussed. The measurement of bandgap energies of organosilicates will be made by examining the onset of inelastic energy loss in core-level atomic spectra using X-ray photoelectron spectroscopy. This energy serves as a reference point from which many other facets of the material can be understood, such as the location and presence of defect states in the bulk or at the interface. A comparison with other measurement techniques reported in the literature is presented.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Magnetostrictive Fe{sub 73}Ga{sub 27} nanocontacts for low-field conductance switching
The electrical conductance G of magnetostrictive nanocontacts made from Galfenol (Fe{sub 73}Ga{sub 27}) can be reproducibly switched between “on” and “off” states in a low magnetic field of ∼20–30 mT at 10 K. The switching behavior is in agreement with the magnetic field dependence of the magnetostriction inferred from the magnetization behavior, causing a positive magnetostrictive strain along the magnetic field. The repeated magnetic-field cycling leads to a stable contact geometry and to a robust contact configuration with a very low hysteresis of ∼1 mT between opening and closing the contact due to a training effect. Non-integral multiples of the conductance quantum G{sub 0} observed for G > G{sub 0} are attributed to electron backscattering at defect sites in the electrodes near the contact interface. When the contact is closed either mechanically or by magnetic field, the conductance shows an exponential behavior below G{sub 0} due to electron tunneling. This allows to estimate the magnetostriction λ = 4 × 10{sup −5} at 10 K. The results demonstrate that such magnetostrictive devices are suitable for the remote control of the conductance by low magnetic fields in future nanotechnology applications.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Recombination and bandgap engineering in CdSeTe/CdTe solar cells
Selenium compositional grading in CdTe-based thin-film solar cells substantively improves carrier lifetime and performance. However, where and how recombination lifetime improves has not been studied significantly. Here, we deposit a CdSe x Te 1-x /CdTe bilayer on MgZnO/SnO 2 /glass, which achieves a short-circuit current density greater than 28 mA/cm 2 and carrier lifetimes as long as 10-20 ns. We analyze the grain structure, composition, and recombination through the thickness of the absorber using electron backscatter diffraction, Auger-electron spectroscopy, cathodoluminescence spectrum imaging, and time-resolved photoluminescence microscopy. Despite small CdSeTe grains near the pn-junction and significantly larger CdTe grains in the rest of the film, both time-resolved photoluminescence and cathodoluminescence reveal that the carrier lifetime in CdSeTe alloy regions is longer than in CdTe regions. The results indicate that Se both passivates grain boundaries and improves grain-interior carrier lifetime. However, these effects occur only where there is significant alloying, which is important for bandgap engineering.
14 SOLAR ENERGY↗
Linear Complexions: Metastable Phase Formation and Coexistence at Dislocations
The unique three-phase coexistence of metastable B2–FeNi with stable L1 0 –FeNi and L1 2 –FeNi 3 is discovered near edge dislocations in body-centered cubic Fe-Ni alloys using atomistic simulations. Stable nanoscale precipitate arrays, formed along the compression side of dislocation lines and defined as linear complexions, were observed for a wide range of compositions and temperatures. Here, by analyzing the thermodynamics associated with these phase transitions, we are able to explain the metastable phase formation and coexistence, in the process defining new research avenues for theoretical and experimental investigations.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Maximizing the external radiative efficiency of hybrid perovskite solar cells
Despite rapid advancements in power conversion efficiency in the last decade, perovskite solar cells still perform below their thermodynamic efficiency limits. Non-radiative recombination, in particular, has limited the external radiative efficiency and open circuit voltage in the highest performing devices. We review the historical progress in enhancing perovskite external radiative efficiency and determine key strategies for reaching high optoelectronic quality. Specifically, we focus on non-radiative recombination within the perovskite layer and highlight novel approaches to reduce energy losses at interfaces and through parasitic absorption. By strategically targeting defects, it is likely that the next set of record-performing devices with ultra-low voltage losses will be achieved.
14 SOLAR ENERGY↗
Prediction of Thermal Transport Properties of Materials with Microstructural Complexity
This project aims at overcoming the major obstacle standing in the way of progress in dynamic multiscale simulation, which is the lack of a concurrent atomistic-continuum method that allows phonons, heat and defects to pass through the atomistic-continuum interface. The research has led to the development of a concurrent atomistic-continuum (CAC) methodology for multiscale simulations of materials microstructural, mechanical and thermal transport behavior. Its efficacy has been tested and demonstrated through simulations of dislocation dynamics and phonon transport coupled with microstructural evolution in a variety of materials and through providing visual evidences of the nature of phonon transport, such as showing the propagation of heat pulses in single and polycrystalline solids is partially ballistic and partially diffusive. In addition to providing understanding on phonon scattering with phase interface and with grain boundaries, the research has contributed a multiscale simulation tool for understanding of the behavior of complex materials and has demonstrated the capability of the tool in simulating the dynamic, in situ experimental studies of nonequilibrium transient transport processes in material samples that are at length scales typically inaccessible by atomistically resolved methods.
