JP2007080799A - Photo cathode and electron tube - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a photo cathode, composed of a compound semiconductor, in which a wide dynamic range is realized and a gate operation at high speed is made possible, as well as an electron tube. <P>SOLUTION: The photo cathode 10 which can be applied for the electron tube is provided with a substrate 11, a light absorbing layer 14 formed on the substrate 11, composed of a compound semiconductor, and emitting photo electron from a photoemission surface 14E corresponding to light incidence, and an electrode layer 15 having an opening 15H formed on the photoemission surface 14E so that a part from the substrate 11 through the light absorbing layer 14 can have the same electric potential, and exposing the photoemission surface 14E. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a photocathode including a light absorption layer composed of a compound semiconductor and an electron tube using the photocathode.

In the case of a so-called field assist type photocathode used for an electron tube such as a photomultiplier tube, it is known that an electrode is formed on the photoelectron emission surface in order to generate an electric field inside the photocathode (Patent Literature). 1 and 2). In addition to the field assist type, it is known that an electrode is provided on the periphery of the light absorption layer (active layer) in order to establish electrical connection with the outside of the photocathode (see Patent Document 3).
US Pat. No. 6,121,612 JP-A-8-255580 JP-A-9-213206

  In an electron tube such as a photomultiplier tube or an image intensifier tube to which a photocathode is applied, so-called gate operation is performed by changing the potential between the anode and the cathode in order to block excessive input light and analyze high-speed phenomena. It is known. However, depending on the compound semiconductor used for the light absorption layer of the photocathode, there has been a problem that gate operation at high speed becomes difficult. In particular, when a nitride semiconductor is used for the light absorption layer, this problem appears more markedly than when other compound semiconductors (for example, GaAs, GaAsP, or InGaAs) are used. In some cases, the gate operation itself could not be performed.

  As a result of intensive studies on this problem, the present inventor has paid attention to the fact that the surface resistance varies depending on the material of the light absorption layer, and the reason why the gate operation becomes difficult is the low sensitivity characteristic due to the size of the surface resistance of the photocathode. It came to know that it was in (linearity). The sheet resistance varies greatly depending on the carrier concentration and thickness of the light absorption layer of the photocathode. For example, although it belongs to a technical field different from the technical field to which the photocathode belongs, a photoelectron mobility transistor (HEMT) which is one of electronic devices includes a GaN layer having a thickness of 1 μm and a surface resistance of 360Ω. / Sq. A device has been reported. However, it was found that when the compound semiconductor is a light absorption layer, the surface resistance of the photocathode may be 1 kΩ or more, which hinders the gate operation. In particular, when the surface resistance value of the photocathode is 10 kΩ or more, it has been found that high-speed gate operation is extremely difficult.

  Therefore, an object of the present invention is to provide a photocathode and an electron tube that are capable of high-speed gate operation even with a photocathode having high surface resistance, based on the above findings.

  The photocathode according to the present invention includes (1) a substrate, (2) a light absorption layer formed on the substrate, made of a compound semiconductor, and emitting photoelectrons from a photoelectron emission surface in response to light incidence; And an electrode layer formed on the photoelectron emission surface and having an opening exposing the photoelectron emission surface so that a portion from the substrate to the light absorption layer has the same potential.

  According to the photocathode of the present invention, the electrode layer is formed on the photoelectron emission surface so that the portion from the substrate to the light absorption layer has the same potential. By comprising in this way, an electric charge is supplied to the whole photocathode and the characteristic which an electron emission surface has is homogenized. Therefore, even in the case of a photocathode using a compound semiconductor with high surface resistance, the entire electron emission surface can be effectively utilized to emit photoelectrons according to the incident light intensity even when the incident light is strong. , Sensitivity characteristics are improved. The photocathode can also be applied to high-speed gate operation in which the potential changes in a short time. Note that the effect of this improvement in sensitivity characteristics becomes significant in the photocathode having a large area of the photoelectron emission surface. In addition, according to the photocathode of the present invention, since the characteristics of the electron emission surface are homogenized, the rise when a high voltage is applied to the photocathode is improved, so a wide dynamic range is realized. Is done.

  The photocathode of the present invention comprises (1) a substrate having a light incident surface, and (2) a photoelectron that is formed on the substrate and emits photoelectrons in response to light passing through the light incident surface, and is composed of a compound semiconductor. And (3) an electrode layer that is formed with a pattern including a plurality of openings on the light absorption layer and has a portion that extends to contact a part of the substrate.

  According to the photocathode of the present invention, the electrode layer is formed on the light absorption layer with a pattern including a plurality of openings, and has a portion extending until it contacts a part of the substrate. By comprising in this way, an electric charge is supplied to the whole photocathode. In addition, the characteristics of the “electron emission surface” as the surface from which the light absorption layer emits electrons can be homogenized. Therefore, in the state before the electrode layer is formed, 1 kΩ / sq. Even with a photocathode having a high surface resistance as described above, good sensitivity characteristics can be realized. In particular, 10 kΩ / sq. In a state before the electrode layer is formed. Even a photocathode having an extremely high sheet resistance as described above can realize a high-speed gate operation.

