LAMINATE, ELECTRON SOURCE AND ELECTRON DEVICE INCLUDING THE LAMINATE, AND METHODS FOR MANUFACTURING AND PURIFYING THE LAMINATE - Patent application

Abstract

To provide a novel low-work function material which has low chemical reactivity to lanthanoid-based boride known as a low-work function material, especially, a low-work function material which can have its surface purified at a lower heating temperature than before after being exposed to atmospheric gas.SOLUTION: The present invention relates to a laminate which has its surface coated with a thin film and includes a lanthanoid-based boride film formed on a substrate, the thin film being a hexagonal boron nitride thin film of a monoatomic layer.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a laminate including a lanthanoid-based boride film formed on a substrate whose surface is covered with a thin film, the thin film being a monolayer hexagonal boron nitride thin film. That is, the present invention relates to a laminate having a structure including a substrate and a lanthanoid-based boride film formed on the substrate, the surface of which is covered with a monolayer hexagonal boron nitride thin film. The present invention also relates to an electron source and an electronic device comprising the laminate. The present invention also relates to a method for producing said laminate. The present invention also relates to a method for cleaning the laminate. [Background technology]

[0002] Low work function materials have high electron emission efficiency and are therefore promising electronic materials for electron sources, electronic devices, etc. For example, a lanthanoid boride film, lanthanum hexaboride (LaB6), has a low work function of 2.3 eV on the (100) plane and exhibits high electron emission efficiency, and its use as an electron source material has been reported (Non-Patent Document 1).

[0003] Furthermore, as mentioned above, lanthanum hexaboride, a lanthanoid boride, is a low work function material, and research is being conducted on the surface structure and work function of its clean (100), (110), and (111) faces (Non-Patent Document 2).

[0004] Furthermore, a sputtering method for forming a LaB6 film has been reported for the production of a lanthanum hexaboride (LaB6) film, a lanthanoid boride (Patent Document 1). Specifically, Patent Document 1 discloses that when forming a LaB6 film by sputtering, a LaB6 target is sputtered in an argon atmosphere containing 0.01 to 5% by volume of nitrogen gas, and then annealed in an inert atmosphere, resulting in a LaB6 film with excellent crystallinity.

[0005] In addition, as shown in Figure 1, various lanthanide borides and alkaline earth metal borides have low work functions and are known to be capable of serving as highly efficient electron emission sources. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5665112 [Non-patent literature]

[0007] [Non-Patent Document 1] R.Nishitani, et al., “SURFACE STRUCTURES AND WORK FUNCTIONS OF THE LaB6(100), (110) and (111) CLEAN SURFACES”, Surf. Sci, 93, 535-549 (1980) [Non-patent document 2] R.Nishitani, et al., “OXYGEN ADSORPTION ON THE LaB6(100),(110) AND (111) SURFACES”, Surf. Sci, 115, 48-60 (1982) Summary of the Invention [Problem to be solved by the invention]

[0008] However, when the above-mentioned low work function materials are actually used as electronic materials for electron sources, electronic devices, etc., the following problems arise. Low-work-function materials have the property of easily emitting electrons, and therefore generally have high chemical reactivity. Therefore, atmospheric gases are easily adsorbed onto the surface of the material, easily contaminating the surface. In other words, compounds are easily formed on the surface of the low-work-function material. As a result, the work function easily increases, which can cause instability and attenuation of the emission current.

[0009] Therefore, in order to restore the high work function state resulting from the contaminated surface state (i.e., the surface state where compounds have formed) to a low work function state, it is necessary to re-clean the surface by vacuum heating. However, in the case of low work function materials, the compounds formed on the surface are stable, so vacuum heating at high temperatures of 1300°C or higher is required to re-clean the surface.

[0010] In fact, Non-Patent Document 2 reports that in lanthanum hexaboride (LaB6), which is known as a low work function material, when the surface atoms bond with oxygen and are oxidized by atmospheric gases such as exposure to the air or residual gas adsorption, the work function increases rapidly, and that in order to restore this high work function state to a low work function state, the surface is cleaned by heating at a high temperature of 1300°C or higher in a vacuum.

[0011] For these reasons, lanthanoid borides, such as lanthanum hexaboride, which are known as low work function materials, are highly chemically reactive. When these materials are used as electronic materials for electron sources or electronic devices, compounds are easily formed on the surface due to atmospheric gases such as exposure to the air or residual gas adsorption, contaminating the surface, resulting in a high work function. Therefore, it is generally recognized that in order to restore this high work function state to a low work function state, it is necessary to clean the surface by vacuum heating at extremely high temperatures of 1300°C or higher.

[0012] However, when used as electronic materials for electron sources and electronic devices that are sensitive to temperature, vacuum heating at such extremely high temperatures of 1300°C or higher can lead to undesirable material degradation and shorten the lifespan of the electronic materials.

[0013] Under these circumstances, there is a demand for the development of new low work function materials with low chemical reactivity for use as electronic materials such as electron sources and electronic devices, particularly low work function materials whose surfaces can be cleaned by vacuum heating at temperatures lower than conventional temperatures after exposure of the surface to atmospheric gases, in relation to lanthanide borides, which are known as low work function materials.

[0014] Therefore, the present invention aims to provide a novel low work function material with low chemical reactivity, particularly a low work function material whose surface can be cleaned by vacuum heating at a temperature lower than conventional temperatures after exposure to an atmospheric gas. Specifically, the present invention aims to provide a laminate including a lanthanoid boride film formed on a substrate whose surface is covered with a thin film, the thin film being a monolayer hexagonal boron nitride thin film.

[0015] Alternatively, the present invention aims to provide a laminate that can be cleaned by vacuum heating at a temperature lower than conventional temperatures (specifically, less than 1300°C) after being exposed to an atmospheric gas.

[0016] Alternatively, the present invention aims to provide a laminate in which the work function of a laminate exposed to an atmospheric gas after cleaning is performed by vacuum heating at a temperature lower than conventional temperatures (specifically, less than 1300°C) is approximately the same as the work function before exposure to the atmospheric gas, and the low work function of the laminate is maintained and preserved without increasing even after cleaning.

[0017] Alternatively, the present invention aims to provide an electron source and an electronic device using a novel low work function material with low chemical reactivity, particularly a low work function material whose surface can be cleaned by vacuum heating at a lower temperature than conventional temperatures (specifically, less than 1300°C) after exposing the surface to an atmospheric gas, among lanthanoid borides known as low work function materials. Specifically, the present invention aims to provide an electron source and an electronic device including the laminate.

