WO2016147603A1

WO2016147603A1 - Thermionic power generation element and method for manufacturing same

Info

Publication number: WO2016147603A1
Application number: PCT/JP2016/001241
Authority: WO
Inventors: 片岡　光浩; 裕治木村; 進祖父江; 直也森岡; 山崎　聡; 竹内　大輔; 宙光加藤; 一之渡辺
Original assignee: 株式会社デンソー; 国立研究開発法人産業技術総合研究所; 学校法人東京理科大学
Priority date: 2015-03-13
Filing date: 2016-03-08
Publication date: 2016-09-22
Also published as: JP2016171246A

Abstract

Provided is a thermionic power generation element having: an emitter (2) for generating thermions, having an emitter substrate (21) that has electrical conductivity, a first layer (22) laminated onto the emitter substrate and comprising an n-type diamond semiconductor containing phosphorus as a donor, a second layer (23) laminated onto the first layer, comprising an n-type diamond semiconductor containing nitrogen as a donor and having a film thickness of 40 nm or less, and a terminal layer (24, 24b) formed on the outermost surface of the second layer and containing one or more specific metal elements M selected from the group consisting of the alkali metals and Mg; and a collector (3) having at least a collector substrate (31) that has electrical conductivity, the collector (3) being arranged facing the emitter across a gap and collecting the thermions.

Description

THERMOELECTRIC POWER GENERATOR AND MANUFACTURING METHOD THEREOF

Cross-reference of related applications

This application is based on Japanese Application No. 2015-511168 filed on Mar. 13, 2015, the contents of which are incorporated herein by reference.

The present disclosure relates to a thermoelectric power generation element that converts thermal energy into electrical energy and a method for manufacturing the same.

As a power generation element that converts thermal energy into electric energy, there is a thermoelectron power generation element that generates electromotive force using thermionic emission. For example, Patent Document 1 discloses an example of an electron emission device in which a first diamond layer and a second diamond layer are formed on a conductive substrate. In the electron emission device of Patent Document 1, P (phosphorus) is used as a dopant added to the first diamond layer, and N (nitrogen) is used as a dopant added to the second diamond layer. We are trying to increase this.

However, the thermoelectron emission device disclosed in Patent Document 1 has a problem in that the thermoelectric current is still insufficient for use as a thermoelectric power generation element, and power generation efficiency is low.

JP 2009-238690 A

This disclosure intends to provide a thermionic power generation element with high power generation efficiency.

The thermoelectric power generating element according to the first aspect of the present disclosure includes an emitter substrate having electrical conductivity, an n-type diamond semiconductor containing phosphorus as a donor, and a first layer stacked on the emitter substrate; One type selected from the group consisting of an n-type diamond semiconductor containing nitrogen as a donor, having a film thickness of 40 nm or less, laminated on the first layer, and an alkali metal element and Mg Two or more kinds of specific metal elements M are contained, and have a termination layer formed on the outermost surface of the second layer, and have at least an emitter for generating thermoelectrons and a collector substrate having electrical conductivity. And a collector which is disposed through a gap so as to face the emitter and collects the thermoelectrons. The thermoelectron power generation device includes the first layer, the second layer having the specific thickness, and the termination layer formed on the outermost surface of the second layer on the emitter substrate. Has an emitter. In the emitter, it is considered that the influence of the second layer having a relatively high resistivity is reduced by reducing the thickness of the second layer to 40 nm or less. As a result, the internal resistance of the emitter in the thickness direction can be reduced.

Further, the specific metal element M is present on the outermost surface of the second layer. Thereby, the work function of the second layer can be greatly reduced.

As described above, the emitter can reduce the internal resistance and work function by having the specific configuration. As a result, the thermoelectron power generation element can significantly increase the thermoelectron current generated from the emitter as compared with the prior art, and can further improve the power generation efficiency.

In the method for manufacturing a thermoelectric generator according to the second aspect of the present disclosure, a first layer made of an n-type diamond semiconductor containing phosphorus as a donor is formed on an emitter substrate having electrical conductivity, A second layer made of an n-type diamond semiconductor containing nitrogen as a donor is formed on the first layer, and then one or two selected from the group consisting of an alkali metal element and Mg on the outermost surface of the second layer A termination layer is formed by adsorbing the specific metal element M described above, and an emitter having the emitter substrate, the first layer, the second layer, and the termination layer is manufactured, and a collector substrate having electrical conductivity. Is prepared separately from the emitter, and the emitter and the collector are opposed to each other with a gap therebetween.

