Search NASA⌕ Search

SEARCH · Search NASA

Results for “semiconductor structures”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Codesign of an integrated metal–insulator–semiconductor photocathode for photoelectrochemical reduction of CO 2 to ethylene

Photoelectrochemical carbon-dioxide reduction (PEC CO 2 R) is a potentially attractive means for producing chemicals and fuels using sunlight, water, and carbon dioxide; however, this technology is in its infancy. To date, most studies of PEC CO 2 R have reported products containing one carbon atom (C 1 products) but the production of valuable products containing two or more carbons (C 2+ products), such as ethylene, ethanol, etc., is rarely demonstrated. Metal–semiconductor–insulator (MIS) photocathode/catalyst structures offer a promising approach for this purpose, since they integrate the functions of light absorption, charge separation, and catalysis. In this study, we have investigated a Cu/TiO 2 /p-Si photocathode/catalyst structure with the aim of establishing the effects of semiconductor–insulator interactions on the performance of the photocathode and the influence of the direction of illumination of the MIS structure on the total current density and the distribution of products formed by on the Cu catalyst. Here, we have also examined the influence of ionomer coatings deposited on the Cu surface on the total current density and the distribution of products formed. A major finding is that for a fixed Cu potential the distribution of products formed by PEC CO 2 R are the same, irrespective of the direction of illumination, and are identical to those obtained by electrochemical reduction of CO 2 (EC CO 2 R). Another important finding is that the total current density and the faradaic efficiency to ethylene are enhanced significantly by deposition of a thin bilayer of Sustainion/Nafion onto the surface of the Cu.

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

Time-Resolved Structural Measurement of Thermal Resistance across a Buried Semiconductor Heterostructure Interface

The precise control and understanding of heat flow in heterostructures is pivotal for advancements in thermoelectric energy conversion, thermal barrier coatings, and efficient heat management in electronic and optoelectronic devices. In this study, we employ high-angular-resolution time-resolved X-ray diffraction to structurally measure thermal resistance in a laser-excited AlGaAs/GaAs semiconductor heterostructure. Our methodology offers femtometer-scale spatial sensitivity and nanosecond time resolution, enabling us to directly observe heat transport across a buried interface. We corroborate established Thermal Boundary Resistance (TBR) values for AlGaAs/GaAs heterostructures and demonstrate that TBR arises from material property discrepancies on either side of a nearly flawless atomic interface. This work not only sheds light on the fundamental mechanisms governing heat flow across buried interfaces but also presents a robust experimental framework that can be extended to other heterostructure systems, paving the way for optimized thermal management in next-generation devices.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Vapor deposition rate modifies anisotropic glassy structure of an anthracene-based organic semiconductor

We control the anisotropic molecular packing of vapor-deposited glasses of ABH113, a deuterated anthracene derivative with promise for future organic light emitting diode materials, by changing the deposition rate and substrate temperature at which they are prepared. We find that at substrate temperatures from 0.65 T g to 0.92 T g , the deposition rate significantly modifies the orientational order in the vapor-deposited glasses as characterized by x-ray scattering and birefringence. Both measures of anisotropic order can be described by a single deposition rate–substrate temperature superposition (RTS). This supports the applicability of the surface equilibration mechanism and generalizes the RTS principle from previous model systems with liquid crystalline order to non-mesogenic organic semiconductors. We find that vapordeposited glasses of ABH113 have significantly enhanced density and thermal stability compared to their counterparts prepared by liquidcooling. For organic semiconductors, the results of this study provide an efficient guide for using the deposition rate to prepare stable glasses with controlled molecular packing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Mapping hidden space-charge distributions across crystalline metal oxide/group IV semiconductor interfaces

