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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Exceptional electronic transport and quantum oscillations in thin bismuth crystals grown inside van der Waals materials

Confining materials to two-dimensional forms changes the behaviour of the electrons and enables the creation of new devices. However, most materials are challenging to produce as uniform, thin crystals. Here we present a synthesis approach where thin crystals are grown in a nanoscale mould defined by atomically flat van der Waals (vdW) materials. Here, by heating and compressing bismuth in a vdW mould made of hexagonal boron nitride, we grow ultraflat bismuth crystals less than 10 nm thick. Due to quantum confinement, the bismuth bulk states are gapped, isolating intrinsic Rashba surface states for transport studies. The vdW-moulded bismuth shows exceptional electronic transport, enabling the observation of Shubnikov–de Haas quantum oscillations originating from the (111) surface state Landau levels. By measuring the gate-dependent magnetoresistance, we observe multi-carrier quantum oscillations and Landau level splitting, with features originating from both the top and bottom surfaces. Our vdW mould growth technique establishes a platform for electronic studies and control of bismuth’s Rashba surface states and topological boundary modes1,2,3. Beyond bismuth, the vdW-moulding approach provides a low-cost way to synthesize ultrathin crystals and directly integrate them into a vdW heterostructure.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Conceptual Design of a Thermal–to-14-MeV Neutron Conversion Device for Use in the Advanced Test Reactor

Here, this paper details the conceptual design of a thermal-to-14-MeV neutron converter consisting of a mixture of lithium and deuterium in a blanket material. Such a device operates a two-step reaction, first generating tritons via thermal neutron absorption in the tritium breeding material, and in the second step, high-energy neutrons are produced either via deuterium-tritium fusion reaction or with tritium reacting with lithium. A thermal-to-14-MeV neutron converter significantly hardens the neutron spectrum by virtually removing thermal neutrons and adding a high-energy 14 MeV component to the neutron spectrum. While similar concepts have been previously proposed and tested in other reactors, the unique characteristics of the Advanced Test Reactor (ATR), namely its important thermal flux (up to $10^{15}$ n.cm$^{-2}$.s$^{-1}$) makes it markedly attractive for obtaining a very large fast neutron flux, usable for irradiation studies under neutron flux conditions prototypical of fusion reactors. The paper provides a description of a new computational scheme developed for handling the coupled neutron-triton transport mechanism using the Geant4 toolkit. Resulting neutron spectra and high-energy neutron yields are summarized for different irradiation positions and potential neutron breeder materials. Maximum predicted thermal-to-14-MeV neutron yields are on the order of $2.10^{-4}$, which is consistent with previous studies found in the literature. Thus, when placed inside the ATR, such a neutron converter will be providing the largest high-energy neutron source available for activation and irradiation studies of materials foreseen for use in fusion reactors. Future steps will involve qualifying the computational scheme using the ATR critical facility using activation foil measurements.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2023

Recent Advancements in Reconfigurable mmWave Devices Based on Phase-Change and Metal Insulator Transition Materials

Chalcogenide Phase Change Materials (PCM) and metal insulator transition (MIT) materials are a group of materials that are capable of switching between low resistance and high resistance states. These emerging materials have been widely used in optical storage media and memory devices. Over the past recent years, there have been interests in exploiting the PCM and MIT materials, especially germanium antimony telluride (GST) alloys and vanadium dioxide (VO 2 ), for radio frequency (RF) applications. The PCM and MIT-based RF devices are expected to bridge the gap between semiconductor switches and microelectromechanical system (MEMS) switches as they combine the low insertion loss performance of MEMS technology and the small size and reliability performance of semiconductor technology. This article presents an overview of the PCM and MIT materials for RF circuits and discusses the recent advancements in reconfigurable millimeter-wave (mmWave) devices based on PCM and MIT materials in depth.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Photogating-driven enhanced responsivity in a few-layered ReSe 2 phototransistor

