WO2008084950A1

WO2008084950A1 - Ohmic electrode and method for forming the same

Info

Publication number: WO2008084950A1
Application number: PCT/KR2008/000091
Authority: WO
Inventors: Jong Lam Lee; Jun Ho Son
Original assignee: Seoul Opto Device Co., Ltd.; Postech Academy-Industry Foundation
Priority date: 2007-01-08
Filing date: 2008-01-08
Publication date: 2008-07-17

Abstract

Provided is an ohmic electrode disposed on a semiconductor layer of an LED and a method for forming the same. The ohmic electrode includes: a reflective layer formed of Ag alloy, wherein a portion of particles of the reflective layer are diffused into an interface between the ohmic electrode and the semiconductor layer and metallized at the interface; and a protective layer disposed on the reflective layer to inhibit an out-diffusion of the reflective layer. Because the portion of Ag particles are metallized at the interface between the ohmic electrode and the semiconductor layer during thermal annealing, the ohmic electrode has a high bonding strength and a low contact resistance. Further, because the protective layer inhibits an excessive out-diffusion of Ag particles during thermal annealing, the ohmic electrode has an excellent thermal stability while maintaining a high light reflectance of Ag.

Description

OHMIC ELECTRODE AND METHOD FOR FORMING THE

SAME

Technical Field

[1] The present disclosure relates to an ohmic electrode and a method for forming the same, and more particularly, to an ohmic electrode disposed on a semiconductor layer of a light emitting device to apply an external driving power to the semiconductor layer and a method for forming the same. Background Art

[2] A light emitting diode (LED) has various advantageous characteristics such as long lifetime, small size, light weight, excellent impact resistance and vibration resistance, and high directivity of emitted light. Further, the LED can be operated with a low voltage, needs neither a preheating time nor a complex driving circuit, and can be packaged in a variety of shapes. Accordingly, the LED is expected to replace a conventional white light source such as an incandescent lamp, a fluorescent lamp, and a mercury arc lamp, in several years.

[3] In particular, a nitride based semiconductor LED can emit light having a wide range of wavelength from red light to ultraviolet ray and has excellent physical/chemical stability because of its wide energy band gap. Accordingly, much attention is being paid on the nitride based semiconductor LED to achieve high efficiency and high output power. However, the nitride semiconductor LED is not satisfactory yet in view of light output, light emitting efficiency, and cost, and therefore more improvement of performance is required. Particularly, the light output of the nitride semiconductor LED is still lower than that of the conventional white light source, which should be improved further. In addition, low thermal stability accompanied therewith should be considered at the same time.

[4] A typical nitride semiconductor LED is manufactured by forming a nitride based n- type layer, a nitride based active layer, and a nitride based p-type layer on a sapphire substrate, and then forming an n-type electrode and a p-type electrode horizontally to apply power to the n-type layer and the p-type layer. Such a horizontal type LED is cost effective since it is manufactured through a relatively simple process. However, the sapphire substrate used in the horizontal type LED is an insulator with poor thermal conductivity. Accordingly, when current is applied to a large area to achieve higher output power, thermal stability is degraded due to heat accumulation.

[5] To overcome such disadvantages, a vertical type LED and a flip chip type LED have been suggested. In these devices, a reflective layer is formed in a p-type electrode so that light generated in an active layer may be emitted out through an n-type electrode, and a metal substrate having high thermal conductivity is used instead of the sapphire substrate so that heat may be rapidly radiated to the outside. In this way, high output power can be achieved by applying current to a large area and degradation of thermal stability can be prevented. Maximum currents applicable to the vertical type and flip chip type LEDs are several times as much as that applicable to the horizontal type LED, so that higher output power can be achieved, which enables the vertical type and flip chip type LEDs to replace the conventional white light source for illumination.

