U.S. patent application number 10/804810 was filed with the patent office on 2005-09-22 for photonic crystal light emitting device.
Invention is credited to Epler, John E., Krames, Michael R., Wierer, Jonathan J. JR..
Application Number | 20050205883 10/804810 |
Document ID | / |
Family ID | 34838945 |
Filed Date | 2005-09-22 |
United States Patent
Application |
20050205883 |
Kind Code |
A1 |
Wierer, Jonathan J. JR. ; et
al. |
September 22, 2005 |
Photonic crystal light emitting device
Abstract
A photonic crystal structure is formed in an n-type region of a
III-nitride semiconductor structure including an active region
sandwiched between an n-type region and a p-type region. A
reflector is formed on a surface of the p-type region opposite the
active region. In some embodiments, the growth substrate on which
the n-type region, active region, and p-type region are grown is
removed, in order to facilitate forming the photonic crystal in an
an-type region of the device, and to facilitate forming the
reflector on a surface of the p-type region underlying the photonic
crystal. The photonic crystal and reflector form a resonant cavity,
which may allow control of light emitted by the active region.
Inventors: |
Wierer, Jonathan J. JR.;
(Fremont, CA) ; Krames, Michael R.; (Mountain
View, CA) ; Epler, John E.; (Milpitas, CA) |
Correspondence
Address: |
PATENT LAW GROUP LLP
2635 NORTH FIRST STREET
SUITE 223
SAN JOSE
CA
95134
US
|
Family ID: |
34838945 |
Appl. No.: |
10/804810 |
Filed: |
March 19, 2004 |
Current U.S.
Class: |
257/98 ;
257/E33.005; 257/E33.068 |
Current CPC
Class: |
H01L 33/382 20130101;
H01L 2924/0002 20130101; H01L 33/22 20130101; H01L 33/62 20130101;
H01L 33/0093 20200501; H01L 2933/0083 20130101; H01L 33/10
20130101; H01L 2924/0002 20130101; H01L 2924/00 20130101 |
Class at
Publication: |
257/098 |
International
Class: |
H01L 033/00 |
Claims
What is being claimed is:
1. A light emitting device comprising: a III-nitride semiconductor
structure including an active region disposed between an n-type and
a p-type region; and a photonic crystal structure formed in at
least a portion of the n-type region; and a reflector disposed on
at least a portion of a surface of the p-type region opposite the
active region.
2. The device of claim 1 wherein the photonic crystal structure
comprises a periodic variation in a thickness of the n-type
region.
3. The device of claim 2 wherein a ratio of the period of the
periodic structure and the wavelength of light emitted by the
active region in air is about 0.1 to about 5.
4. The device of claim 1 wherein the photonic crystal structure
comprises a planar lattice of holes.
5. The device of claim 4 wherein the holes have a depth between
about 0.05.lambda. and about 5.lambda., where .lambda. is a
wavelength in the III-nitride semiconductor structure of light
emitted by the active region.
6. The device of claim 4 wherein a lattice type, lattice constant,
hole diameter, and hole depth are selected to create a
predetermined radiation pattern.
7. The device of claim 6 wherein greater than 50% of radiation
exiting the device is emitted in an exit cone defined by an angle
of 45 degrees to an axis normal to a surface of the device.
8. The device of claim 4 wherein the planar lattice is selected
from the group consisting of a triangular lattice, a square
lattice, a hexagonal lattice, and a honeycomb lattice.
9. The device of claim 4 wherein the planar lattice includes more
than one lattice type.
10. The device of claim 4 wherein the lattice has a lattice
constant a between about 0.1.lambda. and about 10.lambda., where
.lambda. is a wavelength in the III-nitride semiconductor structure
of light emitted by the active region.
11. The device of claim 4 wherein the lattice has a lattice
constant a between about 0.1.lambda. and about 4.lambda., where
.lambda. is a wavelength in the III-nitride semiconductor structure
of light emitted by the active region.
12. The device of claim 4 wherein the lattice has a lattice
constant a and the holes have a diameter between about 0.1 a and
about 0.5 a.
13. The device of claim 4 wherein the holes are filled with a
dielectric.
14. The device of claim 13 wherein the dielectric has a dielectric
constant between about 1 and about 16.
15. The device of claim 1 wherein a distance between the reflector
and the photonic crystal structure is between about .lambda. and
about 5.lambda., where .lambda. is a wavelength in the III-nitride
semiconductor structure of light emitted by the active region.
16. The device of claim 1 wherein a distance between a center of
the active region and the photonic crystal structure is less than
about 4.lambda., where .lambda. is a wavelength in the III-nitride
semiconductor structure of light emitted by the active region.
17. The device of claim 1 wherein a total thickness of III-nitride
semiconductor layers in the device is less than about 1 .mu.m.
18. The device of claim 1 wherein a total thickness of III-nitride
semiconductor layers in the device is less than about 0.5
.mu.m.
19. The device of claim 1 wherein a thickness of the n-type region,
the active region, and the p-type region is less than about 1
.mu.m.
20. The device of claim 1 wherein a thickness of the n-type region,
the active region, and the p-type region is less than about 0.5
.mu.m.
21. The device of claim 1 wherein at least a portion of the
reflector underlies the photonic crystal structure.
