Electron emission element and electron microscope

Abstract

The present invention provides an electron emission element with narrow electron energy divergence and excellent electron emission efficiency, and an electron microscope using the electron emission element. The electron emission element comprises: a stacked structure in which a first electrode, an electron acceleration layer composed of an insulating film, and a second electrode are stacked in sequence, the second electrode is an electrode for electrons to pass through and emit electrons from the surface, and the energy divergence of the emitted electrons is greater than 100 meV and less than 600 meV. For example, as the second electrode, one or more layers and less than 20 layers of graphene can be used, and as the insulating film, hexagonal boron nitride can be used.

Description

Technical Field

[0001] The present invention relates to an electron emitting element for emitting electrons and an electron microscope using the electron emitting element. Background Art

[0002] Conventionally, cold cathode electron sources and hot cathode electron sources are known as electron emission elements (also called "electron sources") that emit electrons from atoms to the outside. Cold cathode electron sources apply a high voltage between the cathode and the anode to provide energy for electrons to fly out of the cathode. Technology related to cold cathode electron sources is being developed in the fields of, for example, flat-panel display devices, lighting, and hydrogen generation devices.

[0003] Furthermore, as electron emitting elements for electron microscopes, the following electron sources have been put into practical use: a thermal electron source that extracts electrons from a solid by raising the temperature, a field emission electron source that extracts electrons from a sharp-tipped tungsten needle by field emission, and a Schottky electron source with a structure in which zirconium oxide is adsorbed on the tip of the tungsten needle. As one of the factors that determine the resolution (resolving power) of an electron microscope, there is chromatic aberration. Chromatic aberration is mainly determined by the energy divergence ΔE of the electrons emitted from the electron source. ΔE is determined by the electron emission mechanism of the electron source, so the range has been roughly determined in various types of electron sources. The operating temperature of the thermal electron source is 1900 to 2800K, and the energy divergence is 2 to 4eV. The operating temperature of the field emission electron source is 300K (room temperature), and the energy divergence of the electrons is 0.3eV. The operating temperature of the Schottky electron source is 1800K, and the energy divergence of the electrons is 0.7 to 1eV.

[0004] On the other hand, a thin film electron source composed of a stacked structure of thin films for use in flat-panel display devices and the like is known (see Patent Document 1). The thin film electron source has a structure in which an upper electrode, an electron acceleration layer, and a lower electrode are stacked, and emits electrons accelerated in the electron acceleration layer into a vacuum. Examples of thin film electron sources include MIM (Metal-Insulator-Metal) type electron sources, MOS (Metal-Oxide-Semiconductor) type electron sources, and ballistic surface electron sources. The upper electrode is also called a surface electrode or an electron transmission electrode, and constitutes an electron emission surface. The thin film electron source is not a needle or filament structure, and is therefore also called a planar electron emission element. Planar electron emission elements are not necessarily limited to flat plates, and also include elements having a thin film stacked structure that is not in the shape of a needle or filament and has a curved surface. Planar electron emitting elements have the following characteristics: high stability of emitted electrons, high straightness of emitted electrons, ability to operate at low voltages below 10V, ability to operate stably even in low vacuum, ability to surface emit electrons, and ability to be manufactured using existing semiconductor processes.

[0005] Figure 3 This is a cross-sectional view schematically showing the structure of a conventional electron emission element 300 (see Patent Document 2 and Non-Patent Document 1). The electron emission element 300 is composed of a lower electrode substrate 301, an insulator layer 302, and an electron transmission electrode layer 303. In order to prevent the work function from increasing due to the influence of oxidation, etc., the material of the electron transmission electrode layer 303 uses precious metals such as gold, platinum, and iridium. A portion of the insulator layer 302 has a thickness of 5nm to 20nm, and functions as an electron acceleration layer. The electron transmission electrode layer 303 is stacked on the insulator layer 302 to form an electron emission surface. The insulator layer 302 other than the electron emission surface is thicker than the electron emission surface, and has a thickness of about tens to hundreds of nm. A contact electrode layer 304 for applying a voltage is provided on the portion of the electron transmission electrode layer 303 that does not overlap with the electron emission surface 305. If a voltage of about 5V to 20V is applied between the lower electrode substrate 301 and the contact electrode layer 304, the potential barrier formed in the insulator layer 302 becomes thinner, and the electrons in the lower electrode substrate 301 tunnel to the conduction band of the insulator layer 302 due to the tunneling effect of quantum mechanics. The electrons reaching the conduction band of the insulator layer 302 lose a part of their energy due to scattering of lattice vibration, but the electrons having higher energy than the work function of the electron transmission electrode layer 303 pass through the electron transmission electrode layer 303 and are emitted into the vacuum.

