U.S. patent application number 12/221304 was filed with the patent office on 2009-06-04 for substrate-free light emitting diode chip.
This patent application is currently assigned to Goldeneye, Inc.. Invention is credited to Karl W. Beeson, William R. Livesay, Richard L. Ross, Scott M. Zimmerman.
Application Number | 20090140279 12/221304 |
Document ID | / |
Family ID | 40674813 |
Filed Date | 2009-06-04 |
United States Patent
Application |
20090140279 |
Kind Code |
A1 |
Zimmerman; Scott M. ; et
al. |
June 4, 2009 |
Substrate-free light emitting diode chip
Abstract
A light emitting diode (LED) chip has a multilayer semiconductor
structure that is at least 10 microns thick and does not require an
attached growth substrate or transfer substrate for structural
rigidity or support. The multilayer semiconductor structure
includes a first doped layer, a second doped layer and an active
region interposed between the first doped layer and the second
doped layer. Optionally, the multilayer semiconductor structure
includes an undoped layer. At least one of the layers of the
multilayer semiconductor structure is at least 5 microns thick and
is preferably deposited by hydride vapor phase epitaxy.
Inventors: |
Zimmerman; Scott M.;
(Basking Ridge, NJ) ; Beeson; Karl W.; (Princeton,
NJ) ; Livesay; William R.; (San Diego, CA) ;
Ross; Richard L.; (Del Mar, CA) |
Correspondence
Address: |
William Propp, Esq.;Goldeneye, Inc.
Suite 233, 9747 Businesspark Avenue
San Diego
CA
92131
US
|
Assignee: |
Goldeneye, Inc.
|
Family ID: |
40674813 |
Appl. No.: |
12/221304 |
Filed: |
July 31, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61005258 |
Dec 3, 2007 |
|
|
|
Current U.S.
Class: |
257/98 ;
257/E33.023 |
Current CPC
Class: |
H01L 33/62 20130101;
H01L 2924/0002 20130101; H01L 25/0753 20130101; H01L 33/32
20130101; H01L 33/508 20130101; H01L 33/648 20130101; H01L
2933/0083 20130101; H01L 33/505 20130101; H01L 33/42 20130101; H01L
2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/98 ;
257/E33.023 |
International
Class: |
H01L 33/00 20060101
H01L033/00 |
Claims
1. An substrate-free, gallium-nitride-based light emitting diode
chip comprising: a multilayer semiconductor structure that has a
first side and an opposing second side and that includes a first
doped layer proximal to said first side, an active region, a second
doped layer proximal to said second side with said active region
interposed between said first doped layer and said second doped
layer; a first electrode in electrical contact with said first
doped layer; and a second electrode in electrical contact with said
second doped layer; wherein said multilayer semiconductor has a
total thickness of at least 10 microns and wherein said active
region emits internally generated light when an electrical voltage
is applied between said first electrode and said second
electrode.
2. The light emitting diode chip as in claim 1, wherein said
multilayer semiconductor structure is at least 20 microns
thick.
3. The light emitting diode chip as in claim 2, wherein said
multilayer semiconductor structure is at least 30 microns
thick.
4. The light emitting diode chip as in claim 1, wherein at least a
portion of said first side of said multilayer semiconductor
structure and at least a portion of said second side of said
multilayer semiconductor structure transmit externally incident
light.
5. The light emitting diode chip as in claim 4, wherein said light
emitting diode chip transmits at least 60 percent of said
externally incident light that is directed to said first side or
said second side of said multiplayer semiconductor structure.
6. The light emitting diode chip as in claim 5, wherein said light
emitting diode chip transmits at least 70 percent of said
externally incident light.
7. The light emitting diode chip as in claim 6, wherein said light
emitting diode chip transmits at least 80 percent of said
externally incident light.
8. The light emitting diode chip as in claim 1, further comprising
a reflective surface covering substantially all of said first side
or substantially all of said second side of said multilayer
semiconductor structure, wherein said reflective surface reflects
externally incident light.
9. The light emitting diode chip of claim 8, wherein said
reflective surface is said first electrode or said second
electrode.
10. The light emitting diode chip as in claim 8, wherein said light
emitting diode chip reflects at least 60 percent of said externally
incident light that is directed to said multiplayer semiconductor
structure.
11. The light emitting diode chip as in claim 10, wherein said
light emitting diode chip reflects at least 70 percent of said
externally incident light.
12. The light emitting diode chip as in claim 11, wherein said
light emitting diode chip reflects at least 80 percent of said
externally incident light.
13. The light emitting diode chip as in claim 1, wherein said
multiplayer semiconductor structure has an absorption coefficient
less that 20 per centimeter.
14. The light emitting diode chip as in claim 13, wherein said
multiplayer semiconductor structure has an absorption coefficient
less that 10 per centimeter.
15. The light emitting diode chip as in claim 14, wherein said
multiplayer semiconductor structure has an absorption coefficient
less that 5 per centimeter.
16. The light emitting diode chip as in claim 15, wherein said
multiplayer semiconductor structure has an absorption coefficient
less that 2 per centimeter.
17. The light emitting diode chip as in claim 1, wherein said first
doped layer or said second doped layer is at least 5 microns thick
and is fabricated by hydride vapor phase epitaxy.
18. The light emitting diode chip as in claim 17, wherein said
first doped layer or said second doped layer is at least 10 microns
thick.
19. The light emitting diode chip as in claim 18, wherein said
first doped layer or said second doped layer is at least 15 microns
thick.
20. The light emitting diode chip as in claim 19, wherein said
first doped layer or said second doped layer is at least 25 microns
thick.
21. The light emitting diode chip as in claim 1, wherein said
multiplayer semiconductor structure includes a substantially
undoped layer adjacent to said first side or said second side,
wherein said undoped layer is at least 5 microns thick and wherein
said undoped layer is fabricated by hydride vapor phase
epitaxy.
22. The light emitting diode chip as in claim 21, wherein said
undoped layer is at least 10 microns thick.
23. The light emitting diode chip as in claim 22, wherein said
undoped layer is at least 15 microns thick.
24. The light emitting diode chip as in claim 23, wherein said
undoped layer is at least 25 microns thick.
25. The light emitting diode chip as in claim 1, wherein said first
doped layer is an n-doped layer and said second doped layer is a
p-doped layer.
Description
REFERENCE TO PRIOR APPLICATION
[0001] This application claims the benefit of U.S. Provisional
Patent Application Ser. No. 61/005,258, which was filed on Dec. 3,
2007, and which is herein incorporated by reference.
TECHNICAL FIELD
[0002] The present invention is a thick light emitting diode chip.
The chip is substrate-free or free-standing and requires no growth
substrate or transfer substrate for structural support.
BACKGROUND OF THE INVENTION
[0003] Conventional light emitting diodes (LEDs) are fabricated by
epitaxially growing multiple layers of semiconductors on a growth
substrate. Inorganic light-emitting diodes can be fabricated from
GaN-based semiconductor materials containing, for example, gallium
nitride (GaN), aluminum nitride (AlN), aluminum gallium nitride
(AlGaN), indium nitride (InN), indium gallium nitride (InGaN) and
aluminum indium gallium nitride (AlInGaN). Other appropriate
materials for LEDs include, for example, aluminum gallium indium
phosphide (AlGaInP), gallium arsenide (GaAs), indium gallium
arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP),
diamond, boron nitride and zinc oxide (ZnO).
[0004] The total thickness of the semiconductor layers for a
conventional GaN-based LED excluding a substrate is less than about
5 microns and usually only about 3 to 4 microns. The layers are
fabricated by epitaxially growing a multilayered semiconductor
structure on a growth substrate using metal organic chemical vapor
deposition (MOCVD), which has a very slow growth rate of
approximately 0.1 micron per hour. This results in deposition times
of tens of hours and makes the growth of thicker layers
prohibitively expensive. The approximately 3 to 4 micron thick
multilayer semiconductor structure is very fragile and will break
easily if removed from the growth substrate to form a free-standing
or substrate-free die. The semiconductor layers must therefore
either remain attached to the growth substrate or, alternatively,
be attached to a transfer substrate, using wafer bonding techniques
before being removed from the growth substrate. The wafer bonding
techniques are expensive and can be unreliable. The added steps
increase the cost of manufacturing LEDs. Removal of the growth
substrate can be done by a laser liftoff process, chemical
processing or mechanical polishing.
[0005] The growth substrate for GaN-based LEDs is usually sapphire
or silicon carbide and is chosen to closely match the
crystallographic structure of the epitaxial layers. A transfer
substrate, if utilized, can be a metal, another semiconductor
material such as silicon, or a ceramic material such as aluminum
nitride. Such growth or transfer substrates may not suitable for
the final LED device. For example, sapphire is a poor thermal
conductor and is therefore not the most effective thermal conductor
to direct heat away from the semiconductor layers. Thermal
considerations are very important for LEDs, which generate a
significant amount of heat during operation. The heat lowers the
light output and operating lifetime of the LED. As LED sizes become
larger, such heating effects become more important and can
seriously degrade the light-output performance and lifetime of the
LEDs.
[0006] Removing the growth substrate and attaching a transfer
substrate to the semiconductor layers also adds thermal resistance
to the device. The increased thermal resistance can come from the
transfer substrate itself and from the bonding layer used to attach
the transfer substrate.
[0007] In addition, the growth or transfer substrate may absorb
some of the light emitted by the LED, thereby lowering the optical
output. The substrate may also trap or reflect some of the light
generated by the LED within the LED, resulting in an additional
loss in optical output. Light trapping is caused by the high
refractive index of the substrate relative to air and results in
total internal reflection of emitted light back through the
substrate and back through the epitaxial layers.
[0008] It would be desirable to develop thick, substrate-free or
free-standing LED chips that do not need the original growth
substrate or an attached transfer substrate for structural support.
