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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 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

Corrugated Quantum Well Infrared Photodetectors and Arrays

Quantum well infrared photodetectors (QWIPs) have many advantages in infrared detection, mainly due to the mature Ill-V material technology. The employment of the corrugation structure further advances the technology by providing a simple, yet efficient light-coupling scheme. A C-QWIP enjoys the same flexibility as a detector with intrinsic normal incident absorption. In this paper, we will discuss the utilities of C-QWIPs in different applications, including two-color detection and polarization-sensitive detection. Besides practical applications, C-QWIPs are also useful in detector characterization. They can be used for measuring the absorption coefficient of light propagating parallel to the layers under bias and providing information on the energy resolved photoconductive gain. These two quantities have never been measured before. Based on the corrugation design, we have made several modifications that further improve the detector sensitivity without increasing its complexity. Other than the C-QWIP structure, we also continue searching for other sensitive detector architectures. In a quantum grid infrared photodetector, 3-dimensional electron confinement can be achieved, with which the detector is able to absorb light in all directions. At the same time, the photoconductive gain can also be improved. We further improve the design using a blazed structure. All the experimental results are supported by a rigorous electromagnetic modal transmission-line theory developed especially for these types of structures. Preliminary thermal imaging using C-QWIP FPAs validates the advantages of the present approach.

Choi, K. K.↗
NASA NTRS2001

Embedded Coplanar Strips Traveling-Wave Photomixers

The electric field distribution in photomixers with electrodes deposited on the surface has already been calculated. It was shown that the strength of the electric field diminishes rapidly with depth. It was argued that the resulting reduction of the effective interaction volume of the device lowers the optical-to-heterodyne conversion. In this paper, we will present the results of our investigation on the influence of the electrode placement on the performance of photomixers. We have fabricated and measured traveling-wave photomixer devices which have both embedded and surface electrodes - the nominal spacing between the electrodes was 2 micrometers. Devices were made using either low-temperature-grown (LTG)-GaAs or ErAs:GaAs as the photoconductive material. The dark current, photocurrent, and radio frequency (RF) emission were measured at nominally 1 THz. The experimental data show a surprising difference in the behavior of ErAs:GaAs devices when the electrodes are embedded. A factor of two increase in RF radiation is observed for electric fields < 20 kV/cm. No such improvement was observed for the LTG-GaAs devices. We argue that the distinctive behavior of the two photoconductive materials is due to differences in the crystal structure - LTG-GaAs is isotropic, while ErAs:GaAs is uniaxial. We find that the carrier mobility in-plane (parallel) to the ErAs layers in the ErAs:GaAs superlattice is larger than orthogonal to these layers. The data indicate that carrier velocity overshoot is responsible for the excess radiation produced for the embedded electrode ErAs:GaAs devices.

Wyss, R. A.↗
NASA NTRS1977

Holographic recording medium

A holographic recording medium comprising a conductive substrate, a photoconductive layer and an electrically alterable layer of a linear, low molecular weight hydrocarbon polymer has improved fatigue resistance. An acrylic barrier layer can be interposed between the photoconductive and electrically alterable layers.

Gange, Robert Allen↗
DOE OSTI2020

Extrinsic Absorption Pathways in Vanadium-Doped SiC Measured Using a Total Internal Reflection Geometry

Vanadium-doped SiC is under study for high-power photoconductive switches and is receiving increased attention as a potential material system for quantum computing, as the V can act as a quantum emitter at telecom wavelengths. Knowledge of the defect-mediated electronic transition pathways from sub-bandgap illumination in this material with low absorption and multi-photon and multi-path transition probability is important in optimizing quantum efficiency. Herein, the optical transitions of vanadium-doped SiC for two polytypes as a function of dopant concentration using a total internal reflection (TIR) geometry are examined. Furthermore, normalized quantum efficiency is reported and correlated with both dopant concentrations (V, N, B, and Al) measured by secondary ion mass spectroscopy (SIMS) and absorption measurements. Although both polytypes respond well to 532 nm light, efficiency at longer wavelengths does not correlate 1:1 with absorption coefficient and strongly depends on the polytype and net background co-dopant concentration. This has important implications for efficient excitation of both carriers for optoelectronics and for the use of V as a quantum emitter.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Monolithic optoelectronic circuit design for on-chip terahertz applications

We demonstrate a monolithic coplanar stripline platform for on-chip terahertz (THz) generation, transmission, and detection, addressing key challenges of mode purity, bandwidth, and referencing. Capacitive coupling of the photoconductive generator switch enforces pure odd-mode propagation, increases THz field strength, and extends the operational frequency range, achieving 0.05–1.4 THz. Our architecture enables fully monolithic fabrication with amorphous silicon switches, in situ field referencing, and galvanic isolation between generation and detection. Finite-element simulations and experiments confirm that suppressing parasitic modes improves signal integrity, providing a robust platform for high-fidelity THz spectroscopy, ultrafast electronics, and nanoscale quantum materials research.

