U.S. patent application number 13/249157 was filed with the patent office on 2013-04-04 for light emitting devices having dislocation density maintaining buffer layers.
This patent application is currently assigned to Bridgelux, Inc.. The applicant listed for this patent is Will Fenwick, Long Yang. Invention is credited to Will Fenwick, Long Yang.
Application Number | 20130082274 13/249157 |
Document ID | / |
Family ID | 47991731 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130082274 |
Kind Code |
A1 |
Yang; Long ; et al. |
April 4, 2013 |
LIGHT EMITTING DEVICES HAVING DISLOCATION DENSITY MAINTAINING
BUFFER LAYERS
Abstract
A method for forming a light emitting device comprises forming a
buffer layer having a plurality of layers comprising a substrate,
an aluminum gallium nitride layer adjacent to the substrate, and a
gallium nitride layer adjacent to the aluminum gallium nitride
layer. During the formation of each of the plurality of layers, one
or more process parameters are selected such that an individual
layer of the plurality of layers is strained.
Inventors: |
Yang; Long; (Union City,
CA) ; Fenwick; Will; (Livermore, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Yang; Long
Fenwick; Will |
Union City
Livermore |
CA
CA |
US
US |
|
|
Assignee: |
Bridgelux, Inc.
|
Family ID: |
47991731 |
Appl. No.: |
13/249157 |
Filed: |
September 29, 2011 |
Current U.S.
Class: |
257/76 ; 118/697;
257/94; 257/E33.027; 257/E33.043; 438/47 |
Current CPC
Class: |
H01L 33/0025 20130101;
H01L 33/12 20130101; H01L 21/02505 20130101; H01L 21/0254 20130101;
H01L 21/02458 20130101; H01L 21/02381 20130101; H01L 33/007
20130101; H01L 2924/0002 20130101; H01L 21/0262 20130101; H01L
33/025 20130101; H01L 2924/00 20130101; H01L 2924/0002 20130101;
H01L 33/32 20130101 |
Class at
Publication: |
257/76 ; 438/47;
118/697; 257/94; 257/E33.027; 257/E33.043 |
International
Class: |
H01L 33/30 20100101
H01L033/30; B05C 11/00 20060101 B05C011/00; B05C 13/00 20060101
B05C013/00; H01L 33/12 20100101 H01L033/12 |
Claims
1. A light emitting device, comprising: a buffer layer comprising
an aluminum gallium nitride layer and a gallium nitride (GaN) layer
adjacent to the aluminum gallium nitride layer; and a light
emitting stack adjacent to the buffer layer, the light emitting
stack including an active layer configured to generate light upon
the recombination of electrons and holes, wherein a combined
thickness of the buffer layer and the light emitting stack is less
than or equal to 5 micrometers (.mu.m).
2. The light emitting device of claim 1, wherein said buffer layer
is adjacent to a silicon substrate.
3. The light emitting device of claim 1, wherein the buffer layer
further comprises an aluminum nitride (AlN) layer.
4. The light emitting device of claim 3, wherein the AlN layer is
adjacent to the silicon substrate and the GaN layer is adjacent to
the light emitting stack.
5. The light emitting device of claim 1, wherein the combined
thickness is less than or equal to 3 .mu.m.
6. The light emitting device of claim 1, wherein the light emitting
device has a radius of curvature (absolute value) that is greater
than 50 m.
7. A light emitting device, comprising: a buffer layer including an
aluminum gallium nitride layer and a gallium nitride (GaN) layer
adjacent to the aluminum gallium nitride layer; and a light
emitting stack adjacent to the GaN layer, the light emitting stack
having an active layer configured to generate light upon the
recombination of electrons and holes, wherein an absolute value of
a radius of curvature of the buffer layer is greater than 50 m.
8. The light emitting device of claim 7, wherein the buffer layer
further comprises an aluminum nitride (AlN) layer.
9. The light emitting device of claim 7, wherein said buffer layer
is adjacent to a silicon substrate.
10. The light emitting device of claim 7, wherein a combined
thickness of the buffer layer and the light emitting stack is less
than or equal to about 5 micrometers (.mu.m).
11. The light emitting device of claim 7, wherein the buffer layer
has a defect density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2.
12. A light emitting device, comprising: a buffer layer comprising:
i) a compressive strained Al.sub.xGa.sub.1-xN layer adjacent to the
AlN layer, wherein `x` is a number between 0 and 1; and ii) a
compressive strained gallium nitride (GaN) layer adjacent to the
strained Al.sub.xGa.sub.1-xN layer; and a light emitting stack
adjacent to the buffer layer, the light emitting stack having an
n-type gallium nitride (n-GaN) layer, a p-type gallium nitride
(p-GaN) layer, and an active layer between the n-GaN and p-GaN
layers, the active layer configured to generate light upon the
recombination of electrons and holes.
13. The light emitting device of claim 12, wherein the buffer layer
further comprises a tensile strained aluminum nitride (AlN)
layer.
14. The light emitting device of claim 13, further comprising an
electrode adjacent to the tensile strained AlN layer.
15. The light emitting device of claim 12, further comprising a
substrate adjacent to the buffer layer or the light emitting
stack.
16. The light emitting device of claim 15, wherein the substrate is
formed of a material selected from the group consisting of silicon,
germanium, silicon oxide, silicon dioxide, titanium oxide, titanium
dioxide, sapphire, silicon carbide (SiC), a ceramic material and a
metallic material.
17. The light emitting device of claim 12, wherein a combined
thickness of the buffer layer and the light emitting stack is less
than or equal to about 5 micrometers (.mu.m).
18. The light emitting device of claim 12, wherein a thickness of
the strained AN layer is less than or equal to about 1 micrometer
(.mu.m).
19. The light emitting device of claim 12, wherein a thickness of
the strained Al.sub.xGa.sub.1-xN layer is less than or equal to
about 1 micrometer (.mu.m).
20. The light emitting device of claim 12, wherein a thickness of
the strained GaN layer is less than or equal to about 4 micrometers
(.mu.m).
21. The light emitting device of claim 12, wherein the n-GaN layer
is adjacent to the strained GaN layer.
22. The light emitting device of claim 12, wherein a thickness of
the buffer layer is less than or equal to about 5 micrometers
(.mu.m).
23. The light emitting device of claim 12, wherein the strained GaN
layer has a defect density between about 1.times.10.sup.8 cm.sup.-2
and 2.times.10.sup.10 cm.sup.-2.
24. The light emitting device of claim 12, wherein the light
emitting stack has a defect density between about 1.times.10.sup.8
cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2.
25. The light emitting device of claim 24, wherein the defects are
V-defects originating from dislocations in the buffer layer.
26. The light emitting device of claim 12, further comprising an
electrode adjacent to the light emitting stack.
27. The light emitting device of claim 12, further comprising a
strained Al.sub.yGa.sub.1-yN layer adjacent to the strained
Al.sub.xGa.sub.1-xN layer, wherein `y` is a number between 0 and
1.
28. A light emitting device, comprising a buffer layer adjacent to
a light emitting stack, the light emitting stack having an active
layer configured to generate light upon the recombination of
electrons and holes, said active layer having an n-type gallium
nitride layer and a p-type gallium nitride layer, wherein the
buffer layer has a radius of curvature (absolute value) that is
greater than 50 m.
29. The light emitting device of claim 28, wherein the buffer layer
comprises aluminum, gallium and nitrogen, wherein the buffer layer
is compositionally graded between aluminum nitride and gallium
nitride.
30. A method for forming a light emitting device, comprising:
forming, over a substrate in a reaction chamber, a light emitting
stack having an active layer configured to generate light upon the
recombination of electrons and holes, wherein the light emitting
stack is formed adjacent to a gallium nitride (GaN) layer, wherein
the GaN layer is formed adjacent to an aluminum gallium nitride
layer under processing conditions selected to form defects in the
GaN layer, wherein the aluminum gallium nitride layer is formed
adjacent to an aluminum nitride (AlN) layer under processing
conditions selected to form defects in the aluminum gallium nitride
layer, and wherein the AlN layer is formed adjacent to said
substrate under processing conditions selected to form defects in
the AlN layer.
31. The method of claim 30, wherein the substrate is formed of a
material selected from the group consisting of silicon, germanium,
silicon oxide, silicon dioxide, titanium oxide, titanium dioxide,
sapphire, silicon carbide (SiC), a ceramic material and a metallic
material.
