JP3565535B2 - Photocathode and electron tube - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a photocathode having sensitivity to light in an ultraviolet region to an infrared region, and an electron tube using the same.
[0002]
[Prior art]
A photocathode having sensitivity to ultraviolet light is disclosed in U.S. Pat. No. 4,616,248. 7 and 8 are cross-sectional views of the reflective and transmissive photocathodes 60 and 70 disclosed in this publication. An active layer for generating photoelectrons by absorbing ultraviolet light on a sapphire substrate 61. 62 is made of AlGaN.
[0003]
More specifically, in the reflective photocathode 60 of FIG. 7, an active layer 62 of AlGaN is formed on a sapphire substrate 61 by epitaxial growth via a buffer layer 63 made of GaN. On the other hand, in the transmission type photocathode 60 of FIG. 8, the active layer 62 is formed on the sapphire substrate 61 by direct epitaxial growth unlike the transmission type photocathode 70 of FIG. In the photocathodes 60 and 70 shown in FIGS. 7 and 8, the surface layer 63 made of Cs is formed on the surface of the active layer 62, and the surface of the active layer 62 is activated. The work function of the active layer 62 activated by the surface layer 63 is reduced. Further, a metal electrode 64 is formed on the periphery of the photocathodes 60 and 70 so that the photocathodes 60 and 70 can be electrically connected to the outside. Therefore, when the light to be detected (hν) enters as shown by the arrows in FIGS. 7 and 8, the electrons in the active layer 62 are excited and the photoelectrons (e ⁻ ) Can be easily released.
[0004]
[Problems to be solved by the invention]
However, US Pat. No. 5,557,167 discloses that although the photocathode disclosed above has sensitivity in the ultraviolet region, it does not satisfy the following five conditions, and the sensitivity is not high. Have been. The five conditions are 1) that the diffusion length of photoelectrons in the active layer of the photocathode is long, and 2) the thickness of the active layer is about the electron diffusion length so that the active layer absorbs light to be detected having a desired wavelength. 3) The conductivity type of the active layer of the photocathode is p-type or i-type, and the active layer surface has a negative electron affinity so that photoelectrons are easily emitted. 4) that the substrate that physically supports the active layer also functions as an entrance window, has high transmittance for a desired wavelength; and 5) crystal defects of the active layer at the interface between the active layer and the substrate. The purpose is to suppress the formation and prevent the electrons excited by the light to be detected from being captured by recombination levels derived from the crystal defects.
[0005]
Also, US Pat. No. 5,557,167 discloses that these conditions can be satisfied and a photocathode with high sensitivity can be obtained. FIG. 9 is a cross-sectional view of a transmission type photocathode 80 disclosed in this publication. In this photocathode 80, single crystal sapphire (Al ₂ O ₃ The active layer 81 is formed of a ternary mixed crystal of AlGaN on a substrate 81 made of ()). Further, an interface layer 82 made of AlGaN is formed between the substrate 81 and the active layer 82 so that generation of crystal defects formed at the interface between the substrate 81 and the active layer 82 is suppressed. I have. A metal electrode 84 is formed on the outer peripheral surface of the photocathode 80 so that the photocathode 80 can be electrically connected to the outside.
[0006]
However, the active layers of the photocathodes disclosed in the above two publications are all formed of a ternary mixed crystal of AlGaN. Therefore, in these photocathodes, even if the active layer is formed of GaN having the largest energy gap, the photocathode is essentially sensitive only to ultraviolet light having a wavelength of 200 to 360 nm. Therefore, the electron tube provided with these photocathodes can also have sensitivity only to ultraviolet light having a wavelength of 200 to 360 nm, and the wavelength range of light to be detected, which is a detection target, is limited.
[0007]
Accordingly, an object of the present invention is to provide a photocathode having high spectral sensitivity over a wide wavelength range and an electron tube using the same.
[0008]
[Means for Solving the Problems]
The photocathode of the present invention has been made in order to achieve the above-mentioned object, and is a photocathode in which photoelectrons are emitted upon incidence of light to be detected, which is a detection target. _x1 (Al _y1 Ga _1-y1 ) _1-x1 N and an active layer for generating photoelectrons by incidence of light to be detected, which is a detection target, and In between the substrate and the active layer. _x2 (Al _y2 Ga _1-y2 ) _1-x2 N, comprising a buffer layer for reducing lattice mismatch between the substrate and the active layer, and forming an active layer. _x1 (Al _y1 Ga _1-y1 ) _1-x1 The composition of N is in the range of x1> 0 and 0 ≦ y1 ≦ 1, and the composition of In forming the buffer layer _x2 (Al _y2 Ga _1-y2 ) _1-x2 The composition of N is characterized in that x2> 0 and 0 ≦ y2 ≦ 1.