A wide variety of two-dimensional (2D) metal dichalcogenide compounds have recently attracted much research interest due to their very high photoresponsivities (R) making them excellent candidates for optoelectronic applications. High R in 2D photoconductors is associated with trap state dynamics leading to a photogating effect, which is often manifested by a fractional power dependence (γ) of the photocurrent (I ph ) at an effective illumination intensity (P eff ). Furthermore, we present photoconductivity studies as a function of gate voltages, over a wide temperature range (20 K to 300 K) of field-effect transistors fabricated using thin layers of mechanically exfoliated Rhenium Diselenide (ReSe 2 ). We obtain very high responsivities R ~ 16500 A/W and external quantum efficiency (EQE) ~ 3.2 × 106% (at 140 K, V g = 60 V and P eff = 0.2 nW). A strong correlation between R and γ was established by investigating the dependence of these two quantities at various gate voltages and over a wide range of temperatures. Such correlations indicate the importance of trap state mediated photogating and its role in promoting high photo-responsivities in these materials. We believe such correlations can offer valuable insights for the design and development of high-performance photoactive devices using 2D materials.

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

Vertical GaN Superjunction Diode on Sapphire with Kilovolt Dynamic Breakdown Voltage

The development of superjunction structures for use in vertical wide bandgap power devices promise to break the 1-D material limits. Additionally, the possibility of utilizing heteroepitaxial GaN-on-Sapphire wafer for vertical devices can significantly trim the material and device cost. This work introduces a quasi-vertical GaN-on-Sapphire superjunction PN diode design utilizing sputtered p-NiO on the etched GaN fins for superjunction formation. DC breakdown voltage is shown to vary with superjunction charge imbalance and significantly exceed the expected 1-D planar limit of 350V given the epilayer design used. A maximum breakdown voltage of 840 V is extracted for near charge balance conditions limited by leakage current. Dynamic breakdown of the device is characterized as a function of reverse voltage slew rate. A maximum dynamic breakdown voltage of 1160 V under a reverse voltage slew rate of 2000 V/μs is found.

Porter, Matthew↗
DOE OSTI2020

Engineering of Lead Selenide Quantum Dot Based Devices and Core/Shell Heterostructures

Near infrared (NIR) emitting colloidal quantum dots (QDs) such as PbSe are interesting materials for implementation in various optoelectronic devices such as solar cells, photodetectors, and radiation detectors. Material properties like size tunable emission wavelengths, facile solution processability, the possibility of carrier multiplication and Auger assisted up-conversion, and high Z number provide PbSe QDs with unique advantages over many currently available commercial materials used in these devices. In this dissertation, after an introduction to QDs in general, we first focus on synthesis of PbSe-based heterostructured QDs for study via optical spectroscopy. PbSe/CdSe QDs synthesized via cation exchange reaction were used as seeds to study the proper conditions for further CdSe or CdS shell growth. Optical spectroscopy studies show that growth of thicker CdSe shells leads to tunable visible emission, while growth of thick CdS shells can greatly increase the photoluminescence quantum yield (PLQY) observed from the intermediate CdSe shell. NIR PL lifetimes can be greatly enhanced by the growth of CdS shells, due to strong delocalization of the hole wavefunction within the PbSe core, while the electron is delocalized throughout the QD including in the CdS shell, resulting in radiative lifetimes in the tens of microseconds. Two pulse delay measurements show that Auger assisted up conversion is also taking place in these engineered QD dot heterostructures. In the sections that follow, we look at QD devices, first with a focus on novel devices fabricated from PbSe QDs, using an amine synthesis method that allows for facile in-solution ligand exchange. QD film mobilities are measured for PbSe QDs capped with various short ionic ligands. Carrier densities in PbSe QD films are calculated from C-V measurements, showing for the first time that carrier densities can be modulated by simply changing the capping ligand. vii Radiation detectors utilizing PbSe QDs capped with KI and NH4I ligands show tangible response to incident alpha radiation. Current output response to alpha radiation is dependent on strength of radiation source and amount of bias voltage applied. In the final section a critical review of hybrid layered 2D-QD photodetectors is presented. In a typical example, PbSe and PbS QDs act as the absorbing layer in phototransistor devices which utilize layered 2D materials such as graphene and transition metal dichalcogenides as transport layers. Resulting devices display high gains and improved directivities compared to QD only and 2D only devices.

