JP2006156590A - Light emitting diode - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve such a problem that, though light emission becomes most intense in the projection area of a metal film in the conventional GaN-based LED provided with an electrode combined with a light-transmissive electrode film and a metal film, the light is prevented from being picked up by the metal film that is located just above, and light emission efficiency is lowered as a result. <P>SOLUTION: The GaN-based LED is provided with a current diffusion layer made of a metallic material, and an electrode combined with a light-transmissive electrode layer. A material for the current diffusion layer and the light transmission electrode layer is selected to match for the conductive type of the nitride semiconductor layer, so that a contact resistance on the boundary between the current diffusion layer and a nitride semiconductor layer may be larger than that on the boundary between the light-transmissive electrode layer and the nitride semiconductor layer. Therefore, current supplied to the nitride semiconductor layer concentrates on a route crossing the boundary between the light-transmissive electrode layer and the nitride semiconductor layer, so that light emission can be prevented in the projection area of the current diffusion area. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a light emitting diode, and in particular, a technique for improving the light emission efficiency of a light emitting diode (hereinafter referred to as “GaN-based LED”) having a laminate composed of a plurality of nitride semiconductor layers including an n-type layer and a p-type layer. About.

  A GaN-based LED is a light-emitting element having a pn junction diode structure in which an n-type nitride semiconductor layer and a p-type nitride semiconductor layer are bonded as a basic structure, and the composition of the nitride semiconductor used in the light-emitting region is selected. By doing so, it is possible to generate light in the visible to ultraviolet region. In particular, GaN-based LEDs that generate light having a visible short wavelength (blue) to near-ultraviolet wavelengths are attracting attention as excitation light sources for white LEDs used as illumination light sources and light sources for full-color display devices.

The nitride semiconductor is a compound semiconductor represented by a general formula Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1). Examples are ternary GaN, AlN, InN, ternary AlGaN, InGaN, InAlN, quaternary AlInGaN, and the like. Here, a part of the group 3 element is substituted with B (boron), Tl (thallium) or the like, or a part of N (nitrogen) is P (phosphorus), As (arsenic), Sb (antimony), Those substituted with Bi (bismuth) or the like are also included in the nitride semiconductor.
Since nitride semiconductors are supplied with electrons from nitrogen vacancies contained as defects, they exhibit n-type conductivity even when they are undoped, but when used as n-type, they are usually Si (silicon), Ge (germanium) , Se (selenium), Te (tellurium), C (carbon) and the like are doped as n-type impurities.
A p-type nitride semiconductor is obtained by doping elements such as Mg (magnesium), Zn (zinc), Be (beryllium), Ca (calcium), Sr (strontium), Ba (barium) as p-type impurities. It is done. When a nitride semiconductor crystal is grown by the MOVPE (organometallic compound vapor phase epitaxy) method or the HVPE (hydride vapor phase epitaxy) method, if these p-type impurities are doped, hydrogen passivation occurs. Thereafter, it is preferable to activate the p-type impurity by performing an annealing process or an electron beam irradiation process in an inert gas atmosphere.

In general, a GaN-based LED is formed by sequentially growing an n-type layer and a p-type layer made of a nitride semiconductor on a substrate by using a vapor phase growth method such as a MOVPE method, an HVPE method, or an MBE (molecular beam epitaxy) method. After the stacked body is formed, an n-side electrode and a p-side electrode are formed on the n-type layer and the p-type layer, respectively. When a light-emitting layer having a band gap smaller than those of the n-type layer and the p-type layer is inserted between the n-type layer and the p-type layer, a double hetero structure is formed and the light emission efficiency is improved.
When forming the n-side electrode, if the substrate is insulative, the n-type layer is partially exposed by dry etching from the p-type layer side and formed on the exposed surface. When a conductive substrate is used, the n-side electrode can be formed on the back surface (the surface on which the nitride semiconductor layer is not formed) of the substrate.
The p-side electrode is formed so as to cover the surface of the p-type layer that is the upper surface of the multilayer body. A light-transmitting p-side electrode has been put into practical use, and a GaN-based LED using such a p-side electrode can take out light emission through the p-side electrode, so that the back surface of the substrate is bonded to the mount surface. Therefore, it can be easily mounted.