  Further, the electrode layer may have a portion that extends until it contacts a part of the substrate via the side portion of the light absorption layer. By comprising in this way, an electric charge can be supplied more uniformly with respect to a photocathode.

  Further, the pattern including a plurality of openings may be a pattern including a portion where each opening is provided at a substantially constant interval. By comprising in this way, an electric charge can be supplied more uniformly with respect to a photocathode.

  The photocathode may further include a window layer interposed between the light absorption layer and the substrate, made of an AlGaAs-based material, and having a wider energy band gap than the light absorption layer. With such a configuration, lattice mismatch between the light absorption layer and the substrate can be reduced. In addition, a window layer made of an AlGaAs material can act as a barrier against photoelectrons.

  The photocathode may further include an antireflection film interposed between the substrate and the window layer. By interposing the antireflection film, the reflectance of a desired wavelength is reduced for the light incident on the light absorption layer, and the efficiency of emitting photoelectrons can be increased.

  In the photocathode, the compound semiconductor constituting the light absorption layer may be a nitride semiconductor. With this configuration, by using a nitride semiconductor with extremely high surface resistance for the light absorption layer, 1 kΩ / sq. In the state before the electrode layer is formed. Even a photocathode having such a high sheet resistance can achieve a wide dynamic. Further, in the state before the electrode layer is formed, 10 kΩ / sq. Even with a photocathode having an extremely high surface resistance as described above, a photocathode capable of high-speed gate operation can be obtained.

  Further, when the compound semiconductor constituting the light absorption layer is a nitride semiconductor, the compound semiconductor is interposed between the light absorption layer and the substrate, is composed of the nitride semiconductor, and has an energy band gap larger than that of the light absorption layer. A wide buffer layer may be further provided. With such a configuration, lattice mismatch between the light absorption layer and the substrate can be reduced. In addition, a buffer layer having a wider energy band gap than the light absorption layer can act as a barrier for photoelectrons.

  The electron tube of the present invention includes the photocathode, an anode that collects photoelectrons emitted from the photocathode, and a vacuum container that houses the photocathode and the anode. The electron tube may further include voltage applying means for applying a pulsed voltage between the photocathode and the anode.

  According to the electron tube of the present invention, the electrode layer is formed on the photoelectron emission surface so that the portion of the photocathode from the substrate to the light absorption layer has the same potential. Therefore, even when a compound semiconductor with high surface resistance is used for the photocathode, the charge characteristic is supplied to the entire photocathode, thereby improving the sensitivity characteristic of the photocathode and enabling high-speed gate operation. . Further, the homogenization of the characteristics of the photoelectron emission surface is improved, so that the rise when a high voltage is applied to the photocathode is improved, so that a wide dynamic range is realized.

  According to the present invention, the entire electron emission surface is effectively utilized by forming the electrode layer on the photoelectron emission surface so that the region from the photocathode substrate to the light absorption layer has the same potential. Even when the incident light is strong, photoelectrons corresponding to the incident light intensity can be emitted, and a photocathode and an electron tube capable of high-speed gate operation can be obtained.

  Hereinafter, a photocathode and an electron tube according to embodiments of the present invention will be described with reference to the accompanying drawings. In addition, the same code | symbol is used for the same element and the overlapping description is abbreviate | omitted.

(First embodiment)
FIG. 1 is a plan view of the photocathode 10 according to the first embodiment, and FIG. 2 is a sectional view taken along line II-II in FIG.

  The photocathode 10 includes a substrate 11, an antireflection film 12, a window layer 13, a light absorption layer 14, and an electrode layer 15. The light absorption layer 14 has a photoelectron emission surface 14E that emits photoelectrons in response to the incidence of light. Further, on the photoelectron emission surface 14E side, an active layer 16 is formed on the photoelectron emission surface 14E exposed by the opening 15H and the electrode layer 15.

  The substrate 11 is a plate-like member having a light incident surface on one surface. Further, it is a portion that becomes an incident window of the photocathode 10 and is made of a material (for example, borosilicate glass) having a thermal expansion coefficient close to that of GaAs constituting the light absorption layer 14. The thickness of the substrate 11 is not particularly limited. However, when the photocathode according to the present embodiment is applied to an electron tube such as an image intensifier tube, the substrate 11 becomes a part of the vacuum container, so that the mechanical strength is maintained. The thickness can be determined.

The antireflection film 12 is interposed between the substrate 11 and the window layer 13 and includes a first antireflection layer 12A and a second antireflection layer 12B. The antireflection layers 12A and 12B are made of materials such as SiO ₂ and Si ₃ N ₄ , and the materials used for the respective layers are selected and the thicknesses are adjusted so that the reflectance at a desired wavelength is minimized. Yes.

