JP2007150075A - Nitride semiconductor light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitride semiconductor light emitting element which can form one electrode on the rear surface of a substrate while forming a vertical light emitting element by using an SiC substrate as the substrate, and can increase external quantum efficiency by effectively utilizing also light advancing toward a substrate side. <P>SOLUTION: A nitride semiconductor laminate 9 including an active layer 5 formed therein with at least a light emitter is provided on an SiC substrate 1, and a pair of upper and lower electrodes 11 and 12 are provided on the rear surface of the SiC substrate 1 and on the front surface of the semiconductor laminate 9, respectively. The active layer 5 has a laminated structure of a high refractive-index layer 51 and a low refractive-index layer 52 each having a thickness of λ/(4n) and stacked on each other, where λ denotes the wavelength of emitted light, n denotes the refractive index of the semiconductor layer (n<SB>1</SB>being the refractive index of the high refractive-index layer, n<SB>2</SB>being the refractive index of the low refractive-index layer). <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a light emitting device using a nitride semiconductor. More specifically, a nitride that can improve the external quantum efficiency by using a semiconductor substrate made of SiC as the substrate and effectively taking out light emitted from the active layer while providing one electrode on the back surface of the substrate. The present invention relates to a semiconductor light emitting device.

  In a conventional light emitting device using a nitride semiconductor, for example, a nitride semiconductor laminated portion including a buffer layer and a light emitting layer forming portion is grown on a sapphire substrate, and a part of the semiconductor laminated portion is etched to form a semiconductor laminated portion. The lower conductive layer is exposed, a lower electrode is provided on the exposed lower conductive layer surface, and an upper electrode is provided on the upper surface side of the semiconductor stack. When such sapphire is used as a substrate, the sapphire substrate is an insulator, and therefore, an electrode connected to the conductive type layer on the lower layer side of the semiconductor stacked portion cannot be formed on the back surface of the substrate, as described above. A part of the semiconductor laminated portion must be removed by etching to expose the lower conductive type layer. Therefore, the manufacturing process becomes very complicated, and contamination due to etching adheres to the light emitting surface and the electrode forming surface, resulting in problems with electrical characteristics such as a decrease in light output and an increase in the electrical resistance of the electrode connection part. ing.

In order to solve such a problem, a semiconductor light emitting device having a structure in which a semiconductor substrate made of SiC is used as a substrate and a nitride semiconductor is used thereon to grow a double heterojunction light emitting layer forming portion via a buffer layer Is also considered. That is, as shown in FIG. 7, a buffer layer 102 is provided on a SiC substrate 101, and a light emission comprising a double heterojunction in which an active layer 104 is sandwiched between a lower semiconductor layer 103 and an upper semiconductor layer 105. By laminating the layer forming portion 106, forming the upper electrode 107 on the front surface and forming the lower electrode 108 on the back surface of the SiC substrate 101, without etching a part of the semiconductor laminated portion such as the light emitting layer forming portion 106. A lower electrode 108 connected to the lower conductivity type layer of the semiconductor laminated portion is formed on the back surface of the semiconductor substrate 101 (see, for example, Patent Document 1).
US Pat. No. 5,523,589

  As described above, it is preferable to use SiC as a substrate because one electrode can be formed on the back surface of the substrate without etching the semiconductor stacked portion. However, the light emitted from the light emitting part is emitted uniformly in all directions, and the light having the same intensity as that of the upper surface side of the semiconductor laminated part from which the light is extracted proceeds to the SiC substrate side, but SiC is emitted from the nitride semiconductor. Absorbs light Therefore, for example, in the case of a sapphire substrate, even the light traveling toward the substrate side can use the light emitted from the side surface of the substrate or the light reflected on the back surface side of the substrate. Almost no light can be used, and theoretically half of the emitted light is wasted.

  On the other hand, the active layer is formed with a bulk layer or a quantum well structure, but even if the active layer is formed with a quantum well structure, light is not emitted from all well layers, but light is emitted only from a part of the well layers close to the p-type layer. It is known that no light is emitted from other quantum well structures. In this case, how much light is emitted is related to manufacturing problems, and if the thickness of the active layer itself is reduced, the light emission characteristics are deteriorated. Therefore, the active layer needs to have a certain thickness.

