|Publication number||USRE42636 E1|
|Application number||US 12/662,196|
|Publication date||Aug 23, 2011|
|Filing date||Apr 5, 2010|
|Priority date||Jul 26, 2000|
|Also published as||CA2412416A1, CA2412416C, DE60143120D1, EP1320894A1, EP1320894A4, EP1320894B1, US6420736, US6642549, US20030010994, WO2002009185A1|
|Publication number||12662196, 662196, US RE42636 E1, US RE42636E1, US-E1-RE42636, USRE42636 E1, USRE42636E1|
|Inventors||John Chen, Bingwen Liang, Robert Shih|
|Original Assignee||Dalian Lumei Optoelectronics Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Classifications (15), Legal Events (7)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates to an improved window for a gallium nitride (GaN)-based light-emitting diode (LED).
A semiconductor light-emitting diode (LED) includes a substrate, a light emitting region, a window structure, and a pair of electrodes for powering the diode. The substrate may be opaque or transparent. Light-emitting diodes which are based on gallium nitride (GaN) compounds generally include a transparent, insulating substrate, i.e., a sapphire substrate. With a transparent substrate, light may be utilized from either the substrate or from the opposite end of the LED which is termed the “window”.
The amount of light generated by an LED is dependent on the distribution of the energizing current across the face of the light emitting region. It is well known in semiconductor technology that the current flowing between the electrodes tends to concentrate in a favored path directly under the electrode. This current flow tends to activate corresponding favored portions of the light-emitting region to the exclusion of portions which fall outside the favored path. Further since such favored paths fall under the opaque electrode, the generated light reaching the electrode is lost. Prior art GaN LEDs have employed conductive current spreading layers formed of nickel/gold (Ni/Au), and have a gold (Au) window bond pad mounted on such layers. In such arrangements, the Ni/Au layer and/or the Au bond pad tend to peel during the wire bonding operation to the pad.
In one embodiment consistent with the present invention, light is utilized at the output of the window structure, which includes a very thin, semi-transparent nickel oxide/gold (NiOx/Au) contact layer formed on a p-doped nitride compound window layer; a semi-transparent amorphous conducting top window layer; and a p electrode structure formed of a titanium layer with a covering Au bond pad. The amorphous top layer, by way of example, may be formed of indium tin oxide (ITO), tin oxide (TO), or zinc oxide (ZnO). Layers of other amorphous, conductive, and semi-transparent oxide compounds also may be suitable for construction of the top window layer.
Advantageously, the thin NiOx/Au layer provides an excellent ohmic connection to both the amorphous current spreading conducting layer and to the magnesium (Mg)-doped GaN window layer. The highly conductive amorphous layer efficiently spreads current flowing between the electrodes across the light-emitting region to improve the efficiency of the device.
Additionally, the titanium electrode passes through both the amorphous conducting layer and the underlying Ni/Au to: (a) form an ohmic contact with those layers; (b) contact the p-doped top widow layer and form a Schottky diode connection therewith; and (c) provide good adhesion between the titanium (Ti) and the magnesiusm (Mg)-doped window layer. The Schottky diode connection forces current from the electrode into the amorphous conducting layer and eliminates the tendency of the prior art structures to concentrate current in a path directly under the electrode.
The FIGURE is a schematic depicting a cross-sectional view of an LED according to one embodiment consistent with the present invention.
The Figure depicts an LED according to one embodiment consistent with the present invention, as a GaN-based device in which light exits through window 109.
The LED of the Figure includes a sapphire substrate 101, buffer region 102, GaN substitute substrate layer 103, n cladding layer 104, active region 106, p cladding layer 107, window layers 108, 109, n electrode 105, and a window structure which includes window layers 108, 109, a thin NiOx/Au semi-transparent layer 110, a semi-transparent amorphous conducting layer 111, a titanium electrode 112, and a bond pad 113.
Layers 101 through 104, and layers 106 through 109, are grown in a Metal Organic Chemical Vapor Deposition (MOCVD) reactor. The details of MOCVD growth of the stated layers are well known in the semiconductor industry and will not be discussed herein.
The remaining components of the illustrative LED, namely, layers NiOx/Au layer 110, amorphous conducting layer 111, n electrode 105, p electrode 112, and bond pad 113, are formed by evaporation in an apparatus other than a MOCVD reactor. Such processes are well known in the semiconductor industry and are not described herein.
The Light-emitting Structure
The illustrative light-emitting structure of the Figure includes an n cladding layer 104, active region 106, and p cladding layer 107.