36 MATERIALS SCIENCE↗
DOE OSTI2024

From wide to ultrawide-bandgap semiconductors for high power and high frequency electronic devices

Abstract Wide and ultrawide-bandgap (U/WBG) materials have garnered significant attention within the semiconductor device community due to their potential to enhance device performance through their substantial bandgap properties. These exceptional material characteristics can enable more robust and efficient devices, particularly in scenarios involving high power, high frequency, and extreme environmental conditions. Despite the promising outlook, the physics of UWBG materials remains inadequately understood, leading to a notable gap between theoretical predictions and experimental device behavior. To address this knowledge gap and pinpoint areas where further research can have the most significant impact, this review provides an overview of the progress and limitations in U/WBG materials. The review commences by discussing Gallium Nitride, a more mature WBG material that serves as a foundation for establishing fundamental concepts and addressing associated challenges. Subsequently, the focus shifts to the examination of various UWBG materials, including AlGaN/AlN, Diamond, and Ga 2 O 3 . For each of these materials, the review delves into their unique properties, growth methods, and current state-of-the-art devices, with a primary emphasis on their applications in power and radio-frequency electronics.

Materials Science↗
DOE OSTI2023

Post-growth Recrystallization by Halides for High Throughput CIGS Photovoltaics

The objective of this project in broad terms was to address the capital equipment expense associated with Cu(In,Ga)Se 2 (CIGS) as a material for photovoltaic devices. This project has demonstrated a new method for processing CIGS using metal halides to recrystallize and enhance the quality of the final materials. A wide spectrum of metal halides were examined. Of these Cu and In chlorides and AgBr were found to be most effective; and, of these, AgBr was found to produce exemplary results. Cu and In chlorides were found to produce undesirable changes in film composition. The project demonstrated over 16% efficiency in a CIGS device deposited at 10x the standard rate of ~1 μm/hr and with a >100°C reduction in maximum temperature to 450°C. The entire deposition of the absorber layer including the recrystallization step required only 15 minutes, which is a 4x decrease in processing time.

14 SOLAR ENERGY↗
DOE OSTI2022

Growth Optimization and Device Integration of Narrow‐Bandgap Graphene Nanoribbons

Abstract The electronic, optical, and magnetic properties of graphene nanoribbons (GNRs) can be engineered by controlling their edge structure and width with atomic precision through bottom‐up fabrication based on molecular precursors. This approach offers a unique platform for all‐carbon electronic devices but requires careful optimization of the growth conditions to match structural requirements for successful device integration, with GNR length being the most critical parameter. In this work, the growth, characterization, and device integration of 5‐atom wide armchair GNRs (5‐AGNRs) are studied, which are expected to have an optimal bandgap as active material in switching devices. 5‐AGNRs are obtained via on‐surface synthesis under ultrahigh vacuum conditions from Br‐ and I‐substituted precursors. It is shown that the use of I‐substituted precursors and the optimization of the initial precursor coverage quintupled the average 5‐AGNR length. This significant length increase allowed the integration of 5‐AGNRs into devices and the realization of the first field‐effect transistor based on narrow bandgap AGNRs that shows switching behavior at room temperature. The study highlights that the optimized growth protocols can successfully bridge between the sub‐nanometer scale, where atomic precision is needed to control the electronic properties, and the scale of tens of nanometers relevant for successful device integration of GNRs.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Will Nontoxic High‐Performance Perovskite Photovoltaics Ever Be Possible?

Lead-based halide perovskite solar cells represent a significant advancement in photovoltaic technology, achieving certified power conversion efficiencies of over 27%. However, the toxicity of lead poses a major barrier to widespread commercialization. The demand for environmentally safe alternatives has driven extensive research into Pb-free perovskites. Current efforts include replacing Pb with Sn or Ge; forming double perovskites in which Pb is substituted by a monovalent–trivalent cation pair; and developing chalcogenide perovskites where the B site (ABX 3 ) adopts tetravalent cations (rather than Pb) to balance the charge. This concept examines the recent progress in developing Pb-free alternatives, revealing fundamental performance bottlenecks, inherent material limitations, and persistent development challenges. Through a comparative assessment of material properties and device performance limitations, this work highlights the underlying dilemma between environmental safety and efficiency in perovskite photovoltaics. The analysis identifies fundamental material constraints that create substantial barriers to simultaneously achieving both objectives.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Computational discovery of two-dimensional rare-earth iodides: promising ferrovalley materials for valleytronics

