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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 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Top-gated chemical vapor deposited MoS{sub 2} field-effect transistors on Si{sub 3}N{sub 4} substrates

We report the electrical characteristics of chemical vapor deposited (CVD) monolayer molybdenum disulfide (MoS{sub 2}) top-gated field-effect transistors (FETs) on silicon nitride (Si{sub 3}N{sub 4}) substrates. We show that Si{sub 3}N{sub 4} substrates offer comparable electrical performance to thermally grown SiO{sub 2} substrates for MoS{sub 2} FETs, offering an attractive passivating substrate for transition-metal dichalcogenides (TMD) with a smooth surface morphology. Single-crystal MoS{sub 2} grains are grown via vapor transport process using solid precursors directly on low pressure CVD Si{sub 3}N{sub 4}, eliminating the need for transfer processes which degrade electrical performance. Monolayer top-gated MoS{sub 2} FETs with Al{sub 2}O{sub 3} gate dielectric on Si{sub 3}N{sub 4} achieve a room temperature mobility of 24 cm{sup 2}/V s with I{sub on}/I{sub off} current ratios exceeding 10{sup 7}. Using HfO{sub 2} as a gate dielectric, monolayer top-gated CVD MoS{sub 2} FETs on Si{sub 3}N{sub 4} achieve current densities of 55 μA/μm and a transconductance of 6.12 μS/μm at V{sub tg} of −5 V and V{sub ds} of 2 V. We observe an increase in mobility at lower temperatures, indicating phonon scattering may dominate over charged impurity scattering in our devices. Our results show that Si{sub 3}N{sub 4} is an attractive alternative to thermally grown SiO{sub 2} substrate for TMD FETs.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Deposition of high permeability FeCoN films on mica substrates

In this study, sheet mica was explored as an alternative substrate for thin film deposition due to its unique layered structure and atomically flat surface. Static magnetic properties and microwave permeability of sputtered FeCoN films on mica with and without SiO{sub 2} under-layer were examined. In-plane isotropy and a large coercivity of 70 Oe were observed for the film directly deposited on mica substrate. With the insertion of 40 nm SiO{sub 2} under-layer between the film and mica substrate, well-defined in-plane anisotropy is formed due to the significant reduce in film residual stress, associated with a 3 Oe coercive force along easy axis. As a result, excellent microwave properties have been achieved and are comparable with the same film on Si substrate. Moreover, the permeability spectra are surprisingly stable from room temperature up to 390 K. Mica is proved as a promising substrate for magnetic films with high microwave performance, which is a great advantage for practical applications. Furthermore, our results provide an effective way to reduce the residual stress in sputtered films.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

High temperature annealing effects on deep-level defects in a high purity semi-insulating 4H-SiC substrate

Effects of high-temperature annealing on deep-level defects in a high-purity semi-insulating 4H silicon carbide substrate have been studied by employing current-voltage, capacitance-voltage, junction spectroscopy, and chemical impurity analysis measurements. Secondary ion mass spectrometry data reveal that the substrate contains boron with concentration in the mid 10{sup 15 }cm{sup −3} range, while other impurities including nitrogen, aluminum, titanium, vanadium and chromium are below their detection limits (typically ∼10{sup 14 }cm{sup −3}). Schottky barrier diodes fabricated on substrates annealed at 1400–1700 °C exhibit metal/p-type semiconductor behavior with a current rectification of up to 8 orders of magnitude at bias voltages of ±3 V. With increasing annealing temperature, the series resistance of the Schottky barrier diodes decreases, and the net acceptor concentration in the substrates increases approaching the chemical boron content. Admittance spectroscopy results unveil the presence of shallow boron acceptors and deep-level defects with levels in lower half of the bandgap. After the 1400 °C annealing, the boron acceptor still remains strongly compensated at room temperature by deep donor-like levels located close to mid-gap. However, the latter decrease in concentration with increasing annealing temperature and after 1700 °C, the boron acceptor is essentially uncompensated. Hence, the deep donors are decisive for the semi-insulating properties of the substrates, and their thermal evolution limits the thermal budget for device processing. The origin of the deep donors is not well-established, but substantial evidence supporting an assignment to carbon vacancies is presented.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Effect of substrate rotation on domain structure and magnetic relaxation in magnetic antidot lattice arrays