[6] Meanwhile, to further improve the light output of the vertical type and flip chip type

LEDs, a high-efficiency reflective layer which reflects generated light without absorption should be used as the p-type electrode. Aluminum (Al) and silver (Ag) having excellent light reflectance to a visible light may be used as the p-type electrode to achieve improved light output characteristics. However, contact resistance between Al and nitride based semiconductor layer is high, so that high current can't be supplied. Although contact resistance of Ag is low, bonding strength with other layers and thermal stability are not satisfactory, which causes an agglomeration and an interface void during high-temperature thermal annealing.

[7] Accordingly, a gold-based p-type electrode (e.g., Ni/ Au, Pd/Au and Pt/ Au) used in the horizontal type LED is still widely used in the vertical type LED, in which Au absorbs light and reduces the light reflectance. Therefore, if such a gold-based electrode is used as the p-type electrode in the vertical type LED, sufficient output power and reliability required to replace a conventional white light source can hardly be achieved. Disclosure of Invention

Technical Problem

[8] The present disclosure provides an ohmic electrode having low contact resistance, high light reflectance, and excellent thermal stability, and a method for forming the ohmic electrode in which the ohmic electrode is obtained by forming an electrode which has a laminated structure of a reflective layer of Ag alloy having high reflectance and a protective layer on a semiconductor layer and then by performing thermal annealing thereof. Technical Solution

[9] In accordance with an exemplary embodiment, an ohmic electrode disposed on a semiconductor layer of a LED may include: a reflective layer formed of Ag alloy; a protective layer disposed on the reflective layer to inhibit an out-diffusion of the reflective layer; and an interface between the reflective layer and the semiconductor layer, wherein the interface is formed as a portion of particles of the reflective layer is in-diffused towards the semiconductor layer and metallized.

[10] The Ag alloy may include Ag, and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt,

Pd, and combinations thereof. A composition of the alloying element in the Ag alloy is in a range of 0.01% to 80%.

[11] The protective layer may include one of Ru, Ir, Rh, Pt, W, Ta, Ti, Co and combinations thereof.

[12] A thickness of the reflective layer may be in a range of approximately 50 A to 5,000

A, and a thickness of the protective layer may be in a range of approximately 50 A to 1,000 A.

[13] A total thickness of the reflective layer and the protective layer may be in a range of approximately 100 A to 5,000 A.

[14] In accordance with another exemplary embodiment, a method for forming an ohmic electrode on a semiconductor layer of a light emitting device (LED) includes: forming a reflective layer formed of Ag alloy on the semiconductor layer; forming a protective layer to inhibit out-diffusion of Ag particles; and performing thermal annealing of the reflective layer and the protective layer so that a portion of the Ag particles are metallized at an interface between the reflective layer and the semiconductor layer while the protective layer inhibits the out-diffusion of the Ag particles.

[15] The Ag alloy may include Ag, and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt,

Pd and combinations thereof. A composition of the alloying element in the Ag alloy may be in a range of approximately 0.01% to 80%.

[16] The protective layer may include one of Ru, Ir, Rh, Pt, W, Ta, Ti, Co and combinations thereof.

[17] The thermal annealing may be performed at a temperature of approximately 15O⁰C to

600 ⁰C.

[18] The thermal annealing may be performed in an oxygen containing atmosphere.

[19] The oxygen containing atmosphere may include one of an oxygen atmosphere, an ambient air, a mixture atmosphere of oxygen and nitrogen, a mixture atmosphere of oxygen and argon and combinations thereof.

