JP4055794B2 - Gallium nitride compound semiconductor light emitting device - Google Patents

Description

  The present invention relates to a gallium nitride-based compound semiconductor light-emitting element used for an optical device such as a light-emitting diode or a laser diode.

  Gallium nitride compound semiconductors are widely used as semiconductor materials for visible light emitting devices and high-temperature operating electronic devices. They are put into practical use in the field of blue and green light emitting diodes and in the field of blue-violet laser diodes. Development is progressing. In particular, in the field of light emitting diodes, an improvement in light emission efficiency of several tens to about 100 times that of conventionally used blue light emitting elements made of SiC has been achieved, and high efficiency light emission is required. Since it can be suitably used for outdoor display devices, it has attracted much attention.

  Gallium nitride-based compound semiconductor light-emitting devices that can emit visible light, including those used in light-emitting diodes and laser diodes, basically have an active layer made of InGaN and a GaN or AlGaN having a larger band gap than the active layer. Those containing a double heterostructure sandwiched between p-type and n-type clad layers made of, etc. are the mainstream. A p-type contact layer made of GaN is bonded to the p-type cladding layer, and an n-type contact layer made of GaN is bonded to the n-type cladding layer. These stacked structures are made of a gallium nitride compound semiconductor thin film grown on a substrate made of sapphire, SiC, or the like by a crystal growth method such as metal organic vapor phase epitaxy or molecular beam epitaxy. Nowadays it is mainstream.

  Gallium nitride-based compound semiconductors used for light-emitting diodes such as blue and green that have recently been put into practical use also basically have the above-mentioned laminated structure, and sapphire is used as a substrate, and an n-type contact layer is formed on the substrate , An n-type cladding layer, an active layer, a p-type cladding layer, and a p-type contact layer are sequentially stacked (hereinafter, the side on which the n-type contact layer and the n-type cladding layer are formed with respect to the active layer) May be referred to as the n-side, and the side on which the p-type cladding layer and the p-type contact layer are formed may be referred to as the p-side). Further, a p-side electrode is formed on the surface of the p-type contact layer, and the p-type contact layer, the p-type cladding layer, and the active layer are formed on the n-type contact layer in the direction opposite to the stacking direction from the p-type contact layer side. An n-side electrode is formed on the surface exposed by removing a part of the layer and the n-type contact layer. A voltage is applied to the p-side electrode and the n-side electrode to inject holes from the p-type contact layer side and electrons from the n-type contact layer side into the active layer. In the active layer, light emission corresponding to the band gap energy of the active layer is basically obtained by recombination of injected holes and electrons.

  By the way, as represented by the case where insulating sapphire is used for the substrate, in the light emitting element configured to provide the p-side electrode and the n-side electrode on the same surface side of the substrate, when a voltage is applied, n This includes a process in which electrons injected from the side electrode into the n-type contact layer move in a direction substantially perpendicular to the stacking direction in the n-type contact layer or n-type cladding layer, that is, in the plane direction.

  Here, in the case of a gallium nitride-based compound semiconductor, since the resistivity is higher than that of other Group 3-5 compound semiconductors, there is a feature that current hardly flows and electrons do not easily move. Therefore, electrons injected from the electrode to the n-type contact layer move so that the resistance decreases from the n-type contact layer to the active layer, and tend to concentrate in a region near the electrode under the active layer. is there. For this reason, in the active layer, light emission is concentrated in a region close to the n-type contact layer, and there is a problem that it is difficult to obtain in-plane uniform light emission from the active layer.

  Therefore, in order to solve this problem, a layer having an electron concentration higher than that of the n-type contact layer is formed between the n-type contact layer and the active layer, and the active layer is formed by uniformly spreading electrons in this plane. A structure that can inject electrons uniformly in the plane has been proposed. Such a structure is shown in Patent Document 1, for example.

FIG. 8 is a cross-sectional view showing the structure of a conventional gallium nitride compound semiconductor light emitting device disclosed in the above publication. In FIG. 8, an n-type contact layer 43 is formed on a substrate 41 via a buffer layer 42. An n-type high concentration layer 403 having an electron concentration higher than that of the n-type contact layer 43 is formed on the n-type contact layer 43. An n-type cladding layer 44, an active layer 45, a p-type cladding layer 46, and a p-type contact layer 47 are formed on the n-type high concentration layer 403. A p-side electrode 49 is formed on the surface of the p-type contact layer 47, and a part of the p-type contact layer 47, the p-type cladding layer 46, the active layer 45, the n-type cladding layer 44, and the n-type high concentration layer 403 is formed. An n-side electrode 48 is formed on the surface of the n-type contact layer 43 which is removed in the direction opposite to the stacking direction and exposed. In this way, by providing the n-type high concentration layer 403 having an electron concentration higher than that of the n-type contact layer 43 between the n-type contact layer 43 and the n-type cladding layer 44, the n-type contact layer is formed from the n-side electrode 48. It is said that the electrons injected into 43 easily spread uniformly in the n-type high concentration layer 403, so that uniform surface emission can be obtained from the active layer 45.
JP-A-8-23124

  In the gallium nitride-based compound semiconductor light-emitting device, including improvements due to the above structure, various improvements have been made to increase the light output and reduce the operating voltage. As a result, compared with the conventional blue light-emitting device made of SiC. Thus, an improvement in luminous efficiency of about 100 times has been achieved and it has been put to practical use. However, as the development in many application fields such as outdoor large display devices and various light sources progresses, further improvement in luminous efficiency and reduction in operating voltage are desired in order to improve visibility and reduce power consumption. It has come to be.

  The problem to be solved in the present invention is to provide a structure of a gallium nitride-based compound semiconductor light-emitting device capable of further improving the light emission efficiency and reducing the operating voltage.

  The inventors of the present invention conducted intensive research on a semiconductor multilayer structure made of an n-type gallium nitride compound semiconductor provided between a substrate and an active layer, particularly in a gallium nitride compound semiconductor light emitting device. As a result, the luminous efficiency is improved by providing a semiconductor multilayer structure having a mobility higher than the mobility of electrons in the n-type contact layer forming the electrode for injecting electrons between the active layer and the substrate. And found that the operating voltage can be reduced.

