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DOE OSTI2022

Wide bandgap semiconductor materials and devices

The technological and societal impacts of electronic devices based on Ge, Si, and compound semiconductors like GaAs have been profound, fueling the decades long quest in identifying ever-larger bandgap semiconductors to untap new applications and possibilities. Specifically, an increase in the bandgap leads to shorter wavelength emission and an increased breakdown electric field, which has direct consequences for solid-state lighting like light-emitting diodes (LEDs) and laser diodes (LDs) from the visible (blue-green, blue, and violet) and beyond (UV and deep-UV) spectral range and for radically improved power devices supported by a higher intrinsic breakdown strength. Wide bandgap (WBG) semiconductors represent the frontier of materials that satisfy these criteria and include group IV, III–V, and II–VI material families like SiC (3.2 eV), GaN (3.4 eV), and ZnO (3.4 eV), respectively. With even larger bandgaps exceeding 4 eV, ultrawide bandgap (UWBG) semiconductors include diamond, III-nitrides incorporating Al and B (e.g., AlN, BN, and AlGaN), and sesquioxides like Ga 2 O 3 and (Al,Ga) 2 O 3 . These materials span widely varying stages of technological maturity, with SiC and GaN platforms among the most mature with commercially available devices in RF and high-power electronics, while other platforms such as Ga 2 O 3 rapidly advancing and poised to enable new UV and deep-UV optoelectronic devices. This Special Topic on Wide Bandgap Semiconductor Materials and Devices covers broad research subtopics on WBG and UWBG materials that span bulk crystals, epitaxy and substrate technologies, fundamental defect science, and doping, as well as electronic and optoelectronic device fabrication and characterization. Here, we highlight works from the collection, which we categorize by material platform of SiC, III-nitrides, and Ga 2 O 3 and related alloys.

36 MATERIALS SCIENCE↗

DOE OSTI2022

Low voltage AC electroluminescence in silicon MOS capacitors

Low power silicon based light source and detector are attractive for on-chip photonic circuits given their ease of process integration. However, conventional silicon light emitting diodes emit photons with energies near the band edge where the corresponding silicon photodetectors lack responsivity. On the other hand, previously reported hot carrier electroluminescent silicon devices utilizing a reverse biased diode require high operating voltages. Here, we investigate hot carrier electroluminescence in silicon metal–oxide–semiconductor capacitors operating under transient voltage conditions. Further, during each voltage transient, large energy band bending is created at the edge of the source contact, much larger than what is achievable at a steady state. As a result, electrons and holes are injected efficiently from a single source contact into the silicon channel at the corresponding voltage transient, where they subsequently undergo impact ionization and phonon-assisted interband recombination. Notably, we show low voltage operation down to 2.8 V by using a 20 nm thick high-κ gate dielectric. We show further voltage scaling is possible by reducing the gate dielectric thickness, thus presenting a low voltage platform for silicon optoelectronic integrated circuits.

42 ENGINEERING↗

DOE OSTI2023

Atomic layer etching (ALE) of III-nitrides

Atomic layer etching (ALE) was performed on (Al, In, Ga)N thin films using a cyclic process of alternating Cl 2 gas absorption and Ar + ion bombardment in an inductively coupled plasma etcher system. The etch damage was characterized by comparing photoluminescence of blue single quantum well light-emitting diodes before and after the etch as well as bulk resistivities of etched p-doped layers. It was found that etched surfaces were smooth and highly conformal, retaining the step-terrace features of the as-grown surface, thus realizing ALE. Longer exposures to the dry etching increased the bulk resistivity of etched surfaces layers slightly, with a damaged depth of ∼55 nm. With further optimization and damage recovery, ALE is a promising candidate for controlled etching with atomic accuracy. Finally, it was found that Al 0.1 Ga 0.9 N acts as an etch barrier for the ALE etch, making it a suitable etch to reveal buried V-defects in III-nitride light emitting diodes.