[0006] The operating principle of the thin-film electron source can be explained using an energy band diagram (see Patent Document 2). Figure 4This is an energy band diagram in a state where a voltage is applied to the upper metal electrode 33 in a structure in which a lower semiconductor electrode 31, an electron acceleration layer 32, and an upper metal electrode 33 are stacked. In the case of an MIS type electron source, an insulator is used as the electron acceleration layer 32. An electric field is generated in the electron acceleration layer 32 due to the voltage applied between the upper metal electrode and the lower semiconductor electrode. Through this electric field, electrons flow from the lower semiconductor electrode 31 to the electron acceleration layer 32 due to the tunneling phenomenon. The energy distribution 41 of the electrons just after tunneling from the lower semiconductor electrode 31 to the electron acceleration layer 32 is the same as the energy distribution of the electrons in the lower semiconductor electrode 31 because there is no inelastic scattering of the electrons due to the tunneling phenomenon. The electrons are accelerated by the electric field in the electron acceleration layer 32 and become hot electrons. When being accelerated by the electric field in the electron acceleration layer 32, the electrons lose energy due to inelastic scattering in the electron acceleration layer 32, so the energy distribution is expanded as in the energy distribution 42 of the electrons, and the peak position is also shifted to the low energy side. Moreover, when the hot electrons pass through the upper metal electrode 33, the electrons further lose energy due to inelastic scattering, and the energy distribution further expands as shown in the energy distribution 43 of the electrons, and the peak position also shifts to the lower energy side. At the time point when reaching the upper metal electrode 33-vacuum 34 interface, that is, the surface of the upper metal electrode, only a part of the electrons having energy greater than the work function of the surface are emitted into the vacuum 34. The energy distribution 44 of the electrons emitted into the vacuum becomes a distribution as follows: the energy below the work function of the surface of the upper metal electrode 33 is cut off in the energy distribution 43 of the electrons.

[0007] Furthermore, the inventors have developed a method for forming a film of graphene instead of a film of catalyst metal on an insulator (see non-patent document 2), and proposed a structure of an electron emission element (electron source) that can efficiently emit electrons by using one layer of graphene or a few layers of graphite in the electron transmission electrode layer 303 (see patent document 2 and non-patent document 1).

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent Application Publication No. 2009-43440

[0011] Patent Document 2: Japanese Patent Application Publication No. 2017-45639

[0012] Non-patent literature

[0013] Non-patent document 1: K. Murakami, et al., Applied Physics Letters, Vol. 108, 083506 (2016).

[0014] Non-patent document 2: K. Murakami, et al., Applied Physics Letter, Vol. 106, 093112 (2015). Summary of the invention

[0015] Problems to be solved by the invention

[0016] In the conventional electron emission element that used one layer of graphene or several layers of graphite in the electron transmission electrode layer, the use of light element carbon as the electron transmission electrode can achieve high efficiency and large current of electron emission. It is expected that the scattering caused by the electron transmission electrode layer can be suppressed to some extent, but the energy dispersion of electrons is still around 1eV.

[0017] The present invention is made to solve these problems, and an object of the present invention is to provide an electron emitting element that suppresses the scattering of electrons in the insulating film and the electrons in the electron-transmitting electrode film to the limit and realizes a narrow energy divergence of electrons. In addition, an object of the present invention is to provide an electron microscope using an electron emitting element having a narrow energy divergence.

[0018] Solutions for solving problems

[0019] In order to achieve the above-mentioned object, the present invention has the following features.

