Patent Publication Number: US-11664182-B2

Title: Electron emitting element and power generation element

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2020-183576, filed on Nov. 2, 2020; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to an electron emitting element and a power generation element. 
     BACKGROUND 
     For example, an electron emitting element is used for a power generation element or the like. It is desired to improve the efficiency of the electron emitting element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view illustrating an electron emitting element according to a first embodiment; 
         FIGS.  2 A and  2 B  are schematic views illustrating characteristics of the electron emitting element; 
         FIG.  3    is a graph view illustrating characteristics of the electron emitting element; 
         FIG.  4    is a graph view illustrating characteristics of the electron emitting element; 
         FIG.  5    is a graph view illustrating characteristics of the electron emitting element; 
         FIG.  6    is a schematic cross-sectional view illustrating a power generation element according to a second embodiment; 
         FIGS.  7 A and  7 B  are schematic cross-sectional views showing a power generation module and a power generation device according to the embodiment; and 
         FIGS.  8 A and  8 B  are schematic views showing the power generation device and a power generation system according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, an electron emitting element includes a first region, a second region, and a third region. The first region includes a semiconductor including a first element of an n-type impurity. The second region includes diamond. The diamond includes a second element including at least one selected from the group consisting of nitrogen, phosphorous, arsenic, antimony, and bismuth. The third region is provided between the first region and the second region. The third region includes Al x1 Ga 1-x1 N (0&lt;x1≤1) including a third element including at least one selected from the group consisting of Si, Ge, Te and Sn. A +c-axis direction of the third region includes a component in a direction from the first region toward the second region. 
     According to one embodiment, a power generation element includes the electron emitting element described above, and an opposing member facing the second region. A gap is between the second region and the opposing member. 
     Various embodiments are described below with reference to the accompanying drawings. 
     The drawings are schematic and conceptual; and the relationships between the thickness and width of portions, the proportions of sizes among portions, etc., are not necessarily the same as the actual values. The dimensions and proportions may be illustrated differently among drawings, even for identical portions. 
     In the specification and drawings, components similar to those described previously or illustrated in an antecedent drawing are marked with like reference numerals, and a detailed description is omitted as appropriate. 
     First Embodiment 
       FIG.  1    is a schematic cross-sectional view illustrating an electron emitting element according to a first embodiment. 
     As shown in  FIG.  1   , an electron emitting element  50  according to the embodiment includes a first region  11 , a second region  12 , and a third region  13 . 
     The first region  11  includes a semiconductor  11   s . The semiconductor  11   s  includes a first element of an n-type impurity. The semiconductor  11   s  is an n-type semiconductor. The semiconductor  11   s  in the first region  11  includes, for example, at least one selected from the group consisting of AlGaN, GaAs, Si and SiC. An example of the semiconductor  11   s  will be described later. 
     The second region  12  includes diamond. Diamond includes a second element. The second element includes at least one selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, and bismuth. The second element functions as an n-type impurity. The diamond in the second region  12  is n-type. The diamond in the second region  12  may include, for example, multiple crystal grains. Diamond may include, for example, polycrystals. The diamond may be, for example, a single crystal. Diamond may be, for example, nanocrystals. 
     The third region  13  is provided between the first region  11  and the second region  12 . The third region  13  includes Al x1 Ga 1-x1 N (0&lt;x1≤1) including the third element. The third element includes at least one selected from the group consisting of Si, Ge, Te and Sn. The third element functions as an n-type impurity. Al x1 Ga 1-x1 N (0&lt;x1≤1) included in the third region  13  is an n-type. The third region  13  includes, for example, AlGaN or AlN. As will be described later, the composition ratio x1 of Al is preferably not less than 0.2. The composition ratio x1 of Al may be not less than 0.5. 
     The +c-axis direction of the crystal in the third region  13  includes a component in the direction from the first region  11  toward the second region  12 . For example, the +c-axis direction of the third region  13  is along the direction from the first region  11  to the second region  12 . 
