JP2009094107A - ELECTRODE FOR GaN-BASED LED DEVICE AND GaN-BASED LED DEVICE USING THE SAME - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a GaN-based LED device wherein a transparent electrode is made of a TCO film with micro unevenness on its front surface and the optical reflectivity on the rear surface of a metal film for bonding pads can be also prevented from being degraded. <P>SOLUTION: An electrode for a GaN-based LED device is comprised of a transparent electrode 13 which is formed in contact with the surface of a GaN-based semiconductor film 12 and a metal film 14 which is formed as a bonding pad in a part on the transparent electrode 13. The transparent electrode 13 is made of a first TCO film 13a and a second TCO film 13b which are electrically connected with each other, and both the first TCO film 13a and the second TCO film 13b are exposed over the surface of the transparent electrode 13. The surface of the first TCO film 13a is smoother than that of the second TCO film 13b, and the metal film 14 is formed on the surface of the first TCO film 13a. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an electrode for a GaN-based LED element, and a GaN-based LED element using the same.

A GaN-based semiconductor is a compound semiconductor represented by the chemical formula Al _a In _b Ga _1-ab N (0 ≦ a ≦ 1, 0 ≦ b ≦ 1, 0 ≦ a + b ≦ 1), and is a group III nitride semiconductor. Also called a nitride-based semiconductor. A GaN-based LED element in which a light-emitting element structure such as a pn junction structure, a double hetero structure, or a quantum well structure is formed of a GaN-based semiconductor can generate green to near-ultraviolet light. It is put to practical use in applications such as display devices.

  Recently, TCO films have been used as transparent electrodes in GaN-based LED elements. TCO is a transparent conductive oxide, typically ITO (indium tin oxide), indium oxide, tin oxide, IZO (indium zinc oxide), AZO (aluminum zinc oxide). Material), zinc oxide, and FTO (fluorine-doped tin oxide). Among TCOs, ITO is widely used. Since an ITO film usually has fine irregularities derived from a polycrystalline structure on its surface, a GaN-based LED element using the ITO film as a transparent electrode has a high output. This is because the presence of the fine irregularities suppresses total reflection of light incident on the surface of the transparent electrode from the inside of the element, so that the light generated inside the element can be efficiently transmitted to the outside of the element through the surface of the transparent electrode. It is because it is taken out.

  FIG. 10 shows a cross-sectional structure of a conventional GaN-based LED element using a TCO film as a transparent electrode. This GaN-based LED element has a substrate 101 made of sapphire and a GaN-based semiconductor film 102 formed thereon. In the GaN-based semiconductor film 102, an n-type layer 102-1 made of Si-doped GaN, a p-type layer 102-3 made of Mg-doped AlGaN, and an InGaN well layer and a GaN barrier layer sandwiched therebetween. And a light emitting layer 102-2 having a multiple quantum well structure. On the GaN-based semiconductor film 102, two electrodes are formed on the same surface side. One is a p-electrode formed on the p-type layer 102-3, a transparent electrode composed of the TCO film 103a and the TCO film 103b, and a metal film 104 formed as a bonding pad on a part of the transparent electrode. And is composed of. The TCO film 103b covers substantially the entire upper surface of the p-type layer 102-3, and the TCO film 103a is formed on a part of the TCO film 103b. The metal film 104 is formed on the TCO film 103a. The other electrode is an n-electrode formed on the n-type layer 102-1, and is composed of a TCO film 105 in contact with the n-type layer 102-1, and a metal film 106 laminated thereon. Yes.

In a GaN-based LED element using a TCO film as a transparent electrode, the metal film provided as a bonding pad on the surface of the transparent electrode has a high light reflectivity on the back surface (the surface in contact with the transparent electrode). (Patent Document 1).
JP 2005-317931 A JP 2002-25349 A Japanese Journal of Applied Physics, Vol. 44, No. 4B, 2005, 2525-2527 (Japan Journal of Applied Physics, Vol. 44, No. 4B, 2005, pp. 2525-2527).

