Effect of thermal annealing on the morphology of sol–gel processed barium strontium titanate thin films: Consequences on electrical properties

Abstract

The link between the morphology and the electrical properties of the sol–gel processed Ba0.7Sr0.3TiO3 thin films is investigated. Previous studies have not fully explained the differences in growth morphology as a function of the elaboration conditions. The thin films were investigated by Grazing Incidence Small-Angle X-ray Scattering (GISAXS), x-ray diffraction, and scanning electron microscopy. More precisely, prototype films were studied as a function of the annealing temperature: at low temperatures (140 °C–200 °C) by in situ GISAXS and at high temperatures (600 °C–800 °C) by ex situ GISAXS. At ∼150 °C, self-organized domains with a preferential distance of approximately 14 nm are formed. At high annealing temperatures, the growing domains become either nanoparticles or pores with a preferential distance of approximately 85 nm at 600 °C. This growth evolution is successfully explained by a general model based on convection and evaporation. With thermal annealing, the characteristic lengths parallel to the surface increase due to convection and the characteristic lengths perpendicular to the surface decrease due to evaporation. In addition, two types of annealing were investigated at 700 °C. For annealing after each other layer, a growth with vertically shifted particles occurs with no ferroelectric behavior. On the contrary, for annealing after each deposited layer, a columnar growth occurs and a ferroelectric hysteresis loop is obtained. The ferroelectricity of the sol–gel barium strontium titanate thin films is definitely linked to the complete removal of organic constituents leading to columnar growth.