JPH0533486B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an electron emitting cathode applied to a photocathode, a secondary electron multiplier, or the like.

(Prior Art) Semiconductor crystal wafers made of compound semiconductors such as gallium arsenide and indium gallium arsenide, or semiconductors of the group typified by silicon, have their surfaces coated with alkali metals or alkali metals. When activated with oxygen and oxygen, it provides high photoelectron or secondary electron emission efficiency, so it is used as a photocathode or an electron emission cathode such as an electron multiplier tube.

The difference between photoelectron emission phenomenon and secondary electron emission phenomenon is
It depends on whether the electron emission is caused by incident light or incident electrons, and basically they have many things in common.

Therefore, the electron emission mechanism will be explained mainly using a photoelectron emission cathode as an example.

The photoelectron emission phenomenon occurs through the following three processes.

(1) Excitation process of electrons in the cathode due to light absorption at the photoelectron emission cathode, (2) Transfer process of excited electrons to the emission surface, (3) Escape process of these electrons from the emission surface. Among the electrons excited by the process 1), excited electrons generated too far from the emission surface lose their energy during the movement process before reaching the emission surface and cannot contribute to photoelectron emission.

In other words, excited electrons generated at a location farther than the escape distance do not reach the emission surface and do not contribute to photoelectron emission.

Therefore, in order to increase the photoelectron emission efficiency, that is, the quantum efficiency, increase the light absorption at the cathode,
Moreover, it is necessary to cause this to occur within the escape distance when viewed from the radiation surface.

FIG. 7 is a perspective view showing an example of a conventional cathode for emitting photoelectrons or secondary electrons.

FIG. 8 is an enlarged sectional view illustrating the mechanism of photoelectron emission in the photoelectron or secondary electron emission cathode.

A semiconductor crystal wafer used in a conventional photoelectron emission cathode has a flat plate shape as shown in FIG.

As shown in FIG. 8, consider the case where incident light 1, which is the light to be detected, enters the surface 6 of the photoelectron emission cathode having such a shape.

A part of the incident light is reflected by this surface and becomes reflected light 2, which is dissipated without entering the photoelectron emission cathode again. Therefore, this reflected light 2 does not contribute to photoelectron emission.

In addition, the light 3 that has entered the photoelectron emission cathode 5
excites electrons 4a and 4b in the cathode.

Since the incident surface is the photoelectron emission surface, the excited electrons 4b generated at a location deeper than the escape distance (=diffusion length) cannot contribute to photoelectron emission.

Similarly, in the cathode for secondary electron emission, the incident 1
A part of the secondary electrons is reflected by the surface of the cathode and does not contribute to secondary electron emission.

Furthermore, excited electrons generated at a location deeper than the escape distance cannot contribute to secondary electron emission.

(Problems to be solved by the invention) For the reasons mentioned above, conventional photoelectronic or
The cathode for secondary electron emission has a structure in which there is incident light or primary electrons that are reflected at the surface, and excited electrons generated deeper than the escape distance cannot reach the surface and contribute to electron emission. These factors reduce quantum efficiency.

An object of the present invention is to provide a cathode for emitting photoelectrons or secondary electrons, in which the quantum efficiency is improved by allowing the portions that have caused the reduction in the quantum efficiency to contribute to electron emission.

(Means for Solving the Problem) In order to achieve the above object, the cathode for emitting photoelectrons or secondary electrons according to the present invention is provided with a large number of protrusions on the incident surface of light or primary electrons, and The width is approximately the diffusion length of internally generated electrons.

The incident surface is activated with an alkali metal or an alkali metal and oxygen.

This shape effectively reduces the reflectance for light or primary electrons incident on the surface of the semiconductor crystal wafer, and also makes it possible to emit excited photoelectrons or secondary electrons from a plane different from the incident plane. There is.

The photoelectron emission cathode is used as a reflective photocathode, and the secondary electron emission cathode is used as a photomultiplier or a dynode of an electron multiplier.

(Example) The present invention will be described in more detail below with reference to the drawings and the like.

FIG. 1 is a perspective view showing an embodiment of a cathode for photoelectron emission according to the present invention.

As shown in FIG. 1, a large number of grooves are provided on the light incident surface of a semiconductor crystal wafer (GaAs).

The width and depth of the groove in this example is approximately 3 μm, and the width of the top surface is 1 μm.

Such processing is possible using normal semiconductor processing techniques, photolithography, ion milling, or other processing techniques.

