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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 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Local electric field measurement in GaN diodes by exciton Franz–Keldysh photocurrent spectroscopy

The eXciton Franz–Keldysh (XFK) effect is observed in GaN p–n junction diodes via the spectral variation of photocurrent responsivity data that redshift and broaden with increasing reverse bias. Photocurrent spectra are quantitatively fit over a broad photon energy range to an XFK model using only a single fit parameter that determines the line shape and the local bias (V l ), uniquely determining the local electric field maximum and depletion widths. As expected, the spectrally determined values of V l vary linearly with the applied bias (V) and reveal a large reduction in the local electric field due to electrostatic non-uniformity. The built-in bias (V bi ) is estimated by extrapolating V l at V=0, which, when compared with independent C-V measurements, indicates an overall ±0.31 V accuracy of V l . This demonstrates sub-bandgap photocurrent spectroscopy as a local probe of electric field in wide bandgap diodes that can be used to map out regions of device breakdown (hot spots) for improving electrostatic design of high-voltage devices.

42 ENGINEERING↗
DOE OSTI2020

Carrier recombination and diffusion in high-purity diamond after electron irradiation and annealing

Carrier-transport mechanisms are studied in high-purity diamond irradiated with 6 MeV electrons in the dose range of 10 12 –10 16 cm -2 and annealed at different temperatures up to 1450°C. Lifetimes and diffusion coefficients are extracted using two pump–probe techniques based on free-carrier absorption and transient-grating principles and then correlated with the corresponding defect evolution from spectroscopic measurements. Furthermore, the neutral monovacancy is revealed as the main carrier recombination center in the as-irradiated diamond, providing bipolar carrier lifetimes of a few nanoseconds at the highest irradiation dose. Carrier-capture cross sections are reduced during annealing as vacancies aggregate into divacancies at ≤1000°C and extended vacancy clusters at 1450°C.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Charge collection in h -BN neutron detectors at elevated temperatures

Many of the neutron detector applications are in the environments with harsh conditions such as high temperatures. We report here the measurements of charge collection parameters of hexagonal boron nitride (h-BN) detectors at elevated temperatures, including the electron mobility-lifetime product (μτ) and surface recombination field (s/μ). It was found that μτ is increased, while s/μ is decreased as temperature increases. The temperature dependence of the surface recombination field (s/μ) revealed that electrons trapped in the surface states tend to thermally activate to the bulk region at higher temperatures with an activation energy of about 0.22 eV, leading to a reduction in the density of the charged surface states at elevated temperatures. Consequently, the charge collection efficiency is enhanced at elevated temperatures due to a reduced surface recombination field and increased electron mobility-lifetime product. The results suggested that h-BN neutron detectors are favorable for high temperature operation.

47 OTHER INSTRUMENTATION↗
DOE OSTI2021

Observation of spatially resolved Rashba states on the surface of CH 3 NH 3 PbBr 3 single crystals

Hybrid organic-inorganic perovskites (HOIPs) are prime candidates for studying Rashba effects due to the heavy metal and halogen atoms in their crystal structure coupled with predicted inversion symmetry breaking. Nevertheless, observation of the Rashba effect in cubic CH 3 NH 3 PbBr 3 single crystals that possess bulk inversion symmetry is the subject of extensive debate due to the lack of conclusive experiments and theoretical explanations. In this work, we provide experimental evidence that Rashba state in cubic CH 3 NH 3 PbBr 3 single crystals at room temperature occurs exclusively on the crystal surface and depends on specific surface termination that results in local symmetry breaking. We demonstrate this using a suite of spatially resolved and depth-sensitive techniques, including circular photogalvanic effect, inverse spin Hall effect, and multiphoton microscopy, that are supported by first principle calculations. Our work suggests using surface Rashba states in these materials for spintronic applications.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

The influence of surface impurities on photoelectric currents driven by intense soft x rays