Japanese Unexamined Patent Publication No. 2000-216431 (Patent Document 1) discloses a GaN-based LED using a composite electrode composed of a metal film formed in a lattice pattern and a transparent conductive layer as a light-transmitting p-side electrode. It is disclosed. A sectional view of this GaN-based LED is shown in FIG.
In FIG. 9, 1 is a sapphire substrate, 2 is a buffer layer, 3 is an n-type GaN contact layer made of Si-doped GaN, 4 is an InGaN light-emitting layer having an MQW structure in which undoped GaN layers and undoped InGaN layers are alternately stacked, 5 is a p-type AlGaN cladding layer made of Mg-doped AlGaN, 6 is a p-type GaN contact layer made of Mg-doped GaN, P1 is an n-side electrode, and P2 is a p-side electrode.
The p-side electrode P2 includes a metal film P2A formed in a lattice pattern and a transparent conductive layer P2B formed so as to cover the metal film P2A. The metal film P2A is made of a metal having a good ohmic property with the p-type contact layer 6, and specifically made of Pd (palladium), Pt (platinum), Ni (nickel), or the like. The transparent conductive layer P2B is made of a conductive metal oxide such as SnO ₂ (tin oxide), ITO (indium tin oxide), MgO (magnesium oxide), or ZnO (zinc oxide).
When the current is supplied from the n-side electrode P1 and the p-side electrode P2, the InGaN light emitting layer 4 emits light. This light emission is extracted outside the element through an opening of the metal film P2A (a region where the metal film P2A is not formed and the transparent conductive layer P2B is formed in contact with the surface of the p-type contact layer 6). Since the highly conductive metal film P2A is included, the current flowing through the p-side electrode P2 is sufficiently diffused in the lateral direction (the direction perpendicular to the thickness direction of the nitride semiconductor laminate), and the in-plane of the light emitting layer 4 Uniformity of light emission in is improved.

  By the way, according to Patent Document 1, a metal film made of Pd, Pt, Ni or the like forms a better ohmic contact with a p-type nitride semiconductor than a conductive metal oxide. Therefore, in the GaN-based LED of FIG. 9, the current flowing from the p-side electrode P2 to the p-type contact layer 6 is concentrated on a path crossing the interface between the metal film P2A and the p-type contact layer 6 having a low contact resistance. Therefore, the light emission in the light emitting layer 4 is the strongest in the projection region of the metal film P2A. However, since light emission in the region is easily reflected by the metal film P2A located immediately above the region, taking out to the outside is hindered, thereby causing a problem that the light emission efficiency of the LED is lowered.

JP 2000-216431 A JP 2003-60236 A JP 2001-210867 A JP 2004-179365 A JP 2004-266258 A JP 2004-6991 A

  The present invention solves the above-mentioned problems of the prior art in a GaN-based LED having an electrode in which a light-transmissive electrode film and a metal film are combined, and provides a GaN-based LED with excellent luminous efficiency. For the purpose.

In order to achieve the above object, the GaN-based LED according to the present invention has the following characteristics.
(1) A substrate, a laminate formed on the substrate and including a plurality of nitride semiconductor layers including an n-type layer and a p-type layer, and a metal partially formed on the upper surface of the laminate A transparent electrode layer formed so as to be electrically connected to the current diffusion layer on the upper surface of the stacked body exposed in the region where the current diffusion layer is not formed. And a contact resistance at an interface between the laminate and the current diffusion layer is larger than a contact resistance at an interface between the laminate and the translucent electrode layer.
(2) The uppermost layer of the laminate is made of an n-type nitride semiconductor, the current diffusion layer is made of Ag, Rh, Pd, or Pt, and the translucent electrode layer is made of a conductive metal oxide. The light emitting diode as described in said (1).
(3) The light-emitting diode according to (2), which has a tunnel junction structure.
(4) In the above (2), the uppermost layer is an n-type nitride semiconductor layer having a thickness of less than 1 μm or an n-type nitride semiconductor layer having an electron concentration of less than 1 × 10 ¹⁸ cm ^−3. The light emitting diode as described.
(5) The uppermost layer of the laminate is made of a p-type nitride semiconductor, the current diffusion layer is made of Al, and the translucent electrode layer is made of a conductive metal oxide. Light emitting diode.
(6) The uppermost layer of the laminate is made of a p-type nitride semiconductor, the current diffusion layer is made of Al, and the translucent electrode layer is made of a metal having a work function larger than that of Al. ).
(7) The light-emitting diode according to (6), wherein the translucent electrode layer is made of Ni, Pd, Rh, Au, Ir, or Pt.
(8) The light-emitting diode according to any one of (2) to (5), wherein the translucent electrode layer made of the conductive metal oxide has a thickness of 200 nm or less.
(9) The translucent electrode layer is formed so as to cover the current diffusion layer, and thereby the translucent electrode is formed on the upper surface of the laminate exposed in a region where the current diffusion layer is not formed. The light emitting diode according to any one of (1) to (8), wherein a layer is formed.
(10) Further, a bonding pad electrode formed on the translucent electrode layer, and at least a part of the bonding pad electrode is formed on the upper surface of the laminate so as to fall within the projected region of the pad electrode. A current blocking layer, and the current blocking layer is made of the same material as the current diffusion layer, has substantially the same film thickness as the current diffusion layer, and the upper surface of the stacked body, The light emitting diode according to any one of (1) to (9), wherein 50% or more of the projected area of the pad electrode is covered with the current blocking layer.

