Preparation And Properties Of Oxide Films And Nanofibers Field Effect Transistors

Next generation display requires not only thin film transistors (TFTs) with high performance but also requires them to be transparent and compatible with flexible substrates. Due to the advantages of relative higher mobility and transparency than their amorphous silicon and organic counterparts, oxide semiconductor TFTs attract considerable interest. However, most oxide TFTs are fabricated by vacuum deposition techniques such as r-f magnetron sputtering and pulsed laser deposition, which are costly and complex. Solution process, however, can reduce the manufacturing cost to a great extent if combined with new-born roll-to-roll technique and printing techniques. Recently, solution processed oxide TFTs have got extensive investigation and made a great progress. But most of the reported processes need high temperature annealing to generate functional devices. Therefore, in order to fabricate solution-processed oxide TFTs on flexible substrates, alternative methods with much lower temperature are highly desired. This dissertation aims to investigate the zinc oxide (ZnO) thin film/nanofiber’s fabrication by low temperature solution process and the flexible ZnO-TFTs’ integration.Firstly, we fabricated ZnO thin film by spin-coating an aqueous ammine-hydroxo zinc solution, which can transfer to ZnO at temperatures no higher than100℃. Polycrystalline ZnO can be achieved by spin coating several times, while amorphous ZnO can be achieved by a single spin coating process and subsequent annealing. As-produced one layer ZnO film have a thickness of～7nm and transparency above90%. Based on this, bottom gate TFT has been achieved using silicon dioxide as dielectric layer. Processed at maximum temperature of200℃, the TFT shows a mobility μFE of0.428cm2/V·s, an current ON/OFF ratio of104and subthreshold voltage swing of2.62V/dec.The stability of TFTs is critical for practical use. Unstability of devices could cause threshold voltage shift and driven current decease, thus the pixel will need more complex circuits to achieve reliable operation. Several studies have demonstrated that the surface/environment interactions can impact the bottom-gate TFTs’ stability significantly. Here, we demonstrated that photoresist can help enhance the device stability as a native passivation layer. We also investigated the influence of the annealing gas as well as the temperature on the device properties and stability. Besides, to achieve low power consumption in practical use and reduce the operation voltage of oxide TFTs, gate dielectric layer with large capacitance while good insulation properties are needed. In this work, we investigated aluminum trioxide (Al2O3) obtained by atomic layer deposition (ALD). The Al2O3fabricated at200℃shows high permittivity value of8.4and good insulating properties. The devices using Al2O3as the dielectric layer achieve lower threshold voltage and much higher current ON/OFF ratio.TFTs on flexible substrates offer many advantage such as low cost, light weight and mechanical flexibility and will find wide applications. In this study, we fabricated ZnO-TFTs in bottom-gate and bottom-contact configurations on flexible polyimide (PI) substrate, using Al2O3as the gate dielectric layer and5nmTi/50nmAu as the source and drain electrodes. In the experiment, we used two different annealing technique and investigated the as-prepared devices’ properties. The devices annealing by Rapid Thermal Process (RTP) shows reasonable conductivity modulation and current saturation characterization. While microwave annealing leads to devices with higher performance. Transistors prepared at480W by microwave irradiation have shown enhanced characteristics of3.4×10-4cm2/V·s saturation mobility and a4.8×103current ON/OFF ratio, with VT of6.92V, and S of1.82V/dec.Oxide based nanostructures exhibit unique piezoelectric and gas sensitive properties and will find a wide range of applications such as nanogenerator, gas sensor and piezotronic transistor, etc. Combining the solution process and electrospinning technique, continuous ZnO nanofibers with diameter of100-200nm were achieved with excellent smooth morphology. Field effect transistor using single ZnO nanofiber was fabricated. Pb(Zro.3Tio.7)03(PZT) nanofibers with diameters down to100nm were also realized, with excellent piezoelectric properties demonstrated by piezoelectric force microscopy.