Threshold Voltage Modulation in a Transistor with a Two-Dimensional Electron Gas Channel at the Interface between Al₂O₃ and Sub-5 nm ZnO Films

Recent reports on the emergence of two-dimensional electron gas at the heterointerface of amorphous and polycrystalline oxide thin films, such as amorphous-Al2O3/polycrystalline-TiO2 (a-AO/TO) and a-AO/polycrystalline-ZnO on a Si wafer, open a venue for oxide-based thin-film-transistor research. Due to the higher Hall mobility of ZnO, the device performance was considerably improved in a-AO/ZnO compared with a-AO/TO. However, an excessively high negative threshold voltage (Vth) issue still remains, resulting in a static power consumption problem in the channel-off state. In this study, the Vth value of the a-AO/ZnO device was shifted positively by decreasing the thickness of the ZnO thin film (tZnO) to less than 10 nm, accompanied by a grain size reduction of the polycrystalline ZnO film. The large density of grain boundaries in the ZnO film with thinner thickness trapped the 2D electrons, leading to a decrease in effective carrier density (ns). As ns decreased by decreasing the tZnO from 11 to 4.6 nm, the Vth increased from −2.6 to −0.63 V. With an optimized tZnO of 5.6 nm, the transistor exhibited a Vth of −0.86 V, a saturated field-effect mobility of ∼4.8 cm2/(V s), and a subthreshold swing of 194 mV/dec.