Article

Optical properties of Zr and ZrO2

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Abstract

Optical properties of Zr and its oxide have been measured on the surface of nuclear fuel cladding tubes. It has been shown that ellipsometry with focusing can routinely be used to measure thin layers and surface properties on Zr tubes with a diameter as small as 9.1 mm. Multi-sample and depth profiling models have been used to determine reference dielectric function spectra for both the Zr substrate and its oxide. Temporal behavior of the oxide thickness has been measured for oxidation temperatures of 600 °C and 800 °C. A vertical inhomogeneity of the oxide properties has been found by the optical measurements as well as by depth-profiling X-ray photoelectron spectroscopy investigations that revealed the formation of sub-oxides at the interface region of Zr and its surface oxide.

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The galvanostatic oxidation of hafnium in different electrolytes was investigated by means of in-situ ellipsometry in the range 0–150 V corresponding to oxides thicknesses in the range 0–350 nm. The oxide surfaces were characterized by scanning electron microscopy (SEM) examination. Single and double-layer models were used to interpret the experimental results and it was observed that the nature of the electrolyte affects mainly the properties of the external thin layer close to the solution. The bulk oxide grown in H2SO4. NaOH and H3PO4, has a refractive index of 2.06–2.07 and an absorption coefficient of 0.02–0.03. The Δ- ψ profiles recorded during the oxidation in HNO3 reveal different stages of oxide corrosion. The films formed in this electrolyte have the lowest refractive indices as a consequence of their porous nature. An electric field strength increase with thickness was observed which could be associated to changes from amorphous to crystalline oxide structure. This fact could produce internal stresses during the oxide growth resulting in an anodic fracture. The ellipsometric results of the present work confirm previous ac impedance investigations.
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Optical properties of Zr and ZrO2 films in the energy range from 1.5 to 100eV were obtained by quantitative analysis of reflection electron energy loss spectroscopy (REELS) and ellipsometry. The films were prepared on (111) silicon substrates by reactive laser ablation using a zirconium target. For the growth of ZrO2 films a pressure of 5mTorr of oxygen in the growth chamber was used. The substrate temperature during deposition was 400∘C. The deposits were studied ex situ by X-ray diffraction (XRD) and in situ by X-ray photoelectron spectroscopy (XPS) and REELS. The ZrO2 films were found to be polycrystalline with monoclinic structure. The XPS results showed that the oxygen pressure used is the optimal control to produce ZrO2 films by laser ablation. A gap of 5eV for the ZrO2 film was measured by REELS.
Article
The integration of high-k dielectric materials into semiconductor devices requires nondestructive, fast, and accurate characterization methods. Spectroscopic ellipsometry (SE) is an outstanding candidate for this purpose. A multisample variable-angle SE method was used to characterize ZrO2 samples deposited on Si(100) by high-vacuum chemical vapor deposition. Proper modeling of the optical properties of the interfacial layer is found to be the key to accurate characterization of ZrO2 films. Based on a stacking model consisting of an effective medium approximation surface-roughness layer, a Tauc-Lorentz (TL) layer to represent the ZrO2 layer, and a second TL layer to represent the interfacial layer, we accurately extract both thickness and optical constants of each layer. The extracted surface-roughness and thickness values were confirmed by atomic force microscopy and transmission electron microscopy results. The optical constants of the interfacial layer suggest that the interfacial layer is composed of nonstoichiometric zirconium silicate. (C) 2004 American Vacuum Society.
Article
In this paper results concerning the optical characterization of inhomogeneous thin films of ZrO2 are presented. The optical characterization of these films is performed using the simultaneous interpretation of experimental data corresponding to variable angle of incidence spectroscopic ellipsometry (VASE) and near-normal incidence spectroscopic reflectometry (NNSR). It is shown that the ZrO2 films exhibit a depth inhomogeneity concerning their refractive indices. Further, it is shown that the ZrO2 films studied can be replaced by the four-layer system from the point of view of satisfactory fits of the experimental data. The values of the thicknesses and spectral dependences of the refractive indices of these four films are determined. Moreover, it is proved that the individual methods, i.e. VASE and especially NNSR, cannot be used separately to characterize the ZrO2 films investigated. The simultaneous treatment of the spectral dependences of the ellipsometric parameters and the reflectance measured for different angles of incidence allows complete optical characterization of these inhomogeneous films to be performed, i.e. only a combination of VASE and NNSR enables us to determine the values of both the refractive indices and the thicknesses of the films replacing the inhomogeneous ZrO2 films investigated from the optical point of view. Copyright © 2000 John Wiley & Sons, Ltd.