Atomic-layer etching↗

DOE OSTI2025

Exploring transport mechanisms in atomic precision advanced manufacturing enabled pn junctions

We investigate the different transport mechanisms that can occur in pn junction devices made using atomic precision advanced manufacturing at temperatures ranging from cryogenic to room temperature. We first elucidate the potential cause of the anomalous behavior observed in the forward-bias response of these devices in recent cryogenic temperature measurements, which deviates from the theoretical response of a silicon Esaki diode. These anomalous behaviors include current suppression at low voltages in the forward-bias response and a much lower valley voltage at cryogenic temperatures than theoretically expected for a silicon diode. To investigate the potential causes of these anomalies, we studied the effects of a few possible transport mechanisms, including band-to-band tunneling, bandgap narrowing, potential impact of non-Ohmic contacts, band quantization, impact of leakage, and inelastic trap-assisted tunneling, through semi-classical simulations. We find that a combination of two sets of band-to-band tunneling (BTBT) parameters can qualitatively approximate the shape of the tunneling current at low bias. This can arise from band quantization and realignment due to the strong potential confinement in δ-layers. We also find that the lower-than-theoretically-expected valley voltage can be attributed to modifications in the electronic band structure within the δ-layer regions, leading to a significant bandgap narrowing induced by the high density of dopants. Finally, we extend our analyses to room temperature operation and predict that trap-assisted tunneling (TAT) facilitated by phonon interactions may become significant, leading to a complex superposition of BTBT and TAT transport mechanisms in the electrical measurements.

band gap↗

DOE OSTI2026

Electric field dependence of nanoscale cathodoluminescence inside Cr doped β-Ga 2 O 3 interfaces

Chromium (Cr) is a common impurity in β-Ga 2 O 3 crystals, where its characteristic R1 and R2 luminescence lines are susceptible to both the host crystal field and externally applied fields. Here, in this work, we demonstrate that the Cr cathodoluminescence (CL) quenches toward the bulk of the crystal but enhances with applied reverse bias, reflecting the effect of free carrier depletion with increasing electric field. Furthermore, we illustrate that the R1/R2 CL intensity ratio, measured as the integrated area ratio of R1 to R2, can be used as a direct probe of the electric field in a Ni-β-Ga 2 O 3 : Cr Schottky diode. This optical calibration method provides a complementary approach to conventional C–V and I–V measurements for determining electric field strength in the depletion region of β-Ga 2 O 3 -based Schottky diodes and can be extended to other semiconductors and multilayer device structures.

36 MATERIALS SCIENCE↗

DOE OSTI2020

Aging Effect Analysis of PV Inverter Semiconductors for Ancillary Services Support

PV inverters can provide reactive power while generating active power. An ongoing microgrid implementation at Duke Energy actively engages non-utility PVs to generate/absorb reactive power in support of ancillary services to increase microgrid resiliency during extreme events. PV systems are requested to provide reactive power support: 1) in response to grid voltage variation to better regulate the local voltage; or 2) in response to utility incentives, such as following Transactive Energy System (TES) incentives. However, providing ancillary services might shorten the lifetime expectation of PV inverter semiconductors. This paper summarizes the potential impacts on a PV inverter semiconductor's lifetime when providing ancillary services. The analysis presented in this research work shows that providing reactive power support will increase the mean junction temperature and the junction temperature variation of the inverter diodes. This increased junction temperature will eventually lead to shorter diode lifetime. The lifetime estimation of semiconductors is briefly reviewed. The power losses of PV inverter semiconductors are derived as a support analysis to the junction temperature calculation. In addition, the impact of the filtering inductor on the semiconductor current distribution is discussed. The theoretical analysis presented in this research work is supported by simulation results.

14 SOLAR ENERGY↗

DOE OSTI2023

Plasmon Mediated Near–Field Energy Transfer From Solid–State, Electrically Injected Excitons to Solution Phase Chromophores

An organic diode is demonstrated that near-field energy transfers to molecules in solution via surface plasmon polaritons, in contrast to typical far-field excitation via absorption of traveling photons. Electrically generated excitons couple to surface plasmon modes in the cathode; the plasmons subsequently excite chromophore molecules on top of the cathode. External quantum efficiency and time resolved photoluminescence measurements are used to characterize the diode and the near-field energy transfer process. In addition, it is shown that excited chromophores can charge-transfer to quencher molecules, illustrating the potential of this device to be used for photochemical applications.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2023

Optically Excited Lasing in a Cavity‐Based, High‐Current‐Density Quantum Dot Electroluminescent Device

Abstract Laser diodes based on solution‐processable materials can benefit numerous technologies including integrated electronics and photonics, telecommunications, and medical diagnostics. An attractive system for implementing these devices is colloidal semiconductor quantum dots (QDs). The progress towards a QD laser diode has been hampered by rapid nonradiative Auger decay of optical‐gain‐active multicarrier states, fast device degradation at high current densities required for laser action, and unfavorable competition between optical gain and optical losses in a multicomponent device stack. Here we resolve some of these challenges and demonstrate optically excited lasing from fully functional high‐current density electroluminescent (EL) devices with an integrated optical resonator. This advance has become possible due to excellent optical gain properties of continuously graded QDs and a refined device architecture, which allows for highly efficient light amplification in a thin, EL‐active QD layer.