[0009]
According to such a configuration, the active layer contains at least In. Therefore, by adjusting the composition of each element constituting the active layer, light to be detected having a wavelength of 200 nm to 600 nm is absorbed by the active layer, and photoelectrons are generated. Therefore, the photocathode has sensitivity to light to be detected having a wavelength of 200 nm to 600 nm. The elements constituting the active layer are the same as the elements constituting the buffer layer. Therefore, by adjusting the composition of each element constituting the active layer and the buffer layer, while lattice irregularities between the active layer and the substrate are alleviated, most of the light to be detected in the above wavelength range is absorbed by the active layer and photoelectrons are generated. Will occur. Therefore, a photocathode having high sensitivity to light to be detected in the range of 200 nm to 600 nm is realized.
[0010]
Preferably, the lattice constant of the buffer layer is substantially equal to the lattice constant of the active layer. As a result, the active layer is lattice-matched with the buffer layer, the lattice mismatch between the active layer and the substrate is almost eliminated, and crystal defects due to the lattice mismatch in the active layer hardly occur. Therefore, electrons in the active layer excited by the absorption of the light to be detected are not captured by the recombination levels derived from the crystal defects, so that a more sensitive photocathode can be obtained.
[0011]
Further, in order for the photocathode of the present invention to be of a transmission type, the energy gap of the buffer layer is preferably larger than that of the active layer.
[0012]
Further, in order to reduce lattice irregularity between the buffer layer and the active layer, a sub-layer made of InAlGaN may be formed between the buffer layer and the active layer.
[0013]
In addition, since the photocathode including the sub-layer is of a transmission type, the energy gap of the sub-layer is preferably equal to or larger than the energy gap of the active layer.
[0014]
Also, it is preferable that the lattice constant of the sub-layer is substantially equal to the lattice constant of the active layer. As a result, the active layer is lattice-matched with the buffer layer, and the lattice mismatch between the active layer and the buffer layer is almost eliminated, and crystal defects due to the lattice mismatch in the active layer hardly occur. Therefore, the probability of electrons in the active layer excited by light absorption being captured by the recombination levels derived from the crystal defects is significantly reduced, and a more sensitive photocathode can be obtained.
[0015]
Further, in order to make the active layer surface have a negative electron affinity, a surface layer for lowering the work function of the active layer is formed on the surface of the active layer by an alkali metal or an oxide thereof, a fluoride thereof or an iodide thereof. May be formed.
[0016]
In the electron tube of the present invention, a substrate made of sapphire and an In _x1 (Al _y1 Ga _1-y1 ) _1-x1 N, an active layer for generating photoelectrons by absorption of light to be detected, which is a detection target, and In between the substrate and the active layer. _x2 (Al _y2 Ga _1-y2 ) _1-x2 N, comprising a buffer layer for reducing lattice mismatch between the substrate and the active layer, and forming an active layer. _x1 (Al _y1 Ga _1-y1 ) _1-x1 The composition of N is in the range of x1> 0 and 0 ≦ y1 ≦ 1, and the composition of In forming the buffer layer _x2 (Al _y2 Ga _1-y2 ) _1-x2 A vacuum vessel is formed by supporting the substrate at the side wall end so as to house a photocathode having a composition of N in the range of x2> 0 and 0 ≦ y2 ≦ 1, and an active layer and a buffer layer of the photocathode. A housing and an anode which is installed inside the vacuum vessel and has a positive voltage with respect to the photocathode are provided.
[0017]
In such a configuration, light to be detected having a wavelength of 200 nm to 600 nm is efficiently converted into photoelectrons by the photocathode, and the photoelectrons are collected at the anode. Therefore, this electron tube can efficiently convert an optical signal in such a wavelength range into an electric signal.
[0018]
Further, in order to multiply a photoelectron signal emitted from the photocathode, a multiplying means for multiplying photoelectrons emitted from the photocathode into secondary electrons may be provided between the photocathode and the anode.