[0020] (1) An electron emitting element, characterized in that the electron emitting element comprises: a stacked structure in which a first electrode, an electron acceleration layer composed of an insulating film, and a second electrode are stacked in sequence, the second electrode is an electrode for electrons to pass through and emit electrons from a surface, and the energy divergence of the emitted electrons is greater than 100 meV and less than 600 meV.

[0021] (2) The electron emitting element according to (1) above, wherein the second electrode is a graphene layer having 1 or more and 20 or less layers.

[0022] (3) The electron emitting element according to (1) or (2) above, wherein the insulating film is hexagonal boron nitride.

[0023] (4) The electron emitting element according to any one of (1) to (3), wherein the first electrode is a semiconductor.

[0024] (5) The electron emitting element according to any one of (1) to (3) above, wherein the first electrode is a magnetic body.

[0025] (6) The electron emitting element according to any one of (1) to (3), wherein the first electrode has a quantum well structure.

[0026] (7) An electron microscope comprising the electron emitting element according to any one of (1) to (6), wherein electrons are emitted from the electron emitting element into a vacuum or a gas.

[0027] Effects of the Invention

[0028] According to the electron emission element of the present invention, an electron beam having a narrower energy divergence than the energy divergence of an electron beam obtained by field emission from a metal such as tungsten can be obtained. In the case of a MOS type electron emission element, if it is the prior art, the electrons having the energy distribution of the electrons in the lower electrode tunnel and lose energy due to scattering in the insulating film. Since there are electrons that lose energy, there are also electrons that do not lose energy randomly, so the result is that the energy divergence is widened. On the other hand, according to the electron emission element of the present invention, there is almost no scattering of electrons in the insulating film, and there is almost no scattering of electrons in the electrode, so an energy distribution that directly reflects the electron distribution in the solid can be obtained. To explain in more detail, the electron distribution in the solid is multiplied by the tunneling probability (Tunnel probability) when the triangular potential of the tunneling insulating film is obtained to obtain the energy distribution of the emission. For example, it is known that if the lower electrode is a semiconductor, the energy distribution of the electrons in the semiconductor is based on the product of the Fermi-Dirac distribution function and the state density of the electrons in the semiconductor, and the tunneling probability is superimposed on the energy distribution of the electrons in the conductive band of the semiconductor to obtain the energy distribution of the electrons.

[0029] In the present invention, when a hexagonal boron nitride (hereinafter also referred to as hexagonal boron nitride, h-BN) thin film is used as an accelerating layer composed of an insulating film and graphene or graphite is used as an electron transmission electrode, the change in the energy distribution of electrons caused by electron scattering inside the electron emission element can be suppressed. In this case, electrons can be taken out while maintaining the electronic state of the lower electrode. When n-Si is used as the lower electrode, electrons with an energy dispersion of about 70 meV reflecting the energy distribution of the n-Si conductor can be emitted.

[0030] In the electron emission element of the present invention, when the second electrode is a graphene film or a graphite film, compared with the case where the second electrode is other electrode materials, not only can the scattering of electrons be suppressed and efficient electron emission be achieved, but also the following effects are achieved: since the graphene film or the graphite film is a low-reactive and stable material, the electrode itself does not corrode and has a long life.

[0031] Furthermore, an electron microscope equipped with the electron emitting element of the present invention has almost no chromatic aberration and has a very high resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1It is a schematic diagram showing the structure of the electron emitting element in the first embodiment.

[0033] Figure 2 This is an energy band diagram for explaining the operation of the electron emitting element in the first embodiment.

[0034] Figure 3 It is a schematic diagram showing the structure of an electron emitting element in the related art.

[0035] Figure 4 This is an energy band diagram for explaining the operation of an electron emitting element in the related art. DETAILED DESCRIPTION

[0036] Hereinafter, embodiments of the present invention will be described.

[0037] The present inventors have conducted research and development focusing on materials of a thin film stacked structure in an electron emitting element and an electron microscope using the electron emitting element, and have realized a device that emits electrons with a narrow energy dispersion.

