WO2007105868A1

WO2007105868A1 - Light emitting diode having columns for light extraction and method of fabricating the same

Info

Publication number: WO2007105868A1
Application number: PCT/KR2007/001064
Authority: WO
Inventors: Duck Hwan Oh; Yeo Jin Yoon
Original assignee: Seoul Opto Device Co., Ltd.
Priority date: 2006-03-14
Filing date: 2007-03-05
Publication date: 2007-09-20
Also published as: KR100756842B1

Abstract

The present invention relates to a light emitting diode having columns for light extraction and a method of fabricating the light emitting diode. The light emitting diode comprises a first conductive type (Al, In, Ga)N semiconductor layer, a second conductive type (Al, In, Ga)N semiconductor layer positioned on one region of the first conductive type semiconductor layer, an active layer interposed between the first and second conductive type semiconductor layers. Additionally, columns for light extraction are positioned on other region of the first conductive type semiconductor layer. The columns for light extraction reduce light loss due to internal reflection, thereby enhancing light extraction efficiency.

Description

LIGHT EMITTING DIODE HAVING COLUMNS FOR LIGHT EXTRACTION AND METHOD OF FABRICATING THE SAME

Technical Field

[1] The present invention relates to a light emitting diode and a method of fabricating the same, and more particularly, to a light emitting diode having columns for light extraction and a method of fabricating the same. Background Art

[2] GaN based light emitting diodes (LEDs) have been employed for more than 10 years, and have considerably changed LED technologies. The GaN based LEDs have been currently used in a variety of applications such as full color LED display, LED traffic lights and white LEDs.

[3] Recently, high-efficiency white LEDs are expected to substitute for fluorescent lamps. Particularly, the efficiency of white LEDs has reached a level similar to that of general fluorescent lamps. However, there is room for further improvement in the efficiency of LEDs, and thus, further efficiency improvement is continuously required.

[4] Two major approaches have been attempted to improve the efficiency of LEDs. The first approach is to enhance the internal quantum efficiency determined by the crystal quality and the epitaxial layer structure, and the second approach is to increase light extraction efficiency.

[5] Since the internal quantum efficiency currently reaches 70 to 80%, there is little room for further improvement of the internal quantum efficiency. However, there is room for improvement in the light extraction efficiency. For the improvement in light extraction efficiency, it is important to eliminate internal light loss by employing a heat radiation structure, roughened surfaces, a patterned sapphire substrate, and the like.

[6] The roughened surfaces are employed to prevent total internal reflection due to a difference between refractive indices of a GaN-based LED and the surrounding thereof, e.g. a substrate and the atmosphere. Since a GaN based semiconductor material has a high refractive index of about 2.4, a critical angle thereof is relatively small. Light incident onto a surface of a GaN based LED at an angle larger than the critical angle is totally reflected and then returned again to the interior of the LED. Such light may be again reflected and then radiated to the outside of the LED, but a portion of the light is absorbed into the LED or electrodes and then lost in the form of heat. The roughened surfaces prevent light incident onto the surface of an LED from being returned to the interior of the LED due to the total internal reflection, and thus, allows the light to be radiated to the outside of the LED. [7] Meanwhile, a technology of employing a patterned sapphire substrate to improve light extraction efficiency has been disclosed in "InGaN-based near-ultraviolet and blue-light-emitting diodes with high external quantum efficiency using a patterned sapphire substrate and a mesh electrode", Japanese Journal of Applied Physics, Vol. 41, 2002, pp. L1431-L143, December 15, 2002.

[8] According to the technical paper, a sapphire substrate is etched to form convex hexagons thereon such that light loss due to total reflection of light between an LED and a substrate can be reduced to thereby improve light extraction efficiency.

[9] However, the light extraction efficiency of LEDs has not yet reached a satisfactory level, and thus, continuous efforts to improve the light extraction efficiency are required.

Disclosure of Invention Technical Problem

[10] An object of the present invention is to provide a light emitting diode capable of further improving light extraction efficiency.

[11] Another object of the present invention is to provide a method of fabricating a light emitting diode capable of further improving the light extraction efficiency using conventional processes. Technical Solution

[12] According to an aspect of the present invention for achieving the object, there is provided a light emitting diode having columns for light extraction. The light emitting diode comprises a first conductive type (Al, In, Ga)N semiconductor layer, a second conductive type (Al, In, Ga)N semiconductor layer positioned on one region of the first conductive type semiconductor layer, and an active layer interposed between the first and second conductive type semiconductor layers. Additionally, columns for light extraction are positioned on other region of the first conductive type semiconductor layer. Accordingly, since light totally reflected within the first conductive type semiconductor layer can be radiated to the outside through the columns for light extraction, light extraction efficiency can be increased. The columns for light extraction may be used in combination with a technique for forming a roughened top surface of the second conductive type semiconductor layer and/or employing a patterned substrate, and thus, the light extraction efficiency can be further enhanced as compared with the related art.

