Light emitting element and method for manufacturing light emitting element

Abstract

To improve the quality of an active layer.SOLUTION: A second active layer 316 is a structure consisting of a strain relaxation layer 316A and a quantum well structure layer 316B of SQW or MQW, stacked in sequence. The strain relaxation layer 316A is an SQW structure consisting of barrier layers and well layers stacked in sequence, and a quantum well structure in which the thickness of the well layer is adjusted to be thin so that it does not emit light. The barrier layer is AlGaN and the well layer is InGaN. The wavelength corresponding to the band edge energy of the well layer of the strain relaxation layer 316A should be shorter than the emission wavelength of the quantum well structure layer 316B, for example, 400-460 nm.SELECTED DRAWING: Figure 15

Description

[Technical field]

[0001] The present invention relates to a light-emitting device made of a Group III nitride semiconductor, and more particularly to a light-emitting device having a structure in which active layers emitting different colors of light are stacked via an intermediate layer, and also to a method for manufacturing the same. [Background technology]

[0002] In recent years, there has been a demand for higher resolution displays, and micro LED displays, in which each pixel is a tiny LED on the order of 1 to 100 μm, have been attracting attention. There are various methods for achieving full color, including a method in which three active layers that emit light in blue, green, and red, respectively, are stacked in order on the same substrate. In this case, an intermediate layer must be formed between the active layers in order to drive each active layer individually. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5854419 Summary of the Invention [Problem to be solved by the invention]

[0004] Green and red light emission requires InGaN with a high In composition ratio, but lattice mismatch causes distortion, making it impossible to form high-quality InGaN.

[0005] It is therefore an object of the present invention to improve the quality of the active layer. [Means for solving the problem]

[0006] One aspect of the present invention is In a light-emitting device using a Group III nitride semiconductor, An n-type layer; a first active layer provided on the n-type layer and having a quantum well structure including a first well layer and a first barrier layer; an intermediate layer provided on the first active layer to prevent the first active layer from emitting light; a strain relaxation layer provided on the intermediate layer, the strain relaxation layer being a quantum well structure having a second well layer and a second barrier layer, the thickness of the second well layer being adjusted so as not to emit light; a second active layer having a quantum well structure that emits light and is provided on the strain relaxation layer; a p-type layer provided on the second active layer, a wavelength corresponding to a band edge energy of the second well layer of the strain relaxation layer is set to be shorter than an emission wavelength of the second active layer, The light-emitting device has a laminated structure from the n-type layer to the second active layer in which no p-type Group III nitride semiconductor is present.

[0007] In the present invention, the intermediate layer may be made of a group III nitride semiconductor containing In, and the In composition of the intermediate layer may be set to a band gap that does not absorb the light emitted from the second active layer. The In composition of the intermediate layer may be 10% or less. The intermediate layer may be InGaN. The intermediate layer may be In-doped GaN. The intermediate layer may have a structure in which a non-doped first layer made of a non-doped group III nitride semiconductor and a second layer made of an n-type group III nitride semiconductor are laminated in this order from the first active layer side. In this case, the thickness of the intermediate layer may be 150 nm or less, and the thicknesses of the first layer and the second layer may be 10 nm or more. The emission wavelength of the second active layer may be 570 to 700 nm.

[0008] In the present invention, a wavelength corresponding to the band edge energy of the well layer of the strain relaxation layer may be set to be equal to a wavelength corresponding to the band edge energy of the first well layer of the first active layer.

[0009] In the present invention, the emission wavelength of the strain relaxation layer corresponding to the band edge energy of the well layer may be set to be 40 to 100 nm shorter than the emission wavelength of the light emitting layer.

[0010] In the present invention, the strain relaxation layer may have an SQW structure.

[0011] In the present invention, a ratio of a thickness of the first active layer to a thickness of the second active layer may be 30% or less. Effect of the Invention

[0012] According to the present invention, the quality of the active layer can be improved. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the configuration of a light-emitting element according to a first embodiment. [Diagram 2] FIG. 13 is a diagram showing a configuration of a light-emitting element according to a modified example. [Diagram 3] FIG. 13 is a diagram showing a configuration of a light-emitting element according to a modified example. [Figure 4] 3A to 3C are diagrams illustrating a manufacturing process of the light emitting element according to the first embodiment. [Diagram 5] 3A to 3C are diagrams illustrating a manufacturing process of the light emitting element according to the first embodiment. [Figure 6] 3A to 3C are diagrams illustrating a manufacturing process of the light emitting element according to the first embodiment. [Figure 7] 3A to 3C are diagrams illustrating a manufacturing process of the light emitting element according to the first embodiment. [Figure 8] 1 is an AFM image of the surface of the third active layer 18. [Figure 9] 1 is a graph showing the relationship between driving current and external quantum efficiency. [Figure 10] FIG. 6 is a diagram showing the configuration of a light-emitting element according to a second embodiment. [Figure 11] Graph showing emission spectrum. [Figure 12] Graph showing emission spectrum. [Figure 13] Graph showing emission spectrum. [Figure 14] Graph showing emission spectrum. [Figure 15] FIG. 13 is a diagram showing the configuration of a light-emitting element according to a third embodiment. [Figure 16] 1 is an AFM image of the surface of the third active layer 18. [Figure 17] 1 is a graph showing the relationship between driving current and external quantum efficiency. [Figure 18] FIG. 2 is a diagram showing the configuration of a light-emitting element in Experimental Example 1. [Figure 19] FIG. 2 is a diagram showing an equivalent circuit of the light-emitting element according to the first embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

[0015] (First embodiment) FIG. 1 is a diagram showing the configuration of a light-emitting element according to the first embodiment. The light-emitting element according to the first embodiment is capable of emitting blue, green, and red light. The light-emitting element according to the first embodiment is a flip-chip type that extracts light from the back side of the substrate, and is mounted face-down on a mounting substrate (not shown). Although the first embodiment has a one-chip structure for one pixel, a monolithic type may also be used. In other words, the element structure according to the first embodiment may be arranged in a matrix on the same substrate to form a micro LED display element.

