Patent Publication Number: US-2023138962-A1

Title: Nitride semiconductor, semiconductor device, and method for manufacturing the nitride semiconductor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-176091, filed on Oct. 28, 2021; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a nitride semiconductor, a semiconductor device, and a method for manufacturing the nitride semiconductor. 
     BACKGROUND 
     For example, there is a semiconductor device using a nitride semiconductor such as GaN. It is desired to improve the characteristics of semiconductor devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a schematic cross-sectional view illustrating a nitride semiconductor according to a first embodiment; 
         FIGS.  2 A and  2 B  are graphs illustrating the nitride semiconductor; 
         FIGS.  3 A and  3 B  are images illustrating the nitride semiconductor; 
         FIGS.  4 A and  4 B  are graphs illustrating characteristics of the nitride semiconductor; 
         FIG.  5    is a schematic cross-sectional view illustrating a nitride semiconductor according to the first embodiment; 
         FIG.  6    is a schematic cross-sectional view illustrating a semiconductor device according to a second embodiment; 
         FIG.  7    is a schematic cross-sectional view illustrating a semiconductor device according to the second embodiment; and 
         FIG.  8    is a flow chart illustrating a method manufacturing a nitride semiconductor according to a third embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     According to one embodiment, a nitride semiconductor includes a base body including boron, a first nitride region including Al x1 Ga 1-x1 N (0.98&lt;x1≤1), and a second nitride region including Al x2 Ga 1-x2 N (0≤x2&lt;1, x2&lt;x1). A concentration of boron in the base body is not less than 1×10 19  cm −3 . The first nitride region is between the base body and the second nitride region. The first nitride region includes a first surface facing the base body and a second surface facing the second nitride region. A second concentration of boron in the second surface is not more than 1/8000 of a first concentration of boron in the first surface. 
     According to one embodiment, a method for manufacturing a nitride semiconductor is disclosed. The method can include forming a part of a first nitride region on a base body including boron at a first temperature. The first nitride region includes Al x1 Ga 1-x1 N (0.98&lt;x1≤1). A concentration of boron in the base body is not less than 1×10 19  cm −3 . The method can include forming an other part of the first nitride region on the part of the first nitride region at a second temperature higher than the first temperature. The method can include forming a second nitride region on the other part of the first nitride region. The second nitride region includes Al x2 Ga 1-x2 N (0≤x2&lt;1, x2&lt;x1). 
     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 a nitride semiconductor according to a first embodiment. 
     As shown in  FIG.  1   , a nitride semiconductor  110  according to the embodiment includes a base body  10   s , a first nitride region  10  and a second nitride region  20 . The first nitride region  10  is between the base body  10   s  and the second nitride region  20 . 
     The base body  10   s  includes boron. A concentration of boron in the base body  10   s  is not less than 1×10 19  cm −3 . The base body  10   s  includes silicon. The base body  10   s  is, for example, a silicon substrate. 
     The first nitride region  10  includes Al x1 Ga 1-x1 N (0.98&lt;x1≤1). The first nitride region  10  includes, for example, AlN. The first nitride region  10  is, for example, an AlN layer. 
     The second nitride region  20  includes Al x2 Ga 1-x2 N (0≤x2&lt;1, x2&lt;x1). The second nitride region  20  includes at least one of an AlGaN layer or a GaN layer. 
     In this example, the second nitride region  20  includes a first nitride layer  11 . The first nitride layer  11  includes Al y1 Ga 1-y1 N (0&lt;y1&lt;1, y1&lt;x1). The first nitride layer  11  is, for example, an AlGaN layer. In one example, a composition ratio y1 is not less than 0.2 and not more than 0.5. For example, the first nitride layer  11  is in contact with the first nitride region  10 . A first direction D 1  from the base body  10   s  to the first nitride region  10  is defined as a Z-axis direction. One direction perpendicular to the Z-axis direction is defined as an X-axis direction. A direction perpendicular to the Z-axis direction and the X-axis direction is defined as a Y-axis direction. 
     The base body  10   s , the first nitride region  10  and the second nitride region  20  are along the X-Y plane. The first nitride region  10  and the second nitride region  20  are layered substantially parallel to the X-Y plane. 
