Semiconductor light emitting element and method for manufacturing same

A semiconductor light emitter (A) includes an n-type semiconductor layer (2), a p-type semiconductor layer (4), and an active layer (3) between these two layers (2, 4). The light emitter (A) further includes an n-side electrode (5) on the n-type layer (2) and a p-side electrode (6) on the p-type layer (4). An insulating layer (7) covers the n-type and p-type layers (2),(4), while also partially covering the n-side and p-side electrodes (5),(6), leaving part of the electrodes (5, 6) exposed. The n-side electrode (5) has a first Al layer (51) formed on the n-type layer (2) and a second Ni, W, Zr or Pt layer (52) formed on the first layer (51). The p-side electrode (6) has a first Au layer (61) formed on the p-type layer (4), and a second Ni, W, Zr or Pt layer (62) formed on the first layer (61).

TECHNICAL FIELD

The present invention relates to a semiconductor light emitting element and a method for manufacturing the same.

BACKGROUND ART

Semiconductor lasers or light emitting diodes are examples of conventional semiconductor light emitting elements (see e.g. Patent Document 1 below). Semiconductor light emitting elements can provide in general high luminance with less power consumption, thereby making a suitable light source for a liquid crystal display device, for example.Patent Document 1: JP-A-2003-243773

FIG. 10shows a conventional semiconductor light emitting element. The illustrated light emitting element X includes a substrate101, on which an n-GaN layer102, an active layer103and a p-GaN layer104are laminated. The n-GaN layer102and the p-GaN layer104are covered by an insulating layer107. The insulating layer107is made of e.g. SiO2and formed with two openings107a. The openings107aexpose part of the n-GaN layer102and p-GaN layer104. A wiring108is connected to each of the n-GaN layer102and the p-GaN layer104via the opening107a. The wiring108includes an Ni layer108acontacting the n-GaN layer102or the p-GaN layer104, and an Au layer108bformed on the Ni layer108.

The light emitting element X has the following problems. In manufacturing the light emitting element X, the openings107aare formed by etching the insulating layer covering the n-GaN layer102and the p-GaN layer104. This etching damages the n-GaN layer102and the p-GaN layer104. As a result, the electrical resistance at the interface between each of these layers and the wiring108increases, so that the drive voltage of the light emitting element X becomes unduly high.

As one of the measures against this problem, it may be considered to form a relay electrode (not shown) made of e.g. Au in each of the openings107aand then form the wiring108. In this case, good electrical conduction is expected to be established between the n-GaN layer102or the p-GaN layer104and the wiring108by the relay electrode.

However, this arrangement causes another problem. That is, when the wiring108is formed at a high ambient temperature, Au may diffuse from the relay electrode into the insulating layer107. As a result, at least part of the insulating layer107becomes conductive, which increases the leakage current.

DISCLOSURE OF THE INVENTION

The present invention has been proposed under the circumstances described above. It is, therefore, an object of the present invention to provide a semiconductor light emitting element which is capable of preventing an increase in drive voltage and suppressing leakage current.

According to a first aspect of the present invention, there is provided a semiconductor light emitting element comprising an n-type semiconductor layer, a p-type semiconductor layer, an active layer disposed between the n-type semiconductor layer and the p-type semiconductor layer, an n-side electrode contacting the n-type semiconductor layer, a p-side electrode contacting the p-type semiconductor layer, and an insulating layer covering the n-type semiconductor layer and the p-type semiconductor layer in a manner such that part of the n-side electrode and part of the p-side electrode is exposed. The n-side electrode comprises a first layer made of Al and contacting the n-type semiconductor layer, and a second layer formed on the first layer and made of any one of Ni, W, Zr and Pt. The p-side electrode comprises a first layer made of Au and contacting the p-type semiconductor layer, and a second layer formed on the first layer and made of any one of Ni, W, Zr and Pt.

This arrangement provides the following technical effects. In the process of etching the insulating layer after the n-side electrode and the p-side electrode are formed, only the second layer of each electrode is subjected to the etching. Since the second layer is made of Ni, W, Zr or Pt, the surface is not considerably damaged by the etching. Thus, the resistance at the interface between each electrode and the member electrically connected to the electrode is relatively low. Thus, the semiconductor light emitting element can be driven at a relatively low voltage. Further, in each of the electrodes, the upper surface of the first layer can be entirely covered by the second layer. In this case, of each electrode, the portion which is in contact with the insulating layer mostly comprises the second layer. (For instance, this state can be provided by making the first layer appropriately thin.) Ni, W, Zr or Pt which forms the second layer is unlikely to diffuse into the insulating layer, as compared with Au. Thus, the insulating layer7does not unduly become conductive, whereby leakage current in the semiconductor light emitting element is prevented.

