U.S. patent application number 10/448503 was filed with the patent office on 2003-11-06 for increasing the brightness of iii-nitride light emitting devices.
Invention is credited to Camras, Michael D., Goetz, Werner K., Khare, Reena.
Application Number | 20030205717 10/448503 |
Document ID | / |
Family ID | 25171761 |
Filed Date | 2003-11-06 |
United States Patent
Application |
20030205717 |
Kind Code |
A1 |
Khare, Reena ; et
al. |
November 6, 2003 |
Increasing the brightness of III-Nitride light emitting devices
Abstract
LEDs employing a III-Nitride light emitting active region
deposited on a base layer above a substrate show improved optical
properties with the base layer grown on an intentionally misaligned
substrate with a thickness greater than 3.5 .mu.m. Improved
brightness, improved quantum efficiency, and a reduction in the
current at which maximum quantum efficiency occurs are among the
improved optical properties resulting from use of a misaligned
substrate and a thick base layer. Illustrative examples are given
of misalignment angles in the range from 0.05.degree. to
0.50.degree., and base layers in the range from 6.5 to 9.5 .mu.m
although larger values of both misalignment angle and base layer
thickness can be used. In some cases, the use of thicker base
layers provides sufficient structural support to allow the
substrate to be removed from the device entirely.
Inventors: |
Khare, Reena; (Sunnyvale,
CA) ; Goetz, Werner K.; (Palo Alto, CA) ;
Camras, Michael D.; (Sunnyvale, CA) |
Correspondence
Address: |
PATENT LAW GROUP LLP
2635 NORTH FIRST STREET
SUITE 223
SAN JOSE
CA
95134
US
|
Family ID: |
25171761 |
Appl. No.: |
10/448503 |
Filed: |
May 29, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10448503 |
May 29, 2003 |
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09797770 |
Mar 1, 2001 |
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6576932 |
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Current U.S.
Class: |
257/103 ; 257/14;
257/94; 257/E21.119; 257/E33.003; 438/22 |
Current CPC
Class: |
H01L 33/007 20130101;
H01L 33/16 20130101; H01L 21/0254 20130101; H01L 21/02458 20130101;
H01L 21/0237 20130101; H01L 21/02433 20130101 |
Class at
Publication: |
257/103 ; 257/94;
438/22; 257/14 |
International
Class: |
H01L 033/00; H01L
031/0328; H01L 021/00 |
Claims
What is being claimed is:
1. A method comprising: providing a sapphire substrate having an
upper face wherein said upper face is misaligned from a main
crystal plane of said substrate at least 0.05.degree.; depositing a
base layer above said upper face of said substrate wherein said
base layer has a thickness exceeding about 3.5 micrometers; doping
at least a portion of said base layer with an n-type dopant; and
forming a III-Nitride light emitting region above said base
layer.
2. The method of claim 1 further comprising removing said substrate
following said depositing said base layer thereon.
3. The method of claim 1 wherein said upper face of said sapphire
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 10.degree..
4. The method of claim 1 wherein said upper face of said sapphire
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 5.degree..
5. The method of claim 1 wherein said upper face of said sapphire
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 1.degree..
6. The method of claim 1 wherein said thickness is between about
3.5 micrometers to about 200 micrometers.
7. The method of claim 1 wherein said thickness is between about
3.5 micrometers to about 20 micrometers.
8. The method of claim 1 wherein said thickness is between about
3.5 micrometers to about 10 micrometers.
9. The method of claim 1 wherein said thickness is between about
3.5 micrometers to about 7 micrometers.
10. The method of claim 1 wherein said main crystal plane is the
c-plane.
11. The method of claim 1 wherein said main crystal plane is the
r-plane.
12. The method of claim 1 wherein said main crystal plane is the
a-plane.
13. The method of claim 1 wherein said main crystal plane is the
m-plane.
14. The method of claim 1 wherein doping at least a portion of said
base layer comprises increasing a doping level of the base layer in
a direction towards said light-emitting region.