Such thick, substrate-free or free-standing LED chips could be
easily manipulated, lifted or handled without breaking and could be
subsequently bonded to other surfaces or leadframes in any desired
pattern to form light emitting devices. Different growth techniques
will be required to make such a structure since MOCVD is too slow
to fabricate thick multilayer semiconductor structures.
[0009] In conventional LED designs, the back side of the LED
opposite the light emitting side is a reflective surface. It would
also be desirable to develop LED chips that do not have a back
reflecting surface and that can emit light from all sides.
Eliminating the back reflecting surface can reduce the average
optical pathlength of the emitted light within the LED structure,
thereby reducing optical absorption within the LED, increasing the
external quantum efficiency and increasing the light output of the
LED.
[0010] Conventional LEDs are cooled by a submount or heat sink in
thermal contact with the LED die. The conventional LED die includes
either a growth substrate or a transfer substrate. Heat flows from
the LED semiconductor layers, through the growth or transfer
substrate and through the submount or heat sink to ambient. The
heat sink may include fins or other types of structures to transfer
heat to an ambient fluid, such as air or water. It would be
desirable to develop LED light sources where the LED die does not
include a growth substrate or a transfer substrate and where the
LED die can be cooled directly without having the added thermal
resistance of a growth substrate or a transfer substrate.
[0011] The deficiencies of conventional LEDs described above can be
eliminated by the various embodiments of this invention that are
described below in the summary, the figures and the detailed
descriptions of the preferred embodiments.
SUMMARY OF THE INVENTION
[0012] A substrate-free or free-standing LED chip of this invention
is an LED chip that does not include a growth substrate or transfer
substrate as an element of the LED chip. The growth substrate is
defined as the substrate onto which the multilayer semiconductor
structure is epitaxially grown. For LED chips of this invention,
the growth substrate is removed after the multilayer semiconductor
structure is fabricated and no transfer substrate is permanently
bonded to the multilayer semiconductor structure prior to the
removal of the growth substrate. The LED chips of this invention
have multilayer semiconductor structures that are at least 10
microns thick and do not require an attached growth substrate or
transfer substrate for structural rigidity or support. The LED
chips can be handled without damage and without breaking.
[0013] After the LED chips of this invention are fabricated, the
substrate-free or free-standing LED chips may be packaged into more
complex LED light sources. If desired, the LED chips may be
attached to a surface, a submount, a heat sink, a leadframe or to
any other structure. However, none of these additional surfaces,
submounts, heat sinks, leadframes or other structures are elements
of the LED chip.
[0014] One embodiment of this invention is a thick light emitting
diode chip. The LED chip is substrate-free, which means that the
chip requires no growth substrate or transfer substrate for
structural support. The LED chip includes a multilayer
semiconductor structure that has a first side and an opposing
second side. The multilayer semiconductor structure includes a
first doped layer proximal to the first side, an active region, and
a second doped layer proximal to the second side with the active
region interposed between the first doped layer and the second
doped layer. The first doped layer can be an n-doped layer and the
second doped layer can be a p-doped layer or the polarities of the
two layers can be reversed. The total thickness of the multilayer
semiconductor structure is at least 10 microns, preferably at least
20 microns and more preferably at least 30 microns. The LED chip
also includes a first electrode in electrical contact with the
first doped layer and a second electrode in electrical contact with
the second doped layer. The active region emits internally
generated light when a voltage is applied between the first
electrode and the second electrode.
[0015] An LED chip that has both electrodes on the first side or
the second side of the multilayer semiconductor structure may
optionally include a substantially undoped layer on the side of the
multilayer semiconductor structure opposite the electrodes. The
undoped layer can add additional thickness to the multilayer
semiconductor structure without affecting the electrical properties
of the structure. The additional thickness adds to the structural
strength of the multilayer semiconductor structure. In addition,
the heat transfer characteristics of an additional undoped layer
fabricated from the same semiconductor material (for example,
gallium nitride) as the remainder of the multilayer semiconductor
structure will be better than the heat transfer characteristics of
a growth substrate or transfer substrate of the prior art made from
a different material.
[0016] The multilayer semiconductor structure of the thick LED chip
has at least one thick semiconductor layer to provide structural
support to the chip. The thick semiconductor layer can be the first
doped layer, the second doped layer, the optional undoped layer or
a combination of two or more thick layers. The thick semiconductor
layers preferably are fabricated by hydride vapor phase epitaxy
(HVPE). Each thick semiconductor layer is at least 5 microns thick,
preferably at least 10 microns thick, more preferably at least 15
microns thick and most preferably at least 25 microns thick. When
the LED chip includes at least one thick semiconductor layer to
provide structural support, the growth or transfer substrate is no
longer needed and the LED chip can be handled as a substrate-free
or free-standing device without damage.
[0017] Another embodiment of this invention is a substrate-free LED
chip where at least a portion of the first side of the multilayer
semiconductor structure and a portion of the second side of the
multilayer semiconductor structure transmit externally incident
light. Externally incident light is light that is directed to the
LED chip from another light source or light that is emitted by the
LED chip and that is recycled back to the chip. Preferably the LED
chip transmits at least 60 percent of the externally incident
light, more preferably the LED chip transmits at least 70 percent
of the externally incident light and most preferably the LED chip
transmits at least 80 percent of the externally incident light.
[0018] Another embodiment of this invention is a substrate-free LED
chip where substantially all of the first side of the multilayer
semiconductor structure or substantially all of the second side of
the multilayer semiconductor structure reflects light and where,
respectively, the second side or the first side is a light emitting
side of the LED. Preferably the LED chip reflects at least 60
percent of the externally incident light directed to the light
emitting side of the chip, more preferably the LED chip reflects at
least 70 percent of the externally incident light and most
preferably the LED chip reflects at least 80 percent of the
externally incident light.
[0019] In order for the substrate-free LED chip to have high
external quantum efficiency and to have either high transmission or
high reflectivity to externally incident light, the multilayer
semiconductor structure should have low optical absorption. The
absorption coefficient of the multilayer semiconductor structure
should be less than 20 per centimeter, preferably less than 10 per
centimeter, more preferably less than 5 per centimeter and most
preferably less than 2 per centimeter.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] A more detailed understanding of the present invention, as
well as other objects and advantages thereof not enumerated herein,
will become apparent upon consideration of the following detailed
description and accompanying drawings, wherein:
[0021] FIGS. 1A and 1B are illustrations of conventional LED chip
of the prior art that has two electrodes on the upper side and
includes a growth substrate. FIG. 1A is a top plan view of the LED.
FIG. 1B is a side cross-sectional view along the I-I plane
illustrated in FIG. 1A.
[0022] FIG. 2 is a side cross-sectional view of conventional LED
chip of the prior art that has two electrodes on the lower side and
includes a growth substrate.
[0023] FIG. 3 is a side cross-sectional view of conventional LED
chip of the prior art that has two electrodes on the lower side and
includes a transfer substrate.
[0024] FIGS. 4A and 4B are illustrations of conventional LED chip
of the prior art that has one electrode on the upper side, one
electrode on the lower side and includes a transfer substrate. FIG.
4A is a top plan view of the LED. FIG. 4B is a side cross-sectional
view along the I-I plane illustrated in FIG. 4A.
[0025] FIG. 4C is a heat flow diagram of the conventional LED chip
illustrated in FIGS. 4A and 4B.
[0026] FIGS. 5A, 5B and 5C illustrate a substrate-free LED chip of
the present invention that has two upper electrodes, a thick first
doped layer and a lower reflecting layer. FIG. 5A is a top plan
view of the chip. FIGS. 5B and 5C are side cross-sectional views
along the I-I plane illustrated in FIG. 5A.
[0027] FIG. 5D is a diagram of the heat flow from a substrate-free
LED chip of the present invention to ambient.
[0028] FIGS. 6A and 6B are side cross-sectional views of
substrate-free LED chip of the present invention that has two upper
electrodes and no lower reflecting layer.
[0029] FIG. 7 is a side cross-sectional view of substrate-free LED
chip of the present invention that has two upper electrodes, a
thick second doped layer and a lower reflecting layer.
[0030] FIG. 8 is a side cross-sectional view of substrate-free LED
chip that has two upper electrodes, a thick first doped layer, a
thick second doped layer and a lower reflecting layer.
[0031] FIG. 9 is a side cross-sectional view of a substrate-free
LED chip of the present invention that has two upper electrodes, a
thick undoped layer and no lower reflecting layer.
[0032] FIG. 10 is a side cross-sectional view of a substrate-free
LED chip of the present invention that has two lower
electrodes.
[0033] FIG. 11 is a side cross-sectional view of another
substrate-free LED chip of the present invention that has two lower
electrodes.
[0034] FIGS. 12A and 12B illustrate a substrate-free LED of the
present invention that has one upper electrode and one lower
electrode. FIG. 12A is a top plan view. FIG. 12B is a side
cross-sectional view along the I-I plane illustrated in FIG.
12A.
[0035] FIG. 13 is a side cross-sectional view of another
substrate-free LED of the present invention that has one upper
electrode and one lower electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The preferred embodiments of the present invention will be
better understood by those skilled in the art by reference to the
above listed figures. The preferred embodiments of this invention
illustrated in the figures are not intended to be exhaustive or to
limit the invention to the precise form disclosed. The figures are
chosen to describe or to best explain the principles of the
invention and its applicable and practical use to thereby enable
others skilled in the art to best utilize the invention. For ease
of understanding, the thicknesses of the layers in the
semiconductor structures in the figures are not drawn to scale.
[0037] Light emitting diodes can be fabricated by epitaxially
growing multiple layers of semiconductors on a growth substrate.
Inorganic light-emitting diodes can be fabricated from gallium
nitride (GaN) based semiconductor materials containing, for
example, gallium nitride (GaN), aluminum nitride (AlN), aluminum
gallium nitride (AlGaN), indium nitride (InN), indium gallium
nitride (InGaN) and/or aluminum indium gallium nitride (AlInGaN).
