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  Light-emitting deviceUSPTO Application #: 20080054278Title: Light-emitting device Abstract: A light-emitting device includes a substrate, a first nitride semiconductor stack formed on the substrate, a nitride light-emitting layer formed on the first nitride semiconductor stack, a second nitride semiconductor stack formed on the nitride light-emitting layer, and a first transparent conductive oxide layer formed on the second nitride semiconductor stack. The second nitride semiconductor stack includes a plurality of hexagonal-pyramid cavities formed in an upper surface of the second nitride semiconductor stack. The plurality of hexagonal-pyramid cavities of the second nitride semiconductor stack are filled with the first transparent conductive oxide layer, and a low-resistance ohmic contact is generated at the inner surfaces of the plurality of hexagonal-pyramid cavities so as to decrease the operation voltage and improve light-emitting efficiency of the light-emitting device. (end of abstract) Agent: Quintero Law Office, PC - Santa Monica, CA, US Inventors: Chen Ou, Ting-Yang Lin, Shih-Kuo Lai USPTO Applicaton #: 20080054278 - Class: 257095000 (USPTO) Related Patent Categories: Active Solid-state Devices (e.g., Transistors, Solid-state Diodes), Incoherent Light Emitter Structure, With Heterojunction, With Contoured External Surface (e.g., Dome Shape To Facilitate Light Emission) The Patent Description & Claims data below is from USPTO Patent Application 20080054278. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] The present invention relates to a light-emitting device, and more particularly, to a high efficiency light-emitting device. [0003] 2. Description of the Prior Art [0004] Semiconductor light-emitting devices have been applied widely in optical display devices, traffic signals, data storing devices, communication devices, illumination devices, and medical apparatuses. [0005] The conventional nitride LED includes a thin metallic layer on a top surface of the LED, such as material of the Ni/Au group, regarded as a transparent conductive layer. However, part of LED light still cannot travel through metal. Light generated by the LED is absorbed by the thin metallic layer and the light transmittance is decreased. In order to have a good transmittance, the thickness of the thin metallic layer is limited to be within several tens to several hundreds of angstroms. Although the thickness of the thin metallic layer is limited, the thin metallic layer merely has transmittance of visible light in the range of 60%.about.70%, and the light-emitting efficiency of the LED is still low. [0006] U.S. Pat. No. 6,078,064, which is included herein by reference, discloses an LED structure. The surface of LED includes a transparent conductive oxide layer formed on a p-type contact layer of a high carrier concentration. Generally, the transparent conductive oxide layer has a high transmittance of more than 90%. Therefore, the thickness of such layer can be thicker and the current spreading is better, such that the brightness and light-emitting efficiency of the LED are improved. Note that the transparent conductive oxide layer must contact with the p-type contact layer of a high carrier concentration more than 5.times.10.sup.18 cm.sup.-3, so as to form a better ohmic contact. [0007] Taiwan Patent No. 144,415, which is incorporated herein by reference, discloses a method for forming a reverse tunneling layer. An N+ reverse tunneling contact layer is formed between a transparent oxide electrode layer and a semiconductor light-emitting layer to achieve the purpose of forming a good ohmic contact so as to improve the light-emitting efficiency of the LED and decrease the operation voltage. [0008] In addition, Y. C. Lin also disclosed a related method in the paper "InGaN/GaN Light Emitting Diodes with Ni/Au, Ni/ITO and ITO p-Type Contacts" (Solid-State Electronics Vol. 47 Page 849-853). He disclosed that a thin metallic layer was formed on a p-type contact layer of a nitride LED, and then a transparent conductive oxide layer was formed on the thin metallic layer. This method can efficiently reduce the contact resistance between the p-type contact layer and the transparent conductive oxide layer. However, the transmittance is still decreased by the thin metallic layer and thus the light-emitting efficiency of the LED is still affected by the thin metallic layer. [0009] Therefore, the present invention aims to improve the brightness of an LED, to solve the contact resistance issue occurring between such a contact layer and transparent conductive oxide layer, and to simplify the process complexity. SUMMARY OF THE INVENTION [0010] It is therefore an object of the claim invention to provide a light-emitting device with high transmittance to solve the above-mentioned problems. [0011] The claimed invention discloses a light-emitting device. The light-emitting includes a substrate, a