[0018] Alternatively, the present invention aims to provide a method for producing a novel low work function material with low chemical reactivity, particularly a low work function material in which cleaning of the surface after exposure of the material surface to an atmospheric gas can be carried out by vacuum heating at a lower temperature than conventional (specifically, less than 1300°C), with respect to lanthanide borides known as low work function materials. Specifically, the present invention aims to provide a method for producing the laminate.

[0019] Alternatively, the present invention aims to provide a method for cleaning a surface of a lanthanoid boride, which is known as a low work function material, by exposing the surface to an atmospheric gas and then vacuum heating the surface at a temperature lower than conventional temperatures (specifically, less than 1300°C).Specifically, the present invention aims to provide a method for cleaning the laminate. [Means for solving the problem]

[0020] As a result of extensive research, the present inventors have found for the first time that, with regard to lanthanoid-based borides known as low work function materials, the above-mentioned problems can be solved by covering a lanthanoid-based boride film formed on a substrate with a protective film that has low gas adsorption to atmospheric gases and is so-called chemically stable, and that can maintain the low work function of the lanthanoid-based boride without increasing it (specifically, a thin film of an insulator having low gas adsorption and an extremely thin monoatomic layer), and have thereby completed the present invention.

[0021] Specifically, the present invention has the following aspects [1] to

[12] . [1] A laminate including a lanthanoid-based boride film formed on a substrate, the surface of which is covered with a thin film, The thin film is a monolayer hexagonal boron nitride thin film. Laminate. [2] The laminate according to [1], wherein the lanthanoid-based boride film is a lanthanum hexaboride film. [3] The laminate according to [1] or [2], wherein the lanthanoid boride film has a thickness of 1 nm or more and 100 nm or less. [4] The laminate according to any one of [1] to [3], wherein the work function after exposure to an atmospheric gas and subsequent vacuum heating at 500°C or higher and 600°C or lower is approximately the same as the work function before exposure to the atmospheric gas. [5] An electron source comprising the laminate according to any one of [1] to [4]. [6] An electronic device comprising the laminate according to any one of [1] to [4]. [7] forming a nitrogen-containing lanthanide boride film on a substrate; heating the lanthanoid-based boride film in a vacuum at a temperature range of more than 750°C and less than 1200°C to diffuse nitrogen in the lanthanoid-based boride film, causing the nitrogen to react with boron atoms contained in the lanthanoid-based boride film on the surface of the lanthanoid-based boride film, thereby depositing a monoatomic layer of hexagonal boron nitride thin film on the surface of the lanthanoid-based boride film, and covering the surface of the lanthanoid-based boride film with the deposited monoatomic layer of hexagonal boron nitride thin film; A method for producing the laminate according to any one of [1] to [4], comprising: [8] The method according to [7], wherein the nitrogen-containing lanthanoid boride film is formed by sputtering in an inert gas atmosphere using a nitrogen-containing lanthanoid boride sintered body as a target. [9] the substrate is a lanthanoid-based boride single crystal substrate or a SiO2 substrate provided with a polycrystalline lanthanoid-based boride film; The method according to [7], wherein the lanthanoid-based boride film is formed by irradiating the surface of the substrate with nitrogen radicals.

[10] The surface of the lanthanoid boride film is covered with the monoatomic layer of hexagonal boron nitride thin film by 1×10 -9 Pa or more 1×10 -5The method according to any one of [7] to [9], wherein the method is carried out by heating the mixture in a vacuum of 1000 Pa or less for a period of 5 minutes to 3 hours to diffuse nitrogen.

[11] A method for cleaning a laminate, comprising the steps of: A laminate in which the surface of a lanthanoid-based boride film formed on a substrate is coated with a thin film made of a monoatomic layer of hexagonal boron nitride is exposed to an atmospheric gas, and then the laminate contaminated by the atmospheric gas is subjected to vacuum heating at a low temperature of 500°C or higher and 600°C or lower, thereby cleaning the laminate.

[12] The method for cleaning a laminate according to

[11] , wherein the lanthanoid-based boride film is a lanthanum hexaboride film. [Effects of the Invention]

[0022] According to the present invention, a novel low work function material having low chemical reactivity can be provided, which is a laminate in which the surface of a lanthanoid-based boride film formed on a substrate is covered with a thin film, and the thin film is a chemically stable protective film (specifically, a hexagonal boron nitride thin film having a thickness of a monolayer) that can maintain the work function of the lanthanoid-based boride film (e.g., a lanthanum hexaboride (LaB6) film) without increasing the work function.

[0023] Alternatively, according to the present invention, by using the laminate, cleaning of the laminate exposed to atmospheric gas can be carried out by vacuum heating at a temperature lower than conventional (specifically, less than 1300°C).

[0024] Alternatively, according to the present invention, by using the laminate, even if the laminate exposed to atmospheric gas is cleaned by vacuum heating at a temperature lower than conventional (specifically, less than 1300°C), the work function after the heating is approximately the same as the work function before exposure to atmospheric gas, and the low work function of the laminate can be maintained and preserved without increasing even after cleaning.

[0025] Alternatively, according to the present invention, an electron source or an electronic device including the laminate can be obtained. As described above, the laminate can be cleaned after exposure to an atmospheric gas by vacuum heating at a temperature lower than conventional temperatures (specifically, less than 1300°C), so that according to the present invention, an electron source or an electronic device with less thermal damage can be obtained. Furthermore, as described above, even if the laminate is cleaned after exposure to the atmospheric gas by vacuum heating at a temperature lower than conventional temperatures (specifically, less than 1300°C), the work function after heating is approximately the same as the work function before exposure to the atmospheric gas, and the low work function of the laminate can be maintained and preserved without increasing even after cleaning.Therefore, according to the present invention, a stable emission current can be obtained as an electron source or an electronic device. For these reasons, the present invention is expected to extend the life of electron sources and electronic devices.

[0026] Alternatively, the present invention can provide a novel cleaning method for cleaning a low work function material exposed to an atmospheric gas by vacuum heating at a temperature lower than conventional temperatures (specifically, less than 1300°C).