According to the method for manufacturing a thermionic power generation element, the thermionic power generation element can be easily manufactured.

The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. The drawing
FIG. 1 is an explanatory diagram of a thermionic power generation element in Example 1. FIG. 2 is a partially enlarged sectional view showing an example of a termination layer in Example 1. FIG. 3 is an explanatory diagram of the energy band of the emitter in Example 1. FIG. 4 is an explanatory diagram of an energy band when only the second layer is stacked on the emitter substrate. FIG. 5 is an explanatory diagram of an energy band when only the first layer is stacked on the emitter substrate. FIG. 6 is a partially enlarged cross-sectional view showing an example of a termination layer in which a LiO group is configured in Example 3, and FIG. 7 is a graph showing the thermionic current magnitude of a sample prepared by changing the thickness of the second layer in the reference example.

In the thermoelectric generator, the first layer of the emitter is preferably composed of an n-type diamond semiconductor having a P dopant concentration of 1 × 10 ¹⁹ cm ⁻³ or more. In this case, the internal resistance in the thickness direction of the first layer can be sufficiently reduced, and the thermionic current can be further increased. The higher the dopant concentration of P, the smaller the internal resistance. However, when the dopant concentration exceeds 1 × 10 ²¹ cm ⁻³ , it is difficult to obtain an effect of reducing the internal resistance commensurate with the doping amount.

The second layer is preferably composed of an n-type diamond semiconductor having a dopant concentration of N of 1 × 10 ²⁰ cm ⁻³ or more. In this case, the internal resistance in the thickness direction of the second layer can be sufficiently reduced, and the thermionic current can be further increased. The higher the dopant concentration of N, the lower the internal resistance. However, when the dopant concentration exceeds 1 × 10 ²¹ cm ⁻³ , it is difficult to obtain an effect commensurate with the amount of doping.

The film thickness of the second layer is 40 nm or less as described above. When the thickness of the second layer exceeds 40 nm, it is difficult to increase the thermionic current. On the other hand, when the thickness of the second layer is excessively thin, the second layer is difficult to be formed uniformly, and the first layer may be exposed on the surface of the emitter. In this case, the effect of laminating the second layer cannot be obtained, and the power generation efficiency may be reduced. Therefore, the thickness of the second layer is preferably 1 nm or more and 40 nm or less.

A termination layer containing one or more specific metal elements M selected from the group consisting of alkali metal elements and Mg is formed on the outermost surface of the second layer. Although it is difficult to specify the chemical state of the specific metal element M present in the termination layer, for example, the specific metal element M may be directly bonded to the outermost surface of the n-type diamond semiconductor constituting the second layer. Good. That is, the n-type diamond semiconductor constituting the second layer may be terminated by the specific metal element M present in the termination layer.

The termination layer is formed by, for example, sequentially forming the first layer and the second layer by a method such as microwave plasma CVD (chemical vapor deposition), and then bonding hydrogen to the outermost surface by a method such as heating in a vacuum. Can be easily formed by adsorbing the specific metal element M continuously. Here, “continuously” mentioned above means that after desorbing hydrogen bonded to the outermost surface of the second layer, the specific metal element M is adsorbed without exposing the outermost surface to the atmosphere.

The termination layer may contain specific metal elements M and O. In this case, the specific metal elements M and O present in the termination layer constitute an MO group, and the n-type diamond semiconductor constituting the second layer is terminated by the MO group. preferable.

For example, after forming the first layer and the second layer in the same manner as described above, the termination layer containing the MO group is subjected to surface oxidation treatment to terminate the outermost surface of the second layer with oxygen, and then the second layer. It can be formed by adsorbing the specific metal element M on the outermost surface of the layer. As the surface oxidation treatment described above, for example, an ozone oxidation treatment that oxidizes the outermost surface with ozone can be employed.

As the alkali metal element contained in the termination layer, for example, Li (lithium), Na (sodium), K (potassium), or Cs (cesium) can be employed. When the n-type diamond semiconductor constituting the second layer is terminated with the alkali metal element, the work function can be further reduced by using Na as the alkali metal element.

The amount of the specific metal element M contained in the termination layer is preferably an amount corresponding to 0.2 molecular layer or more and 10 molecular layer or less. In this case, the termination layer can be reliably formed and the thermionic emission characteristics of the emitter can be improved.

