The Effect of Cation/Anion Addition on the electrical performance of Zinc Tin Oxide thin film transistor by solution-processing.

Abstract

Thin film transistors (TFTs) with amorphous zinc tin oxide (ZTO) channel layer were fabricated by a simple and low-cost solution process, prepared by dissolving 0.2 M of zinc acetate dihydrate and tin chloride dihydrate in 20 mL of 2-methoxyethanol. All ZTO thin films showed amorphous phases and no impurities (no carbon and chlorine content) even at process temperature of 350 ℃. As the Sn ratio in ZTO films increased, the values of saturated mobility (u_(sat)) and subthreshold gate swing (S.S) exhibited a parabolic behavior in ZTO TFTs, depicting that the μsat and S.S values were a maximum (3.4 cm^(2)/Vs) and minimum (0.38 V/decade) at Zn/Sn=1 ratio. Interestingly, the x-ray absorption and X-ray photoemission spectroscopy revealed the origin of parabolic behavior, indicating not only to improve a charge transport in conduction bands but also to increase the Sn^(4+)/Sn^(2+) ratio at the peak values (Sn/(Zn+Sn)=0.5). However, the bias stability of ZTO TFT showed weak performance. So, This paper addressed the effect of gallium nitrate hydrate addition in order to improve electrical performance and positive bias stability of by solution processing. Further, the mechanisms responsible for chemical properties and electronic band structure are explored. A broad exothermic peak accompanied by weight loss appeared in the range from about 350 to 570 ℃ for the ZTO solution; the thermal reaction of the Ga-ZTO:N solution was completed at 520 ℃. This is because the gallium nitrate hydrate precursor promoted the decomposition and dehydroxylation reaction for Zn(CH_(3)COO)_(2)·2H_(2)O and/or SnCl_(2)·2H_(2)O precursors. The concentrations of carbon and chloride in gallium nitrate hydrate added ZTO films annealed at 400 °C have a lower value (C 0.65, Cl 0.65 at.%) compared with those of ZTO films (C 3.15, Cl 0.82 at.%). Absorption bands at 416, 1550, and 1350 cm^(−1) for Ga:N ZTO films indicated the presence of ZnGa2O4, N−H, and N=O groups by Fourier transform infrared spectroscopy measurement, respectively. As a result, an inverted staggered Ga-ZTO:N TFT exhibited a mobility of 4.84 cm^(2) /Vs in the saturation region, a subthreshold swing of 0.35 V/decade, and a threshold gate voltage (V_(TH)) of 0.04 V. In addition, the instability of V_(TH) values of the ZTO TFTs under positive bias stress conditions was suppressed by adding Ga and N from 13.6 to 3.17 V, which caused a reduction in the oxygen-related defects located near the conduction band.