U.S. patent application number 14/055596 was filed with the patent office on 2015-03-12 for distributed bragg reflector for reflecting light of multiple wavelengths from an led.
This patent application is currently assigned to TOSHIBA TECHNO CENTER INC.. The applicant listed for this patent is Toshiba Techno Center Inc.. Invention is credited to Chao-Kun LIN.
Application Number | 20150069434 14/055596 |
Document ID | / |
Family ID | 47753072 |
Filed Date | 2015-03-12 |
United States Patent
Application |
20150069434 |
Kind Code |
A1 |
LIN; Chao-Kun |
March 12, 2015 |
DISTRIBUTED BRAGG REFLECTOR FOR REFLECTING LIGHT OF MULTIPLE
WAVELENGTHS FROM AN LED
Abstract
A blue LED device has a transparent substrate and a reflector
structure disposed on the backside of the substrate. The reflector
structure includes a Distributed Bragg Reflector (DBR) structure
having layers configured to reflect yellow light as well as blue
light. In one example, the DBR structure includes a first portion
where the thicknesses of the layers are larger, and also includes a
second portion where the thicknesses of the layers are smaller. In
addition to having a reflectance of more than 97.5 percent for
light of a wavelength in a 440 nm-470 nm range, the overall
reflector structure has a reflectance of more than 90 percent for
light of a wavelength in a 500 nm-700 nm range.
Inventors: |
LIN; Chao-Kun; (San Jose,
CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Toshiba Techno Center Inc. |
Tokyo |
|
JP |
|
|
Assignee: |
TOSHIBA TECHNO CENTER INC.
Tokyo
JP
|
Family ID: |
47753072 |
Appl. No.: |
14/055596 |
Filed: |
October 16, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13587746 |
Aug 16, 2012 |
8624482 |
|
|
14055596 |
|
|
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|
61530385 |
Sep 1, 2011 |
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Current U.S.
Class: |
257/98 |
Current CPC
Class: |
H01L 2224/73265
20130101; H01L 2224/49107 20130101; H01L 33/46 20130101; G02B
5/0833 20130101 |
Class at
Publication: |
257/98 |
International
Class: |
H01L 33/46 20060101
H01L033/46 |
Claims
1. A Light Emitting Diode (LED) device comprising: a substrate; an
active layer formed above the substrate; and a reflector structure
disposed below the substrate, the reflector structure comprising: a
low-index total internal reflection layer (TIR); and a Distributed
Bragg Reflector (DBR) disposed below the substrate, wherein the DBR
comprises a first plurality of periods and a second plurality of
periods, wherein each of the first plurality of periods includes a
first layer of a high index dielectric material with a first
thickness and a second layer of a low index dielectric material
with a second thickness, and wherein each of the second plurality
of periods includes a first layer of the high index dielectric
material with a third thickness and a second layer of the low index
dielectric material with a fourth thickness, and wherein the TIR is
disposed between the substrate and the DBR.
2. The LED device of claim 1, wherein the high index dielectric
material is selected from the group consisting of: TiO.sub.2, ZnSe,
Si.sub.3N.sub.4, Nb.sub.2O.sub.5 and Ta.sub.2O.sub.5; and wherein
the low index dielectric material is selected from the group
consisting of: SiO.sub.2, MgF.sub.2 and CaF.sub.2.
3. The LED device of claim 1, wherein the reflector structure has a
reflectance greater than 90 percent for first light having a
wavelength in a first range from 500 nm to 700 nm and passing from
the substrate to the reflector structure.
4. The LED device of claim 3, wherein the reflector structure has a
reflectance greater than 90 percent for second light having a
wavelength in a second range from 440 nm to 470 nm and passing from
the substrate to the reflector structure.
5. The LED device of claim 1, wherein the first layer of each of
the first plurality of periods is titanium dioxide approximately 75
nm thick, wherein the second layer of each of the first plurality
of periods is silicon dioxide approximately 138 nm thick, wherein
the first layer of each of the second plurality of periods is
titanium dioxide approximately 46 nm thick, and wherein the second
layer of each of the second plurality of periods is silicon dioxide
approximately 85 nm thick.
6. The LED device of claim 5, wherein a titanium dioxide layer of
the first plurality of periods of the DBR is in contact with the
TIR.
7. The LED device of claim 5, wherein a titanium dioxide layer of
the second plurality of periods of the DBR is in contact with a
silicon dioxide layer of the first plurality of periods.