Advantageous Effects

[20] An ohmic electrode in accordance with exemplary embodiments is formed by forming an electrode structure including a reflective layer of Ag alloy and a protective layer on a semiconductor layer and performing thermal annealing thereof. During thermal annealing of the ohmic electrode, some of Ag particles are metallized at an interface between the ohmic electrode and the semiconductor layer, so that the ohmic electrode can have high bonding strength and low contact resistance. Furthermore, the protective layer prevents an excessive out-diffusion of the Ag particles during the thermal annealing to reduce an agglomeration and an interface void. Accordingly, excellent light reflectance of Ag can be maintained and excellent thermal stability can be achieved simultaneously. Brief Description of the Drawings

[21] Exemplary embodiments can be understood in more detail from the following description taken in conjunction with the accompanying drawings, in which:

[22] FIG. 1 is a sectional view illustrating a vertical type LED in accordance with an exemplary embodiment;

[23] FIGS. 2 through 6 are sectional views illustrating a method for manufacturing the

LED in accordance with the exemplary embodiment;

[24] FIG. 7 is a graph illustrating current versus voltage characteristics of ohmic electrodes in accordance with an experimental example and a comparison example;

[25] FIGS. 8(a) and 8(b) are graphs illustrating color level spectrums of Ag3d and NIs by a synchrotron radiation photoelectron spectroscopy (SRPES) analysis before and after thermal annealing of the ohmic electrode in accordance an experimental example;

[26] FIG. 9 is a graph illustrating light reflectance of ohmic electrodes in accordance with an experimental example and a comparison example; and

[27] FIG. 10 is a sectional view illustrating a flip chip type LED in accordance with another exemplary embodiment. Best Mode for Carrying Out the Invention

[28] Hereinafter, specific embodiments will be described in detail with reference to the accompanying drawings. However, the present invention is not limited thereto and may be embodied in a variety of other embodiments by those skilled in the art. The specific embodiment is to be considered illustrative, and is provided only with the aim of making the spirit and the scope of the invention to be easily understood by those skilled in the art.

[29] In the drawings, thicknesses of layers are enlarged to describe the layers and regions clearly and like reference numerals refer to like elements. It will be understood that when a layer, a film, a region, and a plate is referred to as being "on" or "over" another element in the descriptions of the embodiments, the term "on" may denote "directly on", or "indirectly on", and the term "over" may denote "directly over'Or "indirectly over".

[30] FIG. 1 is a sectional view illustrating a vertical type LED in accordance with an exemplary embodiment.

[31] Referring to FIG. 1, the LED includes a semiconductor layer 200, an n-type electrode

400 disposed on one side of the semiconductor layer 200, and a p-type electrode 300 disposed on the other side of the semiconductor layer 200. The p-type electrode 300 has a multilayered structure including a reflective layer 310 and a protective layer 320, and forms an ohmic contact with the semiconductor layer 200.

[32] The semiconductor layer 200 includes an n-type layer 210, an active layer 220 and a p-type layer 230, each of which includes one of a Si layer, an AlN layer, an InGaN layer, an AlGaN layer, an AlInGaN layer and combinations thereof. For example, in the exemplary embodiment of FIG. 1, the n-type layer 210 and the p-type layer 230 include a GaN layer, and the active layer 220 includes an InGaN layer. The n-type layer 210 supplies electrons. The n-type layer 210 may include an n-type semiconductor layer and an n-type clad layer. The n-type semiconductor layer and the n- type clad layer may be formed by injecting an n-type dopant such as Si, Ge, Se, Te and C into the above-described semiconductor layer. The p-type layer 230 supplies holes. The p-type layer 230 may include a p-type semiconductor layer and a p-type clad layer. The p-type semiconductor layer and the p-type clad layer may be formed by injecting a p-type dopant such as Mg, Zn, Be, Ca, Sr and Ba into the above-described semiconductor layer. The electron supplied from the n-type layer 210 and the hole supplied from the p-type layer 230 are recombined in the active layer 220 to thereby emit light of a predetermined wavelength. The active layer 220 may be formed as a multilayered semiconductor layer having a single or multiple quantum well structure by alternately laminating a well layer and a barrier layer. A wavelength of the output light varies depending on the semiconductor material of the active layer 220. Therefore, the semiconductor material for the active layer 220 may be selected depending on a desired output wavelength. For example, in the exemplary embodiment of FIG. 1, the semiconductor layer 200 is formed as follows: a GaN layer is deposited and an n-type impurity is injected into the GaN layer to form an n-type layer 210; a barrier layer of GaN and a well layer of InGaN are alternately deposited on the n-type layer 210 to form an active layer 220 of a multiple quantum well structure; and then a GaN layer is deposited on the active layer 220 and a p-type impurity is injected into the GaN layer to form a p-type layer 230.

