WO2018016291A1

WO2018016291A1 - Electron emitting material and electron emitting element

Info

Publication number: WO2018016291A1
Application number: PCT/JP2017/023982
Authority: WO
Inventors: 直也森岡; 裕治木村; 片岡　光浩; 明久荻野
Original assignee: 株式会社デンソー; 国立大学法人静岡大学
Priority date: 2016-07-18
Filing date: 2017-06-29
Publication date: 2018-01-25
Also published as: JP2018014161A

Abstract

One electron emitting material according to the present invention is provided with: a diamond film (11) which is an N-type diamond semiconductor doped with an impurity; oxygen (12) which is bonded to the surface of the diamond film; and a termination metal (13) which is bonded to the diamond film via the oxygen and contains at least one metal selected from among Na, K, Rb, Cs and Mg. Another electron emitting material according to the present invention is provided with: a diamond film (11) which is an N-type diamond semiconductor doped with an impurity; oxygen (12) which is bonded to the surface of the diamond film; a first metal layer (14) which is bonded to the diamond film via the oxygen and contains at least one metal selected from among Li, Na, K, Rb, Cs and Mg; and a second metal layer (15) which is laminated on the first metal layer and contains at least one metal selected from among Li, Na, K, Rb, Cs and Mg, said at least one metal being different from the metal contained in the first metal layer.

Description

Electron emitting material and electron emitting device

Cross-reference to related applications

This application is based on Japanese Patent Application No. 2016-140962 filed on July 18, 2016, the description of which is incorporated herein by reference.

The present disclosure relates to an electron emission material used when heat energy or light energy is converted into electric energy and an electron emission element using the electron emission material.

Examples of the electron-emitting device include a field emission device and a thermionic power generation device. A field emission device utilizes a phenomenon in which electrons are emitted by applying a high electric field to metal and semiconductor materials, and electrons are emitted from the cathode electrode by applying a voltage to the cathode electrode and the anode electrode. To the anode electrode. The thermoelectric power generation element converts the thermal energy into electric energy by utilizing the phenomenon that the hot electrons are emitted from the surface of the high temperature electrode. The thermoelectrons are emitted by raising the temperature of the emitter electrode, and the collector Reach the electrode.

In order to improve the characteristics of such an electron-emitting device, it is necessary to reduce energy such as an electric field and heat required for emitting electrons.

For example, in the thermoelectric power generation element, various studies have been made to improve the power generation output. Specifically, for example, a thermionic power generation element using diamond that exhibits negative electron affinity (hereinafter referred to as “NEA”) and has an effect of lowering the work function (hereinafter referred to as “lower work function”) is provided. Can be mentioned. According to this, due to the effect of NEA, the energy level of the conduction band of diamond exceeds the vacuum level. In this state, the electrons in the conduction band have sufficient energy to jump out into the vacuum, so that the electrons in the conduction band can be easily released into the vacuum. In other words, by reducing the difference between the vacuum level and the Fermi level, that is, the work function, due to the effect of NEA, extremely high-efficiency thermal electron emission from the electrode surface becomes possible, which is more efficient at lower temperatures than metal. Power generation is possible.

Also, for example, photoexcited thermionic power generation (hereinafter referred to as “PETE”) using electrons excited by light in order to perform highly efficient thermionic power generation at a low temperature. In photoexcited thermionic power generation, electrons are emitted from an emitter electrode by irradiating the emitter electrode with light and once exciting the electrons of the electrode material with the light, followed by heating. That is, the photo-excited thermionic power generation emits electrons at a lower temperature than the thermionic power generation element, and the power generation output is further improved.

In addition, in recent years, the possibility of PETE using a diamond film on a silicon substrate has also been studied. Electrons in the silicon substrate are photoexcited, and the electrons on the excited silicon substrate are emitted from the diamond film as thermal electrons. It has been clarified that it contributes to Further, in the electron emission material disclosed in Patent Document 1, it is known that when the diamond film surface is terminated with lithium (Li) via oxygen, the work function is smaller than that of the diamond surface terminated with hydrogen. It has been. The electron emission material having such a low work function is expected to have an effect of further improving the power generation output because the energy required for emitting electrons from the electrode material surface is reduced.

US8617651 gazette

Here, by coating the surface of the diamond film with an alkali metal such as Li, the work function is made lower than that of the uncoated diamond film surface. However, in that case, the interaction between the diamond film and the alkali metal is weak because the alkali metal does not terminate the surface of the diamond film by a chemical bond. Therefore, when diamond whose surface is coated with an alkali metal is used as an electrode material, when the electrode temperature is increased, the alkali metal covering the diamond surface is detached from the diamond surface. That is, diamond whose surface is coated with an alkali metal cannot maintain the work function of the surface at a high temperature at a low temperature.