8. The LED device of claim 1, wherein the TIR is a single layer of
silicon dioxide.
9. The LED device of claim 5, wherein the TIR is a single layer of
silicon dioxide that is thicker than any silicon dioxide layer of
the DBR.
10. The LED device of claim 1, wherein the substrate is a
transparent substrate.
11. The LED device of claim 1, wherein the second thickness is
larger than the first thickness, and wherein the fourth thickness
is larger than the third thickness.
12. The LED device of claim 11, wherein the high index dielectric
material is TiO.sub.2, and the low index dielectric material is
SiO.sub.2.
13. The LED device of claim 1, wherein: the active layer is
configured to emit a first light of a wavelength less than 500 nm;
and the reflector structure has a reflectance greater than 90.0
percent for a second light having a wavelength in a range from 500
nm to 700 nm and passing from the substrate to the reflector
structure.
14. The LED device of claim 1, further comprising an active layer
comprising indium and gallium and configured to emit light of a
wavelength less than 500 nm, wherein an overall LED device exhibits
a Photon Recycling Efficiency (PRE) of more than 85 percent for
light having a wavelength in a range of 500-700 nm.
15. The LED device of claim 1, further comprising an active layer
configured to emit a first light of a wavelength of approximately
440-470 nm, wherein the reflector structure has a reflectance
greater than 90.0 percent for a second light having a wavelength of
approximately 500-700 nm and passing from the substrate to the
reflector structure.
16. The LED device of claim 1, further comprising a reflective
metal layer disposed below the DBR.
17. The LED device of claim 16, wherein the reflective metal layer
is made of a metal taken from the group consisting of: aluminum,
silver, rhodium, platinum and nickel.
18. The LED device of claim 1, wherein: the active layer is
configured to emit a light of a first wavelength less than 500 nm;
the first layer of the first plurality of periods has a first
quarter-wave optical thickness (QWOT) value with respect to the
light of the first wavelength; the first layer of the second
plurality of periods has a second QWOT value with respect to the
light of the first wavelength; and the second QWOT is closer to one
than the first QWOT.
19. The LED device of claim 1, wherein: the active layer is
configured to emit a light of a first wavelength less than 500 nm;
the second layer of the first plurality of periods has a third
quarter-wave optical thickness (QWOT) value with respect to the
light of the first wavelength; the second layer of the second
plurality of periods has a fourth QWOT value with respect to the
light of the first wavelength; and the fourth QWOT is closer to one
than the third QWOT.
20. The LED device of claim 1, wherein: the active layer is
configured to emit a light of a first wavelength less than 500 nm;
the TIR has a first quarter-wave optical thickness (QWOT) value
with respect to the light of the first wavelength; the first layer
of the first plurality of periods has a second quarter-wave optical
thickness (QWOT) value with respect to the light of the first
wavelength; the first layer of the second plurality of periods has
a third QWOT value with respect to the light of the first
wavelength; the first QWOT is larger than the second QWOT; and the
second QWOT is larger than the third QWOT.
21. The LED device of claim 1, wherein: the active layer is
configured to emit a light of a first wavelength less than 500 nm;
the TIR has a first quarter-wave optical thickness (QWOT) value
with respect to the light of the first wavelength; the second layer
of the first plurality of periods has a fourth quarter-wave optical
thickness (QWOT) value with respect to the light of the first
wavelength; the second layer of the second plurality of periods has
a fifth QWOT value with respect to the light of the first
wavelength; the first QWOT is larger than the fourth QWOT; and the
fourth QWOT is larger than the fifth QWOT.
22. An LED assembly comprising: the LED device of claim 1; and a
printed circuit board (PCB) formed below the reflector
structure.
23. The LED assembly of claim 22, wherein the PCB is formed of
aluminum.
24. The LED assembly of claim 22, further comprising a phosphor
covering the LED device of claim 1, wherein the active layer emits
a light of a first wavelength less than 500 nm, and the phosphor
converts the light of the first wavelength to a light of a second
wavelength in a range of 500-700 nm.
25. The LED assembly of claim 24, wherein the LED assembly is
configured to emit white light.