22. The device of claim 1 further comprising a host substrate
bonded to the reflector.
23. The device of claim 22 further comprising a metal bonding layer
disposed between the host substrate and the reflector.
24. The device of claim 23 wherein the metal bonding layer
comprises gold.
25. The device of claim 22 wherein the host substrate comprises one
of Si, GaAs, Cu, Mo, W, and alloys thereof.
26. The device of claim 1 wherein the reflector comprises
silver.
27. The device of claim 1 wherein the photonic crystal structure is
formed in a first portion of the n-type region, the device further
comprising a contact formed on a second portion of the n-type
region, the second portion being substantially free of the photonic
crystal structure.
28. The device of claim 27 wherein the contact surrounds the
photonic crystal structure.
29. The device of claim 1 further comprising: a trench extending
through the p-type region and the active region to the n-type
region; and a contact disposed on the n-type region within the
trench.
30. The device of claim 29 wherein the contact and the photonic
crystal structure are formed on opposite surfaces of the n-type
region.
31. The device of claim 1 wherein the n-type region comprises a
first n-type region, the device further comprising: a second n-type
region disposed between the photonic crystal structure and the
active region.
32. The device of claim 1 wherein the photonic crystal structure
extends into the active region.
33. The device of claim 32 wherein the photonic crystal structure
extends into the p-type region.
34. A method of forming a semiconductor light emitting device, the
method comprising: growing a III-nitride semiconductor structure on
a growth substrate, the III-nitride semiconductor structure
including an active region disposed between an n-type and a p-type
region; bonding the III-nitride semiconductor structure to a host
substrate; removing the growth substrate; and forming a photonic
crystal structure in the n-type region of the III-nitride
semiconductor structure.
35. The method of claim 34 wherein forming a photonic crystal
structure comprises etching the photonic crystal structure in a
surface of the n-type region exposed by removal of the growth
substrate.
36. The method of claim 34 wherein the n-type region is a first
n-type region and where forming a photonic crystal structure
comprises: etching the photonic crystal structure in the first
n-type region after growth of the first n-type region; growing a
second n-type region over the photonic crystal structure; and
growing the active region and the p-type region over the second
n-type region.
37. The method of claim 34 wherein: growing a III-nitride
semiconductor structure on a growth substrate comprises growing the
p-type region overlying the growth substrate, growing the active
region overlying the p-type region, and growing the n-type region
overlying the active region; the host substrate is a first host
substrate; and the first host substrate is bonded to the n-type
region; the method further comprising: after removing the growth
substrate, bonding a second host substrate to the p-type region;
and removing the first host substrate.
Description
BACKGROUND
[0001] 1. Field of Invention
[0002] The present invention relates to semiconductor light
emitting devices including photonic crystal structures.
[0003] 2. Description of Related Art
[0004] Light emitting diodes ("LEDs") are technologically and
economically advantageous solid state light sources. LEDs are
capable of reliably providing light with high brightness, hence in
the past decades they have come to play a critical role in numerous
applications, including flat-panel displays, traffic lights, and
optical communications. An LED includes a forward biased p-n
junction. When driven by a current, electrons and holes are
injected into the junction region, where they recombine and release
their energy by emitting photons. The quality of an LED can be
characterized, for example, by its extraction efficiency, which
measures the intensity of the emitted light for a given number of
photons generated within the LED chip. The extraction efficiency is
limited, for example, by the emitted photons suffering multiple
total internal reflections at the walls of the high refractive
index semiconductor medium. As a result, the emitted photons do not
escape into free space, leading to poor extraction efficiencies,
typically less than 30%.
[0005] In the past thirty years, various approaches have been
proposed to enhance the extraction efficiency of LEDs. The
extraction efficiency can be increased, for example, by enlarging
the spatial angle in which the emitted photons can escape by
developing suitable geometries, including cubic, cylindrical,
pyramidal, and dome like shapes. However, none of these geometries
can entirely eliminate losses from total internal reflection.
[0006] A further source of loss is the reflection caused by the
refractive index mismatch between the LED and the surrounding
media. While such losses could be reduced with an anti-reflection
coating, complete cancellation of reflection can be achieved only
at a specific photon energy and one angle of incidence.
[0007] U.S. Pat. No. 5,955,749, entitled "Light Emitting Device
Utilizing a Periodic Dielectric Structure," granted to J.
Joannopoulos et al., describes an approach to the problem of
enhancing the extraction efficiency. According to U.S. Pat. No.
5,955,749, a photonic crystal is created by forming a lattice of
holes completely through the semiconductor layers of the light
emitting diode. The lattice of holes creates a medium with a
periodically modulated dielectric constant, affecting the way light
propagates through the medium. The photons of the light emitting
diode can be characterized by their spectrum or dispersion
relation, describing the relation between the energy and the
wavelength of the photons. The relationship may be plotted,
yielding a photonic band diagram consisting of energy bands, or
photonic bands, separated by band gaps. Though the photonic band
diagram is analogous to the spectrum of electrons in crystalline
lattices as expressed in an electronic band diagram, the photonic
band diagram is unrelated to the electronic band diagram. When a
photonic crystal is formed in an LED it affects how light
propagates in the structure. Therefore if the proper lattice
spacing is chosen, light that would otherwise have been trapped in
the structure by total internal reflection can now escape,
increasing the extraction of the LED. Also, alternative lattices
can reduce the photon mode volume in the LED structure increasing
the radiative rate or internal efficiency of the LED active
layer.