[0038] The electron emission element of the embodiment of the present invention has a structure in which a first electrode (also referred to as a "lower electrode"), an electron acceleration layer composed of an insulating film, and a second electrode (also referred to as an "upper electrode, a surface electrode, and an electron transmission electrode") are stacked in sequence. The second electrode is an electrode for electrons to pass through, and the surface of the second electrode constitutes an electron emission surface. The electron emission surface in the electron emission element of the present invention is a substantially flat surface.

[0039] As an insulating film that functions as an electron acceleration layer, boron nitride is preferred. A hexagonal boron nitride (hexagonal boron nitride, h-BN) film is more preferred. h-BN has a two-dimensional structure like graphene, so the film thickness can be controlled by controlling the number of layers. In addition, since the atomic number is small, the scattering of electrons becomes smaller, so compared with the previous silicon dioxide film, a significant improvement in electron emission efficiency can be expected. By using h-BN, electrons with a narrow energy divergence can be emitted. Narrowing the energy divergence of the emitted electrons is particularly important in applications such as electron microscopy. The electron emission surface of the electron emission element used for the electron microscope is preferably a plane.

[0040] As the electron permeable electrode, a graphene film or a graphite film is preferably used. The electron permeable electrode is preferably a polycrystalline graphene film having a carbon layer and a surface electrode having a carbon layer of 1 or a polycrystalline graphite film having about 20 or less carbon layers.

[0041] The lower electrode is not particularly limited as long as it is a conductive material or semiconductor material that can be used as an electrode and can realize the stacked structure of the present invention. For example, a silicon electrode can be cited. Silicon is preferably silicon with high conductivity, and more preferably N-type silicon with electrons in the conduction band. Silicon can be any of single crystal, polycrystalline, and amorphous. In addition, as the lower electrode, for example, a magnetic metal electrode can be cited.

[0042] (First Embodiment)

[0043] Below, refer to Figure 1 This embodiment is described. The electron emitting element 10 includes: a lower electrode 1 (first electrode) formed of a conductive substrate, a thick insulating film 2 defining an electron emitting surface 5, a thin insulating film 6 functioning as an electron acceleration layer of the electron emitting surface 5, an electron transmission electrode 3 (surface electrode, upper electrode, second electrode), and a contact electrode 4 (also referred to as a third electrode). As the lower electrode 1, a silicon semiconductor substrate is used in this embodiment.

[0044] In order to suppress the scattering of electrons to the limit, the material of the insulating film 6 is preferably composed of light elements. In the present embodiment, boron nitride (BN) is used. The thickness of the insulating film 6 is preferably appropriately selected to be the thickness most suitable for the electron acceleration layer. A film thickness of about 4nm or more and about 20nm or less makes the electron emission efficiency higher, so it is preferred. If it is thinner than 4nm, the electrons directly tunnel (direct tunnel) the insulating film under a bias voltage below the work function of the electron transmission electrode. The electrons that directly tunnel do not have the energy to cross the work function of the electron transmission electrode, so they are recovered by the electron transmission electrode, so the electron emission efficiency is significantly reduced. In addition, when a bias voltage above the work function of the electron transmission electrode is applied, the electric field applied to the insulating film 6 becomes very high, so it is easy to produce dielectric breakdown, and the component life is shortened, so it is not preferred. When it is thicker than 20nm, the contribution of electron scattering in the insulating film 6 is large, and the energy of most of the tunneling electrons is reduced to below the work function of the electron transmission electrode, and is recovered by the electron transmission electrode, so the electron emission efficiency is significantly reduced.

[0045] The electron permeation electrode is a graphene film with 1 carbon layer or a graphite film with about 20 layers or less. In order to suppress the scattering of electrons in the electron permeation electrode as much as possible, a single-layer graphene film is more preferred. However, the experimental results show that if the graphite film is about 20 layers or less, no significant degradation of the electron emission efficiency is observed. A graphite film of 7nm corresponds to about 20 layers. Therefore, a film thickness of more than 2nm and less than 7nm is more preferred. The graphene film or graphite film can be a single crystal or a polycrystalline. For example, as a graphene film, a polycrystalline film with a grain size of about 50nm or more and less than 300nm is used to produce it, and good electron emission can be obtained. In the case of a single layer of graphene, in order to emit electrons from the entire surface, a continuous film is required.