That is, the present invention has at least a substrate, an n-type electrode on the upper surface, an n-type contact layer disposed above the substrate, and an active layer disposed above the n-type contact layer. A gallium nitride-based compound semiconductor light emitting device, wherein a gallium nitride-based compound semiconductor is provided between one or both of the n-type contact layer and the substrate, and the active layer and the n-type contact layer. The n-type contact layer has a mobility in the surface direction of electrons in the semiconductor stack structure higher than the mobility in the surface direction of electrons in the n-type contact layer. The distance between the surface of the semiconductor and the semiconductor laminated structure is in the range of 0.01 μm to 0.3 μm. The semiconductor stacked structure includes a first n-type layer made of undoped GaN, and Al _x Ga _1-x N (which is formed in contact with the first n-type layer and doped with an n-type impurity. However, the second n-type layer composed of 0 ≦ x ≦ 1) is included (hereinafter referred to as “Al _x Ga _1-x N” or similarly expressed with a subscript in this specification) The gallium nitride compound semiconductor shown may be simply referred to as AlGaN.)

  In the present invention, the semiconductor multilayer structure is a multilayer structure in which two or more pairs of a first n-type layer made of undoped GaN and a second n-type layer doped with n-type impurities are laminated. It can also be included.

  According to such a configuration, electrons injected from the n-side electrode into the n-type contact layer easily move in the n-side layer structure and easily spread in the entire surface of the n-side layer structure. Therefore, it is possible to improve the uniformity of electron injection into the active layer.

  According to the present invention, it is possible to realize further improvement in luminous efficiency and reduction in operating voltage as compared with a gallium nitride compound semiconductor light emitting device having a conventional structure. An advantageous effect is obtained that it can be suitably applied to an outdoor display device, various light sources, and the like in which power reduction is desired. Also, when used in a laser diode, the efficiency of electron injection into the active layer is improved, so that an advantageous effect that the light emission efficiency can be increased and the operating voltage can be reduced is obtained.

The invention according to claim 1 includes a substrate, an n-side electrode on the upper surface, an n-type contact layer disposed above the substrate, an active layer disposed above the n-type contact layer, A gallium nitride compound semiconductor light emitting device having at least an undoped first n-type layer and an n-type impurity between the n-type contact layer and the substrate; A semiconductor multilayer structure comprising a second n-type layer having a band gap larger than that of the first layer and having an n-type impurity concentration higher than that of the first n-type layer, wherein the semiconductor multilayer structure is provided in the n-type contact layer A first n-type layer and a second n-type layer are sequentially laminated from the near side, and the distance between the surface of the n-type contact layer on which the n-side electrode is formed and the semiconductor multilayer structure is 0.01 μm to It is the range of 0.3 μm By providing a semiconductor stacked structure having an electron mobility higher than the surface mobility in the n-type contact layer in the n-side layer structure, the spread of electrons in the n-side layer structure is increased. It has the effect that it can be improved. Here, the n-type contact layer means a layer in which an n-side electrode for injecting electrons by being electrically connected to the light emitting element is formed.

The invention according to claim 2 is the invention according to claim 1, wherein the first n-type layer is made of GaN, and the second n-type layer is in contact with the first n-type layer. A first n-type layer made of undoped GaN and an AlGaN doped with an n-type impurity, characterized by comprising formed Al _x Ga _1-x N (where 0 ≦ x ≦ 1) Due to the junction with the second n-type layer made of the above, the semiconductor layer structure has an effect that electrons are easily moved.

The invention according to claim 3 is the invention according to claim 2, wherein the semiconductor laminated structure is a multilayer in which two or more pairs of the first n-type layer and the second n-type layer are laminated. It is characterized by including a structure, and by making the junction of the first n-type layer and the second n-type layer multi-layered, electrons move more easily in these semiconductor stacked structures. It has the effect of becoming.

  The invention according to claim 4 is the invention according to claim 2 or 3, wherein the semiconductor multilayer structure is a third n-type layer made of undoped GaN formed in contact with the second n-type layer. And further forming a third n-type layer made of undoped GaN in contact with the second n-type layer, whereby electrons move further in these semiconductor stacked structures. It has the effect of becoming easier.

The invention according to claim 5 is the invention according to any one of claims 2 to 4, wherein the second n-type layer has an electron concentration of 4 × 10 ¹⁸ / cm ³ or more and 1 × 10 ²⁰ / cm ^3. The second n-type layer and the first n-type layer or the first n-type layer are identified by specifying the electron concentration in the second n-type layer. In addition, electrons can easily move in the semiconductor multilayer structure formed by bonding with the third n-type layer, and the crystallinity of the semiconductor multilayer structure can be favorably maintained.

  The invention according to claim 6 is the invention according to any one of claims 2 to 5, wherein the thickness of the second n-type layer is adjusted to a range of 10 nm to 100 nm. Therefore, by specifying the film thickness of the second n-type layer, the amount of electrons supplied to the first n-type layer can be appropriately secured.

  The invention according to claim 7 is the invention according to any one of claims 2 to 6, wherein the second n-type layer has a film thickness adjusted to a range of 1 nm to 100 nm. By specifying the thickness of the first n-type layer, the ease of movement of electrons in the semiconductor multilayer structure is ensured, and the series resistance of the semiconductor multilayer structure increases excessively. It has the effect | action that can be prevented.

The invention according to claim 8 is the invention according to any one of claims 1 to 4, wherein the second n-type layer is the first n-type layer and / or the third n-type layer. The second n-type layer is characterized in that an impurity diffusion prevention region made of undoped Al _x Ga _1-x N (where 0 ≦ x ≦ 1) is provided in a layered form on the contact side, The n-type impurities can be prevented from diffusing into the first n-type layer to lower the electron mobility in the first n-type layer.

  The invention according to claim 9 is characterized in that, in the invention according to claim 8, the layer thickness of the impurity diffusion prevention region is adjusted to a range of 1 nm to 10 nm. By adjusting the layer thickness of the region to a specific range, it is possible to appropriately prevent diffusion of n-type impurities into the first n-type layer and to prevent an increase in series resistance in the second n-type layer. Has the effect of being able to.

  Hereinafter, specific examples of embodiments of the present invention will be described with reference to FIGS. In these drawings, the same elements and the same members are denoted by the same reference numerals.

(Embodiment 1)
FIG. 1 is a sectional view showing the structure of a gallium nitride-based compound semiconductor light-emitting device according to the first embodiment of the present invention.

  In FIG. 1, an n-type contact layer 3 a, a semiconductor multilayer structure 300, an n-type contact layer 3 b, an n-type cladding layer 4, an active layer 5, and a p-type layer are formed on a substrate 1 via a buffer layer 2. A mold cladding layer 6 and a p-type contact layer 7 are sequentially stacked. A p-side electrode 9 is formed on the surface of the p-type contact layer 7, and from the surface side of the p-type contact layer 7, the p-type contact layer 7, the p-type cladding layer 6, the active layer 5, and the n-type cladding layer. An n-side electrode 8 is formed on the surface of the n-type contact layer 3b exposed by removing a portion of 4 by etching.

