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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Study of phase decoherence in GeSn (8%) through measurements of the weak antilocalization effect

Alloying germanium with tin offers a means to modulate germanium's electronic structure, enabling a greater degree of control over quantum properties such as the retention of the phase or spin of the electron wave. However, the extent to which the presence of high dopant concentrations in GeSn alters these quantum behaviors is poorly understood. Here, we investigate the role of dopant concentrations on phase coherence through measurements of the weak antilocalization (WAL) effect at temperatures between 30 mK and 10 K in p-GeSn (8%) thin films, which were doped to a series of carrier densities on the order of 10 12 cm –2 . Phase coherence and spin–orbit lengths were extracted from the magnetoconductivities using the 2D Hikami–Larkin–Nagaoka model. Phase coherence lengths peaked at 577, 593, and 737 nm for the low-, mid-, and high-density samples, while upper limits on the spin–orbit lengths of less than 25 nm were relatively independent of carrier density and temperature. The phase coherence lengths increased as the temperature decreased but changed only minimally with carrier density, contrary to common models of temperature-dependent inelastic scattering. Saturation of the phase coherence lengths occurred below 600 mK. Based on these findings, intrinsically generated inelastic scattering mechanisms such as two-level systems or impurity band scattering likely contribute to phase decoherence in these alloys. Our results provide insight into the inelastic scattering mechanisms of GeSn, while suggesting a need for further investigation into phase decoherence mechanisms in doped group-IV alloys.

42 ENGINEERING↗
DOE OSTI2025

Local charge-screening and polarization switching in a ferroelectric ionic-superconductor

For ferroelectric ionic-conductors, polarization switching is complicated by the interplay between ion mobility and charge screening effects. When the ionic charge carriers also play a key role in the domain reversal, such as in Rb-doped KTiOPO 4 (RKTP), a higher level of complexity is introduced. RKTP provides an ideal platform for investigating the relationship between ionic conductivity and polarization reversal because its highly anisotropic crystal properties allow selective modification of material characteristics through diffusive cation doping. Here, we use indiffused Ba/K doping to create a significant increase in the ionic conductivity. Time-of-flight secondary ion mass spectrometry is employed to map Ba/K doping within the RKTP crystal and correlate it to changes in ionic mobility and polarization switching characteristics under an external field applied to the nonpolar face. Using band-excitation piezoresponse force microscopy, we demonstrate a selective switching-inhibition mechanism driven by the enhanced charge screening.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Selectively patterned Mg-doped GaN by SiN x -driven hydrogen injection

In this work, we demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN x ) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiN x films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance. The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.

42 ENGINEERING↗
DOE OSTI2022

First-principles calculation of gate-tunable ferromagnetism in magic-angle twisted bilayer graphene under pressure

Magic-angle twisted bilayer graphene (MATBG) is notable as a highly tunable platform for investigating strongly correlated phenomena such as unconventional superconductivity and quantum spin liquids, due to easy control of doping level through gating and sensitive dependence of the magic angle on hydrostatic pressure. Experimental observations of correlated insulating states, unconventional superconductivity and ferromagnetism in MATBG indicate that this system exhibits rich exotic phases. In this work, using density functional theory calculations in conjunction with the effective screening medium method, we find the MATBG under pressure at a twisting angle of 2.88 °and simulate how its electronic states evolve when doping level and electric field perpendicular to plane are tuned by gating. Our calculations show that, at doping levels between two electrons and four holes per moiré unit cell, a ferromagnetic (FM) solution with spin density localized at AA stacking sites is lower in energy than the nonmagnetic solution. The magnetic moment of this FM state decreases with both electron and hole doping and vanishes at four electrons/holes doped per moiré unit cell. Hybridization between the flat bands at the Fermi level and the surrounding dispersive bands can take place at finite doping. On increasing the out-of-plane electric field at zero doping, a transition from the FM state to the nonmagnetic one is seen. An investigation of impurity effects shows that both absorption of O 2 molecules and occurrence of Stone–Wales impurities suppress the FM state, and the mechanisms are understood from our calculations. We also analyze the interlayer bonding character due to flat bands via Wannier functions. Finally, we report trivial band topology of the flat bands in the FM state at a certain doping level.

Physics↗
DOE OSTI2021

Electrical, structural and thermomechanical properties of doped-LaCrO3 ceramics for high temperature electronics and sensing applications.

Many important electrical and sensor applications require stable conductors under working temperatures up to 1500oC various redox environments. Common high temperature conductors such as nitrides and carbides are not stable under oxidizing atmospheres, for this reason low redox behavior materials would be more attractive. In this work, further dopant studies of the LaCrO3 system were completed in order to control the electronic and ionic conductivity, as well as the relative chemical and microstructural stability. Neodymium and Niobium dopants were initially studied to control the p-type, and potentially n-type conductive mechanisms, respectively, but other dopants strategies were also investigated. The electrical conductivity and Seebeck coefficient were characterized up to 1500oC under varying oxygen partial pressures. The phase and microstructural evolution of each material was also investigated for these compositions by XRD and SEM. The materials showing the best electrical and thermomechanical performance were included within thick films sensors and were tested.

