Electronic source

Abstract

The disclosed electron source includes: (A) a graphene layer superficially having an electron-emitting area; and (B) a p-type semiconductor layer that is in direct contact with the graphene layer and that forms a Schottky junction with the graphene layer. In the thus-configured electron source, electrons are accelerated in the depletion layer in the p-type semiconductor layer. Furthermore, unlike with an MIS-type electron source, electrons are not accelerated in the insulating layer; therefore, dielectric breakdown due to a large number of electrons travelling through the valence bands of an insulation object does not occur, allowing a long-life electron source to be realized. It is preferable that the above-described p-type semiconductor layer have a depletion layer having a width of 2.91-30 nm.

Description

[Technical Field] 【0001】 This invention relates to a planar electron source. [Background technology] 【0002】 As electron sources (also called electron emission elements) that emit electrons into a vacuum, gas, or liquid, planar electron sources having an MIS (Metal / Insulator / Semiconductor) structure and an MIM (Metal / Insulator / Metal) structure are disclosed, for example, in Patent Documents 1 to 3. 【0003】 Figure 1 shows a schematic cross-sectional view illustrating an example of the configuration of an electron-emitting element. The electron-emitting element 1000 includes a lower electrode substrate 1100, an insulating layer 1200, and an electron-transmitting electrode layer 1300. Conventionally, noble metals such as Au, Pt, and Ir, which have low reactivity, are used as the material for the electron-transmitting electrode layer 1300. The insulating layer 1200 is formed so that a portion of it has a thickness of 5 nm to 20 nm and functions as an electron-emitting surface 1500. The insulating layer 1210 in the portion other than the electron-emitting surface is usually formed to be thicker than the electron-emitting surface 1500, with a thickness of several tens to several hundreds of nm. An upper electrode layer 1400 for applying voltage is formed on the portion of the electron-transmitting electrode layer 1300 that does not overlap with the electron-emitting surface 1500. 【0004】 When using such an electron emission element 1000, applying a voltage of about 5V to 20V between the lower electrode substrate 1100 and the upper electrode layer 1400 thins the potential barrier formed in the insulator layer 1200, causing electrons in the lower electrode substrate 1100 to tunnel into the conduction band of the insulator layer 1200 due to the quantum mechanical tunneling effect. Electrons that enter the conduction band of the insulator layer 1200 lose some of their energy due to scattering by lattice vibrations, but electrons with energy higher than the work function of the electron-transmitting electrode layer 1300 pass through the electron-transmitting electrode layer 1300 and are emitted into the vacuum. 【0005】 The operating principle of a thin-film electron source, such as the electron emission element 1000, is known to be explainable using an energy band diagram (see, for example, Patent Document 2). Figure 2 is an energy band diagram of a structure in which a lower electrode substrate 1100, an insulating layer 1200 functioning as an electron acceleration layer, and an upper electrode layer 1300 are stacked, with a voltage applied to the upper electrode layer 1400. An electric field 32 is generated in the insulating layer 1200 by the voltage applied between the upper and lower electrodes. This electric field 32 causes electrons to flow from the lower electrode substrate 1100 into the insulating layer 1200 by tunneling. The energy distribution of electrons immediately after tunneling is as shown in distribution 41 in the figure. These electrons are accelerated by the electric field 32 in the insulating layer 1200, and some lose energy through interaction with atoms in the insulating layer 1200, becoming hot electrons as shown in energy distribution 42. When these hot electrons pass through the upper electrode layer 1300, some electrons lose even more energy due to inelastic scattering, etc., resulting in a spread-out energy distribution as shown in distribution 43. At the interface between the upper electrode layer 1300 and the vacuum 2000, i.e., when the electrons reach the surface of the upper electrode layer 1300, electrons with energy less than the work function of the surface remain in the upper electrode layer 1300, while electrons with energy greater than the work function, i.e., electrons included in distribution 44, are released into the vacuum 2000. In Figure 2, the energy levels 33 and vacuum level 34 of the upper electrode layer 1300 are also shown, where Ec represents the conduction band, Ef represents the Fermi level, and Ev represents the valence band. 