Patent Publication Number: US-8980658-B2

Title: Light-emitting element

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a Divisional of U.S. application Ser. No. 12/372,585, filed Feb. 17, 2009, which claims priority to Japanese Patent Application No. 2008-035661 filed on Feb. 18, 2008. The entire contents of each are incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Field 
     The present specification relates to a light-emitting element and a method for manufacturing the light-emitting element, and particularly to a light-emitting element using quantum dots and a method for manufacturing the light-emitting element. 
     2. Description of the Related Art 
     A semiconductor light-emitting element using a semiconductor island structure (quantum dot) has been known (Japanese Unexamined Patent Application Publication No. 2003-332695). Such the semiconductor light-emitting element has a structure of n-type AlGaAs/n-type GaAs/InGaAs island structure/nitrogen-containing compound semiconductor/p-type GaAs/p-type AlGaAs. 
     The InGaAs island structure has internal stress that comes from compressive stress whereas the nitrogen-containing compound semiconductor has tensile stress. Thus, the internal stress of the InGaAs island structure is reduced by disposing the nitrogen-containing compound semiconductor so as to be in contact with the InGaAs island structure. 
     As a result, the internal stress of the InGaAs island structure, which is a light-emitting layer, is reduced and an emission spectrum of 1.55 μm is achieved at room temperature. 
     However, since a quantum dot itself, which is a light-emitting layer, is not controlled to be p-type or n-type in known semiconductor light-emitting elements, the amount of carriers (electrons and holes) injected into the light-emitting layer is small, which causes a problem in that light-emitting efficiency is low. 
     SUMMARY 
     Representative embodiments described herein provide a light-emitting element in which light-emitting efficiency can be improved. 
     The embodiments also provide a method for manufacturing the light-emitting element in which light-emitting efficiency can be improved. 
     According to a representative embodiment, a light-emitting element includes first and second insulators. The first insulator includes first quantum dots with a first conduction type. The second insulator disposed on the first insulator includes second quantum dots with a second conduction type that is different from the first conduction type. 
     Preferably, the first insulator includes a plurality of the first quantum dots, and the second insulator includes a plurality of the second quantum dots. 
     Preferably, the plurality of first quantum dots are irregularly arranged in a thickness direction of the first insulator, and the plurality of second quantum dots are irregularly arranged in a thickness direction of the second insulator. 
     Preferably, the first conduction type is n-type, and the second conduction type is p-type. 
     Preferably, a barrier energy against holes is larger than a barrier energy against electrons in the first insulator, and a barrier energy against electrons are larger than a barrier energy against holes in the second insulators. 
     Preferably, the first quantum dots and the second quantum dots are composed of silicon dots; the first insulator is composed of a silicon oxide film; and the second insulator is composed of a silicon nitride film. 
     Preferably, the light-emitting element further includes a third insulator formed on the second insulator and including third quantum dots with the second conduction type. The third insulator preferably has a larger barrier energy against electrons than the second insulator. 
     Preferably, the third insulator includes a plurality of the third quantum dots. 
     Preferably, the plurality of third quantum dots are irregularly arranged in a thickness direction of the third insulator. 
     Preferably, the at least one first quantum dot, the second quantum dots, and the third quantum dots are composed of silicon dots; the first insulator is composed of a silicon oxide film; the second insulator is composed of a silicon nitride film; and the third insulator is composed of a silicon oxynitride film. 
     According to a representative embodiment, a light-emitting element includes a light-emitting layer, a first insulator, and a second insulator. The first insulator supplies electrons to the light-emitting layer through n-type quantum dots. The second insulator supplies holes to the light-emitting layer through p-type quantum dots. 
     Preferably, the first insulator is composed of a silicon oxide film and the second insulator is composed of a silicon nitride film. 
     According to a representative embodiment, a method for manufacturing a light-emitting element includes a first step of depositing a first insulator including quantum dots on a principal surface of a semiconductor substrate; a second step of depositing a second insulator including quantum dots on the first insulator; a third step of introducing an impurity of a first conduction type into the first insulator; a fourth step of introducing an impurity of a second conduction type that is different from the first conduction type into the second insulator; and a fifth step of heat-treating the first insulator including the impurity of the first conduction type and the second insulator including the impurity of the second conduction type. 
     Preferably, in the first step, the first insulator that is composed of a silicon oxide film is deposited on the principal surface by adjusting a flow rate ratio of a second material gas including silicon relative to a first material gas including oxygen, to higher than or equal to a first standard flow rate ratio. Preferably, in the second step, the second insulator that is composed of a silicon nitride film is deposited on the first insulator by adjusting a flow rate ratio of the second material gas relative to a third material gas including nitrogen, to higher than or equal to a second standard flow rate ratio. 
     Preferably, an n-type impurity is introduced into the first insulator in the third step, and a p-type impurity is introduced into the second insulator in the fourth step. 
     Preferably, in the fifth step, the first insulator including the n-type impurity and the second insulator including the p-type impurity are heat-treated in a nitrogen atmosphere. 
     According to a representative embodiment, a method for manufacturing a light-emitting element includes a first step of depositing a first insulator including quantum dots on a principal surface of a semiconductor substrate; a second step of depositing a second insulator including quantum dots on the first insulator; a third step of depositing a third insulator including quantum dots on the second insulator, the third insulator having a larger barrier energy against electrons than the second insulator; a fourth step of introducing an impurity of a first conduction type into the first insulator; a fifth step of introducing an impurity of a second conduction type that is different from the first conduction type into the second and third insulators; and a sixth step of heat-treating the first insulator including the impurity of the first conduction type and the second and third insulators including the impurity of the second conduction type. 
     In the light-emitting element of the present invention, one of electrons and holes are supplied to the boundary between the first insulator and the second insulator through one of quantum dots in the first insulator and quantum dots in the second insulator, and the other one of electrons and holes are supplied to the boundary between the first insulator and the second insulator through the other one of quantum dots in the first insulator and quantum dots in the second insulator. The electrons and holes supplied to the boundary between the first insulator and the second insulator recombine with each other to emit light. In other words, the light-emitting element of the present invention emits light by supplying both the electrons and holes to the boundary between the first insulator and the second insulator. 
