U.S. patent application number 12/579494 was filed with the patent office on 2010-02-11 for edge-emitting led assembly.
This patent application is currently assigned to Avago Technologies ECBU IP (Singapore) Pte. Ltd. Invention is credited to Steven D. Lester, Jeffrey N. Miller, Virginia M. Robbins.
Application Number | 20100032703 12/579494 |
Document ID | / |
Family ID | 37892772 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100032703 |
Kind Code |
A1 |
Miller; Jeffrey N. ; et
al. |
February 11, 2010 |
EDGE-EMITTING LED ASSEMBLY
Abstract
A light-emitting diode (LED) in accordance with the invention
includes an edge-emitting LED stack having an external emitting
surface from which light is emitted, and a reflective element that
is located adjacent to at least one external surface of the LED
stack other than the external emitting surface. The reflective
element receives light that is generated inside the LED stack and
reflects the received light back into the LED stack. At least a
portion of the reflected light is then emitted from the external
emitting surface.
Inventors: |
Miller; Jeffrey N.; (Los
Altos Hills, CA) ; Lester; Steven D.; (Palo Alto,
CA) ; Robbins; Virginia M.; (Los Gatos, CA) |
Correspondence
Address: |
Kathy Manke;Avago Technologies Limited
4380 Ziegler Road
Fort Collins
CO
80525
US
|
Assignee: |
Avago Technologies ECBU IP
(Singapore) Pte. Ltd
Singapore
SG
|
Family ID: |
37892772 |
Appl. No.: |
12/579494 |
Filed: |
October 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11235592 |
Sep 26, 2005 |
7635874 |
|
|
12579494 |
|
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Current U.S.
Class: |
257/98 ; 257/99;
257/E33.058; 257/E33.072 |
Current CPC
Class: |
H01L 33/46 20130101 |
Class at
Publication: |
257/98 ; 257/99;
257/E33.072; 257/E33.058 |
International
Class: |
H01L 33/00 20100101
H01L033/00 |
Claims
1-10. (canceled)
11. A light emitting diode (LED) assembly comprising: an
edge-emitting LED stack comprising a first external emitting
surface from which light is emitted; and a reflective enclosure
enclosing at least two external surfaces other than the first
external emitting surface, wherein the reflective element is
operable to reflect light that is generated inside the LED stack
back into the LED stack, whereby at least a portion of the
reflected light is emitted from the first external emitting
surface.
12. The LED assembly of claim 11, wherein the edge-emitting LED
stack further comprises: a second external surface comprising a
p-type contact; a third external surface comprising an n-type
contact; and wherein the reflective enclosure further encloses at
least a portion of the second external surface and at least a
portion of the third external surface.
13. The LED assembly of claim 12, wherein the edge-emitting LED
stack further comprises: a substrate; and a reflective layer
located adjacent to a major internal surface of the substrate.
14-23. (canceled)
Description
DESCRIPTION OF THE RELATED ART
[0001] Light-emitting diodes (LEDs) may be broadly classified under
two categories: front-emitting LEDs and edge-emitting LEDs. While
some of these LEDs are used as wide-angle illumination sources,
others are used for producing directional light that is coupled
into an optical fiber for example. In LEDs that produce directional
light, the amount of emitted light can be increased in two ways: 1)
by increasing the overall light efficiency of the LED and 2) by
making the emitted light more directional.
[0002] An edge-emitting LED is typically constructed to incorporate
both these solutions for increasing the amount of emitted light.
The active junction region, which is the source of incoherent light
in the edge-emitting LED, is sandwiched between cladding layers.
The refractive index of the cladding layers is lower than the
refractive index of the active junction region but higher than the
refractive index of the material immediately surrounding the
cladding layers. Such a structure operates as an asymmetric,
dielectric waveguide to channel light towards the edge of the
edge-emitting LED. Attention is drawn to the following manuscript,
which provides one example of such an approach: "Very High Radiance
Edge-Emitting LED," by Michael Ettenberg et al. published in the
IEEE Journal of Quantum Electronics, Vol. QE-12, No. 6 Jun.
1976.