【0006】 Against this backdrop, for example, Patent Document 4 discloses an electron emission element that enables highly efficient electron emission by using graphene in the upper electrode layer 1300. Specifically, the material used for the electron-transmitting electrode layer 1300 is either one layer of graphene or several layers of graphite. By using carbon, a light element, as the electron-transmitting electrode, the energy distribution is prevented from spreading from distribution 42 to distribution 43 in Figure 2, increasing the number of electrons that exceed the work function, and thus enabling highly efficient electron emission and high current. 【0007】 However, electron sources that emit electrons by tunneling through solids have a problem: they have a short lifetime. This is because the dielectric strength deteriorates as electrons travel through the insulating layer, which acts as an electron accelerating layer, and the leakage current gradually increases. 【0008】 Incidentally, a Schottky junction device method is known as a surface electron source in which a semiconductor layer forming a Schottky junction is sandwiched between upper and lower electrodes, and hot electrons are generated by causing avalanche breakdown with a high electric field (for example, Patent Document 5). Although electron sources employing such a Schottky junction device method are disclosed in Patent Documents 6 and 7, for example, it is not necessarily clear what kind of Schottky junction made of what material is preferable to employ as an electron source. [Prior art documents] [Patent Documents] 【0009】 [Patent Document 1] Japanese Patent Publication No. 2010-244735 [Patent Document 2] Japanese Patent Publication No. 2003-162956 [Patent Document 3] Japanese Patent Publication No. 2001-23511 [Patent Document 4] Japanese Patent Publication No. 2017-45639 [Patent Document 5] Japanese Patent Application Publication No. 10-241553 [Patent Document 6] Patent No. 2721278 [Patent Document 7] Japanese Patent Application Publication No. 3-129633 [Overview of the project] [Problems that the invention aims to solve] 【0010】 Therefore, one aspect of the present invention is to provide a new electron source with a long lifespan. 【Means for Solving the Problem】 【0011】 The electron source according to the present invention has: (A) a graphene layer having an electron emission area on the surface; and (B) a p-type semiconductor layer that is in direct contact with the graphene layer and forms a Schottky junction with the graphene layer. 【Brief Description of the Drawings】 【0012】 [Figure 1] FIG. 1 is a diagram showing a configuration example of a conventional electron source. [Figure 2] FIG. 2 is a diagram showing the energy distribution of a conventional electron source. [Figure 3] FIG. 3 is a diagram schematically showing a cross section of an electron source according to the first embodiment. [Figure 4] FIG. 4 is a diagram showing the energy distribution according to the first embodiment. [Figure 5] FIG. 5 is a diagram showing the dependence of depletion layer width and emission current on applied voltage for a certain sample. [Figure 6] FIG. 6 is a diagram showing the dependence of depletion layer width and emission current on applied voltage for another sample. [Figure 7] FIG. 7(a) shows the lifetime according to the embodiment, and (b) shows the lifetime in the prior art. [Figure 8] FIG. 8 is a diagram showing the manufacturing process of an electron source according to the first embodiment. [Figure 9] FIG. 9(a) schematically shows a cross section of an electron source according to the second embodiment, and (b) shows its energy distribution. [Figure 10] FIG. 10 schematically shows a cross section of an electron source according to the third embodiment, and (b) shows its energy distribution. [Figure 11] FIG. 11 is a diagram showing an application example of an electron source according to the third embodiment. 【Modes for Carrying Out the Invention】 【0013】 [Embodiment 1] Figure 3 shows a cross-sectional view of the electron source according to this embodiment. The electron emission element 100 according to this embodiment includes a lower electrode substrate 110 which is a p-type semiconductor and a graphene layer 130, and the portion that is in direct contact with the lower electrode substrate 110 and forms a Schottky junction functions as an electron emission area 150. In the portion other than the electron emission area 150, an insulating layer 121 is provided between the lower electrode substrate 110 and the graphene layer 130, and a contact electrode 140 is provided on the graphene layer 130. 