     Thus, light-emitting efficiency can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a light-emitting element according to an embodiment. 
         FIG. 2  is an enlarged sectional view of an n-type silicon oxide film and a p-type silicon nitride film shown in  FIG. 1 . 
         FIG. 3  is an energy band diagram, at zero bias, of the light-emitting element shown in  FIG. 1 . 
         FIG. 4  is an energy band diagram of the light-emitting element shown in  FIG. 1  when an electric current is applied. 
         FIG. 5  is a schematic view of a plasma chemical vapor deposition (plasma CVD) apparatus used for manufacturing the light-emitting element shown in  FIG. 1 . 
         FIG. 6  is a first process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 1 . 
         FIG. 7  is a second process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 1 . 
         FIG. 8  is a graph showing light-emitting characteristics of the light-emitting element shown in  FIG. 1 . 
         FIGS. 9A and 9B  are sectional views of light-emitting elements, which are comparative examples of the light-emitting element shown in  FIG. 1 . 
         FIG. 10  is a sectional view of another light-emitting element according to an embodiment of the present invention. 
         FIG. 11  is a first process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 10 . 
         FIG. 12  is a second process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 10 . 
         FIG. 13  is a third process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 10 . 
         FIG. 14  is a fourth process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 10 . 
         FIG. 15  is a sectional view of still another light-emitting element according to an embodiment of the present invention. 
         FIG. 16  is an enlarged sectional view of a p-type silicon oxynitride film shown in  FIG. 15 . 
         FIG. 17  is an energy band diagram, at zero bias, of the light-emitting element shown in  FIG. 15 . 
         FIG. 18  is an energy band diagram of the light-emitting element shown in  FIG. 15  when an electric current is applied. 
         FIG. 19  is a first process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 15 . 
         FIG. 20  is a second process chart for describing a method for manufacturing the light-emitting element shown in  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS 
     Representative embodiments are described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and the descriptions are not repeated. 
       FIG. 1  is a sectional view of a light-emitting element according to an embodiment of the present invention. Referring to  FIG. 1 , a light-emitting element  10  according to the embodiment of the present invention includes a substrate  1 , an n-type silicon oxide film  2 , a p-type silicon nitride film  3 , p + -type polysilicon (p +  poly-Si) films  4 , electrodes  5 , and an electrode  6 . 
     The substrate  1  is made of n + -type silicon (n +  Si) with a resistivity of about 0.1 Ω·cm. The n-type silicon oxide film  2 , as described below, includes a plurality of quantum dots made of n-type Si and is formed on one principal surface of the substrate  1 . The n-type silicon oxide film  2  has a film thickness of about 150 nm. 
     The p-type silicon nitride film  3 , as described below, includes a plurality of quantum dots made of p-type Si and is formed on the n-type silicon oxide film  2 . The p-type silicon nitride film  3  has a film thickness of about 100 nm. 
     The p +  poly-Si films  4  are constituted by p +  poly-Si films  41  to  44  and formed on the p-type silicon nitride film  3 . The p +  poly-Si films  4  have a boron (B) concentration of about 10 20  cm −3  and a film thickness of about 50 nm. 
     The electrodes  5  are constituted by electrodes  51  to  54 , which are formed on the p +  poly-Si films  41  to  44 , respectively. Each of the electrodes  51  to  54  is made of aluminum (Al). 
     The electrode  6  is made of Al and is formed on the rear surface of the substrate  1  (a surface opposite the surface in which the n-type silicon oxide film  2 , etc. are formed). 
       FIG. 2  is an enlarged sectional view of the n-type silicon oxide film  2  and the p-type silicon nitride film  3  shown in  FIG. 1 . Referring to  FIG. 2 , the n-type silicon oxide film  2  includes a plurality of quantum dots  21 , each of which is composed of an n-type Si dot and has a phosphorus (P) concentration of about 10 19  cm −3 . The plurality of quantum dots  21  are irregularly arranged in the n-type silicon oxide film  2 . 
     The p-type silicon nitride film  3  includes a plurality of quantum dots  31 , each of which is composed of a p-type Si dot and has a B concentration of about 10 19  cm −3 . The plurality of quantum dots  31  are irregularly arranged in the p-type silicon nitride film  3 . 
     As described above, the n-type silicon oxide film  2  and the p-type silicon nitride film  3  include the quantum dots  21  each composed of an n-type Si dot and the quantum dots  31  each composed of a p-type Si dot, respectively, and form a p-n junction. 
       FIG. 3  is an energy band diagram, at zero bias, of the light-emitting element  10  shown in  FIG. 1 . Referring to  FIG. 3 , a conduction band Ec 1  and a valence band Ev 1  are present in the substrate  1  made of n +  Si, and n +  Si has an energy band gap Eg 1  of 1.12 eV. 
     A conduction band Ec 2  and a valence band Ev 2  are present in the p +  poly-Si film  4 , and the p +  poly-Si film  4  has an energy band gap Eg 1  of 1.12 eV. 
     Since the substrate  1  made of n +  Si is doped with high-concentration P and the p +  poly-Si film  4  is doped with high-concentration B, the energy level of the conduction band Ec 1  edge of n +  Si is close to that of the valence band Ev 2  edge of the p +  poly-Si film  4 . 
     Since the n-type silicon oxide film  2  includes the plurality of quantum dots  21  as described above, it has a layered structure of the quantum dots  21  and silicon dioxide (SiO 2 ) layers  22  not including the quantum dots  21 . Thus, each of the quantum dots  21  is sandwiched by the SiO 2  layers  22 . 
     The SiO 2  layers  22  have an energy band gap of about 9 eV. Each of the quantum dots  21  sandwiched by two of the SiO 2  layers  22  has a sub-level L sub   1  on the conduction band Ed side of n +  Si and a sub-level L sub   2  on the valence band Ev 1  side of n +  Si due to a quantum size effect. 
     The sub-level L sub   1  is higher than the energy level of the conduction band Ec 1  of n +  Si and the sub-level L sub   2  is higher than the energy level of the valence band Ev 1  edge of n +  Si. As a result, the energy difference between the sub-level L sub   1  and the sub-level L sub   2  is larger than the energy band gap Eg 1  of n +  Si. 
     The energy difference ΔE 1  between the conduction band Ec 1  edge of n +  Si and the conduction band edge of the SiO 2  layers  22  is about 3.23 eV, and the energy difference ΔE 2  between the valence band Ev 1  edge of n +  Si and the valence band edge of the SiO 2  layers  22  is about 4.65 eV. Accordingly, the n-type silicon oxide film  2  has a barrier energy (=ΔE 1 ) against electrons in n +  Si that is smaller than a barrier energy (=ΔE 2 ) against holes in n +  Si. 
