Patent Publication Number: US-2022230860-A1

Title: Photocathode, electron tube, and method for manufacturing photocathode

Description:
TECHNICAL FIELD 
     The present disclosure relates to a photocathode, an electron tube, and a method for manufacturing a photocathode. 
     BACKGROUND ART 
     Patent Literature 1 describes a photocathode. This photocathode includes a supporting substrate, a photoelectron emitting layer provided on the supporting substrate, and an underlayer provided between the supporting substrate and the photoelectron emitting layer. The underlayer contains an oxide of a beryllium alloy or a beryllium oxide. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Patent No. 5342769 
     SUMMARY OF INVENTION 
     Technical Problem 
     In the photocathode described in Patent Literature 1, by providing the underlayer containing a beryllium element between the supporting substrate and the photoelectron emitting layer, an improvement in an effective quantum efficiency is tried to be attained. On the other hand, in the above-described technical field, an improvement in productivity is demanded. 
     An object of the present disclosure is to provide a photocathode, an electron tube, and a method for manufacturing a photocathode which are capable of improving productivity. 
     Solution to Problem 
     The present inventor has conducted intensive studies in order to solve the above-described problem, and thus has attained the following finding. That is, an underlayer containing a nitride of beryllium has a higher productivity (is more efficiently manufactured) than an underlayer of an oxide of a beryllium alloy or a beryllium oxide. The present disclosure is made based on such a finding. 
     That is, a photocathode according to the present disclosure includes a substrate, a photoelectric conversion layer provided on the substrate and configured to generate photoelectrons in response to incidence of light, and an underlayer provided between the substrate and the photoelectric conversion layer and containing beryllium, in which the underlayer has a first underlayer containing a nitride of beryllium. 
     In this photocathode, the underlayer containing beryllium is provided between the substrate and the photoelectric conversion layer. Further, the underlayer has the first underlayer containing a nitride of beryllium. Therefore, as shown in the above finding, the underlayer is efficiently manufactured. Thus, according to this photocathode, the productivity can be improved. 
     In the photocathode according to the present disclosure, the underlayer may have a second underlayer provided between the first underlayer and the photoelectric conversion layer and containing an oxide of beryllium. In this case, the quantum efficiency is improved. 
     In the photocathode according to the present disclosure, an amount of the oxide of beryllium may be larger than an amount of the nitride of beryllium in the second underlayer. In this case, the quantum efficiency is reliably improved. 
     In the photocathode according to the present disclosure, the underlayer may be in contact with the substrate. In this case, since the underlayer can formed directly on the substrate, the productivity is further improved. 
     In the photocathode according to the present disclosure, the photoelectric conversion layer may be in contact with the underlayer. In this case, the quantum efficiency is further improved. 
     In the photocathode according to the present disclosure, the substrate may be composed of a material that transmits the light. In this case, a transmissive photocathode can be configured. 
     In the photocathode according to the present disclosure, the amount of the oxide of beryllium may be larger than the amount of the nitride of beryllium in the underlayer. In this case, the quantum efficiency of the photocathode is improved, and the underlayer can function as the underlayer in a wider wavelength range. 
     In the photocathode according to the present disclosure, in the underlayer, the amount of at least one of the nitride of beryllium and the oxide of beryllium may be unevenly distributed in a thickness direction of the underlayer. At this time, in the underlayer, the amount of the nitride of beryllium may be larger on the substrate side than on the photoelectric conversion layer side, and the amount of the oxide of beryllium may be larger on the photoelectric conversion layer side than on the substrate side. 
     Alternatively, in the photocathode according to the present disclosure, in the underlayer, the amount of the nitride of beryllium may be substantially uniformly distributed in the thickness direction of the underlayer, and the amount of the oxide of beryllium may be substantially uniformly distributed in the thickness direction of the underlayer. In any of these cases, the quantum efficiency of the photocathode is further improved, and the underlayer can function as the underlayer in a wider wavelength range. 