Two-dimensional Ferrovalley materials with intrinsic valley polarization are rare but highly promising for valley-based nonvolatile random access memory and valley filter devices. These ferromagnetic materials exhibit valleys at or near the Fermi level with intrinsic magnetism. The strong coupling between magnetism and spin–orbit coupling induces intrinsic valley polarization. Using Kinetically Limited Minimization, an unconstrained crystal structure prediction algorithm, and prototype sampling based on first-principles calculations, we have discovered new Ferrovalley materials, rare-earth iodides RI 2 , where R is a rare-earth element belonging to Sc, Y, or La-Lu, and I is Iodine. The rare-earth iodides are layered and demonstrate either 2H, 1T, or 1T d phase as the ground state in bulk, analogous to transition metal dichalcogenides (TMDCs). The calculated exfoliation energy of monolayers (MLs) is comparable to that of graphene and TMDCs, suggesting possible experimental synthesis. The MLs in the 2H phase exhibit ferromagnetism due to unpaired electrons in d and f orbitals. Throughout the rare-earth series, d bands have valley polarization at K and $\overline{K}$ points in the Brillouin zone in the vicinity of the Fermi level. Large intrinsic valley polarization in the range of 15–143 meV without external stimuli is observed in these Ferrovalley materials, which can be enhanced further by applying an in-plane bi-axial strain. These valleys can selectively be probed and manipulated for information storage and processing, potentially offering superior performance beyond conventional electronics and spintronics. Here we further show that the 2H ferromagnetic phase of RI 2 MLs possesses non-zero Berry curvature and exhibits anomalous valley Hall effect with considerable anomalous Hall conductivity. Our work will incite exploratory synthesis of the predicted Ferrovalley materials and their application in valleytronics and beyond.

2D materials↗
DOE OSTI2024

Role of mechanical stress localizations on the radiation hardness of AlGaN/GaN high electron mobility transistors

Multi-material, multi-layered systems such as AlGaN/GaN high electron mobility transistors (HEMTs) contain residual mechanical stresses that arise from sharp contrasts in device geometry and materials parameters. These stresses, which can be either tensile or compressive, are difficult to detect and eliminate because of their highly localized nature. We propose that their high-stored internal energy makes potential sites for defect nucleation sites under radiation, particularly if their locations coincide with the electrically sensitive regions of a transistor. In this study, we validate this hypothesis with molecular dynamic simulation and experiments exposing both pristine and annealed HEMTS to 2.8 MeV Au +3 irradiation. Our unique annealing process uses mechanical momentum of electrons, also known as the electron wind force (EWF) to mitigate the residual stress at room temperature. High-resolution transmission electron microscopy and cathodoluminescence spectra reveal the reduction of point defects and dislocations near the two-dimensional electron gas region of EWF-treated devices compared to pristine devices. The EWF-treated HEMTs showed relatively higher resilience with approximately 10% less degradation of drain saturation current and ON-resistance and 5% less degradation of peak transconductance. Both mobility and carrier concentration of the EWF-treated devices were less impacted compared to the pristine devices. Our results suggest that the lower density of nanoscale stress localization contributed to the improved radiation tolerance of the EWF-treated devices. Intriguingly, the EWF is found to modulate the defect distribution by moving the defects to electrically less sensitive regions in the form of dislocation networks, which act as sinks for the radiation induced defects and this assisted faster dynamic annealing.

AlGaN/GaN HEMTs↗
DOE OSTI2020

Synergistically integrated phosphonated poly(pentafluorostyrene) for fuel cells

Modern electrochemical energy conversion devices require more advanced proton conductors for their broad applications. Phosphonated polymers have been proposed as anhydrous proton conductors for fuel cells. However, the anhydride formation of phosphonic acid functional groups lowers proton conductivity and this prevents the use of phosphonated polymers in fuel cell applications. Here, we report a poly(2,3,5,6-tetrafluorostyrene-4-phosphonic acid) that does not undergo anhydride formation and thus maintains protonic conductivity above 200 °C. We use the phosphonated polymer in fuel cell electrodes with an ion-pair coordinated membrane in a membrane electrode assembly. We find that this synergistically integrated fuel cell reached peak power densities of 1,130 mW cm -2 at 160 °C and 1,740 mW cm -2 at 240 °C under H2/O2 conditions, substantially outperforming polybenzimidazole- and metal phosphate-based fuel cells. Our result indicates a pathway towards using phosphonated polymers in high-performance fuel cells under hot and dry operating conditions.