     Since the p-type silicon nitride film  3  includes the plurality of quantum dots  31  as described above, it has a layered structure of the quantum dots  31  and silicon nitride (Si 3 N 4 ) layers  32  not including the quantum dots  31 . Thus, each of the quantum dots  31  is sandwiched by the Si 3 N 4  layers  32 . 
     The Si 3 N 4  layers  32  have an energy band gap of about 5.2 eV. Each of the quantum dots  31  sandwiched by two of the Si 3 N 4  layers  32  has a sub-level L sub   3  on the conduction band Ec 2  side of the p +  poly-Si film  4  and a sub-level L sub   4  on the valence band Ev 2  side of the p +  poly-Si film  4  due to a quantum size effect. 
     The sub-level L sub   3  is higher than the energy level of the conduction band Ec 2  edge of the p +  poly-Si film  4  and the sub-level L sub   4  is higher than the energy level of the valence band Ev 2  edge of the p +  poly-Si film  4 . As a result, the energy difference between the sub-level L sub   3  and the sub-level L sub   4  is larger than the energy band gap Eg 1  of the p +  poly-Si film  4 . 
     The energy difference ΔE 3  between the conduction band Ec 2  edge of the p +  poly-Si film  4  and the conduction band edge of the Si 3 N 4  layers  32  is about 2.3 eV, and the energy difference ΔE 4  between the valence band Ev 2  edge of the p +  poly-Si film  4  and the valence band edge of the Si 3 N 4  layers  32  is about 1.78 eV. Accordingly, the p-type silicon nitride film  3  has a barrier energy (=ΔE 4 ) against holes in the p +  poly-Si film  4  that is smaller than a barrier energy (=ΔE 3 ) against electrons in the p +  poly-Si film  4 . 
       FIG. 4  is an energy band diagram of the light-emitting element  10  shown in  FIG. 1  when an electric current is applied. When a voltage is applied between the electrode  5  and the electrode  6 , assuming that the electrode  5  side is positive and the electrode  6  side is negative, the energy band of n +  Si constituting the substrate  1  is raised. Consequently, electrons  11  in n +  Si flow in the n-type silicon oxide film  2  through the plurality of quantum dots  21  included in the n-type silicon oxide film  2 , and are injected into a quantum dot  31 N positioned closest to the boundary between the n-type silicon oxide film  2  and the p-type silicon nitride film  3 . 
     In contrast, holes  12  in the p +  poly-Si film  4  flow in the p-type silicon nitride film  3  through the quantum dots  31  included in the p-type silicon nitride film  3 , and are stored in the quantum dot  31 N positioned closest to the boundary between the n-type silicon oxide film  2  and the p-type silicon nitride film  3 , because a silicon oxide film functions as a high barrier against holes. 
     Then, electrons  13  stored in the quantum dot  31 N and holes  14  stored in the quantum dot  31 N recombine with each other to emit light. 
     In the light-emitting element  10 , as described above, holes are stored in the boundary between the n-type silicon oxide film  2  and the p-type silicon nitride film  3  because a silicon oxide film functions as a high barrier against holes. As a result, light emission occurs from the boundary between the n-type silicon oxide film  2  and the p-type silicon nitride film  3  in the light-emitting element  10 . 
     In the light-emitting element  10 , the plurality of quantum dots  21  in the n-type silicon oxide film  2  are doped to be n-type and the plurality of quantum dots  31  in the p-type silicon nitride film  3  are doped to be p-type. Therefore, the sub-level L sub   1  of the plurality of quantum dots  21  decreases compared with the case where they are not doped to be n-type, while the sub-level L sub   4  of the plurality of quantum dots  31  decreases compared with the case where they are not doped to be p-type. Consequently, electrons in n +  Si easily flow in the n-type silicon oxide film  2  compared with the case where the plurality of quantum dots  21  are not doped to be n-type, such that more electrons are stored in the quantum dot  31 N. Similarly, holes in the p +  poly-Si film  4  easily flow in the p-type silicon nitride film  3  compared with the case where the plurality of quantum dots  31  are not doped to be p-type, such that more holes are stored in the quantum dot  31 N. 
     Thus, light-emitting efficiency can be improved. 
     Furthermore, the n-type silicon oxide film  2  irregularly includes the plurality of quantum dots  21  and the p-type silicon nitride film  3  irregularly includes the plurality of quantum dots  31 , whereby the injection efficiency of electrons and holes is improved due to the electric-field enhancement effect caused by the irregularly arranged quantum dots  21  and  31 . 
     Thus, light-emitting efficiency can be improved. 
       FIG. 5  is a schematic view of a plasma chemical vapor deposition (plasma CVD) apparatus used for manufacturing the light-emitting element  10  shown in  FIG. 1 . Referring to  FIG. 5 , a plasma CVD apparatus  100  includes a reaction chamber  101 , an electrode plate  102 , a sample holder  103 , a heater  104 , a radio frequency (RF) power supply  105 , pipes  106  to  108 , and gas cylinders  109  to  111 . 
     The reaction chamber  101  is a hollow container and has an outlet  101 A. The electrode plate  102  and the sample holder  103  each having a diameter of 200 mmφ are plate-shaped and disposed in the reaction chamber  101  so as to be spaced 50 mm apart and substantially parallel. The heater  104  is disposed in the sample holder  103 . 
     The RF power supply  105  is connected to the electrode plate  102  and the sample holder  103 . The pipe  106  has one end connected to the reaction chamber  101  and the other end connected to the gas cylinder  109 . The pipe  107  has one end connected to the reaction chamber  101  and the other end connected to the gas cylinder  110 . The pipe  108  has one end connected to the reaction chamber  101  and the other end connected to the gas cylinder  111 . 
     The sample holder  103  holds a substrate  1 . The heater  104  heats the substrate  1  to a desired temperature. The RF power supply  105  applies an RF power of 13.56 MHz between the electrode plate  102  and the sample holder  103 . 
     The gas cylinders  109 ,  110 , and  111  hold N 2 O (100%) gas, 10% SiH 4  gas diluted with hydrogen (H 2 ) gas, and NH 3  (100%) gas, respectively. 
     The N 2 O gas, the SiH 4  gas, and the NH 3  gas are supplied to the reaction chamber  101  through the pipes  106 ,  107 , and  108 , respectively. The N 2 O gas, the SiH 4  gas, and the NH 3  gas supplied to the reaction chamber  101  are exhausted through the outlet  101 A using an exhaust device (not shown) such as a rotary pump. Thus, a determined pressure is achieved in the reaction chamber  101 . 
     In the plasma CVD apparatus  100 , a silicon oxide film is deposited on the substrate  1  by applying the RF power between the electrode plate  102  and the sample holder  103  using the RF power supply  105  while the N 2 O gas and the SiH 4  gas are supplied to the reaction chamber  101 . Alternatively, a silicon nitride film is deposited on the substrate  1  by applying the RF power between the electrode plate  102  and the sample holder  103  using the RF power supply  105  while the NH 3  gas and the SiH 4  gas are supplied to the reaction chamber  101 . 
       FIGS. 6 and 7  are respectively a first process chart and a second process chart for describing a method for manufacturing the light-emitting element  10  shown in  FIG. 1 . Referring to  FIG. 6 , in the manufacturing of the light-emitting element  10 , the substrate  1  made of n +  Si is prepared (see process (a)), cleaned, and placed on the sample holder  103  of the plasma CVD apparatus  100 . 
     A silicon oxide film  11  is deposited on one principal surface of the substrate  1  under the reaction conditions shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
            