[0019]
The anode may be a fluorescent film that emits light when a two-dimensional electron image emitted corresponding to a two-dimensional optical image of the light to be detected incident on the photocathode enters. Thus, a two-dimensional optical image of the detected light can be directly observed.
[0020]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.
[0021]
FIG. 1 is a side sectional view schematically showing a transmissive photocathode 10 to which the present invention is applied, having a mesa-shaped cross section, and light to be detected having a wavelength in a range of 200 nm to 600 nm. High sensitivity is obtained.
[0022]
In this photocathode 10, single-crystal sapphire (Al ₂ O ₃ ) Is also used as a window material for the light to be detected (hν) indicated by the arrow in FIG. The reason for using sapphire as the substrate 11 is that sapphire is translucent from ultraviolet light having a wavelength of about 150 nm to visible light having a wavelength of about 600 nm, in other words, translucent in a wavelength range wider than the wavelength range of the light to be detected. It is because it has. The reason why a single crystal is used instead of a polycrystal is that it is difficult to epitaxially grow a high-quality active layer on the polycrystal.
[0023]
On the substrate 11, an active layer 12 for generating photoelectrons by absorbing the light to be detected is formed of a single crystal. In this embodiment, the active layer 12 is formed of In _x1 (Al _y1 Ga _1-y1 ) _1-x1 It is formed by a mixed crystal of N, and its composition is in the range of x1> 0 and 0 ≦ y1 ≦ 1. In forming the active layer 12 _x1 (Al _y1 Ga _1-y1 ) _1-x1 When the composition of N changes in the above range, the energy gap and the fundamental absorption edge wavelength, and the range in which they change, will be apparent by considering FIG.
[0024]
FIG. _x Al _1-x N, In _x Ga _1-x N, Al _x Ga _1-x 3 shows an energy gap Eg and a fundamental absorption edge wavelength λe with respect to the composition of each element of N. According to this figure, depending on the composition of In, Al and Ga, the energy gap Eg changes from 2 to 6 eV as shown by the solid lines in FIGS. 2 (a) to 2 (c). Further, the fundamental absorption edge wavelength λe changes from 200 nm in the ultraviolet region to 600 nm in the visible region as shown by the thick solid line in FIGS. 2A to 2C. Therefore, the active layer 12 of the present embodiment, which is formed by arbitrarily combining these ternary mixed crystals, can absorb the detection light having a wavelength of 200 nm to 600 nm and generate photoelectrons.
[0025]
On the other hand, the buffer layer called an active layer and an interface layer in the above-mentioned US Pat. No. 5,557,167 is formed of an AlGaN ternary mixed crystal containing no In. For this reason, the range of the fundamental absorption edge wavelength λe that changes according to the composition of AlGaN is limited to the ultraviolet region from 200 nm to 360 nm as shown by the thick solid line in FIG. The layer absorbs only light to be detected having a wavelength of 200 nm to 360 nm.
[0026]
In addition, a buffer layer 13 is formed between the substrate 11 and the active layer 12 in lattice matching with the active layer 12, and alleviates the lattice mismatch between the substrate 12 and the active layer 12 to reduce the crystallinity of the active layer 12. Is increasing. The present embodiment is characterized in that the lattice matching is easily maintained even when the composition of each element forming the active layer 12 changes. This is because the buffer layer 13 is made of In which is made of the same element as the active layer 12. _x2 (Al _y2 Ga _1-y2 ) _1-x2 This is because it is formed of N and its composition is in the range of x2> 0 and 0 ≦ y2 ≦ 1. At this time, the lattice constant and the energy gap of the active layer 12 and the buffer layer 13 can be independently changed by appropriately changing the composition of the elements forming both. Therefore, the lattice constants of the active layer 12 and the buffer layer 13 can be easily adjusted, and the two can be easily lattice-matched. At the interface between the active layer 12 and the buffer layer 13 that are lattice-matched, generation of crystal defects such as dislocations is suppressed. Therefore, recombination levels derived from the crystal defects are hardly formed. Therefore, the probability that the electrons of the active layer 12 excited by the light to be detected are captured by the recombination levels while moving to the emission surface is significantly reduced.