36 MATERIALS SCIENCE↗
Development of Corrosion Resistant Coatings for Structural Materials for Liquid Fueled Molten Salts Reactors Applications (Final Report)
Current structural alloys, code certified for the MSRs temperature ranges, contain high levels of chromium, making them highly susceptible to molten halide salt corrosion. One potential solution to circumvent the need for code certification of novel alloys, which is expensive and time consuming, is to design claddings that will protect the underlying code certified materials from corrosion damage during operation. In this work, we examined the corrosion of Ni and Cu electroplated, Ni and CuNi weld overlay, Mo-laser clad, and carburized claddings on SS316H for use in molten salt reactor environments. After characterization of the cladded materials (Task 1), static corrosion tests were used to assess corrosion resistance of the claddings in typical molten fluoride salts (Task 2). The corrosion tests were performed in molten FLiNaK at 700°C up to 1000 hours. Pre- and post-corrosion Scanning Electron Microscopy (SEM), energy-dispersive-spectroscopy (EDS), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD) and glow-discharge-optical-emission-spectroscopy (GDOES) were performed on cladding cross-sections and surfaces to evaluate degradation. The Cu and Ni electroplated samples, as well as the carburized samples, showed excellent corrosion resistance relative to the bare SS316H. To assess high temperature cladding stability, ageing experiments were performed at temperatures up to 900 C in inert atmosphere for the electroplated and carburized samples and a diffusion model was developed to predict long term cladding behavior (Task 3). It was found that the Cu cladding was basically insensitive to the high temperature ageing, except for small secondary phases forming at the interface. On the other hand, the Ni electroplated cladding experienced significant interdiffusion. Nevertheless, the gain in corrosion resistance for a 100μm Ni electroplated cladding is phenomenal, with more than 50% reduction in chromium dissolution from the substrate material for the first 15 years of salt exposure at 700°C. To assess radiation resistance and phase stability, high-temperature 4MeV Ni heavy ion irradiation was performed across the cladding/substrate interface up to 50 displacement-per-atom (DPA) at 500°C and 700°C to assess the phase stability and irradiation behaviors of the electroplated systems (Task 4). The Ni and Cu electroplated systems did not experience void swelling at the contrary to the SS316H substrate due to their nanocrystalline nature. Some level of recrystallization was observed in the cladding, as well as radiation induced segregation and enhanced diffusion. The interface acted as a potent sink for point defects with the presence of a void denuded zone. The mechanical properties of the claddings were assessed using thermal shock, micro-indentation, nano-indentation, and four-point bend testing experiments (Task 5). These experiments were performed on the claddings in as-received, corroded, irradiated, and thermally aged states to determine the effects of typical molten salt reactor environments on cladding integrity. No significant mass loss was observed after repeated thermal shocks. While the electroplated samples softened after high-temperature ageing, irradiation hardening compensate this effect, such that there is little different with the as-received materials. The results of these experiments suggest that the electroplated (copper/nickel) show the greatest promise for application in molten salt reactor development. These claddings prevented any chromium dissolution from occurring during the static corrosion experiments. Additionally, they demonstrated favorable properties for high temperature diffusion, phase stability, interfacial mechanical properties, and irradiation resistance. Weld-overlay cladding are also of great interest and could reach properties similar to the electroplated samples upon optimization (especially with multiple weld passes). The carburized SS316H showed excellent behavior as well, but more work needs to be done to assess their long-term stability. Finally, the Mo-laser clad system was not pursued further due to the manufacturing challenges. It is also worth noting that the Ni cladding systems should behave relatively well in terms of weldability since the Ni-weld overlay microstructure and associated corrosion rate are sound. While the NiCu weld-overlay has even lower corrosion rates than the Ni-weld overlay, more studies on welding of Cu-electroplated systems are necessary since Cu clusters are known to embrittle steels.
