LED display element and manufacturing method thereof

Abstract

To provide a monolithic LED display element with improved contrast.SOLUTION: An LED display element is made of a group III nitride semiconductor and is of a monolithic type. An n-electrode 20 of the LED display element includes an n-contact layer 201 provided in contact with an n-layer 11, an n-side low reflection layer 202 provided on the n-contact layer 201, and an n-side bonding layer 203 provided on the n-side low reflection layer 202. The n-side low reflection layer 202 is formed by alternately laminating two or more pairs of light-transmitting metal films 202A and transparent conductive films 202B made of transparent conductive oxide. This n-side low reflection layer 202 reduces the reflectance of the n-electrode 20.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to an LED display element and 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 have been attracting attention (Patent Document 1). A micro LED display is a display in which tiny LEDs on the order of 1 to 100 μm are arranged in a matrix. Each tiny LED forms one pixel or subpixel. Known micro LED displays have a structure in which the micro LEDs are individual chips, and a monolithic structure in which multiple micro LEDs are fabricated on a single substrate (Patent Document 1). Monolithic structures are superior in terms of miniaturization.

[0003] Patent Document 2 describes covering the side and top surfaces of the light-emitting element with a passivation layer made of black epoxy molding compound. It also describes that the passivation layer prevents light from leaking from the side surfaces of the light-emitting element and suppresses light interference from adjacent light-emitting elements. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2020-88383 [Patent Document 2] Special Publication No. 2022-532327 Summary of the Invention [Problem to be solved by the invention]

[0005] However, conventional monolithic microLEDs have problems such as reduced contrast due to the light emitted from the active layer or external light being reflected at the electrode interface, or unwanted areas appearing to emit light. For example, pixels that do not emit light appear to emit light due to the light reflection at the electrode, or the display may not be dark enough.

[0006] Furthermore, the method of covering with a passivation layer, as in Patent Document 2, becomes less effective when the pixels of a micro LED display element are miniaturized. For example, if the semiconductor layer is made continuous without separating each pixel with a groove in order to miniaturize each pixel, the side surface of the light-emitting layer cannot be covered, and interference between each pixel cannot be sufficiently suppressed. Furthermore, as a result of miniaturizing each pixel, the ratio of the electrode area to each pixel increases, which reduces the ratio of the passivation layer and prevents sufficient light absorption.

[0007] The present invention has been made in view of the above background, and aims to provide a monolithic LED display element with improved contrast and a method for manufacturing the same. [Means for solving the problem]

[0008] One aspect of the present invention is A monolithic LED display element having an n-layer, an active layer, a p-layer, an n-electrode, and a p-electrode, wherein a region of the active layer facing the p-electrode is a light-emitting portion, and a plurality of the light-emitting portions are arranged, At least one of the p-electrode and the n-electrode has a low-reflection layer in which a transparent conductive film made of a transparent conductive oxide and a light-transmitting metal film that can transmit light are alternately laminated. death, The low-reflection layer includes a light-opaque metal film laminated on the opposite end of the n-layer. , in the LED display element.

[0009] Another aspect of the present invention is a method for manufacturing the LED display element of the above-mentioned one aspect, a combination setting step of setting a plurality of combinations of thicknesses of the low-reflection layers; an integration step of integrating the reflectance of the p-electrode or the n-electrode provided with the low-reflection layer for the plurality of combinations at a wavelength of 400 to 700 nm and an incident angle of 0 to 16°; a minimum integral value extraction step of extracting a minimum integral value from the plurality of integral values ​​obtained in the integration step; a combination extraction step of extracting the combination corresponding to an integral value that is equal to or less than twice the minimum integral value from the plurality of integral values ​​acquired by the integration step; a forming step of forming each of the layers so that the thickness of each of the layers is the combination extracted in the combination extraction step; The present invention relates to a method for manufacturing an LED display element, comprising: [Effects of the Invention]

[0010] The LED display element and its manufacturing method described above can reduce light reflection by the p-electrode and n-electrode, thereby preventing a decrease in display contrast and preventing unwanted areas from appearing to be emitting light. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram showing the configuration of an LED display element according to a first embodiment. [Figure 2] A diagram showing the planar pattern of grooves and electrodes. [Figure 3] FIG. 2 is a diagram showing the configuration of an n-electrode 20. [Figure 4] FIG. 2 is a diagram showing the configuration of p-electrodes 21A to 21C. [Figure 5] 2A to 2C are diagrams illustrating a manufacturing process of the LED display element according to the first embodiment. [Figure 6] 2A to 2C are diagrams illustrating a manufacturing process of the LED display element according to the first embodiment. [Figure 7] 2A to 2C are diagrams illustrating a manufacturing process of the LED display element according to the first embodiment. [Figure 8] 2A to 2C are diagrams illustrating a manufacturing process of the LED display element according to the first embodiment. [Figure 9] 1 is a table showing the thickness of each layer of the n-electrode 20 and a graph showing the wavelength dependence of reflectance. [Figure 10] 1 is a table showing the thickness of each layer of the n-electrode 20 and a graph showing the wavelength dependence of reflectance. [Figure 11] 1 is a table showing the thickness of each layer of the n-electrode 20 and a graph showing the wavelength dependence of reflectance. [Figure 12] 1 is a table showing the thickness of each layer of the n-electrode 20 and a graph showing the wavelength dependence of reflectance. [Figure 13] 1 is a table showing the thickness of each layer of p-electrodes 21A to 21C, and a graph showing the wavelength dependency of reflectance. [Figure 14] 1 is a table showing the thickness of each layer of p-electrodes 21A to 21C, and a graph showing the wavelength dependency of reflectance. [Figure 15] 1 is a table showing the thickness of each layer of p-electrodes 21A to 21C, and a graph showing the wavelength dependency of reflectance. [Figure 16] 1 is a table showing the thickness of each layer of p-electrodes 21A to 21C, and a graph showing the wavelength dependency of reflectance. DETAILED DESCRIPTION OF THE INVENTION

[0012] The LED display element has an n-layer, an active layer, a p-layer, an n-electrode, and a p-electrode. The region of the active layer facing the p-electrode forms a light-emitting section. The LED display element is a monolithic type in which a plurality of the light-emitting sections are arranged. At least one of the p-electrode and the n-electrode has a low-reflection layer in which transparent conductive films made of transparent conductive oxides and light-transmitting metal films that can transmit light are alternately stacked. For example, the LED display element may have a configuration in which a plurality of the light-emitting sections are arranged in a matrix. The thickness of the light-transmitting metal film is set to a thickness that allows light to pass through.