[0003] Unfortunately, this dielectric waveguide structure is not
ideal, because the cladding layers and the other surrounding layers
do not completely confine the light in the active junction region
of the edge-emitting LED. A portion of light is lost due to
radiation through these layers and out of the various external
surfaces other than the external emitting surface from which light
is designed to be emitted out of the edge-emitting LED.
SUMMARY
[0004] A light-emitting diode (LED) in accordance with the
invention includes an edge-emitting LED stack having an external
emitting surface from which light is emitted, and a reflective
element that is located adjacent to at least one external surface
of the LED stack other than the external emitting surface. The
reflective element receives light that is generated inside the LED
stack and reflects the received light back into the LED stack. At
least a portion of the reflected light is then emitted from the
external emitting surface.
[0005] Clearly, some alternative embodiments may exhibit advantages
and features in addition to, or in lieu of, those mentioned above.
It is intended that all such alternative embodiments be included
within the scope of the present invention, and be protected by the
accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Many aspects of the invention can be better understood with
reference to the following drawings. The components in the drawings
are not necessarily to scale. Instead, emphasis is placed upon
clearly illustrating the principles of the invention. Moreover, in
the drawings, like reference numerals designate corresponding parts
throughout the several views.
[0007] FIG. 1 shows various layers of a first exemplary
edge-emitting LED stack in accordance with the invention.
[0008] FIG. 2 shows the LED stack of FIG. 1 together with a
reflective element located adjacent to an external surface other
than the external emitting surface from which light is emitted.
[0009] FIG. 3 shows a first exemplary embodiment of a reflective
element.
[0010] FIG. 4 shows a second exemplary embodiment of a reflective
element.
[0011] FIG. 5 shows the LED stack of FIG. 1 together with two
reflective elements in accordance with the invention.
[0012] FIG. 6 shows the LED stack of FIG. 1 enclosed in a
reflective enclosure in accordance with the invention.
[0013] FIG. 7 shows various layers of a second exemplary
edge-emitting LED stack in accordance with the invention.
[0014] FIG. 8 shows the LED stack of FIG. 7 enclosed in a first
exemplary reflective enclosure in accordance with the
invention.
[0015] FIG. 9 shows the LED stack of FIG. 7 enclosed in a second
exemplary reflective enclosure in accordance with the
invention.
[0016] FIG. 10 is a flowchart of an exemplary method of light
emission in accordance with the invention.
DETAILED DESCRIPTION
[0017] An exemplary embodiment in accordance with the invention
describes an edge-emitting light-emitting diode (LED) incorporating
one or more reflective elements that are located adjacent to one or
more external surfaces of the LED stack other than the external
emitting surface from which light is designed to be emitted. As a
result of this reflection, at least a portion of the reflected
light is additionally emitted from the external emitting surface
thereby increasing the efficiency of the edge-emitting LED.
[0018] FIG. 1 shows various layers of a first exemplary
edge-emitting LED stack 100 in accordance with the invention. LED
stack 100 is formed of multiple layers that operate to generate and
transmit incoherent light. The incoherent light is generated in
active region layer 120 where holes and electrons combine to emit
photons. Active region layer 120 is sandwiched between a first
cladding layer 115 and a second cladding layer 125. A p-type
contact layer 110 is formed on a major surface of cladding layer
115, with a p-type contact 105 located on at least a portion of the
major external surface of contact layer 110.
[0019] Substrate 130 is located adjacent to cladding layer 125.
Typically, substrate 130 is composed of a conductive semiconductor
such as GaN. An n-type contact layer 135 is formed on a major
surface of substrate 130 with an n-type contact 140 located on at
least a portion of the major external surface of contact layer
135.
[0020] The refractive index of cladding layer 115 is lower than the
refractive index of the active region layer 120 but higher than the
refractive index of the p-type contact layer 110. Similarly, the
refractive index of cladding layer 125 is lower than the refractive
index of the active region layer 120, but higher than the
refractive index of the substrate 130 and the n-type contact layer
135.
[0021] Active region layer 120 in conjunction with the cladding
layers and other layers of LED stack 100 operates as a dielectric
waveguide to channel light towards an emitting edge located on
external emitting surface 145 of LED stack 100. LED light is
generated in the active region layer 120 when a suitable voltage is
applied between p-type contact 105 and n-type contact 140.
