JPWO2006082687A1 - GaN-based light emitting diode and light emitting device - Google Patents

Abstract

Provided are a GaN-based light emitting diode with improved luminous efficiency and a light emitting device using the same. On the n-type GaN-based semiconductor layer 2, a light-emitting layer 3 made of a GaN-based semiconductor and a p-type GaN-based semiconductor layer 4 are formed in this order. The ohmic electrode P2 is formed in a pattern having a window portion, and a metal reflective layer P5 that reflects light reaching from the light emitting layer through the window portion is a p-type ohmic contact with the p-type GaN-based semiconductor layer 4. A GaN-based light emitting diode, which is formed so as to sandwich the electrode P2, and a protective film P4 made of an insulator is interposed between the reflective layer P5 and the p-type ohmic electrode P2. In a preferred embodiment of the GaN-based light emitting diode, the outermost surface layer of the reflective layer P5 is used as a bonding layer, or a bonding layer is further formed on the reflective layer P5. Are bonded with a conductive bonding material.

Description

  The present invention relates to a GaN-based light emitting diode with improved luminous efficiency and a light emitting device using the same.

A GaN-based light emitting diode (hereinafter also referred to as a “GaN-based LED”) is a semiconductor light emitting device having a pn junction diode structure in which p-type and n-type GaN-based semiconductors are joined with a light-emitting layer made of a GaN-based semiconductor interposed therebetween. By selecting the composition of the GaN-based semiconductor that constitutes the light-emitting layer of the device, light ranging from red to ultraviolet can be emitted.
A GaN-based semiconductor is a compound semiconductor made of a group III nitride determined by the chemical formula Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1). For example, those having an arbitrary composition such as GaN, InGaN, AlGaN, AlInGaN, AlN, and InN are exemplified. Further, in the above chemical formula, a part of the group III element is substituted with boron (B), thallium (Tl), or the like, and a part of N (nitrogen) is phosphorus (P), arsenic (As), antimony ( Those substituted with Sb) or bismuth (Bi) are also included in the GaN-based semiconductor.
A GaN-based LED is formed on a crystal substrate made of sapphire or the like by using a vapor phase growth method such as an organic metal compound vapor phase growth (MOVPE) method, a hydride vapor phase growth (HVPE) method, or a molecular beam epitaxy (MBE) method. An n-type GaN-based semiconductor layer, a light-emitting layer, and a p-type GaN-based semiconductor layer are sequentially stacked, and then an electrode is formed on each of the n-type GaN-based semiconductor layer and the p-type GaN-based semiconductor layer. .
In this specification, when vapor-phase growth of the GaN-based semiconductor layer, it is assumed that the crystal substrate is on the lower side and the GaN-based semiconductor layer is stacked thereon, and this distinction between the upper and lower sides is described for the element structure. This also applies to A direction orthogonal to the vertical direction (also the thickness direction of the substrate and the semiconductor layer) is also referred to as a horizontal direction.
The p-type GaN-based semiconductor layer is also simply referred to as a p-type layer, and the n-type GaN-based semiconductor layer is also simply referred to as an n-type layer.
An ohmic electrode for a p-type GaN-based semiconductor is also called a p-type ohmic electrode, and an ohmic electrode for an n-type GaN-based semiconductor is also called an n-type ohmic electrode.
A p-type ohmic electrode formed by laminating an n-type layer, a light-emitting layer, and a p-type layer in order on a transparent substrate, and transmitting light generated in the light-emitting layer on the p-type layer, and a reflection A GaN-based LED provided with a layer is known (Japanese Patent Laid-Open No. 2004-119996).

In the GaN-based LED described in Japanese Patent Application Laid-Open No. 2004-119996, a p-type ohmic electrode made of Pd (palladium) is partially formed on the surface of the p-type layer, and a reflection made of Al (aluminum) is formed thereon. The layer directly covers. Since Al is one of the metals with the highest reflectance in the green to near-ultraviolet region, which is a typical emission wavelength of GaN-based LEDs, adoption of such a structure improves the light extraction efficiency of GaN-based LEDs. Is expected to do. However, on the other hand, in this structure, Al diffuses in the inside of the p-type ohmic electrode or the interface between the p-type ohmic electrode and the p-type layer by the heat treatment usually performed at the time of forming the ohmic electrode. There is a concern that the contact resistance with the p-type layer increases. When this contact resistance increases, the operating voltage of the LED increases and the conversion efficiency decreases, so that the overall light emission efficiency decreases even when the light extraction efficiency increases.
An object of the present invention is to solve the above-described problems of the prior art and provide a GaN-based LED with improved luminous efficiency.
The present invention has the following features.
(1) An n-type GaN-based semiconductor layer, a light-emitting layer made of a GaN-based semiconductor formed on the n-type GaN-based semiconductor layer, and a p-type GaN-based semiconductor layer formed on the light-emitting layer A p-type ohmic electrode formed in a pattern having a window on the surface of the p-type GaN-based semiconductor layer, and the p-type ohmic electrode sandwiched between the p-type GaN-based semiconductor layer, GaN-based light emission having a metal reflective layer that reflects light that reaches the light-emitting layer through the window, and a protective film made of an insulator interposed between the reflective layer and the p-type ohmic electrode diode.
(2) The GaN-based light emitting diode according to (1), wherein the outermost surface layer of the reflective layer is a bonding layer, or a metal bonding layer is further formed on the reflective layer.
(3) The GaN-based light emitting diode according to (2), wherein the bonding layer is a layer made of Au, Au alloy, Sn, or Sn alloy.
(4) The GaN-based light emitting diode according to (3), wherein the reflective layer is formed of Ag, an Ag alloy, Al, an Al alloy, or a platinum group element at least at a portion that reflects light reaching from the light emitting layer. .
(5) In the reflective layer, at least a portion that reflects light reaching the light emitting layer is formed of Ag, an Ag alloy, Al, or an Al alloy, and a barrier is provided between the portion and the bonding layer. The GaN-based light emitting diode according to (3), wherein a layer is interposed.
(6) The GaN-based light emitting diode according to (2), wherein the protective film has a thickness of 0.1 μm to 1 μm.
(7) The GaN-based light-emitting diode according to (1), wherein the protective film has a lower refractive index than the p-type GaN-based semiconductor layer.
(8) The reflective layer is formed of Ag, Ag alloy, Al, or Al alloy at least at a portion that reflects light reaching from the light emitting layer, and the p-type ohmic electrode includes p-type ohmic The GaN-based light emitting diode according to (1), which is an electrode.
(9) The GaN-based light emitting diode according to (1), wherein the p-type ohmic electrode includes a portion made of Ni, Ti, or Cr at a portion in contact with the protective film.
(10) The GaN-based light emitting diode according to (1), wherein the p-type ohmic electrode has a thickness of 60 nm to 1 μm.
(11) The GaN-based light emitting diode according to (10), wherein the thickness of the p-type ohmic electrode is 100 nm or more.
(12) The area ratio of the window part in the pattern having the window part is 60 to 80%, and the light emitting layer includes a case of In _x Ga _1-x N (x = 0) having an emission wavelength of 420 nm or less. GaN-based light emitting diode according to (1).
(13) The GaN-based light emitting diode according to (2) is fixed on the mounting surface of the mounting base material with the bonding layer facing the mounting surface. The bonding layer and the mounting base material And a light-emitting device, which are bonded by a conductive bonding material.
(14) The light emitting device according to (13), wherein the bonding by the conductive bonding material is formed by brazing.
(15) The light emitting device according to (13), wherein the bonding by the conductive bonding material is formed by eutectic bonding.

1A and 1B are diagrams showing a GaN-based light emitting diode according to an embodiment of the present invention. FIG. 1A is a top view, and FIG. 1B is a cross-sectional view taken along line XY in FIG. is there.
FIG. 2 is a diagram illustrating a manufacturing process of the GaN-based light emitting diode shown in FIG.