If the amount of the specific metal element M contained in the termination layer is less than 0.2 molecular layer, the effect of reducing the work function may be insufficient. On the other hand, when the amount of the specific metal element M exceeds 10 molecular layers, the specific metal element M contained in the termination layer may be excessive. In some cases, a film of the specific metal element M is formed on the surface of the emitter, which may hinder the emission of thermoelectrons in a relatively low temperature range.

From the above, from the viewpoint of sufficiently obtaining the action and effect of the specific metal element M, the amount of the specific metal element M is preferably an amount corresponding to 0.2 to 10 molecular layers.

The amount of the specific metal element M contained in the termination layer is more preferably an amount corresponding to 0.2 molecular layer or more and 1 molecular layer or less. In this case, the stability of the specific metal element M in the termination layer is improved, and the specific metal element M is less likely to be detached from the termination layer when heated to a high temperature. In this case, the work function of the emitter can be effectively reduced. This is because the specific metal element M or the MO group is easily bonded directly to the resurface of the second layer by reducing the amount of the specific metal element M to one molecular layer or less. As a result, the specific metal element M is adsorbed. This is because the surface properties can be effectively changed as the energy increases.

In a conventional thermoelectric power generation element in which the outermost surface of the diamond layer is terminated with hydrogen, it has been confirmed that when the diamond layer is heated to 700 ° C. or higher, hydrogen begins to desorb from the outermost surface. Therefore, it is difficult to use the conventional thermoelectric power generation element at a high temperature of, for example, 700 ° C. or more, and there is a limit to improving the power generation efficiency. Further, the conventional thermoelectron power generation device has a problem that the thermoelectron emission characteristics gradually deteriorate when used over a long period of time.

On the other hand, the thermoelectric power generation element can suppress detachment of the specific metal element M from the termination layer by setting the amount of the specific metal element M in the specific range. As a result, the thermoelectron power generation element has excellent thermoelectron emission characteristics even at a higher temperature than conventional thermoelectron power generation elements, and can maintain excellent thermoelectron emission characteristics over a long period of time.

The emitter and collector preferably have an internal resistance in the thickness direction of 1 Ωcm ² or less. In this case, it is possible to reduce the voltage drop when the current derived from the thermoelectrons passes through the emitter and collector, and to sufficiently reduce the loss derived therefrom. As a result, the power generation efficiency of the thermionic power generation element can be further improved.

The emitter substrate is any one of Si (silicon), Ti (titanium), Mo (molybdenum), Ir (iridium), Ta (tantalum), W (tungsten), Ru (ruthenium), Cr (chromium), or Pt (platinum). It is preferable that it is comprised. An emitter substrate made of these materials is likely to generate diamond nuclei when the first layer is formed. Moreover, the diamond semiconductor produced on the emitter substrate made of these materials is difficult to peel from the emitter substrate in the temperature region where the diamond semiconductor is grown. Therefore, in this case, the first layer made of an n-type diamond semiconductor with few defects and good film quality can be produced. As a result, the internal resistance of the emitter in the thickness direction can be further reduced, and the power generation efficiency can be further improved.

Of the materials described above, it is more preferable to use Si as the emitter substrate. Si has few impurities, crystal defects, and the like, and a high-quality material having a large area can be easily obtained. Therefore, the manufacturing cost of the thermoelectric power generation element can be reduced more easily.

The emitter may have an interface intermediate layer between the emitter substrate and the first layer. In the interface intermediate layer, the sum of the resistance in the thickness direction, the interface resistance between the emitter substrate and the interface resistance between the first layer is larger than the interface resistance between the emitter substrate and the first layer. It is preferable to be configured to be small. In this case, the internal resistance of the emitter in the thickness direction can be further reduced. As a result, the power generation efficiency of the thermionic power generation element can be further improved.

For example, a metal carbide can be used as the interface intermediate layer described above. Examples of the metal carbide include titanium carbide, tantalum carbide, tungsten carbide, molybdenum carbide, silicon carbide, and chromium carbide. Among these, it is more preferable to use titanium carbide.
(Example 1)
Examples of the thermoelectric power generation element will be described with reference to FIGS. As shown in FIG. 1, the thermoelectric generator 1 includes an emitter 2 that generates thermoelectrons, and a collector 3 that is disposed through a gap d so as to face the emitter 2 and collects thermoelectrons. . The emitter 2 is formed on the outermost surface of the emitter layer 21 having electrical conductivity, the first layer 22 laminated on the emitter substrate 21, the second layer 23 laminated on the first layer, and the second layer 23. And a terminated layer 24.

