U.S. patent application number 11/860486 was filed with the patent office on 2008-01-10 for light emitting devices with improved light extraction efficiency.
This patent application is currently assigned to PHILIPS LUMILEDS LIGHTING COMPANY, LLC. Invention is credited to Michael D. Camras, Gerard Harbers, William R. Imler, Matthijs H. Keuper, Paul S. Martin, Douglas W. Pocius, Frank M. Steranka, Helena Ticha, Ladislav Tichy, R. Scott West.
Application Number | 20080006840 11/860486 |
Document ID | / |
Family ID | 33541554 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080006840 |
Kind Code |
A1 |
Camras; Michael D. ; et
al. |
January 10, 2008 |
Light Emitting Devices with Improved Light Extraction
Efficiency
Abstract
A device includes a light emitting semiconductor device bonded
to an optical element. In some embodiments, the optical element may
be elongated or shaped to direct a portion of light emitted by the
active region in a direction substantially perpendicular to a
central axis of the semiconductor light emitting device and the
optical element. In some embodiments, the semiconductor light
emitting device and optical element are positioned in a reflector
or adjacent to a light guide. The optical element may be bonded to
the first semiconductor light emitting device by a bond at an
interface disposed between the optical element and the
semiconductor light emitting device. In some embodiments, the bond
is substantially free of organic-based adhesives.
Inventors: |
Camras; Michael D.;
(Sunnyvale, CA) ; Harbers; Gerard; (Sunnyvale,
CA) ; Imler; William R.; (Oakland, CA) ;
Keuper; Matthijs H.; (San Jose, CA) ; Martin; Paul
S.; (Pleasanton, CA) ; Pocius; Douglas W.;
(Sunnyvale, CA) ; Steranka; Frank M.; (San Jose,
CA) ; Ticha; Helena; (Srch, CZ) ; Tichy;
Ladislav; (Srch, CZ) ; West; R. Scott; (Morgan
Hill, CA) |
Correspondence
Address: |
PATENT LAW GROUP LLP
2635 NORTH FIRST STREET
SUITE 223
SAN JOSE
CA
95134
US
|
Assignee: |
PHILIPS LUMILEDS LIGHTING COMPANY,
LLC
San Jose
CA
|
Family ID: |
33541554 |
Appl. No.: |
11/860486 |
Filed: |
September 24, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11328964 |
Jan 9, 2006 |
7276737 |
|
|
11860486 |
Sep 24, 2007 |
|
|
|
10633054 |
Jul 31, 2003 |
7009213 |
|
|
11328964 |
Jan 9, 2006 |
|
|
|
Current U.S.
Class: |
257/98 ;
257/E33.073 |
Current CPC
Class: |
H01L 33/58 20130101;
H01L 33/60 20130101; H01L 33/44 20130101; H01L 33/505 20130101;
H01L 2924/0002 20130101; H01L 2924/0002 20130101; H01L 2924/00
20130101 |
Class at
Publication: |
257/098 |
International
Class: |
H01L 29/22 20060101
H01L029/22; H01L 33/00 20060101 H01L033/00 |
Claims
1. A device comprising: a first semiconductor light emitting device
comprising a stack of semiconductor layers including an active
region; and an optical element bonded to the first semiconductor
light emitting device; wherein the optical element is elongated in
a first direction.
2. The device of claim 1 wherein the first semiconductor light
emitting device is elongated in the first direction.
3. The device of claim 1 further comprising a second semiconductor
light emitting device comprising a stack of layers including an
active region bonded to the optical element next to the first
semiconductor light emitting device.
4. The device of claim 3 wherein the active region of the first
semiconductor light emitting device and the active region of the
second semiconductor light emitting device emit light of different
colors.
5. The device of claim 1 wherein the optical element is bonded to
the first semiconductor light emitting device by a bond at an
interface disposed between said optical element and said first
semiconductor light emitting device, wherein said bond is
substantially free of organic-based adhesives.
6. The device of claim 1 wherein the optical element is an optical
concentrator.
7. The device of claim 1 wherein: the optical element has a first
surface adjacent to the bond connecting the optical element to the
first semiconductor light emitting device, a second surface
substantially parallel to the first surface, and a substantially
parabolic cross section; and an area of the first surface is
smaller than an area of the second surface.
8. The device of claim 7 wherein the optical element has a side
surface.
9. The device of claim 8 wherein the side surface connects the
first surface to the second surface.
10. The device of claim 8 further comprising a mirror on at least a
portion of the side surface.
11. The device of claim 1 further comprising a light guide adjacent
to the optical element.
12. The device of claim 11 wherein the light guide is in contact
with the optical element.
13. The device of claim 11 wherein the light guide is spaced apart
from the optical element.
14. A device comprising: a semiconductor light emitting device
comprising a stack of semiconductor layers including an active
region; and an optical element bonded to the semiconductor light
emitting device; wherein the semiconductor light emitting device
and the optical element are positioned in a reflector.
15. The device of claim 14 further comprising a heat sink attached
to the semiconductor light emitting device.
16. The device of claim 14 wherein the optical element is shaped to
direct a portion of light emitted by the active region in a
direction substantially perpendicular to a central axis of the
semiconductor light emitting device and the optical element.
17. The device of claim 16 wherein the reflector is shaped to
direct a portion of the light exiting the optical element in a
direction substantially parallel to the central axis.
18. The device of claim 14 wherein the optical element is bonded to
the semiconductor light emitting device by a bond at an interface
disposed between said optical element and said semiconductor light
emitting device, wherein said bond is substantially free of
organic-based adhesives.
19. The device of claim 14 further comprising a light guide
adjacent to the reflector.
20. The device of claim 19 wherein the light guide is in contact
with the reflector.
21. The device of claim 19 wherein the light guide is spaced apart
from the reflector.
22. A device comprising: a first semiconductor light emitting
device comprising a stack of semiconductor layers including an
active region; an optical element bonded to the semiconductor light
emitting device; and a light guide adjacent to the optical
element.
23. The device of claim 22 wherein the light guide is in contact
with the optical element.
24. The device of claim 22 wherein the light guide does not touch
the optical element.
25. The device of claim 22 wherein the light guide comprises: a
first surface adjacent to the optical element; and a second angled
surface, wherein and angle between the first and second is greater
than 90 degrees.
26. A device comprising: a semiconductor light emitting device
comprising a stack of semiconductor layers including an active
region; and an optical element bonded to the first semiconductor
light emitting device; wherein the optical element comprises a
material selected from the group of an oxide of tellurium, aluminum
oxynitride, cubic zirconia, transparent alumina, and spinel.
27. The device of claim 26 wherein the optical element is bonded to
the semiconductor light emitting device by a bond at an interface
disposed between said optical element and said semiconductor light
emitting device, wherein said bond is substantially free of
organic-based adhesives.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Division of U.S. patent application
Ser. No. 11/328,964, filed on Jan. 9, 2006, which is a Division of
U.S. patent application Ser. No. 10/633,054, filed on Jul. 31,
2003, now U.S. Pat. No. 7,009,213, granted Mar. 7, 2006. Both U.S.
patent application Ser. No. 11/328,964 and U.S. Pat. No. 7,009,213
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Description of the Related Art
[0003] The light extraction efficiency of a light emitter is
defined as the ratio of the LED's external quantum efficiency to
the LED's internal quantum efficiency. Typically, the light
extraction efficiency of a packaged LED is substantially less than
one, i.e. much of the light generated in the LED's active region
never reaches the external environment.
