Influence of NF Plasma-Treated HfO Gate Insulator Surface on Tin Oxide Thin-Film Transistors.

We studied the impact of NF plasma treatment on the HfO gate insulator of amorphous tin oxide (a-SnO) thin-film transistors (TFTs). The plasma treatment was for 0, 10, or 30 s. The HfO insulator demonstrated a slightly higher breakdown voltage, whereas the capacitance value remained almost constant (~150 nF/cm). The linear mobility slightly increased from ~30 to ~35 cm/Vs when the treatment time increased from 0 to 10 s, whereas a 30 s-treated TFT demonstrated a decreased mobility of ~15 cm/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 remained amorphous, whereas more metal-oxygen bonds were formed with a 10 s NF 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 HfO/a-SnO interface quality.