Electrode structure, and semiconductor light-emitting device having the same

Abstract

A semiconductor light emitting device including: a transparent substrate; an electron injection layer which is formed on the transparent substrate; an active layer which is formed on a first region of the electron injection layer; a hole injection layer which is formed on the active layer; a first electrode structure which is formed on the hole injection layer and concurrently provides a high reflectivity and a low contact resistance; a second electrode structure which is formed on a second region of the electron injection layer; and a circuit substrate which is electrically connected with the first and second electrode structures, the first electrode structure includes: a contact metal structure which has any one selected from the group consisting of nickel, palladium, platinum and ITO (Indium Tin Oxide) that have low contact resistance; and a reflective layer which has aluminum or silver.

Description

BACKGROUND OF THE INVENTION

This application claims the priority of Korean Patent Application No. 2003-75220, filed on Oct. 27, 2003, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

1. Field of the Invention

The present invention relates to a semiconductor light-emitting device using a nitride semiconductor or a like material, and more particularly, to a high reflective electrode structure concurrently satisfying a low contact resistance and a high reflectivity, and a flip-chip light emitting device having the same.

2. Description of the Related Art

A nitride-based compound semiconductor, which is generally used for a visible light emitting device, is being currently advanced to an ultraviolet light region for a white light emitting device going through a visible light region of blue and green. The nitride-based compound semiconductor is mainly classified into a structure of extracting an upward light from light emitting from the active layer, and a structure of extracting a downward light passing through a transparent substrate such as a sapphire substrate.

In a flip-chip light emitting device having the structure of extracting light through the transparent substrate, reflectivity at an interface of a P-type electrode is of importance to again reflect the upward light to direct downward.

In the meantime, it is advantageous that a light emitting device has a low operation voltage. At present, the most general method for lowering an operation voltage of the light emitting device decreases resistance of a material layer formed between an electrode layer and an active layer. Especially, since a hole injection layer and a P-type electrode are Ohmic contacted each other in the flip-chip light emitting device, it is very desirable that the hole injection layer and the P-type electrode have low Ohmic contact resistance formed therebetween so as to reduce the operation voltage.

FIG. 1 is a schematic sectional view illustrating a conventional nitride semiconductor light-emitting device.

As illustrated in FIG. 1, the conventional flip-chip nitride semiconductor light emitting device 10 includes a sapphire substrate 11; an N-type GaN layer 12 sequentially formed on the sapphire substrate 11; an active layer 16 formed of InGaN; a P-type GaN layer 18; a nickel layer 20; a P-type reflective electrode 22; and an N-type electrode 14 formed on one side surface of the N-type GaN layer 12. The light emitting device 10 has a dual hetero structure where the N-type GaN layer 12 functions as a cladding layer for a first conductive type, and the P-type GaN layer 18 functions as a cladding layer for a second conductive type.

Further, the nickel layer 20 is formed on the P-type GaN layer 18 to have a thickness of below about 10 nm, and functions as a contact metal layer for forming the Ohmic contact. Since the P-type reflective electrode 22 is formed of aluminum (Al) or silver (Ag), light transmitting the nickel layer 20 that is the contact metal is reflected at an interface between the P-type reflective electrode 22 and the nickel layer 20.

The conventional light emitting device 10 can directly extract light from the P-type reflective electrode that is formed of material such as aluminum (Al) or silver (Ag) with a high reflectivity, and can obtain a high efficiency of light extraction. However, the conventional light emitting device has a disadvantage in which contact resistance is increased when the P-type reflective electrode 22 with the high reflectivity is directly employed. Accordingly, the nickel layer 20 is formed as the contact metal for forming the Ohmic contact, thereby reducing the contact resistance.

However, in the flip-chip nitride semiconductor light emitting device 10 having the nickel layer 20 as the contact metal, since light emitting from the active layer 16 formed of InGaN passes through the nickel layer 20, and then is reflected at the interface of the nickel layer 20 and the P-type reflective electrode 22, and then again passes through the nickel layer 20 and the sapphire substrate 11 for emission, a large amount of light is absorbed by the nickel layer 20. Therefore, the conventional flip-chip nitride semiconductor light emitting device 10 has a drawback in that it is very difficult to increase the reflectivity.

In other words, since the nickel layer 20, which is the contact metal, is used to be in reliable contact with the P-type GaN layer 18, the thicker nickel layer 20 can provide a better contact with the P-type GaN layer 18. However, if the nickel layer 20 has a thickness of above 10 nm, it is difficult to have enough reflectivity.