[0016] As shown in FIG. 1, the light-emitting element of the first embodiment has a substrate 10, an n layer 11, an ESD layer 12, an underlayer 13, a first active layer 14, a first intermediate layer 15, a second active layer 16, a second intermediate layer 17, a third active layer 18, a protective layer 19, regrown layers 20A-20C, electron blocking layers 21A-21C, p layers 22A-22C, an n electrode 23, and p electrodes 24A-24C.

[0017] The substrate 10 is a growth substrate for growing a Group III nitride semiconductor, such as sapphire, Si, or GaN.

[0018] The n-layer 11 is an n-type semiconductor provided on the substrate 10 via a low-temperature buffer layer or a high-temperature buffer layer (not shown). However, the buffer layer may be provided as necessary, and if the substrate is GaN, the buffer layer need not be provided. The n-layer 11 is, for example, n-GaN or n-AlGaN. The Si concentration is, for example, 1×10 18 ~100×10 18 cm -3 It is.

[0019] The ESD layer 12 is a semiconductor layer provided on the n-layer 11 to improve electrostatic breakdown voltage. The ESD layer 12 may be omitted if necessary. The ESD layer 12 is, for example, non-doped or lightly Si-doped GaN, InGaN, or AlGaN.

[0020] The underlayer 13 is a semiconductor layer with a superlattice structure provided on the ESD layer 12, and serves to relieve lattice distortion of the semiconductor layer formed on the underlayer 13. The underlayer 13 may be provided as necessary, and may be omitted. The underlayer 13 is formed by alternately laminating III-nitride semiconductor thin films having different composition ratios (for example, two of GaN, InGaN, and AlGaN), and the number of pairs is, for example, 3 to 30. It may be undoped, or may be doped with 1×10 Si. 17 ~100×10 17 cm -3 The doping may be to a certain extent. Also, as long as the strain can be relaxed, it is not necessary to have a superlattice structure. Any material that reduces the lattice constant difference at the heterointerface with the first active layer 14 may be used, for example, an InGaN layer, an AlInN layer, or an AlGaIn layer.

[0021] The first active layer 14 is a light emitting layer having an SQW or MQW structure provided on the underlayer 13. The emission wavelength is blue, 430 to 480 nm. The first active layer 14 has a structure in which 1 to 7 pairs of barrier layers made of AlGaN and well layers made of InGaN are alternately laminated, more preferably 1 to 5 pairs, and even more preferably 1 to 3 pairs.

[0022] The first intermediate layer 15 is a semiconductor layer provided on the first active layer 14, and is located between the first active layer 14 and the second active layer 16. The first intermediate layer 15 is a layer provided to enable individual control of the light emission from the first active layer 14 and the light emission from the second active layer 16. The first intermediate layer 15 also serves to protect the first active layer 14 from etching damage when forming the second grooves 31 described below.

[0023] The material of the first intermediate layer 15 is a group III nitride semiconductor containing In, and is preferably InGaN, for example. The surfactant effect of In can suppress roughness of the surface of the first intermediate layer 15 and improve the surface flatness. In addition, lattice distortion can be alleviated. The In composition ratio of the first intermediate layer 15 (the molar ratio of In to the total group III metal of the group III nitride semiconductor) may be set to a band gap that does not absorb the light emitted from the first active layer 14 and the second active layer 16. The In composition ratio is preferably 10% or less, more preferably 5% or less, and further preferably 2% or less. If the In composition ratio is greater than 10%, it causes the surface of the first intermediate layer 15 to become rough. In can be any amount greater than 0%, and may be a doping level (a level at which a mixed crystal is not formed). For example, if the In concentration is 1×10 14 cm -3 More than 1×10 22 cm -3 The following is GaN.

[0024] The first intermediate layer 15 may be doped with an impurity. Preferably, the impurity is an n-type impurity. For example, the Si concentration is 1×10 17 ~1000×10 17 cm -3 , preferably 10 x 10 17 ~100×10 17 cm -3 , and more preferably 20×10 17 ~80×10 17 cm -3 may be also possible.

[0025] The thickness of the first intermediate layer 15 is preferably 20 to 150 nm. If it is thicker than 150 nm, the surface of the first intermediate layer 15 may become rough. If it is thinner than 20 nm, it may be difficult to control the depth of the second grooves 31 to be formed within the first intermediate layer 15, as described below. The thickness is more preferably 30 to 100 nm, and even more preferably 50 to 80 nm.

[0026] The second active layer 16 is a light emitting layer of SQW or MQW structure provided on the first intermediate layer 15. The emission wavelength is green, and is 510 to 570 nm. The second active layer 16 has a structure in which 1 to 7 pairs of barrier layers made of GaN and well layers made of InGaN are alternately laminated. More preferably, the number is 1 to 5 pairs, and even more preferably, 1 to 3 pairs. Moreover, the number of pairs is preferably equal to or less than the number of pairs in the first active layer 14, and more preferably, less.

[0027] The second intermediate layer 17 is a semiconductor layer provided on the second active layer 16, and is located between the second active layer 16 and the third active layer 18. The second intermediate layer 17 is provided for the same reason as the first intermediate layer 15, and is a layer provided to enable individual control of the light emission from the second active layer 16 and the light emission from the third active layer 18. In addition, the second intermediate layer 17 also serves to protect the second active layer 16 from etching damage when forming a third groove 32 described below.

