  Electrical conductivity, Hall effect and magnetoresistance of In$_2$O$_3$:Sn
thin films deposited on a glass substrates at different temperatures and oxygen
pressures, as well as the films treated in a hydrogen plasma, have been
investigated in the temperature range 1.5-300 K. The observed temperature
dependences of resistivity were typical for metallic transport of electrons
except temperature dependence of resistivity of the In$_2$O$_3$:Sn film
deposited in the oxygen deficient atmosphere. The electron concentration and
mobility for the film deposited at 230$^\circ$C was larger than that for the
film deposited nominally at room temperature. Short (5 minutes) treatment of
the films in hydrogen plasma leads to the enhancement of electrical
conductivity while longer (30 minute) treatment has the opposite effect. The
electrical measurements were accompanied by AFM and SEM studies of structural
properties, as well as by XPS analysis. Basic on structural and electrical
measurements we conclude the reduction process initiated by the hydrogen plasma
provides essential modification of the ITO films surface. At the same time,
electrical properties of the remaining (located beneath the surface layer)
parts of the ITO films remain mostly unchangeable. XPS analysis shows that
grown in situ oxygen deficient ITO films have enhanced DOS between the Fermi
level and the valence band edge. The extra localized states behave as acceptors
leading to a compensation of $n$-type ITO. That can explain lower $n$-type
conductivity in this material crossing over to a Mott-type hopping.