     An electron tube according to the present disclosure includes any of the above-described photocathodes and an anode configured to collect electrons. According to this electron tube, the productivity can be improved by the aforementioned reasons. 
     A method for manufacturing a photocathode according to the present disclosure includes a first step of preparing a substrate, a second step of forming an underlayer containing beryllium on the substrate, and a third step of forming a photoelectric conversion layer configured to generate photoelectrons in response to incidence of light on the underlayer, in which the second step has a forming step of forming an intermediate layer containing a nitride of beryllium on the substrate, and a treatment step of performing an oxidation treatment with respect to the intermediate layer so as to form a first underlayer provided on the substrate and containing a nitride of beryllium and a second underlayer provided on the first underlayer and containing an oxide of beryllium as the underlayer. 
     In this manufacturing method, after the intermediate layer containing a nitride of beryllium is formed on the substrate, by the oxidation treatment of this intermediate layer, the underlayer including a first underlayer containing a nitride of beryllium and a second underlayer containing an oxide of beryllium is formed. Therefore, as shown in the above finding, the underlayer is efficiently manufactured. Furthermore, the quantum efficiency is improved. Thus, according to this manufacturing method, the productivity of the photocathode with improved quantum efficiency is improved. 
     In the method for manufacturing a photocathode according to the present disclosure, in the forming step, the intermediate layer may be formed by evaporation or sputtering of beryllium in a nitrogen atmosphere. In this way, by evaporation or sputtering of beryllium in a nitrogen atmosphere, the underlayer (intermediate layer) can be efficiently manufactured. 
     In the method for manufacturing a photocathode according to the present disclosure, in the forming step, the intermediate layer may be formed by evaporation or sputtering of beryllium in a state of mixing an inert gas different from nitrogen in a nitrogen atmosphere. In this case, the underlayer (intermediate layer) can be more efficiently manufactured. 
     In the method for manufacturing a photocathode according to the present disclosure, the oxidation treatment may include a heating treatment and/or a discharge treatment. In this way, as the oxidation treatment for the second underlayer, the heating treatment or the discharge treatment is effective. 
     In the method for manufacturing a photocathode according to the present disclosure, in the treatment step, the oxidation treatment may be performed so that an amount of the oxide of beryllium is larger than an amount of the nitride of beryllium in the second underlayer. In this case, a photocathode with reliably improved quantum efficiency can be manufactured. 
     In the method for manufacturing a photocathode according to the present disclosure, in the second step, the underlayer may be formed directly on the substrate. In this case, the productivity is further improved. 
     In the method for manufacturing a photocathode according to the present disclosure, in the third step, the photoelectric conversion layer may be formed directly on the underlayer. In this case, a photocathode with further improved quantum efficiency can be manufactured. 
     In the method for manufacturing a photocathode according to the present disclosure, the substrate may be composed of a material that transmits the light. In this case, a transmissive photocathode can be manufactured. 
     Advantageous Effects of Invention 
     According to the present disclosure, it is possible to provide a photocathode, an electron tube, and a method for manufacturing a photocathode which are capable of improving productivity. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating an electron tube (photomultiplier) according to the present embodiment. 
         FIG. 2  is a partial cross-sectional view of a photocathode illustrated in  FIG. 1 . 
         FIG. 3  is a schematic cross-sectional view for describing a method for manufacturing the photocathode illustrated in  FIGS. 1 and 2 . 
         FIG. 4  is a schematic cross-sectional view for describing the method for manufacturing the photocathode illustrated in  FIGS. 1 and 2 . 
         FIG. 5  is a schematic cross-sectional view for describing the method for manufacturing the photocathode illustrated in  FIGS. 1 and 2 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment will be specifically described with reference to the drawings. Note that, in each drawing, the same or equivalent elements are denoted by the same reference numerals, and duplicate description may be omitted. 