Capacitive coupling↗
DOE OSTI2025

Bipolar transport in diamond under photo-excitation: Evidence of free charge scattering by excitons

The fundamental characteristics of the Coulomb e-h interactions in ultra-pure diamond are investigated through time-resolved photoconductivity measurements in homogeneously photoexcited samples with ohmic-like low resistive B + sandwich contacts. Here, we demonstrate that, in the electric field, the bipolar charge mobility exhibits drastic reduction for photo-injected densities of 10 14 ‐10 16 cm ‐3 that cannot be explained only by mutual scattering between free e-h particles. This phenomenon is clarified by adding scattering caused by excitons whose density grows in the photo-exited system. The comparison of drift mobilities at 300 K in diamond obtained in this study and in pure Si and Ge obtained from literature, where exciton scattering is insignificant, is presented. We derive the empirical expression for the density dependences of bipolar mobility for the electric fields up to 1 kV/cm that could be useful for the development of diamond electronic devices.

Bipolar drift↗
DOE OSTI2024

Elucidating the Role of InGaAs and InAlAs Buffers on Carrier Dynamics of Tensile-Strained Ge Double Heterostructures

Extensive research efforts of strained germanium (Ge) are currently underway due to its unique properties, namely, (i) possibility of band gap and strain engineering to achieve a direct band gap, thus exhibiting superior radiative properties, and (ii) higher electron and hole mobilities than Si for upcoming technology nodes. Realizing lasing structures is vital to leveraging the benefits of tensile-strained Ge (ε-Ge). Here, we use a combination of different analytical tools to elucidate the effect of the underlying InGaAs/InAlAs and InGaAs overlaying heterostructures on the material quality and strain state of ε-Ge grown by molecular beam epitaxy. Using X-ray analysis, we show the constancy of tensile strain in sub-50 nm ε-Ge in a quantum-well (QW) heterostructure. Further, effective carrier lifetime using photoconductive decay as a function of buffer type exhibited a high (low) defect-limited carrier lifetime of ~68 ns (~13 ns) in 0.61% (0.66%) ε-Ge grown on an InGaAs (InAlAs) buffer. These results correspond well with the measured surface roughness of 1.289 nm (6.303 nm), consistent with the surface effect of the ε-Ge/III–V heterointerface. Furthermore, a reasonably high effective lifetime of ~78 ns is demonstrated in a QW of ~30 nm 1.6% ε-Ge, a moderate reduction from ~99 ns in uncapped ε-Ge, alluding to the surface effect of the overlying heterointerface. Thus, the above results highlight the prime quality of ε-Ge that can be achieved via III–V heteroepitaxy and paves a path for integrated Ge photonics.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Interplay between microstructure, defect states, and mobile charge generation in transition metal dichalcogenide heterojunctions

Two-dimensional transition metal dichalcogenides (2D-TMDCs) have gained attention for their promise in next-generation energy-harvesting and quantum computing technologies, but realizing these technologies requires a greater understanding of TMDC properties that influence their photophysics. To this end, we discuss here the interplay between TMDC microstructure and defects with the charge generation yield, lifetime, and mobility. As a model system, we compare monolayer-only and monolayer-rich MoS 2 grown by chemical vapor deposition, and we employ the TMDCs in Type-II charge-separating heterojunctions with semiconducting single-walled carbon nanotubes (s-SWCNTs). Our results suggest longer lifetimes and higher yields of mobile carriers in samples containing a small fraction of defect-rich multilayer islands on predominately monolayer MoS 2 . Compared to the monolayer-only heterojunctions, the carrier lifetimes increase from 0.73 μs to 4.71 μs, the hole transfer yield increases from 23% to 34%, and the electron transfer yield increases from 39% to 59%. We reach these conclusions using a unique combination of microwave photoconductivity (which probes only mobile carriers) along with transient absorption spectroscopy (which identifies spectral signatures unique to each material and type of photoexcited quasiparticle, but does not probe mobility). Overall, our results highlight the substantial changes in photophysics that can occur from small changes in TMDC microstructure and defect density, where the presence of defects does not necessarily preclude improvements in charge generation.