In an x-ray driven cavity experiment, an intense flux of soft x rays on the emitting surface produces significant emission of photoelectrons having several kiloelectronvolts of kinetic energy. At the same time, rapid heating of the emitting surface occurs, resulting in the release of adsorbed surface impurities and subsequent formation of an impurity plasma. This numerical study explores a simple model for the photoelectric currents and the impurity plasma. In this work, attention is given to the effect of varying the composition of the impurity plasma. The presence of protons or hydrogen molecular ions leads to a substantially enhanced cavity current, while heavier plasma ions are seen to have a limited effect on the cavity current due to their lower mobility. Additionally, it is demonstrated that an additional peak in the current waveform can appear due to the impurity plasma. A correlation between the impurity plasma composition and the timing of this peak is elucidated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2022

Origin of layered perovskite device efficiencies revealed by multidimensional time-of-flight spectroscopy

Mixtures of layered perovskite quantum wells with different sizes form prototypical light-harvesting antenna structures in solution-processed films. Gradients in the bandgaps and energy levels are established by concentrating the smallest and largest quantum wells near opposing electrodes in photovoltaic devices. Whereas short-range energy and charge carrier funneling behaviors have been observed in layered perovskites, our recent work suggests that such light-harvesting processes do not assist long-range charge transport due to carrier trapping at interfaces between quantum wells and interstitial organic spacer molecules. Here, we apply a two-pulse time-of-flight technique to a family of layered perovskite systems to explore the effects that interstitial organic molecules have on charge carrier dynamics. In these experiments, the first laser pulse initiates carrier drift within the active layer of a photovoltaic device, whereas the second pulse probes the transient concentrations of photoexcited carriers as they approach the electrodes. The instantaneous drift velocities determined with this method suggest that the rates of trap-induced carrier deceleration increase with the concentrations of organic spacer cations. Overall, our experimental results and model calculations suggest that the layered perovskite device efficiencies primarily reflect the dynamics of carrier trapping at interfaces between quantum wells and interstitial organic phases.

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

Breakdown characteristics of deep-ultraviolet Al 0.6 Ga 0.4 N p-i-n avalanche photodiodes

A top-illuminated deep-ultraviolet Al 0.6 Ga 0.4 N p-i-n avalanche photodiode (APD) structure was designed and grown by metalorganic chemical vapor deposition on an AlN bulk substrate and on two different quality AlN/sapphire templates, and APDs were fabricated and tested. The APD devices with a circular diameter of 20 μm have demonstrated a distinctive reverse-bias breakdown behavior. The reverse breakdown voltage of the APDs is approximately -140 V, which corresponds to a breakdown electric field of 6–6.2 MV/cm for the Al 0.6 Ga 0.4 N material as estimated by Silvaco TCAD simulation. The APDs grown on the AlN bulk substrate show the lowest leakage current density of < 1 × 10 -8 A/cm 2 (at low reverse bias) compared to that of the devices grown on the AlN templates. From the photocurrent measurement, a maximum gain (current limited) of 1.2 × 10 4 is calculated. The average temperature coefficients of the breakdown voltage are negative for APD devices fabricated from both the AlN bulk substrate and the AlN templates, but these data show that the coefficient is the least negative for the APD devices grown on the low-dislocation-density AlN bulk substrate.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2021

Charge collection and trapping mechanisms in hexagonal boron nitride epilayers

Understanding charge collection and trapping mechanisms is crucial for using hexagonal boron nitride (h-BN) as active layers for many photonic and electronic devices such as deep UV detectors and emitters, neutron detectors, and single photon emitters. Charge collection and trapping mechanisms in h-BN epilayers have been investigated by probing impurity related optical emissions under an applied electrical field. Our results suggested that the existence of oxygen impurities affects the charge collection efficiency and results in an additional emission peak at 3.75 eV, corresponding to a donor-acceptor pair (DAP) recombination involving O N (oxygen residing on the nitrogen site) donors and the V B -H (boron vacancy bonded with hydrogen complex) deep level acceptors. Experimental results further revealed that the applied electric field induces an anti-correlation between the emission intensity of the DAP transition and the charge collection efficiency from which it was shown that it is possible to find an expression to quantitatively measure the maximum charge collection efficiency in h-BN. The results introduce not only a coherent picture for the relationship between common impurities in h-BN and charge collection and trapping mechanisms but also useful insights into possible approaches to improve the quality, purity, and charge collection of the h-BN epilayers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Probing drift velocity dispersion in MAPbI 3 photovoltaic cells with nonlinear photocurrent spectroscopy

