JP3615857B2 - Photocathode and electron tube using the same - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a photocathode made of a group III-V compound semiconductor, in particular, a so-called reflective photocathode that emits photoelectrons from an incident surface of light to be detected, and an electron tube using the photocathode.
[0002]
[Prior art]
Conventionally, since GaAs constituting the active layer has a negative electron affinity, the photocathode provided with it has high performance. FIG. 7 shows a so-called reflection type photocathode as an example, and an active layer 21 made of p-type GaAs is epitaxially grown on a semiconductor substrate 10 made of GaAs. A surface layer 22 made of Cs ₂ O is formed on the surface of the active layer 21 that has been heated and cleaned in a vacuum vessel (not shown) in order to lower its work function. The reflection type photocathode having high sensitivity has been obtained, and a photomultiplier tube having the same has already been put into practical use.
[0003]
[Problems to be solved by the invention]
On the other hand, a photocathode having an active layer 21 made of GaAsP having a larger energy gap than GaAs is also disclosed, and has sensitivity to wavelengths shorter than 900 nm, particularly in the visible light region. However, even if the composition of GaAsP constituting the active layer 21 is changed, there is no suitable semiconductor substrate 10 that lattice matches with that. Therefore, when GaAs or GaP that is not lattice-matched with the GaAsP active layer 21 is used as the semiconductor substrate 10, the photocathode whose crystallinity of the GaAsP active layer 21 is lowered as a result cannot obtain sensitivity higher than expected. . Actually, as is apparent from the relationship between the energy gap and the lattice constant of the III-V compound semiconductor and mixed crystal thereof shown in FIG. 8, when the active layer 21 is made of GaAsP, the semiconductor substrate 10 having the same lattice constant as that of the semiconductor substrate 10 is obtained. It is essentially difficult to obtain a high quality GaAsP active layer 21 that does not exist.
[0004]
In order to alleviate the lattice irregularity between the GaAsP active layer 21 and the semiconductor substrate 10, the active layer 21 may have a GaAsP composition inclined as shown in FIG. 9, or a GaAs / As shown in FIG. Although the GaAsP superlattice layer 11 is interposed between the semiconductor substrate 10 and the GaAsP active layer 21, no substantial solution has been achieved. Therefore, when a photocathode having the above-described problems is incorporated in an electron tube such as a photomultiplier tube, it is difficult to detect faint light efficiently.
[0005]
Therefore, as a result of studying various combinations of material configurations in which there is almost no lattice irregularity between the semiconductor substrate and the epitaxial layer, the inventor essentially solved the above problems by using a GaInP-based material. I found out that I can do it. The present invention has been completed based on such knowledge, and provides a highly sensitive photocathode in the visible light region and an electron tube using the photocathode.
[0006]
[Means for Solving the Problems]
The photocathode according to the present invention includes a group III-V compound semiconductor substrate containing Ga and As as main components, and a carrier concentration of Ga _x In _1-x P on the semiconductor substrate of 1 × 10 ¹⁸ cm ⁻³ or more. An active layer formed of a p-type compound semiconductor, which absorbs light to be detected and generates photoelectrons, and is formed on the active layer central portion with an alkali metal or an oxide thereof or a fluoride thereof. A photocathode having a surface layer for lowering a work function, wherein the active composition has an atomic composition ratio x in a range of 0 <x ≦ 0.75. As a result, there is little lattice mismatch between the semiconductor substrate and the active layer epitaxially grown thereon, and the GaInP active layer efficiently generates photoelectrons by visible light, so that the photocathode has high efficiency. Photoelectrons can be emitted to the outside.
[0007]
Moreover, the semiconductor substrate has a buffer layer made of (Al _y Ga _1-y ) _{x ′} In _{1-x ′} P having an energy gap larger than that of the active layer on the upper surface, and the atomic composition ratio x ′ of the buffer layer is It may be characterized by being equal to the atomic composition ratio x of the active layer. The buffer layer makes it difficult for crystal defects to be propagated to the active layer of the semiconductor substrate, so that crystal defects in the active layer can be reduced.
[0008]
Further, the range of the atomic composition ratio x of the active layer may be 0.45 <x <0.55. Thereby, crystal defects in the active layer are further suppressed.