[33] The n-type electrode 400 may have a single layered structure or a multilayered structure formed of at least one of Pb, Sn, Au, Ge, Cu, Bi, Cd, Zn, Ag, Ni, Ti and alloys including at least one of the foregoing. The n-type electrode supplies a negative potential to the semiconductor layer 200 and serves as a light-emitting surface through which light generated in the semiconductor layer 200 is emitted to the outside.

[34] The p-type electrode 300 may be an ohmic electrode having a multilayered structure including a reflective layer 310 and a protective layer 320. The reflective layer 310 may be formed of Ag alloy. The Ag alloy may include Ag, and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt, Pd and combinations thereof. A composition of the alloying element may be in a range of approximately 0.01% to 80%. When the composition of the alloying element is lower than approximately 0.01%, the diffusion of Ag is not effectively suppressed. When the composition of the alloying element is higher than approximately 80%, the reflectance is significantly decreased, so that the Ag alloy can not be used for the reflective layer. The protective layer 320 may be formed of one of Ru, Ir, Rh, Pt, W, Ta, Ti, Co and combinations thereof. For example, in the exemplary embodiment of FIG. 1, the reflective layer 310 is formed of Cu-Ag alloy where the content of Cu ranges from approximately 0.01% to approximately 80%, and the protective layer 320 is formed of Ru. The p-type electrode 300 applies a positive potential to the semiconductor layer 200, and serves as a reflection plane to reflect light so that most of light is emitted to the outside through a light-emitting surface 400, i.e., the n-type electrode 400.

[35] A manufacturing method of a LED having the above described configurations will now be described with reference to FIGS. 2 through 6. FIGS. 2 through 6 are sectional views illustrating a method for manufacturing the LED in accordance with the exemplary embodiment of FIG. 1.

[36] Referring to FIG. 2, an n-type layer 210, an active layer 220 and a p-type layer 230 are sequentially stacked on a substrate 100 to form a semiconductor layer 200 having a multilayered structure, and a patterning process is performed using a predetermined mask to separate cells from each other. The separated cells are used as a light emitting unit of an LED on a basis of a single cell or a batch of cells.

[37] The substrate 100 may be a sapphire substrate, a silicon carbide (SiC) substrate, a silicon (Si) substrate, a zinc oxide (ZnO) substrate, a gallium arsenide (GaAs) substrate or a gallium phosphide (GaP) substrate. In particular, a sapphire substrate may be used as the substrate 100.

[38] The semiconductor layer 200 may include one of a Si layer, a GaN layer, an AlN layer, an InGaN layer, an AlGaN layer, an AlInGaN layer and combinations thereof. In the exemplary embodiment, a GaN layer is deposited and then an n-type impurity is injected into the GaN layer to form an n-type layer 210. A barrier layer of GaN and a well layer of InGaN are alternately deposited on the n-type layer 210 to form an active layer 220 having a multiple quantum well structure. Thereafter, a GaN layer is deposited on the active layer 220 and then a p-type impurity is injected into the GaN layer to form a p-type layer 230. Though not shown, a buffer layer may be further formed between the substrate 100 and the n-type layer 210 to relieve stress caused by a lattice mismatch between the substrate 100 and the n-type layer 210, so that the n-type layer 210 can grow effectively in a following process.

