U.S. patent number RE42,636 [Application Number 12/662,196] was granted by the patent office on 2011-08-23 for window for gallium nitride light emitting diode.
This patent grant is currently assigned to Dalian Lumei Optoelectronics Corporation. Invention is credited to John Chen, Bingwen Liang, Robert Shih.
United States Patent |
RE42,636 |
Chen , et al. |
August 23, 2011 |
Window for gallium nitride light emitting diode
Abstract
A window structure for a gallium nitride (GaN)-based light
emitting diode (LED) includes a Mg+ doped p window layer of a GaN
compound; a thin, semi-transparent metal contact layer; and an
amorphous current spreading layer formed on the contact layer. The
contact layer is formed of NiO.sub.x/Au and the current spreading
layer is formed of Indium Tin Oxide. The p electrode of the diode
includes a titanium adhesion layer which forms an ohmic connection
with the current spreading layer and a Schottky diode connection
with the Mg+ doped window layer.
Inventors: |
Chen; John (Rowland Heights,
CA), Liang; Bingwen (Sunnyvale, CA), Shih; Robert
(Cernitos, CA) |
Assignee: |
Dalian Lumei Optoelectronics
Corporation (Dalian, CN)
|
Family
ID: |
24510400 |
Appl.
No.: |
12/662,196 |
Filed: |
April 5, 2010 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
09626445 |
Jul 26, 2000 |
6420736 |
Jul 16, 2002 |
|
|
Current U.S.
Class: |
257/99; 257/96;
257/103; 257/E33.07; 257/94 |
Current CPC
Class: |
H01L
33/42 (20130101); H01L 33/14 (20130101); H01L
33/32 (20130101); H01L 33/02 (20130101) |
Current International
Class: |
H01L
33/00 (20100101) |
Field of
Search: |
;257/99,94,96,103,E33.07 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
0622858 |
|
Nov 1994 |
|
EP |
|
10012921 |
|
Jan 1998 |
|
JP |
|
Primary Examiner: Ho; Tu-Tu V
Attorney, Agent or Firm: Edwards, Esq.; Jean C. Edwards
Neils PLLC
Claims
What is claimed is:
1. A light emitting diode comprising: a substrate; a light emitting
region; a window structure.[.; and first and second electrodes;
wherein said window structure comprises.]. .Iadd.comprising: an Mg+
doped window layer; .Iaddend. a semi-transparent .[.metal.].
.Iadd.NiO.sub.x/Au .Iaddend.contact layer.[.,.]. .Iadd.disposed on
said Mg+ doped window layer; .Iaddend.and a semi-transparent,
conductive amorphous current spreading layer formed directly on an
exposed face of said contact layer; .[.and wherein an opening is
formed through said contact layer and said current spreading layer
and said first electrode comprises a layer of titanium formed on
said current spreading layer and through said opening to contact an
upper surface of said Mg+ doped window layer..]. .Iadd.a first
electrode having a layer of titanium and formed on said current
spreading layer and forming an ohmic connection with said
current-spreading layer; a second electrode; and an opening formed
through said contact layer and said current spreading layer, such
that said layer of titanium of said first electrode contacts an
upper surface of said Mg+ doped window layer. .Iaddend.
.[.2. The light emitting diode in accordance with claim 1, wherein
said contact layer is a NiO.sub.x/Au layer..].
3. The light emitting diode in accordance with claim 1, wherein
said amorphous current spreading layer is formed of Indium Tin
Oxide.
.[.4. The light emitting diode diode in accordance with claim 2,
wherein said amorphous current spreading layer is formed of Indium
Tin Oxide..].
5. The light emitting diode in accordance with claim 1, wherein
said window structure comprises: a .[.Mg+ doped.]. .Iadd.further
.Iaddend.window layer.[.; and wherein said Ni/Au contact layer is
formed on said Mg+ doped window layer and said first electrode
forms an ohmic connection with said current spreading layer.]..
6. The light emitting diode in accordance with claim 5, wherein
said first electrode forms a Schottky diode connection with said
Mg+ doped window layer.
.[.7. The light emitting diode in accordance with claim 2, wherein
after heat treatment, said contact layer comprises a Ni oxide/Au
layer..].
8. A light emitting diode comprising: a substrate; a light emitting
region; a window structure; and first and second electrodes;
wherein said window structure comprises: an Mg+ doped window layer;
a semi-transparent NiO.sub.x/Au contact layer formed on said Mg+
doped window layer; and a semi-transparent, conductive amorphous
current spreading layer formed of indium tin oxide directly on an
exposed face of said contact layer; wherein said first electrode
forms an ohmic connection with said current spreading
layer.[.;.]..Iadd., .Iaddend.and forms a Schottky diode connection
with said Mg+ doped window layer; wherein an opening is formed
through said contact layer and said current spreading layer; and
wherein said first electrode comprises a layer of titanium formed
on said current spreading layer and through said opening to contact
an upper surface of said Mg+ doped window layer.
9. A light emitting diode comprising: a substrate; a buffer region;
a GaN substitute substrate layer; an n cladding layer; an active
region; a p cladding layer; .[.a double window layer structure;.].
an n electrode; a window structure comprising: .Iadd.a double
window layer structure; .Iaddend. a semi-transparent metal contact
layer.[.,.]..Iadd.; .Iaddend.and a semi-transparent, conductive
amorphous current spreading layer formed directly on an exposed
face of said contact layer; a titanium electrode; and a bond pad;
wherein an opening is formed through said contact layer and said
current spreading layer to said double window layer structure
.[.for said titanium electrode.]..Iadd., such that said titanium
electrode contacts an upper surface of said double window layer
structure.Iaddend..
Description
The present invention relates to an improved window for a gallium
nitride (GaN)-based light-emitting diode (LED).
BACKGROUND OF THE INVENTION
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.
SUMMARY OF THE INVENTION
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 (NiO.sub.x/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 NiO.sub.x/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.
BRIEF DESCRIPTION OF THE DRAWING
The FIGURE is a schematic depicting a cross-sectional view of an
LED according to one embodiment consistent with the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
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
.Iadd.window layers 108, 109, .Iaddend.a thin NiO.sub.x/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
NiO.sub.x/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 NiO.sub.x/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 (H.sub.2) as a carrier gas,
NH.sub.3 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. 1. The ambient gas of
the reactor is switched from H.sub.2 to nitrogen (N.sub.2)
immediately after completion of the LED structure; 2. The reactor
temperature is ramped down from the growth temperature to about 900
degrees C. in about 2 minutes; 3. The flow of NH.sub.3 is
terminated; 4. The reactor temperature is further ramped down to
about 750 degrees C. in about 2 minutes; 5. A temperature of about
750 degrees C. is held for about 20 minutes; 6. The heater of the
reactor is shut off and the reactor is allowed to complete
cool-down naturally. Experience shows that cool-down to 120 degrees
C. occurs in about 30 minutes after heater shut off.
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
NiO.sub.x/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.]. .Iadd.110
.Iaddend.and the Ti and Au contact structures are heated in an
atmosphere of molecular nitrogen and air. Thus.Iadd., .Iaddend.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.
* * * * *