[0008] In an effort to explore the usefulness of photonic crystals
for light generation, U.S. Pat. No. 5,955,749 gives a partial
description of a theoretical structure of a photonic crystal
device.
[0009] U.S. Pat. No. 5,955,749 describes an n-doped layer, an
active layer, a p-doped layer, and a lattice of holes formed in
these layers. However, the device of U.S. Pat. No. 5,955,749 is not
operational and therefore is not a LED. First, electrodes are not
described, even though electrodes are needed for the successful
operation of a photonic crystal LED ("PXLED"). Though the
fabrication of electrodes in regular LEDs is known in the art, for
PXLEDs neither the fabrication of electrodes, nor their influence
on the operation of the PXLED is obvious. For example, suitably
aligning the mask of the electrode layer with the lattice of holes
may require new fabrication techniques. Also, electrodes are
typically thought to reduce the extraction efficiency as they
reflect a portion of the emitted photons back into the LED, and
absorb another portion of the emitted light.
[0010] Second, U.S. Pat. No. 5,955,749 proposes fabricating
photonic crystal light emitting devices from GaAs. GaAs is indeed a
convenient and hence popular material for fabricating regular LEDs.
However, it has a high surface recombination velocity of about
10.sup.6 cm/sec as described, for example, by S. Tiwari in
"Compound Semiconductor Devices Physics," Academic Press (1992).
The surface recombination velocity expresses the rate of the
recombination of electrons and holes on the surface of the diode.
Electrons and holes are present in the junction region of the LED,
originating from the n-doped layer and the p-doped layer,
respectively. When electrons and holes recombine across the
electronic band gap, the recombination energy is emitted in the
form of photons and generates light. However, when electrons and
holes recombine through intermediate electronic states in the
electronic band gap, then the recombination energy is emitted in
the form of heat instead of photons, reducing the light emission
efficiency of the LED. In an ideal crystal there are no states in
the electronic band gap. Also, in today's high purity semiconductor
crystals there are very few states in the electronic band gap in
the bulk material. However, on the surface of semiconductors
typically there are a large number of surface states and defect
states, many of them in the electronic band gap. Therefore, a large
fraction of electrons and holes that are close to the surface will
recombine through these surface and defect states. This surface
recombination generates heat instead of light, considerably
reducing the efficiency of the LED.
[0011] This problem does not result in a serious loss of efficiency
for regular LED structures. However, PXLEDs include a large number
of holes, thus PXLEDs have a much larger surface area than regular
LEDs. Therefore, the surface recombination may be capable of
reducing the efficiency of the PXLED below the efficiency of the
same LED without the photonic crystal structure, making the
formation of photonic crystal structure pointless. Since GaAs has a
high surface recombination velocity, it is not a promising
candidate for fabricating photonic crystal LEDs. The seriousness of
the problem is reflected by the fact that so far, to Applicants'
knowledge, no electrically operated LED with the photonic crystal
through the active region has been reported in the literature that
uses GaAs and claims an enhanced extraction, or internal,
efficiency. In particular, U.S. Pat. No. 5,955,749 does not
describe the successful operation of a photonic crystal LED. Also,
U.S. Pat. No. 5,955,749 does not describe the influence of the
photonic crystal on the emission process, which can affect the
internal efficiency of the LED.
[0012] While photonic crystals are promising for light extraction
for the reasons described above, there are problems with the
design. There are several publications describing experiments on a
lattice of holes having been formed in a slab of a semiconductor.
An enhancement of the extraction rate at photon energies in the
photonic band gap has been reported by R. K. Lee et al. in
"Modified Spontaneous Emission From a Two-dimensional Photonic
Bandgap Crystal Slab," in the Journal of the Optical Society of
America B, vol. 17, page 1438 (2000). Lee et al. not only shows the
extraction benefits of a photonic crystal in a light emitting
design, but also shows that the photonic lattice can influence the
spontaneous emission. However, Lee et al. do not show how to form
and operate a light emitting device with this design. A photonic
crystal LED can be formed from Lee et al.'s light emitting design
by including electrodes. The addition of the electrodes, however,
will substantially affect the extraction and the spontaneous
emission of the LED. Since this effect is unknown, it cannot be
disregarded in the design of a LED. Since the Lee et al. design
does not include such electrodes, the overall characteristics of an
LED formed from that design are unclear. This questions the
usefulness of the design of Lee et al.
SUMMARY
[0013] In accordance with embodiments of the device, a photonic
crystal structure is formed in an n-type region of a III-nitride
semiconductor structure including an active region sandwiched
between an n-type region and a p-type region. A reflector is formed
on a surface of the p-type region opposite the active region. In
some embodiments, the growth substrate on which the n-type region,
active region, and p-type region are grown is removed, in order to
facilitate forming the photonic crystal in an n-type region of the
device, and to facilitate forming the reflector on a surface of the
p-type region underlying the photonic crystal. The photonic crystal
and reflector form a resonant cavity, which may allow control of
light emitted by the active region.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a cross sectional view of a photonic crystal light
emitting diode.
