The Study Of Oxide Thin-film Transistor And Active Material

Thin film transistor(TFT) is the core components of liquid crystal display(LCD) and active matrix organic light-emitting diode(AMOLED). And TFT based on oxide semiconductors active layer is considered as the most suitable technology for AMOLED due to its advantages of relatively high mobility, low cost, relatively low process temperature, good uniformity, high transparent to visible light and compatible with the production line of a-Si TFT. As the flat panel display technique develops towards ultra-high definition, ultra-large scale and flexible display, it raises high demand for the performance and process temperature of metaloxide TFT(MO-TFT). Therefore, it needs to further improve the mobility(> 20 cm2V-1s-1) and stability of MO-TFT to meet the requirement of the ultra-high definition and ultra-large scale display. Besides, it needs to further decrease the process temperature to satisfy the flexible substrate.In this paper, a method of using solvent to modify the back channel of MO-TFT to improve the performance of device was proposed for the first time. Because the oxide is sensitive to the water and oxygen molecules in the atmosphere, the absorption/desorption effect of water and oxygen molecules on the back channel will degrade the performance of MO-TFT greatly. The surface energy of InGaZnO(IGZO) layer was reduced dramatically after using solvent to modify the back channel of TFT, which could suppress the absorption/desorption effect of water and oxygen molecules on the back channel significantly and reduce the impurity oxygen attached on the surface of the back channel. After using solvent to modify the back channel of TFT, the device shows higher mobility and better stability.Based on the above-mentioned solvent treatment, in order to further protect the oxide back channel from the effects of the atmosphere, an inorganic passivation is the commonly used method to improve the device stability. But the plasma damage or hydrogen doping was usually induced in the manufacture of the inorganic passivation, which will severely degrade the device performance. To solve this problem, based on the inspiration of the previous job, an IGZO-TFT with back channel modified by well-ordered self-assembled monolayers(SAMs) was successfully fabricated. The device exhibited excellent electrical properties with a high mobility of 26.6 cm2V-1s-1, a threshold voltage of 0 V, a hysteresis between forward and reverse sweep of 0.06 V, a threshold voltage shift of 0.14 V under the bias stress(VG =10 V,VD = 10.1 V) for 2.5 h. This method avoids the damage during the manufacture of passivation layer and greatly reduces the cost. Besides, we further study the effect of alkyl chain length of self-assemble molecules on the TFT device and explore the related mechanism, which provide a theoretical guidance for attaining high-performance MO-TFT by choosing the proper self-assemble molecules.Even though modifying oxide back channel with self-assemble monolayers can greatly improve the performance of the device as mentioned above, the treated-oxide semiconductor is still very sensitive to acid and easily etched by the acid solution during etching the source and drain electrodes. Therefore, it still needs to add an etch-stop layer(ESL) to protect the oxide from being etched. In order to realize a back-channel etching type TFT(BCE TFT) without ESL, Au nanoparticles are chose to modify the surface of oxide layer. After introducing Au nanoparticles on the surface of oxide layer, the film shows high etch resistance ability, which is helpful to reduce the damage during the patterning of the source and drain electrodes. Using this method, we successfully fabricated the BCE-type IGZO-TFT. The microphotograph shows that there is no obvious damage in the oxide layer and the device shows good electrical properties. This method is not limited to the type of oxide semiconductor material and doesn’t need additional patterning steps. It provides a new approach for the realization of low-cost MOTFT array.The previous part is aiming to improve the performance of MO-TFT from the perspective of device and good results have been achieved in terms of device’s stability, but it is difficult to further improve the mobility. As a result, we developed a high mobility Zr-In-O material system through theoretical analysis and simulation verification and the TFT was successfully fabricated. The device exhibited an excellent electrical properties with a high mobility of 38.8 cm2V-1s-1, an on-to-off current ratio of 3 × 108, a subthreshod swing of 0.37 V/decade, the threshold voltage shift was 1.03 V and 2.13 V under positive(VG = + 20 V) and negative(VG =- 20 V) gate bias stress, respectively. The maximum process temperature is kept below 150 oC, indicating that the Zr-In-O material has great potential in the flexible TFT. In order to further improve the device stability, ZrIn O/IGZO double active layer structure was adopted. The device with ZrInO/IGZO double active layer exhibited a more excellent performance with a high mobility of 36.2 cm2V-1s-1, an on-to-off current ratio of 3.5 × 109, a subthreshold swing of only 0.09 V/decade. Furthermore, the threshold shifted only 0.2 V and 0.97 V for the device under positive(VG = + 20 V) and negative(VG =- 20 V) gate bias stress, respectively.Since the maximum process temperature of Zr In O-TFT mentioned above is 150 oC, we consider further to realize flexible ZrInO-TFT on the plastic substrate. But the surface of plastic substrate is too rough, and the ability to block water and oxygen molecules is very poor. To solve this problem, the combination of multi-layer block structure with inorganic-organicinorganic(SiNx/Photoresist/SiNx) was adopted and the flexible ZrInO-TFT was successfully fabricated on the PEN substrate. The flexible device exhibited excellent electrical properties with a high mobility of 22.6 cm2V-1s-1, a subthreshold swing of 0.39 V/decade, an on-to-off current ration of 2.51 × 107. Furthermore, these good TFT performances were not degraded until under the mechanical bending situation at a curvature radius of 20 mm, which demonstrated its huge application prospect in the next generation of flexible AMOLED display.