[0028] The material of the second intermediate layer 17 is the same as that of the first intermediate layer 15. The first intermediate layer 15 and the second intermediate layer 17 may be made of the same material. The second intermediate layer 17 may also be doped with an impurity like the first intermediate layer 15. The thickness of the second intermediate layer 17 is also the same as that of the first intermediate layer 15, and the thickness of the first intermediate layer 15 and the second intermediate layer 17 may be the same. However, it is preferable that the second intermediate layer 17 is thinner than the first intermediate layer 15 and has a higher In composition ratio than the first intermediate layer 15. This is because the second active layer 16 emitting green light is more susceptible to heat damage than the first active layer 14 emitting blue light, and is more susceptible to the effect of strain at the interface.

[0029] The third active layer 18 is a light emitting layer of SQW or MQW structure provided on the second intermediate layer 17. The emission wavelength is red, and is 590 to 700 nm. The third active layer 18 has a structure in which 1 to 7 pairs of barrier layers made of InGaN and well layers made of InGaN are alternately laminated. More preferably, the number is 1 to 5 pairs, and even more preferably, 1 to 3 pairs. Also, the number of pairs is preferably equal to or less than the number of pairs in the second active layer 16, and more preferably, less.

[0030] The protective layer 19 is a semiconductor layer provided on the third active layer 18. The protective layer 19 protects the active layer and also functions as an electron blocking layer. The protective layer 19 may be made of a material having a wider band gap than the well layer of the third active layer 18, such as AlGaN, GaN, or InGaN. The thickness of the protective layer 19 is preferably 2.5 to 50 nm, and more preferably 5 to 25 nm. The protective layer 19 may be doped with an impurity, or may be doped with Mg. In this case, the Mg concentration is 1×10 18 ~1000×10 18 cm -3 It is better to say:

[0031] A portion of the protective layer 19 is etched to provide grooves, which include a third groove 32 that extends from the protective layer 19 to the second intermediate layer 17, a second groove 31 that extends to the first intermediate layer 15, and a first groove 30 that extends to the n-layer 11.

[0032] Regrowth layers 20A-20C are provided on protective layer 19, on second intermediate layer 17 exposed at the bottom surface of third groove 32, and on first intermediate layer 15 exposed at the bottom surface of second groove 31. The configuration of regrown layers 20A-20C is similar to that of protective layer 19.

[0033] The electron blocking layers 21A to 21C are semiconductor layers provided on the regrown layers 20A to 20C, respectively, and serve to block electrons from exceeding the third active layer 18. The electron blocking layer may be a single layer of GaN or AlGaN, or may have a structure in which two or more of AlGaN, GaN, and InGaN are laminated, or a structure in which layers are laminated with only different composition ratios. A superlattice structure may also be used. The thickness of the electron blocking layers 21A to 21C is preferably 5 to 50 nm, and more preferably 5 to 25 nm. The Mg concentration of the electron blocking layers 21A to 21C is 1×10 19 ~100×10 19 cm -3 It is better to say:

[0034] The p-layers 22A to 22C are semiconductor layers provided on the electron blocking layers 21A to 21C, respectively, and are composed of a first layer and a second layer in that order from the electron blocking layer 21 side. The first layer is preferably p-GaN or p-InGaN. The thickness of the first layer is preferably 10 to 500 nm, more preferably 10 to 200 nm, and further preferably 10 to 100 nm. The Mg concentration of the first layer is 1×10 19 ~100×10 19 cm -3 The second layer is preferably p-GaN or p-InGaN. The thickness of the second layer is preferably 2 to 50 nm, more preferably 4 to 20 nm, and further preferably 6 to 10 nm. The Mg concentration of the second layer is 1×10 20 ~100×10 20 cm -3 It is better to say:

[0035] In the first embodiment, the regrown layers 20A to 20C, the electron blocking layers 21A to 21C, and the p-layers 22A to 22C are provided separately from each other, but may be continuous (see FIG. 2). In this case, the regrown layers, the electron blocking layers, and the p-layers are also formed on the side surfaces of the third groove 32 and the second groove 31, but this has almost no effect on the operation of the device. The reason is as follows. If the p-electrodes 24A, 24B, and 24C are sufficiently spatially separated from each other, the resistance of the p-layer connecting the p-electrodes 24A, 24B, and 24C is very high, so that almost no current flows. In addition, since the mobility of holes is low, the holes do not spread laterally from the region in contact with the electrodes, but flow predominantly vertically through the pn junction directly below the electrodes. Therefore, even if the regrown layers 20A to 20C, the electron blocking layers 21A to 21C, and the p-layers 22A to 22C are continuous, the operation of the device is not affected. In other words, when a current is passed through the p electrode 24A, the current flows directly below the p electrode 24A, resulting in the active layer directly below the p electrode 24A emitting light, while almost no current flows through the active layers directly below the p electrodes 24B and 24C, causing them to emit light.

[0036] 3, an insulating film 27 may be provided on the side surface of the third groove 32 and the side surface of the second groove 31. This insulating film 27 is a remaining mask used when selectively growing the regrown layers 20A-20C, the electron blocking layers 21A-21C, and the p-layers 22A-22C.

[0037] N-electrode 23 is an electrode provided on n-layer 11 exposed on the bottom surface of first groove 30. If substrate 10 is made of a conductive material, n-electrode 23 may be provided on the back surface of substrate 10 without providing first groove 30. The material of n-electrode 23 is, for example, Ti / Al.

[0038] The p electrodes 24A to 24C are electrodes provided on the p layers 22A to 22C, respectively. The material of the p electrodes 24A to 24C is, for example, Ag, Ni / Au, Co / Au, ITO, or the like.