     In the electron emitting element  50  according to the embodiment, electrons are emitted from a surface  12   f  of the second region  12 . The electrons are, for example, thermions. For example, the surface  12   f  is exposed to a space  18 . The electrons are emitted into the space  18 . In the embodiment, by providing the above-mentioned third region  13 , electrons can be emitted with high efficiency. Examples of electron emission characteristics will be described later. 
     The direction from the first region  11  toward the second region  12  is taken as a Z-axis direction. One direction perpendicular to the Z-axis direction is taken as an X-axis direction. The direction perpendicular to the Z-axis direction and the X-axis direction is taken as a Y-axis direction. 
     The Z-axis direction corresponds to the stacking direction of the first region  11 , the third region  13 , and the second region  12 . The first region  11  and the third region  13  extend along the X-Y plane. In one example, the second region  12  extends along the X-Y plane. For example, the diamond included in the second region  12  may have multiple island shapes. Multiple island-shaped diamonds may be arranged along the X-Y plane. 
     The +c-axis direction of the crystal in the third region  13  is, for example, along the Z-axis direction. The absolute value of the angle between +c and the Z-axis direction is not more than 45 degrees. The absolute value of the angle between +c and the Z-axis direction may be not more than 10 degrees or less. If the absolute value of the angle is small, high efficiency can be easily obtained. 
     A thickness t1 (see  FIG.  1   ) of the first region  11  along the Z-axis direction is, for example, not less than 100 nm or more and not more than 200 μm. A thickness t3 of the third region  13  along the Z-axis direction is, for example, not less than 5 nm and not more than 50 nm or less. A thickness t2 of the second region  12  along the Z-axis direction is, for example, not less than 5 nm and not more than 50 nm. 
     In the following, an example of the simulation result of the characteristics of the electron emitting device will be described. 
       FIGS.  2 A and  2 B  are schematic views illustrating characteristics of the electron emitting element. 
     The horizontal axis of these figures is the position pZ along the Z-axis direction. The vertical axis of these figures is the energy Ec. These figures illustrate the energy Ec of the conduction band. In  FIG.  2 A , the +c-axis direction of the third region  13  is along the direction from the first region  11  toward the second region  12 . This state is referred to as “+c-axis crystal orientation”. In  FIG.  2 B , the −c axis of the third region  13  is along the direction from the first region  11  toward the second region  12 . This state is taken as the “crystal orientation of the −c axis”. 
     In these figures, the first region  11  is GaN including Si as the first element. The temperature of the first region  11  is 600° C. The second region  12  includes an n-type diamond including N (nitrogen) as a second element. The thickness t2 of the second region  12  is 20 nm. The composition ratio x1 of Al in Al x1 Ga 1-x1 N of the third region  13  is 0.25. The concentration of the third element (Si) in the third region  13  is 1×10 14 /cm 3 . The thickness t3 of the third region  13  is 20 nm. 
     As shown in  FIGS.  2 A and  2 B , the energy Ec becomes the highest at the boundary between the third region  13  and the second region  12 . The energy Ec at the boundary between the third region  13  and the second region  12  is taken as the energy E 1 . The energy E 1  in the case of “+c-axis crystal orientation” is lower than the energy E 1  in the “−c-axis crystal orientation”. At the low energy E 1 , electrons are efficiently emitted. 
       FIG.  3    is a graph view illustrating characteristics of the electron emitting element. 
     The horizontal axis of  FIG.  3    is the composition ratio x1 of Al in the third region  13 . The vertical axis is the energy E 1 . In the simulation of  FIG.  3   , the conditions described with respect to  FIGS.  2 A and  2 B  are adopted as the conditions other than the Al composition ratio x1.  FIG.  3    shows the result in the case of “+c-axis crystal orientation” and the result in the case of “−c-axis crystal orientation”. 
