| Recently, oxide-based thin-film transistors (TFTs) have attracted a lot of attention. The channels of oxide-based TFTs have not only a good transparency, but also a high carrier mobility, even up to10cm2/Vs for oxide-based thin films deposited at room temperature. However, in most of the reports, the operating voltages of oxide TFTs is rather high(>10V), leading to a larger power consumption. Thus, an active research topic is to reduce the device operating voltages, improve the electrostatic modulation by the gate electrical field, and design the new device structures and study the relavent device physics, etc. In this PhD dissertation, my work can be summarized as follows:(1) Electric-double-layer(EDL) effect has been observed in SiO2nanogranular films deposited by plasma-enhance-chemical-vapor-deposition(PECVD) method. The C-f test indicates that such dielectric shows a large capacitance(>1.0μF/cm2) even for a dielectric thickness of4μm. Based on these results, a mobile-proton-induced EDL model has been proposed, and further experiments have been performed for verifying our suggested model. The findings of SiO2nanogranular film and corresponding EDL theory play an important role in our next work for novel TFT investigation.(2) Using SiO2nanogranular film as dielectric for EDL electrostatic gating, transparent full-room-temperature TFTs have been fabricated based on InGaZnO4channels. The devices show an ultra-low operating voltage of only1.0V as well as a good device performance with a large current on-off ratio of1.1×106, a small subthreshold swing of110mV/dec, and a high mobility of28.5cm2/Vs.(3) The EDL capacitance of SiO2nanogranular film has been greatly enhanced using solution treating such LiCl, H3PO4. Base on these dielectrics, vertical indium-tin-oxide(ITO) channel TFTs have been fabricated. The main advance of such vertical TFTs is eaily acheiveable short channels which are simply defined by the deposited ITO thickness. At the same time, we find that the operating voltage of vertical TFTs can be further decreased using such solution-treated composite electrolyte dielectric. The vertical TFTs by using30%LiCl solution treating exhibit a good performance with a subthreshold swing of72mV/dec, a current on-off ratio of1.4×106, and a low operating voltage of0.8V. While the vertical TFTs treated with10%H3PO4solution also show a good performance with a subthreshold swing of78mV/dec, a current on-off ratio of2×106, and a low operating voltage of0.8V. (4) We find that, the channel can be defined in a self-assembly manner using shadow-mask diffraction during radio frequency magnetron sputtering of an ITO target. Based on such a process, a flexible transparent self-assembled channel TFT has been fabricated at a conducting plastic substrate. The device exhibits a good performance with a subthreshold swing of65mV/dec, a current on-off ratio of107, and a low operating voltage of1.0V. The self-assembled channel process can greatly simplify the traditional device fabrication processes, and attract much research attention to the low-cost electronics applications.(5) Based on the self-assembly process, we have successfully demonstrated a single in-plane gate and dual-in-plane gate TFT. All of the gate, channel and source/drain can be performed using a shadow mask during the radio frequency magnetron sputtering of ITO. The transparent in-plane-gate self-assembled channel TFT shows a nice device performance with a subthreshold swing of125mV/dec, a current on-off ratio of2×106. While for the dual-in-plane gate TFT, the threshold voltage can be tuned from the-0.55V to0.76V with the second in-plane-gate voltage changing from3.0V to-2.0V. According to the results, a two-serial-capacitor coupling model has been proposed to explain the electrostatic gating. Both of the experiments and analys indicate that the conducting layer between the gate dielectric and substrate plays an important role in the TFT operation. It is the conducting layer that connects the two EDL capacitors of the gate and channel. In addition, an electrostatic coupling model based on three capacitors has also been proposed to explain the threshold voltage modulation with changing the second in-plane-gate bias.(6) By simply decreasing the ITO thickness during the radio frequency magnetron sputtering, a type of junctionless ITO-channel TFTs has been successfully fabricated. The channel, source and drain can be fabricated with homogeneous doping type and uniform carrier concentration. No junctions between the channel and source/drain are required. We have successfully demonstrated the junctionless flexible paper TFTs, and juncitonless in-plane-gate transparent TFTs. Our results indicate that the devices show a good TFT performance if the ITO thickness is decreased to about20nm. The junctionless ITO TFT will provide a new opportunity for the design of low-cost electronics device. |
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