       FIG. 1  is a schematic cross-sectional view illustrating a photomultiplier as an example of an electron tube according to the present embodiment. A photomultiplier (electron tube)  10  illustrated in  FIG. 1  includes a photocathode  1 , a container  32 , a focusing electrode  36 , an anode  38 , a multiplication unit  40 , a stem pin  44 , and a stem plate  46 . The container  32  has a tubular shape and is configured as a vacuum housing by sealing one end by an input window  34  (herein, a substrate  100  of the photocathode  1 ) and sealing the other end by the stem plate  46 . The focusing electrode  36 , the anode  38 , and the multiplication unit  40  are disposed in the container  32 . 
     The input window  34  transmits incident light hv. The photocathode  1  emits photoelectrons e −  in response to the incident light hv from the input window  34 . The focusing electrode  36  guides the photoelectrons e −  emitted from the photocathode  1  to the multiplication unit  40 . The multiplication unit  40  includes a plurality of dynodes  42  and multiplies secondary electrons generated in response to incidence of the photoelectrons e−. The anode  38  collects the secondary electrons generated by the multiplication unit  40 . The stem pin  44  is provided to penetrate through the stem plate  46 . The corresponding focusing electrode  36 , anode  38 , and dynodes  42  are electrically connected to the stem pin  44 . 
       FIG. 2  is a partial cross-sectional view of a photocathode illustrated in  FIG. 1 .  FIG. 2( b )  is an enlarged view of a region A of  FIG. 2( a ) . As illustrated in  FIG. 2 , the photocathode  1  is configured as a transmissive type. The photocathode  1  has the substrate  100 , an underlayer  200 , and a photoelectric conversion layer  300 . The substrate  100  is composed of a material that transmits the light (incident light hv). The substrate  100  includes a surface  101   a  and a surface (first surface)  102   a  on a side opposite to the surface  101   a . The surface  101   a  is a surface facing the outside of the container  32  and is an incident surface of the incident light  1   v  herein. The underlayer  200  is provided on the surface  102   a . The underlayer  200  is in contact with the surface  102   a . That is, the underlayer  200  is formed directly on the substrate  100  (surface  102   a ). 
     The underlayer  200  has a surface  200   a  on a side opposite to the surface  102   a . The photoelectric conversion layer  300  is provided on the surface (second surface)  200   a . In other words, the photoelectric conversion layer  300  is provided on the substrate  100 , and the underlayer  200  is provided between the substrate  100  and the photoelectric conversion layer  300 . The photoelectric conversion layer  300  is in contact with the surface  200   a  of the underlayer  200 . That is, the photoelectric conversion layer  300  is provided directly on the underlayer  200  (surface  200   a ). In this way, in the photocathode  1 , the underlayer  200  and the photoelectric conversion layer  300  are sequentially stacked on the substrate  100 . The photoelectric conversion layer  300  receives the incidence of the incident light hv through the substrate  100  and the underlayer  200  and generates the photoelectrons e −  in response to this incident light hv. That is, herein, the photocathode  1  is a transmissive photocathode. 
     Herein, a first specific example of the configuration of the underlayer  200  will be described. In this first specific example, the underlayer  200  contains a nitride of beryllium (for example, beryllium nitride). More specifically, the underlayer  200  includes a first underlayer  210  containing a nitride of beryllium and a second underlayer  220  containing an oxide of beryllium (for example, beryllium oxide). The first underlayer  210  has a surface (third surface)  210   a  on a side opposite to the surface  102   a  of the substrate  100 . The second underlayer  220  is provided on the surface  210   a . In other words, the second underlayer  220  is provided between the first underlayer  210  and the photoelectric conversion layer  300 . Herein, the second underlayer  220  is in contact with the surface  210   a  of the first underlayer  210 . Note that, as described below, the surface  210   a  is not limited to a surface having a clear boundary as illustrated in the drawing, and may be an imaginary surface. 