32. The method of claim 30, wherein the light emitting stack is
formed under processing conditions selected to generate V-defects
originating from dislocations in the GaN layer.
33. The method of claim 30, wherein the GaN layer is formed under
processing conditions selected to generate compressive strain in
the GaN layer.
34. The method of claim 30, wherein the aluminum gallium nitride
layer is formed under processing conditions selected to generate
compressive strain in the aluminum gallium nitride layer.
35. The method of claim 30, wherein the AlN layer is formed under
processing conditions selected to generate tensile strain in the
AlN layer.
36. The method of claim 30, wherein the processing conditions are
selected from the group consisting of reaction space chamber,
precursor flow rate, carrier gas flow rate and growth
temperature.
37. The method of claim 30, wherein said defects are
dislocations.
38. A method for forming a light emitting device, comprising: (a)
providing a substrate in a reaction chamber; (b) forming an
aluminum nitride (AlN) layer adjacent to the substrate under
processing conditions selected to generate defects in the AlN
layer; (c) forming an aluminum gallium nitride layer adjacent to
the AlN layer under processing conditions selected to generate
defects in the aluminum gallium nitride layer; and (d) forming a
gallium nitride (GaN) layer adjacent to the aluminum gallium
nitride layer under processing conditions selected to generate
defects in the GaN layer.
39. The method of claim 38, further comprising (e) forming a light
emitting stack adjacent to the GaN layer under processing
conditions selected to generate V-defects originating from
dislocations in the GaN layer.
40. The method of claim 38, wherein the aluminum gallium nitride
layer is Al.sub.xGa.sub.1-xN, wherein `x` is a number between 0 and
1.
41. The method of claim 38, further comprising forming an
additional aluminum gallium nitride layer between the aluminum
gallium nitride layer and the GaN layer.
42. The method of claim 38, further comprising forming a light
emitting stack adjacent to the GaN layer, the light emitting stack
comprising an active layer configured to generate light upon the
recombination of electrons and holes.
43. The method of claim 38, wherein the light emitting stack
comprises an n-type gallium nitride (n-GaN) layer, a p-type gallium
nitride (p-GaN) layer, and said active layer between the n-GaN and
the p-GaN layers.
44. The method of claim 43, wherein the n-GaN layer is adjacent to
the GaN layer.
45. A method for forming a light emitting device, comprising:
forming a plurality of layers adjacent to a substrate, said
plurality of layers including i) an aluminum nitride layer adjacent
to the substrate, ii) an aluminum gallium nitride layer adjacent to
the aluminum nitride layer and iii) a gallium nitride layer
adjacent to the aluminum gallium nitride layer, wherein, during the
formation of each of said plurality of layers, one or more process
parameters are selected such that an individual layer of said
plurality of layers has a strain that is nonzero with increasing
thickness of said individual layer.
46. The method of claim 45, wherein the substrate is formed of a
material selected from the group consisting of silicon, germanium,
silicon oxide, silicon dioxide, titanium oxide, titanium dioxide,
sapphire, silicon carbide (SiC), a ceramic material and a metallic
material.
47. The method of claim 45, wherein during the formation of the
aluminum nitride layer, one or more process parameters are selected
such that the aluminum nitride layer has a tensile strain that is
nonzero with increasing thickness of the aluminum nitride
layer.
48. The method of claim 45, wherein during the formation of the
aluminum gallium nitride layer, one or more process parameters are
selected such that the aluminum gallium nitride layer has a
compressive strain that is nonzero with increasing thickness of the
aluminum gallium nitride layer.
49. The method of claim 45, wherein during the formation of the
gallium nitride layer, one or more process parameters are selected
such that the gallium nitride layer has a compressive strain that
is nonzero with increasing thickness of the gallium nitride
layer
50. The method of claim 45, wherein said individual layer of said
plurality of layers has a strain that is nonzero with increasing
thickness of said individual layer at a growth temperature of said
individual layer.
51. A system for forming a light emitting device, comprising: a
reaction chamber for holding a substrate; a pumping system in fluid
communication with the reaction chamber, the pumping system
configured to purge or evacuate the reaction chamber; and a
computer system having a processor for executing machine readable
code implementing a method for forming a buffer layer adjacent to
the substrate, the method comprising: forming a plurality of layers
adjacent to said substrate, said plurality of layers including i)
an aluminum nitride layer adjacent to the substrate, ii) an
aluminum gallium nitride layer adjacent to the aluminum nitride
layer and iii) a gallium nitride layer adjacent to the aluminum
gallium nitride layer, wherein, during the formation of each of
said plurality of layers, one or more process parameters are
selected such that an individual layer of said plurality of layers
has a strain that is nonzero with increasing thickness of said
individual layer.
Description
BACKGROUND
[0001] Lighting applications typically use incandescent or
gas-filled bulbs. Such bulbs typically do not have long operating
lifetimes and thus require frequent replacement. Gas-filled tubes,
such as fluorescent or neon tubes, may have longer lifetimes, but
operate using high voltages and are relatively expensive. Further,
both bulbs and gas-filled tubes consume substantial amounts of
energy.
[0002] A light emitting diode (LED) is a device that emits light
upon the recombination of electrons and holes. An LED typically
includes a chip of semiconducting material doped with impurities to
create a p-n junction. Current flows from the p-side, or anode, to
the n-side, or cathode. Charge-carriers--electrons and holes--flow
into the p-n junction from electrodes with different voltages. When
an electron meets a hole, the electron recombines with the hole in
a process that may result in the radiative emission of energy in
the form of a photon (h.nu.). The photons, or light, are
transmitted out of the LED and employed for use in various
applications, such as, for example, lighting applications and
electronics applications.
[0003] LED's, in contrast to incandescent or gas-filled bulbs, are
relatively inexpensive, operate at low voltages, and have long
operating lifetimes. Additionally, LED's consume relatively little
power and are compact. These attributes make LED's particularly
desirable and well suited for many applications.
[0004] Despite the advantages of LED's, there are limitations
associated with such devices. Such limitations include materials
limitations, which may limit the efficiency of LED's; structural
limitations, which may limit transmission of light generated by an
LED out of the device; and manufacturing limitations, which may
lead to high processing costs. Accordingly, there is a need for
improved LED's and methods for manufacturing LED's.
SUMMARY
[0005] In an aspect, light emitting devices, such as light emitting
diodes (LED's), are provided. In an embodiment, a light emitting
device comprises a buffer layer comprising an aluminum gallium
nitride layer and a gallium nitride (GaN) layer adjacent to the
aluminum gallium nitride layer. The light emitting device further
comprises a light emitting stack adjacent to the buffer layer, the
light emitting stack having an active layer configured to generate
light upon the recombination of electrons and holes, wherein a
combined thickness of the buffer layer and the light emitting stack
is less than or equal to 5 micrometers (.mu.m). In some cases, the
buffer layer includes an aluminum nitride (AlN) layer. The AlN
layer can be adjacent to the aluminum gallium nitride layer. In
some situations, the AlN layer is between a substrate, such as a
silicon substrate, and the aluminum gallium nitride layer.
[0006] In another embodiment, a light emitting device comprises a
buffer layer having an aluminum nitride (AlN) layer, an aluminum
gallium nitride layer adjacent to the AlN layer, and a gallium
nitride (GaN) layer adjacent to the aluminum gallium nitride layer;
and a light emitting stack adjacent to the GaN layer. The light
emitting stack includes an active layer configured to generate
light upon the recombination of electrons and holes. An absolute
value of a radius of curvature of the buffer layer is greater than
50 m.
[0007] In another embodiment, a light emitting device comprises a
buffer layer comprising i) a tensile strained aluminum nitride
(AlN) layer, ii) a compressive strained Al.sub.xGa.sub.1-xN layer
adjacent to the AlN layer, wherein `x` is a number between 0 and 1,
and iii) a compressive strained gallium nitride (GaN) layer
adjacent to the strained Al.sub.xGa.sub.1-xN layer. The light
emitting device further comprises a light emitting stack adjacent
to the buffer layer. The light emitting stack includes an n-type
gallium nitride (n-GaN) layer, a p-type gallium nitride (p-GaN)
layer, and an active layer between the n-GaN and p-GaN layers. The
active layer configured to generate light upon the recombination of
electrons and holes.
[0008] In another embodiment, a light emitting device comprises a
buffer layer adjacent to a light emitting stack. The light emitting
stack includes an active layer configured to generate light upon
the recombination of electrons and holes. The active layer includes
an n-type gallium nitride layer and a p-type gallium nitride layer.