Accordingly, the semiconductor light emitting device is required to have a reflection structure for maintaining the high reflectivity while maintaining the low contact resistance.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor light-emitting device having a P-type electrode structure concurrently satisfying a low contact resistance and a high reflectivity.

Further, the present invention provides an electrode structure concurrently satisfying a low contact resistance and a high reflectivity in a semiconductor light-emitting device.

According to an aspect of the present invention, there is provided a semiconductor light emitting device including: a transparent substrate; an electron injection layer which is formed on the transparent substrate; an active layer which is formed on a first region of the electron injection layer; a hole injection layer which is formed on the active layer; a first electrode structure which is formed on the hole injection layer and concurrently provides a high reflectivity and a low contact resistance; a second electrode structure which is formed on a second region of the electron injection layer; and a circuit substrate which is electrically connected with the first and second electrode structures, wherein the first electrode structure includes: a contact metal structure which has any one selected from the group consisting of nickel, palladium, platinum and ITO (Indium Tin Oxide) that have low contact resistance; and a reflective layer which has aluminum or silver.

According to another aspect of the present invention, there is provided an electrode structure including: a transparent substrate; an electron injection layer which is formed on the transparent substrate; a contact metal structure which has any one selected from the group consisting of nickel, palladium, platinum and ITO that have low contact resistance to be used in a semiconductor light emitting device having an active layer and a hole injection layer; and a reflective layer having the high reflectivity such as aluminum or silver.

Here, the contact metal structure may be island-type or mesh-type.

An area ratio of the contact metal structure to the reflective layer may be from 1% to 90%, and the thickness of the contact metal structure is less than 200 nm.

Further, the reflective layer may be formed of aluminum (Al) or silver (Ag) that has the high reflectivity.

Furthermore, the transparent substrate may be formed of sapphire or silicon carbide.

Also, the electron injection layer may be formed of N-type GaN, the active layer may be formed of InGaN, and the hole injection layer may be formed of P-type GaN.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

FIG. 1 is a schematic sectional view illustrating a conventional nitride semiconductor light-emitting device;

FIG. 2 is a sectional view illustrating a semiconductor light-emitting device according to a preferred embodiment of the present invention;

FIGS. 3A and 3B are sectional views illustrating an electrode structure used in a semiconductor light-emitting device of FIG. 2 according to a preferred embodiment of the present invention;

FIGS. 4A through 4F are plane views illustrating electrode structures depending on varied area ratios of a palladium layer to a silver layer according to the present invention; and

FIGS. 5A and 5B are graphs illustrating the correlation of light emission and respective meshed regions shown in FIGS. 4A through 4F, and the correlation of operation voltage and the respective meshed regions in a semiconductor light-emitting device.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. In the drawings, the thicknesses of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present.

FIG. 2 is a sectional view illustrating a semiconductor light-emitting device according to a preferred embodiment of the present invention.

As shown in FIG. 2, the semiconductor light-emitting device 100 includes a transparent substrate 102 formed of transparent material such as sapphire (Al₂O₃) or silicon carbide (SiC); an electron injection layer 104 formed of an N-type GaN on the transparent substrate 102; an active layer 106 formed of InGaN; and a hole injection layer 108 formed of a P-type GaN. The electron injection layer 104 includes a first portion and a second portion, and step-shaped with the first portion thinner than the second portion. The active region 106 and the hole injection layer 108 are formed on the second portion.

Further, the semiconductor light-emitting device 100 further includes a P-type electrode structure 110 also functioning as a reflective layer formed on the hole injection layer 108 according to a preferred embodiment of the present invention, the P-type electrode structure 110 may include a contact metal structure functioning as a contact metal forming Ohmic contact to reduce contact resistance; and a reflective layer formed of metal such as silver (Ag) or aluminum (Al) with the high reflectivity.

FIGS. 3A and 3B are sectional views illustrating an electrode structure used in the semiconductor light-emitting device of FIG. 2 according to a preferred embodiment of the present invention.

Referring first to FIG. 3A, the P-type electrode structure 110 according to a first embodiment of the present invention is formed on the hole injection layer 108, and includes a contact metal structure 110A that functions as the contact metal forming the Ohmic contact to reduce the contact resistance; and a reflective layer 110B. Additionally, the contact metal structure 110A may be formed of any one selected from the group consisting of nickel (Ni), palladium (Pd), platinum (Pt) or indium tin oxide (ITO) that have a low contact resistance. The reflective layer 110B may be formed of metal having the high reflectivity such as aluminum (Al) or silver (Ag).