For the substrate 1, sapphire, GaN, SiC, spinel (MgAl ₂ O), or the like can be used.

  For the buffer layer 2, GaN, AlN, AlGaN, AlInN, or the like can be used. For example, a layer formed at a temperature of 900 ° C. or less and a thickness of several nm to several tens nm can be preferably used. Here, since the buffer layer 2 has a function of relaxing the lattice mismatch between the substrate 1 and the laminated structure made of the gallium nitride compound semiconductor formed thereon, like GaN, When a substrate having a lattice constant close to that of the gallium nitride-based compound semiconductor formed above is used, the formation of the buffer layer 2 can be omitted depending on the growth method and growth conditions.

  The n-type contact layer 3a and the n-type contact layer 3b are formed of a gallium nitride compound semiconductor, and are particularly preferably formed of GaN or AlGaN. Gallium nitride-based compound semiconductors tend to exhibit n-type conductivity even in an undoped state where n-type impurities are not doped. In particular, when used as an n-type contact layer for providing the n-side electrode 8, Si or Ge If GaN doped with an n-type impurity such as n-type is used, an n-type layer having a high electron concentration can be obtained, and the contact resistance with the n-side electrode 8 can be reduced.

  For the n-side electrode 8 formed on the surface of the n-type contact layer 3b, a metal such as Al, Ti, Au or the like having a good ohmic property with the n-type contact layer 3b is used in a single layer, a multilayer or an alloy state. be able to.

The n-type cladding layer 4 is formed of a gallium nitride compound semiconductor and is formed of Al _a Ga _1-a N (where 0 ≦ a ≦ 1) doped with an n-type impurity such as Si or Ge. Although it is preferable, in the case of a light emitting element used for a light emitting diode, the formation of the n-type cladding layer 4 can be omitted.

The active layer 5 formed on the n-type cladding layer 4 is formed of a gallium nitride compound semiconductor having a band gap smaller than that of the n-type cladding layer 4. In particular, a gallium nitride compound semiconductor containing indium, that is, In _p Al _q Ga _1-pq N (where 0 <p ≦ 1, 0 ≦ q ≦ 1, 0 <p + q ≦ 0), among them, In _r Ga _1-r N (where 0 <r <1) is preferable (hereinafter referred to as In _r Ga _1-r N or a gallium nitride-based material similarly represented by a subscript in this specification) A compound semiconductor is sometimes simply referred to as InGaN.) The active layer 5 can be configured to obtain a desired emission wavelength by doping n-type impurities and p-type impurities at the same time or only one of them, but the film thickness is reduced to about 10 nm or less. It is particularly preferable in terms of improving the light emission efficiency that the layer has a quantum well structure so that the active layer 5 has good color purity and high light emission efficiency. When the active layer 5 has a quantum well structure, a single quantum well structure in which a well layer made of InGaN is sandwiched between barrier layers having a larger band gap than the well layer may be used. In this case, the barrier layer is the active layer. The p-type and n-type clad layers formed on both sides can be shared. Alternatively, a multiple quantum well structure in which well layers and barrier layers are alternately stacked may be used.

A p-type cladding layer 6 is formed on the active layer 5. The p-type cladding layer 6 is formed of a gallium nitride compound semiconductor having a band gap larger than the band gap of the active layer 5, and in particular Al _b Ga _1-b N doped with a p-type impurity such as Mg (provided that Preferably, it is formed by 0 ≦ b ≦ 1). Usually, the p-type cladding layer 6 is often formed at a growth temperature higher than the temperature suitable for the growth of the active layer 5 in order to form it with good crystallinity. While the mold cladding layer 6 is heated to the growth temperature, the crystallinity of the active layer 5 may be degraded due to dissociation of constituent elements such as indium and nitrogen constituting the active layer 5. Therefore, a part of the p-type cladding layer 6 on the side in contact with the active layer 5 is continuously grown while raising the temperature after the active layer 5 is grown, and the remaining temperature is continuously increased at the growth temperature of the p-type cladding layer 6. When the p-type cladding layer 6 is grown, it is possible to effectively prevent the crystallinity of the active layer 5 from deteriorating. At this time, a part of the p-type cladding layer 6 grown while raising the temperature is preferably formed of Al _c Ga _1-c N (where 0 ≦ c <1, c <b), particularly GaN. This is because the effect as a cladding layer formed in contact with the active layer 5 can be sufficiently achieved, and at the same time, the effect of preventing the deterioration of crystallinity due to dissociation of the constituent elements of the active layer 5 can be enhanced.

A p-type contact layer 7 is formed on the p-type cladding layer 6. The p-type contact layer 7 is formed of a gallium nitride compound semiconductor doped with a p-type impurity. In particular, In _d Ga _1-d N (0 ≦ d ≦ 0) doped with a p-type impurity such as Mg in order to reduce the contact resistance with the p-side electrode 9 formed on the surface of the p-type contact layer 7. ).

  Here, the semiconductor multilayer structure 300 is formed in the n-side multilayer structure between the active layer 5 and the substrate 1, and particularly in this embodiment, the n-side electrode 8 is formed on the surface. The structure is formed between the n-type contact layer 3 b and the substrate 1.

  FIG. 2 is a cross-sectional view schematically showing a part of the gallium nitride compound semiconductor light emitting device according to the first embodiment of the present invention, showing the layer structure from the n-type contact layer 3a to the active layer 5. Yes. In the present embodiment, the semiconductor multilayer structure 300 is formed between the n-type contact layer 3 b and the substrate 1.

  The semiconductor multilayer structure 300 includes a first n-type layer 31 made of undoped GaN and a second n-type layer made of AlGaN formed in contact with the first n-type layer 31 and doped with n-type impurities. 32. In the present invention, it is essential that the first n-type layer 31 is made of undoped GaN, and the second n-type layer 32 is made of AlGaN doped with an n-type impurity such as Si or Ge.

  By forming the semiconductor multilayer structure 300 in which the first n-type layer 31 made of undoped GaN and the AlGaN doped with the n-type impurity are laminated in the n-side layer structure, electrons are formed in the semiconductor multilayer structure 300. Although it is not clear why it becomes easy to move, it can be estimated as follows.

  That is, in the gallium nitride compound semiconductor doped with n-type impurities, the mobility of electrons in the n-type layer depends on the n-type impurities ionized by generating electrons, as in other Group 3-5 compound semiconductors. It is thought that it is often greatly influenced by scattering.