In addition, with respect to the material of Patent Document 1, the work function can be kept low while suppressing the separation of the alkali metal from the diamond surface at high temperature by terminating the diamond film surface with Li via oxygen. However, it is necessary to drive at a high temperature in order to emit thermoelectrons from the material and obtain a sufficient thermoelectron current. From this, it is considered that a diamond film terminated with Li via oxygen can provide stability at high temperature driving but is still not sufficient for lowering the work function of the diamond film surface.

The present disclosure has been made in view of the above points, and an object thereof is to provide an electron-emitting material that can maintain a low work function during high-temperature driving and an electron-emitting device using the material.

An electron emission material according to a first aspect of the present disclosure is an electron emission material including a diamond film, oxygen bonded to the surface of the diamond film, and a termination metal bonded to the diamond film via oxygen, The film is an N-type diamond semiconductor doped with impurities, and the termination metal contains at least one of Na, K, Rb, Cs, and Mg.

The work function of the diamond film can be lowered by terminating the surface of the diamond film with at least one metal of Na, K, Rb, Cs, and Mg via oxygen. In the following description, the metal that terminates the diamond film is referred to as “termination metal” for convenience.

In addition, when the terminal metal is bonded to the diamond film through oxygen, a Coulomb force acts between the terminal metal and oxygen, and these are Coulomb bonded. As a result, even if the termination metal is heated to a high temperature, the termination metal is hardly separated from the diamond film by the above-described Coulomb coupling. Therefore, by adopting such a configuration, the terminal metal is stable without being detached from the surface of the diamond film even when driven at a high temperature, and the electron emission material can maintain the work function of the surface low. In the following description, for the sake of convenience, the property that the terminal metal does not separate from the surface of the diamond film even when the temperature is raised is referred to as “high temperature stability”.

The electron emission material according to the second aspect is laminated on the diamond film, oxygen bonded to the surface of the diamond film, a first metal layer bonded to the diamond film via oxygen, and the first metal layer. The diamond film is an N-type diamond semiconductor doped with impurities, and the first metal layer is composed of Li, Na, K, Rb, Cs, and Mg. The second metal layer includes at least one of Li, Na, K, Rb, Cs, and Mg, which is different from the first metal layer.

The above-mentioned first metal layer is provided on the surface of the diamond film, and the above-mentioned second metal layer is laminated on the first metal layer. In addition, high temperature stability and low work function can be achieved by combining a metal containing at least one of the alkali metal and Mg of the first metal layer with oxygen through oxygen. Further, the termination metal of the second metal layer can provide an electron emission material in which the surface of the diamond film is further reduced in work function while ensuring the high temperature stability by the first metal layer.

The electron-emitting device according to the third aspect includes a pair of electrodes consisting of an emitter electrode and a collector electrode arranged so as to face each other. In such a configuration, the emitter electrode is an electrode that emits electrons, and is formed of, for example, an electron-emitting material according to the first aspect.

The work function can be kept low while ensuring high temperature stability. For example, by applying the electron emission material according to the first aspect as the emitter electrode, the efficiency of electron emission from the emitter electrode can be increased. Thereby, an electron-emitting device with high electron emission efficiency can be obtained.

It is sectional drawing which shows the electrode provided with the electron emission material of 1st Embodiment. It is each energy band figure when the surface of a diamond semiconductor thin film is terminated with oxygen and when it is terminated with an alkali metal via oxygen. It is sectional drawing which shows the electrode provided with the electron emission material of 2nd Embodiment. It is the schematic which shows the thermoelectric power generation element of 3rd Embodiment.

Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be described with the same reference numerals.

(First embodiment)
The electron emission material 10 of 1st Embodiment is demonstrated with reference to FIG. In FIG. 1, the example which applied the electron emission material 10 of this embodiment with respect to the electrode to the board | substrate 1 is shown. As the substrate 1, for example, Mo or Si can be used. The electron emission material 10 is a material that emits electrons from the material surface, and can be used as an emitter electrode that emits electrons in, for example, a thermionic power generation element, a photoexcitation electron emission element, a photoexcitation thermoelectron generation element, or the like.

The electron-emitting material 10 of the present embodiment is bonded to the diamond film 11 which is a diamond semiconductor, oxygen bonded to the surface of the diamond film 11 (hereinafter referred to as “bonded oxygen 12”), and the diamond film 11 via the oxygen. It is comprised with the termination | terminus metal 13 to perform. Further, M in FIG. 1 indicates a termination metal 13.

