JP2005228935A - Multilayer structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer structure for emitting electrons, in which the work function is reduced beyond the conventional limit. <P>SOLUTION: The multilayer structure has an intermediate layer comprising at least one atom layer of an element selected from among a group of B, C, O, N, and Sn and a surface layer comprising one atom layer or two atom layers of Li or Na on the intermediate layer on a metal substrate, and the work function is 1eV or smaller. The intermediate layer preferably has a crystal structure made of B or Sn, or a random constituted of B, C, O, N, or Sn. The metal substrate is preferably made of a metal belonging to the Group 1b in the periodic table of elements or its alloy. Cu is preferable as the Group 1b metal. The intermediate layer is preferably formed on the plane (111) of the metal substrate of Cu or a Cu alloy. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a multilayer structure having a reduced work function and an increased electron emission capability.

  In recent years, development of devices (elements) using an electron emission phenomenon has been promoted in various fields. For example, thermionic energy converter (TEC) uses the phenomenon that free electrons in a metal are released to the outside of the metal as thermal electrons when a thermal energy higher than the work function is applied. Is a power generation device that converts electricity into electrical energy. At that time, primary energy (thermal energy) is directly converted to secondary energy (electrical energy), so high power generation efficiency is obtained, and no moving parts are required, resulting in high quietness and no mechanical wear. There are various advantages such as being small and light and capable of taking various shapes.

  As another representative example of an apparatus using the electron emission phenomenon, a field emission display (FED) is attracting attention. This is a display device that utilizes the phenomenon that when a strong electric field is applied to the solid surface, electrons confined on the solid surface are likely to jump out into the vacuum due to the tunnel effect. In order to generate this phenomenon, a very large voltage must be applied to the solid surface. For example, this can be achieved by minimizing the voltage application area using an electrode whose tip is sharpened like a needle. . The electrons emitted from the minimum area of the solid surface are collided with each minute area of the phosphor to cause each pixel to emit light, and an image is displayed as the entire light emitting surface. In principle, this is the same as a conventional CRT that uses an electron gun (thermoelectron emission), so it has a wider field of view and response speed than a liquid crystal display, and at the same time is as thin as a liquid crystal display. Is possible. Thus, FED is attracting attention as an excellent display having the advantages of both CRT and liquid crystal.

  Currently, various devices using the electron emission phenomenon are being developed for practical use, but the most basic requirement is to ensure high electron emission efficiency. It is necessary to lower the work function.

  Conventionally known low work function materials include single element materials such as alkali metals and alkaline earth metals, but they react with the ambient atmosphere in an electron emission environment such as high temperature or high electric field. It is easy to do and is not suitable for an electron-emitting device.

  The work function obtained so far is φ> 1.2 eV. However, in order to ensure practical electron emission efficiency, the work function is desired to be 1 eV or less.

  Furthermore, if it is going to solve the limit of the work function reduction by a material by making the material acicular, it is necessary to use a special material such as a carbon nanotube.

  An object of the present invention is to provide a multilayer structure for electron emission having a work function reduced beyond the conventional limit.

  In order to achieve the above object, as shown in FIG. 1, a multilayer structure 10 of the present invention is formed on a metal substrate 12 with one of elements selected from the group consisting of B, C, O, N and Sn. An intermediate layer 14 composed of atomic layers or more and a surface layer 16 composed of one or two atomic layers of Li or Na on the intermediate layer 14 have a work function of 1 eV or less.

  Preferably, the intermediate layer has a crystal structure composed of B or Sn, or a random structure composed of B, C, O, N, or Sn.

  The metal substrate is preferably made of a metal belonging to group 1b of the periodic table or an alloy thereof, and more preferably made of Cu or a Cu alloy. Desirably, the said intermediate | middle layer is formed on the (111) surface of the metal substrate which consists of Cu or Cu alloy.

  The multilayer structure of the present invention can maximize the surface normal component (μz) of the dipole moment by constituting an intermediate layer and a surface layer by a combination of prescribed elements on a metal substrate, and has been conventionally obtained. The low work function φ ≦ 1 eV that was not present can be realized. However, the positive dipole moment is the direction from the substrate to the surface.

  The reasons for various limitations in the multilayer structure of the present invention will be described.

[Elements constituting the intermediate layer and surface layer]
The reason for combining the elements constituting the intermediate layer as B, C, O, N, or Sn and the elements constituting the surface layer as Li or Na is that the electronegativity is low and the work is low. The combination of the alkali metals Li and Na known as functional materials and the elements B, C, O, N, and Sn on the opposite side of the periodic table with respect to them is (+) from an electrical viewpoint. And (−), the surface perpendicular component (μz) of the dipole moment can be increased. In particular, B, C, O, N, and Sn can increase the surface normal component (μz) of the dipole moment because the distance between the substrate and the alkali metals Li and Na can be increased by forming a random structure. Can do.