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

High Carrier Lifetimes in Epitaxial Germanium-Tin/Al(In)As Heterostructures with Variable Tin Compositions

Group IV-based germanium-tin (Ge1-ySny) compositional materials have recently shown great promise for infrared detection, light emission and ultra-low power transistors. High carrier lifetimes are desirable for enhancing the detection limit and efficiency of photodetectors, low threshold current density in lasers, and low tunneling barrier height by lowering defects and dislocations at the heterointerface of a source and a channel. Here, carrier lifetimes in epitaxial germanium (Ge) and variable tin (Sn) compositional Ge1-ySny materials were experimentally determined on GaAs substrates using the contactless microwave photoconductive decay (u-PCD) technique at an excitation wavelength of 1500 nm. Sharp (2 x 2) reflection high energy electron diffraction patterns and low surface roughness were observed from the surface of the Ge0.97Sn0.03 epilayer. X-ray rocking curves from Ge0.97Sn0.03 and Ge0.94Sn0.06 layers demonstrated the pseudomorphic and lattice-matched growth on AlAs and In0.12Al0.88As buffers, respectively, further substantiated by reciprocal space maps and abrupt heterointerfaces evident from the presence of Pendellosung oscillations. High effective carrier lifetimes of 150 ns to 450 ns were measured for Ge1-ySny epilayers as a function of Sn composition, surface roughness, growth temperature, and layer thickness. The observed increase in the carrier lifetime with an increasing Ge layer thickness and a reducing surface roughness, by incorporating Sn, were explained. The enhancement of the carrier lifetime with an increasing Sn concentration was achieved by controlling the defects with lattice-matched Ge0.94Sn0.06/In0.12Al0.88As heterointerfaces or the pseudomorphic growth of Ge0.94Sn0.06 on GaAs. Therefore, our monolithic integration of variable Sn alloy compositional Ge1-ySny materials with high carrier lifetimes opens avenues to realize electronic and optoelectronic devices.

carrier lifetimes↗
DOE OSTI2022

Carrier recombination dynamics and temperature dependent optical properties of InAs–GaSb heterostructures

Heterostructures with two dissimilar materials could offer unprecedented properties if one can carefully synthesize these heterostructures with atomically smooth interfaces and reduced number of recombination centers. InAs/GaSb-based heterostructures have technological importance for long wavelength infrared photodetectors if one can synthesize these materials with high-optical quality and high-carrier lifetime. In this work, the InAs/GaSb heterostructures with a different number of heterointerfaces and growth conditions were grown by solid source molecular beam epitaxy using valved cracker sources for both arsenic and antimony. Precise control of growth parameters and shutter sequences enabled abrupt InAs/GaSb heterointerfaces, as supported by a high-resolution transmission electron microscopic study. The temperature and power-dependent optical properties by photoluminescence (PL) spectroscopic analysis of InAs/GaSb heterostructures with 4 and 28 heterointerfaces displayed donor to the acceptor and the exciton bound to complex defects (V Ga Ga Sb ) 0 . Since the optical transition in PL measurements serves to determine the quality of the material, and the observed excitonic transitions from these InAs/GaSb heterostructures is an indication of high-quality materials. The high-carrier lifetimes of 139 ns to 185 ns from InAs/GaSb heterostructures were measured using microwave photoconductivity decay (μ-PCD) technique at room temperature. The observed increase in carrier lifetime is due to the decreasing number of Ga-related carrier recombination centers or defect complexes. This is further supported by the PL spectroscopic study. In addition, the carrier lifetime with different injection levels is supported by Shockley–Read–Hall recombination. We report these InAs/GaSb heterostructures with high-optical quality and high-carrier lifetimes would offer a path for the development of high-performance infrared photodetectors.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Nanoshell quantum dots: Quantum confinement beyond the exciton Bohr radius