The window layer 13 is interposed between the light absorption layer 14 and the substrate 11 and is formed on the surface of the second antireflection layer 12B of the antireflection film 12. The window layer 13 is made of an AlGaAs material and has an energy band gap wider than that of the light absorption layer 14. As the AlGaAs-based material used for the window layer 13, a material expressed by a composition formula Al _x Ga _(1-x) As (0 <x ≦ 1) can be used. The thickness of the window layer 13 is preferably less than 10 μm, and more preferably 30 nm to 5 μm. This is because when the thickness is less than 30 nm, the window layer 13 does not act as a barrier against photoelectrons, and when the thickness exceeds 5 μm, the growth required for epitaxial growth to form the window layer 13 is required. This is because it is not practical in terms of time. The window layer 13 is preferably doped p-type.

The light absorption layer 14 is a compound semiconductor layer that emits photoelectrons from the photoelectron emission surface 14 </ b> E according to the light hν that has passed through the light incident surface of the substrate 11. The light absorption layer 14 is made of GaAs, for example, and is doped p-type in a range where the doping concentration (atomic concentration) is 10 ¹⁷ cm ⁻³ or more and 10 ²⁰ cm ⁻³ or less. The carrier concentration is more preferably in the range of 10 ¹⁸ or more and 3 × 10 ¹⁹ cm ⁻³ or less. Further, the thickness of the light absorption layer 14 is preferably 0.5 μm or more and 5 μm or less, and more preferably 1 μm or more and 2.5 μm or less. This thickness can be determined by the photoelectron diffusion length of the light absorption layer 14 and the light absorption rate.

  The electrode layer 15 is formed so that the part from the substrate 11 to the light absorption layer 14 has the same potential. Specifically, as shown in FIG. 1, on the photoelectron emission surface 14E that is the upper surface of the photocathode 10, the electrode layer 15 is formed in a stripe shape, and an opening 15H that exposes a part of the photoelectron emission surface 14E is formed. The shape has. In other words, the light absorption layer 14 is formed with a pattern including a plurality of openings 15H.

  As shown in FIG. 2, the electrode layer 15 is formed so as to cover the side surfaces of the substrate 11, the antireflection film 12, the window layer 13, and the light absorption layer 14. In other words, it has a portion that extends through a side portion of the light absorption layer 14 until it contacts a part of the substrate 11.

  As a material for the electrode layer 15, it is preferable to use a material that can make ohmic contact with the light absorption layer 14 that is a semiconductor layer. The ohmic contact here does not need to be an ideal ohmic contact, but may be anything that can be called an ohmic contact. As such a material, refractory metals such as tungsten, tantalum, chromium, and titanium and alloys thereof such as silicide and carbide can be preferably used. In addition, when GaAs is used for the light absorption layer 14, it is preferable to use chromium as the material of the electrode layer 15. Further, the thickness of the electrode layer 15 is preferably 10 μm or less, and more preferably 3 nm or more and 1 μm or less. When the electrode layer 15 is formed by vapor deposition and photolithography using the above-described chromium, if the thickness is less than 3 nm, the formed thin film does not have sufficient conductivity as an electrode, and the thickness exceeds 10 μm. In this case, it becomes impractical from the viewpoint of the deposition rate.

  The line width of the electrode layer 15 is preferably 50 nm or more and less than 100 μm, and more preferably 100 nm or more and less than 10 μm. The interval is preferably 50 nm or more and less than 1000 μm, and more preferably 100 nm or more and less than 200 μm. By setting it as such a range, even if it does not use special apparatuses, such as an electron beam lithography, it can form with sufficient reproducibility by general optical lithography.

  The active layer 16 is a layer formed in order to lower the work function necessary for the emission of photoelectrons, and has a thickness of about 0.5 monomolecular layer. CsO can be used as the material of the active layer 16, but K, Rb, Na, or the like may be used as the alkali metal in addition to Cs, and F or the like may be used instead of O.

  Next, a method for manufacturing the photocathode according to the first embodiment will be described. Note that the photocathode according to the present embodiment can be manufactured by the same method as the transmission type photocathode described in Patent Document 3.

  First, a semiconductor substrate made of GaAs is prepared. Next, an etching stop layer (not shown) made of AlGaAs, a light absorption layer 14 made of GaAs, and a window layer 13 made of AlGaAs are sequentially deposited thereon using an epitaxial growth apparatus (not shown). A semiconductor multilayer film having the above is manufactured.

After forming the semiconductor multilayer film, the second antireflection layer 12B (for example, Si ₃ N ₄ ) and the first antireflection film are formed on the upper surface of the window layer 13 with a film thickness corresponding to the wavelength of the detected light by using the CVD method. A layer 12A (eg, SiO ₂ ) is sequentially deposited to form the antireflection film 12.

