JP6521750B2 - Nitride semiconductor light emitting device - Google Patents

Description

  The present invention relates to a nitride semiconductor light emitting device.

  Light emitting devices combining semiconductor light emitting elements and phosphors are attracting attention as next-generation light emitting devices that are expected to have low power consumption, small size, high brightness, and broad color reproducibility, and are actively researched and developed. It has been done. The primary light emitted from the light emitting element is generally in the range of long wavelength ultraviolet light to blue, that is, 380 to 480 nm. Then, a light emitting device is realized in which these light emitting elements are combined with wavelength conversion units using various phosphors.

  In particular, a white light emitting device in which a blue light emitting element having a light emission peak at around 450 nm and a phosphor are combined is widely used for general illumination applications and backlights for liquid crystal displays (LCDs (Liquid Crystal Display)). .

These white LED light emitting devices are required to have better color reproducibility, higher conversion efficiency (brightness), and better temperature characteristics, for example, Eu _a Si _b Al _c O _d N divalent and europium-activated oxynitride green light emitting phosphor is a β-type SiAlON, such as _{e, (Ca, Eu) AlSiN} 3 2 divalent europium activated nitride such as red light emitting phosphor and _K 2 ( By using a white light emitting device combined with a manganese-activated fluoride red phosphor such as Si, Mn) F ₆ etc., color reproducibility is better than before, conversion efficiency (brightness) is high and temperature characteristics are good White light emitting devices have been realized (for example, Japanese Patent Application Laid-Open No. 2007-180483 (Patent Document 1), International Publication No. 2009/110285 (Patent Document 2), etc.).

  On the other hand, as a light emitting element to be combined with the above-mentioned phosphor, a nitride semiconductor light emitting element formed of a III-V group compound semiconductor material containing nitrogen is generally used. A nitrogen-containing III-V compound semiconductor material (hereinafter, referred to as "nitride semiconductor material") has a band gap corresponding to the energy of light having a wavelength in the infrared region to the ultraviolet region. Therefore, the nitride semiconductor material is useful as a material of a light emitting element that emits light having a wavelength in the infrared region to an ultraviolet region, a material of a light receiving element that receives light having a wavelength of that region, and the like.

  In a nitride semiconductor light emitting device using a nitride semiconductor material having the above characteristics, a quantum well structure is generally employed in the light emitting layer. When a voltage is applied to a nitride semiconductor light emitting device having a quantum well structure in the light emitting layer, electrons and holes are recombined in the quantum well layer of the light emitting layer to generate light. The light emitting layer having a quantum well structure may be a single quantum well (SQW) structure, but multiple quantum wells (quantum quantum well) in which quantum well layers and barrier layers are alternately stacked. MQW) In many cases.

  For nitride semiconductor light emitting devices in which a quantum well structure is adopted for the light emitting layer, several attempts have been made to obtain desired characteristics by devising the MQW structure of the light emitting layer. For example, Japanese Patent Application Laid-Open No. 2007-142426 (Patent Document 3) describes variations in color distribution generated from a nitride semiconductor light emitting device having a plurality of active layers emitting light of different wavelengths with respect to the arrangement and number of active layers. A nitride semiconductor light emitting device aimed at solving the problem is disclosed. In Patent Document 3, an active layer having a short wavelength among a plurality of active layers is disposed adjacent to the p-type nitride layer side, and the number of quantum well layers is smaller than the number of active layers having a long wavelength. It is stated that by doing so, the intrinsic emission of multiple active layers with different wavelength light can have the desired level of distribution, and that a monolithic white light emitting device can be realized.

Unexamined-Japanese-Patent No. 2007-180483 WO 2009/110285 JP, 2007-142426, A

  In the white light emitting device combining the conventional blue light emitting element and the green light emitting phosphor / red light emitting phosphor, although improvement has been made, further improvement of color reproducibility and conversion efficiency (brightness) is required. It is done.

  In view of the above circumstances, an object of the present invention is a nitride semiconductor capable of improving color reproducibility and conversion efficiency (brightness) even with a combination of conventional green light emitting phosphor / red light emitting phosphor It is providing a light emitting element.

  The inventors of the present invention have a combination of the blue light emitting element and the green light emitting phosphor / red light emitting phosphor, and a light emitting element which emits ultraviolet light or blue violet light with a shorter emission wavelength than the blue light emitting element. It has been found that, by using the above, high conversion efficiency (brightness) can be obtained without increasing the input energy to the light emitting device.

  The nitride semiconductor light emitting device according to the present invention comprises an n-type nitride semiconductor layer, a p-type nitride semiconductor layer, and a multiple quantum provided between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer. A well light emitting layer, wherein the multiple quantum well light emitting layer comprises a second light emitting layer, an intermediate well layer, and a first light emitting layer from the side close to the p-type nitride semiconductor layer; One light emitting layer includes a plurality of first quantum well layers, and a first barrier layer provided between the plurality of first quantum well layers, and the second light emitting layer includes a plurality of second quantum well layers. And a second barrier layer provided between the plurality of second quantum well layers, wherein the band gap energy of the second quantum well layer is larger than the band gap energy of the first quantum well layer. It is characterized by

  In the nitride semiconductor light emitting device according to the present invention, the band gap energy of the intermediate well layer is a value between the band gap energy of the first quantum well layer and the band gap energy of the second quantum well layer. preferable.

  In the nitride semiconductor light emitting device according to the present invention, the layer thickness of the intermediate well layer is preferably larger than the layer thickness of the first quantum well layer or the second quantum well layer.

  In the nitride semiconductor light emitting device according to the present invention, the number of layers of the second quantum well layer is smaller than the number of layers of the first quantum well layer, or the number of layers of the second quantum well layer is the first quantum well layer. It is preferable that the number of layers is larger than the number of layers.

  In the nitride semiconductor light emitting device of the present invention, the emission spectrum has an emission peak from a wavelength emitted from the first quantum well layer and an emission peak from a wavelength emitted from the second quantum well layer. Is preferred.