[0027] According to the present invention, a novel low work function material with low chemical reactivity can be provided. The low gas adsorption characteristic of the material can suppress residual gas adsorption, which is a cause of instability in the emission current. This can improve the stability of the emission current. Therefore, for example, electron sources including the above-described laminates can be used to improve the performance of electron microscopes. In particular, the low gas adsorption of such electron microscopes allows them to operate in low-vacuum environments, where surface contamination typically occurs rapidly. As a result, high-resolution electron microscope images of materials that require observation in low-vacuum environments, such as biological samples, can be expected. [Brief explanation of the drawings]

[0028] [Figure 1] FIG. 1 is a table listing the work functions of representative rare earth and alkaline earth metal hexaborides. [Figure 2]FIG. 2 shows the results of Auger electron spectroscopy (AES) measurements, showing the change in surface composition depending on the heating temperature of a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by sputtering. [Figure 3] FIG. 3 shows the results of high-resolution electron energy loss spectroscopy (HREELS) measurements, showing the change in surface composition depending on the heating temperature of a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by sputtering. [Figure 4] Figure 4 shows the results of X-ray absorption near edge structure (XANES) measurements on the surface structure of a laminate obtained by subjecting a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by sputtering to vacuum heating at 660°C (note that in the figure, TEY indicates the spectrum obtained by the total electron yield method, and TFY indicates the spectrum obtained by the total fluorescence yield method). [Figure 5] FIG. 5 shows the results of Auger electron spectroscopy (AES) measurements, showing the change in surface composition of a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by nitrogen radicals, due to vacuum heating at 800°C. [Figure 6] FIG. 6 shows the results of high-resolution electron energy loss spectroscopy (HREELS) measurements, showing the change in the surface composition of a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by nitrogen radicals, due to vacuum heating at 800°C. [Figure 7] FIG. 7 shows the results of scanning tunneling microscope (STM) measurements on the surface of a laminate obtained by vacuum heating at 800°C after a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by sputtering was exposed to the atmosphere and then vacuum heating at 450°C to clean the laminate. (Note that (a) in the figure shows a topographic image depicting the geometrical irregularities, (b) shows a work function mapping of the same region measured simultaneously with (a), and (c) shows the work function profile along the dotted line in (b).) [Figure 8]FIG. 8 shows the results of scanning tunneling microscope (STM) measurements on the surface of a laminate obtained by vacuum heating at 800°C after a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate by sputtering was exposed to the atmosphere and then vacuum heating at 550°C to clean the laminate. (Note that (a) in the figure shows a topographic image depicting the geometrical irregularities, (b) shows a work function mapping of the same region measured simultaneously with (a), and (c) shows the work function profile along the dotted line in (b).) DETAILED DESCRIPTION OF THE INVENTION

[0029] Hereinafter, embodiments for carrying out the present invention will be described in detail. It should be noted that the present invention is not limited to the following embodiments, and various modifications can be made within the scope of the present invention.

[0030] One aspect of the present invention is a laminate including a lanthanoid-based boride film formed on a substrate, the surface of which is covered with a thin film, the thin film being a monolayer hexagonal boron nitride thin film. That is, the laminate of the present invention has a structure including a substrate and a lanthanoid-based boride film formed on the substrate, the surface of which is covered with a monolayer hexagonal boron nitride thin film.

[0031] The substrate for forming the lanthanoid-based boride film is not particularly limited as long as it can achieve the object of the present invention, but it is preferable to use a lanthanoid-based boride single crystal substrate or an SiO2 substrate provided with a polycrystalline lanthanoid-based boride film (so-called SiO2 substrate with a polycrystalline lanthanoid-based boride film). Furthermore, it is more preferable that the polycrystalline lanthanoid boride film provided on the lanthanoid boride single crystal substrate or SiO2 substrate is made of the same material as the lanthanoid boride film formed on the substrate. This means that, for example, if the lanthanoid boride film formed on the substrate is a lanthanum hexaboride (LaB6) film, it is more preferable to use a lanthanum hexaboride single crystal substrate or an SiO2 substrate provided with a polycrystalline lanthanum hexaboride film as the substrate. Here, "having a polycrystalline lanthanum hexaboride film" means that a polycrystalline lanthanum hexaboride (LaB6) film is provided on an SiO2 substrate in a laminated or other form, and the method for forming the film is not particularly limited as long as the object of the present invention can be achieved, and examples include vapor deposition or deposition on an SiO2 substrate. Regarding the polycrystalline lanthanoid boride film provided on the SiO2 substrate, the thickness of the polycrystalline lanthanum hexaboride film is not particularly limited as long as the object of the present invention can be achieved, but it is preferably 20 to 60 nm.

[0032] In the present invention, the lanthanoid boride is used as a substrate material or a material for a lanthanoid boride film formed on a substrate. The lanthanoid boride used in the present invention is not particularly limited as long as it is a compound containing a lanthanoid atom and a boron atom and has a low work function, and examples thereof include lanthanoid borides such as LaB6, CeB6, PrB6, NdB6, GdB6, etc. From the viewpoint of versatility and electron emission efficiency, LaB6 (work function: 2.3 eV) and CeB6 (work function: 2.6 eV), which have a low work function, are preferred, and LaB6 is more preferred. Here, lanthanide atoms refer to atoms corresponding to atomic numbers 57 to 71 in the periodic table, i.e., the 15 atoms from lanthanum to lutetium.

[0033] The low work function in this application may be any energy (unit: eV) that is generally recognized as a low work function, but specifically, it is preferably 4 eV or less. More preferably, it is 3.5 eV or less, and even more preferably, it is 3 eV or less. From the viewpoint of high electron emission efficiency, the lower the work function, the better.

[0034] The lanthanoid boride film of the present invention may or may not contain nitrogen as long as the object of the present invention can be achieved, but it is preferable that the film contain nitrogen from the viewpoint that the surface of the laminate of the present invention can be self-repaired by reheating if it becomes deteriorated. When the lanthanoid boride film contains nitrogen, the content (wt %) is preferably 0.1 wt % or more and 2 wt % or less, and more preferably 0.2 wt % or more and 0.5 wt % or less.

[0035] The thickness of the lanthanoid boride film formed on the substrate is not particularly limited as long as the object of the present invention can be achieved, but from the viewpoint of electron emission efficiency, it is preferably in the range of 1 nm to 100 nm, more preferably in the range of 5 nm to 100 nm, and even more preferably in the range of 20 nm to 60 nm.

[0036] In the present invention, the thin film that covers the lanthanoid-based boride film formed on the substrate is a monoatomic layer of hexagonal boron nitride thin film. The thin film material, "hexagonal boron nitride," is a layered substance and an inactive material (i.e., an insulating material) with no dangling bonds. Therefore, "hexagonal boron nitride" has the property of suppressing bonding with atmospheric gases and preventing the formation of undesirable compounds such as oxides. In other words, hexagonal boron nitride is chemically stable with low gas adsorption to atmospheric gases, making it an effective material for a protective film covering a lanthanide-based boride film formed on a substrate.