[0039] The operation of the light-emitting device of the first embodiment will be described. In the light-emitting device of the first embodiment, red light can be emitted from the third active layer 18 by applying a voltage between the p-electrode 24A and the n-electrode 23, green light can be emitted from the second active layer 16 by applying a voltage between the p-electrode 24B and the n-electrode 23, and blue light can be emitted from the first active layer 14 by applying a voltage between the p-electrode 24C and the n-electrode 23. In addition, two or more of blue, green, and red can be emitted simultaneously. In this way, in the light-emitting device of the first embodiment, blue, green, and red light emission can be controlled by selecting the electrodes to which the voltage is applied, and it can be used as one pixel of a display.

[0040] FIG. 19 shows an equivalent circuit of the light-emitting element of the first embodiment. As shown in FIG. 19, the light-emitting element of the first embodiment has a structure in which blue, green, and red LEDs are formed in one element, and full-color light emission can be realized with one element. Therefore, it is possible to make the size of one element much smaller than when blue, green, and red LEDs are prepared separately and arranged on the same substrate to fabricate a full-color light-emitting element of one pixel. Furthermore, with the structure of the first embodiment, the process of preparing and arranging blue, green, and red LEDs separately can be omitted, and the manufacturing cost can be significantly reduced, making it possible to realize a very low-cost full-color light-emitting element and a light-emitting display using the same.

[0041] In the first embodiment, since the first intermediate layer 15 and the second intermediate layer 17 contain In, the surfactant effect of In can improve the surface flatness of the first intermediate layer 15 and the second intermediate layer 17, and can also improve the surface flatness of the second active layer 16 and the third active layer 18. In addition, the lattice distortion caused by the difference in lattice constant between the underlayer 13 and the first active layer 14 can also be alleviated. As a result, the light emitting efficiency of the light emitting device of the first embodiment can be improved.

[0042] Next, a manufacturing process of the light emitting device of the first embodiment will be described with reference to the drawings.

[0043] First, the substrate 10 is prepared, and hydrogen or nitrogen, and if necessary, ammonia, are added to the substrate, and the substrate is subjected to a heat treatment.

[0044] Next, a buffer layer is formed on the substrate 10, and an n-layer 11, an ESD layer 12, an underlayer 13, a first active layer 14, a first intermediate layer 15, a second active layer 16, a second intermediate layer 17, a third active layer 18, and a protective layer 19 are formed in this order on the buffer layer (see FIG. 4). The preferred growth temperatures for each layer are as follows:

[0045] The growth temperature of the first active layer 14 is preferably 700 to 950°C. This can improve the crystal quality and the light emission efficiency. The first active layer 14 is composed of a well layer and a barrier layer, and the well layer and the barrier layer may be formed at the same temperature or at different temperatures within the above temperature range. If they are formed at different temperatures, it is preferable that the growth temperature of the well layer is lower than the growth temperature of the barrier layer.

[0046] The growth temperature of the first intermediate layer 15 is preferably 700 to 1000°C. This is to suppress thermal damage to the first active layer 14. If the temperature is lower than 700°C, pits and point defects due to threading dislocations are likely to occur. The growth temperature is more preferably 800 to 950°C, and further preferably 850 to 950°C.

[0047] The growth temperature of the second active layer 16 is preferably 650 to 950°C. This can improve the crystal quality and the light emission efficiency. The second active layer 16 is composed of a well layer and a barrier layer, and the well layer and the barrier layer may be formed at the same temperature or at different temperatures within the above temperature range. When they are formed at different temperatures, it is preferable that the growth temperature of the well layer is lower than the growth temperature of the barrier layer. In addition, it is preferable that the growth temperature of the second active layer 16 is lower than the growth temperature of the first active layer 14.

[0048] The growth temperature of the second intermediate layer 17 is preferably in the same range as the growth temperature of the first intermediate layer 15. However, the growth temperature of the second intermediate layer 17 is preferably lower than the growth temperature of the first intermediate layer 15. This is because the green-emitting second active layer 16 is more susceptible to thermal damage than the blue-emitting first active layer 14, and is more susceptible to strain at the interface.

[0049] The growth temperature of the third active layer 18 is preferably 500 to 950°C. This can improve the crystal quality and the light emission efficiency. The third active layer 18 is composed of a well layer and a barrier layer, and the well layer and the barrier layer may be formed at the same temperature or at different temperatures within the above temperature range. When they are formed at different temperatures, it is preferable that the growth temperature of the well layer is lower than the growth temperature of the barrier layer. In addition, it is preferable that the growth temperature of the third active layer 18 is lower than the growth temperature of the second active layer 16.

[0050] The growth temperature of the protective layer 19 is preferably 500 to 950°C in order to suppress thermal damage to the first active layer 14, the second active layer 16, and the third active layer 18. To improve the crystallinity of the protective layer 19, a higher growth temperature is preferable, more preferably 600 to 900°C, and further preferably 700 to 900°C.

[0051] Next, a portion of the surface of the protective layer 19 is dry etched until it reaches the second intermediate layer 17 to form a third groove 32, and then dry etched until it reaches the first intermediate layer 15 to form a second groove 31 (see FIG. 5). The third groove 32 and the second groove 31 are preferably etched until they reach a thickness midway between the second intermediate layer 17 and the first intermediate layer 15.

[0052] Next, regrown layers 20A-20C are formed on protective layer 19, on second intermediate layer 17 exposed by third groove 32, and on first intermediate layer 15 exposed by second groove 31. The growth temperature is the same as that of protective layer 19. Here, regrown layers 20A-20C may be formed to be continuous as shown in Fig. 2. Alternatively, as shown in Fig. 3, insulating film 27 may be formed on the side surfaces of third groove 32 and second groove 31, and regrown layers 20A-20C may be selectively grown using this as a mask, so that regrown layers 20A-20C are formed separately from one another.

