Unintended Carbon-Related Impurity and Negative Bias Instability in High-Mobility Oxide TFTs

Amorphous oxide semiconductors (AOS) with high-mobility (>40 cm ² /Vs) have recently become common for next-generation displays. While several candidates have been proposed, their practical use remains unclear because of their low reliability originating from bias stress instability such as negative bias stress (NBS) and positive bias stress (PBS). This study shows that CO-related impurity is a major source of NBS instability in amorphous InSnZnO (ITZO), a representative high-mobility AOS. CO-related impurity is formed at the surface of the ITZO layer after the photolithography process, according to thermal desorption spectroscopy and hard X-ray photoemission spectroscopy studies. It is shown that by using a simple method of UV ozone treatment at room temperature to remove these CO-related impurities, a high performance ITZO TFT with mobility of ~ 50 cm ² /Vs and excellent bias stability, NBS (at -20 V, 3600 s): -0.25 V, PBS (+20 V, 3600 s): 0.15 V, can be realized. Almost the same results are obtained for Sn-free IZO-TFTs with mobility of ~70 cm ² /Vs but the NBS instability caused by CO-related impurity does not appear for IGZO TFTs. This finding suggests that the bias stress instability by CO-related species is sensitive to high mobility AOS-TFTs.