Conventional time-of-flight (TOF) measurements yield charge carrier mobilities in photovoltaic cells with time resolution limited by the RC time constant of the device, which is on the order of 0.1–1 µs for the systems targeted in the present work. We have recently developed an alternate TOF method, termed nonlinear photocurrent spectroscopy (NLPC), in which carrier drift velocities are determined with picosecond time resolution by applying a pair of laser pulses to a device with an experimentally controlled delay time. In this technique, carriers photoexcited by the first laser pulse are “probed” by way of recombination processes involving carriers associated with the second laser pulse. Here, we report NLPC measurements conducted with a simplified experimental apparatus in which synchronized 40 ps diode lasers enable delay times up to 100 µs at 5 kHz repetition rates. Carrier mobilities of ∼0.025 cm 2 /V/s are determined for MAPbI 3 photovoltaic cells with active layer thicknesses of 240 and 460 nm using this instrument. Here, our experiments and model calculations suggest that the nonlinear response of the photocurrent weakens as the carrier densities photoexcited by the first laser pulse trap and broaden while traversing the active layer of a device. Based on this aspect of the signal generation mechanism, experiments conducted with co-propagating and counter-propagating laser beam geometries are leveraged to determine a 60 nm length scale of drift velocity dispersion in MAPbI 3 films. Contributions from localized states induced by thermal fluctuations are consistent with drift velocity dispersion on this length scale.

14 SOLAR ENERGY↗
DOE OSTI2023

Study of perovskite CsPbBr 3 detector polarization and its mitigation with ultrahigh x-ray flux

High-flux capable semiconductor x-ray detectors are essential in various applications, but the detrimental effects of detector polarization limit their use in many cases. Here, for this work, we studied the polarization of perovskite CsPbBr 3 semiconductor detectors using ultrahigh flux synchrotron x rays (10 6 –10 12 photons s –1 mm –2 at 58.61 keV). The CsPbBr 3 detectors did not show immediate polarization prominently until a flux higher than 10 10 photons s –1 mm –2 . Using the pump-and-probe technique, we visualized the spatial and temporal effects of polarization. The polarized region, represented by reduced photocurrent, extended beyond the area under direct irradiation, and the reduced photocurrent persisted after potential de-polarization treatments. We found that stronger applied electric fields and fewer carrier traps can mitigate polarization, represented by less photocurrent deficit. By examining the detectors’ current response under controlled ambient light, low, and high-flux x rays, we studied the trap filling and release behavior of CsPbBr 3 . We discovered that the polarization is caused by partial detector damage due to deep defects generated by ultrahigh flux x-ray irradiation. Our work provides insight into CsPbBr 3 polarization under extremely intense x-ray radiation and shows that reducing crystal defects and increasing detector bias voltage are effective solutions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Progress toward absolute calibrations of DC x-ray sources from 1 keV to >30 keV for Z diagnostics applications

Numerous x-ray diagnostics at the Z facility, time-resolved and time-integrated, would benefit from a local calibrated x-ray source. For many years, Z has used low-power Manson x-ray sources to provide some characterization information for components of Z diagnostics below 30 keV. More recently, we have acquired TruFocus x-ray sources, which operate at voltages up to 100 kV depending on the anode material and model. These DC sources have demonstrated good stability and repeatability. Absolute calibrations of either sources or detectors usually involve another calibrated detector from another source, which has not been readily available locally. Therefore, we have adopted the strategy of characterizing a sensitive semiconductor diode detector by controlling or inferring the active volume. This characterized detector is then used to scale the Manson x-ray spectral shape as measured by an Amptek energy-resolving detector. Here, the result agrees well with the modeled source spectrum. We can apply this technique to the TruFocus as well.