     As shown in  FIG.  3   , the energy E 1  in the case of “+c-axis crystal orientation” is lower than the energy E 1  in the case of “−c-axis crystal orientation”. As described above, the low energy E 1  can be obtained in a case where the crystal orientation in the third region  13  is the “+c-axis crystal orientation”. In a case where the +c-axis direction of the third region  13  is the direction from the first region  11  toward the second region  12 , electrons are emitted with high efficiency. According to the embodiment, it is possible to provide an electron emitting element which is possible to improve efficiency. 
     As shown in  FIG.  3   , in the case of the “+c-axis crystal orientation”, when the Al composition ratio x1 in the third region  13  is high, the energy E 1  is low. In the embodiment, the composition ratio x1 of Al is preferably high. In the embodiment, the composition ratio x1 of Al in the third region  13  is preferably not less than 0.2. The composition ratio x1 of Al may be not less than 0.5. The composition ratio x1 of Al may be not less than 0.8. When the composition ratio x1 of Al is high, high efficiency can be easily obtained. The composition ratio of Al may be set from the viewpoint of crystallinity and impurity concentration. 
     In the following, an example of the characteristics in the case of “+c-axis crystal orientation” will be described. 
       FIG.  4    is a graph view illustrating characteristics of the electron emitting element. 
     The horizontal axis of  FIG.  4    is a concentration C 1  of the third element in the third region  13 . The vertical axis is the energy E 1 . In the simulation of  FIG.  4   , the composition ratio x1 of Al is 0.75. The concentration C 1  is a concentration of Si in the third region  13 . As the conditions other than the concentration C 1 , the conditions described with respect to  FIG.  2 A  are adopted. 
     As shown in  FIG.  4   , in the case where the concentration C 1  of the third element is high, the energy E 1  is low. When the concentration C 1  of the third element is high, high efficiency can be easily obtained. In the embodiment, the concentration C 1  of the third element in the third region  13  is preferably not less than 1×10 14 /cm 3 . The concentration C 1  is preferably not less than 1×10 16 /cm 3 . High efficiency is easy to obtain. In the embodiment, the concentration C 1  is, for example, not more than 1×10 20 /cm 3 . If the concentration C 1  exceeds 1×10 20 /cm 3 , for example, the crystallinity in the third region  13  may decrease and the electrical resistance may increase. When the concentration C 1  is not more than 1×10 20 /cm 3 , low electrical resistance can be stably obtained. 
       FIG.  5    is a graph view illustrating characteristics of the electron emitting element. 
     The horizontal axis of  FIG.  5    is the thickness t3 of the third region  13 . In the example of  FIG.  5   , the composition ratio x1 of Al is 0.75, and the concentration C 1  of the third element is 1×10 17 /cm 3 . The vertical axis is the energy E 1 . As the conditions other than these, the conditions described with respect to  FIG.  2 A  are adopted. 
     As shown in  FIG.  5   , when the thickness t3 of the third region  13  is thick, the energy E 1  is low. In the embodiment, the thickness t3 is preferably not less than 5 nm. The thickness t3 may be not less than 10 nm. The thickness t3 may be not less than 20 nm. Low energy E 1  is obtained. High efficiency can be obtained. The thickness t3 is, for example, not more than 50 nm. When the thickness t3 exceeds 50 nm, the electrical resistance tends to increase. When the thickness t3 is not more than 50 nm, low electrical resistance can be stably obtained. 
     In the embodiment, for example, the third region  13  including Al x1 Ga 1-x1 N is provided on the first region  11  including the semiconductor  11   s . The second region  12  including diamond is provided on such the third region  13 . For example, a region including a high concentration of carriers is formed in a region including the interface between the first region  11  and the third region  13 . Regions including high concentrations of carriers include, for example, two-dimensional electron gases. High-concentration carriers move in the third region  13  and are released to the outside (space  18 ) from the surface  12   f  of the second region  12 . The third region  13  is in contact with, for example, the second region  12 . 