Microdimensional triangular magnetic antidot lattice arrays were prepared by varying the speed of substrate rotation. The pre-deposition patterning has been performed using photolithography technique followed by a post-deposition lift-off. Surface morphology taken by atomic force microscopy depicted that the growth mechanism of the grains changes from chain like formation to island structures due to the substrate rotation. Study of magnetization reversal via magneto optic Kerr effect based microscopy revealed reduction of uniaxial anisotropy and increase in domain size with substrate rotation. The relaxation measured under constant magnetic field becomes faster with rotation of the substrate during deposition. The nature of relaxation for the non-rotating sample can be described by a double exponential decay. However, the relaxation for the sample with substrate rotation is well described either by a double exponential or a Fatuzzo-Labrune like single exponential decay, which increases in applied field.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Electrical properties of n-type GaSb substrates and p-type GaSb buffer layers for InAs/InGaSb superlattice infrared detectors

Lightly doped n-type GaSb substrates with p-type GaSb buffer layers are the preferred templates for growth of InAs/InGaSb superlattices used in infrared detector applications because of relatively high infrared transmission and a close lattice match to the superlattices. We report here temperature dependent resistivity and Hall effect measurements of bare substrates and substrate-p-type buffer layer structures grown by molecular beam epitaxy. Multicarrier analysis of the resistivity and Hall coefficient data demonstrate that high temperature transport in the substrates is due to conduction in both the high mobility zone center Γ band and the low mobility off-center L band. High overall mobility values indicate the absence of close compensation and that improved infrared and transport properties were achieved by a reduction in intrinsic acceptor concentration. Standard transport measurements of the undoped buffer layers show p-type conduction up to 300 K indicating electrical isolation of the buffer layer from the lightly n-type GaSb substrate. However, the highest temperature data indicate the early stages of the expected p to n type conversion which leads to apparent anomalously high carrier concentrations and lower than expected mobilities. Data at 77 K indicate very high quality buffer layers.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Selective-area growth and magnetic characterization of MnAs/AlGaAs nanoclusters on insulating Al{sub 2}O{sub 3} layers crystallized on Si(111) substrates

We report on selective-area metal-organic vapor phase epitaxy and magnetic characterization of coupled MnAs/AlGaAs nanoclusters formed on thin Al{sub 2}O{sub 3} insulating layers crystallized on Si(111) substrates. Cross-sectional transmission electron microscopy reveals that poly-crystalline γ-Al{sub 2}O{sub 3} grains are formed after an annealing treatment of the amorphous Al{sub 2}O{sub 3} layers deposited by atomic layer deposition on Si(111) substrates. The 〈111〉 direction of the γ-Al{sub 2}O{sub 3} grains tends to be oriented approximately parallel to the 〈111〉 direction of the Si substrate. We observe that hexagonal MnAs nanoclusters on AlGaAs buffer layers grown by selective-area metal-organic vapor phase epitaxy on partially SiO{sub 2}-masked Al{sub 2}O{sub 3} insulator crystallized on Si(111) substrates are oriented with the c-axis along the 〈111〉 direction of the substrates, but exhibit a random in-plane orientation. A likely reason is the random orientation of the poly-crystalline γ-Al{sub 2}O{sub 3} grains in the Al{sub 2}O{sub 3} layer plane. Magnetic force microscopy studies at room temperature reveal that arrangements of coupled MnAs nanoclusters exhibit a complex magnetic domain structure. Such arrangements of coupled MnAs nanoclusters may also show magnetic random telegraph noise, i.e., jumps between two discrete resistance levels, in a certain temperature range, which can be explained by thermally activated changes of the complex magnetic structure of the nanocluster arrangements.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Thin-film X-ray filters on microstructured substrates and their thermophysical properties