FIG. 3 is a diagram for explaining a manufacturing process of the GaN-based light emitting diode shown in FIG.
FIG. 4 is a diagram illustrating a p-type ohmic electrode pattern (pattern having a window).
FIG. 5 is a cross-sectional view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 6 is a cross-sectional view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 7 is a top view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 8 is a cross-sectional view showing a mounting example of the GaN-based light emitting diode shown in FIG.
FIG. 9 is a cross-sectional view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 10 is a cross-sectional view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 11 is a diagram illustrating a manufacturing process of the GaN-based light emitting diode according to the embodiment of the present invention.
FIG. 12 is a cross-sectional view showing a GaN-based light emitting diode according to an embodiment of the present invention.
FIG. 13 is a sectional view showing a GaN-based light emitting diode according to the prior art.

Hereinafter, the present invention will be specifically described with reference to the drawings.
1A and 1B are schematic views showing the structure of a GaN-based LED according to an embodiment of the present invention. FIG. 1A is a top view, and FIG. 1B is a cross-sectional view taken along line XY in FIG. It is.
In FIG. 1, 1 is a crystal substrate, 2 is an n-type layer, 3 is a light emitting layer, 4 is a p-type layer, P1 is an n-type ohmic electrode, P2 is a p-type ohmic electrode, P3 is a p-side bonding electrode, and P4 is insulated. A protective film P5, which is a body, is a reflective layer.
The crystal substrate 1 is a sapphire substrate, for example.
For example, the n-type layer 2 is made of 5 × 10 5 of Si (silicon). ¹⁸ cm ^-3 3 μm thick GaN layer doped at a concentration of
The light emitting layer 3 is, for example, an MQW (multiple quantum well) layer formed by laminating 10 layers each of an GaN barrier layer having a thickness of 8 nm and an InGaN well layer having a thickness of 2 nm.
The p-type layer 4 has, for example, 1 × 10 Mg (magnesium) on the side in contact with the light emitting layer 3. ¹⁹ cm ^-3 30 nm thick Al doped at a concentration of _0.1 Ga _0.9 The N layer and the side in contact with the p-type ohmic electrode P2 are 5 × 10 5 Mg (magnesium). ¹⁹ cm ^-3 A GaN layer having a film thickness of 200 nm doped with a concentration of
A buffer layer (not shown) made of GaN, AlGaN or the like is preferably provided between the crystal substrate 1 and the n-type layer 2.
The n-type ohmic electrode P1 is, for example, in order from the side in contact with the n-type layer 2, 30 nm thick Al, 100 nm thick Pd (palladium), 100 nm thick Au (gold), and 100 nm thick Pt (platinum). ), An Au film having a thickness of 400 nm is stacked and heat-treated.
The p-type ohmic electrode P2 is formed by, for example, sequentially stacking Pd with a thickness of 20 nm, Au with a thickness of 100 nm, and Ni (nickel) with a thickness of 10 nm from the side in contact with the p-type layer 4 and performing heat treatment. . Since the electrode film having such a thickness becomes opaque, the p-type ohmic electrode P2 is formed in a pattern having a window portion so that light generated in the light emitting layer 3 can be transmitted. The window portion is a portion where no electrode film is present. In the example of FIG. 1, the p-type ohmic electrode P2 is formed in a lattice pattern. The size of the lattice pattern is, for example, a square having a window portion of 8 μm on a side, and the interval between adjacent window portions (the width of the electrode film portion) is 2 μm vertically and horizontally.
Note that the Ni thin film formed on the outermost surface of the p-type ohmic electrode P2 is an adhesion reinforcing layer for improving the adhesion with the protective film P4. A Ti (titanium) layer or a Cr (chromium) layer can also be used for such an adhesion reinforcing layer.
The p-side bonding electrode P3 is formed, for example, by laminating 20 nm-thick Ti and 600 nm-thick Au sequentially from the side in contact with the p-type layer 4 and the p-type ohmic electrode P2 and heat-treating them.
The protective film P4 made of an insulator is, for example, a 300 nm thick SiO film formed by a plasma CVD method. ₂ It is.
The reflective layer P5 is, for example, a stacked body in which Al having a thickness of 100 nm, Pd having a thickness of 100 nm, and Au having a thickness of 100 nm are stacked in this order from the side in contact with the protective film P4.
In the GaN-based LED shown in FIG. 1, light generated in the light emitting layer 3 is emitted out of the element mainly from the lower surface of the crystal substrate 1. Light that travels upward from the light emitting layer 3 directly or through internal reflection is reflected on the lower surface of the p-type ohmic electrode P2 and the lower surface of the reflective layer P5, and changes the traveling direction downward. Protective film P4 is made of SiO ₂ When formed with SiO ₂ Since the refractive index of is lower than that of the GaN-based semiconductor, reflection due to the difference in refractive index occurs at the interface between the p-type layer 4 and the protective film P4, and the effect of improving the light extraction efficiency is further increased. SiO ₂ Is an insulator and therefore has a small light absorption. Therefore, when light generated in the light emitting layer is transmitted through the protective film P4 or reflected at the interface between the p-type layer 4 and the protective film P4. The loss received is extremely low.
Next, a manufacturing process of the GaN-based LED shown in FIG. 1 will be described.
First, a buffer layer, an n-type layer 2, a light emitting layer 3, and a p-type layer 4 are sequentially grown on the growth surface of the crystal substrate 1 by using MOVPE, HVPE, MBE, or the like. After the p-type layer 4 is grown, an annealing process or the like is performed as necessary to reduce the resistance of the p-type layer 4.
FIG. 2A is a top view of the wafer after the growth of the p-type layer 4 is completed. For convenience, only the region corresponding to one element is displayed, but the actual process is performed in units of wafers. The same applies to FIGS. 2B and 2C and FIGS. 3D to 3F.
First, as shown in FIG. 2B, a p-type ohmic electrode P2 is formed on the wafer in which the growth of the p-type layer 4 is completed. A known vapor deposition method, sputtering method, CVD method, or the like can be used to form the electrode film. The patterning of the electrode film can be performed using a normal photolithography technique. For example, after forming a photoresist film on the surface of the p-type layer 4 and patterning the opening in the shape of the electrode to be formed by photolithography, the electrode film is formed using an electron beam evaporation method. In this method, the photoresist film is lifted off. Further, a method of forming an electrode film on the entire surface first and etching away unnecessary portions later is also possible.
Patterning of the n-type ohmic electrode P1, the p-side bonding electrode P3, the protective film P4, and the reflective layer P5 can also be performed by the same method.
After the p-type ohmic electrode P2 is formed, the p-type layer 4 and a part of the light-emitting layer 3 are removed from the surface side of the p-type layer 4 by a reactive ion etching method using chlorine gas, and FIG. As shown, the n-type layer 2 is exposed. This step may be performed immediately before the formation of an n-type ohmic electrode P1 described later.
Next, as shown in FIG. 3D, a protective film P4 made of an insulator is formed so as to cover a part of the p-type ohmic electrode P2. As the film forming method, a known film forming method such as a CVD method, a sputtering method, a vapor deposition method, or the like can be appropriately used depending on the type of the protective film. It is not hindered to use a wet method such as a sol-gel method. Protective film P4 is made of SiO ₂ A preferable film forming method in the case of forming by is a plasma CVD method in which pinholes are hardly generated.
Next, as shown in FIG. 3E, a reflective layer P5 is formed on the surface of the protective film P4. A known vapor deposition method, sputtering method, CVD method or the like can be used to form the reflective layer P5. In FIG. 3E, the reflective layer P5 is formed such that the edge thereof does not exceed the edge of the protective film P4. However, such a configuration is not essential, and the reflective layer P5 You may form so that it may overlap with the protective film P4, and you may form so that the surface of the p-type layer 4 may be exceeded exceeding the edge part of the protective film P4.
