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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Improved Vertical Carrier Transport for Green III-Nitride LEDs Using ( In , Ga ) N Alloy Quantum Barriers

We report on experimental and simulation-based results using ( In , Ga ) N alloy quantum barriers in c -plane green light-emitting diode (LED) structures as a means to improve vertical carrier transport and reduce forward voltage ( V F ) . Three-dimensional device simulations that include random alloy fluctuations are used to understand carrier behavior in a disordered potential. The simulated current density–voltage ( J - V ) characteristics and modified electron-hole overlap | F mod | 2 indicate that increasing the indium fraction in the ( In , Ga ) N quantum barriers leads to a reduced polarization discontinuity at the interface between the quantum barrier and quantum well, thereby reducing V F and improving | F mod | 2 . Maps of electron and hole current through the device show a relatively homogenous distribution in the X Y plane for structures using Ga N quantum barriers; in contrast, preferential pathways for vertical transport are identified in structures with ( In , Ga ) N barriers as regions of high and low current. A positive correlation between hole (electron) current in the p -side ( n -side) barrier and indium fraction reveals that preferential pathways exist in regions of high indium content. Furthermore, a negative correlation between the strain ε z z and indium fraction shows that high indium content regions have reduced strain-induced piezoelectric polarization in the Z direction due to the mechanical constraint of the surrounding lower indium content regions. Experimentally, multiple quantum well green LEDs with ( In , Ga ) N quantum barriers exhibit lower V F and blue-shifted wavelengths relative to LEDs with Ga N quantum barriers, consistent with simulation data. These results can be used to inform heterostructure design of low V F , long-wavelength LEDs and provide important insight into the nature of carrier transport in III-nitride alloy materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Switching of the electron-phonon interaction in 1 T –VSe 2 assisted by hot carriers

We apply an intense infrared laser pulse in order to perturb the electronic and vibrational states in the three-dimensional charge density wave material 1T-VSe 2 . Ultrafast snapshots of the light-induced hot carrier dynamics and nonequilibrium quasiparticle spectral function are collected using time- and angle-resolved photoemission spectroscopy. Furthermore, the hot carrier temperature and time-dependent electronic self-energy are extracted from the time-dependent spectral function, revealing that incoherent electron-phonon interactions heat the lattice above the charge density wave critical temperature on a timescale of (200 ± 40) fs. Density functional perturbation theory calculations establish that the presence of hot carriers alters the overall phonon dispersion and quenches efficient low-energy acoustic phonon scattering channels, which results in a new quasiequilibrium state that is experimentally observed.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2023

Ab initio calculation of carrier mobility in semiconductors including ionized-impurity scattering

The past decade has seen the emergence of ab initio computational methods for calculating phonon-limited carrier mobilities in semiconductors with predictive accuracy. More realistic calculations ought to take into account additional scattering mechanisms such as, for example, impurity and grain-boundary scattering. Here, in this paper, we investigate the effect of ionized-impurity scattering on the carrier mobility. We model the analytical impurity potential parameterized from first principles by a collection of randomly distributed Coulomb scattering centers, and we include this relaxation channel into the ab initio Boltzmann transport equation, as implemented in the EPW code. We demonstrate this methodology by considering silicon, silicon carbide, and gallium phosphide, for which detailed experimental data are available. Our calculations agree well with experiments over a broad range of temperatures and impurity concentrations. For each compound investigated here, we compare the relative importance of electron-phonon scattering and ionized-impurity scattering, and we critically assess the reliability of Matthiessen's rule. We also show that an accurate description of dielectric screening and carrier effective masses can improve quantitative agreement with experiments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Intrinsic Limits of Charge Carrier Mobilities in Layered Halide Perovskites

Layered halide perovskites have emerged as potential alternatives to three-dimensional (3D) halide perovskites due to their improved stability and larger material phase space, allowing fine tuning of structural, electronic, and optical properties. However, their charge carrier mobilities are significantly smaller than those of 3D halide perovskites, which has a considerable impact on their application in optoelectronic devices. Here, we employ state-of-the-art approaches to unveil the electron-phonon mechanisms responsible for the diminished transport properties of layered halide perovskites. Starting from a prototypical A M X 3 halide perovskite, we model the case of n = 1 and n = 2 layered structures and compare their electronic and transport properties to the 3D reference. The electronic and phononic properties are investigated within density functional theory (DFT) and density functional perturbation theory (DFPT), while transport properties are obtained via the Boltzmann transport equation. The vibrational modes contributing to charge carrier scattering are investigated and associated with polar-phonon scattering mechanisms arising from the long-range Fröhlich coupling and deformation-potential scattering processes. Our investigation reveals that the lower mobilities in layered systems primarily originate from the increased electronic density of states at the vicinity of the band edges, while the electron-phonon coupling strength remains similar. Such an increase is caused by the dimensionality reduction and the break in octahedra connectivity along the stacking direction. Our findings provide a fundamental understanding of the electron-phonon coupling mechanisms in layered perovskites and highlight the intrinsic limitations of the charge carrier transport in these materials. Published by the American Physical Society 2024