15. The method of claim 1 wherein: depositing a base layer
comprises depositing a first sublayer above said upper face and a
second sublayer above said first sublayer; and doping at least a
portion of the base layer comprises doping said second sublayer
more heavily than said first sublayer.
16. The method of claim 15 wherein said first sublayer has a dopant
concentration less than about 5.times.10.sup.18 cm.sup.-3 and said
second sublayer has a dopant concentration of at least about
10.sup.18 cm.sup.-3.
17. The method of claim 15 wherein depositing a base layer further
comprises depositing a third sublayer over the second sublayer.
18. A method comprising: providing a substrate having an upper face
wherein said upper face is misaligned from a main crystal plane of
said substrate at least 0.05.degree.; depositing a base layer above
said upper face of said substrate wherein said base layer has a
thickness exceeding about 5.5 micrometers; and forming a
III-Nitride light emitting region above said base layer.
19. The method of claim 18 further comprising removing said
substrate following said depositing said base layer thereon.
20. The method of claim 18 wherein said thickness is between about
6.5 micrometers to about 200 micrometers.
21. The method of claim 18 wherein said thickness is between about
6.5 micrometers to about 20 micrometers.
22. The method of claim 18 wherein said thickness is between about
6.5 micrometers to about 10 micrometers.
23. The method of claim 18 wherein providing a substrate comprises
providing a substrate selected from the group consisting of
sapphire, silicon carbide, gallium nitride, gallium arsenide, and
gallium phosphide.
24. The method of claim 18 further comprising doping at least a
portion of the base layer.
25. The method of claim 24 wherein doping at least a portion of
said base layer comprises increasing a doping level of the base
layer in a direction towards the light-emitting region.
26. The method of claim 24 wherein: depositing a base layer
comprises depositing a first sublayer above said upper face and a
second sublayer above said first sublayer; and doping at least a
portion of the base layer comprises doping said second sublayer
more heavily than said first sublayer.
27. The method of claim 26 wherein said first sublayer has a dopant
concentration less than about 5.times.10.sup.18 cm.sup.-3 and said
second sublayer has a dopant concentration of at least about
10.sup.18 cm.sup.-3.
28. The method of claim 26 wherein depositing a base layer further
comprises depositing a third sublayer over the second sublayer.
29. The method of claim 18 wherein said upper face of said
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 10.degree..
30. The method of claim 18 wherein said upper face of said
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 5.degree..
31. The method of claim 18 wherein said upper face of said
substrate is misaligned from a main crystal plane of said substrate
at an angle between about 0.05.degree. and about 1.degree..
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is a division of application Ser. No.
09/797,770, filed Mar. 1, 2001, now U.S. Pat. No. 6,576,932, which
is incorporated herein by reference.
BACKGROUND
[0002] 1. Field of Invention
[0003] The present invention relates to increasing the brightness
of III-Nitride light emitting diodes.
[0004] 2. Description of Related Art
[0005] Light emitting diodes ("LEDs") are a highly durable solid
state source of light capable of achieving high brightness and
having numerous applications including displays, illuminators,
indicators, printers, and optical disk readers among others. Direct
bandgap semiconductors are the materials of choice for fabrication
of LEDs, which generate light from electricity. One important class
of light emitting systems are based upon compound alloys of Group
III atoms (particularly In, Ga, Al) and nitrogen N, typically
abbreviated as "III-Nitrides." One family of III-Nitride compounds
has the general composition (In.sub.x Ga.sub.1-x).sub.yA.sub.1-- yN
where 0.ltoreq.(x, y).ltoreq.1. III-Nitrides are capable of
emitting light that spans a large portion of the visible and
near-ultraviolet electromagnetic spectrum including ultraviolet,
blue, green, yellow and red wavelengths. Improving the brightness
and other optical properties of LEDs is an important technological
goal.
[0006] A portion of a typical prior art LED structure is depicted
in FIG. 1. Other components of LEDs as known in the art
(electrodes, window materials, etc.) are omitted for clarity.
