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At least 199 records · Page 11
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Depth-resolved cathodoluminescence and surface photovoltage spectroscopies of gallium vacancies in β-Ga 2 O 3 with neutron irradiation and forming gas anneals

The gallium vacancy is one of the dominant native point defects in β-Ga 2 O 3 , one that, together with its complexes, can have a major effect on free carrier densities and transport in this wide bandgap semiconductor. We used a combination of depth-resolved cathodoluminescence spectroscopy and surface photovoltage spectroscopy to identify the optical and energy-level properties of these defects as well as how their defect densities and spatial distributions vary with neutron irradiation and temperature-dependent-forming gas anneals. These studies reveal optical signatures that align closely with theoretical energy-level predictions. Likewise, our optical techniques reveal variations in these defect densities that are consistent with hydrogen passivation of gallium vacancies as a function of temperature and depth from the free Ga 2 O 3 surface. These techniques can help guide the understanding and control of dominant native point defects in Ga 2 O 3 .

42 ENGINEERING↗
DOE OSTI2021

Coexisting Multiple Martensites in Ni 57-x Mn 21+x Ga 22 Ferromagnetic Shape Memory Alloys: Crystal Structure and Phase Transition

A comprehensive study of the crystal structure and phase transition as a function of temperature and composition in Ni 57–x Mn 21+x Ga 22 (x = 0, 2, 4, 5.5, 7, 8) (at. %) magnetic shape memory alloys was performed by a temperature-dependent synchrotron X-ray diffraction technique and transmission electron microscopy. A phase diagram of this Ni 57–x Mn 21+x Ga 22 alloy system was constructed. The transition between coexisting multiple martensites with monoclinic and tetragonal structures during cooling was observed in the Ni 51.5 Mn 26.5 Ga 22 (x = 5.5) alloy, and it was found that 5M + 7M multiple martensites coexist from 300 K to 160 K and that 5M + 7M + NM multiple martensites coexist between 150 K and 100 K. The magnetic-field-induced transformation from 7M martensite to NM martensite at 140 K where 5M + 7M + NM multiple martensites coexist before applying the magnetic field was observed by in situ neutron diffraction experiments. The present study is instructive for understanding the phase transition between coexisting multiple martensites under external fields and may shed light on the design of novel functional properties based on such phase transitions.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Decoding the Atomic Structure of Ga 2 Te 5 Pulsed Laser Deposition Films for Memory Applications Using Diffraction and First-Principles Simulations

Neuromorphic computing, reconfigurable optical metamaterials that are operational over a wide spectral range, holographic and nonvolatile displays of extremely high resolution, integrated smart photonics, and many other applications need next-generation phase-change materials (PCMs) with better energy efficiency and wider temperature and spectral ranges to increase reliability compared to current flagship PCMs, such as Ge 2 Sb 2 Te 5 or doped Sb 2 Te. Gallium tellurides are favorable compounds to achieve the necessary requirements because of their higher melting and crystallization temperatures, combined with low switching power and fast switching rate. Ga 2 Te 3 and non-stoichiometric alloys appear to be atypical PCMs; they are characterized by regular tetrahedral structures and the absence of metavalent bonding. The sp 3 gallium hybridization in cubic and amorphous Ga 2 Te 3 is also different from conventional p-bonding in flagship PCMs, raising questions about its phase-change mechanism. Furthermore, gallium tellurides exhibit a number of unexpected and highly unusual phenomena, such as nanotectonic compression and viscosity anomalies just above their melting points. Using high-energy X-ray diffraction, supported by first-principles simulations, we will elucidate the atomic structure of amorphous Ga 2 Te 5 PLD films, compare it with the crystal structure of tetragonal gallium pentatelluride, and investigate the electrical, optical, and thermal properties of these two materials to assess their potential for memory applications, among others.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Anisotropic electrical properties of NiO x /β-Ga 2 O 3 p-n heterojunctions on (2̅01), (001), and (010) crystal orientations

Abstract NiO x / β -Ga 2 O 3 p-n heterojunctions fabricated on ( 2 ̅ 01 ) , ( 001 ) , and ( 010 ) β -Ga 2 O 3 substrates show distinctly anisotropic electrical properties. All three devices exhibited excellent rectification ≥10 9 , and turn-on voltages >2.0 V. The ( 010 ) device showed very different turn-on voltage, specific on-resistance, and reverse recovery time compared with ( 2 ̅ 01 ) and ( 001 ) devices. Moreover, it is calculated that the interface trap state densities for ( 2 ̅ 01 ) , ( 001 ) , and ( 010 ) plane devices are 4.3 × 10 10 , 7.4 × 10 10 , and 1.6 × 10 11 eV –1 cm –2 , respectively. These differences in the NiO x / β -Ga 2 O 3 heterojunctions are attributed to the different atomic configurations, the density of dangling bonds, and interface trap state densities.