  Here, as in the present invention, the first n-type layer 31 made of GaN undoped without doping n-type impurities and the second n-type layer 32 made of AlGaN doped with n-type impurities are joined together. When the semiconductor multilayer structure 300 is formed as a structure formed in this manner, electrons generated by ionization of the n-type impurity in the second n-type layer 32 doped with the n-type impurity have a band gap that is larger than that of the second n-type layer 32. It is considered that the small first n-type layer 31 is accumulated and exists. In undoped GaN, since the number of ionized impurities that affect the mobility of electrons is much smaller than that of an n-type layer doped with n-type impurities, the mobility of electrons is usually n-type doped with n-type impurities. Very high compared to the mold layer.

  Accordingly, the first n-type layer 31 that is originally undoped and has a low electron concentration is joined to the first n-type layer 31 and is doped with an n-type impurity. Since the number of electrons supplied and having increased mobility becomes extremely large, it is considered that the electrons easily move in the semiconductor multilayer structure 300 as a whole.

  By these actions, electrons injected from the n-side electrode 8 into the n-type contact layer 3b are supplied to the semiconductor multilayer structure 300 via the n-type contact layer 3b immediately below the n-side electrode 8, and the electron mobility is increased. In the stacked structure of the raised first n-type layer 31 and the second n-type layer 32 doped with n-type impurities, the layer structure is uniformly spread over the entire surface of the layer structure, and the semiconductor stacked structure 300 To the active layer 5 through the n-type contact layer 3b and the n-type cladding layer 4. As a result, in-plane uniform light emission can be obtained from the active layer 5, the luminous efficiency is improved, and electrons can easily move in the n-side layer structure including the semiconductor multilayer structure 300, whereby the n-side layer structure. As a result, the operating resistance can be reduced.

  Here, when the n-side electrode is formed on the exposed n-type contact layer by removing a part of the p-type layer from the p-side by etching, the n-side electrode is injected and then injected into the active layer. The flow of electrons until the n-side electrode tends to be concentrated in a narrow region near the n-side electrode, and the electrons are directly below the n-side electrode in the n-type contact layer having a relatively high resistance in the plane. It proceeds in the vertical direction and reaches the lower part of the active layer that has not been removed by etching. For this reason, the series resistance in the surface direction of the n-type contact layer tends to increase. Moreover, it also tends to be difficult to reduce the series resistance when electrons injected from the n-side electrode move in the plane direction.

  On the other hand, as in the present embodiment, the semiconductor multilayer structure 300 is formed below the n-type contact layer 3b, that is, between the n-type contact layer 3b and the substrate 1, so that the implantation is performed from the n-side electrode 8. In the semiconductor multilayer structure 300, the generated electrons are likely to spread throughout the entire surface of the layer structure, so that the series resistance in the plane direction can be reduced. In this case, the depth of etching from the p side is appropriately adjusted, and the distance between the surface of the n-type contact layer 3b on which the n-side electrode 8 is formed and the semiconductor multilayer structure 300 is in the range of 0.01 μm to 0.3 μm. As a result, electrons can be easily supplied from the n-side electrode 8 to the semiconductor layer structure 300, so that the above-described effects can be easily achieved.

  In the present embodiment, the semiconductor laminated structure 300 has a configuration in which the first n-type layer 31 and the second n-type layer 32 are repeatedly laminated in order from the side in contact with the n-type contact layer 3b. However, the number of repetitions depends on the thickness of the first n-type layer 31 and the second n-type layer 32, but is preferably in the range of 1 to 300 times. In the case of increasing the number of repetitions to increase the number of layers, it is preferable that the film thickness is appropriately reduced to prevent generation of defects and cracks in the layer structure from the viewpoint of increasing the manufacturing yield.

  In the present embodiment, the pair of the second n-type layer 32 and the first n-type layer 31 is laminated twice from the side close to the substrate 1, and the first layer in the repetition of the lamination is , A second n-type layer 32 doped with an n-type impurity is formed, and a third n-type layer made of undoped GaN is further formed in contact with the second n-type layer 32, that is, the substrate 1 The third n-type layer may be formed in contact with the second n-type layer 32 and the n-type contact layer 3a on the side closer to the surface. Electrons generated in the first second n-type layer 32 of the stack are also present in the third n-type layer formed in contact therewith. Due to the same effect as described above, This is because electrons are more likely to move inside.

  In the present embodiment, the n-type contact layer 3a is preferably the same gallium nitride compound semiconductor as the n-type contact layer 3b, ie, GaN doped with Si to increase the electron concentration. When the n-type electrode 8 is not in contact with the n-type contact layer 3a, the n-type contact layer 3a is made of undoped GaN or AlGaN and has a laminated structure of gallium nitride compound semiconductors formed thereon. It can also be used as an underlayer.

(Embodiment 2)
FIG. 3 is a cross-sectional view showing the structure of a gallium nitride-based compound semiconductor light-emitting device according to the second embodiment of the present invention, and the same reference numerals as those in FIG. 1 denote the same members.

  In FIG. 3, an n-type contact layer 3a, a semiconductor multilayer structure 300, an n-type contact layer 3b, an n-type cladding layer 4, an active layer 5, and a p-type layer are formed on a substrate 1 via a buffer layer 2. A type cladding layer 6 and a p-type contact layer 7 are sequentially stacked in the same manner as in the gallium nitride-based compound semiconductor light-emitting element shown in FIG. A p-side electrode 9 is formed on the surface of the p-type contact layer 7, and an n-side electrode 8 is formed on the exposed surface of the n-type contact layer 3a.

  3, the substrate 1, the buffer layer 2, the n-type contact layer 3a, the semiconductor multilayer structure 300, the n-type contact layer 3b, the n-type cladding layer 4, the active layer 5, the p-type cladding layer 6, the p-type contact layer 7, The configurations of the p-side electrode 9 and the n-side electrode 8 are the same as those described with reference to FIG. 1 in the first embodiment, but the gallium nitride-based compound semiconductor light-emitting element of the present embodiment is the same as the above-described embodiment. The difference from the first one is that the semiconductor multilayer structure 300 is formed between the n-type contact layer 3 a on which the n-side electrode 8 is formed and the active layer 5.

  FIG. 4 is a cross-sectional view showing a partial structure of a gallium nitride-based compound semiconductor light emitting device according to the second embodiment of the present invention, showing a layer structure from the n-type contact layer 3a to the active layer 5. Yes. As described above, the semiconductor multilayer structure 300 is formed between the n-type contact layer 3 a and the active layer 5.

