H. Mimura

Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP.

Photoconductive dipole antennas fabricated on semi-insulating (SI) GaAs and SI-InP were used to detect terahertz (THz) pulses. The responses of these long-carrier-lifetime photoconductive detectors were compared to that of the photoconductive antenna fabricated on a low-temperature grown GaAs (LT-GaAs) with a subpicosecond carrier lifetime. The SI-InP-based photoconductive detector showed a higher responsivity and a better signal-to-noise ratio (SNR) than the LT-GaAs-based photoconductive detector at low gating laser powers. The SI-GaAs-based detector, however, showed a responsivity comparable to that of the LT-GaAs photoconductive detector only at very weak gating laser power, and the SNR of the SI-GaAs-based detector was poor for overall gating laser powers due to the high background noise originating from a large amount of stray photocurrent.

Point x-ray source using graphite nanofibers and its application to x-ray radiography

A point x-ray emission was obtained from a diode configuration composed of a graphite-nanofiber cold cathode and a conical-shaped copper metal anode. When combined with a highly sensitive charge coupled device (CCD) camera with a scintillator, this x-ray source was then used to obtain x-ray transmission images of a tungsten mesh and an x-ray test chart. The spatial resolution of this x-ray radiography system was on the order of 10 μm and the acquisition time for these images was less than 10 s. This combination of the point x-ray source and the sensitive detection system offers a high-resolution x-ray radiography system.

Optically Induced Absorption in Porous Silicon and Its Application to Logic Gates

We have observed a large optically induced absorption in porous silicon. The figure of merit for third order optical nonlinearity is of the order of 10 -3 (esu. cm), a value as large as that of a photorefractive material such as BaTiO 3 . By exploiting this phenomenon, we have made simple demonstrations of optical hysteresis, parallel image processing, and all-optical logic gates. These results point to the possibility of expanding the applications of Si semiconductors from electronics to all-optical technologies.

Intense electron emission from graphite nanocraters and their application to time-resolved x-ray radiography

A high-density electron emission more than 20 mA/cm2 was achieved from a cold cathode fabricated by simple plasma etching nanocraters onto a graphite substrate. The magnitude of the field enhancement factor was about 30 000, which is similar to that for the highest reported performance for carbon nanotube. The performance of this cathode was demonstrated by obtaining high-intensity pulse x-ray generation. High-speed x-ray radiography images of dynamical processes of the order of 10 μs were successfully shown, thus offering a new technique for nondestructive inspections.

Photoresponse of a p-type Si field emitter

The high-frequency bunched-beam (pulsed beam train) generated directly from a cathode is strongly desired for a compact microwave vacuum tube with high efficiency. The emission current from a p-type semiconductor field emitter saturates in dark with increase of the gate voltage because of the supply limit of the minority carrier. When a p-type emitter is illuminated by light with photon energy larger than the band gap energy, the emission current is remarkably increased because of the additional supply of the photo-excited carriers. It suggests that the bunched beam could be obtained, when the p-type emitter is irradiated with a pulsed laser beam. The frequency of the bunched beam, however, is limited by the photo-response of the emitter. Therefore, we investigated the photo-response of a p-type Si field emitter under the irradiation of a pulsed laser beam.

The density of states in silicon nanostructures determined by space-charge-limited current measurements

Space-charge-limited current (SCLC) flow was investigated as a function of applied potential and specimen thickness in nanocrystalline silicon films prepared by electrochemical anodization. From the analysis of the current–voltage (J–V) characteristics in the SCLC regime, the density of states distribution near the Fermi level was determined. The agreement between the experimental J–V characteristics and the theoretical curve strongly implies that the current flow is entirely controlled by localized states situated at the quasi-Fermi level.

Selective growth of carbon nanotubes on Si microfabricated tips and application for electron field emitters

In this article, we present the results on the synthesis and electron emission characteristics of an individual and a carbon nanotube (CNT) bundle on a Si tip. The Si tip with diameter of about 20 nm at the apex and the tip height of 5 μm was fabricated using Si microfabrication process. Individual and carbon nanotubes bundle were selectively grown at the apex of the Si tip using a hot-filament chemical vapor deposition with a mixture of C2H2 and H2 gases under a negative substrate biasing. Electron field emission characteristics of the Si tips with and without the individual CNT on the same substrate were measured in a vacuum of 1.7×10−4 Pa. Threshold or turn on voltages defined at the emission current of 10 pA of approximately 40 V (4 V/μm) and 200 V (20 V/μm) were found for the Si tips with and without the individual CNT, respectively. Emission current and emission light pattern of the emitted electrons from the carbon nanotubes bundle were also studied.

Electron emission from porous silicon planar emitters

Porous silicon planar emitters were fabricated by depositing a thin Au film on a conventional porous Si and their emission characteristics were examined. The emission currents and energy distributions were measured for the emitters with various Au thicknesses and for cesiated ones. The experimental results suggest that the emission mechanism of the porous silicon emitter studied in this work is conventional field emission, in which electrons are emitted from nanocrystals in the porous silicon directly into a vacuum.

Field emission from porous (100) GaP with modified morphology

Morphology of n-type porous GaP samples with (100) crystal orientation was modified by postchemical etching for field-emitter application. The structures produced have uniform size and shape distribution and are found to show good emission characteristics. The mechanism of the morphology modification is explained, and a strong correlation between field-emission characteristics and postchemical etching time is shown.

Deep level energy states in porous silicon and porous silicon carbide determined by space-charge-limited current measurements

Abstract We determined the density of states distribution near the Fermi level both for porous silicon (PS) and for porous silicon carbide (PSC) from the analysis of the current–voltage ( J – V ) characteristics in the space-charge-limited current (SCLC) regime. The distribution of deep level energy states of PS can be described by a stretched exponential function, whereas that of PSC exhibits a simple exponential shape. Theoretical analysis well explains the J – V characteristics of both porous materials, which suggests that the current flow is entirely controlled by localized states situated at the quasi-Fermi level.