Preferential orientation of fluorine-doped SnO2 thin films: The effects of growth temperature

Polycrystalline fluorine-doped SnO₂ thin films with a given thickness of about 250 nm have been grown by ultrasonic spray pyrolysis with a growth temperature in the range of 360–480 °C. A texture transition from 〈1 0 1〉 to 〈1 0 0〉 and 〈3 0 1〉 crystallographic orientations has experimentally been found by X-ray diffraction measurements as growth temperature is raised, revealing that a process of abnormal grain growth has occurred. The texture effects have been investigated within a thermodynamic approach considering that grain growth is driven by the minimization of total free energy. The anisotropic character of the physical quantities and the effects of growth temperature have been shown both on the surface energy per unit volume through its dependence on the oxygen chemical potential and on the strain energy density through its variation with the elastic strain and biaxial modulus. Importantly, it is demonstrated by thermodynamic simulations that the oxygen chemical potential increases with growth temperature in the spray pyrolysis conditions, showing that the atmosphere is less and less reducing. For low growth temperature, it is revealed that the 〈1 0 1〉 preferred orientation is due to surface energy minimization since the (1 0 1) reduced surfaces have a surface energy lower than the (1 0 0) reduced surfaces. In contrast, as growth temperature is raised, the 〈1 0 0〉 crystallographic orientation becomes predominant owing to strain energy minimization. A texture map is finally determined, revealing the expected texture as a function of elastic strain and oxygen chemical potential.