The GaN-based LED according to the present invention has a current diffusion formed on the upper surface of a multilayer body (hereinafter, also simply referred to as “laminate body”) composed of a plurality of nitride semiconductor layers including an n-type layer and a p-type layer. A layer and a translucent electrode layer. This current spreading layer is electrically connected to the translucent electrode layer, and in order to assist lateral current spreading in the translucent electrode layer, current is supplied to the light emitting region with good uniformity. In addition, the uniformity of light emission in the element plane is improved.
In addition, in the GaN-based LED according to the present invention, the maximum resistance of the laminate is such that the contact resistance at the interface between the current diffusion layer and the laminate is higher than the contact resistance at the interface between the translucent electrode layer and the laminate. The materials for the current diffusion layer and the translucent electrode layer are selected in accordance with the conductivity type of the upper nitride semiconductor layer. For this reason, most of the current supplied from the translucent electrode layer to the laminated body flows through a current path crossing the interface between the translucent electrode layer and the laminated body. As a result, in the light emitting layer, light emission in the projected region of the current diffusion layer is suppressed, while light emission in the projected region of the region where the translucent electrode layer and the laminate form an interface is promoted. The light emission in the latter region is efficiently taken out of the device through the translucent electrode layer because there is no shadowed current diffusion layer directly above, so that the light emission efficiency of the LED is improved.

(First embodiment)
1 to 3 are device configuration diagrams of a GaN-based LED 100 according to the first embodiment of the present invention, FIG. 1 is a plan view, FIG. 2 is a cross-sectional view taken along line AB in FIG. 1, and FIG. It is sectional drawing in the CD line of FIG.
1-3, 101 is a sapphire substrate, 102 is a low-temperature growth buffer layer made of a nitride semiconductor material, 103 is an n-type film having a film thickness of 3 μm made of Si-doped GaN (electron concentration of about 5 × 10 ¹⁸ cm ⁻³ ). GaN cladding layer, 104 is an InGaN light emitting layer having an MQW structure including an InGaN layer as a well layer, 105 is a p-type AlGaN cladding layer made of Mg-doped AlGaN, and has a thickness of 50 nm, and 106 is Mg-doped GaN (Mg concentration is about 1 × 10 ²⁰ 150 cm thick highly doped p-type GaN layer composed of cm ⁻³ ), 107 represents a 5 nm thick n-type GaN contact layer composed of Si doped GaN (electron concentration of about 1 × 10 ²⁰ cm ⁻³ ), and P 101 represents a film thickness. An n-side electrode in which an Al layer having a thickness of 100 nm is laminated on a Ti layer having a thickness of 30 nm, P102 has a thickness of 40 made of Ag (silver) m current spreading layer, P103 is a 100 nm thick transparent electrode layer made of ITO, and P104 and P105 are respectively a n-side and a 300 nm thick Au layer laminated on a 30 nm thick Ti layer. The p-side bonding pad electrode P106 is a 40 nm thick current blocking layer made of Ag.

In the GaN-based LED 100, a pn junction related to light emission is formed between the n-type GaN cladding layer 103 and the p-type AlGaN cladding layer 105, and the junction between the n-type GaN contact layer 107 and the highly doped p-type GaN layer 106 is Not involved in luminescence. The configuration of such a GaN-based LED is disclosed in Japanese Patent Application Laid-Open No. 2003-60236 (Patent Document 2) and the like, and an n-type layer doped with an n-type impurity at a high concentration and a p-type impurity at a high concentration. Utilizing the fact that a tunnel junction structure is formed at the interface with the doped p-type layer, the p-side contact layer can be made an n-type layer, thereby reducing the contact resistance of the p-side electrode. be able to.
As will be described later, in the GaN-based LED 100, the n-type GaN contact layer 107 is grown after the InGaN light-emitting layer 104. However, the InGaN light-emitting layer 104 deteriorates due to heat generated when the n-type GaN contact layer 107 is grown. In order to prevent the light emission efficiency from being lowered, it is desirable to form the n-type GaN contact layer 107 thin. Therefore, it is difficult to make the n-type GaN contact layer 107 have sufficient lateral current diffusibility.

When the n-side and p-side pad electrodes P104 and P105 of the GaN-based LED 100 are energized, the current is spread laterally by the current diffusion layer P102 and the translucent electrode layer P103 on the p-side.
Here, the resistivity of the conductive metal oxide such as ITO or ZnO has the best conductivity and is on the order of 10 ⁻⁴ Ωcm, whereas the resistivity of the metal is 10 ⁻⁶ to 10 ^{−. Since} the thickness is on the order of ⁵ Ωcm, the current diffusion capability of the former is 10 to 100 times that of the latter in the metal thin film and the conductive metal oxide thin film formed to the same thickness. Therefore, the current diffusibility of the translucent electrode layer P103 can be sufficiently supplemented by compositing the translucent electrode layer P103 with the current diffusion layer P102 having a thickness of 1/10 or more.
Since the contact resistance between a conductive metal oxide such as ITO or ZnO and a metal material is generally low, the current diffusion layer P102 and the translucent electrode layer P103 are electrically connected if formed so as to be in contact with each other. can do. Further, the contact resistance between the two layers can be reduced by performing heat treatment after forming the current diffusion layer P102 and the translucent electrode layer P103.