The buffer layer has a radius of curvature (absolute value) that is
greater than 50 m.
[0009] In another aspect, methods for forming light emitting
devices are provided. In an embodiment, a method for forming a
light emitting device comprises forming, over a substrate in a
reaction chamber, a light emitting stack having an active layer
configured to generate light upon the recombination of electrons
and holes. The light emitting stack is formed adjacent to a gallium
nitride (GaN) layer that is, in turn, formed adjacent to an
aluminum gallium nitride layer under processing conditions that
form defects in the GaN layer. The aluminum gallium nitride layer
is formed adjacent to an aluminum nitride (AlN) layer under
processing conditions that form defects in the aluminum gallium
nitride layer. The AlN layer is formed adjacent to the substrate
under processing conditions that form defects in the AlN layer.
[0010] In another embodiment, a method for forming a light emitting
device comprises providing a substrate in a reaction chamber and
forming an aluminum nitride (AlN) layer adjacent to the substrate
under processing conditions selected to generate defects in the AlN
layer. An aluminum gallium nitride layer is formed adjacent to the
AlN layer under processing conditions selected to generate defects
in the aluminum gallium nitride layer. A gallium nitride (GaN)
layer is formed adjacent to the aluminum gallium nitride layer
under processing conditions selected to generate defects in the GaN
layer.
[0011] In another embodiment, a method for forming a light emitting
device comprises forming a plurality of layers adjacent to a
substrate. The plurality of layers include i) an aluminum nitride
layer adjacent to the substrate, ii) an aluminum gallium nitride
layer adjacent to the aluminum nitride layer and iii) a gallium
nitride layer adjacent to the aluminum gallium nitride layer.
During the formation of each of the plurality of layers, one or
more process parameters are selected such that an individual layer
of the plurality of layers has a strain that is nonzero with
increasing thickness of the individual layer.
[0012] In another embodiment, a method for forming a light emitting
device comprises forming, over a substrate in a reaction chamber
(or reaction space if the reaction chamber includes a plurality of
reaction spaces), a light emitting stack having an n-type gallium
nitride (n-GaN) layer, a p-type gallium nitride (p-GaN) layer and
an active layer between the n-GaN layer and the p-GaN layer. The
active layer is configured to generate light upon the recombination
of electrons and holes. The light emitting stack is formed adjacent
to a gallium nitride (GaN) layer. The GaN layer is formed adjacent
to an aluminum gallium nitride layer, the aluminum gallium nitride
is formed adjacent to an aluminum nitride layer, and the AlN layer
is formed adjacent to the substrate. The substrate in some cases is
a silicon substrate.
[0013] In some cases, during the formation of one or more of the
GaN layer, aluminum gallium nitride layer and the AlN layer,
processing conditions are selected to generate defects (or
strain-inducing defects) in one or more of the GaN layer, aluminum
gallium nitride layer and the AlN layer. In some cases, during the
formation of the GaN layer, aluminum gallium nitride layer and the
AlN layer, processing conditions are selected to generate defects
in each of the GaN layer, aluminum gallium nitride layer and the
AlN layer. Processing conditions in some cases are selected to
maintain a predetermined density of defects in the layers. In some
situations, the predetermined defect density is between about
1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2. In some
embodiments, processing conditions are selected such that at a
growth temperature between about 800.degree. C. and 1200.degree.
C., or between about 900.degree. C. and 1100.degree. C., each of
the GaN layer, aluminum gallium nitride layer and the AlN layer has
a non-zero tensile or compressive strain with increasing thickness
of the layer.
[0014] In another embodiment, a method for forming a light emitting
device comprises providing a substrate in a reaction chamber,and
forming an aluminum nitride (AlN) layer adjacent to the substrate
under processing conditions selected to generate strain in the AlN
layer. An aluminum gallium nitride layer is formed adjacent to the
AlN layer under processing conditions selected to generate strain
in the aluminum gallium nitride layer. A gallium nitride (GaN)
layer is formed adjacent to the aluminum gallium nitride layer
under processing conditions selected to generate strain in the GaN
layer.
[0015] In another aspect, systems for forming light emitting
devices are provided. In an embodiment, a system for forming a
light emitting device comprises a reaction chamber for holding a
substrate and a pumping system in fluid communication with the
reaction chamber, the pumping system configured to purge or
evacuate the reaction chamber. The system includes a computer
system having a processor for executing machine readable code
implementing a method for forming a buffer layer adjacent to the
substrate. The method comprises forming a plurality of layers
adjacent to the substrate, the plurality of layers including i) an
aluminum nitride layer adjacent to the substrate, ii) an aluminum
gallium nitride layer adjacent to the aluminum nitride layer and
iii) a gallium nitride layer adjacent to the aluminum gallium
nitride layer. During the formation of each of the plurality of
layers, one or more process parameters are selected such that an
individual layer of the plurality of layers has a strain that is
nonzero with increasing thickness of the individual layer.
[0016] Additional aspects and advantages of the present disclosure
will become readily apparent to those skilled in this art from the
following detailed description, wherein only illustrative
embodiments of the present disclosure are shown and described. As
will be realized, the present disclosure is capable of other and
different embodiments, and its several details are capable of
modifications in various obvious respects, all without departing
from the disclosure. Accordingly, the drawings and description are
to be regarded as illustrative in nature, and not as
restrictive.
[0017] All publications, patents, and patent applications mentioned
in this specification are herein incorporated by reference to the
same extent as if each individual publication, patent, or patent
application was specifically and individually indicated to be
incorporated by reference.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] A better understanding of the features and advantages of the
present invention will be obtained by reference to the following
detailed description that sets forth illustrative embodiments, in
which the principles of the invention are utilized, and the
accompanying drawings of which:
[0019] FIG. 1 schematically illustrates a nascent light emitting
device;
[0020] FIG. 2 schematically illustrates a cross section of a light
emitting diode, in accordance with an embodiment;
[0021] FIG. 3 schematically illustrates a method for forming a
light emitting device, in accordance with an embodiment;
[0022] FIG. 4 schematically illustrates the strain and accumulated
stress on a light emitting device at various stages of formation of
a buffer layer over a silicon substrate, in accordance with an
embodiment;
[0023] FIG. 5 shows simplified cross-sectional side views at
various stages of a process for forming a buffer layer of a nascent
light emitting device over a silicon substrate, in accordance with
an embodiment; and
[0024] FIG. 6 shows a system used to fabricate a light emitting
device, in accordance with an embodiment.
DETAILED DESCRIPTION
[0025] While various embodiments of the invention have been shown
and described herein, it will be obvious to those skilled in the
art that such embodiments are provided by way of example only.
Numerous variations, changes, and substitutions may occur to those
skilled in the art without departing from the invention. It should
be understood that various alternatives to the embodiments of the
invention described herein may be employed in practicing the
invention.
[0026] The term "light emitting device," as used herein, refers to
a device configured to generate light upon the recombination of
electrons and holes in a light emitting region (or "active layer")
of the device, such as upon the application (or flow) of a
forward-biasing electrical current through the light emitting
region. A light emitting device in some cases is a solid state
device that converts electrical energy to light. A light emitting
diode ("LED") is a light emitting device. There are many different
LED device structures that are made of different materials and have
different structures and perform in a variety of ways. Some light
emitting devices (laser diodes) emit laser light, and others
generate non-monochromatic light. Some LED's are optimized for
performance in particular applications. An LED may be a so-called
blue LED comprising a multiple quantum well (MQW) active layer
having indium gallium nitride. A blue LED may emit
non-monochromatic light having a wavelength in a range from about
440 nanometers to 500 nanometers. A phosphor coating may be
provided that absorbs some of the emitted blue light. The phosphor
in turn fluoresces to emit light of other wavelengths so that the
light the overall LED device emits has a wider range of
wavelengths.
[0027] The term "layer," as used herein, refers to a layer of atoms
or molecules on a substrate. In some cases, a layer includes an
epitaxial layer or a plurality of epitaxial layers. A layer may
include a film or thin film. In some situations, a layer is a
structural component of a device (e.g., light emitting diode)
serving a predetermined device function, such as, for example, an
active layer that is configured to generate (or emit) light. A
layer generally has a thickness from about one monoatomic monolayer
(ML) to tens of monolayers, hundreds of monolayers, thousands of
monolayers, millions of monolayers, billions of monolayers,
trillions of monolayers, or more. In an example, a layer is a
multilayer structure having a thickness greater than one monoatomic
monolayer. In addition, a layer may include multiple material
layers (or sub-layers). In an example, a multiple quantum well
active layer includes multiple well and barrier layers. A layer may
include a plurality of sub-layers. For example, an active layer may
include a barrier sub-layer and a well sub-layer.
