Investigating the Effect of Various Acid-Catalysts and Erbium Doping Concentrations on the Properties of Sol–Gel Silica Films: A VASE Study

In this study, we report the deposition of thin layers of erbium-doped silica onto silicon substrates using the sol–gel method. Deposition was performed by spin coating, followed by drying and heat treatment in the range of 500°C up to 900°C. Different acid catalysts with various erbium doping concentrations were investigated to assess their impact on the film properties including thickness and refractive index. Under the same experimental conditions, the acid catalyst used significantly influenced the thickness and shrinkage of the films, irrespective of the erbium doping concentration. The key factors for this behavior are the viscosity of the sols and the gelation time, which vary tremendously depending on the acid used and the doping concentration. Strong acids were found to produce films with the highest resistance to shrinkage during annealing compared to weak acids. From the elliposometric investigation, the interplay among the doping concentration, pore formation, and annealing was found to affect the refractive index of the synthesized film.