[0004] Light extraction efficiency is reduced by total internal
reflection at interfaces between the LED and surrounding material
followed by reabsorption of the totally internally reflected light
in the LED. For example, for a cubic geometry LED on a transparent
substrate encapsulated in epoxy, the refractive index (n) at the
emission wavelength changes from a value of, for example,
n.sub.semi .about.3.5 in the LED semiconductor to n.sub.epoxy
.about.1.5 in the epoxy. The corresponding critical angle for total
internal reflection of light incident on the epoxy encapsulant from
the LED semiconductor of this example is
.theta..sub.C=arcsin(n.sub.epoxy/n.sub.semi) .about.25.degree..
Neglecting scattering and multiple reflections, light emitted over
4.pi. steradians from a point in the active region of the cubic LED
crosses a semiconductor/epoxy encapsulant interface only if it is
emitted into one of six narrow light cones, one for each interface,
with each light cone having a half angle equal to the critical
angle. Additional losses due to total internal reflection can occur
at the epoxy/air interface. Consequently, an efficient conventional
geometry (for example, rectangular parallelepiped) transparent
substrate AlInGaP LED encapsulated in epoxy, for example, may have
an external quantum efficiency of only 40%, despite having an
internal quantum efficiency of nearly 100%.
[0005] The effect of total internal reflection on the light
extraction efficiency of LEDs is further discussed in U.S. Pat.
Nos. 5,779,924; 5,793,062; and 6,015,719, all of them incorporated
in their entirety herein by reference.
[0006] In one approach to improving light extraction efficiency,
LEDs are ground into hemispherical shapes. Light emitted from a
point in the active region of a hemispherically shaped LED
intersects the hemispherical interface at near normal incidence.
Thus, total internal reflection is reduced. However, this technique
is tedious and wasteful of material. In addition, defects
introduced during the grinding process may compromise the
reliability and performance of the LEDs.
[0007] In another approach, LEDs are encapsulated (encased) in a
material with a dome or hemispherically shaped surface. For
example, the epoxy encapsulant of the above example may be dome
shaped to reduce losses due to total internal reflection at the
epoxy encapsulant/air interface. However, shaping the surface of a
low refractive index encapsulant such as epoxy does not reduce
losses due to total internal reflection at the semiconductor/low
index encapsulant interface.
[0008] Moreover, epoxy encapsulants typically have coefficients of
thermal expansion that poorly match those of the semiconductor
materials in the LED. Consequently, the epoxy encapsulant subjects
the LED to mechanical stress upon heating or cooling and may damage
the LED. LEDs are also encapsulated in dome shaped high index
glasses, which increase the critical angle for the
semiconductor/encapsulant interface. Unfortunately, optical
absorption in high index glasses and mechanical stress typically
degrade the performance of an LED encapsulated in such glass.
SUMMARY
[0009] According to embodiments of the invention, a device includes
a light emitting semiconductor device bonded to an optical element.
In some embodiments, the optical element may be elongated or shaped
to direct a portion of light emitted by the active region in a
direction substantially perpendicular to a central axis of the
semiconductor light emitting device and the optical element. In
some embodiments, the semiconductor light emitting device and
optical element are positioned in a reflector or adjacent to a
light guide. The optical element may be bonded to the first
semiconductor light emitting device by a bond at an interface
disposed between the optical element and the semiconductor light
emitting device. In some embodiments, the bond is substantially
free of organic-based adhesives.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] FIG. 1A is a schematic diagram of an optical element and a
light emitter to be bonded to each other in accordance with an
embodiment of the present invention.
[0011] FIG. 1B is a schematic diagram of an optical element bonded
with a bonding layer to a light emitter in accordance with an
embodiment of the present invention.
[0012] FIG. 1C is a schematic diagram of an optical element bonded
to a light emitter in accordance with another embodiment of the
present invention.
[0013] FIG. 1D is a schematic diagram of an optical concentrator
bonded to a light emitter in accordance with another embodiment of
the present invention.
[0014] FIG. 2 is a schematic diagram of an optical element directly
bonded to a light emitter in accordance with an embodiment of the
present invention.
[0015] FIG. 3 is a schematic diagram of an optical element bonded
with a bonding layer to a light emitter having beveled sides in
accordance with an embodiment of the present invention.
[0016] FIG. 4 is a schematic diagram of an optical element bonded
with a bonding layer to a light emitter having substrate and
superstrate layers in accordance with an embodiment of the present
invention
[0017] FIG. 5 is a schematic diagram of an optical element directly
bonded to a light emitter having substrate and superstrate layers
in accordance with an embodiment of the present invention
[0018] FIG. 6 is a schematic diagram of an optical element bonded
with a bonding layer to a light emitter having a "flip chip"
geometry in accordance with an embodiment of the present
invention.
[0019] FIG. 7 is a schematic diagram of an optical element directly
bonded to a light emitter having a "flip chip" geometry in
accordance with an embodiment of the present invention
[0020] FIG. 8 is a schematic diagram of an optical element bonded
with a bonding layer to a light emitter having an active region
substantially perpendicular to the optical element.
[0021] FIG. 9 is a schematic diagram of an optical element bonded
directly to a light emitter having an active region substantially
perpendicular to the optical element.
[0022] FIG. 10 is a schematic diagram of a light emitter located in
a recess of a surface of an optical element to which it is directly
bonded.
[0023] FIG. 11 is a schematic diagram of a light emitter located in
a recess of a surface of an optical element to which it is bonded
with a bonding layer.
[0024] FIG. 12A is a schematic diagram of an optical element bonded
to a light emitter in accordance with an embodiment of the present
invention.
[0025] FIGS. 12B-C are schematic diagrams of optical elements
bonded to light emitters, forming top-emitting devices in
accordance with embodiments of the present invention.
[0026] FIGS. 13A-B are schematic diagrams of elongated optical
elements bonded to light emitters in accordance with embodiments of
the present invention.
[0027] FIGS. 14A-B are schematic diagrams of optical elements
bonded to light emitters and positioned in reflectors in accordance
with embodiments of the present invention.
[0028] FIGS. 15A-C are schematic diagrams of asymmetric optical
elements bonded to light emitters in accordance with embodiments of
the present invention.
[0029] FIG. 16 is a schematic diagram of a column-like optical
element bonded to a light emitter in accordance with embodiment of
the present invention.
[0030] FIGS. 17A-C are schematic diagrams of optical elements
bonded to light emitters and coupled to light guides in accordance
with embodiments of the present invention.
[0031] FIG. 18 is a schematic diagram of the path of light rays in
a tapered light guide in accordance with an embodiment of the
present invention.
[0032] FIG. 19 is a schematic diagram of an optical element
including grooves in the shape of a Fresnel lens bonded to a light
emitter in accordance with an embodiment of the present
invention
DETAILED DESCRIPTION
[0033] FIGS. 1-18 illustrate a light emitting device 1, including a
substantially transparent optical element 2 bonded to a light
emitter 4 according to various embodiments of the invention. The
figures describe various light emitters, various optical elements,
and various ways to bond the light emitter to the optical element.
The particular combinations of light emitter, optical element, and
bond illustrated are not meant to be limiting. In general, any of
the light emitters, optical elements, and bonds described may be
combined. Bonding an optical element to a light emitter may
increase the amount of light extracted from the top of the device
and the ratio of top light to side light, allowing any optics used
with the device to be tailored to top light, possibly resulting in
a more efficient and compact system.
[0034] FIG. 1A illustrates optical element 2 and light emitter 4 to
be attached by a bond at an interface between light emitter 4 and
optical element 2 in accordance with an embodiment of the present
invention. Throughout the application the term "light emitter"
includes, but is not limited to, light emitting diodes and laser
diodes. In addition, various embodiments of the invention can be
used as light detectors and solar cells as well.
[0035] FIG. 1B illustrates optical element 2 attached to light
emitter 4 by a bond, which includes a substantially transparent
bonding layer 6, in accordance with an embodiment of the present
invention.