               
                   
                 Flow rate of SiH 4  (10%, diluted with H 2 ) 
                 86 
                 sccm 
               
               
                   
                 Flow rate of N 2 O (100%) 
                 34 
                 sccm 
               
               
                   
                 Pressure 
                 133 
                 Pa 
               
               
                   
                 RF power 
                 0.32 
                 W/cm 2   
               
               
                   
                 Substrate temperature 
                 300° 
                 C. 
               
               
                   
                 Reaction time 
                 3 
                 minutes 
               
               
                   
                   
               
            
           
         
       
     
     A silicon nitride film  12  is then deposited on the silicon oxide film  11  under the reaction conditions shown in Table 2. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
             
            
               
                   
                 Flow rate of SiH 4  (10%, diluted with H 2 ) 
                 92 
                 sccm 
               
               
                   
                 Flow rate of NH 3  (100%) 
                 28 
                 sccm 
               
               
                   
                 Pressure 
                 133 
                 Pa 
               
               
                   
                 RF power 
                 0.32 
                 W/cm 2   
               
               
                   
                 Substrate temperature 
                 300° 
                 C. 
               
               
                   
                 Reaction time 
                 4 
                 minutes 
               
               
                   
                   
               
            
           
         
       
     
     Subsequently, an amorphous silicon (a-Si) film  13  is deposited on the silicon nitride film  12  using the reaction conditions shown in Table 2 under which the NH 3  gas is stopped (see process (b) in  FIG. 6 ). 
     Phosphorus ions (P + ) are then injected into the silicon oxide film  11  by ion implantation (see process (c) in  FIG. 6 ). In this case, the acceleration voltage of ion implantation is adjusted such that the P +  ions are injected into only the silicon oxide film  11 . Thus, an n-type silicon oxide film  2  is formed (see process (d) in  FIG. 6 ). 
     Boron ions (B + ) are then injected into the silicon nitride film  12  and the a-Si film  13  by ion implantation (see process (d) in  FIG. 6 ). In this case, the acceleration voltage of ion implantation is adjusted such that the B +  ions are injected into only the silicon nitride film  12  and the a-Si film  13 . Thus, a p-type silicon nitride film  3  and a p-type a-Si film  13 A are formed (see process (e) in  FIG. 7 ). 
     The resultant structure including the substrate  1 /n-type silicon oxide film  2 /p-type silicon nitride film  3 /p-type a-Si film  13 A is annealed under the conditions shown in Table 3. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
             
            
               
                   
                 Annealing temperature 
                 1000° C. 
               
               
                   
                 Annealing atmosphere 
                 nitrogen atmosphere 
               
               
                   
                 Pressure 
                 atmospheric pressure 
               
               
                   
                 Annealing time 
                 1 hour 
               
               
                   
                   
               
            
           
         
       