[13] The one region of the first conductive type semiconductor layer may be surrounded by the other region thereof, and the columns for light extraction may be arranged along the circumference of the second conductive type semiconductor layer. Accordingly, light which is radiated from the active layer and then internally reflected within the first conductive type semiconductor layer can be emitted outward through the columns, and thus, the light extraction efficiency can be further enhanced.

[14] Each of the columns for light extraction may have an upper portion of a convex shape. Such an upper convex portion of the column functions as a lens and allows light emitted to the outside through the columns to be radiated in a direction parallel to the columns.

[15] Further, an upper end of each column for light extraction may be formed of the same material as the second conductive type semiconductor layer. Accordingly, the columns may be formed simultaneously when the second conductive type semiconductor layer is formed, and thus, a process of fabricating a light emitting diode can be simplified.

[16] The light emitting diode may further comprise a substrate and a buffer layer interposed between the substrate and the first conductive type semiconductor layer. The substrate may be a patterned substrate, e.g. a patterned sapphire substrate.

[17] According to another aspect of the present invention for achieving the other object, there is provided a method of fabricating a light emitting diode having columns for light extraction. According to the method of the present invention, a buffer layer is formed on a substrate. A first conductive type (Al, In, Ga)N semiconductor layer, an active layer and a second conductive type (Al, In, Ga)N semiconductor layer are formed on the substrate with the buffer layer formed thereon. Then, a mask pattern defining a light emitting region and column regions is formed on top of the second conductive type semiconductor layer. The second conductive type semiconductor layer and the active layer are etched using the mask pattern as an etching mask such that the first conductive type semiconductor layer is exposed. As a result, the columns for light extraction are formed adjacent to the light emitting region.

[18] According to the embodiment of the present invention, since columns for light extraction are formed using a mask pattern defining the column regions, light extraction efficiency can be enhanced using conventional processes without making the fabricating processes complicated.

[19] Furthermore, the column regions may be defined along the circumference of the light emitting region. Accordingly, the columns for light extraction are formed around the circumference of the light emitting region.

Advantageous Effects

[20] According to the present invention, there can be provided a light emitting diode capable of further improving light extraction efficiency as compared with a conventional light emitting diode in which a roughened surface or a patterned sapphire substrate is employed. Further, a method of fabricating a light emitting diode capable of further improving light extraction efficiency using conventional processes can be provided.

Brief Description of the Drawings

[21] Fig. 1 is a plan view illustrating a light emitting diode having columns for light extraction according to an embodiment of the present invention.

[22] Fig. 2 is a sectional view taken along the line A-A' in Fig. 1.

[23] Figs. 3 to 6 are sectional views illustrating a method of fabricating a light emitting diode having columns for light extraction according to an embodiment of the present invention. Best Mode for Carrying Out the Invention

[24] Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. The following embodiment is provided only for illustrative purpose such that the spirit of the present invention can be fully conveyed to those skilled in the art. Therefore, the present invention is not limited to the following embodiment but may be implemented in other forms. In the drawings, the width, length, thickness, etc. of components are exaggerated for the sake of convenience. Throughout the descriptions, like reference numerals designate like elements.

[25] Fig. 1 is a plan view illustrating a light emitting diode (LED) having columns 30 for light extraction according to an embodiment of the present invention, and Fig. 2 is a sectional view taken along line A-A' in Fig. 1.

[26] Referring to Figs. 1 and 2, a first conductive type (Al, In, Ga)N semiconductor layer

25 is positioned on a substrate 21. A buffer layer 23 may be interposed between the first conductive type semiconductor layer 25 and the substrate 21. Furthermore, a second conductive type (Al, In, Ga)N semiconductor layer 29 is positioned on one region of the first conductive type semiconductor layer 25, and an active layer 27 is interposed between the first and second conductive type semiconductor layers 25 and 29.

[27] The substrate 21 may be a substrate made of sapphire, SiC, spinel or the like, and may be a patterned substrate, e.g. a patterned sapphire substrate.