Article
Accurate dielectric function values are essential for spectroscopic ellipsometry data analysis by traditional optical model-based analysis techniques. In this paper, we show that B-spline basis functions offer many advantages for param- eterizing dielectric functions. A Kramers–Kronig consistent B-spline formulation, based on the standard B-spline recursion relation, is derived. B-spline representations of typical semiconductor and metal dielectric functions are also presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Article
The ductile-to-brittle transition of the claddings of PWR and VVER nuclear fuel rods has been investigated in ring compression tests performed at room temperature with Zircaloy-4 and E110 samples oxidised in high temperature steam. These experiments were evaluated on the basis of the form of load–displacement curves. The ductile samples were characterised by a horizontal ductile plateau after the elastic deformation, while in the case of the brittle samples the ductile plateau was missing. The ductility limits of both alloys were expressed in terms of oxidation time and cladding temperature. From this, a numerical procedure was derived for the simulation of Zr cladding embrittlement during loss-of-coolant accidents. The results indicated a faster embrittlement of the E110 alloy than that of the Zircaloy-4 under identical conditions. The new approach is proposed as an alternative of the 17% ECR criterion to evaluate cladding embrittlement in design basis accident (DBA) scenarios.
Article
In this paper spectroscopic ellipsometry is used to determine the dielectric function (refractive index) of three bulk materials: cubic zirconia (c-ZrO2), cubic magnesium oxide (c-MgO) and amorphous (vitreous) arsenic sulfide (a-As2S3) from 130 nm in the vacuum ultraviolet to 33 μ in the infrared. This work utilizes the very wide spectral coverage and sensitivity of modern spectroscopic ellipsometers to determine bulk optical properties of these materials over a wide spectral range. Ellipsometric psi and delta data at multiple angles of incidence were fit to extract the dielectric function of each material. Intensity transmission data were also acquired at normal incidence and fit simultaneously with the psi and delta data when possible. Including transmission data in the analysis greatly improves sensitivity to small absorption features. The ellipsometric delta data were very sensitive to surface quality. Therefore, it was very important to include surface roughness in all models to avoid non-physical absorption artifacts in the optical constants. The experimental data were fit in the transparent spectral range to determine the real part of the dielectric function and the surface roughness. Fixing the surface roughness then allows the optical constants to be determined by a direct fit for ε1 and ε2 at each measured psi-delta data point. Combinations of multiple Gaussian, Lorentz, and Tauc–Lorentz dispersion functions were then used to fit the experimental data. The different shapes of each function allow fitting a wide range of absorption features throughout the ultraviolet, visible and infrared spectral ranges. Combining multiple oscillator types provides a very flexible approach to fitting optical constants over a wide spectral range while simultaneously enforcing Kramers–Kronig consistency in the fitted optical constants.
Article
The optical constants of Zircaloy 2 and 4, which are alloys of zirconium and their thermal oxides, are determined by ellipsometry over a 0.3131-3.39-microm wavelength range. Both the refractive index and extinction coefficient of both types of Zircaloy are similar to those of zirconium. Both components of the complex index increase with wavelength from nominal values of 1.135-il.523 at 0.3131 microm to 4.846-i7.345 at 3.39 microm. The refractive indices of the oxide of Zircaloy 2 and 4 heated in air are similar to that of zirconium oxide and decrease in value as a function of wavelength from ~2.3 at 0.3131 microm to ~2.0 at 3.39 microm. The extinction coefficient is both thickness and temperature dependent and varies between ~0.1 and 0.3.
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