Further, the P-type electrode structure 110 according to a preferred embodiment of the present invention performs a function of uniformly distributing current, which is applied from a circuit board assembled later on, in the hole injection layer 108, as well as a function of contact.

In the meantime, in the active layer 106, electrons injected from the electron injection layer 104 are combined with holes injected from the hole injection layer 108. The combined electrons and holes fall to a low energy band to cause light emission. At this time, the emitting light is reflected at an interface between the reflective layer 110B and the contact metal structure 110A of the P-type electrode structure 110 and at an interface between the reflective layer 110B and the hole injection layer 108. The reflected light sequentially goes through the hole injection layer 108, the active layer 106, the electron injection layer 104 and the transparent substrate 102 while emitting in the direction of an arrow of FIG. 2.

The contact metal structure 110A is island-shaped, and the reflective layer 110B covers the resultants including the hole injection layer 108 and the contact metal structure 110A. Though the contact metal structure 110A is rectangular island-shaped, but can have other shapes such as a semispherical shape or a regular-tetrahedron within a scope or spirit of the present invention.

Referring to FIG. 3B, a P-type electrode structure 210 according to a second embodiment of the present invention is formed on the hole injection layer 208, and includes a contact metal structure 210A that function as the contact metal forming the Ohmic contact to reduce the contact resistance; and a reflective layer 210B. The reflective layer 210B may be formed of metal having the high reflectivity such as aluminum (Al) or silver (Ag).

The contact metal structure 210A is mesh-shaped, and the reflective layer 210B covers the resultants including the hole injection layer 208 and the contact metal structure 210A. Though the mesh-shaped contact metal structure 210A has a square bar shape, but can have other shapes such as a cylindrical shape or a rectangular shape within a scope or spirit of the present invention.

Referring again to FIG. 2, an N-type electrode 112 is formed on the thinner first portion of the electron injection layer 104. The N-type electrode 112 can be also formed to have an electrode structure such as Ti/Al/Pt/Au in which metals are deposited. As described above, after semiconductor light-emitting parts are mounted on the transparent substrate 102, the resultant transparent substrate 102 is aligned on a sub-mount 118 having an Au layer 116 and a solder ball 114 formed on the Au layer 116. The Au layer 116 is wire-shaped such as a lead frame.

Next, flip-chip bonding is performed to assemble the sub-mount 118 with the transparent substrate 102 mounting the semiconductor light-emitting parts thereon, so that the semiconductor light-emitting device 100 is completed. Though not illustrated in detail in the drawings, but a process of forming a bonding metal for bonding the P-type electrode structure 110 and the N-type electrode 112 with the sub-mount 118 can be additionally performed.

FIGS. 4A through 4F are plane views illustrating the electrode structures depending on varied area ratios of the palladium (Pd) layer to the silver (Ag) layer according to the present invention.

FIGS. 4A through 4F illustrate a contact metal layer formed of palladium (Pd) that is formed on the hole injection layer to have a thickness of about 3 nm. This represents experimental results of the electrode structures where the area ratios of the Pd layer to the Ag layer are varied going from FIG. 4A to FIG. 4F so as to describe the effect of the present invention.

Describing in detail, in FIG. 4A, all portions denoted by 402 are formed as the Pd layer that is a standard type with a thickness of about 25-35 Å. In FIGS. 4B through 4E, regions (mesh_1 to mesh_4) 402 respectively correspond to the Pd layer. Each area ratio of the Pd layer to the Ag layer of the regions is 1.25, 0.78, 0.56 and 0.44, respectively. Here, a reference numeral 400 denotes the reflective layer formed of Ag, and a reference numeral 402 denotes the contact metal layer formed of Pd. In FIG. 4F, only the Ag layer is formed without the Pd layer.

FIGS. 5A and 5B are graphs illustrating the correlation of light emission with respective meshed regions shown in FIGS. 4A through 4F and the correlation of operation voltage with respective meshed regions in a semiconductor light-emitting device.

FIG. 5A illustrates the correlation of the light emission of the light emitting device in the standard electrode structure, the mesh_1 to mesh_4 electrode structure and the electrode structure with only the Ag layer that are shown in FIGS. 4A through 4F.

As shown in the drawing, it can be appreciated that luminance is lowest in the standard electrode structure with the Pd layer being entirely formed as the contact metal layer and then, the Al layer being entirely formed on the Pd layer. As described above, this phenomenon occurs because light emitting from the active layer is reflected at an interface between the Pd layer and the P-type electrode structure such that the reflected light emits toward the transparent substrate, thereby being much absorbed into the Pd layer.

