Effect of annealing on the thermoelectricity of indium tin oxide thin film thermocouples

The effect of annealing on the thermoelectricity of indium tin oxide (ITO) thin film thermocouples (TFTCs) was studied. A microstructure characterisation indicate the influence of two different annealing mechanisms on the thermoelectric properties, namely, recrystallisation and carrier concentration. The ITO thin films recrystallised when the annealing temperature reached 1100^∘C in air, which improved the thermoelectric reproducibility, linearity, and stability of the TFTCs. More oxygen vacancies were produced in the ITO thin films after annealing at 400^∘C in vacuum, but were reduced after annealing at 1100^∘C in air. The Sn²⁺ ions transformed into Sn⁴⁺ ions in the ITO 90/10 thin films after annealing, providing additional free electrons in the films. These microstructural transformations are related to changes in the thermoelectric properties of the ITO TFTCs, as described by the thermoelectric theory of degenerated semiconductors. A decrease in the Seebeck coefficient after annealing was attributed to increases in the carrier concentrations and recrystallisation. Additionally, after annealing at 1100^∘C in air, the Seebeck coefficient of the ITO TFTCs agreed with the theoretical value. These results can be used to control thin film thermoelectric properties by different annealing processes.