Next, as shown in FIG. 3F, an n-type ohmic electrode P1 and a p-side bonding electrode P3 are formed. The n-type ohmic electrode P1 is formed on the surface of the n-type layer 2 previously exposed by reactive ion etching. The p-side bonding electrode P3 is formed so as to be electrically connected to the exposed portion of the p-type ohmic electrode P2. Any of these electrodes may be formed first. A known vapor deposition method, sputtering method, CVD method, or the like can be used for film formation.
After the formation of the n-type ohmic electrode P1 and the p-side bonding electrode P3 is completed, the entire wafer is heat-treated at 400 ° C. for 5 minutes to promote adhesion between the electrode and the GaN-based semiconductor layer. This heat treatment also has an effect of improving the adhesion between the protective film P4 and the reflective layer P5.
It should be noted that n-type ohmic electrodes and p-type ohmic electrodes are preferably subjected to such heat treatment in order to reduce the contact resistance with the semiconductor. However, some electrode materials can be used without heat treatment. In addition, there are some which can obtain a sufficiently low contact resistance, and when such an electrode material is used, heat treatment is not essential. Further, heat treatment is not essential for members other than the n-type ohmic electrode and the p-type ohmic electrode.
After the heat treatment, the lower surface of the crystal substrate 1 is ground and / or polished as necessary to reduce the thickness of the crystal substrate 1, and then element isolation is performed using a method such as scribing, dicing, or laser fusing.
As mentioned above, although the structure and manufacturing process of GaN-type LED which concern on embodiment of this invention were demonstrated using FIGS. 1-3, this invention is not limited to the structure which concerns on the said illustration.
The crystal substrate may be any substrate that can be used for epitaxial growth of a GaN-based semiconductor. In addition to a sapphire substrate, Si, SiC, GaN, AlGaN, ZnO, AlN, GaAs, GaP, ZrB ₂ TiB ₂ Spinel, NGO (NdGaO ₃ ), LGO (LiGaO ₂ ), LAO (LaAlO ₃ And the like, and substrates having a crystal layer made of these materials as surface layers are exemplified as preferred crystal substrates.
A lateral growth of the GaN-based semiconductor crystal is generated by processing the crystal growth surface of the crystal substrate into an uneven surface or partially forming a mask on the surface that inhibits the growth of the GaN-based semiconductor crystal. be able to. The crystal grown in the lateral direction becomes a high-quality crystal having a low dislocation density.
The crystal substrate used for the growth of the GaN-based semiconductor can also be removed in the process of manufacturing the element or after mounting the chip-shaped element.
In a stacked structure including an n-type layer, a light-emitting layer, and a p-type layer, light is emitted by recombination of the n-type carrier injected into the n-type layer and the p-type carrier injected into the p-type layer in the light-emitting layer. As long as it is configured as described above, conventionally known techniques may be referred to as appropriate for the crystal composition of each layer, the layer thickness, the kind and concentration of added impurities, and the like. Preferably, the light emitting layer has a double hetero structure sandwiched between an n-type layer and a p-type layer having a larger band gap. The light emitting layer preferably has a single quantum well (SQW) structure or a multiple quantum well (MQW) structure. The n-type layer and the p-type layer can have a multilayer structure in which layers having different functions such as a clad layer and a contact layer are stacked.
As the p-type ohmic electrode, a conventionally known electrode can be appropriately used as an electrode having a low contact resistance with respect to the p-type GaN-based semiconductor.
A p-type ohmic electrode containing Au is known to have a particularly low contact resistance with a GaN-based semiconductor, and is the most preferable p-type ohmic electrode. For example, Au is laminated with Au electrode made of Au alone, Au alloy electrode made of an alloy mainly composed of Au, one or more metals selected from Ni, Pd, Rh (rhodium), Pt, Ti, etc. And an Au-based electrode obtained by heat treatment.
Such a p-type ohmic electrode containing Au is susceptible to the diffusion of the constituent material of the reflective layer because the melting point of Au is relatively low. Therefore, the configuration of the present invention in which the protective film is interposed between the p-type ohmic electrode and the reflective layer is particularly effective when a p-type ohmic electrode containing Au is used.
In particular, when Al and Au react even at a relatively low temperature to form an intermetallic compound, a p-type ohmic electrode containing Au and a reflective layer made of Al are directly laminated, so that heat treatment normally performed at the time of forming the ohmic electrode Therefore, there is a problem that the characteristics of the p-type ohmic electrode are remarkably deteriorated. Therefore, the configuration of the present invention in which the protective film is interposed between the p-type ohmic electrode and the reflective layer is particularly effective when the p-type ohmic electrode containing Au and the reflective layer made of Al are combined.
Other preferable p-type ohmic electrodes include an electrode made of a platinum group element alone or an alloy, and an electrode in which two or more kinds selected from platinum group elements are laminated. Since the platinum group element has excellent reflectivity with respect to visible to near-ultraviolet light, the use of such a p-type ohmic electrode improves the light extraction efficiency of the LED.
When the uppermost layer of the p-type ohmic electrode is a layer made of Au or a platinum group element, the adhesion with the protective film made of an insulator is lowered, and the protective film is easily peeled off. Therefore, it is preferable that the portion of the p-type ohmic electrode that is in contact with the protective film is formed of Ni, Ti, Cr, or the like. Metals such as Ni, Ti, and Cr show good adhesion to metal oxides and metal nitrides. By suppressing the peeling of the protective film, the characteristics of the element are stabilized and the reliability is improved.
By forming the p-type ohmic electrode in a pattern having a window portion, light absorption by the electrode is reduced and loss due to the absorption is reduced, so that the light extraction efficiency of the element is improved.
There is no limitation on the film thickness of the p-type ohmic electrode, and it is not impeded that the metal thin film has a thickness of less than 20 nm, which is highly transparent. However, when a p-type ohmic electrode having such a thickness is formed in a pattern having a window portion, the sheet resistance becomes too high, and the current is sufficiently diffused to the corner of the chip depending on the chip size. The problem of disappearing arises.
In order to reduce the sheet resistance of the p-type ohmic electrode and allow the current to diffuse well in the lateral direction, the thickness of the electrode film is preferably 60 nm or more, and more preferably 100 nm or more. When the thickness is 100 nm or more, most of the light incident on the electrode film portion is reflected without passing through the electrode film, which is preferable for improving the light extraction efficiency of the device. When light passes through the electrode film of the p-type ohmic electrode, in addition to absorption loss accompanying the transmission, a reflection loss occurs when the transmitted light is reflected by the reflective layer.
In addition, in the GaN-based LED of the present invention, the p-type ohmic electrode is formed in a pattern having a window portion, which is caused by the difference in thermal expansion coefficient between the metal constituting the electrode and the p-type layer and protective film sandwiching the metal. In order to suppress the deterioration of the electrode, the thickness of the p-type ohmic electrode is preferably 60 nm or more, more preferably 100 nm or more, as described above.
This is because the linear expansion coefficient of a metal suitable for a p-type ohmic electrode is approximately 1 × 10. ^-5 K ^-1 ~ 2x10 ^-5 K ^-1 In contrast, the linear expansion coefficient of the GaN-based semiconductor is 5.6 × 10 6 in the case of GaN. ^-6 K ^-1 It is said that the linear expansion coefficient of the metal oxide or metal nitride that is smaller than this range and that is the material of the protective film is often 1 × 10. ^-5 K ^-1 Because it is below. Thus, when the p-type ohmic electrode sandwiched between the p-type layer and the protective film made of a material having a smaller linear expansion coefficient than itself is heated during the manufacturing process or when the device is used, it is cooled to room temperature. In that case, it will receive a strong tensile stress. At this time, the smaller the film thickness of the electrode film, the more easily the electrode film is easily deformed or broken due to the stress migration phenomenon. In particular, in a p-type ohmic electrode formed in a pattern having a window portion, the current diffusion function is greatly affected by a large-scale deformation or destruction of the electrode film. If the current spreading function of the electrode is reduced, the operating voltage of the element will increase, light emission will become uneven due to the current being concentrated in part, local heat generation, light emission efficiency will decrease due to this heat generation, element life will decrease, etc. A problem occurs.
