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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

A spectroscopic ellispometric study of the tunability of the optical constants and thickness of GeO{sub x} films with swift heavy ions

Sub-stoichiometric GeO{sub x} films were fabricated by electron-beam evaporation method. The films were irradiated with 100 MeV Ag{sup 7+} ions at fluences between 1 x 10{sup 12} and 1 x 10{sup 14} ions-cm{sup -2}. Spectroscopic ellipsometric measurements were performed in air at room temperature. The values of the layer thickness and refractive index were extracted from ellipsometry using a multilayer analysis and the Tauc Lorentz model. The refractive index (at 633 nm) of the as-deposited GeO{sub x} film was estimated to be 1.860 and decreased to 1.823 for films irradiated at an ion fluence of 1 x 10{sup 14} ions-cm{sup -2}. The thickness of the films also decreased after irradiation and is due to a sputtering induced by the ion beam. The change in the refractive index with ion fluence is attributed to a stoichiometric change and structural transformation represented by GeO{sub x}{yields} Ge + GeO{sub y} (y > x) occurring due to a thermal spike induced by ion irradiation. Swift heavy ions thus provide a scope for modulating the refractive index of GeO{sub x} films. The thickness and stoichiometric changes are supported by Rutherford backscattering measurements.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Optoelectronic properties of Mg{sub 2}Si semiconducting layers with high absorption coefficients

In an attempt to develop a low-cost material for solar cell devices, polycrystalline magnesium silicide (poly-Mg{sub 2}Si) semiconducting layers have been prepared by applying rf magnetron sputtering using a Mg{sub 2}Si target. The optimum substrate temperature for the poly-Mg{sub 2}Si growth was found to be T{sub s} = 200 deg. C; the film deposition at higher temperatures leads to desorption of Mg atoms from the growing surface, while the amorphous phase formation occurs at room temperature. The poly-Mg{sub 2}Si layer deposited at T{sub s} = 200 deg. C shows the (111) preferential orientation with a uniform grain size of {approx}50 nm. The dielectric function of the poly-Mg{sub 2}Si layer has been determined accurately by spectroscopic ellipsometry. From the analysis, quite high absorption coefficients and an indirect gap of 0.77 eV in the poly-Mg{sub 2}Si layer have been confirmed. The above poly-Mg{sub 2}Si layer shows clear photoconductivity and can be applied as a narrow-gap bottom layer in multi-junction solar cell devices.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Low-damage milling of an amino acid thin film with cluster ion beam

In this work, we characterized the surface damage layer and sputtering yield of polycrystalline L-leucine films before and after irradiation with Ar cluster or monomer ion beams with x ray photoelectron spectroscopy and ellipsometry. Irradiation with Ar monomer ion beams induced heavy damage on the surface of L-leucine films, such as bond breaking and carbonization. In contrast, no significant surface damage was observed in the films irradiated with Ar cluster ion beams. The sputtering yield of L-leucine decreased dramatically with increasing fluence of monomer Ar ions and approached the value of the sputtering yield of graphite; but under irradiation with Ar cluster ion beams, the sputtering yield remained constant with fluence. The differences in sputtering yield behavior were explained in relation with the surface damage layer on organic materials. Thus, cluster ion beams could potentially be used to mill down biological materials without significant damage on the surface and could contribute to various applications in the analysis and processing of life matter.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Microstructure, optical property, and electronic band structure of cuprous oxide thin films

Cuprous oxide (Cu{sub 2}O) thin films were grown via radio frequency sputtering deposition at various temperatures. The dielectric functions and luminescence properties of the Cu{sub 2}O thin films were measured using spectroscopic ellipsometry and photoluminescence, respectively. High-energy peaks were observed in the photoluminescence spectra. Several critical points (CPs) were found using second derivative spectra of the dielectric functions and the standard critical point model. The electronic band structure and the dielectric functions were calculated using density functional theory, and the CP energies were estimated to compare with the experimental data. We identified the high-energy photoluminescence peaks to quasi-direct transitions which arose from the granular structures of the Cu{sub 2}O thin films.

36 MATERIALS SCIENCE↗
DOE OSTI2011

Hydrogenation and surface density changes in hydrocarbon films during erosion using Ar/H{sub 2} plasmas

