Electrical Properties of Al2O3/ZnO Metal–Insulator–Semiconductor Capacitors

An Al
₂
O
₃
thin film is assembled on an unintentionally doped ZnO for metal-insulator-semiconductor (MIS) capacitors. Leakage current density (J ), capacitance- voltage (C -V ) properties, and their thermal stabilities are investigated. The J-values for the MIS capacitors before and after annealing at 300 °C are lower than 2.0 × 10
^-7
A/cm
²
at a gate voltage of -10.0~10.0 V. Frequency-dependent C-V characteristic shows a stable maximum capacitance for the MIS capacitor before annealing. However, it decreases dramatically at 1 MHz for that after annealing at 300 °C, which is attributed to the change of ZnO channel resistance. Flat band voltage shifts in the C-V curves after annealing at 300 °C are one order lower than those before annealing, indicating the decrease of positive fixed charge density in the Al
₂
O
₃
film. There are ledge phenomena for hysteresis characteristics in the C-V curves, which result possibly from the delayed evacuation of minority carriers in deep depletion region. By considering the Fowler-Nordheim tunneling model, band configurations of the Al
₂
O
₃
/ZnO heterojunctions are clarified to be straddling gap structures with conduction band offsets larger than 1.3 eV.