The first layer 22 is composed of an n-type diamond semiconductor containing P as a donor. The second layer 23 is made of an n-type diamond semiconductor containing N as a donor and has a thickness of 40 nm or less. As shown in FIG. 2, the termination layer 24 contains the specific metal element M in an amount corresponding to 0.2 to 10 molecular layers. In this example, an alkali metal element is used as the specific metal element M.

The collector 3 has at least a collector substrate 31 having electrical conductivity. Hereinafter, a more detailed configuration of the thermoelectric generator 1 will be described together with a manufacturing method.

The emitter substrate 21 of this example is made of Mo. As will be described later, the emitter substrate 21 also serves as an electrode for connecting the external load 4.

The n-type diamond semiconductor constituting the first layer 22 can be formed by, for example, a microwave plasma CVD method using CH ₄ gas as a carbon source, PH ₃ gas as a phosphorus source, and H ₂ gas as a carrier gas. . The film formation conditions of the first layer 22 are, for example, as follows.

-Substrate temperature: 1000 ° C
-Ratio of CH ₄ gas flow rate to H ₂ gas flow rate (CH ₄ flow rate / H ₂ flow rate): 0.01
-Ratio of PH ₃ gas flow rate to CH ₄ gas flow rate (PH ₃ flow rate / CH ₄ flow rate): 0.05
・ Film pressure: 30 Torr
・ Microwave output: 750W
・ Film thickness 1.0μm
・ P dopant concentration 1 × 10 ²⁰ cm ⁻³
The n-type diamond semiconductor constituting the second layer 23 can be formed by, for example, a microwave plasma CVD method using CH ₄ gas as a carbon source, N ₂ gas as a nitrogen source, and H ₂ gas as a carrier gas. . The film formation of the second layer 23 is normally performed without exposing the first layer 22 to the atmosphere after the film formation of the first layer 22 is completed. The film forming conditions for the second layer 23 are, for example, as follows.

-Substrate temperature: 1000 ° C
-Ratio of CH ₄ gas flow rate to H ₂ gas flow rate (CH ₄ flow rate / H ₂ flow rate): 0.01
-Ratio of N ₂ gas flow rate to CH ₄ gas flow rate (N ₂ flow rate / CH ₄ flow rate): 10
・ Film pressure: 50 Torr
・ Microwave output: 1000W
・ Film thickness 40nm
N dopant concentration 5 × 10 ²⁰ cm ⁻³
After the first layer 22 and the second layer 23 are formed on the emitter substrate 21, the specific metal element M is applied to the outermost surface of the second layer 23 without exposing the first layer 22 and the second layer 23 to the atmosphere. Adsorb. Thereby, the termination layer 24 can be formed on the outermost surface of the second layer 23. It can be estimated that the specific metal element M contained in the termination layer 24 of this example is bonded to the outermost surface of the n-type diamond semiconductor constituting the second layer 23. That is, it can be presumed that the n-type diamond semiconductor constituting the second layer 23 is terminated by the specific metal element M present in the termination layer 24 as shown in FIG.

The adsorption of the specific metal element M to the second layer 23 can be performed using, for example, an alkali metal dispenser. More specifically, the specific metal element M can be adsorbed on the outermost surface of the second layer 23 by exposing the outermost surface of the second layer 23 to the atmosphere of the specific metal element M supplied from the alkali metal dispenser. . The adsorption amount of the specific metal element M can be measured using XPS (X-ray photoelectron spectroscopy). Moreover, as an alkali metal element supplied from the dispenser, for example, any one of Li, Na, K, and Cs can be used.

The collector 3 has a structure in which a 2.5 μm thick first layer 32 and a 20 nm thick second layer 33 are sequentially laminated on a collector substrate 31 made of Mo. The film formation conditions of the first layer 32 and the second layer 33 are the same as those of the first layer 22 and the second layer 23 in the emitter 2.

The size of the gap d between the emitter 2 and the collector 3 is not particularly limited, but in this example, the emitter 2 and the collector 3 are arranged so that the gap d is about 20 to 30 μm. . The space between the emitter 2 and the collector 3 is depressurized to 1 × 10 ⁻⁵ Pa or less.

When operating the thermoelectric generator 1, the emitter substrate 21 and the collector substrate 31 are connected via the external load 4 as shown in FIG. 1, and the emitter 2 is heated in this state. Thereby, thermoelectrons are emitted from the surface of the emitter 2 into the gap d between the emitter 2 and the collector 3 and collected by the collector 3. Then, the electrons collected in the collector 3 flow from the collector substrate 31 to the external circuit (see arrow 101), pass through the external load 4 and return to the emitter 2 (see arrow 102).