We report interactions of low pressure Ar, H{sub 2}, and Ar/H{sub 2} mixture plasmas with a-C:H films. Surface evolution and erosion of a-C:H films were examined for ion energies up to 200 eV by rf biasing the substrates. Film surfaces were characterized using in situ ellipsometry, x-ray photoelectron spectroscopy, and atomic force microscopy. Multilayer models for steady-state modified surface layers are constructed using ellipsometric data and compared with results of molecular dynamics (MD) simulations and transport of ions in matter (TRIM) calculations. We find that Ar plasma causes a modified layer at the surface that is depleted of H atoms. The depth and degree of this modification is strongly depending on Ar ion energies. This depletion saturates quickly during plasma exposure (<1 s) and persists during steady-state erosion. We find that the thickness and density of the H-depleted layer are in good agreement with MD and TRIM simulations. The degree of surface densification decreases when small amounts of H{sub 2} are added to Ar plasmas. When more than 5% H{sub 2} is added to the plasma, long term loss in surface density is observed, indicating rehydrogenation and saturation of H in the film. As the H{sub 2} fraction increases, the near-surface atomic H increases and the ion composition bombarding the surface changes. This causes incorporation of H deeper into the a-C:H film. For a-C:H films exposed to pure H{sub 2} plasmas, H is introduced into the near-surface region to a depth of up to {approx}8 nm from the surface. As the rf bias is increased the ion energy transitions from solely chemical sputtering to one involving physical sputtering, causing the yield of C atoms from the surface to greatly increase. The increasing yield suppresses H incorporation/saturation and decreases the magnitude of the modified surface layer.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Optical and structural characterization of thermal oxidation effects of erbium thin films deposited by electron beam on silicon

Thermal oxidation effects on the structural, compositional, and optical properties of erbium films deposited on silicon via electron beam evaporation were analyzed by x-ray diffraction, x-ray photoelectron spectroscopy, Auger electron spectroscopy, and spectroscopic ellipsometry. A gradual rise in oxidation temperature from 700 to 900 deg. C resulted in a transition from ErO- to Er{sub 2}O{sub 3}-rich phase. Additional increase in oxidation temperature above 1000 deg. C led to the formation of erbium silicate due to further oxygen incorporation, as well as silicon out-diffusion from the substrate. A silicon oxide interfacial layer was also detected, with its thickness increasing with higher oxidation temperature. Additionally, film refractive index decreased, while its Tauc bandgap value increased from {approx}5.2 eV to {approx}6.4 eV, as the oxidation temperature was raised from 700 deg. C to above 900 deg. C. These transformations were accompanied by the appearance of an intense and broad absorption band below the optical gap. Thermal oxidation effects are discussed in the context of film structural characteristics and defect states.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Dielectric Function Spectra and Critical-Point Energies of Cu2ZnSnSe4 from 0.5 to 9.0 eV

We present dielectric function {var_epsilon} = {var_epsilon}{sub 1} + i{var_epsilon}{sub 2} spectra and critical-point energies of Cu{sub 2}ZnSnSe{sub 4} determined by spectroscopic ellipsometry from 0.5 to 9.0 eV. We reduce artifacts from surface overlayers to the maximum extent possible by performing chemical-mechanical polishing and wet-chemical etching of the surface of a Cu{sub 2}ZnSnSe{sub 4} thin film. Ellipsometric data are analyzed by the multilayer model and the {var_epsilon} spectra are extracted. The data exhibit numerous spectral features associated with critical points, whose energies are obtained by fitting standard lineshapes to second energy derivatives of the data. The experimental results are in good agreement with the {var_epsilon} spectra calculated within the GW quasi-particle approximation, and possible origins of the pronounced critical-point structures are identified.

14 SOLAR ENERGY↗
DOE OSTI2012

Refractive Index of Sodium Iodide

The refractive index of sodium iodide, an important scintillator material that is widely used for radiation detection, is based on a single measurement made by Spangenberg at one wavelength using the index-matching liquid immersion method (Z. Kristallogr., 57, 494-534 (1923)). In the present paper, we present new results for the refractive index of sodium iodide as measured by the minimum deviation technique at six wavelengths between 436 nm (n=1.839 0.002) and 633 nm (n=1.786 0.002). These 6 measurements can be fit to a Sellmeier model, resulting in a 2 of 1.02, indicating a good fit to the data. In addition, we report on ellipsometry measurements, which suggest that the near-surface region of the air sensitive NaI crystal seriously degrades, even in a moisture-free environment, resulting in a significantly lower value of the refractive index near the surface. First-principles theoretical calculations of the NaI refractive index that agree with the measured values within 0.025-0.045 are also presented and discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Structural relaxation in amorphous oxide semiconductor, a-In-Ga-Zn-O

Amorphous In-Ga-Zn-O (a-IGZO) is expected as a backplane transistor material to drive next-generation flat-panel and flexible displays. It has been elucidated that thermal annealing even at low temperatures <200 deg. C reduces deep subgap defects and those at {>=}300 deg. C further improve device characteristics, stability, and uniformity. These temperatures are much lower than the reported crystallization temperature (T{sub X}{approx} 600 deg. C). In this work, we investigate effects of thermal annealing on the structural and optical properties of a-IGZO thin films. We performed classical molecular dynamics simulation (CMD) and optical interference analyses including spectroscopic ellipsometry (SE). CMD reproduced the x-ray diffraction pattern of a-IGZO and exhibited a glass transition. Experimentally, it was found that T{sub X} depends largely on deposition methods and conditions, probably due to different chemical compositions. Sputter-deposited a-IGZO films exhibited onset T{sub X}{approx} 600 deg. C and crystalline volume fraction X{sub C} increased linearly from 600 deg. C. 1.2% of film densification occurred even at 600 deg. C due to crystallization. High-temperature in situ SE measurements did not detect a symptom of a glass transition temperature (T{sub g}) presumably because the T{sub X} is close to T{sub g} similar to the case of amorphous metals.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Electrical and optical properties of sputtered amorphous vanadium oxide thin films

