Zinc tin oxide thin film transistors produced by a high rate reactive sputtering: Effect of tin composition and annealing temperatures

Amorphous zinc tin oxides (a-ZTO), which are stoichiometrically close to the Zn2SnO4 and ZnSnO3 phases, have been deposited using remote-plasma reactive sputtering, and incorporated as the channel layers in thin film transistors (TFTs). The influence of tin composition and annealing temperatures on the structural and phase evolutions of the thin films, and the electrical performances of the TFTs are investigated. Zn2SnO4 exhibited randomly oriented polycrystalline peaks at annealing temperatures 700 degrees C, while ZnSnO3 decomposed into Zn2SnO4 and SnO2 at 950 degrees C. TFTs employing a Zn2SnO4 channel, after a post-deposition annealing at 500 degrees C, exhibited a field effect mobility similar to 14cm(2)V(-1)s(-1) and a sub-threshold slope similar to 0.6Vdec(-1). When the tin content was increased in the channel, as in ZnSnO3, TFTs exhibited an increase in field effect mobility similar to 20cm(2)V(-1)s(-1), but with a slight deterioration of sub-threshold slope to similar to 0.8Vdec(-1). When the post-deposition annealing temperature was reduced to 300 degrees C, a mobility as high as similar to 10cm(2)V(-1)s(-1) was still achieved, however, a significant shoulder in the I-DS-V-GS curve, together with a higher off-state current was observed. TFT characteristics are explained by the sub-bandgap defect states measured by photothermal deflection spectroscopy and the extracted Urbach energies.