  In the nitride semiconductor light emitting device according to the present invention, the layer thickness of the second quantum well layer is preferably smaller than the layer thickness of the first quantum well layer.

  The present invention is a nitride semiconductor light emitting device monolithically provided with a first light emitting layer that emits blue light and a second light emitting layer that emits (near) ultraviolet light. With such a configuration, it is possible to provide a nitride semiconductor light emitting device capable of improving color reproducibility and conversion efficiency (brightness). Further, according to the present invention, the light in the large current drive region is provided by providing the intermediate well layer between the first light emitting layer that emits blue light and the second light emitting layer that emits (near) ultraviolet light. It is possible to provide a nitride semiconductor light emitting device capable of reducing the drop in output.

FIG. 1 is a schematic cross-sectional view of a nitride semiconductor light emitting device according to an embodiment which is an example of a nitride semiconductor light emitting device of the present invention. It is an example of the band gap energy figure of the multiple quantum well light emitting layer 14 suitably used for the nitride semiconductor light-emitting device of this invention. FIG. 7 shows an emission spectrum of the nitride semiconductor light emitting device manufactured in Example 1 when driven at IF = 20 mA. FIG. 5 is a graph showing external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 1 in comparison with Comparative Example 1. FIG. FIG. 10 is a band gap energy diagram of the multiple quantum well light emitting layer in the sample of Comparative Example 2. 7 is a graph showing the external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 1 in comparison with Comparative Example 2. FIG. ⁷ is an emission spectrum at a current density of 13.4 A / cm ² of the nitride semiconductor light emitting device manufactured in Example 1. FIG. 7 is a band gap energy diagram showing a multiple quantum well structure in the nitride semiconductor light emitting device produced in Example 2. 7 is an emission spectrum of the nitride semiconductor light emitting device manufactured in Example 2. FIG. FIG. 16 is a graph showing external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 2 in comparison with Comparative Example 2. FIG. FIG. 7 is a band gap energy diagram showing a multiple quantum well structure in the nitride semiconductor light emitting device produced in Example 3. FIG. 16 is a graph showing external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 3 in comparison with Comparative Example 2. FIG. 7 is an emission spectrum of the nitride semiconductor light emitting device manufactured in Example 3.

  Hereinafter, embodiments of the present invention will be described. In the drawings of the present invention, the same reference numerals represent the same or corresponding parts.

  Also, in the present specification, “barrier layer” refers to a layer sandwiched between quantum well layers. The layer not sandwiched by the quantum well layers is referred to as the “first barrier layer” or the “last barrier layer”, and the notation is changed to a layer sandwiched by the quantum well layers.

  Further, in the present specification, the term "dopant concentration" and the term "carrier concentration" which is the concentration of electrons and holes generated with the doping of n-type dopant or p-type dopant are used. The relationship will be described later.

  Furthermore, in the present specification, the “carrier gas” is a gas other than the group III source gas, the group V source gas, and the dopant source gas. The atoms that constitute the carrier gas are not taken into the nitride semiconductor layer or the like.

  Further, in the present specification, the “n-type nitride semiconductor layer” includes therein a low carrier concentration n-type nitride semiconductor layer or an undoped layer having a thickness that does not hinder the flow of electrons practically. It is also good.

  In the present specification, the “p-type nitride semiconductor layer” also includes a low carrier concentration p-type nitride semiconductor layer or an undoped layer having a thickness that does not hinder the flow of holes in practice. Good. The phrase "does not hinder in practical use" means that the operating voltage of the nitride semiconductor light emitting device is at a practical level.

[Configuration of nitride semiconductor light emitting device]
FIG. 1 shows a schematic cross-sectional view of a nitride semiconductor light emitting device 1 according to an embodiment which is an example of the nitride semiconductor light emitting device of the present invention. The nitride semiconductor light emitting device 1 of the example shown in FIG. 1 includes a substrate 3, a buffer layer 5, a nitride semiconductor underlayer 7, an n-type contact layer 8 and a low temperature n sequentially provided on the substrate 3. Nitride semiconductor layer 10 (v pit generation layer), a nitride semiconductor multilayer structure body 120, a multiple quantum well light emitting layer 14, and three p type nitride semiconductor layers 16, 17 and 18 .

  A transparent electrode layer 23 is provided on the p-type nitride semiconductor layer 18, and a p electrode 25 is provided on the transparent electrode layer 23. In addition, an n electrode 21 made of Au or the like is provided on the upper surface of the n-type contact layer 8 exposed by etching, and a nitride semiconductor is provided to expose a part of the surface of the n electrode 21 and a part of the surface of the p electrode 25. The surface of the light emitting element is covered with a transparent insulating protective film 27.

[substrate]
For example, an insulating substrate such as sapphire or a conductive substrate such as GaN, SiC or ZnO can be used as the substrate 3. The thickness of the substrate 3 is not particularly limited, but the thickness of the substrate at the time of growth of the nitride semiconductor layer is preferably 900 μm to 1200 μm, and the thickness of the substrate at the time of use of the nitride semiconductor light emitting device is 50 μm. The thickness is preferably 300 μm or less.

  The convex portion 3 a and the concave portion 3 b may be formed on the upper surface of the substrate 3. The shapes of the convex portions and the concave portions are not particularly limited, but the convex portions are preferably substantially circular disposed at the apexes of substantially regular triangles in plan view, and the distance between the apexes of adjacent convex portions is 1 μm to 5 μm Is preferred. Further, the cross-sectional shape of the convex portion may be trapezoidal, and it is more preferable that the apex of the trapezoidal shape is rounded.

  The nitride semiconductor light emitting device of the present invention may be a nitride semiconductor light emitting device having no substrate by removing the substrate 3 after the growth of the nitride semiconductor layer on the substrate.

[Buffer layer]
The buffer layer 5, for example, _{Al s0 Ga t0 O u0 N 1} -u0 (0 ≦ s0 ≦ 1,0 ≦ t0 ≦ 1,0 ≦ u0 <1, s0 + t0 = 1) the nitride semiconductor layer of the formula consisting of Is preferably used, and more preferably an AlN layer or an AlON layer.

