Study On Fabrication Process And Theorictiacal Modeling Of Amorphous Indium-Gallium-Zinc-Oxide Thin-Film Transistors

Recently, amorphous Indium-Gallium-Zinc-Oxide (a-IGZO) thin-filmtransistors (TFTs) have attracted much attention because of theiradvantages over traditional amorphous silicon (a-Si) TFTs including highelectron mobility, low process temperature, good large-area uniformity,good visible light transparency and so on. However, research onfabrication process and theoretical modeling of IGZO is lacking. In thiswork, we study the fabrication process and theoretical models of a-IGZOTFTs both experimentally and theoretically. A-IGZO thin films and TFTdevices are successfully fabricated with magnetron sputtering and otherprocess with which we investigate the process effects. Firstly, weinvestigate the influence of sputtering process conditions on the propertiesof a-IGZO thin films and TFT devices and special attention is paid on RFsputtering power and deposition temperature. We find that the carrierconcentration in a-IGZO films increased with rising RF power and theoptical band-gap are apparently influenced by RF power due toband-filling effect. Rise of RF power can also affect the performance ofa-IGZO TFTs especially increasing the field effect mobility which isthought to result from the variation of extended states. For substratetemperature during a-IGZO sputtering, with optimized annealing, we findthat IGZO deposition temperature strongly affects the TFT performance.From Hall measurement results, we find the resistivity much lower forintentional heated a-IGZO films due to the higher carrier concentrationwhich may account for the high off-current in the corresponding TFT devices. On the other hand, the variation of field effect mobility of TFTswith deposition temperature does not consist with the tendencies in Hallmobility and we infer that the trap sates at front channel interface may beresponsible for the variations of field effect mobility and subthresholdswing with IGZO deposition temperature. We also study the effect of othersputtering process parameters including mix gas flow rate and ambientpressure which are found to affect the deposition rate, resistivity andoptical properties of a-IGZO thin films. Then we try to integrate thesputtered SiOxpassivation layer into a-IGZO TFT process for betterambient stability. Novel two-state annealing consisting a high-temperatureannealing process and a low-temperature one which clearly enhances theair annealing efficiency, and double-layer active layer structure usingoxygen-rich IGZO as the back channel layer to weaken sputtering plasmadamage are applied in our device to neutralize the side effect during SiOxpassivation sputtering with which we successfully fabricate a-IGZO TFTdevices with good performance and ambient stability. Finally, to study theoff-current mechanism, we use TCAD tools to model the density of statesin a-IGZO. We successfully build an accurate and effective model, withwhich we study the influence of deep donor-like states on a-IGZO TFTsperformance and also the physical mechanism of off-current in a-IGZOTFTs with various source/drain electrodes is obtained.