Next, the function and effect of this example will be described with reference to FIGS. FIG. 3 is an example of the energy band of the emitter 2. The position in the vertical direction in FIG. 3 corresponds to the energy level, and the higher level indicates the higher energy level. Further, the horizontal direction is divided into three

regions

221, 231, and 201 by two vertical lines of a vertical line 200 corresponding to the surface of the emitter and a vertical line 230 corresponding to the boundary between the first layer 22 and the second layer 23. did.

In the left region 221, the lower end 222 of the conduction band of the first layer 22, the impurity level 223, and the upper end 224 of the valence band are shown. In the central region 231, the lower end 232 of the conduction band, the impurity level 233, and the upper end 234 of the valence band of the second layer 23 are shown. Since the thickness of the termination layer 24 is very thin, the boundary between the second layer 23 and the termination layer 24 and the energy band in the termination layer 24 are omitted from FIG. 3 for convenience.

FIG. 4 shows an example of the energy band of the emitter 2 in which only the second layer 23 is laminated on the emitter substrate 21. The vertical position in FIG. 4 corresponds to the energy level as in FIG. In addition, the energy band of the second layer 23 is shown in the region 231 on the left side of the vertical line 200 corresponding to the surface.

Similarly, FIG. 5 is an example of an energy band of an emitter in which only the first layer 22 is stacked on the emitter substrate 21. The vertical position in FIG. 5 corresponds to the energy level as in FIG. Further, the energy band of the first layer 22 is shown in the left region 221 with respect to the vertical line 200 corresponding to the surface. 4 and 5, the same reference numerals as those in FIG. 3 represent the same components as those in FIG. 3 unless otherwise indicated.

As is known from FIGS. 3 and 4, the impurity level 223 of the n-type diamond semiconductor constituting the first layer 22 has a conduction band higher than the impurity level 233 of the n-type diamond semiconductor constituting the second layer 23. It is formed at a position close to the lower ends 222 and 232. Therefore, the first layer 22 is more likely to cause hopping conduction and has a lower resistivity than the second layer 23. Therefore, as compared with the case where the first layer 22 is not provided as shown in FIG. 4, the emitter 2 (see FIG. 3) of this example can reduce the internal resistance in the thickness direction.

As is known from FIG. 5, when the n-type diamond semiconductor constituting the first layer 22 is exposed on the surface of the emitter 2, an upward bend 225 is generated at the lower end 222 of the conduction band in the vicinity of the surface. Therefore, it becomes difficult for the electrons 6 to be emitted from the emitter 2 and increase the thermionic current. This is presumably because the n-type diamond semiconductor containing P as a donor has a property that defect levels are likely to be formed when exposed to the surface of the emitter 2. On the other hand, as shown in FIGS. 3 and 4, in the n-type diamond semiconductor containing N as a donor, defect levels are not easily formed when exposed to the surface of the emitter 2, and the conduction band is bent near the surface. Hard to occur. Therefore, as compared with the case where the second layer 23 is not stacked on the first layer 22 as shown in FIG. 5, the emitter 2 (see FIG. 3) of this example can reduce the barrier in the vicinity of the surface, and the thermionic current. Is easily increased.

In the emitter 2 of this example, the thickness of the second layer 23 is 40 nm or less. Therefore, it is considered that the influence of the second layer 23 having a higher resistivity than the first layer 22 on the internal resistance of the entire emitter 2 can be reduced.

In the emitter 2 of this example, the n-type diamond semiconductor constituting the second layer 23 is terminated by the specific metal element M contained in the termination layer 24. Therefore, the electrons 6 thermally excited inside the emitter 2 are easily emitted from the surface of the emitter 2.

As a result, the thermoelectric power generation element 1 of this example can easily increase the thermionic current, and can further improve the power generation efficiency.
(Example 2)
In this example, the change of the work function of the emitter 2 when the specific metal element M is changed is evaluated by the first principle calculation. The first principle calculation was performed under the following conditions.

The calculation code used for the first principle calculation is the first principle electronic state calculation package ABINIT. The potential function used in the calculation is a norm-preserving pseudopotential.

As the structural model, a slab model having a film portion corresponding to the second layer 23 and the termination layer 24 and a vacuum portion arranged on both sides of the film portion in the thickness direction was employed in the calculation region. The film portion has a structure in which ten diamond unit cells are laminated in the thickness direction, and both surface in the thickness direction is a surface and has a surface unit cell having a period of 2 × 1. Further, the dangling bonds existing on the surface of the film part are terminated by the specific metal element M.