Alternatively, such a shape can be obtained by partially growing a semiconductor crystal on a flat semiconductor crystal wafer by an epitaxial growth method.

The light obliquely incident on the cathode for photoelectron emission having such a shape hits the upper surface 6a of the cathode for photoelectron emission represented by 1a in FIG. Bottom 6c
It is classified into those corresponding to.

Light 1b incident on the side surface 6b of the photoelectron emission cathode
The light is divided into light 3 that enters the photoelectron emission cathode 5 and light 2 that is reflected from the side surface 6b.

The light 3 that has entered the photoelectron emission cathode 5 excites electrons 4a and 4b within the cathode.

Excited electrons 4b generated at a location farther than the escape distance when viewed from the side surface 6b are emitted from the side surface 6d, which is within the escape distance when viewed from the electrons, and contribute to photoelectron emission. The reflected light 2 is incident on the bottom surface 6c, and part of it causes photoelectron emission through the process described above.

The light reflected by the bottom surface 6c enters the side surface 6e again, and part of it causes photoelectron emission through the process described above.

In this way, the quantum efficiency for light incident on the side surface 6b becomes greater than that of the prior art.

Even for light that is directly incident on the bottom surface 6c, the reflected light is incident on the photoelectron emission surface multiple times, resulting in a higher quantum efficiency than in the prior art.

Since electrons excited by the light 1 incident on the top surface 1a can be emitted not only from the top surface but also from the side surfaces 6e and 6f, the quantum efficiency for this light is also higher than in the prior art.

In the case of a cathode for secondary electron emission, if the electron incident surface of a semiconductor crystal wafer is shaped with many grooves as shown in Figure 1, then the incident primary electrons that are reflected on the cathode surface will be absorbed again. The light enters the secondary electron emission cathode and contributes to secondary electron emission.

Furthermore, the secondary electrons generated through excitation can be emitted from a plane different from the incident plane, thereby increasing the secondary electron emission efficiency.

FIG. 3 is a sectional view showing an embodiment of a phototube using the photoelectron emission cathode as a reflective photoelectron emission cathode.

Gallium arsenide (GaAs) is processed into a groove shape on the entrance surface side inside the glass outer cylinder 7, which is a vacuum container.
It consists of a wafer photoelectron emission cathode 5, an anode 8 made of nickel metal, and an inlet 9. The gallium arsenide wafer used was 1×
It is a zinc-doped P-type gallium arsenide wafer with a carrier density of 10 ¹⁹ /cm ³ .

The escape distance of the electrons is about 1 μm, so the width of the convex part that forms the groove is about 1 μm, which is about the same as the escape distance, and the ion milling is done so that the depth of the groove and the width of the bottom are both 3 μm. Created by.

In addition, the surface of the processed gallium arsenide wafer is used for normal NEA photocathode activation.
Activated by an alkali metal or an alkali metal and oxygen.

Incident light 1, which is the light to be detected, enters the photoelectron emission cathode 5, causes photoelectron emission, and becomes a photoelectron flow, which is collected by the anode 8.

A phototube (conventional phototube) having the same shape as the phototube of the above embodiment is used, except that a photoelectron emission cathode 5 whose surface is a gallium arsenide (GaAs) wafer whose surface is not subjected to the above-mentioned processing and activated with an alkali metal or an alkali metal and oxygen is used. Example) will be used to compare with the phototube of the above example.

FIG. 4 shows a comparison of the quantum efficiencies of the photoelectron emission cathodes of the conventional example and the phototube of the above-mentioned example.

The illustrated curve 10 is a curve showing the spectral quantum efficiency of the conventional example, and the curve 11 is a curve showing the spectral quantum efficiency of the example.

From this graph, it can be understood that the quantum efficiency of the photoelectron emission cathode of the example is improved by about 30% over the entire wavelength range.

FIG. 5 is a sectional view showing an embodiment of a photomultiplier tube using the photoelectron emission cathode as a dynode.

A transmission-type antimony cesium photocathode 12 is formed inside a window portion of a glass outer cylinder 7 forming a vacuum container.

Secondary electron emission cathode 1 made of gallium phosphide (GaP) wafer whose photoelectron incident surface side is processed into a groove shape.
3. Anode 8 made of nickel metal, lead-in wire 9a,
9b and an electrode 14 are provided.

The gallium phosphide wafer 13 has a density of 1×10 ¹⁹ /
It is a zinc-doped P-type gallium phosphide wafer with a carrier density of cm ³ .