Conductive metal oxides such as ITO have high transmittance in the visible to near-violet wavelength range, but are not completely absorbed, especially in the wavelength range from the visible short wavelength region to the ultraviolet region due to interband transition. The influence of absorption also appears and the transmittance tends to be low. Further, at the interface between the translucent electrode layer P103 made of ITO having a refractive index of about 2 and the medium surrounding the outside of the device (passivation film made of SiO2, sealing resin, air, etc.), light is emitted due to the difference in refractive index. Is likely to be reflected again toward the inside of the element, and part of the light emission crosses the translucent electrode layer P103 many times. Therefore, the light emission efficiency of the LED can be improved by forming the light-transmitting electrode layer P103 thin to improve the light transmittance.
For example, Japanese Patent Laid-Open No. 2001-210867 (Patent Document 3) discloses a GaN-based LED using an ITO electrode having a film thickness of about 0.5 μm. The current diffusion layer P102 is formed of Ag having a conductivity several tens of times higher than that of the translucent electrode layer P103, so that the current diffusibility of the translucent electrode layer P103 is compensated. It can be as follows.
In the GaN-based LED according to the present invention, a preferable thickness when the translucent electrode layer is formed of a conductive metal oxide is 200 nm or less, and more preferably 100 nm or less.

  The effect of the current diffusion layer P102 in the GaN-based LED 100 becomes more prominent as the size of the LED chip increases. Up to now, the standard chip size of GaN-based LEDs has been 300 to 400 μm square, but especially in the application of lighting devices, it is better to mount a small number of large chips than to mount a large number of small chips. In order to be advantageous in manufacturing, GaN-based LEDs having a chip size of 1 mm square or more have been developed. In such a large-sized chip, since it is important to secure current diffusibility in the lateral direction, the configuration of the present invention that assists current diffusion in the lateral direction by the current diffusion layer is particularly useful.

  It is known that Ag, which is the material of the current diffusion layer P102, does not form a good ohmic contact with the n-type nitride semiconductor, and can be used as an ohmic electrode material for the n-type nitride semiconductor. The contact resistance with the n-type nitride semiconductor is larger than that of ITO or ZnO. Therefore, the current supplied to the n-type GaN contact layer 107 is concentrated on a path crossing the interface between the translucent electrode layer P103 and the n-type contact layer 107, and the current diffusion layer P102 and the n-type GaN contact layer 107 There is little current flowing across the interface. In the n-type GaN contact layer 107 having a small thickness, the highly doped p-type GaN layer 106 and the p-type AlGaN cladding layer 105 having a small thickness and low conductivity, current hardly diffuses in the lateral direction. The InGaN light emitting layer 104 emits light substantially only in the projected region in the region where the n-type GaN contact layer 107 and the translucent electrode layer P103 form an interface. Light generated in the region is efficiently extracted out of the element through the translucent electrode layer P103 because the current diffusion layer P102 does not exist immediately above the region.

  In addition to Ag, Rh (rhodium), Pt, Pd, and the like are preferable as the metal that does not form a good ohmic contact with the n-type nitride semiconductor and is suitable as a material for the current diffusion layer P102. Among them, Ag and Rh are particularly excellent in light reflectivity. Therefore, when the current diffusion layer P102 is formed of Ag and Rh, light absorption by the current diffusion layer P102 is reduced, which is preferable in improving the light emission efficiency of the LED. .

  The current diffusion layer P102 may be formed to a thickness that allows light transmission, thereby improving the light emission extraction property. Generally, when a metal film is formed to a thickness of about 20 nm or less, it shows light transmittance. The film thickness of the metal film in the case of light transmission is preferably 3 nm or more in order to ensure conductivity.

  The pattern of the current diffusion layer P102 (pattern when the chip is viewed from above) is not particularly limited as long as it can compensate the current diffusion in the lateral direction in the translucent electrode layer P103. 1 and 4 to 6 show an example of a pattern of the current diffusion layer P102. In these figures, the dotted lines represent the contour lines of the current diffusion layer P102 and the current blocking layer P106 described later. The reason why the layers are indicated by dotted lines is that these layers are formed under the translucent electrode layer P103.

In the example of FIG. 1, a plurality of linear current diffusion layers P102 are formed on the surface of the n-type GaN contact layer 107 so as to be separated from each other.
In the example of FIG. 4, a plurality of linear current diffusion layers P202 are formed in a radial pattern, and further integrated with the current element layer P206 at one end of the linear portion.
In the example of FIG. 5, the linear current diffusion layer P302 is formed in an annular shape.
In the example of FIG. 6, a plurality of linear current diffusion layers P <b> 402 are formed in a grid pattern. This pattern can also be viewed as a kind of lattice pattern.
The pattern of the current spreading layer may be a pattern obtained by modifying the patterns illustrated in these drawings or combining them. Although not shown, the current diffusion layer can be formed in a net-like, dendritic, comb-like pattern or the like spreading on the upper surface of the n-type GaN contact layer 107.