Physics↗
DOE OSTI2020

Materials Data on Ga(Bi3O5)4 by Materials Project

Ga(Bi3O5)4 crystallizes in the cubic I23 space group. The structure is three-dimensional. Ga2+ is bonded to four equivalent O2- atoms to form GaO4 tetrahedra that share corners with twelve equivalent BiO5 square pyramids. All Ga–O bond lengths are 1.89 Å. Bi+3.17+ is bonded to five O2- atoms to form distorted BiO5 square pyramids that share corners with eight equivalent BiO5 square pyramids, a cornercorner with one GaO4 tetrahedra, and an edgeedge with one BiO5 square pyramid. There are a spread of Bi–O bond distances ranging from 2.11–2.59 Å. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a trigonal planar geometry to three equivalent Bi+3.17+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three equivalent Bi+3.17+ atoms. In the third O2- site, O2- is bonded to one Ga2+ and three equivalent Bi+3.17+ atoms to form distorted corner-sharing OGaBi3 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga(MoSe2)4 by Materials Project

Ga(MoSe2)4 crystallizes in the cubic F-43m space group. The structure is three-dimensional. Mo+3.25+ is bonded to six Se2- atoms to form distorted MoSe6 pentagonal pyramids that share corners with three equivalent GaSe4 tetrahedra and edges with six equivalent MoSe6 pentagonal pyramids. There are three shorter (2.48 Å) and three longer (2.76 Å) Mo–Se bond lengths. Ga3+ is bonded to four equivalent Se2- atoms to form GaSe4 tetrahedra that share corners with twelve equivalent MoSe6 pentagonal pyramids. All Ga–Se bond lengths are 2.45 Å. There are two inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to three equivalent Mo+3.25+ and one Ga3+ atom to form a mixture of distorted corner and edge-sharing SeGaMo3 tetrahedra. In the second Se2- site, Se2- is bonded in a 3-coordinate geometry to three equivalent Mo+3.25+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga(IO3)3 by Materials Project

Ga(IO3)3 crystallizes in the hexagonal P6_3 space group. The structure is three-dimensional. Ga3+ is bonded in an octahedral geometry to six O2- atoms. There are three shorter (2.00 Å) and three longer (2.01 Å) Ga–O bond lengths. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted single-bond geometry to one I5+ atom. The O–I bond length is 1.80 Å. In the second O2- site, O2- is bonded in a distorted bent 120 degrees geometry to one Ga3+ and one I5+ atom. The O–I bond length is 1.89 Å. In the third O2- site, O2- is bonded in a bent 120 degrees geometry to one Ga3+ and one I5+ atom. The O–I bond length is 1.86 Å. I5+ is bonded in a 3-coordinate geometry to three O2- atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga(MoSe2)4 by Materials Project

Ga(MoSe2)4 crystallizes in the trigonal R3m space group. The structure is three-dimensional. there are two inequivalent Mo+3.25+ sites. In the first Mo+3.25+ site, Mo+3.25+ is bonded to six Se2- atoms to form distorted MoSe6 pentagonal pyramids that share corners with three equivalent GaSe4 tetrahedra and edges with six MoSe6 pentagonal pyramids. There are a spread of Mo–Se bond distances ranging from 2.47–2.76 Å. In the second Mo+3.25+ site, Mo+3.25+ is bonded to six Se2- atoms to form distorted MoSe6 pentagonal pyramids that share corners with three equivalent GaSe4 tetrahedra and edges with six equivalent MoSe6 pentagonal pyramids. There are three shorter (2.49 Å) and three longer (2.79 Å) Mo–Se bond lengths. Ga3+ is bonded to four Se2- atoms to form GaSe4 tetrahedra that share corners with twelve MoSe6 pentagonal pyramids. All Ga–Se bond lengths are 2.45 Å. There are four inequivalent Se2- sites. In the first Se2- site, Se2- is bonded to three Mo+3.25+ and one Ga3+ atom to form a mixture of distorted edge and corner-sharing SeGaMo3 tetrahedra. In the second Se2- site, Se2- is bonded in a 3-coordinate geometry to three Mo+3.25+ atoms. In the third Se2- site, Se2- is bonded in a 3-coordinate geometry to three equivalent Mo+3.25+ atoms. In the fourth Se2- site, Se2- is bonded to three equivalent Mo+3.25+ and one Ga3+ atom to form a mixture of distorted edge and corner-sharing SeGaMo3 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga(FeO2)2 by Materials Project