  The present invention has been made in view of such a situation. By using a SiC substrate, one electrode is formed on the back surface of the substrate, and the light traveling toward the substrate side is effectively produced while forming a vertical light emitting element. An object of the present invention is to provide a nitride semiconductor light emitting device having a structure capable of improving the external quantum efficiency.

  Another object of the present invention is to grow a nitride semiconductor layer that cannot achieve perfect lattice matching as a semiconductor layer with as little lattice distortion as possible by sandwiching the superlattice structure, and to effect light emitted from the active layer. An object of the present invention is to provide a nitride semiconductor light emitting device having a structure capable of emitting light to the surface side.

  A nitride semiconductor light emitting device according to the present invention includes a SiC substrate, a nitride semiconductor multilayer portion including an active layer provided on the SiC substrate and having at least a light emitting portion formed thereon, a back surface of the SiC substrate, and the semiconductor multilayer portion. A pair of electrodes respectively provided on the surface side of the active layer, wherein the active layer has an emission wavelength of λ and a refractive index of the semiconductor layer is n, and a high refractive index layer and a low refractive index layer have λ / (4n ) With a thickness of 1).

  Here, the nitride semiconductor means a compound of a group III element Ga and a group V element N or a group III element Ga partially or entirely substituted with other group III elements such as Al and In, and It means a nitride in which a part of N of the group V element is substituted with another group V element such as P or As. Moreover, the low refractive index and the high refractive index mean the relationship of the relative refractive index between both layers, and the absolute value of a refractive index is not ask | required. Furthermore, the refractive index when the layer is formed in a multilayer structure such as a superlattice means an average refractive index.

  At least a part of the high refractive index layer of the active layer has a thickness of λ / (4n) due to a laminated structure of a thin layer (well layer) having a high refractive index and a thin layer (barrier layer) having a low refractive index. As a result, a part of the λ / (4n) layers can be formed into a quantum well structure.

  A buffer layer formed of a nitride semiconductor is provided on the surface of the SiC substrate, and a light reflecting layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided between the buffer layer and the active layer. This makes it possible to relax the lattice mismatch with the SiC substrate by the buffer layer while freely selecting the composition of the reflective layer.

  Between the SiC substrate and the active layer, a light reflecting layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided in contact with the SiC substrate, so that the light reflecting layer is a laminated structure of a thin layer. Therefore, it is easy to increase the carrier concentration, and as a low refractive index layer, an AlGaN-based compound having a large Al mixed crystal ratio (the mixed crystal ratio of Al and Ga is not unambiguous, meaning that the compound can be variously taken. The following “system” is also used in the same meaning), and the matching between the SiC substrate and the nitride semiconductor layer can be improved rather than providing a buffer layer.

  Between the active layer and the SiC substrate, a light reflecting layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided in contact with the active layer, so that it is also reflected by the laminated structure of the active layer. Light that cannot be reflected can be reflected at a position very close to the light emitting portion, and the emitted light can be emitted to the surface side without waste. In this case, by using a material having a large band gap energy, such as an AlGaN-based compound, as the light reflecting layer, the carrier confinement function is sufficiently exerted as in the conventional cladding layer.

At least a part of the laminated structure constituting the light reflecting layer may be formed of a superlattice structure. The low refractive index layer of the light reflecting layer is made of Al _x Ga _1-x N (0 <x <1), and the high refractive index layer is Al _y Ga _1-y N (0 ≦ y < 1, y <x) or In _z Ga _1-z N (0 <z ≦ 1).

  Furthermore, the active layer is formed so as to make the average refractive index on the non-light emitting portion side smaller than the average refractive index on the light emitting portion side of the active layer, thereby forming a band gap in a region that does not contribute to light emission of the active layer. Can be increased, and the carrier confinement effect is exhibited. Here, the average refractive index means a refractive index when acting as a whole of a laminated structure of a low refractive index layer and a high refractive index layer.

  By forming the active layer with a double heterojunction structure in which the active layer is sandwiched between an n-type layer and a p-type layer having a band gap energy larger than that of the active layer, carriers are not limited to the position and composition of the light reflecting layer. Thus, a nitride semiconductor light emitting device with excellent luminous efficiency can be obtained.