  The semiconductor multilayer structure 300 is formed in contact with the first n-type layer 31 made of undoped GaN and the first n-type layer 31 from the side close to the n-type contact layer 3a, and is doped with n-type impurities. And a second n-type layer 32 made of AlGaN. In the present invention, it is essential that the first n-type layer 31 is made of undoped GaN and the second n-type layer 32 is made of AlGaN doped with an n-type impurity such as Si or Ge. The configuration is the same as that of the first embodiment.

  Also in the present embodiment, a semiconductor stacked structure 300 in which a first n-type layer 31 made of undoped GaN and AlGaN doped with an n-type impurity are stacked is formed in the n-side layer structure. The reason why electrons easily move in the stacked structure 300 is considered to be substantially the same as the reason explained in the first embodiment.

  Therefore, in the present embodiment, electrons injected from the n-side electrode 8 into the n-type contact layer 3a are supplied to the semiconductor multilayer structure 300 formed on the n-type contact layer 3a, and the electron mobility is increased. In the stacked structure of the raised first n-type layer 31 and the second n-type layer 32 doped with n-type impurities, the layer structure is uniformly spread over the entire surface of the layer structure, and the semiconductor stacked structure 300 To the active layer 5 through the n-type contact layer 3b and the n-type cladding layer 4. As a result, in-plane uniform light emission is obtained from the active layer 5, luminous efficiency is improved, and electrons easily move in the n-type layer including the semiconductor multilayer structure 300, thereby reducing direct resistance in the n-type layer. As a result, the operating voltage can be reduced.

  Note that the number of repetitions and the film thickness of the first n-type layer 31 and the second n-type layer 32 can be the same as those in the first embodiment.

  Also in the present embodiment, the last layer of the stack of the first n-type layer 31 and the second n-type layer 32, that is, the layer in contact with the n-type contact layer 3b is an n-type impurity. However, a third n-type layer made of undoped GaN may be formed in contact with the second n-type layer 32.

  Furthermore, in this embodiment, as in FIG. 1 in the first embodiment, the n-type contact layer 3a is doped with the same gallium nitride compound semiconductor as the n-type contact layer 3b, that is, Si, to increase the electron concentration. However, when the n-side contact layer 3b is not in contact with the n-side electrode 8, the formation of the n-type contact layer 3b can be omitted.

(Embodiment 3)
FIG. 5 is a cross-sectional view showing the structure of a gallium nitride-based compound semiconductor light emitting device according to the third embodiment of the present invention, and the same reference numerals as those in FIG. 1 denote the same members.

  In FIG. 5, an n-type contact layer 3a, a semiconductor multilayer structure 300, an n-type contact layer 3b, a semiconductor multilayer structure 301, an n-type contact layer 3c, and a buffer layer 2 are disposed on the substrate 1. An n-type cladding layer 4, an active layer 5, a p-type cladding layer 6, and a p-type contact layer 7 are sequentially stacked. A p-side electrode 9 is formed on the surface of the p-type contact layer 7, and an n-side electrode 8 is formed on the exposed surface of the n-type contact layer 3b. Here, the semiconductor multilayer structure 301 formed on the n-type contact layer 3b has the same configuration as the semiconductor multilayer structure 300 formed under the n-type contact layer 3b in the second embodiment.

  5, the substrate 1, the buffer layer 2, the n-type contact layer 3a, the semiconductor laminated structures 300 and 301, the n-type contact layer 3b, the n-type cladding layer 4, the active layer 5, the p-type cladding layer 6, and the p-type contact layer. 7. The configuration of the p-side electrode 9 and the n-side electrode 8 is the same as that described with reference to FIG. 1 or 2 in the first embodiment or the second embodiment, but the gallium nitride system of the present embodiment. The compound semiconductor light emitting element is different from that of the first embodiment or the second embodiment in that the semiconductor stacked structures 300 and 301 are provided between the n-type contact layer 3b on which the n-side electrode 8 is formed and the substrate 1. In addition, it is formed between the n-type contact layer 3b and the active layer 5, respectively.

  FIG. 6 is a cross-sectional view showing a partial structure of a gallium nitride-based compound semiconductor light emitting device according to the third embodiment of the present invention, showing a layer structure from the n-type contact layer 3a to the active layer 5. Yes. As described above, the semiconductor multilayer structures 300 and 301 are formed between the n-type contact layer 3b and the substrate 1 and between the n-type contact layer 3b and the active layer 5, respectively.

  Each of the semiconductor stacked structures 300 and 301 is formed in contact with the first n-type layer 31 made of undoped GaN and the first n-type layer 31 from the side close to the n-type contact layer 3b, and is n-type. And a second n-type layer 32 made of AlGaN doped with impurities. In the present invention, it is essential that the first n-type layer 31 is made of undoped GaN and the second n-type layer 32 is made of AlGaN doped with an n-type impurity such as Si or Ge. The configuration is the same as that of the first embodiment.

  Also in the present embodiment, by forming the semiconductor multilayer structures 300 and 301 in which the first n-type layer 31 made of undoped GaN and AlGaN doped with n-type impurities are laminated in the n-side layer structure. The reason why electrons easily move in the semiconductor stacked structures 300 and 301 is considered to be due to the reason substantially similar to the assumption described in the first embodiment.

  Therefore, in the present embodiment, among the electrons injected from the n-side electrode 8 into the n-type contact layer 3b, electrons going downward through the n-type contact layer 3b are formed under the n-type contact layer 3b. The semiconductor laminated structure 300 is supplied to the semiconductor laminated structure 300, and is uniformly spread over the entire surface of the layer structure. The semiconductor laminated structure 300 is injected into the semiconductor laminated structure 301 through the n-type contact layer 3b, and the n-type contact layer 3b. Together with electrons injected upward into the semiconductor multilayer structure 301, the semiconductor multilayer structure 301 is further spread over the entire surface and injected into the active layer 5 via the n-type contact layer 3 c and the n-type cladding layer 4. Will be. As a result, in-plane uniform light emission is obtained from the active layer 5, the light emission efficiency is improved, and electrons are easily moved in the n-type layer including the semiconductor multilayer structures 300 and 301, whereby the n-side layer structure is obtained. As a result, the operating resistance can be reduced.

  Note that the number of repetitions and the film thickness of the first n-type layer 31 and the second n-type layer 32 can be the same as those in the first embodiment.

  Also in the present embodiment, the last layer in the repeated stacking of the first n-type layer 31 and the second n-type layer 32 from the side close to the n-type contact layer 3b is doped with n-type impurities. Although the second n-type layer 32 is provided, a third n-type layer made of undoped GaN may be further formed in contact with the second n-type layer 32.

