Reflectance anisotropy spectroscopy applied to organic thin films: The role of the substrate

When light is reflected by a solid sample, the collected signal includes the optical response of all the matter within the light penetration depth. Studying films thinner than this length, the substrate thus contributes to the reflected beam, and affects optical spectroscopy measurements. This is particularly true when polarized light is used, as in reflectance anisotropy spectroscopy (RAS), where substrate symmetry properties play a crucial part. A detailed comprehension of the substrate role in the optical response of a sample has important consequences for correctly extracting the optical contribution of the layer grown on it. This problem becomes delicate in organics, where the choice of the substrate is often fundamental for obtaining an overlayer with a desired structure. In particular, the use of organic single crystal substrates, often structurally anisotropic, is a powerful tool for obtaining highly ordered crystalline films by organic epitaxy. To achieve the film optical properties from an experimental RAS spectrum, an interpretative key is necessary: but the most used model describing the interaction of light with the layered system (the so called three-layer model) fails when the substrate is anisotropic. This care in the application of RAS to organics has not been discussed in details yet, in particular to understand the contribution of the substrate to the total anisotropy signal. Within a phenomenological approach to RAS, we have here weighed the substrate contribution, by considering five different and carefully chosen samples, i.e. a crystalline overlayer of the same material and thickness but on five different substrates. The data show that the substrate clearly influences the readability of the spectra by influencing their lineshape, in some complex cases limiting the possibility of extracting the film properties. Consequently, the correct choice of the substrate - that is a fundamental task for obtaining specific properties of the growing layer - is also a delicate element for a correct interpretation of RAS results.