     In the embodiment, the semiconductor  11   s  included in the first region  11  includes, for example, Al x2 Ga 1-x2 N (0≤x2&lt;1, x2&lt;x1). In this case, the first element includes at least one selected from the group consisting of Si, Ge, Te and Sn. High concentrations of carriers can be effectively obtained. For example, in the third region  13 , good crystallinity can be easily obtained. For example, low resistance is easy to obtain. 
     For example, the semiconductor  11   s  may include at least one selected from the group consisting of Si and SiC. In this case, the first element includes at least one selected from the group consisting of nitrogen, phosphorus, arsenic, antimony, and bismuth. High concentrations of carriers can be effectively obtained. For example, low resistance is easy to obtain. 
     For example, the semiconductor  11   s  may include GaAs. In this case, the first element includes at least one selected from the group consisting of S, Se and Te. For example, low resistance is easy to obtain. 
     In embodiments, the surface  12   f  of the second region  12  may be terminated by H or OH. For example, as shown in  FIG.  1   , the second region  12  includes a first surface  12   a  and a second surface  12   b . The second surface  12   b  is between the first surface  12   a  and the third region  13 . The first surface  12   a  is the surface  12   f . The first surface  12   a  includes at least one selected from the group consisting of hydrogen and hydroxyl groups. The first surface  12   a  becomes stable when the first surface  12   a  includes at least one selected from the group consisting of hydrogen and hydroxyl groups. Stable electron emission can be obtained. 
     For example, a concentration of hydrogen on the first surface  12   a  is higher than a concentration of hydrogen on the second surface  12   b . When the concentration of hydrogen on the second surface  12   b  is high, for example, a density of holes tends to be high in the vicinity of the second surface  12   b . When the concentration of hydrogen on the second surface  12   b  is low, for example, it is possible to suppress an increase in the density of holes. This makes it easier for electrons to be emitted from the first surface  12   a.    
     As shown in  FIG.  1   , the electron emitting element  50  may further include a first electrode  15 . There is the first region  11  between the first electrode  15  and the second region  12 . The first electrode  15  is electrically connected to the first region  11 . A current accompanying the emission of electrons flows through the first electrode  15 . 
     Second Embodiment 
       FIG.  6    is a schematic cross-sectional view illustrating a power generation element according to a second embodiment. 
     As shown in  FIG.  6   , a power generation element  110  according to the embodiment includes the electron emitting element  50  according to the first embodiment and an opposing member  20 . The opposing member  20  faces the second region  12 . The opposing member  20  is conductive. For example, there is the second region  12  between the first region  11  and the opposing member  20 . There is a gap  55  between the second region  12  and the opposing member  20 . 
     For example, the first region  11  is set to a high temperature. Electrons are emitted from the surface  12   f  of the second region  12  toward the gap  55 . The opposing member  20  receives electrons. The current flowing between the electron emitting element  50  and the opposing member  20  is taken out as the current of the power generation element  110 . 
     As shown in  FIG.  6   , the power generation element  110  may include a container  60 . The electron emitting element  50  and the opposing member  20  are provided in the container  60 . The pressure inside the container  60  is lower than the atmospheric pressure. For example, the gap  55  is in a reduced pressure state. The electrons emitted from the second region  12  efficiently reach the opposing member  20 . 
     A distance d1 between the second region  12  and the opposing member  20  along the direction from the second region  12  toward the opposing member  20  (for example, the Z-axis direction) is, for example, not less than 100 nm and not more than 1 mm. For example, high power generation efficiency can be obtained. For example, the distance d1 may be not less than 1 μm and not more than 100 μm. Higher power generation efficiency can be obtained. 
