Study on Electrical Characteristics and Physical

• Main Authors: Li, Hung-Wei, 李泓緯
• Other Authors: Tai, Ya-Hsiang
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/51023890582627107516

博士 === 國立交通大學 === 光電工程學系 === 100 === In order to investigate the mechanism of photo leakage current of poly-Si TFT, the poly-Si TFTs with patterned metal shielding layer are used to study. The location of the exposed region in poly-Si layer is defined by employing the proposed structure. The exposed region’s photo leakage current increased at the drain junction location; therefore, the drain junction was the effective area in inducing photo leakage current under light exposure. The drain-exposed poly-Si TFT exhibits a significantly high photo leakage current and a slight subthreshold swing degradation under illumination. However, the exposed region located at the source junction significantly degraded at the sub-threshold swing of TFT under illumination with an increase in drain bias. In other words, the accumulated holes will, however, affect the applied gate voltage at the source junction and cause the degradation of the S.S. in the poly-Si TFTs under illumination. Besides, the analytical model was applied to extract the parameters. Based on the experimental results and analytical parameters, a band diagram was proposed to explain the degradation of subthreshold swing. Next, the interface treatment is applied at the bottom interface surface between poly-Si film and top buffer layer in the poly-Si TFTs. The NH3 is used to serve as the treatment gas. In order to verify the proposed method, the Ar ion implant is also used to treat the interface. From experimental results of the Ar ion implantation device, the maximum degradation of sub-threshold is 46.4% and the photo leakage current is about 8.6pA under the 5620nit back-light illumination. However, the photo leakage current of conventional device exposed to the 2160 back-light brightness was about 13.6pA. By introducing the trap states, the excess electron-hole pairs would have probability to recombine at the defect sites to decrease the photo leakage current and moderate the degradation of sub-threshold swing. Besides the Ar ion implantation is used to treat the interface, the NH3 gas plasma also can be applied at the interface to create defect states. However, the channel doping is usually applied for modulate the threshold of poly-Si TFTs. In this study, the photo leakage current of poly-Si TFT with channel doping modulation is also investigated. By adjusting the implantation energy, the target depth of the implantation set at the interface. The device with the channel doping modulation has the best result of the photo leakage current. Third, the stability of poly-Si TFTs under both darkened and backlight illumination environments is investigated. The stability tests include DC hot-carrier stress and dynamic AC stress. In the DC hot-carrier stress, the experimental results showed the ON-current degradation under the backlight illumination was slightly higher than the darkened environment. The extra electron-hole pairs, generated by the back-light illumination, would influence the electric field near the drain junction and they would also reduce the channel-hot-carrier stress. Besides, the four terminal devices n-channel poly-Si TFT with counter-doped lateral body terminal was applied in order to monitor the hole current generated by the ionization impact near the drain junction. In the other part, both conventional and patterned metal-shielding structures are used to verify the reliability of the poly-Si TFTs under darkened and illuminated dynamic AC stress. Experimental results indicate that during the illuminated stress the competitive mechanisms of carrier increase and electric field reduction are generated. Forth, the lateral body terminal Silicon-Oxide-Nitride-Oxide-Silicon Thin-Film-Transistors (LBT SONOS TFTs) is proposed to enhance the erasing efficiency. This device has superior erasing efficiency by gate as well as lateral body electrode exerting bias. The erasing mechanism of LBT SONOS TFTs has been illustrated by the energy band diagrams. Holes gain sufficient energy by electric field in deep depletion region to surmount the tunneling oxide barrier because of exerting body bias under erasing operation. Also, the lateral body terminal exerting bias can enhance erasing efficiency and which is confirmed by different erasing conditions and structure. In addition, to verify the hole current injecting from the lateral body site, the size effect of LBT SONOS TFTs is also discussed. In addition, the hump phenomenon presents at large width of the LBT SONOS TFTs after erasing operation has also been discussed and explained it with the schematic diagram. Besides, the fabrication of SONOS-TFTs with lateral body terminal is quite easy and involves no additional processes. Such a SONOS-TFT is thereby highly promising for the future system-on-panel display applications. Fifth, the sol-gel spin-coating method is used to fabricate the Zr or Sn doped zinc oxide based thin film transistor. Regardless of the deposition method, all undoped ZnO conducting films have unstable electric properties in the long term. This stability is related to the change in surface conductance of ZnO films under oxygen chemisorptions and desorption. The gas ambient effect on the photo leakage current、the electrical characteristic of bias temperature、gate bias stability and hysteresis of transfer characteristics would be investigated. Last, a brief review of the mechanism of chemisorbed and physically adsorption would be discussed. Based on the mechanism and the above experimental results, a new method would be proposed to remove the chemisorbed and physically adsorbed of oxygen. In the proposed method, the light, heat, and vacuum environment were employed simultaneously. This proposed method is based on that the photo generated holes could easily migrate to the chemisorptions site due to the built-in electric filed and transition from chemisorbed state to a physisorbed state. The heat is used to overcome the van der waal force. Besides, the environment should in a vacuum environment to ensure that the desorbed oxygen could be evacuate from the chamber. By applying the proposed on the ZnO-based devices, the stability of the ZnO-based devices were greatly improved. Furthermore, these results can be as important information for the subsequent passivation process concerning the pre-treatment of the active layer.