[0007] An LED typically has one or more layers epitaxially
deposited on a surface of a substrate prior to the formation of the
light emitting active region. These epitaxial layers form a "base
layer" that can have n-type conductivity. FIG. 1 depicts an example
of a base layer having a GaN layer beneath an n-type GaN layer.
[0008] The light emitting active region in which radiative
recombination of electrons and holes occurs is formed on top of the
base layer, typically in the form of at least one quantum well
although single and double heterostructures and homojunctions can
also be used. Above the active region lie p-type conductive
injection and confinement regions. Positive and negative contacts
(omitted from FIG. 1) are also provided.
[0009] There remains a need for LEDs with improved optical
performance including higher LED brightness and higher quantum
efficiency.
SUMMARY
[0010] The present invention relates to methods of fabricating
light emitting devices, particularly LEDs employing a III-Nitride
light emitting active region deposited on an n-type conductive base
layer. The substrate upon which the base layer is grown is cut
intentionally misaligned from a main crystal plane. In addition to
intentional substrate misalignment, base layers are employed that
are thicker than 3.5 .mu.m. In some embodiments of the present
invention, the presence of a thick base layer provides sufficient
mechanical support for the device such that the substrate can be
removed entirely from the light emitting system, further increasing
the performance of the device.
[0011] Examples are provided for the illustrative case of thick
base layers deposited on a sapphire substrate misaligned from the
c-axis. Misalignment angles are in the range from 0.05.degree. to
approximately 10.degree.. The present invention also employs base
layers thicker than 3.5 .mu.m, preferably in the range of 7
.mu.m-10 .mu.m. The combination of base layers >3.5 .mu.m, grown
on the misaligned substrates leads to surprisingly improved light
emission.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The drawings herein are not to scale.
[0013] FIG. 1: Schematic cross-sectional depiction of a portion of
a prior art LED layered structure.
[0014] FIG. 2: Schematic depiction of a sapphire unit cell.
[0015] FIGS. 3a and 3b: Relative light emitting efficiency for LEDs
grown on mis-oriented c-plane sapphire substrates with
mis-orientation angles of 0.3.degree. and 0.05.degree.. Data are
shown for LED structures with base layers having strain state A (a)
and B (b).
[0016] FIG. 4: Schematic cross-sectional depiction of a portion of
a LED layered structure pursuant to an embodiment of the present
invention.
[0017] FIG. 5: Schematic cross-sectional depiction of a portion of
a LED layered structure pursuant to another embodiment of the
present invention.
[0018] FIG. 6: Brightness as a function of dominant wavelength for
LEDs having thin (3.5 .mu.m) and thick (6.5 .mu.m) base layers
grown on-axis (tilt angle less than 0.05.degree.) and off-axis
(tilt angle in the range from about 0.20.degree. to about
0.40.degree.).
[0019] FIG. 7: Relative light emitting efficiency as a function of
forward current for the thick base layer (6.5 .mu.m) LED depicted
in FIG. 5 grown on-axis (0.03.degree.) and off-axis
(0.39.degree.).
[0020] FIG. 8: Brightness as a function of dominant wavelength for
four experiments of the same device structure grown with a thick
base layer (6.5 .mu.m) on substrates with and without
misalignment.
[0021] FIG. 9: Display device incorporating the high brightness
LEDs of the present invention.
DETAILED DESCRIPTION
[0022] The present invention relates to epitaxial layer thickness
and crystal orientation for light emitting diodes ("LEDs") and, in
particular to the substrate orientation and the base layer
thickness between the substrate and the light emitting active
region resulting in improved light emitting properties. The base
layer is the layer or layers between the substrate and the active
region, including layers close to the substrate, such as buffer or
nucleation layers and layers close to the active region, such as
transition layers. Specific examples are included in connection
with LEDs based upon a double heterostructure multiple quantum well
light emitting active region composed of indium gallium nitride
(InGaN) deposited on an n-type conductive base layer on a sapphire
substrate. These examples are intended to be illustrative only. The
present invention is applicable to homojunctions, to single and
double heterostructures and to single or multiple quantum well
embodiments.