According to a preferred embodiment of the present invention, when the electrode structure employs the combination of the Pd layer and the Ag layer, luminance is improved by above 10%. Further, as the area ratio of the Ag layer to the Pd layer is decreased, the luminance is gradually increased. In the meantime, the standard electrode structure has the lowest luminance, and the electrode structure with only the Ag layer has the highest luminance.

FIG. 5B illustrates the correlation of the operation voltage of the light emitting device in the standard electrode structure, the mesh_1 to mesh_4 electrode structure and the electrode structure with only the Ag layer that are respectively shown in FIGS. 4A through 4F.

As shown in FIG. 5A, the electrode structure having only the Ag layer without the Pd layer has the highest luminance in the light emitting device, but has the operation voltage exceeding 4.99V as shown in FIG. 5B. Accordingly, the electrode structure has the operation voltage greatly exceeding 3.80V that can be applied to an actual device. This is because the contact resistance is increased in the electrode structure with only the Ag layer.

However, when the electrode structure employs the combination of the Pd layer and the Ag layer according to a preferred embodiment of the present invention, the electrode structure has the operation voltage that is not so large in comparison with the conventional electrode structure while providing the high reflectivity such that the light emission of the light emitting device can be maintained. Further, the present invention controls the area of the Pd layer to control the operation voltage and the reflectivity of the P-type electrode structure, thereby optimizing light efficiency of the light emitting device.

As described above, the present invention has an effect in that light absorption made by the contact metal layer can be reduced while light efficiency of the semiconductor light emitting device can be improved, by controlling the area of the contact metal layer that is in contact with the hole injection layer formed of the P-type semiconductor.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

1. A semiconductor light emitting device comprising:

a transparent substrate;

an electron injection layer formed on the transparent substrate;

an active layer formed on a first region of the electron injection layer;

a hole injection layer formed on the active layer;

a first electrode structure which is formed on the hole injection layer and concurrently providing a high reflectivity and a low contact resistance;

a second electrode structure formed on a second region of the electron injection layer; and

a circuit substrate electrically connected with the first and second electrode structures.

2. The light emitting device of claim 1, wherein the first electrode structure comprises:

a contact metal structure which has any one selected from the group consisting of nickel, palladium, platinum and ITO (Indium Tin Oxide) that have low contact resistance; and

a reflective layer formed on the contact metal structure.

3. The light emitting device of claim 2, wherein the contact metal structure is island-shaped.

4. The light emitting device of claim 2, wherein the contact metal structure is mesh-shaped.

5. The light emitting device of claim 3, wherein an area ratio of the contact metal structure to the reflective layer is from 1% to 90%.

6. The light emitting device of claim 5, wherein the thickness of the contact metal structure is less than 200 nm.

7. The light emitting device of claim 4, wherein an area ratio of the contact metal structure to the reflective layer is from 1% to 90%.

8. The light emitting device of claim 7, wherein the thickness of the contact metal structure is less than 200 nm.

9. The light emitting device of claim 2, wherein the reflective layer is formed of aluminum (Al) or silver (Ag) that has a high reflectivity.

10. The light emitting device of claim 1, wherein the transparent substrate is formed of sapphire.

11. The light emitting device of claim 1, wherein the hole injection layer is formed of P-type GaN.

12. An electrode structure used in a semiconductor light emitting device having an active layer and a hole injection layer formed on one surface of the active layer, the structure comprising:

a contact metal structure which is formed on one surface of the hole injection layer to face with the active layer, and has any one selected from the group consisting of nickel, palladium, platinum and ITO that have low contact resistance; and

a reflective layer formed on the contact metal structure.

13. The electrode structure of claim 12, wherein the contact metal structure is island-shaped.

14. The electrode structure of claim 12, wherein the contact metal structure is mesh-shaped.

15. The electrode structure of claim 13, wherein an area ratio of the contact metal structure to the reflective layer is from 1% to 90%.

16. The electrode structure of claim 15, wherein the thickness of the contact metal structure is less than 200 nm.

17. The electrode structure of claim 14, wherein an area ratio of the contact metal structure to the reflective layer is from 1% to 90%.

18. The electrode structure of claim 17, wherein the thickness of the contact metal structure is less than 200 nm.

19. The electrode structure of claim 12, wherein the reflective layer is formed of aluminum (Al) or silver (Ag) that has a high reflectivity.

20. The electrode structure of claim 12, wherein the hole injection layer is formed of P-type GaN