25 ENERGY STORAGE↗
DOE OSTI2023

New value of old knowledge: sulphur-based GaAs surface passivation and potential GaAs application in molecular electronics and spintronics

GaAs is well known for its extremely high electron mobility and direct band gap. Owing to the technological advances in silicon-based technology, GaAs has been limited to niche areas. This paper discusses the application of GaAs in molecular electronics and spintronics as a potential field for considering this amazing but challenging material. GaAs is challenging because its surface is characterized by a high density of surface states, which precludes the utilization of this semiconducting material in mainstream devices. Sulfur(S)-based passivation has been found to be significantly useful for reducing the effect of dangling bonds and was researched thoroughly. GaAs applications in molecular spintronics and electronics can benefit significantly from prior knowledge of GaAs and S interactions because S is a popular functional group for bonding molecular device elements with different semiconductors and metals. In this article, the problem associated with the GaAs surface is discussed in a tutorial form. A wide variety of surface passivation methods has been briefly introduced. We attempted to highlight the significant differences in the S-GaAs interactions for different S passivation methods. We also elaborate on the mechanisms and atomic-scale understanding of the variation in surface chemistry and reconstruction due to various S passivation methods. It is envisioned that GaAs and thiol-terminated molecule-based novel devices can exhibit innovative device characteristics and bring the added advantage of S-based passivation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

High thermoelectric figure of merit of porous Si nanowires from 300 to 700 K.

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Long operating lifetime mid-infrared LEDs based on black phosphorus

Abstract Black phosphorus (BP) is a narrow bandgap layered semiconductor promising for mid-infrared optoelectronic applications. BP-based devices have been shown to surpass state-of-the-art mid-infrared detectors and light-emitting diodes (LEDs) in terms of performance. Despite their device advantages, the material’s inherent instability in the air could hinder its use in practical optoelectronic applications. Here, we investigated the impact of passivation on the device lifetime of BP LEDs, which deteriorate in a matter of seconds without using passivation. The lifetime is significantly extended with an Al 2 O 3 passivation layer and nitrogen packaging via atomic layer deposition and ultra-violet curable resin sealing. The operational lifetime (half-life) at room temperature is extrapolated to be ~15,000 h with an initial power density of 340 mW/cm 2 based on accelerated life testing. The present results indicate that efficient BP optoelectronics can be highly robust through simple and scalable packaging technologies, with important practical implications for mid-infrared applications.

42 ENGINEERING↗
DOE OSTI2025

Polarization and domains in wurtzite ferroelectrics: Fundamentals and applications

The 2019 report of ferroelectricity in (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] broke a long-standing tradition of considering AlN the textbook example of a polar but non-ferroelectric material. Combined with the recent emergence of ferroelectricity in HfO2-based fluorites [Böscke et al., Appl. Phys. Lett. 99, 102903 (2011)], these unexpected discoveries have reinvigorated studies of integrated ferroelectrics, with teams racing to understand the fundamentals and/or deploy these new materials—or, more correctly, attractive new capabilities of old materials—in commercial devices. The five years since the seminal report of ferroelectric (Al,Sc)N [Fichtner et al., J. Appl. Phys. 125, 114103 (2019)] have been particularly exciting, and several aspects of recent advances have already been covered in recent review articles [Jena et al., Jpn. J. Appl. Phys. 58, SC0801 (2019); Wang et al., Appl. Phys. Lett. 124, 150501 (2024); Kim et al., Nat. Nanotechnol. 18, 422–441 (2023); and F. Yang, Adv. Electron. Mater. 11, 2400279 (2024)]. We focus here on how the ferroelectric wurtzites have made the field rethink domain walls and the polarization reversal process—including the very character of spontaneous polarization itself—beyond the classic understanding that was based primarily around perovskite oxides and extended to other chemistries with various caveats. The tetrahedral and highly covalent bonding of AlN along with the correspondingly large bandgap lead to fundamental differences in doping/alloying, defect compensation, and charge distribution when compared to the classic ferroelectric systems; combined with the unipolar symmetry of the wurtzite structure, the result is a class of ferroelectrics that are both familiar and puzzling, with characteristics that seem to be perfectly enabling and simultaneously nonstarters for modern integrated devices. The goal of this review is to (relatively) quickly bring the reader up to speed on the current—at least as of early 2025—understanding of domains and defects in wurtzite ferroelectrics, covering the most relevant work on the fundamental science of these materials as well as some of the most exciting work in early demonstrations of device structures.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