[0028] The term "coverage," as used herein, refers to the fraction
of a surface covered or occupied by a species in relation to the
total area of the surface. For example, a coverage of 10% for a
species indicates that 10% of a surface is covered by the species.
In some situations, coverage is represented by monolayers (ML),
with 1 ML corresponding to complete saturation of a surface with a
particular species. For example, a pit coverage of 0.1 ML indicates
that 10% of a surface is occupied by pits.
[0029] The term "active region" (or "active layer"), as used
herein, refers to a light emitting region of a light emitting diode
(LED) that is configured to generate light. An active layer
comprises an active material that generates light upon the
recombination of electrons and holes, such as, for example, with
the aid of a forward-biasing electrical current through the active
layer. An active layer may include one or a plurality of layers (or
sub-layers). In some cases, an active layer includes one or more
barrier layers (or cladding layers, such as, e.g., GaN) and one or
more quantum well ("well") layers (such as, e.g., InGaN). In an
example, an active layer comprises multiple quantum wells, in which
case the active layer may be referred to as a multiple quantum well
("MQW") active layer.
[0030] The term "doped," as used herein, refers to a structure or
layer that is chemically doped. A layer may be doped with an n-type
chemical dopant (also "n-doped" herein) or a p-type chemical dopant
(also "p-doped" herein). In some cases, a layer is undoped or
unintentionally doped (also "u-doped" or "u-type" herein). In an
example, a u-GaN (or u-type GaN) layer includes undoped or
unintentionally doped GaN.
[0031] The term "Group III-V semiconductor," as used herein, refers
to a material having one or more Group III species and one or more
Group V species. A Group III-V semiconductor material in some cases
is selected from gallium nitride (GaN), gallium arsenide (GaAs),
aluminum gallium arsenide (AlGaAs), gallium arsenide phosphide
(GaAsP), aluminum gallium indium phosphide (AlGaInP), gallium
phosphide (GaP), indium gallium nitride (InGaN), aluminum gallium
phosphide (AlGaP), aluminum nitride (AlN), aluminum gallium nitride
(AlGaN), and aluminum gallium indium nitride (AlGaInN).
[0032] The term "dopant," as used herein, refers to a chemical
dopant, such as an n-type dopant or a p-type dopant. P-type dopants
include, without limitation, magnesium, beryllium, zinc and carbon.
N-type dopants include, without limitation, silicon, germanium,
tin, tellurium, and selenium. A p-type semiconductor is a
semiconductor that is doped with a p-type dopant. An n-type
semiconductor is a semiconductor that is doped with an n-type
dopant. An n-type Group III-V material, such as n-type gallium
nitride ("n-GaN"), includes a Group III-V material that is doped
with an n-type dopant. A p-type Group III-V material, such as
p-type GaN ("p-GaN"), includes a Group III-V material that is doped
with a p-type dopant. A Group III-V material includes at least one
Group III element selected from boron, aluminum, gallium, indium,
and thallium, and at least one Group V element selected from
nitrogen, phosphorus, arsenic, antimony and bismuth.
[0033] The term "adjacent" or "adjacent," as used herein, includes
`next to`, `adjoining`, `in contact with`, and `in proximity to`.
In some instances, adjacent components are separated from one
another by one or more intervening layers. For example, the one or
more intervening layers can have a thickness less than about 10
micrometers ("microns"), 1 micron, 500 nanometers ("nm"), 100 nm,
50 nm, 10 nm, 1 nm, or less. In an example, a first layer is
adjacent to a second layer when the first layer is in direct
contact with the second layer. In another example, a first layer is
adjacent to a second layer when the first layer is separated from
the second layer by a third layer.
[0034] The term "substrate," as used herein, refers to any
workpiece on which film or thin film formation is desired. A
substrate includes, without limitation, silicon, germanium, silica,
sapphire, zinc oxide, carbon (e.g., graphene), SiC, AlN, GaN,
spinel, coated silicon, silicon on oxide, silicon carbide on oxide,
glass, gallium nitride, indium nitride, titanium dioxide, aluminum
nitride, a ceramic material (e.g., alumina, AlN), a metallic
material (e.g., molybdenum, tungsten, copper, aluminum), and
combinations (or alloys) thereof.
[0035] The term "injection efficiency," as used herein, refers to
the proportion of electrons passing through a light emitting device
that are injected into the active region of the light emitting
device.
[0036] The term "internal quantum efficiency," as used herein,
refers to the proportion of all electron-hole recombination events
in an active region of a light emitting device that are radiative
(i.e., producing photons).
[0037] The term "extraction efficiency," as used herein, refers to
the proportion of photons generated in an active region of a light
emitting device that escape from the device.
[0038] The term "external quantum efficiency" (EQE), as used
herein, refers to the ratio of the number of photons emitted from
an LED to the number of electrons passing through the LED. That is,
EQE=Injection efficiency.times.Internal quantum
efficiency.times.Extraction efficiency.
[0039] While silicon provides various advantages, such as the
ability to use semiconductor fabrication, the formation of Group
III-V semiconductor based LED's on a silicon substrate poses
various limitations. As an example, the lattice mismatch and
coefficient of thermal expansion between silicon and gallium
nitride leads to structural stresses that generate defects upon the
formation of gallium nitride thin films, such as threading and/or
hairpin dislocations (collectively "dislocations" herein).
[0040] LED's may be formed of various semiconductor device layers.
In some situations, Group III-V semiconductor LED's offer device
parameters (e.g., wavelength of light, external quantum efficiency)
that may be preferable over other semiconductor materials. Gallium
nitride (GaN) is a binary Group III-V direct bandgap semiconductor
that may be used in optoelectronic applications and high-power and
high-frequency devices.
[0041] Group III-V semiconductor based LED's may be formed on
various substrates, such as silicon, germanium and sapphire.
Silicon provides various advantages over certain other substrates,
such as the capability of using current manufacturing and
processing techniques, in addition to using large wafer sizes that
aid in maximizing the number of LED's formed within a predetermined
period of time. However, while silicon provides various advantages,
recognized herein are various limitations and difficulties
associated with forming Group III-V semiconductor-based LED's (such
as gallium nitride-based LED's) on silicon.
[0042] One issue is the formation of a gallium and silicon alloy,
which may be undesirable in circumstances in which high quality GaN
is desired. In some situations, at a temperature greater than about
1000.degree. C., the growth of high quality GaN may be difficult
due to the formation of a silicon-gallium alloy at an interface
between a gallium nitride device layer and the silicon substrate.
Another issue associated with forming Group III-V
semiconductor-based LED's on silicon is the lattice mismatch and
the mismatch in coefficient of thermal expansion (CTE) between
gallium nitride and silicon, which may generate structural stresses
that may lead to cracking issues in LED devices. Cracking of
various device layers of a light emitting device (e.g., LED) may
yield poor device performance and limit the lifetime of the light
emitting device.
[0043] In an example, for an LED having a GaN epitaxial layer (also
"epilayer" herein) on a silicon substrate, the stress in the
epilayer increases with increasing thickness in the GaN epilayer.
The increase in stress may lead to the silicon wafer to bow and in
some cases crack. The cracking issue may be more severe for a GaN
layer that is n-doped with silicon, due at least in part to a high
tensile strain in silicon-doped GaN. While the thickness of the
silicon-doped GaN layer may be selected to avoid cracking, such
thickness limitations may impose performance limitations for GaN
and silicon-based LED devices.
[0044] In some cases, following the formation of a GaN thin film on
a silicon substrate at an elevated growth temperature, during cool
down the silicon substrate contracts at a lower rate than the GaN
thin film, at least partly because GaN has a higher coefficient of
thermal expansion than silicon. Under such circumstances, at room
temperature the GaN thin film is under tensile strain. Conversely,
GaN has a lower coefficient of thermal expansion than sapphire
(Al.sub.2O.sub.3). As a consequence, for a GaN thin film grown on a
sapphire substrate, following thin film formation and cool down to
room temperature, the GaN thin film is under compressive strain.
