Structural anisotropy in amorphous SnO₂ film probed by X-ray absorption spectroscopy

Polarization-dependent X-ray absorption measurements reveal the existence of structural anisotropy in amorphous (a-) SnO₂ film. The anisotropy is readily seen for the second neighbor interaction whose magnitude differs along three measured directions. The differences can be well accounted for by 10%-20% variation in the Debye-Waller factor. Instead of a single Gaussian distribution found in crystalline SnO₂, the Sn-O bond distribution is bimodal in a-SnO₂ whose separation shows a weak angular dependence. The oxygen vacancies, existing in the a-SnO₂ film in the order of 10²¹ cm^-3, distribute preferentially along the film surface direction.