     The second underlayer  220  has a surface on a side opposite to the surface  102   a  of the substrate  100  and the surface  210   a  of the first underlayer  210 . This surface of the second underlayer  220  is the surface  200   a  of the underlayer  200  herein. Furthermore, the first underlayer  210  is in contact with the surface  102   a  of the substrate  100 . That is, herein, the underlayer  200  is in contact with the substrate  100  (surface  102   a ) in the first underlayer  210  and is in contact with the photoelectric conversion layer  300  in the second underlayer  220 . 
     The amount of the oxide of beryllium is larger than the amount of the nitride of beryllium in the second underlayer  220 . In other words, the amount of the oxide of beryllium is equal to or less than the amount of the nitride of beryllium in the first underlayer  210 . The surface  210   a  of the first underlayer  210  may be defined as a boundary between a region where the amount of the oxide of beryllium is larger than the amount of the nitride of beryllium and a region where the amount of the oxide of beryllium is equal to or less than the amount of the nitride of beryllium in a depth direction of the underlayer  200  (a direction intersecting the surface  200   a  of the underlayer  200 ). In this case, the first underlayer  210  and the second underlayer  220  may be continuously formed, and thus the surface  210   a  may be an imaginary surface. 
     A ratio of the amount of the oxide of beryllium and the amount of the nitride of beryllium is, for example, a ratio of the numbers of atoms. In this case, a region which includes the surface  200   a  of the underlayer  200  (in the depth direction from the surface  200   a ) and where the ratio of the numbers of atoms of oxygen is larger than the ratio of the numbers of atoms of nitrogen is regarded as the second underlayer  220 , and a region on the substrate  100  side in relation to this region may be regarded as the first underlayer  210 . Examples of an analysis method of the numbers of atoms include X-ray photoelectron spectroscopy and Auger electron spectroscopy. 
     The thickness of the entire underlayer  200  is, for example, about 200 Å to 800 Å. The thickness of the first underlayer  210  is, for example, about 200 Å to 700 Å. The thickness of the second underlayer  220  is, for example, about 0 to 100 Å. The ratio of the thickness of the second underlayer  220  to the thickness of the first underlayer  210  is, for example, about 0 to 0.5. The oxygen atom percentage in the second underlayer  220  is, for example, about 30 at % to 100 at %. Note that, in the photocathode  1 , the second underlayer  220  may not be provided (that is, “0” may be selected from the above thickness range of the second underlayer  220 ), and in this case, the thickness of the first underlayer  210  may be consistent with the thickness of the entire underlayer  200 . In a case where the second underlayer  220  is provided, the lower limit of the thickness of the second underlayer  220  is, for example, 1 Å. 
     Subsequently, a second specific example of the configuration of the underlayer  200  will be described. In this second specific example, the underlayer  200  contains a nitride of beryllium (for example, beryllium nitride). Furthermore, the underlayer  200  may contain oxygen. The oxygen may be contained as an oxide of beryllium (for example, beryllium oxide) in the underlayer  200 . In a case where the underlayer  200  is considered as a layer including two regions of a first region  210 R on the substrate  100  side and a second region  220 R on the photoelectric conversion layer  300  side (for example, a layer composed of the first region  210 R and the second region  220 R), the distribution of the nitride of beryllium and the oxide of beryllium in the first region  210 R and the second region  220 R may have various forms. 
     For example, in the underlayer  200 , the amount of at least one of the nitride of beryllium and the oxide of beryllium may be unevenly distributed in the thickness direction of the underlayer  200  (that is a direction intersecting the surface  200   a , the direction toward the photoelectric conversion layer  300  from the substrate  100 ). More specifically, in the underlayer  200 , there may be a difference in distribution of the nitride of beryllium and the oxide of beryllium between the first region  210 R and the second region  220 R. 