Nanoshell quantum dots (QDs) represent a novel class of colloidal semiconductor nanocrystals (NCs), which supports tunable optoelectronic properties over the extended range of particle sizes. Traditionally, the ability to control the bandgap of colloidal semiconductor NCs is limited to small-size nanostructures, where photoinduced charges are confined by Coulomb interactions. A notorious drawback of such a restricted size range concerns the fact that assemblies of smaller nanoparticles tend to exhibit a greater density of interfacial and surface defects. This presents a potential problem for device applications of semiconductor NCs where the charge transport across nanoparticle films is important, as in the case of solar cells, field-effect transistors, and photoelectrochemical devices. Furthermore, the morphology of nanoshell QDs addresses this issue by enabling the quantum-confinement in the shell layer, where two-dimensional excitons can exist, regardless of the total particle size. Such a geometry exhibits one of the lowest surface-to-volume ratios among existing QD architectures and, therefore, could potentially lead to improved charge-transport and multi-exciton characteristics. The expected benefits of the nanoshell architecture were recently demonstrated by a number of reports on the CdS bulk /CdSe nanoshell model system, showing an improved photoconductivity of solids and increased lifetime of multi-exciton populations. Along these lines, this perspective will summarize the recent work on CdS bulk /CdSe nanoshell colloids and discuss the possibility of employing other nanoshell semiconductor combinations in light-harvesting and lasing applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

A hybrid optoelectronic Mott insulator

The coupling of electronic degrees of freedom in materials to create “hybridized functionalities” is a holy grail of modern condensed matter physics that may produce versatile mechanisms of control. Correlated electron systems often exhibit coupled degrees of freedom with a high degree of tunability which sometimes lead to hybridized functionalities based on external stimuli. However, the mechanisms of tunability and the sensitivity to external stimuli are determined by intrinsic material properties which are not always controllable. A Mott metal-insulator transition (MIT) is technologically attractive due to the large changes in resistance, tunable by doping, strain, electric fields, and orbital occupancy but not, in and of itself, controllable with light. Here, an alternate approach is presented to produce optical functionalities using a properly engineered photoconductor/strongly correlated hybrid heterostructure. This approach combines a photoconductor, which does not exhibit an MIT, with a strongly correlated oxide, which is not photoconducting. Due to the intimate proximity between the two materials, the heterostructure exhibits giant volatile and nonvolatile, photoinduced resistivity changes with substantial shifts in the MIT transition temperatures. This approach can be extended to other judicious combinations of strongly correlated materials.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Role of defects in ultra-high gain in fast planar tin gallium oxide UV-C photodetector by MBE

Here we report ultra-high responsivity of epitaxial (Sn x Ga 1–x ) 2 O 3 (TGO) Schottky UV-C photodetectors and experimentally identified the source of gain as deep-level defects, supported by first principles calculations. Epitaxial TGO films were grown by plasma-assisted molecular beam epitaxy on (–201) oriented n-type β-Ga 2 O 3 substrates. Fabricated vertical Schottky devices exhibited peak responsivities as high as 3.5 ×10 4 A/W at –5 V applied bias under 250 nm illumination with sharp cutoff shorter than 280 nm and fast rise/fall time in milliseconds order. Hyperspectral imaging cathodoluminescence (CL) spectra were examined to find the mid-bandgap defects, the source of this high gain. Irrespective of different tin mole fractions, the TGO epilayer exhibited extra CL peaks at the green band (~2.20 eV) not seen in β-Ga 2 O 3 along with enhancement of the blue emission-band (~2.64 eV) and suppression of the UV emission-band. Based on hybrid functional calculations of the optical emission expected for defects involving Sn in β-Ga 2 O 3 , V Ga –Sn complexes are proposed as potential defect origins of the observed green and blue emission-bands. Such complexes behave as acceptors that can efficiently trap photogenerated holes and are predicted to be predominantly responsible for the ultra-high photoconductive gain in the Sn-alloyed Ga 2 O 3 devices by means of thermionic emission and electron tunneling. Regenerating the V Ga –Sn defect complexes by optimizing the growth techniques, we have demonstrated a planar Schottky UV-C photodetector of the highest peak responsivity.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Quantitative Prediction of Perovskite Stability Using Accelerated Testing and Machine Learning