Cucco, Bruno (ORCID:0000000331564143)↗
DOE OSTI2024

Assessing Carrier Mobility, Dopability, and Defect Tolerance in the Chalcogenide Perovskite Ba Zr S 3

The chalcogenide perovskite Ba Zr S 3 has attracted much attention as a promising solar absorber for thin-film photovoltaics. Here we use first-principles calculations to evaluate its carrier transport and defect properties. We find that Ba Zr S 3 has a phonon-limited electron mobility of 37 cm 2 / V s, which is comparable to that in halide perovskites, but lower hole mobility of 11 cm 2 / V s. The defect computations indicate that Ba Zr S 3 is intrinsically n -type due to shallow sulfur vacancies, but that strong compensation by sulfur vacancies will prevent attempts to make it p -type. We also establish that Ba Zr S 3 shows some degree of defect tolerance, presenting only few low formation energy, deep intrinsic defects. Among the deep defects, sulfur interstitials are the dominant nonradiative recombination centers but exhibit a moderate capture coefficient. Our work highlights the material’s intrinsic limitations in carrier mobility and p -type doping, and suggests focusing on suppressing the formation of sulfur interstitials to achieve longer carrier lifetime. Published by the American Physical Society 2024

Yuan, Zhenkun (ORCID:0000000272005973)↗
DOE OSTI2024

Picosecond carrier dynamics in InAs and GaAs revealed by ultrafast electron microscopy

Understanding the limits of spatiotemporal carrier dynamics, especially in III-V semiconductors, is key to designing ultrafast and ultrasmall optoelectronic components. However, identifying such limits and the properties controlling them has been elusive. Here, using scanning ultrafast electron microscopy, in bulk n-GaAs and p-InAs, we simultaneously measure picosecond carrier dynamics along with three related quantities: subsurface band bending, above-surface vacuum potentials, and surface trap densities. We make two unexpected observations. First, we uncover a negative-time contrast in secondary electrons resulting from an interplay among these quantities. Second, despite dopant concentrations and surface state densities differing by many orders of magnitude between the two materials, their carrier dynamics, measured by photoexcited band bending and filling of surface states, occur at a seemingly common timescale of about 100 ps. This observation may indicate fundamental kinetic limits tied to a multitude of material and surface properties of optoelectronic III-V semiconductors and highlights the need for techniques that simultaneously measure electro-optical kinetic properties.

42 ENGINEERING↗
DOE OSTI2020

High-sensitivity telecommunication-compatible photoconductive terahertz detection through carrier transit time reduction

We present a telecommunication-compatible photoconductive terahertz detector realized without using any short-carrier-lifetime photoconductor. By utilizing plasmonic contact electrodes on a thin layer of high-mobility photoconductor, the presented detector offers a short transit time for the majority of the photocarriers in the absence of a short-carrier-lifetime photoconductor. Consequently, high-sensitivity terahertz detection is achieved with a record-high signal-to-noise ratio of 122 dB over a 3.6 THz bandwidth under an optical probe power of 10 mW. To achieve such a high sensitivity, the device geometry is chosen to maintain a high resistance and low Johnson Nyquist noise. This design approach can be widely applied for terahertz detection using various semiconductors and optical wavelengths, without being limited by the availability of short-carrier-lifetime photoconductors.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Investigating Local Carrier Dynamics in PERC Patterned CdTe Solar Cells

