Blue sensitive sub-band gap negative photoconductance in SnO2/TiO2 NP bilayer oxide transistor

Abstract

Large negative photoconductance (NPC) of SnO₂/TiO₂ nanoparticles (NPs) heterostructure has been observed with thin film transistor (TFT) geometry and has been investigated using sub-bandgap light (blue) illumination. This negative photoconduction has been detected both in accumulation and depletion mode operation, which effectively reduces the carrier mobility (μ) of the TFT. Moreover, the threshold voltage (V_th) widely shifted in the positive direction under illumination. The combined effects of the reduction of mobility and V_th shifting led to a faster reduction of On (or Off) state current under illumination. The negative photosensitivity of this system is as high as 3.2 A W⁻¹, which has been rarely reported in the earlier literature. Moreover, the variation of On (or Off) current, μ and V_th shift is linear with low-intensity blue light. This SnO₂/TiO₂ NP bilayer channel has been deposited on top of an ionic dielectric (Li-Al₂O₃) that reduces its operating voltage of this TFT within 2 V. Furthermore, the device has achieved a saturation mobility of 0.4 cm² V⁻¹ s⁻¹ with an on/off ratio of 7.4 × 10³ in the dark. An energy band diagram model has been proposed based on the type-II heterostructure formation between SnO₂/TiO₂ semiconductors to explain this NPC mechanism. According to the energy band diagram model, adsorbed H₂O molecules of TiO₂ NPs created a depleted layer in the heterostructure that accelerated the recombination process of photo-generated carriers rather than its transport.