As described above, a TCO film having fine irregularities on the surface, such as an ITO film, has an effect of improving the output of the GaN-based LED element. However, the metal film formed on the surface of the TCO film is inferior in reflectivity on the back surface. This is because the back surface reflects the surface shape of the TCO film and has low smoothness.
Accordingly, the present invention provides an electrode for a GaN-based LED element in which a light reflection on the back surface of a metal film for a bonding pad is prevented from being lowered while forming a transparent electrode with a TCO film having fine irregularities on the surface. The main purpose is to provide

The present invention provides an electrode for a GaN-based LED device having the following characteristics:
A transparent electrode formed in contact with the surface of the GaN-based semiconductor film; and a metal film formed as a bonding pad on a part of the transparent electrode, wherein the transparent electrodes are electrically connected to each other. 1 TCO film and a second TCO film, both of the first TCO film and the second TCO film are exposed on the surface of the transparent electrode, and the first TCO film An electrode for a GaN-based LED element, wherein the surface of the GaN-based LED element is smoother than the surface of the second TCO film, and the metal film is formed on the surface of the first TCO film.

  In the electrode for a GaN-based LED element provided by the present invention, the transparent electrode formed on the surface of the GaN-based semiconductor film is composed of a first TCO film having a relatively high surface smoothness and a relatively low surface. A second TCO film having smoothness is formed, and a metal film for bonding pads is not formed on the surface of the second TCO film, but only on the surface of the first TCO film. For this reason, the smoothness of the back surface of the metal film is prevented from being lowered, and consequently the light reflectivity of the back surface of the metal film is prevented from being lowered. In addition, light incident on the surface of the transparent electrode from the inside of the element efficiently goes out of the element from the surface of the second TCO film having relatively low surface smoothness without being blocked by the metal film. Can do.

  Since the GaN-based LED element using the electrode for the GaN-based LED element provided by the present invention has excellent light emission output, it is suitably used in applications that require high output such as lighting applications. be able to.

(First embodiment)
FIG. 1 is a cross-sectional view showing one structural example of a GaN-based LED element using the electrode of the present invention. This GaN-based LED element has a substrate 11 made of sapphire and a GaN-based semiconductor film 12 formed thereon. In the GaN-based semiconductor film 12, an n-type layer 12-1 made of Si-doped GaN, a p-type layer 12-3 made of Mg-doped AlGaN, and an InGaN well layer and a GaN barrier layer sandwiched therebetween. And a light emitting layer 12-2 having a multiple quantum well structure. On the GaN-based semiconductor film 12, two electrodes are formed on the same surface side. One of them is a p-electrode formed on the p-type layer 12-3, and is a transparent electrode comprising a TCO film 13a ("first TCO film") and a TCO film 13b ("second TCO film"). 13. The p-electrode further has a metal film 14 formed as a bonding pad on a part of the transparent electrode 13. The second TCO film 13b substantially entirely covers the upper surface of the p-type layer 12-3, and the first TCO film 13a is formed on a part of the TCO film 13b. The metal film 14 is formed on the surface of the first TCO film 13a. Both the first TCO film 13a and the second TCO film 13b are exposed on the surface of the transparent electrode, but the exposed surface S13a (covered by the metal film 14) of the first TCO film 13a is 2 has higher smoothness than the exposed surface S13b of the TCO film 13b. The other electrode is an n-electrode formed on the n-type layer 12-1, and is composed of a TCO film 15 in contact with the n-type layer 12-1 and a metal film 16 laminated thereon. .

The GaN-based LED element shown in FIG. 1 can be manufactured, for example, by the following method.
First, a buffer layer (not shown), an n-type layer 12-1, a light emitting layer 12-2, and a p-type layer 12-3 are sequentially formed on a sapphire substrate 11 having a diameter of 2 inches by using the MOVPE method. Then, an epitaxial wafer whose cross section is shown in FIG. 2 is manufactured.