  In the InGaN light emitting layer 104, the projected region of the current diffusion layer P102 emits light weakly (or does not emit light), so that the area of the region covered by the current diffusion layer P102 on the upper surface of the n-type GaN contact layer 107 Is preferably as small as possible in a range where the current diffusibility does not significantly decrease. Since the resistivity of the metal material is low as described above, the current diffusion layer P102 can be formed into a pattern composed of a relatively narrow linear portion. For example, the line width of the linear portion included in each of the exemplified patterns is preferably 0.5 μm to 10 μm.

A current blocking layer P106 may be formed below the p-side bonding pad electrode P105. Light emission in the region of the InGaN light emitting layer 104 located below the pad electrode P105 is lost because most of it is blocked by the large pad electrode P105 and is not extracted outside the device. The current blocking layer P106 is a layer for suppressing the occurrence of such loss by blocking current supply from the translucent electrode layer P103 to the nitride semiconductor layer in the projection region of the pad electrode P105.
The current blocking layer P106 is most preferably formed so as to completely cover the projection region of the pad electrode P105 on the surface of the n-type GaN contact layer 107 as in the example of FIG. 1, but 50% of the projection region If formed so as to cover the above, an effect of reducing loss can be obtained. In this case, it is more effective to cover the portion near the center of the projection region with the current blocking layer P106.
From the viewpoint of manufacturing efficiency, the current blocking layer P106 is preferably formed simultaneously from the same material as the current diffusion layer P102. As shown in FIG. 4, the current spreading layer P202 and the current blocking layer P206 can be integrated on a pattern. If the current blocking layer P106 is formed of the same material as that of the current diffusion layer P102, the current element layer P106 becomes conductive. Therefore, the transparent electrode layer P103 between the p-side pad electrode P105 and the current blocking layer P106 is omitted. The p-side pad electrode P105 can also be formed directly on the surface of the current blocking layer P106.
The current blocking layer is not prevented from being formed of a material different from that of the current spreading layer. For example, various insulating materials can be used.

The GaN-based LED 100 can be manufactured by the following method.
First, the sapphire substrate 101 is set in a MOVPE apparatus, and the surface is thermally etched by heating in a hydrogen stream. Next, after supplying the organometallic raw material and ammonia to form the low-temperature growth buffer layer 102 at 300 to 700 ° C., the substrate temperature is raised to about 1000 ° C., and the n-type GaN cladding layer 103, the InGaN light emitting layer 104, A p-type AlGaN cladding layer 105, a highly doped p-type GaN layer 106, and an n-type GaN contact layer 107 are sequentially grown to a predetermined thickness to form a nitride semiconductor multilayer body. The substrate temperature for growing the InGaN light emitting layer 104 is about 800 ° C.

  Next, etching is performed to a depth reaching the n-type GaN cladding layer 103 from the surface of the n-type GaN contact layer 107 by reactive ion etching (RIE), so that the InGaN light-emitting layer 104, the p-type AlGaN cladding layer 105, Part of the doped p-type GaN layer 106 and the n-type GaN contact layer 107 is removed to expose the n-type GaN cladding layer 103.

Next, an n-side electrode P101 is formed by sequentially laminating a Ti film and an Al film on the exposed surface of the n-type GaN clad layer 103 by using a method such as vacuum deposition or sputtering.
Note that the n-side electrode can be formed using the same conductive metal oxide as the light-transmitting electrode layer. In that case, it is preferable to change the order of the steps and simultaneously form the n-side electrode and the translucent electrode layer.

  Next, a current diffusion layer P102 and a current blocking layer P106 made of Ag are formed at predetermined positions on the surface of the n-type GaN contact layer 107 by using a method such as vacuum deposition or sputtering. At this time, Ag alone may be used, but an Ag alloy having improved chemical stability as compared with Ag alone may also be used. Even when the current spreading layer P102 is formed of Rh, Pt, Pd, or the like, the material is not limited to a simple substance of these metals, and an alloy can be used.

Next, a transparent electrode layer P103 made of ITO is formed on the surface of the n-type GaN contact layer 107 so as to cover the formed current diffusion layer P102. The translucent electrode layer P103 is formed by CVD (thermal CVD, plasma CVD, MOCVD, photo CVD), spray method, sputtering method, vacuum deposition method, cluster beam deposition method, pulse laser deposition method, ion plating method. Sol-gel method, laser ablation method, and other known ITO thin film forming methods can be used as appropriate.
The translucent electrode layer may be formed of a conductive metal oxide other than ITO, such as ZnO or SnO ₂ , but in this case, the above method can be used.

