Toshiaki Kusunoki

Fluctuation-Free Electron Emission from Non-Formed Metal-Insulator-Metal (MIM) Cathodes Fabricated by Low Current Anodic Oxidation

Fluctuation-free electron emission is obtained from MIM (Al-Al2O3-Au) cathodes. The Al2O3 layer is fabricated by anodic oxidation with a reduced electrolysis current density, i.e., a reduced oxidation rate. The slow oxidation process improves the insulating effect of the Al2O3 layer, and enables the MIM cathodes to operate in the non-formed state. The fluctuation-free emission is reproducible when the diode voltage is cut off instantaneously. With a thin Al2O3 layer, the diode voltage reguired for the cathode operation is reduced to values slightly above the work function of the top electrode.

Increasing emission current from MIM cathodes by using an Ir-Pt-Au multilayer top electrode

We investigated the effect of the top electrode materials on the electron emission and durability of metal-insulator-metal (MIM) cathodes. The durability is improved when high sublimation-enthalpy material, such as Ir, Mo, or W, are used; however, the emission current, and the transfer ratio, decrease. The material dependence of the transfer ratio is shown to be dominated by the scattering probability of hot electrons in the metal. The scattering probability was estimated from the metals density-of-states, or more simply, from the number of d-electrons. We found that the multilayer top electrode consisting of Ir, Pt, and Au provides the best top electrode combination for MIM cathodes. The high sublimation-enthalpy Ir layer, which is in contact with the insulator, acts as a barrier metal and improves the durability, whereas the surface Au layer maintains the transfer ratio at a high value. With this top electrode structure, emission current density is increased to 5.8 mA/cm/sup 2/, which is sufficient for field-emission displays. We demonstrated a display consisting of a 30/spl times/30-dot MIM cathode-array with the multilayer top electrodes.

Highly efficient and long life metal–insulator–metal cathodes

The authors improved the emission efficiency and lifetime of metal–insulator–metal cathodes. The drift of the diode current was suppressed by using a thinner tunneling insulator and a lower diode voltage. The cathode with a 7.9-nm-thick tunneling insulator kept the diode current stable at 0.5 A/cm2 for more than 20 000 h, although the initial emission efficiency declined from 2% to less than 0.5%, and the emission current drift increased. The decreased emission efficiency could be enhanced to more than 3% by mixing CsHCO3 into an Au/Pt/Ir multilayer top electrode.

Field-emission display based on nonformed MIM-cathode array

We developed nonformed metal-insulator-metal (MIM) cathodes and demonstrated their suitability for field-emission displays (FEDs). The MIM cathodes have an emission current density of 5.8 mA/cm/sup 2/ at an operation voltage of 9 V. The cathodes are operated in the nonformed state, and they exhibit no fluctuation in the emission current, even without a ballast resistor. At a cathode-anode separation of 2 mm and an acceleration voltage of 4 kV, the divergence of the emitted electron beam is 25 /spl mu/m. These features make nonformed MIM cathodes suitable for high-voltage acceleration-type FEDs. We also fabricated a prototype 3.8-cm diagonal frit-sealed color display, and demonstrated its matrix-multiplexing operation.

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

The relationship between the thickness of the anodized Al/sub 2/O/sub 3/ 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/cm/sup 2/ was thus obtained. The existence of an optimal thickness for the anodized Al/sub 2/O/sub 3/ 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/sub 2/O/sub 3/ 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 Al/sub 2/O/sub 3/ insulator.

Reducing electron energy dispersion of nonformed metal–insulator–metal electron emitters using the near‐threshold drive method

The energy distribution of electrons emitted from an Al/Al2O3/Au metal–insulator–metal (MIM) electron emitter is measured. The thickness of the insulator is 5.5 nm. The energy distribution becomes narrower as the operating voltage Vd decreases since the low energy tail of the distribution is cut off by the potential barrier of the surface work function φ of the emitter. When the emitter is operated in the nonformed state, ΔE, the full width at half‐maximum of the distribution, is 0.32 eV for Vd=5.0 V, which is slightly above φ of Au (4.7 eV). As Vd increases, the high‐energy tail of the distribution broadens whereas the shape of the low‐energy tail remains unchanged. For a formed MIM emitter, ΔE becomes broader by 0.15–0.2 eV more than ΔE of a nonformed emitter at each Vd; thus, operation in the nonformed state is essential to obtain good monochromaticity. The spatial distribution of the work function in the emitter surface is also measured by the retarding potential method. The variation of φ, which limi...

Quantitative relationship between the work function and transfer ratio of a potassium-adsorbed MIM cathode

Abstract The emission current I e , diode current I d and work function Φ of a potassium adsorbed Al-Al 2 O 3 -Au MIM cathode were measured. When Φ was reduced to 2.5 eV by K adsorption, the transfer ratioα =I/ e I d increased to 0.32%. By evaporating K atoms gradually, the quantitative relationship between α and Φ was derived, and this was converted into the energy distribution of hot electrons in an energy range of 2.5 to 6 eV. By simulating the measured distribution and α, we found that the electron-electron inelastic scattering in the top metal determines the shape of the distribution, while the electron-phonon inelastic scattering in the insulator and the electron-phonon elastic scattering reduce α.

14.2: Novel Device Structure of MIM Cathode Array for Field Emission Displays

A new device structure of an MIM-cathode array with the passivation layer has been developed for FED. This design makes possible self-patterning of the top electrode. As the results, high emission current density of 80mA/cm2 was successfully obtained in the 1.5′(3.8-cm)-diameter, 60 × 60 pixels cathode array.

Large‐screen displays using metal—insulator—metal cathode arrays

— Large-screen (32-in. WXGA and 17-in. VGA) displays using metal—insulator—metal (MIM) cathode arrays have been developed. A cathode structure with low-resistance electrodes and low-capacitance emitters shortens signal delay and decreases the voltage drop in large MIM-cathode arrays. By using a dual-scan method, the signal delay was suppressed to less than 30% of the horizontal scan time in the 32-in. WXGA panel. Emission efficiency of the cathode array was improved to 3% by reducing the surface work function of the top electrode from 4.7 to 3.9 eV. The cathode life was also improved to more than 10,000 hours. The display panel incorporating the cathode arrays and high-efficiency P22-phosphor screens with 3-mm spacers showed high screen brightness (average brightness, 378 cd/m2; peak brightness, 832 cd/m2) at an anode voltage of 10 kV.

Characterization of the tunneling insulator in MIM cathodes by low-stress I-V measurement

The low-stress current-voltage (I-V) measurement system was developed and applied to characterization of the insulator in metal-insulator-metal (MIM) tunneling cathodes. The amount of the total charges injected into these devices during the I-V measurement decreases by four orders of magnitude compared to that for the conventional measurement system. The developed I-V measurement, therefore, can be considered to be a damage-free characterization method. The time-reciprocal dependence of the transient current demonstrates that the low-voltage leakage current (LVLC) of an MIM cathode is a good measure of the trap density in the tunneling insulator. The LVLC measurement was used for monitoring the plasma process-induced damage and long-term degradation of MIM cathodes. The charging damage during the sputter-deposition process was reduced to an acceptable level by modifying the sputtering machine and designing the device layout pattern properly according to antenna ratio. A long-term operation experiment showed that the low-stress I-V measurement can detect the latent degradation of insulator quality. The developed I-V measurement is thus considered to be a useful tool that will lead to improvements in device performance and lifetime.