The effect of increasing the thickness of the p-type ohmic electrode is that the protective film material formed thereon is made of SiO. ₂ This is noticeable when using. Because SiO ₂ This is because, among suitable protective film materials, the material has a particularly low linear expansion coefficient.
In addition, this effect becomes prominent when a portion made of Ni, Ti, Cr or the like is provided on the p-type ohmic electrode to enhance the adhesion to the protective film. This is because the stress that the p-type ohmic electrode receives from the protective film due to the difference in thermal expansion coefficient increases as the adhesion with the protective film increases.
On the other hand, if the film thickness of the p-type ohmic electrode is too large, the electrode film tends to be peeled off from the surface of the p-type layer. Therefore, the film thickness of the p-type ohmic electrode is preferably 1 μm or less. , 500 nm or less is more preferable, and 300 nm or less is particularly preferable.
When the film thickness of the p-type ohmic electrode is reduced, when a photoresist film is formed on the p-type ohmic electrode formed in a pattern having a window on the p-type layer in the LED manufacturing process. Further, the occurrence of defects such as peeling and dropping of the photoresist film is suppressed, and the yield is improved. In the step of forming the photoresist film in this way, for example, dry etching is performed on the surface of the p-type layer 4 from the state shown in FIG. 2B, and the n-type layer 2 is then formed as shown in FIG. This is a step of exposing. In this step, a photoresist film can be used as an etching mask. In that case, the portion except the surface of the p-type layer 4 to be etched is covered with the photoresist film. That is, this photoresist film has a concave-convex surface composed of the surface of the p-type layer 4 and the p-type ohmic electrode P2 formed in a pattern having a window portion thereon (the window portion of the p-type ohmic electrode P2 becomes a concave portion). , The electrode film portion becomes a convex portion), but the lower the film thickness of the p-type ohmic electrode P2, the closer the underlying surface becomes to a flat surface. The property is improved, and the peeling and dropping are suppressed.
In order to obtain this effect, the thickness of the p-type ohmic electrode is preferably 500 nm or less, and more preferably 300 nm or less. When the film thickness of the p-type ohmic electrode is reduced in this way, the photoresist film is peeled off when the photoresist film is formed on the protective film formed to cover the p-type ohmic electrode and Dropout can also be suppressed.
Examples of patterns having window portions include patterns in which the electrode film has a net shape, a branched shape, a comb shape, a radial shape, a spiral shape, a meander shape, and the like. 4A shows an example of a net-like pattern having a rectangular window, FIG. 4B shows an example of a net-like pattern having a circular window, and FIG. 4C shows a multiple annular pattern and a radial pattern. FIG. 4D is an example of a meander pattern, FIG. 4E is an example of a comb pattern, and FIG. 4F is an example of a branch pattern. The lattice pattern is one of the net patterns. These patterns can also be mixed.
In any of the patterns, it is preferable to form the electrode film portion and the window portion finely. The band width when the electrode film portion and the window portion are formed in a band shape, and the vertical and horizontal directions of the dot when formed in a dot shape. Is preferably 1 μm to 50 μm, more preferably 2 μm to 25 μm, and particularly preferably 2 μm to 15 μm.
When forming a p-type ohmic electrode in the pattern which has a window part, it is preferable to make the area ratio of the window part which occupies this pattern into the range of 20%-80%. The larger the window area ratio, the higher the light extraction efficiency. On the other hand, since the area of the electrode film portion becomes smaller, a current flows locally at a high current density in the light emitting layer. In _x Ga _1-x In an element using N (0 ≦ x ≦ 1) in the light emitting layer, the lower the In composition x, that is, the shorter the light emission wavelength, the smaller the decrease in light emission efficiency associated with the increase in current density. It is known that it is suitable for driving at a high current density because the shift of the light emitting layer is small. _x Ga _1-x N (including the case where x = 0) and the emission wavelength is in the range of violet to near ultraviolet (about 420 nm to about 365 nm), the window area ratio is 60% to 80%, It is advantageous to improve the light emission efficiency to operate the light emitting layer at a high current density.
If the area ratio of the window portion is 60% or more, the electrode pattern includes many narrow portions of the electrode film, resulting in a difference in thermal expansion coefficient between the p-type layer and the protective film. The electrode film is likely to be deformed or broken due to thermal stress, and the current spreading function of the electrode is significantly reduced when the deformation or breakage occurs. In order to prevent this problem, the thickness of the p-type ohmic electrode is preferably 100 nm or more.
FIG. 5 is a cross-sectional view of a GaN-based LED according to an embodiment of the present invention. In the element shown in FIG. 5, the protective film P4 does not cover the entire surface of the p-type layer 4 exposed at the window portion of the p-type ohmic electrode P2, and the surface of the p-type layer 4 at the central portion of the window portion. Although the reflective layer P5 is in contact with the reflective layer P5, deterioration of the characteristics of the p-type ohmic electrode P2 due to diffusion of the constituent material of the reflective layer P5 can be suppressed even in such an aspect.
In the element shown in FIGS. 1 and 5, when the protective film P4 made of an insulator is formed of a material having a refractive index lower than that of the p-type layer 4, a difference in refractive index occurs at the interface between the p-type layer 4 and the protective film P4. Light reflection occurs. Such reflection is generally preferable for improving the light extraction efficiency of the LED because the loss is smaller than the reflection on the metal surface. Therefore, the protective film P4 is formed of an insulator having a refractive index lower than that of the p-type layer 4, and the p-type layer 4 exposed in the window portion of the p-type ohmic electrode P2 as in the element shown in FIG. The reflective layer P5 is preferably formed so as to be completely separated by the protective film P4.
As a material for the protective film made of an insulator, SiO ₂ In addition to SiN _x ZnO ₂ TiO ₂ Etc. are exemplified. That is, an insulating metal oxide, metal nitride, or metal oxynitride is suitable for such a protective film. ₂ O ₃ , AlN, ZrO ₂ Etc. are exemplified. These insulators can be stacked and used. Since the protective film made of these insulators has low light absorption, the loss received when light is transmitted through the protective film or reflected at the interface between the p-type layer and the protective film is small. can do.
The thickness of the protective film is not particularly limited as long as the object of the present invention is achieved, but is preferably 0.1 μm or more in order to reliably form a film without a pinhole. , 0.2 μm or more is more preferable, and 0.3 μm or more is particularly preferable.
When the thickness of the protective film is 3 μm or less, it becomes easy to perform the patterning using a simple lift-off method.
On the other hand, since the thermal conductivity of the insulator is not good, the outermost surface layer of the reflective layer is used as a bonding layer, or a bonding layer is further formed on the reflective layer, and this bonding layer is used for mounting. When the substrate is bonded and mounted, the thickness of the protective film is preferably 1 μm or less, and 0.5 μm or less so that the effect of improving the heat dissipation of the element obtained by the mounting method is increased. More preferably, it is more preferably 0.3 μm or less.
FIG. 6 is a cross-sectional view of a GaN-based LED according to an embodiment of the present invention. In the element shown in FIG. 6, the protective film P4 is formed to extend so as to cover the end face of the light emitting layer 3 exposed by etching when the n-type layer 2 is exposed. That is, the protective film P4 also serves as the end face protective film of the light emitting layer 3.
The reflective layer is preferably formed of a material having a higher reflectance than the p-type ohmic electrode at the wavelength of light generated in the light emitting layer. A preferable material for the reflective layer is Ag (silver), Al, Rh, Pt or the like having a high reflectance in the visible short wavelength region to the near ultraviolet region, and particularly Ag and Al. Platinum group elements (Ir, Pd, Ru, Os) other than Rh and Pt can also be suitably used. The reflection layer may be formed of a metal having high reflectivity at least a portion that reflects light reaching from the light emitting layer through a window provided in the p-type ohmic electrode. For example, in the element shown in FIG. 1, only this portion of the reflective layer is formed of Al. It is also possible to form the reflective layer only with these highly reflective metals.