first nitride semiconductor stack formed on the substrate, a nitride light-emitting layer formed on the first nitride semiconductor stack, a second nitride semiconductor stack formed on the nitride light-emitting layer having a plurality of hexagonal-pyramid cavities on the surface of the second nitride semiconductor layer opposite to the nitride light-emitting layer, wherein the hexagonal-pyramid cavity extends downward from the surface of the second nitride semiconductor layer, and a first transparent conductive oxide layer formed on the second nitride semiconductor stack. The plurality of hexagonal-pyramid cavities of the second nitride semiconductor stack are filled with the first transparent conductive oxide layer, such that a low-resistance ohmic contact is generated between the transparent conductive oxide layer and the inner surfaces of the plurality of hexagonal-pyramid cavities. [0012] In general, if the second nitride semiconductor stack is p-type material and its surface opposite to the nitride light-emitting layer is flat smooth, and parallel to the substrate surface, the transparent conductive oxide layer cannot directly form a good ohmic contact with the p-type nitride semiconductor stack and thereby increases the operation voltage. [0013] In contrast, the claimed invention provides a plurality of hexagonal-pyramid cavities in the surface of the p-type nitride semiconductor stack opposite to the nitride light-emitting layer, wherein the hexagonal-pyramid cavity extends downward from the surface of the second nitride semiconductor layer, and then forms a transparent conductive oxide layer over the surface, wherein the transparent conductive oxide layer contacts not only the flat region of the surface of the p-type nitride semiconductor without cavity area (hereinafter "flat outer surface"), but also the inner surfaces of the hexagonal-pyramid cavities (below called "cavity inner surface"). The surface energy state of the flat outer surface differs from that of the cavity inner surfaces. The difference between the surface energy states is contributed by the difference of the crystal directions as well as the difference in the surface energy potential between the flat outer surface and the cavity inner surfaces. If the transparent conductive oxide layer is directly formed on the flat outer surface of the p-type nitride semiconductor stack, the interface between the transparent conductive oxide layer and the flat outer surface has a higher potential barrier leading to a higher contact resistance. However, when the transparent conductive oxide layer contacts with the cavity inner surfaces, since the lower potential barrier between interface of the cavity inner surface and the transparent conductive oxide layer, a good ohmic contact can be formed. Therefore, the p-type layer does not need a high carrier concentration as mentioned in the previous prior art. The operation voltage of the device can be reduced to the level as the conventional Ni/Au based LED. [0014] When operation current is applied, current is first spread through the transparent conductive oxide layer, then flowing into the p-type nitride semiconductor stack mainly through the lower resistance contact area of the cavity inner surfaces contacting with the transparent conductive oxide layer, and finally flowing to the light-emitting layer to generate light. [0015] Furthermore, the other advantages of the claimed invention of the hexagonal-pyramid cavities are that the hexagonal-pyramid cavities can effectively reduce both the total reflection effect on the device surface and the light absorption effect of the p-type nitride semiconductor stack. Thus the light-emitting efficiency can be further enhanced. Besides, the light transmittance of the transparent conductive oxide layer is better than that of the conventional thin metallic layer. Constantly, the claimed invention can greatly improve the light-emitting efficiency of the device and can provide the device a low operation voltage. [0016] These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0017] FIG. 1 is a diagram of a first embodiment of the light-emitting device according to the present invention. [0018] FIG. 2 is a diagram of a p-type nitride semiconductor stack of a plurality of hexagonal-pyramid cavities according to the present invention. [0019] FIG. 3 is a graph of brightness of the present invention light-emitting device vs. density of the hexagonal-pyramid cavities. [0020] FIG. 4 is a graph of brightness of the present invention light-emitting device vs. diagonal length of hexagonal-pyramid cavities. [0021] FIG. 5 is a graph of brightness of the present invention light-emitting device vs. depth of hexagonal-pyramid cavities. Continue reading... Full patent description for Light-emitting device Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Light-emitting device patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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