[0027]
By the way, since the photocathode 10 of the present embodiment is of a transmission type, the buffer layer 13 not only improves the crystallinity of the active layer 12 but also allows the active layer 12 to absorb most of the light to be detected incident on the substrate. It is necessary to have a property of transmitting light to be detected. That is, the buffer layer 13 needs to have a larger energy gap than the active layer 12 while lattice-matching with the active layer 12. However, since the active layer 12 and the buffer layer 13 are formed of a quaternary mixed crystal of the same element, the lattice constant of the active layer 12 and the buffer layer 13 can be changed by appropriately changing the composition of the elements forming both. And the energy gap can be changed. Therefore, the energy gap between the active layer 12 and the buffer layer 13 can be easily adjusted, and the buffer layer 13 easily has an energy gap larger than that of the active layer 12 while lattice-matching with the active layer 12.
[0028]
Note that the cross-sectional view of the photocathode 10 of FIG. 1 configured as described above is a schematic diagram, and is different from the cross-section of an actual photocathode. For example, the thicknesses of the active layer and the buffer layer are very thin compared to the thickness of the sapphire substrate. However, it is actually impossible to show the active layer and the buffer layer on the drawing. Accordingly, the thickness of the active layer 12 and the buffer layer 13 is not only shown in FIG. 1 but also in other drawings in an enlarged manner. Further, a metal electrode 14 is formed on the outer peripheral surface of the photocathode 10, and the photocathode 14 enables electrical connection with the outside.
[0029]
When light to be detected (hv) is incident from the substrate 11 side of such a transmissive photocathode 10 as shown by the arrow in FIG. 1, the substrate 11 has translucency with respect to the light to be detected. Therefore, the detected light is transmitted without being absorbed by the substrate 11. Further, since the substrate 11 is made of a single crystal, the light to be detected is not scattered by the crystal grain boundaries and its optical path does not change. The light to be detected transmitted through the substrate 11 reaches the buffer layer 13. As described above, the buffer layer 13 is also made of a single crystal and has a light-transmitting property with respect to the light to be detected, so that the light to be detected is not absorbed or scattered by the buffer layer 13. 13 is transmitted. The light to be detected transmitted through the buffer layer 13 is absorbed by the active layer. At this time, recombination levels derived from crystal defects are hardly formed in the forbidden band of the active layer 12 because generation of crystal defects due to lattice irregularity is suppressed in the active layer 12. Therefore, when the detected light is absorbed by the active layer 12 to excite electrons in the active layer 12, the probability that the electrons are trapped by the recombination level is significantly reduced. Therefore, most of the light to be detected is absorbed by the active layer 12 to excite electrons, and photoelectrons (e ⁻ ) Will be released to the outside.
[0030]
FIG. 3 shows the result of measuring the spectral sensitivity characteristics of the photocathode of the present embodiment. According to FIG. 3, it can be seen that the short wavelength threshold wavelength appears very steeply due to the buffer layer having an energy gap larger than that of the active layer, and the buffer layer plays an excellent role of a short wavelength cut filter. . This is because the buffer layer is lattice-matched with the active layer 12. As long as the buffer layer 13 is lattice-matched with the active layer 12, the fundamental absorption edge wavelength of the buffer layer 13 changes from 200 nm to 600 nm according to the composition of each element forming the buffer layer 13 as described above. It becomes possible. Therefore, the short wavelength side threshold wavelength can be arbitrarily adjusted in the range of 200 nm to 600 nm as shown by the dotted line in FIG.
[0031]
Just as the buffer layer 13 can adjust the short wavelength side threshold wavelength, the active layer 12 can also adjust the steep long wavelength example threshold wavelength as shown in FIG. The active layer 12 is made of the same element as the buffer layer 13, and the basic absorption edge wavelength of the active layer 13 can be changed from 200 nm to 600 nm according to a change in the composition as long as the active layer 12 is lattice-matched with the buffer layer 13. is there. Therefore, the active layer plays a role of an excellent long wavelength cut filter. Further, from the above, the photocathode 10 also has a function as an excellent bandpass filter in the range of 200 nm to 600 nm.
[0032]
As described above, the short wavelength side and the long wavelength side threshold wavelengths are in such a wide range because In is contained in the active layer 12 and the buffer layer 13 constituting the photocathode 10 as described above. It is. On the other hand, the active layer and the buffer layer of the above-mentioned US Pat. No. 5,557,167 are formed of AlGaN ternary mixed crystals and do not contain In. Therefore, the range of the threshold wavelength wavelengths on the short wavelength side and the long wavelength side that can be adjusted according to the composition of AlGaN is limited to the ultraviolet region of 200 nm to 360 nm.