Crystal spectroscopy↗
DOE OSTI2025

Photothermoelectric effect in cadmium arsenide thin films for unbiased, mid-infrared photodetection

Mid-infrared photodetectors that can operate at room temperature are of great interest for a wide range of applications. Here, we demonstrate unbiased, mid-infrared (10.4 μm) photodetection in epitaxial thin films of the three-dimensional Dirac semimetal cadmium arsenide (Cd 3 As 2 ), which are grown on a III–V heterostructure. We show that the photocurrent response of planar metal–Cd 3 As 2 –metal devices is consistent with the photothermoelectric effect, which is due to a temperature gradient that develops under asymmetric illumination of the device. We show that the photoresponsivity of Cd 3 As 2 channel devices is an order of magnitude greater than devices with a III–V semiconductor channel, attesting to the excellent thermoelectric properties of Cd 3 As 2 and promising efficient unbiased, room-temperature mid-infrared photodetection.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Impact of superlattice size on quantum efficiency and polarization in MOCVD-grown strained GaAs/GaAsP photocathodes

Strained superlattice GaAs photocathodes are crucial for providing high photocurrent beams of spin-polarized electrons at several accelerator facilities including the Continuous Electron Beam Accelerator Facility at Thomas Jefferson National Accelerator Facility and the future Electron–Ion Collider at Brookhaven National Laboratory. Here, in this work, we study the effects of varying the number of superlattice pairs on the polarization and photocurrent of the photocathodes. We observe a saturation in quantum efficiency beyond 30 pairs, with additional layers yielding minimal photocurrent improvement while noticeably reducing the polarization of the beam.

Chemical vapor deposition↗
DOE OSTI2021

Terahertz emission spectroscopy of ultrafast exciton shift current in the noncentrosymmetric semiconductor CdS

The charge-neutral exciton has been predicted to carry genuine photocurrent due to the geometric Berry phase of the electronic bands, if the inversion symmetry in a crystal is broken. Here we detect such exciton shift current in a prototypical polar semiconductor CdS by using terahertz emission spectroscopy. A distinct peak emerges in the photocurrent spectra at the energy of the exciton resonance, which is demonstrated to result from the distinct displacements of electrons and holes in real space within the excitons to produce a finite transient charge current at subpicosecond time scale. Our findings elucidate the Berry phase physics of the charge-neutral photoexcitations and also shed light on the novel energy harvesting mechanism by exciton generation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Light-driven permanent transition from insulator to conductor

The transition from insulator to conductor can be realized in some materials but requires modification of both the arrangement of atoms and their electronic configurations. This is often achieved by doping. Here we reveal a different mechanism the lattice may adopt to induce such a transition. Experiments showed the surprising finding that limited exposure to subband-gap light caused a permanent transition from an insulator state to a conductor state in the insulating oxide Ga 2 O 3 , with a nine orders of magnitude increase in electronic conduction. Furthermore, annealing up to 400 °C did not suppress or decrease the induced conductivity. Photoexcitation by light-induced modification in the charge state of defects and subsequent lattice distortion around them was suggested to be the underlying mechanism behind this transition. Density functional theory calculations confirmed that modifying the charge state of defects leads to redistribution of the localized electrons and massive structural distortion in the surrounding lattice, causing large shifts in the density of states and introducing new states with shallower energy levels. Both experimental and theoretical results revealed the introduction of stable shallow energy levels, explaining the mechanism behind the transition from an insulator to a conductor state by light. We suggest that this mechanism may occur in other wide band-gap metal oxides leading to drastic modification in their electronic properties.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Bulk Photovoltaic Effect Driven by Collective Excitations in a Correlated Insulator

We investigate the bulk photovoltaic effect, which rectifies light into electric current, in a collective quantum state with correlation driven electronic ferroelectricity. We show via explicit real-time dynamical calculations that the effect of the applied electric field on the electronic order parameter leads to a strong enhancement of the bulk photovoltaic effect relative to the values obtained in a conventional insulator. The enhancements include both resonant enhancements at sub-band-gap frequencies, arising from excitation of optically active collective modes, and broadband enhancements arising from nonresonant deformations of the electronic order. The deformable electronic order parameter produces an injection current contribution to the bulk photovoltaic effect that is entirely absent in a rigid-band approximation to a time-reversal symmetric material. Furthermore, our findings establish that correlation effects can lead to the bulk photovoltaic effect and demonstrate that the collective behavior of ordered states can yield large nonlinear optical responses.

14 SOLAR ENERGY↗