Preferably, the semiconductor light emitting element further comprises a wiring contacting the n-side electrode (or the p-side electrode). The wiring includes a first layer, and a second layer formed on the first layer. The first layer of the wiring is in contact with the second layer of the n-side electrode (or the p-side electrode) and made of the same material as that of the second layer of the n-side electrode (or the p-side electrode). The second layer of the wiring is made of Au. With this arrangement, the n-side electrode or the p-side electrode and the wiring are bonded to each other at the portions made of the same material. With this arrangement, the resistance at the interface between each electrode and the wiring is lower as compared with the structure in which different kinds of metals are bonded to each other.

According to a second aspect of the present invention, there is provided a method for manufacturing a semiconductor light emitting element comprising an n-type semiconductor layer, a p-type semiconductor layer, and an active layer disposed between the n-type semiconductor layer and the p-type semiconductor layer. The method comprises the steps of forming an n-side electrode in contact with the n-type semiconductor layer, forming a p-side electrode in contact with the p-type semiconductor layer, forming an insulating layer covering the n-type semiconductor layer, the p-type semiconductor layer, the n-side electrode and the p-side electrode, and etching the insulating layer to expose part of the n-side electrode and part of the p-side electrode. The formation of the n-side electrode is performed by forming a first layer of Al on the n-type semiconductor layer and forming a second layer of any one of Ni, W, Zr and Pt on the first layer. The formation of the p-side electrode is performed by forming a first layer of Au on the p-type semiconductor layer and forming a second layer of any one of Ni, W, Zr and Pt on the first layer.

With this arrangement, only the second layer of each of the n-side electrode and the p-side electrode is subjected to etching. Since the second layer is made of Ni, W, Zr or Pt, the surface is not considerably damaged by the etching. Thus, the resistance at the interface between each of the n-side electrode and the p-side electrode and the member electrically connected to the electrode is relatively low. This makes it possible to drive the semiconductor light emitting element at a relatively low voltage.

Other features and advantages of the present invention will become more apparent from the detailed description given below with reference to the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1shows an example of semiconductor light emitting element according to the present invention. The illustrated semiconductor light emitting element A is formed on a substrate1and includes an n-GaN layer2, an active layer3, a p-GaN layer4, an n-side electrode5, a p-side electrode6, an insulating layer7and a wiring8. The semiconductor light emitting element A is designed to emit light through the insulating layer7.

The substrate1is made of e.g. sapphire and supports the n-GaN layer2, the active layer3, the p-GaN layer4and so on. The substrate1has a thickness of e.g. about 350 μm.

The n-GaN layer2is a layer formed by doping Si into GaN. The n-GaN layer2has a thickness of e.g. about 3.5 μm. A buffer layer21and an undoped GaN layer22are laminated between the substrate1and the n-GaN layer2. The buffer layer21and the undoped GaN layer22serve to alleviate lattice deformation between the substrate1and the n-GaN layer2. The thicknesses of the buffer layer21and the undoped GaN layer22are about 0.05 μm and 2.0 μm, respectively.

The active layer3has a multiple quantum well (MQW) structure. In the active layer3, electrons and holes are recombined to emit light, and the light is amplified. In this process, the electrons are supplied from the n-side electrode5, whereas the holes are supplied from the p-side electrode6. The active layer3is made up of a plurality of InGaN layers and a plurality of GaN layers which are alternately laminated. The In composition ratio in each of the InGaN layers is e.g. about 15%. Thus, the band gap of the InGaN layer is smaller than that of the n-GaN layer2, so that the InGaN layer functions as a well layer of the active layer3. Each of the GaN layers is a barrier layer of the active layer3. The number of the InGaN layers and the GaN layers constituting the active layer3is in the range of 3 to 7, for example. The active layer3has a thickness of e.g. about 0.1 μm.

The p-GaN layer4is a layer formed by doping Mg into GaN. The p-GaN layer4has a thickness of e.g. about 0.1 μm.

The n-side electrode5is formed on the n-GaN layer2. The n-side electrode5serves to supply electrons to the active layer3and has a laminated structure made up of an Al layer (first layer)51and an Ni layer (second layer)52. The Al layer51is in contact with the n-GaN layer2and has a thickness of about 4000 Å. The Ni layer52is formed on the Al layer51and has a thickness of about 500 Å. Instead of the Ni layer52, a layer made of W, Zr or Pt may be employed as the second layer.