Ga(FeO2)2 is Spinel-like structured and crystallizes in the orthorhombic Imma space group. The structure is three-dimensional. there are two inequivalent Fe+2.50+ sites. In the first Fe+2.50+ site, Fe+2.50+ is bonded to six O2- atoms to form FeO6 octahedra that share corners with six equivalent FeO4 tetrahedra, edges with two equivalent FeO6 octahedra, and edges with four equivalent GaO6 octahedra. There are four shorter (2.07 Å) and two longer (2.16 Å) Fe–O bond lengths. In the second Fe+2.50+ site, Fe+2.50+ is bonded to four O2- atoms to form FeO4 tetrahedra that share corners with six equivalent FeO6 octahedra and corners with six equivalent GaO6 octahedra. The corner-sharing octahedra tilt angles range from 56–58°. There is two shorter (1.92 Å) and two longer (1.96 Å) Fe–O bond length. Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with six equivalent FeO4 tetrahedra, edges with two equivalent GaO6 octahedra, and edges with four equivalent FeO6 octahedra. All Ga–O bond lengths are 2.03 Å. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a rectangular see-saw-like geometry to three Fe+2.50+ and one Ga3+ atom. In the second O2- site, O2- is bonded in a rectangular see-saw-like geometry to two Fe+2.50+ and two equivalent Ga3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga(CuN)3 by Materials Project

Ga(CuN)3 crystallizes in the cubic Pm-3m space group. The structure is three-dimensional. Cu2+ is bonded in a square co-planar geometry to four equivalent N3- atoms. All Cu–N bond lengths are 2.07 Å. Ga3+ is bonded to six equivalent N3- atoms to form corner-sharing GaN6 octahedra. The corner-sharing octahedral tilt angles are 0°. All Ga–N bond lengths are 2.07 Å. N3- is bonded to four equivalent Cu2+ and two equivalent Ga3+ atoms to form a mixture of edge and corner-sharing NGa2Cu4 octahedra. The corner-sharing octahedral tilt angles are 0°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Defect-mediated metastability and carrier lifetimes in polycrystalline (Ag,Cu)(In,Ga)Se 2 absorber materials

Using a combination of optical and electrical measurements, we develop a model for metastable defects in Ag-alloyed Cu(In,Ga)Se 2 , one of the leading thin film photovoltaic materials. By controlling the pre-selenization conditions of the back contact prior to the growth of polycrystalline (Ag,Cu)(In,Ga)Se 2 absorbers and subsequently exposing them to various stresses (light soaking and dark-heat), we explore in this paper the nature and role of metastable defects on the electro-optical and photovoltaic performance of high-efficiency solar cell materials and devices. Positron annihilation spectroscopy indicates that dark-heat exposure results in an increase in the concentration of the selenium–copper divacancy complex (V Se –V Cu ), attributed to depassivation of donor defects. Deep-level optical spectroscopy finds a corresponding increase of a defect at Ev+0.98 eV, and deep-level transient spectroscopy suggests that this increase is accompanied by a decrease in the concentration of mid-bandgap recombination centers. Time-resolved photoluminescence excitation spectroscopy data are consistent with the presence of the V Se –V Cu divacancy complex, which may act as a shallow trap for the minority carriers. Light-soaking experiments are consistent with the V Se –V Cu optical cycle proposed by Lany and Zunger, resulting in the conversion of shallow traps into recombination states that limit the effective minority carrier recombination time (and the associated carrier diffusion length) and an increase in the doping density that limits carrier extraction in photovoltaic devices.

