| Summary: | 博士 === 國立交通大學 === 電子研究所 === 100 === Recently, higher electron mobility is needed for thin film transistors (TFTs) to twist the liquid crystal in large-size displays. However, the electron mobility of conventional amorphous-silicon (α-Si:H) TFTs is very low (< 1 cm2 / Vs). As a result, amorphous metal-oxide TFTs with high mobility (10~100 cm2 / Vs) is very promising for the application of future displays. Accordingly, the exploration of amorphous metal-oxide TFTs becomes an important topic. Despite higher electron mobility of amorphous metal-oxide TFTs, the device performance is always affected by the ambience, illumination, and long-term bias stress, resulting in the threshold voltage shift and a metastable state.
This dissertation investigated the improvement of device stability by changing fabricating process conditions firstly. In this part, we explored the essential elements of post-deposition annealing to adjust the film stoichiometry for sol-gel derived amorphous indium-zinc-oxide thin film transistors (α-IZO TFTs). Compared with the as-deposited α-IZO TFTs, the electrical characteristics of vacuum-annealed one tends to the conducting properties, while the oxygen-annealed one appears switch characteristics and exhibits better performance as the annealing duration extends. Hence, only the heating energy alone cannot achieve the purpose of annealing, the oxygen-containing environment is also required.
Then, the surrounding ambience, light, and temperature will be altered to monitor the device stability. With increasing relative humidity, the electron mobility and the threshold voltage of sol-gel derived amorphous indium-gallium-zinc-oxide (α-IGZO) TFTs enhance, while the subthreshold swing degrades and the drain-induced-barrier-lowering appears. Hence, a water dipole model is proposed to explain these anomalous deteriorations. Under the light-illumination, the conductivity of sol-gel derived α-IGZO TFTs will increase, due to the oxygen desorption. On the other hand, the oxygen re-adsorption will cause a recovery behavior once the light source is removed.
Furthermore, we compare the temperature sensitivity between sol-gel derived and sputtered α-IGZO TFTs. In the ambience without oxygen, thermal activation dominates and enhances the device performance. In oxygen-containing environment, the varying form of adsorbed oxygen and the combination of oxygen and vacancies dominate the device performance. Then, the adsorbing capability of oxygen on the back-channel in sputtered α-IGZO TFTs were examined at various ambient temperatures. Results imply that higher ambient temperature can assist the oxygen to adsorb on α-IGZO film easily and can further passivate part of the traps in α-IGZO film, leading to a decrease in the density-of-states.
The experiment of bias stress is also performed to simulate the device operation and to find the degradation mechanism. For application in active-matrix organic light-emitting diodes, the anomalous capacitance-voltage degradation of sputtered α-IGZO TFTs under hot carrier stress is explored. In vacuum, both the gate-to-drain capacitance (CGD) and the gate-to-source capacitance (CGS) curves exhibit positive shifts due to the electron trapping in the gate insulator. While in an oxygen-rich environment, the CGD-VG curve shows a significantly positive shift due to the electric-field-induced oxygen adsorption. The degradation in the CGS-VG curve is not only the positive shift, but also the anomalous two-step turn-on behavior. This phenomenon can be ascribed to the electron trapping in the gate insulator and electric-field-induced oxygen adsorption on the channel layer, especially in the area adjacent to the drain terminal. The electron trapping increases the source energy-barrier, with the electric-field-induced oxygen adsorption further raising the energy band near the drain, resulting in a two-step turn-on behavior in the CGS-VG curve.
Lastly, we address some suggestions for future research topic to obtain the amorphous metal-oxide TFTs with high stability and good performance. Finally, we hope that the fully transparent large area electronics for future display will be realized.
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