  Furthermore, in this embodiment, the n-type contact layer 3a and the n-type contact layer 3c are the same gallium nitride compound semiconductor as the n-type contact layer 3b, that is, GaN doped with Si to increase the electron concentration. However, when the n-side contact layer 3a is not in contact with the n-side electrode 8, the n-type contact layer 3a is formed of undoped GaN or AlGaN as described in the first embodiment. The n-type contact layer 3c is not in contact with the n-side electrode 8 and can be used as an underlayer of a laminated structure of gallium nitride compound semiconductors formed thereon. As described in the second embodiment, the formation of the n-type contact layer 3c can be omitted.

  In the first to third embodiments, the surface of the n-type contact layer forming the n-side electrode 8 is configured to be substantially parallel to the plane direction of the layer. For example, in the third embodiment, the p-type layer is removed by etching from the p-side so that the surface is inclined from the n-type contact layer 3c to the n-type contact layer 3a, and the exposed n-type is exposed. When the n-side electrode 8 is formed from the contact layer 3c to the n-type contact layer 3a, the n-side electrode 8 is formed on the n-type contact layer 3c, the semiconductor multilayer structure 301, the n-type contact layer 3b, the semiconductor multilayer structure 300, and the n-type contact layer 3a. Electrons are directly injected from the n-layer, and the easiness of spreading of electrons in these n-side layer structures can be further increased.

According to the knowledge of the present inventors, the second n-type layer 32 constituting the semiconductor multilayer structures 300 and 301 has an electron concentration in the range of 4 × 10 ¹⁸ / cm ^{3 to} 1 × 10 ²⁰ / cm ^3. It was found desirable to be adjusted so that When the electron concentration is lower than 4 × 10 ¹⁸ / cm ^3, it is difficult to obtain a sufficient spread of electrons in the semiconductor multilayer structures 300 and 301. This is presumably because the amount of electrons accumulated in the first n-type layer 31 formed in contact with the second n-type layer 32 is reduced. On the other hand, if it exceeds 1 × 10 ²⁰ / cm ³ , the crystallinity of the second n-type layer 32 tends to deteriorate, and the crystallinity of the laminated structure formed thereon also tends to deteriorate.

  Furthermore, it is preferable that the second n-type layer 32 has a film thickness adjusted to a range of 10 nm to 100 nm. This is because if the thickness of the second n-type layer 32 is smaller than 10 nm, the amount of electrons that can be supplied to the first n-type layer 31 decreases. On the other hand, since the film thickness of the second n-type layer 32 that contributes to the supply of electrons to the first n-type layer 31 is normally considered to be up to 100 nm, the first n-type layer becomes thicker than 100 nm. Since the part which does not contribute to supply of the electron to 31 will increase, it is not preferable.

  Furthermore, it is preferable that the first n-type layer 31 has a film thickness adjusted to a range of 1 nm to 100 nm. When the film thickness of the first n-type layer 31 is smaller than 1 nm, it is difficult to obtain a sufficient spread of electrons in the semiconductor multilayer structures 300 and 301. This is considered to be because it becomes difficult to accumulate the electrons supplied from the second n-type layer 32. On the other hand, since electrons supplied from the second n-type layer 32 are considered to be concentrated in the vicinity of the first n-type layer 31 in contact with the second n-type layer 32, the first n-type layer 31 It is considered that even if the film thickness of 31 is increased excessively, it does not contribute to the ease of spreading of electrons. According to the knowledge of the present inventors, the effect of reducing the in-plane series resistance in the n-side layer structure by increasing the electron mobility and making the electrons easily spread in the layer structure when the thickness is greater than 100 nm. Rather, the increase in series resistance when electrons move vertically across the first n-type layer 31 tends to be greater.

(Embodiment 4)
The structure of the gallium nitride-based compound semiconductor light emitting device in the present embodiment is substantially the same as that of FIG. 1 shown in the first embodiment. The gallium nitride compound semiconductor light emitting device of this embodiment is different from that of the above embodiment in that the second n-type layer 32 doped with an n-type impurity in the semiconductor multilayer structure 300 is the first n-type. An impurity diffusion prevention region that is undoped without being doped with n-type impurities is provided in a layer on the side in contact with the layer 31. Hereinafter, the present embodiment will be described with reference to FIG.

  FIG. 7 is a cross-sectional view showing a partial structure of a gallium nitride-based compound semiconductor light emitting device according to the fourth embodiment of the present invention. The same reference numerals as those in FIG. 1 denote the same members. Yes.

In FIG. 7, a first n-type layer 31 made of undoped GaN is formed on one side or both sides of a second n-type layer 32 made of Al _x Ga _1-x N doped with n-type impurities. In the second n-type layer 32, an impurity diffusion prevention region 34 made of undoped Al _x Ga _1-x N is provided on a part of the side in contact with the first n-type layer 31.

  Here, in the first n-type layer 31 in which electrons generated by ionization of the n-type impurity in the second n-type layer 32 doped with the n-type impurity are in contact with the second n-type layer 32. In the first n-type layer 31 having a very small number of ionized impurities that affect the mobility of electrons, the mobility of electrons is n-type doped with n-type impurities. As described in the first embodiment, it is extremely higher than the mold layer.

  Therefore, in order to maintain high electron mobility in the first n-type layer 31, n-type impurities doped in the second n-type layer 32 are mixed into the first n-type layer 31 by diffusion or the like. It is important to prevent this.

  Therefore, as shown in FIG. 7, in the second n-type layer 32 doped with the n-type impurity, an impurity diffusion prevention region where the n-type impurity is not doped in a part on the side in contact with the first n-type layer 31. When 34 is provided, the mobility of the first n-type layer 31 is prevented from being lowered due to the mixing of n-type impurities, and the mobility of electrons in the first n-type layer 31 can be kept high. . This configuration is particularly useful when the second n-type layer 32 is doped with an n-type impurity at a high concentration.

  The layer thickness of the impurity diffusion preventing region 34 is preferably adjusted to a range of 1 nm to 5 nm. If the thickness is smaller than 1 nm, the effect of preventing the n-type impurity from being mixed into the first n-type layer 31 is reduced. If the thickness is larger than 5 nm, the second n-type layer 32 to the first n-type This is because the supply of electrons to the layer 31 tends to be difficult.

  The configuration of the present embodiment can also be applied to the semiconductor multilayer structures 300 and 301 in the above first to third embodiments.

