Correlating the density of trap-states and the field-effect performance in metal-oxide thin-film transistors with high-k gate dielectrics via a trap-limited conduction method

We performed a quantitative study using a trap-limited conduction method to correlate the density of localized states with the performance of Sm2O3- and HfO2-gated metal-oxide thin-film transistors (TFTs) fabricated at room temperature. We found that TFT performance with a Sm2O3 gate dielectric exceeds that with a HfO2 gate dielectric. Sm2O3-gated TFTs with an In-Ga-Zn-O (In-Zn-O) channel show a saturation mobility of 57.1 (114.9) cm2V-1s-1, effective threshold voltage of 0.9 (1.6) V, and subthreshold swing of 0.116 (0.256) Vdec-1. Based on a trap-limited conduction model, the improved performance correlates with the lower density of localized states in the tail and deep regions. A lower density of tail states leads to higher mobility, and the lower density of deep states tends to reduce both the threshold voltage and the subthreshold swing. Using semi-empirical quantum chemistry simulations, we show that the decrease in the number of localized states is associated with the inherent characteristics of Sm cations, due to their lower electropositivity and larger ionic radius compared to Hf, which leads to less ionic distortion in the channel.