36 MATERIALS SCIENCE↗

DOE OSTI2024

What Matters for the Charge Transport of 2D Perovskites?

Abstract Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light‐emitting diodes, and photodetectors. However, the trade‐off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A′‐site organic spacer cations, A‐site cations, B‐site metal cations, and X‐site halide ions. Finally, an outlook on the future development of improving the 2D perovskites’ charge transport is discussed.

Zhang, Yixin↗

DOE OSTI2023

Quantum Efficiency Gain in 2D Perovskite Photo and X‐Ray Detectors

Abstract The perovskite polycrystalline thin film detector fabricated by solution method is a promising low‐cost, scalable technology for radiation imaging, but its thin volume limits the sensing efficiency for high‐energy X‐ray photons. This work reports 2D perovskite thin film photo‐diodes with a detection gain when sensing visible and X‐ray photons. Detailed power and temperature‐dependent device characterizations reveal a charge multiplication effect to be responsible for the observed high gain. This is caused by a disparity in electron and hole transport, where electron transport is retarded by a trap/de‐trap process via shallow trap states, whereas the hole transport is fast enough to produce a photoconductive gain in satisfying the charge neutrality. The 2D perovskite made with butylamine spacers is also found to exhibit a larger efficiency gain than those made with phenylethylamines because of the higher likelihood of forming shallow traps in the former. The thin film diodes feature a high temporal response over 1 MHz due to the fast charge collection across a thin volume, and the discovery provides device physics mechanisms in connection to material structure‐function relationship for future optoelectronics design that can boost the efficiency of X‐ray sensing and dim light detection.

2D perovskites↗

DOE OSTI2022

Comment on “Introduction of a Novel Figure of Merit for the Assessment of Transparent Conductive Electrodes in Photovoltaics: Exact and Approximate Form”

Anand et al. introduce an implicit equation for transparent conductive electrode (TCE) merit in photovoltaics (PV) by lumping series resistance in with the diode. However, an explicit equation has been previously derived by lumping series resistance external to the diode. For TCEs of practical interest for PV, the choice of where to lump series resistance does not affect merit, so external lumping is preferred. On the other hand, monolith deadspace, metal grid shadowing, and grid resistance shift TCE merit away from the figure of merit behavior published by Anand et al.

14 SOLAR ENERGY↗

DOE OSTI2020

Clean Energy Manufacturing: Renewable Energy Technology Benchmarks

The manufacturing of clean energy technologies has become a global enterprise. An analysis of four clean energy technology manufacturing processes—wind turbine components, crystalline silicon solar photovoltaic modules, light-duty vehicle lithium-ion battery cells, and light-emitting diodes—was conducted to provide a reference point for comparison over time as this sector evolves. The manufacturing supply chain analysis included processing of raw materials, making required subcomponents, and assembling final products, using benchmarks that crossed 12 economies: Brazil, Canada, China, Germany, India, Japan, Malaysia, Mexico, South Korea, Republic of China (Taiwan), the United Kingdom, and the United States. Larger economies, with extensive manufacturing supply chains and high prevailing wages, tend to retain more value added from clean energy manufacturing than small economies. Manufacturing of clean energy technologies drives extensive trade among economies to support the widely distributed supply chain links. Production of wind turbine components and photovoltaic modules is more centralized than production of light-emitting diodes and lithium-ion battery cells. Across the four clean energy technologies evaluated, there was generally an excess of manufacturing capacity relative to global supply.

