Sol–gel processing of TeO2 thin films from citric acid stabilised tellurium isopropoxide precursor

A sol–gel process for TeO2 thin layers synthesis was developed, including the tailoring of the tellurium alkoxide reactivity, their preparation by dip-coating and the effects of heat treatments on their structure and microstructure. High quality thin films were made by dipping silica glass substrates in solutions prepared in the tellurium isopropoxide, isopropanol, citric acid and water system. The structure and microstructure of the films were characterized by X-ray reflectometry, X-ray diffraction, optical and scanning electron microscopies as a function of the chemical parameters of the sol, withdrawal speed, temperature and time of annealing. They were found to be highly dependent on the initial thickness and applied thermal treatment. Indeed, TeO2 thin films could be either quite fully dense and amorphous or made of randomly orientated γ-TeO2 or/and α-TeO2 crystals. Firing at the highest tested temperature (∼450–500 °C) promotes grain growth and islanding, so producing a layer of textured but isolated grains.