[0009]
An electron tube using the above photocathode according to the present invention includes a photocathode, a vacuum vessel that contains the photocathode inside and is kept in a vacuum state, and is installed inside the vacuum vessel and is positive with respect to the photocathode. And an anode that holds the voltage of at least. Thereby, the photoelectron signal from the photocathode can be converted into an electric signal.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
An embodiment of the present invention will be described for each embodiment with reference to the drawings.
[0011]
FIG. 1 is a sectional view of a first embodiment of the photocathode according to the present invention. On a commercially available semiconductor substrate 10 made of p-type GaAs having a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ and having a thickness of 350 μm, p-type Ga _x In _1-x P having a carrier concentration of 1 × 10 ¹⁹ cm ^−3. The active layer 21 having a thickness of 5 μm is formed by vapor phase epitaxial growth in the range of 0 <x ≦ 0.75, and absorbs light to be detected as a detection target to generate photoelectrons. As shown in FIG. 2, the limit value x = 0.75 described above is a limit composition in which the Ga _x In _1-x P active layer 21 can be a direct transition semiconductor, and x> 0.75. In addition, since the Ga _x In _1-x P active layer 21 is an indirect transition semiconductor, the absorption coefficient is abruptly decreased, and the photocathode sensitivity is expected to be rapidly decreased.
[0012]
Therefore, the inventor made a prototype of Ga _x In _1-x P active layer 21 and measured the quantum efficiency, which is one of the indicators of photoelectric sensitivity. FIG. 3 shows the result of measuring the quantum efficiency at a wavelength of 500 nm with respect to the range of 0 ≦ x ≦ 0.8. As theoretically expected, it was confirmed that no photoelectron was generated in the Ga _x In _1-x P active layer at x = 0.8, and the quantum efficiency was drastically decreased at x ≧ 0.75. .
[0013]
Moreover, as shown in the experimental results of FIG. 3, it is clear that the quantum efficiency of the prototype Ga _x In _1-x P active layer 21 is maximized in the range of 0.45 ≦ x ≦ 0.55. It was. Therefore, within the above range, almost no crystal defects are introduced into the Ga _x In _1-x P active layer 21, and a highly sensitive photocathode in the visible light region can be realized.
[0014]
As described above, when the value of the atomic composition ratio x of the Ga _x In _1-x P active layer 21 changes, the energy gap of the active layer 21 changes accordingly. That is, the wavelength at which the photocathode sensitivity falls can be arbitrarily changed on the long wavelength side of the spectral sensitivity characteristic. However, at this time, _if the degree of lattice mismatch between the Ga _x In _1-x P active layer 21 and the GaAs semiconductor substrate 10 exceeds 0.5%, crystal defects due to the lattice imperfection are introduced into the active layer 21, A reduction in surface sensitivity is inevitable. However, if the degree of lattice irregularity between them is within 0.5%, the strain stress existing inside the lattice constituting the active layer 21 is relaxed by the deformation of the lattice, so that crystal defects are introduced. It may not be done. Therefore, according to the above, the x range of the Ga _x In _1-x P active layer 21 of the present invention is 0.45 <x <0.55, which is in good agreement with the experimental results.
[0015]
In the present invention, the GaAs semiconductor substrate 10 is not limited to p-type and may be n-type. Moreover, if the carrier concentration of the p-type Ga _x In _1-x P active layer 21 is 1 × 10 ¹⁸ cm ⁻³ or more, there is no practical problem. And the thickness of the active layer 21 should just be a thickness which can fully absorb to-be-detected light, and should just be about 1 micrometer or more substantially.
[0016]
A surface layer 22 made of Cs ₂ O for reducing the work function is formed extremely thin on the surface of the active layer 21, and photoelectrons from the active layer can be easily emitted to the outside. FIG. 4 is a cross-sectional view of a second embodiment of the photocathode according to the present invention. A p-type (Al _y ) having an energy gap larger than that of the active layer 21 and having a carrier concentration of 1 × 10 ¹⁹ cm ⁻³ between the semiconductor substrate 10 and the active layer 21 (x = 0.5) described in the first embodiment. A buffer layer 12 (x ′ = 0.5, y = 0.5) made of Ga _1-y ) _{x ′} In _{1-x ′} P is interposed as an epitaxial layer having a thickness of 2 μm. Crystal defects in the active layer 21 are reduced by preventing the defects from propagating to the active layer 21. Further, since the atomic composition ratio x ′ of the buffer layer 12 is equal to the atomic composition ratio x of the active layer 21, a high quality active layer 21 lattice-matched with the buffer layer 12 is formed as shown in FIG. Here, the atomic composition ratio x ′ of the buffer layer 12 and the atomic composition ratio x of the active layer 21 do not have to be completely equal. From the analogy described in the first embodiment, as long as they are substantially equal, No crystal defects are introduced into 21.
