Influence of NF₃ Plasma-Treated HfO₂ Gate Insulator Surface on Tin Oxide Thin-Film Transistors

We studied the impact of NF₃ plasma treatment on the HfO₂ gate insulator of amorphous tin oxide (a-SnO_x) thin-film transistors (TFTs). The plasma treatment was for 0, 10, or 30 s. The HfO₂ insulator demonstrated a slightly higher breakdown voltage, whereas the capacitance value remained almost constant (~150 nF/cm²). The linear mobility slightly increased from ~30 to ~35 cm²/Vs when the treatment time increased from 0 to 10 s, whereas a 30 s-treated TFT demonstrated a decreased mobility of ~15 cm²/Vs. The subthreshold swing and the threshold voltage remained in the 100–120 mV/dec. range and near 0 V, respectively. The hysteresis dramatically decreased from ~0.5 V to 0 V when a 10 s treatment was applied, and the 10 s-treated TFT demonstrated the best stability under high current stress (HCS) of 100 μA. The analysis of the tin oxide thin film crystallinity and oxygen environment demonstrated that the a-SnO_x remained amorphous, whereas more metal–oxygen bonds were formed with a 10 s NF₃ plasma treatment. We also demonstrate that the density of states (DOS) significantly decreased in the 10 s-treated TFT compared to the other conditions. The stability under HCS was attributed to the HfO₂/a-SnO_x interface quality.