Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
A study of Zinc Oxynitride Thin Film Transistor: mobility and stability Kyung-Chul Ok, Hyun-Jun Jeong, Hyun-Mo Lee, and Jin-Seong Park* Division of Materials Science and Engineering, Hanyang University, 222 wangsmini-ro, Seoul, 133-719, Republic of Korea Abstract Amorphous ZnON thin film transistors were investigated in terms of semiconducting behavior, fabrication process and light induced bias stability. The electrical characteristics such as superior mobility and light induced bias stability were systemically analyzed by specific physical and chemical properties. 1. Introduction Amorphous oxide semiconductor thin film transistors (AOS TFTs) have been attracted many attention in active matrix liquid crystal display (AMLCD) and organic light-emitting diode (AMOLED) display because of their good electrical properties and high on/off ratio compared to conventional a-Si TFTs1. In spite of various advantages, AOS TFTs was faced with limitation for the applications of ultra high definition (UHD) display and fast response in the large area. Fig. 1. (a) cross-sectional device configuration, (b) transfer curve and (c) saturation mobility of ZnO and ZnON TFTs Since the report of a-ZnON TFTs in 20092, amorphous zinc-oxynitride semiconductor has been suggested as a new potential candidate material to overcome above issues due to its high mobility (≥ 50cm2/Vs) and good stability3-4. Unfortunately, there are several reports of aZnON in particular due to very short period of investigation. In this paper, we investigate the a-ZnON TFTs focused on the semiconducting behavior and fabrication process such as oxygen partial pressure and effective annealing time. The amorphous phase and low effective mass of ZnON lead to high filed effect mobility in large area uniformity. Figure 1 (b) and (c) show transfer characteristics and saturation mobility of ZnO and ZnON TFTs, respectively. After post annealing process at 250oC in vacuum (~10-1 Torr), electrically metallic properties of as-deposited a-ZnON TFTs was changed into semiconducting properties, while ZnO TFTs was changed from insulating to semiconducting properties. Especially, the two order of saturation mobility of aZnON TFTs (~56.3cm2/Vs) was higher than ZnO TFTs. The main difference between the ZnO and ZnON could be suggested that the amorphous phase and low effective mass of ZnON lead to high filed effect mobility in large area uniformity. As a result of XPS analysis, N-rich ZnxNy states which can be act as a electron donor were effectively suppressed by post annealing process. Therefore, the post annealing process is important to fabricate semiconducting a-ZnON film and is available to control the carrier concentration in the channel layer. 2. 2.2 2.1 Highly stable and high mobility amorphous ZnON TFTs Comparative studies of ZnO and amorphous ZnON TFTs First of all, the fabrication process of a-ZnON TFTs was investigated by comparing with conventional ZnO TFTs. Figure 1. (a) shows the cross-sectional device configuration as a inverted staggered structure. The different Ar/O2 and Ar/O2/N2 gas mixture were used to reactive gas source to fabricated ZnO and ZnON film during the DC reactive sputtering. Effect of Oxygen Partial Pressure: High mobility The effect of oxygen partial pressure of a-ZnON TFTs was investigate in order to optimize fabrication process during the DC reactive sputtering. The device structure and annealing process were carried out identical conditions in 2.1. In this part, 30nm-thick ZnON films were fabricated with different oxygen partial pressures (O2/Mixture gas (Ar+N2+O2) = 0.013: A, 0.026: B, 0.038: C and 0.051: D). Figure 2. (a) shows the representative transfer characteristics of ZnON TFTs as a function of oxygen partial pressure and (b) shows the average TFT parameters at different oxygen partial pressure. 1 Fig. 2 (a) Representative transfer curves of ZnON TFTs as a function of oxygen partial pressure. (b) The average saturation mobility, threshold voltage and subthreshold voltage of seven ZnON TFTs at different oxygen partial pressure. Fig. 3 (a) Representative transfer curves of a-ZnON TFT (5hrs) under NBIS (VG = -20 V, 1500 lux @ white) as a function of stress time. (b) Time-dependent threshold voltage shift (∆Vth) of a-ZnON (1~5 hrs) under NBS and NBIS conditions. Interestingly, as oxygen partial pressure increased from A to B, TFT mobility increased from 47.03 to 71.69 cm2/Vs. However, oxygen partial pressure increased from B to D, mobility was degraded to 17.51cm2/Vs. Generally, in oxide TFT, as oxygen partial pressure is increased, mobility is decreased. However, ZnON TFTs show different conventional tendency, mobility show parabolic trend as a function of oxygen partial pressure. This phenomena explained by XPS O 1s analysis (not shown). The binding energy at 529.8eV (peak 1) and 531.1eV (peak 2) represent metal-oxide (Zn-O) and oxygen related defects (ex. oxygen vacancy), respectively. As oxygen partial pressure increased from A to B, ratio of peak 1 was decreased from 74.42% to 73.2% in spite of increase of peak 2 from 16.64% to 17.64%. However, oxygen partial pressure changed from B to D, peak 1 was increased up to 79.12% and peak 2 was decreased to 14.23%. The optimum oxygen partial ratio of ZnON TFTs leads to suppress the oxygen related defects as well as high TFT performance. After post annealing process as enough time (5hrs), enhancement of bonding properties between Zn metal and two different anions (O, N) can result in the stable O-Zn-N hybridization. 3. The DC reactive sputtered a-ZnON TFTs were investigated in focused on the semiconducting behavior and fabrication process. In order to turn into the semiconducting properties for the as-deposited film, post annealing process is available to suppress the carrier concentration. The optimized oxygen partial pressure during the DC reactive sputtering could affect to the high saturation mobility (~50cm2/Vs). After suitable annealing time (5hrs), considerable improvement of light-induced bias stability of a-ZnON TFTs was investigated with stable O-Zn-N bonding properties. 4. 2.3 Effect of Annealing time: Reliability The effect of annealing process of a-ZnON TFTs was investigated as a function of annealing time (1, 3 and 5 hours). The a-ZnON TFTs were fabricated by optimized Ar/O2/N2 mixture. Figure 3. (a) shows representative transfer curve of a-ZnON (5 hrs) and (b) shows evolution of threshold voltage under negative bias stress (NBS) and negative bias illumination stress (NBIS). All the TFTs were shown as similar device characteristics of Vth (~ -3 V) and μsat (~50cm2/Vs) but the light-induced device instability was significantly improved by increasing the annealing time from 1 hr (-10.88 V) to 5hrs (-2.28 V). These phenomenon could be the stable nitrogen related states near the valence band maximum (VBM) affect to the improved light induced instability due to the screening the oxygen related defects. Conclusions Acknowledgements This work was References 1. J. S. Park, W.J. Maeng, H.-S. Kim and J.-S. Park, Thin Solid Films Vol. 520, p. 1679 (2012) 2. Y. Ye, R. Lim and J. M. White, J. Appl. Phys. Vol. 106, p. 074512 (2009) 3. H.-S. Kim, S. H. Jeon, J. S. Park, T. S. Kim, K. S. Son, J.-B. Seok, J.-S. Seo, J.-S. Kim, E. Lee, J. G. Chung, H. Lee, S. Han, M. Ryu, S. Y. Lee and K. Kim, Scientific Reports Vol. 3, p. 1459 (2013) 4. E. Lee, A. Benayad, T. Shin, H. Lee, D.-S. Ko, T. S. Kim, K. S. Son, M. Ryu, S. Jeon and G.-S. Park, Scientific Reports Vol. 4. p. 4948 (2014) Page No. 2