Photocathode and electron tube

Details Photocathode and electron tube Photocathode and electron tube

2001-04-27

2003-05-13

Patel, Ashok (Department: 2879)

Electric lamp and discharge devices

Photosensitive

Having phosphor screen

C313S525000, C313S542000, C313S523000, C313S544000, C257S010000

Reexamination Certificate

active

06563264

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a photocathode (photoelectron-emitting surface) for emitting a photoelectron in response to a photon incident thereon, and an electron tube using the same. In particular, it relates to a photocathode for detecting light in an infrared region, and an electron tube using the same.
2. Related Background Art
Concerning photocathodes which have a sensitivity in a long wavelength region among those absorbing incident light, exciting a photoelectron, and emitting it, there are techniques such as those described in U.S. Pat. No. 3,958,143 (hereinafter referred to as literature 1), Japanese Pat. Application Laid-Open No. H04-269419 (literature 2), P. E. Gregory, et al., “Field-assisted photoemission to 2.1 microns from a Ag/&rgr;-In
0.77
Ga
0.23
As photocathode,”
Appl. Phys. Lett.
, 36(8), Apr. 15, 1980, pp. 639-640 (literature 3), Japanese Patent Application Laid-Open No. H08-255580 (literature 4), and Japanese Patent Application Laid-Open No. H05-234501 (literature 5).
The technique disclosed in literature 1 is one concerning a transferred electron type photocathode, in which a light-absorbing layer and an electron-emitting layer are stacked on a semiconductor substrate, a bias voltage is applied to both layers, so as to form an electric field within the light-absorbing layer, and photoelectrons are accelerated by this electric field, so as to be emitted into vacuum.
The technique disclosed in literature 2 is a transition electron type photocathode of this kind, in which a Schottky electrode for applying the electrode is formed like a pattern by use of a photolithography technique, so as to enhance the reproducibility of electron emission with respect to incidence of light.
Literature 3 describes results obtained when photoelectron emission was observed with respect to incident light having a wavelength up to 2.1 &mgr;m while using In
0.77
Ga
0.23
As as a light-absorbing layer in this kind of transition electron type photocathode.
The technique disclosed in literature 4 uses a p
junction in place of the Schottky electrode, so as to stabilize the interface state, thereby improving reproducibility.
The technique disclosed in literature 5 uses a multiple quantum well layer as the light-absorbing layer, so as to absorb light among sub-bands, thereby enhancing sensitivity up to a long wavelength region.
SUMMARY OF THE INVENTION
However, even with these techniques, photocathodes having a favorable sensitivity in the infrared region, a region on the longer wavelength side from a wavelength of 1.7 &mgr;m in particular, have not come into practice.
Specifically, according to literature 3 concerned with results of an experiment to which the techniques disclosed in literatures 1, 2 are applied, the photoelectron conversion efficiency upon observation of photoelectron emission up to a wavelength of 2.1 &mgr;m was 0.1%, which was very low, whereas the results of observation were obtained while the photocathode was held at a extremely-low temperature of 125 K.
The technique of literature 4 is problematic in that it is hard to keep lattice matching at a wavelength of 1.7 &mgr;m or longer and acquire reproducibility.
The technique of literature 5 is disadvantageous in that the light absorption among sub-bands yields a lower absorption efficiency than the light absorption among conventional band to band, or inter bands does, thereby resultantly yielding a low photoelectric conversion efficiency.
Hence, a photocathode having a favorable sensitivity in the infrared region, a high photoelectric conversion efficiency, and a favorable reproducibility have not come into practice.
Therefore, in view of the above-mentioned problem, it is an object of the present invention to provide a photocathode having a favorable sensitivity in the infrared region, a high photoelectric conversion efficiency, and a favorable reproducibility; and an electron tube utilizing the same.
For solving the above-mentioned problem, the photocathode of the present invention is a photocathode for emitting a photoelectron in response to light in an infrared region incident thereon, and is characterized in that it comprises (1) a substrate comprising InP of a first conduction type; (2) a buffer layer, formed on the substrate, comprising InAS
x2
P
1−x2
(0<x2<1) of the first conduction type lattice-matching the substrate; (3) a light-absorbing layer, formed on the buffer layer, comprising In
x1
Ga
1−x1
As (1>x1>0.53) of the first conduction type lattice-matching the buffer layer; (4) an electron-emitting layer, formed on the light-absorbing layer, comprising InAS
x3
P
1−x3
(0<x3<1) of the first conduction type lattice-matching the light-absorbing layer; (5) a contact layer, formed on the electron-emitting layer with a predetermined pattern so as to expose the electron-emitting layer with a substantially uniform distribution, comprising InAs
x3
P
1−3
of a second conduction type; (6) an active layer, formed on the exposed surface of the electron-emitting layer, comprising an alkali metal or an oxide or fluoride thereof; (7) a first electrode formed on the contact layer; and (8) a second electrode formed in the substrate.
As a consequence, the light-absorbing layer absorbs light having a wavelength of 2.1 &mgr;m or longer and generates a photoelectron. The buffer layer is disposed between the light-absorbing layer and substrate, and their interfaces are lattice-matched, so that a favorable interface state is achieved, whereby stable light absorption is effected. If a bias voltage is applied between the first and second electrodes, then an electric field is generated within the photocathode, whereas the generated photoelectron is accelerated by this electric field, so as to be released into vacuum by way of the electron-emitting layer. The electron-emitting layer and light-absorbing layer are also lattice-matched at their interface, which is maintained in a favorable state, whereby electrons smoothly reach the surface of light-absorbing layer. The electrons having reached the surface are rapidly emitted into vacuum by the active layer.
Preferably, the As composition ratio x2 of the buffer layer changes stepwise or continuously from the substrate side to the light-absorbing layer side. As a consequence, lattice mismatching is alleviated between the buffer layer and the substrate and light-absorbing layer.
Alternatively, the buffer layer may comprise a superlattice structure formed by stacking a plurality of thin films having As composition ratios x2 different from each other. The lattice mismatching between the buffer layer and the substrate and light-absorbing layer is alleviated in this case as well.
The electron tube of the present invention is characterized in that it is an electron tube constituted by encapsulating any of the above-mentioned photocathodes and an anode into a vacuum envelope.
The term “electron tube” used herein is an apparatus for detecting weak light by use of a photocathode, which encompasses not only photomultiplier tube (photo-tube), but also various kinds of apparatus such as streak tube (streak camera) and image tube. Utilizing the photocathode of the present invention provides an electron tube which favorably detects light in the infrared region.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.


REFERENCES:
patent: 39581

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