benchmarks↗

DOE OSTI2021

Improved multiline HVDC circuit breakers with asymmetric conducting branches

Here, this paper proposes two improved multiline HVDC circuit breakers (DCBs) for HVDC grid applications, including type-1 improved multiline DCB and type-2 improved multiline DCB. The two improved DCBs draw on the core idea of extended H-bridge. m adjacent DC transmission lines share a single main breaker (MB) and surge arrester. Due to the extended H-bridge design, bidirectional current flow can be achieved by a unidirectional MB. An upper conducting branch and a lower conducting branch are connected to a DC line. The upper and lower conducting branches in the two improved multiline DCBs are asymmetric. The upper conducting branch of both two improved DCBs is formed by a load commutation switch (LCS) and an ultra-fast disconnector (UFD). The lower conducting branch of the type-1 improved scheme is composed of a diode and a UFD; and of the type-2 improved scheme is made up of diode stack. Compared with other DCBs, the proposed type-1 DCB and type-2 DCB are more cost effective. Among the five DCBs, the type-2 improved scheme requires the least number of individual switching actions. The effectiveness and feasibility of the proposed schemes are verified through simulation of a bipolar three-terminal HVDC grid in PSCAD/EMTDC.

42 ENGINEERING↗

DOE OSTI2021

Transient absorption spectroscopy to explore cellular pathways to photobiomodulation

Photobiomodulation (PBM) describes the use of low irradiance light in the red to near-infrared wavelength range to stimulate biological effects in tissue, and many biological and spectroscopic techniques are used to study PBM. However, these techniques focus on the products or downstream effects rather than the electronic transitions that initiate the PBM processes. This study presents a novel approach to studying low irradiance light exposures on individual proteins and/or protein complexes by combining a continuous wave (CW) laser diode with femtosecond transient absorption spectroscopy (TAS), coined here as CW-TAS, and tests the system on reduced cytochrome c (Cyt c) for proof of principle. TAS was conducted using a 532-nm excitation pump beam and a 350-600 nm supercontinuum probe. In this work, CW laser diodes with wavelengths of 450 nm, 635 nm, and 808 nm were interchangeably fiber coupled into the HELIOS Fire. Samples of Cyt c were tested by TAS using a pump power of 15 µW, both with and without CW exposure. CW exposures were carried out with irradiances of 1.60 and 3.20 mW/cm 2 , except for 808 nm, which was only tested at 1.60 mW/cm 2 . Both kinetic and global analyses were performed on the TAS data and the time constants for sets with and without CW exposures were compared. The TAS data for Cyt c with the full dosage of CW exposures did not alter the TAS data distinguishably from the control data. No new electronic transient signals were observed beyond the background when testing Cyt c with the CW exposures. Kinetic analysis confirmed that existing transients did not deviate beyond uncertainty. Global time constants for Cyt c were calculated to be 0.25 ± 0.03 ps and 5.1 ± 0.3 ps for the control study, and the time constants for the CW exposed Cyt c were not significantly different. This study concludes that CW irradiation, at doses delivered, does not alter the transient absorption data of Cyt c. The CW-TAS method provides a new tool for studying PBM effects in other proteins and protein complexes, such as Complex IV, in future studies.

59 BASIC BIOLOGICAL SCIENCES↗

DOE OSTI2025

Characterization of 4H-SiC Low Gain Avalanche Detectors (LGADs)

4H-SiC Low Gain Avalanche Detectors (LGADs) have been fabricated and characterized. The devices employ a circular mesa design with low-resistivity contacts and an SiO 2 passivation layer. The I–V and C–V characteristics of the 4H-SiC LGADs are compared with complementary 4H-SiC PiN diodes to confirm a high breakdown voltage and low leakage current. Both LGADs and PiN diodes were irradiated with alpha particles from a $^{210}_{84}$Po source. The charge collected by each device was compared, and it was observed that low-gain charge carrier multiplication is achieved in the 4H-SiC LGAD.

43 PARTICLE ACCELERATORS↗

DOE OSTI2020

Electrical injection and transport in Teflon-diluted hole transport materials

Diluting small molecule organic semiconductor hole transport materials with Teflon AF (TAF) dramatically increases their thermal/morphological stability while counterintuitively also increasing the current density in hole-only diodes. Here, we explore the origin of this current enhancement in co-evaporated blends of TAF and N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPD) and find that it results from improved hole injection and not from a mobility enhancement due to semiconductor dilution effects. Whereas electroabsorption measurements indicate that an interface dipole shift of up to 0.5 eV (depending on TAF concentration) aides hole injection at the indium-tin-oxide anode, time-of-flight and temperature-dependent current-voltage measurements show that the mobility of 25 vol% TAF:NPD blends decreases by roughly an order of magnitude relative to neat NPD despite having slightly lower energetic disorder. For comparison, diluting with 25 vol% high density polyethylene achieves a similar reduction in energetic disorder but only decreases the mobility by a factor of two. Furthermore, these results highlight the tradeoff between reducing energetic disorder and frustrating percolation in diluted organic semiconductors, and represent a step toward thermally-evaporated polymer-small molecule composites that maintain high mobility while offering improved morphological stability and lower refractive index for organic light emitting diodes.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2021