26. A Light Emitting Diode (LED) device comprising: a substrate; an
active layer formed on the substrate and configured to emit second
light of a wavelength less than 500 nm, wherein the active layer
comprises indium and gallium; and a reflector structure formed on
an opposite side of the substrate from the active layer, the
reflector structure comprising a low-index total internal
reflection (TIR) layer and a Distributed Bragg Reflector (DBR),
wherein the reflector structure has a first reflectance greater
than 90 percent for first light having a wavelength in a range of
500 nm to 700 nm and passing from the substrate to the reflector
structure and a second reflectance greater than 90 percent for the
second light, and the TIR is disposed between the substrate and the
DBR.
27. The LED device of claim 26, wherein the DBR comprises: a first
plurality of periods, wherein each of the first plurality of
periods includes a first layer of a first material of a first
thickness and a second layer of a second material of a second
thickness; and a second plurality of periods, wherein each of the
second plurality of periods includes a third layer of the first
material of a third thickness and a fourth layer of the second
material of a fourth thickness.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of and claims priority
upon U.S. application Ser. No. 13/587,746 filed on Aug. 16, 2012
(pending), which is a Non-provisional application of U.S.
Provisional Application No. 61/530,385, filed on Sep. 1, 2011, the
contents of which are all herein incorporated by this reference in
their entireties.
TECHNICAL FIELD
[0002] The present invention relates generally to light-emitting
diodes (LEDs), and more particularly, to a blue LED having a
reflector structure that reflects blue and yellow light well.
BACKGROUND INFORMATION
[0003] FIG. 1 (prior art) is a simplified cross-sectional diagram
of one type of so-called white LED assembly 1. Assembly 1 includes
a lateral blue LED device 2. The active layer 3 of the blue LED
device 2 emits light in all directions, and the light bounces
randomly within the LED device. A substantial amount (about 50%) of
light travels downward. If the light 4 traveling downwards is not
reflected back upward so that it can then escape from the top
surface of LED device, but rather if the light traveling downwards
is absorbed by the die-attach adhesive or by the aluminum core PCB,
then the light generation efficiency of the overall white LED
assembly will suffer.
[0004] The structure of the lateral LED device entails a sapphire
substrate 5 that is substantially transparent to the blue light.
Accordingly, a reflector structure 6 is disposed on the backside
(i.e., bottom side in the diagram) of the transparent substrate 5
to reflect light that was traveling in a downward direction.
Reflector structure 6 reflects the light that travels downwards,
passes this light back up and through the transparent substrate and
through the epitaxial layers of the LED device. The reflected light
then escapes the LED device and reaches phosphor 7 embedded in
encapsulant, such as silicone. The phosphor absorbs some of the
blue light and fluoresces, thereby re-emitting light of longer
wavelengths including green, yellow and red light. The overall
spectrum of light emitted from the overall LED assembly 1 is
therefore said to be white light. This white light is the useful
light produced by the assembly.
[0005] The reflector structure 6 can be a single layer of a highly
reflective metal such as, for example, silver. Unfortunately,
silver has attendant contamination and electromigration issues. For
this and other reasons, LED devices such as the LED device 2 of
FIG. 1 may have reflector structures involving a total internal
reflection (TIR) layer 8, a Distributed Bragg Reflector (DBR)
structure 9, and an underlying layer 10 of reflective metal. The
combination of these layers is superior in terms of reflectivity to
a single mirror layer of a highly reflective metal.
[0006] According to Snell's law, all of the light traveling from a
material having a higher index of refraction toward a material
having a lower index of refraction at an angle greater than the
critical angle will be reflected back into the
higher-index-of-refraction material without experiencing any energy
loss. This mechanism is known as total internal reflection (TIR).
The TIR layer 8 is fashioned to reflect blue light that is passing
toward the reflector at angles greater than the critical angle. The
lower two portions 9 and 10 of the reflector structure (the DBR and
the reflective metal layer) are provided to reflect any remaining
light that passes through the TIR layer.
[0007] In its simplest form, a DBR is a quarter wave stack of
dielectric materials. The quarter wave stack consists of a stack of
layers, where the material from which the layers are made
alternates from layer to layer down the stack. The materials are
selected such that the alternating layers have a high index of
refraction, and then a low index of refraction, and then a high
index of refraction, and so forth down the stack. For a given
wavelength of light entering the stack from the top, the upper
layer is made to have a thickness of one quarter of the wavelength,
where this wavelength is the wavelength of the light when the light
is passing through the layer. The wavelength .lamda., frequency f,
and velocity v of light is given by the equation .lamda.=v/f. When
light leaves one medium and enters another medium, the speed and
wavelength of the light may change but the frequency does not
change. The material from which the upper layer is made therefore
determines the speed of light v in the medium. The material
therefore also influences the wavelength .lamda. of the light in
the upper layer.
