Gate/insulator-interfacial-dipole-controlled current conduction in Al₂O₃ metal-insulator-semiconductor capacitors

Electric dipoles at a metal-gate/Al₂O₃ interface are found to control the current conduction in negatively biased Al₂O₃ metal-insulator-semiconductor (MIS) capacitors by effectively increasing the Al₂O₃ electron affinity near the gate and thereby reducing the barrier height against electron field emission from the gate. By carrying out space-charge-controlled field emission analysis, the Al₂O₃ effective electron affinity in Al-gate capacitors was found to be larger than that for the Au gate by 0.38 eV, and the value for the Ni gate was similar to that for the Au gate. The cross-sectional transmission-electron-microscope images of the samples revealed the presence of an approximately 3-nm-thick layer intervening between the Al gate and the Al₂O₃ film. This layer is likely to have formed Al/Al₂O₃ interfacial dipoles that caused the aforementioned shift of the Al₂O₃ effective electron affinity. It was also confirmed that the conventional Fowler-Nordheim tunneling analysis yields remarkably erroneous results under the presence of these dipoles. These findings not only form the basis for investigating the band alignment of metal-gate MIS capacitors, but also alert us to a possibility of unexpectedly large leakage currents in negatively biased metal-gate MIS field-effect transistors.