  According to the present invention, since the active layer is formed with a structure in which layers having different refractive indexes are alternately laminated with a thickness of λ / (4n), light is emitted from a part of the laminated structure and emitted from another part. Even if it does not, the laminated structure part which is not issued contributes as a reflective layer. That is, in the nitride semiconductor, the lifetime of holes is short, so holes injected from the p-type layer do not reach the entire active layer, but recombine with electrons in the active layer portion close to the p-type layer to emit light. To do. Therefore, a region that does not emit light is formed on the side close to the substrate. Since this non-light emitting region has the above-mentioned laminated structure, it becomes a distributed Bragg reflection layer, and light emitted from the light emitting part is reflected immediately on the substrate side of the light emitting part. Therefore, the emitted light can be emitted to the surface side without waste. Of this active layer, the part of λ / (4n) of the high refractive index layer of the part that becomes the light emitting part (the part where holes reach) has a multilayer structure of a high refractive index layer and a low refractive index layer. By doing so, a quantum well structure of a well and a barrier can be obtained, and a highly efficient light emitting portion having a multiple quantum well structure can be obtained.

When the active layer has the multilayer structure described above, since the active layer forms a light emitting portion, the composition of the nitride semiconductor is limited by a desired emission wavelength and cannot be set freely. Since the layer is a layer in which carriers are confined, its thickness is limited, and the number of pairs in the laminated structure is also limited, so that the reflectivity cannot be sufficiently increased. However, since, for example, In _0.1 Ga _0.9 N and GaN are alternately stacked with a thickness of λ / (4n), the reflectance can be increased more than before, and light is emitted toward the substrate. More than half of the light can be directed to the surface side. As a result, absorption by the substrate can be suppressed, the external quantum efficiency can be greatly improved, and a nitride semiconductor light emitting device having excellent light emission efficiency can be obtained.

  In addition, as described above, the active layer multilayer structure may not be able to achieve a sufficiently high reflectance. In that case, part of the emitted light travels to the substrate side and is made by the substrate made of SiC. The light emitted from the active layer is absorbed and wasted. However, by forming a light reflection layer between the active layer and the substrate, part of the leaked light can also be completely reflected on the surface side and used effectively. Since this light reflecting layer is provided anywhere between the active layer and the substrate, there is no loss of light unless a layer that absorbs light emitted between them is provided. When serving as a layer (cladding layer), the layer is provided in contact with the active layer. When serving as a relaxation layer of the SiC substrate and the nitride semiconductor layer, the layer is provided in contact with the SiC substrate. The light that cannot be reflected by the active layer is also reflected by the light reflecting layer and can be efficiently emitted from the surface side. As a result, a nitride semiconductor light emitting device having excellent external quantum efficiency can be obtained without absorbing light emitted from the substrate while forming one electrode on the back surface of the substrate using the SiC substrate.

  Furthermore, by forming at least a part of the light reflecting layer with a superlattice structure, it is possible to prevent distortion due to lattice mismatch and thermal expansion coefficient difference between the low refractive index layer and the high refractive index layer. A good nitride semiconductor layer can be obtained. Furthermore, by making the refractive index of the low refractive index layer on the non-light emitting portion side smaller than the refractive index of the low refractive index layer on the light emitting portion side of the active layer, it is possible to confine carriers only in the light emitting region, and further increase the luminous efficiency. Can be high.

Next, a nitride semiconductor light emitting device according to the present invention will be described with reference to the drawings. The nitride semiconductor light-emitting device according to the present invention has an active layer 5 in which at least a light-emitting portion is formed on a SiC substrate 1 as shown in FIG. And a pair of upper electrodes 11 and a lower electrode 12 are provided on the back surface of the SiC substrate 1 and on the front surface side of the semiconductor multilayer portion 9, respectively. The active layer 5 has a high refractive index where the emission wavelength is λ, the refractive index of the semiconductor layer is n (the refractive index of the high refractive index layer 51 is n ₁ , and the refractive index of the low refractive index layer 52 is n ₂ ). The layers 51 and the low refractive index layers 52 are alternately stacked with a thickness of λ / (4n).

  In the example shown in FIG. 1, the active layer 5 is a double heterojunction light emitting layer forming portion 7 sandwiched between an n-type GaN layer 4 and a p-type GaN layer 6 having a larger band gap than the active layer 5. Further, on the SiC substrate 1, the buffer layer 2, the light reflecting layer (n-type DBR layer) 3 between the buffer layer 2 and the light emitting layer forming portion 7, and the p-type contact layer made of GaN on the light emitting layer forming portion 7. By providing 8, the semiconductor stacked portion 9 is formed. However, the structure is not limited to this, and the buffer layer 2 and the n-type layer 4 may be omitted as in the example described later, and at least the active layer 5 that forms the light emitting portion is provided. That's fine.

