By inserting H 2 O treatment steps during atomic layer deposition of a ZnO layer, the turn-on voltage shift from negative bias stress (NBS) under illumination was reduced considerably compared to that of a device that has a continuously grown ZnO layer without any treatment steps. Meanwhile, treatment steps without introducing reactive gases, and simply staying under a low working pressure, aggravated the instability under illuminated NBS due to an increase of oxygen vacancy concentration in

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... ZnO layer. From the experiment results, additional oxidation of the ZnO channel layer is proven to be effective in improving the stability against illuminated NBS.