JP3580973B2 - Photocathode - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a photocathode, and more particularly to a photocathode made of diamond.
[0002]
[Prior art]
As a semiconductor photocathode having sensitivity to light longer than the wavelength of light corresponding to the energy gap, for example, a semiconductor photocathode disclosed in JP-A-2-260349 is known. FIG. 5 shows an energy band diagram of the semiconductor photocathode 50. In the semiconductor photocathode 50, a semi-insulating GaAs substrate 51 having a high-density impurity level (shown by a dashed line in the figure) between a valence band (VB) and a conduction band (CB). And the n-type GaAs layer 52 are joined. Further, a reverse bias voltage V ₀ is applied to the GaAs substrate 51 and the GaAs layer 52 via the electrodes 53 and 54.
[0003]
In the semiconductor photocathode 50, the impurity level electrons (e ⁻ ) are excited into the conduction band by the incidence of the detection light (hν) longer than the wavelength of the light corresponding to the energy gap. When the electrons reach the junction between the GaAs substrate 51 and the n-type GaAs layer 52 diffuses, it is accelerated become hot electrons by the electric field of the depletion layer formed by the reverse bias voltage V _0, beyond the surface barrier Emitted into the vacuum as photoelectrons.
[0004]
[Problems to be solved by the invention]
However, the above-described semiconductor photocathode 50 needs a reverse bias voltage of several hundred volts in order to emit hot electrons to the outside. For this reason, even when there is no incident light to be detected, a leak current that causes dark current from the electrode is generated to a considerable extent. As a result, when the semiconductor photocathode 50 is applied to a phototube, a photomultiplier, or the like, a defect such as deterioration of the SN ratio occurs.
[0005]
Incidentally, according to a report by Himpsel et al. (Physical Review B, 20, 2, (1979) 624), the (111) plane of a natural single crystal diamond doped with boron (B) was atomically cleaned. Has a negative electron affinity. In the photocathode using diamond as a material, the quantum efficiency is 20% when the photon energy is in the range of 5.5 eV to 9 eV, and 40 to 70% in the range of 13 eV to 35 eV, which is relatively high.
[0006]
Also, diamonds reported by Sakaemori et al. (Diamond and Related Material (4) (1995) 806, Japan Journal of Applied Physics (Jpn. J. Appl. Phys.) 33, (1994) 6312) also found that the electron affinity was negative. According to this report, the surface of a single crystal diamond film formed on a high pressure synthesized single crystal diamond (100) substrate by microwave plasma CVD (Chemical Vapor Deposition) is terminated by hydrogen. Have a negative electron affinity. Thus, the electron affinity of the photocathode becomes negative not only on the (111) plane but also on the (100) plane of the single crystal diamond film.
[0007]
However, the above-mentioned photoelectric surface must have a wavelength corresponding to the energy gap (5.5 eV) of single crystal diamond in order for the detection light to enter and excite electrons from the valence band to the conduction band. . On the contrary, on the photocathode of these single crystal diamonds, the detected light having photon energy smaller than the energy gap, that is, the detected light having a wavelength longer than 225 nm in terms of wavelength has no sensitivity. means. Therefore, such a photocathode is only applied in a very limited range.
[0008]
Therefore, an object of the present invention is to provide a photocathode having sensitivity to light having a wavelength longer than 225 nm.
[0009]
[Means for Solving the Problems]
The photocathode according to the present invention has been made in order to achieve the above-mentioned object. In the photocathode where photoelectrons are emitted to the outside due to the incidence of light, the photocathode has a granular shape composed of polycrystalline diamond or polycrystalline diamond as a main component. The material has a diamond material formed in a film shape, and is doped with an impurity that forms an impurity level that excites electrons into a conduction band by light incidence in the forbidden band of the diamond material, thereby being larger than 225 nm. It is characterized by having sensitivity to light of a wavelength and having a zero or negative electron affinity on a surface from which photoelectrons are emitted to the outside.
