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At least 235 records · Page 13

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DOE OSTI2020

A genetic algorithm approach for reconstructing spectral content from filtered x-ray diode array spectrometers

Filtered diode array spectrometers are routinely employed to infer the temporal evolution of spectral power from x-ray sources, but uniquely extracting spectral content from a finite set of broad, spectrally overlapping channel spectral sensitivities is decidedly nontrivial in these under-determined systems. Here, we present the use of genetic algorithms to reconstruct a probabilistic spectral intensity distribution and compare to the traditional approach most commonly found in the literature. Unlike many of the previously published models, spectral reconstructions from this approach are neither limited by basis functional forms nor do they require a priori spectral knowledge. While the original intent of such measurements was to diagnose the temporal evolution of spectral power from quasi-blackbody radiation sources—where the exact details of spectral content were not thought to be crucial—we demonstrate that this new technique can greatly enhance the utility of the diagnostic by providing more physical spectra and improved robustness to hardware configuration for even strongly non-Planckian distributions.

47 OTHER INSTRUMENTATION↗

DOE OSTI2020

Non-uniform longitudinal current density induced power saturation in GaAs-based high power diode lasers

The output power of modern 975 nm GaAs-based broad area diode lasers is limited by increasing carrier and photon losses at high bias. Here, we use experiment and one-dimensional calculations on these devices to reveal that higher current densities (and hence higher local recombination rates and higher losses) arise near the front facet due to spatial hole burning and that the non-uniformity is strongly affected by laser geometry, which is more severe for longer resonators and less severe for higher front facet reflectivity. Specifically, we use devices with a segmented p-contact to directly measure the current distribution along the resonator and compare this with laser simulation. Devices with a 6 mm resonator show 29% more current at the front than back, twice as large as the 15% current non-uniformity in devices with a 3 mm resonator. In contrast, increased front facet reflectivity (20% rather than 0.8%) is shown to almost halve the current non-uniformity from 29% to 18% in devices with a 6 mm resonator and reduces power saturation. Although the magnitude of current non-uniformity in experiment and theory is broadly consistent, in experiment, an additional divergence is seen in current flow (and hence recombination rate) near the facets, and earlier power saturation occurs. Finally, we discuss the possible saturation mechanisms that are not included in the simulation.

47 OTHER INSTRUMENTATION↗

DOE OSTI2020

Directional shock diode behavior through the interaction of geometric voids in engineered polymer assemblies

With the advent of additive manufacturing (AM) techniques, a new class of shockwave mitigation and structural supports has been realized through the hierarchical assembly of polymer materials. To date, there have been a limited number of studies investigating the role of structure on shockwave localization and whether AM offers a means to tailor shockwave behavior. Of particular interest is whether the mesoscopic structure can be tailored to achieve shockwave properties in one direction of impact vs the other. In this work, we illustrate directional response in engineered polymer foams. In situ time-resolved x-ray phase contrast imaging at the Advanced Photon Source was used to characterize these diode-like structures. This work offers a breakthrough in materials technology for the development of protective structures that require augmentation of shock in one direction while diminishing transmission in the opposite direction.

42 ENGINEERING↗

DOE OSTI2020

Stable and efficient blue and green organic light emitting diodes employing tetradentate Pt(II) complexes

PtON1-tBu and PtON6-tBu-doped blue organic light-emitting diodes (OLEDs) demonstrated estimated operational lifetimes LT 70 of 174 and 380 h at an initial luminance of 1000 cd/cm 2 with CIE coordinates of (0.188, 0.256) and (0.197, 0.309), respectively. Modifying the molecular structure of the lumiphore resulted in a PtON12-tBu-based green OLED, which realized an estimated operational lifetime LT 70 of 1410 h at an initial luminance of 1000 cd/cm 2 with CIE coordinates of (0.281, 0.559). These studies demonstrate that the tetradentate Pt(II) complexes bearing phenyl-pyrazole or benzopyrazole can act as stable phosphorescent emitters for the development of efficient and stable blue and green OLEDs.