【0014】 The p-type semiconductor is, for example, p-Si, and there are no particular restrictions on the thickness of the lower electrode substrate 110. The insulating layer 121 is, for example, SiO2, Al2O3, or TiO2. The contact electrode 140 is made of, for example, Ti or Ni. The graphene layer 130 is preferably composed of 1 to 10 layers of graphene, and more preferably 1 to 3 layers of graphene. The thickness of one graphene layer is 0.355 nm. 【0015】 Thus, no insulating layer is provided between the graphene layer 130 and the lower electrode substrate 110, which is a p-type semiconductor, and the graphene layer 130 and the lower electrode substrate 110 are in direct contact to form a Schottky junction. As schematically shown in Figure 4, when a positive voltage is applied to the contact electrode 140 provided on the upper part of the graphene layer 130, the conduction band (E) on the surface of the p-type semiconductor p-Si is formed. c Electrons present in the valence band (E) flow into the graphene layer 130, and a depletion layer is formed on the surface of the p-type semiconductor (e.g., p-Si). Heat and light cause the valence band (E) to change. v ) to the conduction band (E c Electrons excited to the ) penetrate the formed depletion layer, are accelerated by the internal electric field, and reach the surface of the p-type semiconductor (e.g., p-Si). Some electrons flow into the graphene layer 130, but since the graphene layer 130 is formed to be very thin, only about one to a few atomic layers thick, some electrons pass through the graphene layer 130 and are emitted into the vacuum. 【0016】 In this electron source 100, electrons are accelerated in the depletion layer. Unlike MIS-type electron sources, electrons are not accelerated through the insulating layer, and dielectric breakdown that occurs when a large number of electrons travel through the valence band of the insulator does not occur, thus enabling a long-life electron source. 【0017】 The width of the depletion layer formed in a p-type semiconductor for electron emission is important. If the p-type semiconductor is p-Si and the doping level is 1 × 10⁻⁶ 20 cm -3 Figure 5 shows the dependence of the depletion layer width and emission current on the applied voltage for the given sample. In the example in Figure 5, the horizontal axis represents the applied voltage [V], and the vertical axis represents the emission current [A] and the corresponding depletion layer width [nm]. In this sample, as shown by the dotted line, electron emission begins at an applied voltage of 11.5V, and the emission current increases up to 50V. 【0018】 The depletion layer width W [m] is determined by the applied voltage V and the carrier density N. D [m -3 The following relationships exist with respect to ]. 【number】 Here, ε represents the permittivity of a p-type semiconductor, ε0 represents the permittivity of vacuum, and V D represents the internal potential, and q represents the elementary charge. 【0019】 From equation (1) above, as shown by the solid line in Figure 5, in this sample, the depletion layer width at the applied voltage V = 11.5 V, when electron emission begins, is 12.5 nm, and the depletion layer width at the applied voltage V = 50 V is 25.8 nm. 【0020】 On the other hand, for other samples even with the same doping level, the figures showing the dependence of the depletion layer width and emission current on the applied voltage are shown in Fig. 6. In the case of this sample, as shown by the dotted line, it can be seen that electron emission starts when the applied voltage is 11.5 V, and the emission current increases up to 70 V. In the case of this sample, from equation (1), the depletion layer width at the applied voltage V = 11.5 V at which electron emission starts is 17.9 nm, and the depletion layer width at the applied voltage V = 70 V is 30.44 nm. 【0021】 From such experimental data, it can be seen that electron emission is possible up to a depletion layer width of 30 nm. On the other hand, in order to emit electrons, a voltage of at least the work function of the electrode must be applied. Since the work function of a general electrode material is around 5 eV, about 6 V of voltage application is required for electron emission. On the other hand, in the case of Si, even if the doping level is high, it is 10 21 cm -3 Therefore, when a reverse bias of 6 V is applied to a Schottky junction with p-Si having such a doping level, the depletion layer width is calculated to be 2.91 nm. From the above, for the purpose of electron emission, it is preferable to form a depletion layer having a width of 2.91 nm or more and 30 nm or less. 