Solution synthesis of thin films in the SnO2-In2O3 system:: A case study of the mixing of sol-gel and metal-organic solution processes

Thin films with nominal molar compositions ranging from 98% In2O3-2% SnO2 to 2% In2O3-98% SnO2 were prepared by mixing solutions of Sn(II) 2-ethylhexanoate in butanol with In2O3 sols prepared by a chemical complexation-based sol-gel process. The starting solutions were characterized by Fourier transform infrared spectroscopy, while the thin films, prepared by spin coating on silicon substrates and subsequent heat treatments up to 500 or 800 degrees C, were characterized by X-ray diffraction, field emission scanning electron microscopy, Auger spectroscopy, Fourier transform infrared spectroscopy, high-resolution transmission electron microscopy, and electron energy loss spectroscopy. It was concluded that the main factor governing the film formation is the lack of cross-linking reactions between the Sn and In species in the starting solutions. This may result in phase separation already during the spinning stage, in particular, for compositions with comparable concentrations of the two components. During the heat treatment, In2O3 crystallization is favored and is able to prevent that of SnO2 even when the concentration of the former is 10%. This result is explained by referring to the different chemistry involved in the solution processing of the Sn precursor with respect to the sol-gel processing of In2O3. Upon heat treatment up to 800 degrees C, a peculiar structure forms in the film with 30% In2O3, constituted by a mixture of nanocrystals of SnO2, In2O3, and a mixed phase.