  Here, as the AlON layer constituting the buffer layer 5, it is preferable that a very small part (0.5 atomic% or more and 2 atomic% or less) of N is replaced by oxygen. In this case, the buffer layer is formed to extend in the normal direction of the growth surface of the substrate, so that the buffer layer 5 composed of an aggregate of columnar crystals with uniform crystal grains can be obtained.

  The thickness of the buffer layer 5 is not particularly limited, but is preferably 3 nm or more and 100 nm or less, and more preferably 5 nm or more and 50 nm or less.

  The crystallinity of the nitride semiconductor base layer described later is improved by using an AlON layer formed by a known sputtering method. The crystallinity of the nitride semiconductor underlayer can be confirmed by the half width of the X-ray rocking curve.

[Nitride semiconductor underlayer]
The nitride semiconductor base layer 7 can be formed on the surface of the buffer layer 5 by, for example, the MOCVD (Metal Organic Chemical Vapor Deposition) method.

As the nitride semiconductor underlying layer 7, for _{example, Al x0 Ga y0 In z0 N} (0 ≦ x0 ≦ 1,0 ≦ y0 ≦ 1,0 ≦ z0 ≦ 1, x0 + y0 + z0 = 1) III -nitride represented by the formula A layer made of a semiconductor can be used.

  As the nitride semiconductor underlayer 7, it is preferable to use a nitride semiconductor layer containing Ga as a group III element so as not to inherit crystal defects such as dislocations in a buffer layer composed of aggregates of columnar crystals. .

As nitride semiconductor base layer 7, for example, an n-type dopant may be doped in a range of 1 × 10 ¹⁷ pieces / cm ³ or more and 1 × 10 ¹⁹ pieces / cm ³ or less. However, in terms of maintaining good crystallinity, it is preferable that the nitride semiconductor underlayer 7 be undoped.

  For example, at least one selected from the group consisting of Si, Ge, and Sn can be used as the n-type dopant doped into the nitride semiconductor foundation layer 7. Among them, it is preferable to use Si as the n-type dopant. When Si is used as the n-type dopant doped in the nitride semiconductor underlayer, it is preferable to use silane or disilane as the n-type doping gas.

  By making the layer thickness of the nitride semiconductor base layer 7 as large as possible, the defects in the nitride semiconductor base layer decrease, but even if the layer thickness of the nitride semiconductor base layer 7 is increased to a certain degree or more, the nitride semiconductor base layer Defects saturate. Thereby, the layer thickness of the nitride semiconductor underlayer 7 is preferably 1 μm to 8 μm, and more preferably 3 μm to 5 μm.

[N-type contact layer]
The n-type contact layer 8 is, for example, a group III nitride semiconductor represented by the formula Al _{x 1} Ga _{y 1} In _{z 1} N (0 ≦ x1 ≦ 1, 0 ≦ y1 ≦ 1, 0 ≦ z1 ≦ 1, x1 + y1 + z1 = 1). A layer doped with an n-type dopant can be used as the layer consisting of Among them, as the n-type contact layer 8, an n-type dopant may be used in the Al _{x 2} Ga _{1-x 2} N (0 ≦ x2 ≦ 1, preferably 0 ≦ x2 ≦ 0.5, more preferably 0 ≦ x2 ≦ 0.1) layer. It is more preferable to use a layer doped with

The n-type dopant to be doped into the n-type contact layer 8 is not particularly limited, and, for example, at least one selected from the group consisting of Si, P, As and Sb can be used. Among them, it is preferable to use Si as the n-type dopant. The same is true for the layers described below. The n-type dopant concentration of the n-type contact layer is not particularly limited, but is preferably 1 × 10 ¹⁹ / cm ³ or less.

  When the layer thickness of the n-type contact layer 8 is as large as possible, the resistance of the n-type contact layer decreases. On the other hand, when the layer thickness of the n-type contact layer 8 is increased, the manufacturing cost of the nitride semiconductor light emitting device is increased. Although it is preferable that the layer thickness of the n-type contact layer 8 is 1 micrometer or more and 10 micrometers or less from balance of both, it is not specifically limited.

  In the embodiment to be described later, the n-type contact layer 8 is formed by two growth steps of temporarily interrupting the growth of the n-type GaN layer and then re-growing the same n-type GaN layer. However, the method of forming the n-type contact layer and the configuration thereof are not limited to this.

  For example, the n-type contact layer 8 may be a single layer or a plurality of two or more layers. When the n-type contact layer is a plurality of layers, each layer may have the same composition, or at least one layer may have a different composition. When the n-type contact layer is a plurality of layers, each layer may have the same layer thickness, or at least one layer may have a different layer thickness.

[Low temperature n-type nitride semiconductor layer (V pit generation layer)]
For example, a nitride represented by the general formula Al _{x 2} Ga _{y 2} In _{z 2} N (0 ≦ x 2 ≦ 1, 0 ≦ y 2 ≦ 1, 0 ≦ z 2 ≦ 1, x 2 + y 2 + z 1) as the low temperature n-type nitride semiconductor layer 10 A layer doped with an n-type dopant can be used for the layer made of a semiconductor material. Preferably, it is composed of a nitride semiconductor material represented by the general formula Al _{x 2} Ga _{1-x 2} N (0 ≦ x2 ≦ 1, preferably 0 ≦ x2 ≦ 0.3, more preferably 0 ≦ x2 ≦ 0.1). Layer or a layer formed of a nitride semiconductor material represented by the general formula _{Inz2Ga1 -z2N} (0 ≦ z2 ≦ 1, preferably 0 ≦ z2 ≦ 0.3, more preferably 0 ≦ z2 ≦ 0.1) Use a layer doped with n-type dopants.

