Patent Application
20200234935
The present disclosure relates to a transmissive photocathode and an electron tube.
There is a transmissive photocathode including a light transmitting substrate that has a first surface on which light is incident and a second surface which emits the light incident from the first surface side, a photoelectric conversion layer that is provided on a light emission side of the light transmitting substrate and converts the light emitted from the second surface into photoelectrons, and a light transmitting conductive layer that is provided between the light transmitting substrate and the photoelectric conversion layer and is constituted of a graphene (for example, refer to Patent Literature 1).
Patent Literature 1: Japanese Patent No. 5899187
In transmissive photocathodes as described above, a light transmitting conductive layer constituted of a graphene having both excellent light transmissivity and high conductivity is provided between a light transmitting substrate and a photoelectric conversion layer, and therefore both retention of sufficient sensitivity and improvement of linearity can be achieved. In order to further enhance the sensitivity in such a transmissive photocathode, it is conceivable that the light transmitting conductive layer be constituted of a single-layered graphene. However, depending on the types of the light transmitting substrate and the photoelectric conversion layer, there are cases where defects such as creases or breakage occur in the light transmitting conductive layer at the time of manufacturing, and sensitivity is degraded at positions where the defects have occurred.
Therefore, an object of an aspect of the present disclosure is to provide a transmissive photocathode and an electron tube, in which occurrence of defects in a light transmitting conductive layer can be curbed even when a single-layered graphene is used as the light transmitting conductive layer.
According to an aspect of the present disclosure, there is provided a transmissive photocathode including a light transmitting substrate that has a first surface on which light is incident and a second surface which emits the light incident from the first surface side, a photoelectric conversion layer that is provided on the second surface side of the light transmitting substrate and converts the light emitted from the second surface into photoelectrons, a light transmitting conductive layer that is provided between the light transmitting substrate and the photoelectric conversion layer and is composed of a single-layered graphene, and a thermal stress alleviation layer that is provided between the photoelectric conversion layer and the light transmitting conductive layer and has light transmissivity. A thermal expansion coefficient of the thermal stress alleviation layer is smaller than a thermal expansion coefficient of the photoelectric conversion layer and larger than a thermal expansion coefficient of the graphene.
In this transmissive photocathode, the light transmitting conductive layer is constituted of a single-layered graphene. Accordingly, compared to a case where the light transmitting conductive layer is constituted of a plurality of graphene layers, light transmittance of the light transmitting conductive layer can be enhanced, and sensitivity can be enhanced. In addition, the inventors have found that defects in the light transmitting conductive layer as described above occur due to a difference between the thermal expansion coefficients of the graphene and the photoelectric conversion layer when the photoelectric conversion layer is formed on the light transmitting conductive layer. Based on this knowledge, in this transmissive photocathode, the thermal stress alleviation layer having a thermal expansion coefficient smaller than the thermal expansion coefficient of the photoelectric conversion layer and larger than the thermal expansion coefficient of the graphene is provided between the photoelectric conversion layer and the light transmitting conductive layer. Accordingly, it is possible to alleviate thermal stress acting on the light transmitting conductive layer when the photoelectric conversion layer is formed. As a result, occurrence of defects in the light transmitting conductive layer can be curbed even when a single-layered graphene is used as the light transmitting conductive layer.
In the transmissive photocathode according to the aspect of the present disclosure, the thermal expansion coefficient of the thermal stress alleviation layer may be within a range of 0.0×10−6/K to 10.0×10−6/K. In this case, occurrence of defects in the light transmitting conductive layer can be curbed reliably.
In the transmissive photocathode according to the aspect of the present disclosure, the thermal stress alleviation layer may be composed of oxide or fluoride. In this case, occurrence of defects in the light transmitting conductive layer can be curbed more reliably.
In the transmissive photocathode according to the aspect of the present disclosure, the thermal stress alleviation layer may be composed of aluminum oxide, hafnium oxide, chromium oxide, gallium oxide, silicon oxide, or magnesium fluoride. In this case, occurrence of defects in the light transmitting conductive layer can be curbed still more reliably.
In the transmissive photocathode according to the aspect of the present disclosure, the light transmitting substrate may be formed of an UV ray transmitting material. In this case, in the transmissive photocathode which is highly sensitive in a wavelength range including UV rays, occurrence of defects in the light transmitting conductive layer can be curbed.
In the transmissive photocathode according to the aspect of the present disclosure, the photoelectric conversion layer may be constituted by including antimony or tellurium and an alkali metal. In this case, in the transmissive photocathode which is highly sensitive in a wavelength range including UV rays, occurrence of defects in the light transmitting conductive layer can be curbed.
