Drop-cast deposition of sol-gel ZnO for transparent thin-film transistor application

Abstract

ZnO thin films have been studied extensively as a useful material for blue/UV light emitting diodes and lasers, photo detectors, chemical sensors, and solar cells since ZnO is a direct wide-bandgap (3.37eV) semiconductor with large exciton binding energy (60meV). Several techniques were employed to produce pure ZnO films, including pulsed laser deposition, sputtering, chemical vapor deposition, spray pyrolysis, and the sol-gel process. Among the many preparation techniques, the sol-gel route represents an easy low cost and efficient method for large surface depositon. We present here the fabrication and operation of a ZnO thin-film transistor(TFT) utilizing drop-cast deposited sol-gel processed ZnO. Drop-cast deposition technique, instead of spin-coating, was developed for the possibility of using sol-gel ZnO for inkjet deposition. We pre-fabricated source-drain electrodes on a silicon substrate with 50 nm thermal oxide. The ZnO sol-gel (0.005 l) was dropped in the channel formed between the source-drain contacts separated by 20 m. Low temperature annealing was performed for 1 min to remove the solvents and form a continuous film. Then the sample was annealed in a furnace at 500℃ for 1 hour. SEM investigation showed that the ZnO thin-film was granular with and average 50 nm grain size and the thickness was relatively uniform at around 100 nm. The transfer characteristics measured using the silicon substrate as the back gate, showed that the device conductance increased with ZnO grain size and thin-film thickness. We will report on sol-gel ZnO TFT characteristics with differing deposition and annealing conditions and discuss their effects on device performance.