The electronic structures of semiconducting heterojunctions are critically dependent on composition including the presence and concentrations of dopants, both intended and unintended. Dopant profiles in the interfacial region can have major effects on band energies which in turn drive transport properties. Here, in this study, we use core-level photoelectron line shapes excited with hard x rays to extract information about electric fields resulting from internal charge transfer in epitaxial La 0.03 Sr 0.97 Zr x Ti 1–x O 3 /Ge(001) (0.1≤x≤0.7) heterostructures. Experiments were carried out for heterojunctions involving both n- and p-type Ge substrates. These heterojunctions were not amenable to electronic characterization of all regions by transport measurements because the doped substrates act as electrical shunts, precluding probing the more resistive films and masking interface conductivity. However, the core-level line shapes were found to be a rich source of information on built-in potentials that exist throughout the heterostructure, and yielded valuable insight into the impact of band bending on band alignment at the buried interfaces. The electronic effects expected for Ge with uniform n- and p-type doping are eclipsed by those of unintended oxygen dopants in the Ge near the interface. This study illustrates the power of hard x-ray photoemission spectroscopy and related modeling to determine electronic structure in material systems for which insight from traditional transport measurements is limited.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Light-induced Weyl semiconductor-to-metal transition mediated by Peierls instability

Elemental tellurium is a strongly spin-orbit coupled Peierls-distorted semiconductor whose band structure features topologically protected Weyl nodes. Here, using time-dependent density functional theory calculations, we show that impulsive optical excitation can be used to transiently control the amplitude of the Peierls distortion, realizing a mechanism to switch tellurium between three states: Weyl semiconductor, Weyl metal and non-Weyl metal. Further, we present experimental evidence of this inverse-Peierls distortion using time-resolved optical second harmonic generation measurements. These results provide a pathway to multifunctional ultrafast Weyl devices and introduce Peierls systems as viable hosts of light-induced topological transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Atomic Structure, Dynamics, Changes in Chemical Bonding and Semiconductor-Metal Transition in Sb 2 Se 3 : A Remarkable Material for Quantum Networks and Energy Applications

Antimony sesquiselenide has become an outstanding functional material for photovoltaics, energy storage and transformation, memory and photonic applications. Sb 2 Se 3 is one of the most successful emerging solar light absorbers and has also been identified as a highly promising ultralow-loss phase-change material (PCM) for next-generation coherent nanophotonic processors, photonic tensor cores, quantum and neuromorphic networks. Unlike benchmark telluride PCMs, Sb 2 Se 3 features a quasi-one-dimensional (1D) crystalline structure consisting of (Sb 4 Se 6 ) ∞ ribbons, lacks the typical PCM chemical bonding, and undergoes an extended semiconductor-metal transition above the melting point. Consequently, the origin of high optical contrast between crystalline (SET) and amorphous (RESET) logic states remains elusive and presents a significant challenge. Using high-energy X-ray diffraction and Raman spectroscopy over a wide temperature range, supported by first-principles simulations and complemented by thermal, optical and electrical measurements, as well as by 121 Sb-Mossbauer spectroscopy, the quasi-1D network of orthorhombic antimony sesquiselenide was found to undergo significant evolution in amorphous and supercooled Sb 2 Se 3 , leading to lower coordination, shorter interatomic distances and a higher p-electron density on antimony, indicating changes in chemical bonding. The observed novel Sb 2 Se 3 nanocrystalline polymorph, characterized by trigonal antimony coordination and more isolated Sb-Se ribbons, could help reduce multiple trapping defect states in the bandgap, which are typical of orthorhombic Sb 2 Se 3 , thereby enhancing the power-conversion efficiency of photovoltaic devices. Semimetallic and metallic liquid Sb 2 Se 3 exhibit a gradual transformation into a denser 2D and/or 3D network with higher antimony coordination. Localized electron states in the pseudogap are becoming extended, leading to an increase in electronic conductivity σ following the relationship σ ∝ N(E F ) 2 . Liquid Sb 2 Se 3 also appears to be strongly fragile, with a nonmonotonic change in viscosity and higher atomic mobility in the metallic liquid. Furthermore, these results explain extraordinary functionalities of Sb 2 Se 3 for photonic and energy applications.

antimony↗
DOE OSTI2021

Manipulation and statistical analysis of the fluid flow of polymer semiconductor solutions during meniscus-guided coating