Amorphous vanadium oxide (VO{sub x}) is a component found in composite nanocrystalline VO{sub x} thin films. These types of composite films are used as thermistors in pulsed biased uncooled infrared imaging devices when containing face centered cubic vanadium monoxide phase crystallites, and substantial fractions of amorphous material in the composite are necessary to optimize device electrical properties. Similarly, optoelectronic devices exploiting the metal-to-semiconductor transition contain the room-temperature monoclinic or high-temperature (>68 deg. C) rutile vanadium dioxide phase. Thin films of VO{sub x} exhibiting the metal-to-semiconductor transition are typically polycrystalline or nanocrystalline, implying that significant amounts of disordered, amorphous material is present at grain boundaries or surrounding the crystallites and can impact the overall optical or electronic properties of the film. The performance of thin film material for either application depends on both the nature of the crystalline and amorphous components, and in this work we seek to isolate and study amorphous VO{sub x}. VO{sub x} thin films were deposited by pulsed dc reactive magnetron sputtering to produce amorphous materials with oxygen contents {>=}2, which were characterized electrically by temperature dependent current-voltage measurements and optically characterized by spectroscopic ellipsometry. Film resistivity, thermal activation energy, and complex dielectric function spectra from 0.75 to 6.0 eV were used to identify the impact of microstructural variations including composition and density.

36 MATERIALS SCIENCE↗
DOE OSTI2012

Synergistic etch rates during low-energetic plasma etching of hydrogenated amorphous carbon

The etch mechanisms of hydrogenated amorphous carbon thin films in low-energetic (<2 eV) high flux plasmas are investigated with spectroscopic ellipsometry. The results indicate a synergistic effect for the etch rate between argon ions and atomic hydrogen, even at these extremely low kinetic energies. Ion-assisted chemical sputtering is the primary etch mechanism in both Ar/H{sub 2} and pure H{sub 2} plasmas, although a contribution of swift chemical sputtering to the total etch rate is not excluded. Furthermore, ions determine to a large extent the surface morphology during plasma etching. A high influx of ions enhances the etch rate and limits the surface roughness, whereas a low ion flux promotes graphitization and leads to a large surface roughness (up to 60 nm).

36 MATERIALS SCIENCE↗
DOE OSTI2012

Ellipsometrically determined optical properties of nickel-containing tungsten oxide thin films: Nanostructure inferred from effective medium theory

Films of Ni{sub x}W{sub 1-x} oxide with 0.05 {<=} x {<=} 0.53 were produced by reactive dc magnetron co-sputtering onto Si. Such films have documented electrochromism. Spectroscopic ellipsometry was used to extract accurate data on the dielectric function in the photon range 0.062 to 5.62 eV. The results for 0.62 to 5.62 eV were compared with computations from the Bruggeman effective medium theory applied to two nanostructural models: one representing a random mixture of structural entities characterized by the dielectric functions of WO{sub 3} and NiWO{sub 4} and the other describing a random mixture of WO{sub 3} and NiO. Unambiguous evidence was found in favor of the former model, and hence the films are composed of nanosized tungsten oxide and nickel tungstate. This agrees excellently with an earlier investigation of ours on Ni{sub x}W{sub 1-x} oxide films, where nanostructure was inferred from Raman spectroscopy, x-ray photoelectron spectroscopy, and x-ray diffraction.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2012

Microstructural evolution of thin film vanadium oxide prepared by pulsed-direct current magnetron sputtering