42 ENGINEERING↗

DOE OSTI2021

High-resolution planar electron beam induced current in bulk diodes using high-energy electrons

Understanding the impact of high-energy electron radiation on device characteristics remains critical for the expanding use of semiconductor electronics in space-borne applications and other radiation harsh environments. Here in this paper, we report on in situ measurements of high-energy electron radiation effects on the hole diffusion length in low threading dislocation density homoepitaxial bulk n-GaN Schottky diodes using electron beam induced current (EBIC) in high-voltage scanning electron microscopy mode. Despite the large interaction volume in this system, quantitative EBIC imaging is possible due to the sustained collimation of the incident electron beam. This approach enables direct measurement of electron radiation effects without having to thin the specimen. Using a combination of experimental EBIC measurements and Monte Carlo simulations of electron trajectories, we determine a hole diffusion length of 264 ± 11 nm for n-GaN. Irradiation with 200 kV electron beam with an accumulated dose of 24 × 10 16 electrons cm -2 led to an approximate 35% decrease in the minority carrier diffusion length.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2021

Light extraction in tandem organic light emitting diodes

Since the invention of organic light emitting diodes (OLEDs), great research efforts have been dedicated to improving their efficiency and lifetime. For high-brightness applications, tandem OLED structures have advantages because of the lower current densities required to achieve high brightness. With the successful development of highly efficient charge generation layers, high brightness tandem OLEDs are used in displays and lighting. However, the major challenge for tandem OLEDs is the low light extraction efficiency, because about 50% of the light is trapped inside the device as waveguide modes. In this Perspective, we first review the recent works done on light extraction, analyze different waveguide mode extraction structures, and then identify the key factors determining the extraction efficiencies in tandem OLEDs.

42 ENGINEERING↗

DOE OSTI2022

Edge termination in vertical GaN diodes: Electric field distribution probed by second harmonic generation

We characterized the electric field distribution of GaN-on-GaN p–n diodes with partially compensated ion-implanted edge termination (ET) using an electric field induced second harmonic generation technique (EFISHG). The distributed electric field from the anode to the outer edge of the ET demonstrates the effectiveness of the ET structure. However, EFISHG also shows that its effectiveness is strongly dependent on the acceptor charge distribution in the ET's partially compensated layer (PC). A generally lower amount of acceptor charge can be inferred from the measured electric field distribution resulting from excessive ion implantation energy or dose during ET fabrication and causing lower than optimal breakdown voltage. Localized field crowding can be observed when the remaining acceptors uncompensated by the implant in the PC layer are nonuniformly distributed around the periphery of the devices. Important information can be obtained from these direct electric field measurements and used for optimizing the device design and fabrication process.

Cao, Yuke↗

DOE OSTI2022

GaN-on-GaN p-i-n diodes with avalanche capability enabled by eliminating surface leakage with hydrogen plasma treatment

Traditional mesa terminations require precise angle design to reduce the electric field at the edge and surface treatment to reduce etch damage. Otherwise, the device usually suffers a premature breakdown. This work proposes the use of easy-to-implement hydrogen plasma treatment to solve the premature breakdown caused by mesa and demonstrates the avalanche capability in GaN-on-GaN p-i-n diodes. The breakdown electric field when the avalanche occurred was ∼2.3 MV/cm at room temperature for a GaN drift layer with a doping concentration of ∼7 × 1015 cm−3, which is consistent with the theoretical value. The temperature coefficient of the avalanche breakdown voltage of the devices was 4.64–4.85 × 10−4 K−1. This work shows a simple and effective approach to achieve avalanche capability in vertical GaN power devices, which can serve as an important reference for the future development of efficient and robust GaN power electronics.

Fu, Kai (ORCID:0000000294057512)↗

DOE OSTI2023

Imaging suprathermal x-rays from a laboratory plasma jet using PIN-diode-based and scintillator-based 1D pinhole/coded aperture cameras

A PIN-diode-based 1D x-ray camera and a scintillator-based 1D x-ray camera, both with a microsecond to submicrosecond time resolution, have been developed to perform time-resolved imaging of transient, low-intensity, suprathermal x-rays associated with magnetohydrodynamic instabilities disrupting a plasma jet. These cameras have a high detection efficiency over a broad x-ray band, a wide field of view, and the capability to produce >50 time-resolved frames with a ≤1 μs time resolution. The x-ray images are formed by a pinhole or by a coded aperture placed outside a vacuum chamber in which the plasma jet is launched. The 1D imaging shows that the location of the x-ray source is either a few centimeters away from an inner disk electrode or near a spatially translatable metal frame that is 30–40 cm away from the electrode. Compared to a pinhole, a coded aperture increases the signal collection efficiency but also introduces unwanted artifacts.