14 SOLAR ENERGY↗
DOE OSTI2025

Synergistic effects of Al, Ga, and In doping on ZnO nanorod arrays grown via citrate-assisted hydrothermal technique for highly efficient and fast scintillator screens

To be used as efficient alpha particle scintillator in the fields of nuclear security, nuclear nonproliferation and high-energy physics, scintillator screens must have high light output and fast decay properties. While there has been a great deal of progress in scintillation efficiency, achieving fast decay time properties are still a challenge. In this work, the near band edge (NBE) UV luminescence and alpha particle induced scintillation properties of vertically aligned densely packed ZnO nanorods (NRs) doped with Al, Ga, and In have been thoroughly investigated. The high crystalline hexagonal wurtzite structure with a strong orientation through the c -axis plane (002) and aspect ratios in the range 13–22 have been observed for all ZnO NRs. Electron paramagnetic resonance (EPR) analysis exhibited paramagnetic signals at g ≈ 1.96 for all ZnO NRs. A cost effective green hydrothermal synthesis technique was employed to grow well-aligned NRs. Using citrate as an additive acting as a strong reducing agent in the solution during the crystal growth, defects on the surface are significantly suppressed, thereby enhancing the NBE UV emission. Significantly higher NBE UV emission was observed from the top surface of ZnO NRs in cathodoluminescence (CL) microscopy. Results show that citrate assisted donor doping of ZnO NRs not only reduces the defect emission and NBE self-absorption, but also induces fast decay time (~ 600–700 ps), which makes ZnO NRs a good candidate for fast alpha particle scintillator screens used in associated particle imaging for time and direction tagging of individual neutrons generated in D–T and D–D neutron generators.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Radiation Response of Ga 2 O 3 MOSFETs Probed via Focused Particle Beams

Gallium Oxide (Ga 2 O 3 ), particularly in its β-phase, is attracting lots of interest for high-power and high-voltage electronics due to its wide bandgap, high breakdown field, and thermal stability. This study investigates the radiation response of Ga 2 O 3 Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs) under Total Ionizing Dose (TID) and Displacement Damage (DD) conditions, which are critical for applications in radiation environments. Utilizing a dual-beam Focused Ion Beam and Scanning Electron Microscope setup, microscale analysis of radiation effects on individual devices is performed. The findings reveal distinct behaviors under TID and DD + TID conditions, with TID leading to threshold voltage shifts due to trapped charges, while DD results in decreased drive current attributed to increased carrier scattering from lattice defects. Notably, it is demonstrated that the TID effect can be mitigated through dynamic threshold voltage adjustments and that the predicted TID from ions calculated by Monte Carlo simulations overestimates actual TID due to unaccounted charge yield effects. In conclusion, this research enhances the understanding of Ga2O3 MOSFETs' performance in harsh radiation environments, providing insights for the design of robust electronic devices for space and nuclear applications.

MOSFET↗
DOE OSTI2020

Behavior of Na and RbF-Treated CdS/Cu(In,Ga)Se 2 Solar Cells with Stress Testing under Heat, Light, and Junction Bias

In this work, the effects of Na and RbF alkali treatment on the metastability behavior of CdS/Cu(In,Ga)Se 2 solar cells are investigated with stress factors of heat, junction bias, and illumination. Four device types with and without Na or RbF treatments are subjected to heat- and light-soaking under open- and short-circuit (OC, SC) junction bias. Low-Na devices show a higher bandgap due to increased minimum Ga content, higher recombination current, and lower open-circuit voltage (V OC ). Devices with RbF post-deposition treatment (PDT) show an improvement in net doping density ≈10 16 cm –3 , V OC , and efficiency. Heat- and light-soaking under OC junction bias provokes an increase in net carrier concentration and V OC irrespective of the alkali treatments. After SC stress, a decrease in V OC and net carrier concentration is observed, which can be stabilized by RbF-PDT. An increase in Na and oxygen concentration in CIGS is observed for baseline and low-Na devices, respectively, after OC stress. The oxygen concentration in CdS decreases after heat- and light-soaking for devices without RbF-PDT, whereas it remains unchanged for devices with RbF-PDT. The atomic concentration profiles in CIGS significantly stabilize as a function of stress with the addition of RbF-PDT.