[0022] The direction of emitted incoherent light is indicated by
arrow 150. External emitting surface 145 has an anti-reflective
coating, while the external surface opposing external emitting
surface 145 has a reflective coating. The reflective coating
operates to reflect light generated in active region layer 120
towards external emitting surface 145. Consequently, light is
inhibited from escaping out the external surface opposing external
emitting surface 145.
[0023] FIG. 2 shows LED stack 100 of FIG. 1 together with an
exemplary reflective element 205 located adjacent to external
surface 210 of LED stack 100. Reflective element 205 is described
below in more detail using FIGS. 3 and 4. In a first exemplary
embodiment, reflective element 205 is attached to external surface
210 using an adhesive, such as an optical quality epoxy. In a
second exemplary embodiment, reflective element 205 is located
adjacent to external surface 210 using mechanical fasteners. In a
third exemplary embodiment, reflective element 205 is placed
adjacent to external surface 210 and LED stack 100 is encapsulated
together with reflective element 205 using a suitable material such
as plastic or epoxy.
[0024] Reflective element 205 reflects light generated in active
region layer 120 and emitted from external surface 210 back towards
the interior of LED stack 100. Reflective element 205 also reflects
light emanating out of external surface 210 from other layers of
LED stack 100 in addition to active region layer 120. For example,
light emitted from external surface 210 associated with cladding
layers 115 and 125, substrate 130, and conducting layers 110 and
135 is reflected back towards the interior of LED stack 100.
[0025] In alternative embodiments, one or more reflective elements
are located adjacent to one or more external surfaces excluding
external emitting surface 145 of LED stack 100. These external
surfaces include external surface 215, external surface 220,
external surface 225, external surface 230, and external surface
235. Typically, reflecting elements are not located adjacent to the
two contacts--n-type contact 140 and p-type contact 105, which are
made of metal such as gold.
[0026] FIG. 3 shows a first exemplary embodiment of a reflective
element 300 that is located adjacent to an external surface of LED
stack 100 other than the external emitting surface from which light
is emitted. In this exemplary embodiment, reflective element 300 is
a low-loss mirror which reflects light back into LED stack 100 in a
direction indicated by arrow 305. Reflective element 300 is
composed of material that has a refractive index lower than the
refractive index of the material contained in the various layers of
LED stack 100 and has a quarter-wavelength thickness "t" that is
derived from a selected wavelength in a spectrum of light emitted
by LED stack 100.
[0027] For example, when LED stack 100 is designed for producing
red light, the quarter wavelength thickness "t" is derived from a
wavelength in the center of the red visible spectrum. Similarly,
when LED stack 100 is designed for producing blue light, the
quarter wavelength thickness "t" is derived from a wavelength in
the center of the blue visible spectrum. In alternative
embodiments, visible, near-visible, and infra-red wavelengths are
used for determining the deriving the thickness "t" of reflective
element 300. In another alternative embodiment, the refractive
index of the material contained in reflective element 300 varies
linearly or non-linearly from one major surface to an opposing
major surface.
[0028] FIG. 4 shows a second exemplary embodiment of a reflective
element 400 that is located adjacent to an external surface of LED
stack 100 other than the external emitting surface from which light
is emitted. In this exemplary embodiment, reflective element 400 is
composed of multiple layers of material each having a different
thickness. Each of the multiple layers has a refractive index lower
than the refractive index of material contained in the various
layers of LED stack 100. The thickness of each of the multiple
layers bears a quarter-wave relationship to one or more wavelengths
of light emitted by LED stack 100.
[0029] For example, when LED stack 100 is designed for producing
red light, the thickness of layer 405 is derived by using a
wavelength in the low-end of the red visible spectrum. The
thickness of layer 410 is derived by using a wavelength in the
middle of the red visible spectrum, and the thickness of layer 415
is derived by using a wavelength in the upper-end of the red
visible spectrum.