     In this example, the second nitride region  20  includes a second nitride layer  12  and a third nitride layer  13 . The second nitride layer  12  is between the first nitride layer  11  and the third nitride layer  13  in the first direction D 1 . The second nitride layer  12  includes Al y2 Ga 1-y2 N (0≤y2&lt;1). A composition ratio y2 is, for example, not less than 0 and not more than 0.1. The second nitride layer  12  is, for example, a GaN layer. The third nitride layer  13  includes Al y3 Ga 1-y3 N (y2&lt;y3&lt;x1). A composition ratio y3 is, for example, not less than 0.15 and not more than 0.3. The third nitride layer  13  is, for example, an AlGaN layer. 
     The second nitride layer  12  includes a region facing the third nitride layer  13 . A carrier region is formed in this region. The carrier region is, for example, a two-dimensional electron gas. 
     The nitride semiconductor  110  is used as a semiconductor device  120 . The semiconductor device  120  includes the nitride semiconductor  110 . The carrier region is used in the operation of the semiconductor device  120 . The second nitride layer  12  and the third nitride layer  13  are, for example, functional layers. 
     The first nitride layer  11  functions, for example, as a part of a buffer layer. The first nitride region  10  functions, for example, as at least a part of the buffer layer. By providing the buffer layer, warpage is suppressed in the structure including the base body  10   s , the first nitride region  10  and the second nitride region  20 . Good crystallinity is obtained in the functional layer. 
     As described above, the base body  10   s  includes boron. Thereby, for example, in the structure including the base body  10   s , the first nitride region  10  and the second nitride region  20 , the warpage is suppressed more effectively. When the base body  10   s  includes boron, conductivity is generated in the base body  10   s . For example, the influence of static electricity is suppressed. Practical nitride semiconductors and semiconductor devices can be easily obtained. 
     When the base body  10   s  includes boron, the boron may diffuse toward the functional layer. For example, when boron diffuses into the buffer layer including Al and Ga (for example, the first nitride layer  11 ), the buffer layer tends to be non-uniform. It is preferable that the boron included in the base body  10   s  is blocked in the first nitride region  10  and the diffusion of boron is suppressed. 
     As shown in  FIG.  1   , the first nitride region  10  includes a first surface  10   f  and a second surface  10   g . The first surface  10   f  faces the base body  10   s . The second surface  10   g  faces the second nitride region  20 . A concentration of boron on the first surface  10   f  is high due to the influence of boron included in the base body  10   s . On the other hand, a concentration of boron is low on the second surface of  10   g . For example, in the first nitride region  10 , the concentration of boron drops sharply. 
     It was found that the concentration of boron changes depending on the formation conditions of the first nitride region  10 . Hereinafter, an example of the profile of the concentration of boron in the structure including the first nitride region  10  and the second nitride region  20  will be described. 
       FIGS.  2 A and  2 B  are graphs illustrating the nitride semiconductor. 
       FIG.  2 A  corresponds to a sample of a first configuration SPL 1 .  FIG.  2 B  corresponds to a sample of a second configuration SPL 2 . These figures are results of SIMS (Secondary Ion Mass Spectrometry) analysis of the sample. The horizontal axis is a position Pz in the Z-axis direction. The vertical axis on the left side is the boron concentration C(B). The vertical axis on the left side is the Al ion intensity Int (Al). 
     The nitride region is formed by, for example, a MOCVD method or the like using a gas including a raw material including aluminum and a raw material including nitrogen. In the first configuration SPL 1 , the first nitride region  10  is formed by being divided into two portions. As shown in  FIG.  1   , a part of the first nitride region  10  (first portion  10   a ) is formed at a low first temperature. Another part of the first nitride region  10  (second portion  10   b ) is formed at a second temperature higher than the first temperature. In this example, the first temperature is 830° C. The second temperature is 1040° C. A thickness t 10   a  of the first portion  10   a  is about 20 nm. A thickness t 10   b  of the second portion  10   b  is about 180 nm. A thickness t 10  of the first nitride region  10  is about 200 nm. In the second configuration SPL 2 , the entire first nitride region  10  is formed at the second temperature. In these configurations, the first nitride region  10  is AlN. 
     In these samples, the first nitride layer  11  (at least a part of the second nitride region  20 ) is formed on the first nitride region  10 . In this example, the first nitride layer  11  includes Al y1 Ga 1-y1 N (0&lt;y1&lt;1, y1&lt;x1). In this example, the composition ratio y1 is 0.48. 