[0037] Additionally, the "thin film" in the hexagonal boron nitride thin film means that the thickness of the insulating material, hexagonal boron nitride, is so thin that it is physically adsorbed without charge transfer when adsorbed onto the lanthanoid-based boride film that coats the substrate. Specifically, it is two atomic layers or less, preferably a monoatomic layer. In the case of physical adsorption, the bonding occurs so that the vacuum levels are aligned, so the low work function of the lanthanoid-based boride film that coats the substrate is maintained and preserved as the work function of the system. In other words, the low work function of the lanthanoid-based boride film is maintained and preserved even when coated with the hexagonal boron nitride thin film. Therefore, in the present application, given that the work function of the laminate of the present invention (i.e., the system work function) means the work function of the surface of the laminate, the work function of the lanthanoid-based boride film that constitutes the laminate is the work function of the laminate of the present invention (i.e., the system work function). In this way, the hexagonal boron nitride thin film has an extremely thin thickness as a "thin film" that allows for the physical adsorption (specifically, two atomic layers or less, preferably a monoatomic layer), and therefore serves as an effective protective film for maintaining and preserving the low work function of the lanthanoid boride film that coats it as an underlayer.

[0038] Therefore, the "monolayer hexagonal boron nitride thin film" used in the present invention is a chemically stable protective film (specifically, a protective film with low gas adsorption to atmospheric gases) for covering a low work function lanthanoid-based boride film formed on a substrate, and also a protective film for maintaining and preserving the low work function of the lanthanoid-based boride film without increasing its work function.

[0039] Incidentally, graphene, which is monoatomic layer graphite, cannot be used as a substitute for the "monoatomic layer hexagonal boron nitride thin film" used in the present invention. Because graphene is a conductive material, charge transfer occurs when graphene is adsorbed onto a lanthanoid-based boride film that coats the substrate. As a result, the low work function of the lanthanoid-based boride film that coats the substrate cannot be maintained or preserved as the work function of the system. In other words, when a thin film that coats a lanthanoid-based boride film formed on a substrate is graphene, the low work function of the lanthanoid-based boride film increases when coated with graphene, making it impossible to maintain or preserve the low work function.

[0040] The "hexagonal boron nitride thin film" used in the present invention is preferably a monoatomic layer with good crystallinity. Therefore, it is desirable for the "hexagonal boron nitride thin film" used in the present invention to self-form on the surface of the low work function material that is used as a coating underlayer, and from this perspective, the low work function material that is used as a coating underlayer is preferably a lanthanoid boride film. As described above, the material for the lanthanoid boride film is not particularly limited as long as it is a compound containing a lanthanoid atom and a boron atom and has a low work function, and examples thereof include lanthanoid borides such as LaB6, CeB6, PrB6, NdB6, GdB6, etc. Among the lanthanoid boride films made from these lanthanoid borides, a lanthanum hexaboride (LaB6) film or a cerium hexaboride (CeB6) film is preferred, with a lanthanum hexaboride (LaB6) film being more preferred, also from the viewpoint of self-formation of a "hexagonal boron nitride thin film."

[0041] As described above, the laminate of the present invention has a structure in which the surface of the lanthanoid-based boride film is coated with a monolayer of a hexagonal boron nitride thin film, and the surface of the laminate can be cleaned by vacuum heating at a temperature lower than 1300°C (for example, a temperature range of 500°C to 600°C) after exposure to an atmospheric gas. Furthermore, the work function of the laminate before and after the cleaning is approximately the same, and the low work function of the laminate is maintained and preserved without increasing even when the cleaning is performed after exposure to the atmospheric gas.

[0042] Thus, although the work function of the laminate of the present invention when vacuum heated for cleaning after exposure to atmospheric gas is approximately the same as the work function before exposure to the atmospheric gas, the fact that cleaning by vacuum heating can be carried out at a lower heating temperature than conventional means that the gas adsorption property of the laminate of the present invention is low. Here, "approximately the same" means that the work function after cleaning is within +0.5 eV or less of the work function before cleaning. Incidentally, the fact that the work function after cleaning is smaller than the work function before cleaning is not particularly problematic, since, as described above, this is in line with the object of the present invention to provide a laminate that maintains and preserves the low work function of the laminate after cleaning without increasing it.

[0043] In view of the object of the present invention to perform heating at a lower temperature than conventionally used (specifically, a temperature lower than 1300°C), the heating temperature after exposure to the atmospheric gas may be lower than 1300°C, but is preferably 600°C or lower. Lower temperatures are preferable, but if the heating temperature is too low, the laminate cannot be sufficiently cleaned and it becomes difficult to maintain a low work function of the laminate, so from the perspective of performing sufficient cleaning, it is desirable to set the temperature to above 450°C. Preferably, the temperature is 500°C or higher, more preferably 550°C or higher.

[0044] The atmosphere during heating after exposure to the atmospheric gas is preferably a vacuum, but is not limited to a vacuum and may be an inert atmosphere as long as the object of the present invention can be achieved. In the case of vacuum heating, the degree of vacuum is 1 x 10 -5 The range of 5×10 Pa or less is preferable. -7 It is more preferable that the range is 0.1 Pa or less.

[0045] Hereinafter, the present invention will be described in relation to an embodiment other than the laminate which is one embodiment of the present invention, but the explanation already given for the laminate which is one embodiment of the present invention will apply in the same manner unless otherwise specified.

[0046] Another aspect of the present invention is an electron source or an electronic device including the laminate. An electron source is an electron emission source, and electron emission methods include thermionic emission, photoelectron emission, field emission, and secondary electron emission due to electron or ion bombardment, depending on the type of energy applied. Electron sources are used in scanning electron microscopes (SEMs), transmission electron microscopes (TEMs), electron probe microanalyzers (EPMAs) surface analysis devices, metal 3D printers, microfocus X-ray sources, lithography devices, etc. Electronic devices are anything that is generally recognized as an electronic device, and can be broadly divided into three types: semiconductors such as electronic circuits, electronic displays such as liquid crystal devices, and general electronic components other than semiconductors and electronic displays (e.g., capacitors, power supplies).Electronic devices are widely used in electronic equipment, etc.

[0047] When the electron source including the laminate is a generally well-known field emission electron source or thermal emission electron source, for example, the following embodiment can be mentioned. An example of a field emission electron source is a laminate structure in which a metal needle for field electron emission (corresponding to the "substrate" in this application) is used, a lanthanum hexaboride layer (corresponding to the "lanthanoid-based boride film" in this application) is formed on the metal needle, and the layer is then covered with a monoatomic layer of a hexagonal boron nitride thin film. In this case, the laminate is heated to 500 to 600°C and used in this state as a field emission electron source. Heating to 500 to 600°C maintains a clean surface, preventing the attenuation of the emission current due to the adsorption of residual gases, which is a problem with conventional field emission sources. An example of a thermal emission electron source is a laminate structure in which a lanthanum hexaboride layer (corresponding to the "lanthanoid-based boride film" of the present application) is formed on a substrate and then coated with a monolayer of a hexagonal boron nitride thin film. In this case, the temperature is increased by electrically heating the lanthanum hexaboride layer or by heating the substrate, thereby emitting thermoelectrons. Conventional thermal emission electron sources using lanthanum hexaboride, as described above, require operation at 1300°C or higher for cleaning. However, the thermal emission electron source of the present invention can be cleaned at temperatures lower than 1300°C, thereby reducing the operating temperature. As a result, evaporation of surface atoms is suppressed, and the lifetime of the electron source is expected to be extended.