  Hereinafter, specific examples of the method for producing a gallium nitride-based compound semiconductor light emitting device of the present invention will be described with reference to the drawings. The following examples mainly show a growth method of a gallium nitride-based compound semiconductor using a metal organic vapor phase growth method, but the growth method is not limited to this, and a molecular beam epitaxy method or an organic metal It is also possible to use a molecular beam epitaxy method or the like.

Example 1
In this example, the gallium nitride compound semiconductor light emitting device shown in FIG. 1 was produced.

  First, after placing the sapphire substrate 1 having a mirror-finished surface on the substrate holder in the reaction tube, the surface temperature of the substrate 1 is kept at 1100 ° C. for 10 minutes, and the substrate is heated while flowing hydrogen gas, Cleaning was performed to remove dirt such as organic substances adhering to the surface of the substrate 1 and moisture.

  Next, the surface temperature of the substrate 1 is lowered to 600 ° C., and a TMA carrier gas containing nitrogen gas, ammonia, and trimethylaluminum (TMA) as a main carrier gas is allowed to flow, and a buffer layer 2 made of AlN. Was grown to a thickness of 25 nm.

Next, after stopping only the carrier gas of TMA and raising the temperature to 1050 ° C., while flowing nitrogen gas and hydrogen gas as the main carrier gas, a carrier gas for TMG newly containing trimethylgallium (TMG), and Si An n-type contact layer 3a made of GaN doped with Si was grown to a thickness of 1.5 μm while flowing SiH ₄ (monosilane) gas as a source.

After the n-type contact layer 3a was grown and formed, the semiconductor laminated structure 300 was grown by keeping the substrate temperature at 1050 ° C. In forming the semiconductor multilayer structure 300, first, while flowing nitrogen gas and hydrogen gas as the main carrier gas, growth is performed while newly flowing a carrier gas for TMG, a carrier gas for TMA, and a SiH ₄ gas. Then, a second n-type layer 32 made of Al _0.1 Ga _0.9 N doped with Si was grown to a thickness of 30 nm. The second n-type layer 32 had an electron concentration of about 6 × 10 ¹⁸ / cm ³ . Next, the first n-type layer 31 made of undoped GaN was grown to a thickness of 20 nm by growing a new TMG carrier gas at the same substrate temperature. Thereafter, the second n-type layer 32 and the first n-type layer 31 were repeatedly laminated in the same manner. In this way, a semiconductor multilayer structure 300 was formed in which pairs of the second n-type layer 32 and the first n-type layer 31 were laminated twice.

After forming the semiconductor multilayer structure 300, the carrier gas for TMA is stopped, the substrate temperature is continuously maintained at 1050 ° C., and a new TMG carrier gas and SiH ₄ gas are allowed to flow while flowing. An n-type contact layer 3b made of GaN doped with is grown to a thickness of 0.5 μm.

After growing the n-type contact layer 3b, the carrier gas for TMG is stopped, the substrate temperature is continuously maintained at 1050 ° C., and a carrier gas for TMG, a carrier gas for TMA, and a SiH ₄ gas are newly added. The n-type cladding layer 4 made of Al _0.01 Ga _0.99 N doped with Si was grown to a thickness of 0.05 μm.

After the n-type cladding layer 4 is grown, the TMG carrier gas, the TMA carrier gas, and the SiH ₄ gas are stopped, the substrate temperature is lowered to 750 ° C., and nitrogen gas is flowed as the main carrier gas at 750 ° C. An active layer 5 having a single quantum well structure made of undoped In _0.4 Ga _0.6 N was grown to a thickness of 5 nm while flowing a TMG carrier gas and a TMI carrier gas.

After growing the active layer 5, the carrier gas for TMI is stopped, the substrate temperature is raised toward 1050 ° C. while flowing the carrier gas for TMG, and subsequently, undoped GaN is grown to a thickness of 4 nm. When the temperature reaches 1050 ° C., it is newly grown while flowing a nitrogen gas and a hydrogen gas as main carrier gases, a carrier gas for TMA, and a carrier gas for Cp ₂ Mg as an Mg source. Al _0.15 Ga _0.85 N doped with is grown to a thickness of 0.1 μm. Thereafter, the carrier gas for TMG and the carrier gas for TMG were stopped, the substrate temperature was kept at 1050 ° C., and Mg was diffused from undoped Al _0.15 Ga _0.85 N doped with Mg. In this way, the p-type cladding layer 6 made of GaN doped with Mg and AlGaN doped with Mg was formed to a thickness of about 0.1 μm.

After forming the p-type cladding layer 6, the substrate temperature is maintained at 1050 ° C., and a new TMG carrier gas and a Cp ₂ Mg carrier gas are allowed to flow while flowing, and Mg is doped. The resulting p-type contact layer 7 was grown to a thickness of 0.1 μm.

After growing the p-type contact layer 7, the carrier gas for TMG and the carrier gas for Cp ₂ Mg were stopped, the main carrier gas and ammonia were allowed to flow as they were, and the wafer was removed from the reaction tube. .

Next, an SiO ₂ film was deposited on the surface of the p-type contact layer 7 by a CVD method, and then patterned into a predetermined shape by photolithography to form an etching mask. Then, a part of the p-type contact layer 7, the p-type cladding layer 6, the active layer 5, and the n-type cladding layer 4 is formed at a depth of about 0.55 μm in the direction opposite to the stacking direction by reactive ion etching. To remove the surface of the n-type contact layer 3b. Then, an n-side electrode 8 made of Al was vapor-deposited on the surface of the n-type contact layer 3b exposed by photolithography and vapor deposition. Further, a p-side electrode 9 made of Ni and Au was deposited on the surface of the p-type contact layer 7 in the same manner.

  Thereafter, the back surface of the sapphire substrate 1 was polished to a thickness of about 100 μm and separated into chips by scribing. In this manner, the gallium nitride compound semiconductor light emitting device shown in FIG. 1 was obtained.

  The chip was bonded to the stem with the electrode forming surface facing upward, and then the n-side electrode and the p-side electrode of the chip were each connected to the electrode on the stem with a wire, and then resin molded to produce a light emitting diode. When this light emitting diode was driven with a forward current of 20 mA, it emitted blue light with a peak light emission wavelength of 480 nm. The light emission output at this time was 1210 μW, and the forward operation voltage was 3.4V.