Vanadium oxide (VO{sub x}) thin films have been deposited by pulsed-DC magnetron sputtering using a metallic vanadium target in a reactive argon and oxygen environment. While the process parameters (power, total pressure, oxygen-to-argon ratio) remained constant, the deposition time was varied to produce films between 75 {+-} 6 and 2901 {+-} 30 A thick, which were then optically and electrically characterized. The complex dielectric function spectra ({epsilon} = {epsilon}{sub 1} + i{epsilon}{sub 2}) of the films from 0.75 to 5.15 eV were extracted by ex situ, multiple-angle spectroscopic ellipsometry (SE) measurements for the series of varied thickness VO{sub x} samples. Significant changes in {epsilon} and resistivity occur as a function of thickness, indicating the correlations exist between the electrical and the optical properties over this spectral range. In addition, in situ measurements via real time SE (RTSE) were made on the film grown to the largest thickness to track optical property and structural variations during growth. RTSE was also used to characterize changes in the film occurring after growth was completed, namely during post sputtering in the presence of argon and oxygen while the sample is shielded, and atmospheric exposure. RTSE indicates that the exposure of the film to the argon and oxygen environment, regardless of the shutter isolating the target, causes up to 200 A of the top surface of the deposited film to become more electrically resistive as evidenced by variations in {epsilon}. Exposure of the VO{sub x} thin film to atmospheric conditions introduces a similar change in {epsilon}, but this change occurs throughout the bulk of the film. A combination of these observations with RTSE results indicates that thinner, less ordered VO{sub x} films are more susceptible to drastic changes due to atmospheric exposure and that microstructural variations in this material ultimately control its environmental stability.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Improved passivation of the ZnO/Si interface by pulsed laser deposition

Zinc oxide thin-films were grown on crystalline silicon employing magnetron sputtering and pulsed laser deposition. Bulk and interface properties were investigated using scanning electron microscopy, Raman backscattering, photoluminescence, and infrared spectroscopic ellipsometry. Sputter deposited ZnO samples reveal a large degree of disorder and an interface defect density of Almost-Equal-To 10{sup 12} cm{sup -2}. A significant improvement of the structural quality is observed in samples grown by pulsed laser deposition. The bulk defect density is further reduced, when introducing monatomic oxygen during deposition. Simultaneously, the defect density at the ZnO/Si interface decreases by about a factor of five. Implications for devices containing ZnO/Si interfaces are discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2013

Synchrotron radiation-based far-infrared spectroscopic ellipsometer with full Mueller-matrix capability

We developed far-IR spectroscopic ellipsometer at the U4IR beamline of the National Synchrotron Light Source in Brookhaven National Laboratory. This ellipsometer is able to measure both, rotating analyzer and full-Mueller matrix spectra using rotating retarders, and wire-grid linear polarizers. We utilize exceptional brightness of synchrotron radiation in the broad spectral range between about 20 and 4000 cm{sup -1}. Fourier-transform infrared (FT-IR) spectrometer is used for multi-wavelength data acquisition. The sample stage has temperature variation between 4.2 and 450 K, wide range of {theta}-2{theta} angular rotation, {chi} tilt angle adjustment, and X-Y-Z translation. A LabVIEW-based software controls the motors, sample temperature, and FT-IR spectrometer and also allows to run fully automated experiments with pre-programmed measurement schedules. Data analysis is based on Berreman's 4 Multiplication-Sign 4 propagation matrix formalism to calculate the Mueller matrix parameters of anisotropic samples with magnetic permeability {mu}{ne} 1. A nonlinear regression of the rotating analyzer ellipsometry and/or Mueller matrix (MM) spectra, which are usually acquired at variable angles of incidence and sample crystallographic orientations, allows extraction of dielectric constant and magnetic permeability tensors for bulk and thin-film samples. Applications of this ellipsometer setup for multiferroic and ferrimagnetic materials with {mu}{ne} 1 are illustrated with experimental results and simulations for TbMnO{sub 3} and Dy{sub 3}Fe{sub 5}O{sub 12} single crystals. We demonstrate how magnetic and electric dipoles, such as magnons and phonons, can be distinguished from a single MM measurement without adducing any modeling arguments. The parameters of magnetoelectric components of electromagnon excitations are determined using MM spectra of TbMnO{sub 3}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2013

Optical properties of SrTiO{sub 3} on silicon(100)

Epitaxial buffer layers enable the many functionalities found in perovskites to be integrated with silicon. However, epitaxial growth of SrTiO{sub 3} on silicon is tricky and has so far only been achieved by molecular beam epitaxy. Nonetheless, previous investigations of these films were limited by the amorphous layer occurring at the interface. Through a combination of improved interface quality and an improved model, we report the optical properties of SrTiO{sub 3} films on Si(100) investigated by spectroscopic ellipsometry. We find that the data are best described by a model with two different SrTiO{sub 3} layers, potentially resulting from variations in the oxygen content.

36 MATERIALS SCIENCE↗
DOE OSTI2013

AlP/GaP distributed Bragg reflectors

Distributed Bragg reflectors with high reflectivity bands centered at wavelengths from 530 to 690 nm (green to red) based on AlP/GaP quarter-wave stacks are prepared on (001)GaP using gas-source molecular-beam epitaxy. Additionally, the complex refractive index of AlP is measured using spectroscopic ellipsometry within the range of 330-850 nm in order to facilitate an accurate reflector design. Structures consisting of 15 quarter-wave stacks reach a peak reflectance between 95% and 98%, depending on the spectral position of the maximum.

36 MATERIALS SCIENCE↗