Effects of helium annealing in low-temperature and solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors

In this paper, low-temperature, solution-processed amorphous indium-gallium-zinc-oxide thin-film transistors (a-IGZO TFTs) with annealing temperatures as low as 300°C were fabricated using different annealing gases of He, N2, and O2 and their electrical characteristics and long-term stability were analyzed. The field-effect mobility was improved by nearly two times for the a-IGZO TFTs annealed in helium ambient (He sample) in comparison with those annealed in oxygen and nitrogen ambient environments. The subthreshold slope and threshold voltage were also improved for the a-IGZO TFTs annealed in the helium ambient environment due to the low defect density of states in the sub-bandgap region. However, X-ray photoelectron spectroscopy measurements indicate that the electrical characteristics of the low-temperature solution-processed a-IGZO TFTs show severe channel-length dependencies due to the oxygen vacancy variations in the active region. In addition, long-term stability measurements up to seven days reveal that due to the inherently low electron density and high defect density of states in the active region, an increase in the carrier density in the active region induces a large negative shift of the threshold voltage without changing the field-effect mobility or subthreshold slope. It is understood that low-temperature solution-processed a-IGZO TFTs strongly depend on the formation of oxygen vacancies, which can be resolved by controlling the indium mole fraction under a helium annealing condition.