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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Characterization of multilayer GaAs/AlGaAs transistor structures by variable angle spectroscopic ellipsometry

Variable angle of incidence spectroscopic ellipsometry (VASE) has been implemented as a means of determining layer thickness, alloy composition, and growth quality of GaAs/AlGaAs samples composed of relatively thick layers as well as superlattices. The structures studied in this work contained GaAs/AlGaAs multilayers with a superlattice 'barrier' and were grown for later formation of modulation-doped field effect transistors (MODFETs). Sample modeling was performed by treating the superlattice as a bulk AlGaAs layer of unknown composition. Extremely good data fits were realized when five layer thicknesses and two alloy ratios were allowed to vary in a regression analysis. Room temperature excitonic effects associated with the e-hh(1), e-lh(1) and e-hh(2) transitions were observed in the VASE data.

Merkel, Kenneth G.↗
NASA NTRS1990

Ellipsometric study of InGaAs MODFET material

In(x)Ga(1-x)As based MODFET (modulation doped field effect transistor) material was grown by molecular beam epitaxy on semi-insulating InP substrates. Several structures were made, including lattice matched and strained layer InGaAs. All structures also included several layers of In(0.52)Al(0.48)As. Variable angle spectroscopic ellipsometry was used to characterize the structures. The experimental data, together with the calibration function for the constituent materials, were analyzed to yield the thickness of all the layers of the MODFET structure. Results of the ellipsometrically determined thicknesses compare very well with the reflection high energy electron diffraction in situ thickness measurements.

Alterovitz, S. A.↗
NASA NTRS1987

Rapidly solidified nickel aluminide of improved stoichiometry and ductilization

It has been found that tri-nickel aluminide compositions are quite sensitive to the ratio of nickel to aluminum in their ability to receive boron as a dopant and that compositions which are relatively poor in the aluminum component can be doped more effectively with boron. Further, it has been found for the nickel aluminides which have lower concentrations of aluminum that the percentage of boron which can be added to the composition to effectively increase the strength of the alloys is . favored by the lower aluminum ratio so that higher concentrations of boron are addable. The compositions which result have significant strength properties based on tensile tests of the compositions.

Alan I Taub↗
NASA NTRS1990

Healing of voids in the aluminum metallization of integrated circuit chips

The thermal stability of GaAs modulation-doped field effect transistors (MODFETs) is evaluated in order to identify failure mechanisms and validate the reliability of these devices. The transistors were exposed to thermal step-stress and characterized at ambient temperatures to indicate device reliability, especially that of the transistor ohmic contacts with and without molybdenum diffusion barriers. The devices without molybdenum exhibited important transconductance deterioration. MODFETs with molybdenum diffusion barriers were tolerant to temperatures above 300 C. This tolerance indicates that thermally activated failure mechanisms are slow at operational temperatures. Therefore, high-reliability MODFET-based circuits are possible.

Cuddihy, Edward F.↗
NASA NTRS1991

New materials and techniques for improved mm wave devices

Current research on microwave and mm wave three terminal semiconductor devices is summarized with particular attention given to the development of the pseudomorphic InGaAs modulation-doped field effect transistor (MODFET). Application of the high-indium-concentration MODFET grown on InP in the temperature range of 120-150 K is also described.

Alterovitz, Samuel A.↗
NASA NTRS1996

Effect of variations in the doping profiles on the properties of doped multiple quantum well avalanche photodiodes

The purpose of this study is to use both theoretical and experimental evidence to determine the impact of doping imbalance and symmetry on the physical and electrical characteristics of doped multiple quantum well avalanche photodiodes (APD). Theoretical models have been developed to calculate the electric field valence and conduction bands, capacitance-voltage (CV), and carrier concentration versus depletion depth profiles. The models showed a strong correlation between the p- and n-doping balance inside the GaAs wells and the number of depleted stages and breakdown voltage of the APD. A periodic doping imbalance in the wells has been shown to result in a gradual increase (or decrease) in the electric field profile throughout the device which gave rise to partially depleted devices at low bias. The MQW APD structures that we modeled consisted of a 1 micron top p(+)-doped (3 x 10(exp 18) cm(exp -3)) GaAs layer followed by a 1 micron region of alternating layers of GaAs (500 A) and Al(0.42)Ga(0.58)As (500 A), and a 1 micron n(+) back layer (3 x 10(exp 18) cm(exp -3)). The GaAs wells were doped with p-i-n layers placed at the center of each well. The simulation results showed that in an APD with nine doped wells, and where the 50 A p-doped layer is off by 10% (p = 1.65 x 10(exp 18) cm(exp -3), n = 1.5 x 10(exp 18) cm(exp -3)), almost half of the MQW stages were shown to be undepleted at low bias which was a result of a reduction in the electric field near the p(+) cap layer by over 50% from its value in the balanced structure. Experimental CV and IV data on similar MBE grown MQW structures have shown very similar depletion and breakdown characteristics. The models have enabled us to better interpret our experimental data and to determine both the extent of the doping imbalances in the devices as well as the overall p- or n-type doping characteristics of the structures.