[0010]
According to this configuration, the photocathode of the present invention has sensitivity to light having a wavelength longer than the wavelength of light corresponding to the energy gap of the diamond material, that is, light having a wavelength greater than 225 nm. Also, since the surface from which photoelectrons are emitted has zero or negative electron affinity, the photocathode has high sensitivity, and a high bias voltage for electrons to cross the surface barrier needs to be applied to the photocathode. Disappears. As a result, there is no dark current generated by the leak current injected from the electrode. As a result, in the present invention, a photocathode having high sensitivity and low noise is obtained in addition to the specific wavelength corresponding to the energy gap of the diamond material. Become like
[0011]
In addition, in order to effectively lower the work function of the diamond surface, it is preferable that the surface of the diamond material is terminated with hydrogen. This not only suppresses the formation of surface levels and facilitates the control of impurity levels, but also increases the electron affinity of the diamond surface terminated by positively polarized hydrogen due to its lower electronegativity than carbon. It is easily considered to be zero or negative.
[0012]
In order to further reduce the work function of the diamond surface, it is more preferable that a layer of an electrically positive alkali metal or a compound thereof is formed on the surface of the diamond material.
[0013]
Further, even if the surface of the diamond material is terminated by oxygen or fluorine and the layer of an alkali metal or a compound thereof is formed, the electron affinity becomes zero or negative. This is because, for example, a surface dipole such as Cs ⁺ O ⁻ or Cs ⁺ F ⁻ is formed, and the presence of the surface dipole effectively lowers the work function of the diamond surface.
[0014]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same elements are denoted by the same reference numerals.
[0015]
The cross-sectional view of the photoelectric tube 10 in FIG. 1 shows a state where the photoelectric surface 20 to which the present invention is applied is used. As shown in the figure, the photoelectric tube 10 includes a vacuum container 11, and the vacuum container 11 has a cylindrical cylindrical portion 12 having an opening formed on a side surface, a dome-shaped top portion 13, and a flat bottom portion 14. . The open end of the side surface of the cylindrical portion 13 projects outward, and an entrance window 15 made of, for example, magnesium fluoride (MgF ₂ ) is attached here.
[0016]
At the bottom of the vacuum vessel 11, metal stem pins 30a and 30b are fixed so as to pass therethrough. The upper end of the stem pin 30a protrudes into the vacuum chamber 11 and is connected to the rectangular frame-shaped anode 40. The upper end of the stem pin 40 b also projects into the vacuum chamber 11 and is connected to the photocathode 20. Thus, the photocathode 20 faces the entrance window 15, and the anode 40 is supported inside the vacuum vessel 11 so as to be located between the entrance window 15 and the photocathode 20.
[0017]
The vacuum chamber 11 stem pins 30a protruding outward, between the lower end of the 30b, the DC power supply _{V 1} is the electric leads 31a, is connected via 31b, high anode 40 is only approximately 100V than the photocathode 20 Voltage To have. At this time, the ammeter 32 is interposed between the stem pins 31a and the DC power supply V _1, and is capable of detecting the output signal from the photoelectric tube 10.
[0018]
The photocathode 20 is formed by forming a polycrystalline diamond (diamond material) 22 on an Si substrate 21 in a film shape. As schematically shown in FIG. 2, a layer 23 made of a single atom of hydrogen is formed on the surface of the film of the polycrystalline diamond 22, and forms a boundary between the polycrystalline diamond 22 and its external space.
[0019]
In the present embodiment, the polycrystalline diamond 22 contains boron (B) and nitrogen (N) by a predetermined amount as impurity elements. Therefore, the energy band diagram of the polycrystalline diamond 22 is as shown in FIG. 2, and an impurity level (indicated by a dashed line) is formed in the forbidden band between the conduction band (CB) and the valence band (VB). Is done. The position of the impurity level at this time depends on the concentrations of B and N in the film of the polycrystalline diamond 22. When the concentration of B is relatively higher than the concentration of N, a relatively shallow impurity level is formed near the upper end of the valence band, as shown in FIG. On the other hand, when the concentration of B is lower than the concentration of N, a relatively deep impurity level is formed near the center of the forbidden band as shown in FIG. Thus, the position of the above-described impurity level can be controlled. In any of the above cases, the impurity level is formed in a state almost coincident with the Fermi level (E _F ), so that the impurity level is almost occupied by bound electrons. As a result, transition of an arbitrary electron other than between bands between the conduction band and the valence band is possible. This means that the photocathode made of diamond, which has been sensitive only to light in the ultraviolet region of 225 nm or less, is also sensitive to light in the visible region.
[0020]
The optimum values of the concentrations of B and N are determined by the surface state of the polycrystalline diamond 22 which affects the work function (φ), which is the difference between the vacuum level (VL) and the Fermi level, and within the polycrystalline diamond 22. It depends on the mean free path of the excited electrons. Since the work function and the mean free path depend on the method of forming the polycrystalline diamond 21 and the conditions of the surface treatment to be described later, specific optimum values should be determined in consideration of such conditions.