[0013] The low-reflection layer may include a light-opaque metal film laminated on the opposite end of the n-layer.

[0014] The light-transmitting metal film of the low-reflectivity layer that is closest to the n-layer may be thinner than the transparent conductive film that is closest to the n-layer.

[0015] The p-electrode may have a p-contact layer in contact with the p-layer and the low-reflection layer laminated on the p-contact layer. In this case, the total thickness of the low-reflection layer laminated on the p-contact layer may be thicker than that of the p-contact layer. The p-contact layer may be made of a transparent conductive oxide, and the layer of the low-reflection layer closest to the p-contact layer may be the light-transmitting metal film.

[0016] The n-electrode may have an n-contact layer in contact with the n-layer and the low-reflection layer laminated on the n-contact layer. In this case, the low-reflection layer laminated on the n-contact layer may be thicker than the n-contact layer. The n-contact layer may be made of metal, and the layer of the low-reflection layer on the n-contact layer side may be the transparent conductive film. The light-transmitting metal film constituting the low-reflection layer laminated on the n-contact layer may be thicker than the n-contact layer.

[0017] The transparent conductive film may be ITO or IZO.

[0018] The light-transmitting metal film may be at least one selected from Cr, Ir, Mo, Ni, Pt, Rh, Ta, Ti, V, W, and TiN.

[0019] (Embodiment 1) 1. Overview of element configuration FIG. 1 is a diagram showing the configuration of an LED display element of embodiment 1. The LED display element of embodiment 1 is a monolithic micro LED display element made of a group III nitride semiconductor. A monolithic type has a structure in which multiple light-emitting element structures are provided on the same substrate, and the light-emitting element structures are arranged in a matrix to form a display. The light-emitting element structure corresponds to one pixel in the display, and one pixel is composed of sub-pixels that can emit blue, green, and red, respectively. The LED display element of embodiment 1 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). For ease of explanation, the LED display element of embodiment 1 is assumed to be a 2 × 2 pixel display.

[0020] As shown in FIG. 1, the LED display element of embodiment 1 includes a substrate 10, an n-layer 11, a first active layer 12, a first intermediate layer 13, a second active layer 14, a second intermediate layer 15, a third active layer 16, a protective layer 17, a p-layer 18, an n-electrode 20, and p-electrodes 21A to 21C.

[0021] 2.About each component Next, each component of the LED display element of the first embodiment will be described in more detail.

[0022] The substrate 10 is a growth substrate for growing a group III nitride semiconductor, such as sapphire, Si, or GaN.

[0023] The n-layer 11 is an n-type semiconductor layer 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 needed, and if the substrate is GaN, the buffer layer may 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 is.

[0024] An ESD layer or an underlayer may be provided on the n layer 11. The ESD layer is a layer provided to improve electrostatic breakdown voltage. The ESD layer is, for example, undoped or lightly Si-doped GaN, InGaN, or AlGaN. The underlayer is a semiconductor layer with a superlattice structure provided on the ESD layer, and is a layer for alleviating lattice distortion of the semiconductor layer formed on the underlayer. The underlayer is formed by alternately laminating III-nitride semiconductor thin films with 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 Si at a concentration of 1×10 17 ~100×10 17 cm -3 The layer may be doped to some extent. Also, as long as it can relax the strain, it does not need to have a superlattice structure. Any material that reduces the lattice constant difference at the heterointerface with the first active layer 12 may be used, such as an InGaN layer, an AlInN layer, or an AlGaIn layer.

[0025] The first active layer 12 is a light emitting layer of SQW or MQW structure provided on the n-layer 12. The emission wavelength is blue, 430 to 480 nm. The first active layer 12 has a structure in which 1 to 7 pairs of barrier layers made of AlGaN and well layers made of InGaN are alternately stacked. The number of pairs is more preferably 1 to 5, and even more preferably 1 to 3.

[0026] The first intermediate layer 13 is a semiconductor layer provided on the first active layer 12, and is located between the first active layer 12 and the second active layer 14. The first intermediate layer 13 is a layer provided to enable separate control of the light emission from the first active layer 12 and the light emission from the second active layer 14. The first intermediate layer 13 also serves to protect the first active layer 12 from etching damage when forming second grooves 31, which will be described later.

[0027] The material of the first intermediate layer 13 is GaN or InGaN. The first intermediate layer 13 may be undoped, but is preferably doped with n-type impurities. For example, when 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.

[0028] The thickness of the first intermediate layer 13 is preferably 20 to 150 nm. If it is thicker than 150 nm, the surface of the first intermediate layer 13 may become rough. If it is thinner than 20 nm, it may be difficult to control the depth of the second grooves 31, described below, within the first intermediate layer 13 when forming the second grooves 31. The thickness is more preferably 30 to 100 nm, and even more preferably 50 to 80 nm.

[0029] The second active layer 14 is a light-emitting layer of SQW or MQW structure provided on the first intermediate layer 13. The emission wavelength is green, 510 to 550 nm. The second active layer 14 has a structure in which 1 to 7 pairs of barrier layers made of GaN and well layers made of InGaN are alternately stacked. The number of pairs is more preferably 1 to 5 pairs, and even more preferably 1 to 3 pairs. The number of pairs is preferably equal to or less than the number of pairs in the first active layer 12, and more preferably less.

[0030] The second intermediate layer 15 is a semiconductor layer provided on the second active layer 14, and is located between the second active layer 14 and the third active layer 16. The second intermediate layer 15 is provided for the same reason as the first intermediate layer 13, and is a layer provided to enable separate control of the light emission from the second active layer 14 and the light emission from the third active layer 16. The second intermediate layer 15 also serves to protect the second active layer 14 from etching damage when forming the third grooves 32 described below.