As shown in the band diagram of FIG. 1B, the active layer 5 has a high refraction made of, for example, In _r Ga _1-r N (0 <r ≦ 1, eg, r = 0.1). The refractive index layer 51 is formed with a thickness of λ / (4n ₁ ) where the refractive index is n ₁ and the wavelength of light to be emitted is λ, and, for example, In _s Ga _1-s N (0 ≦ s ≦ A low dielectric constant refractive index layer 52 of 0.5, s <r, for example, s = 0) is formed with a thickness of λ / (4n ₂ ) where the refractive index is n _2. About 1 to 10 pairs of low dielectric constant layers 52 are alternately stacked to form a total thickness of about 0.08 to 0.8 μm. In the active layer 5, the portion that actually emits light is several high refractive index layers on the p-type layer 6 side, and it is confirmed by experiments that the active layer 5 on the n-type layer 4 side does not emit light. Has been. The composition of the high-refractive index layer 51 that emits light is determined according to the wavelength of light to be emitted. For example, the above-described composition is used when light having a wavelength of 450 nm is emitted, but light having other wavelengths is emitted. In some cases, this composition changes. In the present invention, since the layer having different refractive index is laminated with a thickness of λ / (4n) even in the non-light emitting portion, it becomes a distributed Bragg reflector, which emits light and proceeds toward the substrate side. Is reflected to the p-type layer 6 side. The active layer may be non-doped, n-type, or p-type, but is preferably formed so that the light emitting portion (pn junction) is on the upper surface side.

Since the high refractive index layer 51 and the low refractive index layer 52 are each formed with a thickness of λ / (4n), for example, when emitting blue light having a wavelength of 450 nm with the above-described composition, the high refractive index layer The thickness of the layer 51 is 46 nm, which is an intermediate layer structure between the bulk structure and the quantum well structure. Therefore, although light emission having both characteristics is performed, the high refractive index layer 52 can also be formed with a quantum well structure. An example is shown in FIG. 2 in a band diagram. That is, the example shown in FIG. 2 is an example in which only the portion constituting the light emitting portion has a multiple quantum well structure (MQW), and the n-type layer 4 side has the same structure as that shown in FIG. The high refractive index layer 51 and the low refractive index layer 52 not having the quantum well structure have the same composition as the example shown in FIG. 1 and are similarly formed. In the multiple quantum well structure, for example, about 5 sets of well layers 511 made of In _0.1 Ga _0.9 N are formed with a period of 3 nm and barrier layers 512 made of GaN are 6 nm. The entire thickness is λ / (4n).

  In the above example, the light emitting portion and the non-light emitting portion are formed with substantially the same composition. However, since the light emitting wavelength is not limited in the non-light emitting portion, for example, as shown in FIG. The band gap can be increased. 6A is a diagram corresponding to FIG. 1B of the active layer portion, and FIG. 6B is a diagram corresponding to FIG. In each figure, the purpose is to show the layer structure, and the number of layers is conceptually shown in any figure, and is not the exact number of layers, but in FIG. 1, FIG. 2, and FIG. The difference in the number of layers is meaningless. In this way, by making the refractive index in the non-light emitting portion smaller than the refractive index in the light emitting portion, the band gap in the non-light emitting portion can be increased, and the carrier confinement effect is exhibited, and only the light emitting portion is exhibited. You can confine your carrier.

  Therefore, if the non-light-emitting portion is made thick to increase the difference in refractive index between the high refractive index layer 51 and the low refractive index layer 52 or increase the number of sets to sufficiently increase the reflectance, light emission The light emitted from the part can be reflected immediately to the surface side by the active layer, and the band gap of the non-light-emitting part can be increased to have a carrier confinement effect, and the conventional cladding layer is unnecessary. Become.

As the substrate, a single crystal SiC substrate 1 is used, and in the example shown in FIG. A buffer layer 2 made of, for example, Al _0.2 Ga _0.8 N is formed on the SiC substrate 1. The buffer layer 2 is for relaxing a lattice mismatch between SiC and the nitride semiconductor layer to grow a nitride semiconductor layer having good crystallinity, and preferably has a lattice constant close to that of SiC. From this point of view, AlN has the closest lattice constant to SiC, but AlN is an insulator, and in order to provide one electrode on the back surface of the substrate, it is necessary to make it highly conductive. Therefore, an AlGaN-based compound is used. However, when the Al mixed crystal ratio is increased, the lattice constant is preferably close to that of the SiC substrate 1, but the carrier concentration cannot be increased as the Al mixed crystal ratio is increased. The buffer layer 2 is provided with the above-described composition from both the constants.