For GaN thin films formed on silicon and GaN thin films formed on
sapphire, the differences in lattice constants between GaN and
silicon and sapphire imposes tensile strain on GaN thin films at
room temperature. For GaN formed on sapphire, the tensile strain
due to the mismatch in lattice constants is counterbalanced by the
compressive strain due to mismatch in coefficient of thermal
expansion between GaN and sapphire, preventing GaN thin films on
from cracking For GaN formed on silicon, on the other hand, the
tensile strain due to the mismatch in coefficient of thermal
expansion and mismatch in lattice constant generate tensile strain
at room temperature, which typically leads to the GaN thin film to
bow and in some cases crack at room temperature. At least in some
situations, this provides a disincentive for forming LED's having
GaN thin films on silicon substrates.
[0045] In an example, FIG. 1 schematically illustrates simplified
cross-sectional views showing the formation of a light emitting
device 100 having silicon substrate 105 and a GaN thin film 110
formed thereon. The light emitting device 100 in some cases is a
nascent light emitting device; additional processing operations may
be required to form a completed light emitting device. The silicon
substrate 105 is heated to a growth temperature, as illustrated in
the top view of FIG. 1. At the growth temperature, the GaN film 110
is formed on the silicon substrate 105, which causes the silicon
substrate 105 and the GaN film 110 to bow, as illustrated in the
middle view of FIG. 1. After the GaN film 110 is formed on the
silicon substrate 105, the structure is allowed to cool down to
room temperature. However, the stress produced by the GaN film 110
on the substrate 105 leaves a bow on the structure, as illustrated
in the lower of view of FIG. 1.
[0046] In some cases, the GaN film 110 is formed on a
monocrystalline (or single crystal) substrate, such as Si(111), in
which case the GaN film 110 is an epilayer. Due to the mismatch of
coefficient of thermal expansion between the silicon substrate 105
and the GaN thin film 110, at the growth temperature the GaN thin
film 110 is under tensile strain, leading the GaN thin film 110 and
the silicon substrate 105 to bow. At the growth temperature, the
GaN thin film 110 and the silicon substrate 105 are bowed by an
angle .theta. in relation to an axis parallel to a bottom surface
of the silicon substrate 105. The angle .theta. is greater than
0.degree.. The GaN thin film 110 and the silicon substrate 105 have
a concave configuration in relation to the axis. The mismatch in
lattice constants between GaN and silicon leads to additional
tensile strain. In such a case, upon cool-down to room temperature,
the GaN thin film 110 is under tensile strain, which may lead to
cracking in various device layers of the light emitting device
100.
[0047] In some cases, the bowing and cracking issues in GaN thin
films on silicon substrates may be addressed by minimizing the
defect density of GaN thin film during formation. This helps
provide low defect density, high quality GaN thin films on silicon
substrates. However, the formation of low defect density GaN thin
films on silicon substrates has posed manufacturing challenges.
[0048] Structures, devices and methods described in various
embodiments of the invention help address the issues described
above in regards to the formation of GaN thin films on silicon
substrates. In some embodiments, structures and methods are
provided to reduce the strain in GaN thin films formed on silicon
substrates. This minimizes, if not eliminates, bowing and cracking
of GaN thin films on silicon substrates following cool down from a
growth temperature to room temperature.
[0049] Structures, devices and methods are based, at least in part,
on the unexpected realization that any tensile strain in a GaN thin
film on a silicon substrate--due, for example, to the mismatch in
coefficient of thermal expansion)--may be counterbalanced by an
opposing strain generated in the GaN thin film. The opposing strain
in some cases is a compressive strain. In some embodiments, a
GaN-containing buffer layer having on a silicon substrate is
strained at a growth temperature to have compressive strain, which
may balance the tensile strain in the GaN-containing buffer layer,
thereby minimizing, if not eliminating bowing and crack
formation.
[0050] In some embodiments, various device layers of a light
emitting device are formed by introducing or maintaining
dislocations in the various device layers. The dislocations, which
may give rise to V-pits (or V-defects) under unique (or otherwise
predetermined) growth conditions, help maintain strain (compressive
or tensile) in each of the various device layers at the growth
temperature. In some embodiments, device layers of a light emitting
device are formed over a silicon substrate to have a predetermined
dislocation density in order to generate a compressive strain at
the growth temperature that balances the tensile strain in the
device layers.
[0051] As device layers grow in thickness, dislocations may
decrease. For instance, with increasing thickness of a device layer
on silicon, the density of dislocations decreases with increasing
thickness of the device layer. In some embodiments, the thickness
of the device layers, such as a buffer layer (including the various
layers of the buffer layer), is selected to maintain a
predetermined dislocation density in the device layers at the
growth temperature. That is, certain device layers are formed to
have a thickness that provides a predetermined dislocation density.
In an example, a device layer is formed at a thickness selected to
maintain a dislocation density between about 1.times.10.sup.8
cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2.
[0052] In some embodiments, dislocations have at least two
functions. One function is to balance stresses in the various
layers of the light emitting device. Another function is to
generate V-pits (or V-defects) in the light emitting device. The
active layer may be formed in the V-pits during the formation of
the light emitting device.
Light Emitting Devices and Buffer Layers
[0053] An aspect of the invention provides light emitting devices,
such as light emitting diodes. In some embodiments, a light
emitting device comprises a plurality of layers formed on a silicon
substrate. In some cases, the plurality of layers include a buffer
layer. One or more of the plurality of layers are strained. In some
cases, one or more of the plurality of layers are intentionally
strained--e.g., during the formation of the plurality of layers,
processing conditions are selected to generate strain in the
plurality of layers, such as by way of defects. In some
embodiments, the strain generates a compressive strain that
balances any tensile strain--due, for example, to the mismatch in
coefficient of thermal expansion between the silicon substrate and
overlying device layers--in the light emitting device, which
provides a light emitting device that has little to no net strain
at room temperature.
[0054] In some embodiments, the buffer layer is compressively
strained at a growth temperature. Upon cool down from the growth
temperature (such as, for example, to room temperature), the
compressive strain balances the tensile strain in the buffer
layer.
[0055] In some embodiments, one or more layers of the light
emitting device are strained with the aid of dislocations formed in
the one or more layers during growth. The dislocations aid in
maintaining (or generating) strain in the one or more layers at a
growth temperature and upon cool-down from the growth
temperature.
[0056] In some embodiments, a light emitting device includes a
buffer layer formed on a silicon substrate and a light emitting
stack formed on the buffer layer. The light emitting stack includes
a light emitting active layer. The buffer layer is strained to have
a net compressive strain that balances any tensile strain in the
buffer layer. This provides a buffer layer having little to no
overall strain at room temperature.
[0057] At room temperature, the light emitting device may be
concave, flat or substantially flat. In cases in which the light
emitting device is concave, the substrate bends toward the buffer
layer. In some embodiments, the light emitting device has a radius
of curvature (absolute value) that is greater than about 30 meters
("m"), or 40 m, or 50 m, or 100 m, or 200 m, or 300 m, or 400 m, or
500 m, or 1000 m, or 10,000 m. In some cases, the radius of
curvature (or degree of bowing) is substantially zero or less than
zero (i.e., the substrate and various device layers are convex). In
some situations, the light emitting device has a radius of
curvature (degree of bowing) that is less than about -50 m, or -100
m, or -200 m, or -300 m, or -400 m, or -500 m, or -1000 m, or
-10,000 m.
[0058] At a growth temperature, the light emitting device may be
convex--i.e., the substrate bends away from the buffer layer (see
FIG. 5). In some embodiments, at the growth temperature the light
emitting device has a radius of curvature (absolute value) that is
greater than about 3 m, or 4 m, or 5 m, or 6 m, or 7 m, or 8 m, or
9 m, or 10 m, or 15 m, or 20 m, or 25 m, or 30 m, or 35 m, or 40 m,
or 45 m. In some embodiments, at the growth temperature the light
emitting device has a radius of curvature (absolute value) that is
between about 0.1 m and 50 m, or 0.5 m and 20 m, or 1 m and 6 m.
The radius of curvature at the growth temperature may be
predetermined by regulating one or more growth conditions (see
below).
[0059] The radius of curvature may be calculated by calculating the
degree to which light directed to a surface scatters, such as, for
example, with the aid of a deflectometer. By measuring the
scattering of light during device layer formation, any change in
strain may be calculated. The radius of curvature is inversely
proportional to the strain--the more strained a layer, the lower
the radius of curvature; conversely, the less strained a layer, the
higher the radius of curvature. In the case of a substantially flat
surface (i.e., little to no bowing), the radius of curvature
approaches infinity.
