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Hafnium dioxide (HfO 2) thin films are generally used as high refractive index layers in optical coating devices. Aging effects on optical and mechanical properties of the films are very crucial for long-standing performance and stability of the devices, therefore knowledge of the aging effects is very important. In the present work, HfO 2 thin film has been prepared using electron beam evaporation. The optical properties and residual stress of the film have been investigated in six months interval over a period of one-year in ambient condition to probe its aging behaviour. The measured refractive index increases with aging and its change is Δ n = 0.011 at wavelength of 550 nm. The dispersion parameters such as ε L , N / m *, ε s , and E d increase with aging, while E 0 decreases. The measured film residual stress is tensile in nature. The residual stress decreases with aging and the change in stress in the film is around 78% in a year. A simple model has been utilized to establish and explain a correlation between the measured stress and the film porosity which varies with aging time. The results suggest that the change in the residual stress is significant in contrast to a very small change in the refractive index with aging. [ABSTRACT FROM AUTHOR]

Abstract: A comparative morphology, grain structure and optical properties studies of reactive electron beam co-evaporated mixed thin films of hafnia–silica (HfO2:SiO2) and zirconia–silica (ZrO2:SiO2) systems have been carried out using atomic force microscopy and phase modulated ellipsometry. The addition of silica, especially with small fractions, has demonstrated altogether different types of evolutions in the microstructure and morphology in these binary thin film systems possibly forming new solid solution phases. Such morphological evolutions are probed through RMS roughness, power spectral density, height–height correlation and autocorrelation analyses of the topographic data acquired through atomic force microscopy. The present investigations indicated that with composition-control morphological quality improvement is more favourable in composite hafnia–silica over the zirconia–silica films. So for ultraviolet optical coating applications which demand low light scattering thin film microstructure, hafnia–silica binary composite system has a definite edge over the zirconia–silica counterpart. [Copyright &y& Elsevier]

Abstract: Tailoring of the refractive index of optical thin films has been a very fascinating as well as challenging topic for developing new generation optical coatings. In the present work a novel Gd2O3/SiO2 composite system has been experimented and probed for its superior optical properties through phase modulated spectroscopic ellipsometry, spectrophotometry and atomic force microscopy. The optical parameters of the composite films have been evaluated using Tauc–Lorentz (TL) formulations. In order to derive the growth dependent refractive index profiles, each sample film has been modeled as an appropriate multilayer structure where each sub-layer was treated with the above TL parameterizations. All codeposited films demonstrated superiority with respect to the band gap and morphological measurements. At lower silica mixing compositions such as in 10–20% level, the composite films depicted superior spectral refractive index profile, band gap as well as the morphology. This aspect highlighted the fact that microstructural densifications in composite films can override the chemical compositions while deciding the refractive index and optical properties in such thin films. [Copyright &y& Elsevier]

Abstract: Several powerful effective medium formulations/approximation (EMA) and associated theories with different origins and concepts have been discussed and utilized here in order to model the experimental refractive index evolutions of ZrO2–SiO2 and Gd2O3–SiO2 composite films with respect to their compositional mixings. Amongst these formulations, the Böttcher''s generalized theory has been noticed to have more versatility and can simulate varieties of experimental observations incorporating a form factor parameter to account for the grain structure and morphology to a great extent. The refractive index modeling results of most of the available theories were compared with respect to their functional evolutions and limitations. It was noticed that at higher silica fractions (>20%) in our composite films, the effective experimental refractive index parameters have remained close to the most modeling results and Böttcher''s expression has shown to fit the observable parameters very accurately. However, under low silica compositions (<20%) the refractive index values of the composite films depicted different functional evolutions. Such deviations have been attributed to the various morphological, grain structure and band gap supremacies observed in these specific composite films which are not accounted by the effective medium formulations and approximations. These observations are well supported by the atomic force microscopy results. [Copyright &y& Elsevier]

Abstract: Composites are pragmatic choices for tailoring the material to have a desired property. Besides, such thin films have scopes to display superior optical, microstructural and morphological properties which are otherwise not possible to obtain from the pure component films. Vapor-phase-mixed binary composite Gd2O3/SiO2 thin film is one such interesting system where band gap as well as refractive index superiority is observed simultaneously under certain compositional mixings. Such and similar observations in composites cannot be explained by Moss empirical rule. Our systematic study on the microstructure of this composite system based on ellipsometry and scanning probe microscopy has satisfactorily provided the information that can explain such optical properties supremacy. Morphological measurements and its derived parameters like autocorrelation and height–height correlation functions have provided several clues that represent the superior grain structures of the composites. Besides, refractive index modeling through effective single oscillator model has strongly supported such analysis results favoring the superior microstructure in composite films. [Copyright &y& Elsevier]

Abstract: Composite optical thin-film materials have received a significant amount of interest in order to relieve the material constraints on refractive indices as well as reducing the number of layers required in optical coating design. Amongst others binary zirconia–silica composite thin films have attracted considerable attentions due to their several favorable opto-mechanical properties. In the present studies such a composite system under certain compositional mixings displayed both refractive index and band gap supremacy over pure zirconia films violating the most popular Moss rule. This unexpected evolution has several practical applications one of which can be directly employed in extending the range of tunability of the refractive index. Besides, the probing of such a novel evolution through the analysis of ellipsometric refractive index modeling and morphological correlation functions has revealed several novel as well as superior microstructural properties in the composite thin film systems. All these characterization and analysis techniques distinctly indicate a strong interrelation between the microstructural ordering and superior optical properties of the present zirconia–silica codeposited composites. [Copyright &y& Elsevier]

Abstract: Aging-related multilayer spectral instabilities can pose severe performance limiting constraints to optical multilayer devices. In this study such instabilities of some periodic Gd2O3/SiO2 optical multilayer systems have been explored using scanning probe force–distance microscopy and spectrophotometric techniques. In the present case, a strong correlation between the spectral instabilities and the viscoelastic properties of the associated thin film layers has been distinctly noticed. From the experimental analysis it was quite evident that the spectral instability, which starts during the nucleation and growth stage in thin films, continues to persist at a much longer time scale following aging processes. In this study it is shown that the elastic properties of the constituent thin films, the layer design and the bilayer thickness have established a strong interrelation which ultimately contributes to the multilayer instabilities. These spectral instabilities also have strong interconnections with the morphological and viscoelastic changes in such multilayers. Other multilayer parameters like the total number of layers, the layer structure, the microroughness evolutions, related stiffness factors and the adhesion properties of the periodic layer systems contribute substantially to this instability process. [Copyright &y& Elsevier]

Abstract: Depositing optical coatings at ambient substrate temperature has been stimulating interest as it has a range of potential applications. However, developing such optical coatings especially using refractory oxides is challenging since they yield stable thin films mostly at elevated substrate temperatures. Gadolinia films (Gd2O3), in the present experiment, however, observed to form stable films even at ambient substrate temperatures when deposited through reactive electron beam deposition. In addition, the films exhibited interesting optical properties such as reasonably high refractive index and wide band gap, which are rarely obtainable with conventional oxide materials. During the deposition, rate and oxygen pressure have been systematically varied. The deposited films have subsequently been characterized for optical and band gap related properties using spectrophotometric as well as ellipsometric spectral measurements. Atomic force microscopy has also been employed to study the surface topography and morphological changes under various deposition conditions. Both oxygen pressure and rate of evaporation have observed to greatly influence both the optical constant and surface topography of the gadolinia films. [Copyright &y& Elsevier]