Crystal structure development of vanadium oxide thin films deposited by a magnetron sputtering technique

Abstract

Crystal structure of vanadium oxide films with different thickness, deposited on glass and F:SnO2 coated (FTO) substrates, was studied. It was found that all the films annealed at 450 °C developed a metastable β-V2O5 monoclinic crystal structure, represented by crystals of up to 100 nm. At higher temperatures, the original crystals of relatively thick films of 400 nm on the glass substrate first increased in size and improved their crystal structure symmetry from the monoclinic to tetragonal, then gradually intergrew into spectacular superstructures of crystal “leaves” (500 °C) and “needles” (550 °C). Thin films of 130 nm exhibited a similar crystallization processes but at a higher rate. It was found that the FTO substrate supported the slowest crystal growth dynamics with annealing temperature. The original crystals did not grow in size and retained their low β monoclinic symmetry in the whole temperature range up to 550 °C. The superstructure appeared around film imperfections in the form of plate macroscopic crystals with improved tetragonal symmetry, and it covered only one third of the film surface. The metastable β phase growth and its development were discussed and explained in terms of the combined effect of high crystal/substrate surface tension and crystal surface energy.