[0060] In some embodiments, one or more layers of a light emitting
device are strained at a growth temperature. The growth temperature
is elevated in relation to room temperature. The strain at the
elevated growth temperature aids in balancing any opposing strain
(e.g., compressive strain) at the elevated growth temperature. In
such a case, upon cool down to room temperature, the one or more
layers of the light emitting device have little to no strain, which
advantageously minimizes, if not eliminates bowing and, in some
cases, the formation of cracks.
[0061] In some embodiments, a light emitting device comprises a
buffer layer adjacent to a light emitting stack. The buffer layer
comprises a strained aluminum nitride (AlN) layer, a strained
Al.sub.xGa.sub.1-xN (wherein `x` is a number between 0 and 1) layer
adjacent to the AlN layer, and a strained gallium nitride (GaN)
layer adjacent to the strained Al.sub.xGa.sub.1-xN layer. In some
situations, the strained AlN layer may be precluded. The light
emitting stack comprises an n-type gallium nitride (n-GaN) layer, a
p-type gallium nitride (p-GaN) layer, and an active layer between
the n-GaN and p-GaN layers. The active layer is configured to
generate light upon the recombination of electrons and holes, such
as upon the application of a forward-biasing electrical current
through the active layer. In some cases, the n-GaN layer is
adjacent to the strained GaN layer. The n-GaN layer is configured
to aid in the flow of electrical current to the active layer. The
p-GaN layer is configured to aid in the flow of holes to the active
layer.
[0062] In some situations, the buffer layer of the light emitting
device has at most one AlN layer, at most one Al.sub.xGa.sub.1-xN
layer adjacent to the at most one AlN layer, and at most one GaN
adjacent to the at most one Al.sub.xGa.sub.1-xN layer. In an
example, the light emitting device has one AlN layer, one
Al.sub.xGa.sub.1-xN layer adjacent to the AlN layer, and one GaN
layer adjacent to the Al.sub.xGa.sub.1-xN layer. The light emitting
device in such a case does not include any additional AlN layers,
Al.sub.xGa.sub.1-xN layers, and GaN layers.
[0063] In some cases, the light emitting device include one or more
additional strained aluminum gallium nitride layers between the
strained Al.sub.xGa.sub.1-xN layer and the strained GaN layer. In
some embodiments, the light emitting device includes a strained
Al.sub.yGa.sub.1-yN layer (wherein `y` is a number between 0 and 1)
between the Al.sub.xGa.sub.1-xN layer and the strained GaN layer.
The strained Al.sub.yGa.sub.1-yN layer may be compositionally
graded between the composition of an outermost sub-layer of the
strained Al.sub.xGa.sub.1-xN layer (adjacent to the strained
Al.sub.yGa.sub.1-yN layer) and the internationally strained GaN
layer.
[0064] The light emitting device further includes a substrate
adjacent to the buffer layer or the light emitting stack. In some
cases, the substrate is adjacent to the buffer layer. In an
example, the substrate is adjacent to the AlN layer of the buffer
layer. In other cases, the substrate is adjacent to the light
emitting stack, such as adjacent to the p-GaN layer of the light
emitting stack. The substrate includes one or more of silicon,
germanium, silicon oxide, silicon dioxide, titanium oxide, titanium
dioxide, sapphire, silicon carbide (SiC), a ceramic material (e.g.,
alumina, AlN) and a metallic material (e.g., molybdenum, tungsten,
copper, aluminum).
[0065] In some embodiments, a thickness of a light emitting device
is selected to generate and/or maintain a predetermined defect
density (e.g., dislocation density) in the light emitting device,
including the buffer layer of the light emitting device. The
defects in turn induce strain (e.g., compressive or tensile
strain). The defect density in some cases can be a function of the
thickness of the buffer layer. In an example, the thicker the
buffer layer, the lower the defect density, and the thinner the
buffer layer, the higher the defect density. Devices described in
certain embodiments are based on the unexpected realization that by
carefully selecting the thickness of individual layers of the light
emitting device and the growth conditions, various issues describe
above, such as cracking upon cool-down to room temperature, may be
mitigated, if not eliminated.
[0066] In some embodiments, a thickness of the light emitting
device is less than or equal to about 5 micrometers (".mu.m"), or
less than or equal to about 4 .mu.m, or less than or equal to about
3 .mu.m. In some embodiments, a combined thickness of the buffer
layer and the light emitting stack is less than or equal to about 5
micrometers (".mu.m"), or less than or equal to about 4 .mu.m, or
less than or equal to about 3 .mu.m. In some embodiments, a
thickness of the strained AlN layer is less than or equal to about
1 .mu.m, or less than or equal to about 0.5 .mu.m, or less than or
equal to about 0.4 .mu.m. In some embodiments, a thickness of the
strained Al.sub.xGa.sub.1-xN layer is less than or equal to about 1
.mu.m, or less than or equal to about 0.8 .mu.m, or less than or
equal to about 0.7 .mu.m. In some embodiments, a thickness of the
strained GaN layer is less than or equal to about 4 .mu.m, or less
than or equal to about 3 .mu.m, or less than or equal to about 2.5
.mu.m. In some embodiments, a thickness of the buffer layer is less
than or equal to about 5 .mu.m, or less than or equal to about 4
.mu.m, or less than or equal to about 3 .mu.m.
[0067] Various layers of the light emitting device are strained
during growth by having a predetermined density of defects. In some
embodiments, the strained AlN layer has a defect density (e.g.,
dislocation density) between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2, the strained Al.sub.xGa.sub.1-xN layer
has a defect density between about 1.times.10.sup.8 cm.sup.-2 and
2'10.sup.10 cm.sup.-2, and the strained GaN layer has a defect
density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2. In some cases, the light emitting
stack has a defect density between about 1.times.10.sup.8 cm.sup.-2
and 2.times.10.sup.10 cm.sup.-2.
[0068] In some embodiments, the dislocation density of the strained
GaN layer is less than those of the strained AlGaN and AlN layers.
The dislocation density of the strained AlGaN layer may be less
than the dislocation density of the AlN layer. In some situations,
the addition of a new material during the growth of the buffer
layer is accompanied by a release of strain for the first 10-150
monolayers of the layer.
[0069] In some cases, the buffer layer has a dislocation density
between about 1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10
cm.sup.-2, which facilitates in the formation of V-defects (or
V-pits) in the GaN layer and the LED layers. In such cases,
straining the buffer layer--including AlN, Al.sub.xGa.sub.1-xN and
GaN layers of the buffer layer--by maintaining a density of
dislocations facilitates the formation of V-defects in the buffer
layer and the LED layers. By selecting one or more growth
conditions, the size of V-defect can be controlled. Furthermore,
the active region, where the light is generated, can be grown
selectively only at the areas between V-defects. This is an
effective way to grow high-efficiency LED materials. The selective
growth of the active layer, thus, tolerates the existing of
dislocations which, then, is utilized to engineer the stress of the
overall grown layers.
[0070] In some embodiment, the light emitting device includes
additional layers. In some cases, the light emitting device
includes an electron blocking layer between the active layer and
the p-GaN layer. In some embodiment, the light emitting device
include a first electrode in electrical communication with the
n-GaN layer and a second electrode in electrical communication with
the p-GaN layer. The light emitting device may include a layer of
an optically reflective material (also "optical reflector" herein)
adjacent to the p-GaN layer. The layer of the optically reflective
material may be formed of one or more of silver, platinum, gold and
nickel, rhodium and indium.
[0071] FIG. 2 shows an LED 200, in accordance with an embodiment.
The LED 200 comprises a first substrate 205, an AlN layer 210
adjacent to the first substrate 205, an AlGaN layer 215 adjacent to
the AlN layer 210, a GaN layer 220 adjacent to the AlGaN layer 215,
an n-type GaN ("n-GaN") layer 225 adjacent to the GaN layer 220, an
active layer 230 adjacent to the n-GaN layer 225, an electron
blocking (e.g., AlGaN) layer 235 adjacent to the active layer 230,
and a p-type GaN ("p-GaN") layer 240 adjacent to the electron
blocking layer 235.