In the example shown in FIG. 1, a light reflection layer (DBR layer) 3 is formed on the buffer layer 2. The light reflecting layer 3 has the same conductivity type as that of the substrate 1 and is a DBR low refractive index layer (active layer layer) made of, for example, n-type Al _x Ga _1-x N (0 <x <1, for example, x = 0.3). 31 and n-type Al _y Ga _1-y N (0 ≦ y <1, y <x, for example y = 0) or In _z Ga _1-z The DBR high refractive index layer 32 made of N (0 <z ≦ 1) is divided into λ / (4n ₃ ) and λ / (4n ₄ ) (n ₃ and n ₄ are a DBR low refractive index layer and a DBR high refractive index, respectively. The layers are alternately stacked with a thickness of each refractive index), and about 10 to 50 sets are stacked. Thus, by alternately stacking two layers having different refractive indexes, a distributed Bragg reflection layer (DBR) is formed, and in particular, by forming each layer to a thickness of λ / (4n), Reflect well.

The light reflecting layer 2 includes a DBR low refractive index layer 31 made of, for example, Al _x Ga _1-x N (0 <x <1, eg, x = 0.3) and Al _y Ga _1-y N (0 ≦ y < 1 and y <x, for example, y = 0), it is possible to grow both at a high temperature of about 700 to 1200 ° C. and to be crystalline at a high temperature. A good nitride semiconductor layer can be stacked, and even when both layers are stacked alternately, it can be grown without changing the temperature, which is preferable. In this case, since it is difficult to increase the refractive index difference between the two layers, it is necessary to increase the number of layers or increase the Al mixed crystal ratio in the DBR low refractive index layer 31 in order to increase the reflectance. Even if the mixed crystal ratio of Al is increased, the carrier concentration can be increased to form a laminated structure with the GaN layer, instead of forming a bulky thick layer like the buffer layer, and the lattice with the SiC substrate can be increased. It is easy to form a high-quality film as the constant approaches.

However, if the refractive index difference is still increased while the Al mixed crystal ratio is not increased or the Al mixed crystal ratio of the DBR low refractive index layer 31 is increased, an InR is used as the DBR high refractive index layer 32. _{z Ga 1-z N (0} <z ≦ 1, for example z = 0.02) can also be used. In this case, the AlGaN-based compound grows at the aforementioned high temperature, and the InGaN-based compound grows at a low temperature of about 400 to 1000 ° C.

In addition, the DBR low refractive index layer 31 and the DBR high refractive index layer 32 described above are not the bulk layer described above, and both layers are further refracted as shown in the band diagram of FIG. 5, for example. A thin layer having a different rate, for example, In _v Ga _1-v N (0 <v <1, for example, v = 0.01, the adjustment layer 321 is about 0.5 to 2 nm, for example, Al _w Ga _1-w N ( Even if the relaxation layer 322 having 0 ≦ w <1, for example, w = 0, is formed in a superlattice structure of about 1 to 3 nm, the average refractive index of each superlattice structure is low and high. By alternately stacking as described above, it is possible to suppress the generation of crystal defects based on the difference in the lattice constant due to the superlattice structure while similarly contributing as the light reflecting layer. Both the DBR high refractive index layer 32 are limited to the above example. The invention is not only to be laminated to be easy to reflect light at different refractive index layers.

In the example shown in FIG. 1, a light emitting layer forming portion 7 having a double heterojunction structure in which an active layer 5 is sandwiched by an n-type layer 4 and a p-type layer 6 having a larger band gap is formed. In the example shown in FIG. 1, the light emitting layer forming portion 7 is represented by a general formula of Al _a Ga _b In _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1). For example, an n-type layer 4 made of n-type GaN having a thickness of about 0.5 μm and a p-type layer 6 made of p-type GaN having a thickness of about 0.5 μm so that the active layer 5 is sandwiched between the nitride materials. It consists of a double hetero structure provided with. The n-type layer 4 and the p-type layer 6 are preferably made of a material having a band gap larger than that of the active layer 5 from the viewpoint of confining carriers in the active layer 5, and may be an AlGaN-based compound instead of the aforementioned GaN. The n-type layer 4 and the p-type layer 6 may be formed in different compositions from each other, and the present invention is not necessarily limited thereto.