[28] The buffer layer 23 is employed to reduce lattice mismatch between the first conductive type semiconductor layer 25 and the substrate 21. An undoped semiconductor layer (not shown) may be further formed on the buffer layer 23.

[29] The first and second conductive type semiconductor layers 25 and 29 may be N- type and P-type, or P-type and N-type, respectively. Here, an (Al, In, Ga)N semiconductor represents a binary to quaternary Group III nitride. Si and Mg may be doped to form N-type and P-type semiconductor layers, respectively. The active layer 27 may also be formed of an (Al, In, Ga)N semiconductor compound semiconductor, and may be formed into a single or multiple quantum well structure. Additionally, the second conductive type semiconductor layer 29 may have a roughened top surface.

[30] Furthermore, columns 30 for light extraction are positioned on other region of the first conductive type semiconductor layer 25. Each of the columns 30 takes the shape of a rod with a length relatively longer than a diameter, and an upper portion thereof may have a convex shape.

[31] As shown in Fig. 1, the one region of the first conductive type semiconductor layer

25 may be surrounded by the other region. Meanwhile, the columns 30 may be positioned at one side of the second conductive type semiconductor layer 29, but may be arranged along the circumference of the second conductive type semiconductor layer 29 as shown in this figure.

[32] Meanwhile, an electrode 31 is positioned on the second conductive type semiconductor layer 29. The electrode 31 may be a transparent electrode, e.g. Ni/ Au or indium tin oxide (ITO). The contact of the electrode 31 to the second conductive type semiconductor layer 29 exhibits an ohmic characteristic, and an additional means may be employed if necessary. For example, in a case where the electrode 31 is formed of ITO, a high concentration N-type tunneling layer (not shown) may be interposed between the second conductive type semiconductor layer 29 and the electrode 31.

[33] In addition, a P-type electrode pad 33 is positioned on the electrode. The P-type electrode pad 33 may be brought into contact with the second conductive type semiconductor layer 29 through the electrode 31. Further, an N-type electrode pad 35 is positioned on the other region of the first conductive type semiconductor layer 25. The N-type electrode pad 35 may be formed such that the columns 30 are positioned further outward than the N-type electrode pad as shown in Fig. 1, but the present invention is not limited thereto. That is, the N-type electrode pad 35 may be formed further outward than the columns 30 such that the columns 30 are positioned closer to the second conductive type semiconductor layer 29. Alternatively, the columns 30 may not be formed on a region of the first conductive type semiconductor layer 25 which is extended from the N-type electrode pad 35 to an edge of the LED. Preferably, the P- type and N-type electrode pads 33 and 35 are positioned at opposite sides for current distribution.

[34] According to a related art, light radiated from the active layer 27 to the substrate 21 is internally reflected either on an interface between the first conductive type semiconductor layer 25 and the substrate 21 due to a difference of refractive indices between them, or on an interface between the substrate 21 and the air due to a difference of refractive indices between them. The internally reflected light may be reflected toward the second conductive type semiconductor layer 29 and emitted to the outside of the LED through the roughened surface. Alternatively, light reflected to a side of the LED with an angle less than a critical angle may be emitted through a side surface of the LED. However, light that is neither reflected toward the second conductive type semiconductor layer 29 nor emitted through the side surface of the LED is lost in the form of heat while internal reflections are repeated. Further, a patterned sapphire substrate is employed to allow light to be scattered on patterns of the sapphire substrate, thereby randomly changing the traveling directions of light. However, all the light cannot be emitted to the outside of the LED. On the other hand, according to the embodiment of the present invention, the columns 30 for light extraction are provided in the periphery of the second conductive type semiconductor layer 29, so that light of which internal reflections are repeated can be emitted to the outside of the LED through the columns 30 for light extraction. The columns 30 decrease the number of internal reflections and prevent light loss due to the internal reflections to thereby enhance light extraction efficiency.

[35] Further, an upper portion of each of the columns 30 is formed into a convex shape, so that light radiated through the columns 30 can be emitted in a direction generally parallel to the columns 30.

[36] Meanwhile, the upper end of each column 30 may be formed of the same material as the second conductive type (Al, In, Ga)N semiconductor layer 29. Additionally, a lower portion of each column 30 may be formed of the first conductive type semiconductor layer 25. The same material as the active layer 27 may be interposed between the upper and lower portions of the columns.

[37] Figs. 3 to 6 are sectional views illustrating a method of fabricating a light emitting diode having columns 30 for light extraction according to an embodiment of the present invention.