[39] A surface treatment may be performed after forming the semiconductor layer 200 to improve a quality of a succeeding layer and bonding strength of the interface. For example, in the exemplary embodiment, a first surface treatment is performed after forming the p-type layer 230. The first surface treatment includes: dipping the surface of the semiconductor, i.e., the p-type layer 230 in aqua regia (HCl : HNO₃ = 3 : 1) for about 10 minutes; washing it with deionized water; and drying it with nitrogen gas. Before depositing the succeeding layer, i.e., p-type electrode, a second surface treatment is performed. The second surface treatment includes: dipping the surface of the semiconductor for approximately 1 minute in a mixture solution of HCl and deionized water at a ratio of approximately 1:1; and drying it. Of course, the first and second surface treatments may be performed selectively and may be omitted. [40] Referring to FIG. 3, a reflective layer 310 and a protective layer 320 are sequentially stacked on the semiconductor layer 200. The reflective layer may be formed to have a thickness of approximately 50 A to 5,000 A using a metal alloy including Cu-Ag. The protective layer 320 may be formed to have a thickness of approximately 50 A to 1,000

A using a metal including Ru. The total thickness of the reflective layer 310 and the protective layer 320 may be in a range of approximately 100 A to approximately 5,000

A. For example, in the exemplary embodiment, Cu and Ag is charged in a crucible to form Cu-Ag alloy, and then the Cu-Ag alloy is evaporated in an e-beam evaporator to deposit a Cu-Ag layer 310, i.e., the reflective layer 310, to have a thickness of approximately 1,500 A. Thereafter, Ru is evaporated in an e-beam evaporator to form a Cu layer 320, i.e., the protective layer 320, to have a thickness of approximately 500 A.

[41] Referring to FIG. 4, a rapid thermal annealing is performed for the reflective layer

310 of Cu-Ag alloy and the protective layer 320 of Ru to form a p-type electrode 300. The rapid thermal annealing may be performed in an atmosphere containing oxygen, for example, in an oxygen atmosphere, an ambient air, a mixture atmosphere of oxygen and nitrogen, or a mixture atmosphere of oxygen and argon. A pressure of the oxygen containing atmosphere may be equal to or lower than atmospheric pressure. The rapid thermal annealing may be performed at a temperature of approximately 15O⁰C to 600⁰C.

[42] When the rapid thermal annealing is performed for the ohmic electrode 300, i.e., the p-type electrode 300 having a multilayered structure of Cu-Ag/Ru in an oxygen atmosphere, Ag particles begin to diffuse in the Cu-Ag reflective layer 310, and then diffuse into an interface between the Cu-Ag reflective layer 310 and the p-type layer 230. The Ag particles are metallized in the entire interface, so that the ohmic electrode 300 has high bonding strength and low contact resistance. An excessive in-diffusion of the Ag particles is inhibited by the Cu layer included in the Cu-Ag reflective layer 310, and an excessive out-diffusion also is inhibited by the Ru protective layer 330 covering the Cu-Ag reflective layer 310, whereby an interfacial void and an agglomeration of the Ag layer can be prevented. Therefore, the ohmic electrode 300 can have excellent thermal stability while maintaining high light reflectance which is an inherent characteristic of Ag metal.

[43] Typically, an n-type electrode (not shown) and a p-type electrode are disposed horizontally in the above-described LED. In detail, a mesa etching is performed on a portion of the semiconductor layer 200 to expose a portion of the n-type layer 210, and the n-type electrode is formed on the exposed top surface of the n-type layer 210 while maintaining a base substrate 100 for forming the semiconductor layer 200. However, the electrodes may be disposed vertically. That is, all or a portion of the base substrate 100 is removed and then an n-type electrode is formed on a bottom surface of the n- type layer 210. Hereinafter, a method for forming the vertical type LED will be described for example.

[44] Referring to FIG. 5, a lift-off process is performed to remove a substrate 100 from an n-type layer 210 by irradiating laser on a bottom surface of the substrate 100 where the semiconductor layer 200 is formed. Afterwards, an n-type electrode 400 is formed on the n-type layer 210 from which the substrate 100 has been removed, as shown in FIG. 6. The n-type electrode 400 may include a single layer or multiple layers formed of one of Pb, Sn, Au, Ge, Cu, Bi, Cd, Zn, Ag, Ni, Ti and alloys including at least one of the foregoing. The n-type electrode 400 may be formed of a transparent metal so that light generated from the semiconductor layer 200 can be emitted to the outside through the n-type electrode 400.