  The formed antireflection film 12 is thermocompression bonded to the substrate 11. At this time, the material of the substrate 11 is relatively close to the thermal expansion coefficient of the light absorption layer 14 made of GaAs and has a lower transition temperature than the constituent material of the antireflection film 12, such as Corning 7056 glass. Is used.

  After the substrate 11 is thermocompression bonded to the antireflection film 12, if the semiconductor substrate is selectively etched using an ammonia-based solution, the etching removal is automatically stopped when the etching stop layer is exposed.

  When etching is performed on the exposed etching stop layer using a hydrochloric acid solution, the etching automatically stops in the light absorption layer 14 and the surface of the light absorption layer 14 is exposed. Then, by using a predetermined mask, chromium is vapor-deposited as the electrode layer 15 having an opening 15H exposing a part of the photoelectron emission surface 14E of the light absorption layer 14, as shown in FIGS.

Finally, Cs and O ₂ are deposited on the photoelectron emission surface 14E of the light absorption layer 14 exposed through the opening 15H of the electrode layer 15. Thereby, the photocathode 10 shown in FIG.1 and FIG.2 is obtained.

  According to the photocathode 10 according to the first embodiment described above, the electrode layer is formed on the photoelectron emission surface 14E so that the portion from the substrate 11 to the light absorption layer 14 has the same potential. With this configuration, charges are supplied to the entire photocathode 10 and the characteristics of the photoelectron emission surface 14E are homogenized. Therefore, in the state before the electrode layer is formed, 1 kΩ / sq. Even with a photocathode having a high surface resistance value as described above, the entire photoelectron emission surface 14E can be effectively utilized to emit photoelectrons corresponding to the intensity of incident light even with strong incident light. Sensitivity characteristics are improved. Furthermore, 10 kΩ / sq. In the state before the electrode layer is formed. Even a photocathode having an extremely high surface resistance as described above can be operated at a high speed. In addition, since the characteristics of the photoelectron emission surface 14E are homogenized, the rise when a high voltage is applied to the photocathode 10 is improved, so that a wide dynamic range is realized.

  The photocathode 10 is interposed between the light absorption layer 14 and the substrate 11, is composed of an AlGaAs-based material, and includes a window layer 13 having an energy band gap wider than that of the light absorption layer. With this configuration, not only the lattice mismatch between the light absorption layer 14 and the substrate 11 can be reduced, but also the window layer 13 can act as a wall against photoelectrons.

  In addition, since the anti-reflection film 12 is interposed between the substrate 11 and the window layer 13 in the photocathode 10, the reflectance at a desired wavelength is reduced with respect to the light incident on the light absorption layer 14, and photoelectrons are emitted. Efficiency can be increased.

(Second Embodiment)
FIG. 3 is a cross-sectional view of the photocathode 20 according to the second embodiment along the line II-II in FIG. In addition, since the top view of the photocathode 20 becomes a figure similar to FIG. 1, it abbreviate | omits by attaching | subjecting the code | symbol corresponding to a corresponding element in FIG.

  The photocathode 20 includes a substrate 21, a buffer layer 22, a light absorption layer 23, and an electrode layer 24 having an opening 24H. The light absorption layer 23 has a photoelectron emission surface 23E that emits photoelectrons in response to the incidence of light. On the photoelectron emission surface 23E side, an active layer 25 is formed on the photoelectron emission surface 23E and the electrode layer 24 exposed by the opening 24H. The nitride semiconductor in the present embodiment is a nitride semiconductor containing at least one of In, Ga, and Al as a group III element and at least N as a group V element, or a mixed crystal thereof, and a stacked layer thereof. Means a semiconductor structure consisting of

  The substrate 21 is a plate-like member having a light incident surface on one surface, and may be any material that is transparent to the wavelength of the light to be detected and can epitaxially grow a nitride semiconductor of good quality. An example of such a material is sapphire. In addition to sapphire, single crystal substrates such as AlN and AlGaN, oxide substrates such as ZnO, LAO, and LGO can be used. Further, the material of the substrate 21 may be a combination of these materials.

  The buffer layer 22 is interposed between the substrate 21 and the light absorption layer 23, is made of a nitride semiconductor, and has an energy band gap wider than that of the light absorption layer 23. Since the buffer layer 22 is a layer for relaxing the lattice mismatch with the light absorption layer 23, the buffer layer 22 is not necessarily a single layer and may be a multi-layer. The film thickness is not particularly limited, and the shape may be flat or may have a concavo-convex structure.