  Next, as shown in FIG. 3, a second TCO film 13b made of ITO is formed on the upper surface of the p-type layer 12-3 of the semiconductor wafer by using an electron beam evaporation method. Irregularities based on the polycrystalline structure are spontaneously formed on the surface of the second TCO film 13b.

  Next, as shown in FIG. 4, the second TCO film 13b is patterned into a predetermined shape by using a photolithography technique.

  Next, part of the p-type layer 12-3 and the light emitting layer 12-2 is removed by RIE (reactive ion etching), and the n-type layer 12-1 is partially exposed as shown in FIG.

  Next, a lift-off mask having an opening of a predetermined shape is formed at a predetermined position on the second TCO film 13b and the exposed surface of the n-type layer 12-1, and then a sputtering method is used. Then, an ITO film to be the first TCO film 13a and the TCO film 15 is formed. Subsequently, a metal film is formed on the ITO film. When the mask is lifted off after the formation of the metal film, as shown in FIG. 6, a laminated film composed of the first TCO film 13 a and the metal film 14 is formed on the second TCO film 13 b and the n-type layer 12. A structure in which a laminated film (n electrode) composed of the TCO film 15 and the metal film 16 is formed on the exposed surface of −1 is obtained. By using the lift-off method, the planar shape and size of the TCO film and metal film included in each laminated film are the same.

  Here, the reason why the sputtering method is used as the film formation method of the first TCO film 13a while the electron beam evaporation method is used as the film formation method of the second TCO film 13b is that in the case of the ITO film, the electron beam evaporation method is used. This is because the surface formed by the sputtering method is smoother than the film formed by the sputtering method (Patent Document 2).

  When the formation of the p-electrode and the n-electrode is completed, the GaN-based LED element formed on the wafer is separated into chips using a method usually used in this field. Note that before the wafer is cut, the exposed portions of the GaN-based semiconductor film 12 and the TCO film 13b may be covered with a transparent insulating protective film made of a metal oxide such as silicon oxide.

  As mentioned above, although an example of the manufacturing method of the GaN-type LED element shown in FIG. 1 was demonstrated, the manufacturing method of this LED element is not limited to the method demonstrated above.

  In order to make the surface smoothness of the first TCO film 13a higher than that of the second TCO film 13b, the second TCO film may be a polycrystalline film and the first TCO film may be an amorphous film. An example of a TCO film that easily becomes an amorphous film is an IZO (indium zinc oxide) film. A preferred method for forming the IZO film is sputtering.

  In order to make the smoothness of the surface of the second TCO film 13b particularly low, the surface of the second TCO film 13b can be roughened by etching after film formation. Thereby, irregular reflection of light occurs on the surface of the second TCO film 13b, and the light extraction efficiency of the LED element is improved. In the case of a polycrystalline TCO film, when a liquid capable of dissolving the TCO film is brought into contact, the crystal grain boundary is etched quickly, so that the surface becomes rough. In the case of an ITO film, an acid such as hydrochloric acid can be used as such a liquid. As another method for roughening the surface of the TCO film, there is a method in which fine particles of metal or polymer are attached on the surface of the film, and a dry etching process using the fine particles as an etching mask is performed. For a specific procedure for roughening the surface of the ITO film by this method, for example, Non-Patent Document 1 can be referred to.

  In a preferred embodiment, as shown in FIG. 7, the second TCO film 13 b is removed from between the metal film 14 and the GaN-based semiconductor film 12. In other words, a portion is provided in which the front surface of the GaN-based semiconductor film 12 and the back surface of the metal film 14 face each other with only the first TCO film 13a interposed therebetween. By doing so, the back surface of the metal film 14 can be made smoother. This is because the surface of the first TCO film 13a is smoother when formed directly on the GaN-based semiconductor film 12 having excellent surface smoothness than when it is formed via the second TCO film 13b. is there. In the example of FIG. 7, a part of the second TCO film 13b is removed from between the metal film 14 and the GaN-based semiconductor film 12, but the entire second TCO film 13b may be removed.

