Metal-oxide thin-film transistors (TFT) fabricated by spray pyrolysis are of increasing interest because of its simple process and scalability. A bottleneck issue is to get a bubble-free and dense material. We studied the effect of ammonium acetate (AA) addition in the oxide precursor solution on the performance of spray-coated ZnO TFTs. AA acts as a stabilizer, which increases the solubility of the solution and enhances the film quality by reducing the defects. With AA addition in ZnO precursor, the films are coffee ring free with high mass density and better grain orientation. The ZnO TFT with AA exhibit a remarkable improvement of its device performance such as saturation mobility increasing from 5.12 to 41.53 cm²V⁻¹s⁻¹, the subthreshold swing decreasing from 340 to 162 mV/dec and on/off current ratio increasing from ~10⁵ to 10⁸. Additionally, the TFTs show excellent stability with a low threshold voltage shift of 0.1 V under gate bias stress. Therefore, the addition of AA is a promising approach to achieve high-performance ZnO TFTs for low-cost manufacturing of displays.

Zinc sulphate (ZnSO4.7H2O), an inorganic material has been crystallized by an isothermal evaporation method. ZnSO4.7H2O is highly soluble in water and the solubility is found to be increased almost linearly with the increase of temperature. At room temperature around 34°C, the solubility was found to be 92.41 gm/50 ml. The FT-IR spectroscopy was performed on pure zinc sulphate crystals to identify the presence of functional groups. The grown crystals have been subjected to powder X-ray diffraction to determine the unit cell dimensions and the crystal structure. The lattice parameters are found to be, a = 9.9810 Å, b = 7.2500 Å and c = 24.2800 Å, respectively. The values were in good agreement with those of the reported values. The TGA and DTA study revealed that the grown crystals have good thermal stability. The UV-vis spectrum showed that the material has wide optical transparency in the Ultra-violet region. Key words: Isothermal evaporation; Crystatization; Zinc sulphate; Structural characterization DOI: http://dx.doi.org/10.3329/jbas.v35i2.9426 JBAS 2011; 35(2): 203-210

We report a highly stable, nanocrystalline, zinc oxide (ZnO) thin-film transistor (TFT) fabricated by spray pyrolysis using purified ZrOₓ as a gate insulator. The crystalline ZnO layer shows an average grain size of 30 nm and a smooth surface with an average rms roughness value of 1.21 nm. The smoothness of the ZnO film is due to the alignment of all grains with the (002) c-axis. The TFTs exhibit a saturation mobility of 12.76 cm²/V· s, a subthreshold swing of 260 mV/dec, and an on/off current ratio of ~10⁸ with negligible hysteresis voltage. The ZnO TFT shows excellent stability under bias stress because of the high-quality ZnO film and its excellent interface with ZrOₓ. The conduction band tail width of ZnO was obtained from the TCAD fitting to the measurement data and was found to be 65 meV.