[0021]
Polycrystalline diamond 22 has many crystal grain boundaries. Unbound carbon having no binding partner is contained at the interface of the grain boundary. Such unbonded carbon also exists on the surface of the polycrystalline diamond 22. This unbonded carbon forms a localized level in the forbidden band. At this time, depending on the density of the localized levels, the Fermi level is fixed near the center of the forbidden band, and it may be difficult to control the position of the impurity level by the impurity element. However, in the present embodiment, the hydrogen of the monoatomic layer 23 that forms a boundary between the polycrystalline diamond 22 and its external space is bonded to unbonded carbon of the polycrystalline diamond 22. Thereby, the localized level is greatly reduced, and the above-described control can be sufficiently performed.
[0022]
This hydrogen can be considered to be chemically adsorbed on the polycrystalline diamond 22. At this time, it is presumed that electrons are exchanged between unbonded carbon and hydrogen having a lower electronegativity than carbon, the degree of exudation of electrons changes, and the diamond surface 22 is positively charged. As a result, a surface potential is formed, and the work function (φ) in FIG. 2 is effectively reduced. The polycrystalline diamond 22 is a kind of a wide gap semiconductor. Therefore, as shown in FIG. 3, the energy gap (Eg ₂ ) of the polycrystalline diamond 22 is larger than the energy gap (Eg ₁ ) of the GaAs 51 of FIG. 5, which is well known as a semiconductor. At this time, when comparing the electron affinity, which is the difference in vacuum level (VL) with respect to the bottom of the conduction band, the electron affinity (χ ₂ ) of the polycrystalline diamond 22 is smaller than the electron affinity (χ ₁ ) of GaAs 51. . Accordingly, FIG. 3 as shown in (b), when the work function of the polycrystalline diamond 22 becomes ₂ phi and effectively reduced the electron affinity energy gap larger polycrystalline diamond 22 (chi ₂ ) Is easily zero or negative. On the other hand, in the GaAs 51 of FIG. 3A, since the surface level is formed, the Fermi level (E _F1 ) is pinned near the center of the forbidden band on the surface regardless of the doping of the impurity. I will. In addition, GaAs has a smaller energy gap than diamond. Therefore, the electron affinity (χ ₁ ) does not easily become zero or negative unless a strong dipole is formed on the surface.
[0023]
Although not shown, the production of the photoelectric surface 20 is performed by the following method. First, after the Si substrate 21 is degreased and cleaned, a predetermined etching process is performed. Thereafter, for example, a polycrystalline diamond 22 is formed on the Si substrate 21 by a microwave plasma CVD method in a plasma discharge chamber excited by microwaves. That is, the Si substrate 21 is placed in a plasma discharge chamber, and for example, carbon monoxide (CO) and methane (CH ₄ ) as source gases and hydrogen (H ₂ ) as a carrier gas are introduced into the plasma discharge chamber. . In this state, when a raw material gas or the like in the plasma discharge chamber is subjected to discharge decomposition using microwaves, polycrystalline diamond 22 is deposited on Si substrate 21. In this embodiment, a predetermined amount of a gas of diborane (B ₂ H ₆ ) or ammonia (NH ₃ ) is introduced as a dopant gas during the above-mentioned deposition, and a predetermined concentration of B and N is doped into the polycrystalline diamond 22. Thus, impurity levels are formed in the polycrystalline diamond 22. Thereafter, the polycrystalline diamond 22 on the Si substrate 21 is left for a few minutes in a hydrogen plasma atmosphere to terminate the surface with hydrogen. In this embodiment, a microwave plasma CVD method is used to form the thin film of the polycrystalline diamond 22. However, the method of forming the thin film is not limited to this, and the film may be formed by, for example, a hot filament method.
[0024]
Further, in order to use the above-described photoelectric surface 20 as a part of the photoelectric tube 10 shown in FIG. 1, light to be detected (hv) to be detected is made to enter the entrance window 15 of the photoelectric tube 10. The detected light that has entered the incident window 12 passes through the incident window 12 and enters the photoelectric surface 20. In the photocathode 20, since B and N are contained in the polycrystalline diamond 22, an impurity level is formed in a forbidden band as shown in FIG. 2 or 3B. Therefore, the detected light having an energy lower than the energy gap of 5.5 eV of the diamond among the detected light incident on the photocathode 20 is absorbed. Due to this absorption, an impurity level electron (e ⁻ ) is excited to the conduction band (CB).