[38] Referring to Fig. 3, the first conductive type (Al, In, Ga)N semiconductor layer 25, the active layer 27 and the second conductive type (Al, In, Ga)N semiconductor layer 29 are formed on the substrate 21. Before the first conductive type semiconductor layer 25 is formed, a buffer layer 23 may be formed and an undoped (Al, In, Ga)N semiconductor layer (not shown) may be formed on the buffer layer 23.

[39] The respective semiconductor layers may be formed through techniques including metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE) and the like.

[40] Additionally, the roughened surfaces may be formed by etching the second conductive type semiconductor layer 29 through a photoelectrochemical (PEC) etching technique.

[41] Referring to Fig. 4, a mask pattern defining a light emitting region 39a and column regions 39b is formed on the second conductive type semiconductor layer 29. The light emitting region 39a is not limited to a specific shape but may be generally defined as a rectangular shape. The column regions 39b may be defined to be arranged around the circumference of the light emitting region 39a. As shown in Fig. 1, the light emitting region 39a may be formed except a pad region on which the N-type electrode pad 35 is formed.

[42] The mask pattern may be formed using a photoresist, oxide film or the like. In a case where the mask pattern is formed using a photoresist, a bottom surface of the mask pattern may be formed broader than a top surface thereof through a reflow process.

[43] Furthermore, the regions other than the light emitting region 39a and the column regions 39b are exposed to the outside.

[44] Referring to Fig. 5, the second conductive type semiconductor layer 29 and the active layer 27 are etched using the mask pattern as an etching mask such that the first conductive type semiconductor layer 25 are exposed. At this time, the first conductive type semiconductor layer 25 may be overetched.

[45] The second conductive type semiconductor layer 29 within the light emitting region

39a and the column regions 39b remains unetched, and the second conductive type semiconductor layer 29 within the other regions is etched such that the first conductive type semiconductor layer 25 is exposed to the outside. Thus, the columns 30 for light extraction are formed in the column regions 39b.

[46] Meanwhile, in a case where the mask pattern is formed using the reflow process, an upper portion of each of the columns 30 may be formed into a convex shape as shown in this figure.

[47] Referring to Fig. 6, the electrode 31 is formed on the second conductive type semiconductor layer 29. The electrode 31 may be formed using a lift-off technique, and may be formed of, for example, Ni/Au, ITO or the like.

[48] The electrode 31 may be formed after the first conductive type semiconductor layer

25 has been exposed to the outside. Alternatively, the electrode may be formed by forming an electrode layer on the second semiconductor layer 29 using an e-beam evaporation technique and then patterning the layer using a photolithographic process before exposing the first conductive type semiconductor layer 25.

[49] Subsequently, the P- type electrode pad 33 is formed on the electrode 31, and the N- type electrode pad 35 is formed on the first conductive type semiconductor layer 25. Each of the electrode pads 33 and 35 may be formed using a lift-off technique. Accordingly, the light emitting diode of Fig. 2 is completed.

Claims

[1] A light emitting diode, comprising: a first conductive type (Al, In, Ga)N semiconductor layer; a second conductive type (Al, In, Ga)N semiconductor layer positioned on one region of the first conductive type semiconductor layer; an active layer interposed between the first and second conductive type semiconductor layers; and columns for light extraction positioned on other region of the first conductive type semiconductor layer. [2] The light emitting diode as claimed in claim 1, wherein one region of the first conductive type semiconductor layer is surrounded by the other region thereof, and the columns for light extraction are arranged along a circumference of the second conductive type semiconductor layer. [3] The light emitting diode as claimed in claim 1, wherein each of the columns for light extraction has an upper portion of convex shape. [4] The light emitting diode as claimed in claim 1, wherein an upper end of each column for light extraction is formed of the same material as the second conductive type semiconductor layer. [5] The light emitting diode as claimed in claim 1, further comprising: a substrate; and a buffer layer interposed between the substrate and the first conductive type semiconductor layer. [6] A method of fabricating a light emitting diode, comprising: forming a buffer layer on a substrate; forming a first conductive type (Al, In, Ga)N semiconductor layer, an active layer and a second conductive type (Al, In, Ga)N semiconductor layer on the buffer layer; forming a mask pattern defining a light emitting region and column regions over the second conductive type semiconductor layer; and etching the second conductive type semiconductor layer and the active layer using the mask pattern as an etching mask to expose the first conductive type semiconductor layer. [7] The method as claimed in claim 6, wherein the column regions are arranged along a circumference of the light emitting region.