First-principles calculations were performed using the above conditions and structural model. The Hamiltonian calculation result of the Kohn-Sham equation was extracted from the obtained results, and the work function was calculated using the effective potential therein. Specifically, first, the calculation area was virtually divided in the thickness direction, and the average value of effective potentials in each area was calculated. And the value which deducted the Fermi energy obtained by calculation from the average value of the effective potential in the center part of a vacuum area | region was made into the work function.

Table 1 shows the result of the above calculation performed by changing the element terminating the film part. In this example, for each element, two types of structural models were created for the case where the adsorption amount was equivalent to 0.25 molecular layer and that corresponding to one molecular layer, and the calculation was performed. In addition, for comparison with the case where the dangling bond is terminated with the specific metal element M, the structural model in which the dangling bond is terminated with hydrogen and the structural model of the clean surface where the dangling bond is not terminated are also calculated. It was.

As is known from Table 1, the work function of the structural model in which the film part is terminated with Li, Na, and K is smaller than the work function of the structural model in which the film part is terminated with H (hydrogen).

The power generation characteristics of the thermoelectric power generation element 1 are, for example, G. W. As described in Sutton “direct energy conversion” (Kogakusha, 1968), it can be expressed as the following formulas (1) and (2).

W _o = AV _o T _E ² exp {−e (V _o + Φ _E ) / kT _E } (1)
J _o = AT _E ² exp {−e (V _o + Φ _E ) / kT _E } (2)
In addition, the meaning of the symbol used in the said Formula (1) and Formula (2) is as follows.

W _o (W / cm ² ): Maximum power density per unit area J _o (A / cm ² ): Current density of thermoelectron current when maximum power density is obtained V _o (V): Maximum power density is obtained Voltage T _E (K): Emitter temperature Φ _E (eV): Emitter work function A (A / cm ² K ² ): Richardson constant e (C): Elementary quantity of electricity k (J / K) : Boltzmann constant As known from the above formulas (1) and (2), by reducing the value of the work function Φ _E of the emitter, the maximum output density W _o and the current density J _o of the thermoelectron current at that time Can be increased. Therefore, the emitter 2 having the surface of the second layer 23 terminated by the specific metal element M can easily increase the thermionic current as compared with the conventional case, and can further improve the power generation efficiency.
(Example 3)
This example is an example of the thermoelectric generator 1 in which the surface of the second layer 23 is terminated with an MO group. In this example, the first layer 22 and the second layer 23 are formed on the emitter substrate 21 under the same conditions as in the first embodiment. Next, by performing a surface oxidation treatment, a functional group F containing oxygen (see FIG. 6) is formed on the outermost surface of the second layer 23, and the n-type diamond semiconductor constituting the second layer 23 is terminated with oxygen. As the surface oxidation treatment, for example, a method such as UV ozone treatment in which the surface of the second layer 23 is irradiated with ultraviolet light while being exposed to an oxygen atmosphere can be employed.

After the surface oxidation treatment is performed on the outermost surface of the second layer 23, Li is adsorbed on the outermost surface of the second layer 23. Thereby, the termination layer 24 b containing Li and O can be formed on the outermost surface of the second layer 23. It is considered that Li contained in the termination layer 24b of this example reacts with the functional group F described above to form a LiO group as shown in FIG. That is, it can be estimated that the n-type diamond semiconductor constituting the second layer 23 is terminated by a LiO group composed of Li and O existing in the termination layer 24b.

Others are the same as in Example 1. Of the reference numerals used in this example, the same reference numerals as those used in the first embodiment represent the same components as in the first embodiment unless otherwise specified.

In this example, the work function of the emitter 2 when the surface of the second layer 23 was terminated by the LiO group was evaluated by the first principle calculation. Specifically, the work function was calculated by the same method as in Example 2 except that dangling bonds in the structural model of Example 2 were terminated with a LiO group. As a result, the work function of the structural model terminated with the LiO group was 1.78 eV.

From this result, it can be understood that the thermoelectric power generation element 1 in which the surface of the second layer 23 is terminated with a LiO group can further reduce the work function of the emitter 2 and has excellent thermoelectron emission characteristics.
(Reference example)
This example is a reference example in which the thickness of the second layer 23 is changed to various thicknesses. The n-type diamond semiconductor constituting the first layer 22 of this example and the n-type diamond semiconductor constituting the second layer 23 were formed by microwave plasma CVD using the same conditions as in Example 1. The film thickness of the first layer 22 was 2.5 μm, and the dopant concentration of P was 1 × 10 ²⁰ cm ⁻³ . The dopant concentration of N in the second layer 23 was 3 × 10 ²⁰ cm ⁻³ .