     As shown in  FIG.  2 A , in the first configuration SPL 1 , the concentration of boron on the first surface  10   f  of the first nitride region  10  (first concentration C 1 ) is 7×10 19  cm −3 . The concentration of boron on the second surface  10   g  of the first nitride region  10  (second concentration C 2 ) is 4×10 15  cm −3 . The concentration of boron in the first nitride layer  11  (third concentration C 3 ) is 6×10 14  cm −3 . For example, the third concentration C 3  is not more than 6×10 14  cm −3  (not more than detection limit). In the first configuration SPL 1 , the boron concentration C(B) drops sharply in the first nitride region  10 . Boron diffusion is effectively suppressed. 
     As shown in  FIG.  2 B , in the second configuration SPL 2 , the concentration of boron on the first surface  10   f  of the first nitride region  10  (first concentration C 1 ) is 7×10 19  cm −3 . The concentration of boron on the second surface  10   g  of the first nitride region  10  (second concentration C 2 ) is 5×10 16  cm −3 . The concentration of boron in the first nitride layer  11  (third concentration C 3 ) is 9×10 14  cm −3 . In the second configuration SPL 2 , the decrease in the boron concentration C(B) is insufficient in the first nitride region  10 . Suppression of the diffusion of boron in the second configuration SPL 2  is insufficient as compared with the first configuration SPL 1 . 
       FIGS.  3 A and  3 B  are images illustrating the nitride semiconductor. 
     These figures are AFM (Atomic Force Microscope) images of the sample.  FIG.  3 A  corresponds to the first configuration SPL 1 .  FIG.  3 B  corresponds to the second configuration SPL 2 . These AFM images are AFM images on the surface of the first nitride region  10 . In these AFM images, the first nitride layer  11  is not formed. 
     As shown in  FIG.  3 B , in the second configuration SPL 2 , multiple dark spots are observed in the image. The multiple dark spots are pits. On the other hand, as shown in  FIG.  3 A , no dark spot is observed in the first configuration SPL 1 . In the first configuration SPL 1 , high flatness is obtained on the surface of the first nitride region  10 . 
     In the first configuration SPL 1 , the first nitride region  10  is formed at two stages of temperature. It is believed that the formation of the first portion  10   a  at the lower first temperature continuously and uniformly covers the surface of the boron-including base body  10   s . Then, formation of the second portion  10   b  at the high second temperature gives AlN with high crystal quality. It is considered that the diffusion of boron is suppressed by continuously and uniformly covering the surface of the base body  10   s  including boron. 
     On the other hand, in the second configuration SPL 2 , the entire first nitride region  10  is formed at the high second temperature. In this case, it is considered that AlN is continuously formed on the surface of the base body  10   s  including boron. For example, the nucleus that is the source of the pit is formed, and the pit is formed. Boron included in the base body  10   s  is considered to diffuse upward through, for example, pits. 
     As described above, there is a difference in the uniformity (for example, pits) of the first nitride region  10  between the first configuration SPL 1  and the second configuration SPL 2 . The difference in uniformity is the difference in the profile of boron. 
     As will be described below, such a difference in the first nitride region  10  causes a difference in leakage current.  FIGS.  4 A and  4 B  are graphs illustrating characteristics of the nitride semiconductor. 
     These figures illustrate electrical characteristics of the sample. In the sample, a first electrode electrically connected to the base body  10   s  is provided. A second electrode electrically connected to the second nitride region  20  is formed on an upper surface of the second nitride region  20 . The current (leakage current) when a voltage is applied to these electrodes is measured. The horizontal axis of  FIGS.  4 A and  4 B  is an applied voltage Vl. The vertical axis is a current density  31  of the leak current.  FIG.  4 A  corresponds to the first configuration SPL 1 .  FIG.  4 B  corresponds to the second configuration SPL 2 . The characteristics of the six measurement samples are illustrated in each of the first configuration SPL 1  and the second configuration SPL 2 . 
     As shown in  FIG.  4 A , in the first configuration SPL 1 , the current density  31  is relatively low and fluctuation is small. As shown in  FIG.  4 B , in the second configuration SPL 2 , the current density  31  is high and fluctuation is large. As described above, the leakage current can be suppressed in the first configuration SPL 1 . 
