Charge transport at high temperatures in solution-processed zinc-tin-oxide thin-film transistors

We report charge transport studies at temperatures in the range of 303–402 K for solutiondeposited amorphous zinc-tin-oxide (a-ZTO) thin-film transistors (TFTs) operating in the subthreshold region. The developed TFTs, which had a non-patterned bottom gate and top contact structure, employed a heavily-doped Si wafer and a SiO2 as a gate electrode and a gate insulator layer, respectively. In a-ZTO, the trap activation energy (ETAC) was estimated using the Maxwell-Boltzmann approximation. The decreasing ETAC with increasing gate-voltage-induced sheet carrier density (ns) in the a-ZTO channel can be understood as being due to a shift of the Fermi level (EF) toward the conduction band edge (EC) with increasing gate voltage. Samples with low ns, which exhibited thermally-activated behavior, revealed multiple trap and release phenomena. In samples with high ns, on the other hand, we observed decreasing mobility/conductivity with increasing temperature at temperatures higher than 348 K. This suggests that the ETAC can drop to zero, implying a shift of EF beyond EC, where the crossover from the thermal activation to band transport is observed. The temperature-dependent characteristics also revealed that the density of subgap trap states at EF exhibited thermally-activated behavior with an activation energy of 0.7 eV, suggesting that subgap trap states existed near 0.7 eV below the EC.