[0048] Another aspect of the present invention is a method for producing the laminate. A method for producing the laminate of the present invention includes, for example, forming a nitrogen-containing lanthanoid boride film on a substrate, heating the lanthanoid boride film in a vacuum at a temperature range of more than 750°C and less than 1200°C to diffuse the nitrogen in the lanthanoid boride film, causing the nitrogen to react with boron atoms contained in the lanthanoid boride film on the surface of the lanthanoid boride film, depositing a monoatomic layer of hexagonal boron nitride thin film on the surface of the lanthanoid boride film, and covering the surface of the lanthanoid boride film with the deposited monoatomic layer of hexagonal boron nitride thin film.

[0049] Here, the method for forming a nitrogen-containing lanthanoid boride film on a substrate is not particularly limited as long as the object of the present invention can be achieved, but examples thereof include a film formation method using sputtering and a film formation method using nitrogen radicals.

[0050] Sputtering is a method of forming a thin film by colliding plasmatized inert gas (mainly argon) ions with high energy and depositing the ejected fine particles of the target material onto a distant object. Specifically, this method uses a nitrogen-containing lanthanoid boride sintered body as a target and sputters it under an inert gas atmosphere of argon gas to form a film of the nitrogen-containing lanthanoid boride on a substrate. Generally, a nitrogen-containing lanthanoid boride film is deposited by RF sputtering. When formed by RF sputtering deposition, the lanthanoid boride film can have a high degree of shape freedom and high scalability that are not possible with single-crystal materials. The inert gas is not particularly limited as long as it can achieve the object of the present invention, and examples thereof include generally chemically inert gases such as nitrogen gas and rare gases. In the present invention, when forming a film by sputtering using a nitrogen-containing lanthanoid-based boride sintered body, it is preferable not to use nitrogen gas, which will result in excessive supply of impurities. Therefore, it is preferable to use a rare gas, and it is more preferable to use argon gas from the viewpoint of versatility. However, the above-mentioned film formation by sputtering can also be performed using a nitrogen-free lanthanoid-based boride sintered body. In this case, nitrogen may be used as the inert gas, for example, a nitrogen-containing inert gas (e.g., nitrogen-containing argon gas) may be used.

[0051] The film formation method using nitrogen radicals is a method of forming a nitrogen-containing lanthanoid boride film on a surface of a target material by irradiating the surface with nitrogen radicals in a vacuum. Specifically, the method involves irradiating the surface of a lanthanoid boride single crystal substrate or a SiO2 substrate surface provided with a polycrystalline lanthanoid boride film with nitrogen radicals in a vacuum at room temperature, thereby forming a nitrogen-containing lanthanoid boride film on the substrate surface.

[0052] A method for coating a nitrogen-containing lanthanoid boride film formed on a substrate with a monoatomic layer hexagonal boron nitride thin film includes, for example, heating the nitrogen-containing lanthanoid boride film formed on a substrate by the above-mentioned method in a vacuum at a temperature range of more than 750°C and less than 1200°C, and causing the nitrogen in the lanthanoid boride film to thermally diffuse and react with the boron atoms contained in the lanthanoid boride film on the surface of the lanthanoid boride film, thereby depositing a monoatomic layer hexagonal boron nitride thin film on the surface of the lanthanoid boride film to coat it. The degree of vacuum and heating conditions are not particularly limited as long as the object of the present invention can be achieved, but the following ranges are preferred from the viewpoint of thermal diffusion of nitrogen. The vacuum level is 1 x 10 -9 Pa or more 1×10 -5 The range of 1×10 Pa or less is preferable. -9 Pa or more 5×10 -7 Pa or less is more preferable. The heating temperature is in the range of more than 750° C. and less than 1200° C., preferably in the range of 800° C. to 1100° C. The heating time is preferably 5 minutes to 3 hours.

[0053] Another method for forming a nitrogen-containing lanthanoid boride film on a substrate is to form a nitrogen-free lanthanoid boride film on the substrate by a method such as sputtering, and then form a nitrogen-containing lanthanoid boride film by the above-mentioned film formation method using nitrogen radicals. Specifically, for example, a material that is not doped with nitrogen <100> One example of such a method is to deposit an oriented polycrystalline lanthanum hexaboride (LaB6) film on a substrate by sputtering, and then irradiate the undoped polycrystalline lanthanum hexaboride film with nitrogen radicals to deposit a nitrogen-containing lanthanoid boride film on the substrate.

[0054] Furthermore, in order to coat a nitrogen-free lanthanoid boride film formed on a substrate with a monolayer hexagonal boron nitride thin film, the surface may be exposed to, for example, borazine gas (B3N3H6), and then coated with a monolayer hexagonal boron nitride thin film by chemical vapor deposition.

[0055] As described above, the method for producing a laminate of the present invention involves forming a nitrogen-containing lanthanoid-based boride film on a substrate and then coating the lanthanoid-based boride film with a monoatomic layer of a hexagonal boron nitride thin film. Therefore, when the nitrogen-containing lanthanoid-based boride film is formed on a substrate by the sputtering deposition method or the nitrogen radical deposition method, no hexagonal boron nitride is present. In other words, the nitrogen does not exist as hexagonal boron nitride. Subsequent heating in a vacuum at a temperature range of more than 750°C and less than 1200°C causes thermal diffusion of the nitrogen in the lanthanoid-based boride film, resulting in reaction with boron atoms contained in the lanthanoid-based boride film on the surface of the lanthanoid-based boride film to form a monoatomic layer of hexagonal boron nitride. As a result, the lanthanoid-based boride film is coated with a monoatomic layer of a hexagonal boron nitride thin film. When the thickness of a hexagonal boron nitride thin film is a monolayer, the surface is not equivalent to the surface of bulk hexagonal boron nitride, but is treated as a physically adsorbed system of an extremely thin insulating film. As a result, the surface electronic properties of the laminate of the present invention are highly dependent on the electronic properties of the lanthanoid boride film underlying the hexagonal boron nitride, and the work function of this system (i.e., the work function of the laminate of the present invention) is given by the work function of the underlying lanthanoid boride film.