[0030] As a further example, when LED stack 100 is designed for
producing blue light, the thickness of layer 405 is derived by
using a wavelength in the low-end of the blue visible spectrum. The
thickness of layer 410 is derived by using a wavelength in the
middle of the blue visible spectrum, and the thickness of layer 415
is derived by using a wavelength in the upper-end of the blue
visible spectrum.
[0031] It will be understood that FIG. 4 merely shows one exemplary
arrangement of the multiple layers of reflective element 400. Other
arrangements are used in other alternative embodiments. For
example, in a first alternative embodiment, the thickness of each
of the layers is identical to one another. In a second alternative
embodiment, the thickness of each of the multiple layers decreases
with location farther away from the LED stack 100. In a third
alternative embodiment, the thickness of each of the multiple
layers bears a linear relationship to the thickness of other
layers. In a fourth alternative embodiment, the thickness of each
of the multiple layers bears a non-linear relationship to the
thickness of other layers--for example, a logarithmic relationship.
In a fifth alternative embodiment, the refractive index of one or
more of the layers is different than the refractive index of one or
more of the other layers. In a sixth alternative embodiment, the
refractive index of the material in one or more of the layers
varies linearly or non-linearly from one major surface to an
opposing major surface of the layer.
[0032] FIG. 5 shows LED stack 100 together with two exemplary
reflective elements 505 and 510 located adjacent to external
surfaces other than the emitting surface, in accordance with the
invention. Reflective element 505 is located adjacent to external
surface 510 and reflective element 520 is located adjacent to
external surface 515. In a first embodiment, reflective element 505
is similar to reflective element 520. For example, the refractive
index and the thickness of both reflectors are identical. In a
second embodiment, reflective element 505 is different from
reflective element 520. For example, reflective element 505 has a
thickness that is derived from a certain wavelength of light
emitted from LED stack 100, while reflective element 520 has a
different thickness that is derived from a different wavelength of
light emitted from LED stack 100.
[0033] FIG. 6 shows LED stack 100 enclosed in a reflective
enclosure 605 in accordance with the invention. In this exemplary
embodiment, reflective enclosure 605 is formed of two
sections--reflective section 605A and reflective section 605B.
Reflective section 605A is located adjacent to external surfaces
625, 610, and 635, while reflective section 605B is located
adjacent to external surfaces 620, 615, and 630.
[0034] In a first exemplary embodiment, each of the reflective
sections 605A and 605B is attached to the respective sides of LED
stack 100 using an adhesive such as an optical quality epoxy. In a
second exemplary embodiment, each of the reflective sections 605A
and 605B are attached to LED stack 100 using mechanical fasteners.
In a third exemplary embodiment, each of the reflective sections
605A and 605B are placed in contact with LED stack 100, and LED
stack 100 is encapsulated together with the two reflective sections
605A and 605B, using a suitable material such as plastic or
epoxy.
[0035] FIG. 7 shows various layers of a second exemplary
edge-emitting LED stack 700 in accordance with the invention. It
will be understood that terms such as "top," "bottom," and "beside"
are used below merely for purposes of description and are not
intended to limit the positional relationship of the various
elements of FIG. 7.
[0036] LED stack 700 is formed of multiple layers that operate to
generate and transmit incoherent light. The incoherent light is
generated in active region layer 720 where holes and electrons
combine to emit photons. Active region layer 720 is sandwiched
between a first cladding layer 715 and a second cladding layer 725.
A p-type contact layer 710 is formed on a major surface of cladding
layer 715, with a p-type contact 705 located on at least a portion
of the major external surface of contact layer 710.
[0037] An n-type contact layer 730 is located below cladding layer
725, with a buffer layer 705 located below contact layer 730. An
n-type contact 740 is located on at least a portion of the major
external surface of contact layer 730. Substrate 740 is located
below buffer layer 735. In this embodiment, substrate 740 is
composed of a non-conductive material such as sapphire.
[0038] The refractive index of cladding layer 715 is lower than the
refractive index of the active region layer 720, but higher than
the refractive index of the p-type contact layer 710. The
refractive index of cladding layer 725 is lower than the refractive
index of the active region layer 720, but higher than the
refractive index of the layers below.