20 FOSSIL-FUELED POWER PLANTS↗
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 OSTI2024

Electron irradiation effects on the optical properties of Hf- and Zn-doped β -Ga 2 O 3

Optical and electrical properties of Hf- and Zn-doped β-Ga 2 O 3 samples, which are n-type and insulating, respectively, were altered via high-energy electron irradiation at 2.5 or 0.5 MeV. The β-Ga 2 O 3 :Hf samples irradiated with 2.5 MeV electrons experienced a color change from blue to yellow and a large drop in conductivity, attributed to the creation of gallium vacancies, which compensate donors. This irradiation resulted in the absence of free carrier absorption and the presence of Cr 3+ photoluminescence (PL). PL mapping prior to irradiation revealed optically active ZnO precipitates that formed during the growth of β-Ga 2 O 3 :Zn. These precipitates have a 384 nm (3.23 eV) stacking fault emission in the core; in the outer shell of the precipitate, the PL blue-shifts to 377 nm (3.29 eV) and a broad defect band is observed. After 0.5 MeV electron irradiation, the defect band broadened and increased in intensity. The blue PL band (435 nm) of β-Ga 2 O 3 was enhanced for both Hf- and Zn-doped samples irradiated with 0.5 MeV. This enhancement is correlated with an increase in oxygen vacancies.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Local p‐ and n‐Type Doping of an Oxide Semiconductor via Electric‐Field‐Driven Defect Migration

Layered oxides exhibit high ionic mobility and chemical flexibility, attracting interest as cathode materials for lithium-ion batteries and the pairing of hydrogen production and carbon capture. Recently, layered oxides emerged as highly tunable semiconductors. For example, by introducing anti-Frenkel defects, the electronic hopping conductance in hexagonal manganites is increased locally by orders of magnitude. Here, local acceptor and donor doping in Er(Mn,Ti)O3 is demonstrated, facilitated by the controlled splitting of anti-Frenkel defects under applied d.c. voltage. By combining density functional theory calculations, scanning probe microscopy, atom probe tomography, and scanning transmission electron microscopy, it is shown that the oxygen defects can readily be moved through the layered crystal structure, leading to nano-sized interstitial-rich (p-type) and vacancy-rich (n-type) regions. The resulting pattern is comparable to dipolar npn-junctions and stable on the timescale of days. These findings reveal the possibility of temporarily functionalizing oxide semiconductors at the nanoscale, giving additional opportunities for the field of oxide electronics and the development of transient electronics in general.

He, Jiali↗
DOE OSTI2022

Revealing uncommon transport in the previously unascertained very low cation clathrate-I Eu 2 Ga 11 Sn 35

In this study, high occupancy of cation sites is typical for clathrate-I compositions allowing limited tunability of the electrical properties beyond doping and elemental substitution. Herein, we report on the structure and electrical transport of single-crystal Eu 2 Ga 11 Sn 35 , the sole example of a very low (25%) cation concentration clathrate-I material with atypical transport directly attributable to the structure and stoichiometry.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Spatial Interactions in Hydrogenated Perovskite Nickelate Synaptic Networks

A key aspect of how the brain learns and enables decision-making processes is through synaptic interactions. Electrical transmission and communication in a network of synapses are modulated by extracellular fields generated by ionic chemical gradients. Emulating such spatial interactions in synthetic networks can be of potential use for neuromorphic learning and the hardware implementation of artificial intelligence. Here, in this work, we demonstrate that in a network of hydrogen-doped perovskite nickelate devices, electric bias across a single junction can tune the coupling strength between the neighboring cells. Electrical transport measurements and spatially resolved diffraction and nanoprobe X-ray and scanning microwave impedance spectroscopic studies suggest that graded proton distribution in the inhomogeneous medium of hydrogen-doped nickelate film enables this behavior. We further demonstrate signal integration through the coupling of various junctions.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Giant optical nonlinearity of Fermi polarons in atomically thin semiconductors

In this study, realizing strong nonlinear optical responses is a long-standing goal of both fundamental and technological importance. Recently, substantial efforts have been focused on exploring excitons in solids to achieve nonlinearities even down to few-photon levels. However, a crucial tradeoff arises as strong light–matter interactions require large oscillator strength and short radiative lifetime of excitons, which limits their nonlinearity. Here we experimentally demonstrate strong nonlinear optical responses with large oscillator strength by exploiting the coupling between excitons and carriers in an atomically thin semiconductor. By controlling the electric field and electrostatic doping of trilayer WSe 2 , we observe the hybridization between intralayer and interlayer excitons and the formation of Fermi polarons. Substantial optical nonlinearity is observed under continuous-wave and pulsed laser excitation, where the Fermi polaron resonance blueshifts by as much as ~10 meV. Intriguingly, we observe a remarkable asymmetry in the optical nonlinearity between electron and hole doping, which is tunable by the applied electric field. We attribute these features to the optically induced valley polarization due to the interactions between excitons and free charges. Our results establish atomically thin heterostructures as a highly versatile platform for engineering nonlinear optical response with applications to classical and quantum optoelectronics.