It is shown that structured substrates having micron- or submicron-sized through holes and coated with an ultrathin organic film can be used for the fabrication of thin-film X-ray filters via direct growth of functional layers on a substrate by sputter deposition, without additional complex processing steps. An optimised process is considered for the fabrication of X-ray filters on support structures in the form of electroplated fine nickel grids and on track-etched polymer membranes with micron- and submicron-diameter through pores. ‘Optimisation’ is here taken to mean matching the sputter deposition conditions with the properties of substrates so as to avoid overheating. The filters in question are intended for both imaging and single-channel detectors operating in the soft X-ray and vacuum UV spectral regions, at wavelengths from 10 to 60 nm. Thermal calculations are presented for the heating of ultrathin layers of organic films and thin-film support substrates during the sputter deposition of aluminium or other functional materials. The paper discusses approaches for cooling thin-film composites during the sputter deposition process and the service of the filters in experiments and gives a brief overview of the works that utilised filters produced by the described technique on microstructured substrates, including orbital solar X-ray research in the framework of the CORONAS programme and laboratory laser plasma experiments. (invited paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Structural basis for substrate binding and specificity of a sodium–alanine symporter AgcS

The amino acid, polyamine, and organocation (APC) superfamily is the second largest superfamily of membrane proteins forming secondary transporters that move a range of organic molecules across the cell membrane. Each transporter in the APC superfamily is specific for a unique subset of substrates, even if they possess a similar structural fold. The mechanism of substrate selectivity remains, by and large, elusive. As such, here, we report two crystal structures of an APC member from Methanococcus maripaludis , the alanine or glycine:cation symporter (AgcS), with L - or D -alanine bound. Structural analysis combined with site-directed mutagenesis and functional studies inform on substrate binding, specificity, and modulation of the AgcS family and reveal key structural features that allow this transporter to accommodate glycine and alanine while excluding all other amino acids. Mutation of key residues in the substrate binding site expand the selectivity to include valine and leucine. These studies provide initial insights into substrate selectivity in AgcS symporters.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2021

Understanding Improvements in Coalesced Epilayers Grown over Nanopatterned Substrates

Nanopatterned substrates provide opportunities for reducing costs of high-efficiency III-V devices through methods such as substrate reuse and growth on cheaper substrates. In this work, we aim to better understand some of the fundamental challenges of growing high quality III-V material on nanopatterned substrates. Through specific pattern orientations and control of overgrowth morphology, we report that smooth and defect-free coalesced III-V epilayers are attainable over nanopatterned GaAs substrates.

coalescense↗
DOE OSTI2016

Role of low-energy ion irradiation in the formation of an aluminum germanate layer on a germanium substrate by radical-enhanced atomic layer deposition

Radical-enhanced atomic layer deposition uses oxygen radicals generated by a remote microwave-induced plasma as an oxidant to change the surface reactions of the alternately supplied trimethylaluminum precursor and oxygen radicals on a Ge substrate, which leads to the spontaneous formation of an aluminum germanate layer. In this paper, the effects that low-energy ions, supplied from a remote microwave plasma to the substrate along with the oxygen radicals, have on the surface reactions were studied. From a comparative study of aluminum oxide deposition under controlled ion flux irradiation on the deposition surface, it was found that the ions enhance the formation of the aluminum germanate layer. The plasma potential measured at the substrate position by the Langmuir probe method was 5.4 V. Assuming that the kinetic energy of ions arriving at the substrate surface is comparable to that gained by this plasma potential, such ions have sufficient energy to induce exchange reactions of surface-adsorbed Al atoms with the underlying Ge atoms without causing significant damage to the substrate. This ion-induced exchange reaction between Al and Ge atoms is inferred to be the background kinetics of the aluminum germanate formation by radical-enhanced atomic layer deposition.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Substrate temperature control for the formation of metal nanohelices by glancing angle deposition

The targets of this study are to develop a device to precisely control the temperature during glancing angle deposition, to make films consisting of low melting temperature metal nanoelements with a controlled shape (helix), and to explore the substrate temperature for controlling the nanoshapes. A vacuum evaporation system capable of both cooling a substrate and measurement of its temperature was used to form thin films consisting of arrays of Cu and Al nanohelices on silicon substrates by maintaining the substrate temperature at T{sub s}/T{sub m} < 0.22 (T{sub s} is the substrate temperature and T{sub m} is the melting temperature of target material). The critical T{sub s}/T{sub m} to produce Cu and Al nanohelices corresponds to the transitional homologous temperature between zones I and II in the structure zone model for the solid film, where surface diffusion becomes dominant. X-ray diffraction analysis indicated that the Cu and Al nanohelix thin films were composed of coarse oriented grains with diameters of several tens of nanometers.