[0056] Another aspect of the present invention is a method for cleaning the laminate, specifically, the method comprises exposing the laminate, which comprises a lanthanoid-based boride film formed on a substrate and a monolayer of a thin film of hexagonal boron nitride, to an atmospheric gas, and then vacuum-heating the laminate contaminated by the atmospheric gas at a low temperature of 500°C to 600°C. As described above, vacuum heating the laminate at a low temperature of 500°C or more and 600°C or less is desirable because it means that the laminate can be purified at a heating temperature lower than conventionally after exposure to atmospheric gas (specifically, a temperature lower than 1300°C). Furthermore, according to this cleaning method, even if the cleaning of the laminate exposed to the atmospheric gas is carried out at a temperature between 500°C and 600°C, which is much lower than conventional temperatures, the work function of the laminate before and after cleaning remains roughly the same, and the low work function of the laminate can be maintained without increasing even after cleaning, which is desirable for electronic materials as described above.

[0057] Conditions not specified in this application are not particularly limited as long as the object of the present invention can be achieved. [Example]

[0058] Next, the embodiments of the present invention will be described in more detail with reference to examples. However, the embodiments of the present invention are not limited to the following examples as long as they do not depart from the gist of the invention.

[0059] The laminate of the present invention, which includes a lanthanoid-based boride film formed on a substrate whose surface is covered with a thin film, the thin film being a monolayer hexagonal boron nitride thin film, was produced by the following two methods using lanthanum hexaboride (LaB6) as a representative example of a lanthanoid-based boride. The first method involves forming a nitrogen-containing lanthanum hexaboride film on a substrate using a sputtering film formation method, and the second method involves forming a nitrogen-containing lanthanum hexaboride film on a substrate using a nitrogen radical film formation method.

[0060] Example 1 First, a nitrogen-containing lanthanum hexaboride (LaB6) film was formed on a substrate using a sputtering method.

[0061] <Film formation by sputtering> A nitrogen-containing lanthanum hexaboride (LaB6) film was deposited by RF sputtering on a lanthanum hexaboride single crystal substrate or a SiO2 substrate with a polycrystalline lanthanum hexaboride film laminated thereon. Specifically, a Canon ANELVA E200-S RF sputtering system (i.e., the system for depositing the lanthanum hexaboride film) was used, and a sintered lanthanum hexaboride (LaB6) compact containing 0.4 wt% nitrogen was used as the target. The pressure inside the system during sputtering was maintained between 0.3 and 0.5 Pa while maintaining an argon gas atmosphere with an argon gas flow rate of 10 sccm. The sputtering power was 20 to 50 W, and the substrate temperature during sputtering was room temperature. However, the substrate temperature could be in the range of room temperature to 150°C. The thickness of the deposited lanthanum hexaboride film was measured using a stylus profiler (KLA-Tencor Alpha-Step D-500) and found to be in the range of 20 to 60 nm. As mentioned above, the sputtering was carried out in an atmosphere of argon gas only, and no nitrogen gas was supplied during the sputtering. Furthermore, annealing was not performed immediately after film formation.

[0062] Next, the nitrogen-containing lanthanum hexaboride film formed on the substrate by the sputtering film formation method described above was coated with a monolayer of hexagonal boron nitride thin film to produce a laminate of the present invention. Specifically, the process is as follows.

[0063] <Coating with a monolayer of hexagonal boron nitride thin film> The nitrogen-containing lanthanum hexaboride (LaB6) film formed on the substrate by the sputtering film formation method or the nitrogen radical film formation method was 1 × 10 -5The laminate was heated in a temperature range of 800 to 1100°C in a vacuum of 100 Pa or less, and a monolayer hexagonal boron nitride thin film was deposited and coated on the surface of the lanthanum hexaboride film. This resulted in the production of a laminate of the present invention. The heating temperature was measured using a pyrometer (Impac 8 Pro Series, manufactured by Advanced Energy Industries). Heating was carried out by electron beam impact heating. However, this heating may be performed by electrical heating, electron beam impact heating, or radiation heating. The heating time was determined at each set temperature as the time required for the base pressure to recover or 2 x 10 to obtain an equilibrium state on the surface of the laminate. -7 The time required for the pressure to reach or exceed 100 Pa was defined as the time required for the pressure to reach or exceed 100 Pa. Specifically, the time ranged from 30 minutes to 3 hours. The vacuum heating for coating the surface of the lanthanum hexaboride film with a monoatomic layer of a hexagonal boron nitride thin film does not need to be performed in the film formation apparatus immediately after the lanthanum hexaboride film is formed, but may be performed after the nitrogen-containing lanthanum hexaboride film formed on the substrate is once exposed to the atmosphere.

[0064] The laminate obtained as described above was confirmed by the following method to have a monoatomic layer of hexagonal boron nitride thin film formed on the surface thereof.

[0065] <Confirmation of coating by Auger electron spectroscopy (AES) measurement> In regard to vacuum heating for coating a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate with a monoatomic layer of a hexagonal boron nitride thin film, a cylindrical mirror electron analyzer (manufactured by Yamamoto Vacuum Laboratory) and an electron gun (R-Dec RDA001 model) were used as an Auger electron spectroscopy (AES) measuring device to evaluate the change in the surface composition of the lanthanum hexaboride film due to changes in heating temperature. The degree of vacuum in the device was 2 x 10 -8 From 1×10 -7The measured values ​​were Pa. Measurements were performed on the lanthanum hexaboride film immediately after deposition before vacuum heating, and on the lanthanum hexaboride film after vacuum heating at various temperatures ranging from 500 to 1200°C. The lanthanum hexaboride (LaB6) film after vacuum heating was measured at room temperature after vacuum heating. The electron beam energy was 15 keV, and the irradiation angle was approximately 70° from the surface normal. The results are shown in Figure 2. From the results in Figure 2, it was confirmed that the signal corresponding to nitrogen, which appears around 390 eV and was not observed up to 750°C, was detected at 800°C, which is above 750°C. At the same time, it was also confirmed that the signal corresponding to nitrogen was detected even at 1100°C, but was no longer detected at 1200°C. Therefore, it was found that in order for a nitrogen-containing coating to appear on the nitrogen-containing lanthanum hexaboride film formed on the substrate, vacuum heating must be performed at a temperature range higher than 750°C and lower than 1200°C.