     It is considered that such a difference in the leakage current is based on the difference in the first nitride region  10  as described above (for example, the presence or absence of a pit). It is considered that such a difference in the leakage current is due to the difference in the profile of boron in the first nitride region  10 . 
     As described with respect to  FIG.  2 A , in the first configuration SPL 1 , the boron concentration C(B) drops sharply in the first nitride region  10 . In this example, the second concentration C 2  is 1/14000 of the first concentration C 1 . In the embodiment, the second concentration C 2  may be not more than 1/10000 of the first concentration C 1 . The second concentration C 2  may be not more than 1/8000 of the first concentration C 1 . In the first nitride region  10  where the boron concentration C(B) drops sharply, for example, the pits can be suppressed and the leakage current can be suppressed. 
     As shown in  FIGS.  2 A and  2 B , in this example, in the first configuration SPL 1  and the second configuration SPL 2 , a boron concentration C 0  in the base body  10   s  is 1.5×10 19  cm −3 . Even at such a high boron concentration C 0 , in the first configuration SPL 1 , the boron concentration C(B) sharply decreases in the first nitride region  10 . This is due to the fact that the diffusion of boron is effectively suppressed in the first configuration SPL 1 . The second concentration C 2  may be not more than 1/2500 of the boron concentration C 0  in the base body  10   s.    
     On the other hand, a reference example in which the boron concentration C 0  in the base body  10   s  is less than 1×10 19  cm −3  can be considered. In the reference example, the second concentration C 2  on the second surface  10   g  may be low. It is considered that this is due to the rate-determining supply of boron because the boron concentration C 0  in the base body  10   s  is low. However, in the reference example in which the boron concentration C 0  in the base body  10   s  is low, it is difficult to obtain desired characteristics (suppression of warpage and appropriate conductivity). 
     In the embodiment, even in the case of the base body  10   s  having the boron concentration C 0  of not less than 1×10 19  cm −3 , the diffusion of boron can be effectively suppressed by the appropriate first nitride region  10 . 
     As shown in  FIG.  2 A , the first concentration C 1  of boron on the first surface  10   f  is higher than the boron concentration C 0  on the base body  10   s . The first surface  10   f  corresponds to the interface between the base body  10   s  and the first nitride region  10 . Boron is likely to be localized at the interface. 
     As shown in  FIG.  2 A , the third concentration C 3  of boron in the second nitride region  20  (in this example, the first nitride layer  11 ) is not more than the second concentration C 2 . In the embodiment, the third concentration C 3  is preferably not more than 1×10 16  cm −3 . In the embodiment, the second concentration C 2  is preferably not more than 8×10 16  cm −3 . In the embodiment, the first concentration C 1  is preferably not less than 5×10 19  cm −3 . As shown in  FIG.  1   , the thickness t 10  of the first nitride region  10  is preferably not less than 100 nm and not more than 250 nm. The thickness t 10  is the thickness of the first nitride region  10  in the first direction D 1  from the base body  10   s  to the first nitride region  10 . 
       FIG.  5    is a schematic cross-sectional view illustrating a nitride semiconductor according to the first embodiment. 
     As shown in  FIG.  5   , in a nitride semiconductor  111  according to the embodiment, the second nitride region  20  includes is a fourth nitride layer  14  in addition to the first nitride layer  11 , the second nitride layer  12 , and the third nitride layer  13 . In this example, the second nitride region  20  further includes a fifth nitride layer  15 . Except for the above, the configuration of the nitride semiconductor  111  may be the same as that of the nitride semiconductor  110 . 
     The fourth nitride layer  14  is between the first nitride layer  11  and the second nitride layer  12 . In this example, the fourth nitride layer  14  is between the first nitride layer  11  and the fifth nitride layer  15 . 
