Fermi-Level Engineering of Atomic Layer-Deposited Zinc Oxide Thin Films for a Vertically Stacked Inverter

Abstract

We report here atomic layer-deposited ZnO FETs and an effective electron doping method based on Al2O3 layer for high-performance transistors and its circuit application. Through the atomic layer deposition, an ultrathin and flat ZnO film (5 nm) has been deposited successfully where our ZnO FETs initially show n-type enhancement-mode transfer characteristics. The threshold voltage and the electron field-effect mobility of the device are 42 V and 2.8 cm2 V–1 s–1, respectively. After the deposition of the top Al2O3 layer, our ZnO FETs exhibit depletion-mode transfer characteristics with enhanced mobility. The threshold voltage (Vth) shifts to −38 V and the electron field-effect mobility increases by a factor of 2. Analyzed by a four-point probe and Hall and temperature-variable current–voltage measurements, Al2O3-induced doping is considered the main reason for the noticeable changes by minimizing the trap states in our ZnO films. Furthermore, a vertically stacked low-voltage n-type ZnO inverter was successfully fabricated utilizing the enhancement- and depletion-mode ZnO FETs with a voltage gain of ∼1.5 at low supplied voltage. We thus conclude that the atomic layer-deposited ZnO FETs with Al2O3-induced doping offers practical benefits for the high-performance transistors and its circuit applications.