[0031] The material of the second intermediate layer 15 is the same as that of the first intermediate layer 13. The first intermediate layer 13 and the second intermediate layer 15 may be made of the same material. Furthermore, the second intermediate layer 15 may also be doped with impurities, similar to the first intermediate layer 13. Furthermore, the first intermediate layer 13 and the second intermediate layer 15 may have the same thickness.

[0032] The third active layer 16 is a light-emitting layer of SQW or MQW structure provided on the second intermediate layer 15. The emission wavelength is red, 590 to 700 nm. The third active layer 16 has a structure in which barrier layers made of InGaN and well layers 7 made of InGaN are alternately stacked in 1 to 7 pairs. The number of pairs is more preferably 1 to 5 pairs, and even more preferably 1 to 3 pairs. The number of pairs is preferably equal to or less than the number of pairs in the second active layer 14, and more preferably less.

[0033] The protective layer 17 is a semiconductor layer provided on the third active layer 16. The protective layer 17 protects the active layer and also functions as an electron blocking layer. The protective layer 17 may be made of a material having a wider band gap than the well layer of the third active layer 16, such as AlGaN, GaN, or InGaN. The thickness of the protective layer 17 is preferably 2.5 to 50 nm, and more preferably 5 to 25 nm. The protective layer 17 may be doped with an impurity, or may be doped with Mg. In this case, the Mg concentration should be 1×10 18 ~1000×10 18 cm -3 It is best to do so.

[0034] A portion of the protective layer 17 is etched to form grooves, including a third groove 32 extending from the protective layer 17 to the second intermediate layer 15, a second groove 31 extending to the first intermediate layer 13, and a first groove 30 extending to the n-layer 11. The planar patterns of the first groove 30, the second groove 31, and the third groove 32 are as shown in FIG. 2 . The first groove 30 is provided in a ring-shaped pattern along the periphery of the LED display element in a peripheral region thereof. The second groove 31 and the third groove 32 are provided in a striped pattern within the region surrounded by the first groove 30, in the following order from one end: a region without a groove, the third groove 32, the second groove 31, a region without a groove, the third groove 32, and the second groove 31. Thus, in the LED display element of embodiment 1, each pixel is not separated by a groove, and the semiconductor layer is provided continuously.

[0035] The p-layer 18 is a semiconductor layer provided in a continuous film shape on the protective layer 17, on the second intermediate layer 15 exposed at the bottom of the third groove 32, and on the first intermediate layer 13 exposed at the bottom of the second groove 31, and is composed of a first layer and a second layer in that order from the substrate 10 side. Hereinafter, the region of the p-layer 18 on the protective layer 17 will be referred to as p-layer 18A, the region on the second intermediate layer 15 as p-layer 18B, and the region on the first intermediate layer 13 as p-layer 18C. 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 even more 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 best to do so.

[0036] In the first embodiment, the p-layer 18 is provided continuously on the protective layer 17, the second intermediate layer 15, and the first intermediate layer 13. However, these layers may be provided separately. A regrown layer having the same structure as the protective layer 17 or an electron blocking layer may be provided between the p-layer 18A and the protective layer 17, between the p-layer 18B and the second intermediate layer 15, and between the p-layer 18C and the first intermediate layer 13. The electron blocking layer blocks electrons from passing through the third active layer 16 and entering the p-layer 18. The electron blocking layer may be a single layer of GaN or AlGaN, or may have a laminated structure of two or more of AlGaN, GaN, and InGaN, or a laminated structure of AlGaN or the like with only the composition ratio changed. A superlattice structure may also be used. The electron blocking layer may have a thickness of, for example, 5 to 50 nm, and a Mg concentration of, for example, 1×10 19 ~100×10 19 cm -3 is.

[0037] The n-electrode 20 is an electrode provided on the n-layer 11 exposed at the bottom of the first groove 30. As shown in FIG. 2, the n-electrode 20 has a rectangular ring-shaped pattern that follows the outer periphery of the LED display element. There is one n-electrode, which serves as a common electrode for each pixel and each sub-pixel. The n-electrode 20 has a structure with low reflectivity (reflectivity when light is incident on the n-electrode 20 from the GaN; hereinafter, the same applies to the reflectivity of the n-electrode 20 unless otherwise specified). The structure and materials of the n-electrode 20 will be described in more detail below.

[0038] The p-electrodes 21A-21C are electrodes separately provided on the p-layers 18A-18C. A pair of the p-electrodes 21A-21C is provided on an area that will become a pixel. The p-electrodes 21A-21C are also provided on an area that will become a sub-pixel. In each active layer, the area directly below the p-electrodes 21A-21C serves as a light-emitting portion. Specifically, the area directly below the p-electrode 21A (the area facing the p-electrode 21A) of the third active layer 16 emits red light and serves as a red sub-pixel; the area directly below the p-electrode 21B (the area facing the p-electrode 21B) of the second active layer 14 emits green light and serves as a green sub-pixel; and the area directly below the p-electrode 21C (the area facing the p-electrode 21C) of the first active layer 12 emits blue light and serves as a blue sub-pixel. The p electrodes 21A to 21C have a structure with low reflectivity (reflectivity when light is incident on the p electrodes 21A to 21C from the GaN; hereinafter, the same applies to the reflectivity of the p electrodes 21A to 21C unless otherwise specified). The detailed structure and materials of the p electrodes 21A to 21C will be described later.

[0039] 3. Configuration of n-electrode 20 Next, the configuration of n-electrode 20 will be described in detail. Fig. 3 is a diagram showing the configuration of n-electrode 20. As shown in Fig. 3, n-electrode 20 has n-contact layer 201 provided on and in contact with n-layer 11, n-side low-reflection layer 202 provided on n-contact layer 201, and n-side junction layer 203 provided on n-side low-reflection layer 202.

[0040] N-contact layer 201 is a layer provided to make ohmic contact with n-layer 11 and reduce the contact resistance of n-electrode 20 with n-layer 11. N-contact layer 201 is made of a material such as Ti, V, Cr, or Ni.