     For example, in the underlayer  200 , the amount of the nitride of beryllium may be larger in the first region  210 R than in the second region  220 R, and the amount of the oxide of beryllium may be larger in the second region  220 R than in the first region  210 R. Further, there may be a difference in amount between the nitride of beryllium and the oxide of beryllium to the extent that the first region  210 R and the second region  220 R can be identified as different layers from each other with the surface  210   a  interposed therebetween. In this case, the first region  210 R can be regarded as a nitride layer of beryllium and the second region  220 R can be regarded as an oxide layer of beryllium. 
     On the other hand, in the underlayer  200 , the amount of the nitride of beryllium may be substantially uniformly distributed in the thickness direction of the underlayer  200 , and the amount of the oxide of beryllium may also be substantially uniformly distributed in the thickness direction of the underlayer  200 . In other words, over at least two regions of the first region  210 R and the second region  220 R, the amount of the nitride of beryllium may be substantially uniformly distributed in the thickness direction thereof, and the amount of the oxide of beryllium may also be substantially uniformly distributed in the thickness direction thereof. 
     Further, also in any cases, the amount of the oxide of beryllium may be larger than the amount of the nitride of beryllium. Furthermore, also in any cases, the above-described distribution is not necessarily reliably shown over the entire underlayer  200 , and basically, it is determined that the above-described distribution is subjectively shown but a region showing a different tendency may also slightly exist. 
     Furthermore, the above-described first and second specific examples may be arbitrarily combined with each other. For example, the first region  210 R and the second region  220 R in the second specific example can be replaced by the first underlayer  210  and the second underlayer  220  in the first specific example. In this case, the ranges of the thicknesses of the first underlayer  210  and the second underlayer  220  in the first specific example may be applied to the first region  210 R and the second region  220 R in the second specific example. 
     The photoelectric conversion layer  300  is, for example, composed of a compound of antimony (Sb) and an alkali metal. The alkali metal may include, for example, at least any of cesium (Cs), potassium (K), and sodium (Na). The photoelectric conversion layer  300  functions as an active layer of the photocathode  1 . The thickness of the photoelectric conversion layer  300  is, for example, about 100 Å to 2500 Å. The thickness of the entire photocathode  1  is, for example, about 300 Å to 3300 Å. 
     Subsequently, a method for manufacturing the photocathode  1  will be described.  FIGS. 3 to 5  are schematic cross-sectional views for describing a method for manufacturing the photocathode illustrated in  FIGS. 1 and 2 .  FIG. 3( c )  is an enlarged view of a region F of  FIG. 3( b ) .  FIG. 4( b )  is an enlarged view of a region G of  FIG. 4( a ) . In this manufacturing method, first, as illustrated in  FIG. 3( a ) , the substrate  100  is prepared (first step). Herein, the container  32  configured by sealing one end by the substrate  100  is prepared. Subsequently, the underlayer  200  containing beryllium is formed on the substrate  100  (surface  102   a ) (second step). The second step will be specifically described. 
     In the second step, first, an intermediate layer  400  containing a nitride of beryllium (for example, beryllium nitride) is formed on the substrate  100  (surface  102   a ) (forming step). More specifically, first, the container  32  (substrate  100 ) subjected to a washing treatment is disposed in a chamber B. Furthermore, a beryllium source C is disposed in the chamber B to face the substrate  100  (surface  102   a ). Then, while the atmosphere inside the chamber B is replaced by a nitrogen atmosphere, the intermediate layer  400  is formed directly on the substrate  100  (surface  102   a ) by evaporation or sputtering of beryllium in that nitrogen atmosphere (see  FIGS. 3( b ) and 3( c ) ). The atmosphere inside the chamber B at this time may be composed of only nitrogen or may be mixed with an inert gas different from nitrogen. As the inert gas, for example, argon, helium, neon, krypton, xenon, hydrogen, and the like are mentioned. 
     As an evaporation method, resistive heating vapor deposition, chemical vapor deposition, and the like can be used. As the sputtering, DC magnetron reactive sputtering, RF magnetron sputtering (non-reactive), RF magnetron reactive sputtering, or the like can be used. 