[0072] The GaN layer 220 may be formed of u-GaN (i.e., undoped or
unintentionally doped GaN). The AlN layer 210, AlGaN layer 215 and
GaN layer 220, in some cases, at least partly define a buffer layer
of the LED 200. The n-GaN layer 225, active layer 230, and p-GaN
layer 240 define a light emitting stack 245 of the LED 200. The
light emitting sack 245 may include other layers, such as the
electron blocking layer 235. The electron blocking layer 235 is
configured to minimize the recombination of electrons with holes in
the p-GaN layer 240.
[0073] The first substrate 205 may be formed of silicon. In some
situations, the LED 200 includes a second substrate 250 (Substrate
2) adjacent to the p-GaN layer 240. In such a case, the first
substrate 205 may be precluded. The second substrate 250 may be
included in the final LED 200.
[0074] In some embodiments, the AlN layer 210, AlGaN layer 215 and
the GaN layer 220 are strained layers. In some cases, the AlN layer
210 is under tensile strain, the AlGaN layer 215 is under
compressive strain and the GaN layer 220 is under compressive
strain.
[0075] The AlGaN layer 215 may have an aluminum and gallium
composition selected to effect desirable (or predetermined) device
properties. In some cases, the aluminum and gallium composition is
selected to generate strain in the AlGaN layer 215. The AlGaN layer
215 may have the formula Al.sub.xGa.sub.1-xN, wherein `x` is a
number between 0 and 1. In some situations, the AlGaN layer 215 is
compositionally graded in aluminum and gallium. In an example, at
the interface between the AlN layer 210 and the AlGaN layer 215,
the aluminum content of the AlGaN layer 215 is greater than the
gallium content (i.e., x>1-x), and at the interface between the
AlGaN layer 215 and the GaN layer 220, the gallium content of the
AlGaN layer 215 is greater than the aluminum content (i.e.,
1-x>x). In another example, at the interface between the AlN
layer 210 and the AlGaN layer 215, the aluminum content of the
AlGaN layer 215 is less than the gallium content (i.e., x<1-x),
and at the interface between the AlGaN layer 215 and the GaN layer
220, the gallium content of the AlGaN layer 215 is greater than the
aluminum content (i.e., 1-x>x).
[0076] In some embodiments, the AlN layer 210 has a defect density
between about 1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10
cm.sup.-2, the AlGaN layer 215 has a defect density between about
1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2, and the
GaN layer 220 has a defect density between about 1.times.10.sup.8
cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2. In some cases, the light
emitting stack 245 has a defect density between about
1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2.
[0077] The LED 200 may include a first electrode in electrical
communication with the n-GaN layer 225 and a second electrode in
electrical communication with the p-GaN layer 240. In some cases,
the first electrode is in electrical contact with the n-GaN layer
225. The second electrode may be in electrical contact with the
p-GaN layer 240.
[0078] In some cases, The LED 200 includes a layer of an optically
reflective material adjacent to the p-GaN layer. In an example, the
Led 200 includes layer of an optically reflective material (e.g.,
silver) between the p-GaN layer 240 and the second substrate
250.
Methods for Forming Light Emitting Devices
[0079] Another aspect of the invention provides methods for forming
light emitting devices, such as light emitting diodes. In some
embodiments, methods for forming a light emitting device comprise
forming a barrier layer adjacent to a substrate, the barrier layer
including i) an aluminum nitride (AlN) layer adjacent to the
silicon substrate, ii) an aluminum gallium nitride layer adjacent
to the AlN layer, and iii) a gallium nitride (GaN) layer adjacent
to the aluminum gallium nitride layer. In some embodiments, during
the formation of the barrier layer, one or more process parameters
are selected such that an individual layer of the barrier layer has
a tensile strain or compressive strain that is nonzero with
increasing thickness of the layer. The tensile strain and
compressive strain in the barrier layer can be adjusted such that
the barrier layer has a net compressive strain at a growth
temperature.
[0080] The strain (compressive or tensile) in device layers (e.g.,
AlN layer, aluminum gallium nitride layer, GaN layer) of the light
emitting device may be at least partially dependent on the defect
density in the device layers. In some embodiments, during the
formation of the barrier layer, one or more process parameters are
selected such that an individual layer of the barrier layer has a
predetermined concentration of defects (e.g., dislocations). In
some situations process parameters are selected such that an
individual layer of the barrier layer has a defect density between
about 1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10
cm.sup.-2.
[0081] In some embodiments, the substrate is formed of a material
including silicon, germanium, silicon oxide, silicon dioxide,
titanium oxide, titanium dioxide, sapphire, silicon carbide (SiC),
a ceramic material and a metallic material. In some
implementations, the substrate is formed of silicon.
[0082] Process parameters (or growth conditions) are adjustable
based upon the selection of one or more process parameters for
forming a light emitting device. Growth conditions may include
growth temperature, carrier gas flow rate, precursor flow rate,
growth rate, reaction chamber pressure and susceptor (or platten)
rotation rate.
[0083] In some embodiments, one or more layers of a light emitting
device are formed at a growth temperature between about 750.degree.
C. and 1200.degree. C., or between about 900.degree. C. and
1100.degree. C. Individual layers may be formed at growth
temperatures selected to effect a predetermined defect density.
[0084] In some cases, during the formation of one or more of the
GaN layer, aluminum gallium nitride layer and the AlN layer,
processing conditions are selected to generate defects in one or
more of the GaN layer, aluminum gallium nitride layer and the AlN
layer. In some cases, during the formation of the GaN layer,
aluminum gallium nitride layer and the AlN layer, processing
conditions are selected to generate defects in the GaN layer,
aluminum gallium nitride layer and the AlN layer. The defects aid
in maintaining a predetermined level of strain in the layers at the
growth temperature.
[0085] In an embodiment, the AlN layer is formed under growth
conditions selected to generate tensile strain in the AlN layer. In
another embodiment, the aluminum gallium nitride layer is formed
under growth conditions selected to generate compressive strain in
the aluminum gallium nitride layer. In another embodiment, the GaN
layer is formed under growth conditions selected to generate
compressive strain in the GaN layer.
[0086] In some embodiments, various device layers, such as a buffer
layer, are under tensile strain or compressive strain by virtue of
defects (e.g., dislocations). Process conditions are selected to
form a layer having a predetermined defect density. In an example,
an AlN layer is formed under process conditions selected such that
the AlN layer is under tensile strain due at least in part to
defects in the AlN layer. The AlN layer in some cases is under
tensile strain at a growth temperature that is elevated with
respect to the tensile strain it exhibits at room temperature. The
density of defects is selected to generate a predetermined level of
tensile strain. In some cases, the defect density is between about
1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2. In
other examples, an aluminum gallium nitride layer and GaN layer are
formed under process conditions selected such that the aluminum
gallium nitride and GaN layers are under compressive strain due at
least in part to defects in the aluminum gallium nitride and GaN
layers. The aluminum gallium nitride and GaN layers in some cases
are under compressive strain at a growth temperature that is
elevated with respect to room temperature. The density of defects
is selected to generate a predetermined level of compressive
strain. In some cases, the defect density is between about
1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2. In
other examples, process conditions are selected such that a buffer
layer having AlN, aluminum gallium nitride and GaN layers is under
compressive strain at a growth temperature, due at least in part to
defects in the buffer layer. In some situations, the defect density
in the buffer layer (including the individual layers) is between
about 1.times.10.sup.8 cm.sup.-2 and 2.times.10.sup.10
cm.sup.-2.
[0087] Various source gases (or precursors) may be used with
methods described herein. A gallium precursor may include
trimethylgallium (TMG), triethylgallium, diethylgallium chloride
and coordinated gallium hydride compounds (e.g., dimethylgallium
hydride). An aluminum precursor may include tri-isobutyl aluminum
(TIBAL), trimethyl aluminum (TMA), triethyl aluminum (TEA), and
dimethylaluminum hydride (DMAH). An indium precursor may include
trimethyl indium (TMI) and triethyl indium (TEI). A nitrogen
precursor may include ammonia (NH.sub.3), nitrogen (N.sub.2), and
plasma-excited species of ammonia and/or N.sub.2. A p-type dopant
precursor may be selected from a boron precursor (e.g.,
B.sub.2H.sub.6), a magnesium precursor (e.g., biscyclopentadienyl
magnesium), an aluminum precursor, to name a few examples. An
n-type precursor may be selected from a silicon precursor (e.g,
SiH.sub.4), a germanium precursor (e.g., tetramethylgermanium,
tetraethylgermanium, dimethyl amino germanium tetrachloride,
isobutylgermane) and a phosphorous precursor (e.g., PH.sub.3), to
name a few examples.