When Ag is used as an anode, there is a problem that electrochemical migration is likely to occur. Therefore, forming a reflective layer made of Ag directly on a p-type ohmic electrode may cause deterioration of the p-type ohmic electrode or LED. May cause deterioration.
Since the linear expansion coefficient of Al is about four times the linear expansion coefficient of GaN-based semiconductors, when a reflective layer made of Al is formed directly on a p-type layer as in the prior art, the difference in thermal expansion coefficient is different. The reflective layer is likely to be deformed by the thermal stress generated by. When this deformation spreads to the p-type ohmic electrode, the contact state between the p-type ohmic electrode and the p-type layer deteriorates, and the contact resistance of the p-type ohmic electrode increases. In addition, Al has a low melting point, so that it is easily diffused during heat treatment of the electrode, and stress migration due to the thermal stress is also likely to occur. Al diffused at the interface between the p-type ohmic electrode and the p-type layer increases the contact resistance of the p-type ohmic electrode. Further, Al has a property of forming an intermetallic compound with a metal that is a material of the p-type ohmic electrode.
For these reasons, forming the reflective layer made of Al directly on the p-type ohmic electrode may cause deterioration of the p-type ohmic electrode.
For this reason, in particular, when a reflective layer made of Ag or Al is used, the configuration of the present invention in which a protective film is interposed between the p-type ohmic electrode and the reflective layer is effective.
When the reflective layer is formed of Ag or Al, it is preferable to use a single substance in terms of reflectivity, but in order to improve heat resistance and weather resistance, the reflectivity at the wavelength of light generated in the light-emitting layer is significantly reduced. An alloy to which other elements are added can also be used within a range that does not (for example, a range that does not become less than 80% of the simple substance). As such an alloy, a highly reflective Ag alloy or Al alloy, which has been developed for wiring of various semiconductor light emitting devices and liquid crystal display devices, can be preferably used. As a suitable Al alloy, an alloy obtained by adding Ti, Si, Nd, Cu or the like to Al is exemplified.
As a method of forming a reflective layer made of an alloy of Ag or Al, in addition to alloy sputtering, etc., a thin film made of an element to be added is formed on the surface of the protective film, and after laminating Al or Ag, A heat treatment method can also be used.
In the element of FIG. 1, when viewed from the upper surface side, the n-type ohmic electrode P1 and the p-side bonding electrode P3 are both rectangular and are formed along two opposing sides of the rectangular element. The shape and arrangement of the n-type ohmic electrode and the p-side bonding electrode are not limited to this. For example, the shape of these electrodes may be a square shape as in the element shown in FIG. 7A or a circular shape as in the element shown in FIG. 7B. Like the element shown to (a), it is good also as diagonal arrangement | positioning.
Etching from the surface of the p-type layer exposes the n-type layer and forms an n-type ohmic electrode on the exposed surface of the n-type layer, in an n-type ohmic electrode, p-type ohmic electrode, p-side JP, 2000-164930, A, etc. can also be referred for the shape of a bonding electrode.
Note that a transparent conductive substrate such as a SiC substrate, a ZnO substrate, or a GaN substrate can be used as the crystal substrate. In that case, an n-side ohmic electrode can be formed on the lower surface of the crystal substrate.
As the n-type ohmic electrode, an electrode conventionally known as an electrode having a low contact resistance with respect to the n-type GaN-based semiconductor can be appropriately used. As such an electrode, for example, an electrode in which a portion in contact with the n-type layer is made of Al, Ti, Cr, W, or an alloy thereof can be given.
A preferable n-type ohmic electrode is one in which a portion in contact with the n-type layer is made of Al. Such an n-type ohmic electrode can have the same cross-sectional structure as the reflective layer. In that case, since the n-type ohmic electrode and the reflective layer can be formed simultaneously, the number of manufacturing steps can be reduced.
The n-type ohmic electrode can be used as a bonding electrode if the layer thickness is about 200 nm or more, but if necessary, an n-side bonding electrode is separately formed on the n-type ohmic electrode. May be.
FIG. 8 is a cross-sectional view showing a mounting example of the GaN-based LED shown in FIG.
In FIG. 8, S is a mounting base material, for example, a pattern of lead electrodes S2, S3, S4 made of Au formed on the surface of a substrate S1 made of AlN. In the GaN-based LED, the reflective layer P5 is directed to the mounting substrate S side, the n-type ohmic electrode P1 is the lead electrode S2, the p-side bonding electrode P3 is the lead electrode S3, the reflective layer P5 is the lead electrode S4, Each is bonded to the mounting substrate S by bonding with the conductive bonding material C. The conductive bonding material C is, for example, a conductive paste in which a brazing material such as Au—Sn solder or conductive fine particles are dispersed in a resin binder.
In the example shown in FIG. 8, since the reflective layer P5 of the LED does not have a function as an electrode, the lead electrode S4 of the mounting substrate S does not function as an electrode. I'm calling. The reason why the reflective layer P5 is bonded to the mounting substrate S is to release heat generated by the LED. For this purpose, the conductive bonding material is a suitable bonding material. This is because the conductive bonding material is made of a metal material itself, like a brazing material, or contains a high content of fine particles of metal, carbon, etc. like a conductive paste. This is because is good.
In the present invention, a layer used for bonding with a bonding material when an element is mounted, such as the outermost surface layer of the reflective layer P5 in FIG. 8, is referred to as a bonding layer.
In FIG. 8, the reason why the reflective layer P5 is joined to the lead electrode S4 is to release heat generated by the LED to the base material S. For this purpose, it is particularly preferable to use a brazing material as the conductive bonding material C.
As in the example of FIG. 8, when the outermost surface layer of the reflective layer P5 is used as a bonding layer and the bonding layer and the mounting substrate S are bonded with a conductive bonding material, the heat generated in the light emitting layer during LED operation. However, since it is efficiently transmitted to the mounting substrate, the temperature rise of the element is suppressed. As a result, a decrease in light emission efficiency and wavelength variation are suppressed, and the lifetime and reliability of the element are improved.
Such an effect can also be obtained by forming a metal bonding layer on the reflective layer and bonding the bonding layer and the mounting substrate with a conductive bonding material.
In the example of FIG. 8, when a brazing material is used as the conductive bonding material, the outermost surface layer (= bonding layer) of the reflective layer P5 is formed of Au so that the wettability with the brazing material is good. Is preferred. Since Au is not easily oxidized, the surface made of Au shows good wettability with respect to various brazing materials. If the surface of the bonding layer has good wettability with respect to the brazing material, a bonding interface is formed in which the bonding layer and the brazing material are in close contact with each other without any gap, and the thermal resistance of the interface is reduced.
Eutectic solder is most often used as the brazing material, and Sn (tin) is often used as the component metal of the eutectic solder. When the bonding layer is formed of Sn or Sn alloy (Sn alloy containing the same component as the Sn-based eutectic solder to be used), the bonding layer and the eutectic solder are formed when eutectic solder containing Sn as a component is used. It can be tightly joined.
In addition, Au alloy-based solders such as Au-Si alloy, Au-Ge alloy, Au-Sn alloy, Au-Sb alloy, etc. have good electrical and thermal conductivity and are chemically stable. For this reason, it is widely used for joining semiconductor components. When the bonding layer is formed of Au or an Au alloy (an Au alloy containing the same components as the Au alloy solder used), the bonding layer and the Au alloy solder are closely bonded when using the Au alloy solder. Can do.
The bonding layer and the lead electrode can be eutectic bonded. In eutectic bonding, for example, an Au layer is used as the bonding layer, and an Sn layer is formed on the surface of the lead electrode. With these layers in contact, energy is applied in the form of heat, vibration, etc. Bonding is performed by forming an Au—Sn eutectic alloy in the part. Eutectic bonding is also a preferable bonding method for improving the heat dissipation of the element because it is bonding with a metal material.