(Example 2)
In the present embodiment, the n-type contact layer 3a is formed with a thickness of 1.95 μm, the n-type contact layer 3b is formed with a thickness of 0.05 μm, and the first n-type layer 31 and the second n-type in the semiconductor multilayer structure 300 are formed. A gallium nitride compound semiconductor light emitting device shown in FIG. 3 was produced in the same manner as in Example 1 except that the layer 32 was formed in the reverse order. Further, after that, a light emitting diode was produced in the same manner as in Example 1 and driven with a forward current of 20 mA. As a result, light was emitted in blue with a peak wavelength of 480 nm. The light emission output at this time was 1180 μW, and the forward operation voltage was 3.5V.

(Example 3)
In this embodiment, the n-type contact layer 3a is formed with a thickness of 1.50 μm, the n-type contact layer 3b is formed with a thickness of 0.35 μm, and the n-type contact layer 3c is formed with a thickness of 0.05 μm. The semiconductor multilayer structure 301 is the same as the semiconductor multilayer structure 300 of the second embodiment, and between the n-type contact layer 3a and the n-contact layer 3b and between the n-type contact layer 3b and the n-type contact layer, respectively. A gallium nitride-based compound semiconductor light emitting device shown in FIG. 5 was produced in the same manner as in Example 1 except that it was formed between 3c. Further, after that, a light emitting diode was produced in the same manner as in Example 1 and driven with a forward current of 20 mA. As a result, light was emitted in blue with a peak wavelength of 480 nm. The light emission output at this time was 1290 μW, and the forward operation voltage was 3.3V.

(Comparative example)
For comparison, a gallium nitride compound semiconductor light emitting device shown in FIG. 8 was produced.

  Specifically, the n-type contact layer 43 is formed to a thickness of 2.0 μm under the same conditions as the n-type contact layer 3 a of the first embodiment, and the n-type high concentration layer 403 is formed from the n-type contact layer 43. The n-type cladding layer 44 is formed under the same conditions as the n-type cladding layer 4 of the first embodiment. Then, the buffer layer 42, the active layer 45, the p-type cladding layer 46, the p-type contact layer 47, the n-side electrode 48, and the p-side electrode 49 are formed in accordance with the buffer layer 2, the active layer 5, and the p-type in the first embodiment. The formation was performed under the same conditions as the formation of the cladding layer 6, the p-type contact layer 7, the n-side electrode 8 and the p-side electrode 9. Using the gallium nitride-based compound semiconductor light-emitting device thus obtained, a light-emitting diode was produced in the same manner as in Example 1 and driven with a forward current of 20 mA. As a result, light was emitted in blue with a peak emission wavelength of 480 nm. However, the light emission output at this time tended to decrease to 1020 μW, and the forward operation voltage was 3.7 V, which was higher than the light emitting diode of the above example.

  It is suitable for a semiconductor light emitting element that needs to increase luminous efficiency and reduce operating voltage.

Sectional drawing which shows the structure of the gallium nitride type compound semiconductor light-emitting device based on 1st embodiment of this invention Sectional drawing which shows the one part structure of the gallium nitride type compound semiconductor light-emitting device based on 1st Embodiment of this invention Sectional drawing which shows the structure of the gallium nitride type compound semiconductor light-emitting device based on 2nd embodiment of this invention Sectional drawing which shows the one part structure of the gallium nitride type compound semiconductor light-emitting device based on 2nd embodiment of this invention Sectional drawing which shows the structure of the gallium nitride type compound semiconductor light-emitting device based on 3rd embodiment of this invention. Sectional drawing which shows the structure of a part of gallium nitride-based compound semiconductor light emitting device according to the third embodiment of the present invention Sectional drawing which shows the structure of a part of gallium nitride compound semiconductor light-emitting device based on 4th Embodiment of this invention Sectional drawing which shows the structure of the conventional gallium nitride compound semiconductor light emitting element

Explanation of symbols

1 substrate 2 buffer layer 3a, 3b, 3c n-type contact layer 4 n-type cladding layer 5 active layer 6 p-type cladding layer 7 p-type contact layer 8 n-side electrode 9 p-side electrode 31 first n-type layer 32 second N-type layer 33 Third n-type layer 34 Impurity diffusion prevention region 300, 301 Semiconductor laminated structure

Claims (9)

Gallium nitride-based compound semiconductor light-emitting device having at least a substrate, an n-side electrode on the upper surface, an n-type contact layer disposed above the substrate, and an active layer disposed above the n-type contact layer An element that is doped with an undoped first n-type layer and an n-type impurity between the n-type contact layer and the substrate, and has a larger band gap than the first n-type layer; A semiconductor multilayer structure including a second n-type layer having an n-type impurity concentration higher than that of the first n-type layer, and the semiconductor multilayer structure includes a first n-type from the side close to the n-type contact layer. A layer and a second n-type layer are sequentially laminated, and a distance between the surface of the n-type contact layer on which the n-side electrode is formed and the semiconductor laminated structure is in a range of 0.01 μm to 0.3 μm. Characteristic gallium nitride compound semiconductor The light-emitting element. The first n-type layer is made of GaN, and the second n-type layer is Al _x Ga _1-x N (where 0 ≦ x ≦ 1) formed in contact with the first n-type layer. The gallium nitride-based compound semiconductor light-emitting device according to claim 1, comprising: 3. The gallium nitride compound according to claim 2, wherein the semiconductor stacked structure includes a multilayer structure in which two or more pairs of the first n-type layer and the second n-type layer are stacked. Semiconductor light emitting device. 4. The gallium nitride compound semiconductor according to claim 2, wherein the semiconductor multilayer structure further includes a third n-type layer made of undoped GaN formed in contact with the second n-type layer. Light emitting element. 5. The method according to claim 2, wherein the second n-type layer is adjusted to have an electron concentration in a range of 4 × 10 ¹⁸ / cm ^{3 to} 1 × 10 ²⁰ / cm ^3. 2. A gallium nitride compound semiconductor light emitting device according to claim 1. 6. The gallium nitride-based compound semiconductor light-emitting device according to claim 2, wherein the second n-type layer has a thickness adjusted to a range of 10 nm to 100 nm. 7. The gallium nitride-based compound semiconductor light-emitting element according to claim 2, wherein the first n-type layer has a film thickness adjusted to a range of 1 nm to 100 nm. The second n-type layer is formed of undoped Al _x Ga _1-x N (where 0 ≦ x ≦ 1) on the side in contact with the first n-type layer and / or the third n-type layer. The gallium nitride-based compound semiconductor light-emitting device according to claim 2, wherein the impurity diffusion prevention region is provided in a layered form. 9. The gallium nitride compound semiconductor light emitting device according to claim 8, wherein the thickness of the impurity diffusion preventing region is adjusted to a range of 1 nm to 5 nm.

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