[0033]
Note that, in the present embodiment, the buffer layer has a fixed composition of In. _x2 (Al _y2 Ga _1-y2 ) _1-x2 The case where N is formed has been described as an example. However, the buffer layer is not limited to this. For example, a gradient composition layer whose composition changes gradually from the sapphire substrate to the active layer, a plurality of buffer layers having different compositions, or a combination of these may be used. Further, a surface layer made of Cs and O is formed on the surface of the active layer, the work function of the surface of the active layer is reduced, and the surface of the active layer has a negative electron affinity (NEA). May be. At this time, it goes without saying that the same effect can be obtained even when Cs of the surface layer is other alkali metal such as Rb, K or the like. Further, other than O, for example, F, I, etc. may be used.
[0034]
The photocathode of the present invention is not limited to the above. FIG. 4 schematically shows a second embodiment of the photocathode 10 of the present invention. In the present embodiment, unlike the first embodiment in which the single crystal buffer layer is formed, the single crystal sub-layer 15 is formed of InAlGaN on the amorphous buffer layer 13 formed at a low temperature. . The reason why the sub-layer 15 is provided between the active layer 12 and the buffer layer 13 is that the amorphous buffer layer 13 contains many defects, so that the interface in the case where the active layer 12 is directly formed on the buffer layer 13 is formed. This is because crystal defects may be generated in the active layer 12. This crystal defect forms a recombination level in the forbidden band of the active layer 12, and electrons excited by absorption of the light to be detected by the active layer are captured by the recombination level. Therefore, the sensitivity of the photocathode 10 may be significantly reduced. Therefore, by forming the sub-layer 15 between the active layer 12 and the buffer layer 13, lattice irregularities between the active layer 12 and the buffer layer 13 are reduced. At this time, the lattice constants of the active layer 15 and the sub-layer 15 are substantially matched so that the generation of crystal defects at the interface between the active layer 12 and the sub-layer 15 is suppressed as much as possible. Is desirable.
[0035]
Further, since the photocathode 10 of the second embodiment is of a transmission type similarly to the first embodiment, the sub-layer 15 needs to have a light-transmitting property with respect to the wavelength of the light to be detected. is there. Therefore, it is necessary that InAlGaN forming the sub-layer 15 has a predetermined composition such that the energy gap of the sub-layer 15 is also larger than the energy gap corresponding to the wavelength of the light to be detected.
[0036]
The photocathode 10 of the second embodiment configured as described above is manufactured by a so-called organometallic vapor phase epitaxy method using a highly reactive organometallic compound and ammonia (NH ₃ ) And pyrolysis. The fabrication of the photocathode 10 of the first embodiment is performed in substantially the same manner as in the second embodiment.
[0037]
First, a single-crystal sapphire substrate having a predetermined size is degreased and cleaned, and then placed in a reaction chamber of a metal organic chemical vapor deposition apparatus (not shown). After that, ammonia gas (NH ₃ ) And hydrogen gas (H ₂ ) And introduce. In this state, the sapphire substrate 11 is heated and maintained at a temperature of about 1000 ° C. for a predetermined time to clean its surface, thereby removing water and volatile components on the surface.
[0038]
Next, the sapphire substrate 11 is maintained at about 450 ° C., and trimethylindium (TMI) as a source material of In, trimethylaluminum (TMA) as a source gas of Al, and trimethylgallium (TMG) as a source material of Ga are supplied to a reaction chamber. To be introduced. At this time, the buffer layer 13 made of InAlGaN and having a thickness of about 20 nm becomes amorphous. Thereafter, the sapphire substrate 11 is heated and maintained at about 900 ° C., and the sublayer 15 made of single crystal InAlGaN and having a thickness of about 200 nm is formed. At this time, in order to make the conductivity type of the sub-layer 15 a p-type doped with a predetermined amount of Mg, a predetermined amount of cyclopentadienyl magnesium which is a raw material of Mg is introduced.