Other appropriate materials for LEDs include, for example, aluminum
gallium indium phosphide (AlGaInP), gallium arsenide (GaAs), indium
gallium arsenide (InGaAs), indium gallium arsenide phosphide
(InGaAsP), diamond, boron nitride (BN) and zinc oxide (ZnO).
[0038] Especially important LEDs for this invention are GaN-based
LEDs that utilize epitaxially-grown layers that can include, for
example, GaN, AlN, AlGaN, InN, InGaN or AlInGaN. Depending on the
composition of the semiconductor layers, GaN-based LEDs emit light
in the ultraviolet, blue, cyan or green regions of the optical
spectrum. The growth substrate for GaN-based LEDs is typically
sapphire (Al.sub.2O.sub.3), silicon carbide (SiC), bulk gallium
nitride or bulk aluminum nitride. Although the embodiments of this
invention will be described using GaN-based LEDs, other types of
LEDs including, but not limited to, AlGaInP and ZnO LEDs may also
be utilized in the embodiments.
[0039] Typical epitaxial growth methods for thin semiconductor
layers of GaN-based materials include chemical vapor deposition
(CVD), metal-organic chemical vapor deposition (MOCVD), vapor phase
epitaxy (VPE), hydride vapor phase epitaxy (HVPE) and molecular
beam epitaxy (MBE). MOCVD is the most common method for
conventional GaN-based LEDs where the total thickness of the
epitaxial layers is less than about 5 microns. MOCVD is a
relatively slow deposition method with growth rates of
approximately 0.1 micron per hour. HVPE has much higher growth
rates (10 microns per hour is possible). HVPE can be used to
fabricate substrate-free LEDs that are described in this invention
and where the thickness of one of the layers is at least 5 microns
and could be as much as 30 microns or more.
[0040] Conventional GaN-based LED chips of the prior art are
fabricated so that the total thickness of the epitaxial
semiconductor layers, which include a first doped layer, an active
region, and a second doped layer, with the active region interposed
between the first doped layer and the second doped layer, is less
than about 5 microns thick. The complete set of epitaxial
semiconductor layers is denoted in this application as the
multilayer semiconductor structure. The first doped layer and the
second doped layers of the multilayer semiconductor structure can
be, respectively, an n-doped layer and a p-doped layer or the
layers can be reversed so that the first doped layer is a p-doped
layer and the second doped layer is an n-doped layer.
[0041] When the multilayer semiconductor structure is less that
about 5 microns thick, it is too fragile to form a self-supporting
device. To provide structural support, a conventional LED chip of
the prior art retains the growth substrate upon which the
multilayer semiconductor structure is fabricated or includes a
transfer substrate that is bonded to the multilayer semiconductor
structure opposite the growth substrate during the fabrication
process. The transfer substrate, if present, provides structural
support to the epitaxial layers once the growth substrate is
removed.
[0042] A conventional LED chip of the prior art usually emits light
predominately from one side of the chip. The opposing side is
substantially covered by one or more reflecting layers and emits
little, if any, light.
[0043] Examples of conventional GaN-based LED chips of the prior
art are illustrated in FIGS. 1 to 4. FIGS. 1A and 1B are
illustrations of conventional LED chip 100. FIG. 1A is a top plan
view of the chip. FIG. 1B is a side cross-sectional view along the
I-I plane illustrated in FIG. 1A. Conventional LED chip 100 in FIG.
1 has both the n-electrode and p-electrode on the "top" surface of
the device and the chip includes a growth substrate. Conventional
LED chip 100 includes a first electrode 102, a multilayer
semiconductor structure 104, a second electrode 114, a growth
substrate 106 and a back reflector 115. The multilayer
semiconductor structure 104 includes a first doped layer 108, an
active region 110 and a second doped layer 112, which is on the
opposite side of the active region 110 from the first doped
semiconductor layer 108. Consequently, the active region is
interposed between the first doped layer and the second doped
layer. The active region is in electrical contact with the first
doped layer and the second doped layer and the active region emits
light in a first wavelength range when a current is applied through
the first and second electrodes.
[0044] The first electrode 102 is in electrical contact with the
first doped layer 108 and the second electrode 114 is in electrical
contact with the second doped layer 112. The first electrode and
the second electrode may be fabricated from reflecting metals.
[0045] The multilayer semiconductor structure 104 of the LED chip
100 can be fabricated from GaN-based semiconductor materials
containing GaN, AlN, AlGaN, InN, InGaN and/or AlInGaN.
[0046] The active region 110 of the multilayer semiconductor
structure 104 is a p-n homojunction, a p-n heterojunction, a single
quantum well or a multiple quantum well of the appropriate
semiconductor material for the LED.
[0047] A multilayer semiconductor structure 104 is fabricated on a
growth substrate 106 of sapphire. The multilayer semiconductor
structure includes a first doped layer 108, an active region 110
and a second doped layer 112. The growth substrate 106 has a first
surface 120 and a second surface 122 opposite the first
surface.
[0048] The first doped layer 108 is an n-doped GaN layer, which is
epitaxially deposited or otherwise conventionally fabricated on the
second surface 122 of the growth substrate. The first doped layer
108 has a first surface 124 and a second surface 126 opposite the
first surface. The first surface 124 of the first doped layer is in
contact with surface 122 of the growth substrate.
[0049] The active region 110 is a GaN-based multiple quantum well
structure, which is epitaxially deposited or otherwise
conventionally fabricated on the second surface 126 of the first
doped layer 108. The active region 110 has a first surface 128 and
a second surface 130 opposite the first surface. The first surface
128 of the active region is in electrical contact with the second
surface 126 of the first doped layer.
[0050] The second doped layer 112 is a p-doped GaN layer, which is
epitaxially deposited or otherwise conventionally fabricated on the
second surface 130 of the active region 110. The second doped layer
has a first surface 132 and a second surface 134 opposite the first
surface. The first surface 132 of the second doped layer is in
electrical contact with the second surface 130 of the active
region.
[0051] A portion 116 of the second doped layer 112 and the active
region 110 is removed to expose a portion 116 of the second surface
126 of the first doped layer. The first electrode 102 and the
second electrode 114 are fabricated from aluminum. An aluminum
layer is deposited on the second surface 134 of the second doped
layer and the exposed portion 116 of the second surface 126 of the
first doped layer. The aluminum layer is patterned by standard
photolithographic techniques to form the first electrode 102 and
the second electrode 114. First electrode 102 has a first surface
136 and a second surface 138. The first surface 136 of the first
electrode is in electrical contact with the second surface 126 of
the first doped layer. Second electrode 114 has a first surface 140
and a second surface 142. First surface 140 of the second electrode
is in electrical contact with the second surface 134 of the second
doped layer.
[0052] The first electrode 102 only partially covers the exposed
portion 116 of the second surface 126 of the first doped layer. The
second electrode 114 only partially covers the second surface 134
of the second doped layer. The remaining portion of the exposed
portion 116 of the second surface 126 of the first doped layer and
the second surface 134 of the second doped layer are an output or
exit surface for the light emitted by the LED 100. To form a back
reflector 115, a layer of silver is deposited on the first surface
120 of the growth substrate.
[0053] In summary, LED chip 100 has a first electrode 102, a
multilayer semiconductor structure 104 that includes first-doped,
active and second-doped layers, a growth substrate 106 and a second
electrode 114. LED chip 100 has a first side 152 and a second side
154. The first side 152 is substantially adjacent to the first
doped layer 108. The second side 154 is substantially adjacent to
the second doped layer 112. The active region 110 emits internally
generated light in a first wavelength range when a current is
applied through the first electrode 102 and the second electrode
114. The light is emitted from the second side 154 of the LED.
[0054] The total thickness 150 of the multilayer semiconductor
structure 104 for conventional LED chip 100 of the prior art is
less than 5 microns. For example, the thickness of the first doped
layer (the n-doped layer) is typically 3 microns, the thickness of
the active region (a multi-quantum well structure) is typically 0.5
microns and the thickness of the second doped layer (the p-doped
layer) is typically 0.5 microns, resulting in a total thickness of
4 microns. In prior art LEDs, the semiconductor layers are usually
grown by MOCVD.
[0055] Another conventional LED design of the prior art is
illustrated in FIG. 2. Conventional LED chip 200 in FIG. 2 is
similar to LED chip 100 in FIG. 1 except that the LED chip 200
structure is inverted relative to LED chip 100 and has both the
first electrode and second electrode on the lower surface of the
device. This configuration is sometimes called a flip-chip design.
LED 200 also includes a growth substrate.
[0056] Except for the back reflecting surfaces, most of the
elements of LED chip 200 of the prior art are the same as LED chip
100. Conventional LED chip 200 includes a first electrode 102, a
multilayer semiconductor structure 104, a second electrode 114 and
a growth substrate 106. The multilayer semiconductor structure 104
includes a first doped layer 108, an active region 110 and a second
doped layer 112, which is on the opposite side of the active region
110 from the first doped semiconductor layer 108.
[0057] The first electrode 102 is in electrical contact with the
first doped layer 108 and the second electrode 114 is in electrical
contact with the second doped layer 112. For LED chip 200, the
second electrode is a reflecting electrode and covers substantially
all of surface 134 of the second doped layer 112. The first
electrode and the second electrode may be fabricated from
reflecting metals.
[0058] The multilayer semiconductor structure 104 of LED chip 200
can be fabricated from GaN-based semiconductor materials containing
GaN, AlN, AlGaN, InN, InGaN and/or AlInGaN. Alternatively, the
multilayer semiconductor structure can be fabricated from any
appropriate light-emitting semiconductor material.
[0059] The active region 110 of the multilayer semiconductor
structure 104 is a p-n homojunction, a p-n heterojunction, a single
quantum well or a multiple quantum well of the appropriate
semiconductor material for the LED chip 200.