[0023] Pursuant to the present invention, the substrate is cut
intentionally misaligned from a main crystal plane and thick base
layers are grown on the misaligned substrates. Higher brightness
and improved efficiency are among the improvements demonstrated in
various embodiments by off-axis growth of a thick base layer. One
embodiment is the growth of a thick n-type GaN base layer on an
off-axis sapphire substrate.
[0024] Substrate Orientation and Base Layers
[0025] The substrates upon which LEDs can be fabricated include
sapphire, SiC, GaN, GaAs, and GaP among others. Examples are
included for the specific case of misaligned base layer growth on a
sapphire substrate. However, sapphire is described herein as an
illustrative example, not intended as a limitation on the various
embodiments.
[0026] Sapphire, or Al.sub.2O.sub.3, has a hexagonal structure
belonging to the space group R3c. The basic structure consists of
hexagonal close-packed planes of oxygen intercalated with planes of
aluminum atoms. FIG. 2 depicts the structure of a unit cell of the
sapphire crystal and the planes commonly designated a, c, m and r.
The {0001} plane is designated the "c-plane," and the "c-axis" is
perpendicular to the c-plane. However, sapphire and III-Nitrides
have a large lattice mismatch. To deposit III-Nitride layers on top
of sapphire substrates a thin III-Nitride nucleation layer, also
called a buffer layer, must be deposited first. The rest of the
base layer then can be deposited. The base layer provides for
carrier transport to the light emitting layer. The base layer
typically comprises one or more III-Nitride materials (undoped,
n-type or p-type).
[0027] We consider the example of an n-doped GaN base layer
including doped, modestly doped, undoped and/or unintentionally
doped GaN sublayers, recognizing that other materials can be
employed for the base layer. A base layer with a graded doping can
also be used. The doping level of the base layer can be lower in
the direction towards the substrate and can be higher in the
direction towards the active region, although the region close to
the active region or the substrate may not follow this doping
grade. Typical procedures for depositing n-type base layers,
fabricating the MQW active region, and depositing p-type layers are
described in several standard references including Introduction to
Nitride Semiconductor Blue Lasers and Light Emitting Diodes, Eds.
S. Nakamura and S. F. Chichibu, (Taylor & Francis, 2000) and
"InGaN Light emitting Diodes with Quantum-Well Structures" by S.
Nakamura, appearing in Materials Research Society Symposium
Proceedings Vol. 395, Gallium Nitride and Related Materials, Eds,
F. A. Ponce, R. D. Dupuis, S. Nakamura, J. A. Edmond (Materials
Research Society, 1996), pp. 879-887.
[0028] Conventional fabrication techniques for LEDs involve the
growth of one or more layers collectively comprising a base layer
and providing a transition from the substrate to the active region.
In prior art LEDs employing a sapphire substrate, base layers are
conventionally grown on the sapphire substrate along the c-axis,
typically called "on-axis" or "aligned" growth. "On-axis"
emphasizes that the sapphire crystal is cut as precisely as is
feasible along the c crystallographic plane (or other main crystal
plane) and the base layer is grown substantially perpendicular to
the c-axis.
[0029] In the present invention, the sapphire (or other) substrate
upon which the base layer is grown is cut not precisely
perpendicular to the c-axis depicted in FIG. 2 (or other main
crystallographic axis) but is inclined at a small deviation from
perpendicularity. The substrate surface on which the base layer is
deposited pursuant to these embodiments is thus not precisely the
c-plane depicted in FIG. 2. We use "misalignment" or "tilt angle"
to denote the angle between the normal to the growth plane and the
c-axis. Misaligned, off-axis growth thus denotes growth of a base
layer on a substrate surface that is misaligned from such a main
crystal plane.