The diamond film 11 is composed of an N-type diamond thin film doped with impurities such as nitrogen and phosphorus at a high concentration, for example, 1 × 10 ²⁰ cm ⁻³ to 1 × 10 ²¹ cm ⁻³ . About this dope concentration, the higher one is preferable. This is because, when the doping concentration is low, the number of excited thermal electrons is small and the efficiency of electron emission is low. Moreover, as a semiconductor impurity added to the diamond film 11, N (nitrogen), P (phosphorus), As (arsenic), Sb (antimony), S (sulfur), etc. are used, for example.

Next, although the bonded oxygen 12 does not exist at the time of forming the diamond film 11, it was formed on the surface of the diamond film 11 by ozone oxidation treatment, oxygen plasma surface treatment, or oxygen radical treatment in the manufacturing process described later. Is.

Next, the terminating metal 13 is formed by forming bonded oxygen 12 on the surface of the diamond film 11 and then performing vapor deposition in a manufacturing process described later, and terminating the diamond film 11 through the bonded oxygen 12. It is. The termination metal 13 is a metal containing at least one of, for example, Na (sodium), K (potassium), Rb (rubidium), Cs (cesium), and Mg (magnesium). The termination metal 13 may be a combination of two or more kinds of alkali metals, a combination of alkali metals and Mg, or a combination of two or more kinds of alkali metals and Mg.

Here, the effect of terminating at the surface of the diamond film 11 with at least one of an alkali metal and Mg via oxygen is the case where it is terminated with oxygen and the case where it is terminated with an alkali metal such as Cs. A comparison will be described with reference to FIG.

2A shows a case where the surface of the diamond film 11 constituting the electron emission material is terminated with oxygen, and FIG. 2B shows a case where the surface of the diamond film 11 constituting the electron emission material is terminated with oxygen via Cs. FIG.

First, when the surface of the diamond film 11 constituting the electron emission material is terminated with oxygen, as shown in FIG. 2A, the vacuum level becomes higher than the conduction band due to positive electron affinity (ΔE> 0). ). For this reason, energy is required to discharge electrons in the conduction band to the vacuum, and in this state, the work function, which is the difference between the vacuum level and the Fermi level, becomes high. That is, the electron emission material in this state has high energy required for emitting electrons, and the efficiency of electron emission is poor. On the other hand, as shown in FIG. 2B, when the surface of the diamond film 11 constituting the electron emitting material is terminated with, for example, Cs via oxygen, the vacuum level is lower than the conduction band by NEA ( ΔE <0). For this reason, electrons in the conduction band can be emitted to vacuum with energy = 0, and the electron emission material in this state has a low work function. In other words, highly efficient electron emission can be realized by adjusting the work function to be low.

Thus, the polarity of the electron affinity can be changed by the termination structure on the surface of the diamond film 11 constituting the electron emission material 10. When the surface of the diamond film 11 terminated with a metal containing at least one of an alkali metal and Mg via oxygen is used as a termination structure, extremely stable NEA can be obtained, and highly efficient thermoelectrons can be obtained. Release can be realized in a long time.

Here, the present inventors have conducted intensive studies on the termination metal 13 from the viewpoint of achieving both a low work function on the diamond film surface and high-temperature stability. As a result, when the termination metal 13 is an alkali metal, the work function of the diamond film 11 tends to decrease as the atomic weight of Li, Na, K, Rb, Cs increases, and Na, K, Rb It has been found that the efficiency of electron emission is increased by selecting Cs as compared with Li. On the other hand, when the termination metal 13 is an alkali metal, the alkali metal tends to be detached from the diamond film 11 at high temperatures as the number of Li, Na, K, Rb, Cs and their atomic numbers increases. I found out. That is, there is a trade-off relationship between lowering the work function and stability at high temperature, and the work function can be lowered by appropriately selecting the termination metal 13 according to the driving temperature, and electron emission with excellent electron emission efficiency. Material 10 can be used.

Next, an example of a method for manufacturing the electron emission material according to the present disclosure will be described.

The diamond film 11 is formed, for example, as an N-type diamond film doped with nitrogen by, for example, a microwave CVD (abbreviation of chemical vapor deposition) method. At this time, for example, a mixed gas obtained by diluting CH ₄ with H ₂ is used as a reactive gas, and for example, N ₂ gas is used as a dopant. And the doping amount of nitrogen is adjusted by adjusting these gas flow rates.