According to another aspect of the present disclosure, there is provided an electron tube including the transmissive photocathode described above. According to this electron tube, for the reasons described above, occurrence of defects in a light transmitting conductive layer can be curbed even when a single-layered graphene is used as the light transmitting conductive layer.
According to the aspects of the present disclosure, occurrence of defects in a light transmitting conductive layer can be curbed even when a single-layered graphene is used as the light transmitting conductive layer.
Hereinafter, with reference to the drawings, an embodiment of a transmissive photocathode according to an aspect of the present disclosure will be described. In the following description, terms such as “up” and “down” are for convenience based on the state illustrated in the drawings. In each of the diagrams, the same reference signs are applied to parts which are the same or corresponding, and duplicate description will be omitted. In the drawings, there are parts which are exaggerated partially in order to make description of characteristic parts easy to understand, and dimensions of the parts differ from actual dimensions. In the present embodiment, as an example, a transmissive photocathode 2 used as a transmissive photocathode in a photomultiplier tube 1 will be described.
As illustrated in
As illustrated in
An electron multiplier 14 for multiplying photoelectrons released from the photoelectric conversion layer 9 is accommodated inside the sealed container 13 formed in this manner. This electron multiplier 14 is formed to have a block shape due to thin plate-shaped dynode plates 15 which have a number of electron multiplier holes and are stacked in a plurality of layers, and is installed on the upper surface of the stem 10. As illustrated in
Moreover, as illustrated in
As illustrated in
Subsequently, with reference to
The seal member 5 is formed of a metal such as aluminum, for example, to have a circular ring shape corresponding to the shape of the upper end portion of the side tube 3. The contact portion 6 is a metal film formed of a metal such as chromium, for example, to have a circular ring shape. The contact portion 6 has a film thickness of approximately 100 mm, for example, and is electrically connected to the seal member 5. The contact portion 6 is provided on the inner surface 4b of the light transmitting substrate 4 by vapor deposition, for example. An outer edge of the contact portion 6 is laid along the outer edge of the light transmitting substrate 4, and an inner edge of the contact portion 6 surrounds a photoelectric conversion region 4c disposed in a central portion of the light transmitting substrate 4. In other words, the photoelectric conversion region 4c is defined by the inner edge of the contact portion 6 in the central portion of the light transmitting substrate 4.
The light transmitting conductive layer 7 is provided in a direct contact state on the photoelectric conversion region 4c that is a circular region in which the contact portion 6 is not provided on the inner surface 4b of the light transmitting substrate 4. The light transmitting conductive layer 7 is constituted of a single-layered graphene. The thickness of the light transmitting conductive layer 7 is approximately 0.3 nm, for example. The light transmitting conductive layer 7 covers the photoelectric conversion region 4c in its entirety, is disposed to be laid over the contact portion 6 in the outer edge portion thereof, and is electrically connected to the contact portion 6. More specifically, the light transmitting conductive layer 7 is disposed to be laid over the inner edge portion of the contact portion 6 over the entire circumference of the outer edge portion, and the outer edge portion of the light transmitting conductive layer 7 and the inner edge portion of the contact portion 6 overlap each other over the entire circumference. It is preferable that the light transmitting conductive layer 7 in its entirety be directly covered with the thermal stress alleviation layer 8 described below. Therefore, it is preferable that the light transmitting conductive layer 7 be disposed to be laid over the contact portion 6 as in the present embodiment, without being disposed to be sandwiched between the light transmitting substrate 4 and the contact portion 6. In the present embodiment, the light transmitting conductive layer 7 is disposed to be laid over the contact portion 6 over the entire circumference of the outer edge portion. However, the embodiment is not limited thereto. The photoelectric conversion region 4c in its entirety need only be covered with the light transmitting conductive layer 7, and the light transmitting conductive layer 7 and the contact portion 6 need only be electrically connected to each other. For example, the light transmitting conductive layer 7 may be disposed to be laid over the contact portion 6 in a part in the circumferential direction. However, from the viewpoint of improvement in cathode uniformity, when the light transmitting conductive layer 7 is disposed to be laid over the contact portion 6 over the entire circumference of the outer edge portion, this is preferable because a distribution of electrical resistance inside the photoelectric conversion region 4c becomes uniform easily.