[0036] The term "transparent" is used herein to indicate that an
optical element so described, such as a "transparent optical
element," a "transparent bonding layer," a "transparent substrate,"
or a "transparent superstrate" transmits light at the emission
wavelengths of light emitting device with a less than about 50%
single pass loss, preferably less than about 10% single pass loss
due to absorption or scattering. The emission wavelengths of light
emitting device 4 may lie in the infrared, visible, or ultraviolet
regions of the electromagnetic spectrum. One of ordinary skill in
the art will recognize that the conditions "less than 50% single
pass loss" and "less than 10% single pass loss" may be met by
various combinations of transmission path length and absorption
constant. As used herein, "optical concentrator" includes but is
not limited to total internal reflectors, and includes optical
elements having a wall coated with a reflective metal, a dielectric
material, a reflective coating, or a total internal reflector to
reflect or redirect incident light. Here an example of a reflective
coating is the "white reflectance coating," produced by the Munsell
Color company, which includes barium sulfate.
[0037] FIGS. 1A and 1B illustrate embodiments where light emitter 4
includes a stack of layers. The stack includes semiconductor layers
and an active region, capable of emitting light. In detail, light
emitter 4 includes a first semiconductor layer 8 of n-type
conductivity (n-layer) and a second semiconductor layer 10 of
p-type conductivity (p-layer). Semiconductor layers 8 and 10 are
electrically coupled to active region 12. Active region 12 is, for
example, a p-n diode junction associated with the interface of
layers 8 and 10. Alternatively, active region 12 includes one or
more semiconductor layers that are doped n-type or p-type or are
undoped. Active region 12 can also include quantum wells. N-contact
14 and p-contact 16 are electrically coupled to semiconductor
layers 8 and 10, respectively. Active region 12 emits light upon
application of a suitable voltage across contacts 14 and 16. In
alternative implementations, the conductivity types of layers 8 and
9, together with contacts 14 and 16, are reversed. That is, layer 8
is a p-type layer, contact 14 is a p-contact, layer 10 is an n-type
layer, and contact 16 is an n-contact.
[0038] Semiconductor layers 8 and 10 and active region 12 are
formed from III-V semiconductors including but not limited to AlN,
AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb, II-VI
semiconductors including but not limited to ZnS, ZnSe, CdSe, CdTe,
group IV semiconductors including but not limited to Ge, Si, SiC,
and mixtures or alloys thereof. These semiconductors have indices
of refraction ranging from about 2.4 to about 4.1 at the typical
emission wavelengths of light emitting devices in which they are
present. For example, III-Nitride semiconductors such as GaN have
refractive indices of about 2.4 at 500 nm, and III-Phosphide
semiconductors such as InGaP have refractive indices of about
3.6-3.7 at 600 nm.
[0039] Contacts 14 and 16 are, in one implementation, metal
contacts formed from metals including but not limited to gold,
silver, nickel, aluminum, titanium, chromium, platinum, palladium,
rhodium, rhenium, ruthenium, tungsten, and mixtures or alloys
thereof. In another implementation, one or both of contacts 14 and
16 are formed from transparent conductors such as indium tin
oxide.
[0040] Although the figures illustrate specific light emitting
device structures, the present invention is independent of the
structure and number of semiconductor layers in light emitter 4 and
of the detailed structure of active region 12. Also, light emitter
4 may include, for example, transparent substrates and superstrates
not illustrated in FIGS. 1A and 1B. Further, dimensions of the
various elements of light emitter 4 illustrated in the various
figures are not to scale.
[0041] In some embodiments transparent optical element 2 includes a
metallization layer 20. Metallization layer 20 is electrically
coupled to contact 14 of light emitter 4. Metallization layer 20 is
formed from metals including but not limited to gold, silver,
nickel, aluminum, titanium, chromium, platinum, palladium, rhodium,
rhenium, ruthenium, tungsten, and mixtures or alloys thereof. In
another implementation, metallization layer 20 is formed from
transparent conductors such as indium tin oxide. In one embodiment
metallization layer 20 is a mesh, in other embodiments a continuous
or patterned layer. The thickness of metallization layer 20 is in
the range of, for example, about 2 .ANG. to about 5000 .ANG..
Metallization layer 20 transmits greater than about 10%, preferably
greater than about 50%, of the incident light. Metallization layer
20 is formed on surface 22 of transparent optical element 2. In
some embodiments surface 22 is substantially flat.
[0042] Transparent optical element 2 is bounded by surface 24. The
shape of surface 24 reduces the reflection of the light, emitted by
light emitter 4, as discussed in detail below. In addition, losses
at surface 24 can be further reduced by applying a conventional
antireflection coating 25 to surface 24.
[0043] FIG. 1B illustrates an embodiment where a layer of bonding
material is applied on a top surface 18 of light emitter 4 to form
transparent bonding layer 6. Transparent bonding layer 6 bonds
optical element 2 and light emitter 4. Transparent bonding layer 6
is, for example, about 10 Angstroms (.ANG.) to about 100 microns
(.mu.m) thick. The bonding material is applied, for example, by
conventional deposition techniques including but not limited to
spinning, sputtering, evaporation, chemical vapor deposition (CVD),
or as part of material growth by, for example, metal-organic
chemical vapor deposition (MOCVD), vapor phase epitaxy (VPE),
liquid phase epitaxy (LPE), or molecular beam epitaxy (MBE).
Transparent bonding layer 6 is optionally patterned with, for
example, conventional photolithographic and etching techniques to
leave contact 14 uncovered by bonding material and thus to permit
contact 14 to make electrical contact with optional metallization
layer 20 on optical element 2.
[0044] In an alternative embodiment, transparent bonding layer 6 is
formed on substantially flat surface 22 of optical element 2 or on
the surface of metallization layer 20 and optionally patterned
with, for example, conventional photolithographic and etching
techniques to permit electrical contact between contact 14 and
metallization layer 20. In another embodiment transparent bonding
layers such as bonding layer 6 are formed on both surface 18 of
light emitter 4 and surface 22 of optical element 2. In other
embodiments, contact 14 is not separately provided, and bonding
layer 6 is patterned to permit electrical contact between
metallization layer 20 and n-layer 8.
[0045] FIG. 1C illustrates an embodiment without contact 14 and
bonding layer 6. Here metallization layer 20 additionally functions
as a contact layer for light emitter 4 and as a bonding layer. In
other embodiments, contact 14 is not located on surface 18 and the
metallization layer 20 is not used.
[0046] In the subsequent embodiments bonding layer 6 is formed on
light emitter 4. However, in alternative embodiments bonding layer
6 can be formed on surface 22 of optical element 2, or on both
light emitter 4 and surface 22.
[0047] In some implementations, the bonding material of transparent
bonding layer 6 is a high index optical glass, i.e. an optical
glass having a refractive index greater than about 1.5 in the range
of wavelengths emitted by active region 12. Preferably, the
refractive index of bonding layer 6 is greater than about 1.8.
[0048] In some implementations the bonding material of bonding
layer 6 has a suitably low softening temperature. A definition of
the softening temperature is the temperature, where the glass
starts to appreciably deform under its own weight. The softening
temperature can be higher than the glass transition temperature.
The softening temperature of the bonding material can be in the
range of about 250.degree. C. to about 500.degree. C., for example,
within the range of 300.degree. C. to 400.degree. C. Bonding layer
6 is formed by heating the bonding material to a temperature in the
proximity of the softening temperature T.sub.S, as described
below.
[0049] In some implementations, the transparency of bonding layer 6
is high. Transparency is controlled, among others, by the bandgap
of the bonding material of bonding layer 6. Bonding materials with
larger bangdap often have better transparency.