Menkara, H. M.↗
NASA NTRS1997

The First National Student Conference: NASA University Research Centers at Minority Institutions

The conference includes contributions from 13 minority universities with NASA University Research Centers. Topics discussed include: leadership, survival strategies, life support systems, food systems, simulated hypergravity, chromium diffusion doping, radiation effects on dc-dc converters, metal oxide glasses, crystal growth of Bil3, science and communication on wheels, semiconductor thin films, numerical solution of random algebraic equations, fuzzy logic control, spatial resolution of satellite images, programming language development, nitric oxide in the thermosphere and mesosphere, high performance polyimides, crossover control in genetic algorithms, hyperthermal ion scattering, etc.

Daso, Endwell O.↗
NASA NTRS2003

High Electron Mobility in SiGe/Si n-MODFET Structures on Sapphire Substrates

For the first time, SiGe/Si n-Modulation Doped Field Effect Transistors (n-MODFET) structures have been grown on sapphire substrates. Room temperature electron mobility value of 1271 square centimeters N-sec at an electron carrier density (n(sub e) = 1.33x10(exp 12) per square centimeter)) of 1.6 x 10(exp 12) per square centimeter was obtained. At 250 mK, the mobility increases to 13,313 square centimeters/V-sec (n(sub e)=1.33x10(exp 12) per square centimeter)) and Shubnikov-de Haas oscillations appear, showing excellent confinement of the two-dimensional electron gas.

Mueller, Carl H.↗
NASA NTRS2004

High Mobility SiGe/Si n-MODFET Structures and Devices on Sapphire Substrates

Si/Ge/Si n-type modulation doped field effect structures and transistors (n-MODFET's) have been fabricated on r-plane sapphire substrates. Mobilities as high as 1380 cm(exp 2)/Vs were measured at room temperature. Excellent carrier confinement was shown by Shubnikov-de Haas measurements. Atomic force microscopy indicated smooth surfaces, with rm's roughness less than 4 nm, similar to the quality of SiGe/Si n-MODFET structures made on Si substrates. Transistors with 2 micron gate lengths and 200 micron gate widths were fabricated and tested.

Mueller, Carl↗
NASA NTRS2004

High Electron Mobility Transistor Structures on Sapphire Substrates Using CMOS Compatible Processing Techniques

System-on-a-chip (SOC) processes are under intense development for high-speed, high frequency transceiver circuitry. As frequencies, data rates, and circuit complexity increases, the need for substrates that enable high-speed analog operation, low-power digital circuitry, and excellent isolation between devices becomes increasingly critical. SiGe/Si modulation doped field effect transistors (MODFETs) with high carrier mobilities are currently under development to meet the active RF device needs. However, as the substrate normally used is Si, the low-to-modest substrate resistivity causes large losses in the passive elements required for a complete high frequency circuit. These losses are projected to become increasingly troublesome as device frequencies progress to the Ku-band (12 - 18 GHz) and beyond. Sapphire is an excellent substrate for high frequency SOC designs because it supports excellent both active and passive RF device performance, as well as low-power digital operations. We are developing high electron mobility SiGe/Si transistor structures on r-plane sapphire, using either in-situ grown n-MODFET structures or ion-implanted high electron mobility transistor (HEMT) structures. Advantages of the MODFET structures include high electron mobilities at all temperatures (relative to ion-implanted HEMT structures), with mobility continuously improving to cryogenic temperatures. We have measured electron mobilities over 1,200 and 13,000 sq cm/V-sec at room temperature and 0.25 K, respectively in MODFET structures. The electron carrier densities were 1.6 and 1.33 x 10(exp 12)/sq cm at room and liquid helium temperature, respectively, denoting excellent carrier confinement. Using this technique, we have observed electron mobilities as high as 900 sq cm/V-sec at room temperature at a carrier density of 1.3 x 10(exp 12)/sq cm. The temperature dependence of mobility for both the MODFET and HEMT structures provides insights into the mechanisms that allow for enhanced electron mobility as well as the processes that limit mobility, and will be presented.