[0008] Each material has an index of refraction .eta.. The index of
refraction .eta. is the ratio of the speed of light in a vacuum to
the speed of light in the medium. The wavelength of light in a
medium is given by the equation .lamda.=.lamda.o/.eta., where
.eta.o is the wavelength in a vacuum. Light traveling through air
is traveling at close to the speed of light in a vacuum, so the
wavelength of light in air is close to wavelength of the light in a
vacuum. The design wavelength .eta.o for the DBR is usually longer
than the LED emitting wavelength when the reflectivity of the DBR
for the light with incident angles between zero degrees and the
critical angle is considered. For example, the optimal DBR design
wavelength for a 450 nm LED is around 510 nm. The relationship
QWOT=.lamda.o/4.eta. is used to determine the quarter wavelength in
the medium of a layer, where .eta. is the refractive index of the
material from which the layer is made. In this way, the refractive
indices of the materials of the various layers of the stack are
used to determine how thick each layer of the stack should be so
that it is one quarter wavelength in thickness.
[0009] Light passes into the stack and through the upper layer, and
then some of the light reflects off the interface between the upper
layer and the next layer down in the stack. Part of the light
proceeds down into the next layer of the stack to the next
interface. If the interface is one from a low-index medium to a
high-index medium, then any light reflected from the interface will
have a phase shift of 180 degrees. If, on the other hand, the
interface is one from a high-index medium to a low-index medium,
then any reflected light will have no phase shift. Each interface
causes a partial reflection of the light wave passing into the
stack. The phase shifts, in combination with the thicknesses of the
layers of the stack, are such that the portions of light reflecting
off interfaces all return to the upper surface of the stack in
phase with each other. The many reflections off the many interfaces
all combine at the top of the stack with constructive interference.
The result is that the Distributed Bragg Reflector has a high
reflectivity within a finite spectral range known as the stop-band.
Then lastly at the bottom of the reflector structure 6 is the layer
10 of reflective metal.
[0010] FIG. 2 (prior art) is a table that sets forth the
thicknesses and materials of the various layers of the Distributed
Bragg Reflector of the prior art LED device 2 of FIG. 1 based on a
design wavelength of 510 nm. The .pi. notation above the line
between two rows indicates that the light reflected by the
interface between the materials of the two rows is phase shifted by
180 degrees. The upper SiO.sub.2 layer has a thickness of 4101
angstroms and is the TIR layer 8. The DBR structure 9 includes
three periods, where each period has a first layer of TiO.sub.2
that is 447 angstroms thick and a second layer of SiO.sub.2 that is
820 angstroms thick.
[0011] FIG. 3 is a diagram that shows the normal-incident
reflectivity spectrum with the reflector design described in FIG.
2. The stop-band of the spectrum centers around 510 nm, and the
short wavelength side of the stop-band is aligned to 450 nm.
According to theoretical calculation, the reflectivity spectrum
blue-shifts toward the short wavelength when the light incident
angle increases from surface normal toward grazing angle to the
reflector. The reflector was optimized to ensure high reflectivity
for the light with wavelength of 450 nm over a broad range of
incident angles. FIG. 4A is a diagram that charts the reflectivity
of the reflector structure 6 versus the angle of incidence of light
with a wavelength of 450 nm reaching a point 11 on the reflector.
The light with incident angles between 0 and 58 degree are
reflected by the DBR and the metal reflector, while the light with
incident angle greater than 58 degree is reflected by the TIR
layer. To evaluate the total reflectivity of the reflector with all
incident angles, a normalized angular reflectance is defined.