The light absorption layer 23 is made of a nitride semiconductor and emits photoelectrons from the photoelectron emission surface 23E according to the light hν that has passed through the light incident surface of the substrate 21. For example, (Al) GaN can be used as the nitride semiconductor used for the light absorption layer 23, but a desired composition (In, Ga, Al) N mixed crystal may be selected according to the wavelength of the light to be detected. . The film thickness of the light absorption layer 23 can be set to, for example, 100 nm as the diffusion length of photoelectrons. However, the diffusion length depends on the crystallinity of the light absorption layer, and is selected in the range of 50 nm to 500 nm. Good. The light absorption layer 23 is preferably doped p-type, and the doping concentration (atomic concentration) is preferably in the range of 10 ¹⁷ cm ^{−3 to} 10 ²⁰ cm ⁻³ .

  The electrode layer 24 is formed so that the part from the substrate 21 to the light absorption layer 23 has the same potential. Specifically, as shown in FIG. 1, on the photoelectron emission surface 23E of the photocathode 20, the electrode layer 24 is formed in a stripe shape, and has a shape having an opening 24H exposing a part of the photoelectron emission surface 23E. Has been. In this embodiment, as shown in FIG. 3, the electrode layer 24 is not formed so as to cover the entire side surface of the photocathode as in the first embodiment, and the substrate 21 has an electrode layer on the side surface. Without being formed, the electrode layer 24 is formed only on the end face of the portion wider than the width of the buffer layer 22 and the light absorption layer 23.

  The shape of the electrode layer 24 can be, for example, a thickness of 500 nm, a line width of 2 μm, and an interval of 100 μm. The shape of the electrode layer 24 is not particularly limited. However, if the thickness is too thin, the resistance of the electrode is increased, and if the thickness is too thick, the electrode layer 24 is easily peeled off. If the line width is too thick, the ratio of the surface area of the opening 24H to the surface area of the photoelectron emission surface 23E (aperture ratio) is reduced, and photoelectrons cannot be emitted sufficiently. The spacing depends on the resistivity of the nitride semiconductor layer, but if it is too wide, the entire photoelectron emission surface 23E cannot be used effectively, and a high dynamic range and high-speed gate operation cannot be achieved. The rate decreases and photoelectrons cannot be emitted sufficiently.

  The thickness of the electrode layer 24 is preferably 10 μm or less, and more preferably 3 nm or more and 1 μm or less. The line width is preferably 50 nm or more and less than 100 μm, and more preferably 100 nm or more and less than 10 μm. The interval is preferably 50 nm or more and less than 1000 μm, and more preferably 100 nm or more and less than 200 μm. By setting it as such a range, even if it does not use special apparatuses, such as an electron beam lithography, it can form with sufficient reproducibility by general optical lithography and vapor deposition.

  As the material of the electrode layer 24, it is preferable to use a material that can make ohmic contact with the light absorption layer 23, which is a semiconductor layer, as in the first embodiment. As such a material, for example, Cr can be used, but there is no particular limitation as long as the material has a low resistance. However, it is preferable to select a material that hardly reacts with an alkali metal such as Cs.

  Further, when the light absorption layer 23 is a nitride semiconductor, a refractory metal such as tungsten or tantalum or an alloy thereof such as silicide or carbide is preferable. This is because a photocathode made of a nitride semiconductor requires a higher temperature than a photocathode made of GaAs or the like in the surface cleaning step before forming the active layer 25 as will be described later.

  The active layer 25 is a layer formed in order to lower the work function necessary for the emission of photoelectrons, like the active layer 16 of the first embodiment, and the same material as that of the first embodiment can be selected. it can.

  Next, a case where sapphire is used for the substrate 21 and tungsten (W) is used for the electrode layer 24 will be described for the method for manufacturing the photocathode according to the second embodiment.

  First, a substrate 21 made of sapphire is prepared, and a buffer layer 22 and a light absorption layer 23 are sequentially deposited thereon using an epitaxial growth apparatus (not shown). Next, W is deposited on the photoelectron emission surface 23 </ b> E of the light absorption layer 23, the side surfaces of the buffer layer 22 and the light absorption layer 23, and the end surface of the substrate 21.

Next, after applying a photoresist to the surface of W formed on the photoelectron emission surface 23E and performing a predetermined heat treatment, a stripe-shaped photomask (in the above example, a line width of 2 μm and an interval of 100 μm) is formed. Used for exposure and development. The W is processed into a predetermined shape by etching, and then the photoresist is removed to form the electrode layer 24 having a predetermined shape on the photoelectron emission surface 23E of the light absorption layer 23. Finally, Cs and O ₂ are deposited on the photoelectron emission surface 23E of the light absorption layer 23 exposed through the opening 24H of the electrode layer 24 and the electrode layer 24 formed on the photoelectron emission surface 23E. By forming the active layer 25 in this way, the work function on the surface of the photoelectron emission surface 23E is lowered. Thereby, the photocathode 20 shown in FIG. 3 is obtained.