     The fourth nitride layer  14  includes multiple first films  14   a  and multiple second films  14   b . The multiple first films  14   a  include Al z1 Ga 1-z1 N (0≤z1&lt;1). The multiple second films  14   b  include Al z2 Ga 1-z2 N (0&lt;z2≤1, z1&lt;z2). The multiple first films  14   a  are, for example, GaN films or AlGaN films. The multiple second films  14   b  are, for example, AlN films. One of the multiple second films  14   b  is between one of the multiple first films  14   a  and another one of the multiple first films  14   a  in the first direction D 1 . One of the multiple first films  14   a  is between one of the multiple second films  14   b  and another one of the multiple second films  14   b  in the first direction D 1 . For example, the first film  14   a  and the second film  14   b  are alternately provided. In this example, one of the multiple second films  14   b  is in contact with the first nitride layer  11 . In this example, another one of the multiple second films  14   b  is in contact with the fifth nitride layer  15 . One of the multiple first films  14   a  and one of the multiple second films  14   b  may be in contact with the first nitride layer  11 . One of the multiple first films  14   a  and one of the multiple second films  14   b  may be in contact with the fifth nitride layer  15  or the second nitride layer  12 . 
     A thickness t 14   a  of the multiple first films  14   a  in one first direction D 1  is not less than 20 nm or more and not more than 30 nm or less. A thickness t 14   b  of the multiple second films  14   b  in one first direction D 1  is not less than 3 nm and not more than 8 nm. The fourth nitride layer  14  is, for example, a superlattice layer. By providing the fourth nitride layer  14 , for example, high crystallinity can be easily obtained. 
     The fifth nitride layer  15  is provided between the fourth nitride layer  14  and the second nitride layer  12 . The fifth nitride layer  15  includes Al y5 Ga 1-y5 N (0≤y5&lt;1, y5&lt;x1). The fifth nitride layer  15  includes, for example, carbon. The fifth nitride layer  15  is, for example, a GaN layer including carbon. The fifth nitride layer  15 , for example, suppresses dislocations and provides higher crystallinity. 
     Second Embodiment 
     The second embodiment relates to a semiconductor device. As shown in  FIG.  1   , the semiconductor device  120  according to the embodiment includes at least a part of the nitride semiconductor  110  according to the embodiment. As shown in  FIG.  5   , a semiconductor device  121  according to the embodiment includes at least a part of the nitride semiconductor  111  according to the embodiment. As described below, the semiconductor device may include electrodes. 
       FIG.  6    is a schematic cross-sectional view illustrating a semiconductor device according to a second embodiment. 
     As shown in  FIG.  6   , a semiconductor device  122  according to the second embodiment includes a nitride semiconductor  112  according to the first embodiment, first to third electrodes  51  to  53 , and an insulating member  61 . In the nitride semiconductor  112 , the fifth nitride layer  15  is omitted. Similar to the nitride semiconductor  111 , the fifth nitride layer  15  may be provided in the nitride semiconductor  112 . 
     A direction from the first electrode  51  to the second electrode  52  is along a second direction D 2 . The second direction D 2  crosses the first direction D 1 . The second direction D 2  is, for example, the X-axis direction. 
     A position of the third electrode  53  in the second direction D 2  is between a position of the first electrode  51  in the second direction D 2  and a position of the second electrode  52  in the second direction D 2 . 
     The second nitride layer  12  includes a first partial region  12   a , a second partial region  12   b , a third partial region  12   c , a fourth partial region  12   d , and a fifth partial region  12   e . A direction from the first partial region  12   a  to the first electrode  51  is along the first direction D 1 . A direction from the second partial region  12   b  to the second electrode  52  is along the first direction D 1 . A position of the third partial region  12   c  in the second direction D 2  is between a position of the first partial region  12   a  in the second direction D 2  and a position of the second partial region  12   b  in the second direction D 2 . A direction from the third partial region  12   c  to the third electrode  53  is along the first direction D 1 . The fourth partial region  12   d  is between the first partial region  12   a  and the third partial region  12   c  in the second direction D 2 . The fifth partial region  12   e  is between the third partial region  12   c  and the second partial region  12   b  in the second direction D 2 . 
     The third nitride layer  13  includes a sixth partial region  13   f  and a seventh partial region  13   g . A direction from the fourth partial region  12   d  to the sixth partial region  13   f  is along the first direction D 1 . A direction from the fifth partial region  12   e  to the seventh partial region  13   g  is along the first direction D 1 . The insulating member  61  includes a first insulating region  61   p . At least a part of the first insulating region  61   p  is provided between the third partial region  12   c  and the third electrode  53  in the first direction D 1 . 