The n-type dopant concentration of the low temperature n-type nitride semiconductor layer 10 is preferably lower than the n-type dopant concentration of the n-type contact layer, and more preferably 1 × 10 ¹⁹ / cm ³ or less. The low temperature n-type nitride semiconductor layer may be an undoped layer.

  The layer thickness of the low temperature n-type nitride semiconductor layer 10 is not particularly limited, but is preferably 50 nm or more and 500 nm or less.

  The low temperature n-type nitride semiconductor layer 10 may have a laminated structure, and in this case, the n-type dopant concentration of the low temperature n-type nitride semiconductor layer 10 corresponds to each of the layers constituting the low temperature n-type nitride semiconductor layer It is determined by dividing the total amount of n-type dopants contained in by the volume of the low temperature n-type nitride semiconductor layer.

  In addition, the low temperature n-type nitride semiconductor layer 10 has a function as a start point of the generation of the V pit structure. Here, “V pit structure” is generated due to threading dislocation, and from the inside of the low temperature n-type nitride semiconductor layer to the upper surface of the light emitting layer (the surface of the light emitting layer located on the p-type nitride semiconductor layer side) It means a crystal defect having a shape that is expanded toward the diameter. This V pit structure is partially formed in the low temperature n-type nitride semiconductor layer, the nitride semiconductor multilayer structure, and the multiple quantum well layer.

[Nitride semiconductor multilayer structure]
Nitride semiconductor multilayer structure 120, for example, the general formula _{Al x4 Ga y4 In z4 N (} 0 ≦ x4 ≦ 1,0 ≦ y4 ≦ 1,0 ≦ z4 ≦ 1, x4 + y4 + z4 = 1) and _Al x5 _Ga y5 _{In z5} N It is formed by repeatedly laminating two layers having different band gap energies consisting of (0 ≦ x5 ≦ 1, 0 ≦ y5 ≦ 1, 0 ≦ z5 ≦ 1, x5 + y5 + z5 = 1). The composition of the nitride semiconductor multilayer structure does not absorb the light emitted from the multiple quantum well light emitting layer 14, that is, any material capable of having a band gap energy higher than that of the multiple quantum well light emitting layer 14 is acceptable. The structure in which the wide band gap layer and the narrow band gap layer made of Si-doped InGaN are repeatedly stacked is more preferable because the crystallinity of the multiple quantum well light emitting layer 14 is improved.

[Multi-quantum well light emitting layer]
FIG. 2 shows an example of a band gap energy diagram of the multiple quantum well light emitting layer 14 suitably used for the nitride semiconductor light emitting device of the present invention. The horizontal axis in FIG. 2 indicates the layer thickness in the stacking direction, and the left direction means the side closer to the p-type nitride semiconductor layer. Further, the vertical axis of the figure indicates the magnitude of the band gap energy, which means that the band gap energy is large in the upward direction.

  In the example shown in FIG. 2, the multiple quantum well light emitting layer 14 is a light emitting layer (second light emitting layer) 142 of the second wavelength, a middle well layer 14 M, and a first wavelength from the side near the p-type nitride semiconductor layer. A light emitting layer (first light emitting layer) 141 of a wavelength is provided.

  In the first light emitting layer 141, a plurality of first quantum well layers 14W (14W1, 14W2, 14W3, 14W4) and a plurality of first barrier layers 14A (14A1, 14A2, 14A3, 14A4) are alternately stacked.

  In the second light emitting layer 142, a plurality of second quantum well layers 14V (14V1, 14V2, 14V3) and a plurality of second barrier layers 14B (14B1, 14B2, 14B3) are alternately stacked.

  In the example shown in FIG. 2, the intermediate well layer 14M is stacked between the first barrier layer 14A4 and the second barrier layer 14B1.

  The first barrier layer 14AZ is provided immediately above the nitride semiconductor multilayer structure 120, that is, in a layer in contact with the first quantum well layer 14W1. The last barrier layer 14B0 is provided directly on the second quantum well layer 14V3 located closest to the p-type nitride semiconductor layer 16 side.

The material of the first light emitting layer 141, the second light emitting layer 142, the intermediate well layer 14M, the first barrier layer 14AZ, and the last barrier layer 14B0 can be set arbitrarily, for example, general formula Al _x Ga _y In _z N ( It consists of the nitride semiconductor shown by 0 <= x <= 1, 0 <= y <= 1, 0 <= z <= 1, x + y + z = 1). The band gap energy of each layer can be adjusted by changing the values of x, y and z. For example, the band gap energy is decreased by increasing the In concentration to GaN, and the Al concentration is increased to the GaN. Band gap energy increases.

  With regard to the band gap energies of the second quantum well layer 14V (14V1, 14V2, 14V3) and the first quantum well layer 14W (14W1, 14W2, 14W3, 14W4), the band gap energy of the second quantum well layer 14V is the first quantum. It can be arbitrarily set on condition that it is larger than the band gap energy of the well layer 14W. In the nitride semiconductor light emitting device 1 of the present invention, the band gap energy of the second quantum well layer 14V is larger than the band gap energy of the first quantum well layer 14W, so that the light emitting wavelength of the first quantum well layer 14W is higher than that of the first quantum well layer 14W. The light emission wavelength of the quantum well layer 14V becomes a short wavelength, and among the light emitted from the first quantum well layer 14W, the light emitted to the upper surface of the chip, that is, the p-type nitride semiconductor layer side becomes difficult to be absorbed by the second quantum well layer, The luminous efficiency is improved. In addition, since the energy band gap of the second quantum well layer close to the p-type nitride semiconductor layer is large, holes are hardly trapped, and the light emission efficiency is improved.