Remarkable progress has been achieved in CdTe photovoltaics (PVs) to further improve cell performance while reducing manufacturing costs. Researchers optimized the front contact that leads the short-circuit current density (J sc ) over 31mA/cm 2 (e.g., (Zn, Mg)O buffer layer and/or alloying CdTe with Se). Reese et al. reported an open-circuit voltage (V oc ) over 1 V with group-V doped single crystal, showing the feasibility of increasing V oc in CdTe PV that is fixed to less than 900 mV in most cases. Recent studies suggested that the V oc improvement in polycrystalline CdTe PVs requires well-passivated back contact. One possible strategy is to use CdMgTe alloy (E g > 1.8 eV), where the band-gap offset (ΔE CB ≈ 0.2 eV) reflects minority carrier electrons, thereby decreasing surface recombination. Another strategy is to utilize a stable Al 2 O 3 layer as an electron reflector; experimental works confirmed the improved performance with a conformal Al 2 O 3 layer. In this configuration, the precise control of Al 2 O 3 (≈1 nm) over the entire CdTe layer is essential because the J sc greatly depends on the tunneling current. While promising, both CdMgTe and Al 2 O 3 passivation on CdTe introduce unfavorable valence band offset that blocks the hole transport. Kephart et al. proposed a patterned passixc vation layer (≈ 20 nm Al 2 O 3 ) with point contacts (≈ 3 μm in diameter) that extract hole carriers. This configuration is similar to the passive emitter and rear contact (PERC) design that has been extensively studied in Si PVs. While remarkably high carrier lifetime and photoluminescence (> 10 X) were measured, consistent improvement in V oc and efficiency in this CdTe PERC were not yet observed. At this time, it is unclear how the surface potential is distributed in the presence of patterned Al 2 O 3 reflectors and how this additional field impacts the PV performance for Cu-doped and group-V doped CdTe PERC devices. To tackle this challenge, we develop robust nanofabrication and characterization platform to study CdTe PERC devices consisting of a patterned Al 2 O 3 layer on CdSe( 1-x )Te x (Cu-doped, GrV-doped; x = 0 to 1; CST). First Solar supplies high-quality CST materials. The dark and light I-Vs under 1-sun illumination are extracted for a series of each set of devices. We perform quantitative and qualitative PV analysis for a series of Cu-doped and Gr-V doped PERC devices, showing notably different V oc changes for GrV-doped compared to Cu-doped PERC devices. This project attempts to produce individual contacts on single grains and grain boundaries to measure lateral transport. As a model system, we use a CdSe( 1-x )Te x film on glass prepared by a colossal grain growth (CGG) technique at National Renewable Research Laboratories (NREL). Our preliminary results confirm that intergranular transport is hindered by the electrical barrier formed near grain boundaries. The PERC fabrication and the lateral transport measurement platform developed in this project can easily be applied to other types of advanced CdSe( 1-x )Te x architectures to better understand local photocarrier transport, in turn, providing the fundamental knowledge to improve the performance of CdTe PVs.

14 SOLAR ENERGY↗
DOE OSTI2024

Microwave and rf Spectroscopy of 2D Carrier Systems and Bilayers (Final Technical Report)

We performed microwave spectroscopic studies of two dimensional electron systems (2DES) exhibiting the quantum Hall effect (QHE) in GaAs/Al${_x}$Ga$_{1-x}$As heterostructures. A common theme of the proposed work is electron solids, of which the simplest is the Wigner crystal, a triangular lattice of individual carriers. The solids are stabilized by carrier-carrier interaction and occur under conditions when the kinetic energy of the system is "frozen out" by high magnetic field, $B$. One of the main reasons for microwave spectroscopic study of 2DES is that the solid phases have striking microwave or rf spectra. In the low wave vector ($q$) limit these spectra exhibit a resonance that is understood as a pinning mode, a small collective oscillation of pieces of the solid within the disorder potential. Pinning modes can be very sharp, with quality factors up to around 30, and are valuable as signatures of the electron solids, and can reveal a solid even when standard dc transport shows an insulator, an integer QHE, or even a fractional QHE The pinning mode also is sensitive to the solid properties, and can reveal transitions between solids.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Picosecond Carrier Dynamics in InAs and GaAs Revealed by Ultrafast Electron Microscopy

Understanding the limits of spatio-temporal carrier dynamics, especially in III-V semiconductors, is key to designing ultrafast and ultra-small optoelectronic components. However, identifying such limits and the properties controlling them has been elusive. Here, using scanning ultrafast electron microscopy (SUEM), in bulk n-GaAs and p-InAs, we simultaneously measure picosecond carrier dynamics along with three related quantities: sub-surface band bending, above-surface vacuum potentials, and surface trap densities. We make two surprising observations. First, we uncover a negative-time contrast in secondary electrons resulting from an interplay among these quantities. Second, despite dopant concentrations and surface state densities differing by many orders of magnitude between the two materials, their carrier dynamics, measured by photo-excited band bending and filling of surface states, occur at a seemingly common timescale of about 100 ps. This observation may indicate fundamental kinetic limits tied to a multitude of material and surface properties of optoelectronic III-V semiconductors, and highlights the need for techniques that simultaneously measure electro-optical kinetic properties.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Commercial Building HVAC Technology Challenge - Carrier (Final Report)