[0025]
The electrons excited into the conduction band diffuse and reach the surface of the polycrystalline diamond 22.
[0026]
At this time, since the photocathode 20 is made of polycrystalline diamond 22, irregularities are formed on the surface thereof. As a result, the light to be detected is optically refracted and scattered by the unevenness, so that the optical path length is increased and the substantial light absorption efficiency is increased, so that more photoelectrons are generated. In addition, since the film is made of granular materials, the electron traveling distance of photoelectrons emitted from each granular material is shortened, and the arrival efficiency of photoelectrons reaching the emission surface is also increased. In addition, since unbonded carbon on the surface of the polycrystalline diamond 22 is bonded to hydrogen in the monoatomic layer 23, the work function is reduced, and the electron affinity (χ ₂ ) as shown in FIG. Is zero or negative. Therefore, most of the electrons that efficiently reach the surface of the polycrystalline diamond 22 are easily emitted as photoelectrons into the vacuum.
[0027]
The photoelectrons (e ⁻ ) emitted into the vacuum are collected at the anode 40 to which a positive voltage is applied to the photocathode 20, as shown in FIG. The photoelectrons collected by the anode 40 are extracted outside by the stem pins 30a and 30b, and are detected by the ammeter 32 as a photocurrent.
[0028]
As described above, in the present embodiment, since the photoelectrons are easily emitted over the surface barrier, it is not necessary to apply a bias voltage for increasing the dark current in the photocathode 20. As a result, the photocathode 20 can be used without cooling. Therefore, in a phototube using this, not only high sensitivity and low noise can be obtained, but also handling of a small current becomes easy, and weak light can be detected.
[0029]
In order to further reduce the work function of the photocathode 20, as shown in FIG. 4, an adsorption layer 24 made of, for example, an alkali metal such as Cs, Rb or K, which is electrically positive, is further provided on the surface. It may be formed. However, the adsorptive layer 24 is not limited to a layer composed of only an alkali metal, but may be composed of a compound such as an oxide of the alkali metal.
[0030]
The photoelectric surface of the present invention is not limited to the above embodiment. That is, even if a monoatomic layer of oxygen or fluorine is formed instead of the monoatomic layer of hydrogen 23 of FIG. 4 formed on the surface of the polycrystalline diamond 22, the photoelectric surface 20 having a negative or zero electron affinity is formed. can get. This oxygen and fluorine alkali metal and combine with e.g. Cs ⁺ O ^- and Cs ⁺ F ^- since the surface dipole or the like is formed, by the presence of the surface dipole, decreases the effective work function of diamond surface Because you do. It should be noted that merely terminating the surface of the polycrystalline diamond 22 with oxygen or fluorine does not make the electron affinity zero or negative. This is because oxygen or fluorine that terminates the diamond surface to suppress the formation of surface levels is negatively polarized because it has a higher electronegativity than carbon, unlike hydrogen.
[0031]
The photocathode of the above embodiment has a polycrystalline diamond formed on a Si substrate, but the present invention is not limited to this. Obviously, a photocathode having the same effect can be obtained even if a material containing polycrystalline diamond as a main component is formed on another semiconductor, metal, or the like. In addition, the photoelectric surface is not limited to polycrystalline diamond or one composed mainly of polycrystalline diamond, and it is needless to say that the photoelectric surface may be composed of single crystal diamond or diamond-like carbon. However, in a single crystal diamond, the traveling distance of the electrons excited by the incidence of light becomes longer, so that the crystal lattice may collide with the electrons and lose kinetic energy. For this reason, the sensitivity may be lower than that of polycrystalline diamond. In addition, the synthesis of single crystal diamond requires an expensive single crystal diamond substrate at present, and there is a problem from the viewpoint of industrial availability and economy. Further, it is difficult to produce diamond-like carbon having a certain quality. Therefore, at present, a polycrystalline diamond or the like that can use a large-area Si substrate or the like that is inexpensive and has stable quality is suitable.