     A current flowing between the first electrode  51  and the second electrode  52  can be controlled by a potential of the third electrode  53 . The potential of the third electrode  53  may be, for example, a potential based on a potential of the first electrode  51 . The first electrode  51  functions, for example, as a source electrode. The second electrode  52  functions, for example, as a drain electrode. The third electrode  53  functions, for example, as a gate electrode. The first insulating region  61   p  functions, for example, as a gate insulating film. The semiconductor device  122  is, for example, a transistor. 
     As described above, the second nitride layer  12  includes a region facing the third nitride layer  13 . A carrier region (for example, a two-dimensional electron gas) is formed in this region. The semiconductor device  122  is, for example, a HEMT (High Electron Mobility Transistor). 
     In this example, at least a part of the third electrode  53  is between the sixth partial region  13   f  and the seventh partial region  13   g . The semiconductor device  122  is, for example, a normally-off type transistor. 
       FIG.  7    is a schematic cross-sectional view illustrating a semiconductor device according to the second embodiment. 
     As shown in  FIG.  7   , in a semiconductor device  123  according to the embodiment, the third electrode  53  does not overlap the third nitride layer  13  in the second direction D 2 . Except for this, the configuration of the semiconductor device  123  may be the same as the configuration of the semiconductor device  122 . The semiconductor device  123  is, for example, a normally-on type transistor. 
     The first electrode  51  includes, for example, at least one selected from the group consisting of aluminum, titanium, nickel, and gold. The second electrode  52  includes, for example, at least one selected from the group consisting of aluminum, titanium, nickel, and gold. The third electrode  53  includes, for example, at least one selected from the group consisting of TiN, WN, Ni, Au, Pt and Ti. 
     Third Embodiment 
       FIG.  8    is a flow chart illustrating a method manufacturing a nitride semiconductor according to a third embodiment. 
     As shown in  FIG.  8   , a method for manufacturing the nitride semiconductor according to the embodiment includes forming a part of a first nitride region  10  (for example, the first portion  10   a ) including Al x1 Ga 1-x1 N (0.98&lt;x1≤1) on the base body  10   s  including boron at the first temperature (step S 110 ). A concentration of boron (concentration C 0 ) in the base body  10   s  is not less than 1×10 19  cm −3 . 
     The manufacturing method includes forming an other part of the first nitride region  10  (second portion  10   b ) on the part of the first nitride region  10  (first portion  10   a ) at a second temperature (Step S 120 ). The second temperature is higher than the first temperature. 
     The manufacturing method includes forming a second nitride region  20  including Al x2 Ga 1-x2 N (0≤x2&lt;1, x2&lt;x1) on the other part of the first nitride region  10  (step S 130 ). 
     By such a manufacturing method, for example, the profile of boron illustrated in  FIG.  2 A  can be obtained. Even when the target high boron concentration C 0  is applied to the base body  10   s , the diffusion of boron is suppressed. The first nitride region  10  in which the pits are suppressed is obtained. For example, the leakage current is suppressed. 
     In the embodiment, the first temperature is, for example, not less than 800° C. and not more than 900° C. The second temperature is, for example, not less than 1000° C. and not more than 1100° C. The formation of the first nitride region  10  is performed by, for example, a MOCVD method or the like using a gas including a raw material including aluminum (trimethylaluminum: TMAI) and a raw material including nitrogen (ammonia: NH 3 ). 
     For example, the formed first nitride region  10  includes the first surface  10   f  facing the base body  10   s  and the second surface  10   g  facing the second nitride region  20 . The second concentration C 2  of boron on the second surface  10   g  is, for example, not more than 1/8000 of the first concentration C 1  of boron on the first surface  10   f.    
     Information on the concentration or composition of an element can be obtained by, for example, SIMS (Secondary Ion Mass Spectrometry) or EDX (Energy dispersive X-ray spectroscopy). Information on the thickness can be obtained by observing with an electron microscope. 
     According to the embodiment, it is possible to provide a nitride semiconductor, a semiconductor device, and a method for manufacturing the nitride semiconductor, which can improve the characteristics. 
     In the specification, “a state of electrically connected” includes a state in which multiple conductors physically contact and a 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 a 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 nitride semiconductor such as nitride regions, 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 nitride semiconductors, semiconductor devices, and methods for manufacturing nitride semiconductors practicable by an appropriate design modification by one skilled in the art based on the nitride semiconductors, the semiconductor devices, and the methods for manufacturing nitride semiconductors described above as embodiments of the invention also are within the scope of the invention to the extent that the purport 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.