Specifically, for example, when the diamond film 11 is formed on the substrate 1 such as Mo or Si, the substrate 1 is set in a vacuum chamber of microwave CVD. Then, the vacuum chamber is decompressed, and the substrate 1 is heated within a range of 600 to 1100 ° C., for example. Thereafter, a mixed gas obtained by diluting CH ₄ with H ₂ (CH ₄ concentration: 0.05 to 5%) and N ₂ gas are introduced into the vacuum chamber. At this time, the mixing ratio of the N ₂ gas and the mixed gas is adjusted so that the nitrogen of N ₂ becomes 1 to 200, for example, when the carbon of CH _{4 in} the mixed gas is 1. Further, the film forming conditions at this time are adjusted, for example, within a range of a reaction pressure of 40 to 150 Torr and an input power of a microwave power source within a range of 500 to 1500 W. Then, a diamond film 11 having a thickness of, for example, 100 nm is formed while these mixed gases are reacted in the microwave. Thereby, an N-type diamond thin film doped with nitrogen at a high concentration is obtained. In the case where the diamond film 11 is formed from a thin film of N-type diamond doped with phosphorus, a thin film of N-type diamond doped with phosphorus at a high concentration can be obtained by replacing the N ₂ gas with a PH ₃ gas. .

The diamond film 11 has no particular upper limit because the film thickness dependence on thermoelectric emission has not been confirmed. However, the diamond film 11 has a film thickness of at least 100 nm and the same thickness with no unevenness over the entire surface of the substrate. Preferably it is formed. This is because a defect site that is not formed as a film occurs in the diamond film having a film thickness of less than 100 nm, and when used as the emitter electrode 3, the efficiency of electron emission may be reduced. On the other hand, the diamond film 11 has no reason to increase the film thickness more than necessary, but rather increases the manufacturing cost.

Next, for example, the surface of the diamond film 11 is oxidized using ozone generated by decomposing oxygen with ultraviolet rays, and the surface of the diamond film 11 is terminated with oxygen. Subsequently, the diamond film 11 terminated with oxygen is vacuum-deposited with an alkali metal such as Cs to obtain the electron emission material 10 terminated with Cs via oxygen.

When the termination metal 13 is composed of a plurality of metals, a plurality of metals can be terminated on the surface of the diamond film 11 by co-evaporating alkali metal or the like from a plurality of deposition sources.

By such a manufacturing method, it is possible to manufacture the electron emission material 10 excellent in high-temperature stability while reducing the work function.

(Second Embodiment)
Next, the electron emission material 10 of 2nd Embodiment is demonstrated with reference to FIG. FIG. 3 shows an example in which the electron-emitting material 10 of the present embodiment is formed as an electrode on the substrate 1 as in FIG. 1, and M1 is a first metal layer 14 to be described later, and M2 is a second metal to be described later. Layer 15 is shown. The electron-emitting material 10 of this embodiment includes a diamond film 11, a bonded oxygen 12 bonded to the diamond film 11, a first metal layer 14 bonded to the diamond film 11 via the bonded oxygen 12, and the first metal layer. 14 and a second metal layer 15 stacked on top of each other. Moreover, the 1st metal layer 14 and the 2nd metal layer 15 are comprised by the metal which mentions mutually different later. That is, in the electron emission material 10 of the present embodiment, not only the first metal layer 14 is bonded via the diamond film 11 and the bonded oxygen 12, but in addition, the second metal layer 15 is replaced with the first metal layer 15. 14 is different from that of the first embodiment in that the configuration is stacked on top of 14. Further, the metals that can be the terminal metal 13 of the electron emission material 10 of the first embodiment are Na, K, Rb, Cs, and Mg, but the first metal layer 14 and the second metal layer of the present embodiment. 15 is different from the first embodiment in that it includes Li in addition to these. In the present embodiment, these differences will be mainly described.

Here, although there is no restriction | limiting about the metal used for the 1st metal layer 14 and the 2nd metal layer 15, For the 1st metal layer 14, metals with high high temperature stability, such as Li, Na, Mg, are used, and the 2nd metal For the layer 15, it is preferable to use a metal such as Rb or Cs that can lower the work function. As a result of intensive studies by the present inventors, it has been found that high temperature stability can be improved while lowering the work function as compared with the electron emission material 10 of the first embodiment in which Rb and Cs capable of lowering the work function are used as termination metals. This is because the.

Specifically, for example, Li is bonded to the diamond film 11 via the bonded oxygen 12 as the first metal layer 14, and Cs is stacked as the second metal layer 15 on the first metal layer 14. An example will be described. When Li is used for the first metal layer 14, the work function is not sufficiently lowered, but the high temperature stability is excellent. Further, by stacking Cs as the second metal layer 15 on the first metal layer 14 and terminating it, the work surface can be further reduced in work function. Moreover, a stable work function can be obtained by laminating the same alkali metal Cs on the alkali metal Li. That is, even when the Cs is the same, the case of the second metal layer 15 of the present embodiment from the surface of the diamond film 11 at the time of high temperature driving is more than the case of the termination metal 13 as in the first embodiment. The withdrawal becomes difficult. In this way, the electron emission material 10 of the present embodiment has higher temperature stability while lowering the work function than the electron emission material 10 of the first embodiment.