2d materials↗
DOE OSTI2024

Growth and characterization of ferromagnetic Ga 2 O 3 :(Cr, Mn)

The goal of this Exploratory Express project was to explore the possibility of tunable ferromagnetism in Mn or Cr incorporated epitaxial Ga 2 O 3 films. Tunability of magnetic properties can enable novel applications in spintronics, quantum computing, and magnetism-based logics by allowing control of magnetism down to the nanoscale. Carriers (electrons or holes) mediated ferromagnetic ordering in semiconductor can lead to tunable ferromagnetism by leveraging the tunability of carrier density with doping level, gate electric field, or optical pumping of the carriers. The magnetic ions (Cr or Mn) in Ga 2 O 3 act as localized spin centers which can potentially be magnetically coupled through conduction electrons to enable ferromagnetic ordering. Here we investigated tunable ferromagnetism in beta Ga 2 O 3 semiconductor host with various n-doping levels by incorporating 2.4 atomic percent Mn or Cr. The R&D approach involved growth of epitaxial Ga 2 O 3 film on sapphire or Ga 2 O 3 substrate, implantation of Mn or Cr ions, annealing of the samples post implantation, and magnetic measurements. We studied magnetic behavior of Mn:Ga 2 O 3 as a function of different n-doping levels and various annealing temperatures. The vibrating sample magnetometry (VSM) measurement exhibited strong ferromagnetic signals from the annealed Mn:Ga 2 O 3 sample with n-doping level of 5E19 cm -3 . This ferromagnetic behavior disappears from Mn:Ga 2 O 3 when the n-doping level is reduced to 5E16 cm -3 . Although these results are to be further verified by other measurement schemes due to the observation of background ferromagnetism from the growth substrate, these results indicate the possibility of tunable ferromagnetism in Mn:Ga 2 O 3 mediated by conduction electrons.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Evaluation of Doped-LaCrO3 ceramics for high temperature sensor applications

The main objective of this work was to investigate the applicability of the doped-LaCrO3 system for electrical interconnects and sensor applications at temperatures >800°C. Various A- and B-site doping strategies were investigated, and the effect on their electrical conductivity, Seebeck coefficient, and thermal coefficient of expansion were characterized up to 1500ºC. Solid-state, co-precipitation and sol gel methods were used to synthesize the compositions, and the chemical/thermal stability, microstructural evolution, sintering and grain growth kinetics were investigated for these compositions by XRD, SEM and Rietveld methods. The materials showing the greatest promise were included within thick film thermistor and thermocouple sensor designs that were tested to 1500ºC in various gas environments (various pO2 concentrations).

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2020

Tunneling electroresistance effects in epitaxial complex oxides on silicon

Complex oxide-based ferroelectric tunnel junctions (FTJs) show excellent nonvolatile memory characteristics promising for emerging technology. However, integration of these epitaxially grown FTJs electrically with a silicon substrate remains challenging due to their incompatible lattice structures and poor electronic interfaces resulting from the direct synthesis techniques. Here, we present an epitaxial SrRuO 3 /PbZr 0.2 Ti 0.8 O 3 /SrRuO 3 FTJ integrated electrically with a doped silicon substrate after a layer transfer process. The tunnel currents of the FTJ on silicon show a large tunneling electroresistance (~1×10 5 %) effect, which is explained by a numerical FTJ model incorporating pinned dipoles at the interfaces. This proof of concept of the integration of functional oxide heterostructures with silicon opens a pathway to beyond-CMOS computing devices using unconventional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effect of Dopant Compensation on the Conductivity of the Intrinsic poly-Si Isolation Region in Passivated IBC Silicon Solar Cells

The performance of IBC solar cells depends on the ability of this region to electrically isolate the doped fingers so that shunting does not occur and reduce the cell fill factor. We report on simulations which model the region between the p- and n-type poly-Si fingers of interdigitated back contact (IBC) solar cells as a series of resistors extending from one doped finger to the other. We demonstrate that the existence of a well compensated region between the doped fingers is enough to prevent shunting and loss of cell performance, despite contamination of dopants from the opposing dopant fingers. We show through these simulations that a net doping concentration below ~1018 cm-3 will enable a high resistivity and identify the conditions under which this highly resistive region forms despite imperfect finger edges. Additionally, we apply this analysis to tails measured by time-of-flight secondary ion mass spectrometry and confirm this hypothesis for our experimentally measured tails.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2026

Point defects in semiconductors: Friends and foes for quantum technologies

Point defects in semiconductors are both a curse and a blessing in microelectronics: they enable the control of electrical conductivity through doping, yet can also act as trapping and recombination centers that degrade device performance. In quantum information science, defects play a similarly dual role. They can be harnessed as spin–photon interfaces enabling the coupling of electronic and nuclear spins to light and the creation of distributed entanglement for quantum networks or used as atomistic scale sensors for quantum sensing. At the same time, defects are a major source of decoherence for superconducting qubits, one of the leading quantum computing platforms. This article discusses how a deeper materials-level understanding of defects can guide the design of improved quantum devices for communication, sensing, and computation.

Zhu, Yizhi [Rice University, Houston, TX (United S↗