Effects of oxygen contents on the physical properties of MgO films and carrier transport of Au/MgO/ZnO MIS diodes

Abstract

MgO films with various oxygen contents were deposited on ZnO using radio-frequency magnetron sputtering, and Au/MgO/ZnO metal-insulator-semiconductor (MIS) diodes were fabricated. The effects of various oxygen contents on the physical properties of MgO and the carrier transport in MIS diodes were studied. The crystallinity of MgO films improved with increased oxygen content up to 66%; however, the crystallinity degraded upon further increase of oxygen content up to 100%. X-ray photoelectron spectroscopy showed that the oxygen vacancies were compensated by the introduced oxygen atoms; however, the excess oxygen atoms could not fill the lattice sites and were instead absorbed on the MgO surface. The optical bandgaps of MgO films increased from 5.16 to 5.22 eV for 0 and 100% oxygen content, respectively, resulted by the suppressed oxygen vacancies. The average roughness of MgO films increased from 4.02 to 6.86 nm for 0 and 100% oxygen content, respectively. As compared to MIS diodes without oxygen content, the leakage current was reduced by approximately 300 times in MIS diodes with 66% oxygen content. At a high bias voltage (V ≥ 0.4V), MIS diodes without oxygen exhibited ohmic conduction. In contrast, MIS diodes with oxygen exhibited Fowler-Nordheim tunnelling at high bias voltages. With increasing oxygen content, the tunnelling barrier height increased to its maximum of 0.46 eV in MIS diodes with 66% oxygen content. Nevertheless, the barrier height reduced to 0.29 eV for the excess oxygen content up to 100%. However, at low-bias voltages, all MIS diodes demonstrated a direct tunnelling mechanism owing to their rectangular barriers.