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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 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Spectroscopic ellipsometry study of thickness and porosity of the alteration layer formed on international simple glass surface in aqueous corrosion conditions

The porosity and pore-size distribution in the glass play an important role in glass corrosion; however, such information is difficult to be obtained non-destructively. Here we report the use of spectroscopic ellipsometry (SE) under controlled humidity environments to determine those structural parameters in the alteration layers formed on international simple glass (ISG), a model nuclear waste glass, in aqueous corrosion conditions. The SE results show that the ISG sample corroded for nearly 4.5 years develops pores larger than 2 nm in diameter, while the alteration layers formed initially in less than 1 month do not have such large pores. The development of larger pores over a long period of corrosion time, while the overall thickness remains relatively constant, implies structural rearrangements of the silicate network occurring within the alteration layer, which could eventually affect the transport of reactants and products as well as the structural stability.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Characterization of an active metasurface using terahertz ellipsometry

Switchable metasurfaces fabricated on a doped epi-layer have become an important platform for developing techniques to control terahertz (THz) radiation, as a DC bias can modulate the transmission characteristics of the metasurface. To model and understand this performance in new device configurations accurately, a quantitative understanding of the bias-dependent surface characteristics is required. In this work, we perform THz variable angle spectroscopic ellipsometry on a switchable metasurface as a function of DC bias. By comparing these data with numerical simulations, we extract a model for the response of the metasurface at any bias value. Using this model, we predict a giant bias-induced phase modulation in a guided wave configuration. Lastly, these predictions are in qualitative agreement with our measurements, offering a route to efficient modulation of THz signals.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Spectroscopic ellipsometry extraction of optical constants for materials from oxide covered samples: Application to the plutonium/oxides system

Spectroscopic ellipsometry is combined with scanning electron microscopy and multisample analysis of the measured oxide thickness difference from one sample at two different times during the oxide growth phase or from two separate samples to obtain the optical constants of the oxides. With the obtained thickness and optical constants of the oxides, the optical properties of the substrate material can be uniquely deduced. This methodology has been applied to obtain, the refractive indices and extinction coefficients of delta plutonium (δ-Pu) metal with 0.5 wt. % Ga in the wavelength region from 435 nm to 850 nm, without the influence of surface oxides. Lastly, the presented methodology can also be applied to metals and semiconductors from surface oxidized samples, even when the oxides do not possess a common set of optical characteristics.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As determined by variable-angle spectroscopic ellipsometry

The optical constants of single-crystal Al x Ga 1-x P and Al x Ga 1-x As have been widely studied at various Al compositions but have not yet been thoroughly investigated as polycrystalline materials. Using variable-angle spectroscopic ellipsometry, we estimate and analyze the optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As that were non-epitaxially deposited at deposition temperatures of either 250 or 420 °C, and with various dopant species, dopant concentrations, and V/III flux ratios. Here, for wavelengths of 350–400 nm, decreasing the V/III ratio increases the refractive index of Be-doped polycrystalline Al 0.25 Ga 0.75 P. For wavelengths >500 nm, as the targeted Be dopant concentration increases for polycrystalline Al 0.25 Ga 0.75 P, the amount the extinction coefficient increases and the indirect bandgap decreases depends on the V/III ratio. Furthermore, Si-doped polycrystalline Al 0.25 Ga 0.75 P has significantly smaller extinction coefficient values than when it is Be-doped, for the same V/III ratio and targeted doping concentration, at wavelengths >500 nm. Based on our findings, we hypothesize that the dopant species, dopant concentration, and the V/III ratio dictate various types of defect concentrations, which then affect the optical constants and indirect bandgap of polycrystalline Al 0.25 Ga 0.75 P. Additionally, for wavelengths >500 nm, we find that polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As have the lowest extinction coefficient values when deposited at 420 °C, as opposed to 250 °C. Thus, we hypothesize that the deposition temperature is the most significant factor in dictating the optical constants of these polycrystalline III-Vs. Ultimately, this work demonstrates alternative methods to tune the optical constants of polycrystalline Al 0.25 Ga 0.75 P and Al 0.9 Ga 0.1 As, as opposed to tuning the Al composition.

42 ENGINEERING↗
DOE OSTI2014

Evaluation of the SEI using a multilayer spectroscopic ellipsometry model

A multilayer spectroscopic ellipsometry (SE) model has been developed to characterize SEI formation. The model, which consists of two Cauchy layers, is constructed with an inner layer meant to model primarily inorganic compounds adjacent to an electrode and an outer layer which mirrors polymeric, organic constituents on the exterior of the SEI. Comparison of 1:1 EC:EMC and 1:4 EC:EMC with 1.0 M LiPF₆ shows distinct differences in the two modeled layers. The data suggest that the thickness of both layers change over a wide potential range. These changes have been linked with other reports on the growth of the SEI.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Assessing atomically thin delta-doping of silicon using mid-infrared ellipsometry

Hydrogen lithography has been used to template phosphine-based surface chemistry to fabricate atomic-scale devices, a process we abbreviate as atomic precision advanced manufacturing (APAM). In this work, we use mid-infrared variable angle spectroscopic ellipsometry (IR-VASE) to characterize single-nanometer thickness phosphorus dopant layers (δ-layers) in silicon made using APAM compatible processes. A large Drude response is directly attributable to theδ-layer and can be used for nondestructive monitoring of the condition of the APAM layer when integrating additional processing steps. Furthermore, the carrier density and mobility extracted from our room temperature IR-VASE measurements are consistent with cryogenic magneto-transport measurements, showing that APAM δ-layers function at room temperature. Finally, the permittivity extracted from these measurements shows that the doping in the APAM δ-layers is so large that their low-frequency in-plane response is reminiscent of a silicide. However, there is no indication of a plasma resonance, likely due to reduced dimensionality and/or low scattering lifetime.