The p-side electrode6is formed on the p-GaN layer4. The p-side electrode6serves to supply holes to the active layer3and is made up of an Au layer (first layer)61and an Ni layer (second layer)62. The Au layer61is in contact with the p-GaN layer4and has a thickness of about 4000 Å. The Ni layer62is formed on the Au layer61and has a thickness of about 500 Å. Preferably, the Ni layer62covers the entirety of the upper surface of the Au layer61. Instead of the Ni layer62, a layer made of W, Zr or Pt may be employed as the second layer. In the present invention, it is preferable that the second layer of the n-side electrode5and the second layer of the p-side electrode6are made of the same material.

The insulating layer7is made of e.g. SiO2and covers the n-GaN layer2and the p-GaN layer4. The insulating layer7is formed with two openings7a. The n-side electrode5or the p-side electrode6is provided in each of the openings7a, so that the upper surface of each electrode is exposed at the insulating layer7.

The wiring8is provided for electrically connecting the semiconductor light emitting element A to an adjacent semiconductor light emitting element or non-illustrated terminal, for example. The wiring8is made up of an Ni layer (first layer)81and an Au layer (second layer)82. The Ni layer81is in contact with the Ni layer52of the n-side electrode5or the Ni layer62of the p-side electrode6and made of the same material as that of the Ni layers52,62. The Au layer82is formed on the Ni layer81. The thicknesses of the Ni layer81and the Au layer82are about 500 Å and 8000 Å, respectively. It is preferable that the material of the first layer of the wiring8is the same as that of the second layer of the p-side electrode5or n-side electrode6which is in contact with the wiring.

A method for manufacturing the semiconductor light emitting element A will be described below with reference toFIGS. 2-5.

First, as shown inFIG. 2, a buffer layer21, an undoped GaN layer22, an n-GaN layer2, an active layer3and a p-GaN layer4are laminated on a substrate1. For instance, these layers are formed by metal-organic chemical vapor deposition (MOCVD).

Then, as shown inFIG. 3, an n-side electrode5and a p-side electrode6are formed. Specifically, an Al layer51, an Au layer61and Ni layers52,53are formed by e.g. vapor deposition and lift-off. In this process, the Al layer51and the Au layer61are formed to have a thickness of 4000 Å, whereas the Ni layers52,53are formed to have a thickness of 500 Å.

Then, as shown inFIG. 4, an insulating layer7A is formed to cover the n-GaN layer2, the p-GaN layer4, the n-side electrode5and the p-side electrode6. Specifically, the insulating layer7A is formed by vapor deposition using SiO2, for example.

Then, as shown inFIG. 5, two openings7aare formed by etching the insulating layer7A via a mask (not shown) formed by e.g. photolithography. This etching may be ion etching. Specifically, for example, the ion etching is performed by supplying CF4as the etching gas at the flow rate of 40 sccm (volume flow rate (cc=cm3) per minute in a standard state) under the pressure of about 3.0 Pa and at a high frequency power of about 100 W. By forming the two openings7a, the Ni layer52of the n-side electrode5and the Ni layer62of the p-side electrode6are exposed. Thus, the formation of the insulating layer7is completed.

Thereafter, a wiring8having a laminated structure made up of an Ni layer81with a thickness of about 500 Å and an Au layer82with a thickness of about 8000 Å is formed by e.g. vapor deposition and lift-off. Thus, the semiconductor light emitting element A is obtained.

The advantages of the semiconductor light emitting element A will be described below.

FIG. 6shows the forward current If relative to the forward voltage Vf, which was measured with respect to the semiconductor light emitting element A and comparative examples 1, 2. In this figure, the graph GAindicates the measurements of the semiconductor light emitting element A, whereas the graphs GXand GYindicate the measurements of the comparative example 1 and the comparative example 2, respectively. The structure of the comparative example 1 is the same as that of the semiconductor light emitting element X shown inFIG. 10. The structure of the comparative example 2 is basically similar to that of the comparative example 1 (semiconductor light emitting element X) but differs from the comparative example 1 in that a relay electrode is provided in each of the openings107a. Each of the relay electrodes has a two-layer structure made up of a lower layer made of Ni and an upper layer made of Au. The lower layer is in contact with the layer102or104of the semiconductor light emitting element X, whereas the upper layer is in contact with the wiring108.