[45] Hereinafter, an experimental example and a comparison example will be described to illustrate the characteristics of the ohmic electrode 300 forming an ohmic contact with the semiconductor layer 200 in the LED in accordance with the exemplary embodiment. An ohmic electrode 300 used in the experimental example has a mul- tilayered structure of Cu-Ag/Ru formed by stacking a Cu-Ag alloy layer 310 and a Ru layer 320 on the semiconductor layer 200 and performing a rapid thermal annealing for approximately 1 minute at a temperature of approximately 400 ⁰C in ambient air.

[46] FIG. 7 is a graph illustrating current versus voltage of the ohmic electrode in accordance with the experimental example and the comparison example. The line A represents current versus voltage graph of the ohmic electrode having a multilayered structure of Ni/Au in accordance with the comparison example. The line B represents current versus voltage graph of the ohmic electrode having a multilayered structure of Cu-Ag/Ru in accordance with the experimental example.

[47] A contact resistance is calculated using a transmission line measurement (TLM) method proposed by Shottky to evaluate electrical characteristics of the ohmic electrode. According to the TLM method, a resistance R_τ at 0 V is obtained by measuring current (I)-voltage (V) relationships between two metal electrodes separated by distances dl, d2, d3 and d4, respectively. After drawing a graph of the measured R_τ depending on the distance, a contact resistance can be calculated from the following equations through an extrapolation: [48]

r_c = R_c x Z(μm) x 10 k-4 ι [Ωcm]

[49] where R_x is a resistance [Ω] between the two metal electrodes, R_s [Ω] is a sheet resistance of the semiconductor layer, d is a distance between the two electrodes, Z is a width of the metal electrode, and p_c is a contact resistance.

[50] A contact resistance of the ohmic electrode in accordance with the comparison example calculated from the current- voltage graph in FIG. 3 and the TLM method is approximately 5 x 10³ Ωcm², and a contact resistance of the ohmic electrode in accordance with the experimental example is approximately 2 x 10^~4 Ωcm². The contact resistance of the ohmic electrode in accordance with the experimental example is lower than that in accordance with the comparison example because Ag particles are diffused and metallized during the thermal annealing process to provide a high bonding strength.

[51] FIGS. 8 (a) and 8(b) are graphs illustrating core level spectrums of Ag3d and NIs by a synchrotron radiation photoelectron spectroscopy (SRPES) analysis before and after thermal annealing of an ohmic electrode in accordance with an experimental embodiment. The graphs are obtained before the thermal annealing, and after the thermal annealing at 300⁰C and 400⁰C, respectively.

[52] Referring to FIG. 8 (a), it can be seen that a peak intensity of Ag3d is lowered after the thermal annealing, which indicates that Ag particles are in-diffused during the thermal annealing. Furthermore, Ag-O and Ag-Ga bondings are mainly detected after the thermal annealing whereas Ag-Ag bond is mainly detected right after deposition, which indicates that Ag oxide and Ag-Ga solid solution are formed during the thermal annealing. Referring to FIGS. 8(a) and 8(b), peaks of Ag3d and NIs are shifted towards a lower binding energy as the thermal annealing temperature increases, which indicates that Fermi level is shifted towards a valence band. Accordingly, a Shottky barrier between the p-type layer 230 and the Cu-Ag reflective layer 310 is lowered, which is advantageous for ohmic contact.

[53] FIG. 9 is a graph illustrating light reflectance of ohmic electrodes in accordance with an experimental example and a comparison example. The light reflectance at a wavelength band around 460 nm is measured. The curve B represents light reflectance of an ohmic electrode having Cu-Ag/Ru structure after thermal annealing in accordance with the experimental example, and the curve A represents light reflectance of an ohmic electrode having Ag/Ru structure after thermal annealing in accordance with the comparison example. The curve I is a reference curve representing a light reflectance of Ag.