[0053] Next, electron blocking layers 21A to 21C are formed on the regrown layers 20A to 20C. The growth temperature of the electron blocking layers 21A to 21C is preferably 750 to 1000°C in order to suppress thermal damage to the first active layer 14, the second active layer 16, and the third active layer 18. The growth temperature is more preferably 750 to 950°C, and even more preferably 800 to 900°C.

[0054] Next, p-layers 22A to 22C are formed on electron blocking layers 21A to 21C (see FIG. 6). The growth temperature of p-layers 22A to 22C is preferably 650 to 1000°C, more preferably 700 to 950°C, and further preferably 750 to 900°C.

[0055] Next, a partial area of ​​the surface of p-layer 22C is dry-etched until it reaches n-layer 11 to form first groove 30 (see FIG. 7). Then, n-electrode 23 is formed on n-layer 11 exposed at the bottom of first groove 30, and p-electrodes 24A-24C are formed on p-layers 22A-22C. In this manner, the light-emitting element of the first embodiment is manufactured.

[0056] Second embodiment As shown in FIG. 10, the light emitting device of the second embodiment is obtained by replacing the first intermediate layer 15 and the second intermediate layer 17 of the light emitting device of the first embodiment with a first intermediate layer 215 and a second intermediate layer 217.

[0057] The first intermediate layer 215 has a structure in which an undoped layer 215A and an n-type layer 215B are laminated in this order from the first active layer 14 side. The undoped layer 215A and the n-type layer 215B are made of the same material, which is GaN or InGaN. The same material as that of the first intermediate layer 15 in the first embodiment is preferable. The undoped layer 215A is undoped, and the n-type layer 215B is Si-doped. The Si concentration of the n-type layer 215B is 1×10 17 ~1000×10 17 cm -3 It is preferable that the thickness of the first intermediate layer 215 is the same as that of the first intermediate layer 15. That is, it is preferable that the thickness is 20 to 150 nm. It is also preferable that the thickness of the non-doped layer 215A is 10 nm or more. This is for the purpose of controlling the etching depth and avoiding etching damage to the first active layer 14. It is also preferable that the thickness of the n-type layer 215B is 10 nm or more. This is for the purpose of independently controlling the light emission characteristics of each active layer. The n-type layer 215B may be modulation doped with Si, and a part of the n-type layer 215B may have a non-doped region.

[0058] The second intermediate layer 217 has a structure in which a non-doped layer 217A and an n-type layer 217B are laminated in this order from the second active layer 16 side. The non-doped layer 217A and the n-type layer 217B have the same structure as the non-doped layer 215A and the n-type layer 215B. However, it is preferable that the second intermediate layer 217 is thinner than the first intermediate layer 215 and has a higher In composition ratio than the first intermediate layer 215. This is for the same reason as in the case of the second intermediate layer 17. That is, the second active layer 16 emitting green light is more susceptible to thermal damage than the first active layer 14 emitting blue light, and is more susceptible to the effect of strain at the interface.

[0059] The third groove 32 has a depth that reaches the non-doped layer 217A of the second intermediate layer 217. In this way, by removing the n-type layer 217B of the second intermediate layer 17 below the p-electrode 24B, an n-type layer is prevented from being positioned above the second active layer 16, so that the second active layer 16 emits light. The second groove 31 has a depth that reaches the non-doped layer 215A of the first intermediate layer 215. This is for the same reason, and by removing the n-type layer 215B of the first intermediate layer 15 below the p-electrode 24C, an n-type layer is prevented from being positioned above the first active layer 14, so that the first active layer 14 emits light.

[0060] Here, we will explain the distance between pn junctions. The distance between pn junctions corresponds to the thickness of the film that is depleted at zero bias. In LEDs, it corresponds to the total thickness of the non-doped or lightly doped active layer sandwiched between a p-layer with a high concentration of acceptor impurities and an n-layer with a high concentration of donor impurities.

[0061] When the first intermediate layer 215 and the second intermediate layer 217 are non-doped, the pn junction distance (thickness of the depletion layer) corresponds to the distance from the electron block layer 21A highly doped with acceptor impurities to the n layer 11 highly doped with donor impurities in the region below the p electrode 24A, that is, the film thickness including the first active layer 14, the second active layer 16, the third active layer 18, the first intermediate layer 15, and the second intermediate layer 17. Also, the p electrode 24B corresponds to the distance from the electron block layer 21B highly doped with acceptor impurities to the n layer 11, that is, the film thickness including the first active layer 14, the second active layer 16, the first intermediate layer 15, and a part of the second intermediate layer 17. Also, the p electrode 24C corresponds to the distance from the electron block layer 21C highly doped with acceptor impurities to the n layer 11, that is, the film thickness including the first active layer 14 and a part of the first intermediate layer 15.

[0062] Therefore, the distance between the pn junctions differs in each of these three cases, and the driving voltage, current injection efficiency, and reverse current also differ. Furthermore, when applying a voltage to the p-electrode 24A to make the third active layer 18 emit light, electron and hole carriers are supplied to all active layers, and light may also be emitted from the second active layer 16 and the first active layer 14. Similarly, when applying a voltage to the p-electrode 24B to make the second active layer 16 emit light, light may also be emitted from the first active layer 14.