[0046] A method for manufacturing the electron emitting element of this embodiment will be described.

[0047] (Step 1) A single crystal silicon substrate is prepared.

[0048] (Step 2) The silicon substrate is cleaned to remove impurities. A cleaning method used in the semiconductor field can be used.

[0049] (Step 3) Form a hexagonal boron nitride film on a silicon substrate. The following methods can be used for the film formation: a method of transferring a hexagonal boron nitride film formed on another substrate such as Cu or Ni-Fe alloy; a method of transferring a hexagonal boron nitride film formed on another substrate such as Cu or Ni-Fe alloy; 6 NB), borazine (B 3 N 3 H 6 ), or diborane (B 2 H 6 ) and ammonia (NH 3 ) as a raw material by CVD method, etc. The film forming method is not particularly limited.

[0050] Next, an example of the CVD method for forming a h-BN film on a silicon substrate in (step 3) is described. A heating device is prepared in which a quartz tube is inserted into a horizontal electric furnace. The pressure in the quartz tube is reduced to 5×10 －4 After the temperature reaches about 1000°C, the temperature of the electric furnace is set to about 1050°C. 3 N 3 H 6 ) vapor, and the pressure in the quartz tube was adjusted to about 30 Pa. The borazine vapor was introduced for 30 minutes to form a h-BN film.

[0051] (Step 4) Next, a surface electrode of graphene or graphite is formed on the surface of the boron nitride film. The method for forming the graphene or graphite film is not particularly limited, and an example of a method suitable for forming the surface electrode film is thermal CVD (thermal chemical vapor deposition) using a metal vapor catalyst.

[0052] The following is a description of a method for forming a graphite film on a first electrode using a metal vapor catalyst. A heating device is prepared in which a quartz tube is inserted into a horizontal electric furnace. The temperature of the electric furnace is set to about 1050°C, and a mixed gas of methane gas and argon gas is introduced from one side of the quartz tube. Inside the quartz tube, metal gallium is set on a quartz porcelain boat. A first electrode with an insulating film formed on the surface where the film is to be formed is set downwind of the metal gallium. Graphene of the desired film thickness is uniformly formed on the surface of the boron nitride film on the first electrode. For example, in a 1-inch quartz tube, the flow rate of argon gas is set to 100 sccm, the flow rate of methane gas is set to 1 sccm, and the film is formed for 30 minutes, a polycrystalline graphite film with a film thickness of 7 nm is uniformly formed on the boron nitride film. By shortening the film formation time, a single-layer polycrystalline graphene film can also be formed. It should be noted that the set temperature of the electric furnace, the film formation time, the gas flow ratio, etc. are not limited to the values ​​disclosed herein and can vary according to the desired film thickness and crystallinity. In addition, the introduced gas is not limited to argon and methane, and a combination of an inert gas serving as a carrier gas and a gas serving as a carbon supply source can be arbitrarily selected. Gallium is used as an example of a material for the metal vapor catalyst, but it is not limited to gallium. In addition to gallium, metals such as indium can also be used.

[0053] [Measurement of characteristics of electron emission element]