[0039] Active region layer 720 in conjunction with the cladding
layers and other layers of LED stack 700 operates as a dielectric
waveguide to channel light towards an emitting edge located on
external emitting surface 745 of LED stack 700. LED light is
generated in the active region layer 720 when a suitable voltage is
applied between p-type contact 705 and n-type contact 740.
[0040] External emitting surface 745 has an anti-reflective
coating, while the external surface opposing external emitting
surface 745 has a reflective coating. The reflective coating
operates to reflect light generated in active region layer 720
towards external emitting surface 745. Consequently, light is
inhibited from escaping out the external surface opposing external
emitting surface 745.
[0041] FIG. 8 shows LED stack 700 enclosed in a first exemplary
reflective enclosure 805 in accordance with the invention. In this
exemplary embodiment, reflective enclosure 805 is formed of three
sections--reflective section 805A, reflective section 805B, and
reflective section 805C. Reflective section 805A is located
adjacent to external surface 810 and a first portion of external
surface 815. Reflective section 805B is located adjacent to
external surface 825, a first portion of external surface 820, and
a second portion of external surface 815. Reflective section 805C
is located adjacent to external surface 830 and a second portion of
external surface 820.
[0042] In a first exemplary embodiment, each of the reflective
sections 805A, 805B, and 805C are attached to the respective sides
of LED stack 700 using an adhesive such as an optical quality
epoxy. In a second exemplary embodiment, each of the reflective
sections 805A, 805B, and 805C are attached to LED stack 700 using
mechanical fasteners. In a third exemplary embodiment, each of the
reflective sections 805A, 805B, and 805C are placed in contact with
LED stack 700, and LED stack 700 is encapsulated together with the
three reflective sections using a suitable material such as plastic
or epoxy.
[0043] Furthermore, in the embodiment shown in FIG. 8, a reflective
buffer layer 845 is located above substrate 850. In addition to
operating as a buffer layer between substrate 850 and n-type
contact layer 840, reflective buffer layer 845 is composed of
material having a refractive index that is lower than the
refractive index of the material of n-type contact layer 730. The
refractive index of reflective buffer layer 840 is selected to
provide a large degree of reflection to light transmitted from the
active region.
[0044] Reflective sections 805A, 805B, and 805C in conjunction with
reflective buffer layer 845 operate to reflect light emitted from
the active region layer and other layers of LED stack 700 back
towards the interior of LED stack 700. The reflected light is
transmitted together with directly radiated light out of the
external emitting edge of LED stack 700.
[0045] FIG. 9 shows LED stack 700 enclosed in a second exemplary
reflective enclosure 905 in accordance with the invention. In this
exemplary embodiment, reflective enclosure 905 is formed of two
sections--reflective section 905A and reflective section 905B.
Reflective section 905B encloses the two external side surfaces, a
portion of the two external top surfaces, and the entire external
bottom surface. In this exemplary embodiment, buffer layer 935
operates primarily as a buffer between substrate 940 and n-type
contact layer 930.
[0046] FIG. 10 is a flowchart of an exemplary method of light
emission in accordance with the invention. In block 1005,
incoherent light is generated in an active region of an
edge-emitting LED. This is carried out by applying a suitable
voltage between the n-type and p-type contacts of the edge-emitting
LED. The incoherent light is propagated by waveguide action in an
active region layer and is emitted from the external emitting
surface. For purposes of description, this emitted light is
referred to as waveguide light.
[0047] In block 1010, a first reflective element is provided. The
first reflective element is located adjacent to at least one
external surface of the LED stack other than the external emitting
surface from which light is emitted from the LED stack. In block
1015, the reflective element is used to reflect light generated in
the active region back into the LED stack such that at least a
portion of the reflected light is emitted from the external
emitting surface. This portion of reflected light is emitted
together with the waveguide light, thereby leading to an increase
in the intensity of light emitted out of the edge-emitting LED.
Consequently, the efficiency of the edge-emitting LED is
increased.
[0048] The above-described embodiments are merely set forth for a
clear understanding of the principles of the disclosure. Many
variations and modifications may be made without departing
substantially from the disclosure. All such modifications and
variations are included herein within the scope of this
disclosure.
* * * * *