PV technologies based on hybrid perovskites offer the potential for reducing solar cell costs, but they are particularly vulnerable to degradation by environmental factors such as moisture, oxygen, and illumination. Commercialization will require not only stable materials and device architectures but also accelerated testing protocols and models that can predict degradation from the accelerated testing data. Here, we report results from in situ photoluminescence (PL) and photoconductivity (PC) measurements during perovskite degradation with simultaneous optical transmittance (Tr) measurements or reflected dark field (DF) imaging. From PL, PC, Tr, and DF, we determine (respectively) the steady-state quasi-Fermi level splitting, the mean effective carrier diffusion length, the extent of conversion of perovskite to higher bandgap degradation products, and the extent of scattering from domains with different orientation or composition, all as a function of time during degradation. Simultaneous measurement of PL-PC-Tr or PL-PC-DF on perovskite absorbers in an environmental chamber over a wide range of humidity, oxygen, temperature, and illumination levels yields a rich data set. We use machine learning to develop a model that accurately predicts (within 10%) the time for the diffusion length to decrease to 85% of its initial value. The model takes the environmental conditions and the first few measurements from the PL-PC-Tr or PL-PC-DF experiment as input. Thus, the model provides a framework to interpret the results of accelerated testing of absorber materials. One of the dominant features in the model of degradation for CH 3 NH 3 PbI 3 is the initial rate at which transmittance increase. For devices with opaque contacts, the pixel-averaged rate of change of the intensity in dark-field images can be used in place of transmittance. Results are also presented on full PV devices with in-situ current-voltage (JV) measurements. Here, the data from simultaneous PL-DF-JV under environmental stresses reveal intimate connection between degradation and shunts and provide a framework to extend accelerated testing to devices. Further, the presentation reveals underlying universal behavior in degradation pathways over a broad range of environmental stresses and perovskite compositions.

14 SOLAR ENERGY↗
DOE OSTI2023

Persistent optical phenomena in oxide semiconductors

The interaction of transparent oxide semiconductors with light is critically important for a range of applications. Persistent effects could be exploited for holographic memory or optically defined circuits. Conversely, they may also be detrimental to device operation. Large, room-temperature persistent photoconductivity (PPC) was discovered in strontium titanate (SrTiO 3 , STO) after annealing in a hydrogen-containing atmosphere. Barium titanate (BaTiO 3 , BTO), a ferroelectric material, was recently found to also exhibit PPC. Room-temperature photodarkening was observed in Cu-doped gallium oxide (β-Ga 2 O 3 ) after exposure to sub-bandgap light. Hydrogen is believed to play a central role in these persistent phenomena. In the proposed model, a photon excites substitutional hydrogen (a proton inside an oxygen vacancy), making the defect unstable. The proton leaves and binds to a host oxygen atom, forming an O-H bond that is observed with infrared spectroscopy. Here, an oxygen vacancy is left behind. Because oxygen vacancies in STO and BTO are shallow donors, this process results in PPC. In β-Ga 2 O 3 :Cu, however, the oxygen vacancy neighbors a Cu acceptor. In that case, photoexcitation results in the rare Cu 3+ state, which absorbs visible light. The effect can be “erased” by annealing at 300-400°C.

Annealing↗
DOE OSTI2024

ARC-SAFE: Accelerated Response semiconducting Contactors and Surge Attenuation For DC Electrical systems (Final Scientific/Technical Report)

The landscape of power transmission and distribution is quickly evolving as more power conversion is done through power electronics and transmitted or distributed at medium voltage direct current (MVDC). As power electronics tend to store less energy and be less resilient to faults than conventional power transformers, a reliable and fast protection against faults is critical to protect power electronics (PE) based infrastructure, especially for medium- and high-voltage applications. This motivates the development of PE based protection circuits to replace slower contemporary electromechanical breakers. In this project a novel PE based MVDC circuit breaker is developed; the new design is comprised of 1) a normally-on leg made of commercially-available, cascaded SiC junction field effect transistors (JFETs) with a passive balancing network, and 2) a normally-off leg based on an optically-triggered gallium nitride (GaN) photoconductive semiconductor switch (PCSS). The normally-off leg was designed to be quickly turned on, to divert current to an auxiliary dissipative circuit, as the normally-on leg is turned off. This approach, using solid-state devices, was selected for a high-performance, fast-switching operation for the DC circuit breaker as compared to approaches that use slower mechanical switches. To be practical, the circuit breaker must have low conduction loss (low Ron) in the normally-on leg and fast coordinated triggering of the normally-off leg to avoid damage from inductive flyback, which could be considerable for long lengths of cable.

24 POWER TRANSMISSION AND DISTRIBUTION↗