2D material↗
DOE OSTI2014

Self-consistent iteration procedure in analyzing reflectivity and spectroscopic ellipsometry data of multilayered materials and their interfaces

For multilayered materials, reflectivity depends on the complex dielectric function of all the constituent layers, and a detailed analysis is required to separate them. Furthermore, for some cases, new quantum states can occur at the interface which may change the optical properties of the material. In this paper, we discuss various aspects of such analysis, and present a self-consistent iteration procedure, a versatile method to extract and separate the complex dielectric function of each individual layer of a multilayered system. As a case study, we apply this method to LaAlO{sub 3}/SrTiO{sub 3} heterostructure in which we are able to separate the effects of the interface from the LaAlO{sub 3} film and the SrTiO{sub 3} substrate. Our method can be applied to other complex multilayered systems with various numbers of layers.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Optical functions of silicon from reflectance and ellipsometry on silicon-on-insulator and homoepitaxial samples

The optical properties of silicon have been determined from 0.2 to 6.5 eV at room temperature, using reflectance spectra of silicon-on-insulator (SOI) and ellipsometric spectra of homoepitaxial samples. Optimized Fabry-Perot-type SOI resonators exhibit high finesse even in near ultraviolet. Very high precision values of the real part of the refractive index are obtained in infrared up to a photon energy of 1.3 eV. The spectra of the extinction coefficient, based on observations of light attenuation, extend to 3.2 eV due to measurements on SOI layers as thin as 87 nm. These results allowed us to correct spectroellipsometric data on homoepitaxial samples for the presence of reduced and stabilized surface layers.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Spectroscopic ellipsometry-based study of optical properties of amorphous and crystalline ZnSnO alloys and Zn{sub 2}SnO{sub 4} thin films grown using sputtering deposition: Dielectric function and subgap states

We investigated the optical properties of amorphous and crystalline zinc tin oxide (ZTO) thin films grown on SiO{sub 2}/Si substrates with varying compositions via a co-sputtering deposition method at room temperature. The co-sputtering targets consist of SnO{sub 2} and ZnO. By varying the relative power ratio of the two targets, we demonstrate the ability to control the Sn and Zn composition of the resulting ZTO thin films. The ratio of [Sn]/([Sn] + [Zn]) atomic compositions was estimated at 11%, 29%, 42%, 54%, and 60%. Using a 600 °C annealing process, the as-grown amorphous ZTO films were transformed into crystalline ZTO films. The dielectric functions were obtained based on the measured ellipsometric angles, ψ and Δ. We determined the dielectric functions, absorption coefficients, and optical gap energies of ZTO thin films with varying compositions. The dielectric functions, absorption coefficients, and optical gap energies of amorphous and crystalline Zn{sub 2}SnO{sub 4} thin films were obtained at 29 at. % of Sn. Subgap states at 1.6 eV (A) and 2.8 eV (B) of ZnSnO alloys and Zn{sub 2}SnO{sub 4} films were found in the imaginary part of the dielectric function spectra. The subgap state intensities were reduced via a nitrogen gas annealing. Possible origins of the observed subgap states will be discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Determining Infrared Optical Constants for Dolomite using Single Angle Reflectance and Spectroscopic Ellipsometry

The optical constants, namely the real (n) and imaginary (k) parts of the complex refractive index, are of particular interest to generate the infrared (IR) spectra of liquid and solid materials in different morphologies. To obtain n/k, however, most materials are typically not found in the monolithic forms necessary to easily measure n/k, and thus require the use of other methods such as pressing powders to form planar, specularly-reflective pellets. In this work dolomite crystals are measured using fixed-angle IR reflectance spectroscopy in both 1) a monolithic form with the crystal fixed in epoxy and polished, but data are also recorded from 2) pressed-pellet forms of powders of the same mineral using the same spectroscopic protocols. First results for the two methods are compared. It was found that the measured reflectance can vary by as much as a factor of two between the dolomite crystals and the pressed powder forms. For the two-sample preparation approaches the preliminary spectra are compared and the implications and limitations of each method for determining the optical constants of given materials are discussed. Comparison to literature data suggest that polarization effects likely account for the differing amplitude results for reflectance (and hence the k -vectors): Dolomite is known to be biaxial with significantly differing ordinary and extraordinary rays; the pressed pellet method measures an ensemble of microcrystals in randomly oriented positions, whereas the single crystal maintains just one orientation relative to the optical axis.

ellipsometry, dolomite, complex refractive index, ↗