The Surface Chemistry and Structure of Colloidal Lead Halide Perovskite Nanocrystals

Since the initial discovery of colloidal lead halide perovskite nanocrystals, there has been significant interest placed on these semiconductors because of their remarkable optoelectronic properties, including very high photoluminescence quantum yields, narrow size- and composition-tunable emission over a wide color gamut, defect tolerance, and suppressed blinking. These material attributes have made them attractive components for next-generation solar cells, light emitting diodes, low-threshold lasers, single photon emitters, and X-ray scintillators. While a great deal of research has gone into the various applications of colloidal lead halide perovskite nanocrystals, comparatively little work has focused on the fundamental surface chemistry of these materials. While the surface chemistry of colloidal semiconductor nanocrystals is generally affected by their particle morphology, surface stoichiometry, and organic ligands that contribute to the first coordination sphere of their surface atoms, these attributes are markedly different in lead halide perovskite nanocrystals because of their ionicity. Herein, emerging work on the surface chemistry of lead halide perovskite nanocrystals is highlighted, with a particular focus placed on the most-studied composition of CsPbBr 3 . We begin with an in-depth exploration of the native surface chemistry of as-prepared, 0-D cuboidal CsPbBr 3 nanocrystals, including an atomistic description of their surface termini, vacancies, and ionic bonding with ligands. We then proceed to discuss various post-synthetic surface treatments that have been developed to increase the photoluminescence quantum yields and stability of CsPbBr 3 nanocrystals, including the use of tetraalkylammonium bromides, metal bromides, zwitterions, and phosphonic acids, and how these various ligands are known to bind to the nanocrystal surface. To underscore the effect of post-synthetic surface treatments on the application of these materials, we focus on lead halide perovskite nanocrystal-based light emitting diodes, and the positive effect of various surface treatments on external quantum efficiencies. We also discuss the current state-of-the-art in the surface chemistry of 1-D nanowires and 2-D nanoplatelets of CsPbBr 3 , which are more quantum confined than the corresponding cuboidal nanocrystals but also generally possess a higher defect density because of their increased surface area-to-volume ratios.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2025

Heterovalent Substitution of K 2 SrP 2 O 7 :Cr 3+ to Achieve Anti-Thermal-Quenching Broadband Near-Infrared Luminescence

Broadband near-infrared (NIR) light sources based on phosphor-converted light-emitting diodes are highly desirable for biochemical analysis and medical diagnosis applications. However, thermal quenching remains a demanding challenge for developing efficient NIR phosphors. Herein, we report the enhancement of both quantum efficiency and thermal stability in Cr 3+ -activated K 2 SrP 2 O 7 phosphors through a heterovalent substitution strategy by replacing Sr 2+ with Al 3+ in K 2 Sr 1–x Al x P 2 O 7 (0.05 ≤ x ≤ 0.2) to obtain optimized broadband NIR emission. Structural modulation via Al 3+ substitution leads to the optimized composition, K 2 Sr 0.88 Al 0.1 P 2 O 7 :0.02Cr 3+ , which emits across a broad NIR range of 650–1100 nm peaking at 807 nm with a full width at half-maximum of ∼130 nm under 448 nm excitation. Remarkably, its emission intensity at 150 °C remains 120% of the initial value at room temperature, demonstrating a rare antithermal-quenching behavior. Temperature-dependent XRD studies further reveal that Al 3+ substitution effectively suppresses lattice expansion at elevated temperatures, indicating enhanced lattice stability under thermal excitation. Detailed structural and spectral analyses show that the substitution enhances local site symmetry, reduces electron–phonon coupling, increases thermally induced absorption probability, and fortifies energetic barriers against nonradiative transitions. These synergistic effects collectively endow this NIR phosphor with a superior thermal stability. Furthermore, NIR light-emitting diodes fabricated with this phosphor exhibit strong potential for applications in information identification, nondestructive detection, and night vision technologies. This study demonstrates a local structure engineering strategy for designing thermally robust Cr 3+ -activated NIR phosphors, offering valuable insights into material discovery and NIR spectroscopy device development.

Cr3+↗