Further, after the first layer 22 and the second layer 23 are formed on the emitter substrate 21, a hydrogen plasma process is performed on the outermost surface of the second layer 23, and a process of hydrogenating the outermost surface of the second layer 23 is performed. It was. Further, following the hydrogen plasma treatment, a treatment for terminating the outermost surface of the second layer 23 with hydrogen was performed by placing the emitter 2 in a hydrogen atmosphere.

In this example, as shown in Table 2, four types of samples (samples E1 to E2 and samples C1 to C2) having different film thicknesses of the second layer 23 were produced. Further, in this example, for comparison with the samples E1 to E2 and the samples C1 to C2, the sample C3 in which the emitter substrate 21 and the second layer 23 are stacked, the emitter substrate 21 and the first layer 22 are stacked. Sample C4 was prepared. The thickness of the second layer 23 in the sample C3 is 2.0 μm, and the dopant concentration of N is 3 × 10 ²⁰ cm ⁻³ . The thickness of the first layer 22 in the sample C4 is 2.5 μm, and the dopant concentration of P is 1 × 10 ²⁰ cm ⁻³ .

Moreover, the sample E1 of this example had an internal resistance per unit area in the thickness direction of about 0.7 Ωcm ² . The internal resistance was measured using a two-terminal method. In measuring the internal resistance, a metal electrode was formed by vapor deposition on the outermost surface of the second layer 23 in the sample E1, and this metal electrode and the emitter substrate 21 were used as terminals used for the measurement in the two-terminal method.

The six samples obtained as described above were evaluated for thermionic emission performance by the following method.

First, a sample was attached to the cathode electrode arranged in the vacuum vessel, and the cathode electrode and the emitter substrate 21 were brought into electrical contact. Next, the vacuum container was evacuated until the pressure in the vacuum container became 1 × 10 ⁻⁵ Pa or less. After the evacuation in the vacuum apparatus is completed, the sample is heated to 600 ° C., and a voltage is applied between the cathode electrode and the anode electrode disposed facing the cathode electrode through a gap. An electric field having an electric field strength of 0.025 V / μm was formed. And the thermionic current generated from the sample was measured.

The magnitude of the thermionic current obtained by the above-described method is considered to be approximately proportional to the magnitude of the thermionic current when the thermoelectric power generating element 1 is configured with each sample as the emitter 2.

Table 2 and FIG. 7 show the results of converting the magnitude of the thermoelectron current generated from each sample into the current density per unit area on the surface of the emitter 2. Note that the vertical axis in FIG. 7 is the current density of the thermoelectron current, and the horizontal axis is the film thickness of the second layer 23.

As can be seen from Table 2 and FIG. 7, the sample E1 and the sample E2 in which the film thickness of the second layer 23 is 40 nm or less include the sample C1 and the sample C2 having a film thickness exceeding 40 nm, or only the second layer 23. Compared to the sample C3 and the sample C4 having only the first layer 22, the current density of the thermionic current was significantly increased.

As can be seen from Table 2, the sample C1 in which the second layer 23 having a film thickness of 60 nm was stacked on the first layer 22 showed the same current density as the sample C3 having only the second layer 23. From this, when the film thickness of the second layer 23 is 60 nm or more, the internal resistance of the second layer 23 affects the thermionic emission performance, and the effect of laminating the first layer 22 and the second layer 23 is obtained. Can be guessed. Therefore, in order to obtain the effect of increasing the thermionic current by laminating the first layer 22 and the second layer 23 on the emitter substrate 21, the thickness of the second layer 23 needs to be 40 nm or less. It can be understood that.

In this example, the surface of the second layer 23 is hydrogen-terminated instead of the formation of the termination layers 24 and 24b containing the specific metal element M. The influence on the size is essentially the same as when the termination layers 24 and 24b including the specific metal element M are formed.

In the above-described Examples 1 to 3 and the reference example, the example in which the first layer 22 is directly laminated on the emitter substrate 21 is shown, but an interface intermediate layer is provided between the emitter substrate 21 and the first layer 22. It can also take the structure to comprise. For example, when an interface intermediate layer made of titanium carbide is formed, the following method can be employed.