  Alkali metals Li and Na are too active with elemental elements, and are liable to react with the atmosphere and cannot be maintained as a stable structure. In particular, it cannot be used in an electron emission environment exposed to a high temperature or a high electric field. In the present invention, these alkali metals Li and Na are not elemental elements, but are in a chemically bonded state with the intermediate layer elements B, C, O, N, and Sn, thereby providing practically sufficient environmental stability. Realized a low work function material.

[Intermediate layer is 1 atomic layer or more]
If the intermediate layer composed of any one of B, C, O, N, and Sn is one monolayer or more, its effect can be obtained, but preferably 1 to 10 atomic layers, more preferably 1 or It is a diatomic layer.

[The surface layer is a single atomic layer or a two atomic layer]
The surface layer made of either Li or Na is a monolayer or a monolayer. This is because the surface normal component (μz) of the dipole moment faces the inside when the number of layers is three monolayers or more, thus preventing electron emission.

  The above effect can be obtained by forming the intermediate layer and the surface layer on the metal substrate. From this point of view, it is desirable to use the group 1b metal of the periodic table as the metal substrate. When a metal substrate made of Cu or a Cu alloy is used, the above-mentioned effects can be obtained more easily. When an intermediate layer and a surface layer are sequentially formed on the (111) surface of a metal substrate made of Cu or a Cu alloy, the above-mentioned effects are most easily obtained. Become.

  When the intermediate layer has a crystal structure composed of B or Sn or a random structure composed of B, C, O, N, or Sn, the above-described effects can be obtained particularly easily. Here, the “random structure” means not only “amorphous” but also a “random and gap structure” more positively. By using a random structure having “irregular and gaps”, as described above, the surface vertical component (μz) of the dipole moment can be increased by separating the distance between the alkali metals Li and Na in the surface layer and the substrate.

  Samples of various multilayer structures according to the present invention were prepared, and the surface normal component (μz) of the dipole moment was obtained as a physical property value corresponding to the work function φ. This is based on the empirical fact that there is a substantially linear correlation as shown in FIG. 2 between the work function φ and the surface normal component (μz) of the dipole moment. As can be seen from the figure, that the work function φ is 1 eV or less substantially corresponds to the fact that the surface perpendicular component (μz) of the dipole moment is 10 debye or more. As shown in the figure, in the conventional material, the work function φ is larger than 1 eV, that is, the surface normal component (μz) of the dipole moment is less than 10 debye.

  Determination of an object to be created as a sample satisfying the specified range of the present invention was performed using the search procedure and the search system shown in FIGS. That is, on the premise that Cu or Cu / Ag alloy is used as the substrate, the surface normal component of the dipole moment (by changing the combination of the constituent elements of each layer and the combination of bonding states between the layers for the intermediate layer and the surface layer ( [mu] z) was determined by calculation, and a structure having a surface perpendicular component of the dipole moment [mu] z≥10 debye (≈ work function≤ 1 eV) was selected.

  Based on these results, seven types of samples shown in Table 1 were actually created to determine the surface normal component (μz) of the dipole moment. Samples were prepared according to the method described in Japanese Patent No. 3373357 according to the following procedure.

[Sample creation procedure]
First, an atomic layer of boron (B) crystal layer (samples 1 and 2) on the (111) plane of a Cu substrate (samples 1 and 3-7) or a Cu-Ag alloy substrate (sample 2 only) , Amorphous carbon (a-C) layer (sample 3), monoatomic tin (Sn) crystal layer (samples 4 and 5), amorphous nitrogen (a-N) layer (sample 6) or amorphous oxygen (a-O) ) Layer (Sample 7) was formed. This film was formed using an Ar ion transfer method at a gun voltage of 5 kV, a gun current of 0.5 mA, a sample rotation speed of 2 rpm, an Ar ion irradiation angle of 10 degrees, and an irradiation time of 6 seconds.

  Next, an Ar ion transfer method is performed on the intermediate layer under the same conditions as described above, and one atomic layer of lithium (Li) layer (samples 1 to 4 and 6) or one atomic layer of sodium (Na) is used as a surface layer. Layers (Samples 5, 7) were formed.

  The layer configuration and characteristics (μz value) of the obtained samples 1 to 7 are shown in Table 1, and the structure of each sample is schematically shown in FIGS.