     
     As a result, P atoms injected into the n-type silicon oxide film  2  and B atoms injected into the p-type silicon nitride film  3  are electrically activated. Furthermore, the p-type a-Si film  13 A is converted to a p +  poly-Si film  4  (see process (f) in  FIG. 7 ). 
     The p +  poly-Si film  4  is patterned to p +  poly-Si films  41  to  44  by photolithography (see process (g) in  FIG. 7 ). 
     After that, the electrodes  5  ( 51  to  54 ) are formed on the p +  poly-Si films  41  to  44  by sputtering Al. The electrode  6  is formed on the rear surface of the substrate  1  by sputtering Al (see process (h) in  FIG. 7 ). Thus, the light-emitting element  10  is completed. 
     As described above, the silicon oxide film  11  including quantum dots is formed by using the reaction conditions shown in Table 1 while the silicon nitride film  12  including quantum dots is formed by using the reaction conditions shown in Table 2. Accordingly, the silicon oxide film  11  including quantum dots or the silicon nitride film  12  including quantum dots can be formed in a single film formation. 
     The flow rate ratio of the SiH 4  gas relative to the N 2 O gas used in the conditions shown in Table 1 under which the silicon oxide film  11  is formed is higher than the flow rate ratio (standard flow rate ratio) of the SiH 4  gas relative to the N 2 O gas used to form a SiO 2  film as an insulating film. In other words, the silicon oxide film  11  is formed with a flow rate of SiH 4  gas higher than that of the standard in the present invention; therefore, it is called a silicon-rich oxide film. 
     The flow rate ratio of the SiH 4  gas relative to the NH 3  gas used in the conditions shown in Table 2 under which the silicon nitride film  12  is formed is higher than the flow rate ratio (standard flow rate ratio) of the SiH 4  gas relative to the NH 3  gas used to form a Si 3 N 4  film as an insulating film. In other words, the silicon nitride film  12  is formed with a flow rate of SiH 4  gas higher than that of the standard; therefore, it is called a silicon-rich nitride film. 
     Accordingly, the silicon oxide film  11  including quantum dots composed of Si dots is formed using the conditions under which the silicon-rich oxide film is formed, whereas the silicon nitride film  12  including quantum dots composed of Si dots is formed using the conditions under which the silicon-rich nitride film is formed. 
     The density of quantum dots  21  included in the n-type silicon oxide film  2  and quantum dots  31  included in the p-type silicon nitride film  3  can be increased by increasing the flow rate ratio of the SiH 4  gas relative to the N 2 O gas and the flow rate ratio of the SiH 4  gas relative to the NH 3  gas, respectively, and by shortening the heat treatment time in the process (e) of  FIG. 7  to about a few seconds. 
     The density of quantum dots  21  included in the n-type silicon oxide film  2  and quantum dots  31  included in the p-type silicon nitride film  3  can be decreased by reducing the flow rate ratio of the SiH 4  gas relative to the N 2 O gas and the flow rate ratio of the SiH 4  gas relative to the NH 3  gas, respectively, and by lengthening the heat treatment time in the process (e) of  FIG. 7  to several tens of minutes or more. 
     As described above, the density of the quantum dots  21  included in the n-type silicon oxide film  2  and the quantum dots  31  included in the p-type silicon nitride film  3  can be controlled with the flow rate ratio of the SiH 4  gas relative to the N 2 O gas and the NH 3  gas and the heat treatment time in the process (e) of  FIG. 7 . 
     In the method for manufacturing the light-emitting element  10  shown in  FIGS. 6 and 7 , it has been described that after the silicon oxide film  11  including quantum dots and the silicon nitride film  12  including quantum dots are formed by plasma CVD, the P +  ions and B +  ions are respectively injected into the silicon oxide film  11  and the silicon nitride film  12  by ion implantation to form the n-type silicon oxide film  2  and the p-type silicon nitride film  3 . However, the possible embodiments are not limited to this method. The n-type silicon oxide film  2  and the p-type silicon nitride film  3  may be formed by plasma CVD. 
     In this case, the n-type silicon oxide film  2  is formed by plasma CVD using PH 3  gas as a source gas of P whereas the p-type silicon nitride film  3  is formed by plasma CVD using B 2 H 6  gas as a source gas of B. 
     Reaction conditions under which the n-type silicon oxide film  2  is formed are specified by adding a flow rate of the PH 3  gas to the reaction conditions shown in Table 1. Reaction conditions under which the p-type silicon nitride film  3  is formed are specified by adding a flow rate of the B 2 H 6  gas to the reaction conditions shown in Table 2. 
     Furthermore, although it has been described that the n-type silicon oxide film  2  is formed using P in the above description, the present invention is not limited to this. The n-type silicon oxide film  2  may be formed using arsenic (As). In this case, As ions are injected into only the silicon oxide film  11  by ion implantation in the process (c) of  FIG. 6 . If the n-type silicon oxide film  2  is formed by plasma CVD using As, AsH 3  gas is used as a source gas of As. 
       FIG. 8  is a graph showing light-emitting characteristics of the light-emitting element  10  shown in  FIG. 1 .  FIGS. 9A and 9B  are sectional views of light-emitting elements, which are comparative examples of the light-emitting element  10  shown in  FIG. 1 . Referring to  FIG. 9A , a light-emitting element  200  is the same as the light-emitting element  10  shown in  FIG. 1  except that the n-type silicon oxide film  2  of the light-emitting element  10  is removed. Referring to  FIG. 9B , a light-emitting element  210  is the same as the light-emitting element  10  except that the p-type silicon nitride film  3  of the light-emitting element  10  is removed. That is to say, the light-emitting element  200  uses only the p-type silicon nitride film  3  as a light-emitting layer whereas the light-emitting element  210  uses only the n-type silicon oxide film  2  as a light-emitting layer. 
       FIG. 8  shows light intensity per unit current and unit film thickness as a function of wavelength. A solid curve k 1 , a dotted curve k 2 , and a chain curve k 3  denote light-emitting intensities of the light-emitting element  10  shown in  FIG. 1 , the light-emitting element  200  shown in  FIG. 9A , and the light-emitting element  210  shown in  FIG. 9B , respectively. When the light intensity of the light-emitting element  10  is normalized in terms of film thickness, the film thickness of the p-type silicon nitride film  3  was used, but that of the n-type silicon oxide film  2  was not. This is because 95% of light emission in the light-emitting element  10  occurs from the p-type silicon nitride film  3 . 
     In  FIG. 8 , SRO in the vertical axis represents a silicon-rich oxide film formed under the silicon-rich reaction conditions shown in Table 1. SRN represents a silicon-rich nitride film formed under the silicon-rich reaction conditions shown in Table 2. The curves k 1 , k 2 , and k 3  show light-emitting intensities of the light-emitting elements  10 ,  200 , and  210 , respectively, when 20 V is applied between the electrodes  5  and the electrode  6 . 