[0050] Transparent bonding layer 6 is formed, for example, from
Schott glass SF59, a dense flint glass which has refractive index
(n).about.11.95 at .about.600 nanometers (nm) and a glass
transition temperature .about.362.degree. C. Alternatively,
transparent bonding layer 6 is formed from high index optical
glasses including but not limited to Schott glass LaSF 3, with
n.about.1.81 at .about.600 nm and glass transition temperature
.about.630.degree. C.; Schott glass LaSF N18, with n.about.1.91 at
.about.600 nm and glass transition temperature .about.660.degree.
C.; and mixtures thereof. These glasses are available from Schott
Glass Technologies Incorporated, of Duryea, Pa. Bonding layer 6 may
also be formed from a high index chalcogenide glass, such as
(Ge,As,Sb,Ga)(S,Se,Te) chalcogenide glasses, for example or from a
high index chalcohalide glass, such as
(Ge,As,Sb,Ga)(S,Se,Te,F,Cl,Br,I) chalcohalide glasses, for
example.
[0051] In other implementations, bonding layer 6 is formed from
III-V semiconductors including but not limited to GaP (n.about.3.3
at 600 nm), InGaP (n.about.3.7 at 600 nm), GaAs (n.about.3.4 at 500
nm), and GaN(n.about.2.4 at 500 nm); II-VI semiconductors including
but not limited to ZnS (n.about.2.4 at 500 nm), ZnSe (n.about.2.6
at 500 nm), ZnTe (n.about.3.1 at 500 nm), CdS (n.about.2.6 at 500
nm), CdSe (n.about.2.6 at 500 nm), and CdTe (n.about.2.7 at 500
nm); group IV semiconductors and compounds including but not
limited to Si (n.about.3.5 at 500 nm), SiC, and Ge (n.about.4.1 at
500 nm); organic semiconductors, metal and rare earth oxides
including but not limited to tungsten oxide, tellurium oxide, lead
oxide, titanium oxide (n.about.2.9 at 500 nm), nickel oxide
(n.about.2.2 at 500 nm), zirconium oxide (n.about.2.2 at 500 nm),
indium tin oxide, chromium oxide, antimony oxide (n.about.2.1),
bismuth oxide (n.about.1.9-2.5), gallium oxide (n.about.1.94),
germanium oxide (n.about.2.0), molybdenum oxide (n.about.2.4),
cadmium oxide (n.about.2.47), cobalt oxide (n.about.2.4), cerium
oxide (n.about.2.3), indium oxide (n.about.2.0), neodymium oxide
(n.about.2.0); oxyhalides such as bismuth oxychloride
(n.about.2.15); metal fluorides including but not limited to
magnesium fluoride (n.about.1.4 at 500 nm) and calcium fluoride
(n.about.1.4 at 500 nm); metals including but not limited to Zn,
In, Mg, and Sn; yttrium aluminum garnet (YAG), phosphide compounds,
arsenide compounds, antimonide compounds, nitride compounds, high
index organic compounds; and mixtures or alloys thereof.
[0052] Bonding layer 6 includes, in some implementations,
luminescent material that converts light of wavelengths emitted by
active region 12 to other wavelengths. The luminescent material
includes, for example, conventional phosphor particles, organic
semiconductors, II-VI or III--V semiconductors, II-VI or III-V
semiconductor quantum dots or nanocrystals, dyes, polymers, and
materials such as GaN that luminesce from defect centers. If
bonding layer 6 includes conventional phosphor particles, then
bonding layer 6 should be thick enough to accommodate particles
typically having a size of about 5 microns to about 50 microns.
[0053] In one implementation, bonding layer 6 is formed from a
material with a high refractive index n at the light emitting
device's emission wavelengths. In some embodiments the refractive
index is greater than about 1.5, preferably greater than about 1.8.
In some embodiments the refractive index is less than that of the
top layer of light emitter 4, for example, semiconductor layer 8.
Hence, a critical angle exists for total internal reflection of
light incident on the semiconductor layer 8/bonding layer 6
interface from inside light emitter 4. This critical angle is
increased compared to the critical angle for an interface between
light emitter 4 and epoxy or air, however, and more light is
extracted through surface 18 into bonding layer 6 than would be
extracted into an epoxy encapsulant or air. In another
implementation, the refractive index of bonding layer 6 (for
example, ZnS or ZnSe) is greater than or equal to that of
semiconductor layer 8 (for example, GaN), and none of the light
incident on bonding layer 6 from inside light emitter 4 is totally
internally reflected. Neglecting Fresnel reflection losses, which
can be minimized by approximately matching the refractive indices
of bonding layer 6 and the top layer of light emitter 4, in the
latter case also more light is extracted through surface 18 into
bonding layer 6 than would be extracted into an epoxy encapsulant
or air.
[0054] In some embodiments, the bond between light emitter 4 and
optical element 2 is substantially free of traditional
organic-based adhesives such as epoxies, since such adhesives tend
to have a low index of refraction.
[0055] In another implementation, transparent bonding layer 6 is
formed from a low refractive index bonding material, i.e., a
bonding material having a refractive index less than about 1.5 at
the light emitting device's emission wavelengths. Magnesium
fluoride, for example, is one such bonding material. Low index
optical glasses, epoxies, and silicones may also be suitable low
index bonding materials. One of ordinary skill in the art will
recognize that efficient transmission of light from light emitter 4
across transparent bonding layer 6, formed from a low index
material, to optical element 2 can be achieved if bonding layer 6
is sufficiently thin. Accordingly, in this implementation losses
due to total internal reflection at the light emitter 4/bonding
layer 6 interface are reduced by making the thickness of bonding
layer 6 less than about 500 .ANG., preferably less than about 100
.ANG.. Optical element 2 might bond poorly to light emitter 4 if
the roughness of surface 18 or surface 22 or typical height of
irregularities on surface 18 or surface 22 exceed the thickness of
bonding layer 6. In this embodiment, surfaces 18 and 22 are
optionally polished to achieve a surface roughness of magnitude
less than or equal to the thickness of bonding layer 6.
[0056] If light emitter 4 includes material that absorbs light
emitted by active region 12, and if bonding layer 6 is formed from
a low index material but is not thin as described above, then a
large portion of the light emitted by active region 12 will
typically be trapped in light emitter 4 and lost to absorption even
if bonding layer 6 is itself nonabsorbing. In contrast, a bonding
layer 6 formed from a high index material will typically couple a
larger fraction of light emitted by active region 12 out of light
emitter 4 into optical element 2, even if the high index bonding
material is a material such as a chalcogenide glass, for example,
which absorbs a portion of the emitted light.
[0057] After transparent bonding layer 6 is applied to light
emitter 4, the surface 22 of optical element 2 is positioned
against bonding layer 6. The temperature of bonding layer 6 is then
raised to a temperature between about room temperature and about
1000.degree. C., and optical element 2 and light emitter 4 are
pressed together for a period of time of about one second to about
6 hours at a pressure of about 1 pound per square inch (psi) to
about 6000 psi. The inventors believe that this process bonds
optical element 2 to light emitter 4 by a bond effected at the
interface between optical element 2 and bonding layer 6 (formed on
light emitter 4) by, for example, material transfer via shear
stress, evaporation-condensation, liquification, melting, or
softening, followed by solidification, diffusion, or alloying. The
inventors believe that in some implementations optical element 2 is
bonded to light emitter 4 by a bond similarly effected by, for
example, material transfer between bonding layers formed on each of
optical element 2 and light emitter 4, or between bonding layer 6
(formed on optical element 2) and light emitter 4. Thus, a bonded
interface characterized by material transfer may be disposed
between optical element 2 and light emitter 4. In some
implementations, for example, surface 18 at the interface of
n-layer 8 and bonding layer 6 is such a bonded interface. In
another implementation, an interface of bonding layers formed on
each of optical element 2 and light emitter 4 is a bonded
interface. In another implementation, an interface of optical
element 2 and bonding layer 6 is a bonded interface.