The thermal stress alleviation layer 8 is provided on the lower surface side of the light transmitting conductive layer 7 such that the light transmitting conductive layer 7 in its entirety is covered. More specifically, the thermal stress alleviation layer 8 covers the lower surface of the light transmitting conductive layer 7 in its entirety in a state where it comes into direct contact with the light transmitting conductive layer 7. In addition, the thermal stress alleviation layer 8 is provided such that the outer edge portion thereof is positioned on a side outward from the outer edge of the light transmitting conductive layer 7 and covers a part of the contact portion 6. In other words, the thermal stress alleviation layer 8 is provided in a range such that also a part of the contact portion 6 is covered beyond a boundary between the light transmitting conductive layer 7 and the contact portion 6. In the present embodiment, the thermal stress alleviation layer 8 comes into contact with the seal member 5 in the outer edge portion. The thermal stress alleviation layer 8 need only cover at least the light transmitting conductive layer 7 in its entirety. However, in order to protect an outer end portion of the light transmitting conductive layer 7, it is preferable that the thermal stress alleviation layer 8 be provided to reach the contact portion 6 beyond the light transmitting conductive layer 7 as in the present embodiment. In addition, when the thermal stress alleviation layer 8 in its entirety is disposed on the light transmitting conductive layer 7 and the contact portion 6, that is, on a conductive layer, electric charge is favorably supplied to the photoelectric conversion layer 9 via the thermal stress alleviation layer 8.
The thermal stress alleviation layer 8 is inferior to the light transmitting conductive layer 7 with regard to light transmissivity and conductivity but is superior to the photoelectric conversion layer 9 with regard to light transmissivity. The thermal stress alleviation layer 8 is composed of aluminum oxide (Al2O3), hafnium oxide (HfO2), chromium oxide (Cr2O3), gallium oxide (Ga2O3), silicon dioxide (SiO2), or magnesium fluoride (MgF2), for example. The thermal stress alleviation layer 8 has a film thickness of approximately 10 nm, for example, and is formed to be thicker than the light transmitting conductive layer 7 such that supply of electric charge from the light transmitting conductive layer 7 to the photoelectric conversion layer 9 is not hindered while curbing reflection of incident light. The thermal stress alleviation layer 8 is formed by vapor deposition, for example. Since the thermal stress alleviation layer 8 is disposed under a high temperature environment when the photoelectric conversion layer 9 is formed as described below, it is constituted of a thermally stable material. In addition, since the thermal stress alleviation layer 8 is disposed inside the sealed container 13 (inside a vacuum space), it is formed of a material which releases less gas. Moreover, the thermal stress alleviation layer 8 is formed of a material having a refractive index such that reflection of incident light on a boundary surface with respect to the light transmitting conductive layer 7 and a boundary surface with respect to the photoelectric conversion layer 9 can be curbed. However, since a single-layered graphene constituting the light transmitting conductive layer 7 is extremely thin and an influence of the light transmitting conductive layer 7 on reflection is then relatively small, the thermal stress alleviation layer 8 may be formed of a material having a refractive index between those of the light transmitting substrate 4 and the photoelectric conversion layer 9.
The photoelectric conversion layer 9 is provided on the lower surface side of the thermal stress alleviation layer 8 such that the thermal stress alleviation layer 8 is covered. More specifically, the photoelectric conversion layer 9 covers the lower surface of the thermal stress alleviation layer 8 in its entirety in a state where it does not come into direct contact with the light transmitting conductive layer 7. The photoelectric conversion layer 9 is provided such that the photoelectric conversion region 4c is covered. In other words, the photoelectric conversion layer 9 is provided in a region including the photoelectric conversion region 4c when viewed in the light incident direction (up-down direction in
Here, the thermal expansion coefficient of the thermal stress alleviation layer 8 is smaller than the thermal expansion coefficient of the photoelectric conversion layer 9 and is larger than the thermal expansion coefficient of the graphene (light transmitting conductive layer 7). More specifically, it is preferable that the thermal expansion coefficient of the thermal stress alleviation layer 8 be within a range of 0.0×10−6/K to 10.0×10−6/K. Moreover, it is preferable that the thermal stress alleviation layer 8 be composed of oxide or fluoride. For example, materials constituting the thermal stress alleviation layer 8 include aluminum oxide, hafnium oxide, chromium oxide, gallium oxide, silicon dioxide, and magnesium fluoride. The thermal expansion coefficients for the thermal stress alleviation layer 8 in the cases thereof are set to 7.0×10−6/K, 3.8×10−6/K, 6.2×10−6/K, 8.2 to 8.5×10−6/K, 0.5×10−6/K, and 8.48×10−6/K, respectively. In contrast, for example, in a case of a bialkali photoelectric surface including antimony, the thermal expansion coefficient of the photoelectric conversion layer 9 can be regarded such that it is equivalent to a thermal expansion coefficient of antimony, that is, 12.0×10−6/K. In addition, when the photoelectric conversion layer 9 is constituted of a cesium-tellurium photoelectric surface, the thermal expansion coefficient of the photoelectric conversion layer 9 can be regarded such that it is equivalent to a thermal expansion coefficient of tellurium, that is, 16.8×10−6/K. Furthermore, the thermal expansion coefficient of the graphene is set to (−8.0±0.7)×10−6/K. In addition, when the light transmitting substrate 4 is formed of synthetic quartz, UV ray transmitting glass, and Kovar glass, the thermal expansion coefficients for the light transmitting substrate 4 are set to 0.5×10−6/K, 4.1×10−6/K, and 3.2×10−6/K, respectively, and it is smaller than the thermal expansion coefficient of the photoelectric conversion layer 9 and larger than the thermal expansion coefficient of the graphene (light transmitting conductive layer 7). The thermal expansion coefficient of the graphene is disclosed in the following reference literature, for example.