[0054] The electron emission element of the present embodiment is prepared as follows, which comprises: a lower semiconductor electrode of a silicon semiconductor substrate (n-Si), an insulating film of h-BN (Example 13nm) and an upper graphene electrode (Example 1nm), and the electron emission characteristics of the obtained electron beam are evaluated in an ultra-high vacuum chamber. First, the lower semiconductor electrode (cathode) is grounded, and a voltage is applied to the upper graphene electrode (gate) from 0V to about +20V in 0.1V steps, and +1000V is applied to the SUS plate (anode) that opposes the electron emission element, and the current flowing through the lower semiconductor electrode, the upper graphene electrode, and the SUS plate is measured. According to the measurement results, it was confirmed that electron emission started from the applied voltage of 13V, and the emission current increased with the increase of the applied voltage. It was confirmed that the emission current density reached 2A / cm at an applied voltage of 55V. 2, and obtain sufficient emission current. Next, an electron spectrometer mounted in an ultra-high vacuum chamber is used to measure the energy divergence of electrons emitted from the electron emission element. The electron spectrometer uses, for example, an electrostatic hemispherical energy analyzer. An electrostatic parallel plate energy analyzer can also be used. The working method of the electron emission element is different from the electron emission characteristic evaluation part. The electron emission element is placed opposite to a screen on which a fluorescent substance is coated on a transparent electrode. A hole with a diameter of about 1 mm is pre-set in the center of the screen. A voltage of +2.5 kV is applied to the screen. -100 V is applied to the lower semiconductor electrode and the upper graphene electrode. When electrons are emitted from the electron emission element, the voltage applied to the upper graphene electrode is slowly reduced from -100 V. In short, by adjusting the potential difference between the upper graphene electrode and the lower semiconductor electrode to 0 V to 20 V, the electron emission element can work in the same way as the electron emission characteristic evaluation. The emitted electrons collide with the fluorescent screen, and the electron emission pattern can be observed. While observing the electron emission pattern, adjust the position of the electron emission element so that the emitted electrons pass through the holes on the screen. The electrons passing through the holes on the screen are absorbed by the electron beam energy analyzer, and the energy spectrum of the emitted electrons is measured. The reason for applying -100V to the lower electrode of the semiconductor is to eliminate the influence of secondary electrons. The half-maximum full width of the obtained energy spectrum is defined as energy divergence. In this embodiment, the half-maximum width of the energy of the electrons is 0.275eV.

[0055] [Comparative Example 1] As Comparative Example 1, the following electron emitting element was produced, which includes: a lower electrode 1 of a silicon semiconductor substrate, SiO 2 The insulating film 6 and the electron transmission electrode 3 of graphene or graphite are provided. The material of the insulating film is different from that of the present embodiment. The energy spectrum of the electron beam obtained in Comparative Example 1 was measured. The half-peak width of the energy of the electrons in Comparative Example 1 is 1.6 eV. The insulating film is SiO 2 , so the electrons are scattered while traveling in the insulating film, generating electrons that lose energy. As a result, the energy dispersion of the electrons is expanded, and the half-peak width has a width of about 1.6eV. Even when SiO 2 Even when the thickness of the graphene film as the electron transmissive electrode is reduced to the limit by optimizing the film thickness and suppressing the scattering in the electron transmissive electrode, a half-peak width of 1.0 eV or less cannot be obtained.

[0056] [Comparative Example 2] As Comparative Example 2, the energy spectrum of the electron beam obtained by the tungsten field emission element was measured. In Comparative Example 2, the half-peak width of the electron energy was 0.37 eV. The reason why the energy divergence of the electrons in the tungsten field emission element is narrow is that the electrons tunnel in a vacuum and are not scattered on the way. The energy divergence is obtained by multiplying the energy distribution near the Fermi level of the Fermi distribution by the tunneling probability.

[0057] Compared with Comparative Examples 1 and 2, in the electron emission element of this embodiment, the insulating film through which electrons pass is boron nitride, and the electron transmission electrode is graphene, so the scattering of electrons is suppressed to the limit. As a result, there is almost no energy diffusion in scattering. In addition, the lower electrode is a silicon semiconductor and the Fermi level is located in the band gap, so the distribution function of the area above the Fermi level in the Fermi distribution overlaps with the tunneling probability, so that the energy distribution is narrower than that of the field emission element of tungsten. The half-peak width of 0.3eV or less is obtained.

[0058] [Explanation of working principle]

