Emission current enhancement of MIM cathodes by optimizing the tunneling insulator thickness

The relationship between the thickness of the anodized Al₂ O₃ tunneling insulator and the transfer ratio was investigated for metal-insulator-metal (MIM) cathodes to optimize the thickness in terms of a high transfer ratio and emission current. Combining ellipsometry, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM), we determined the accurate thickness of an anodized Al film less than 20 nm-thick. With the knowledge of accurate thickness, we found that the transfer ratio increases as the insulator thickness increases from 5.2 nm to 10.6 nm, but saturates at 13.3 nm and decreases slightly at 20.1 nm. Optimizing the thickness of the insulator to 13.3 nm raised the transfer ratio of 0.1% for our previous work (Kusunoki and Suzuki, IEEE Trans. Electron Devices, vol. 47, pp. 1667-1672, 2000) to 0.7%. A high emission current of 14 mA/cm² was thus obtained. The existence of an optimal thickness for the anodized Al₂O₃ insulator was also clarified from a theoretical simulation. This is the result of a trade-off, as thickness increases, between the decreasing probability of cut-off at the surface workfunction barrier of the Ir-Pt-Au top electrode and the increased scattering of hot electrons inside the Al ₂O₃ insulator and top electrode. The relationship is discussed on the basis of the absolute distribution of energy of the hot electrons, which we determined by simulating inelastic scattering driven by electron-optical phonon interaction in the Al₂O₃ insulator