     In the subsequent step, as illustrated in  FIG. 3( b ) , the other end of the container  32  is sealed by the stem plate  46  attached with the focusing electrode  36 , the anode  38 , and the multiplication unit  40 . An evaporation source D is disposed in the focusing electrode  36 . Furthermore, in the stem plate  46 , an alkali metal source E is disposed through the stem pin  44 . In this state, as illustrated in  FIG. 4 , the underlayer  200  is formed from the intermediate layer  400  by the oxidation treatment of the intermediate layer  400  (treatment step). More specifically, in the treatment step, the oxidation treatment is performed with respect to the intermediate layer  400  from a side in the intermediate layer  400  opposite to the substrate  100 . Thereby, a film-shaped region, which includes a surface  400   a  in the intermediate layer  400  on a side opposite to the substrate  100  and contains a nitride of beryllium, is substituted with a region containing an oxide of beryllium. As a result, the first underlayer  210  and the second underlayer  220  are formed, and the underlayer  200  is obtained. 
     That is, in the treatment step, the oxidation treatment is performed with respect to the intermediate layer  400  from a side opposite to the substrate  100  (surface  102   a ) so that the first underlayer  210  provided on the substrate  100  (surface  102   a ) and containing a nitride of beryllium and the second underlayer  220  provided on the surface  210   a  in the first underlayer  210  on a side opposite to the substrate  100  (surface  102   a ) and containing an oxide of beryllium are formed as the underlayer  200 . The method of the oxidation treatment is, for example, a heating treatment and/or a discharge treatment. 
     In the case of oxidation by discharge, DC discharge oxidation, AC discharge oxidation (for example, RF discharge oxidation), or the like can be used. In the case of utilizing glow discharge as the method of the oxidation treatment, after oxygen is appropriately enclosed in the container  32  set in a vacuum state, a voltage is applied between the focusing electrode  36  and the container  32  (substrate  100 ), and the region containing a nitride of beryllium is substituted with the region containing an oxide of beryllium from the surface  400   a  side of the intermediate layer  400 . The pressure (gas pressure) in the container  32  at this time is, for example, about 0.01 Pa to 1000 Pa. 
     Note that, in the forming step, the underlayer  200  containing a nitride of beryllium and an oxide of beryllium is formed by using an atmosphere containing nitrogen and oxygen, and thus this oxidation treatment (treatment step) may be omitted. Alternatively, the amount of the oxide of beryllium in the underlayer  200  may be further increased by further executing this oxidation treatment (treatment step). As the oxidation treatment method, in addition to oxidation by discharge or oxidation by heat as mentioned above, oxidation by light, oxidation by an oxidative atmosphere (such as ozone or water-vapor atmosphere) or an oxidant (such as an oxidizing solution), a combination thereof, and the like can be used. Further, by changing conditions of the oxidation treatment method, the underlayer  200  with the distribution as mentioned above can be obtained. 
     In the subsequent step, as illustrated in  FIG. 5 , the photoelectric conversion layer  300  is formed on the surface  200   a  of the underlayer  200  on a side opposite to the substrate  100  (third step). More specifically, in the third step, first, as illustrated in  FIG. 5( a ) , an intermediate layer  500  is formed on the surface  200   a  by evaporation of antimony using the evaporation source D. Subsequently, as illustrated in  FIG. 5( b ) , the intermediate layer  500  is activated by supplying vapor of alkali metal from the alkali metal source E to the intermediate layer  500 . Thereby, the photoelectric conversion layer  300  composed of a compound of antimony and an alkali metal is formed from the intermediate layer  500 . 