Referring to FIG. 4B, light is assumed to be transmitted toward
point 11 on the reflector from all directions with a uniform
angular distribution. The amount of light incident on the point
that is reaching the point 11 with an incident angle .theta. is
considered. Many different light rays may actually reach the point
from this incident angle, where the light rays can be thought of as
passing to the point in a cone shape. The upper lip of the cone 12
illustrated in FIG. 4B represents a circle of origination points
for such rays for the incident angle .theta.. Accordingly, there is
more light incident on point 11 for an incident angle of one degree
than for an incident angle of zero degrees. This larger amount of
light at larger angles is considered, and the corresponding total
amount of reflected light is determined for angles zero
(orthogonal) through 90 degrees (a grazing angle). The normalized
angular reflectance is then calculated by integrating the angular
reflectivity (FIG. 4A) with a sine dependence of incident angle and
normalized to a perfect angular reflectivity spectrum. This
analysis is performed for light of a given wavelength, for example
450 nm, to compare the performance of the reflector for blue light
emitted by the LED in the white LED assembly FIG. 1. When analyzed
this way, the prior art reflector structure of the LED device of
FIG. 1 has a reflectivity of approximately 97 percent for incident
blue light (having a wavelength of 450 nm). Accordingly, most all
of the blue light 4 traveling downward is then reflected back up
the reflector so that it can escape the LED device. The reflector
structure involving DBR 9 is more effective than a simple mirror
layer of a reflective metal such as silver.
SUMMARY
[0012] A blue LED device has an active layer involving indium,
gallium and nitrogen. The active layer is configured to emit blue
light that is quasi-monochromatic and non-coherent. The blue LED
also has a transparent substrate (substantially transparent to
visible light) and a reflector structure disposed on the backside
of the substrate. The reflector structure includes a Distributed
Bragg Reflector (DBR) structure having layers configured to reflect
green, yellow and red light as well as blue light. In one example,
the DBR structure includes a first portion where the thicknesses of
the layers are relatively larger, and also includes a second
portion where the thicknesses of the layers are relatively smaller.
In addition to having a normalized angular reflectance of more than
97.5 percent for light of a wavelength in a first range between 440
nm-470 nm, the overall reflector structure also has a normalized
angular reflectance of more than 95 percent for light of a
wavelength in a second range between 500 nm-700 nm. The reflector
structure reflects light passing from the transparent substrate and
to the reflector structure such that the overall LED device has a
Photon Recycling Efficiency (PRE) of more than 85 percent for light
having a wavelength ranging from 500 nm to 700 nm.
[0013] Further details and embodiments and techniques are described
in the detailed description below. This summary does not purport to
define the invention. The invention is defined by the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, where like numerals indicate like
components, illustrate embodiments of the invention.
[0015] FIG. 1 (prior art) is a simplified cross-sectional diagram
of a conventional so-called white LED.
[0016] FIG. 2 (prior art) is a table that sets forth the
thicknesses and constituent materials of the various layer of the
Distributed Bragg Reflector of the prior art LED device of FIG.
1.
[0017] FIG. 3 (prior art) is a diagram that charts reflectivity
versus the wavelength of incident light at a normal incident angle
for the reflector structure of the prior art LED device of FIG.
1.
[0018] FIG. 4A (prior art) is a diagram that charts the
reflectivity of the reflector structure of the prior art LED device
of FIG. 1 versus the angle of incidence of light of a wavelength of
450 nm reaching a point on the reflector.
[0019] FIG. 4B (prior art) is a conceptual diagram that illustrates
a consideration involved in determining the normalized angular
reflectance.
[0020] FIG. 5 is a diagram of a white LED assembly in accordance
with one novel aspect.
[0021] FIG. 6 is a simplified cross-sectional diagram of a blue LED
device within the white LED assembly of FIG. 5.
[0022] FIG. 7 is a table that sets forth the thicknesses and
constituent materials of the various layers of the novel reflector
structure of FIGS. 5-6.
[0023] FIG. 8 is a diagram that charts reflectivity versus
wavelength of incident light normal to the reflector surface for
the novel reflector structure of FIGS. 5-7.
[0024] FIG. 9 is a table that compares the normalized angular
reflectance at 450 nm and at 580 nm of the novel reflector
structure of FIGS. 5-7 to the prior art reflector structure of
FIGS. 1-4.
[0025] FIG. 10 is a table that compares measured PRE values of the
novel reflector structure of FIGS. 5-7 (at 450 nm, 580 nm, and 630
nm) with calculated PRE values to the prior art reflector structure
of FIGS. 1-4 (at 450 nm, 580 nm, and 630 nm).
[0026] FIG. 11 is a flowchart of a method for forming a reflector
structure on a blue LED that exhibits a high normalized angular
reflectance for light having a wavelength in a range from 500 nm to
700 nm and in another range from 440 nm to 470 nm.
DETAILED DESCRIPTION
[0027] Reference will now be made in detail to some embodiments of
the invention, examples of which are illustrated in the
accompanying drawings.