     As evident from  FIG. 8 , the light-emitting intensity of the light-emitting element  10  is 33% higher than that of the light-emitting element  200  and ten or more times higher than that of the light-emitting element  210 . This is because, as described above, the light-emitting element  10  has a structure including a junction between the n-type silicon oxide film  2  and the p-type silicon nitride film  3 , which respectively supply electrons and holes to a light-emitting layer through the n-type silicon oxide film  2  and the p-type silicon nitride film  3 . 
     In contrast, either electrons or holes are supplied to a light-emitting layer in the light-emitting element  200  or  210 , which produces lower light-emitting intensity than that of the light-emitting element  10 . 
     Therefore, it is experimentally demonstrated that a structure including a junction between the n-type silicon oxide film  2  and the p-type silicon nitride film  3  produces higher light-emitting intensity. 
       FIG. 10  is a sectional view of another light-emitting element according to an embodiment of the present invention. A light-emitting element of the present invention may be a light-emitting element  10 A shown in  FIG. 10 . Referring to  FIG. 10 , the light-emitting element  10 A is the same as the light-emitting element  10  shown in  FIG. 1  except that the n-type silicon oxide film  2  of the light-emitting element  10  is replaced with a silicon oxide film  60  and the p-type silicon nitride film  3  is replaced with a silicon nitride film  70 . 
     The silicon oxide film  60  is formed on the substrate  1 , and the silicon nitride film  70  is formed on the silicon oxide film  60 . 
     The silicon oxide film  60  includes a plurality of SiO 2  films  61  and a plurality of n-type silicon oxide films  62 . The plurality of SiO 2  films  61  and the plurality of n-type silicon oxide films  62  are alternately stacked in a thickness direction of the light-emitting element  10 A. Each of the plurality of n-type silicon oxide films  62  includes a plurality of n-type Si dots  63  irregularly arranged in a film thickness direction thereof. Each of the plurality of SiO 2  films  61  has a film thickness of 1 to 5 nm whereas each of the plurality of n-type silicon oxide films  62  has a film thickness of 3 to 10 nm. 
     The silicon nitride film  70  includes a plurality of Si 3 N 4  films  71  and a plurality of p-type silicon nitride films  72 . The plurality of Si 3 N 4  films  71  and the plurality of p-type silicon nitride films  72  are alternately stacked in a thickness direction of the light-emitting element  10 A. Each of the plurality of p-type silicon nitride films  72  includes a plurality of p-type Si dots  73  irregularly arranged in a film thickness direction thereof. Each of the plurality of Si 3 N 4  films  71  has a film thickness of 1 to 5 nm whereas each of the plurality of p-type silicon nitride films  72  has a film thickness of 3 to 10 nm. 
     Each of the plurality of n-type Si dots  63  includes P concentration that is substantially the same as the P concentration in each of the quantum dots  21 . Each of the plurality of p-type Si dots  73  includes B concentration that is substantially the same as the B concentration in each of the quantum dots  31 . 
     As described above, the light-emitting element  10 A has a structure in which the SiO 2  films  61  not including dopants sandwich each of the n-type silicon oxide films  62  and the Si 3 N 4  films  71  not including dopants sandwich each of the p-type silicon nitride films  72 . Accordingly, a light-emitting element may have a structure in which insulators (SiO 2  films  61  or Si 3 N 4  films  71 ) not including dopants sandwich quantum dots (n-type Si dots  63  or p-type Si dots  73 ). 
     Next, a method for manufacturing the light-emitting element  10 A is described.  FIGS. 11 to 14  are respectively first, second, third, and fourth process charts for describing a method for manufacturing the light-emitting element  10 A shown in  FIG. 10 . Referring to  FIG. 11 , in the manufacturing of the light-emitting element  10 A, a substrate  1  is prepared (see process (a) in  FIG. 11 ), cleaned, and a SiO 2  film  61  is formed on one principal surface of the substrate  1  by plasma CVD using SiH 4  gas and N 2 O gas as a raw material (see process (b) in  FIG. 11 ). In this case, the SiO 2  film  61  is formed under the reaction conditions shown in Table 1 with a SiH 4  gas flow rate of 86 sccm and a N 2 O gas flow rate of 200 sccm. 
     Subsequently, a silicon oxide film  80  is deposited on the SiO 2  film  61  by plasma CVD using the SiH 4  gas and the N 2 O gas as a raw material under the reaction conditions shown in Table 1 (see process (c) in  FIG. 11 ). 
     A plurality of SiO 2  films  61  and a plurality of silicon oxide films  80  are alternately formed on the substrate  1  by repeating the processes (b) and (c) in  FIG. 11  (see process (d) in  FIG. 11 ). 
     After that, a silicon nitride film  90  is deposited on the top layer of the SiO 2  films  61  by plasma CVD using the SiH 4  gas and NH 3  gas as a raw material under the reaction conditions shown in Table 2 (see process (e) in  FIG. 11 ). 
     A Si 3 N 4  film  71  is deposited on the silicon nitride film  90  by plasma CVD using the SiH 4  gas and the NH 3  gas as a raw material (see process (f) in  FIG. 12 ). In this case, the Si 3 N 4  film  71  is formed under the reaction conditions shown in Table 2 with a SiH 4  gas flow rate of 92 sccm and a NH 3  gas flow rate of 150 sccm. A plurality of Si 3 N 4  films  71  and a plurality of silicon nitride films  90  are then alternately formed on the top layer of the SiO 2  films  61  by repeating the processes (e) and (f) in  FIGS. 11 and 12 . An a-Si film  13  is deposited on the top layer of Si 3 N 4  film  71  using the reaction conditions shown in Table 2 under which the NH 3  gas is stopped (see process (g) in  FIG. 12 ). 
     P +  ions are then injected into the plurality of silicon oxide films  80  by ion implantation (see process (h) in  FIG. 12 ). In this case, the acceleration voltage of ion implantation is adjusted such that the P +  ions are injected into only the plurality of silicon oxide films  80 . Thus, a plurality of n-type silicon oxide films  62  are formed (see process (i) in  FIG. 13 ). 
     B +  ions are then injected into the plurality of silicon nitride films  90  and the a-Si film  13  by ion implantation (see process (i) in  FIG. 13 ). In this case, the acceleration voltage of ion implantation is adjusted such that the B +  ions are injected into only the plurality of silicon nitride films  90  and the a-Si film  13 . Thus, the plurality of p-type silicon nitride films  72  and the p-type a-Si film  13 A are formed (see process (j) in  FIG. 13 ). 
     The resultant structure including the substrate  1 /SiO 2  film  61 /n-type silicon oxide film  62 / . . . /SiO 2  film  61 /p-type silicon nitride film  72 /Si 3 N 4  film  71 / . . . /Si 3 N 4  film  71 /p-type a-Si film  13 A is annealed under the conditions shown in Table 3. 
     As a result, P atoms injected into the n-type silicon oxide films  62  and B atoms injected into the p-type silicon nitride films  72  are electrically activated. Furthermore, the p-type a-Si film  13 A is converted to the p +  poly-Si film  4  (see process (k) in  FIG. 13 ). 
     The p +  poly-Si film  4  is patterned to p +  poly-Si films  41  to  44  by photolithography (see process (I) in  FIG. 14 ). 
     After that, the electrodes  5  ( 51  to  54 ) are formed on the p +  poly-Si films  41  to  44  by sputtering Al. The electrode  6  is formed on the rear surface of the substrate  1  by sputtering Al (see process (m) in  FIG. 14 ). Thus, the light-emitting element  10 A is completed. 
