Large mobility modulation in ultrathin amorphous titanium oxide transistors

Recently, ultrathin metal-oxide thin film transistors (TFTs) have shown very high on-off ratio and ultra sharp subthreshold swing, making them promising candidates for applications beyond conventional large-area electronics. While the on-off operation in typical TFTs results primarily from the modulation of charge carrier density by gate voltage, the high on-off ratio in ultrathin oxide TFTs can be associated with a large carrier mobility modulation, whose origin remains unknown. We investigate 3.5 nm-thick titanium oxide based ultrathin TFTs exhibiting 6-decade on-off ratio, predominantly driven by gate induced mobility modulation. The power law behavior of the mobility features two regimes, with a very high exponent at low gate voltages, unprecedented for oxide TFTs. We find that this phenomenon is well explained by the presence of high-density tail states near the conduction band edge, which supports carrier transport via variable range hopping. The observed two-exponent regimes reflect the bi-exponential distribution of the density of band-tail states. This improved understanding would be significant in fabricating high-performance ultrathin oxide devices.

近年来，超薄金属氧化物薄膜晶体管（TFTs）展现出极高的开关比与极陡的亚阈值摆幅，使其成为超越传统大面积电子学应用的极具潜力的候选器件。典型TFT的开关工作机制主要源于栅极电压对载流子浓度的调控，而超薄氧化物TFT的高开关比则可归因于显著的载流子迁移率调制，但其背后的物理机制至今尚不明确。本研究针对厚度为3.5 nm的基于氧化钛的超薄TFTs展开探究，该器件具备6个数量级的开关比，其开关特性主要由栅极诱导的迁移率调制驱动。该器件的迁移率幂律行为呈现两个调控区域，在低栅极电压下具有极高的幂指数，这在氧化物TFT中前所未见。我们发现，这一现象可通过导带边附近存在的高密度带尾态得到合理解释，该带尾态通过变程跃迁辅助载流子输运。实验观测到的双幂指数区域反映了带尾态密度的双指数分布。这一认知的提升对于制备高性能超薄氧化物器件具有重要意义。