[0066] <Confirmation by high-resolution electron energy loss spectroscopy (HREELS) measurement> Based on the results of the Auger electron spectroscopy (AES) measurements, high-resolution electron energy loss spectroscopy (HREELS) measurements were performed to identify the structure of the surface composition of the lanthanum hexaboride (LaB6) film vacuum-heated at temperatures ranging from 750°C to 1200°C. This method evaluates the atomic bonding state from surface atomic vibrations and determines the surface atomic structure. The HREELS measurement was performed using a Delta-0.5 specs GmbH instrument. The electron incident energy was set to 2.0 eV, and measurements were performed under specular reflection conditions. The measurements were performed at room temperature after vacuum heating. The results are shown in Figure 3. As shown in the results of FIG. 3, when vacuum heating was carried out at 850° C. to 1100° C., loss energy peaks of 100, 173, and 180 meV were detected. These peaks were confirmed to be almost identical to the phonon energies (i.e., the vibration peaks of the single-atom-layer hexagonal boron nitride thin film) of the single-atom-layer hexagonal boron nitride thin film reported in “E. Rokuta et al., “Phonon Dispersion of an Epitaxial Monolayer Film of Hexagonal Boron Nitride on Ni(111)”, Phys. Rev. Lett. 79, 4609

[1997] ”. That is, when vacuum heating was carried out at 850° C. to 1100° C., it was confirmed that a single-atom-layer hexagonal boron nitride thin film was formed on the surface of the lanthanum hexaboride film. On the other hand, at 750° C. and 1200° C., loss energy peaks of 100, 173, and 180 meV were not detected. Therefore, it was found that by performing vacuum heating in a temperature range higher than 750° C. and lower than 1200° C., the lanthanum hexaboride film containing nitrogen formed on the substrate can be coated with a single-atom-layer hexagonal boron nitride thin film.

[0067] <Confirmation of Coating by X-Ray Absorption Near-Edge Structure (XANES) Measurement> In order to identify the structure of the surface composition of the lanthanum hexaboride (LaB6) film vacuum-heated in a temperature range higher than 750° C. and lower than 1200° C., as a representative example, the structure of the surface composition when vacuum-heated at 800° C. was also identified by another method. Specifically, X-ray absorption near-edge structure (XANES) measurement was performed using the beamline (BL7U vacuum ultraviolet spectroscopy) apparatus at the Aichi Synchrotron Light Center. This measurement method is a method for observing the electron state unique to a substance and identifying the substance. At that time, hexagonal boron nitride powder was used as a reference sample in order to demonstrate that a single-atom-layer hexagonal boron nitride was formed on the surface. The results are shown in FIG. 4. The spectrum in Figure 4 is a XANES spectrum near the nitrogen K-edge. This measurement was performed using a combination of total electron yield (TEY) spectroscopy, which is sensitive to surface detection, and total fluorescence yield (TFY) spectroscopy, which is sensitive to bulk detection. The measured sample was a laminate obtained by vacuum heating at 800 °C, as described above. After the vacuum heating, the sample was exposed to air, transported to the beamline, and re-cleaned at 660 °C in the same apparatus. As shown by the dashed line in Figure 4, the TEY spectrum of the laminate obtained by vacuum heating (specifically, "h-BN / LaB6 (after heating)" in Figure 4) exhibited peaks at the same energy positions as the TEY and TFY spectra of the reference hexagonal boron nitride (specifically, "h-BN (reference)" in Figure 4). However, the TFY spectrum of the laminate obtained by vacuum heating did not exhibit any peaks indicating such a structure. These results indicate that hexagonal boron nitride is not present inside the laminate obtained by vacuum heating, but is present only on the surface. Furthermore, based on the detection sensitivity of TFY, it was found that the hexagonal boron nitride present on the surface is a monoatomic layer. Therefore, it was found that by performing vacuum heating in the temperature range of higher than 750°C and lower than 1200°C, a nitrogen-containing lanthanum hexaboride film formed on a substrate can be coated with a monolayer of a hexagonal boron nitride thin film.

[0068] Example 2 First, a nitrogen-containing lanthanum hexaboride (LaB6) film was formed on a substrate using a nitrogen radical deposition method.

[0069] <Film formation using nitrogen radicals> A lanthanum hexaboride (LaB6) film containing nitrogen was formed on a lanthanum hexaboride (001) single crystal substrate by irradiating the substrate with nitrogen radicals at room temperature for 3 minutes. Specifically, a radical source device (ER-1000 manufactured by Eiko) was used, and the pressure inside the device was set to 1 × 10 under a nitrogen atmosphere with a nitrogen gas flow rate of 4 sccm. -2 From 2 x 10-2 The pressure was kept in the range of Pa. The power was 280W.

[0070] Next, a monolayer hexagonal boron nitride thin film was coated on the nitrogen-containing lanthanum hexaboride (LaB6) film formed on the substrate by the nitrogen radical film formation method described above, to prepare a laminate of the present invention.

[0071] <Coating with a monolayer of hexagonal boron nitride thin film> A nitrogen-containing lanthanum hexaboride (LaB6) film was formed on a substrate by the sputtering deposition method or the nitrogen radical deposition method, and a monoatomic layer of hexagonal boron nitride thin film was deposited and coated on the surface of the lanthanum hexaboride film by the same method as in Example 1, to obtain a laminate of the present invention.

[0072] The laminate obtained as described above was confirmed by the following method to have a monoatomic layer of hexagonal boron nitride thin film formed on the surface thereof.

[0073] <Confirmation of coating by Auger electron spectroscopy (AES) measurement> Regarding vacuum heating for covering a nitrogen-containing lanthanum hexaboride (LaB6) film formed on a substrate with a monoatomic layer of a hexagonal boron nitride thin film, in order to evaluate the change in the surface composition of the lanthanum hexaboride film due to changes in heating temperature, Auger electron spectroscopy (AES) measurements were carried out at room temperature using the same method as in Example 1. Therefore, the measurement conditions were the same as in Example 1. One of the results (the vacuum heating condition was 1×10 -6 Figure 5 shows the results of heating at 800°C for 15 minutes in a vacuum of 100 Pa. 5, it was confirmed that, as in Example 1, a signal corresponding to nitrogen, which appeared near 390 eV and was not observed up to 750° C., was detected at 800° C., which is above 750° C. Although not shown in the figure, it was also confirmed that the signal corresponding to nitrogen was detected even at 1100° C., but was no longer detected at 1200° C. Therefore, as in Example 1, it was found that in order for a nitrogen-containing coating to appear on the nitrogen-containing lanthanum hexaboride film formed on the substrate, vacuum heating must be performed at a temperature range higher than 750°C and lower than 1200°C.