[0060] For purposes of illustration, LED chip 200 is assumed to be
a GaN-based LED. The important fabrication steps for this
GaN-based, illustrative example will be briefly summarized. Many of
the fabrication steps are identical to the steps for LED chip 100
and will not be repeated.
[0061] First a multilayer semiconductor structure 104 of LED chip
200 is fabricated on a sapphire growth substrate 106 using the same
methods that are described above for LED 100. The growth substrate
106 has a first surface 120 and a second surface 122 opposite the
first surface. The multilayer semiconductor structure includes a
first doped layer 108 that is n-doped GaN, an active region 110
that is a GaN-based multiple quantum well structure and a second
doped layer 112 that is p-doped GaN.
[0062] A portion 116 of the second doped layer 112 and the active
region 110 is removed to expose a portion 116 of the second surface
126 of the first doped layer. The first electrode 102 and the
second electrode 114 are fabricated from aluminum. An aluminum
layer is deposited on the second surface 134 of the second doped
layer and the exposed portion 116 of the second surface 126 of the
first doped layer. The aluminum layer is patterned by standard
photolithographic techniques to form the first electrode 102 and
the second electrode 114. The first surface 136 of the first
electrode is in electrical contact with the second surface 126 of
the first doped layer. First surface 140 of the second electrode is
in electrical contact with the second surface 134 of the second
doped layer.
[0063] The first electrode 102 partially covers the exposed portion
116 of the second surface 126 of the first doped layer. The second
electrode 114 substantially covers the second surface 134 of the
second doped layer. Surface 136 of the first electrode and surface
140 of the second electrode form the back reflector for LED chip
200.
[0064] In summary, LED chip 200 has a first electrode 102, a
multilayer semiconductor structure 104 that includes first-doped,
active and second-doped layers, a growth substrate 106 and a second
electrode 114. LED chip 200 has a first side 252 and a second side
254. The first side 252 is substantially adjacent to the first
doped layer 108. The second side 254 is substantially adjacent to
the second doped layer 112. The active region 110 emits internally
generated light in a first wavelength range when a current is
applied through the first electrode 102 and the second electrode
114. The light is emitted from the first side 252 of the LED.
[0065] The total thickness 150 of the multilayer semiconductor
structure 104 for LED chip 200 of the prior art is less than 5
microns. For example, the thickness of the first doped layer (the
n-doped layer) is typically 3 microns, the thickness of the active
region (a multi-quantum well structure) is typically 0.5 microns
and the thickness of the second doped layer (the p-doped layer) is
typically 0.5 microns, resulting in a total thickness of 4 microns.
The semiconductor layers are usually grown by MOCVD.
[0066] When utilized as in a light source, LED chip 200 is normally
attached to a submount (not shown). The submount acts as a heat
transfer element or heatsink to remove heat generated by the device
during operation. The submount also includes electrical
interconnections that attach to the first electrode and second
electrode.
[0067] Another conventional LED design of the prior art is
illustrated in FIG. 3. Conventional LED chip 300 in FIG. 3 is
similar to LED chip 100 in FIG. 1 and LED chip 200 in FIG. 2. The
LED chip 300 structure is inverted and has both the n-electrode and
p-electrode on the lower side of the device. This configuration is
another version of a flip-chip structure. However, for the LED chip
300 design, the LED structure is bonded to a transfer substrate 302
that includes electrical connections (not shown) to the n-electrode
and the p-electrode. The original growth substrate has been
removed.
[0068] Except for the removal of the growth substrate and the
addition of a transfer substrate 302, most of the elements of LED
chip 300 are the same as LED chip 200. Conventional LED chip 300 of
the prior art includes a first electrode 102, a multilayer
semiconductor structure 104, a second electrode 114 and a transfer
substrate 302 on the second side 354 of the chip. The multilayer
semiconductor structure 104 includes a first doped layer 108 on the
first side 352 of the chip, an active region 110 and a second doped
layer 112, which is on the opposite side of the active region 110
from the first doped semiconductor layer 108.
[0069] The first electrode 102 is in electrical contact with the
first doped layer 108 and the second electrode 114 is in electrical
contact with the second doped layer 112. For LED 300, the second
electrode is a reflecting electrode and covers substantially all of
surface 134 of the second doped layer 112. The first electrode and
the second electrode may be fabricated from reflecting metals.
[0070] The active region 110 of the multilayer semiconductor
structure 104 can be a p-n homojunction, a p-n heterojunction, a
single quantum well or a multiple quantum well of the appropriate
semiconductor material for the LED chip 300.
[0071] For purposes of illustration, LED chip 300 is assumed to be
a GaN-based LED. The important fabrication steps for this
GaN-based, illustrative example will be briefly summarized. Many of
the fabrication steps are identical to the steps for LED chip 100
and will not be repeated.
[0072] First a multilayer semiconductor structure 104 of LED chip
300 is fabricated on a sapphire growth substrate (not shown) using
the same methods that are described above for LED 100.
[0073] A portion 116 of the second doped layer 112 and the active
region 110 is removed to expose a portion 116 of the second surface
126 of the first doped layer. To form the electrodes, a metal layer
is deposited on the second surface 134 of the second doped layer
and the exposed portion 116 of the second surface 126 of the first
doped layer. The metal layer is patterned by standard
photolithographic techniques to form the first electrode 102 and
the second electrode 114. The first electrode 102 partially covers
the exposed portion 116 of the second surface 126 of the first
doped layer. The second electrode 114 substantially covers the
second surface 134 of the second doped layer. Surface 136 of the
first electrode and surface 140 of the second electrode form the
back reflector for LED chip 300.
[0074] A transfer substrate 302 is bonded to surface 138 of the
first electrode and surface 142 of the second electrode. The
transfer substrate includes electrical vias (not shown) in order to
make electrical connections to the electrodes. After the transfer
substrate is attached, the growth substrate is removed by standard
processing steps. For example, the growth substrate can be removed
by a laser liftoff process, a chemical process or by mechanical
polishing.
[0075] The total thickness 150 of the multilayer semiconductor
structure 104 for LED chip 300 of the prior art is less than 5
microns. For example, the thickness of the first doped layer (the
n-doped layer) is typically 3 microns, the thickness of the active
region (a multi-quantum well structure) is typically 0.5 microns
and the thickness of the second doped layer (the p-doped layer) is
typically 0.5 microns, resulting in a total thickness of 4 microns.
The semiconductor layers are usually grown by MOCVD.
[0076] When utilized as in a light source, LED chip 300 may also
include a submount (not shown), which acts as a heat transfer
element or heatsink to remove heat generated by the device during
operation.
[0077] Another conventional LED of the prior art has one electrode
on the upper side of the device and one electrode on the lower
side. LED chip 400 illustrated in FIGS. 4A and 4B is one example of
such a device of the prior art.
[0078] FIGS. 4A and 4B are illustrations of a conventional LED chip
400 of the prior art that has a upper electrode, a lower electrode
and includes a transfer substrate. FIG. 4A is a top plan view of
the chip. FIG. 4B is a side cross-sectional view along the I-I
plane illustrated in FIG. 4A. The multilayer semiconductor
structure of LED chip 400 is inverted relative to the LED chip 100
structure. However, LED chip 400 has one electrode, in this case
the n-electrode or first electrode 102, on the upper side of the
device and the other electrode, the p-electrode or the second
electrode 114, on the lower side of the device. In a similar manner
as the LED chip 300 design, the LED chip 400 structure is bonded to
a transfer substrate. The transfer substrate 402 is either an
electrical conductor or includes an electrical connection (not
shown) to the second or p-electrode. The original growth substrate
has been removed.
[0079] Except for the arrangement of the electrodes, most of the
elements of LED chip 400 of the prior art are the same as for LED
chip 300. Conventional LED chip 400 includes a first electrode 102
on the first side 452 of the device, a multilayer semiconductor
structure 104, a second electrode 114 proximal to the second side
454 of the device and a transfer substrate 402. The multilayer
semiconductor structure 104 includes a first doped layer 108, an
active region 110 and a second doped layer 112, which is on the
opposite side of the active region 110 from the first doped
semiconductor layer 108.
[0080] The first electrode 102 is in electrical contact with the
first doped layer 108 and the second electrode 114 is in electrical
contact with the second doped layer 112. For LED chip 400, the
second electrode is a reflecting electrode and covers substantially
all of surface 134 of the second doped layer 112. The first
electrode and the second electrode may be fabricated from
reflecting metals.
[0081] The active region 110 of the multilayer semiconductor
structure 104 can be a p-n homojunction, a p-n heterojunction, a
single quantum well or a multiple quantum well of the appropriate
semiconductor material for the LED chip 400.
[0082] For purposes of illustration, LED chip 400 is assumed to be
a GaN-based LED. The important fabrication steps for this
GaN-based, illustrative example will be briefly summarized. Many of
the fabrication steps, including the fabrication of the multilayer
semiconductor structure, are identical to the steps for LED chip
100, LED chip 200 and LED chip 300 and will not be repeated.
[0083] The second electrode 114 is fabricated from a metal. A metal
layer is deposited on the second surface 134 of the second doped
layer of the multilayer semiconductor structure. The first surface
140 of the second electrode is in electrical contact with the
second surface 134 of the second doped layer. Surface 140 of the
second electrode also forms the back reflector for LED chip
400.
[0084] A transfer substrate 402, and in particular surface 404 of
the transfer substrate, is bonded to surface 142 of the second
electrode. The transfer substrate 402 is either an electrical
conductor or includes an electrical interconnect (not shown) to the
second electrode.
[0085] After the transfer substrate is attached, the growth
substrate is removed by standard processing steps, exposing the
first surface 124 of the first doped layer. For example, the growth
substrate can be removed by a laser liftoff process, a chemical
process or by mechanical polishing.