  In the nitride semiconductor light emitting device 1 of the present invention, the band gap energy of the intermediate well layer 14M can be set arbitrarily, but the band gap energy of the second quantum well layer 14V and the band gap energy of the first quantum well layer 14W It is preferable that it is a value between. Here, the “value between” may be an intermediate value of the band gap energies of the second quantum well layer 14V and the first quantum well layer 14W, or a value closer to the second quantum well layer 14V. Or the value of the first quantum well layer 14W. In the nitride semiconductor light emitting device 1 of the present invention, the band gap energy of the intermediate well layer 14M is set to a value between the band gap energy of the second quantum well layer 14V and the band gap energy of the first quantum well layer 14W. The holes supplied from the side of the p-type nitride semiconductor layer are once trapped by the intermediate well layer 14M between the second quantum well layer 14V and the first quantum well layer 14W. Holes are easily diffused again into the well layer closer to the n-type nitride semiconductor layer, and light emission recombination is facilitated in the well layer closer to the n-type nitride semiconductor layer, and the light emission efficiency is improved.

  In the nitride semiconductor light emitting device 1 of the present invention, the layer thickness of the intermediate well layer 14M can be set arbitrarily, but the layer thickness of the second quantum well layer 14V (not the total thickness (total thickness of each layer) It is more preferable to be larger than the thickness of each layer of the quantum well layer. In this case, when the layer thicknesses of the plurality of second quantum well layers 14V are different from each other, the layer thickness of the intermediate well layer 14M may be larger than the maximum value of the layer thicknesses of the second quantum well layers 14V. preferable. By making the layer thickness of the intermediate well layer 14M larger than that of the second quantum well layer 14V, holes supplied from the p-type nitride semiconductor layer side are different between the second quantum well layer 14V and the first quantum well layer 14W. Once trapped in the middle well layer 14M with a large layer thickness between them, holes are easily diffused again from the middle well layer 14M to the well layer on the side closer to the n-type nitride semiconductor layer, thereby n-type nitride semiconductor Luminescent recombination in the well layer on the side closer to the layer is facilitated, and the luminous efficiency is improved.

  In the example shown in FIG. 2, the number of first quantum well layers 14W is four (ie, the number of first quantum well layers 14W1, 14W2, 14W3, 14W4) and the number of second quantum well layers 14V are three. (Ie, the second quantum well layers 14V1, 14V2 and 14V3), but the number of layers in either well layer can be set arbitrarily, and the number of layers of the first quantum well layer 14W and the number of layers of the second quantum well layer 14V And may be identical.

  When the number of layers of the second quantum well layer 14V is smaller than the number of layers of the first quantum well layer 14W, there is an advantage that the external quantum efficiency is improved at the time of large current driving.

  On the other hand, when the number of layers of the second quantum well layer 14V is larger than the number of layers of the first quantum well layer 14W, the external quantum efficiency can not be expected during large current driving, but the emission intensity of the second wavelength can be improved. There is an advantage.

  Although the layer thicknesses of the first quantum well layer 14W and the second quantum well layer 14V can be set arbitrarily, the layer thickness of the second quantum well layer 14V (not the total thickness (total thickness of layers) but the second quantum It is more preferable that the thickness of each layer of the well layer 14W be smaller than the layer thickness of the first quantum well layer 14W (not the total thickness (total of the thicknesses of the layers) but the thickness of each layer of the first quantum well layer 14W). In this case, when the layer thicknesses of the plurality of first quantum well layers 14W are different from each other and the plurality of second quantum well layers 14V are different from each other, the maximum value among the layer thicknesses of the second quantum well layers 14V is the first quantum It is preferable to be smaller than the minimum value of the layer thickness of the well layer 14W. Since the first quantum well layer 14W has a larger In concentration than the second quantum well layer 14V, the ratio of Auger recombination which is non-radiative recombination is high, so the layer thickness of the first quantum well layer 14W is increased. Hole injection efficiency is preferably high. On the other hand, since the second quantum well layer 14V has a low In concentration, the rate of Auger recombination may be low and the layer thickness may be small. By further reducing the layer thickness of the second quantum well layer 14V, the overlap of the wave function of electrons and holes is improved, and the light emission recombination rate is also improved.

  The holes trapped in the intermediate well layer 14M are diffused again to the well layer mainly on the side near the n-type nitride semiconductor layer, and are radiatively recombined with the electrons in the first quantum well layer of the first light emitting layer. Therefore, the nitride semiconductor light emitting device of the present invention has two emission peaks from the wavelength at which the emission spectrum is emitted from the first quantum well layer and the wavelength emitted from the second quantum well layer.

  Although some of the holes trapped in the middle well layer 14M may lead to recombination of light and electrons in the middle well layer 14M, they do not become the main light emission, and the light emission spectrum may be broadened, or Since the emission intensity of either one or both of the energy band gaps in the one quantum well layer 14W or the second quantum well layer 14V is increased, the emission spectrum has the wavelength emitted from the first quantum well layer 14W, and It has two emission peaks from the wavelength emitted from the quantum well layer 14V.

[P-type nitride semiconductor layer]
The p-type nitride semiconductor layer 16, 17, 18, each independently, for example, _{Al s4 Ga t4 In u4 N (} 0 ≦ s4 ≦ 1,0 ≦ t4 ≦ 1,0 ≦ u4 ≦ 1, s4 + t4 + u4 = 1) layer It is preferable to use a layer doped with a p-type dopant for the _{Al.sub.s4Ga.sub. (1-s4)} N (0 <s4.ltoreq.0.4, preferably 0.1.ltoreq.s4.ltoreq.0.3) layer. It is more preferable to use a layer doped with

The p-type dopant is not particularly limited, but it is preferable to use, for example, magnesium. The carrier concentration in the p-type nitride semiconductor layers 16, 17, 18 is preferably 1 × 10 ¹⁷ / cm ³ or more. Since the activation rate of the p-type dopant is about 0.01, the p-type dopant concentration (different from the carrier concentration) in the p-type nitride semiconductor layers 16, 17, 18 is 1 × 10 ¹⁹ / cm ³ or more Is preferred. However, the p-type dopant concentration in the p-type nitride semiconductor layer 16 located on the multiple quantum well light emitting layer 14 side among the p-type nitride semiconductor layers 16, 17, 18 is less than 1 × 10 ¹⁹ / cm ³ Is preferred.