Mueller, Carl↗
NASA NTRS2007

Thermal Properties of Oxides With Magnetoplumbite Structure for Advanced Thermal Barrier Coatings

Oxides having magnetoplumbite structure are promising candidate materials for applications as high temperature thermal barrier coatings because of their high thermal stability, high thermal expansion, and low thermal conductivity. In this study, powders of LaMgAl11O19, GdMgAl11O19, SmMgAl11O19, and Gd0.7Yb0.3MgAl11O19 magnetoplumbite oxides were synthesized by citric acid sol-gel method and hot pressed into disk specimens. The thermal expansion coefficients (CTE) of these oxide materials were measured from room temperature to 1500 C. The average CTE value was found to be approx.9.6x10(exp -6)/C. Thermal conductivity of these magnetoplumbite-based oxide materials was also evaluated using steady-state laser heat flux test method. The effects of doping on thermal properties were also examined. Thermal conductivity of the doped Gd0.7Yb0.3MgAl11O19 composition was found to be lower than that of the undoped GdMgAl11O19. In contrast, thermal expansion coefficient was found to be independent of the oxide composition and appears to be controlled by the magnetoplumbite crystal structure. Thermal conductivity testing of LaMgAl11O19 and LaMnAl11O19 magnetoplumbite oxide coatings plasma sprayed on NiCrAlY/Rene N5 superalloy substrates indicated resistance of these coatings to sintering even at temperatures as high as 1600 C.

Bansal, Narottam P.↗
NASA NTRS1993

Characterization of Si (sub X)Ge (sub 1-x)/Si Heterostructures for Device Applications Using Spectroscopic Ellipsometry

Spectroscopic ellipsometry (SE) characterization of several complex Si (sub X)Ge (sub 1-x)/Si heterostructures prepared for device fabrication, including structures for heterojunction bipolar transistors (HBT), p-type and n-type heterostructure modulation doped field effect transistors, has been performed. We have shown that SE can simultaneously determine all active layer thicknesses, Si (sub X)Ge (sub 1-x) compositions, and the oxide overlayer thickness, with only a general knowledge of the structure topology needed a priori. The characterization of HBT material included the SE analysis of a Si (sub X)Ge (sub 1-x) layer deeply buried (600 nanometers) under the silicon emitter and cap layers. In the SE analysis of n-type heterostructures, we examined for the first time a silicon layer under tensile strain. We found that an excellent fit can be obtained using optical constants of unstrained silicon to represent the strained silicon conduction layer. We also used SE to measure lateral sample homogeneity, providing quantitative identification of the inhomogeneous layer. Surface overlayers resulting from prior sample processing were also detected and measured quantitatively. These results should allow SE to be used extensively as a non-destructive means of characterizing Si (sub X)Ge (sub 1-x)/Si heterostructures prior to device fabrication and testing.

Sieg, R. M.↗
DOE OSTI2022

Computational Study of Variable Fuel-Air Ratio and Hydrogen Doping in a Rotating Detonation Engine

Rotating detonation engines (RDEs) hold great promise as a technology for powering aviation engines. The pressure gain that results from continuously traversing detonation waves enables high theoretical performance compared to deflagration-based combustion. However, the harsh operating conditions inside an RDE restrict the availability of suitable experimental diagnostics to study these systems in detail and the range of spatial and temporal scales render numerical simulations of these systems difficult. In this study, we have combined adaptive mesh refinement and a robust shock-capturing approach to study RDE systems computationally using the compressible, reactive solver PeleC. Multi-species transport along with compressible Navier-Stokes equations are solved in the model along with finite-rate chemistry. Complex geometries are represented using an embedded boundary method with second-order spatial accuracy and the system is evolved in time using a second-order Runge-Kutta method. We present studies of a methane-air RDE system that is doped with varying levels of hydrogen. Our analysis focusses on how fuel mixture and doping levels effect the detonation flame structure and the generation of multiple wave modes.

ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2026

Thermoelectric Behavior of Structurally Disordered Lanthanide and Actinide Silicides

Lanthanide and actinide intermetallic materials exhibit intriguing electronic, structural, and magnetic properties arising from their partially filled f orbitals. Strong f-electron correlations can significantly modify the electronic density of states, carrier scattering, and transport behavior, making these systems promising candidates for unconventional thermoelectric materials. In previous studies, single crystals of A1.33T4Al8Si2 (A = Ce, Th, U, Np; T = Co, Ni) were synthesized. This structure consists of ordered TAl2 corrugated double layers separated by disordered An–Si monolayers. The inherent disorder within the An–Si layers has been linked to unusual magnetic behavior, such as spin-glass phenomena, and is expected to strongly influence phonon scattering and charge-carrier transport relevant to thermoelectric performance. However, the thermoelectric properties of this material family have not previously been explored. In this work, DFT calculations were performed to evaluate the electronic density of states, band structures, and thermoelectric properties, including the effects of doping. To validate these predictions, Ce1.33Co4Al8Si2 single crystals were grown and experimentally characterized. Additionally, a new Yb1.33Co4Al8Si2 phase was synthesized and characterized using single-crystal XRD, SEM-EDS, magnetization measurements, and thermoelectric measurements. These results provide insight into structure–property relationships governing thermoelectric behavior in f-electron intermetallics and offer guidance for future thermoelectric materials.