Recent work in structure–processing relationships of polymer semiconductors have demonstrated the versatility and control of thin-film microstructure offered by meniscusguided coating (MGC) techniques. Here, we analyze the qualitative and quantitative aspects of solution shearing, a model MGC method, using coating blades augmented with arrays of pillars. The pillars induce local regions of high strain rates—both shear and extensional—not otherwise possible with unmodified blades, and we use fluid mechanical simulations to model and study a variety of pillar spacings and densities. We then perform a statistical analysis of 130 simulation variables to find correlations with three dependent variables of interest: thin-film degree of crystallinity and transistor field-effect mobilities for charge-transport parallel (Μ para ) and perpendicular (Μ perp ) to the coating direction. Our study suggests that simple fluid mechanical models can reproduce substantive correlations between the induced fluid flow and important performance metrics, providing a methodology for optimizing blade design.

36 MATERIALS SCIENCE↗
DOE OSTI2024

A Scalable & Non-Destructive Characterization Strategy to Study Semiconductor/Dielectric Interfaces and Predict Wafer-Level Device Performance

The defect density present at the dielectric-semiconductor interface in an MOS structure directly influences the channel carrier characteristics in semiconductor devices, especially in wide bandgap material systems used in power devices. While these trap defects are typically quantified through electrical characterization of MOS-capacitor test structures, this treatment offers very little insight into the physical nature of interface defects. Such shortcomings demand a physical characterization strategy to guide fabrication optimization. X-ray photoelectron spectroscopy (XPS) is suggested as a viable technique to determine chemical data for dielectric interfaces formed using atomic layer deposition (ALD) on GaN substrates. Previously, 1-D XPS characterization has confirmed the presence of a Ga x O y interlayer between ALD dielectrics and the GaN substrate. In this work, XPS data is serially collected to form 2-D images of an ALD-Al 2 O 3 /GaN interface as a proof-of-concept experiment for in-situ XPS quality monitoring during ALD processing. The information provided by this work reveals some of the challenges for incorporating XPS characterization as an in-situ strategy during fabrication of GaN-based devices. Separately, electrical mapping of a 2-D array of ALD-Al 2 O 3 /GaN MOS-capacitor devices provide a means to quantify the spatial variations in interface quality across a single wafer. Physical characterization techniques, such as time-of-flight secondary ion mass spectroscopy, provide additional chemical information about the Al 2 O 3 /Ga x O y /GaN structure that complement the electrical mapping results. This analysis shows that a higher Ga x O y content correlates with higher interface state defects for trap energies deep in the band gap.

42 ENGINEERING↗
DOE OSTI2023

Coherent Phonon-Induced Modulation of Charge Transfer in 2D Hybrid Perovskites

Electron–phonon interactions play an essential role in charge transport and transfer processes in semiconductors. For most structures, tailoring electron–phonon interactions for specific functionality remains elusive. Here, it is shown that, in hybrid perovskites, coherent phonon modes can be used to manipulate charge transfer. In the 2D double perovskite, (AE2T) 2 AgBiI 8 (AE2T: 5,5“-diylbis(amino-ethyl)-(2,2”-(2)thiophene)), the valence band maximum derived from the [Ag 0.5 Bi 0.5 I4] 2– framework lies in close proximity to the AE2T-derived HOMO level, thereby forming a type-II heterostructure. During transient absorption spectroscopy, pulsed excitation creates sustained coherent phonon modes, which periodically modulate the associated electronic levels. Thus, the energy offset at the organic–inorganic interface also oscillates periodically, providing a unique opportunity for modulation of interfacial charge transfer. Density-functional theory corroborates the mechanism and identifies specific phonon modes as likely drivers of the coherent charge transfer. These observations are a striking example of how electron–phonon interactions can be used to manipulate fundamentally important charge and energy transfer processes in hybrid perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Mechanical and Ionic Characterization for Organic Semiconductor–Incorporated Perovskites for Stable 2D/3D Heterostructure Perovskite Solar Cells