[0063] In the second embodiment, such a problem is solved by the structure of the intermediate layer. That is, in the second embodiment, the first intermediate layer 15 is made of two layers, a non-doped layer 215A and an n-type layer 215B doped with a high concentration of donor impurities, and the second intermediate layer 17 is made of two layers, a non-doped layer 217A and an n-type layer 217B doped with a high concentration of donor impurities, and the n-type layers 215B and 217B are doped with Si to make them n-type.

[0064] Therefore, the pn junction distance is the distance from the electron block layer 21A to the n-type layer 217B of the second intermediate layer 17 in the region under the p electrode 24A, the distance from the electron block layer 21B to the n-type layer 215B of the first intermediate layer 15 in the region under the p electrode 24B, and the distance from the electron block layer 21C to the n-layer 11 in the region under the p electrode 24C. That is, the pn junction distance under all electrodes does not include the active layers, but corresponds to the total film thickness including one active layer and the non-doped layer of the intermediate layer. Here, by appropriately controlling the thickness of the non-doped layer 215A of the first intermediate layer 215 and the non-doped layer 217A of the second intermediate layer 17, the pn junction distance can be made equal in these three cases. As a result, the variation in the driving voltage, current injection efficiency, and reverse current can be suppressed in these three cases, and uniform control is possible. Furthermore, in these three cases, only one each of the first active layer 14, second active layer 16, and third active layer 18 is included between the pn junctions, and the n-type intermediate layer acts as a barrier layer for holes, making it difficult for holes to be injected beyond the n-type intermediate layer into the lower active layer. As a result, it is possible to prevent active layers other than the one that are intended to emit light and located between the pn junctions from emitting light.

[0065] Third embodiment As shown in FIG. 15, the light emitting device of the third embodiment is obtained by replacing the second active layer 16 and the third active layer 18 of the light emitting device of the first embodiment with a second active layer 316 and a third active layer 318.

[0066] The second active layer 316 has a structure in which a strain relaxation layer 316A and an SQW or MQW quantum well structure layer (light emitting layer) 316B are laminated in this order. The quantum well structure layer 316B has the same structure as the second active layer 16 of the first embodiment.

[0067] The strain relaxation layer 316A has an SQW structure in which a barrier layer and a well layer are laminated in order, and is a quantum well structure in which the thickness of the well layer is adjusted to be thin so as not to emit light. For example, the well layer can be made to be 1 nm or less in thickness so as not to emit light. The barrier layer is AlGaN, and the well layer is InGaN. The wavelength corresponding to the band edge energy of the well layer of the strain relaxation layer 316A may be shorter than the emission wavelength of the quantum well structure layer 316B, and is 400 to 460 nm if the emission wavelength of the second active layer 16 is 500 to 560 nm. It is preferably 40 to 100 nm shorter than the emission wavelength of the quantum well structure layer 316B. In this case, the growth temperature of the strain relaxation layer 316A is 700 to 800°C.

[0068] The wavelength corresponding to the band edge energy of the well layer of the strain relaxation layer 316A may be equal to the emission wavelength of the first active layer 14. In this case, the strain relaxation layer 316A may be grown at the same growth temperature as the first active layer 14.

[0069] The band edge energy in the well layer of the strain relaxation layer 316A can be controlled by the thickness of the well layer. That is, by making the thickness of the well layer of the strain relaxation layer 316A sufficiently thin, the energy of the subband in the well rises and the band edge energy becomes large. As a result, it may be shorter than the emission wavelength of the quantum well structure layer 316B. The growth temperature is arbitrary, but it may be grown at the same growth temperature as that of the quantum well structure layer 316B. Furthermore, by making the thickness of the well layer of the strain relaxation layer 316A thin, the subband further rises and the energy difference with the barrier layer becomes small. That is, it becomes closer to the band edge energy of the barrier layer. As a result, it becomes difficult to confine carriers in the well layer of the strain relaxation layer 316A and it becomes difficult to emit light, so that it functions as a part of the barrier layer of the quantum well structure layer 316B and also has the effect of strain relaxation at the same time. In this way, by forming the strain relaxation layer 316A having a well layer with worse carrier confinement than the well layer of the quantum well structure layer 316B, it is possible to form the strain relaxation layer 316A that does not emit light.

[0070] In short, the material and layer structure of the strain relaxation layer 316A are set so that the effective lattice constant of the entire strain relaxation layer 316A is between the lattice constant of the first intermediate layer 15 and the lattice constant of the quantum well structure layer 316B, and the thickness of the well layer is set so that the strain relaxation layer 316A does not emit light.

[0071] The strain relaxation layer 316A may have an MQW structure in which two or more pairs of a barrier layer and a well layer are stacked, but since the second active layer 316 becomes thicker, an SQW structure is preferable.

[0072] By providing the strain relaxation layer 316A as described above, it is possible to relax the strain in the quantum well structure layer 316B laminated thereon, and it is possible to improve the crystal quality of the well layer of the quantum well structure layer 316B.

[0073] It is preferable to set the ratio of the thickness of the first active layer 14 to the thickness of the second active layer 316 to be 30% or less. This makes it possible to more efficiently relieve the distortion of the quantum well structure layer 316B, and also makes the distance between the pn junctions constant under each of the p electrodes 24A to 24C, thereby making the device characteristics uniform under each of the p electrodes 24A to 24C.

[0074] The third active layer 318 has a structure in which a first strain relaxation layer 318A, a second strain relaxation layer 318B, and an SQW or MQW quantum well structure layer 318C are laminated in this order. The quantum well structure layer 318C has a structure similar to that of the third active layer 18 of the first embodiment.

[0075] The first strain relaxation layer 318A has the same structure as the strain relaxation layer 316A of the second active layer 316. The wavelength corresponding to the band edge energy of the well layer of the first strain relaxation layer 318A should be shorter than the emission wavelength of the quantum well structure layer 316B, and is, for example, 400 to 460 nm.

