Solution Processed Multiple Oxide High-K Thin Film Transistors

Transparent oxide semiconductors (TOSs) are emerging as promising materials for active layers ofthin-film transistors (TFTs) applied in active-matrix liquid crystal display (AMLCD) and active-matrixorganic light-emitting diode display (AMOLED) backplanes. As a representative TOS, oxidesemiconductor has attracted much attention due to advantages of a high field effect mobility, excellentuniformity, and the compatibility with transparent and flexible substrate, as compared to conventionalamorphous and polycrystalline silicon. Compared with the traditional vacuum method, the solutionprocessed technology is a promising fabrication method with low energy and simple equipment, also thecompositions can be accurately controlled.In this study, Indium Zinc oxide (IZO) films were fabricated by spin coated as the active layer.Indium Tin oxide (ITO) films were fabricated as the gate and source drain electrode relatively. SiO2wasfabricated by plasma enhance chemical vapor deposition as the gate insulator. The bottom gate and bottomcontact TFT was fabricated by lithography. In order to get the best ratio of Indium and Zinc, the influenceof Indium Zinc concentration on the electrical properties of IZO TFTs were investigated. Then, the dope ofHafnium on the devices performance were investigated based on the optimal ratio of Indium Zinc. Theexperimental results showed that the increased addition of Hf to the IZO system resulted in suppression ofcarrier generation. HIZO TFT showed lower off currents and higher on-off current ratio. Finally, the high-kZrO2dielectric was also fabricated by solution process. The TFT with the solution processed HIZO activelayer and High-k ZrO2dielectric was achieved successfully.The works of this paper are as follows:1. IZO films with different In concentration were fabricated by solution process. The theory of thefilm’s formation, optical and electrical performance were investigated in this study. All the IZO film wasamorphous, surface was uniform and smooth, grain about20nm, and the visible average opticaltransmittance was more than85%. Result of the electrical characterization of the film revealed that it canbe used as an active layer material in thin film transistors.2. IZO TFTs were successfully fabricated by solution process. The effects of In concentration on theproperties of the device were investigated in the study. The best IZO TFT device properties were obtainedwhen the n concentration is60%: with the saturation mobility of0.24cm2V-1s-1, Ion/Ioffof105, thresholdvoltage of1.3V. The result showed that with the In increase, the saturation mobility of the IZO TFTsincreased. But, when the In concentration is higher than75%, the IZO TFT Ion/Ioffbecome worse. Usingthe In:Zn ratio of3:2, different Hf concentration HIZO TFTs were successfully fabricated at a lowertemperature (300oC). When the Hf concentration is5%, the best device properties were obtain with thesaturation mobility of0.27cm2V-1s-1, Ion/Ioffof106, the threshold voltage is3.7V. In summary, the best Inconcentration for the IZO TFT is60%, and the best Hf concentration is5at.%/Zn.3. The solution-processed zirconium oxide (ZrO2) was fabricated as the gate dielectric ofsolution-processed hafnium-indium-zinc oxide (HIZO) thin flm transistors (TFTs). A good interfacecontact was watched among all the different layers from the SEM. The solution-processed TFTs exhibitedcompetitive device characteristics, including a mobility of9.25cm2V-1s-1, a threshold voltage of0.8V, anda subthreshold gate swing of0.7V/dec, an on-to-off current ratio of over than104. These resultsdemonstrate the potential use of the HIZO/ZrO2thin flm as a promising system materials for the fabrication of all solution processed TFTs.