[0015] FIG. 2 is a cross sectional view of an embodiment of a
photonic crystal light emitting device lacking a growth
substrate.
[0016] FIG. 3 is a plan view of the device of FIG. 2.
[0017] FIG. 4 illustrates a method of fabricating the device of
FIG. 2.
[0018] FIG. 5 illustrates an epitaxial structure prior to bonding
to a host substrate.
[0019] FIG. 6 illustrates a method of bonding an epitaxial
structure to a host substrate.
[0020] FIG. 7 illustrates a method of removing a sapphire substrate
from a III-nitride epitaxial structure.
[0021] FIG. 8 illustrates photoelectrochemical etching to thin the
epitaxial layers after growth substrate removal.
[0022] FIGS. 9-12 illustrate a method of forming a photonic crystal
structure.
[0023] FIG. 13 illustrates an alternative embodiment of the present
invention.
[0024] FIGS. 14A-14D are cut away plan views of the device of FIG.
13.
[0025] FIG. 15 is a plan view of a photonic crystal structure
comprising a planar lattice of holes.
[0026] FIGS. 16A and 16B illustrate a method of forming a photonic
crystal structure.
DETAILED DESCRIPTION
[0027] FIG. 1 illustrates a III-nitride photonic crystal LED
(PXLED) 100, described in more detail in application Ser. No.
10/059,588, "LED Efficiency Using Photonic Crystal Structure,"
filed Jan. 28, 2002 and incorporated herein by reference.
[0028] In PXLED 100 of FIG. 1, an n-type region 108 is formed over
host substrate 102 which may be, for example, sapphire, SiC, or
GaN; an active region 112 is formed over n-type region 108; and a
p-type region 116 is formed over active region 112. Each of regions
108, 112, and 116 may be a single layer or multiple layers of the
same or different composition, thickness, or dopant concentration.
A portion of p-type region 116 and active region 112 are etched
away to expose a portion of n-type region 108, then a p-contact 120
is formed on p-type region 116 and an n-contact 104 is formed on
the exposed portion of n-type region 108.
[0029] Active region 112 includes a junction region where electrons
from n-type region 108 combine with holes of p-type region 116 and
ideally emit energy in the form of photons. Active layer 112 may
include a quantum well structure to optimize the generation of
photons. Many different quantum well structures have been
described, for example, by G. B. Stringfellow and M. George Craford
in "High Brightness Light Emitting Diodes," published by the
Associated Press in 1997. The photonic crystal of PXLED 100 of FIG.
1 is created by forming a periodic structure of holes 122-i in the
LED.
[0030] In the device illustrated in FIG. 1, a usual III-nitride
structure is fabricated with the n-type region formed first on the
substrate, followed by the active region and the p-type region. The
photonic crystal device illustrated in FIG. 1 and the devices
described in U.S. Pat. No. 5,955,749 may have several
disadvantages. First, the photonic crystal structure in the device
of FIG. 1 may be formed by, for example, dry etching into the
p-type region to form a periodic structure. Dry etching could be
reactive ion, inductively coupled plasma, focused ion beam, sputter
etching, electron cyclotron resonance, or chemically assisted ion
beam etching. Dry etching of p-type material is problematic because
etching can damage the crystal, causing vacancies which create
n-type donors. In p-type region 116, the presence of n-type donors
lowers the concentration of holes and, in cases of severe damage to
the crystal, can change the conductivity type of region 116 to
n-type. The inventors have discovered that the damage caused by dry
etching is not limited to a localized area around the etched
region, and may propagate vertically and laterally through the
non-etched areas of the crystal, possibly eliminating the p-n
junction and rendering the device electrically non-operational. The
devices described in U.S. Pat. No. 5,955,749 also etch through
p-type material, and therefore may suffer from the same widespread
damage observed by the inventors. Second, in both the device of
FIG. 1 and the devices in U.S. Pat. No. 5,955,749, portions of the
active region are removed to form the photonic crystal structure,
reducing the amount of active region material and potentially
reducing the amount of light generated in the device. Also, etching
through the quantum wells creates surface recombination,
potentially lowering the efficiency of the device.
[0031] In accordance with embodiments of the invention, a photonic
crystal is formed in an n-type layer of a III-nitride device
attached to a host substrate and from which the growth substrate
has been removed. Such devices may emit light between about 280 and
about 650 nm and usually emit light between about 420 and about 550
nm. FIG. 2 is a cross sectional view of an embodiment of the
invention. FIG. 3 is a plan view of the device of FIG. 2. As
illustrated in FIG. 2, the photonic crystal 122 is formed in n-type
region 108, rather than p-type region 116. N-contact 10 is formed
on a region of n-type region 108 that is not textured with the
photonic crystal, though in other embodiments n-contact 10 may be
formed on the photonic crystal area of n-type region 108. Since the
photonic crystal is formed in an n-type region, the n-type material
is able to laterally inject current from contact 10 to photonic
crystal 122. Light is extracted from the device through photonic
crystal 122, thus the arrangement of n-contact 10 is selected to
maximize the area of the photonic crystal. For example, as
illustrated in FIG. 3, n-contact 10 may surround photonic crystal
region 122-i. N-contact 10 is not limited to a ring contact but
could also be a grid or other structure that facilitates proper
current spreading. To avoid light being absorbed by the n-contact
10, implantation or a dielectric can be used on the epitaxial
material under n-contact 10, preventing current flow and light
generation in that area. A reflective p-contact 12 is formed on
p-type region 116. In contrast to the device illustrated in FIG. 1,
the device of FIG. 2 has the p- and n-contacts formed on opposite
sides of the device. P-contact 12 connects the epitaxial layers 20
to a host substrate 16 either directly or via optional bonding
layers 14. An optional contact 18 may be formed on the surface of
host substrate 16 opposite the device layers 20.