[0076] The second strain relaxation layer 318B has an emission wavelength corresponding to the band edge energy of the well layer of the second strain relaxation layer 318B that is shorter than the emission wavelength of the quantum well structure layer 318C and longer than the emission wavelength corresponding to the band edge energy of the well layer of the first strain relaxation layer 318A (for example, 510 to 570 nm). The rest of the wavelength is the same as that of the first strain relaxation layer 318A.

[0077] It is preferable that the difference between the wavelength corresponding to the band edge energy of the well layer of the first strain relaxation layer 318A and the wavelength corresponding to the band edge energy of the well layer of the second strain relaxation layer 318B, and the difference between the wavelength corresponding to the band edge energy of the well layer of the second strain relaxation layer 318B and the emission wavelength of the quantum well structure layer 318C are 40 to 100 nm.

[0078] The ratio of the thickness of the first active layer 14 to the thickness of the third active layer 318 and the ratio of the thickness of the second active layer 316 to the thickness of the third active layer 318 are preferably set to 30% or less. This makes it possible to more efficiently relieve the distortion of the quantum well structure layer 318C, while making the pn junction distance constant under each of the p electrodes 24A-24C, thereby making the device characteristics uniform under each of the p electrodes 24A-24C.

[0079] By providing the first strain relief layer 318A and the second strain relief layer 318B in this manner, the strain can be relaxed stepwise, and the strain in the quantum well structure layer 318C laminated thereon can be relaxed effectively, thereby improving the quality of the well layer in the quantum well structure layer 318C.

[0080] In the third active layer 318, the strain is relaxed in two stages by the first strain relaxation layer 318A and the second strain relaxation layer 318B, but three or more strain relaxation layers may be provided to relax the strain in three or more stages. Similarly, in the second active layer 316, a plurality of strain relaxation layers 316A may be provided to relax the strain in stages.

[0081] Similarly, a strain relaxation layer may be provided in the first active layer 14. In this case, the growth temperature of the strain relaxation layer is, for example, 800 to 900°C.

[0082] (Other variations) Although the light emitting device of this embodiment has three active layers, namely, the first active layer 14, the second active layer 16, and the third active layer 18, the present invention can be applied to any structure having two or more active layers with different emission wavelengths. Furthermore, the emission color is not limited to blue, green, and red, but may be any color with different emission wavelengths.

[0083] The light emitting element of this embodiment is preferably driven by a PWM circuit to control the light emission. The light intensity can be easily controlled by the pulse width and pulse period, and wavelength shifts due to differences in drive current can also be suppressed.

[0084] (Experimental Results) Next, experimental results relating to this embodiment will be described.

[0085] (Experiment 1) A light-emitting device was fabricated by omitting the second intermediate layer 17 and the third active layer 18 from the light-emitting device of the first embodiment, further omitting the regrown layer 20B, the electron blocking layer 21B, the p-layer 22B, and the p-electrode 24B, replacing the first intermediate layer 15 with the first intermediate layer 215 of the second embodiment, and replacing the second active layer 16 with the second active layer 316 of the third embodiment (see FIG. 18, hereinafter referred to as the light-emitting device of Experimental Example 1). The first intermediate layer 215 was InGaN with an In composition ratio of 5%. The wavelength corresponding to the band edge energy of the well layer in the strain relaxation layer 316A of the second active layer 316 was set to be the same as the emission wavelength of the first active layer 14, and a SQW structure was formed.

[0086] FIG. 8 is an AFM image of the surface of the second active layer 16 of the light-emitting device of Experimental Example 1. In FIG. 8, the upper part shows a 10 μm square range, and the lower part shows a 2 μm square range. For comparison, the first intermediate layer 215 is made of GaN, and the rest is the same as in Experimental Example 2 (Experimental Example 2). As shown in FIG. 8, the density of pits was lower in Experimental Example 1 than in Experimental Example 2. In addition, the surface flatness RMS was 0.88 nm in the 10 μm square range for Experimental Example 2 and 2.6 nm for the comparative example, and in the 2 μm square range for Experimental Example 1 and 3.1 nm for Experimental Example 2, and in both cases, Experimental Example 1 was smaller than the comparative example. As a result, it can be seen that In in the first intermediate layer 215 acts as a surfactant, improving the surface flatness of the first intermediate layer 215, and thus improving the surface flatness and crystal quality of the second active layer 16 thereon.

[0087] 9 is a graph showing the relationship between the drive current and the external quantum efficiency for the light-emitting elements of Experimental Examples 1 and 2. The external quantum efficiency is obtained when a voltage is applied to the p-electrode 24B to cause the second active layer 316 to emit light. As shown in FIG. 9, the external quantum efficiency of Experimental Example 1 is higher than that of Experimental Example 2. This shows that the external quantum efficiency is improved by improving the crystal quality of the second active layer 16.

[0088] (Experiment 2) For the light emitting device of the first embodiment (hereinafter, the light emitting device of Experimental Example 3) in which the first active layer 14 had an emission wavelength of 430 nm, the second active layer 16 had an emission wavelength of 520 nm, and the third active layer 18 had an emission wavelength of 630 nm, a current was injected into the p-electrode 24A, and the emission spectrum was measured. The Si concentration of the n-type layer 215B and the n-type layer 217B was 1×10 18 cm -3 , 2×10 18 cm -3 , 3×10 18 cm -3 For comparison, the emission spectrum was also measured when n-type layer 215B and n-type layer 217B were changed to non-doped layers.