Hybrid metal halide perovskite (MHP) materials, while being promising for photovoltaic technology, also encounter challenges related to material stability. Combining 2D MHPs with 3D MHPs offers a viable solution, yet there is a gap in the understanding of the stability among various 2D materials. The mechanical, ionic, and environmental stability of various 2D MHP ligands are reported, and an improvement with the use of a quater-thiophene-based organic cation (4TmI) that forms an organic-semiconductor incorporated MHP structure is demonstrated. It is shown that the best balance of mechanical robustness, environmental stability, ion activation energy, and reduced mobile ion concentration under accelerated aging is achieved with the usage of 4TmI. It is believed that by addressing mechanical and ion-based degradation modes using this built-in barrier concept with a material system that also shows improvements in charge extraction and device performance, MHP solar devices can be designed for both reliability and efficiency.

14 SOLAR ENERGY↗
DOE OSTI2020

H 2 Pc and pentacene on Cu(110)-(2×1)O: A combined STM and nc-AFM study

Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) with a CuO tip is used to investigate adsorption of metal-free phthalocyanine (H 2 Pc) and pentacene on the Cu(110)-(2×1)O striped-phase reconstructed surface. We show that the combination of STM and nc-AFM is necessary to reveal the detailed adsorption geometry, necessary for interpreting the observed STM contrast. Comparison of the nc-AFM images with simulations shows that some of the H 2 Pc molecules are deformed out-of-plane on this surface, while pentacene retains its molecular geometry regardless of adsorption site. Our research highlights how the stiffness of CuO tips makes it possible to probe the structure of organic semiconductor / metal interfaces.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2025

Dominant end-tunneling effect in two distinct Luttinger liquids coexisting in one quantum wire

Luttinger liquids occupy a notable place in physics as one of the most understood classes of quantum many-body systems. The experimental mission of measuring its main prediction, power laws in observable quantities, has already produced a body of exponents in different semiconductor and metallic structures. Here, we combine tunneling spectroscopy with density-dependent transport measurements in the same quantum wires over more than two orders of magnitude in temperature to very low electron temperatures down to ∼40 mK. This reveals that, when the second 1D subband becomes populated, the temperature dependence splits into two ranges with different exponents in the power-law dependence of the conductance, both dominated by the finite-size effect of the end-tunneling process. This result demonstrates the importance of measuring the Luttinger parameters as well as the number of modes independently through spectroscopy in addition to the transport exponent in the characterization of Luttinger liquids. This opens a pathway to unambiguous interpretation of the exponents observed in quantum wires.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Origin of Rashba Spin-Orbit Coupling in 2D and 3D Lead Iodide Perovskites

isaWhittaker-Brooks2 & Tho D. Nguyen1* We studied spin dynamics of charge carriers in the superlattice-like Ruddlesden-Popper hybrid lead iodide perovskite semiconductors, 2D (BA)2(MA)Pb2I7 (with MA= CH3NH3, and BA= CH3(CH2)3NH3), and 3D MAPbI3 using the magnetic field effect (MFE) on conductivity and electroluminescence in their light emitting diodes (LEDs) at cryogenic temperatures. The semiconductors with distinct structural/ bulk inversion symmetry breaking, when combined with colossal intrinsic spin–orbit coupling (SOC), theoretically give rise to giant Rashba-type SOC. We found that the magneto-conductance (MC) magnitude increases monotonically with the emission intensity and saturates at ≈0.05% and 0.11% for the MAPbI3 and (BA)2(MA)Pb2I7, respectively. The magneto-electroluminescence (MEL) response with similar line shapes as the MC response has a significantly larger magnitude, and essentially stays constant at ≈0.22% and ≈0.20% for MAPbI3 and (BA)2(MA)Pb2I7, respectively. The sign and magnitude of the MC and MEL responses can be quantitatively explained in the framework of the Δg-based excitonic model using rate equations. Remarkably, the width of the MEL response in those materials linearly increases with increasing the applied electric field, where the Rashba coefficient in (BA)2(MA) Pb2I7 is estimated to be about 7 times larger than that in MAPbI3. Our studies might have significant impact on future development of electrically-controlled spin logic devices via Rashba-like effects

