Polymorphic solid phase epitaxy of amorphous SnO2 thin films deposited on sapphire(0001) substrates

Abstract

The polymorphic solid phase epitaxy of amorphous tin oxide (SnO₂) thin films deposited on sapphire(0001) substrates through radio-frequency powder sputtering was investigated. A multimodal X-ray probe station was utilized to simultaneously investigate the crystallization and electronic conductivity changes during the in situ annealing of amorphous SnO₂ thin films under vacuum conditions. Crystallization was initiated at 200 °C, and most of the amorphous phase was converted to the crystalline phase upon annealing to 500 °C. This trend was consistent with the temperature-dependent electrical conductance variation, demonstrating metal-like behavior. Off-specular X-ray diffraction results confirmed the polymorphism of the SnO₂ thin films, i.e., the coexistence of orthorhombic columbite (C–SnO₂) and tetragonal rutile (R–SnO₂) phases and their epitaxial relationship with the sapphire(0001) substrate. In the 35-nm-thick sample, both C–SnO₂ and R–SnO₂ phases were formed epitaxially, while an additional polycrystalline R–SnO₂ phase was observed with increasing film thickness. This indicated that the formation of the metastable C–SnO₂ phase is determined by the strain field induced by the sapphire(0001) substrate. The result also revealed that the volume effect in the absence of interfacial strain favors the formation of a polycrystalline R–SnO₂ phase in the bulk region of amorphous thin films.