[0032] FIGS. 13 and 14A-14D illustrate an alternative embodiment of
the present invention. FIGS. 14A, 14B, 14C, and 14D are cut away
plan views along axes 90, 91, 92, and 93, respectively, illustrated
in the cross sectional view of FIG. 13. In the device of FIG. 13,
both p and n-contacts 12 and 10 are on the host substrate side of
the device, eliminating absorption of light by a top side
n-contact, as in the device of FIGS. 2 and 3. One or more vias are
etched down to n-type region 108 through p-type region 116 and
active region 112 to make n-contact 10. Host substrate structure 49
is fabricated in a layered structure to electrically isolate the p-
and n-contacts. An example of the layer structure is illustrated by
FIGS. 14A-14D, which show plan view slices of the host substrate
along axes 90, 91, 92, and 93 of FIG. 13. N-metal 301 and p-metal
303 are routed such that at the bottom of the host substrate the
positive and negative contacts are separate and can be easily
soldered to another structure. N-metal 301 and p-metal 303 may be
electrically isolated by dielectric 305. Depending on the area of
the LED one or more n-contact vias may be necessary to provide
sufficient current spreading. Bonding the patterned LED to the
patterned host can be accomplished using a flip-chip bonder.
[0033] Bonding the epitaxial layers of the device to a host
substrate, then removing the growth substrate allows the photonic
crystal structure of the device to be formed in an n-type region.
Etching the photonic crystal structure in an n-type region rather
than a p-type region avoids the type-conversion problem associated
with p-type III-nitrides, described above. Also, vacancies
introduced in the n-type region from etching do not affect the
conductivity of the material. In addition, since the photonic
structure in n-type region 108 is separated from p-type region 116
and active region 112, damage to these regions caused by etching
the photonic structure is avoided. The exposed top n-type layer
allows for formation of the photonic crystal proximal to the active
region. In alternative embodiments where surface recombination is
low the photonic crystal may penetrate the active region and p-type
region.
[0034] Alternatively, rather than bonding the epitaxial layers to a
host, then removing the growth substrate, a device with an exposed
top n-type region may be formed by growing the p-type region first
on a growth substrate, followed by an active region and n-type
region. Ignoring the growth difficulties, this would present n-type
layer on the surface just as in FIG. 2, such that etching damage is
not a concern. Contacts to the p-GaN layers would have to be formed
on the surface by first exposing the p-type layers by etching a
mesa. Therefore current would have to spread laterally along
resistive p-type layers, creating a device with high operating
voltage, a result that is undesirable in many applications.
Altenatively the substrate could be removed from this structure so
that the operating voltage is not high. This is done by first
bonding a host to the top n-type layers and then removing the
growth substrate. Next etching is performed to remove the initial
growth layers and expose the p-type region. Then a second bonding
step with a second host is performed on the now-exposed p-type
layers. The first host is removed re-exposing the n-type region for
photonic crystal formation. The resulting structure is the same as
FIG. 2.
[0035] The photonic crystal structure can include a periodic
variation of the thickness of n-type region 108, with alternating
maxima and minima. An example is a grating (one-dimensional
lattice) or planar lattice of holes 122 (two-dimensional lattice).
The lattice is characterized by the diameter of the holes, d, the
lattice constant a, which measures the distance between the centers
of nearest neighbor holes, the depth of the holes w, and the
dielectric constant of the dielectric, disposed in the holes,
.epsilon..sub.h. Parameters a, d, w, and .epsilon..sub.h influence
the density of states of the bands, and in particular, the density
of states at the band edges of the photonic crystal's spectrum.
Parameters a, d, w, and .epsilon..sub.h thus influence the
radiation pattern emitted by the device, and can be selected to
enhance the extraction efficiency from the device. Alternatively,
when the proper photonic crystal parameters are chosen, the
radiation pattern of the emitted light can be narrowed, increasing
the radiance of the LED. This is useful in applications where light
at only specific angles is useful. In one embodiment, the photonic
crystal parameters are chosen such that greater than 50% of
radiation exiting the device is emitted in an exit cone defined by
an angle of 45 degrees to an axis normal to a surface of the
device.