[0039]
Subsequently, on this sub-layer 15, In _x1 (Al _y1 Ga _1-y1 ) _1-x1 An active layer 12 made of N and having a thickness of about 300 nm is epitaxially grown. At this time, since most of the light to be detected entering from the sapphire substrate 11 reaches the active layer 12, the energy gap of the buffer layer 13 and the sub-layer 15 is set to be larger than the energy gap of the active layer 12. It is important that the InAlGaN quaternary mixed crystal to be formed has a predetermined composition. Further, the composition of each element constituting the active layer 12 and the sub-layer 15 is adjusted so that generation of crystal defects at the interface between the active layer 13 and the sub-layer 15 is suppressed, so that the lattice constants of the two are substantially equal. It is important to match.
[0040]
In U.S. Pat. No. 5,557,167, Zn is deposited on the active layer. ₃ N ₂ Although a method of forming a protection layer made of is disclosed, it is not necessary to form such a protection layer unless the Al composition of the active layer is extremely high. This protection layer is for suppressing the oxidation of the active layer surface, and it is needless to say that the protection layer is not limited to the above as long as it is released by raising the temperature in a vacuum.
[0041]
As described above, the photocathode is manufactured by the metal organic chemical vapor deposition method, but this growth method is not essential. Therefore, the growth method is not limited to the metal organic chemical vapor deposition method as long as it can produce a high-quality photocathode. The thickness of each layer is also a typical value, and the present invention is not necessarily limited to the above thickness.
[0042]
Next, after the photocathode produced in the reaction chamber of the metalorganic chemical vapor deposition apparatus is once taken out into the atmosphere, the surface of the active layer 12 is partially masked and arranged in a vacuum chamber, and the ultrahigh vacuum is applied. Evacuate. In this state, an electrode material is deposited to form an electrode on the outer peripheral surface of the photocathode 10.
[0043]
Thereafter, the inside of the vacuum chamber is set to atmospheric pressure, the photocathode 10 is taken out of the vacuum chamber, and the mask on the surface of the active layer 12 is removed. Next, the photocathode 10 is placed in the vacuum chamber again, and evacuated to ultra-high vacuum. In this state, the photocathode 10 is heated to about 800 ° C. to clean the surface of the photocathode 10. The work function of the active layer surface of the photocathode may be lowered by forming only a monolayer of a surface layer (not shown) made of Cs and O. As a result, the active layer has a so-called negative electron affinity, and a photocathode with extremely high sensitivity is manufactured. However, the surface layer is not limited to those made of Cs and O. Of course, similar effects can be obtained with other alkali metals other than Cs, such as Rb and K. Also, for example, F, I, etc., other than O, can be used. If such a photocathode 10 is housed in a vacuum vessel, an electron tube having extremely high photoelectric conversion efficiency is manufactured.
[0044]
The photocathode 10 configured as described above is actually used as a cathode of an electron tube such as a photomultiplier tube, and can be used to detect weak light to be detected having a wavelength of 200 nm to 600 nm.
[0045]
The side sectional view of FIG. 1 schematically shows an electron tube 20 including the photocathode 10 according to the second embodiment, and a photoelectron multiplier capable of detecting weak detection light in the above-described wavelength range by photoelectric conversion. It is a double tube.
[0046]
In this photomultiplier tube, a substrate 11 constituting the photocathode 10 is hermetically supported so as to accommodate the active layer 12 and the like in the housing 21 at the upper end of the housing. The anode 30 is hermetically supported to form the vacuum container 22. Although not shown, in this photomultiplier tube 20, one end of the electric lead is connected to the electrode of the photocathode 10 and the other end extends to the outside, and one end of the electric lead is also connected to the anode. Extends outside. A DC power supply is connected to the other ends of these electric leads so that the anode has a positive voltage with respect to the photocathode.
[0047]
In the vacuum vessel 22, a microchannel plate (hereinafter, referred to as [MCP]) 40 configured by bundling a large number of glass holes having a diameter of about 10 μm is provided as an electron multiplier. To the MCP 40, a positive voltage is applied to the photocathode 10 and a negative voltage is applied to the anode 30 via electric leads (not shown). Further, one end of each of the electrical leads 41a and 41b is connected to the upper surface side (hereinafter, referred to as “input side”) and the lower surface side (hereinafter, referred to as “output side”) of the MCP 40, and the other end penetrates the side wall of the housing. Extending. A DC power supply (not shown) is connected to the other end of the electric lead, so that a voltage for multiplication is applied between the input side of the MCP 40 and the output side of the MCP 40. The multiplying means in the present embodiment is not limited to the MCP 40, but may be, for example, a secondary electron surface made of SbCs or the like, or an electron-implanted diode.