[54] Referring to FIG. 9, the ohmic electrode in accordance with the comparison example shows low light reflectance of approximately 71% (curve A) while the ohmic electrode in accordance with the experimental example shows high light reflectance of approximately 88% (curve B). In the ohmic electrode having Cu-Ag/Ru structure in accordance with the experimental example, the Cu layer and the Ru layer inhibits the excessive diffusion Ag particles during thermal annealing, so that an interface void and an agglomeration can be prevented. Therefore, the excellent light reflectance which is inherent characteristic of Ag metal can be maintained without degradation even after thermal annealing. Similar results are obtained when the Cu-Ag reflective layer 310 and the Ru protective layer 320 of the ohmic electrode are replaced by the above- described layers.

[55] As described above, the ohmic electrode 300 of the LED in accordance with the exemplary embodiment has high bonding strength, low contact resistance, high light reflectance and excellent thermal stability. Therefore, the ohmic electrode can be employed in an LED of high output power requiring high current, for example, the above-described vertical type LED and a below-described flip-chip type LED. In this case, very excellent light emitting characteristics can be expected.

[56] Hereinafter, a flip-chip type LED in accordance with another exemplary embodiment will be described. Elements and configurations similar to those of the exemplary embodiment will be described briefly or omitted.

[57] FIG. 10 is a sectional view illustrating an LED having a flip chip structure in accordance with another exemplary embodiment.

[58] Referring to FIG. 10, the LED includes: a multilayered semiconductor layer 510 having an n-type layer 210, an active layer 220 and a p-type layer 230; an n-type electrode 520 disposed on a predetermined region of the n-type layer 210; a p-type electrode 530 disposed on the p-type layer 230; and a sub-mount substrate 540 connected to the electrodes 520 and 530 by metal bumps 541 and 542. Furthermore, the LED may include a diffusion layer 550 disposed under the n-type layer 210.

[59] The p-type electrode 300 includes a reflective layer 310 of Ag alloy and a protective layer 320. The p-type electrode 300 may be the ohmic electrode in accordance with the foregoing exemplary embodiment. The reflective layer 310 of Ag alloy may be formed of Ag, and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt, Pd and combinations thereof. The protective layer 320 is formed of one of Ru, Ir, Rh, Pt, W, Ta, Ti, Co and combinations thereof. For example, in this exemplary embodiment of FIG. 10, the reflective layer 310 is formed of Cu-Ag alloy in which Cu content is approximately 0.01% to 80%, and the protective layer 320 is formed of Ru. The p-type electrode 530 obtains excellent bonding strength and low contact resistance, because Ag particles diffuse into the interface between the p-type electrode and the semiconductor layer 510 during thermal annealing. Further, the Cu layer and the Ru layer inhibits an excessive diffusion of Ag particles during the thermal annealing to prevent an interface void and an agglomeration of the Ag layer, so that the p-type electrode 300 has high light reflectance and excellent thermal stability.

[60] The diffusion layer 550 uniformly distributes current applied to the n-type electrode

520 into the n-type layer 510 and effectively radiates heat transferred through the n- type layer 510, which allows high output power and improves reliability of the LED.

[61] A method for forming the LED having the above described configurations will now be described.

[62] An n-type layer 210, an active layer 220 and a p-type layer 230 are sequentially stacked on a base substrate (not shown) to form a semiconductor layer 510. A separate cell is formed by patterning process using a predetermined mask. A Cu-Ag reflective layer 310 and a Ru protective layer 320 are sequentially stacked on the p-type layer 230, and annealed in an air at approximately 400⁰C for approximately 1 minute to form an ohmic electrode, i.e., a p-type electrode 530 having low contact resistance and excellent light reflectance. Then an n-type electrode is formed on a portion of the n- type layer. A sub-mount substrate 540 is connected to the electrodes 520 and 530 by metal bumps 541 and 542. The base substrate is removed by a lift-off process using laser. A diffusion layer 550 is attached to a lower portion of the base substrate. The diffusion layer 550 may be formed of a material having excellent conductivity and excellent transparency such as indium tin oxide (ITO) and indium zinc oxide (IZO) to effectively diffuse current applied to the n-type electrode 520 and effectively emit light generated by the active layer 220 to the outside.