The n cladding layer 104 is formed of silicon-doped GaN.
In the illustrative example depicted by the Figure, active region 106 is a silicon-doped n-type gallium indium nitridie/gallium nitride (GaInN/GaN) multi-quantum well (MQW) structure. However, other forms of active regions may be utilized with the illustrative window structure.
The p cladding layer 107 is formed of Mg-doped aluminum gallium nitride (AlGaN).
The Window Layers
The first window layer 108 is formed of Mg-doped GaN. The window layer 108 has a nominal thickness of 300 nm.
The second window layer 109 is similarly formed of Mg-doped GaN. However, window layer 109 is more highly doped to permit an ohmic contact between layer 109 and the very thin NiOx/Au layer 110.
Completion of the MOCVD Growth Process
Growth of the p-type GaN layers is achieved with the introduction of gaseous flows of TMG with hydrogen (H2) as a carrier gas, NH3 as a group V material, and Mg as a dopant. In the absence of an appropriate cool down protocol, hydrogen passivation of the Mg may occur, in which case, the conductivity of the Mg-doped layer is reduced.
In order to avoid hydrogen passivation of the Mg-doped layers 107, 108, and 109, the following described cool-down protocol has been adopted upon completion of the MOCVD growth.
The resulting product exhibits the expected desired physical and electrical characteristics.
Formation of the Electrode Structures
The embodiment consistent with the present invention as depicted by the Figure, illustrates the locations of both p electrode layers 111, 112 and n electrode 105.
Layer 110 is a very thin, semi-transparent contact layer of NiOx/Au which is deposited over the entire exposed face of window layer 109. Opening 114 is formed in layers 110 and 111 to permit the deposit of a titanium adhesion layer 112 to contact window layer 109. Titanium forms a strong physical bond with layer 109 and thus tends to eliminate peeling during wire bonding. In addition to reaching through to layer 109, titanium structure 112 is deposited through and on top of amorphous layer 111. Titanium electrode 112 forms ohmic contacts with layers 110 and 111, and forms a Schottky diode contact with window layer 109. The Schottky diode connection to window layer 109 eliminates the current path directly under the electrode and forces current flowing between the electrodes into conducting layer 111.
The p electrode Au bond pad 113 is deposited on top of titanium layer 112 to form an ohmic contact.
Since the Mg-doped layers do not suffer from hydrogen passivation, it is not necessary to heat treat the structure to activate the Mg doping in those layers. However, Ni/Au layer 111 110 and the Ti and Au contact structures are heated in an atmosphere of molecular nitrogen and air. Thus, the Ni is converted to a form of nickel oxide. The described heat treatment improves the quality of the contact structures.
The invention has been described with particular attention to its preferred embodiment. However, it should be understood that variations and modifications within the spirit and scope of the invention may occur to those skilled in the art to which the invention pertains.
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|US6287947||Jun 8, 1999||Sep 11, 2001||Lumileds Lighting, U.S. Llc||Method of forming transparent contacts to a p-type GaN layer|
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|JPH1012921A||Title not available|
|U.S. Classification||257/99, 257/103, 257/E33.07, 257/94, 257/96|
|International Classification||H01L33/00, H01L33/14, H01L33/42, H01L33/32, H01L33/02|
|Cooperative Classification||H01L33/32, H01L33/42, H01L33/14, H01L33/02|
|Nov 8, 2010||AS||Assignment|
Owner name: AMERICAN XTAL TECHNOLOGY, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:CHEN, JOHN;LIANG, BINGWEN;SHIH, ROBERT;REEL/FRAME:025329/0013
Effective date: 20000724
|Dec 22, 2010||AS||Assignment|
Owner name: AXT, INC., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:AMERICAN XTAL TECHNOLOGY, INC.;REEL/FRAME:025761/0927
Effective date: 20011031
|Feb 22, 2011||AS||Assignment|
Effective date: 20030927
Owner name: LUMEI OPTOELECTRONICS CORP., CALIFORNIA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:AXT, INC.;REEL/FRAME:025842/0118
|Mar 17, 2011||AS||Assignment|
Owner name: DALIAN LUMEI OPTOELECTRONICS, CHINA
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:LUMEI OPTOELECTRONICS CORP.;REEL/FRAME:025973/0912
Effective date: 20071112
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Year of fee payment: 11
|Jul 14, 2014||FPAY||Fee payment|
Year of fee payment: 12