DC CONDUCTION MECHANISMS IN Al–SnO2–Al STRUCTURE DEPOSITED BY THERMAL EVAPORATION

The DC conduction mechanisms in metal–insulator–metal sandwich structure based on amorphous thin films of SnO2 have been studied in the thickness range 100–400 nm, in the substrate temperature range 293–543 K, and in the annealing temperature range 473–773 K, and the results are discussed in terms of current theory. It is observed that at low field and low temperature the conduction mechanism is found to obey the hopping model, at higher temperature the conduction takes place by transport in the extended states but at high field the main barrier lowering effect is associated with localized centers. The increase in electrical conductivity with film thickness is caused by the oxygen vacancies and ${\rm SnO}_{2-x}({\rm V}_{\rm \ddot{O}})_{x}\, {\rm e}_{2x}^\prime$ defects which generate carriers in the films. The increase in electrical conductivity due to an increase in substrate temperature is ascribed to the increasing concentration of ionized donors and hoping of electrons between metal ions in two different valence states. The formation of tin species of lower valence states and doubly ionized oxygen vacancies are thought to be responsible for the increase in electrical conductivity at higher annealing temperature.