14 SOLAR ENERGY↗
DOE OSTI2024

Rigidity‐Driven Structural Isomers in the NaCl–Ga 2 S 3 System: Implications for Energy Storage

Alternative energy sources require the search for innovative materials with promising functionalities. Systems with unusual chemical properties represent an insufficiently explored domain, concealing unexpected features. Using diffraction and Raman spectroscopy over a wide temperature range, supported by first‐principles simulations, a rare phenomenon is unveiled: phase‐dependent chemical interactions between binary components in the NaCl–Ga 2 S 3 system. In this unique occurrence, previously intact binary crystalline species transform upon melting into mixed liquid structural isomers, forming bonds with new partners. The chemical combinatorics appears to be fully reversible for stable crystals and liquids. Despite this, rapidly frozen glasses out of thermodynamic equilibrium remain in a metastable isomeric state, offering remarkable properties, particularly a high room‐temperature Na + conductivity, comparable to the best sodium halide superionic conductors and therefore encouraging for sodium solid‐state batteries and energy applications. A rigidity paradigm is responsible for the observed phenomenon, as the extremely constrained Ga 2 S 3 crystal lattice does not survive viscous flow, breaking up at a short‐range level. The removal of rigidity constraints and dense packing leads to a significant increase in empty space, which is the origin of high sodium diffusivity. Broadly, the rigidity‐driven structural isomerism opens up an inspiring path to the discovery of atypical materials.

25 ENERGY STORAGE↗
DOE OSTI2023

Nonradiative Recombination Dominates Voltage Losses in Cu(In,Ga)Se 2 Solar Cells Fabricated using Different Methods

Voltage losses reduce the photovoltaic conversion efficiency of thin‐film solar cells and are a primary efficiency limitation in Cu(In,Ga)Se 2 . Herein, voltage loss analysis of Cu(In,Ga)Se 2 solar cells fabricated at three institutions with variation in process, bandgap, absorber structure, postdeposition treatment (PDT), and efficiency is presented. Nonradiative voltage losses due to Shockley–Read–Hall charge carrier recombination dominate and constitute >75% of the total compared to <25% from radiative voltage losses. The radiative voltage loss results from nonideal absorption and carriers in band tails that stem from local composition‐driven potential fluctuations. It is shown that significant bulk lifetime improvements are achieved for all alkali PDT processed absorbers, chiefly associated with reductions in nonradiative recombination. Primary voltage loss contributions (radiative and nonradiative) change little across fabrication processes, but variation in submechanisms (bulk lifetime, net acceptor concentration, and interface recombination) differentiate nonradiative loss pathways in this series of solar cells.

14 SOLAR ENERGY↗
DOE OSTI2025

Interstitial Atoms and the Frustrated and Allowed Structural Transitions Principle: Tunability in the Electronic Structure of AuCu 3 ‐type Frameworks in Dy 4 T 1− x Ga 12 (T = Ag, Ir)

In this Article, we explore how the chemical pressure (CP) features of an intermetallic phase may provide opportunities to couple perturbations in electron count with the stabilization of the underlying geometrical structure. AuCu 3 ‐type LnGa 3 (Ln = lanthanide or group 3 metal) phases contain octahedral cavities of negative CP held open by overly compressed Ln–Ga interactions, leading to a series of transition metal‐stuffed derivatives. We present new additions to this family with the synthesis and crystal structures of Dy 4 T 1−x Ga 12 with (T, x) = (Ag, 0.29) and (Ir, 0.15), adopting Y 4 PdGa 12 ‐type superstructures of the AuCu 3 ‐type. density functional Ttheory (DFT)‐CP calculations, when adjusted to avoid dipolar CP features, affirm that T atom incorporation provides a mechanism for the relief of packing tensions, while electronic density of states distributions illustrate that the T atoms serve largely as electron or hole donors to the band structure, as needed for them to attain d 10 configurations. The maximum obtainable value for x may be limited by a mismatch between the Fermi energy and pseudogap, in line with the balance of factors envisioned by the frustrated and allowed structural transitions principle. Furthermore, trends in resistivity measurements on T = Ir, Pd, and Ag compounds are interpretable in terms of the varying degrees of disorder arising from x < 1.0.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Corrosion in Other Liquid Metals (Li, PbLi, Hg, Sn, Ga)

A wide range of liquid metals have been considered for application in nuclear fission and fusion reactors. Liquid mercury (Hg) was tested as a coolant and working fluid for nuclear fission reactors and as a neutron source target. Liquid lithium and lead lithium eutectic (Li and PbLi) have been extensively studied for fusion reactor designs including plasma facing components (PFCs). Liquid tin and gallium (Sn and Ga) have recently gained attention as alternative PFCs due to their low vapor pressure and chemical stability. To enable successful application of these less common liquid metals, corrosion compatibility with containment materials needs to be investigated. For this purpose, this article reviews corrosion behavior and structural material compatibility, including ferrous alloys, refractory metals, and ceramics, by liquid Li, PbLi, Sn, Ga, and Hg.

Jun, Jiheon↗