[0017]
If the carrier concentration of the p-type (Al _y Ga _1-y ) _{x ′} In _{1-x ′} P buffer layer 12 is 1 × 10 ¹⁸ cm ⁻³ or more, there is no practical problem, and the thickness is It is only necessary that the photoelectron cannot pass through, and it may be substantially about 1 μm or more. Further, the Cs ₂ O surface layer 22 is formed extremely thin on the surface of the Ga _x In _1-x P active layer 21 as in the first embodiment.
[0018]
The electron emission mechanism of the photocathode of the second embodiment configured as described above will be described as follows based on the energy band diagram shown in FIG.
[0019]
When the detected light (hν) enters the active layer 21, the photoelectrons (e ⁻ ) excited from the valence band (VB) to the conduction band (CB) diffuse in the active layer 21. Moving. At this time, the buffer layer 12 having an energy gap larger than that of the active layer 21 serves as a potential barrier with respect to the active layer 21, so that photoelectrons travel toward the surface layer 22. The probability of the photoelectrons reaching the surface layer 22 side depends on the mean free path of the active layer 21, that is, the diffusion length of the photoelectrons. The diffusion length depends on the recombination speed of the active layer 21, and the recombination speed depends on the crystal defect density of the active layer 21. Therefore, since the probability that the photoelectrons reach the surface layer 22 side depends on the crystal defect density of the active layer 21, many photoelectrons reach the vicinity of the surface layer 22 in this embodiment. Since the surface of the active layer 21 where the Fermi level (F.L.) has approached to the lower end of the forbidden band because it is p-type Ga _x In _1-x P, the work function is reduced to the vacuum level (V.L. ), The photoelectrons mentioned above are easily emitted from there.
[0020]
As described above, the photocathode of the present embodiment from which photoelectrons are emitted has a quantum efficiency of 50% at a wavelength of 500 nm, which is a high value as in the first embodiment.
[0021]
As described above, since the high-quality active layer 21 in which crystal defects are suppressed is formed on the semiconductor substrate 10 by epitaxial growth, the photocathode of the present invention has high quantum efficiency. That is, the photocathode of the present invention emits more photoelectrons than the prior art for the same detected light.
[0022]
Next, an embodiment of an electron tube using the photocathode according to the present invention will be described.
[0023]
FIG. 6 shows a horizontal sectional view of a so-called circular cage type photomultiplier tube. In FIG. 6, a cylindrical glass bulb with an entrance window 31 constituting the translucent vacuum vessel 30 is inclined with a certain angle with respect to the detected light (hν) incident from the entrance window 31. The above-described photocathode 20, a dynode unit 40 including a plurality of dynodes 40 a to 40 h for sequentially multiplying photoelectrons emitted from the photocathode 20, and an anode 50 for collecting output signals are disposed. Yes. Although not shown, the photocathode 20, the dynode unit 40, and the anode 50 have a positive bleeder voltage with respect to the photocathode 20 through the bleeder circuit and the electrical lead so as to increase step by step as the anode 50 is approached. Applied in a distributed manner.
[0024]
The light to be detected enters the Ga _x In _1-x P active layer 21 through the incident window 31 of the vacuum container 30 of the photomultiplier tube, is absorbed, and photoelectrons e ⁻ are generated and emitted into the vacuum. The photoelectrons e ⁻ emitted into the vacuum are accelerated and incident on the first dynode 40a, generate secondary electrons multiplied by the photoelectrons, and are emitted again into the vacuum. The secondary electrons emitted from the first dynode 40a are accelerated again, enter the second dynode 40b, and further generate and emit secondary electrons. By repeating this eight times, the photoelectrons emitted from the photocathode 20 are emitted at the eighth dynode 40h by a secondary electron multiplication of about 1 million times. The secondary electrons multiplied and emitted from the eighth dynode 40h are collected by the anode 50 and taken out as an output signal current.
