Effects of Active Layer Thickness on the Electrical Characteristics of Solution-Processed In-Ga-Zn-O Thin Film Transistors

Abstract

In recent years, amorphous indium gallium zinc oxide (a-IGZO) thin-film transistor (TFT) received great attention as one of the most promising candidates for the next generation of transparent and flexible electronics for displays due to favorable mobility in amorphous state, high large-area uniformity and so on. Moreover, to overcome problems of low performance and low uniformity of hydrogenated amorphous silicon (a-Si:H) and p-Si, respectively which were used widely as a switching and driving devices in the flat panel displays for the past few years, a-IGZO TFTs have been also suggested as potential replacements. The film deposition methods based on vacuum process, such as radio frequency sputtering (rf sputtering), are generally used to deposit oxide semiconductor layer, but fabricating oxide TFTs using vacuum process is quite demanding because of high cost originated from expensive equipment. In contrast, solution process provides many advantages, such as low-cost, simplicity and high throughput, so several research groups have used sol-gel derived multicomponent oxide TFTs as active channel layer. Thus, many experiment of solution-processed a-IGZO TFTs have been reported to investigate their performances. However, further investigation about factors controlling electrical performances of solution-processed a-IGZO TFTs is still required to examine TFT characteristics. In this thesis, the active layer thickness (tact) was investigated as one of these factors. We fabricated a-IGZO TFTs through two methods to change tact, which were multi-stacking of active layers and depositing increased molarity of IGZO solution with decreased speed of spin-coating. Corresponding changes of electrical properties with different tact were observed in both methods. As tact increases, the a-IGZO TFTs showed both decreased turn on voltage and threshold voltage owing to the increased free carriers in the channel and degraded field-effect mobility, on/off ratio and subthreshold swing because of increased surface roughness and trap density. These changes of performance of two different methods exhibited same dependence as changed tact, however, the extent of changes was shown fairly different each other, to be more in detail, the result of second method exhibited more degraded one. Thus, in order to investigate the reason why these results were occurred, origins of change of electrical properties with different tact were investigated by X-ray reflection (XRR) and extraction of contact resistance (RC) and comparison of results between two methods was also done. By analyzing this, the effect of tact on the electrical properties of solution-processed a-IGZO TFTs could be clarified to optimize electrical properties suitable for each applications and it could be also compared with that of vacuum-based TFTs. In addition, the results of solution-processed a-IGZO TFTs showed the significant improvement in the possibility of optimizing parameters controlling the electrical performances by changing tact.