37 - INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL C↗
DOE OSTI2024

Impurity induced confinement effects in size-separated Mn-doped CsPbCl 3 nanocrystals

Beyond quantum confinement, impurity doping of semiconductor nanocrystals (NCs) offers a degree of control over their optical and electronic properties, which are of interest for potential applications. Manganese (Mn)-doped CsPbCl 3 NCs, for example, show synergistic effects in the visible, with large enhancements in both blue (exciton) and red (dopant) emission over a narrow range of dopant concentration. Although Mn concentration is the primary parameter, NC size offers an additional degree of control over the number of dopants per NC. Size-resolved CsPbCl 3 NCs prepared in the limit of strong Mn doping highlight the emergence of defective NCs with deviations from anticipated confinement trends. In conclusion, the transition correlates with a redshift and increase in the amplitude of dopant emission, and we discuss these observations in the context of atomistic simulations of Ruddlesden-Popper defects in CsPbCl 3 .

36 MATERIALS SCIENCE↗
DOE OSTI2023

Space charge trap-assisted recombination suppressing layer for low-voltage diode operation

Shockley-Read-Hall (SRH) generation and/or recombination in heterojunction devices is suppressed by unconventional doping at or near the heterointerface. The effect of this doping is to shift SRH generation and/or recombination preferentially into the wider band gap material of the heterojunction. This reduces total SRH generation and/or recombination in the device by decreasing the intrinsic carrier concentration ni at locations where most of the SRH generation and/or recombination occurs. The physical basis for this effect is that the SRH generation and/or recombination rate tends to decrease as ni around the depletion region decreases, so decreasing the effective ni in this manner is a way to decrease SRH recombination.

Santhanam, Parthiban↗
DOE OSTI2019

Enhancing Chemical Interaction of Polysulfide and Carbon through Synergetic Nitrogen and Phosphorus Doping

The shuttling effect of the polysulfide (PS) is the most detrimental factor that causes capacity fading of Li–S batteries. By substituting carbon atoms with heteroatoms, the host materials can provide a chemical interaction with PS to trap it within the electrochemically active range of the cathode. In this study, first-principles-based calculations identify the most stable nitrogen (N) and phosphorus (P) codoped atomic structures and their corresponding adsorption energy toward different PS species. Based on the simulated predictions, we introduce a glucosamine-derived carbon nanotube aerogel (CNA) with a secondary dopant of P during the synergetic polymerization and carbonization process. The N–P codoped CNA and S composite (NPCNA-S) renders a highly stable performance even at a sulfur loading of 3.5 mg cm –2 . In conclusion, the specific capacity is >900 mAh g –1 and the stabilized capacity retention is >70% over 450 cycles at 0.2 C rate (1 C = 1675 mA g –1 ).

25 ENERGY STORAGE↗
DOE OSTI2019

In situ and operando investigation of the dynamic morphological and phase changes of a selenium-doped germanium electrode during (de)lithiation processes

To understand the effect of selenium doping on the good cycling performance and rate capability of a Ge 0.9 Se 0.1 electrode, the dynamic morphological and phase changes of the Ge 0.9 Se 0.1 electrode were investigated by synchrotron-based operando transmission X-ray microscopy (TXM) imaging, X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). The TXM results show that the Ge 0.9 Se 0.1 particle retains its original shape after a large volume change induced by (de)lithiation and undergoes a more sudden morphological and optical density change than pure Ge. The difference between Ge 0.9 Se 0.1 and Ge is attributed to a super-ionically conductive Li–Se–Ge network formed inside Ge 0.9 Se 0.1 particles, which contributes to fast Li-ion pathways into the particle and nano-structuring of Ge as well as buffering the volume change of Ge. The XRD and XAS results confirm the formation of a Li–Se–Ge network and reveal that the Li–Se–Ge phase forms during the early stages of lithiation and is an inactive phase. The Li–Se–Ge network also can suppress the formation of the crystalline Li 15 Ge 4 phase. Finally, these in situ and operando results reveal the effect of the in situ formed, super-ionically conductive, and inactive network on the cycling performance of Li-ion batteries and shed light on the design of high capacity electrode materials.

25 ENERGY STORAGE↗