Reversible Charge Transfer Doping in Atomically Thin In₂O₃ by Viologens

Atomically thin oxide semiconductors are emerging as potential materials for their potentiality in monolithic 3D integration and sensor applications. In this study, a charge transfer method employing viologen, an organic compound with exceptional reduction potential among n-type organics, is presented to modulate the carrier concentration in atomically thin In₂O₃ without the need of annealing. This study highlights the critical role of channel thickness on doping efficiency, revealing that viologen charge transfer doping is increasingly pronounced in thinner channels owing to their increased surface-to-volume ratio. Upon viologen doping, an electron sheet density of 6.8 × 10¹² cm^-2 is achieved in 2 nm In₂O₃ back gate device while preserving carrier mobility. Moreover, by the modification of the functional groups, viologens can be conveniently removed with acetone and an ultrasonic cleaner, making the viologen treatment a reversible process. Based on this doping scheme, we demonstrate an n-type metal oxide semiconductor inverter with viologen-doped In₂O₃, exhibiting a voltage gain of 26 at V_D = 5 V. This complementary pairing of viologen and In₂O₃ offers ease of control over the carrier concentration, making it suitable for the next-generation electronic applications.