The purpose of this Cooperative Research and Development Agreement (CRADA) was to test two rooftop commercial heating, ventilation, and air conditioning (HVAC) systems supplied by Carrier Corporation to determine if they met the laboratory test requirements of the United States Department of Energy (DOE) Commercial Building HVAC Technology Challenge (hereafter noted as “the Challenge”). Results of the laboratory testing completed at Oak Ridge National Laboratory (ORNL) demonstrated that the systems met all requirements of the Challenge. Carrier Corporation has successfully completed this aspect of the Challenge. This CRADA report will briefly describe the Challenge, the test plan created to measure the success of systems provided by Challenge participants, and the results of laboratory testing of the Carrier commercial rooftop systems.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2021

Door assembly with collapsible carrier and water shield

A door assembly for a motor vehicle comprised of an outer door panel and an inner door panel interconnected to define a door panel structure having an internal door cavity. A collapsible carrier module is configured to be oriented in a collapsed arrangement to facilitate installation into the door cavity through an opening in the inner door panel. The collapsible carrier module can be moved from its collapsed arrangement into an expanded arrangement upon installation within the door cavity to facilitate subsequent mounting to the door panel structure. The collapsible carrier module includes a barrier configured to close off the opening in the inner panel.

Fortin, Raymond Edward↗
DOE OSTI2020

Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe 2 with Femtosecond Core-Level Spectroscopy

In this study, we employ few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to reveal simultaneously the intra- and interband carrier relaxation and the light-induced structural dynamics in nanoscale thin films of layered 2H-MoTe 2 semiconductor. By interrogating the valence electronic structure via localized Te 4 d (39-46 eV) and Mo 4 p (35-38 eV) core levels, the relaxation of the photoexcited hole distribution is directly observed in real time. We obtain hole thermalization and cooling times of 15 ± 5 fs and 380 ± 90 fs, respectively, and an electron-hole recombination time of 1.5 ± 0.1 ps. Furthermore, excitations of coherent out-of-plane A 1g (5.1 THz) and in-plane E 1g (3.7 THz) lattice vibrations are visualized through oscillations in the XUV absorption spectra. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A 1g coordinate. By directly and simultaneously probing the excited carrier distribution dynamics and accompanying femtosecond lattice displacement in 2H-MoTe 2 within a single experiment, our work provides a benchmark for understanding the interplay between electronic and structural dynamics in photoexcited nanomaterials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Resource Allocation for Single Carrier Massive MIMO Systems

Resource allocation in orthogonal frequency division multiplexing (OFDM) systems is performed through allocating blocks of subcarriers to each user. Even though OFDM is the primary waveform for 5G NR systems, research reports have noted that single carrier modulation (SCM) offers several advantages over OFDM in massive multiple input multiple output (MIMO) systems, making it a preferred candidate for some future applications such as massive machine type communications (mMTC). This paper presents a method for SCM resource allocation and the relevant information recovery algorithms at the receiver. Our emphasis is on cyclic prefixed SCM, where highly flexible and efficient frequency domain detection algorithms enable the operation of many simultaneous users in a massive MIMO uplink scenario. The proposed resource allocation method allows the number of users to exceed the number of antennas at the base station (BS). Each single carrier transmission is partitioned into L interleaved streams, and each user is allocated a number of such streams. One major benefit of SCM is that each data symbol is spread over the entire bandwidth. As such, the receiver performance is dictated by the average channel gain across the transmission band rather than the channel gain at a given frequency bin or a small group of frequencies. In the proposed setup, each stream may be thought of as a resource block in SCM, analogous to resource blocks in OFDM. Hence, in the context of this paper, the terms resource blocks and streams may be used interchangeably.

5G and Beyond Communications↗
DOE OSTI2022

Frequency Domain Detection and Precoding for Massive MIMO With Single Carrier Modulation

Single carrier modulation (SCM) schemes are attractive for uplink (UL) transmissions due to improved power efficiency at the user equipment (UE) transmitter compared with multi-carrier modulation schemes. In a massive MIMO scenario with SCM, the UL detection must mitigate the effects of inter-symbol interference and multiuser interference. This processing is effectively performed in the frequency domain (FD) using a minimum mean squared error (MMSE) detector when the transmission is framed with a cyclic prefix. This paper presents a MMSEbased detector that is compatible with distributed processing in a time-division duplex (TDD) system. The matrix inverses computed for the UL detection are then reused to perform multi-user precoding for the downlink (DL). We find that this scheme yields a tremendous savings in computational complexity compared to commonly used zero-forcing (ZF) precoding without sacrificing any performance. Since MMSE processing introduces a bias to the estimates, we then derive the scalar coefficients necessary to cancel the MMSE bias. The impact of channel estimation errors are analyzed for both the UL and DL cases in conjunction with a power-efficient approach to SCM channel estimation. Moreover, extensive simulations are performed to confirm our theoretical findings.