Instruments & Instrumentation↗

DOE OSTI2023

β -Ga 2 O 3 trench Schottky diodes by low-damage Ga-atomic beam etching

β-Ga 2 O 3 trench Schottky barrier diodes fabricated through a Gallium atomic beam etching technique with excellent field strength and power device figure of merit are demonstrated. Trench formation was accomplished by a low-damage Ga flux etch that enables near-ideal forward operating characteristics that are independent of fin orientation. The reverse breakdown field strength of greater than 5.10 MV/cm is demonstrated at a breakdown voltage of 1.45 kV. This result demonstrates the potential for Ga atomic beam etching and high-quality dielectric layers for improved performance in β-Ga 2 O 3 vertical power devices.

42 ENGINEERING↗

DOE OSTI2023

Ferroelectric Schottky diodes of CuInP 2 S 6 nanosheet

Ferroelectricity in van der Waals (vdW) layered material has attracted a great deal of interest recently. CuInP 2 S 6 (CIPS), the only vdW layered material whose ferroelectricity in the bulk was demonstrated by direct polarization measurements, was shown to remain ferroelectric down to a thickness of a few nanometers. However, its ferroelectric properties have just started to be explored in the context of potential device applications. Here, we report here the preparation and measurements of metal-ferroelectric semiconductor-metal heterostructures using nanosheets of CIPS obtained by mechanical exfoliation. Four bias voltage and polarization dependent resistive states were observed in the current–voltage characteristics, which we attribute to the formation of ferroelectric Schottky diode, along with switching behavior.

36 MATERIALS SCIENCE↗

DOE OSTI2024

Effect of molecular permanent dipole moment on guest aggregation and exciton quenching in phosphorescent organic light emitting diodes

This study explores the effect of molecular permanent dipole moment (PDM) on aggregation of guest molecules in phosphorescent host–guest organic light-emitting diodes (OLEDs). Through a combination of photoluminescence measurements, high-angle annular dark-field scanning transmission electron microscopy analysis, and an Ising model based physical vapor-deposition simulation, we show that higher PDM of tris[2-phenylpyridinato-C2,N]iridium(III) guest can actually lead to a reduced aggregation relative to tris[bis[2-(2-pyridinyl-N)phenyl-C] (acetylacetonato)iridium(III) when doped into a non-polar host 1,3,5-tris(carbazol-9-yl)benzene. This study further explores the effect of host polarity by using a polar host 3′,5′-di(carbazol-9-yl)-[1,1′-biphenyl]-3,5-dicarbonitrile, and it is shown that the polar host leads to reduced guest aggregation. In conclusion, this study provides a comprehensive understanding of the impact of molecular PDM on OLED material efficiency and stability, providing insights for optimizing phosphorescent OLED materials.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2024

Imaging of dark line defect growth in high-power diode laser cavities using broadband near infrared light emission from the laser cavity

An in situ and nondestructive technique is developed to image the formation and evolution of dark line defects in the cavity of a high-power diode laser. Here, the technique uses broadband near infrared emission that originates in the laser's core layers and enables defects to be imaged with high spatial resolution through the substrate. In particular, it enables defect imaging through the substrate of shorter wavelength lasers, even when the substrate is opaque near the lasing wavelength. The evolution of dark line defects during aging is studied in several devices, with correlations established between the observed characteristics of defect growth and changes in device parameters such as optical power, operating wavelength, threshold current, and slope efficiency. Gradual degradation is found to be associated with dark line defects that slowly propagate from dark spots that are present in the device interior in its fresh (unaged) condition, rather than propagating from spots that are formed during aging, suggesting a strategy to screen fresh devices for expected reliability. This defect growth phenomenon is found to be particularly evident in the dark spots near the output facet.

47 OTHER INSTRUMENTATION↗

DOE OSTI2025

Quantitative analysis of leakage current in III-nitride micro-light-emitting diodes

In this study, the electrical characteristics under forward- and reverse-bias conditions of III-nitride blue and green micro-light-emitting diodes (μLEDs) are analyzed. A fitting model is proposed to determine the contributions of reverse leakage current and the effectiveness of sidewall treatments. Moreover, the forward-bias currents of the μLEDs are examined using the extracted ideality factor to examine the impacts of sidewall defects. The results show that sidewall treatments are highly effective for suppression of leakage currents. From the efficiency perspective, higher wall-plug efficiency (WPE) than external quantum efficiency (EQE) is observed when the operating voltage is lower than the photon voltage in both blue and green 20 × 20 μm 2 devices. This enhancement of the WPE over the EQE is due to the suppression of Shockley–Read–Hall (SRH) nonradiative recombination. These observations indicate that μLEDs with sidewall treatments not only improve optical performance but also further enhance the electrical performance of devices by suppressing the leakage current paths due to SRH nonradiative recombination processes.

