Preparation And Properties Of Solution-processed Crystalline Metal Oxide Thin Films And Transistors

Solution-processed metal oxide thin films and transisotrs are promising for active-matrix flat panel displays due to their advantages of simple processing,easy control of chemical compositions,low cost,and good compatibility with mass production.The past few years have seen rapid progress in device peroformance of solution-processed metal oxide thin-film transistors(TFT).However,most of the high-quality oxide semiconductors contain element indium,which is expensive and rare in the earth crust.Moreover,many previously reported high-mobility TFTs based on oxide semiconductors worked in depletion mode,which rendered them unsuitable for active-matrix driving.In this study,we focused on the preparation and properties of solution-processed indium-free crystalline oxide semiconducting thin films and TFTs.The conversion process from precursor solutions to oxides,the microstructures of metal oxide thin films,the surface and interface properties of dielectric/semiconductor structures were systematically investigated.High-performance TFTs based on zinc tin oxide and tin oxide were achieved by novel precursor solutions and optimized processing conditions.The main results and conclusions are as follows:1.The influence of water addition in precursor solution on the structure of zinc tin oxid thin films as well as TFT device performance was reported for the first time.Zinc acetate dihydrate and tin(II)chloride were chosen as precursor materials to fabricate zinc tin oxide thin films.It was found that step heating treatment induces crystallization process,while adequate water added to precursor solution further facilitates alloying and densification process of zinc tin oxide thin films,charaterizated by the enhancement of spinel Zn2SnO4 phase and the growth of crystallites.Top-gate bottom-contact TFTs incorporating atomic-layer-deposited Al2O3 dielectric were fabricated.With the addition of 1.67 M H2 O,the extracted charge carrier mobility of zinc tin oxide TFT devices increased by 130%,from 0.92 to 2.11 cm2 V-1 s-1.The doping effect of yttrium or lanthanum was also investigated.With 5% yttrium or lanthanum doped into precursor solution,the threshold voltage shift of the corresponding TFT device under positive bias stress condition after 1 h decreased from 1.05 V to 0.33 V and 0.17 V,respectively.2.High-performance zinc tin oxide TFT devices were achieved by ultilizing solution-processed Al2O3 dielectric.Because of the synergistic effect of water addition in zinc tin oxide precursor solution and step heating treatment during annealing,smooth and densely packed polycrystalline zinc tin oxide thin films with well-ordered carrier pahtways were fabricated on the surface of Al2O3 dielectric.With Zn/Sn molar ratio of1:1 in the precursor solution,bottom-gate top-contact TFT devices exhibited an average charge carrier mobility of 52.5 cm2 V-1 s-1,a current on/off ratio of 2.1 × 105 and a threshold voltage of 2.3 V.3.The adoption of tin(II)2-ethylhexanoate as precursor material to deposit semiconducting layer aiming at tin oxide TFT was reported for the first time.High-performance SnO2 TFT devices were achieved.Dense-packed polycrystalline tin oxide thin films with only a few oxygen defects were obtained via systematically optimizing processing temperature and precursor concentration.The employment of solution-processed Al2O3 dielectric further contributed to high-perpormance tin oxide TFT devices working in enhancement mode.The driving capability was demonstrated by implementing a single tin oxide TFT device to tune the brightness of an organic light-emitting diode.These SnO2 TFTs exhibited an average charge carrier mobility of96.4 cm2 V-1 s-1,a current on/off ratio of 2.2 × 106 and a threshold voltage of 1.7 V.The mobility is the highest value among solution-processed SnO2 TFTs reported so far.