The first metal layer 14 may be a combination of two or more alkali metals, a combination of alkali metals and Mg, or a combination of Mg and two or more alkali metals. Also good. Further, the second metal layer 15 is the same as the first metal layer 14.

The electron emission material 10 of the present embodiment is manufactured by forming the second metal layer 15 by, for example, vapor deposition after the electron emission material 10 of the first embodiment is manufactured.

(Third embodiment)
Next, the element of the third embodiment using the electron emission material 10 will be described with reference to FIG. As shown in FIG. 4, the thermoelectric generator 20 of the present embodiment is disposed between two

substrates

1 and 2 that are disposed so as to face each other and between the two

substrates

1 and 2 that are separated from each other. And an emitter electrode 3 and a collector electrode 4 laminated on the two substrates, respectively.

Next, a specific configuration example when the electron emitting material 10 is applied to the thermoelectric power generation element 20 will be described.

In the case of the thermoelectric generator 20, the substrate 1 is a conductive substrate made of, for example, molybdenum. The substrate 2 is an insulating substrate made of, for example, Al ₂ O ₃ or the like. Here, the insulating substrate refers to a substrate made of a high resistance material such as an oxide, nitride, ceramic, or a high resistance semiconductor material.

The substrate 1 has an emitter electrode 3 laminated on the substrate 1 as shown in FIG. The emitter electrode 3 is a cathode electrode that emits electrons when heat is applied, and is composed of an electron emission material 10 that is a diamond semiconductor.

The substrate 2 has a collector electrode 4 laminated on the substrate 2 as shown in FIG. The collector electrode 4 is an anode electrode that captures electrons emitted from the electrode emitter 3, and is made of a conductive material such as a metal or a low-resistance semiconductor material. In the present embodiment, the collector electrode 4 is made of molybdenum.

As shown in FIG. 4, the emitter electrode 3 and the collector electrode 4 are arranged to face each other with a predetermined distance therebetween so that the electrodes face each other as shown in FIG. The interval between these electrodes is set to an interval suitable for thermionic power generation, for example, 5 to 50 μm. This interval may be maintained by arranging the emitter electrode 3 and the collector electrode 4 apart from each other with a space therebetween, but may be maintained by using a spacer between these two electrodes. Good. In the case of using a spacer, for example, an insulator (not shown) having a film thickness corresponding to this interval is provided between the emitter electrode 3 and the collector electrode 4 or the substrate 2 or between the substrate 1 and the collector electrode 4 or the substrate 2. Sandwich. By fixing through the spacer in this way, the interval can be more reliably maintained. In addition, when using a spacer, the thermoelectron power generation element 20 of this embodiment should just be the structure where the emitter electrode 3 and the collector electrode 4 are electrically insulated.

Here, the operating principle of the thermoelectric generator 20 will be described. As described in FIG. 2B, the thermoelectric power generation element 20 converts thermal energy into electrical energy by utilizing a phenomenon in which thermoelectrons are emitted from the electrode surface. Specifically, when heat is applied to the emitter electrode 3 from an external heat source, electrons are excited from the impurity level of the diamond semiconductor that is the emitter electrode 3 to the conduction band. As shown in FIG. 2B, since the conduction band of the diamond semiconductor is higher in energy level than the vacuum level by NEA, the thermoelectrons excited in the conduction band jump out into the vacuum without a barrier. Thus, the thermoelectrons are emitted from the surface of the emitter electrode 3 that has become high temperature, and reach the collector electrode 4. In this way, since the thermoelectrons that have reached the collector electrode 4 generate an electromotive force when returning to the emitter electrode 3 via the load 5, the thermoelectric generator 20 can supply power to the load 5. it can.

Thus, in this embodiment, since the electron emission material 10 that maintains a low work function even at a high temperature is used as the emitter electrode 3, a decrease in the amount of emitted thermoelectrons is suppressed even at a high temperature. Thus, the thermionic power generation element 20 with high power generation efficiency and output is obtained.

Next, the effects of using the electron emission material 10 shown in the first, second, and third embodiments will be described with reference to the following Examples 1 to 5, Comparative Examples 1 to 4 and Table 1. I will say more specifically. Table 1 shows the result of measuring the thermoelectron current at a predetermined electrode temperature by using Mo for the substrate 1 and forming the electron emission material according to the configuration on the substrate 1 as the emitter electrode 3 of the thermoelectron emitting element.