First, the graph GA(semiconductor light emitting element A) and the graph GX(comparative example 1) will be compared. For instance, to obtain the forward current If of about 1.0×10−5A, which is a standard for achieving proper light emission in industrial use, the comparative example 1 requires the forward voltage Vf of about 12V. As compared to this, the semiconductor light emitting element of the present invention, only requires the forward voltage Vf of about 7V. That is, according to the present invention, the drive voltage can be reduced as compared with the comparative example 1. The reason for this is considered to be as follows. As shown inFIG. 10, in the comparative example 1, the wiring108is directly bonded to the n-GaN layer102and the p-GaN layer104. Generally, the openings107aprovided at the location of this bonding are formed by etching the insulating layer107. By this etching process, the surfaces of the n-GaN layer102and p-GaN layer104are damaged. Since the wiring8is formed on the damaged surfaces, the electrical resistance at the interface between each of the n-GaN layer102and the p-GaN layer104and the wiring108is relatively high. As a result, a high drive voltage is required. On the other hand, as will be understood fromFIG. 5, the n-GaN layer2and the p-GaN layer4of the present invention are not subjected to etching. Further, although the Ni layers52and62are subjected to etching, the etching speed is relatively low, so that the surfaces are unlikely to be damaged. As a result, the drive voltage of the semiconductor light emitting element A is relatively low.

Next, the graph GA(semiconductor light emitting element A) and the graph GY(comparative example 2) will be compared. In the comparative example 2, the forward current If of about 1.0×10−7A flows even at a relatively low forward voltage Vf of not more than 1.0 V. By the studies performed by the inventors, it has been found that this current is leakage current and hardly contributes to the light emission in the active layer103(seeFIG. 10). The cause of the leakage current is considered to be as follows. As noted before, in the comparative example 2, a relay electrode is provided in each opening107a, and the upper layer of the relay electrode is made of Au. Au diffuses into the insulating layer107at a temperature of about 600° C. As a result, part of the insulating layer107becomes conductive, whereby leakage current flows. Unlike this, of the n-side electrode5and the p-side electrode6of the semiconductor light emitting element A, the portions which are in contact with the insulating layer7are mostly the Ni layers52and62. Ni is unlikely to diffuse into the insulating layer7made of SiO2. Thus, the insulating layer7does not unduly become conductive, whereby leakage current is prevented.

FIG. 7shows the forward current If relative to the forward voltage Vf (Vf-If characteristic) of the semiconductor light emitting element A, which was measured a plurality of times. Specifically, with measurement probes fixed to predetermined positions of the n-side electrode5and the p-side electrode6, the forward current If was measured while changing the forward voltage Vf (first measurement). Then, after the positions of the probes were changed, the measurement was performed in the same manner (second measurement). Then, after the positions of the probes were changed again, the measurement was performed in the same manner (third measurement). As shown in the figure, there are almost no variations in the three measurement results of the Vf-If characteristic. The same measurement was performed with respect to the comparative example 2, and the results are shown inFIG. 8. In the comparative example 2, the Vf-If characteristic varied largely when the positions of the measurement probes were changed. Conceivably, the reason for this difference is that the Ni layers52and62of the semiconductor light emitting element A are hardly damaged by etching, and hence, the surfaces of the p-side electrode5and the n-side electrode6are entirely smooth.

The bonding of the n-side electrode5and the p-side electrode6to the wiring8is achieved by the bonding of the Ni layers52,62to the Ni layer81. Such bonding of the members made of the same material is suitable for reducing the resistance at the bonding portions, as compared with the bonding of the members made of different kinds of metals, and hence, advantageous for reducing the drive voltage of the semiconductor light emitting element A.

The Al layer51of the n-side electrode5can easily form an ohmic contact with the n-GaN layer2. Further, the Au layer61of the p-side electrode6can easily form an ohmic contact with the p-GaN layer4. This is also advantageous for reducing the drive voltage of the semiconductor light emitting element A.

FIG. 9shows an example of illuminator using the semiconductor light emitting element A. In this illuminator, a plurality of semiconductor light emitting elements A are arranged in a matrix. In this figure, the illustration of the substrate1and the insulating substrate7shown inFIG. 1is omitted. Adjacent ones of the semiconductor light emitting elements A arranged in a matrix are connected to each other via a wiring8. In this embodiment, the n-side electrode5of each semiconductor light emitting element A is connected to the p-side electrode6of the adjacent semiconductor light emitting element A. Thus, the semiconductor light emitting elements A are connected in series to each other. The illuminator having this structure is capable of performing surface emission at a low drive voltage, while preventing leakage current.