36 MATERIALS SCIENCE↗
Atomic scale analysis of Nb3Sn on Nb prepared by a vapor-diffusion process for superconducting radiofrequency cavity applications
We report an atomic-scale analysis of the microstructure of Nb3Sn coating on Nb prepared by vapor diffusion process for superconducting radiofrequency (SRF) cavity application using transmission electron microscopy (TEM). Epitaxial growth of Nb3Sn on the Nb substrate is found and four types of orientation relationships at the Nb3Sn/Nb interface are identified by electron diffraction or high-resolution scanning transmission electron microscopy (STEM) analysis. Thin Nb3Sn grains are observed in regions with low Sn flux and they have the specific orientation relationship, Nb3Sn (1-20)//Nb (-111) and Nb3Sn (002)//Nb (0-11). The Nb3Sn/Nb interface of thin grains had a large lattice mismatch, 12.3 at.%, and a high density of misfit dislocations was observed by HR-STEM. Based on our microstructural analysis of the thin grains, we conclude that the thin regions are probably a result of a slow interfacial reaction with this particular orientation relationship at the interface. The Sn-deficient regions are seen to form initially at the Nb3Sn/Nb interface and remain in the grains due to the slow diffusion of Sn in bulk Nb3Sn. The formation of Sn-deficient regions and the effects of strain and interfacial energies on the formation of Sn-deficient regions at various interfaces were also estimated by first-principle calculation. The finding of orientation relationships at the Nb3Sn/Nb interface provides important information about the formation of defects in Nb3Sn coatings such as large thin regions, Sn-deficient regions, which are critical to the performance of Nb3Sn superconducting radiofrequency cavities for accelerators.
43 PARTICLE ACCELERATORS↗
Understanding the Effect of Lead Iodide Excess on the Performance of Methylammonium Lead Iodide Perovskite Solar Cells
The presence of unreacted lead iodide in organic-inorganic lead halide perovskite solar cells is widely correlated with an increase in power conversion efficiency. We investigate the mechanism for this increase by identifying the role of surfaces and interfaces present between methylammonium lead iodide perovskite films and excess lead iodide. We show how type I and II band alignments arising under different conditions result in either passivation of surface defects or hole injection. Through first-principles simulations of solid-solid interfaces, we find that lead iodide captures holes from methylammonium lead iodide and modulates the formation of defects in the perovskite, affecting recombination. Using surface-sensitive optical spectroscopy techniques, such as transient reflectance and time-resolved photoluminescence, we show how excess lead iodide affects the diffusion and surface recombination velocity of charge carriers in methylammonium lead iodide films. Our coupled experimental and theoretical results elucidate the role of excess lead iodide in perovskite solar cells.
14 SOLAR ENERGY↗
Extended Electronic Structure Inhomogeneity Created by Double Chain Layer Defects Surrounding Columnar Tracks in Heavy-Ion Irradiated YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}.
In YBa2Cu3O7-delta (YBCO), heavy-ion irradiation creates continuous amorphous tracks that are highly effective for vortex pinning. However, the electronic structure landscape of defects and consequently their vortex pinning roles are unclear. Here, we show double chain layer (DCL) defects which intersect the columnar tracks are additionally produced by high energy Pb ion irradiation in YBCO. The DCL defects are similar to 29 nm long, about three times the columnar defect diameter. The electronic structures of the DCL and columnar track defects were determined using atomic-resolution scanning transmission electron microscopy (STEM) and high resolution electron energy loss spectroscopy (EELS). Results show a decrease in the oxygen and copper content at the interface between the track and the YBCO matrix, resulting in interfacial strain. For the DCL defects, the STEM/EELS study revealed a localized electron doped CuO2 plane next to the DCL defect. The DCL defects thus further extend the electronic inhomogeneity into YBCO.
critical current↗
Insights into ultraviolet-induced electrical degradation of thermally grown SiO{sub 2}/4H-SiC(0001) interface
The harmful impact of ultraviolet (UV) light irradiation on thermally grown SiO{sub 2}/4H-SiC structures was investigated by means of electrical measurements of metal-oxide-semiconductor (MOS) capacitors. Unlike Si-based MOS devices, significant electrical degradation, such as positive flatband voltage (V{sub FB}) shift and hysteresis in the capacitance-voltage (C-V) curves of SiC-MOS capacitors was induced by UV irradiation with a low-pressure mercury lamp. The interfacial fixed charge density increased with UV-irradiation (22.6 mW/cm{sup 2} for 16 h) to 1.7 × 10{sup 12} cm{sup −2}, which was an order of magnitude larger than that of the as-grown SiO{sub 2}/SiC interface. A detailed study based on single wavelength solid-state UV lasers revealed that there was a threshold photon energy at around 5 eV and a moderate dependence of UV-induced degradation on temperature. These experimental findings imply that pre-existing inactive defects accumulated at the thermally grown SiO{sub 2}/SiC interface were transformed to active carrier traps with high-energy UV irradiation through transparent SiO{sub 2} layers.
36 MATERIALS SCIENCE↗