[0041] The n-side low-reflection layer 202 is formed by alternately laminating two or more pairs of light-transmitting metal films 202A and transparent conductive films 202B. Preferably, there are two to six pairs. The light-transmitting metal film 202A is a metal film with a thickness that allows light to pass through. The n-side low-reflection layer 202 reduces the reflectance of the n-electrode 20. Both the light-transmitting metal film 202A and the transparent conductive film 202B are made of conductive materials, and the n-side low-reflection layer 202 as a whole is conductive.

[0042] The reasons why the n-side low-reflection layer 202 reduces the light reflectance of the n-electrode 20 are as follows. First, the n-side low-reflection layer 202 weakens the overall reflection at the interface between the n-layer 11 and the n-contact layer 201, the reflection at the interface between the n-contact layer 201 and the n-side low-reflection layer 202, and the reflection at each layer interface in the n-side low-reflection layer 202. Second, the light-transmitting metal films 202A of the n-side low-reflection layer 202 absorb light. Even though the thickness of each of the light-transmitting metal films 202A is thin, the total thickness is sufficient to sufficiently absorb light.

[0043] The light-transmitting metal film 202A may be made of a material such as Cr, Ir, Mo, Ni, Pt, Rh, Ta, Ti, V, W, or TiN. A material with high absorption and low reflectance for visible light, particularly for the emission wavelength, is preferred. The light-transmitting metal film 202A may be an alloy, such as an alloy primarily composed of the metal elements listed above. The multiple light-transmitting metal films 202A may all be made of the same material, or may be made of different materials.

[0044] The material of the transparent conductive film 202B may be any transparent conductive oxide that has high transmittance to visible light, particularly at the emission wavelength, and high conductivity, such as ITO or IZO. The transparent conductive films 202B may all be made of the same material, or may be made of different materials.

[0045] The first layer of the n-side low-reflection layer 202 (the layer closest to the n-contact layer 201) is preferably a transparent conductive film 202B. Because light reflection increases at the interface between the n-contact layer 201 and the n-side low-reflection layer 202, the reflection at the n-electrode 20 can be further reduced by canceling out this reflection with the reflection at the interfaces of each layer of the n-side low-reflection layer 202. The last layer of the n-side low-reflection layer 202 (the uppermost layer and the layer farthest from the n-layer 11) may be either a light-transmitting metal film 202A or a transparent conductive film 202B.

[0046] Furthermore, in order to reduce the reflectance of the n-electrode 20, the thickness of the light-transmitting metal film 202A closest to the n-layer 11 may be thinner than the thickness of the transparent conductive film 202B closest to the n-layer 11. Similarly, the total thickness of the n-side low-reflectivity layer 202 may be thicker than the n-contact layer 201.

[0047] Furthermore, the light-transmitting metal film 202A is a metal film with a thickness that transmits light (for example, a transmittance of 5% or more in the visible light range). For example, the light-transmitting metal film 202A is 100 nm or less. If the light-transmitting metal film 202A is made to a thickness that does not transmit light, light may not be transmitted to the subsequent n-side low-reflectivity layer 202 side, and the subsequent structure may not function. However, if the last layer of the n-side low-reflectivity layer 202 (the layer farthest from the n-layer 11 side) is made to be a light-transmitting metal film 202A, the last light-transmitting metal film 202A may be made to be a light-opaque metal film 202C and may be made to a thickness that does not transmit light, for example, a thickness greater than 100 nm.

[0048] The thickness of each layer of n-contact layer 201 and n-side low-reflective layer 202 is preferably set so that the incidence angle dependence and wavelength dependence of the reflectance of n-electrode 20 satisfy the following ranges. Regarding wavelength dependence, the maximum reflectance is 5% or less, preferably 3% or less, in the range of visible light (wavelength 400 to 700 nm) at an incidence angle of 0° (normal incidence). Regarding incidence angle dependence, the maximum reflectance is 10% or less, preferably 5% or less, in the range of incidence angle of 16° or less for light of the emission wavelength of first active layer 12, second active layer 14, and third active layer 16. The thickness of n-contact layer 201 may be fixed, and the thickness of each layer of n-side low-reflective layer 202 may be set so as to satisfy the above.

[0049] The thickness of each layer of the n-contact layer 201 and the n-side low-reflection layer 202 is preferably set as follows. First, multiple combinations of the thickness of each layer of the n-contact layer 201 and the n-side low-reflection layer 202 are set. Then, for each of these multiple combinations, the reflectance of the n-electrode 20 is integrated at a wavelength of 400 to 700 nm and an incident angle of 0 to 16°. Next, the minimum integral value is extracted from the multiple integral values ​​obtained. Next, an integral value that is equal to or less than twice the minimum integral value is extracted from the multiple integral values ​​obtained, and a combination of thicknesses of each layer corresponding to this integral value is extracted. The combination of thicknesses of each layer extracted in this way is set as the thickness of each layer of the n-contact layer 201 and the n-side low-reflection layer 202. Note that although both the n-contact layer 201 and the n-side low-reflection layer 202 are considered in the above, it is also possible to fix the thickness of the n-contact layer 201 and consider only the n-side low-reflection layer 202.

[0050] The n-side bonding layer 203 is a layer that connects to the outside of the device and is made of Au or an Au alloy that mainly contains Au.

[0051] The n-side low reflective layer 202 may be provided directly on the n-layer 11, omitting the n-contact layer 201. In this case, the first layer in the n-side low reflective layer 202 (the layer in contact with the n-layer 11) is preferably the light-transmitting metal film 202A. In this case, the material of the first layer of the light-transmitting metal film 202A may be the same as that of the n-contact layer 201. A barrier layer or the like for preventing metal diffusion may be inserted between the n-contact layer 201 and the n-side low reflective layer 202, or between the n-side low reflective layer 202 and the n-side bonding layer 203.

[0052] 4. Configuration of p-electrodes 21A to 21C Next, the configuration of the p-electrodes 21A to 21C will be described in detail. Fig. 4 is a diagram showing the configuration of the p-electrodes 21A to 21C. As shown in Fig. 4, the p-electrodes 21A to 21C have a p-contact layer 211 provided on and in contact with the p-layers 18A to 18C, a p-side low-reflection layer 212 provided on the p-contact layer, and a p-side bonding layer 213 provided on the p-side low-reflection layer 212.