  When this light emitting layer forming portion 7 is used as a vertical resonator such as a surface emitting laser, for example, the resonator is adjusted to have an overall thickness of λ / n. Easy and preferable. In this case, the thickness is adjusted by the thickness of the n-type layer 4 and the p-type layer 6. In the case of a vertical resonator, a second light reflecting layer is formed on the light emitting layer forming portion 7 (not shown). In this sense, the composition and film thickness of the n-type layer 4 and the p-type layer 6 are also adjusted as appropriate so as to function as a spacer layer or as an electron barrier layer or a hole barrier layer.

  As shown in FIG. 6 described above, by increasing the band gap of the non-light emitting portion of the active layer 5, for example, the structure shown in FIG. 1 has the functions of the n-type layer 4 and the DBR layer 3. The layer may be omitted. Furthermore, if the Al mixed crystal ratio of the low refractive index layer 52 of the non-light emitting portion is increased, the lattice mismatch with the SiC substrate 1 is alleviated to some extent, and the buffer layer 2 may be omitted. That is, the active layer 5 having a structure as shown in FIG. 6 may be directly laminated on the SiC substrate 1. In this case, the conductivity type may be the same conductivity type as that of the SiC substrate 1, that is, the n-type in the example shown in FIG.

  By providing the contact layer 8 made of, for example, p-type GaN on the p-type layer 6, the semiconductor multilayer portion 9 is configured together with the buffer layer 2, the DBR layer 3, and the light emitting layer forming portion 7. However, the semiconductor laminated portion 9 does not need all these layers as described above, and it is sufficient that the semiconductor laminated portion 9 has at least the active layer 5. The contact layer 8 is provided to improve the ohmic contact with the electrode (translucent conductive layer) provided on the upper portion of the contact layer 8 and to diffuse the current from the upper electrode 11 throughout the light emitting element chip. A translucent conductive layer may be provided directly on the surface of the shape layer 6. In particular, if a GaN layer is used as the p-type layer 6, the same layer can be shared, but it is better not to increase the impurity concentration in the vicinity of the active layer, but on the upper electrode 11 side, the carrier concentration should be as high as possible. It is preferable from the viewpoint of ohmic contact and current diffusion, and it is preferable to provide a different layer by changing the carrier concentration, or to change the carrier concentration sequentially to give a gradient. When AlGaN is used as the p-type layer 6, it is preferable to provide the contact layer 8 with GaN separately from the p-type layer 6 because GaN can easily increase the carrier concentration. However, it may be formed of an AlGaN compound layer or an InGaN compound layer.

  On this contact layer 8, a translucent conductive layer 10 is formed, for example, with a Ni—Au layer having a thickness of about 2 to 100 nm, or with a ZnO layer or ITO layer having a thickness of about 0.1 to 2 μm. It is formed by. Although the ZnO layer or the ITO layer is transparent even if it is thick, the Ni—Au layer is thin because it does not transmit light when it is thick. In the example shown in FIG. 1, the ZnO layer is formed to a thickness of about 0.3 μm. The translucent conductive layer 10 is a nitride semiconductor layer, particularly a p-type nitride semiconductor layer, in which it is difficult to increase the carrier concentration, it is difficult to diffuse current over the entire surface of the chip, and an upper portion made of a metal film as an electrode pad. It is provided in order to improve the problem that it is difficult to make ohmic contact with the electrode 11, and it does not matter if these problems can be solved.

  In the example shown in FIG. 1, the upper electrode 11 is formed as a p-side electrode because the upper surface side of the semiconductor multilayer portion 9 is a p-type layer. For example, Ti / Au, Pd / Au, Ni—Au, etc. In general, the thickness is about 0.1 to 1 μm. Further, a lower electrode (n-side electrode) 12 is formed on the back surface of SiC substrate 1 with a thickness of about 0.1 to 1 μm as a whole, for example, with a Ti—Al alloy or Ti / Au laminated structure.

