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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 1,711 records · Page 95
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Effect of oxygen deficiency on electronic properties and local structure of amorphous tantalum oxide thin films

Highlights: • The effect of oxygen flow rate on electronic properties and local structure of tantalum oxide thin films was studied. • The oxygen deficiency induced the nonstoichiometric state a-TaOx. • A small peak at 1.97 eV above the valence band side appeared on nonstoichiometric Ta{sub 2}O{sub 5} thin films. • The oxygen flow rate can change the local electronic structure of tantalum oxide thin films. - Abstract: The dependence of electronic properties and local structure of tantalum oxide thin film on oxygen deficiency have been investigated by means of X-ray photoelectron spectroscopy (XPS), Reflection Electron Energy Loss Spectroscopy (REELS), and X-ray absorption spectroscopy (XAS). The XPS results showed that the oxygen flow rate change results in the appearance of features in the Ta 4f at the binding energies of 23.2 eV, 24.4 eV, 25.8, and 27.3 eV whose peaks are attributed to Ta{sup 1+}, Ta{sup 2+}, Ta{sup 3+}/Ta{sup 4+}, and Ta{sup 5+}, respectively. The presence of nonstoichiometric state from tantalum oxide (TaOx) thin films could be generated by the oxygen vacancies. In addition, XAS spectra manifested both the increase of coordination number of the first Ta-O shell and a considerable reduction of the Ta-O bond distance with the decrease of oxygen deficiency.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Hard x-ray nanotomography of amorphous aluminosilicate cements.

Nanotomographic reconstruction of a sample of low-CO{sub 2} 'geopolymer' cement provides the first three-dimensional view of the pore structure of the aluminosilicate geopolymer gel, as well as evidence for direct binding of geopolymer gel onto unreacted fly ash precursor particles. This is central to understanding and optimizing the durability of concretes made using this new class of binder, and demonstrates the value of nanotomography in providing a three-dimensional view of nanoporous inorganic materials.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Spectral Performance of Multilayer Amorphous Selenium and Selenium–Tellurium Photodetectors

The ability to robustly and with scalability detect single photons in the visible spectrum with wavelength resolution would transform many imaging applications. Theoretical studies propose an array of carbon nanotubes (CNTs) functionalized with semiconductor quantum dots (QDs) as a physical realization of such photon sensors. In this work, we report approaches to synthesize these CNT-QD nanostructures using DNA as a smart glue to connect CNTs to QDs.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Short-range order in amorphous SiO{sub x} by x ray photoelectron spectroscopy

The Si 2p x ray photoelectron spectra of SiO{sub x} with a different composition of 0 {<=} x {<=} 2 have been studied experimentally and theoretically. The SiO{sub x} films were prepared by low-pressure chemical vapor deposition from SiH{sub 4} and N{sub 2}O source at 750 deg. C. Neither random bonding nor random mixture models can adequately describe the structure of these compounds. The interpretation of the experimental results is discussed according to a large scale potential fluctuation due to the spatial variation of chemical composition in SiO{sub x}.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Hydrogen plasma treatments for passivation of amorphous-crystalline silicon-heterojunctions on surfaces promoting epitaxy

The impact of post-deposition hydrogen plasma treatment (HPT) on passivation in amorphous/crystalline silicon (a-Si:H/c-Si) interfaces is investigated. Combining low temperature a-Si:H deposition and successive HPT, a high minority carrier lifetime >8 ms is achieved on c-Si <100>, which is otherwise prone to epitaxial growth and thus inferior passivation. It is shown that the passivation improvement stems from diffusion of hydrogen atoms to the heterointerface and subsequent dangling bond passivation. Concomitantly, the a-Si:H hydrogen density increases, leading to band gap widening and void formation, while the film disorder is not increased. Thus, HPT allows for a-Si:H band gap and a-Si:H/c-Si band offset engineering.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Study of spontaneous and induced absorption in amorphous Ta{sub 2}O{sub 5} and SiO{sub 2} dielectric thin films

