Advanced IR polarimetry to study morphology and structure of ultrathin films

We demonstrate an infrared (IR) polarimetric approach for the detailed analysis of structural and morphological interface properties via the inspection of bands related to a pair of vibrations with perpendicular transition dipole moments. Performing polarization dependent IR spectroscopy and supplementary X-ray photoelectron spectroscopy (XPS) measurements, freshly prepared H/Si(111) surfaces as well as their long-term degradation in dry air environment and after exposure to humid air are investigated. Vibrational bands related to Si-H bending and Si-H stretching modes are discussed in conjunction with analytical optical simulations, elucidating the status of passivation and influence of varying domain sizes and oscillator densities. Notably, the vibration related to a strong out-of-plane directed transition dipole moment is extremely sensitive to the morphology and structure of the surface, whereas the in-plane directed band serves as a sensitive sensor for the overall status of the functionalization or passivation. Studies limited to only one of these bands may result in misleading conclusions on the passivation status, the pathway of degradation or structural properties. The analytic concept is also applied for the interpretation of vibrational bands of a methyl monolayer on Si(111). IR polarimetry is performed in a single reflection geometry which, compared to often used ATR geometries, is not restricted to specifically designed IR transparent substrates and also gives higher flexibility to potential adaptions to measurement chambers and cells. Our approach offers advanced possibilities for analysis in the field of interface science and contributes to a deeper understanding of interface properties. It is relevant for applications relying on specific interface properties, such as sensing, catalysis, photovoltaics, and electronics.