     As described above, in the photocathode  1  according to the present embodiment, the underlayer  200  containing beryllium is provided between the substrate  100  and the photoelectric conversion layer  300 . Further, the underlayer  200  has the first underlayer  210  containing a nitride of beryllium. According to the finding of the present inventor, the film formation rate of a film containing a nitride of beryllium becomes higher than the film formation rate of a film containing an oxide of beryllium, for example, by sputtering in a nitrogen atmosphere, or the like. That is, the underlayer  200  is efficiently manufactured. Thus, according to this photocathode  1 , the productivity is improved. Note that, according to the finding of the present inventor, in the case of using the underlayer  200  containing a nitride of beryllium, sufficient sensitivity (quantum efficiency) can also be secured. 
     Furthermore, in the photocathode  1  according to the present embodiment, the underlayer  200  has the second underlayer  220  provided between the first underlayer  210  and the photoelectric conversion layer and containing an oxide of beryllium. Therefore, the quantum efficiency is improved. 
     Furthermore, in the photocathode  1  according to the present embodiment, the amount of the oxide of beryllium is larger than the amount of the nitride of beryllium in the second underlayer  220 . Therefore, the quantum efficiency is reliably improved. Furthermore, in the photocathode  1  according to the present embodiment, the underlayer  200  is in contact with the substrate  100 . Therefore, since the underlayer  200  can be formed directly on the substrate  100 , the productivity is further improved. 
     Furthermore, in the photocathode  1  according to the present embodiment, the photoelectric conversion layer  300  is in contact with the underlayer  200 . Therefore, the quantum efficiency is further improved. More specifically, when the underlayer  200  containing beryllium is provided in a state of being in contact with the photoelectric conversion layer  300 , the diffusion of an alkali metal (for example, potassium or cesium) contained in the photoelectric conversion layer  300  is effectively suppressed in the manufacturing process, and as a result, it is considered to realize a highly effective quantum efficiency. Moreover, the underlayer  200  functions so as to reverse a direction of, out of photoelectrons generated in the photoelectric conversion layer  300 , photoelectrons traveling toward the substrate  100  side to the photoelectric conversion layer  300  side, and as a result, it is considered to improve the quantum efficiency of the photocathode  1  as a whole. 
     Note that, the photocathode  1  includes the underlayer  200  containing beryllium. In this way, by using the underlayer  200  containing beryllium, an effective quantum efficiency is further improved and the sensitivity is improved. 
     Furthermore, in the photocathode  1 , the underlayer  200  may contain an oxide of beryllium. In this case, the quantum efficiency of the photocathode  1  is improved, and the underlayer can function as the underlayer  200  in a wider wavelength range. 
     Furthermore, in the photocathode  1 , the amount of the oxide of beryllium may be larger than the amount of the nitride of beryllium in the underlayer  200 . In this case, the quantum efficiency of the photocathode  1  is further improved, and the underlayer can function as the underlayer in a wider wavelength range. 
     Furthermore, in the photocathode  1 , in the underlayer  200 , the amount of at least one of the nitride of beryllium and the oxide of beryllium may be unevenly distributed in the thickness direction of the underlayer  200 , the amount of the nitride of beryllium may be substantially uniformly distributed in the thickness direction of the underlayer  200 , and the amount of the oxide of beryllium may be substantially uniformly distributed in the thickness direction of the underlayer  200 . In the case of uneven distribution, when the underlayer  200  is regarded as a layer including two regions of the first region  210 R on the substrate  100  side and the second region  220 R on the photoelectric conversion layer  300  side, in the underlayer  200 , the amount of the nitride of beryllium may be larger on the first region  210 R side (substrate  100  side) than on the second region  220 R side (photoelectric conversion layer  300  side), and the amount of the oxide of beryllium may be larger on the second region  220 R side (photoelectric conversion layer  300  side) than on the first region  210 R side (substrate  100  side). Further, the first region  210 R and the second region  220 R may be the first underlayer and the second underlayer stacked alternately, and the second underlayer may be positioned on the photoelectric conversion layer  300  side in relation to the first underlayer and may contain an oxide of beryllium. Also in any cases, the quantum efficiency of the photocathode  1  is further improved, and the underlayer can function as the underlayer in a wider wavelength range. 