[0089] 11 to 14 are graphs showing emission spectra. FIG. 11 shows the emission spectrum of 18 cm -3 , Figure 12 is 2 × 10 18 cm -3 , Figure 13 is 1 × 10 18 cm -3 14 shows the non-doped state. As shown in FIG. 14, in the non-doped state, not only the third active layer 18 emits red light, but also the first active layer 14 emits blue light, and it was found that the red light was weak and the blue light was strong. On the other hand, as shown in FIGS. 11 to 13, in the Si-doped state, the red light was as strong as or stronger than the blue light, and the intensity of the blue light was reduced as the Si concentration increased. Instead of the intensity of the blue light from the second active layer 16 decreasing, some green light was also emitted, but as shown in FIG. 11, the intensity of the green light was reduced when the Si concentration was sufficiently high. As a result, it was found that by introducing the Si-doped n-type layer 215B and n-type layer 217B into the first intermediate layer 15 and the second intermediate layer 17, it was possible to suppress the light emission from the active layers (the first active layer 14 and the second active layer 16) other than the third active layer 18, which is the active layer to be emitted.

[0090] (Experiment 3) FIG. 16 is an AFM image of the surface of the quantum well structure layer 316C of the light-emitting device of Experimental Example 2 and Experimental Example 4. Experimental Example 4 is the case where the strain relaxation layer 316A was not provided in the second active layer 316 in Experimental Example 2. In FIG. 16, the upper part shows a 10 μm square range, and the lower part shows a 2 μm square range. As shown in FIG. 16, the surface flatness RMS was 2.6 nm in Experimental Example 2 and 3.8 nm in Experimental Example 4 in the 10 μm square range, and 3.1 nm in Experimental Example 2 and 3.3 nm in Experimental Example 4 in the 2 μm square range, and in both cases, Experimental Example 2 was smaller than Experimental Example 4. That is, the surface flatness was improved. As a result, it can be seen that the introduction of the strain relaxation layer 316A into the second active layer 316 relaxed the strain of the quantum well structure layer 316B thereon, and the surface flatness and crystal quality were improved.

[0091] Fig. 17 is a graph showing the relationship between the drive current and the external quantum efficiency for the light-emitting devices of the third embodiment and the comparative example. The external quantum efficiency is obtained when a voltage is applied to the p-electrode 24A to cause the second active layer 316 to emit light. As shown in Fig. 17, the external quantum efficiency of the second experimental example is higher than that of the fourth experimental example. This shows that the external quantum efficiency is improved by alleviating the distortion of the second active layer 316 and improving the surface flatness and crystal quality. [Industrial Applicability]

[0092] The light-emitting device of the present invention can be applied to full-color displays and the like. [Explanation of symbols]

[0093] 10: Substrate 11:n layer 12:ESD layer 13: Base layer 14: 1st active layer 15, 215: First middle tier 16, 316: 2nd active layer 17, 217: Second middle class 18, 318: 3rd active layer 19:Protective layer 20A~20C: Regrowth layer 21A to 21C: Electron blocking layer 22A~22C:p layer 23:n electrode 24A~24C:p electrode 215A, 217A: Non-doped layers 215B, 217B: n-type layer 316A: Strain relaxation layer 316B, 318C: Quantum well structure layer 318A: First strain relaxation layer 318B: Second strain relaxation layer

Claims

1. In a light emitting device using a Group III nitride semiconductor, An n-type layer; a first active layer provided on the n-type layer and having a quantum well structure including a first well layer and a first barrier layer; an intermediate layer provided on the first active layer to prevent the first active layer from emitting light; a strain relaxation layer provided on the intermediate layer, the strain relaxation layer being a quantum well structure having a second well layer and a second barrier layer, the thickness of the second well layer being adjusted so as not to emit light; a second active layer having a quantum well structure that emits light and is provided on the strain relaxation layer; a p-type layer provided on the second active layer, a wavelength corresponding to a band edge energy of the second well layer of the strain relaxation layer is set to be shorter than an emission wavelength of the second active layer, a p-type Group III nitride semiconductor is not present in a laminate structure from the n-type layer to the second active layer;

2. 2. The light-emitting device according to claim 1, wherein a wavelength corresponding to a band edge energy of the second well layer of the strain relaxation layer is set to be equal to a wavelength corresponding to a band edge energy of the first well layer of the first active layer.

3. 2. The light-emitting device according to claim 1, wherein a wavelength corresponding to the band edge energy of said second well layer of said strain relaxation layer is set to be shorter than the emission wavelength of said second active layer by 40 to 100 nm.

4. The light-emitting device according to claim 1 , wherein the strain relaxation layer has a SQW structure.

5. The light-emitting device according to claim 1 , wherein a ratio of a thickness of the first active layer to a thickness of the second active layer is 30% or less.

6. the intermediate layer is made of a group III nitride semiconductor containing In, 2 . The light emitting device according to claim 1 , wherein the In composition of said intermediate layer is set to have a band gap that does not absorb light emitted from said second active layer.

7. The light emitting device according to claim 6 , wherein the In composition of the intermediate layer is 10% or less.

8. The light-emitting device according to claim 6 , wherein the intermediate layer is InGaN.

9. The light-emitting device according to claim 6 , wherein the intermediate layer is In-doped GaN.

10. 2. The light-emitting device according to claim 1, wherein the intermediate layer has a structure in which a first undoped layer made of a non-doped Group III nitride semiconductor and a second layer made of an n-type Group III nitride semiconductor are laminated in this order from a side of the first active layer.

11. The light-emitting device according to claim 10 , wherein the intermediate layer has a thickness of 150 nm or less, and the first layer and the second layer have a thickness of 10 nm or more.

12. The light emitting device according to claim 1, wherein the emission wavelength of the second active layer is 570 to 700 nm.

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