[0059] Reference Figure 2 , the working principle of the implementation mode of the present invention is explained. Figure 2 This is an energy band diagram in a state where a voltage is applied to the upper graphene electrode 13 in a structure in which a lower semiconductor electrode 11, h-BN12, and an upper graphene electrode 13 are stacked. An electric field is generated in h-BN12 due to the voltage applied between the upper metal electrode (upper graphene electrode) and the lower semiconductor electrode. Through this electric field, electrons flow from the lower semiconductor electrode 11 into the h-BN12 due to the tunneling phenomenon. The energy distribution 21 of the electrons just after tunneling from the lower semiconductor electrode 11 to the h-BN12 is the same as the energy distribution in the lower semiconductor electrode 11 because there is no inelastic scattering of the electrons due to the tunneling phenomenon. The electrons are accelerated by the electric field in h-BN12 and become hot electrons. In the case of h-BN, the elements that constitute the insulating layer are B and N, which is different from the conventional SiO 2 、Al 2 O 3 Compared to the atomic number of the constituent elements, the atomic number of the constituent elements is smaller. The inelastic scattering cross-sectional area of ​​electrons is generally proportional to the atomic number, so by using h-BN in the insulating material of the electron acceleration layer, the inelastic scattering of electrons in the electron acceleration layer can be suppressed. Therefore, the energy distribution 22 of the electrons accelerated in h-BN and reaching the upper graphene electrode 13 is the same as the energy distribution 21 of the electrons just after tunneling through the lower semiconductor electrode 11. Moreover, as for the upper graphene electrode, the element constituting the electrode is C, which has a smaller atomic number than the elements (Al, Au, Pt, etc.) of the upper metal electrode material used in the past, so the inelastic scattering of electrons in the upper graphene electrode can also be suppressed. At the time point when reaching the upper graphene electrode 13-vacuum 14 interface, that is, the surface of the upper graphene electrode, all electrons have an energy greater than the work function of the surface, so all electrons other than the electrons elastically scattered backward at the upper graphene electrode 13-vacuum 14 interface are emitted into the vacuum 14. In this series of processes, since inelastic scattering of electrons does not occur, the energy distribution 23 of the electrons emitted into the vacuum maintains the energy distribution 21 of the electrons immediately after tunneling through the lower semiconductor electrode 11 .

[0060] [Range of energy dispersion of emitted electrons]

[0061] The energy divergence of electrons emitted by the electron emitting element of the embodiment of the present invention is in the range of 100 meV or more and 600 meV or less. The upper limit is preferably 600 meV for the following reasons. It is known that even if the voltage applied between the lower semiconductor electrode and the upper graphene electrode is increased, the distance traveled by the electrons in the electron acceleration layer h-BN is sufficiently lengthened, and the influence of the inelastic scattering of the electrons in h-BN is increased, the increase in the energy divergence of the emitted electrons is saturated at about 600 meV. In addition, the lower limit is preferably 100 meV for the following reasons. The energy divergence of electrons in the lower semiconductor electrode at room temperature is about 70 meV. If the film thickness of the electron acceleration layer h-BN is reduced to 4 nm, the distance traveled by the electrons that tunnel through the barrier between the lower semiconductor electrode and h-BN in the electron acceleration layer h-BN is extremely short, and they are emitted into the vacuum through the upper graphene electrode. In this case, in the process of emission from the lower semiconductor electrode into the vacuum, electron scattering is almost suppressed, and the energy dispersion becomes about 100 meV which is slightly wider than the energy distribution of the electrons in the lower semiconductor electrode.

[0062] It is known that in the conventional electron emission element of the stacked structure of [upper metal electrode / thermal oxide film / Si semiconductor substrate], electrons that tunnel through the barrier at the interface between the Si semiconductor substrate and the thermal oxide film, are accelerated in the thermal oxide film by the electric field, and pass through the upper metal electrode, become in thermal equilibrium due to electron scattering in the thermal oxide film and the upper metal electrode, and become a Maxwell-Boltzmann distribution. Therefore, even if the energy divergence of electrons in the n-type Si substrate is narrowed to 70meV, when electrons are extracted into a vacuum using the conventional electron emission element, the energy divergence is expanded to more than 1eV.

[0063] (Second Embodiment)

[0064] In this embodiment, the case where a magnetic metal material (Co, Cr, Fe, etc.) different from that in the first embodiment is used as a lower electrode is described. By using a magnetized magnetic metal as a lower electrode, electrons that maintain the spin state in the magnetic body can be taken out, and different effects other than the effect of small energy divergence can be obtained. The electron emission element of this embodiment can be used in a spin-polarized scanning electron microscope or a spin-resolved photoelectron spectroscopy device for observing the state of the spin. In this embodiment, since the energy distribution of the electrons in the lower electrode is different from that of the semiconductor, the energy distribution becomes the same as that of the field-emitted electrons from a conventional tungsten needle, and the energy divergence is a minimum of 300meV and a maximum of about 600meV.