【0022】 However, the shorter the distance that electrons travel through the depletion layer width, the less the energy reduction due to electron scattering in the depletion layer and the recombination with holes, and the higher the energy monochromaticity and emission current density of the emitted electrons can be. The preferable range is 3.1 nm or more and 30 nm or less. For increasing the emission current density, the more preferable range is 3.1 nm or more and 20 nm or less. For further improving the energy monochromaticity of the emitted electrons, the preferable range is 3.1 nm or more and 10 nm or less, which allows electrons to be accelerated in the depletion layer almost without scattering from the mean free path of the electrons. 【0023】 Regarding the extension of the lifespan of the electron source 100, experimental results are shown in Figures 7(a) and (b). In Figures 7(a) and (b), the horizontal axis represents time [h] and the vertical axis represents emission current [A]. In Figure 7(a), the line in the upper section, which shows that the emission current flows for approximately 500 hours (specifically 497 hours), represents an experimental example of the electron source 100 according to this embodiment, and the line in the lower section, which is interrupted at approximately 0 hours, represents an experimental example of an electron source having the structure shown in Patent Document 4. Figure 7(b) is an enlarged view of the circled area in Figure 7(a), showing that electron emission occurs at approximately 10 hours. -9 [A] has decayed. Thus, it can be seen that the electron source 100 according to this embodiment has a dramatically longer lifespan and emits a large number of electrons. 【0024】 Next, a method for manufacturing the electron source 100 according to this embodiment (when p-Si is used as a p-type semiconductor) will be briefly explained using Figure 8. As shown in Figure 8(a), the p-Si substrate is cleaned. Then, as shown in Figure 8(b), a 300 nm thermal oxide film (SiO2) is deposited. After that, as shown in Figure 8(c), the oxide film of the electron emission area 150 is removed by lithography and BHF (Buffered Hydrofluoric Acid) wet etching. The electron emission area 150 has a size of, for example, 10 μm × 10 μm. 【0025】 Then, as shown in Figure 8(d), the graphene layer 130 is deposited by plasma CVD (Chemical Vapor Deposition). The plasma CVD conditions are, for example, 800°C, 18 Pa, 2 W, Ar: 20 sccm, CH4: 0.5 sccm, and a deposition time of 60 minutes. Subsequently, as shown in Figure 8(e), the graphene layer 130 is patterned by lithography and oxygen plasma etching (100 W, 10 minutes) to obtain the desired shape. 【0026】 Then, as shown in Figure 8(f), the contact electrode 140 is formed in the desired shape by lithography, electron beam deposition, and a lift-off process. Although not shown in Figure 3, a back contact electrode 141 (e.g., Ni or Ti) is fabricated on the back surface of the p-Si substrate by electron beam deposition. 【0027】 When such p-Si is used as a p-type semiconductor, electrons in the valence band are excited to the conduction band by thermal excitation, so it can operate even without light irradiation. 【0028】 On the other hand, when using materials other than p-Si, electrons may be emitted when irradiated with light. Therefore, the material must be selected according to the wavelength of the irradiated light. This is because p-type semiconductors basically absorb light with wavelengths greater than or equal to their band gap, generating electron holes. Specifically, the relationship between the wavelength of the irradiated light and the material to be used is as follows. • In the case of far-infrared rays (4 μm or larger (0.31 eV or less)): InSb(0.17eV) • Mid-infrared (2.5 μm or more and less than 4 μm (above 0.31 eV and below 0.496 eV)) InAs(0.36eV) • Near-infrared light (0.7 μm or more and less than 2.5 μm (above 0.496 eV and below 1.77 eV)) Ge (0.67 eV), Si (1.11 eV), InP (1.34 eV), GaAs (1.43 eV), MoS2 (bulk) (1.2 eV), WS2 (bulk) (1.2 eV) • Visible light (380nm to less than 700nm (over 1.77eV and 3.26eV or less)) SiC (2.8-3.2 eV), GaP (2.25 eV), ZnO (3.2 eV), ZnSe (2.58 eV), MoS2 (monolayer) (2.8 eV), WS2 (monolayer) (2 eV) • Ultraviolet light (below 380nm (exceeding 3.26eV)) GaN (3.4 eV), GaO3 (4.7-4.9 eV), diamond (5.47 eV), h-BN (6 eV), c-BN (6.12 eV), AlN (6.2 eV) 【0029】 [Embodiment 2] In this embodiment, as shown in the schematic cross-sectional view in Figure 9(a), for example, the electron source 100b includes a p-type semiconductor layer 110b, which is, for example, p-Si; a graphene layer 130 that forms a Schottky junction with the p-type semiconductor layer 110b and is in direct contact with the p-type semiconductor layer 110b; and a lower substrate 160 of an n-type semiconductor (for example, n-Si) that is in contact with the side of the p-type semiconductor layer 110b opposite to the graphene layer 130. The contact electrodes and insulating layers are the same as in the first embodiment, so their description is omitted. 【0030】 As shown in Figure 9(a), when a negative voltage is applied to the lower substrate 160 and a positive voltage to the graphene layer 130 to cause electrons to be emitted from the electron emission area of ​​the graphene layer 130, a forward bias is applied to the pn junction of p-Si and n-Si. As a result, as shown in Figure 9(b), the conduction band (E) of the n-type semiconductor (e.g., n-Si) is affected. c The electrons from the n-type semiconductor (e.g., p-Si) begin to diffuse into the conduction band of the p-type semiconductor. Thus, by adding the n-type semiconductor lower substrate 160 as in this embodiment, electrons are supplied from the n-type semiconductor, making it possible to emit electrons without irradiating with light. 【0031】 There are no particular restrictions on the thickness of the n-type semiconductor lower substrate 160, however, the thickness of the p-type semiconductor layer 110b is preferably thin, as it is preferable that carriers diffused from the n-type semiconductor lower substrate 160 do not recombine in the p-type semiconductor. On the other hand, since the depletion layer should be formed in the same manner as in the first embodiment, it is preferable that the p-type semiconductor layer 110b has a thickness of at least the same as the width of the depletion layer. 【0032】 [Embodiment 3] In this embodiment, as shown in Figure 10(a), for example, the electron source 100c has a lower substrate 110c of a p-type semiconductor, such as p-Si, and a graphene layer 130 that forms a Schottky junction with the lower substrate 110c and is in direct contact with the lower substrate 110c. The contact electrodes and insulating layer are the same as in the first embodiment, so their description is omitted. In this embodiment, a recess 115 is formed on the side of the lower substrate 110c opposite to the graphene layer 130. This recess 115 is formed to at least overlap with the electron emission area of ​​the graphene layer 130. That is, the recess 115 is formed to be identical to or encompass the electron emission area. As a result, the recess 115 provided in the lower substrate 110c forms a membrane structure of the p-type semiconductor layer 111 and the graphene layer 130. Light is irradiated into this recess 115 to emit electrons. 【0033】 For example, since light is absorbed by the p-type semiconductor layer 111 in the recess 115, if the p-type semiconductor layer 111 is p-Si, the thickness will be approximately 2.5 μm to 100 nm when using light from visible light (600 nm) to ultraviolet light (400 nm). On the other hand, as will be described later, when emitted electrons are used in lithography, ultraviolet light is used, so a thickness of 100 nm to 200 nm is preferable. On the other hand, for infrared light, a thickness of approximately 2.5 μm to 3 μm is preferable. 【0034】 Such an electron source 100c can be used in an electron beam lithography apparatus. In this case, for example, as shown in Figure 11, a digital mirror device (DMD) and an optical lens are provided, and the electron source 100c is placed between the electron lens and the semiconductor substrate to be exposed. The back side of the electron source 100c (i.e., the side opposite to the electron emission area) is directed towards the optical lens. 【0035】 In this way, the circuit layout pattern is converted into a light irradiation pattern using a digital mirror device and irradiated onto the back of the electron source 100c. This causes the electron source 100c to emit electrons corresponding to the light irradiation pattern across a surface, and by reducing the emitted electrons with an electron lens, it becomes possible to expose a fine circuit layout all at once. Conventionally, drawing was done with a single electron beam, resulting in poor throughput, but this method allows for simultaneous exposure across a surface, which is expected to improve throughput. 