Note that “terminal atoms” in Table 1 are atoms that terminate the surface of the diamond film 11. The “electrode temperature (° C.)” in Table 1 is the electrode temperature of the substrate when the thermionic current is measured. Furthermore, “Thermionic current (A / cm ² )” in Table 1 means that a pair of electrode substrates are arranged opposite to each other in a vacuum chamber of a vacuum vapor deposition machine, and one of the Mos formed with the diamond film 11 under vacuum. The measured value of the current between the electrodes at the temperature when the substrate is heated. In addition, “Rb + Cs” in the column of terminal atoms in Example 5 of Table 1 means a stacked state in which Rb is formed as the first metal layer 14 and Cs is formed as the second metal layer 15. In addition, “O” in the column of terminal atoms in Comparative Example 1 in Table 1 means that only bonded oxygen 12 is formed on the surface of the diamond film 11. Further, “H” in the column of terminal atoms in Comparative Example 4 in Table 1 means that the surface of the diamond film 11 is terminated with hydrogen. Further, “not measurable” in the column of the thermionic current in Table 1 means that the measurement limit value is less than 1.0 × 10 ⁻⁹ A / cm ² .

Example 1
The electron-emitting material of Example 1 was formed on the substrate 1 made of Mo, and was used as the diamond film 11, the bonded oxygen 12 bonded to the surface of the diamond film 11, and the termination metal 13 that terminates the bonded oxygen 12. And Na. Specifically, the diamond film 11 is formed, and the surface of the diamond film 11 is provided with bonded oxygen 12 by ozone oxidation treatment. Then, the substrate is heated to 500 ° C. and Na is supplied to the bonded oxygen 12. On the other hand, Na was supplied and terminated. A voltage was applied to the electrode facing the substrate on which the electron-emitting material of Example 1 was formed, and the thermoelectron current flowing between these electrodes was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was 7.1 × 10 ⁻⁸ A / cm ² .

Further, when the electron emission material of Example 1 was used, a thermoelectron current of the same level as that obtained by hydrogen-termination of the surface of the diamond film 11 of Comparative Example 4 to be described later flowed. From this, it was confirmed that the electron emission material of Example 1 was able to reduce the work function to the same extent as when hydrogen terminated. It was also confirmed that stable electron emission at a high temperature of 500 ° C., that is, a low work function can be maintained.

Even when the surface of the diamond film 11 was produced by oxidation treatment using microwave plasma, an equivalent measurement result was obtained.

(Example 2)
In the electron emission material of Example 2, the surface of the diamond film 11 was terminated with K through the bonded oxygen 12 as a result of supplying K by the same procedure as in Example 1. The substrate on which the electron-emitting material of Example 2 was formed was heated at 500 ° C., and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermoelectron current at an electrode temperature of 500 ° C. was 2.3 × 10 ⁻⁶ A / cm ² .

Thus, when the electron emission material of Example 2 was used, it was confirmed that a thermionic current exceeding Example 1 flows and that the work function can be further reduced as compared with the case of Na termination. Moreover, it was confirmed that the low work function and the high temperature stability can be achieved in the same manner as in Example 1.

(Example 3)
In the electron emission material of Example 3, as a result of supplying Rb by the same procedure as in Example 1, the surface of the diamond film 11 was terminated with Rb through the bonded oxygen 12. And the board | substrate with which the electron emission material of Example 3 was formed was heated at 500 degreeC, and the thermoelectron current which flows between the above-mentioned pair of electrodes in that case was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was 3.6 × 10 ⁻⁴ A / cm ² .

Thus, it was confirmed that when the electron emission material of Example 3 was used, a thermionic current further exceeded that of Example 2 and the work function could be further reduced as compared with the case of K termination. Moreover, it was confirmed that the low work function and the high temperature stability can be achieved in the same manner as in Example 1.

(Example 4)
In the electron emission material of Example 4, as a result of supplying Cs by the same procedure as in Example 1, the surface of the diamond film 11 was terminated with Cs through the bonded oxygen 12. The substrate on which the electron-emitting material of Example 4 was formed was heated at 500 ° C., Cs was always supplied, and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermoelectron current at an electrode temperature of 500 ° C. was 4.0 × 10 ⁻³ A / cm ² .

Thus, it was confirmed that when the electron emission material of Example 4 was used, a thermionic current further exceeded that of Example 3 and that the work function could be further reduced as compared with the case of Rb termination. Moreover, it was confirmed that the low work function and the high temperature stability can be achieved in the same manner as in Example 1.

(Example 5)
In the electron emission material of Example 5, Rb is supplied by the same procedure as that of Example 3, Rb is bonded to the surface of the diamond film 11 through the bonded oxygen 12, and Cs is further supplied. Are obtained by laminating and terminating. The substrate on which the electron-emitting material of Example 5 was formed was heated at 500 ° C., Cs was always supplied, and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was 2.7 × 10 ⁻³ A / cm ² .