[0074] <Confirmation of coating by high-resolution electron energy loss spectroscopy (HREELS) measurement> Based on the Auger electron spectroscopy (AES) measurement results, in order to identify the structure of the surface composition of the lanthanum hexaboride (LaB6) film heated in vacuum at each temperature in the range of 750°C to 1200°C, high-resolution electron energy loss spectroscopy (HREELS) measurement was carried out in the same manner as in Example 1. Therefore, the measurement conditions were the same as in Example 1. One of the results (the vacuum heating condition was 1×10 -6 The results are shown in Figure 6 (when heated at 800°C for 15 minutes in a vacuum of 100 Pa). As shown in Figure 6, when vacuum heating was performed at 800°C, loss energy peaks of 100, 173, and 180 meV were detected, as in Example 1. These peaks were confirmed to be approximately identical to the phonon energy of a monolayer hexagonal boron nitride thin film (i.e., the vibration peaks of a monolayer hexagonal boron nitride thin film) reported in "E. Rokuta et al., 'Phonon Dispersion of an Epitaxial Monolayer Film of Hexagonal Boron Nitride on Ni(111)'," Phys. Rev. Lett. 79, 4609

[1997] ." Although not shown, it was also confirmed that these peaks were detected even at a vacuum heating temperature of 1100°C but disappeared at 1200°C. This confirms that a monolayer hexagonal boron nitride thin film is formed on the surface of the lanthanum hexaboride film when vacuum heating is performed between 850°C and 1100°C. Therefore, it was found that by performing vacuum heating in the temperature range of higher than 750°C and lower than 1200°C, a nitrogen-containing lanthanum hexaboride film formed on a substrate can be coated with a monolayer of a hexagonal boron nitride thin film.

[0075] Example 3 <Purification evaluation> The laminates finally obtained in Examples 1 and 2 were exposed to the atmosphere and then vacuum-heated. As a representative example, for the laminate finally obtained in Example 1, when the vacuum heating temperature for depositing and coating a monolayer hexagonal boron nitride thin film on the surface of the lanthanum hexaboride (LaB6) film was 800°C, after exposure to the atmosphere, 3 × 10 -7 The results of cleaning the laminate at heating temperatures of 450°C and 550°C in a vacuum of 100 Pa are shown in Figures 7 and 8, respectively. Here, the laminate was heated by electrical heating, and the temperature was measured with a pyrometer. The heating time was set at the set temperature (450°C) and the base pressure (2 x 10 -8 Pa) or 1 × 10 -7 The time required for the pressure to reach less than 100 Pa was approximately 30 minutes to 2 hours. The surface condition of the laminate after vacuum heating was evaluated by scanning tunneling microscopy (STM). An Omicron LT-STM was used for the measurements. Using this STM, topographic images and local work function mapping were simultaneously measured. The tunneling conditions were a sample bias of -3.5 V, a tunneling current of 0.5 nA, and a probe amplitude of 1 Å and 1 kHz. Figures 7(a) and 8(a) show topographic images of the geometrical irregularities. Figures 7(b) and 8(b) show work function mappings of the same region simultaneously measured. Figures 7(c) and 8(c) show work function profiles along the dotted lines in Figure 7(b) and Figure 8(b), respectively. The results in Figure 7 confirm that vacuum heating at 450°C did not result in a uniform work function distribution, and that the work function value was approximately 0.2 to 0.6 eV larger than the inherently low work function of the lanthanum hexaboride film (i.e., 2.3 eV, shown by the dashed line in Figure 7(c)). In other words, vacuum heating at 450°C was found to be insufficient in cleaning the laminate after exposure to the atmosphere. On the other hand, the results in Figure 8 confirm that vacuum heating at 550°C, which is higher than 450°C, results in a uniform work function distribution, and that the work function value is fully restored to the low work function inherent to the lanthanum hexaboride film (i.e., 2.3 eV, shown by the dashed line in Figure 8(c)). In other words, vacuum heating at 550°C or higher, 600°C, can sufficiently purify the laminate after exposure to the atmosphere, and a similar effect can be expected at 500°C, which is significantly higher than 450°C. [Industrial Applicability]

[0076] The present invention is expected to be useful as an electronic material for electron sources, electronic devices, etc., that use lanthanoid borides, which are low-work function materials that easily emit electrons and have high chemical reactivity. In particular, the present invention is capable of maintaining a clean surface when heated to 500 to 600°C, and therefore is highly expected to be useful in the fields of field emission electron sources and thermal emission electron sources and fields that utilize them (for example, in the field of electron microscopes).

Claims

1. A laminate including a lanthanoid-based boride film formed on a substrate, the surface of which is covered with a thin film, The thin film is a monolayer hexagonal boron nitride thin film. Laminate.

2. The laminate of claim 1 , wherein the lanthanoid-based boride film is a lanthanum hexaboride film.

3. 3. The laminate according to claim 1, wherein the lanthanoid boride film has a thickness of 1 nm or more and 100 nm or less.

4. 4. The laminate according to claim 1, wherein the work function after vacuum heating at 500°C or higher and 600°C or lower after exposure to an atmospheric gas is approximately the same as the work function before exposure to the atmospheric gas.

5. An electron source comprising the stack according to claim 1 .

6. An electronic device comprising the laminate according to any one of claims 1 to 4.

7. forming a nitrogen-containing lanthanide boride film on a substrate; heating the lanthanoid-based boride film in a vacuum at a temperature range of higher than 750°C and lower than 1200°C to diffuse nitrogen in the lanthanoid-based boride film, causing the nitrogen to react with boron atoms contained in the lanthanoid-based boride film on the surface of the lanthanoid-based boride film, thereby depositing a monoatomic layer of hexagonal boron nitride thin film on the surface of the lanthanoid-based boride film, and covering the surface of the lanthanoid-based boride film with the deposited monoatomic layer of hexagonal boron nitride thin film; A method for producing the laminate of claim 1 , comprising:

8. 8. The method according to claim 7, wherein the nitrogen-containing lanthanoid boride film is formed by sputtering in an inert gas atmosphere using a nitrogen-containing lanthanoid boride sintered body as a target.

9. The substrate is a lanthanide boride single crystal substrate or a SiO.sub.2 substrate with a polycrystalline lanthanide boride film. ２ a substrate, 8. The method according to claim 7, wherein the lanthanoid-based boride film is formed by irradiating the surface of the substrate with nitrogen radicals.

10. The surface of the lanthanoid-based boride film is coated with the monoatomic layer of hexagonal boron nitride thin film at a rate of 1×10 －９ Pa or more 1×10 －５ The method according to any one of claims 7 to 9, wherein the method is carried out by diffusing nitrogen by heating in a vacuum ranging from 0.1 Pa or less for a period of from 5 minutes to 3 hours.

11. A method for cleaning a laminate, comprising the steps of: A laminate in which the surface of a lanthanoid-based boride film formed on a substrate is coated with a thin film made of a monoatomic layer of hexagonal boron nitride is exposed to an atmospheric gas, and then the laminate contaminated by the atmospheric gas is subjected to vacuum heating at a low temperature of 500°C or higher and 600°C or lower, thereby cleaning the laminate.

12. 12. The method of claim 11, wherein the lanthanide-based boride film is a lanthanum hexaboride film.

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