[0086] The first electrode 102 is fabricated from a metal. A metal
layer is deposited on the first surface 124 of the first doped
layer that was exposed by removing the growth substrate. The metal
layer is patterned by standard photolithographic techniques to form
the first electrode 102. The first electrode 102 partially covers
surface 124 of the first doped layer.
[0087] In summary, LED chip 400 has a first electrode 102, a
multilayer semiconductor structure 104 that includes first-doped,
active and second-doped layers, a transfer substrate 402 and a
second electrode 114. LED chip 400 has a first side 452 and a
second side 454. The first side 452 is substantially adjacent to
the first doped layer 108. The second side 454 is substantially
adjacent to the second doped layer 112. The active region 110 emits
internally generated light in a first wavelength range when a
current is applied through the first electrode 102 and the second
electrode 114. The light is emitted from the first side 452 of the
LED.
[0088] The total thickness 150 of the multilayer semiconductor
structure 104 for LED chip 400 is typically less than 5 microns.
For example, the thickness of the first doped layer (the n-doped
layer) is typically 3 microns, the thickness of the active region
(a multi-quantum well structure) is typically 0.5 microns and the
thickness of the second doped layer (the p-doped layer) is
typically 0.5 microns, resulting in a total thickness of 4 microns.
The semiconductor layers are usually grown by MOCVD.
[0089] When utilized as in a light source, LED chip 400 may also
include a submount or heatsink (neither are shown) that acts as a
heat transfer element to remove heat generated by the device during
operation.
[0090] The heat flow from the multilayer semiconductor structure of
LED chip 400 of the prior art is illustrated schematically in FIG.
4C. Heat flows from the multilayer semiconductor structure to the
transfer substrate with thermal resistance 482, then from the
transfer substrate to the submount or heatsink with thermal
resistance 484 and finally from the submount or heatsink with
thermal resistance 486 to the ambient 488. The transfer substrate,
the bonding material (not shown) used to bond the transfer
substrate to the multilayer semiconductor structure and the
submount/heatsink increase the thermal resistance of the
device.
[0091] FIGS. 1 to 4 illustrate conventional LED chips of the prior
art that include multilayer semiconductor structures that are less
than 5 microns thick and that include either a growth substrate or
a transfer substrate.
[0092] In contrast to conventional LED chips of the prior are, the
LED chips of this present invention are substrate-free.
Alternatively, the LED chips of this invention may also be
described as free-standing. A substrate-free or free-standing LED
chip of this invention is an LED chip that does not include a
growth substrate or transfer substrate as an element of the LED
chip. The growth substrate is defined as the substrate onto which
the multilayer semiconductor structure is epitaxially grown. For
LED chips of this present invention, the growth substrate is
removed after the multilayer semiconductor structure is fabricated
and no transfer substrate is permanently bonded to the multilayer
semiconductor structure prior to the removal of the growth
substrate. The LED chips of this present invention have multilayer
semiconductor structures that are at least 10 microns thick and do
not require an attached growth substrate or transfer substrate for
structural rigidity or support. The LED chips can be handled
without damage and without breaking.
[0093] The substrate-free or free-standing LED chips of this
present invention may later be packaged into more complex LED light
sources. If desired, the LED chips may be attached to a surface, a
submount, a heat sink, a leadframe or to any other structure.
However, none of these additional surfaces, submounts, heat sinks,
leadframes or other structures are elements of the LED chip.
[0094] Embodiments of this present invention are substrate-free LED
chips that do not include a growth substrate or a transfer
substrate. Substrate-free LED chips of this invention have
multilayer semiconductor structures that are at least 10 microns
thick. It is also within the scope of this invention that the
multilayer semiconductor structures can have a thickness of at
least 20 microns or at least 30 microns. The multilayer
semiconductor structures utilized for the substrate-free LED chips
are thick enough so that the LED chips can be handled as
free-standing structures without breaking.
[0095] For the substrate-free LED chips of this invention, at least
one layer of the multilayer semiconductor structure is at least 5
microns thick. The at least one thick layer in the multilayer
semiconductor structure can be the first doped layer, the second
doped layer or an undoped layer. Preferably the first doped layer,
the second doped layer or the updoped layer is at least 10 microns
thick. More preferably, the first doped layer, the second doped
layer or the undoped layer is at least 15 microns thick. Most
preferably, the first doped layer, the second doped layer or the
undoped layer is at least 25 microns thick. Alternatively, two or
more layers of the multilayer semiconductor structure are each at
least 5 microns thick.
[0096] The total thickness of the multilayer semiconductor
structure for the substrate-free LED chips of this invention is at
least 10 microns thick. More preferably the total thickness of the
multilayer semiconductor structure is at least 20 microns thick.
Most preferably the total thickness of the multilayer semiconductor
structure is at least 30 microns thick.
[0097] Since thicker semiconductor layers are utilized for the
substrate-free LED chip, the optical absorption coefficients for
the various layers must be low in order to prevent the absorption
of a significant fraction of the internally generated light that is
emitted by the active region of the chip and transmitted through
the thicker semiconductor layers before exiting the chip. Lower
optical absorption within the LED chip will result in higher light
extraction from the chip, higher external quantum efficiency and
higher light output from the LED.
[0098] In some applications, it is also important that the LED chip
be highly reflective or highly transmissive to any externally
incident light that comes from other light sources or from recycled
light that is directed or reflected back toward the chip. The
optical absorption coefficients for the various semiconductor
layers must also be low in these cases so that any externally
incident light that enters the chip will not undergo significant
absorption by the semiconductor layers before exiting the chip.
Lowering the optical absorption coefficients of the semiconductor
layers will increase the reflectivity or transmissivity of the LED
chip to externally incident light.
[0099] The multilayer semiconductor structure of the LED chip can
absorb light and has an absorption coefficient that depends on
wavelength. In many cases, the absorption coefficient is not
uniform across the different semiconductor layers of the multilayer
semiconductor structure. If the different semiconductor layers that
make up the multilayer semiconductor structure have different
absorption coefficients, the absorption coefficient for the
multilayer semiconductor structure is defined in this specification
as the thickness-weighted average absorption coefficient. The
weighting function is the fractional thickness of each
semiconductor layer in the multilayer semiconductor structure. For
example, if 100 percent of the thickness of the multilayer
semiconductor structure has a uniform absorption coefficient of 40
per centimeter in the emitting wavelength range of the internally
generated light, then the thickness-weighted average absorption
coefficient is 40 per centimeter. If 50 percent of the thickness of
the multilayer semiconductor structure has an absorption
coefficient of 30 per centimeter and 50 percent of the thickness of
the multilayer semiconductor structure has an absorption
coefficient of 50 per centimenter, then the thickness-weighted
average absorption coefficient is also 40 per centimeter.
[0100] In order to improve the light extraction efficiency and
external quantum efficiency of an LED chip and to improve the
reflectivity or transmissivity of the LED chip to externally
incident light, the absorption coefficient (i.e. the
thickness-weighted average absorption coefficient) of the
multilayer semiconductor structure in the emitting wavelength range
of the internally generated light is less than 20 per centimeter.
Preferably the absorption coefficient of the multilayer
semiconductor structure in the emitting wavelength range of the
internally generated light is less than 10 per centimeter. More
preferably, the absorption coefficient of the multilayer
semiconductor structure in the emitting wavelength range is less
than 5 per centimeter. Most preferably, the absorption coefficient
of the multilayer semiconductor structure in the emitting
wavelength range is less than 2 per centimeter.
[0101] Minimizing the absorption coefficient of the multilayer
semiconductor structure in the emitting wavelength range of the
internally generated light can be accomplished by improving the
deposition processes for the different semiconductor layers in
order to reduce impurities or defects and to improve the
crystalline structure of the layers.
[0102] Thick semiconductor layers can be grown by methods
including, but not limited to, CVD, MOCVD, VPE, HVPE and MBE. MOCVD
is the most common method for conventional GaN-based LEDs but it
has relatively slow deposition rates of approximately 0.1 micron
per hour. MOCVD deposited layers also have relatively high optical
absorption coefficients due to impurities and defects. HVPE has
much faster growth rates and is the preferred method for this
invention in order to grow GaN layers that are more than 5 microns
thick. HVPE can have growth rates of up to 10 microns per hour or
more and can produce GaN-based LED layers that have optical
absorption coefficients significantly less than 25 per
centimeter.
[0103] For example, HVPE can be used to epitaxially grow the first
doped layer, the second doped layer, both the first and the second
doped layers, any undoped layer or the entire multilayer
semiconductor structure of the LED. HVPE does not have the carbon
impurities that can be present in the MOCVD processes normally used
in GaN LED fabrication. MOCVD may optionally be used to grow active
regions that are single- or multiple quantum wells. If the active
region of the LED chip is a p-n homojunction or a p-n
heterojunction, preferably the entire multilayer semiconductor
structure is fabricated by HVPE.
[0104] A substrate-free LED chip may have two electrodes on one
side of the chip, either the upper side or the lower side.
Alternatively, the substrate-free LED chip may have one electrode
on the upper side of the chip and one electrode on the lower side
of the chip.
[0105] Both electrodes should be highly reflective to internally
generated light to prevent excessive light absorption inside the
chip. In addition, making the external surfaces of the electrodes
highly reflective will result in an LED chip that has higher
reflectivity to externally incident light.
[0106] Gold is a common electrode material. Gold has very good
electrical properties, but is a poor optical reflector for visible
light in the range of 400 mn to 550 mn. For LEDs that emit light in
the 400 to 550 nm range or thereabouts, it is advantageous to
replace gold with a more reflective material. Suitable electrode
materials include, but are not limited to, aluminum and silver. The
electrodes may also be omni-directional reflectors that include a
dielectric layer and a metal layer and have electrically conducting
pathways through the dielectric layer. Preferably the reflectivity
of the electrodes is greater than 90 percent in the emitting
wavelength range. More preferably, the reflectivity of the
electrodes is greater than 95 percent in the emitting wavelength
range. Most preferably, the reflectivity of the electrodes is
greater than 98 percent in the emitting wavelength range.