[0048]
When light to be detected (hν) to be detected is incident on the sapphire substrate 11 of the photomultiplier tube 20 configured as described above, almost all of the active layer is an active layer, as described in the description of the embodiment of the photocathode 10. Reach twelve. In addition, most of the light to be detected that has reached the active layer 12 is absorbed by the active layer 12 and contributes to photoelectric conversion. ⁻ ) Are emitted outside the photocathode 10 having a negative electron affinity. The photoelectrons emitted to the outside of the photocathode 10 enter the input side of the MCP 40 while accelerating. The photoelectrons are about 1 × 10 ⁶ The secondary electrons are multiplied by a factor of two, and the secondary electrons (e ⁻ ) Is released as shown in FIG. The secondary electrons are accelerated and incident on the anode 30 applied with a positive voltage to the MCP 40, and are taken out as an output signal current. Therefore, when it is considered that the light to be detected is composed of photons, the photomultiplier tube 20 reduces the number of photons counted at the photocathode. For this reason, in a predetermined wavelength range determined by the photocathode, that is, in a wavelength range of 200 nm to 600 nm, the detection of weak detection light, in other words, the photon counting measurement can be performed very efficiently.
[0049]
The electron tube of the present invention is not limited to the one that simply detects weak detection light as described above. The electron tube 20 shown in the side sectional view of FIG. 6 is a so-called image intensifier tube which can detect a weak two-dimensional optical image as light to be detected, unlike the photomultiplier tube of FIG. In this image intensifier tube, unlike the photomultiplier tube of FIG. 5, the anode 30 is made of a transparent electrode, and a phosphor (fluorescent film) 31 is formed on the inner surface thereof. Further, a fiber plate 50 is supported at a lower portion of the vacuum vessel.
[0050]
When a weak two-dimensional optical image (hv) as light to be detected is incident on the substrate 11 of such an image intensifier as shown by an arrow in FIG. 6, a two-dimensional photoelectron image corresponding to the two-dimensional optical image (E ⁻ ) Is efficiently released from the photocathode to the internal space of the vacuum vessel 20. Thereafter, when the two-dimensional photoelectron image is accelerated and incident on the input side of the MCP 40, the two-dimensional photoelectron image is approximately 1 × 10 ⁶ Secondary electron multiplication is doubled. At this time, the two-dimensional photoelectron image (e ⁻ ) Is emitted from the output side position corresponding to the input side incident position. The two-dimensional electronic image is accelerated and incident on the phosphor 31 formed on the inner surface of the anode 30, and a two-dimensional image corresponding to the two-dimensional electronic image is displayed on the phosphor 31 in an enhanced manner. The two-dimensional image is taken out through a fiber plate 50 provided below the phosphor 31 and observed. Therefore, in the image intensifier tube of the present embodiment, the above-described photocathode 10 is coupled to the MCP 40, the phosphor 31, and the fiber plate 50, so that a weak two-dimensional optical image can be detected in a wavelength range determined by the photocathode 10. It becomes possible.
[0051]
Note that the electron tube is not limited to a photomultiplier tube or an image intensifier tube, and may be another light detection device such as an imaging tube or a streak tube.
[0052]
【The invention's effect】
In the photocathode of the present invention, the active layer formed on the sapphire substrate and generating photoelectrons by absorbing the light to be detected is In. _x1 (Al _y1 Ga _1-y1 ) _1-x1 It is formed by N, and its composition is in the range of x1> 0 and 0 ≦ y1 ≦ 1. Since the active layer contains In as described above, light to be detected having a wavelength of 200 nm to 600 nm is absorbed by the active layer, and photoelectrons can be generated. In addition, since the buffer layer is formed between the sapphire substrate and the active layer using the same element as the active layer, lattice irregularities between the sapphire substrate and the active layer are easily alleviated. Therefore, crystal defects in the active layer which occur at the interface between the buffer layer and the active layer and adversely affect the sensitivity of the photocathode are reduced, and a photocathode having high sensitivity in such a wavelength range is realized.
[Brief description of the drawings]
FIG. 1 is a sectional view schematically showing a first embodiment of a photocathode according to the present invention.