[0036] Holes 122-i can be arranged to form triangular, square,
hexagonal, honeycomb, or other well-known two-dimensional lattice
types. In some embodiments, different lattice types are formed in
different regions of the device. Holes 122-i can have circular,
square, hexagonal, or other cross sections. In some embodiments,
the lattice spacing a is between about 0.1 .lambda. and about 10
.lambda., preferably between about 0.1 .lambda. and about 4
.lambda., where .lambda. is the wavelength in the device of light
emitted by the active region. In some embodiments, holes 122 may
have a diameter d between about 0.1 a and about 0.5 a, where a is
the lattice constant. Holes 122-i can be filled with air or with an
optional dielectric 11 (FIG. 2) of dielectric constant
.epsilon..sub.h, often between about 1 and about 16. Possible
dielectrics include silicon oxides.
[0037] Photonic crystal 122 and the reflection of the photonic
crystal from reflective p-contact 12 form a GaN resonant cavity.
The resonant cavity offers superior control of the light. As the
GaN cavity is thinned the optical mode volume is reduced. Fewer
waveguided modes can be trapped in the cavity increasing the
chances for the light to exit the device. This can be explained in
the following discussion. The photonic crystal can affect the
waveguided modes by scattering them out of the crystal. As the
number of waveguided modes is reduced the more efficient the light
extraction of the LED. For example if the epitaxial layers are thin
enough to support only one waveguided mode (m), then initially 50%
of the light would exit the GaN (L.sub.out) and 50% would be
waveguided in the epitaxial layers (L.sub.in). For this argument we
assume that we form a photonic crystal that is able to extract an
additional 40% of this waveguided light (S.sub.eff). The extraction
efficiency (C.sub.ext) can be written as:
C.sub.ext=L.sub.out+m*(L.sub.in.times.S.sub.eff)
[0038] Therefore the extraction efficiency of this structure is
50%+1*(50%*40%)=70%. Compare this to an epitaxial structure that
supports 4 waveguided modes with a photonic crystal again with
S.sub.eff=40%. If the light goes equally into all modes then each
mode including the one exit mode has 20% of the light. This
structure would only have an extraction efficiency of
20%+4*(20%*40%)=52%. In this argument the photonic crystal is not
100% efficient scattering out the light. In some embodiments the
photonic crystal is etched deep enough and has the proper lattice
dimensions so that a photonic band gap is created in the plane of
the LED inhibiting waveguide modes, (S.sub.eff=100%). The thinner
the epitaxial layers the easier it is to create a photonic
band-gap. The thickness of the cavity (i.e. the thickness of
epitaxial layers 20) is selected such that the epitaxial layers are
as thin as possible to reduce the number of waveguided modes, but
thick enough to efficiently spread current. In many embodiments,
the thickness of epitaxial layers 20 is less than about 1 .mu.m,
and preferably less than about 0.5 .mu.m.
[0039] In some embodiments, the thickness of epitaxial layers 20 is
between about .lambda. and about 5 .lambda., between about 0.18
.mu.m and about 0.94 .mu.m for a device that emits 450 nm light.
Holes 122 have a depth between about 0.05 .lambda. and the entire
thickness of n-type region 108. Generally, holes 122 are formed
entirely within n-type region 08 and do not penetrate into the
active region. N-type region 108 usually has a thickness of about
0.1 microns or more. The depth of holes 122 is selected to place
the bottoms of holes 122 as close to the active region as possible
without penetrating the active region. In alternative embodiments
the photonic crystal penetrates the active layers and p-type
layers.
[0040] The radiation pattern emitted from the device can be tuned
by changing the lattice type, distance between the active region
and the photonic crystal, lattice parameter a, diameter d, depth w,
and epitaxial thickness (20). The lattice parameter a and diameter
d are illustrated in FIG. 15. In some embodiments, the radiation
pattern may be adjusted to emit light preferentially in a chosen
direction.
[0041] In some embodiments the periodic structure is a variation of
the thickness of one or more selected semiconductor layers. The
periodic structure can include variations of the thickness along
one direction within the plane of the semiconductor layers, but
extending along a second direction without variation, in essence
forming a set of parallel grooves. Two-dimensional periodic
variations of the thickness include various lattices of
indentations.
[0042] The device illustrated in FIGS. 2 and 3 may be fabricated by
the method illustrated in FIG. 4. In stage 31, epitaxial layers 20
of FIG. 2 are grown on a conventional growth substrate. The
epitaxial layers are then attached to a host substrate in stage 33,
such that the growth substrate can be removed in stage 35. The
epitaxial layers may be thinned in optional stage 37, then a
photonic crystal structure is formed on the exposed surface of the
epitaxial layers in stage 39.
[0043] FIG. 5 illustrates stage 31 of FIG. 4 in more detail.
Epitaxial layers 20 of the device of FIG. 2 are grown on a
substrate 40 such as sapphire, SiC, or GaN. Optional preparation
layers 41, which may include, for example, buffer layers or
nucleation layers, may be grown first on substrate 40 to provide a
suitable growth substrate. One or more optional etch stop layers 42
may then be grown. Etch stop layers 42 may facilitate release of
the growth substrate or facilitate thinning of the epitaxial
layers, as described below. The epitaxial layers 20 are grown over
etch stop layers 42 and include n-type region 108, active region
112, and p-type region 116. Usually, the n-type region is grown
first, followed by the active region and the p-type region. A
p-contact 12, often reflective, is formed on the surface of p-type
region 116. P-contact 12 may be a single layer or may include
multiple layers such as an ohmic contact layer, a reflective layer,
and a guard metal layer. The reflective layer is usually silver or
aluminum. The guard metal may include, for example, nickel,
titanium, or tungsten. The guard metal may be chosen to prevent the
reflective metal layer from migrating, particularly in the case of
a silver reflective layer, and to provide an adhesion layer for a
bonding layer 14A, used to bond the epitaxial structure to a host
substrate.
