Multinary metal oxide semiconductors - A study of different material systems and their application in thin-film transistors

Metal oxide semiconductors are emerging class of semiconductors with tremendous potential to replace existing silicon-based semiconductors in a wide variety of next generation electronic applications. Solution processing of metal oxide semiconductor offers a low-cost fabrication of oxide thin film transistors for large area film coatings. Use of traditional metal oxide precursors such metal salts require the addition of several additives to optimize the formation of the desired multinary metal oxide semiconductors. Within this thesis, the use of well-defined, specifically tailored molecular precursor compounds with desired properties such air-stable complexes with solubility in desired aqueous or organic solvents, microwave synthesis of oxide nanoparticles, reduction of process temperature for the formation of metal oxides as well as direct photo-patterning of the multinary oxides are demonstrated. Firstly, indium zinc oxide nanoparticles were synthesized by a rapid microwave-assisted decomposition employing solutions of molecular air stable In and Zn Schiff-base type oximato precursors with methoxyiminopropionato ligands which led to the stable suspensions of IZO nanoparticles with a consistent particle size of ~ 5nm. The removal of adherent organic and hydroxy moieties by annealing at 450°C thus led to an excellent semiconducting behaviour of the finally resulting high quality IZO TFTs with a mobility of 8.7 cm2 /V.s, an Ion/off ratio of 2.8 × 105 and a threshold voltage Vth of +3.3 V. This strategy was also extended towards the synthesis of the a bio-inorganinc ZnO/TMV hybrid semiconductor, where a slightly modified precursor formulation with optimal amount of the base (TEAOH) provides mild basic conditions to ensure an efficient microwave based decomposition of the zinc complex and the in-situ formation of crystalline zinc oxide nanoparticles at a low temperature of 60°C . The ZnO nanoparticles mineralize selectively onto the TMV scaffold with and any structural damage to the TMV. For an optimum number (6 cycles) of ZnO mineralization, a field-effect mobility (µsat) of 6.7 × 10-4 cm2/V.s, Vth of +4.7 V and an Ion/off of 9.0 × 105 without the need for any post-process thermal annealing. Similarly, employemnt of a new Sn(II) oximato precursor in combination with the zinc oximato precursor was employed for the formation of ZTO thin fim semiconductors. XRD analysis shows that the decomposition the tin(II) precursor alone, at temperatures as low as 350°C. The EPR investigations revealed only surface defects and not bulk defect sites with a higher defect concentration observed for the SnO2 compared to the ZnO. Hence, the precursor chemical composition with higher tin content with a Sn:Zn ratio of 7:3 delivered the optimum performance of the of the ZTO TFTs with a µsat of 5.18 cm2/V.s, Vth of 7.5 V and a high Ion/off of 6 x 108 when annealed at a moderate temperature of 350oC. The oximato precursors were also explored for their direct DUV-based photo-patterning of IZO and ZTO semiconductors. This is enabled by accessing the intrinsic ability of the precursor thin film to undergo selective decomposition under UV irradiation and generate site-selective patterning of oxide semiconductors, thereby eliminating the need for the traditional photolithography process. At annealing temperatures of 350°C, high performance TFTs with a µsat of 7.8 cm2/V.s, Vth of 0.3 and a high Ion/off of 3.5 x 108 for IZO and 3.6 cm2/V.s, Vth of 2.4 and a high Ion/off of 5.3 x 107 for ZTO TFTs was achieved. In order to achieve low -temperature solution processing, tailored multimetallic zinc and indium coordination compounds, [Zn4O(dmm-NO)6] and In3O3(dmm-NO2)3·(toluene) with nitro- and nitroso-functionalized dimethylmalonato ligands were investigated for the combustion synthesis of semiconducting indium zinc oxide (IZO) thin films at low processing temperatures. Devices annealed only at 250°C show an active TFT performance and at 300°C already demonstrate a robust FET performance with a µsat of 2.1 cm2/V.s, Vth of +11.5 V and an Ion/off of 3.3 × 107 greater than that of the conventional amorphous hydrogenated silicon and also displays its potential to use integrated with plastic compatible temperatures ≤ 300oC, towards flexible electronics. Another approach based on water-soluble, well‐defined urea nitrate coordination compounds of indium(III), gallium(III) and zinc(II) for the formation of amorphous IGZO thin films was succefully demonstrated. DSC analysis confirms the exothermic decomposition of all three precursors arising from the urea-nitrate (fuel-oxidiser) combination. Interestingly, IGZO TFTs processed even at 200oC show active TFT perfortmance and TFTs annealed at 300°C and 350°C exhibit a good device performance with charge‐carrier mobilities μsat of 1.7 cm2/V⋅s and 3.1 cm2/V⋅s, respectively as well as current on‐off ratios of >107 in both cases. Such precursors are highly suitable for use in aqueous (non-toxic) solution processing of IGZO semiconductors. Lastly, an ALD based In2O3/ZnO heterostructure design delivering high performance TFTs was successfully demonstrated using trimethyl indium and diethyl zinc as molecular precursors. Generation of an optimised In2O3/ZnO heterostructure based on sequential deposition of the individual oxides, processed at a reasonably low temperature regime (250–300°C) deliver high performance TFTs by the likely formation of 2D electron gas transport at the heterostructure interface. Devices based on such a fabrication process demonstrated an average saturation field-effect mobility μsat of 6.5 cm2/V.s and a high current on/off ratio of 4.6 x 107 and a low subthreshold swing (SS) of 0.7 V/dec. respectively, at a reasonable processing temperature of 300°C with potential applications in the field of large-area oxide electronics