  The layer thickness of the entire p-type nitride semiconductor layer 16, 17, 18 (the sum of the layer thicknesses) is not particularly limited, but is preferably 50 nm or more and 300 nm or less. By reducing the thickness of the entire p-type nitride semiconductor layers 16, 17, 18, the heating time at the time of growing the p-type nitride semiconductor layers 16, 17, 18 can be shortened. Thereby, the diffusion of the p-type dopant in the p-type nitride semiconductor layers 16, 17, 18 can be suppressed.

[N electrode, transparent electrode, p electrode]
The n electrode 21 and the p electrode 25 are electrodes for supplying driving power to the nitride semiconductor light emitting device. As shown in the figure, the n electrode 21 and the p electrode 25 are constituted only by the pad electrode portion, but for example, an elongated protrusion (branch electrode) or the like for the purpose of current diffusion is used as the n electrode 21 and / or the p electrode It may be connected to 25.

  In addition, an insulating layer for preventing current from being injected into the p electrode 25 is preferably provided below the p electrode 25. Thereby, the amount of light emission blocked by the p electrode 25 is reduced.

  For example, it is preferable that the n-electrode 21 be configured by laminating a titanium layer, an aluminum layer, and a gold layer in this order. Assuming that wire bonding is performed on the n-electrode 21, the thickness of the n-electrode 21 is preferably 1 μm or more.

  The p electrode 25 is preferably formed by, for example, stacking a nickel layer, an aluminum layer, a titanium layer and a gold layer in this order, and may be made of the same material as the n electrode 21. Assuming that wire bonding is performed on the p electrode 25, the thickness of the p electrode 25 is preferably 1 μm or more.

  The transparent electrode layer 23 is preferably made of, for example, a transparent conductive film such as ITO (Indium Tin Oxide) or IZO (Indium Zinc Oxide), and preferably has a thickness of 20 nm to 200 nm.

[About carrier concentration and dopant concentration]
The carrier concentration means the concentration of electrons or holes, and is not determined only by the amount of n-type dopant or the amount of p-type dopant. Such carrier concentration is calculated based on the result of voltage vs. capacitance characteristics of the nitride semiconductor light emitting device, and refers to the carrier concentration in the state where current is not injected, and ionized impurities, donors It is the sum of carriers generated from the crystallized crystal defects and the acceptor-crystal defects.

  However, the n-type carrier concentration can be considered to be approximately the same as the n-type dopant concentration because the activation rate of Si or the like which is an n-type dopant is high. The n-type dopant concentration can be easily determined by measuring the concentration distribution in the depth direction with SIMS (Secondary Ion Mass Spectroscopy). Furthermore, the relative relationship (ratio) of the dopant concentration is substantially the same as the relative relationship (ratio) of the carrier concentration. From these facts, in the present specification, the dopant concentration which is actually easy to measure is defined. The average n-type dopant concentration can be obtained by averaging the n-type dopant concentrations obtained by the measurement in the thickness direction.

  EXAMPLES The present invention will be described in more detail by way of the following examples, but the present invention is not limited thereto.

Example 1
First, a sapphire substrate 3 (with a diameter of 150 mm) was prepared, in which a concavo-convex shape including a convex portion and a concave portion was formed on the upper surface. The protrusion has a cross-sectional shape of the protrusion 3a shown in FIG. 1 and has a conical tip with a height of about 0.6 μm (the diameter of the circle at the bottom of the cone is 1.2 μm) Each convex part was provided in the position used as the vertex of a substantially triangle in planar view, and the adjacent vertex space | interval was 2 micrometers. Moreover, the recess had a cross-sectional shape of the recess 3 b shown in FIG. 1.

  RCA cleaning was performed on the upper surface of the sapphire substrate 3 in which the convex portions and the concave portions were formed. The sapphire substrate 3 after RCA cleaning was placed in a chamber and heated. A buffer layer 5 (25 nm in thickness) made of AlN crystal was formed on the upper surface of the substrate by reactive sputtering using an Al target in an argon atmosphere containing nitrogen.

The sapphire substrate 3 on which the buffer layer 5 was formed was placed in a MOCVD apparatus, and the temperature of the sapphire substrate 3 was set to 1000.degree. The nitride semiconductor underlayer 7 made of undoped GaN is grown on the upper surface of the buffer layer 5 by the MOCVD method, and then the n-type contact layer 8 made of Si-doped GaN is grown on the upper surface of the nitride semiconductor underlayer 7 The The layer thickness T1 of the nitride semiconductor underlayer 7 (see FIG. 1, a linear distance between the upper surface of the nitride semiconductor underlayer 7 and the recess 3b on the upper surface of the sapphire substrate 3) is 6 μm, and the layer thickness T2 of the n-type contact layer 8 (See FIG. 1) was 3 μm. The n-type dopant concentration of the n-type contact layer was 1 × 10 ¹⁹ / cm ³ .

After the temperature of the sapphire substrate was lowered to 801 ° C., a low temperature n-type nitride semiconductor layer 10 (V pit generation layer) (30 nm thick) made of Si-doped GaN was grown on the upper surface of the n-type contact layer 8 . The concentration of the n-type impurity in the low temperature n-type nitride semiconductor layer 10 (V pit generation layer) was 9 × 10 ¹⁹ / cm ³ .

The nitride semiconductor multilayer structure 120 was grown while maintaining the temperature of the sapphire substrate at 801 ° C. Specifically, 20 sets of alternating wide band gap layers (thickness 1.55 nm) made of Si-doped GaN and narrow band gap layers (thickness 1.55 nm) made of Si-doped InGaN were grown. The concentration of the n-type impurity in any of the layers constituting the nitride semiconductor multilayer structure 120 was 7 × 10 ¹⁸ / cm ³ . The composition of each narrow band gap layer was In _y Ga _1-y N (y = 0.04).

  Next, the temperature of the sapphire substrate was lowered to 769 ° C. to form a multiple quantum well light emitting layer 14 having a structure as shown in FIG.