Thus, it was confirmed that when the electron emission material of Example 5 was used, a thermionic current exceeding Example 3 flowed, and the work function could be further reduced as compared with the case where it terminated with Rb. Moreover, it was confirmed that the low work function and the high temperature stability can be achieved in the same manner as in Example 1.

(Comparative Example 1)
Unlike Example 1, the electron emission material of Comparative Example 1 is one in which the surface of the diamond film 11 is terminated with oxygen as a result of not performing evaporation of alkali metal or the like. The substrate on which the electron-emitting material of Example 1 was formed was heated at 500 ° C., and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was less than the measurement limit of 1.0 × 10 ⁻⁹ A / cm ² . Furthermore, even when the electrode temperature was increased to 730 ° C., the thermoelectron current at 730 ° C. could not be measured.

For this reason, when an electron emitting material in which the surface of the diamond film 11 is terminated with oxygen is used, NEA does not occur in the electron emitting material and the work function is large as described with reference to FIG. In addition, it is considered that the thermionic current did not flow.

(Comparative Examples 2 and 3)
In the electron emission materials of Comparative Examples 2 and 3, Li was vapor-deposited by the same procedure as in Example 1, and the surface of the diamond film 11 was terminated with Li via the bonded oxygen 12. The difference between Comparative Example 2 and Comparative Example 3 is the electrode temperature. First, for Comparative Example 2, the substrate on which the electron-emitting material was formed was heated at 500 ° C., and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was less than the measurement limit of 1.0 × 10 ⁻⁹ A / cm ² . In Comparative Example 3, when the electrode temperature was further increased to 730 ° C., the thermionic current at the electrode temperature of 730 ° C. was 5.7 × 10 ⁻⁷ A / cm ² .

When the electron emission materials having the configurations of Comparative Example 2 and Comparative Example 3 were used, a measurable thermoelectron current did not flow at an electrode temperature of 500 ° C., and the electrode temperature was 730 ° C. in order to measure the thermoelectron current. It was necessary to raise to. From this, the electron-emitting material terminated with Li has a lower work function than that with oxygen termination, but has the same thermionic current unless the electrode temperature is higher than those in Examples 1 to 5. Does not flow, it is thought that low work function is not enough.

(Comparative Example 4)
The electron-emitting material of Comparative Example 4 is produced by the same procedure as that of Comparative Example 1, and the alkali metal is not deposited and the surface of the diamond film 11 is terminated with hydrogen. The substrate on which the electron-emitting material of Comparative Example 4 was formed was heated at 500 ° C., and the thermoelectron current flowing between the pair of electrodes at that time was measured. As a result, the thermionic current at an electrode temperature of 500 ° C. was 2.2 × 10 ⁻⁷ A / cm ² .

From this, when a hydrogen-terminated electron emission material is used, the thermoelectron current is equal to or less than that of Examples 1 to 5, and therefore, the work function can be made lower than that of the electron emission material of Comparative Example 1. However, it seems that low work function is not enough.

As a result, as described in the first embodiment, the second embodiment, and the third embodiment, the electron emission material 10 terminated with an alkali metal on the diamond film 11 through the bonded oxygen 12 has a low work function. It was confirmed that the high temperature stability was excellent.

Although not shown in Table 1, in the electron-emitting materials 10 of Examples 1 to 5, when the alkali metal deposition amount was further increased, a phenomenon in which the thermionic current increased was confirmed. This result is considered to indicate that the ratio of the area covered by the terminal metal 13 to the surface area of the diamond film 11, that is, the coverage is not sufficiently optimal. That is, for the termination metal 13 in Table 1, it is considered that increasing the coverage of the surface of the diamond film 11 increased the number of sites that emit electrons, leading to an increase in thermionic current. This means that the electron emission material 10 has a low work function while ensuring high temperature stability even if the surface of the diamond film 11 is not 100% coated with the termination metal 13, and electron emission is achieved by optimizing the coverage. This suggests that the electron emission characteristics of the material 10 can be further improved.

(Other embodiments)
Note that the present disclosure is not limited to the above-described embodiment, and can be modified as appropriate.

For example, as the electron-emitting device, a thermionic power generation device has been described as an example. However, other examples include a photo-excited electron-emitting device and a photo-excited thermionic power generation device, and the electron-emitting material 10 is used as the emitter electrode 3 in these devices. Can be applied. Since the electron emission material 10 can maintain a low work function even when driven at a high temperature, the electron emission material 10 can be used as an emitter electrode 3 such as a photoexcited electron emission element or a photoexcitation thermoelectron generation element, thereby improving the electron emission efficiency and the power generation efficiency. A high element can be obtained. In this case, the configuration of a photoexcited electron-emitting device can be a known device configuration.