     An energy band diagram of the light-emitting element  10 A shown in  FIG. 10  at zero bias is the one shown in  FIG. 3 . An energy band diagram of the light-emitting element  10 A when an electric current is applied is the one shown in  FIG. 4 . Accordingly, the light-emitting element  10 A emits light through the same mechanism as the light-emitting element  10  described above. 
     Therefore, light-emitting efficiency can also be improved in the light-emitting element  10 A. 
       FIG. 15  is a sectional view of still another light-emitting element according to an embodiment of the present invention. A light-emitting element according to an embodiment may be a light-emitting element  10 B shown in  FIG. 15 . Referring to  FIG. 15 , the light-emitting element  10 B is the same as the light-emitting element  10  shown in  FIG. 1  except that the p-type silicon nitride film  3  of the light-emitting element  10  is replaced with a p-type silicon nitride film  30  and a p-type silicon oxynitride film  8  is further added. 
     The p-type silicon nitride film  30  with a film thickness of about 10 nm has the same composition as the p-type silicon nitride film  3 . 
     The p-type silicon oxynitride film  8  is formed between the p-type silicon nitride film  30  and the p +  poly-Si film  4  so as to be in contact with both of them. The p-type silicon oxynitride film  8  with a film thickness of about 100 nm, as described below, includes a plurality of quantum dots composed of p-type Si and has a composition of SiO 1 N 0.33 . 
       FIG. 16  is an enlarged sectional view of the p-type silicon oxynitride film  8  shown in  FIG. 15 . Referring to  FIG. 16 , the p-type silicon oxynitride film  8  includes a plurality of quantum dots  81 , each composed of a p-type Si dot and having a B concentration of about 10 19  cm −3 . The plurality of quantum dots  81  are irregularly arranged in the p-type silicon oxynitride film  8 . 
       FIG. 17  is an energy band diagram, at zero bias, of the light-emitting element  10 B shown in  FIG. 15 . Referring to  FIG. 17 , the energy band diagram of the n-type silicon oxide film  2  and the p-type silicon nitride film  30  is as described in  FIG. 3 . 
     Since the p-type silicon oxynitride film  8  includes the plurality of quantum dots  81  as described above, it has a layered structure of the quantum dots  81  and silicon oxynitride film layers  82  not including the quantum dots  81 . As a result, each of the quantum dots  81  is sandwiched by the silicon oxynitride film layers  82 . 
     The silicon oxynitride film layers  82  have an energy band gap of about 7.1 eV. Each of the quantum dots  81  sandwiched by two of the silicon oxynitride film layers  82  has a sub-level L sub   5  on the conduction band Ec 2  side of p +  Si and a sub-level L sub   6  on the valence band Ev 2  side of p +  Si due to a quantum size effect. 
     The sub-level L sub   5  is higher than the energy level of the conduction band Ec 2  of the p +  Si and the sub-level L sub   6  is higher than the energy level of the valence band Ev 2  edge of the p +  Si. As a result, the energy difference between the sub-level L sub   5  and the sub-level L sub   6  is larger than the energy band gap Eg 1  of the p +  Si. 
     The energy difference ΔE 5  between the conduction band Ec 2  edge of the p +  Si and the conduction band edge of the silicon oxynitride film layers  82  is about 4.2 eV, and the energy difference ΔE 6  between the valence band Ev 2  edge of the p +  Si and the valence band edge of the silicon oxynitride film layers  82  is the same as the energy difference ΔE 4 . Accordingly, the p-type silicon oxynitride film  8  has a barrier energy (=ΔE 6 ) against holes in the p +  Si that is smaller than a barrier energy (=ΔE 5 ) against electrons in the p +  Si. 
       FIG. 18  is an energy band diagram of the light-emitting element  10 B shown in  FIG. 15  when an electric current is applied. When a voltage is applied between the electrodes  5  and the electrode  6 , assuming that the electrodes  5  side is positive and the electrode  6  side is negative, the energy band of n +  Si constituting the substrate  1  is raised. Consequently, electrons  11  in n +  Si flow in the n-type silicon oxide film  2  through the plurality of quantum dots  21  included in the n-type silicon oxide film  2 , and are injected into the p-type silicon nitride film  30 . 
     Since the p-type silicon oxynitride film  8  has a higher barrier energy against electrons than the p-type silicon nitride film  30 , the electrons injected into the p-type silicon nitride film  30  are blocked by the p-type silicon oxynitride film  8  and stored in the quantum dots  31  of the p-type silicon nitride film  30 . 
     In contrast, holes  12  in the p +  poly-Si film  4  flow in the p-type silicon oxynitride film  8  through the quantum dots  81  included in the p-type silicon oxynitride film  8 , and are injected into the p-type silicon nitride film  30 . Since the n-type silicon oxide film  2  has a higher barrier energy against holes than the p-type silicon nitride film  30 , the holes injected into the p-type silicon nitride film  30  are blocked by the n-type silicon oxide film  2  and stored in the quantum dots  31  of the p-type silicon nitride film  30 . 
     Then, electrons  13  stored in the quantum dots  31  and holes  14  stored in the quantum dots  31  recombine with each other to emit light. 
     In the light-emitting element  10 B, the electrons injected into the p-type silicon nitride film  30  from the substrate  1  made of n +  Si are confined in the p-type silicon nitride film  30  with the p-type silicon oxynitride film  8 , while at the same time the holes injected into the p-type silicon nitride film  30  from the p +  poly-Si film  4  are confined in the p-type silicon nitride film  30  with the n-type silicon oxide film  2 . That is to say, in the light-emitting element  10 B, both the holes and electrons are confined in the p-type silicon nitride film  30  by adding the p-type silicon oxynitride film  8  to the light-emitting element  10 . As a result, in the light-emitting element  10 B, light-emitting efficiency higher than that of the light-emitting element  10  can be achieved. 
     Furthermore, the n-type silicon oxide film  2 , the p-type silicon nitride film  30 , and the p-type silicon oxynitride film  8  irregularly include the plurality of quantum dots  21 , the plurality of quantum dots  31 , and the plurality of quantum dots  81 , respectively, whereby the injection efficiency of electrons and holes is improved due to the electric-field enhancement effect caused by the irregularly arranged quantum dots  21 ,  31 , and  81 . 
     Thus, light-emitting efficiency can be improved. 
       FIGS. 19 and 20  are respectively a first process chart and a second process chart for describing a method for manufacturing the light-emitting element  10 B shown in  FIG. 15 . Referring to  FIG. 19 , in the manufacturing of the light-emitting element  10 B, a substrate  1  made of n +  Si is prepared (see process (a) in  FIG. 19 ), cleaned, and placed on the sample holder  103  of the plasma CVD apparatus  100 . 
     A silicon oxide film  11  is deposited on one principal surface of the substrate  1  under the reaction conditions shown in Table 1. A silicon nitride film  12  is then deposited on the silicon oxide film  11  under the reaction conditions shown in Table 2. 
     A silicon oxynitride film  15  is then deposited on the silicon nitride film  12  under the reaction conditions shown in Table 4. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
             