[0030] The direction of misalignment with respect to a designated
crystallographic axis can also be specified. For the examples
considered herein of growth on a sapphire substrate, it is
convenient to indicate the direction of misalignment of the c-axis
towards the m-plane as "m-plane tilt," or towards the a-plane as
"a-plane tilt." However, off-axis growth planes can have any
orientation, not limited to purely m-plane or purely a-plane tilts.
Off-axis growth for other substrates can be defined in a completely
analogous manner in terms of the direction of the misalignment with
respect to a main crystallographic axis.
[0031] Experiments suggest that optical performance improves as
tilt angles larger than about 0.05.degree. are used in combination
with thick base layers. In general, it is found that growth of a
thick base layer on a substrate with a misalignment of more than
0.05.degree. improves LED brightness and other optical properties
without a clear upper limit to the misalignment angle. It is
possible that the tilt angle that yields LEDs with improved
brightness depends on the strain state of the III-Nitride base
layers. Strain denotes the deviation of the lattice constants of an
epitaxial layer with respect to bulk crystal. III-Nitride layers
grown on c-plane sapphire substrates are generally "in compression"
(the lateral lattice constant is smaller than for a bulk crystal).
However, the introduction of Si into a III-Nitride crystal can
reduce the degree of compression and, at high doping levels, cause
the crystal to be "in tension" (the lateral lattice constant is
larger than for a bulk crystal). In FIG. 3 relative light output
efficiency is shown for LEDs that have a strain state "A" and a
strain state "B" (3a and 3b; respectively). While for strain state
"A" a misalignment angle of 0.3.degree. is favorable, for strain
state "B" a mis-alignment angle of 0.05.degree. gives improved
brightness. Strain state "B" denotes more tension with respect to
strain state "A" and was accomplished by higher Si doping. It is
expected that for even higher Si doping concentration and or
thicker base layers tilt angles >0.05.degree. can be favorable
and improved brightness has been observed for a tilt angle of
1.degree..
[0032] Under a certain set of growth conditions on a lattice
mismatched substrate, the layer being grown may crack. The cracking
limit is the maximum thickness that the layer (of a particular
doping) can be grown without significant cracking, such that device
performance is not adversely affected. There can be a trade-off
between doping and thickness, the lighter the doping, the thicker
the layer can be grown before it cracks.
[0033] M-plane, a-plane and intermediate direction tilts from the
c-plane have been investigated with respect to base layer growth on
sapphire. No significant variation in optical performance has been
observed with variation in tilt direction to the accuracy of the
measurements reported herein. Most tilt angles given herein are
m-plane tilts from the c-plane. Tilt angles less than 0.05.degree.
are not significantly different from on-axis. Thus, "on-axis" is
used herein to indicate tilt angles from 0 to 0.05.degree. in any
direction.
[0034] In the fabrication of light emitting devices on a substrate,
a base layer comprising one or more constituent layers is typically
grown on the substrate as a transition region between the substrate
the light emitting active region. Typically, Metal-Organic Chemical
Vapor Deposition ("MOCVD") is used to grow the sublayers comprising
the base layer, although other deposition techniques can be used
and are within the scope of the present invention. To be concrete
in our discussion, we describe the particular example of growth of
a base layer on a sapphire substrate, not intending thereby to
exclude other substrates such as SiC, GaN, GaAs, and GaP among
others.
[0035] FIG. 4 is a schematic depiction of the cross-section of a
portion of an LED device pursuant to an embodiment of the present
invention. The device comprises a base layer of AlInGaN grown above
an off-axis substrate to a thickness greater than about 3.5 .mu.m.
