Hydrothermal processing of barium strontium titanate sol-gel composite thin films

Abstract

Stoichiometric barium strontium titanate (BST) films of composition ${\rm Ba}_{0.7}{\rm Sr}_{0.3}{\rm TiO}_{3}$ with thickness >2 μm have been fabricated on Si/SiO₂/Pt substrates by hydrothermal sol-gel composite processing. This film deposition technique involves the treatment of a spun-on sol-gel composite film, formed from a suspension of a powder in an aqueous BST sol-gel, at temperatures from 100–200°C at a pressure of 1–15 atm. An initial hydrolysis procedure eliminates dissolution of the dried sol-gel during the hydrothermal treatment. Glancing angle X-ray diffraction shows excellent crystallinity and stoichiometry in the BST films with no evidence of new phases created during processing. Scanning electron micrography and atomic force microscopy show densification of the film structure and the development of a bridging microstructure. Transmission electron micrography indicates that while much of the sol-gel derived matrix phase is amorphous a more crystalline interface occurs with the powder particles. The relative permittivity and loss tangent of the films are measured using a parallel plate capacitor technique in the frequency range 1–100 kHz. At 100 kHz relative permittivities of the films range from ɛ _r = 400–1200 and loss tangents lie in the range 0.05 < tan δ < 0.10, depending on the parameters of film preparation. The film structure and morphology and the electrical studies suggest that the microstructure of the films evolves by deposition of the sol-gel derived BST on the underlying powder, resulting in an electrically interconnected microstructure in which the sol-gel derived material bridges between the high permittivity powder particles.