Adjustable structural, optical and dielectric characteristics in sol-gel PMMA-SiO₂ hybrid films

The structural, optical and dielectric characteristics of PMMA-TMSPM-SiO₂ hybrid thin films, synthesized by sol-gel, were studied as a function of the molar concentration of the coupling agent (TMSPM) in the hybrid precursor solution. It was possible to obtain very homogenous films, with improved thermal stability and variable organic and inorganic composition. The hybrid films showed high uniformity, excellent optical transparency and tunable refractive index from 1.447 to 1.490 at 633 nm depending on the PMMA fraction in the films. The formation of both organic and inorganic components and their interaction in the hybrid material were confirmed by the analysis of the ATR-FTIR spectra, suggesting changes in the film structure caused by the variations of the TMSPM concentration. The dielectric constant of the hybrid films, determined from capacitance-voltage (C-V) measurements performed in metal-insulator-metal (MIM) configuration, had unexpectedly high values between 5 and 14, increasing with decreasing TMSPM molar ratio. Such high dielectric constant values were attributed to highly polarizable OH and CC groups in the hybrid films, OH being part of the residual precursor solvents and CC the result of incomplete conversion of precursor monomer (MMA) into polymer (PMMA), as shown by FTIR measurements. Furthermore, the dielectric constant of the hybrid films was modeled according to an effective media approximation model, taking into account the constituting organic and inorganic phases, residual OH and CC groups.