AC CONDUCTION IN MIXED OXIDES Al–In2O3–SnO2–Al STRUCTURE DEPOSITED BY CO-EVAPORATION

Conductivity-frequency and capacitance-frequency characteristics of mixed oxides Al–In2O3–SnO2–Al structure are examined to elicit any correlation with the conduction mechanisms most often observed in thin film work. The existence of Schottky barriers is believed to be due to a strong donor band in the insulator established during the vacuum evaporation when a layer of mixed oxides In2O3–SnO2 system is sandwiched between two metal electrodes. Low values of activation energy at low temperatures indicate that the transport of the carriers between localized states is mainly due to electronic hopping over the barrier separating the two nearest neighbor sites. The increase in the formation of ionized donors with increase in temperature during electrical measurements indicates that electronic part of the conductivity is higher than the ionic part. The initial increase in conductivity with increase in Sn content in In2O3 lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The decrease in electrical conductivity above the critical Sn content (10 mol% SnO2) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The increase in electrical conductivity with film thickness is caused by the increase in free carriers density, which is generated by oxygen vacancy acting as two electron donor. The increase in conductivity with substrate and annealing temperatures is due to either the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and formation of indium species of lower oxidation state (In2+). Calculations of C and σac from tan δ measurements suggest that there is some kind of space-charge polarization in the material, caused by the storage of carriers at the electrodes. Capacitance decreases not only with the rise of frequency but also with the lowering of temperature. At low temperatures the major contribution to capacitance arises from the ionic polarization, however, with the increase of temperature the contribution from orientation polarization would considerably increase. The decrease in capacitance with the increase in frequency may be attributed to interfacial polarization.