During brazing, the reflective layer P5 is exposed to a high temperature. In order to suppress the alloying reaction between the Al layer and the Au layer included in the reflective layer P5, which is caused by the heat at this time, these layers are controlled. It is preferable to interpose a barrier layer made of a metal material having a higher melting point than Au between the layers. This is because when the reaction between Au and Al occurs, the reflectivity of the Al layer is lowered, and an alloy layer with inferior strength is formed and voids are formed, so that the lifetime and reliability of the device are lowered. Such a reaction gradually progresses even at the operating temperature of the device or at a lower temperature, but this can be suppressed by the intervention of the barrier layer. The same applies when an Al alloy layer is used instead of the Al layer and an Au alloy layer is used instead of the Au layer.
The barrier layer is formed so as to include a layer made of a metal having a higher melting point than a metal having a higher melting point among the metals constituting the two layers to be separated by the barrier layer. Preferred barrier layer materials include so-called refractory metals such as W, Mo, Ta, Nb, V, and Zr, platinum group elements, simple substances such as Ti, Ni, and alloys. The barrier layer may be a multilayer film in which a plurality of layers made of these materials are stacked. A multilayer film in which Pt layers and Au layers are alternately stacked is suitable as a barrier layer.
Depending on the configuration of the reflective layer P5, between the Al (alloy) layer and the Sn (alloy) layer, between the Ag (alloy) layer and the Au (alloy) layer, between the Ag (alloy) layer and the Sn (alloy) layer, A barrier layer is preferably interposed between the layers for the same reason.
The reflective layer P5 and the p-type ohmic electrode P2 may be electrically short-circuited in order to prevent the destruction of the protective film P4 due to static electricity or the like and the accompanying deterioration of the LED. This short circuit can also be performed by short-circuiting the lead electrode S3 and the lead electrode S4 on the mounting substrate S side.
FIG. 9 is a cross-sectional view of a GaN-based LED according to an embodiment of the present invention. In the element of FIG. 9, the p-type ohmic electrode P2 and the reflective layer P5 are short-circuited in the element. Specifically, the p-side bonding electrode P3 is formed extending to the surface of the reflective layer P5. This GaN-based LED can be mounted with good heat dissipation by using as the bonding layer the outermost surface layer of the layered p-side bonding electrode P3 formed on the reflective layer P5.
In the element shown in FIG. 9, when the p-type ohmic electrode P2 and the reflective layer P5 are short-circuited, a portion where the reflective layer P5 is laminated on the p-type ohmic electrode P2 without the protective film P4 does not occur. Structure is adopted. Such a structure is particularly preferable when the reflective layer includes an Al layer or an Ag layer. The reason is that if an Al layer or an Ag layer is provided on the p-type ohmic electrode without a protective film, Al or Ag diffuses and reacts with the material of the p-type ohmic electrode, This is because it easily enters the interface of the p-type layer. This is because the p-type ohmic electrode is only separated from the active layer, which is a heat generating part of the element, by a thin p-type layer, and thus is exposed to a relatively high temperature for a long time. is there. In particular, when an Al layer is formed on a p-type ohmic electrode containing Au, it is desirable to always include a protective film in order to suppress the formation of an alloy having undesirable properties.
FIG. 10 is a cross-sectional view of a GaN-based LED according to an embodiment of the present invention. In the element of FIG. 10, the n-type ohmic electrode P1 and the reflective layer P5 are short-circuited in the element. Specifically, the n-type ohmic electrode P1 is formed extending to the surface of the reflective layer P5. This GaN-based LED can be mounted with good heat dissipation by using the outermost surface layer of the layered n-type ohmic electrode P1 formed on the reflective layer P5 as a bonding layer.
In particular, when Ag is used for the reflective layer, if the potential of the reflective layer is increased, there is a possibility that a problem of Ag electrochemical migration may occur. Therefore, in order to prevent this, it is preferable to short-circuit the reflective layer with an electrode for supplying current to the n-type GaN-based semiconductor as in the element shown in FIG.
FIG. 11 is a diagram for explaining a mode in which the crystal substrate used for the growth of the GaN-based semiconductor layer, which is an embodiment of the present invention, is finally removed from the element.
FIG. 11A shows a wafer in which an n-type layer 2, a light emitting layer 3, and a p-type layer 4 are grown on a crystal substrate 1, and a p-type ohmic electrode P2, a protective film P4, and a reflective layer P5 are further formed. It is sectional drawing. The reflective layer P5 is electrically connected to the p-type ohmic electrode P2 at the periphery of the element.
Here, a barrier layer for suppressing an alloying reaction between the metal materials constituting each of the p-type ohmic electrode P2 and the reflective layer P5 may be interposed. In addition, the reflective layer P5 has a multi-layer structure, the lowermost layer is made of Al or Ag, the uppermost layer is made of Au (alloy) or Sn (alloy), and a barrier layer is interposed therebetween to protect the lowermost layer. It may be formed only on the surface of the film P4, and at least one of the barrier layer and the uppermost layer may be formed extending on the exposed portion of the p-type ohmic electrode P2.
FIG. 11B shows a state where the holding substrate B is bonded to the reflective layer P5 via the conductive bonding material C. The conductive bonding material C is, for example, a brazing material or a conductive paste. The holding substrate B may be a conductive substrate, and is, for example, a Si substrate, a GaAs substrate, a GaP substrate, a SiC substrate, a GaN substrate, a ZnO substrate, or various metal substrates. Further, instead of bonding the holding substrate B using the conductive bonding material C, a thick film made of a metal such as Ni is deposited by electroplating using the reflective layer P5 as an electrode, and this is used as a holding substrate. You can also.
In order to join the reflective layer P5 and the conductive substrate with an Au-based or Sn-based brazing material or by eutectic bonding, the uppermost layer of the reflective layer P5 is made of Au (alloy) or Sn (alloy). It is preferable to form.
FIG. 11C shows that the crystal substrate 1 is removed and an n-type ohmic electrode P1 is formed on the exposed surface of the n-type layer 2. The removal of the crystal substrate 1 can be performed by grinding all or most of the crystal substrate 1 by grinding and polishing, or by peeling the interface between the crystal substrate 1 and the n-type layer 2 using a laser lift-off technique. It can be carried out. The n-type ohmic electrode P1 may be formed on the exposed surface of the n-type layer exposed by etching from below the n-type layer.
In another preferred embodiment of the present invention, a GaN-based semiconductor layer is grown using LEPS (Lateral Epitaxy on a Patterned Substrate), which is a method for growing a GaN-based semiconductor crystal disclosed in European Patent Application Publication No. EP1184897A1. The electrode film portion of the p-type ohmic electrode is selectively formed on a region formed on the surface of the p-type layer and having a relatively low threading dislocation density. By doing in this way, the luminous efficiency (internal quantum efficiency) in a light emitting layer can be improved.
In one aspect of LEPS, a number of stripe-shaped recesses extending in the <11-20> direction of sapphire (the <1-100> direction of a GaN-based semiconductor crystal grown on the substrate) are formed on the surface of the C-plane sapphire substrate. By forming the (groove) by etching, a striped uneven pattern is formed on the surface, and a GaN-based semiconductor crystal is grown thereon. Then, lateral crystal growth starting from the surface of the convex portion occurs, and eventually, crystals grown from the convex portions are united to obtain a crystal layer having a flat surface. When this crystal layer is used as a base layer and an n-type layer, a light emitting layer, and a p-type layer are sequentially grown thereon to produce an LED wafer, the surface of the p-type layer has a striped dislocation density that is particularly low. Appears. That is, the region above the concave portion formed on the surface of the sapphire substrate, that is, the region above the portion where the crystal grown laterally from the convex portion of the sapphire substrate constitutes the base layer. The density of threading dislocations in the region is 10 ⁷ cm ^-2 It can be as low as a table or less.