[0053] P-contact layer 211 is a layer provided to make ohmic contact with p-layers 18A-18C and reduce the contact resistance of p-electrodes 21A-21C with p-layers 18A-18C. Materials for p-contact layer 211 include ITO, IZO, Ni / Au, Co / Au, Ru / Au, Ni, Co, and Ru. Here, / means stacking, and A / B means a structure in which A and B are stacked in this order.

[0054] The p-side low-reflection layer 212 is formed by alternately laminating two or more pairs of light-transmitting metal films 212A and transparent conductive films 212B. Preferably, three to six pairs are laminated. The light-transmitting metal film 212A is a metal film with a thickness that allows light to pass through. The p-side low-reflection layer 212 reduces the reflectance of the p-electrodes 21A to 21C. Both the light-transmitting metal film 212A and the transparent conductive film 212B are made of conductive materials, and the p-side low-reflection layer 212 as a whole is conductive. The p-side low-reflection layer 212 and the n-side low-reflection layer 202 may have the same structure.

[0055] The reason why the p-side low reflective layer 212 reduces the light reflectance of the p-electrodes 21A to 21C is the same as that of the n-side low reflective layer 202.

[0056] The material of the light-transmitting metal film 212A and the material of the transparent conductive film 212B are the same as those of the n-side low-reflection layer 202. When the last layer (the uppermost layer and the layer farthest from the n-layer 11) of the p-side low-reflection layer 212 is the light-transmitting metal film 212A, the light-transmitting metal film 212A of the last layer may be a light-opaque metal film 212C having a thickness that does not transmit light, for example, greater than 100 nm.

[0057] When a transparent conductive film such as ITO or IZO is used as the p-contact layer 211, the first layer of the p-side low-reflection layer 212 (the layer closest to the p-contact layer 211) is preferably a light-transmitting metal film 212A. Because light reflection increases at the interface between the p-contact layer 211 and the p-side low-reflection layer 212, the reflection of the p-electrodes 21A to 21C can be further reduced by counteracting this reflection with the reflection at the interfaces of each layer of the p-side low-reflection layer 212. The last layer (top layer) of the p-side low-reflection layer 212 may be either a light-transmitting metal film 212A or a transparent conductive film 212B.

[0058] The thickness of each layer of the p-contact layer 211 and the p-side low-reflective layer 212 is the same as that of the n-contact layer 201 and the n-side low-reflective layer 202. However, since it is more difficult to make contact with the p-layer 18 than with the n-layer 11, the thickness of the p-contact layer 211 may be fixed to a thickness that allows contact, and the thickness of each layer of the p-side low-reflective layer 212 may be set so that the incidence angle dependence and wavelength dependence of the reflectance of the p-electrodes 21A to 21C satisfy the same conditions as those of the n-electrode 20.

[0059] That is, the thickness of each layer of the p-side low-reflective layer 212 may be set as follows. First, multiple combinations of thicknesses of each layer of the p-side low-reflective layer 212 are set. Then, for each of these multiple combinations, the reflectance of the p-electrodes 21A to 21C is integrated at wavelengths of 400 to 700 nm and at angles of incidence of 0 to 16°. Next, the minimum integral is extracted from the multiple integrals obtained. Next, an integral that is twice or less of the minimum integral is extracted from the multiple integrals obtained, and a combination of thicknesses of each layer corresponding to this integral is extracted. The combination of thicknesses of each layer extracted in this way is set as the thickness of each layer of the p-side low-reflective layer 212.

[0060] In order to reduce the reflectance of the p-electrode 20, the thickness of the light-transmitting metal film 212A closest to the p-layer 18 may be thinner than the thickness of the transparent conductive film 212B closest to the p-layer 18. Similarly, the total thickness of the p-side low-reflectivity layer 212 may be thicker than the p-contact layer 211.

[0061] In order to reduce the reflectance of the p-electrodes 21A to 21C, the thickness of the light-transmitting metal film 212A closest to the n-layer 11 may be thinner than the thickness of the transparent conductive film 212B closest to the n-layer 11. Similarly, the total thickness of the p-side low-reflective layer 212 may be thicker than the p-contact layer 211.

[0062] The p-side junction layer 213 is similar to the n-side junction layer 203 and is a layer that connects to the outside of the device.

[0063] A barrier layer or the like may be inserted between the p-contact layer 211 and the p-side low reflective layer 212, or between the p-side low reflective layer 212 and the p-side bonding layer 213, in order to prevent metal diffusion.

[0064] 5. Operation of LED display elements Next, the operation of the LED display element of Embodiment 1 will be described. In the LED display element of Embodiment 1, applying a voltage between the p-electrode 21A and the n-electrode 20 of a pixel can cause red light to be emitted from the region of the third active layer 16 directly below the p-electrode 21A. Applying a voltage between the p-electrode 21B and the n-electrode 20 of a pixel can cause green light to be emitted from the region of the second active layer 14 directly below the p-electrode 21B. Applying a voltage between the p-electrode 21C and the n-electrode 20 of a pixel can cause blue light to be emitted from the region of the first active layer 12 directly below the p-electrode 21C. The emission of each of these colors can be controlled independently, and two or more of blue, green, and red can also be emitted simultaneously. In this way, the LED display element of this embodiment can control the blue, green, and red emission of a desired pixel (i.e., the emission of each subpixel) by selecting the electrodes to which the voltage is applied, allowing it to be used as a full-color display.

[0065] Here, a portion of the light emitted from a certain subpixel travels directly or is reflected inside the device and travels toward the p electrodes 21A to 21C and the n electrode 20. In addition, some of the external light that enters the device from outside travels toward the p electrodes 21A to 21C and the n electrode 20. Conventional p electrodes 21A to 21C and n electrode 20 that do not use a low-reflection layer have a reflectivity of about 30%, so some of the light is reflected by the p electrodes 21A to 21C and the n electrode 20, and some of the light travels toward non-emitting regions. This causes problems such as non-emitting pixels appearing to emit light or the display not being dark enough.