Specifically, first, the well layer of undoped _{In x Ga (1-x)} N (x = 0.2) (first quantum well layer 14W) was 3.4nm laminate, a barrier made of undoped GaN A layer (first barrier layer 14A) was stacked 4 nm, and this was repeated four times to form a multiple quantum well structure (first light emitting layer 141) of a first wavelength (450 nm).

Next, the substrate temperature was raised to 781 ° C., and an intermediate well layer 14M made of undoped In _z Ga _(1-z) N was stacked to 5 nm. The In composition z of the intermediate well layer 14M was set by adjusting the flow rate of TMI so that the PL emission wavelength of the multiple quantum well of the first wavelength (450 nm) would be 448 nm.

Next, the substrate temperature is raised to 794 ° C., and a well layer (second quantum well layer 14 V) composed of undoped In _y Ga _(1-y) N (y = 0.09) is stacked to a thickness of 3.4 nm, and undoped The barrier layer (second barrier layer 14B) made of GaN is stacked 4 nm, and this is repeated three times to form a multiple quantum well structure (second light emitting layer 142) of the second wavelength (405 nm).

Next, a barrier layer (final barrier layer 14B0) (7 nm thick) made of undoped GaN was grown on the top surface of the light emitting layer (specifically, the top surface of the topmost well layer). After raising the temperature of the sapphire substrate to 1000 ° C., a p-type Al _0.18 Ga _0.82 N layer (p-type nitride semiconductor layer) 16 and a p-type GaN layer (p-type GaN layer) are formed on the top surface of the last barrier layer 14B0. A nitride semiconductor layer 17 and a p-type contact layer (p-type nitride semiconductor layer) 18 were sequentially grown.

The p-type contact layer 18, the p-type GaN layer 17, the p-type Al _0.18 Ga _0.82 N layer 16, the multiple quantum well light emitting layer 14, the nitride semiconductor multilayer structure such that a part of the n-type contact layer is exposed. The body 120, the low temperature n-type nitride semiconductor layer (V pit generation layer) 10 and the n-type contact layer 8 were etched. An n electrode 21 made of Au or the like was formed on the upper surface of the n-type contact layer 8 exposed by this etching. Further, on the upper surface of the p-type contact layer 18, the transparent electrode layer 23 made of ITO and the p electrode 25 made of Au or the like were formed in order. A transparent insulating protective film 27 made of SiO ₂ was formed so as to mainly cover the transparent electrode layer 23 and the side surfaces of the layers exposed by the etching.

  Next, the substrate was divided into chips of 190 μm × 990 μm in size. Thereby, the nitride semiconductor light emitting device 1 of Example 1 was obtained.

(Evaluation)
The nitride semiconductor light emitting device obtained by the above method was mounted on a TO-18 stem, and the emission spectrum and light output of the nitride semiconductor light emitting device were measured without sealing with a resin. FIG. 3 shows an emission spectrum of the nitride semiconductor light emitting device manufactured in Example 1 when driven at IF = 20 mA. Emission peaks at two wavelengths of 405 nm and 450 nm were confirmed. Moreover, the peak intensity ratio of 405 nm and 450 nm became 16:84.

FIG. 4 is a graph showing the external quantum efficiency (EQE) and the applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 1 in comparison with Comparative Example 1, and the vertical axis represents EQE, horizontal axis is current density J (A / cm ² ). For comparison, a sample of Comparative Example 1 in which an undoped In _x Ga _(1-x) N (x = 0.2) well layer having no second wavelength and a barrier layer composed of undoped GaN are repeatedly laminated eight times. Also made.

As a result, although the EQE at less than 10 A / cm ² is lower than the sample of Comparative Example 1 compared to the sample of Comparative Example 1, the EQE at 10 A / cm ² or more, which is a practical area, becomes high, and a large current drive area The so-called droop phenomenon has been improved. Although the detailed cause is unknown, in the sample of Comparative Example 1 which is the conventional structure, the holes supplied from the p-type nitride semiconductor layer side at the time of large current driving are electrons and electrons in the well layer near the p-type nitride semiconductor layer. It is considered that although light emission is recombined, holes are not diffused in the well layer near the n-type nitride semiconductor layer and light emission does not occur. On the other hand, in the structure of the nitride semiconductor light emitting device manufactured in Example 1, since the energy band gap of the well layer close to the p-type nitride semiconductor layer is large, holes are hardly trapped. Furthermore, in the nitride semiconductor light emitting device manufactured in Example 1, the holes supplied from the p-type nitride semiconductor layer side have a multiple quantum well structure of the second wavelength (405 nm) and a multiple quantum of the first wavelength (450 nm). The structure is to be trapped once in the middle well layer with a large layer thickness between the well structures. Therefore, holes are easily diffused again from the intermediate well layer to the well layer on the side closer to the n-type nitride semiconductor layer, and the light emission recombination in the well layer on the side closer to the n-type nitride semiconductor layer is facilitated, and the light emission efficiency is increased. Is considered to improve.

The multiple quantum well light emitting layer has a well structure in which the first quantum well layer (450 nm) is one layer and the second quantum well layer (405 nm) is repeated seven times from the side close to the p-type nitride semiconductor layer, A sample of Comparative Example 2 having the same structure as that of Example 1 was produced (the band gap energy diagram of the multiple quantum well light emitting layer in the sample of Comparative Example 2 is as shown in FIG. 5), and the light emission efficiency was similarly confirmed. FIG. 6 is a graph showing the external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 1 in comparison with Comparative Example 2, and the vertical axis is EQE and the horizontal axis is current density J. (A / cm ² ). As shown in FIG. 6, in the nitride semiconductor light emitting device manufactured in Example 1, compared to the sample of Comparative Example 2, EQE improvement is improved in both the low current density region and the high current density region. It could be confirmed.