[0107] Examples of substrate-free LED chips for this invention that
have at least one thick epitaxial layer and that do not have either
a growth substrate or a transfer substrate are illustrated in FIGS.
5 to 13. In the first set of examples illustrated in FIGS. 5 to 9,
the LED chips each have both electrodes on the upper side of the
chip. In the second set of examples illustrated in FIGS. 10 and 11,
the LED chips each have both electrodes on the lower side of the
chip. In the third set of examples illustrated in FIGS. 12 and 13,
the LED chips each have one electrode on the upper side of the chip
and one electrode on the lower side of the chip.
[0108] FIGS. 5 to 9 illustrate substrate-free LED chips that have
both electrodes on the upper side. FIGS. 5 and 6 illustrate LED
chips having a thick first doped layer. FIG. 7 illustrates an LED
chip with a thick second doped layer. FIG. 8 illustrates an LED
chip with both a thick first doped layer and a thick second doped
layer. FIG. 9 illustrated an LED chip that has a thick undoped
layer.
[0109] FIGS. 10 to 11 illustrate substrate-free LED chips that have
both electrodes on the lower side of the chip and that have a thick
first doped layer. It is also within the scope of this invention
that chips that have both electrodes on the lower side may
alternatively have a thick second doped layer or have both a thick
first doped layer and a thick second doped layer or have a thick
undoped layer. Only examples with a thick first doped layer are
illustrated in the figures.
[0110] FIGS. 12 to 13 illustrate substrate-free LED chips that have
one electrode on the upper side of the chip, one electrode on the
lower side of the chip and have a thick first doped layer. It is
also within the scope of this invention that chips that have one
electrode on the upper side and one electrode on the lower side may
alternatively have a thick second doped layer or have both a thick
first doped layer and a thick second doped layer. Only examples
with a thick first doped layer are illustrated in the figures.
[0111] First, examples of substrate-free LED chips that have two
electrodes on the upper side of the chip are now described. The
chips are illustrated in FIGS. 5 to 9. If the lower electrode or a
reflector substantially covers the lower surface of the chip, the
substrate-free LED chip will emit light from the upper side and
emit little or no light from the lower side. If the lower electrode
or reflector covers only a portion of the lower side of the chip,
the chip will emit light from both the upper and lower sides,
thereby increasing the extraction efficiency, the external quantum
efficiency and the light output of the chip.
[0112] Substrate-free LED chip 500 illustrated in FIGS. 5A, 5B and
5C has both the first electrode and the second electrode on the
upper surface of the device and has neither a growth substrate nor
a transfer substrate. FIG. 5A is a top plan view of the chip. FIGS.
5B and 5C are side cross-sectional views along the I-I plane
illustrated in FIG. 5A.
[0113] Substrate-free LED chip 500 includes a first electrode 102,
a multilayer semiconductor structure 504, a second electrode 114
and a back reflector 115. The multilayer semiconductor structure
504 includes a first doped layer 108, an active region 110 and a
second doped layer 112, which is on the opposite side of the active
region 110 from the first doped semiconductor layer 108.
Consequently, the active region is interposed between the first
doped layer and the second doped layer. The active region is in
electrical contact with the first doped layer and the second doped
layer and the active region emits internally generated light when a
voltage is applied between the first and second electrodes.
[0114] The first electrode 102 is in electrical contact with the
first doped layer 108 and the second electrode 114 is in electrical
contact with the second doped layer 112. The first electrode and
the second electrode may be fabricated from reflecting metals. For
example, the first electrode and the second electrode may be formed
from one or more metals or metal alloys containing, but not limited
to, silver, aluminum, nickel, titanium, chromium, platinum,
palladium, rhodium, rhenium, ruthenium and tungsten. Preferred
metals are aluminum and silver.
[0115] The multilayer semiconductor structure 504 of the LED chip
500 can be fabricated from GaN-based semiconductor materials
containing, for example, GaN, AlN, AlGaN, InN, InGaN and/or
AlInGaN. Alternatively, the multilayer semiconductor structure can
be fabricated from any appropriate light-emitting semiconductor
material.
[0116] The active region 110 of the multilayer semiconductor
structure 504 is a p-n homojunction, a p-n heterojunction, a single
quantum well or a multiple quantum well of the appropriate
semiconductor material for the LED.
[0117] For purposes of illustration, LED chip 500 is assumed to be
a GaN-based LED chip. The important fabrication steps for a
GaN-based, illustrative example will be briefly summarized.
[0118] First a multilayer semiconductor structure 504 is fabricated
on a sapphire growth substrate (not shown). The multilayer
semiconductor structure illustrated in FIGS. 5B and 5C includes a
first doped layer 108, an active region 110 and a second doped
layer 112.
[0119] The first doped layer 108 is an n-doped GaN-based layer,
which is epitaxially deposited or otherwise conventionally
fabricated on a growth substrate. The first doped layer 108 has a
first surface 124 and a second surface 126 opposite the first
surface. The first doped layer is at least 5 microns thick.
Preferably the first doped layer is at least 10 microns thick. More
preferably, the first doped layer is at least 15 microns thick.
Most preferably, the first doped layer is at least 25 microns
thick. The first doped layer may be deposited by any standard GaN
growth technique. Preferably, the first doped layer is deposited by
HVPE. In this illustrative example, the first doped layer is 20
microns thick and is deposited by HVPE.
[0120] The active region 110 is a GaN-based multiple quantum well
structure, which is epitaxially deposited or otherwise
conventionally fabricated on the second surface 126 of the first
doped layer 108. The active region 110 has a first surface 128 and
a second surface 130 opposite the first surface. The first surface
128 of the active region is in electrical contact with the second
surface 126 of the first doped layer. The active region may be
deposited by any standard GaN growth technique. Preferably, the
multiple quantum well structure is deposited by MOCVD or HVPE. In
this illustrative example, the multiple-quantum-well active region
is approximately 0.5 micron thick and is deposited by MOCVD.
[0121] The second doped layer 112 is a p-doped GaN-based layer,
which is epitaxially deposited or otherwise conventionally
fabricated on the second surface 130 of the active region 110. The
second doped layer has a first surface 132 and a second surface 134
opposite the first surface. The first surface 132 of the second
doped layer is in electrical contact with the second surface 130 of
the active region. The second doped layer may be deposited by any
GaN growth technique. In this illustrative example, the second
doped layer is approximately 0.5 micron thick and is deposited by
MOCVD.
[0122] A portion 116 of the second doped layer 112 and the active
region 110 are removed to expose a portion 116 of the second
surface 126 of the first doped layer. The first electrode 102 and
the second electrode 114 are fabricated from aluminum. An aluminum
layer is deposited on the second surface 134 of the second doped
layer and the exposed portion 116 of the second surface 126 of the
first doped layer. The aluminum layer is patterned by standard
photolithographic techniques to form the first electrode 102 and
the second electrode 114. First electrode 102 has a first surface
136 and a second surface 138. The first surface 136 of the first
electrode is in electrical contact with the second surface 126 of
the first doped layer. Second electrode 114 has a first surface 140
and a second surface 142. First surface 140 of the second electrode
is in electrical contact with the second surface 134 of the second
doped layer.
[0123] The first electrode 102 only partially covers the exposed
portion 116 of the second surface 126 of the first doped layer. The
second electrode 114 only partially covers the second surface 134
of the second doped layer. The remaining portion of the exposed
portion 116 of the second surface 126 of the first doped layer and
the second surface 134 of the second doped layer are output or exit
surfaces for the light emitted by the LED chip 500. The first
electrode is not in physical or electrical contact with the second
doped layer and the active region. The air gap between the first
electrode and the second doped layer and the active region may be
filled with a non-conducting material or an insulating material
(not shown).
[0124] The growth substrate (not illustrated) is removed by any
conventional process including laser liftoff, chemical processes
and mechanical polishing. Removing the growth substrate exposes
first surface 124 of the first doped layer. For this illustrative
example, the growth substrate is removed using an
frequency-quadrupled Nd-YAG laser operating at 266 nanometers.
[0125] To form a lower reflector 115, a layer of silver or other
reflective metal is deposited on the exposed first surface 124 of
the first doped layer following the removal of the growth
substrate. Optionally, the lower reflector may also be an
omni-directional reflector that includes a dielectric layer (not
shown) and a metal layer such as silver or aluminum.
[0126] In summary, substrate-free LED chip 500 illustrated in FIGS.
5A, 5B and 5C has a first electrode 102, a multilayer semiconductor
structure 504 that includes a first doped layer, an active region
and a second doped layer and a second electrode 114. LED chip 500
has neither a growth substrate nor a transfer substrate. LED chip
500 has a first side 552 and a second side 554. The first side 552
is proximal to the first doped layer 108. The second side 554 is
proximal to the second doped layer 112. The active region 110 emits
internally generated light when a voltage is applied across the
first electrode 102 and the second electrode 114. Second side 554
of LED chip 500 is the light emitting side for internally generated
light.
[0127] The total thickness 550 of the multilayer semiconductor
structure 504 for LED chip 500 is at least 10 microns. In this
illustrative example, the thickness 556 of the first doped layer
(the n-doped layer) is approximately 20 microns, the thickness of
the active region (a multi-quantum well structure) is approximately
0.5 microns and the thickness 558 of the second doped layer (the
p-doped layer) is approximately 0.5 microns, resulting in a total
thickness of 21 microns. In this example, the first doped layer is
grown by HVPE and the remainder of the semiconductor layers are
grown by MOCVD.
[0128] Example light rays 560, 562 and 564 in FIG. 5B illustrate
internally generated light that is emitted by the active region 110
of the LED. Internally generated light ray 560 is emitted by active
region 110 toward output surface 134 of the LED chip. Internally
generated light ray 560 is directed at an angle to surface 134 that
is less than the critical angle, which allows light ray 560 to exit
the LED chip through surface 134.
