The Study on the Electrical Stability of Bottom-Gate MgZnO Thin-Film Transistors
Date Issued
2011
Date
2011
Author(s)
Tsai, Yi-Shiuan
Abstract
This thesis reports the experimental studies on the gate-bias temperature stability of inverted staggered bottom-gate Mg0.05Zn0.95O thin-film transistors (TFT). According to literatures, the addition of Mg into ZnO related materials can reduce the oxygen vacancies due to higher ionic character of Mg-O than Zn-O bonds. In addition, indium is rare in earth. Therefore, we chose indium free MgZnO TFTs as our research target.
ZnO films crystallize easily, even when grown at room temperature. With the application of post-deposition annealing at 200°C, the (002) peak increased slightly, indicating the substitution of a small portion of Mg for Zn in the ZnO crystals. This substitution reduced the lattice constant of wurtzite ZnO, caused by the slightly smaller ionic radius of Mg2+ than that of Zn2+. Moreover, as the annealing temperature increased to 350°C, grains grew and the crystallinity of Mg0.05Zn0.95O improved, as denoted by a decrease of full width at half maximum (FWHM) of (002) peak. Furthermore, (002) the peak shifted even higher, indicating the substitution of more Zn by Mg in the ZnO crystals.
In the positive gate-bias stability test at room temperature, the subthreshold swing was nearly unchanged for the devices annealed at two different annealing conditions, revealing that the main mechanism for the threshold voltage (Vth) shift was charge trapping. The 350°C-annealed TFT showed less Vth shift, indicating better device stability. As the positive gate-bias stress applied to TFTs at elevated temperatures, humps occurred in the subthreshold region of the transfer curves in the 200°C-annealed TFT, and became severe as temperature and stressing time increases. The hump phenomenon was much less significant in 350°C annealed TFTs; merely a degradation of SS was observed at 80°C, the highest testing temperature in this study. The hump disappeared shortly after removing the positive gate-bias, suggesting that this phenomenon was meta-stable and was resulted from gate-bias induced electric field. This hump phenomenon might have been due to the creation of meta-stable oxygen vacancies in which the neutral vacancies were thermally excited into ionized states and released electrons into the active layer to form a leakage path, when TFTs were subjected to gate-bias stressing at elevated temperatures.
The humps were not identified in the transfer curves when TFTs were subjected to negative bias stress. Instead, a turn-around of Vth shift occurred in the 200°C-annealed TFT. It was attributed to the competing mechanisms of the defect creation and the charge trapping. In the AC bias stress, Vth shift was less severe to the DC bias stress. Nevertheless, the Vth shift increased with the increasing frequency of AC bias stress. It may come from the slow recovery of trapped charges.
In conclusion, the 350°C-annealed TFT showed a better bias temperature stability. The more substitution of Zn by Mg and better crystallinity help improve the stability of MgZnO TFTs.
Subjects
oxide TFTs
MgZnO
MgO
gate-bias stability
thermal stability
Type
thesis
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