(Reference literature) Duhee Yoon, Young-Woo Son, and Hyeonsik Cheong, “Negative Thermal Expansion Coefficient of Graphene Measured by Raman Spectroscopy”, NANO LETTERS, 2011, 11(8), pp. 3227-3231
Therefore, for example, when the thermal stress alleviation layer 8 is composed of aluminum oxide, hafnium oxide, chromium oxide, gallium oxide, silicon dioxide or magnesium fluoride, and the photoelectric conversion layer 9 is constituted of a bialkali photoelectric surface or a cesium-tellurium photoelectric surface, the thermal expansion coefficient of the thermal stress alleviation layer 8 is smaller than the thermal expansion coefficient of the photoelectric conversion layer 9 and larger than the thermal expansion coefficient of the graphene. In these cases, the thermal expansion coefficient of the thermal stress alleviation layer 8 is within a range of 0.0×10−6/K to 10.0×10−6/K. In addition, at this time, when the light transmitting substrate 4 is formed of synthetic quartz, an UV ray transmitting material, or Kovar glass, the difference between the thermal expansion coefficient of the thermal stress alleviation layer 8 and the thermal expansion coefficient of the light transmitting substrate 4 is equivalent to or smaller than 8.0×10−6/K. When the thermal stress alleviation layer 8 is composed of aluminum oxide, hafnium oxide, chromium oxide, gallium oxide, or magnesium fluoride; the light transmitting substrate 4 is formed of synthetic quartz, an UV ray transmitting material, or Kovar glass; and the photoelectric conversion layer 9 is constituted of a bialkali photoelectric surface or a cesium-tellurium photoelectric surface, the thermal expansion coefficient of the thermal stress alleviation layer 8 is larger than a value obtained by dividing the sum total of the thermal expansion coefficients of the light transmitting substrate 4, the thermal expansion coefficient of the graphene, and the thermal expansion coefficient of the photoelectric conversion layer 9 by six and is equivalent to or smaller than 10.0×10−6/K. When the light transmitting substrate 4 is formed of synthetic quartz, and the thermal stress alleviation layer 8 is composed of silicon dioxide, both the light transmitting substrate 4 and the thermal stress alleviation layer 8 are constituted by including silicon dioxide.
Subsequently, an example of a method of manufacturing the transmissive photocathode 2 will be described. First, the contact portion 6 is formed by vapor-depositing chromium in an outer circumferential edge portion on the inner surface 4b of the light transmitting substrate 4. Subsequently, the light transmitting conductive layer 7 constituted of a graphene is disposed such that it covers the photoelectric conversion region 4c in its entirety on the inner surface 4b of the light transmitting substrate 4 and is laid over the inner edge portion of the contact portion 6 over the entire circumference of the outer edge portion. This graphene is disposed, for example, by forming a film-shaped single-layered graphene on a copper foil through CVD and transferring the formed graphene such that the photoelectric conversion region 4c in its entirety on the inner surface 4b of the light transmitting substrate 4 is covered. Subsequently, the light transmitting substrate 4 and the side tube 3 are air-tightly joined to each other with the seal member 5 interposed therebetween by joining the seal member 5 to the lower surface of the contact portion 6. Subsequently, the thermal stress alleviation layer 8 is formed, for example, by vapor-depositing aluminum oxide such that the lower surface side of the contact portion 6 exposed to the inside of the side tube 3 and the lower surface side of the light transmitting conductive layer 7 in its entirety are covered. Subsequently, for example, antimony is vapor-deposited such that the lower surface side of the thermal stress alleviation layer 8 in its entirety is covered. Furthermore, a bialkali photoelectric surface is formed as the photoelectric conversion layer 9 by causing an alkali metal such as potassium or cesium to react with antimony to be activated using a transfer device. Thereafter, the sealed container 13 is formed by welding the flange portion 12a of the ring-shaped side tube 12, to which the stem 10 having the electron multiplier 14 installed therein is air-tightly fixed, to the flange portion 3a of the side tube 3. Accordingly, the photomultiplier tube 1 is obtained.