[0028] FIG. 5 is a simplified cross-sectional diagram of a white
LED assembly 20 in accordance with one novel aspect. White LED
assembly 20 includes a blue LED device 21, an aluminum core PCB 22,
a pair of wire bonds 23 and 24, and an amount of phosphor 25.
Particles of phosphor are suspended in a dome structure of silicone
as illustrated. LED device 21 includes an epitaxial layer portion
that includes, among other parts not illustrated, a p-type layer
26, an active layer 27, an n-type layer 28, a buffer layer 29, and
two metal electrodes 30 and 31. The layers 26-28 are made of
gallium nitride materials and the active layer includes indium such
that the active layer emits so-called blue light as is known in the
GaN blue LED art. The light is quasi-monochromatic and
non-coherent. In the present example, the wavelength of the light
emitted by the active layer 27 has a relatively narrow bandwidth
and is centered at approximately 450 nm.
[0029] The epitaxial layers are disposed on a transparent substrate
32. Transparent substrate 32 is made of a transparent material,
such as sapphire, SiC, GaN or AlN. In the present example, the
transparent substrate 32 is a sapphire substrate. Below substrate
32 is a novel reflector structure 34. Reflector structure 34
includes a total internal reflection (TIR) layer 35, a multi-layer
Distributed Bragg Reflector (DBR) structure 36, and a reflective
metal layer 37. TIR layer 35 and the low refractive index layers of
DBR 36 can be made of low index dielectric material, such as SiO2,
MgF2 or CaF2, and the high index layers of DBR 36 can be made of
high index dielectric material, such TiO2, ZnSe, Si3N4, Nb2O5 or
Ta2O5. Reflective metal layer 37 can be made of any reflective
metal, such as aluminum, silver, rhodium, platinum or nickel.
Reflector structure 34 is disposed on the "backside" of the
substrate on the opposite side of the substrate from the epitaxial
layers. FIG. 6 is a more detailed cross-sectional diagram of the
blue LED device 21 of the white LED assembly 20 of FIG. 5.
[0030] As is conventionally recognized, half of the light emitted
from the active layer of an LED travels downward. This light, which
in the present example has a wavelength of approximately 450 nm,
should be reflected back upward by the reflector structure as
described above in the background section. This light is
represented in FIG. 5 by rays 38 and 39.
[0031] In accordance with one novel aspect, it is now recognized
that some of the light 40 traveling upwards escapes the LED device
and reaches the phosphor 25 but is then down-converted by the
phosphor into light of longer wavelengths. Some of this converted
light 41 then travels back towards the LED device in such a way
that it passes into the LED device. The light that is emitted back
at the LED device by the phosphor is generally in the range of from
500 nm to 700 nm and is referred to here for simplicity purposes as
"yellow" light. This light is represented in FIG. 5 by rays 41-42.
Whereas in the prior art reflector structure described above in
connection with FIGS. 1-4 the reflector structure was not optimized
to reflect light of this yellow wavelength, the novel reflector
structure 34 of FIG. 5 is designed to improve the reflectivity of
light of this wavelength. The novel reflector structure 34 is not
optimized for reflecting only blue light, and is not optimized for
reflecting only yellow light, but rather the layers of the novel
reflector structure are configured to reflect both blue and yellow
light with high reflectivity. Thus, the novel reflector structure
34 has a DBR that is substantially optimized for reflecting both
blue light of approximately 450 nm and yellow light of
approximately 580 nm. In one example, the reflector structure 34
has a normalized angular reflectance of more than 95.5 percent for
first light having a wavelength in a range from 500 nm to 700 nm
(referred to here as yellow light), and also has a normalized
angular reflectance of more than 97.5 percent second light having a
wavelength in a range from 440 nm to 470 nm (referred to here as
blue light). The photon efficacy (lumens per watt) of the overall
novel LED assembly 20 of FIG. 5 is improved as compared to the
photon efficacy of the overall conventional LED assembly 1 of FIG.
1 largely due to the improved reflectivity of the reflector
structure 34 in reflecting the light in the 500 nm to 700 nm
range.
[0032] Designing the DBR structure 34 is not as simple as designing
a first DBR optimized for reflecting yellow light, and designing a
second DBR optimized for reflecting blue light, and then combining
the two DBRs into a single composite DBR structure. Light passing
through the DBR structure from one portion to the next is affected
in complex ways that complicates the determination of the
thicknesses of the various layers, and the DBR is not entirely
optimized for either yellow or blue light, but in a simplistic
explanation a first portion 43 of the DBR 34 functions primarily to
reflect yellow light, whereas a second portion 44 of the DBR 34
functions primarily to reflect blue light. The thicknesses of the
layers of the first portion 43 are larger, whereas the thicknesses
of the layers of the second portion 44 are smaller.
