Improved mobility and bias stability of Hf-doped IGZO/IZO/Hf-doped IGZO thin-film transistor

Amorphous In–Ga–Zn–O (a-IGZO) thin-film transistor (TFTs) have attracted considerable attention owing to their advantages such as low leakage current and high optical transparency. However, because of their low mobility and bias instability, a-IGZO TFTs have limited applications in next-generation displays. Here, a TFT with Hf-doped a-IGZO (IGZO:Hf) and IZO multichannel layers is proposed to solve this problem. Hf easily combines with surrounding oxygen ions; thus, the IGZO:Hf layer improves the stability of TFT by reducing the oxygen vacancies and suppressing the oxygen-related defects. The IZO layer increases the number of free carriers in the channel layer owing to the formation of oxygen vacancies during the annealing process. Further, a two-dimensional electron gas is formed at the interface between the IGZO:Hf and IZO layers, accumulating free carriers near the interface. The optimized TFT with IGZO:Hf/IZO/IGZO:Hf channel layers exhibited a high field-effect mobility (≈40 cm2/V s), high on/off ratio (>107), and low subthreshold swing (≈222 mV/dec). Moreover, it showed excellent stability with a threshold voltage shift of less than 1 V, even under various bias stress conditions. This approach provides a crucial avenue to improve the overall performance of metal oxide-based TFTs.