Chemical composition and temperature dependent performance of ZnO-thin film transistors deposited by pulsed and continuous spray pyrolysis

Zinc oxide thin film transistors (TFTs) deposited by continuous and pulsed spray pyrolysis were investigated to analyze process kinetics which make reduction of process temperature possible. Thus, fluid mechanics, chemical composition, electrical performance, and deposition and annealing temperature were systematically analyzed. It was found that ZnO layers continuously deposited at 360 degrees C contained zinc oxynitrides, CO3, and hydro carbonate groups from pyrolysis of basic zinc acetate. Statistically, every second wurtzite ZnO unit cell contained an impurity atom. The purity and performance of the ZnO-TFTs increased systematically with increasing deposition temperature due to an improved oxidation processes. At 500 degrees C the zinc to oxygen ratio exceeded a high value of 0.96. Additionally, the ZnO film was not found to be in a stabilized state after deposition even at high temperatures. Introducing additional subsequent annealing steps stabilizes the film and allows the reduction of the overall thermal stress to the substrate. Further improvement of device characteristics was obtained by pulsed deposition which allowed a more effective transport of the by-products and oxygen. A significant reduction of the deposition temperature by 140 degrees C was achieved compared to the same performance as in continuous deposition mode. The trap density close to the Fermi energy could be reduced by a factor of two to 4 x 10(17) eV(-1) cm(-3) due to the optimized combustion process on the surface. The optimization of the deposition processes made the fabrication of TFTs with excellent performance possible. The mobility was high and exceeded 12 cm(2)/V s, the subthreshold slope was 0.3 V dec(-1), and an on-set close to the ideal value of 0 V was achieved. (C) 2013 AIP Publishing LLC.