  According to the photocathode 20 according to the second embodiment described above, the electrode layer 24 is formed on the photoelectron emission surface 23E so that the portion from the substrate 21 to the light absorption layer 23 has the same potential. With this configuration, charges are supplied to the entire photocathode 10 and the characteristics of the photoelectron emission surface 23E are homogenized. Therefore, by using a nitride semiconductor that causes the surface resistance of the photocathode to be increased in the light absorption layer 23, the state before the electrode layer is formed is 1 kΩ / sq. Even with a photocathode having a high surface resistance as described above, the entire photoelectron emission surface 23E can be effectively utilized to emit photoelectrons corresponding to the intensity of incident light even with strong incident light. Sensitivity characteristics are improved. Furthermore, by using a nitride semiconductor for the light absorption layer 23, 10 kΩ / sq. In the state before the electrode layer is formed. Even a photocathode having an extremely high surface resistance as described above can be operated at a high speed. In addition, since the characteristics of the photoelectron emission surface 23E are homogenized, the rise when a high voltage is applied to the photocathode 20 is improved, so that a wide dynamic range is realized.

  In addition, since the buffer layer 22 having a wider energy band gap than the light absorption layer 23 is interposed between the light absorption layer 23 and the substrate 21, the light absorption layer and the substrate Can be relaxed. In addition, a buffer layer having a wider energy band gap than the light absorption layer can act as a barrier for photoelectrons.

(Image intensifier tube)
Next, an image intensifier tube will be described as an electron tube to which the photocathode described in the above embodiment is applied.

  FIG. 4 is a schematic cross-sectional view of an image intensifier tube 100 including the photocathode 20 according to the first embodiment as the photocathode. The image intensifier tube 100 includes a photocathode 20, a microchannel plate (MCP) 101, a phosphor 102, and a glass plate 103, and each component is housed in a vacuum vessel 110. The photocathode 20, the MCP 101, and the phosphor 102 are provided with a plurality of electrodes 121, 122, 123, and 124 for applying a desired potential. The photocathode 20 and the vacuum vessel 110 are made of In or the like. It is sealed with a seal portion 104.

The image intensifier tube 100 is manufactured by the following procedure. First, in the vacuum device (not shown), together with the photocathode 10, the side tube portion of the vacuum vessel 110 to which the MCP 101, the phosphor 102, and the electrodes 121, 122, 123, 124 are attached is installed. The photocathode 20 installed in the vacuum apparatus is heated to about 800 ° C. after being evacuated and subjected to surface cleaning treatment. At this time, the active layer 25 is not formed in the photocathode 20, but after performing the surface cleaning treatment, the vacuum apparatus is cooled to room temperature, and Cs and O ₂ are introduced, so that An active layer 25 is formed on the surface of the cathode 20. Then, in the vacuum apparatus, the photocathode 20 is attached to the side tube portion of the vacuum vessel 110 that has been installed in advance, and the image intensifier tube 100 is manufactured by sealing with In at the seal portion 104.

  Next, the operation of the image intensifier tube 100 will be described with reference to FIGS. When the light to be detected hν is incident on the substrate 21 of the photocathode 20 serving as the entrance window of the image intensifier tube 100, the light to be detected hν passes through the substrate 21 and the buffer layer 22 and is absorbed by the light absorption layer 23 to excite photoelectrons. Is done. The excited photoelectrons are diffused in the light absorption layer 23 and then emitted into the vacuum from the photoelectron emission surface 23E of the light absorption layer 23 exposed by the opening 24H of the electrode layer 24 through the active layer 25. The photoelectrons emitted into the vacuum enter the phosphor 102 and emit light. Light emitted from the phosphor 102 is extracted outside the image intensifier tube 100 through the glass plate 103.

  4 shows the case where the photocathode 20 according to the second embodiment is applied as the photocathode, the photocathode 10 according to the first embodiment may be applied instead of the photocathode 20.

  In the image intensifier tube, in order to perform image sampling, a gate operation is performed in which a short pulse voltage is applied to the electrode by changing the potential of the electrode in the image intensifier tube. Next, the configuration and operation for performing the gate operation of the image intensifier tube will be described.

FIG. 5 is a schematic diagram showing the configuration of a voltage application circuit (voltage application means) that applies a pulsed voltage to the anode. The voltage application circuit 150 shown in FIG. 5 includes a power supply unit 151, a switch 152, an ammeter 153, a control unit 154, and resistors R1, R2, and R3. The image intensifier tube 100 shown in FIG. The electrode is electrically connected. V _K , V ₋ , V ₊ , and _VA shown in FIG. 5 indicate the potential of each part, and electrodes 121, 122, 123, and 124 to which V _K , V ₋ , V ₊ , and _VA are attached in FIG. 4. And the circuit current is measured by an ammeter 153.

