The In and Sn 3d3/2 and 3d5/2 ESCA peaks and the oxygen 1s peak of Sn-doped In2O3 films were compared with those for In2O3 films and In2O3, SnO, SnO2, and Sn3O4 powders. Comparison of as-grown with sanded surfaces revealed Sn-rich surface layers in.those films having good optical and transport properties. These experimental fins are interpreted with a schematic energy-band model and the assumption that film darkening in Sn-doped In2O3 films is caused by the formation and growth of an Sn3O4-like second phase in the bulk. Suppression of these phase could be accomplished by higher substrate temperatures, which permit equilibrium conditions to be attained. Sn-rich phases to migrate to the films surface, and the tine disproportionates to Sn2+ and Sn4+ ions.

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