[0065] (Third Embodiment)

[0066] In this embodiment, the case where a nanostructure such as a quantum well structure is used as a lower electrode is described. By controlling the electronic state of the lower electrode, electrons can be extracted with an electron distribution that reflects the artificially controlled electronic state. For example, a quantum well structure consisting of a laminate of an insulator-metal (or semiconductor)-insulator with a film thickness of less than 10nm is used as the lower electrode, and the energy distribution of electrons in the lower electrode is quantized, thereby extracting only electrons with a specific energy level from the upper electrode via an electron acceleration layer. The energy divergence of electrons emitted by the electron emitting element of an embodiment of the present invention is in the range of more than 100meV and less than 600meV.

[0067] It should be noted that the examples shown in the above-mentioned embodiments and the like are described for easy understanding of the invention, and are not limited to the present embodiments.

[0068] Industrial Applicability

[0069] The electron emitting element of the present invention has a narrow energy spread of electrons and a high electron emission efficiency, and therefore can be used in devices that require a narrow energy spread of electrons, such as electron microscopes, and is industrially useful.

[0070] Description of Reference Numerals

[0071] 1: lower electrode; 2: thick insulating film; 3: electron transmission electrode; 4: contact electrode; 5, 305: electron emission surface; 6: insulating film; 10, 300: electron emission element; 11, 31: lower semiconductor electrode; 12: h-BN; 13: upper graphene electrode; 14, 34: vacuum; 21, 22, 23, 41, 42, 43, 44: energy distribution of electrons; 32: electron acceleration layer; 33: upper metal electrode; 301: lower electrode substrate; 302: insulator layer; 303: electron transmission electrode layer; 304: contact electrode layer.

Claims

1. An electron emitting element, characterized in that: The electron emitting element comprises a stacked structure in which a first electrode, an electron acceleration layer composed of an insulating film, and a second electrode are stacked in sequence. The second electrode is an electrode for electrons to pass through and emit electrons from the surface. The energy spread of the emitted electrons is greater than 100 meV and less than 600 meV. The first electrode is a semiconductor, The insulating film is hexagonal boron nitride, and the film thickness of the insulating film is greater than or equal to 4 nm and less than or equal to 20 nm. The second electrode is a graphene having at least one layer and no more than 20 layers.

2. The electron emitting element according to claim 1, characterized in that The energy dispersion of the emitted electrons is greater than or equal to 100 meV and less than or equal to 300 meV.

3. An electron emitting element for use in an electron microscope, wherein: The electron microscope is used to observe the state of spin, and the electron emission element is characterized in that: The electron emitting element comprises a stacked structure in which a first electrode, an electron acceleration layer composed of an insulating film, and a second electrode are stacked in sequence. The second electrode is an electrode for electrons to pass through and emit electrons from the surface. The energy spread of the emitted electrons is greater than 100 meV and less than 600 meV. The first electrode is a magnetic body, The insulating film is hexagonal boron nitride, and the film thickness of the insulating film is greater than or equal to 4 nm and less than or equal to 20 nm. The second electrode is a graphene having at least one layer and no more than 20 layers.

4. An electron emitting element, characterized in that: The electron emitting element comprises a stacked structure in which a first electrode, an electron acceleration layer composed of an insulating film, and a second electrode are stacked in sequence. The second electrode is an electrode for electrons to pass through and emit electrons from the surface. The energy spread of the emitted electrons is greater than 100 meV and less than 600 meV. The first electrode has a quantum well structure, The insulating film is hexagonal boron nitride, and the film thickness of the insulating film is greater than or equal to 4 nm and less than or equal to 20 nm. The second electrode is a graphene having at least one layer and no more than 20 layers.

5. An electron microscope, characterized in that The electron microscope includes the electron emitting element according to any one of claims 1 to 4, and the electron emitting element emits electrons into a vacuum or a gas.

Images