Precursor-route ZnO films from a mixed casting solvent for high performance aqueous electrolyte-gated transistors

Abstract

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A New Precursor Route to Semiconducting Zinc Oxide

“…We find saturation for VD > VG in the output characteristics, and estimate a threshold voltage of Vth = 0.24 V from the (linear) transfer characteristic for W gate, similar as in previous reports [4][5][6]. Such low thresholds are typical for electrolyte-gated TFTs, resulting from the high capacitance of electrolytic gate media [14].…”

“…2b) is split into the Zn 2p3/2 and Zn 2p½ doublet at 1021 eV / 1045 eV; both O 1s and Zn 2p peaks are very similar to the XPS signatures of ZnO derived via the Zinc acetate precursor route, e.g. [4]. However, the Zn 2p XPS doublet is rather insensitive between metallic zinc and ZnO, this is better differentiated by the Zn LMM Auger peak (inset Fig.…”

“…1). Pyrolysis of ZnAc is well established to lead to Zinc oxide [3,4,6,7]. Both films show very similar spectra, and both are no longer soluble in water albeit both precursors (ZnCl2 and ZnAc) are.…”

“…We cannot strictly establish the crystal structure (wurtzite vs. zincblende), but the thermodynamically stable wurtzite structure is most likely. To test the semiconducting properties of spray pyrolysed ZnCl2 films, we sprayed aqueous ZnCl2 onto thin film transistor (TFT) substrates held at 450 o C. Transistor substrates consist of silica-coated glass substrates with evaporated Au / Cr adhesion layer (10 nm Cr / 120 nm Au) contact pairs of width W = 1 mm separated by a gap ('channel') of L = 30 m, W/L =33; we have used same substrates previously for water-gated TFTs [4,13]. We electrically addressed contact pairs on sprayed substrates as TFT 'Source' (S) and 'Drain' (D) and gated via a droplet of deionised water contacted with a tungsten (W) needle bent into L-shape to overlap the width of the channel, similar as in our previous work on water-gated ZnO TFTs [5].…”

“…Therefore, inorganic semiconductors that can be processed from solution-based precursors have recently attracted significant attention as an alternative. In 2007, Ong et al [3] have demonstrated that semiconducting films of the II-VI semiconductor Zinc oxide (ZnO) can be prepared by pyrolysis of the organic precursor, Zinc acetate (ZnAc), which dissolves in relatively benign solvents (alcohols, ketones, mixtures thereof [4]). Since water-gating of precursor-route ZnO was demonstrated [5], it has become a popular semiconductor for application in electrolyte-gated thin film transistors (TFTs) [6].…”