[0044] FIG. 6 illustrates stage 33 of FIG. 4, attaching the
epitaxial layers to a host substrate, in more detail. Bonding
layers 14A and 14B, typically metal, serve as compliant materials
for thermo-compression or eutectic bonding between the epitaxial
structure and the host substrate. Examples of suitable bonding
layer metals include gold and silver. Host substrate 16 provides
mechanical support to the epitaxial layers after the growth
substrate is removed, and provides electrical contact to p-contact
12. Host substrate 16 is selected to be electrically conductive
(i.e. less than about 0.1 .OMEGA.cm), to be thermally conductive,
to have a coefficient of thermal expansion (CTE) matched to that of
the epitaxial layers, and to be flat (i.e. with an root mean square
roughness less than about 10 nm) enough to form a strong wafer
bond. Suitable materials include, for example, metals such as Cu,
Mo, Cu/Mo, and Cu/W; semiconductors with metal contacts (layers 46
and 18 of FIG. 6), such as Si with ohmic contacts and GaAs with
ohmic contacts including, for example, one or more of Pd, Ge, Ti,
Au, Ni, Ag; and ceramics such as compressed diamond. The table
below lists the properties of some suitable host substrates, as
well as the properties of GaN and Al.sub.2O.sub.3 for
comparison:
1 CTE Thermal conductivity Material (10.sup.-6/K) (W/m .multidot.
K) Electrical resistance (.OMEGA. cm) GaN 2.4 130 0.01
Al.sub.2O.sub.3 6.8 40 Very high Si 2.7 150 0.01 plus contact
resistance GaAs 6.0 59 0.01 plus contact resistance Mo 4.8 140 5
.times. 10.sup.-6
[0045] Host substrate structure 49 and epitaxial structure 48 are
pressed together at elevated temperature and pressure to form a
durable metal bond between bonding layers 14A and 14B. In some
embodiments, bonding is done on a wafer scale, before a wafer with
an epitaxial structure is diced into individual devices. The
temperature and pressure ranges for bonding are limited on the
lower end by the strength of the resulting bond, and on the higher
end by the stability of the host substrate structure and the
epitaxial structure. For example, high temperatures and/or high
pressures can cause decomposition of the epitaxial layers in
structure 48, delamination of p-contact 12, failure of diffusion
barriers, for example in p-contact 12, or outgassing of the
component materials in the epitaxial layers. A suitable temperature
range is, for example, about 200.degree. C. to about 500.degree. C.
A suitable pressure range is, for example, about 100 psi to about
300 psi.
[0046] FIG. 7 illustrates a method of removing a sapphire growth
substrate, stage 35 in FIG. 4. Portions of the interface between
sapphire substrate 40 and the III-nitride layers 45 are exposed,
through the sapphire substrate, to a high fluence pulsed
ultraviolet laser 70 in a step and repeat pattern. The photon
energy of the laser is above the band gap of the III-nitride layer
adjacent to the sapphire (GaN in some embodiments), thus the pulse
energy is effectively converted to thermal energy within the first
100 nm of epitaxial material adjacent to the sapphire. At
sufficiently high fluence (i.e. greater than about 1.5 J/cm.sup.2)
and a photon energy above the band gap of GaN and below the
absorption edge of sapphire (i.e. between about 3.44 and about 6
eV), the temperature within the first 100 nm rises on a nanosecond
scale to a temperature greater than 1000.degree. C., high enough
for the GaN to dissociate into gallium and nitrogen gasses,
releasing the epitaxial layers 45 from substrate 40. The resulting
structure includes epitaxial layers 45 bonded to host substrate
structure 49.
[0047] Exposure to the laser pulse results in large temperature
gradients and mechanical shock waves traveling outward from the
exposed region, resulting in thermal and mechanical stress within
the epitaxial material sufficient to cause cracking of the
epitaxial material and failure of wafer bond 14, which limits the
yield of the substrate removal process. The damage caused by
thermal and mechanical stresses may be reduced by patterning the
epitaxial structure down to the sapphire substrate or down to a
suitable depth of the epitaxial structure, to form trenches between
individual devices on the wafer. The trenches are formed by
conventional masking and dry etching techniques, before the wafer
is bonded to the host substrate structure. The laser exposure
region is then matched to the pattern of trenches on the wafer. The
trench isolates the impact of the laser pulse to the semiconductor
region being exposed.
[0048] Growth substrates other than sapphire may be removed with
ordinary chemical etchants, and thus may not require the laser
exposure substrate removal procedure described above. For example,
a suitable growth substrate may include a thin layer of SiC grown
or processed on to a thick layer of Si or SiO.sub.x. The Si base
layer and/or oxide layer may be easily removed by conventional
silicon processing techniques. The remaining SiC layer may be thin
enough to be removed entirely by known etching techniques.
N-contact 10 may then be formed on the exposed surface of the
epitaxial layers. Alternatively, N-contact 10 may be formed in the
holes in the SiC layer.
