A novel AZO capping approach on SnO thin-film transistors for efficient threshold voltage modulation and electrical performance enhancement

A p-type oxide semiconductor, SnO, is promising candidate for the channel material of monolithic 3D integration due to its low-temperature process compatibility and high hole mobility. In this study, we demonstrate the novel capping approach enabling passivation and doping, based on Al-doped ZnO (AZO), to the SnO channel. This AZO capping technique successfully enhanced the overall electrical performance of SnO thin-film transistors (TFTs) through oxygen diffusion driven by the oxygen areal density difference, compared to SnO TFTs without a capping layer. The field effect mobility increased from 1.6 to 2.4 cm²/V·s, achieving one of the best values reported. I_ON/I_OFF was increased from 3.5 × 10² to 2.6 × 10³, subthreshold swing reduced from 15.2 to 9.3 V/dec. Moreover, the AZO capping also enables wide-range threshold voltage (V_TH)control via Al-induced modulation of hole concentration in the SnO through AZO thickness control. The V_TH shift per gate voltage sweep range was tunable up to 13 %, reaching one of the highest values reported and enhancing design flexibility. This V_TH controllability significantly improved the noise margin of SnO–IGZO inverters. Therefore, the proposed AZO capping technique addresses fundamental limitations in SnO TFTs, enabling overall performance enhancement and improved design flexibility for oxide semiconductor circuits.