[0032]
Also, an example in which B and N are doped to form impurity levels has been described as an example, but the type of impurity element is not limited to B and N as long as impurity levels can be formed at predetermined positions. Needless to say, a rare earth element such as Er may be doped. However, if the energy difference between the impurity level and the conduction band bottom becomes too small to achieve the state of zero or negative electron affinity, the sensitivity is significantly reduced, and the effect of the present invention is also significantly reduced. It should be noted that In addition, the value of the electron affinity of the photocathode, like the value of the work function described above, strongly depends on the surface state of diamond, and thus cannot be determined only by the type and concentration of the impurity element.
[0033]
In order to explain the effect of the photocathode of the present invention, the photocathode is used for the phototube in the above embodiment. However, a device incorporating the photocathode of the present invention having a wide detection range and a high detection sensitivity is not limited to the phototube, but may be another weak light detection device. For example, a photomultiplier combined with a secondary electron multiplier of extremely low noise may be used. As a result, extremely weak light can be detected. Further, by making the secondary electron multiplier a micro-channel plate, it becomes possible to detect two-dimensional light of extremely weak light, so that the performance of night-vision sights and the like is dramatically improved. Furthermore, if the photocathode of the present invention is applied to a streak camera or the like using electron deflection, it becomes possible to measure ultra-high-speed faint light with extremely high detection efficiency.
[0034]
【The invention's effect】
In the photocathode of the present invention, since the diamond material has an impurity level, electrons can transition from the impurity level to the conduction band. That is, the photoelectric surface of the present invention has sensitivity to light having a wavelength longer than the wavelength of light corresponding to the energy gap of the diamond material. Further, the position of the impurity level can be controlled by the concentration of the impurity element that forms the impurity level. Therefore, the applicable range of such a photocathode is wide.
[0035]
Further, by appropriately treating the diamond material of the photocathode of the present invention with hydrogen or the like, it becomes easy to have zero or negative electron affinity. Therefore, it is not necessary to apply a high bias voltage for electrons to cross the surface barrier, and there is no dark current generated by a leak current injected from the electrode. Therefore, according to the present invention, a photocathode having high sensitivity and low noise can be obtained over a wide wavelength range.
[Brief description of the drawings]
FIG. 1 is a sectional view of a phototube using a photocathode according to a first embodiment of the present invention.
FIG. 2 is an energy band diagram of polycrystalline diamond doped with an impurity element.
FIG. 3 is an energy band diagram of the photocathode of FIG. 1 shown in comparison with an energy band diagram of GaAs.
FIG. 4 is a cross-sectional view illustrating a second embodiment of the photocathode of the present invention.
FIG. 5 is an energy band diagram of a conventional semiconductor photocathode made of GaAs.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 ... Phototube, 20 ... Photocathode, 21 ... Si substrate, 22 ... Polycrystalline diamond, 24 ... Adsorption layer, 30, 30a, 30b ... Stem pin, 40 ... Anode.

Claims (5)

On the photocathode where photoelectrons are emitted outside by the incidence of light
A polycrystalline diamond or a granular material composed mainly of polycrystalline diamond has a diamond material formed in a film shape,
In the forbidden band of the diamond material, by being doped with an impurity that forms an impurity level that excites electrons into a conduction band by the incidence of the light, it has sensitivity to light having a wavelength greater than 225 nm,
A photoelectric surface, wherein the surface on which the photoelectrons are emitted to the outside has an electron affinity of zero or negative. The photocathode according to claim 1, wherein the surface of the diamond material is terminated by hydrogen. The photoelectric surface according to claim 1, wherein a layer of an alkali metal or a compound thereof is formed on a surface of the diamond material. The photocathode, on which photoelectrons are emitted to the outside by the incidence of light, is made of polycrystalline diamond or a diamond material composed mainly of polycrystalline diamond, and conducts electrons during the forbidden band by the incidence of the light. Impurities that form impurity levels that excite the band are doped,
The electron affinity of the surface from which the photoelectrons are emitted to the outside is zero or negative,
A photocathode, wherein the surface of the diamond material is terminated with oxygen and a layer of an alkali metal or a compound thereof is formed. The photocathode, on which photoelectrons are emitted to the outside by the incidence of light, is made of polycrystalline diamond or a diamond material composed mainly of polycrystalline diamond, and conducts electrons during the forbidden band by the incidence of the light. Impurities that form impurity levels that excite the band are doped,
The electron affinity of the surface from which the photoelectrons are emitted to the outside is zero or negative,
A photocathode, wherein the surface of the diamond material is terminated with fluorine and a layer of an alkali metal or a compound thereof is formed.

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