The Study Of Oxide Thin-film Transistior

Recently, oxide thin-film transistors （TFTs） have been acctracting greatanntention due to the application in active matrix organic light emitting diode（AMOLED）displays. Because of the low mobility and serious threshold voltage (Vth)shift, the traditional amorphous silicion (a-Si) is insufficient to drive the OLED whichdriven by current. To be the poly-silicion (p-Si), although it has high field-effectmobility, good electrical stability, suffering from the low uniformity caused by theexistence of grain boundaries, which are restricted to application in AMOLED,especially for the lager-size displays. On the other hand, oxide semiconductor exhibitreasonably high mobility, transparent in visible light, good uniformity and excellentstability, especially of the low-temperature deposition, which shows the potential torealize the low-cost flexible displays. The oxide TFTs combine the advantages of a-Siand p-Si TFTs, and might be an alternative approach to realize reliable and highresolution AMOLEDs.Firstly, we explore the performance of the TFTs influenced by the preparation ofthe active layer. Consider to the characteristics of the oxide semiconductor, oxygen inthe sputtering atmosphere and sputtering pressure were mainly studied. Combinationof passivation layer and annealing ambient, the influence of the annealing temperatureand ambient were studied also. Then, we found that as the increase of the sputteringpower of molybdenum (Mo), the Vthshift negative and the mobility decrease. Bymeans of X-ray photoelectron spectroscopy (XPS), we analyzed the diffusionphenomenon in the interface of Mo/IZO. These parts provide the guidance forfabrication high performance TFTs.Secondly, the TFT with back-channel-etch structure has great attention due to therelatively simple process and low-cost. But, differ to the conventional silicon-basedmaterials; the oxide semiconductors are very weak in most of the wet etchants. Basedon the results of the previous section in this paper, a stacked structure of Mo/Al/Mowas adopted as the source/drain electrodes and patterned by a wet-etch-method. TheMo and Al were etched by H2O2and KOH, respectively. Good etching profile withfew residues on the channel was obtained. The TFT exhibited a field effect mobility ofas high as11.3cm2V-1s-1, a sub-threshold swing of only0.24V/decade, and anon-to-off current ratio of larger than108. Furthermore, the threshold voltage shiftedonly0.4V and-0.2V for the device under positive (+10V) and negative (-10V) gate bias stress for2.5h, respectively. These results indicate that the method of wet etchingS/D electrodes is feasible for IZO-TFT and has the potential to reduce themanufacturing cost of oxide semiconductor-based TFT backplanes.In addition, aiming at the problems existing in the preparation of high qualitydielectric at low-temperature, we try to form the alumina (Al2O3) on flexible substrateby anode-oxidization. The Al2O3on polyethylene naphthalate (PEN) substrate wasfabricated at room temperature, and the key process factors were inverstigated indetail. The anolized Al2O3shown better side face coverage characteristics than theSiO2deposited by plasma enhanced chemical vapor deposition (PECVD), especiallyfor the thick gate electrode. Additionally, the anolized Al2O3exhibited excellentelectrical and mechanical characteristics. In all, it shown the comparable performanceto the Al2O3thin film formed by ALD. Meanwhile, we fabricated the TFTs with theanolidze Al2O3as gate insulator, which demonstrates the promising application of theTFTs in flexible AMOLED displays. Based on the previous research, we succefullyfabricated a5inch color flexible AMOLED display on PEN substrate, which exhibitsgood image quality in both flat and bent states. It is demostrated that the anodizedAl2O3with tremendous potential in low-temperature and flexible electronics.Finally, transparent conductor oxides (TCO) play an important role in AMOLEDdisplays. As we all known, as the increase of the thickness, the ITO thin-film tends tobe crystalline. Unfortunately, the crystallized ITO film will encounter a patterningproblem, because it can only be etched by strong acids, such as aqua regia, which aredestructive to the underlying films. In order to prepare the low-roughness andeasily-etched transparent electrodes, a layer of ultra-thin IZO was inserted betweenITO films. It was found that this as-deposited TCO film with IZO insertion layers wasamorphous and easily-etched even in weak acids. Furthermore, the surface roughnessof this multilayer TCO film was only0.52nm, much lower than that of the ITOmonolayer film with the same thickness. After annealing at250oC in air, a low sheetresistance of～9.6Ω/and a reasonably transmittance of～85%in visible range wereobtained. This method expanded the application of the TCOs significantly.