[0058] If transparent bonding layer 6 is formed on light emitter 4
from an optical glass, for example, then in one implementation the
temperature of bonding layer 6 is raised to about the strain point
temperature of the optical glass. The strain point temperature is
the temperature at which the optical glass has a viscosity of about
10.sup.14.5 poises. The strain point temperature corresponds to the
first nonlinearity in a plot of expansion versus temperature for
the optical glass, and thus represents the lower limit of the
annealing range. The strain point is near to but less than the
glass transition temperature. The viscosity at the glass transition
temperature is about 10.sup.13 poises. Finally, the softening
temperature is the temperature, where the material deforms
appreciably under its own weight. The viscosity at the softening
temperature is about 10.sup.7.65 poises. Heating the optical glass
to near or above the softening temperature increases its
flexibility and lowers its surface tension, allowing the optical
glass to microscopically conform to surface 22 and to effect a bond
between optical element 2 and bonding layer 6.
[0059] The process of bonding optical element 2 to light emitter 4
described above may be performed with devices disclosed in U.S.
Pat. Nos. 5,502,316 and 5,376,580, incorporated herein by
reference. The disclosed devices bond semiconductor wafers to each
other at elevated temperatures and pressures. These devices may be
modified to accommodate light emitters and optical elements, as
necessary. Alternatively, the bonding process described above may
be performed with a conventional vertical press.
[0060] Transparent optical element 2 can be formed, for example,
from SiC (n.about.2.7 at 500 nm), aluminum oxide (sapphire,
n.about.1.8 at 500 nm), diamond (n.about.2.4 at 500 nm), cubic
zirconia (ZrO.sub.2), aluminum oxynitride (AlON) by Sienna
Technologies Inc., polycrystalline aluminum oxide (transparent
alumina), spinel, Schott glass LaFN21, Schott glass LaSFN35, LaF2,
LaF3, and LaF10 available from Optimax Systems Inc. of Ontario,
N.Y., or any of the materials listed above for use as bonding
materials in transparent bonding layer 6, excluding thick layers of
the metals. A mismatch between the thermal expansion coefficients
of optical element 2 and light emitter 4 to which optical element 2
is bonded can cause optical element 2 to detach from light emitter
4 upon heating or cooling. Also, approximately matching thermal
expansion coefficients reduces the stress induced in light emitter
4 by bonding layer 6 and optical element 2. Hence, in one
implementation optical element 2 is formed from a material having a
thermal expansion coefficient approximately matching the thermal
expansion coefficient of light emitter 4 to which optical element 2
is bonded. Spinel is desirable as an optical element material
because the coefficient of thermal expansion of spinel is the same
for every crystallographic orientation.
[0061] In one embodiment, transparent optical element 2 has a shape
and a size such that light entering optical element 2 from light
emitter 4 will intersect surface 24 of optical element 2 at angles
of incidence near normal incidence. Total internal reflection at
the interface of surface 24 and the ambient medium, typically air,
is thereby reduced. In addition, since the range of angles of
incidence is narrow, Fresnel reflection losses at surface 24 can be
reduced by applying a conventional antireflection coating 25 to
surface 24. The shape of optical element 2 is, for example, a
portion of a sphere such as a hemisphere, a Weierstrass sphere
(truncated sphere), or a portion of a sphere less than a
hemisphere. Alternatively, the shape of optical element 2 is a
portion of an ellipsoid such as a truncated ellipsoid. The angles
of incidence at surface 24 for light entering optical element 2
from light emitter 4 more closely approach normal incidence as the
size of optical element 2 is increased. Hence, the smallest ratio
of a length of the base of transparent optical element 2 to a
length of the surface 18 of light emitter 4 is preferably greater
than about 1, more preferably greater than about 2. For example, in
various embodiments a light emitter 1 mm long may be bonded to a
hemispherical optical element 2.5 mm in diameter and 1.25 mm tall
at the center; a light emitter 2 mm long may be bonded to a
hemispherical optical element 5 mm in diameter and 2.5 mm tall at
the center; and a light emitter 3 mm long may be bonded to a
hemispherical optical element 7.5 mm in diameter and 3.75 mm tall
at the center. Typically, the optical element is much thicker than
the light emitter. For example, a III-nitride light emitter of the
sizes described above may be 100 microns thick, and a III-phosphide
light emitter of the sizes described above may be 250 microns
thick.
[0062] One of ordinary skill in the art will recognize that the
maximum size for which an optical element 2 of a particular
material continues to be transparent as defined above is determined
by the absorption constant of the optical element material at the
emission wavelengths of the light emitting device. In one
implementation, optical element 2 is a Fresnel lens, formed on a
flat surface or a curved surface. Fresnel lenses are typically
thinner than, for example, spherical optical elements of comparable
focal lengths, and hence are less absorptive. Fresnel lenses or
grooved surfaces can also be formed on curved surfaces such as a
dome or cylindrical surface of the light emitter or optical
element. Light emitters utilizing Fresnel lenses have been
described in U.S. patent application Ser. No. 09/823,841, entitled
"Forming an optical element on the surface of a light emitting
device for improved light extraction," by Douglas W. Pocius,
Michael D. Camras, and Gloria E. Hofler, hereby incorporated in its
entirety by reference.
[0063] Optical element 2 includes, in one implementation,
luminescent material that converts light of wavelengths emitted by
active region 12 to other wavelengths. In another implementation, a
coating on surface 22, for example, includes luminescent material.
The luminescent material includes, for example, conventional
phosphor particles, organic semiconductors, II-VI or II-V
semiconductors, II-VI or III-V semiconductor quantum dots or
nanocrystals, dyes, polymers, and materials such as GaN that
luminesce from defect centers. Alternatively, a region of optical
element 2 near surface 22 is doped, for example, with a luminescent
material.
[0064] The magnitudes of the refractive indices of optical element
2 (n.sub.optical element), bonding layer 6 (n.sub.bond), and the
top layer of light emitter 4 (n.sub.LED) can be ordered in six
permutations. If n.sub.LED.ltoreq.n.sub.bond.ltoreq.n.sub.optical
element or n.sub.LED.ltoreq.n.sub.optical
element.ltoreq.n.sub.bond, then losses due to total internal
reflection are eliminated, but Fresnel losses may occur. In
particular, if n.sub.LED=n.sub.bond=n.sub.optical element, then
light enters optical element 2 from light emitter 4 without losses
due to Fresnel or total internal reflection. Alternatively, if
n.sub.bond.ltoreq.n.sub.LED.ltoreq.n.sub.optical element but either
n.sub.bond>n.sub.epoxy or bonding layer 6 is thin as described
above, then, neglecting Fresnel reflection losses, more light is
extracted into optical element 2 than would be extracted into an
epoxy encapsulant or air. Similarly, if
n.sub.bond.ltoreq.n.sub.optical element.ltoreq.n.sub.LED but either
n.sub.bond>n.sub.epoxy or bonding layer 6 is thin as described
above and n.sub.optical element>n.sub.epoxy, then, neglecting
Fresnel reflection losses, more light is extracted into optical
element 2 than would be extracted into an epoxy encapsulant or air.