42 ENGINEERING↗
DOE OSTI2016

Sputtering of silicon and glass substrates with polyatomic molecular ion beams generated from ionic liquids

The effect of irradiating 1-ethyl-3-methylimidazolium positive (EMIM{sup +}) or dicyanamide negative (DCA{sup –}) ion beams using an ionic liquid ion source was characterized concerning its sputtering properties for single crystalline Si(100) and nonalkaline borosilicate glass substrates. The irradiation of the DCA{sup –} ion beam onto the Si substrate at an acceleration voltage of 4 and 6 kV exhibited detectable sputtered depths greater than a couple of nanometers with an ion fluence of only 1 × 10{sup 15} ions/cm{sup 2}, while the EMIM{sup +} ion beam produced the same depths with an ion fluence 5 × 10{sup 15} ions/cm{sup 2}. The irradiation of a 4 kV DCA{sup –} ion beam at a fluence of 1 × 10{sup 16} ions/cm{sup 2} also yields large etching depths in Si substrates, corresponding to a sputtering yield of Si/DCA{sup – }= 10, and exhibits a smoothed surface roughness of 0.05 nm. The interaction between DCA{sup –} and Si likely causes a chemical reaction that relates to the high sputtering yield and forms an amorphous C-N capping layer that results in the smooth surface. Moreover, sputtering damage by the DCA{sup –} irradiation, which was estimated by Rutherford backscattering spectroscopy with the channeling technique, was minimal compared to Ar{sup +} irradiation at the same condition. In contrast, the glass substrates exhibited no apparent change in surface roughnesses when sputtered by the DCA{sup –} irradiation compared to the unirradiated glass substrates.

42 ENGINEERING↗
DOE OSTI2015

Growth of light-emitting SiGe heterostructures on strained silicon-on-insulator substrates with a thin oxide layer

The possibility of using substrates based on “strained silicon on insulator” structures with a thin (25 nm) buried oxide layer for the growth of light-emitting SiGe structures is studied. It is shown that, in contrast to “strained silicon on insulator” substrates with a thick (hundreds of nanometers) oxide layer, the temperature stability of substrates with a thin oxide is much lower. Methods for the chemical and thermal cleaning of the surface of such substrates, which make it possible to both retain the elastic stresses in the thin Si layer on the oxide and provide cleaning of the surface from contaminating impurities, are perfecte. It is demonstrated that it is possible to use the method of molecular-beam epitaxy to grow light-emitting SiGe structures of high crystalline quality on such substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Photoluminescence Studies of Si-Doped Epitaxial GaAs Films Grown on (100)- and (111)A-Oriented GaAs Substrates at Lowered Temperatures

The electrical properties and photoluminescence features of uniformly Si-doped GaAs layers grown on GaAs substrates with the (100) and (111)A crystallographic orientations of the surface are studied. The samples are grown at the same As{sub 4} pressure in the growth temperature range from 350 to 510°C. The samples grown on GaAs(100) substrates possess n-type conductivity in the entire growth temperature range, and the samples grown on GaAs(111)A substrates possess p-type conductivity in the growth temperature range from 430 to 510°C. The photoluminescence spectra of the samples exhibit an edge band and an impurity band. The edge photoluminescence band corresponds to the photoluminescence of degenerate GaAs with n- and p-type conductivity. The impurity photoluminescence band for samples on GaAs(100) substrates in the range 1.30–1.45 eV is attributed to V{sub As} defects and Si{sub As}–V{sub As} defect complexes, whose concentration varies with sample growth temperature. Transformation of the impurity photoluminescence spectra of the samples on GaAs(111)A substrates is interpreted as being a result of changes in the V{sub As} and V{sub Ga} defect concentrations under variations in the growth temperature of the samples.