  Next, by sequentially laminating a Ti film and an Au film at predetermined positions on the n-side electrode P101 and the translucent electrode layer P103 by a method such as vacuum deposition and sputtering, bonding on the n-side and the p-side is performed. Pad electrodes P104 and P105 for forming are formed. The p-side pad electrode P105 is formed on the previously formed current blocking layer P106. These two pad electrodes can be formed simultaneously.

(Second Embodiment)
7 and 8 are device configuration diagrams of a GaN-based LED 500 according to the second embodiment of the present invention. FIG. 7 is a plan view, and FIG. 8 is a cross-sectional view taken along line EF in FIG.
7 and 8, 501 is a sapphire substrate, 502 is a low-temperature growth buffer layer made of a nitride semiconductor material, 503 is an n-type film having a thickness of 3 μm made of Si-doped GaN (electron concentration of about 5 × 10 ¹⁸ cm ⁻³ ). GaN cladding layer, 504 is an InGaN light-emitting layer having an MQW structure including an InGaN layer as a well layer, 505 is a p-type AlGaN cladding layer made of Mg-doped AlGaN, and has a thickness of 50 nm, and 506 is Mg-doped GaN (Mg concentration is about 1 × 10 ²⁰ cm- ³ ) p-type GaN contact layer having a thickness of 150 nm, P501 is an n-side electrode in which a 300 nm-thickness Au layer is laminated on a 30 nm-thickness Ti layer, and P502 is a thickness of Al (aluminum). 40 nm current spreading layer, P503 is a 100 nm thick transparent electrode layer made of ITO, P505 is a film thickness p-side bonding pad electrode formed by laminating an Au layer having a thickness of 300nm on the Ti layer of 0 nm, P506 is a current blocking layer having a thickness of 40nm made of Al.

A characteristic configuration of the GaN-based LED 500 is a current diffusion layer P502 made of Al formed on the surface of the p-type GaN contact layer 506 and a translucent electrode layer P503 made of ITO formed so as to cover it. is there.
Since Al has a resistivity on the order of 10 ⁻⁶ Ωcm, while ITO has a resistivity on the order of 10 ⁻⁴ Ωcm, the current diffusion layer P502 sufficiently supplements the current diffusion of the translucent electrode layer P503. be able to.
In addition, Al is known not to form good ohmic contact with p-type nitride semiconductors, and conductive metal oxides such as ITO and ZnO that are also used as ohmic electrode materials for p-type nitride semiconductors. The contact resistance with a p-type nitride semiconductor is large compared with a thing. Accordingly, the current supplied to the p-type GaN contact layer 506 is concentrated on a path crossing the interface between the translucent electrode layer P503 and the p-type GaN contact layer 506, and the current diffusion layer P502, the p-type GaN contact layer 506, The current flowing across the interface is small. Therefore, light emission occurs in the InGaN light emitting layer 504 substantially only in the projected region in the region where the p-type GaN contact layer 106 and the translucent electrode layer P503 form an interface. The light generated in the region is efficiently extracted out of the element through the translucent electrode layer P503 because the current diffusion layer P502 does not exist immediately above the region.

Al is optimal as a material for the current diffusion layer P502 formed on the p-type GaN contact layer 506 because it is excellent in light reflectivity, has high electrical conductivity, and is chemically stable.
Add elements such as Si (silicon), Ti (titanium), Cu (copper), Nd (neodymium) to Al to suppress thermal deformation in the electrode heat treatment process and the passivation film formation process on the element surface Thus, an Al alloy with improved heat resistance can also be used. The Al alloy film is formed by alloy sputtering, multi-source deposition, etc., or a multilayer film in which a layer made of Al alone and a layer made of a metal element to be added are laminated, and heat treatment is performed at a temperature below the melting point of Al. It can also be obtained by doing.

  In the GaN-based LED 500, as shown in FIG. 7, a current diffusion layer P502 is formed in a lattice pattern, and a continuous current element layer P506 is provided in a part thereof. This is an example of the pattern of the current diffusion layer P502. Also in the second embodiment, the current diffusion layer can be formed in various patterns exemplified in the description of the first embodiment.

As a material for the translucent electrode layer, in addition to ITO, a conductive metal oxide such as ZnO or SnO ₂ can also be used.
In order to reduce the contact resistance between the conductive metal oxide and the p-type nitride semiconductor, the following method may be used.
(A) The conductive metal oxide is doped with the same p-type impurity as the p-type impurity doped in the p-type nitride semiconductor. For details, JP-A-2004-179365 (Patent Document 4) and the like can be referred to.
(B) On the surface of the p-type nitride semiconductor layer, as a first layer, a conductive metal oxide layer having a thickness of 100 Å or more by a method other than sputtering, for example, vacuum deposition, laser ablation, sol-gel, or the like After that, the film is thickened. For details, the above-mentioned Patent Document 3 can be referred to.
(C) When the conductive metal oxide is ZnO, Ga (gallium) or B (boron) is doped. For details, JP-A-2004-266258 (Patent Document 5) and the like can be referred to.

