Electro-optical properties of thin indium tin oxide films: Limitations on performance

This work has been concerned with the measurement and analysis of the electro-optical properties of thin films of indium tin oxide deposited by ion beam sputtering. In the present context, the principal use of these films is for window layer solar cells, and it is necessary to have an accurate knowledge of the optical and electrical properties, and of their dependence on deposition conditions, to enable realistic predictions of the internal quantum efficiency to be made. We have measured the properties of films as a function of deposition rate, partial pressure of oxygen in the sputtering gas and annealing temperature. The properties studied are the variation with wavelength of the optical constants, the Hall mobility, the film structure and composition. The compositional data (from X-ray photoemission spectroscopy) suggest that heat treatment changes the state of oxidation of tin from SnO to SnO_2-x, the oxygen vacancies acting as doubly ionized donors. At high partial pressures of oxygen and low densities of free carriers, the mobility is limited by grain boundary scattering, the magnitude of which is determined by the deposition conditions. For ennealed films with higher carrier densities, the optical and electrical data suggest that ionized impurity scattering is the mechanism limiting the mobility. These two mechanisms thus appear to impose a lower limit on the resistivity achievable in this material.