【0036】 The embodiments described above can be summarized as follows: 【0037】 The electron source according to this embodiment comprises (A) a graphene layer having an electron emission area on its surface, and (B) a p-type semiconductor layer that is in direct contact with the graphene layer and forms a Schottky junction with the graphene layer. 【0038】 In this way, electrons are accelerated in the depletion layer within the p-type semiconductor layer. Furthermore, unlike MIS-type electron sources, electrons are not accelerated in the insulating layer, so dielectric breakdown, which occurs when a large number of electrons travel through the valence band of the insulator, does not occur, resulting in a long-lived electron source. Moreover, if the p-type semiconductor is p-Si, thermal excitation occurs, so electron emission occurs even without light irradiation. 【0039】 Furthermore, the p-type semiconductor layer described above preferably has a depletion layer with a width of 2.91 nm to 30 nm. It is even more preferable that this p-type semiconductor layer has a depletion layer with a width of 3.1 nm to 20 nm. Moreover, it is even more preferable that this p-type semiconductor layer has a depletion layer with a width of 3.1 nm to 10 nm. In this embodiment, the intended use is not as a general semiconductor element other than an electron emission source, but as an electron source that emits electrons, so such depletion layer widths are preferred. The reason for narrowing the range is to enable the emission of electrons with higher energy monochromaticity at a higher current density. 【0040】 Furthermore, the electron source described above may also have an n-type semiconductor layer on the side of the p-type semiconductor layer opposite to the side in contact with the graphene layer. By forming a PN junction between the n-type semiconductor layer and the p-type semiconductor layer, electrons are supplied from the n-type semiconductor, and electron emission occurs without the need for light irradiation. For example, the p-type semiconductor layer is preferably thin, but the recesses allow it to have a thickness of at least the width of the depletion layer required for electron emission. 【0041】 Furthermore, the p-type semiconductor layer described above may have a recess on the side opposite to the surface in contact with the graphene layer, in a portion that overlaps with the electron emission area. When light is shone onto the recess, electrons are emitted from the electron emission area in the graphene layer. 【0042】 As mentioned above, various materials can be used for such p-type semiconductors depending on the light being irradiated. In addition to the layer configuration described above, the electron source may also have an insulating layer (e.g., SiO2) formed in the area other than the electron emission area to insulate the p-type semiconductor layer from the graphene layer and provide contact electrodes. However, the materials and thicknesses of these components can be changed depending on the circumstances without any problems. 【0043】 Furthermore, all values ​​contain a certain degree of error that inevitably arises from the standard deviation observed in each measurement. Also, all numerical ranges presented throughout this specification include narrower ranges as if they were explicitly stated herein.

Claims

[Claim 1] A graphene layer having an electron emission area on its surface, A p-type semiconductor layer that is in direct contact with the graphene layer and forms a Schottky junction with the graphene layer, It has, The aforementioned p-type semiconductor layer is 2.91 nm to 30 nm width, 3. A width of 3.1 nm or more and 20 nm or less, 3. Width between 3.1 nm and 10 nm Having a depletion layer electron source. [Claim 2] A graphene layer having an electron emission area on its surface, A p-type semiconductor layer that is in direct contact with the graphene layer and forms a Schottky junction with the graphene layer, On the side of the p-type semiconductor layer opposite to the side in contact with the graphene layer, an n-type semiconductor layer is provided. An electron source. [Claim 3] A graphene layer having an electron emission area on its surface, A p-type semiconductor layer that is in direct contact with the graphene layer and forms a Schottky junction with the graphene layer, It has, The p-type semiconductor layer has a recess on the surface opposite to the surface in contact with the graphene layer, in a portion that overlaps with the electron emission area. electron source.

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