The surface was also activated with an alkali metal or an alkali metal and oxygen.

As mentioned above, the escape distance of the electrons is about 1 μm, so the width of the convex part forming the groove is about 1 μm, which is about the same as the escape distance, and the depth and width of the groove are both 3 μm. , was prepared by ion milling in the same manner as in the previous example.

Incident light 1, which is the light to be detected, is converted into photoelectrons 15 at the photocathode 12. These photoelectrons are accelerated and focused and incident on the secondary electron emission cathode 13, where they are multiplied and collected at the anode 8.

A gallium phosphide (GaP) wafer whose surface was not subjected to the above-mentioned processing was used, and a photomultiplier tube (conventional example) that was otherwise the same as the photomultiplier tube of the above example was used, and compared with the photomultiplier tube of the above example. do.

FIG. 6 shows a comparison of the secondary electron emission efficiency of the conventional example and the photomultiplier tube of the above embodiment.

The illustrated curve 16 shows the secondary electron emission efficiency of the conventional example. Curve 17 shows the secondary electron emission efficiency of the photomultiplier tube of the example.

From this graph, it can be seen that the secondary electron emission efficiency is significantly improved.

Various modifications can be made to the embodiments described in detail above within the scope of the present invention. In the above embodiment, an example in which groove-like unevenness is provided is shown, but it is also possible to provide an infinite number of cylindrical protrusions.

(Effects of the Invention) As explained in detail above, the cathode for emitting photoelectrons or secondary electrons according to the present invention has small asperities on the surface on which light or primary electrons are incident, and the width of the convex portions is about the same as the diffusion length. It is configured.

Therefore, it is possible to effectively reduce the reflectance of light or primary electrons incident on the surface of the semiconductor crystal wafer, and also to emit excited photoelectrons or secondary electrons from a plane different from the incident plane. The electron emission efficiency can be significantly improved.

[Brief explanation of the drawing]

FIG. 1 is a perspective view showing an embodiment of a cathode for emitting photoelectrons or secondary electrons according to the present invention. FIG. 2 is an enlarged sectional view illustrating the mechanism of photoelectron emission in the photoelectron or secondary electron emission cathode. FIG. 3 is a sectional view showing an example of a phototube using the cathode of the example. FIG. 4 is a graph showing spectral sensitivity curves showing the quantum efficiency of a conventional photoelectron emission cathode and the quantum efficiency of a photoelectron emission cathode improved according to the present invention. FIG. 5 is a sectional view showing an embodiment of a photomultiplier tube using the cathode of the embodiment. FIG. 6 is a graph showing a comparison between the secondary electron emission efficiency of a conventional cathode for secondary electron emission and the secondary electron emission efficiency of a cathode for secondary electron emission using the electrode according to the present invention. FIG. 7 is a perspective view showing an example of a conventional cathode for emitting photoelectrons or secondary electrons. FIG. 8 is an enlarged sectional view illustrating the mechanism of photoelectron emission in the photoelectron or secondary electron emission cathode. 1... Incident light, 2... Reflected light, 3... Light that has entered the cathode, 4... Electrons, 5... Cathode, 6... Cathode surface, 7... Vacuum vessel, 8... Anode, 9 ... Leading line, 10 ... Quantum efficiency of the cathode for photoelectron emission according to the prior art, 11 ... Quantum efficiency of the cathode for photoelectron emission according to the present invention, 12 ... Antimony cesium photocathode, 13 ... Cathode for secondary electron emission , 14... Electrode, 16... Secondary electron emission efficiency of the cathode for secondary electron emission according to the prior art, 17... Secondary electron emission efficiency of the cathode for secondary electron emission according to the present invention.

Claims (1)

[Claims] 1. A device for emitting photoelectrons or secondary electrons, in which a large number of protrusions are provided on the incident surface of light or primary electrons, and the width of the protrusions is set approximately as the diffusion length of internally generated electrons. cathode. 2. The cathode for emitting photoelectrons or secondary electrons according to claim 1, wherein the incident surface is activated with an alkali metal. 3. The cathode for emitting photoelectrons or secondary electrons according to claim 1 or 2, wherein the cathode for emitting photoelectrons or secondary electrons is used as a reflective photocathode. 4. The cathode for emitting photoelectrons or secondary electrons according to claim 1 or 2, wherein the cathode for emitting photoelectrons or secondary electrons is used as a photomultiplier tube or a dynode of an electron multiplier tube.