[0129] Internally generated light ray 562 is emitted by active
region 110 toward the lower reflector 115 of the LED. Internally
generated light ray 562 is reflected by reflector 115 and directed
to the output surface 134 at an angle less than the critical angle.
Internally generated light ray 562 exits the LED chip through
surface 134.
[0130] Internally generated light ray 564 is directed to surface
134 at an angle that is greater than the critical angle. Internally
generated light ray 564 is reflected by total internal reflection
and is redirected toward the rear reflector 115 of the LED
chip.
[0131] Substantially all of the first side 552 of LED 500 is
covered by lower reflector 115. Due to the reflectivity of
reflector 115, the reflectivity of surface 138 of first electrode
102 and the reflectivity of surface 142 of second electrode 114,
substrate-free LED chip 500 can reflect externally incident light.
Externally incident light is light that is directed to the light
emitting side of the LED from another light source or light that is
emitted by the LED and is reflected back to the light emitting side
of the LED as recycled light. For some applications, for example
applications utilizing light recycling to increase the effective
brightness of the LED, it is important that the LED have high
reflectivity to externally incident light. High reflectivity to
externally incident light will exist if the reflecting layers of
the LED have high reflectivity (e.g. greater than 70%) and if the
absorption coefficient of the multilayer semiconductor structure is
low (e.g. less than 20 per centimeter). Preferably LED 500 reflects
at least 60 percent of externally incident light directed to the
light emitting side (second side 554) of the LED. More preferably,
LED 500 reflects at least 70 percent of externally incident light.
Most preferably, LED 500 reflects at least 80 of externally
incident light.
[0132] Example light rays 570, 572 and 574 in FIG. SC illustrate
externally incident light that is incident on the light emitting
side or second side 554 of LED chip 500 and is reflected by the
chip. Externally incident light ray 570 is incident on surface 134
of the LED chip. Externally incident light ray 570 passes through
surface 134, passes through the multilayer semiconductor structure
504 a first time, is reflected by reflector 115, passes through the
multilayer semiconductor structure 504 a second time and exits LED
chip 500 through surface 134. Externally incident light ray 572 is
directed to LED chip 500 and is reflected by surface 142 of the
second electrode 114. Externally incident light 574 is directed to
LED chip 500 and is reflected by surface 138 of the first electrode
102.
[0133] In the illustrative example in FIG. 5, the first doped
semiconductor layer 108 is an n-doped layer and the second doped
semiconductor layer 112 is a p-doped layer. However, the two layers
can in principle be reversed. If the first doped semiconductor
layer 108 is a p-doped layer, then the second doped semiconductor
layer 112 is an n-doped layer. The two doped semiconductor layers
108 and 112 will have opposite n and p conductivity types.
[0134] It is well known by those skilled in the art that the
multilayer semiconductor structure 504 may include additional
layers in order to adjust and improve the operation of the LED chip
500. For example, a current spreading layer (not shown) may be
inserted between surface 136 of the first electrode 102 and surface
126 the first doped layer 108. Such a current spreading layer will
have the same conductivity type as the first doped layer and will
improve the uniformity of current injection across the entire
active region. In addition, a current spreading layer (not shown)
may be inserted between surface 134 of the second doped layer and
surface 140 of the second electrode 114. The latter current
spreading layer will have the same conductivity type as the second
doped layer. As another example, an electron blocking layer or a
hole blocking layer (neither are shown) may be inserted either
between surface 126 of the first doped layer 108 and surface 128 of
the active region 110 or between surface 130 of the active region
110 and surface 132 of the second doped layer 112. An electron
blocking layer reduces the escape of electrons from the active
region into either doped layer. A hole blocking layer reduces the
transfer of holes through the doped layer into the active
region.
[0135] The substrate-free LED chips of this invention, including
LED chip 500, preferably include light extraction elements (not
shown) to aid in extracting internally generated light from the
chips. The light extraction elements may be fabricated by any
means, including chemical means, mechanical means such as grinding,
or optical means such as laser ablation.
[0136] Substrate-free LED chip 500 does not have a growth substrate
or a transfer substrate that can retard heat flow from the chip. If
LED chip 500 is bonded to a surface, a submount, a heat sink or a
leadframe, the thermal resistance for heat transfer is illustrated
in FIG. 5D. Heat will flow from the LED chip to the surface,
submount, heat sink or leadframe with thermal resistance R.sub.4 or
582. Heat will flow from the surface, submount, heat sink or
leadframe to ambient with thermal resistance R.sub.5 or 584. The
total thermal resistance, R.sub.4 plus R.sub.5 or, equivalently,
the sum of the thermal resistances 582 and 584 of the
substrate-free LED chip 500 will be less than for an LED chip such
as LED chip 400 that includes a transfer substrate. For LED chip
400, the total thermal resistance illustrated in FIG. 4C is R.sub.1
plus R.sub.2 plus R.sub.3 or, equivalently, the sum of the thermal
resistances 482, 484 and 486.
[0137] FIGS. 6 to 9 illustrate substrate-free LED chips that are
variations of substrate-free LED chip 500 shown in FIG. 5.
[0138] Substrate-free LED chip 600 in FIGS. 6A and 6B is nearly
identical to LED chip 500 except that LED chip 600 does not have a
reflector on the first side 652. Internally generated light emitted
by the active region 110 can exit from both the upper or second
side 654 and the lower or first side 652. For example, light ray
660 illustrated in FIG. 6A and emitted by the active region exits
LED chip 600 through surface 134 on the second side. Light ray 662
exits LED chip 600 through surface 124 on the first side. Light ray
664 undergoes total internal reflection at surface 134.
[0139] No portion of the first side 652 of LED 600 is covered by a
reflecting layer. Only a portion of the second side 654 of LED 600
is covered by the first electrode and the second electrode. Both
sides of the multilayer semiconductor structure are light emitting
sides and will emit internally generated light. At least a portion
of the first side of the multilayer semiconductor structure and at
least a portion of the second side of the multilayer semiconductor
structure will also transmit externally incident light. Externally
incident light is light that is directed to a light emitting side
of the LED from another light source or light that is emitted by an
LED and is reflected back to the light emitting side of the LED as
recycled light. For some applications, where it is desirable for
light from a phosphor or light from another LED to pass through the
LED, the LED should transmit a large portion of externally incident
light. High transmissivity will exist if the absorption coefficient
of the multilayer semiconductor structure is low (e.g. less than 20
per centimeter). Preferably LED 600 transmits at least 60 percent
of externally incident light directed to a light emitting side
(either the first side 652 or the second side 654) of the LED. More
preferably, LED 600 transmits at least 70 percent of externally
incident light. Most preferably, LED 600 transmits at least 80 of
externally incident light.
[0140] FIG. 6B illustrates externally incident light rays 670 and
672 that are transmitted by LED chip 600. Externally incident light
ray 670 is incident on surface 124 of the first side 652 of LED
600. Externally incident light ray 670 passes through the
multilayer semiconductor structure 504 and exits LED chip 600
through surface 134 on the second side 654. Externally incident
light ray 672 is incident on surface 134 of the second side 654 of
LED 600. Externally incident light ray 672 passes through the
multilayer semiconductor structure 504 and exits LED chip 600
through surface 124 on the first side 652.
[0141] In LED chip 600, the thick epitaxial layer is the first
doped layer 108. Alternatively, the second doped layer 112 may be a
thick epitaxial layer as illustrated by LED 700 in FIG. 7.
[0142] LED chip 700 in FIG. 7 illustrates a substrate-free LED chip
that has a thick second doped layer 112. The thick second doped
layer 112 is a p-doped GaN layer, which is epitaxially deposited or
otherwise conventionally fabricated on the active region. The thick
second doped layer 112 has a first surface 132 and a second surface
134 opposite the first surface. The first surface 132 of the second
doped layer is in electrical contact with the second surface 130 of
the active region. The thickness 558 of the second doped layer 112
is at least 5 microns. Preferably the second doped layer is at
least 10 microns thick. More preferably, the second doped layer is
at least 15 microns thick. Most preferably, the second doped layer
is at least 25 microns thick. The thick second doped layer may be
deposited by any standard GaN growth technique. In this
illustrative example, the second doped layer 20 microns thick and
is deposited by HVPE. The first doped layer 108 and the active
region 110 may be deposited by any standard GaN growth technique.
In this illustrative example, the first doped layer and the active
region are deposited by MOCVD.
[0143] Example light rays 760, 762 and 764 illustrate internally
generated light that is emitted by the active region 110 of the LED
chip 700. Internally generated light ray 760 is emitted by active
region 110 toward output surface 134 of the LED chip. Internally
generated light ray 760 is directed at an angle to surface 134 that
is less than the critical angle, which allows light ray 760 to exit
the LED chip through surface 134.
[0144] Internally generated light ray 762 is emitted by active
region 110 toward the lower reflector 115 of the LED. Internally
generated light ray 762 is reflected by reflector 115 and directed
to the output surface 134 at an angle less than the critical angle.
Internally generated light ray 762 exits the LED chip through
surface 134.
[0145] Internally generated light ray 764 is directed to surface
134 at an angle that is greater than the critical angle. Internally
generated light ray 764 is reflected by total internal reflection
and is redirected toward the rear reflector 115 of the LED
chip.
[0146] FIG. 8 illustrates a side cross-sectional view of a
substrate-free LED chip 800 that has both a thick first doped layer
and a thick second doped layer. In this illustrative example, the
thick first doped layer 108 is an n-doped layer and the thick
second doped layer 112 is a p-doped layer. The thickness 556 of the
first doped layer and the thickness 558 of the second doped layer
are each at least 5 microns. Preferably the first doped layer and
the second doped layer are each at least 10 microns thick. More
preferably, the first doped layer and the second doped layer are
each at least 15 microns thick. Most preferably, the first doped
layer and the second doped layer are each at least 25 microns
thick. The first doped layer and the second doped layer may have
the same thickness or have different thicknesses. In this
illustrative example, the first doped layer is 20 microns thick and
the second doped layer is 5 microns thick. The first doped layer
and the second doped layer may be deposited by any standard GaN
growth technique. In this illustrative example, the first doped
layer and the second doped layer are deposited by HVPE. For LED
chip 800, the active region 110 may be deposited by any standard
GaN growth technique. In this illustrative example, the active
region is deposited by MOCVD.