Subsequently, with reference to
As illustrated in
Subsequently, with reference to
Here, regarding the foregoing photomultiplier tube 1, Example 1 was a sample equivalent to a case where the light transmitting substrate 4 was formed of an UV ray transmitting material, the thermal stress alleviation layer 8 was composed of aluminum oxide, and the photoelectric conversion layer 9 was constituted of a bialkali photoelectric surface. Comparative Example was a sample equivalent to a case where the thermal stress alleviation layer 8 was not formed in Example 1.
As illustrated in
As illustrated in
In the graph of
As described above, in the transmissive photocathode 2 according to the present embodiment, the light transmitting conductive layer 7 is constituted of a single-layered graphene. Accordingly, compared to a case where the light transmitting conductive layer 7 is constituted of a plurality of graphene layers, light transmittance of the light transmitting conductive layer 7 can be enhanced, and sensitivity can be enhanced.
In addition, the inventors have found that defects occurring in the light transmitting conductive layer 7 occur due to a difference between the thermal expansion coefficients of the graphene (light transmitting conductive layer 7) and the photoelectric conversion layer 9 when a metal layer (for example, a layer composed of antimony) is formed on the light transmitting conductive layer 7 and the photoelectric conversion layer 9 is formed by causing an alkali metal (for example, potassium and cesium) to react with the metal layer. That is, when the photoelectric conversion layer 9 is formed, for example, each of the members is cooled after being placed under a high temperature environment heated up to approximately 220° C. through vacuum baking treatment. If the thermal stress alleviation layer 8 is not provided between the light transmitting conductive layer 7 and the photoelectric conversion layer 9, the photoelectric conversion layer 9 and the light transmitting substrate 4 expand and the light transmitting conductive layer 7 meanwhile contracts at the time of heating. Therefore, there is concern that tensile stress acts on the light transmitting conductive layer 7 and breakage such as fracture occurs. In addition, the photoelectric conversion layer 9 and the light transmitting substrate 4 contract and the light transmitting conductive layer 7 meanwhile expands at the time of cooling. Therefore, there is concern that compressive stress acts on the light transmitting conductive layer 7 and the light transmitting conductive layer 7 is flocculated, thereby causing creases.
Based on the knowledge, in the transmissive photocathode 2, the thermal stress alleviation layer 8 having a thermal expansion coefficient smaller than the thermal expansion coefficient of the photoelectric conversion layer 9 and larger than the thermal expansion coefficient of the graphene is provided between the photoelectric conversion layer 9 and the light transmitting conductive layer 7. Accordingly, it is possible to alleviate thermal stress acting on the light transmitting conductive layer 7 when the photoelectric conversion layer 9 is formed. As a result, occurrence of defects in the light transmitting conductive layer 7 can be curbed even when a single-layered graphene is used as the light transmitting conductive layer 7.
In addition, in the transmissive photocathode 2, the thermal expansion coefficient of the thermal stress alleviation layer 8 is within a range of 0.0×10−6/K to 10.0×10−6/K. In addition, the thermal stress alleviation layer 8 is composed of oxide or fluoride. In addition, the thermal stress alleviation layer 8 is composed of aluminum oxide, hafnium oxide, chromium oxide, gallium oxide, silicon oxide, or magnesium fluoride. Consequently, occurrence of defects in the light transmitting conductive layer 7 can be curbed reliably.
In addition, in the transmissive photocathode 2, the light transmitting substrate 4 is formed of an UV ray transmitting material. In addition, the photoelectric conversion layer 9 is constituted by including antimony or tellurium and an alkali metal. Consequently, in the transmissive photocathode 2 which is highly sensitive in a wavelength range including UV rays, occurrence of defects in the light transmitting conductive layer 7 can be curbed.
Subsequently, with reference to
As illustrated in
The present disclosure is not limited to the foregoing embodiment. For example, the material and the shape of each constitution is not limited to the materials and the shapes described above, and various materials and shapes can be employed. In addition, for example, the transmissive photocathode according to the present disclosure can be used as a transmissive photocathode in an electron tube such as a photoelectric tube, an image intensifier, a streak tube, and an X-ray image intensifier, in addition to a photomultiplier tube.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017-151870 | Aug 2017 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2018/018520 | 5/14/2018 | WO | 00 |