(Second Embodiment)
FIG. 8 is a drawing showing another structural example of a GaN-based LED element using the electrode of the present invention, FIG. 8 (a) is a plan view, and FIG. 8 (b) is a PQ of FIG. 8 (a). It is sectional drawing in the position of a line. In the GaN-based LED element shown in this figure, not only the p-electrode but also the n-electrode has a transparent electrode composed of the TCO film 15a and the TCO film 15b. The n-electrode further has a metal film 16 formed as a bonding pad on a part of the transparent electrode. The TCO film 15a has higher surface smoothness than the TCO film 15b, and the metal film 16 is formed on the surface of the TCO film 15a.

  In the preferable manufacturing method of the GaN-based LED element shown in FIG. 8, after the formation of the epi-wafer, first, a part of the p-type layer 12-3 and the light emitting layer 12-2 is removed by RIE, and the n-type layer 12-1 Is partially exposed. Thereafter, the formation of the TCO film 13b on the p-type layer 12-3 and the formation of the TCO film 15b on the exposed surface of the n-type layer 12-1 are simultaneously performed. Preferably, after the TCO film is formed on the entire surface of the epi-wafer, unnecessary portions are removed, thereby leaving the TCO film 13b on the p-type layer 12-3 and the TCO film on the exposed surface of the n-type layer 12-1. Leave 15b. The formation of the TCO film 13a and the metal film 14 on the TCO film 13b and the formation of the TCO film 15a and the metal film 16 on the TCO film 15b are performed by the lift-off method as in the first embodiment. Can be used.

(Third embodiment)
FIG. 9 is a sectional view showing another structural example of the GaN-based LED element using the electrode of the present invention. This GaN-based LED element has a vertical element structure, and one electrode is formed on the back surface of the substrate 11. The substrate 11 is a conductive substrate made of SiC, GaN, ZnO or the like, and a GaN-based semiconductor film 12 is formed thereon. In the GaN-based semiconductor film 12, an n-type layer 12-1 made of Si-doped GaN, a p-type layer 12-3 made of Mg-doped AlGaN, and an InGaN well layer and a GaN barrier layer sandwiched therebetween. And a light emitting layer 12-2 having a multiple quantum well structure. A p-electrode is formed on the p-type layer 12-3. The p-electrode has a transparent electrode 13 composed of a TCO film 13a and a TCO film 13b. The p-electrode further has a metal film 14 formed as a bonding pad on a part of the transparent electrode. The TCO film 13b covers substantially the entire upper surface of the p-type layer 12-3, and the TCO film 13a is formed on a part of the TCO film 13b. The TCO film 13a has higher surface smoothness than the TCO film 13b, and the metal film 14 is formed on the surface of the TCO film 13a. Further, in order to further increase the smoothness of the back surface of the metal film 14, a portion where the GaN-based semiconductor film 12 and the metal film 14 face each other with only the TCO film 13a interposed therebetween is provided. On the back surface of the substrate 11, an n-electrode composed of a TCO film 15 in contact with the back surface and a metal film 16 stacked thereon is formed.

(Preferred embodiment)
In the electrode of the present invention, the thickness of the transparent electrode formed by TCO is not particularly limited, but may be, for example, 0.1 μm to 1 μm. Preferably, it is 0.2 micrometer-0.5 micrometer. The transparent electrode is preferably formed so that the transmittance at the wavelength of the light emitted from the light emitting layer is 80% or more. This transmittance is more preferably 85% or more, and still more preferably 90% or more. The TCO film used for the transparent electrode is preferably formed so that the resistivity is 1 × 10 ⁻⁴ Ωcm or less. Examples of TCO film formation methods include sputtering, reactive sputtering, vacuum deposition, ion beam assisted deposition, ion plating, laser ablation, CVD, spraying, spin coating, and dipping. Is done.