[0147] FIG. 9 illustrates a side cross-sectional view of a
substrate-free LED chip 900 that has a thick, substantially-undoped
layer 902 adjacent to the first side 952 of the multilayer
semiconductor structure. As illustrated in FIG. 9, an LED chip that
has both electrodes on the first side or the second side of the
multilayer semiconductor structure may optionally include a
substantially undoped layer on the side of the multilayer
semiconductor structure opposite the electrodes. The undoped layer
adds additional thickness to the multilayer semiconductor structure
without affecting the electrical properties of the structure. The
additional thickness adds to the structural strength of the
multilayer semiconductor structure. In addition, because the
thermal resistance to heat transfer tends to be high at interfaces
of dissimilar materials, a GaN-based multilayer semiconductor
structure that contains a thick, substantially-undoped GaN layer
will usually have lower thermal resistance and higher heat transfer
than a prior art LED structure that includes a growth substrate or
a transfer substrate of a different material. Furthermore, a thick,
substantially undoped layer of GaN will have a lower optical
absorption coefficient than, for example, a silicon carbide growth
substrate of the prior art.
[0148] The thick substantially undoped layer 902 has a first or
lower surface 904 and a second or upper surface 906. The first or
lower surface 904 of the undoped layer is also the first or lower
surface of the multilayer semiconductor structure. The lower
surface 124 of the first doped layer is in contact with the second
surface 906 of the undoped layer. In this illustrative GaN-based
example, the first doped layer 108 is an n-doped layer with a
thickness 556 of approximately 3 microns, the active region is a
multiple quantum well approximately 0.5 microns thick and the
second doped layer 112 is a p-doped layer with a thickness 558 of
approximately 0.5 micron. The first doped layer, the active region
and the second doped layer may be deposited by any standard GaN
growth technique. In this illustrative example, the three layers
are deposited by MOCVD. The thickness 910 of the undoped layer is
at least 5 microns. Preferably the undoped layer is at least 10
microns thick. More preferably, the undoped layer is at least 15
microns thick. Most preferably, the undoped layer is at least 25
microns thick. In this illustrative example, the undoped layer is
20 microns thick. The undoped layer may be deposited by any
standard GaN growth technique. In this illustrative example, the
undoped layer is deposited by HVPE.
[0149] Example light rays 960, 962 and 964 illustrate internally
generated light that is emitted by the active region 110 of the LED
chip 900. Internally generated light ray 960 is emitted by active
region 110 toward output surface 134 of the LED chip. Internally
generated light ray 960 is directed at an angle to surface 134 that
is less than the critical angle, which allows light ray 960 to exit
the LED chip through surface 134 on the second side 954.
[0150] Internally generated light ray 962 is emitted by active
region 110 toward the lower surface 904 of the LED at an angle less
than the critical angle. Internally generated light ray 962 exits
the LED chip through surface 904 on the first side 952.
[0151] Internally generated light ray 964 is directed to surface
134 at an angle that is greater than the critical angle. Internally
generated light ray 964 is reflected by total internal reflection
and is redirected toward the lower surface 904 of the LED chip.
[0152] For the remainder of this specification, the substrate-free
LED chips will be illustrated as having a thick first doped layer.
However, it will be apparent from the above discussion that any of
the substrate-free chips illustrated in the following diagrams may
instead have a thick second doped layer or may have both a thick
first doped layer and a thick second doped layer. In addition, LED
chips with both electrodes on the lower side of the chip may also
have a thick undoped layer on the upper side of the chip.
[0153] FIGS. 10 and 11 illustrate substrate-free LED chips that
have both electrodes on the lower side of the chip. LED chip 1000
and LED chip 1100 are flip-chip designs, but neither design
includes a growth substrate nor a transfer substrate. The thick
layer in FIGS. 10 and 11 is illustrated to be the first doped
layer. However, the thick layer could also be the second doped
layer or both the first doped layer and the second doped layer or
an undoped layer. These latter examples are not illustrated.
[0154] LED chip 1000 in FIG. 10 includes a thick first doped layer
108. In this example design, the first doped layer is on the upper
or first side 1052 of the chip. The first doped layer 108 is an
n-doped GaN layer, which is epitaxially deposited or otherwise
conventionally fabricated on a growth substrate (not shown). The
growth substrate is later removed by a standard technique such as
laser liftoff, chemical processing or mechanical polishing, thereby
exposing the first surface 124 of the first doped layer. It is
preferably to utilize a substrate removal process that results in
an exposed surface 124 that is rough. A rough surface improves
light extraction form LED chip 1000. The first doped layer 108 has
a first surface 124 and a second surface 126 opposite the first
surface. The thickness 556 of the first doped layer is at least 5
microns. Preferably the first doped layer is at least 10 microns
thick. More preferably, the first doped layer is at least 15
microns thick. Most preferably, the first doped layer is at least
25 microns thick. The first doped layer may be deposited by any
standard GaN growth technique. Preferably, the first doped layer is
deposited by HVPE.
[0155] The first electrode 102 of LED chip 1000 is fabricated on a
portion 116 of the second surface 126 of the first doped layer that
was previously exposed by an etching process. The second electrode
114 is fabricated on the second surface 134 of the second doped
layer. The second electrode substantially covers the second surface
134. Substantially all of the light emitted by LED chip 1000 is
emitted through the upper or first side 1052 of the chip. For
example, light ray 1060 is emitted through surface 124. Light ray
1062 is initially directed to the second side 1054 but is reflected
by surface 140 of the second electrode. Light ray 1062 exits LED
1000 through the upper or first side 1052.
[0156] FIG. 11 illustrates example substrate-free LED chip 1100.
LED chip 1100 is nearly identical to LED chip 1000 except that the
second electrode 114 of LED chip 1100 covers only a portion of
surface 134 of the second doped layer 112. Second electrode 114 is
fabricated by depositing a layer of metal on surface 134 of the
second doped layer followed by patterning the metal layer by
standard photolithographic techniques. Light can exit LED chip 1100
through both the upper or first side 1152 and the lower or second
side 1154 of the chip, thereby increasing the extraction
efficiency, the external quantum efficiency and the light emission
of the chip. For example, internally generated light ray 1160 exits
LED chip 1100 on the upper or first side 1152 of the chip. Light
ray 1162 exits LED chip 1100 on the lower or second side 1154.
Preferably, surface 136 of the first electrode 102 and surface 140
of the second electrode 114 are reflective to internally generated
light and to light externally incident on the first side 1152 of
LED chip 1100.
[0157] FIGS. 12 and 13 illustrate substrate-free LED chips that
have one electrode on the upper side of the chip and one electrode
on the lower side of the chip. The thick layer in FIGS. 12 and 13
is illustrated to be the first doped layer. However, the thick
layer could also be the second doped layer or both the first doped
layer and the second doped layer.
[0158] LED chip 1200 in FIG. 12 includes a thick first doped layer
108. In this example design, the first doped layer is on the upper
or first side 1252 of the chip. The first doped layer 108 is an
n-doped GaN layer, which is epitaxially deposited or otherwise
conventionally fabricated on a growth substrate (not shown). The
growth substrate is later removed by a standard technique such as
laser liftoff, chemical processing or mechanical polishing, thereby
exposing the first surface 124 of the first doped layer. The
thickness 556 of the first doped layer is at least 5 microns.
Preferably the first doped layer is at least 10 microns thick. More
preferably, the first doped layer is at least 15 microns thick.
Most preferably, the first doped layer is at least 25 microns
thick. The first doped layer may be deposited by any standard GaN
growth technique. Preferably, the first doped layer is deposited by
HVPE.
[0159] The first electrode 102 of LED chip 1200 is fabricated on
the first surface 124 of the first doped layer. The first surface
124 was previously exposed by removing the growth substrate. The
first electrode is fabricated by depositing a metal layer and
patterning the layer using standard photolithographic techniques.
The second electrode 114 is fabricated on the second surface 134 of
the second doped layer. In this example, the second electrode
substantially covers the second surface 134. Substantially all of
the light emitted by LED chip 1200 is emitted through the upper or
first side 1252 of the chip. For example, light ray 1260 is emitted
through surface 124. Light ray 1262 is initially directed to the
second side 1254 but is reflected by surface 140 of the second
electrode. Light ray 1262 exits LED 1200 through the upper or first
side 1252.
[0160] Substrate-free LED chip 1300 illustrated in FIG. 13 is
nearly identical to LED chip 1200 except that the second electrode
114 for LED chip 1300 covers only a portion of the second surface
134 of the second doped layer 112. Second electrode 114 is
fabricated by depositing a layer of metal on surface 134 of the
second doped layer followed by patterning the metal layer by
standard photolithographic techniques. Light can exit LED chip 1300
through both the upper or first side 1352 and the lower or second
side 1354 of the chip, thereby increasing the extraction
efficiency, the external quantum efficiency and the light emission
of the chip. For example, internally generated light ray 1360 exits
LED chip 1300 on the upper or first side 1352 of the chip. Light
ray 1362 exits LED chip 1300 on the lower or second side 1354.
[0161] While the invention has been described in conjunction with
specific embodiments and examples, it is evident to those skilled
in the art that many alternatives, modifications and variations
will be apparent in light of the foregoing description.
Accordingly, the invention is intended to embrace all such
alternatives, modifications and variations as fall within the
spirit and scope of the appended claims.
* * * * *