[63] When a power is applied to the electrodes 520 and 530 of the LED having the above- described configurations, electrons are injected into the active layer 220 from the n- type layer 210, and holes are injected into the active layer 220 from the p-type layer 230. The electrons and holes injected into the active layer 220 combine or recombine with each other to output excitation energy as light. The light is emitted to the outside through a light-emitting surface, i.e., a diffusion layer 550. Since the light emitted through a top surface of the active layer 220, i.e., through the p-type layer 230, is reflected back by the reflective layer 320 of the p-type electrode 530 and then emitted to the outside through the light-emitting surface (that is, the diffusion layer 550) to enhance utilization efficiency of the light.

[64] Although the ohmic electrode and method for forming the same have been described with reference to the specific embodiments, it is not limited thereto. Therefore, it will be readily understood by those skilled in the art that various modifications and changes can be made thereto without departing from the spirit and scope of the present invention defined by the appended claims.

Claims

[I] An ohmic electrode disposed on a semiconductor layer of a light emitting device (LED), the ohmic electrode comprising: a reflective layer formed of Ag alloy; a protective layer disposed on the reflective layer to inhibit an out-diffusion of the reflective layer; and an interface between the reflective layer and the semiconductor layer, wherein the interface is formed as a portion of particles of the reflective layer is in-diffused towards the semiconductor layer and metallized. [2] The ohmic electrode of claim 1, wherein the Ag alloy comprises Ag, and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt, Pd and combinations thereof. [3] The ohmic electrode of claim 2, wherein a composition of the alloying element in the Ag alloy is in a range of approximately 0.01% to 80%. [4] The ohmic electrode of claim 1, wherein the protective layer comprises one of

Ru, Ir, Rh, Pt, W, Ta, Ti, Co and combinations thereof. [5] The ohmic electrode of claim 1, wherein a thickness of the reflective layer is in a range of approximately 50 A to 5,000 A, and a thickness of the protective layer is in a range of approximately 50 A to 1,000 A. [6] The ohmic electrode of claim 5, wherein a total thickness of the reflective layer and the protective layer is in a range of approximately 100 A to 5,000 A.

[7] A method for forming an ohmic electrode on a semiconductor layer of a light emitting device (LED), the method comprising: forming a reflective layer formed of Ag alloy on the semiconductor layer; forming a protective layer to inhibit out-diffusion of Ag particles; and performing thermal annealing of the reflective layer and the protective layer so that a portion of the Ag particles are metallized at an interface between the reflective layer and the semiconductor layer while the protective layer inhibits the out-diffusion of the Ag particles.

[8] The method of claim 7, wherein the Ag alloy comprises Ag and one alloying element of Cu, Al, Ir, In, Ni, Mg, Pt, Pd and combinations thereof.

[9] The method of claim 8, wherein a composition of the alloying element in the Ag alloy is in a range of approximately 0.01% to 80%.

[10] The method of claim 7, wherein the protective layer comprises one of Ru, Ir, Rh,

Pt, W, Ta, Ti, Co and combinations thereof.

[I I] The method of claim 7, wherein the thermal annealing is performed at a temperature of approximately 15O⁰C to 600 ⁰C.

[12] The method of claim 11, wherein the thermal annealing is performed in an oxygen containing atmosphere.

[13] The method of claim 12, wherein the oxygen containing atmosphere comprises one of an oxygen atmosphere, an ambient air, a mixture atmosphere of oxygen and nitrogen, a mixture atmosphere of oxygen and argon and combinations thereof.