     Herein, in the method for manufacturing the photocathode  1  according to the present embodiment, after the intermediate layer  400  containing a nitride of beryllium is formed on the substrate  100 , by the oxidation treatment of this intermediate layer  400 , the underlayer  200  including the first underlayer  210  containing a nitride of beryllium and the second underlayer  220  containing an oxide of beryllium is formed. Therefore, as shown in the above finding, the underlayer  200  is efficiently manufactured. Furthermore, the quantum efficiency is improved. Thus, according to this manufacturing method, the productivity of the photocathode  1  with improved quantum efficiency is improved. 
     Furthermore, in the method for manufacturing the photocathode  1  according to the present embodiment, in the forming step, the intermediate layer  400  is formed by evaporation or sputtering of beryllium in a nitrogen atmosphere. In this way, by evaporation or sputtering of beryllium in a nitrogen atmosphere, the underlayer  200  (intermediate layer  400 ) can be efficiently manufactured. 
     Furthermore, in the method for manufacturing the photocathode  1  according to the present embodiment, in the forming step, the intermediate layer  400  is formed by evaporation or sputtering of beryllium in a state of mixing an inert gas different from nitrogen in a nitrogen atmosphere. Therefore, the underlayer  200  (intermediate layer  400 ) can be more efficiently manufactured. 
     Furthermore, in the method for manufacturing the photocathode  1  according to the present embodiment, as the oxidation treatment for forming the second underlayer  220 , a heating treatment or a discharge treatment is effective. According to the finding of the present inventor, by utilizing oxidation by glow discharge as the oxidation treatment, an improvement in the sensitivity (quantum efficiency) can be attained as compared to oxidation by heat. 
     Furthermore, in the method for manufacturing the photocathode  1  according to the present embodiment, in the treatment step, the oxidation treatment is performed so that the amount of the oxide of beryllium is larger than the amount of the nitride of beryllium in the second underlayer  220 . Thereby, a photocathode with reliably improved quantum efficiency can be manufactured. 
     Furthermore, in the method for manufacturing the photocathode  1  according to the present embodiment, in the second step, the underlayer  200  is formed directly on the substrate  100 . Therefore, the productivity is further improved. Moreover, in the method for manufacturing the photocathode  1  according to the present embodiment, in the third step, the photoelectric conversion layer  300  is formed directly on the underlayer  200 . Therefore, as shown in the above finding, the photocathode  1  with further improved quantum efficiency can be manufactured. 
     The above embodiment is to describe an embodiment of the present disclosure. Thus, the present disclosure is not limited to the above-described embodiment, and various modifications may be made. For example, in the above-described embodiment, the photocathode  1  has been described as a transmissive type, but the photocathode  1  can also be configured as a reflective type. Furthermore, another layer may be interposed between the substrate  100  (surface  102   a ) and the underlayer  200  and/or between the underlayer  200  (surface  200   a ) and the photoelectric conversion layer  300 . 
     Furthermore, in the above-described embodiment, the first underlayer  210  and the second underlayer  220  were formed by the oxidation treatment of the intermediate layer  400  containing a nitride of beryllium. On the other hand, the first underlayer  210  and the second underlayer  220  may be formed by forming a film containing a nitride of beryllium (a layer that becomes the first underlayer  210 ) and then newly forming a film containing an oxide of beryllium (a layer that becomes the second underlayer) with respect to that film. In this case, the surface  210   a  between the first underlayer  210  and the second underlayer  220  may be an actually existing surface. 
     INDUSTRIAL APPLICABILITY 
     A photocathode, an electron tube, and a method for manufacturing a photocathode which are capable of improving productivity are provided. 
     REFERENCE SIGNS LIST 
       1 : photocathode,  10 : photomultiplier (electron tube),  100 : substrate,  200 : underlayer,  210 : first underlayer,  220 : second underlayer,  300 : photoelectric conversion layer,  400 ,  500 : intermediate layer.