For example, a p-type ohmic electrode having a comb-like pattern shown in FIG. 4E is formed on the surface of the p-type layer, and each of the stripe-like portions corresponding to the teeth of the comb has a low threading dislocation density. A stripe-shaped region, that is, a region above the concave portion of the sapphire substrate can be provided. In this way, most of the current supplied from the p-type ohmic electrode through the p-type layer to the light-emitting layer can be concentrated in a region where the threading dislocation density is low, and thus the probability of light-emitting recombination of carriers in the light-emitting layer. And the luminous efficiency is improved.
This effect is, among other things, that the light emitting layer has a low In composition x. _x Ga _1-x It becomes conspicuous in LEDs having an emission wavelength of purple (about 420 nm) to near-ultraviolet consisting of N (including the case of x = 0). The reason is that, in a light emitting layer made of a GaN-based semiconductor having a low In composition, the adverse effect of dislocations on the light emission efficiency is increased.
In the embodiment using this LEPS, 60% or more, preferably 70% or more, more preferably 80% or more of the area of the electrode film portion of the p-type ohmic electrode is used as the low dislocation density region above the concave portion of the substrate. It is preferable to form in this area.
Here, the pattern of the p-type ohmic electrode is not limited to a comb pattern. Further, the uneven pattern formed on the substrate surface is not limited to a stripe shape. The concavo-convex pattern may be a pattern in which the direction of the boundary line between the concave portion and the convex portion is the <1-100> direction of the GaN-based semiconductor crystal grown on the substrate. For example, the convex portion is formed in an island shape. Pattern. Also, even if the direction of the boundary line is another direction, the lateral growth rate of the GaN-based semiconductor crystal can be increased by doping Mg, etc., thereby penetrating above the recess of the substrate. A region with a low dislocation density can be formed.
By the way, in this embodiment, before the crystal grown in the lateral direction from the convex part of the substrate is connected to the crystal grown in the concave part, it merges with the crystal grown in the lateral direction from the adjacent convex part. In some cases, a gap may remain between the GaN-based semiconductor crystal layer grown on the substrate. Since such voids are filled with a gaseous substance having a low refractive index, the light reaching from the light emitting layer is easily reflected, and the light generated in the light emitting layer is taken out from the lower surface side of the substrate. It becomes an obstacle. This problem can be solved by joining a new holding substrate on the reflective layer and removing the substrate used for LEPS (the substrate with the concavo-convex pattern formed on the surface) as in the embodiment shown in FIG. can do.

Example 1
As an example of the present invention, a GaN-based LED having the cross-sectional structure shown in FIG. 12 was produced and evaluated by the following procedure.
(Production of LED wafer)
A stripe mask pattern made of a photoresist was periodically formed on one main surface of a C-plane sapphire substrate. The width of the stripe-shaped mask is 3 μm, the period (mask width + the width of the portion where the substrate surface is exposed in a stripe shape between adjacent masks) is 6 μm, and the stripe direction is the <1-100> direction of sapphire (on the substrate) It becomes the <11-20> direction of the GaN-based semiconductor crystal to be grown). By performing reactive ion etching on the mask, a groove having a depth of 1 μm was processed on the exposed surface of the sapphire substrate. Then, the sapphire processing board | substrate 1 with which the striped uneven | corrugated pattern was formed in the surface was obtained by removing a photoresist. Note that, on the sapphire processed substrate having the stripe direction, the lateral growth of the GaN-based semiconductor crystal is suppressed, and the concave portion on the substrate surface is easily embedded.
Next, an n-type layer 2, a light emitting layer 3, and a p-type layer 4 are sequentially grown on the sapphire processed substrate 1 using the MOVPE method, followed by annealing using a rapid thermal annealing (RTA) apparatus. By performing the treatment, an LED wafer provided with a GaN-based semiconductor laminate having an LED structure was obtained. Here, the n-type layer 2 has a two-layer structure of an undoped GaN layer and a Si-doped GaN layer, and the surface of the sapphire processed substrate 1 is buried with the undoped GaN layer, and then a Si-doped GaN layer is grown. The light emitting layer 3 has an MQW structure in which an InGaN well layer whose In composition is adjusted so that an emission wavelength is 400 nm and an InGaN barrier layer having a larger band gap than the well layer are alternately stacked. The p-type layer 4 has a two-layer structure including an Mg-doped AlGaN cladding layer and an Mg-doped GaN contact layer laminated thereon.
(Formation of electrodes, etc.)
Next, the p-type ohmic electrode P2 was formed on the surface of the p-type layer 4 in a lattice pattern having square windows. The area ratio of the window portion in the lattice pattern was about 70%. The p-type ohmic electrode P2 was formed by stacking Rh with a thickness of 30 nm, Au with a thickness of 100 nm, and Ti with a thickness of 20 nm in this order from the side in contact with the p-type layer 4 by vapor deposition. The p-type ohmic electrode P2 was patterned by a lift-off method using a normal photolithography technique. The p-type ohmic electrode P2 after patterning was subjected to heat treatment at 500 ° C. for 1 minute.
Next, in order to partially expose the Si-doped GaN layer included in the n-type layer 2, the reactive ion etching method is used to form the p-type layer 4 and the light emitting layer 3 from the upper surface side of the p-type layer 4. Part was removed. Subsequently, an n-type ohmic electrode P1 was formed by sequentially stacking Ti and Al on the exposed surface of the Si-doped GaN layer formed by the etching using a vapor deposition method. The n-type ohmic electrode P1 was also heat-treated at 500 ° C. for 1 minute.
Next, a 300 nm-thickness protective film P4 made of SiO ₂ is formed by plasma CVD so as to cover the entire upper surface of the wafer, and then the protective film P4 is partially removed by dry etching. An opening was formed, and the upper surface of the n-type ohmic electrode P1 and a part of the p-type ohmic electrode P2 were exposed.
Next, a 200 nm-thick reflective layer P5 made of Al was formed on the protective film P2 by vapor deposition.
Next, the p-side bonding electrode P3 was formed using a vapor deposition method. The p-side bonding electrode is a laminate in which the lowermost layer is Ti having a thickness of 10 nm, on which three layers of Pt having a thickness of 80 nm and Au having a thickness of 80 nm are alternately laminated so that the uppermost layer is an Au layer. The body. The p-side bonding electrode P3 was formed so as to be in contact with the p-type ohmic electrode P2 exposed at the opening of the protective film P4 and to cover the entire reflective layer P5 formed on the protective film P4.
Simultaneously with the formation of the p-side bonding electrode P3, an n-side bonding electrode P6 having the same laminated structure as the p-side bonding electrode P3 was formed on the n-type ohmic electrode P1 exposed at the opening of the protective film P4. .
Next, the lower surface of the sapphire processed substrate 1 was polished to reduce its thickness to 100 μm, and then scribing was performed to cut out an LED chip from the wafer. The size of the LED chip was 350 μm square.
(Evaluation)
The obtained LED chip (bare chip) was fixed on a mounting substrate having positive and negative lead electrodes formed on the surface. For fixing, the upper side of the element (the side on which the GaN-based semiconductor laminate is formed as viewed from the sapphire substrate 1) is directed to the mounting surface of the mounting substrate, and the p-side bonding electrode P3 is set to the positive lead electrode. The n-side bonding electrode P6 was bonded to the negative lead electrode by Au—Sn solder. For the p-side bonding electrode P3, the surface of the portion formed on the reflective layer P5 was used for bonding with Au—Sn solder.
The forward voltage (Vf) and output of the mounted LED chip were measured at a current of 20 mA. An integrating sphere was used to measure the output. As a result, Vf was 3.8 V, and the output was 10.7 mW.
Example 2
An LED chip was fabricated and evaluated in the same manner as in Example 1 except that Rh contained in the p-type ohmic electrode P2 was replaced with Pd. As a result, Vf was 3.4 V, and the output was 9.7 mW.
Comparative Example 1
As an example related to the prior art, a GaN-based LED having a cross-sectional structure shown in FIG. 13A was fabricated and evaluated. This GaN-based LED does not have a p-type ohmic electrode formed in a pattern having a window, and does not have a reflective layer made of Al.