In order to form the semiconductor laminated portion 9 in the n-type, it is possible to mix Se, Si, Ge, and Te into the reaction gas as an impurity source gas such as H ₂ Se, SiH ₄ , GeH ₄ , and TeH _4. In order to obtain p-type, Mg or Zn is mixed into the raw material gas as an organometallic gas such as cyclopentadienylmagnesium (Cp ₂ Mg) or dimethylzinc (DMZn). However, in the case of the n-type, even if impurities are not mixed, since N easily evaporates at the time of film formation and easily becomes an n-type, the property can be used.

Next, a method for producing the nitride semiconductor light emitting device of the present invention will be briefly described with specific examples. First, the SiC substrate 1 is set in, for example, an MOCVD (Metal Organic Chemical Vapor Deposition) apparatus, and component gases of a semiconductor layer to be grown, such as trimethyl gallium, trimethyl aluminum, trimethyl indium, ammonia gas, and n-type dopant gas, are used. Any one of H ₂ Se, SiH ₄ , GeH ₄ , TeH ₄ and a necessary gas of DMZn or Cp ₂ Mg as a p-type dopant gas is introduced together with a carrier gas H ₂ gas or N ₂ gas, For example, at a temperature of about 700 to 1200 ° C., the n-type Al _0.2 Ga _0.8 N buffer layer 2, the DBR low refractive index layer 31 made of Al _0.3 Ga _0.7 N, and the high refractive index layer 32 made of GaN are each λ / (4n). The light reflecting layer 3 is formed by alternately stacking about 20 sets with a thickness of λ.

Next, an n-type layer 4 made of, for example, GaN and having a thickness of about 0.5 μm, for example, a high-refractive index layer 51 made of In _0.1 Ga _0.9 N and a low-refractive index layer 52 made of GaN are each λ / (4n) thick. Then, two active layers 5 alternately laminated, for example, a p-type layer 6 made of GaN and having a thickness of about 0.5 μm, and a p-type contact layer 8 made of p-type GaN and having a thickness of about 0.05 to 2 μm are sequentially epitaxially grown. Thus, the semiconductor stacked portion 9 is formed.

After that, a SiO ₂ protective film is provided on the entire surface of the contact layer 8 and annealed at 400 to 800 ° C. for about 20 to 60 minutes to activate the contact layer 8 and the p-type layer 6. When the annealing is completed, the wafer is put into a sputtering apparatus or a vacuum evaporation apparatus, and a light-transmitting conductive layer 10 made of ZnO is formed on the surface of the contact layer 8 to a thickness of about 0.3 μm. 11 is formed. Thereafter, by wrapping the back surface side of the SiC substrate 1, the SiC substrate 1 is thinned, and a metal film such as Ti or Au is similarly formed on the back surface of the substrate 1 to form the lower electrode 12. . Finally, a chip of a nitride semiconductor light emitting device is obtained by scribing to make a chip.

  According to the present invention, two types of nitride semiconductor layers having different refractive indexes are used as active layers, each having a thickness of λ / (4n), while using a SiC substrate to laminate the nitride semiconductor layers. Since the layers are alternately stacked, the emitted light can be efficiently extracted from the surface side without being absorbed by the SiC substrate that easily absorbs the emitted light. That is, in the active layer, light is actually emitted near the pn junction, and if the active layer is formed non-doped or n-type, light is emitted on the p-type layer side, and in the portion away from the p-type layer, the active layer emits light. In the present invention, since the active layer has a structure in which two layers having different refractive indexes are alternately laminated, the portion that does not emit light functions as a reflective layer and emits light to the substrate side. The light which is going to travel to the surface side is reflected by the active layer. As a result, even if a SiC substrate is used as the substrate, the emitted light can be extracted from the surface side very efficiently, the external quantum efficiency can be greatly improved, and a very bright nitride semiconductor light emitting device can be obtained. .

  In this case, if it is difficult to make a reflective layer that reflects light sufficiently with only the active layer, the non-light-emitting portion of the active layer is thickened to increase the laminated structure of the low-refractive index layer and the high-refractive index layer. Reflects almost all of the light that is emitted and travels to the substrate side by providing a semiconductor layer (n-type layer) that forms a heterojunction structure with the layer or a light reflecting layer on the substrate side. Can be used. Further, by using a nitride semiconductor having a large band gap in the non-light emitting portion of the active layer, a carrier confinement effect is exhibited, and a double heterojunction structure is formed in this portion, and it is necessary to provide an n-type layer or a p-type layer separately. The use of a nitride semiconductor with a high Al mixed crystal ratio in the layer that constitutes the light reflection layer eliminates the need for a buffer layer, grows the light reflection layer directly on the SiC substrate, The layer can be grown directly on the SiC substrate.

