Kazutaka Tsuji

Impact ionization process in amorphous selenium

Abstract Temperature and electric field dependences of impact ionization rates α (for electron) and β (for hole) in a-Se have been determined by photo-multiplication measurements on sandwich type photo-cells. It has been found that both α and β decrease with decreasing temperature, in contrast to those of crystalline semiconductors. The results obtained at room temperature have been fitted to those calculated by Baraffs theory taking the geminate recombination process into account. The observed temperature dependences suggest characteristic mechanisms of the impact ionization process in this material.

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.

Ultra-High-Sensitive Image Pickup Tubes Using Avalanche Multiplication in a-Se.

Extremely high-sensitive image pickup tubes with sensitivities 1000 times higher than those of conventional tubes are fabricated using the avalanche phenomenon in a-Se as photoconductive targets. The excess avalanche noise of a video signal is found to be much less than that expected, based on the carrier ionization rates. The frequency spectra of the noise currents of both the pickup tubes and sandwich-type photocells are examined. The results are compared with those of a simulation, and it is found that the excess noise can be reduced by the charge-storage operation of imaging devices.

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.

Excess Noise in Amorphous Selenium Avalanche Photodiodes

Excess noise in amorphous Selenium avalanche photodiodes (a-Se APD) has been measured in a frequency range from 3 kHz to 30 kHz. The deduced excess noise factors, including dependences on photocurrent, frequency, applied electric field and the a-Se layers thickness, agreed with McIntyres theoretical values.

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.

Ultrahigh-sensitivity new super-HARP camera

NHK led the world in developing a high-sensitivity Super-HARP pickup tube using the avalanche multiplication effect. The authors have now developed an improved version (2/3- inch, with electromagnetic focusing and electromagnetic deflection) which is eight times more sensitive and has much better lag characteristics. The handy New Super-HARP color camera which uses this newly-developed pickup tube has higher sensitivity than human vision (2000 lux, F/110 equivalent), negligible low lag, and a limiting resolution of over 700 TV lines. It will be a powerful tool in emergency news gathering at night, the production of scientific programs, and other applications.