If n.sub.optical element.ltoreq.n.sub.bond.ltoreq.n.sub.LED or
n.sub.optical element.ltoreq.n.sub.LED.ltoreq.n.sub.bond but
n.sub.optical element>n.sub.epoxy, then, neglecting Fresnel
reflection losses, more light is extracted into optical element 2
than would be extracted into an epoxy encapsulant or air. Thus,
transparent optical element 2 preferably has a refractive index at
the emission wavelengths of light emitter 4 greater than about 1.5,
more preferably greater than about 1.8. A similar analysis applies
if the ambient medium is air (n.sub.air.about.1) rather than an
epoxy encapsulant, with n.sub.air substituted for epoxy.
[0065] If bonding layer 6 includes phosphor particles and the
refractive index of the bonding material, preferably a high index
chalcogenide glass, approximately matches the refractive index of
the phosphor particles, then scattering by the phosphor particles
of light emitted by the active region or by the phosphor particles
will be negligible. Preferably, the refractive indices of the
phosphor particles, the bonding material, the top layer (for
example n-layer 8) of light emitter 4, and the optical element are
all about equal. This is the case if the top layer of light emitter
4 is InGaN, the phosphor particles are SrS:Eu and/or SrGaS:Eu, and
the optical element is ZnS.
[0066] FIG. 2 illustrates an alternative embodiment, where
transparent optical element 2 is bonded directly to a top surface
18 of light emitter 4 without the use of a separate bonding layer.
The inventors believe that a bond is effected between optical
element 2 and light emitter 4 by, for example, material transfer
via shear stress, evaporation-condensation,
dissolution-condensation, liquification (or melting or softening)
followed by solidification, diffusion, or alloying. Metallization
layer 20, if present, is patterned to allow surface 22 of optical
element 2 to directly contact surface 18. Surface 22 is optionally
also patterned by etching, for example. In one implementation of
this embodiment, transparent optical element 2 is formed from a
material, such as those listed above, which could be used to form a
separate bonding layer. In another implementation, the material
from which the top layer of light emitter 4 (for example, n-layer 8
in FIG. 2) is formed is suitable as a bonding material. Thus either
optical element 2 or a top layer of light emitter 4 functions
additionally as a bonding layer, and no separate bonding layer is
necessary. In one implementation, the interface of optical element
2 and light emitter 4 at surface 18, for example, is a bonded
interface characterized by mass transfer between optical element 2
and light emitter 4.
[0067] For optical element 2 directly bonded to light emitter 4, if
n.sub.LED.ltoreq.n.sub.optical element or if n.sub.optical
element<n.sub.LED but n.sub.optical element>n.sub.epoxy,
then, neglecting Fresnel reflection losses, more light is extracted
into optical element 2 than would be extracted into an epoxy
encapsulant. A similar analysis applies if the ambient medium is
air (n.sub.air.about.1) rather than an epoxy encapsulant, with
n.sub.air substituted for n.sub.epoxy.
[0068] Transparent optical element 2 is directly bonded to light
emitter 4 at temperatures and pressures as described above for the
bonding process utilizing bonding layer 6. In one implementation,
surface 18 of light emitter 4 or surface 22 of optical element 2 is
doped with a material exhibiting a high diffusivity such as, for
example, Zn or Si. Such doping can be accomplished during materials
growth by MOCVD, VPE, LPE or MBE, for example, or after materials
growth by, for example, implantation. In another implementation, a
thin layer of a high diffusivity material is disposed between
optical element 2 and light emitter 4 by deposition on at least one
of surfaces 18 and 22. Deposition can be accomplished, for example,
by conventional means such as evaporation or sputtering. The
inventors believe that during the bonding process the high
diffusivity material diffuses across the interface between optical
element 2 and light emitter 4 and enhances material transfer
between optical element 2 and light emitter 4. The amount of high
diffusivity material used should be sufficiently low to maintain
the transparency of, for example, optical element 2 and the top
layer of light emitter 4.
[0069] Application of the bonding method is not limited to a
pre-made optical element. Rather, transparent optical element 2 may
be a block of transparent optical element material that is bonded
to light emitter 4 in the manner described above, and then formed
into optical element 2. Optical element 2 may be formed using
etching, perhaps in conjunction with photolithography or other
lithographic techniques, electron beam lithography, ion beam
lithography, X-ray lithography, or holographic lithography. Wet or
dry chemical etching techniques such as plasma etching, reactive
ion etching (RIE), and chemically-assisted ion beam etching (CAIBE)
may be used as well. Also, optical element 2 may be milled into a
surface of the transparent optical element material using ion beam
milling or focused ion beam milling (FIB), ablated into the surface
with a scanning electron or a laser beam, or mechanically machined
into the surface by sawing, milling, or scribing. In addition,
optical element 2 may be stamped into the block of transparent
optical element material using the method disclosed in U.S. patent
application Ser. No. 09/823,841.
[0070] It is advantageous to bond light emitter 4 to optical
element 2 rather than to conventionally encase light emitter 4 in
an encapsulant. For example, the light extraction efficiency
through surface 18 of light emitter 4 bonded to optical element 2
with or without bonding layer 6, as described above, is improved
compared to a conventional epoxy encapsulated light emitters. In
addition, light emitter 4 need not be subject to the damaging
stress experienced by epoxy encapsulated (encased) light emitters.
Moreover, in the absence of epoxy encapsulants, which degrade at
relatively low temperatures, light emitter 4 can be run at higher
temperatures. Consequently, the light output of light emitter 4 can
be increased by running the light emitter at higher current
densities.
[0071] If desired, however, light emitter 4 bonded to optical
element 2 could be additionally encapsulated in, for example, epoxy
or silicone. Such encapsulation of light emitter 4 bonded to
optical element 2 would not effect the light extraction efficiency
through surface 18 of light emitter 4 into optical element 2. Total
internal reflection at the interface of surface 24 and the
encapsulant would be minimized, as described above, by the shape
and size of optical element 2.
[0072] In some implementations, light emitter 4 includes, for
example, metallization for electrical contacts that degrades at
elevated temperatures. In other implementations, light emitter 4 is
bonded to a submount, not shown, with solders or silver bearing
die-attach epoxies that degrade at high temperatures. (Note that
die-attach epoxy is to be distinguished from an epoxy encapsulant.)
Consequently, in one implementation the process of bonding optical
element 2 to light emitter 4, with or without a bonding layer 6,
occurs at temperatures less than about 500.degree. C. in order to
avoid, for example, degrading the metallization or the die-attach
epoxy. In another implementation, optical element 2 is bonded to an
incomplete light emitting device, for example, a light emitting
device missing some or all metallization. In the latter
implementation, fabrication of the light emitting device is
completed after the optical element bonding process.
[0073] FIG. 3 illustrates an embodiment, where sides 26 and 28 of
light emitter 4 are beveled such that they intersect bonding layer
6 at angles .alpha. and .beta., respectively, less than 90.degree.
and intersect active region 12 at angles .gamma. and .delta.,
respectively, greater than 90.degree.. Though two beveled sides are
shown in FIG. 3, in other embodiments light emitter 4 has more or
less than two beveled sides and is, for example, substantially
conical or pyramidal in shape.
[0074] Beveled sides 26 and 28 reflect light emitted from active
region 12 toward bonding layer 6. Light that might otherwise have
been trapped in light emitter 4 or lost out the sides of the light
emitter is thereby advantageously extracted through bonding layer 6
and optical element 2. In one embodiment, light emitter 4 is
surrounded by a low refractive index medium such as air, and a
portion of the light incident on beveled sides 26 and 28 from
active region 12 is totally internally reflected toward bonding
layer 6. In another embodiment, beveled sides 26 and 28 are coated
with a reflective coating, in one implementation a metal layer and
in another implementation a dielectric layer, that reflects light
toward bonding layer 6.
[0075] In one embodiment, contact 16 is highly reflective.