The LED wafer was manufactured in the same manner as in Example 1.
The p-type ohmic electrode P12 is the same as the p-type ohmic electrode of Example 1 except that the surface of the p-type layer 14 of the obtained wafer has a pattern (that is, a flat plate shape) in which no window is provided. Formed.
Next, formation of the n-type ohmic electrode P11 and subsequent formation of the protective film P14 on the entire upper surface of the wafer were performed in the same manner as in Example 1.
Next, an opening in which the protective film P14 was partially removed was formed by dry etching to expose the upper surface of the n-type ohmic electrode P11 and the upper surface of the p-type ohmic electrode P12. Subsequently, the n-side bonding electrode P16 and the p-side bonding having the same laminated structure as that formed in Example 1 on each of the n-type ohmic electrode P11 and the p-type ohmic electrode P12 exposed in the opening. The electrode P13 was formed at the same time.
Thereafter, an LED chip was cut out from the wafer in the same manner as in Example 1, fixed on the mounting substrate, and evaluated.
As a result, Vf was 3.8 V, and the output was 9.5 mW.
When the element of Example 1 and the element of Comparative Example 1 are compared, it is considered that Vf is equivalent because an electrode having the same laminated structure made of Rh and Au is used as the p-type ohmic electrode. On the other hand, in the device of Example 1, at least a part of the light generated in the light emitting layer is reflected to the substrate side by the reflective layer made of Al having a reflectance higher than Rh for light having a wavelength of 400 nm. On the other hand, since the element of Comparative Example 1 does not have a reflective structure using such a reflective layer, it is considered that the output of the element of Example 1 was higher than that of the element of Comparative Example 1.
Further, when comparing the element of Example 2 with the element of Comparative Example 1, the output of the element of Example 2 is Pd, which is less reflective than Rh, although it is used for the p-type ohmic electrode. It exceeds the element of Comparative Example 1. And since the element of Example 2 has Vf lower than the element of the comparative example 1, the luminous efficiency is better than the element of the comparative example 1.
Comparative Example 2
As an example related to the prior art, a GaN-based LED having a cross-sectional structure shown in FIG. 13B was fabricated and evaluated. In this GaN-based LED, a protective film made of an insulator is not interposed between the p-type ohmic electrode and the reflective layer made of Al.
The LED wafer was manufactured in the same manner as in Example 1.
A p-type ohmic electrode P12 was formed on the surface of the p-type layer 14 of the obtained wafer in the same manner as in Example 1.
Next, a 200 nm-thick reflective layer P15 made of Al was formed so as to directly cover the p-type ohmic electrode P12.
Next, formation of the n-type ohmic electrode P11 and subsequent formation of the protective film P14 on the entire upper surface of the wafer were performed in the same manner as in Example 1.
Next, an opening in which the protective film P14 was partially removed was formed by dry etching to expose the upper surface of the n-type ohmic electrode P11 and the upper surface of the reflective layer P15. Subsequently, the n-side bonding electrode P16 and the p-side bonding electrode P13 having the same laminated structure as that formed in Example 1 on each of the n-type ohmic electrode P11 and the reflective layer P15 exposed in the opening. Were formed simultaneously.
Thereafter, an LED chip was cut out from the wafer in the same manner as in Example 1, fixed on the mounting substrate, and evaluated.
As a result, Vf was 4.5 V, and the output was 7.5 mW.

In the GaN-based light emitting diode according to the embodiment of the present invention, a protective film made of an insulator is interposed between the p-type ohmic electrode formed to transmit light generated in the light emitting layer and the reflective layer. An increase in the contact resistance of the p-type ohmic electrode due to the diffusion of the material of the reflective layer is suppressed. Moreover, the fall of the reflectance of a reflection layer by the material of a p-type ohmic electrode diffusing is also suppressed. That is, each of the p-type ohmic electrode and the reflective layer can be optimized by separating the p-type ohmic electrode and the reflective layer by the protective film made of an insulator. Therefore, the light extraction efficiency using the reflective layer can be improved without lowering the conversion efficiency, and the light emission efficiency is improved.
In the GaN-based light emitting diode according to the embodiment of the present invention, since the p-type ohmic electrode is formed in a pattern having a window portion, the loss due to the light generated in the light-emitting layer being absorbed by the p-type ohmic electrode is small. Thus, the light extraction efficiency is improved.
In the GaN-based light emitting diode according to the embodiment of the present invention, preferably, the outermost surface layer of the reflective layer is used as a bonding layer or a bonding layer is formed on the reflective layer, and the bonding layer and the mounting substrate are formed. Are mounted by bonding with a conductive bonding material. When mounted in this way, the heat generated in the light emitting layer during operation of the element is efficiently transferred to the mounting substrate, so that the temperature rise of the element is suppressed, resulting in a decrease in light emission efficiency and wavelength fluctuations. Is suppressed, and the lifetime and reliability of the device are improved.
This application is based on patent application Nos. 2005-031155, 2005-284375, and 2005-317781 filed in Japan, the contents of which are incorporated in full herein.

Claims (15)

an n-type GaN-based semiconductor layer;
A light emitting layer made of a GaN-based semiconductor formed on the n-type GaN-based semiconductor layer;
A p-type GaN-based semiconductor layer formed on the light emitting layer;
A p-type ohmic electrode formed in a pattern having a window on the surface of the p-type GaN-based semiconductor layer;
A metal reflective layer that is formed so as to sandwich the p-type ohmic electrode with the p-type GaN-based semiconductor layer and reflects light reaching the light-emitting layer through the window;
A protective film made of an insulator interposed between the reflective layer and the p-type ohmic electrode;
A GaN-based light emitting diode having: The GaN-based light-emitting diode according to claim 1, wherein the outermost surface layer of the reflective layer is a bonding layer, or a metal bonding layer is further formed on the reflective layer. The GaN-based light-emitting diode according to claim 2, wherein the bonding layer is a layer made of Au, Au alloy, Sn, or Sn alloy. 4. The GaN-based light emitting diode according to claim 3, wherein at least a portion that reflects light reaching from the light emitting layer is formed of Ag, an Ag alloy, Al, an Al alloy, or a platinum group element. The reflection layer is formed of Ag, Ag alloy, Al, or Al alloy at least at a portion that reflects light reaching from the light emitting layer, and a barrier layer is interposed between the portion and the bonding layer. The GaN-based light emitting diode according to claim 3, wherein The GaN-based light emitting diode according to claim 2, wherein the protective film has a thickness of 0.1 μm to 1 μm. The GaN-based light-emitting diode according to claim 1, wherein the protective film has a lower refractive index than the p-type GaN-based semiconductor layer. The reflective layer is formed of Ag, an Ag alloy, Al, or an Al alloy at least at a portion that reflects light reaching from the light emitting layer, and the p-type ohmic electrode is a p-type ohmic electrode containing Au. The GaN-based light emitting diode according to claim 1. The GaN-based light-emitting diode according to claim 1, wherein the p-type ohmic electrode includes a portion made of Ni, Ti, or Cr at a portion in contact with the protective film. The GaN-based light emitting diode according to claim 1, wherein the p-type ohmic electrode has a thickness of 60 nm to 1 μm. The GaN-based light emitting diode according to claim 10, wherein the thickness of the p-type ohmic electrode is 100 nm or more. The area ratio of the window portion in the pattern having the window portion is 60% to 80%, and the light emitting layer includes In _x Ga _1-x N (including the case where x = 0) having an emission wavelength of 420 nm or less. The GaN-based light emitting diode according to claim 1. The GaN-based light emitting diode according to claim 2 is fixed on the mounting surface of the mounting base material with the bonding layer facing the mounting surface, and the bonding layer and the mounting base material are electrically conductive. A light-emitting device bonded by a bonding material. The light emitting device according to claim 13, wherein the bonding by the conductive bonding material is formed by brazing. The light emitting device according to claim 13, wherein the bonding by the conductive bonding material is formed by eutectic bonding.

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