  As shown in Table 1, the surface normal component (μz) value of a large dipole moment exceeding 10 debye, that is, a low work function φ value of less than 1 eV was obtained in any sample.

〔比較例〕
本発明の規定範囲を満たさないサンプルについても種々実際に作成して特性評価したが、いずれも１０debyeを超える大きな双極子モーメントの表面垂直成分（μｚ）（１ｅＶ以下の低仕事関数）は得られなかった。一例として、実施例サンプルと同様のＣｕ基板（１１１）面上に、実施例と同様の条件にて中間層を塩素（Ｃｌ）で形成し、表層をルビジウム（Ｒｂ）で形成した比較サンプルは、双極子モーメントの表面垂直成分がμｚ＝−１８．５debyeと負の値であり、１ｅＶ以下の低仕事関数を実現できない。

[Comparative example]
Various samples were actually prepared and evaluated for characteristics that did not satisfy the specified range of the present invention. However, none of the surface normal components (μz) (low work function of 1 eV or less) having a large dipole moment exceeding 10 debye were obtained. It was. As an example, a comparative sample in which an intermediate layer is formed of chlorine (Cl) and a surface layer is formed of rubidium (Rb) on the same Cu substrate (111) surface as in the example sample under the same conditions as in the example, The surface normal component of the dipole moment is a negative value of μz = −18.5 debye, and a low work function of 1 eV or less cannot be realized.

  The present invention provides a multilayer structure for electron emission capable of realizing a low work function of 1 eV or less exceeding the conventional limit.

FIG. 1 is a cross-sectional view schematically showing a multilayer structure of the present invention. FIG. 2 is a graph showing the interrelationship between the work function φ and the surface normal component (μz) of the dipole moment. FIG. 3 is a flowchart showing a procedure for searching for a low work function material. FIG. 4 is a block diagram of the entire system for searching for low work function materials. FIG. 5 shows a boron (B) crystal layer of 1 atomic layer (1 mL) as an intermediate layer and lithium (Li) of 1 atomic layer (1 mL) as a surface layer on the Cu substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the layer. FIG. 6 shows a boron (B) crystal layer of one atomic layer (1 mL) as an intermediate layer and a lithium of one atomic layer (1 mL) as a surface layer on the Cu—Ag alloy substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the (Li) layer. FIG. 7 shows a carbon (C) amorphous layer of 1 atomic layer (1 mL) as an intermediate layer and lithium (Li) of 1 atomic layer (1 mL) as a surface layer on the Cu substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the layer. FIG. 8 shows that according to the present invention, a single atomic layer (1 mL) tin (Sn) crystal layer as an intermediate layer and a single atomic layer (1 mL) lithium (Li) layer on the Cu substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the layer. FIG. 9 is a graph showing a structure in which a tin (Sn) crystal layer of 1 atomic layer (1 mL) as an intermediate layer and sodium (Na) of 1 atomic layer (1 mL) as a surface layer on the Cu substrate (111) surface according to the present invention It is a schematic diagram which shows the structure of the sample which formed the layer. FIG. 10 shows a nitrogen (N) amorphous layer of 1 atomic layer (1 mL) as an intermediate layer and a lithium (Li) of 1 atomic layer (1 mL) as a surface layer on the Cu substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the layer. FIG. 11 shows an oxygen (O) amorphous layer of one atomic layer (1 mL) as an intermediate layer and a single atomic layer (1 mL) of sodium (Na) as a surface layer on the Cu substrate (111) surface according to the present invention. It is a schematic diagram which shows the structure of the sample which formed the layer.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Multilayer structure 12 of this invention ... Metal substrate 14 ... Intermediate | middle layer (Select from B, C, O, N, Sn)
16 ... surface layer (selected from Li and Na)

Claims (6)

  On a metal substrate, from an intermediate layer composed of one or more atomic layers of an element selected from the group consisting of B, C, O, N and Sn, and from an atomic layer or two atomic layers of Li or Na on the intermediate layer And a work layer having a work function of 1 eV or less.   2. The multilayer structure according to claim 1, wherein the intermediate layer has a crystal structure made of B or Sn.   2. The multilayer structure according to claim 1, wherein the intermediate layer has a random structure made of B, C, O, N, or Sn.   The multilayer structure according to any one of claims 1 to 3, wherein the metal substrate is made of a metal belonging to group 1b of the periodic table or an alloy thereof.   5. The multilayer structure according to claim 4, wherein the metal belonging to Group 1b of the periodic table is Cu.   6. The multilayer structure according to claim 5, wherein the intermediate layer is formed on a (111) plane of a metal substrate made of Cu or an alloy thereof.

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