36 MATERIALS SCIENCE↗
DOE OSTI2018

On the Application of Strain-Compensating GaAsP Layers for the Growth of InGaAs/GaAs Quantum-Well Laser Heterostructures Emitting at Wavelengths above 1100 nm on Artificial Ge/Si Substrates

Stressed InGaAs/GaAs quantum-well laser structures are grown by gas-phase epitaxy from organometallic compounds on GaAs substrates and artificial Ge/Si substrates based on Si(001) with an epitaxial metamorphic Ge layer. To suppress the relaxation of elastic stresses during the growth of InGaAs quantum wells with a high fraction of In, strain-compensating GaAsP layers are applied. The structural and radiative properties of the samples grown on substrates of various types are compared. Stimulated radiation at wavelengths up to 1.24 μm at 300 K is obtained for structures grown on GaAs substrates and at wavelengths up to 1.1 μm at 77 K, for structures grown on Ge/Si substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Structural and Morphological Properties of Hybrid Heterostructures Based on GaN Grown on a Compliant por-Si(111) Substrate

The possibility of synthesizing integrated GaN/por-Si heterostructures by plasma-assisted molecular beam epitaxy without an A1N/Si buffer layer is demonstrated. The beneficial effect of the high-temperature nitridation of a silicon substrate before GaN growth on the crystal quality of the GaN/Si layers is shown. It is established that, to obtain two-dimensional GaN layers on Si(111), it is reasonable to use compliant por-Si substrates and low-temperature GaN seed layers with a 3D morphology synthesized by plasma-assisted molecular beam epitaxy at relatively low substrate temperatures under stoichiometric conditions and upon enrichment with nitrogen. In this case, a self-assembled array of GaN seed nanocolumns with a fairly uniform diameter distribution forms on the por-Si substrate surface. The basic GaN layers, in turn, should be grown at a high temperature under stoichiometric conditions upon enrichment with gallium, upon which the coalescence of nucleated GaN nanocolumns and growth of a continuous two-dimensional GaN layer are observed. The use of compliant Si substrates is a relevant approach for forming GaN-based semiconductor device heterostructures by plasma-assisted molecular beam epitaxy.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Studying the Characteristics of Transistors Based on Gallium Nitride Heterostructures Grown by Ammonia Molecular Beam Epitaxy on Sapphire and Silicon Substrates

Abstracts: Ammonia molecular-beam epitaxy has been used to grow gallium nitride (GaN) transistor heterostructures on sapphire and silicon substrates. GaN transistors with a 1.2-mm periphery fabricated on substrates of both types exhibited similar high static characteristics: saturation current density above 0.75 A/mm, transconductance above 300 mS/mm, and breakdown voltage above 120 V. Measurements of the small-signal parameters showed that transistors based on silicon substrates possessed high gain in a frequency range up to 5 GHz; the specific output power at 1 GHz amounted to 5 W/mm for transistors on sapphire substrate and 2 W/mm for transistors on silicon substrate.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Evaluation of Low-Pressure-Sintered Multi-Layer Substrates for Medium-Voltage SiC Power Modules: Preprint

Direct-bonded aluminum (DBA) multi-layer substrates have been fabricated using low-pressure silver sintering. These multi-layer substrates can be used to reduce the peak electric field strength inside power modules. This benefit is particularly important for medium-voltage silicon carbide (SiC) MOSFETs due to their higher operating voltages. The voiding content and defect density of the bond used to create the multi-layer substrate stackup is critical to the thermal performance and reliability of the power module. This work two low-pressure-assisted sintering techniques; one using nano-silver preform and the other using nano-silver paste to bond 23 mm by 49 mm DBA substrates. Scanning acoustic microscopy (C-SAM) is used to evaluate the bond quality after sintering. To evaluate the reliability of the sintered multi-layer substrates, passive thermal cycling from –40 °C to 200 °C was performed. Cross-sections were cut at pre-determined intervals and imaged with Scanning Electron Microscopy (SEM). After 1,000 thermal cycles, minor cracking was observed, but no failures have occurred.

28 EE - Advanced Manufacturing Office (EE-5A)↗