Amorphous InGaZnO thin-film transistor with optimized high-k gate dielectric

Abstract

In recent years, amorphous InGaZnO (a-IGZO) thin-film transistor (TFT) has been drawing worldwide attention for being an excellent candidate for driving the pixels in next-generation flat-panel displays (FPD). In addition, various high-k materials are adopted as the gate dielectric of the TFT in order to reduce its operating voltage, and thus decrease the power consumption of electronic products. Furthermore, the optimization of the high-k dielectrics leads to higher quality for the dielectric/IGZO interface. Accordingly, the main purpose of this research is to investigate new high-k dielectrics as well as optimization approaches for upgrading the electrical performance and stability of a-IGZO TFTs. Firstly, the effects of Ta incorporation in Y2O3 gate dielectric on the electrical characteristics of a-IGZO TFT are investigated. With an appropriate Ta content in the Y2O3 gate dielectric, the carrier mobility of the TFT can be significantly increased, about three times that of the control sample with Y2O3 gate dielectric. Accordingly, the sample with a Ta/(Ta+Y) atomic ratio of 68.6% presents a high saturation mobility of 33.5 cm2V-1s-1, small threshold voltage of 2.0 V, large on/off current ratio of 2.8×107, and suppressed hysteresis. However, excessive Ta incorporation in the Y2O3 gate dielectric leads to degraded device performance. Then, a-IGZO TFT with high-k Nb2O5 as gate dielectric is prepared for the first time, showing typical field-effect characteristics. By adding La in the Nb2O5 gate dielectric, the electrical performance of the device is significantly improved. Consequently, the sample with appropriate La content has a high saturation mobility of 28.0 cm2V-1s-1, small threshold voltage of 1.84 V, small subthreshold swing of 0.17 V/dec, and negligible hysteresis. Next, the study is focused on the improving effects of Ti incorporation in La2O3 gate dielectric on the electrical performance and stability of a-IGZO TFT. Compared to the control sample with La2O3 gate dielectric, the device performance can be significantly improved with an appropriate Ti dose. Accordingly, the sample with a Ti/(Ti+La) ratio of 6.7% presents a high saturation mobility of 28.1 cm2V-1s-1, small subthreshold slope of 0.17 V/dec, large on/off current ratio of 7.2×107, and acceptable hysteresis. However, excessive Ti incorporation leads to device degradation. Moreover, the effects of Hf incorporation in La2O3 gate dielectric on the electrical characteristics of a-IGZO TFT are studied. With an appropriate dose of Hf, the device performance can be significantly improved, resulting in high saturation mobility of 30.5 cm2V-1s-1, small subthreshold slope of 0.15 V/dec, and negligible hysteresis (0.05 V). However, excessive Hf incorporation results in device degradation. Finally, a comparative study on the electrical characteristics of a-IGZO TFTs with HfLaO and HfLaON gate dielectrics is conducted. It is found that appropriate nitrogen incorporation in HfLaO can obtain a saturation mobility of 31.3 cm2V-1s-1, which is more than twice that of the control sample without nitrogen incorporation. However, the electrical characteristics of the device tend to deteriorate with excessive nitrogen incorporation. (473 words)