FIG. 2 shows In _x Al _1-x N, In _1-x Ga _x N and Al _x Ga _1-x FIG. 3 is a diagram showing an energy gap and a fundamental absorption edge wavelength for each composition of N.
FIG. 3 is a diagram illustrating spectral sensitivity characteristics of the photocathode of the first embodiment.
FIG. 4 is a sectional view schematically showing a photocathode according to a second embodiment of the present invention.
FIG. 5 is a cross-sectional view schematically showing a first embodiment of the electron tube of the present invention, that is, a photomultiplier tube.
FIG. 6 is a sectional view schematically showing a second embodiment of the electron tube of the present invention, that is, an image intensifier tube.
FIG. 7 is a cross-sectional view schematically showing a conventional reflective photocathode.
FIG. 8 is a cross-sectional view schematically showing a conventional transmission-type photocathode.
9 is a cross-sectional view schematically showing a conventional photocathode which has solved the problem of the photocathode of FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Photocathode, 11 ... Substrate, 12 ... Active layer, 13 ... Buffer layer, 14 ... Electrode, 20 ... Electron tube, 21 ... Housing, 22 ... Vacuum container, 30 ... Anode, 31 ... Phosphor, 40 ... MCP, 50: Fiber plate.

Claims (10)

A photocathode in which photoelectrons are emitted by incidence of light to be detected, which is a detection target,
A substrate made of sapphire,
An active layer formed of _Inx1 ( _{Aly1Ga1 -y1} ) _1-x1N on the substrate to generate photoelectrons by absorbing light to be detected;
A buffer layer formed of _Inx2 ( _{Aly2Ga1 -y2} ) _1-x2N between the substrate and the active layer, for reducing lattice irregularity between the substrate and the active layer;
With
The composition of _Inx1 ( _{Aly1Ga1 -y1} ) _1-x1N forming the active layer is in the range of x1> 0 and 0≤y1≤1,
A photocathode, wherein the composition of _Inx2 ( _{Aly2Ga1 -y2} ) _1-x2N forming the buffer layer is in the range of x2> 0 and 0≤y2≤1. The photocathode according to claim 1, wherein a lattice constant of the buffer layer is substantially equal to a lattice constant of the active layer. The photocathode according to claim 1, wherein an energy gap of the buffer layer is equal to or larger than an energy gap of the active layer. 4. The photocathode according to claim 1, wherein a sub-layer made of InAlGaN is formed between the buffer layer and the active layer. 5. The photocathode according to claim 4, wherein a lattice constant of the sub-layer is substantially equal to a lattice constant of the active layer. 6. The photocathode according to claim 4, wherein the energy gap of the sub-layer is equal to or larger than the energy gap of the active layer. The surface of the active layer, wherein a surface layer for reducing the work function of the surface of the active layer is formed of an alkali metal or an oxide thereof, a fluoride thereof or an iodide thereof. Item 7. The photocathode according to any one of Items 1 to 6. A substrate made of sapphire,
An active layer formed of _Inx1 ( _{Aly1Ga1 -y1} ) _1-x1N on the substrate and configured to generate photoelectrons by absorbing detected light to be detected;
A buffer layer formed of _Inx2 ( _{Aly2Ga1 -y2} ) _1-x2N between the substrate and the active layer, for reducing lattice irregularity between the substrate and the active layer;
With
The composition of _Inx1 ( _{Aly1Ga1 -y1} ) _1-x1N forming the active layer is in the range of x1> 0 and 0≤y1≤1,
A photocathode in which the composition of _Inx2 ( _{Aly2Ga1 -y2} ) _1-x2N forming the buffer layer is in the range of x2> 0 and 0≤y2≤1;
A housing that constitutes a vacuum container by supporting the substrate at an end of a side wall so as to house the active layer and the buffer layer of the photocathode therein,
An anode that is installed inside the vacuum vessel and that holds a positive voltage with respect to the photocathode,
An electron tube comprising: 9. The electron tube according to claim 8, further comprising a multiplication unit for multiplying photoelectrons emitted from the photocathode into secondary electrons between the photocathode and the anode. The anode includes a fluorescent film that emits light when a two-dimensional electron image emitted corresponding to a two-dimensional optical image of light to be detected incident on the photocathode is incident. Item 10. The electron tube according to item 8 or 9.

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