The first layer or region of the base layer is typically a buffer
layer or nucleation layer (not shown). The last layer or region of
the base layer can be a transition layer (not shown), that can
provide a transition between the previous base layer growth and the
active region. An active region for emitting light is grown above
the base layer. The active region can be a homojunction, a single
or a double heterostructure, or a single or multiple quantum well
structure. An AlInGaN confinement layer is grown above the active
region. The AlInGaN layers can be any composition of AlInGaN,
including GaN, AlGaN, and InGaN, and can be n-type, p-type,
undoped, or have a graded doping profile. The two AlInGaN layers
can have different compositions from one another. The AlInGaN base
layer can have a graded doping level that generally decreases in
the direction towards the substrate and increases in the direction
towards the active region, although as previously mentioned, other
regions or layers close to the substrate or close to the active
region may not follow this doping grade. The AlInGaN base layer can
be composed of sublayers including a sublayer that is closer to the
substrate and a sublayer that is closer to the active region such
that the sublayer closer to the active region is more heavily doped
than the sublayer closer to the substrate. The sublayer closer to
the active region can be more heavily doped n-type or p-type than
the sublayer closer to the substrate. Examples of sublayer dopings
include: a sublayer closer to the active region more heavily doped
n-type than a n-type sublayer closer to the substrate; a sublayer
closer to the active region more heavily p-type than a p-type
sublayer closer to the substrate; a sublayer closer to the active
region more heavily doped p-type than a n-type sublayer closer to
the substrate; and a sublayer closer to the active region more
heavily doped n-type than a p-type sublayer closer to the
substrate. P-type base layers between the substrate and the active
region can occur, for example, in tunnel junction devices and in
n-up devices that that have an opposite polarity electric field
than the more conventional p-up devices. All of these devices are
included within the scope of the present invention.
[0036] FIG. 5, one embodiment of the present invention, is a
schematic depiction of the cross-section of a portion of an LED
device showing a sapphire substrate 1 and the base layer 3 between
the substrate and the light emitting active region 5. A base layer
of n-type GaN 2 comprising a sublayer of the base layer 3 deposited
on a buffer layer (not shown) on a sapphire substrate 1 misaligned
from a main crystal plane: The buffer layer growth of nitride on
sapphire (on-axis or off-axis) is recognized not to be precisely
epitaxial due to the lattice mismatch. Rather, the initial stages
in the growth of nitride on sapphire seem to proceed by solid phase
crystallization from an amorphous phase of GaN as deposited on the
sapphire. Subsequent base layers are deposited epitaxially on the
buffer layer. In one embodiment described below the base layers are
n-type GaN. If no special precautions are taken, deposited GaN
tends to be n-type conductive. That is, if GaN is deposited without
the introduction of specific dopants, n-type material typically
results. This "unintentional" n-type doping may result from the
incorporation of n-type impurities (for example, silicon and
oxygen) from background gases into the GaN. However, n-doped GaN
can also be used as base sublayer 2 in which modest amounts of
dopants are specifically introduced into the GaN. Specific
introduction of modest amounts of dopant can result in a more
controlled, reproducible. LED structure than unintentional doping.
In the examples presented herein, sublayer 2 is unintentionally
doped n-type.
[0037] In practice, doping levels have an effect on the thickness
to which sublayer 2 can be grown before the cracking limit is
reached. We use the term "lightly doped GaN" to indicate a GaN
layer that is unintentionally or modestly doped having a doping
level sufficiently low, typically less than about 5.times.10.sup.18
dopant atoms per cubic centimeter. Using a lightly doped sublayer
allows the base layer to be grown to the desired thickness without
reaching the cracking limit. "Lightly doped" applied to material
other than GaN used for sublayer 2 likewise denotes doping levels
that allow growth to the desired thickness before the cracking
level is reached. In one embodiment, the lightly doped GaN 2
sublayer is 4.5 .mu.m thick.
[0038] In this embodiement, a sublayer of n-doped GaN 4 is grown
over the lightly doped GaN 2. In this embodiment, n-doped GaN 4 is
2 .mu.m thick. N-CaN 4 is typically doped to a concentration in the
range of approximately 10.sup.18-10.sup.20 dopant atoms per cubic
centimeter. In this embodiment, n-doped GaN 4 has a dopant
concentration on the order of approximately 10.sup.19 dopant atoms
per cubic centimeter. A Si dopant is used for the examples
presented herein but is not a limitation on the scope of the
present invention. Si, Ge, Sn, O are among the dopant atoms used to
dope the III-Nitrides n-type. P-type dopants include Mg, Zn, Be, C,
and Cd. Another layer, layers, or regions of the base layer 3 may
precede the active region 5. This transition layer or region (not
shown) is part of base layer 3 and can be lightly doped and serves
as a transition between the previous part of the base layer and the
active region 5.