     As shown in  FIG.  6   , the opposing member  20  includes a second electrode  25  and a facing layer  21 . The facing layer  21  is provided between the second region  12  and the first electrode  25 . The opposing layer  21  includes, for example, at least one selected from the group consisting of diamond, AlN, AlGaN, SiC, Mo, W, LaB 6 , and tungsten. The above-mentioned tungsten may include thorium oxide. Electrons can enter the facing layer  21  with high efficiency. The above-mentioned tungsten may include thorium oxide. For example, a region including an alkali metal may be provided on the surface of the facing layer  21  on the gap  55  side. The alkali metal includes, for example, at least one selected from the group consisting of Ba and Cs. As a result, electrons can be incident on the facing layer  21  with even higher efficiency. When the opposing layer  21  includes diamond, a region (for example, a terminal region) including at least one selected from the group consisting of hydrogen and hydroxyl groups may be provided on the surface of the opposing layer  21  on the gap  55  side. 
     In the following, an example of application of the power generation element will be described. 
       FIGS.  7 A and  7 B  are schematic cross-sectional views illustrating a power generation module and a power generation device according to the embodiment. 
     As shown in  FIG.  7 A , a power generation module  210  according to the embodiment includes the power generation element (for example, the power generation element  110 ) according to the second embodiment. In this example, multiple power generation elements  110  are arranged on a substrate  120 . 
     As shown in  FIG.  7 B , a power generation device  310  according to the embodiment includes the power generation module  210  described above. Multiple power generation modules  210  may be provided. In this example, the multiple power generation modules  210  are arranged on a substrate  220 . 
       FIGS.  8 A and  8 B  are schematic views showing the power generation device and a power generation system according to the embodiment. 
     As shown in  FIGS.  8 A and  8 B , the power generation device  310  according to the embodiment (that is, the power generation element  110  according to the embodiment) can be applied to solar thermal power generation. 
     As shown in  FIG.  8 A , for example, the light from the sun  61  is reflected by a heliostat  62  and incident on the power generation device  310  (power generation element  110  or power generation module  210 ). The light raises the temperature of the electron emitting device. Heat is converted into a current. The current is transmitted by the electric line  65  or the like. 
     As shown in  FIG.  8 B , for example, the light from the sun  61  is collected by a condensing mirror  63  and incident on the power generation device  310  (power generation element  110  or power generation module  210 ). The heat from the light is converted into a current. The current is transmitted by the electric line  65  or the like. 
     For example, a power generation system  410  includes the power generation device  310 . In this example, multiple power generation devices  310  are provided. In this example, the power generation system  410  includes power generation devices  310  and a drive device  66 . The drive device  66  causes the power generation device  310  to track the movement of the sun  61 . Efficient power generation can be carried out by tracking. 
     By using the power generation element according to the embodiment (for example, the power generation element  110 ), high-efficiency power generation can be performed. 
     The electron emitting device according to the embodiment may be used, for example, in a light emitting device, a display, an X-ray source, a magnetron, or a discharge tube (for example, a vacuum discharge tube). 
     According to the embodiment, an electron emitting element and a power generation element which are possible to improve efficiency can be provided. 
     In the specification, “a state of electrically connected” includes a state in which multiple conductors physically contact and current flows between the multiple conductors. “A state of electrically connected” includes a state in which another conductor is inserted between the multiple conductors and current flows between the multiple conductors. 
     Hereinabove, exemplary embodiments of the invention are described with reference to specific examples. However, the embodiments of the invention are not limited to these specific examples. For example, one skilled in the art may similarly practice the invention by appropriately selecting specific configurations of components included in electron emitting elements such as first to third regions, opposing members included in the electron emitting element, etc., from known art. Such practice is included in the scope of the invention to the extent that similar effects thereto are obtained. 
     Further, any two or more components of the specific examples may be combined within the extent of technical feasibility and are included in the scope of the invention to the extent that the purport of the invention is included. 
     Moreover, all electron emitting elements, and power generation elements practicable by an appropriate design modification by one skilled in the art based on the electron emitting elements and power generation elements described above as embodiments of the invention also are within the scope of the invention to the extent that the spirit of the invention is included. 
     Various other variations and modifications can be conceived by those skilled in the art within the spirit of the invention, and it is understood that such variations and modifications are also encompassed within the scope of the invention. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.