Impact of AlSnO Back-Channel Layer on the Performance of AlSnO/InSnZnO Heterojunction Thin-Film Transistors

In this study, Al0.1Sn0.9O and Al0.3Sn0.7O are used as the back-channel materials of AlxSn1-xO/InSnZnO heterojunction thin-film transistors (HTFTs) to investigate the effect of Al/Sn mole ratio on the electrical performance of AlxSn1-xO/InSnZnO HTFTs. The Tauc plot, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) are used to characterize the bandgap energy, crystallinity, and oxygen vacancy content of the films. The results show that Al0.3Sn0.7O has a wider bandgap energy and fewer oxygen vacancies than Al0.1Sn0.9O. In addition, Al0.1Sn0.9O is polycrystalline, while Al0.3Sn0.7O is amorphous. This difference in crystallinity results in Al0.1Sn0.9O having higher electron mobility than Al0.3Sn0.7O. Therefore, Al0.1Sn0.9O/InSnZnO HTFT exhibits better electrical performance, including a higher field-effect mobility of 71.33 ± 6.13 cm $^{{2}}\text{V}^{-{1}}\text{s}^{-{1}}$ and a steeper subthreshold swing (SS) of 151.23 ± 14.06 mV/dec. In contrast, Al0.3Sn0.7O/InSnZnO HTFT exhibits worse field-effect mobility and SS than InSnZnO TFT, which is used as a reference device in this study. These results suggest that the mobility of AlxSn1-xO and the conduction band offset between AlxSn1-xO and InSnZnO are important parameters to design high-performance HTFTs. Moreover, both Al0.1Sn0.9O/InSnZnO and Al0.3Sn0.7O/InSnZnO HTFTs exhibit relatively insignificant threshold voltage shift compared to InSnZnO TFT during negative bias illumination stress (NBIS). This indicates that the present AlxSn1-xO back-channel design is helpful to enhance the stability of the HTFTs.