36 MATERIALS SCIENCE↗
DOE OSTI2024

Quantum mechanical model of crossing and anti-crossing points in 3D full-band Monte Carlo simulations

This work presents a 3D quantum mechanics based model to address the physics at band structure crossing/anti-crossing points in full band Monte Carlo (FBMC) simulations. The model solves the Krieger and Iafrate (KI) equations in real time using pre-computed coefficients at k-points spatially sampled within the first Brillouin zone. Solving the KI equations in real time makes this model applicable for all electric fields, which enables its use in FBMC device simulations. In this work, a two-level refinement scheme is used to aggressively sample regions in proximity to band crossings for accurate solutions to the KI equations and coarsely sample everywhere else to limit the number of k-points used. The presented sampling method is demonstrated on the band structure of silicon but is effective for the band structure of any semiconductor material. Next, the adaptation of the fully quantum KI model into the semi-classical FBMC method is discussed. Finally, FBMC simulations of hole transport in 4H silicon carbide with and without the KI model are performed. Results along different crystallographic directions for a wide range of electric fields are compared to previously published simulation and experimental values.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Quantum dot polarisation converter in an optomechanical cavity

We propose a scheme of a quantum photon polarisation converter, which is based on controlled electron – photon – phonon transitions in a hybrid semiconductor nanostructure. This structure consists of a GaAs/InAs quantum dot (QD) that has a parallelepiped shape and contains a single electron, and an optomechanical microcavity (MC) based on a photonic crystal (PC) that supports two orthogonally polarised photonic modes and one mechanical (phonon) mode. Within the framework of the microscopic theory, the QD and MC performance characteristics are found. Populations of states of the system as functions of time and its parameters are calculated. The principal possibility of photon polarisation conversion using transitions in a five-level resonance scheme for coherent (single-photon) and steady-state (subphoton) regimes is shown. The MC optical and mechanical spectra are simulated, and the PC structure parameters are selected to ensure the efficient operation of the converter. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Colossal piezoresistance in narrow-gap Eu 5 In 2 Sb 6

Piezoresistance, the change of the electrical resistance (R) of a material in response to an applied mechanical stress (σ), is the driving principle of electromechanical devices such as strain gauges, accelerometers, and cantilever force sensors. Enhanced piezoresistance has been traditionally observed in two classes of uncorrelated materials: nonmagnetic semiconductors and composite structures. In this study, we report the discovery of a remarkably large piezoresistance in Eu 5 In 2 Sb 6 single crystals, wherein anisotropic metallic clusters naturally form within a semiconducting matrix due to electronic interactions. Eu 5 In 2 Sb 6 shows a highly anisotropic piezoresistance, and uniaxial pressure along [001] of only 0.4 GPa leads to a resistivity drop of >99.95%, which results in a colossal piezoresistance factor of 5000×10 –11 Pa –1 . Our result not only reveals the role of interactions and phase separation in the realization of colossal piezoresistance, but it also highlights a route to multifunctional devices with large responses to both pressure and magnetic field.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Colloquium : Physical properties of group-IV monochalcogenide monolayers

The state-of-the-art knowledge of ferroelectric and ferroelastic group-IV monochalcogenide monolayers is surveyed. These semiconductors feature remarkable structural and mechanical properties, such as a switchable in-plane spontaneous polarization, soft elastic constants, structural degeneracies, and thermally driven two-dimensional structural transformations. Additionally, these 2D materials display selective valley excitations, valley Hall effects, and persistent spin helix behavior. After a description of their Raman spectra, a discussion of optical properties arising from their lack of centrosymmetry (such as an unusually strong second-harmonic intensity, large bulk photovoltaic effects, photostriction, and tunable exciton binding energies) is provided as well. Here, the physical properties observed in these materials originate from (correlate with) their intrinsic and switchable electric polarization, and the physical behavior hereby reviewed could be of use in nonvolatile memory, valleytronic, spintronic, and optoelectronic devices: these 2D multiferroics enrich and diversify the 2D material toolbox.

2D ferroelectrics↗