Accordingly, in this embodiment light incident on contact 16 is
reflected toward bonding layer 6 and optical element 2 either
directly or after additional reflection from side 26 or side 28.
The light extraction efficiency of light emitter 4 is consequently
increased.
[0076] FIGS. 4 and 5 illustrate embodiments, where light emitter 4
includes conducting transparent superstrate 30 electrically coupled
to metallization layer 20 and electrically coupled to n-layer 8,
and conducting, optionally transparent, substrate 32 electrically
coupled to p-layer 10 and to contact 16. Superstrate 30 and
(optionally) substrate 32 are formed, for example, from
semiconductors having a band gap energy greater than the energy of
photons emitted by light emitter 4. Superstrate 30 and (optionally)
substrate 32 can also be formed from sapphire, SiC (silicon
carbide), aluminum oxynitride (AlON) of Sienna Technologies Inc.,
GaN, AlN, spinel, and polycrystalline aluminum oxide (transparent
alumina).
[0077] Superstrate 30 is formed from a material having an index of
refraction at the emission wavelengths of active region 12
preferably greater than about 1.5, more preferably greater than
about 1.8. In other implementations, sides 26 and 28 of light
emitter 4 are beveled and highly reflective and contact 16 is
highly reflective, as described above. In the embodiment
illustrated in FIG. 4, transparent optical element 2 is bonded to
superstrate 30 with bonding layer 6, and n-layer 8 is electrically
coupled to metallization layer 20 by n-contact 14.
[0078] FIG. 5 illustrates an embodiment where transparent optical
element 2 is directly bonded to superstrate 30 and n-contact 14 is
not separately provided.
[0079] Contact 14 and contact 16 are disposed on the same side of
light emitter 4 in the "flip chip" embodiments illustrated in FIGS.
6 and 7. Since optical element 2 is bonded to the opposite side of
light emitter 4 from contacts 14 and 16, no metallization layer is
required on optical element 2 in these embodiments. The light
extraction efficiency into optical element 2 is improved by the
absence of the metallization layer. In other implementations, sides
26 and 28 of light emitter 4 are beveled and highly reflective and
contact 16 is highly reflective, as described above. Transparent
superstrate 34 is formed from a material such as, for example,
sapphire, SiC, GaN, AlN, or GaP, having an index of refraction at
the emission wavelengths of active region 12 preferably greater
than about 1.5, more preferably greater than about 1.8. Transparent
superstrate 34 can also be formed from aluminum oxynitride (AlON)
of Sienna Technologies Inc., spinel, an oxide of tellurium, an
oxide of lead, an oxide of tungsten, and polycrystalline aluminum
oxide (transparent alumina).
[0080] FIG. 6 illustrates an embodiment where optical element 2 is
bonded with bonding layer 6 to transparent superstrate 34. FIG. 7
illustrates an embodiment where optical element 2 is directly
bonded to transparent superstrate 34.
[0081] In one implementation of the embodiments illustrated in
FIGS. 6 and 7, optical element 2 is formed from ZnS, superstrate 34
is formed from SiC or GaN, and n-layer 8 is formed from a
III-Nitride semiconductor such as GaN. In another implementation,
optical element 2 is formed from GaP, superstrate 34 is formed from
GaP, and n-layer 8 is formed from a III-Phosphide semiconductor
such as an AlInGaP alloy. If present, transparent bonding layer 6
is formed, for example, from ZnS.
[0082] FIGS. 8 and 9 illustrate embodiments where the orientation
of n-layer 8, p-layer 10, and active region 12 is substantially
perpendicular to optical element 2. As in the embodiments
illustrated in FIGS. 6 and 7, no metallization layer is required on
optical element 2. In the embodiment illustrated in FIG. 8, optical
element 2 is bonded with bonding layer 6 to light emitter 4. In the
embodiment illustrated in FIG. 9, optical element 2 is directly
bonded to light emitter 4. In one implementation, light emitter 4
is sawn ("diced") from a wafer with cuts made in a direction
substantially perpendicular to layers 8 and 10 and to active region
12. In this implementation, the surface of light emitter 4 to be
bonded to optical element 2 is optionally polished to reduce its
roughness. In other implementations, sides 26 and 28 of light
emitter 4 are beveled, contact 14 and contact 16 are highly
reflective, and reflective layer 36 is located to reflect light
into optical element 2.
[0083] FIGS. 10 and 11 illustrate embodiments where light emitter 4
is located in a recess 38 in surface 22 of optical element 2 to
which light emitter 4 is bonded. In the embodiment illustrated in
FIG. 10, optical element 2 is directly bonded to light emitter 4.
In the embodiment of FIG. 11, optical element 2 is bonded to light
emitter 4 with bonding layer 6.
[0084] FIGS. 12A-13 illustrate top-emitter embodiments, adapted to
direct the generated light through a top area of light emitter 4.
FIGS. 12A-C illustrate cylindrical embodiments. In these
embodiments a cross-section of optical element 2 with a plane
parallel to the semiconductor layers of light emitter 4 is
substantially a circle. Cylindrical embodiments have an axis 39,
which is substantially perpendicular to the plane of the circle. In
some embodiments axis 39 may not be perpendicular to the plane of
the circle.
[0085] FIG. 12A illustrates a basic embodiment, where a dome shaped
optical element 2 is bonded to light emitter 4 at surface 22.
[0086] FIG. 12B illustrates a top-emitter embodiment, adapted to
direct the emitted light through a top area of the embodiment. In
some embodiments a mirror layer 40 is formed on a substantial
portion of surface 24 of optical element 2, leaving a smaller
aperture 41 close to axis 39 of the embodiment. This embodiment
narrows the light emitting surface, as preferred in some
applications. The light of light emitter 4, which reaches aperture
41, leaves the embodiment through aperture 41. The light of light
emitter 4, which reaches surface 24 where it is coated with mirror
layer 40 may be reflected back into optical element 2. A portion of
the reflected light reenters light emitter 4, where it can excite
electrons and holes, which later may recombine to emit photons.
This process is sometimes referred to as photon recycling. This
recycled light may leave the embodiment through aperture 41. In
some embodiments, aperture 41 can be anywhere on the surface of
optical element 2. Optical element 2 is attached by a bond to light
emitter 4 at surface 22. In some embodiments there is a
metallization layer 20 on surface 22 to facilitate electrical
connection to the top of light emitter 4, in other embodiments
other contact solutions are used. The device of FIG. 12B is
described in more detail in U.S. application Ser. No. 10/283,737,
titled "Enhanced Brightness Light Emitting Device Spot Emitter,"
filed Oct. 29, 2002 and incorporated herein by reference.
[0087] Mirror layer 40 can be formed from a metallic layer, a
dielectric, a reflective coating, or in general from any
substantially reflective layer, which have a refractive index
sufficiently different from the refractive index of optical element
2.
[0088] In both embodiments of FIGS. 12A-B optical element 2 may
have a Fresnel pattern etched into it, as illustrated in FIG. 19.
The Fresnel pattern includes a set of grooves, often arranged in
concentric pattern. The Fresnel pattern can be etched on the whole
surface 24, or only on the top region, or only on the side region
of surface 24.
[0089] FIG. 12C illustrates another top-emitter embodiment. Here
optical element 2 is an optical concentrator with optional mirror
layer 40 formed on the sides of the concentrator. In some
embodiments the shape of the optical concentrator is similar to a
paraboloid. Although FIG. 12C depicts the optical concentrator as
having a paraboloid shape, surface 24 may have any shape designed
to concentrate light, such as a cone-shape or a beveled shape. The
optical concentrator described here is also known in the field of
non-imaging optics.