[0039] Above the base layer 3 lies the active light emitting region
5. In some embodiments a multi-quantum-well ("MQW") comprising
several quantum well layers separated by barrier layers of higher
bandgap material. For InGaN quantum wells, typical barrier layers
include higher bandgap InGaN, GaN, AlGaN, and AlInGaN. Although the
present invention is described in terms of InGaN MQWs on a sapphire
substrate with n-type GaN buffer regions, the present invention is
not inherently limited to this LED structure.
[0040] Layers of p-type conductivity known as "confinement layers"
and "injection layers" lie opposite the active region from the
n-type base layer, depicted as 6 in FIG. 4. Typical materials,
dimensions and dopant concentrations for the p-layers arc known in
the art and given in the references previously cited and can be,
for example, 100-1000 .ANG. of p-type Al.sub.xGa.sub.1-xN
(0<x<0.25) followed by 100-3000 .ANG. of p-type GaN. A more
heavily doped p-type layer may be formed above the p-GaN to ensure
good ohmic contact of the p-electrode.
[0041] In typical prior art LEDs as depicted in FIG. 1, the base
layers are grown on on-axis substrates with a total thickness of
less than 3.5 .mu.m. One embodiment of the present invention, FIG.
5 shows the base layer 3, grown on off-axis substrates, to be
thicker than that of FIG. 1. That is, the present invention uses
base layers 3 having a thickness greater than about 3.5 .mu.m grown
on off-axis substrates. A preferred thickness of base layer 3 is
from approximately 6.5 to approximately 9.5 .mu.m grown on off-axis
substrates. Improved brightness is one favorable result from the
use of thick base layers 3 in combination with off-axis epitaxial
growth. For economy of language, we refer to "thin" and "thick"
base layers to indicate the general ranges below 3.5 .mu.m for
"thin" and above 3.5 .mu.m for "thick."
[0042] The examples presented herein maintain the N-GaN layer 4 at
a thickness of about 2 .mu.m and cause the base layer 3 to thicken
by causing the lightly doped GaN layer 2 to thicken. This is
illustrative only and the thickening of the base layer described
herein can be achieved by thickening any or any combination of
sublayers comprising the base layer.
[0043] Although base layers 3 thicker than about 3.5 .mu.m and,
advantageously, in the range from approximately 6.5 .mu.m to about
9.5 .mu.m grown on off-axis substrates give adequate results in the
practice of the present invention, considerably thicker layers up
to approximately 200 .mu.m grown off-axis are also feasible.
Brightness increases with increasing thickness of the off-axis
grown base layer and the present invention has no upper limit on
thickness. However, the increased thickness of the overall LED
structure tends to increase manufacturing complexities, for example
throughput and device singulation.
[0044] Substrate Removal
[0045] The primary function of the substrate is to provide a
platform upon which the various layers of the complete light
emitting device can be fabricated. The substrate thus provides
mechanical strength and stability during fabrication and operation.
However, during operation of the light emitting device, the optical
properties of the substrate may interfere with effective light
extraction (among other properties) and thus hinder device
performance. The thick base layers used herein provide, in some
cases, sufficient mechanical stability to allow separation of the
substrate from the remainder of the device following fabrication of
the thick base layers.
EXAMPLES
[0046] Several examples compare brightness and other optical
properties of the LED for various off-axis tilt angles and for
various thicknesses of n-type base layers. The data relates to
InGaN MQW LEDs as generally depicted in FIG. 5.
