Ellipsometry refractive features

Infrared ellipsometry is typically performed in the mid-infrared range of 400 to 5000 cm , but also in the near- and far-infrared. The resonances of molecular vibrations or phonons in the solid state generate typical features in the tanT and A spectra in the form of relative minima or maxima and dispersion-like structures. For the isotropic bulk calculation of optical constants - refractive index n and extinction coefficient k - is straightforward. For all other applications (thin films and anisotropic materials) iteration procedures are used. In ellipsometry only angles are measured. The results are also absolute values, obtained without the use of a standard. 

For an optically isotropic Langmuir monolayer, the phase retardation depends only on the thickness of the film. There is an additional contribution, however, if the refractive index of the monolayer is anisotropic. This feature of ellipsometry has recently been exploited by Bercegol et al. to study the molecular alignment in monolayers of octadecanoic acid. 

An obvious attractive feature of ellipsometry is that two parameters are obtained in a single measurement. If there is only one perfectly homogeneous surface with a complex refractive index, the real and complex part of the refractive index can be calculated from the two ellipsometric angles. The situation becomes more complicated if the assumed system becomes more complicated every thin film has a thickness and a complex refractive index, which means three additional parameters. 

Light is an electromagnetic wave and all its features relevant for ellipsometry can be described within the framework of Maxwell s theory [1]. The relevant material properties are described by the complex dielectric function e or alternatively by the corresponding refractive index n. 

Although the same features in (A) curves are found for Si02 films synthesized by sol-gel and by the top-down thermal oxide (TO) methods (provided the thermal treatment is conducted at the same temperature), the absolute values of the refractive indices differ [7], for example, (633 nm) = 1.48 and sol-gel (533 nm) = 1.44. This difference determined by ellipsometry is a consequence of the higher (micro)porosity present in the sol-gel thin films. A similar behavior is found for Ti02 refractive index for which the Lorentz term is considered. Again, the functional shape is maintained between both synthesis methods, while the (A) values for the film synthesized by sol-gel are noticeably lower than the thermal oxide top-down way. 

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