  In the electrode of the present invention, the material of the metal film provided as a bonding pad on the TCO film is not particularly limited, but preferably, at least a portion in contact with the TCO film is a platinum group (Rh, Pt, Pd, Ir, Ru, Os). , Ni (nickel), Ti (titanium), W (tungsten), Ag (silver), Al (aluminum), or the like. On the back side of the metal film, it is preferable to provide a reflective layer composed mainly of Al, Ag, or a platinum group element, which is a metal having a good light reflectance. When such a reflective layer is provided, a light-transmitting property made of Ni, Ti, W, Ti—W, or the like is provided between the reflective layer and the TCO film so that the adhesive force between the metal film and the TCO film does not decrease. The thin film may be sandwiched. This thin film is formed so that the film thickness is 20 nm or less, more preferably 10 nm or less, and still more preferably 5 nm or less. The surface layer of the metal film is made of Ag (silver), Au (gold), Sn (tin), In (indium), Bi (bismuth), Cu (copper), Zn (so that it can be easily connected to the external electrode. Zinc) or the like is preferable. The film thickness of the metal film can be, for example, 0.2 μm to 10 μm, and preferably 0.5 μm to 2 μm. For forming the metal film, a vacuum deposition method, a sputtering method, a CVD method, or the like can be used.

  In the electrode of the present invention, the planar shape of the metal film provided as a bonding pad on the TCO film is not particularly limited, but is preferably a circular or regular polygon. The size is not particularly limited, but from the viewpoint of workability at the time of mounting, it is desirable to form a size including a circle having a diameter of 60 μm.

  The present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the spirit of the invention.

It is sectional drawing which shows the structural example of the GaN-type LED element using the electrode of this invention. It is sectional drawing for demonstrating the manufacturing process of the GaN-type LED element shown in FIG. It is sectional drawing for demonstrating the manufacturing process of the GaN-type LED element shown in FIG. It is sectional drawing for demonstrating the manufacturing process of the GaN-type LED element shown in FIG. It is sectional drawing for demonstrating the manufacturing process of the GaN-type LED element shown in FIG. It is sectional drawing for demonstrating the manufacturing process of the GaN-type LED element shown in FIG. It is sectional drawing which shows the structural example of the GaN-type LED element using the electrode of this invention. It is a figure which shows the structural example of the GaN-type LED element using the electrode of this invention, Fig.8 (a) is a top view, FIG.8 (b) is sectional drawing in the position of the PQ line of Fig.8 (a). It is. It is sectional drawing which shows the structural example of the GaN-type LED element using the electrode of this invention. It is sectional drawing which shows the structure of the conventional GaN-type LED element.

Explanation of symbols

11 substrate 12 GaN-based semiconductor film 12-1 n-type layer 12-2 light emitting layer 12-3 p-type layer 13 transparent electrode 13a TCO film 13b TCO film 14 metal film 15 TCO film 16 metal film

Claims (5)

A transparent electrode formed in contact with the surface of the GaN-based semiconductor film;
A metal film formed as a bonding pad on a part of the transparent electrode,
The transparent electrode is composed of a first TCO film and a second TCO film electrically connected to each other,
The first TCO film and the second TCO film are both exposed on the surface of the transparent electrode,
The surface of the first TCO film is smoother than the surface of the second TCO film;
The metal film is formed on the surface of the first TCO film;
An electrode for a GaN-based LED element. The electrode according to claim 1, wherein the first TCO film and the metal film have the same planar shape and size. 3. The electrode according to claim 1, wherein a surface of the GaN-based semiconductor film and a back surface of the metal film have a portion facing each other with only the first TCO film interposed therebetween. A GaN-based LED element having the electrode according to claim 1 as a p-electrode. A planar GaN-based LED element, wherein the electrode according to claim 1 is used as a p-electrode and an n-electrode on the same surface side of the LED element.

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