  As a result, the lower electrode connected to the lower conductive type layer of the stacked semiconductor layer can be formed on the back surface of the substrate, and a part of the stacked semiconductor stacked portion is removed by etching to form the lower conductive type layer. There is no need to expose, and there is no adverse effect of contamination caused by etching a part of the semiconductor stacked portion, so that an electrode can be easily formed. Thus, a vertical nitride semiconductor light emitting device having one electrode formed on the back surface of the substrate is obtained, and a high-performance nitride semiconductor light emitting device that is easy to manufacture and easy to use is obtained.

  In the above example, the substrate is n-type and the lower semiconductor layer is n-type. However, the substrate may be p-type and the p-type layer may be lower. In addition, the light emitting layer forming portion has a double hetero structure having a sandwich structure in which an active layer is sandwiched between an n-type layer and a p-type layer, but another semiconductor layer is inserted between any of the layers, or a single heterostructure. A structure in which a laminated structure of λ / (4n) is formed in one layer of the junction structure or the homo pn junction may be employed.

It is explanatory drawing of the cross section and band figure of the nitride semiconductor light-emitting device by this invention. It is a band figure which shows the other structural example of the active layer shown by FIG. It is sectional explanatory drawing of the other structural example of the nitride semiconductor light-emitting device by this invention. FIG. 6 is a cross-sectional explanatory view showing still another configuration example of the nitride semiconductor light emitting device according to the present invention. It is a band figure which shows the other structural example of the light reflection layer shown by FIG. FIG. 6 is a band diagram showing still another configuration example of the active layer shown in FIG. 1. It is sectional explanatory drawing which shows an example of the conventional nitride semiconductor light-emitting device.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 SiC substrate 2 Buffer layer 3 Light reflection layer 4 N-type layer 5 Active layer 6 P-type layer 9 Semiconductor laminated part 11 Upper electrode 12 Lower electrode 31 DBR low refractive index layer 32 DBR high refractive index layer 51 High refractive index layer 52 Low Refractive index layer

Claims (9)

  A SiC substrate, a nitride semiconductor multilayer portion including an active layer provided on the SiC substrate and having at least a light emitting portion formed thereon, and a pair of electrodes provided on the back surface of the SiC substrate and the front surface side of the semiconductor multilayer portion, respectively. The active layer is alternately laminated with a high refractive index layer and a low refractive index layer at a thickness of λ / (4n), where λ is the emission wavelength and n is the refractive index of the semiconductor layer. A nitride semiconductor light emitting device having a structure.   2. At least a part of the high refractive index layer of the active layer is formed to a thickness of λ / (4n) by a laminated structure of a high refractive index thin layer and a low refractive index thin layer. The nitride semiconductor light emitting device described.   A buffer layer formed of a nitride semiconductor is provided on the SiC substrate surface, and a light reflection layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided between the buffer layer and the active layer. The nitride semiconductor light-emitting device according to claim 1 or 2.   The nitride semiconductor according to claim 1, wherein a light reflecting layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided between the SiC substrate and the active layer in contact with the SiC substrate. Light emitting element.   5. The light reflecting layer having a laminated structure of a low refractive index layer and a high refractive index layer is provided between the active layer and the SiC substrate in contact with the active layer. The nitride semiconductor light emitting device described.   6. The nitride semiconductor light emitting device according to claim 3, 4 or 5, wherein at least a part of the laminated structure constituting the light reflecting layer is formed by a superlattice structure. The low refractive index layer of the light reflecting layer is made of Al _x Ga _1-x N (0 <x <1), and the high refractive index layer is Al _y Ga _1-y N (0 ≦ y <1, y <x) or _{in z Ga 1-z N (} 0 <z ≦ 1) nitride semiconductor light emitting device according to claim 3, 4 or 5, wherein a layer having a.   The nitride semiconductor according to claim 1, wherein the active layer is formed so that an average refractive index on the non-light emitting portion side is smaller than an average refractive index on the light emitting portion side of the active layer. Light emitting element.   5. The nitride semiconductor light emitting device according to claim 1, wherein the active layer is a double heterojunction structure in which an n-type layer and a p-type layer having a larger band gap energy than the active layer are sandwiched.

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