When an operation trigger (in FIG. 5, an arrow directed to the control unit 154 from the outside) is input to the voltage application circuit 150 from the outside, the switch 152 is turned on and the power supply unit 151 is turned on. Is operated, and potentials V _K , V ₋ , V ₊ , and _VA are applied to the electrodes 121, 122, 123, and 124 of the image intensifier tube 100, respectively. At this time, the power supply unit 151 changes the potential between the photocathode 20 and the MCP 101 by applying a pulsed voltage to the photocathode 20. When the potential of the MCP 101 is higher than the potential of the photocathode 20, the gate operation is turned on, and the photoelectron image converted by the photocathode 20 is attracted to the MCP 101 having a higher potential and multiplied. It is guided to the body 102 and output as an optical image. On the other hand, when the potential of the MCP 101 is lower than the potential of the photocathode 20, the gate operation is turned off, and the photoelectron image converted by the photocathode 20 is repelled from the low potential MCP 101 and blocked.

  Here, the gate operation performed on the image intensifier tube has been described as an example. However, an electron tube such as a photomultiplier tube can be gated using a similar voltage application circuit.

(Photomultiplier tube)
Next, a photomultiplier tube will be described as an electron tube to which the photocathode described in the above embodiment is applied.

  FIG. 6 is a schematic cross-sectional view of a photomultiplier tube 200 including the photocathode 10 according to the first embodiment as the photocathode. The photomultiplier tube 200 includes a photocathode 10, an electron multiplier 201, and a readout electrode (anode) 202, and each component is housed in a vacuum vessel 210 made of metal. Furthermore, the electron multiplier 201 and the vacuum vessel 210 are provided with a plurality of electrodes 222, 223, and 224 for applying a desired potential. Further, also for the photomultiplier tube 200, as in the case of the image intensifier tube 100 described above, by attaching the photomultiplier tube 200 to the voltage application circuit 150 shown in FIG. 5 and applying a pulsed voltage, The photomultiplier tube 200 can be gated.

  Although FIG. 6 shows the case where the photocathode 10 according to the first embodiment is applied as the photocathode, the photocathode 20 according to the second embodiment may be applied instead of the photocathode 10.

  So far, the image intensifier tube and the photomultiplier tube have been described as examples of the electron tube, but the electron tube to which the photocathode of the present invention is applied is not limited thereto. Specifically, the photocathode of the present invention is also applied to a so-called bipolar tube type that does not include an electron multiplier, or a point sensor having a secondary electron multiplier such as a general photomultiplier. It is possible to apply. Further, the present invention can also be applied to a so-called electron-injection type electron tube in which a photo diode is accelerated and applied by applying a high voltage to the anode using a photodiode or a charge coupled device (CCD).

  The electron tubes such as the image amplifying tube 100 and the photomultiplier tube 200 described above include the photocathodes 10 and 20 according to the first or second embodiment. In the photocathodes 10 and 20, the light absorption layers are formed from the substrates 11 and 21. Electrode layers 15 and 24 are formed on the photoelectron emission surfaces 14E and 23E so that portions reaching 14 and 23 have the same potential. With this configuration, even in the case where a compound semiconductor having a high surface resistance is used for the light absorption layers 14 and 23 as in the case of the photocathode according to the first or second embodiment, Sensitivity characteristics are improved and high-speed gate operation becomes possible. Furthermore, since the characteristics of the photoelectron emission surfaces 14E and 23E are homogenized, the rise when a high voltage is applied to the photocathodes 10 and 20 is improved, so that a wide dynamic range is realized.

  Note that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the present invention.

For example, a material having the following characteristics can be selected as the compound semiconductor constituting the light absorption layer of the photocathode.
GaAs (first embodiment): having sensitivity from ultraviolet to a wavelength near 900 nm. There is no problem with ordinary weak light detection. However, in the high-speed gate operation, when the effective diameter is large, the sensitivity is lowered at the center and the dynamic range is lowered.
GaAsP: has a higher sensitivity than GaAs in the vicinity of a wavelength of 500 to 600 nm. Similar to GaAs, in the high-speed gate operation, when the effective diameter is large, the sensitivity is lowered at the center and the dynamic range is lowered.
InGaAs: Sensitivity on the longer wavelength side than GaAs. Similar to GaAs, in the high-speed gate operation, when the effective diameter is large, the sensitivity is lowered at the center and the dynamic range is lowered.
(Al) GaN (second embodiment): High sensitivity in the ultraviolet region. Although doped in p-type, the resistivity is much higher than that of GaAs. For this reason, since the dynamic range is a problem even in normal weak light detection as well as high-speed gate operation, the present invention is preferably applied. By increasing the Al composition, visible light is cut and so-called solar blind characteristics are obtained. In this case, the resistivity of the photocathode is further increased.
Other: The present invention can be applied to a so-called negative electron affinity photocathode (NEA photocathode) in which an alkali metal compound is applied to a semiconductor crystal to lower the work function.