[0033] FIG. 7 is a table that sets forth the thicknesses and
compositions of the various layers of the reflector structure 34 in
one specific embodiment. Row 45 corresponds to the TIR layer 35.
Rows 46 correspond to the first portion 43 of the DBR structure 36,
and rows 47 correspond to the second portion 44 of the DBR
structure 36. Row 48 corresponds to the layer 37 of reflective
metal. The values in the table are for a design wavelength of 480
nm. Accordingly, the quarter-wave optical thickness (QWOT) values
close to one in rows 47 indicate that the second portion 44 of the
DBR structure will reflect blue light well.
[0034] FIG. 8 is a chart of the reflectivity 49 versus wavelength
for a normal incident angle for the overall reflector structure 34.
The chart compares the reflectivity spectrum of the prior art
reflector to that of the novel reflector. There are two distinct
stop-band features for the novel reflector indicating the
complexity of the reflector design. Dashed curve 50 is the
reflectivity versus wavelength curve 50 of FIG. 3 that is
reproduced in FIG. 8 for comparison purposes.
[0035] FIG. 9 is a table that sets forth the comparison. For first
light having a wavelength of 580 nm (generally referred to herein
as yellow light) passing from the substrate and into the reflector
structure, the novel reflector structure 34 of FIGS. 5-7 has a
reflectivity greater than 95.0 percent. For second light having a
wavelength of 450 nm (generally referred to herein as blue light)
passing from the substrate and into the reflector structure, the
novel reflector structure 34 of FIGS. 5-7 has a reflectivity
greater than 97.5 percent.
[0036] Referring to white LED assembly 20 of FIG. 5, the phosphors
absorb the blue light emitted from the LED device 21 and
down-convert it to longer wavelength (500 nm-700 nm) light. The
long wavelength light re-emitted isotropically from the phosphor
particles and some portion of long wavelength light will inevitably
return to the LED surface. The probability of the returned light to
escape the LED device 21 is referred as the Photon Recycling
Efficiency (PRE). The un-absorbed blue light emitted from the LED
device may also be back-scattered by the phosphors and return to
the LED device. A comprehensive ray-tracing model was employed to
estimate the PRE for various wavelengths light. The absorption of
the Indium Tin Oxide (ITO), the metal electrode, GaN material loss,
the scattering structure and the reflector were all included in the
simulation.
[0037] The simulation was performed using 450 nm light, 580 nm
light, and 630 nm light. The percentage of light reflected (or
"PRE") is set forth in the table of FIG. 10. The relatively small
differences in reflectivity between the novel reflector structure
and the conventional reflector structure indicated in the table of
FIG. 9 are amplified in the real device due to light within the LED
device often making multiple bounces within the device. Simulation
indicates that switching from the conventional reflector structure
6 of FIG. 1 to the novel reflector structure 34 of FIG. 5 results
in more than a 5.0 percent improvement in Photon Recycling
Efficiency for both 580 nm light and 630 nm light.
[0038] FIG. 11 is a flowchart of a method 100 in accordance with
one novel aspect. A reflector structure is formed (step 101) on the
backside of a substrate of a blue LED device. The active layer of
the blue LED device is configured to emit light having a wavelength
of approximately 440-470 nm, whereas the reflector structure has a
normalized angular reflectance greater than 95.0% for light having
a wavelength in a range from 500 nm to 700 nm. In one specific
example, the reflector structure also has a normalized angular
reflectance greater than 97.5% for light having a wavelength of
440-470 nm. In one specific example, the reflector structure formed
in step 101 is the reflector structure 34 of FIGS. 5 and 6, where
this reflector structure 34 has a TIR layer, a DBR structure, and
an underlying layer of metal of the thicknesses and constituent
materials set forth in FIG. 7.
[0039] Although certain specific embodiments are described above
for instructional purposes, the teachings of this patent document
have general applicability and are not limited to the specific
embodiments described above. Accordingly, various modifications,
adaptations, and combinations of various features of the described
embodiments can be practiced without departing from the scope of
the invention as set forth in the claims.
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