42 ENGINEERING↗

DOE OSTI2025

Laser beam pointing stabilization using analog position-sensitive diodes

Laser wire scanners have been used for ion beam profile and emittance measurements at the Spallation Neutron Source linear accelerator. Due to propagation distances exceeding 100 m, reliable measurements require laser beam stabilization, previously accomplished with a feedback control loop using a digital camera as a position sensor [Hardin et al., Opt. Express 19, 2874 (2011)]. Here, this paper presents an upgraded pointing stabilization system utilizing analog position-sensitive diodes (PSDs) as the position detector, optimized with an optical diffuser. The new system significantly surpasses the previous system by offering an order-of-magnitude improvement in radiation tolerance and a bandwidth limited only by the burst repetition rate of the laser beam. Operating with a 60-Hz burst mode laser, the new system effectively suppresses laser beam drifts up to 30 Hz, compared to the previous system’s 4 Hz limit. In addition, the amplitude of spectral components below 0.1 Hz was reduced by a factor of more than 100, which is over seven times greater than was achieved by the previous system.

Data acquisition↗

DOE OSTI2025

Measurement of electrical characteristics of Ge-based diodes with thermal GeO2 films exposed to controlled humidity conditions

We investigated the impact of the interaction between gas-phase water molecules and thermally oxidized GeO2 on Ge on the performance of metal–oxide–semiconductor (MOS) structures. A vacuum-integrated setup was developed to form MOS diodes with a GeO2/Ge structure that had been exposed to controlled humidity conditions and to subsequently measure their electrical characteristics in situ. The capacitance–voltage (C–V) curves exhibited a significant negative shift, indicating the generation of positive charges at the GeO2/Ge interface when the GeO2 surface was exposed to humidity levels above approximately 1%. According to a previous study using electron spectroscopy, this threshold corresponds to the humidity level at which molecular water begins to grow on a GeO2/Ge structure. It is likely that gas-phase water molecules infiltrating the GeO2 film bind to local OH sites via hydrogen bonding, leading to the formation of positive fixed charges at the GeO2/Ge interface.

Sano, Shuto↗

DOE OSTI2025

Growth of tetragonal PtO by molecular-beam epitaxy and its integration into β-Ga 2 O 3 Schottky diodes

We demonstrate the epitaxial growth of tetragonal platinum monoxide (PtO) on MgO, TiO 2 , and β-Ga 2 O 3 single-crystalline substrates by ozone molecular-beam epitaxy. We provide synthesis routes and derive a growth diagram under which PtO films can be synthesized by physical vapor deposition. A combination of electrical transport and photoemission spectroscopy measurements, in conjunction with density functional theory calculations, reveal PtO to be a degenerately doped p-type semiconductor with a bandgap of E g ≈ 1.6 eV. Spectroscopic ellipsometry measurements are used to extract the complex dielectric function spectra, indicating a transition from free-carrier absorption to higher photon energy transitions at E ≈ 1.6 eV. Using tetragonal PtO as an anode contact, we fabricate prototype Schottky diodes on n-type Sn-doped β-Ga 2 O 3 substrates and extract Schottky barrier heights of ϕ B > 2.2 eV.

Hensling, Felix V. E. [Cornell Univ., Ithaca, NY (↗

DOE OSTI2025

Extending the Mott–Gurney law to one-dimensional nonplanar diodes using point transformations

Recent studies have applied variational calculus, conformal mapping, and point transformations to generalize the one-dimensional (1D) space-charge limited current density (SCLCD) and electron emission mechanisms to nonplanar geometries; however, these assessments have focused on extending the Child–Langmuir law (CLL) for SCLCD in vacuum. Since the charge in the diode is independent of the coordinate system (i.e., covariant), we apply bijective point transformations to extend the Mott–Gurney law (MGL) for the SCLCD in a collisional or semiconductor gap to nonplanar 1D geometries. This yields a modified MGL that replaces the Cartesian gap distance with a canonical gap distance that may be written generally in terms of geometric scale factors that are known for multiple geometries. We tabulate results for common geometries. Such an approach may be applied to any current density, including non-space-charge limited gaps and SCLCD that may fall between the CLL and MGL.

Garner, Allen L. [Purdue Univ., West Lafayette, IN↗