As the light-transmitting electrode layer, a metal film formed to a thickness that generates light transmittance can also be used.
Since a p-type nitride semiconductor has a large work function (referred to as about 7 eV in p-type GaN), a Schottky barrier is formed at the junction when any metal is used for an electrode. Therefore, the higher the work function of the metal used as the electrode material, the lower the barrier height and the lower the contact resistance.
Therefore, if the translucent electrode layer P503 is formed of a metal having a work function larger than that of Al, the contact resistance between the current diffusion layer P502 and the p-type GaN contact layer 506 is reduced, and the translucent electrode layer P503 and the p-type GaN contact are formed. The contact resistance with the layer 506 can be larger. As such a metal, Ni, Pd, Rh, Au (gold), Ir (iridium), Pt, and the like, and alloys thereof can be suitably used. These metals are known to be used for ohmic electrodes for p-type nitride semiconductors, and among these, Ni—Au-based thin films are widely used as light transmissive electrodes with low contact resistance. .

  Although the first embodiment and the second embodiment of the present invention have been described above, the present invention is not limited to these embodiments, and various modifications can be made without departing from the spirit of the present invention. Is possible.

As an example of the first embodiment, a GaN-based LED including a tunnel junction structure is taken as an example, and an embodiment in which a current diffusion layer and a translucent electrode layer are formed on an n-type nitride semiconductor layer has been described. It is not limited to GaN-based LEDs including a tunnel junction structure.
In JP 2004-6991 A (Patent Document 6), after forming a stacked body in which an n-type layer made of a nitride semiconductor, a light emitting layer, and a p-type layer are sequentially stacked on a sapphire substrate, A separately prepared conductive support substrate is bonded to the surface via an ohmic electrode, and then the sapphire substrate is removed by polishing or laser lift-off to expose the n-type layer. On the exposed n-type layer surface A method of manufacturing a GaN-based LED that forms an n-side electrode is described. In the GaN-based LED manufactured by this method, since the uppermost layer of the laminate formed on the support substrate is an n-type nitride semiconductor layer, a current similar to that in the first embodiment is formed on the surface thereof. A diffusion layer and a transparent electrode layer can be formed.

By the way, in the method of Patent Document 6, since the n-type layer is grown before the light-emitting layer is grown, there is no problem that the light-emitting layer deteriorates during the growth of the n-type layer, and the n-type layer is formed to an arbitrary thickness. Can grow. And if an n-type layer is doped with an n-type impurity so that the electron concentration is 1 × 10 ¹⁸ cm ⁻³ or more and is formed to have a film thickness of 1 μm or more, the sheet resistance becomes considerably low, and the lateral direction The current diffusibility is improved. For this reason, the n-type layer is made thick and the impurity is doped to a concentration at which the electron concentration becomes sufficiently high. On the other hand, if the n-type layer is formed thick, the production efficiency decreases. In addition, when the impurity concentration is increased, the crystallinity of the n-type layer, the light-emitting layer and the p-type layer grown on the n-type layer is lowered, and problems such as a reduction in light emission efficiency and an increase in leakage current appear.
On the other hand, if an electrode in which a translucent electrode layer and a current diffusion layer made of metal are combined is used, lateral current diffusion is achieved by the electrode, so that the n-type layer can be made thin. In addition, the doping amount of the n-type impurity can be reduced. And when such an electrode is used, the configuration of the present invention in which the contact resistance between the translucent electrode layer and the n-type layer is made higher than the contact resistance between the current diffusion layer and the n-type layer is preferably used. obtain.
That is, according to the present invention, in the GaN-based LED in which the stacked body is bonded so that the crystal growth substrate is removed and the n-type layer is the uppermost layer on the support substrate, the thickness of the n-type layer is increased. When it is less than 1 μm, particularly less than 500 nm, it can be suitably used, and when the electron concentration of the n-type layer is less than 1 × 10 ¹⁸ cm ⁻³ , particularly less than 1 × 10 ¹⁷ cm ^−3. Can be preferably used.

The substrate included in the GaN-based LED of the present invention does not need to be a substrate used for growing a nitride semiconductor layer, and the substrate used for the growth was removed by polishing, laser lift-off, or the like during the process. The bonded support substrate may be used later.
For the growth of the nitride semiconductor layer, a substrate made of SiC, GaN, AlN, Si, spinel, ZnO, GaAs, NGO, LGO, or the like can be used in addition to the sapphire substrate.
When a support substrate is bonded after forming a laminate made of a nitride semiconductor, the support substrate is appropriately selected according to the purpose from the viewpoint of conductivity, thermal conductivity, thermal expansion coefficient, cleavage, and the like. You can choose. The support substrate may be a thick metal layer grown on the surface of the laminate by a method such as plating or vapor phase growth.