[0066] In contrast, the LED display element of embodiment 1 has an n-side low-reflection layer 202 and a p-side low-reflection layer 212 provided on the n-electrode 20 and the p-electrodes 21A-21C, respectively, resulting in a structure with extremely low reflectance. This reduces light reflection by the n-electrode 20 and the p-electrodes 21A-21C, thereby preventing the above-mentioned problems. Furthermore, the n-electrode 20 and the p-electrodes 21A-21C have low and almost constant reflectance over the entire wavelength range of visible light, so they do not affect the emitted light color. As a result, the LED display element of embodiment 1 can improve the contrast of the display and prevent non-emitting areas from appearing to emit light.

[0067] As described above, in the LED display element of embodiment 1, the reflectance of n-electrode 20 and p-electrodes 21A to 21C is low, and therefore light reflection by n-electrode 20 and p-electrodes 21A to 21C is reduced. As a result, the LED display element of embodiment 1 can improve the contrast of the display.

[0068] 6. LED display element manufacturing method Next, the manufacturing process of the LED display element of the first embodiment will be described with reference to the drawings.

[0069] First, n-layer 11, first active layer 12, first intermediate layer 13, second active layer 14, second intermediate layer 15, third active layer 16, and protective layer 17 are formed in this order on substrate 10 (see FIG. 5).

[0070] Next, a portion of the surface of the protective layer 17 is dry-etched until it reaches the second intermediate layer 15 to form a third groove 32, and then dry-etched until it reaches the first intermediate layer 13 to form a second groove 31 (see FIG. 6). The third groove 32 and the second groove 31 are preferably etched to a thickness midway between the second intermediate layer 15 and the first intermediate layer 13.

[0071] Next, p-layers 18A to 18C are formed on protective layer 17, on second intermediate layer 15 exposed by third groove 32, and on first intermediate layer 13 exposed by second groove 31 (see FIG. 7). P-layers 18A to 18C may be continuous or separated.

[0072] Next, a portion of the surface of p-layer 18C is dry-etched until it reaches n-layer 11, thereby forming first groove 30 (see FIG. 8). Then, n-electrode 20 is formed on n-layer 11 exposed at the bottom of first groove 30, and p-electrodes 21A-21C are formed on p-layers 18A-18C. In this manner, the LED display element of Embodiment 1 is manufactured. Either n-electrode 20 or p-electrodes 21A-21C may be formed first.

[0073] In forming n-electrode 20 and p-electrodes 21A-21C, sputtering or vapor deposition is used for film formation, and lift-off is used for pattern formation. Sputtering is preferred for film formation. This allows each layer in n-electrode 20 and p-electrodes 21A-21C to be formed flat and without irregularities, thereby improving the transparency of transparent conductive films 202B and 212B.

[0074] In general, it is more difficult to achieve electrode contact with the p-layers 18A to 18C than with the n-layer 11. Therefore, when forming the p-electrodes 21A to 21C, it is preferable to form the p-contact layer 211 on the p-layers 18A to 18C, and then anneal the p-contact layer 211 to reduce the contact resistance with the p-layers 18A to 18C, and then laminate the p-side low-reflective layer 212 and the p-side junction layer 213 in this order. Furthermore, it is preferable to make the width of the p-side low-reflective layer 212 narrower than that of the p-contact layer 211 so that the p-side low-reflective layer 212 can be formed on the p-contact layer 211 even if there is a positional or dimensional misalignment. Meanwhile, the n-electrode 20 does not need to be annealed after the n-contact layer 201 is formed and before the n-side low-reflective layer 202 is formed.

[0075] Furthermore, if the n-side low reflective layer 202, the n-side bonding layer 203, the p-side low reflective layer 212, and the p-side bonding layer 213 have the same structure, they can be formed simultaneously in the same process. Furthermore, if parts of the n-side low reflective layer 202 and the p-side low reflective layer 212 have the same structure, these parts may be formed simultaneously.

[0076] For example, the n-side low-reflection layer 202 and the p-side low-reflection layer 212 may be formed simultaneously as follows by using the same structure with the light-transmitting metal films 202A and 212A as the first layers and omitting the n-contact layer 201 from the n-electrode 20: First, the p-contact layer 211 is formed on the p-layers 18A to 18C and annealed. Next, the n-side low-reflection layer 202 and the n-side bonding layer 203 are formed on the n-layer 11, and the p-side low-reflection layer 212 and the p-side bonding layer 213 are formed on the p-contact layer 211 simultaneously.

[0077] (Experimental example) Experimental Example 1 Next, the results of a simulation regarding the n-electrode 20 of the first embodiment will be described.

[0078] Experimental Example 1-1 For n-electrode 20, which has n-contact layer 201, n-side low-reflection layer 202, and n-side junction layer 203 stacked in this order on a GaN layer, the reflectance of light incident on n-electrode 20 from the GaN layer was calculated by simulation. n-contact layer 201 was Ti, and n-side low-reflection layer 202 had a structure in which ITO and Ti were alternately stacked in three pairs, with the first layer being ITO. n-side junction layer 203 was Au and had an infinite thickness. The reflectance of n-electrode 20 was then integrated with respect to wavelength (400 to 700 nm) and incident angle (0 to 16°), and the thickness of each layer of n-contact layer 201 and n-side low-reflection layer 202 was set so as to minimize the integrated value.

[0079] FIG. 9 shows a table (FIG. 9(a)) of the thickness of each layer of n-electrode 20 and a graph (FIG. 9(b)) showing the wavelength dependence of the reflectance of n-electrode 20. The wavelength dependence of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 9(b), the reflectance was 1% or less in the visible light region of wavelengths from 400 to 700 nm. Furthermore, in the incident angle range of 0 to 16°, the reflectance was 1% or less in the visible light region.

[0080] Experimental Example 1-2 A simulation was performed in the same manner as in Experimental Example 1-1, except that the n-contact layer 201 was replaced with Ti and the n-side low-reflection layer 202 was replaced with a structure in which three pairs of ITO and W were alternately stacked, with the first layer being ITO. The thicknesses of the n-contact layer 201 and the n-side low-reflection layer 202 were set in the same manner as in Experimental Example 1-1.