[0047] Several batches of LEDs were fabricated with different
dominant emission wavelengths. FIG. 6 depicts the LED brightness in
lumens as a function of this dominant wavelength for LED's having
thin and thick base layers. "Thin" base layers are about 3.5 .mu.m
thick while "thick" base layers are about 6.5 .mu.m thick in FIG.
6. Surprisingly, the improved light emission achieved by using
thick and off-axis base layers in combination markedly exceeds the
sum of the individual improvements from each effect considered
separately. For example, at approximately 510 nm, FIG. 6 depicts
the improvement obtained from thin on-axis to thick on-axis as
segment 100. The improvement achieved from thin on-axis to thin
off-axis is depicted as 100+101. The improvement obtained by the
combination of thick and off-axis base layers is 100+101+102, which
markedly exceeds the sum of contributions from thick and off-axis
effects considered separately (100+101+100=flux level 200). Thus,
the combination of thick base layer together with growth on a on a
off-axis substrate achieves an improvement in light emission that
unexpectedly and clearly exceeds the sum of its individual
parts.
[0048] FIG. 7 depicts the relative efficiency in producing light as
a function of the forward current driving the LED. The data is not
calibrated in terms of absolute light output (lumens) but rather
compares the LED driving current with current generated by the
particular photodetector employed to measure total light emitted.
Thus, relative variations in light emitting efficiency from data
point to data point and curve-to-curve may be extracted from FIG.
7.
[0049] The data of FIG. 7 relates to thick base layers as generally
depicted in FIG. 5 (layer 3 approximately 6.5 .mu.m), for two
angles of misorientation from the c-axis towards the m-plane. The
top curve is measured from a device grown on a substrate with a
misalignment angle of approximately 0.39.degree. and a base layer
thickness of about 6.5 .mu.m. The bottom curve is measured from a
device grown on a on-axis substrate with a base layer thickness of
about 6.5 .mu.m. We see in FIG. 7 that for the two devices both
having thick base layers of comparable thickness the off-axis
deposition has a higher maximum in the efficiency curve than does
on-axis deposition. Additionally, off-axis deposition is seen to
peak at a lower current value than does on-axis deposition, 7.9
milliamp (mA) compared to 12.6 mA.
[0050] Achieving higher efficiency for off-axis deposition as
depicted in FIG. 7 is certainly preferable, giving much brighter
LEDs for the same current. However, achieving maximum efficiency at
a lower current value is also evidence of a more favorable LED
structure. Light emitting efficiency is determined, in part, by
radiative electron-hole recombination and non-radiative loss
mechanisms. Non-radiative losses tend to dominate the performance
of the LED at lower currents. Higher currents tend to cause the
non-radiative losses to saturate, leading to increasing light
emitting efficiency at higher currents. Thus, peak efficiency at a
lower current is evidence of fewer non-radiative loss mechanisms,
indicating less defects and overall a better LED material.
[0051] FIG. 8 depicts four experiments in which on-axis and
off-axis sapphire substrates were loaded into the same reactor and
LEDs fabricated under otherwise identical conditions. All
experiments depicted in FIG. 8 employ thick base layers
approximately 6.5 .mu.m thick. Thus, FIG. 8 allows a clear
comparison of the effect on light emission of tilted vs. untilted
substrates for thick base layers pursuant to the present invention,
removing effects of other experiment-to-experiment variations. FIG.
8 clearly depicts the enhanced brightness resulting from the growth
of thick base layers on tilted substrates.
[0052] The resulting high brightness LEDs formed in accordance with
the invention are particularly suitable for color display panels
using red, green, and blue LEDs as the pixel elements. Such
displays are well known and are represented in FIG. 9. A display
panel 300 has an array of red, green, and blue LEDs, respectively,
that are selectively illuminated by well known circuitry to display
an image. Only three pixels are shown in FIG. 9 for simplicity. In
one embodiment, each primary color is arranged in columns. In other
embodiments, the primary colors are arranged in other patterns,
such as triangles. The high brightness LEDs may also be used for
backlighting an LCD display.
[0053] 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 and preferred embodiments illustrated and described.
* * * * *