[0049] After the growth substrate is removed, the remaining
epitaxial layers may optionally be thinned to form a cavity between
the photonic crystal and p-contact 12 of optimal depth and of
uniform thickness, usually with thickness variations less than
about 20 nm. The epitaxial layers may be thinned by, for example,
chemical mechanical polishing, conventional dry etching, or
photoelectrochemical etching (PEC). PEC is illustrated in FIG.
8.
[0050] As illustrated in FIG. 8, the host substrate and epitaxial
layers (structure 53) are immersed in a basic solution 50. An
example of a suitable basic solution is 0.1 M KOH, though many
other suitable basic solutions may be used and typically depend on
the composition of the material to be etched. The epitaxial surface
of structure 53, often an n-type GaN layer, is exposed to light
with energy greater than the band gap of the surface layer. In the
example illustrated in FIG. 8, ultraviolet light with a wavelength
of about 365 nm and an intensity between about 10 and about 100
mW/cm.sup.2 is used. Exposure to the light generates electron-hole
pairs in the surface semiconductor layer. The holes migrate to the
surface of the epitaxial layers under the influence of the electric
field in the n-type semiconductor. The holes then react with the
GaN at the surface and basic solution 50 to break the GaN bonds,
according to the equation
2GaN+60H.sup.-+6e.sup.+=2Ga(OH).sub.3+N.sub.2. An external electric
potential may be applied across electrodes 51 and 52 to accelerate
and control the etching process.
[0051] In some embodiments, an etch stop layer is incorporated into
the epitaxial layers, as described above in FIG. 4. The etch stop
layer may have a band gap greater than the layer to be etched. For
example, the etched layer may be GaN, and the etch stop layer may
be AlGaN. The light sources used to expose structure 53 is selected
to have an energy greater than the band gap of the layer to be
etched, but less than the band gap of the etch stop layer.
Accordingly, exposure to the light does not generate electron-hole
pairs in the etch stop layer, effectively halting the etch once the
etch stop layer is reached. In some embodiments, InGaN may be used
as the etch stop layer. Indium oxide, formed as the InGaN
decomposes, is insoluble in the etchant solution and coats the
surface of the etched layer, terminating the etch.
[0052] Though the embodiment illustrated in FIG. 2 shows an n-type
region with the same thickness in the photonic crystal region and
in the region underlying contact 10, in some embodiments a three
dimensional structure may be formed on n-type region 108 during
thinning. For example, n-type region 108 may be patterned such that
the portion under contact 10 is thicker than the portion forming
the photonic crystal, in order to minimize the thickness of the
cavity, while providing enough n-type material under contact 10 for
adequate current spreading, optimal contact resistance, and
mechanical strength.
[0053] After thinning, the photonic crystal structure is formed on
the exposed surface of the epitaxial layers. FIGS. 9-12 illustrate
a method of fabricating the photonic crystal structure of the
device of FIG. 2. One or more resist, metal, or dielectric layers
202 are formed over the top surface of the epitaxial layers, as
illustrated in FIG. 9. Resist layers 202 are patterned to form a
lattice of openings in FIG. 10, using a high resolution lithography
technique such as electron beam lithography, nano-imprint
lithography, deep X-ray lithography, interferometric lithography,
hot embossing, or microcontact printing. In FIG. 11, epitaxial
layers 200 are etched using known etching techniques. Damage caused
by dry etching can be mitigated by a subsequent short wet chemical
etch, anneal, a combination thereof, or other surface passivation
techniques. The remaining resist layer 202 is then removed in FIG.
12. Other techniques for forming a photonic crystal, such as
epitaxial lateral overgrowth, are described in more detail in
application Ser. No. 10/059,588, "LED Efficiency Using Photonic
Crystal Structure." As illustrated in FIG. 2, a portion of the
surface of the exposed n-type layer may not be textured with a
photonic crystal, such that n-contact 10 may be formed on a planar
layer. After the photonic crystal is formed, n-contact 10 is
deposited by conventional techniques.
[0054] FIGS. 16A and 16B illustrate an alternative method for
forming the photonic crystal. Rather than etching the photonic
crystal after removing the growth substrate, a buried photonic
crystal is formed during epitaxial growth. In FIG. 16A, epitaxial
growth is stopped before the active layers are grown. A photonic
crystal is then formed in n-type region 108, for example, by
etching as illustrated above in FIGS. 9-12. The material is then
placed back into the growth reactor and first a smoothing n-type
layer 310, often GaN, is grown. The depth of the photonic crystal
holes is greater than the diameter of the holes. The growth
parameters of smoothing layer 310 are chosen so lateral growth is
faster than vertical growth, ensuring that the photonic crystal
holes are not filled. Once the smoothing layer 310 is closed over
the photonic crystal holes in n-type region 108, active region 112
and p-type region 116 are grown. A contact may then be formed on
p-type region 116, and the growth substrate removed as described
above.
[0055] Having described the invention in detail, those skilled in
the art will appreciate that, given the present disclosure,
modifications may be made to the invention without departing from
the spirit of the inventive concept described herein. Therefore, it
is not intended that the scope of the invention be limited to the
specific embodiments illustrated and described.
* * * * *