[0081] FIG. 10 shows a table (FIG. 10(a)) of the thickness of each layer of n-electrode 20 and a graph (FIG. 10(b)) showing the wavelength dependence of the reflectance of n-electrode 20. The wavelength dependence of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 10(b), the reflectance was 1% or less in the visible light region of wavelengths from 400 to 700 nm. Furthermore, in the incident angle range of 0 to 16°, the reflectance was 1% or less in the visible light region.

[0082] Experimental Example 1-3 In Experimental Example 1-1, the n-contact layer 201 was replaced with V, and the n-side low reflective layer 202 was replaced with a structure in which two pairs of ITO and Ta were alternately stacked, with the first layer being ITO, and a simulation was performed in the same manner as in Experimental Example 1-1. The thicknesses of the n-contact layer 201 and the n-side low reflective layer 202 were set in the same manner as in Experimental Example 1-1.

[0083] FIG. 11 shows a table (FIG. 11(a)) of the thickness of each layer of n-electrode 20 and a graph (FIG. 11(b)) showing the wavelength dependence of the reflectance of n-electrode 20. The wavelength dependence of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 11, in the visible light region of wavelengths from 400 to 700 nm, the reflectance was 1% or less except for the region immediately near 700 nm. Furthermore, in the incident angle range of 0 to 16°, the reflectance was 1% or less except for the region immediately near 700 nm in the visible light region.

[0084] Experimental Examples 1-4 The thickness of the top layer (Ta layer) of the n-side low reflective layer 202 in Experimental Example 1-3 was changed from 396.6 nm to 100 nm. The thickness of each of the other layers was not changed from the table in Fig. 11. The reflectance was calculated in this case.

[0085] FIG. 12 is a table (FIG. 12(a)) of the thickness of each layer of n-electrode 20, and a graph (FIG. 12(b)) showing the wavelength dependence of the reflectance of n-electrode 20. The wavelength dependence of reflectance is at an incident angle of 0°. As shown in FIG. 12, there was no change in reflectance compared to FIG. 11. Even if Ta was 100 nm, it was sufficiently thick and no light was transmitted, so it is thought that there was no effect on the reflectance.

[0086] Experimental Example 2 Next, the results of a simulation regarding the p-electrodes 21A to 21C of the first embodiment will be described.

[0087] Experimental Example 2-1 For p-electrodes 21A-21C, each consisting of a p-contact layer 211, a p-side low-reflection layer 212, a barrier layer, and a p-side bonding layer 213 stacked in this order on a GaN layer, the reflectivity of light incident from the GaN layer side was calculated by simulation. The p-contact layer 211 was made of ITO and had a thickness of 100 nm, and the p-side low-reflection layer 212 was made of 4.5 alternating pairs of Ti and ITO, with the first and last layers being Ti. The barrier layer was made of Pt and had a thickness of 100 nm. The n-side bonding layer 203 was made of Au and had an infinite thickness. The reflectivity of the p-electrodes 21A-21C was integrated over wavelength (400-700 nm) and incident angle (0-16°), and the thickness of each layer in the p-side low-reflection layer 212 was set to minimize the integrated value. However, the thickness of the last layer (Ti) of the p-side low-reflection layer 212 was fixed at 100 nm.

[0088] FIG. 13 shows a table (FIG. 13(a)) of the thickness of each layer of p-electrodes 21A to 21C, and a graph (FIG. 13(b)) showing the wavelength dependence of the reflectance of p-electrodes 21A to 21C. The wavelength dependence of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 13(b), in the visible light wavelength range of 400 to 700 nm, the reflectance was 1% or less in the range excluding the immediate vicinity of 400 nm, and was 1% or less over almost the entire range. Furthermore, in the incident angle range of 0 to 16°, the reflectance was 1% or less over almost the entire visible light range.

[0089] Experimental Example 2-2 In Experimental Example 2-1, the barrier layer was omitted, and the p-side low reflective layer 212 was changed from 4.5 pairs to 3.5 pairs, and a simulation was performed in the same manner as in Experimental Example 2-1. However, the thickness of the last layer (Ti) of the p-side low reflective layer 212 was not fixed but was used as a parameter. The thickness of each layer of the p-side low reflective layer 212 was set in the same manner as in Experimental Example 2-1.

[0090] FIG. 14 shows a table (FIG. 14(a)) of the thickness of each layer of the p electrodes 21A to 21C, and a graph (FIG. 14(b)) showing the wavelength dependence of the reflectance of the p electrodes 21A to 21C. The wavelength dependence of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 14(b), the reflectance was 2% or less in the visible light region of wavelengths 400 to 700 nm. Furthermore, in the incident angle range of 0 to 16°, the reflectance was 2% or less in the visible light region except for the region immediately near 700 nm.

[0091] Experimental Example 2-3 In Experimental Example 2-1, the p-side low-reflective layer 212 was changed to a structure in which four pairs of Cr and ITO were alternately stacked, with Ti stacked last, and a simulation was performed in the same manner as in Experimental Example 2-1. As in Experimental Example 2-1, the thickness of the last layer (Ti) of the p-side low-reflective layer 212 was fixed at 100 nm. The thickness of each layer of the p-side low-reflective layer 212 was set in the same manner as in Experimental Example 2-1.

[0092] FIG. 15 shows a table (FIG. 15(a)) of the thickness of each layer of the p electrodes 21A to 21C, and a graph (FIG. 15(b)) showing the wavelength dependency of the reflectance of the p electrodes 21A to 21C. The wavelength dependency of the reflectance is at an incident angle of 0°. As shown in the graph of FIG. 15(b), the reflectance was 2% or less in the visible light region of wavelengths 400 to 700 nm. Furthermore, the reflectance was 2% or less in the visible light region in the incident angle range of 0 to 16°.

[0093] Experimental Example 2-4 In Experimental Example 2-3, the ninth layer (ITO), the tenth layer (Ti), and the eleventh layer (Pt) of the p electrodes 21A to 21C, counting from the p contact layer 211, were omitted. The thicknesses of the other layers were unchanged from the table in Fig. 15. In this case, the reflectance was calculated in the same manner as in Experimental Example 2-1.