[0090] FIGS. 13A-B illustrate elongated embodiments. FIG. 13A
illustrates an embodiment, where light emitter 4 is elongated and
optical element 2 is correspondingly elongated. Optical element 2
is an optical concentrator. A cross section of the optical
concentrator can be substantially a parabola. The cross sectional
parabola has an axis 39. In some embodiments axis 39 can be
substantially normal to surface 22. In embodiments with elongated
parabolic optical concentrators the light emitted by light emitter
4 is directed substantially parallel to axis 39, after reflecting
from mirror layer 40.
[0091] Although FIGS. 13A-B depict the optical concentrator as
having a parabolic cross section, surface 24 may have any shape
designed to concentrate light, such as elongated cone-shapes or
beveled shapes.
[0092] FIG. 13B illustrates an elongated embodiment. In this
embodiment multiple light emitters 4 are bonded to optical element
2. Light emitters 4 can be bonded to a bottom surface of shared
optical element 2. Light emitters 4 can have similar or different
characteristics. Some embodiments have light emitters 4 emitting
light with different color.
[0093] FIG. 14A illustrates an embodiment, where light emitter 4 is
bonded to optical element 2, this assembly being disposed inside
reflector 42. Reflector 42 is a structure adapted to direct the
light towards a top surface of the optical element.
[0094] In some embodiments light emitting device 1 can be further
coupled to a heat sink 46. There can be additional structures
between light emitting device 1 and heat sink 46. Also, heat sink
46 can be disposed in many ways relative to reflector 42, different
from the arrangement of FIG. 14A. Heat sink 46 can be formed from
materials with good thermal conductance, for example, from metals
or certain ceramics. Some embodiments utilize different methods for
controlling the heat production of light emitter 4.
[0095] FIG. 14B illustrates a side-emitting embodiment. In this
side-emitting embodiment optical element 2 is adapted to direct the
emitted light at large angles relative to axis 39. This light,
emitted at large angles, is then reflected by reflector 42. In this
side-emitting embodiment light emitter 4 is once again bonded to
optical element 2. In some embodiments light emitter 4 is also
coupled to heat sink 46.
[0096] FIGS. 15A-C illustrate various side-emitting embodiments
adapted to direct the light at large angles relative to axis
39.
[0097] FIG. 15A illustrates an embodiment, where optical element 2
has a cross section which is thinner in a central region than in a
peripheral region. A substantial portion of surface 24 can be
covered by mirror layer 40. The light emitted by light emitter 4
propagates towards mirror layer 40, where it can be reflected
sideways. This reflected light can leave optical element 2 through
sides 48. Embodiments with differently shaped surfaces 24 direct
the reflected light differently. Embodiments where surface 24 has
two half-parabola-like cross sections, direct the light
substantially horizontally. In some embodiments optical element 2
is cylindrical, i.e. its cross section with a horizontal plane is
essentially a circle.
[0098] FIG. 15B illustrates an embodiment with an asymmetric
optical element 2. Mirror layer 40 on surface 24 reflects the light
towards the right, so that the light can leave optical element 2
through right side 48. Surface 24 can be curved. Its cross section
can be, for example, a parabola.
[0099] FIG. 15C illustrates an embodiment with an asymmetric
optical element 2. Mirror layer 40 on surface 24 may reflect the
light towards the right, so that the reflected light can leave
optical element 2 through right side 48. Surface 24 can be
essentially flat.
[0100] FIG. 16 illustrates an asymmetric elongated embodiment.
Optical element 2 is shaped as a column or slab. In some
embodiments mirror layer 40 is formed on the top of the column. In
some embodiments a mirror layer 40 is formed on one of the sides 24
of the column. In some other embodiments no mirror layer formed on
the sides. In some embodiments, a scattering layer is formed on the
top of the column. The light rays enter optical element 2 through
surface 22. Ascending light rays 55 and descending light rays 65
are repeatedly reflected by mirror layer 40 on the side and on the
top. A portion of light rays 55 and 65, reaching right surface 48,
exit as exiting light rays 75, an other portion is reflected. In
the embodiments, where there is no mirror layer on any of the sides
of optical element 2, light rays 55 and 65 may exit on any of the
sides of the column like optical element 2. In embodiments, where
there is no mirror layer of the top of the column, light rays 55
and 65 may exit also through the top of the column. Therefore,
these embodiments can be used as a source of a "light-slab,"
"light-column," or "light-sheet." Such a light source can be used
in a wide variety of applications.
[0101] FIGS. 17A-C illustrate embodiments, where light emitting
device 1 is positioned in the proximity of a light guide. Some
light guides have a low index of refraction and can be made of, for
example, Teflon, acrylic, or PMMA. Light guide 50 may be in contact
with light emitting device 1 or can be spaced apart from light
emitting device 1 with, for example, air, silicone, epoxy, one or
more filters, or a coating such as an antireflective coating
between light emitting device 1 and light guide 50. Light guides
are capable of directing light to a far away target or application
areas. Light guides can be incorporated into displays, such as
indicators and liquid crystal displays. In all the subsequent light
guide designs of FIGS. 17 and 18 any of the light emitting devices
of FIGS. 1-16 can be utilized.
[0102] FIG. 17A illustrates an embodiment, where the designs of
FIGS. 14A-B are positioned in the proximity of light guide 50,
separated by air, silicone, or epoxy, for example. Reflector 42
directs the light towards light guide 50.
[0103] FIG. 17B illustrates an embodiment, where optical element 2
is an optical concentrator, as in the embodiment of FIG. 12C. In
this embodiment the optical concentrator directs the light towards
light guide 50.
[0104] FIG. 17C illustrates an embodiment, where optical element 2
is an elongated optical concentrator, as in the embodiments of
FIGS. 13A-B. In this embodiment the elongated optical concentrator
directs the light towards elongated light guide 50.
[0105] FIG. 18 illustrates an embodiment, where light guide 50 has
tapered side surfaces 48, broadening away from light emitting
device 1. In some embodiments mirror layer 40 is formed at the end
of light guide 50. In some embodiments a mirror layer 40 is formed
on one of the sides 24 of light guide 50. In some embodiments no
mirror layer is formed on the sides of light guide 50. In some
embodiments, a scattering surface is formed at the end of light
guide 50. Ascending light rays 55 are reflected or scattered into
descending light rays 65 at the top mirror layer 40 or scattering
surface.
[0106] The tapering angle can be small. Ascending light rays 55 can
experience a total internal reflection (TIR) at sides 24 because of
the broadening taper and stay within light guide 50. This extra
path length can mix different colors of light. Returning light rays
65 do not experience TIR because they experience a narrowing taper.
Therefore, at every reflection returning light rays 65 partially
exit light guide 50 as exiting light rays 75. In these embodiments
the repeatedly reflected light may be distributed sufficiently
evenly that the intensity of exiting light rays 75 has a
substantially uniform intensity along light guide 50, providing a
sheet of light. In some embodiments only one of the sides may be
tapered.
[0107] In some embodiments light guide 50 is coupled to the design
of FIG. 13B, with two or more light emitting devices 4. In various
embodiments light emitting devices with different colors can be
employed. In some embodiments the two or more emitted colors are
chosen so that they can combine into a color close to white. Some
embodiments combine amber and blue, some cyan and red, and some
combine red, blue and green. After repeated reflections from
surfaces 48 of light guide 50 the rays of different colors become
sufficiently mixed to appear as close to white light. Embodiments
with such light guides can be applied as sources of a sheet of
white light. Such a white sheet light source can be utilized, for
example, for back-lighting in liquid crystal displays.
[0108] While the present invention is illustrated with particular
embodiments, the invention is intended to include all variations
and modifications falling within the scope of the appended
claims.
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