  This is a case where a conductive metal oxide is used as the translucent electrode layer, but the film formation of ITO, ZnO or the like can be performed at 150 ° C. to 300 ° C. by a physical method such as a vacuum evaporation method or a sputtering method. Even chemical methods such as spraying can be performed at a substrate temperature of 350 ° C. to 500 ° C., which is considerably lower than that of nitride semiconductor crystal growth. Therefore, a relatively thick film can be formed without causing large thermal damage to the InGaN light emitting layer. Therefore, the translucent electrode layer is formed thick in advance, and then a recess having a depth of 1/4 or more of the emission wavelength in the translucent electrode layer is formed on the surface by a method such as etching. The surface of the translucent electrode layer can be made light scattering. If the surface of the translucent electrode layer is made light-scattering, the light generated in the InGaN light-emitting layer is scattered and diffracted on the surface and easily extracted to the outside of the device, which can improve the light extraction efficiency of the LED. it can.

  The stacked body formed of the nitride semiconductor formed on the substrate only needs to include a pn junction diode structure capable of emitting light. With reference to a conventionally known technique, the first embodiment and the second embodiment described above. The structure exemplified as the embodiment can be added or omitted, and various modifications can be made to the thickness, composition, band gap, impurity type, carrier concentration, etc. of each nitride semiconductor layer.

1 is a plan view showing a GaN-based LED according to a first embodiment of the present invention. 1 shows a GaN-based LED according to a first embodiment of the present invention, and is a cross-sectional view taken along line AB in FIG. FIG. 2 shows a GaN-based LED according to the first embodiment of the present invention, and is a cross-sectional view taken along line CD in FIG. 1. 1 is a plan view showing a GaN-based LED according to a first embodiment of the present invention. 1 is a plan view showing a GaN-based LED according to a first embodiment of the present invention. 1 is a plan view showing a GaN-based LED according to a first embodiment of the present invention. It is a top view which shows GaN-type LED which concerns on the 2nd Embodiment of this invention. FIG. 8 shows a GaN-based LED according to the second embodiment of the present invention, and is a cross-sectional view taken along line E-F in FIG. 7. It is sectional drawing which shows the conventional GaN-type LED.

Explanation of symbols

DESCRIPTION OF SYMBOLS 101 Sapphire substrate 102 Low-temperature growth buffer layer 103 N-type GaN clad layer 104 InGaN light emitting layer 105 P-type AlGaN clad layer 106 Highly doped p-type GaN layer 107 N-type GaN contact layer P101 N side electrode P102 Current diffusion layer P103 Translucent electrode Layer P104 n-side bonding pad electrode P105 p-side bonding pad electrode P106 current blocking layer

Claims (10)

A substrate,
A laminate formed of a plurality of nitride semiconductor layers including an n-type layer and a p-type layer, formed on the substrate;
A current diffusion layer made of metal partially formed on the upper surface of the laminate;
A translucent electrode layer formed on the top surface of the laminate exposed in a region where the current diffusion layer is not formed and electrically connected to the current diffusion layer;
The light emitting diode characterized by the contact resistance of the interface of the said laminated body and the said electric current diffusion layer being larger than the contact resistance of the interface of the said laminated body and the said translucent electrode layer.   2. The uppermost layer of the laminate is made of an n-type nitride semiconductor, the current diffusion layer is made of Ag, Rh, Pd, or Pt, and the light-transmitting electrode layer is made of a conductive metal oxide. A light emitting diode according to 1.   The light emitting diode according to claim 2, which has a tunnel junction structure. The light emitting diode according to claim 2, wherein the uppermost layer is an n-type nitride semiconductor layer having a thickness of less than 1 μm or an n-type nitride semiconductor layer having an electron concentration of less than 1 × 10 ¹⁸ cm ^−3. .   2. The light emitting diode according to claim 1, wherein an uppermost layer of the stacked body is made of a p-type nitride semiconductor, the current diffusion layer is made of Al, and the translucent electrode layer is made of a conductive metal oxide.   The uppermost layer of the laminate is made of a p-type nitride semiconductor, the current diffusion layer is made of Al, and the translucent electrode layer is made of a metal having a work function larger than that of Al. Light emitting diode.   The light-emitting diode according to claim 6, wherein the translucent electrode layer is made of Ni, Pd, Rh, Au, Ir, or Pt.   The light emitting diode according to any one of claims 2 to 5, wherein a thickness of the translucent electrode layer made of the conductive metal oxide is 200 nm or less.   The translucent electrode layer is formed so as to cover the current diffusion layer, whereby the translucent electrode layer is formed on the upper surface of the stacked body exposed in a region where the current diffusion layer is not formed. The light emitting diode according to any one of claims 1 to 8, wherein the light emitting diode is configured as described above.   Further, a bonding pad electrode formed on the translucent electrode layer, and a current blocking layer formed on the upper surface of the laminate so that at least a part of the pad electrode enters the projected region of the pad electrode. And the current blocking layer is made of the same material as the current diffusion layer, has substantially the same film thickness as the current diffusion layer, and the pad electrode on the upper surface of the stacked body. The light emitting diode according to claim 1, wherein 50% or more of the projected area is covered with the current blocking layer.

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