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

Abstract

An Al2O3 thin film is assembled on an unin- tentionally 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 $2.0 \times 10^{-7}$ A/cm2 at a gate voltage of −10.0~10.0V. 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–curves after annealing at 300 C are one order lower than those before annealing, indicating the decrease of positive fixed charge density in the Al2O3 film. There are ledge phenomena for hysteresis characteristics in the 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 Al2O3/ZnO heterojunctions are clarified to be straddling gap structures with conduction band offsets larger than 1.3 eV.