[0025]
In this embodiment, since more photoelectrons are emitted from the photocathode 20, the signal current finally output from the anode 50 also becomes larger, and is more sensitive and weaker than a conventional circular cage photomultiplier tube. Therefore, a photomultiplier tube having a sensitivity about 5 times that of the prior art can be obtained.
[0026]
【The invention's effect】
According to the photocathode of the present invention, by using an active layer composed of Ga _x In _1-x P having an appropriate composition or a buffer layer composed of (Al _y Ga _1-y ) _{x ′} In _{1-x ′} P, Since the lattice irregularity between the active layer and the semiconductor substrate can be suppressed as much as possible, a highly sensitive photocathode can be obtained in the visible light region.
[0027]
According to the electron tube using the photocathode of the present invention, the electron tube using the photocathode of the present invention can detect weak light with higher sensitivity than before.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view of a photocathode according to a first embodiment of the present invention.
FIG. 2 is a diagram illustrating a relationship between a lattice constant (broken line) and a forbidden band width (solid line) of (Al _y Ga _1-y ) _{x ′} In _{1-x ′} P quaternary mixed crystal.
FIG. 3 is a graph showing the relationship between the atomic composition ratio x of the active layer and the quantum efficiency of the photocathode at a wavelength of 500 nm.
4 is a side sectional view of a second embodiment of a photomultiplier tube provided with the photocathode of FIG. 1. FIG.
FIG. 5 is an energy band diagram of the second embodiment of the photocathode according to the present invention.
6 is a horizontal sectional view of an embodiment of a photomultiplier tube using the photocathode of FIG.
FIG. 7 is a cross-sectional view of a reflective photocathode obtained by epitaxially growing a p-type GaAs active layer on a GaAs semiconductor substrate.
FIG. 8 shows the relationship between the energy gap and the lattice constant of a III-V compound semiconductor and its mixed crystal, and the mixed crystal of two types of III-V compound semiconductors shows a direct transition semiconductor with a solid line. It is a diagram showing the relationship between the indirect transition semiconductors with dotted lines.
FIG. 9 is a cross-sectional view of a reflective photocathode having an active layer in which the composition of GaAsP is inclined on a GaAs semiconductor substrate.
FIG. 10 is a sectional view of a reflective photocathode in which a GaAs / GaAsP superlattice layer is interposed between a semiconductor substrate and a GaAsP active layer.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Semiconductor substrate, 11 ... Superlattice layer, 12 ... Buffer layer, 20 ... Photocathode, 21 ... Active layer, 22 ... Surface layer, 30 ... Vacuum container, 40 ... dynode part, 40a ... 1st dynode, 40b ... 2nd dynode, 40c ... 3rd dynode, 40d ... 4th dynode, 40e ... 5th dynode, 40f ... Sixth dynode, 40g ... seventh dynode, 40h ... eighth dynode, 50 ... anode.

Claims (4)

A group III-V compound semiconductor substrate mainly composed of Ga and As, and p-type Ga _x In _1-x P having a carrier concentration of 1 × 10 ¹⁸ cm ⁻³ or more on the semiconductor substrate, An active layer that absorbs certain detected light and generates photoelectrons;
A surface layer formed on the active layer by an alkali metal or an oxide thereof or a fluoride thereof and lowering a work function to the active layer;
A photocathode that emits photoelectrons through the surface layer in response to the light to be detected incident through the surface layer, wherein the range of the atomic composition ratio x of the active layer is 0 <x A photocathode characterized by ≦ 0.75. The semiconductor substrate has a buffer layer made of (Al _y Ga _1-y ) _{x ′} In _{1-x ′} P having an energy gap larger than that of the active layer on the upper surface, and the atomic composition ratio x ′ of the buffer layer The photocathode according to claim 1, wherein is substantially equal to an atomic composition ratio x of the active layer. 3. The photocathode according to claim 1, wherein the range of the atomic composition ratio x of the active layer is 0.45 <x <0.55. A vacuum vessel having an incident window for the light to be detected and maintained in a vacuum state;
The photocathode according to any one of claims 1 to 3, housed in a position facing the entrance window inside the vacuum vessel,
An anode installed inside the vacuum vessel and holding a positive voltage relative to the photocathode;
At least an electron tube.

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