Electrical Characteristic Analysis of Postannealed ZnO Thin-Film Transistors under O2 Ambient

An electrical characteristic analysis of postannealed ZnO thin-film transistors (TFTs) under O2 ambient from 200 to 300 °C for 1 h was carried out. The 250-°C-annealed device showed the best electrical characteristics, which can be explained by crystallinity improvement on the basis of X-ray diffraction (XRD) analysis. However, although having an active layer of the best crystal quality, the electrical properties of the 300-°C-annealed device were degraded, which can be due to the higher ΦB0 resulting from the oxidation of Ti used as the S/D electrodes. Next, the stability of ZnO TFTs was investigated. Under a positive bias stress of 20 V, it was found that the annealed devices showed smaller threshold voltage shifts (ΔVTH) than the as-grown device. In addition, with the stress bias removed, all devices recover their original characteristics, which can be explained by temporary charge trapping into preexisting traps. Therefore, the post-thermal annealing under O2 ambient can improve the electrical and reliability characteristics of ZnO TFTs, which can be explained by a fewer grain boundary traps and a lower grain boundary potential barrier, as determined by 1/f noise analysis, closely related to the crystallinity improvement of the active layer and fewer zinc interstitials and/or oxygen vacancies near the grain boundary in the active layer.

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