[0094] FIG. 16 shows a table (FIG. 16(a)) of the thickness of each layer of the p electrodes 21A to 21C, and a graph (FIG. 16(b)) showing the wavelength dependence of the reflectance of the p electrodes 21A to 21C. The wavelength dependence of the reflectance is at an incident angle of 0°. As can be seen from the graph of FIG. 16(b), there was no change in the reflectance compared to FIG. 15(b). Of the p electrodes 21A to 21C, the eighth layer (Cr) counting from the p contact layer 211 is thick at 177.7 nm, and light does not pass through to the layers subsequent to this. Therefore, it is thought that omitting the ninth to eleventh layers would not have affected the reflectance.

[0095] The results of Experimental Examples 1 and 2 showed that providing the n-side low-reflection layer 202 and the p-side low-reflection layer 212 on the n-electrode 20 and the p-electrodes 21A to 21C can significantly reduce the reflectivity of the n-electrode 20 and the p-electrodes 21A to 21C. Furthermore, as a result of setting the thickness of each layer of the n-side low-reflection layer 202 and the p-side low-reflection layer 212, it was found that if the thickness of the light-transmitting metal films 202A and 212A is too thick to transmit light, reducing the thickness of those layers to a level that does not transmit light does not affect the reflectivity. Furthermore, as a result of setting the thickness of each layer of the n-side low-reflection layer 202 and the p-side low-reflection layer 212, it was found that if the intermediate light-transmitting metal films 202A and 212A are too thick to transmit light, omitting subsequent layers does not affect the reflectivity.

[0096] (Variation) The LED display element of embodiment 1 is a full-color display in which one pixel is a set of blue, green, and red subpixels, but it can also be applied to monochrome or two-color displays. Furthermore, although the LED display element of embodiment 1 has three subpixels that directly emit light of each color, full color may be achieved by any other method. For example, wavelength conversion using a phosphor or the like may be used. Specifically, blue and green may be emitted by micro LEDs, and red may be wavelength-converted using a phosphor. Alternatively, a micro LED that emits ultraviolet light may be used, and the ultraviolet light may be converted into blue, green, and red, respectively.

[0097] Furthermore, in the LED display element of embodiment 1, no grooves are provided between pixels or subpixels to facilitate miniaturization and reduce manufacturing costs, but grooves may be provided. However, the LED display element of embodiment 1 is suitable for a structure in which pixels or subpixels are not separated by grooves. In a structure in which no grooves are used to separate pixels or subpixels, the side surfaces of the active layer are not exposed, making it difficult to increase contrast by covering the side surfaces with an absorbing film. This results in a more pronounced decrease in contrast due to crosstalk and other factors. Even in such cases, contrast can be increased by reducing the reflectance of the n-electrode 20 and p-electrodes 21A to 21C, as in embodiment 1.

[0098] In the LED display element of embodiment 1, low-reflection layers (n-side low-reflection layer 202, p-side low-reflection layer 212) are provided on both the n-electrode 20 and the p-electrodes 21A to 21C, but a low-reflection layer may be provided on only one of them. However, to improve contrast, it is preferable to provide a low-reflection layer on both as in embodiment 1.

[0099] Furthermore, although the first embodiment uses a group III nitride semiconductor as the semiconductor material, the present invention can also be applied to cases where other semiconductor materials are used. [Explanation of symbols]

[0100] 10: Circuit board 11:n layer 12: 1st active layer 13: First middle class 14:Second active layer 15: Second middle class 16: 3rd active layer 17:Protective layer 18:p layer 20:n electrode 21A~21C:p electrode 201:n contact layer 202:N side low reflection layer 202A, 212A: Light-transmitting metal film 202B, 212B: Transparent conductive film 203:n side bonding layer 211:p contact layer 212:p side low reflection layer 213:p side bonding layer

Claims

1. A monolithic LED display element having an n-layer, an active layer, a p-layer, an n-electrode, and a p-electrode, wherein a region of the active layer facing the p-electrode serves as a light-emitting portion, and a plurality of the light-emitting portions are arranged, At least one of the p-electrode and the n-electrode has a low-reflection layer in which transparent conductive films made of transparent conductive oxides and light-transmitting metal films that can transmit light are alternately stacked, The low-reflection layer comprises a light-opaque metal film laminated on an opposite end of the n-layer.

2. 2. The LED display element according to claim 1, wherein the thickness of said light-transmitting metal film closest to said n-layer is thinner than the thickness of said transparent conductive film closest to said n-layer.

3. 3. The LED display element according to claim 1, wherein the p-electrode comprises a p-contact layer in contact with the p-layer, and the low-reflection layer laminated on the p-contact layer.

4. 4. The LED display element according to claim 3, wherein the total thickness of the low-reflection layer laminated on the p-contact layer is greater than that of the p-contact layer.

5. the p-contact layer is made of a transparent conductive oxide; 4. The LED display element according to claim 3, wherein the layer of said low-reflection layer closest to said p-contact layer is said light-transmitting metal film.

6. 3. The LED display element according to claim 1, wherein the n-electrode comprises an n-contact layer in contact with the n-layer, and the low-reflection layer laminated on the n-contact layer.

7. 7. The LED display element according to claim 6, wherein the total thickness of the low-reflection layer laminated on the n-contact layer is greater than that of the n-contact layer.

8. the n-contact layer is made of a metal, 7. The LED display element according to claim 6, wherein the layer of the low-reflectivity layer closest to the n-contact layer is the transparent conductive film.

9. 9. The LED display element according to claim 8, wherein the light-transmitting metal film constituting the low-reflection layer laminated on the n-contact layer has a thickness greater than that of the n-contact layer.

10. 3. The LED display element according to claim 1, wherein the transparent conductive film is ITO or IZO.

11. 3. The LED display element according to claim 1, wherein the light-transmitting metal film is at least one selected from the group consisting of Cr, Ir, Mo, Ni, Pt, Rh, Ta, Ti, V, W, and TiN.

Images