  In the photocathode according to the first embodiment and the second embodiment of the present invention, the case where the electrode layer is formed in a stripe shape on the photoelectron emission surface of the photocathode has been described. It is not limited to. That is, even if the electrode layer is not formed in a stripe shape on the photoelectron emission surface, it may be formed with a pattern including a plurality of openings on the light absorption layer. Further, the pattern including a plurality of openings may be a pattern including a portion where each opening is provided at a substantially constant interval. Here, the “substantially constant interval” includes a state in which a portion that is not a constant interval is generated due to shape restrictions and dimensional accuracy in forming the electrode layer. As such a state, for example, the shape of the opening formed in the vicinity of the edge of the photoelectron emission surface is different from the shape of the opening formed in the center of the photoelectron emission surface. May occur.

  When the formed electrode layer has a shape other than the stripe shape, for example, as shown in FIG. 7, the electrode layer 15 may have a portion in which the electrode layer 15 is formed in a mesh shape. Note that FIG. 7 is a plan view of the photocathode corresponding to FIG. The portion different from FIG. 1 is the pattern shape of the electrode 15H (24H), and the electrode 15H (24H) is formed on the light absorption layer with a mesh pattern. Also in this case, as in the case of the stripe shape, the photoelectron emission surfaces are exposed at substantially constant intervals.

  That is, the pattern shape of the electrode 15H (24H) may be any of a mesh shape, a spiral shape, and a shape in which a plurality of rectangles are formed inside the outer end portion of the photoelectron emission surface. Further, the width of the opening formed between the electrode layers on the photoelectron emission surface (the width of the opening 15H (24H) in FIG. 1) may be determined according to the surface resistance and the required gate speed.

1 is a plan view of a photocathode according to a first embodiment of the present invention. It is sectional drawing of the photocathode along the II-II line | wire in FIG. It is sectional drawing of the photocathode which concerns on 2nd Embodiment of this invention. It is a cross-sectional schematic diagram of the image intensifier tube to which the photocathode according to the embodiment of the present invention is applied. It is a schematic diagram which shows the structure of the voltage application circuit (voltage application means) for carrying out gate operation | movement of the electron tube which concerns on embodiment of this invention. It is a cross-sectional schematic diagram of the photomultiplier tube to which the photocathode which concerns on embodiment of this invention was applied. It is another top view of the photocathode which concerns on embodiment of this invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 10,20 ... Photocathode, 11, 21 ... Substrate, 12 ... Antireflection film, 12A, 12B ... Antireflection layer, 13 ... Window layer, 14, 23 ... Light absorption layer, 14E, 23E ... Photoelectron emission surface, 15, 24 ... Electrode layer, 15H, 24H ... Opening, 16, 25 ... Active layer, 22 ... Buffer layer, 100 ... Image intensifier tube (electron tube), 101 ... Microchannel plate (MCP), 102 ... Phosphor, 103 ... Glass plate , 104, seal part, 110, vacuum vessel, 121, 122, 123, 124, 222, 223, 224, electrode, 150, voltage application circuit (voltage application means), 151, power supply unit, 152, switch, 153, current Total: 154: control unit, R1, R2, R3: resistance, 200: photomultiplier tube (electron tube), 201: electron multiplier unit, 202: readout electrode (anode), 210: vacuum Vessel.

Claims (10)

A substrate,
A light absorption layer formed on the substrate, composed of a compound semiconductor and emitting photoelectrons from a photoelectron emission surface in response to the incidence of light;
An electrode layer formed on the photoelectron emission surface and having an opening exposing the photoelectron emission surface so as to make the portion from the substrate to the light absorption layer have the same potential;
A photocathode comprising: A substrate having a light incident surface;
A light absorption layer formed on the substrate, emitting photoelectrons in response to light passing through the light incident surface, and composed of a compound semiconductor;
An electrode layer formed with a pattern including a plurality of openings on the light absorption layer, and having a portion extending to contact with a part of the substrate;
A photocathode comprising:   3. The photocathode according to claim 2, wherein the electrode layer has a portion extending through a side portion of the light absorption layer until it contacts a part of the substrate.   The photocathode according to claim 2, wherein the pattern includes a portion in which the openings are provided at substantially constant intervals.   2. The structure according to claim 1, further comprising a window layer interposed between the light absorption layer and the substrate, made of an AlGaAs material, and having a wider energy band gap than the light absorption layer. 2. The photocathode according to 2.   The photocathode according to claim 5, further comprising an antireflection film interposed between the substrate and the window layer.   The photocathode according to claim 1 or 2, wherein the compound semiconductor constituting the light absorption layer is a nitride semiconductor.   8. The method according to claim 7, further comprising a buffer layer interposed between the light absorption layer and the substrate, made of a nitride semiconductor, and having a wider energy band gap than the light absorption layer. Photocathode. The photocathode according to any one of claims 1 to 8,
An anode for collecting photoelectrons emitted from the photocathode;
A vacuum vessel containing the photocathode and the anode;
With an electron tube.   The electron tube according to claim 9, further comprising voltage applying means for applying a pulsed voltage between the photocathode and the anode.

Images