Quantum Confinement and Interface States in ZnO Nanocrystalline Thin-Film Transistors

The experimental current-voltage (I
_D
-V
_G
) and capacitance-voltage (C-V
_G
) characteristics of the bottomgate/top-contact ZnO thin-film transistors (TFTs) are analyzed accounting for quantum confinement and interface state effects. All the measurements are performed on the same TFTs composed of a 45-nm-thick nanocrystallineZnO channel, indium tin oxide gate electrode, and a 21-nm-thick Al
₂
O
₃
gate insulator. Interface state density (Dit) derived from the combined high-low frequency capacitance method reveals a large Dit near the conduction band edge. The TFT characteristics are simulated using a Schrodinger- Poisson model and compared to a semiclassical model. The Schrodinger-Poisson model calculates a lower C in accumulation by correctly accounting for the peak carrier concentration being several nanometers away from the ZnO/Al
₂
O
₃
interface. The fit error in C-VG in strong accumulation is only 1.2% compared to 4% in the semiclassical simulations. All simulations are performed using 21 nm of Al
₂
O
₃
. An effective mobility (μeff) that increaseslinearlywith gate voltage is derived from the ratio of the experimental ID and the simulated mobile charge per unit area of the channel. Ignoring quantum confinement overestimates the channel charge and hence underestimates μeff by 40%. An additional Dit profile, at the top interface of the ZnO, is proposed in our analysis to explain the I
_D
-V
_G
characteristics in the subthreshold region.