Temperature dependencies of conductivity of ITO films were investigated. Thin ITO films (100 nm thickness) were synthesized by reactive magnetron sputtering method from 80%In - 20%Sn target onto glass substrates. Resistance of the films was about 20 Ohm/cm² at the normal conditions. We investigated quasistatic temperature dependencies of conductivity with heating rate of 1K/min. Some samples obtained in special sputter regimes had anomalous temperature dependence of conductivity. An irreversible decrease of conductivity in the heating-cooling cycle is shown in Fig.1.

Fig. 1. Quasistatic temperature dependence of conductivity of ITO film in the heating-cooling cycle.

Microstructure of the samples was studied by STM method. Films with anomalous properties had different from others nanoporous microstructure with pore size 2-3 nm. The square of specific surface calculated from STM image was about 700 m²/cm³. Because of high specific surface square we supposed that the adsorption from the gas phase influenced on the films conductivity significantly and electron-donor H₂O molecules had the main influence. Under heating H2O molecules desorb and conductivity decreases respectively. The irreversible decrease of conductivity could be related with adsorption of O₂ molecules instead of H₂O molecules. The films were exposed under UV-irradiation. After UV-irradiation their resistance increased up to 800 Ohm. We supposed that this behavior was related with the formation of electron-acceptor peroxide substances into pore volume.
All these effects have never been observed previously.