            
               
                   
                 Flow rate of SiH 4  (10%, diluted with H 2 ) 
                 96 
                 sccm 
               
               
                   
                 Flow rate of N 2 O (100%) 
                 6 
                 sccm 
               
               
                   
                 Flow rate of NH 3  (100%) 
                 18 
                 sccm 
               
               
                   
                 Pressure 
                 133 
                 Pa 
               
               
                   
                 RF power 
                 0.32 
                 W/cm 2   
               
               
                   
                 Substrate temperature 
                 300° 
                 C. 
               
               
                   
                 Reaction time 
                 4 
                 minutes 
               
               
                   
                   
               
            
           
         
       
     
     Subsequently, the amorphous silicon (a-Si) film  13  is deposited on the silicon oxynitride film  15  using the reaction conditions shown in Table 2 under which the NH 3  gas is stopped (see process (b) in  FIG. 19 ). 
     P +  ions are then injected into the silicon oxide film  11  by ion implantation (see process (c) in  FIG. 19 ). In this case, the acceleration voltage of ion implantation is adjusted such that the P +  ions are injected into only the silicon oxide film  11 . Thus, the n-type silicon oxide film  2  is formed (see process (d) in  FIG. 19 ). 
     B +  ions are then injected into the silicon nitride film  12 , the silicon oxynitride film  15 , and the a-Si film  13  by ion implantation (see process (d) in  FIG. 19 ). In this case, the acceleration voltage of ion implantation is adjusted such that the B +  ions are injected into only the silicon nitride film  12 , the silicon oxynitride film  15 , and the a-Si film  13 . Thus, the p-type silicon nitride film  30 , the p-type silicon oxynitride film  8 , and the p-type a-Si film  13 A are formed (see process (e) in  FIG. 20 ). 
     The resultant structure including the substrate  1 /n-type silicon oxide film  2 /p-type silicon nitride film  30 /p-type silicon oxynitride film  8 /p-type a-Si film  13 A is annealed under the conditions shown in Table 3. 
     As a result, P atoms injected into the n-type silicon oxide film  2  and B atoms injected into the p-type silicon nitride film  30  and the p-type silicon oxynitride film  8  are electrically activated. Furthermore, the p-type a-Si film  13 A is converted to the p +  poly-Si film  4  (see process (f) in  FIG. 20 ). 
     After that, the processes (g) and (h) in  FIG. 7  are conducted to complete the light-emitting element  10 B (see processes (g) and (h) in  FIG. 20 ). 
     The light-emitting element has only to include a light-emitting layer that emits light by recombination of electrons and holes, a first insulator that supplies electrons to the light-emitting layer through n-type quantum dots, and a second insulator that supplies holes to the light-emitting layer through p-type quantum dots. This is because the first and second insulators that respectively supply electrons and holes to the light-emitting layer can contribute to improvement in light-emitting efficiency at the light-emitting layer. 
     Each of the n-type silicon oxide films  2  and  60  constitutes “the first insulator” and each of the p-type silicon nitride films  3  and  70  constitutes “the second insulator”. 
     The p-type silicon oxynitride film  8  constitutes “a third insulator”. 
     Each of the quantum dots  21  and the quantum dots  63  constitutes “first quantum dots”, each of the quantum dots  31  and the quantum dots  73  constitutes “second quantum dots”, and the quantum dots  81  constitutes “third quantum dots”. 
     It should be considered that the embodiments disclosed in this entire specification are mere examples and do not limit the present invention. The scope of the present invention is specified by the Claims but not by the descriptions of the above embodiments, and any modification can be made as long as it is within the scope and the spirit of the Claims.