Further, the diamond film 11 is not limited to the film forming method described above, and may be formed by, for example, a CVD method or a sputtering method, and may be formed by RF plasma CVD, DC plasma CVD, RF plasma sputtering, DC plasma sputtering, or the like. The N-type diamond constituting the thin film of the diamond film 11 may be either single crystal or polycrystal. For example, when a diamond substrate produced by high-pressure synthesis is used, a single crystal is formed when the diamond film 11 is formed thereon by, for example, the CVD method.

As the oxidation treatment method for the diamond film 11, the ozone treatment has been described in the first embodiment. However, it is sufficient that the surface of the diamond film 11 can be oxidized, and may be an oxygen plasma treatment, an oxygen radical treatment, a thermal mixed acid treatment, or the like. Other methods may be used.

As the oxygen plasma treatment, for example, a method using vacuum plasma will be described. The surface of the diamond film 11 is terminated with oxygen by converting oxygen or an oxygen mixed gas into plasma in a container under reduced pressure and irradiating the diamond film 11 with oxygen. Specifically, a substrate on which the diamond film 11 placed in the processing chamber is formed by introducing microwave power 300 W into the processing chamber in which the mixed gas pressure of argon and oxygen is 30 Pa is introduced into the processing chamber. To process. An electrode for separating charged particles and neutral particles is installed between the diamond film substrate and the plasma generator, and a plurality of openings provided in the electrode with electrically neutral molecules, atoms or radicals. It can be transported through the part to the substrate surface. On the other hand, ions in the plasma hardly diffuse to the substrate side.

As a result of measuring the atomic composition of the diamond film surface after the oxygen plasma treatment by XPS, the oxygen ratio was about 7%. Note that the plasma processing is not limited to the above-described method and processing conditions, and may be RF plasma, DC plasma, or the like, for example.

Also, for example, a method using atmospheric pressure plasma as oxygen radical treatment will be described. The gas in the dielectric tube is ionized by dielectric barrier discharge, and the diamond film 11 is irradiated with a plasma jet ejected from the dielectric tube under atmospheric pressure, and the surface of the diamond film 11 is terminated with oxygen. Specifically, He gas is flowed through a quartz tube having an inner diameter of 4 mm, and plasma is generated by applying 5 to 10 kV to a copper electrode wound around and closely attached to the outside of the quartz tube. The surface of the film is terminated with oxygen by oxygen radicals generated by the reaction of He ions generated in the discharge part and He atoms in a metastable state with oxygen in the atmosphere.

As a result of measuring the atomic composition of the diamond film surface treated at a position 20 mm downstream from the tip of the quartz tube by XPS, the oxygen ratio was about 25%. Note that the gas to be converted into plasma is not limited to He, and an inert gas such as argon, oxygen or air, or a mixed gas thereof may be used.

Claims

A diamond film (11);
Oxygen (12) bound to the surface of the diamond film;
An electron-emitting material comprising a termination metal (13) bonded to the diamond film via the oxygen,
The diamond film is an N-type diamond semiconductor doped with impurities,
The termination metal is an electron emission material containing at least one of Na, K, Rb, Cs, and Mg.
The electron-emitting material according to claim 1, wherein the termination metal includes at least two of Na, K, Rb, Cs, and Mg.
A diamond film (11);
Oxygen (12) bound to the surface of the diamond film;
A first metal layer (14) for bonding to the diamond film via the oxygen;
An electron emission material comprising: a second metal layer (15) laminated on the first metal layer,
The diamond film is an N-type diamond semiconductor doped with impurities,
The first metal layer includes at least one of Li, Na, K, Rb, Cs, and Mg;
The second metal layer is an electron emission material including at least one metal different from the first metal layer among Li, Na, K, Rb, Cs, and Mg.
A pair of electrodes consisting of an emitter electrode (3) and a collector electrode (4) arranged to face each other,
The electron emitter according to any one of claims 1 to 3, wherein the emitter electrode is an electrode that emits electrons.
The electron-emitting device according to claim 4, wherein the electron-emitting device is a thermoelectron generating device that uses thermoelectrons moving between the pair of electrodes.
The electron-emitting device according to claim 4, wherein the electron-emitting device is a photo-excited electron-emitting device that utilizes electrons emitted from the emitter electrode by photo-excitation.
The electron-emitting device according to claim 4, wherein the electron-emitting device is a photoexcited thermoelectron generating device that uses thermal electrons moving between the pair of electrodes.