First, a titanium thin film is formed on the emitter substrate 21 by vapor deposition. Next, the first layer 22 and the second layer 23 are formed by a microwave plasma CVD method or the like. As a result, the titanium thin film reacts with the carbon contained in the first layer 22 to form an interface intermediate layer made of titanium carbide.

When the interface intermediate layer made of titanium carbide is formed, the internal resistance of the emitter 2 in the thickness direction can be further reduced as compared with the case where the interface intermediate layer is not formed. Therefore, the thermoelectron current can be further increased and the power generation efficiency can be further improved.

Although the present disclosure has been described with reference to the embodiments, it is understood that the present disclosure is not limited to the embodiments and structures. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure.

Claims

An emitter (2) for generating thermal electrons;
A thermoelectric generator (1) comprising the collector (3) for collecting the thermoelectrons,
The emitter includes an emitter substrate (21) having electrical conductivity, an n-type diamond semiconductor containing phosphorus as a donor, a first layer (22) stacked on the emitter substrate, and nitrogen as a donor. One type selected from the group consisting of a second layer (23) composed of an n-type diamond semiconductor and having a film thickness of 40 nm or less and laminated on the first layer, and an alkali metal element and Mg A specific metal element defined by two or more kinds of M is included, and has a termination layer (24, 24b) formed on the outermost surface of the second layer,
The said collector has a collector board | substrate (31) which has electrical conductivity at least, The thermoelectron power generation element arrange | positioned through the gap | interval facing the said emitter.
The thermoelectric generator according to claim 1, wherein the n-type diamond semiconductor constituting the second layer is terminated by the specific metal element present in the termination layer.
The thermoelectric generator according to claim 1, wherein the termination layer further contains oxygen.
4. The heat according to claim 3, wherein the specific metal element and the oxygen present in the termination layer constitute an MO group, and the n-type diamond semiconductor constituting the second layer is terminated by the MO group. Electronic power generation element.
The thermoelectric power generation element according to any one of claims 1 to 4, wherein the alkali metal element is any one of Li, Na, K, and Cs.
6. The thermoelectric generator according to claim 1, wherein the amount of the specific metal element contained in the termination layer is an amount corresponding to 0.2 molecular layer or more and 10 molecular layers or less.
The thermoelectron power generation element according to any one of claims 1 to 6, wherein the emitter and the collector have an internal resistance in the thickness direction of 1 Ωcm 2 or less.
The thermoelectric generator according to any one of claims 1 to 7, wherein the emitter substrate is made of any one of Si, Ti, Mo, Ir, Ta, W, Ru, Cr, or Pt.
An interface intermediate layer is provided between the emitter substrate and the first layer, and the interface intermediate layer includes a resistance in the thickness direction, an interface resistance between the emitter substrate, the first layer, 9. The thermionic power generation element according to claim 1, wherein a sum of an interface resistance between the emitter substrate and the first layer is smaller than an interface resistance between the emitter substrate and the first layer. .
The thermoelectric generator according to claim 9, wherein the interface intermediate layer is made of a metal carbide.
Forming a first layer (22) composed of an n-type diamond semiconductor containing phosphorus as a donor on an electrically conductive emitter substrate (21);
Forming a second layer (23) composed of an n-type diamond semiconductor containing nitrogen as a donor on the first layer;
A termination layer (24, 24b) is formed on the outermost surface of the second layer by adsorbing one or more specific metal elements selected from the group consisting of alkali metal elements and Mg. Producing an emitter (2) comprising the emitter substrate, the first layer, the second layer and the termination layer;
A collector (3) having at least a collector substrate (31) having electrical conductivity is prepared separately from the emitter,
A method of manufacturing a thermoelectric generator (1) comprising: causing the emitter and the collector to face each other at an interval.
The thermoelectric generator according to claim 11, further comprising: forming the termination layer (24) by adsorbing the specific metal element onto the second layer after forming the second layer. Method.
After forming the second layer, surface oxidation treatment is performed to terminate the outermost surface of the second layer with oxygen, and the specific metal element is adsorbed on the outermost surface of the second layer. The method for producing a thermoelectric generator according to claim 11, further comprising: forming the termination layer containing oxygen.
14. The method of manufacturing a thermoelectric generator according to claim 11, wherein the alkali metal element is Li, Na, K, or Cs.
The thermoelectron according to any one of claims 11 to 14, wherein the adsorption amount of the specific metal element on the outermost surface of the second layer is an amount corresponding to 0.2 molecular layer or more and 10 molecular layers or less. A method for producing a power generation element.