FIG. 7 is an emission spectrum at a current density of 13.4 A / cm ² of the nitride semiconductor light emitting device manufactured in Example 1. The vertical axis represents light output (au), and the horizontal axis represents wavelength (A) nm). In the nitride semiconductor light emitting device manufactured in Example 1, the peak intensity ratio of 405 nm to 450 nm is 3: 7 when the emission spectrum is confirmed at a current density of 13.4 A / cm ² , and the intensity of (near) ultraviolet light is It was confirmed that even when combined with a green light emitting phosphor / red light emitting phosphor, a higher conversion efficiency (brightness) could be obtained.

Example 2
FIG. 8 is a band gap energy diagram showing a multiple quantum well structure in the nitride semiconductor light emitting device manufactured in Example 2. Example 2 was the same as Example 1 except that the layer thickness of the well layer (second quantum well layer) in the multiple quantum well structure of the second wavelength (405 nm) was 2 nm.

(Evaluation)
The emission spectrum and light output of the nitride semiconductor light emitting device manufactured in Example 2 were evaluated in the same manner as in Example 1. FIG. 9 is a diagram showing the emission spectrum of the nitride semiconductor light emitting device manufactured in Example 2, where the vertical axis is intensity (au) and the horizontal axis is wavelength (nm). As a result, compared to the nitride semiconductor light emitting device manufactured in Example 1, the peak intensity at 405 nm at current density = 13.4 A / cm ² increases, and the peak intensity ratio between 405 nm and 450 nm becomes 4: 6 ( It was confirmed that, even when the combination of the green light emitting phosphor and the red light emitting phosphor is obtained, the higher conversion efficiency (brightness) can be obtained because the intensity of the near ultraviolet light is increased. FIG. 10 is a graph showing the external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 2 in comparison with Comparative Example 2, and the vertical axis is EQE and the horizontal axis is current density J. (A / cm ² ). The nitride semiconductor light emitting device manufactured in Example 2 has EQE in all current density regions as compared with the sample of Comparative Example 2 and in comparison with the nitride semiconductor light emitting device manufactured in Example 1. Improved.

Example 3
FIG. 11 is a band gap energy diagram showing a multiple quantum well structure in the nitride semiconductor light emitting device produced in Example 3. In the third embodiment, the number of well layers (second quantum well layer) in the multiple quantum well structure of the second wavelength (405 nm) is five, and the well layer in the multiple quantum well structure of the first wavelength (450 nm). The same as Example 1 except that the number of layers (first quantum well layer) was changed to two.

(Evaluation)
The emission spectrum and the light output of the nitride semiconductor light emitting device produced in Example 3 were evaluated in the same manner as in Example 1. FIG. 12 is a graph showing the external quantum efficiency and applied current density dependency of the nitride semiconductor light emitting device manufactured in Example 3 in comparison with Comparative Example 2, and the vertical axis is EQE and the horizontal axis is current density J ( A / cm ² ). FIG. 13 is a diagram showing the emission spectrum of the nitride semiconductor light emitting device manufactured in Example 3, where the vertical axis is intensity (au) and the horizontal axis is wavelength (nm). Although the improvement of EQE confirmed in Example 1 was not obtained, the peak intensity of 405 nm at current density = 13.4 A / cm ² is increased, and the peak intensity ratio of 405 nm and 450 nm is 3: 7. Even if the number of layers in the quantum well layer is larger than the number of layers in the first quantum well layer, the peak intensity ratio of the second wavelength becomes large, and the intensity of (near) ultraviolet light becomes strong. It has been confirmed that even higher conversion efficiency (brightness) can be obtained even in combination with the light emitting phosphor.

  Reference Signs List 1 nitride semiconductor light emitting device, 3 substrate, 5 buffer layer, 7 nitride semiconductor underlayer, 8 n-type contact layer, 10 low temperature n-type nitride semiconductor layer, 14 multiple quantum well light emitting layer, 16 p-type nitride semiconductor layer 17 p-type nitride semiconductor layer, 18 p-type nitride semiconductor layer, 23 transparent electrode layer, 25 p electrode, 27 transparent insulating protective film, 120 nitride semiconductor multilayer structure.

Claims (7)

an n-type nitride semiconductor layer,
a p-type nitride semiconductor layer,
A multiple quantum well light emitting layer provided between the n-type nitride semiconductor layer and the p-type nitride semiconductor layer,
The multiple quantum well light emitting layer includes a second light emitting layer, an intermediate well layer, and a first light emitting layer from the side close to the p-type nitride semiconductor layer,
The first light emitting layer includes a plurality of first quantum well layers, and a first barrier layer provided between the plurality of first quantum well layers.
The second light emitting layer includes a plurality of second quantum well layers, and a second barrier layer provided between the plurality of second quantum well layers.
There is an intermediate well layer between the first light emitting layer and the second light emitting layer, and the band gap energy takes a value between the band gap energy of the second quantum well layer and the band gap energy of the first quantum well layer. ,
The nitride semiconductor light emitting device, wherein the band gap energy of the second quantum well layer is larger than the band gap energy of the first quantum well layer.   The nitride semiconductor light emitting device according to claim 1, wherein the band gap energy of the intermediate well layer is a value between the band gap energy of the first quantum well layer and the band gap energy of the second quantum well layer. .   The nitride semiconductor light emitting device according to claim 1, wherein a layer thickness of the intermediate well layer is larger than a layer thickness of the first quantum well layer or the second quantum well layer.   The nitride semiconductor light emitting device according to any one of claims 1 to 3, wherein the number of layers of the second quantum well layer is smaller than the number of layers of the first quantum well layer.   The nitride semiconductor light emitting device according to any one of claims 1 to 3, wherein the number of layers of the second quantum well layer is larger than the number of layers of the first quantum well layer.   The emission spectrum according to any one of claims 1 to 5, wherein an emission peak from a wavelength emitted from the first quantum well layer and an emission peak from a wavelength emitted from the second quantum well layer. The nitride semiconductor light emitting device as described in 4.   The nitride semiconductor light emitting device according to any one of claims 1 to 6, wherein a layer thickness of the second quantum well layer is smaller than a layer thickness of the first quantum well layer.

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