42 ENGINEERING↗
DOE OSTI2024

Roles of Solvent in the Catalytic Hydrogen Release from Liquid Organic Hydrogen Carriers: Chemical, Thermodynamical and Technological Aspects

A Liquid Organic Hydrogen Carrier (LOHC) enables the storage and transport of hydrogen at ambient pressures and temperatures in a safe and convenient form using current infrastructure. However, it is challenging to directly compare reactivity and selectivity for hydrogen release, especially when comparing the catalytic efficiencies of neat LOHCs to highly diluted LOHCs in different solvents, reaction conditions, and catalysts. This work evaluates the role of solvents in catalysis and quantifies the energy efficiency of the overall process. The presence of solvent dilutes the volumetric density of available hydrogen, but may be necessary to achieve optimal catalysts stability, reactivity, and product selectivity. With respect to the reaction conditions as determined by thermodynamics, solvents with higher vapor pressures than that of the carrier can cause the erroneous impression of a more favorable reaction equilibrium. Concerning energy efficiency, solvents can result in increased energy demand for hydrogen release as the inert solvent must be heated to reaction temperatures required for release of H 2 from the LOHC. Further, this work recommends that investigations of catalyst reactivity should be carried out at different ratios of solvent to LOHC to understand how the reactivity changes and what the implications are for maximizing energy density and catalyst stability and reactivity. Investigations should also consider how these implications will affect the technical needs of applications intended for the LOHC system. Based on the results of this study, it is advised to focus research activities on LOHC systems with a gravimetric solvent content below about 50% as the thermodynamic disadvantages become very pronounced beyond this threshold.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Silica-encapsulated Fe 2 O 3 oxygen carriers for selective chemical looping combustion of hydrogen

Methane dehydroaromatization (MDA) combined with chemical looping (CL) combustion of hydrogen can overcome the MDA thermodynamic constraints conventionally limiting conversion. Development of selective hydrogen-oxidation oxygen carriers in the presence of aromatics in the CL is key to improve MDA performance in the coupled reaction system. Here, in this paper, we show that a novel oxygen carrier (silica-encapsulated Fe 2 O 3 with Fe/Si mole ratio of 1.2) yields high overall methane conversion (49%) and aromatics yield (up to 36%) during 4 h of recirculating a stream in the developed system. A silica layer encapsulating Fe 2 O 3 favors hydrogen transport and oxidation, preventing aromatics diffusing to the Fe 2 O 3 surface. A close physical contact between iron oxide particles and silica, and near complete encapsulation of the particles by the silica layer can facilitate hydrogen conversion without full depletion of iron oxides by forming iron silicates. The silica-encapsulated iron oxide particles can be reoxidized in air after depletion of its oxidative capacity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Spin-related phenomena in spin 3/2 charge carrier holes systems

Charge carrier holes provide a remarkable system for spintronics and quantum information technology. In this review paper, I discuss spin-related phenomena in three-dimensional and low-dimensional hole systems. Special attention is paid to the mutual transformation of heavy and light holes at the boundary of quantum wells and wires that governs values of parameters defining hole spectra in quantum wells, wires and dots, such as effective masses, g-factors and Rashba and Dresselhaus spin–orbit constants. Recently, topological phenomena in condensed matter systems, such as emergence of Majorana zero modes and non-Abelian phases in the fractional quantum Hall effect, sparked considerable interest of researchers. Charge carrier holes turn out to be a remarkable setting for possible observation of these phenomena and advancing topological quantum computing. I discuss the spectra and wavefunctions of two-dimensional holes in magnetic field. While there is a semiclassical range of parameters when heavy and light holes can be described by equidistant Landau levels, ground-level holes and holes in a few low-lying excited states behave as species completely different from electrons. Especially interesting are crossings in hole spectra in magnetic field. Hole–hole interactions can substantially differ from electron–electron interactions. Apart from the difference in exchange splitting, this shows in possible emergence of even denominator fractional quantum Hall state in the ground hole level in magnetic field. I also briefly discuss spintronic phenomena, such as mutual transformation of angular momentum (spin) of holes and electric current, as well as spin-related interference effects in hole transport. Recent developments in a system of Ge hole quantum dots offer new perspectives for hole-based systems.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