Tantalum pentoxide (Ta{sub 2}O{sub 5}) and silicon dioxide (SiO{sub 2}) are common high-index and low-index materials used in dielectric optical coatings for high average-power lasers since high-density sputtered oxide films with absorption losses at near- and mid-infrared wavelengths of less than 1 ppm can be obtained. These oxides have been chosen to investigate the spontaneous and optically induced absorption at {lambda}{sub 0} = 1064 nm that occurs due to simultaneous illumination at shorter wavelengths. The effect is measured using the photothermal common-path interferometric technique. This technique is capable of detecting sub-ppm levels of optical absorption and tracking its changes at a given wavelength when a second laser beam is also incident on a thin film oxide sample. In this work, dual beam experiments are employed to assess changes in the optical absorption at {lambda}{sub 0} = 1064 nm in ion beam sputtered Ta{sub 2}O{sub 5} and SiO{sub 2} thin films deposited on fused silica substrates, with stimulating illumination {lambda}{sub 1} ranging from {lambda}{sub 1} = 266 nm to {lambda}{sub 1} = 780 nm. The power and wavelength of the stimulating radiation were found to affect the optical absorption at {lambda}{sub 0} = 1064 nm. Furthermore, the relaxation dynamics of the induced infrared absorption was found to be wavelength dependent and is thought to be associated with various electron traps existing in the forbidden gap that depend essentially on the film's preparation conditions. The significantly greater effect observed in Ta{sub 2}O{sub 5} at {lambda}{sub 1} = 266 nm is attributed to band-to-band transitions.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Ion-beam-induced bending of freestanding amorphous nanowires: The importance of the substrate material and charging

Ion-beam irradiation offers great flexibility and controllability in the construction of freestanding nanostructures with multiple advanced functionalities. Here, we present and discuss the bending of free-standing nanowires, against, towards, and ultimately parallel to a flux of directional ion irradiation. Bending components both along and perpendicular to the incident ion beam were observed, and the bending behavior was found to depend both on the ion beam scanning strategy and on the conductivity of the supporting substrate. This behavior is explained by an ion-irradiation-related electrostatic interaction. Our findings suggest the prospect of exploiting this technique to engineer 3D nanostructures for advanced applications.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Effect of thermal annealing on structure and optical band gap of amorphous Se{sub 72}Te{sub 25}Sb{sub 3} thin films

Thin films of a−Se{sub 72}Te{sub 25}Sb{sub 3} were prepared by vacuum evaporation technique in a base pressure of 10{sup −6} Torr on to well cleaned glass substrate. a−Se{sub 72}Te{sub 25}Sb{sub 3} thin films were annealed at different temperatures below their crystallization temperatures for 2h. The structural analysis of the films has been investigated using X-ray diffraction technique. The optical band gap of as prepared and annealed films as a function of photon energy in the wavelength range 400–1100 nm has been studied. It has been found that the optical band gap decreases with increasing annealing temperatures in the present system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Amorphous silicon enhanced metal-insulator-semiconductor contacts for silicon solar cells

Carrier recombination at the metal-semiconductor contacts has become a significant obstacle to the further advancement of high-efficiency diffused-junction silicon solar cells. This paper provides the proof-of-concept of a procedure to reduce contact recombination by means of enhanced metal-insulator-semiconductor (MIS) structures. Lightly diffused n{sup +} and p{sup +} surfaces are passivated with SiO{sub 2}/a-Si:H and Al{sub 2}O{sub 3}/a-Si:H stacks, respectively, before the MIS contacts are formed by a thermally activated alloying process between the a-Si:H layer and an overlying aluminum film. Transmission/scanning transmission electron microscopy (TEM/STEM) and energy dispersive x-ray spectroscopy are used to ascertain the nature of the alloy. Idealized solar cell simulations reveal that MIS(n{sup +}) contacts, with SiO{sub 2} thicknesses of ∼1.55 nm, achieve the best carrier-selectivity producing a contact resistivity ρ{sub c} of ∼3 mΩ cm{sup 2} and a recombination current density J{sub 0c} of ∼40 fA/cm{sup 2}. These characteristics are shown to be stable at temperatures up to 350 °C. The MIS(p{sup +}) contacts fail to achieve equivalent results both in terms of thermal stability and contact characteristics but may still offer advantages over directly metallized contacts in terms of manufacturing simplicity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