[0088] FIG. 3 shows a method 300 for forming a light emitting
device, in accordance with an embodiment. In operation 305, a
substrate is provided in a reaction chamber. The reaction chamber
may be a vacuum chamber configured for thin film formation, such as
with the aid of chemical vapor deposition (e.g., metal organic
chemical vapor deposition, or MOCVD) or atomic layer deposition
(ALD).
[0089] Next, in operation 310, an aluminum nitride (AlN) layer is
formed adjacent to the substrate. The AlN layer is formed by
heating the substrate to a growth temperature ranging between about
750.degree. C. and 1200.degree. C. in a reaction chamber with
aluminum precursor and nitrogen precursor gas. In one embodiment,
the growth temperature is set to be between about 900.degree. C.
and 1100.degree. C. The aluminum precursor and the nitrogen
precursor may be supplied into the reaction chamber with the aid of
a carrier gas. The carrier gas may include hydrogen (H.sub.2),
argon, neon, and helium. In some embodiments, the reaction chamber
includes both aluminum precursor and nitrogen precursor gas at the
same time so that the substrate is exposed to the aluminum
precursor and the nitrogen precursor simultaneously. In other
embodiments, aluminum precursor gas and nitrogen precursor gas are
provided into the reaction chamber in an alternating fashion so
that the substrate is exposed to the aluminum precursor and the
nitrogen precursor in an alternating fashion.
[0090] In some situations, during the formation of the AlN layer,
one or more process parameters are selected such that the AlN layer
as formed has a thickness selected to maintain tensile strain in
the AlN layer at the growth temperature. In an example, the
hydrogen flow rate and the one or both of the aluminum and nitrogen
precursor flow rates are selected such that the AlN layer has a
finite tensile strain at the growth temperature. The AlN layer in
such a case has a predetermined defect density. In an example, the
AlN layer has a defect density between about 1.times.10.sup.8
cm.sup.-2 and 2.times.10.sup.10 cm.sup.-2.
[0091] Next, in operation 315, with the substrate at the growth
temperature, a first aluminum gallium nitride layer is formed
adjacent to the AlN layer, the first aluminum gallium nitride layer
having the composition Al.sub.xGa.sub.1-xN, wherein `x` is a number
between 0 and 1. The first aluminum gallium nitride layer is formed
by exposing the AlN layer to an aluminum precursor (e.g., TMA), a
gallium precursor (e.g., TMG) and a nitrogen precursor (e.g.,
NH.sub.3). The partial pressure and flow rate of each of the
precursors is selected to provide a desirable aluminum and gallium
content. In some cases, the first aluminum gallium nitride layer is
compositionally graded in aluminum and gallium (i.e., the aluminum
and gallium content of the first aluminum gallium nitride layer
varies along the direction of growth). In some situations, process
parameters (e.g., carrier gas flow rate, precursor flow rates) are
selected such that the first aluminum gallium nitride layer has a
net compressive strain at the growth temperature. Without a proper
selection of the growth conditions, the AlGaN layer can relax
quickly and the overall stress of the grown layers may level out.
Conventionally, relaxed layers may be desirable because new layers
grown on such relaxed layers are free of strain and may be of
higher crystal quality. However, a layer free of compressive stress
(or strain) at a growth temperature may not be desirable upon
cool-down to room temperature. In some cases, layers that are
otherwise free of compressive strain at a growth temperature have
strain (e.g., tensile strain) at or near room temperature, leading
to bowing and in some cases cracking
[0092] Next, in operation 320, with the substrate at the growth
temperature, a second aluminum gallium nitride layer is formed
adjacent to the first aluminum gallium nitride layer, the second
aluminum gallium nitride layer having the composition
Al.sub.yGa.sub.1-yN, wherein `y` is a number between 0 and 1. The
second aluminum gallium nitride layer is formed by exposing the
first aluminum gallium nitride layer to an aluminum precursor, a
gallium precursor and a nitrogen precursor. The partial pressure
and flow rate of each of the precursors is selected to provide a
desirable aluminum and gallium content. In some cases, the second
aluminum gallium nitride layer is compositionally graded in
aluminum and gallium (i.e., the aluminum and gallium content of the
first aluminum gallium nitride layer varies along the direction of
growth). In some situations, process parameters (e.g., carrier gas
flow rate, precursor flow rates) are selected such that the second
aluminum gallium nitride layer has a net compressive strain at the
growth temperature.
[0093] Next, in operation 325, with the substrate at the growth
temperature, a gallium nitride (GaN) layer is formed adjacent to
the second aluminum gallium nitride layer. The GaN layer is formed
by supplying into the reaction chamber a gallium precursor (e.g.,
TMG) and a nitrogen precursor (e.g., NH.sub.3), and exposing the
second aluminum gallium nitride layer to the gallium precursor and
the nitrogen precursor. In some situations, process parameters
(e.g., carrier gas flow rate, precursor flow rates) are selected
such that the gallium nitride layer has a net compressive strain at
the growth temperature.
[0094] In some cases, the second aluminum gallium nitride layer is
precluded. In such cases, the GaN layer is formed adjacent to the
first aluminum gallium nitride layer.
[0095] Next, in operation 330, a device stack is formed adjacent to
the GaN layer. In some cases, the device stack includes an n-type
gallium nitride (n-GaN) layer adjacent to the GaN layer formed in
operation 325, an active layer adjacent to the n-GaN layer, and a
p-type gallium nitride (p-GaN) layer adjacent to the active layer.
In some embodiments, the GaN layer is exposed to a gallium
precursor (e.g., TMG), a nitrogen precursor (e.g., NH.sub.3) and a
precursor of an n-type dopant (e.g., silane) to form the n-GaN
layer. The n-GaN layer in some cases is formed at a growth
temperature ranging between about 750.degree. C. and 1100.degree.
C. In some embodiments, the growth temperature ranges between about
800.degree. C. and 1050.degree. C. In other embodiments, the growth
temperature ranges between about 850.degree. C. and 1000.degree.
C.
[0096] The active layer is then formed adjacent to the n-GaN layer.
In some cases, the active layer is formed of one or more well
layers (e.g., indium gallium nitride, aluminum gallium nitride,
aluminum indium gallium nitride) and one or more barrier layers
(e.g., gallium nitride) layers, with the well layers and barrier
layers distributed in an alternating configuration. For instance,
with the well layer formed of indium gallium nitride, the well
layer is formed by supplying an indium precursor (e.g., TMI), a
gallium precursor (e.g., TMG) and a nitrogen precursor (e.g.,
NH.sub.3) into the reaction chamber. As another example, a well
layer having aluminum gallium nitride is formed by supplying an
aluminum precursor (e.g., TMA), a gallium precursor (e.g., TMG) and
a nitrogen precursor (e.g., NH.sub.3) into the reaction
chamber.
[0097] One or a plurality of well layers may be separated with
barrier layers, such as barrier layers having gallium nitride. In
an example, a gallium nitride barrier layer is formed by supplying
into the reaction chamber a gallium precursor and a nitrogen
precursor. The active layer is formed to have a predetermined
period of well-barrier stacks. In an example, the active layer has
1, or 2, or 3, or 4, or 5, or 6, or 7, or 8, or 9, or 10, or more
periods of well-barrier stacks. In an example, the active layer is
a multiple quantum well (MQW) active layer having, for example, 10
or more periods.
[0098] In some cases, the quantum well ("well") layer is formed at
temperatures ranging between about 750.degree. C. and 790.degree.
C. In some embodiments, the well is formed at temperatures ranging
between about 770.degree. C. and 780.degree. C. The barrier layer
may be formed at temperatures ranging between about 790.degree. C.
and 850.degree. C. In some embodiments, the barrier layer is formed
at temperatures ranging between about 810.degree. C. and
840.degree. C.
[0099] The p-GaN layer is then formed adjacent to the active layer.
In some cases, the p-GaN layer is formed by supplying a gallium
precursor (e.g., TMG), a nitrogen precursor (e.g., NH.sub.3) and a
precursor of a p-type dopant (e.g., biscyclopentadienyl magnesium,
or Cp2Mg) into the reaction chamber. The p-GaN layer in some cases
is formed at a temperature ranging between about 700.degree. C. and
1100.degree. C. In some embodiments, the temperature ranges between
about 800.degree. C. and 1050.degree. C., while in other
embodiments the temperature ranges between about 850.degree. C. and
1000.degree. C.
[0100] Next, a second substrate may be provided adjacent to the
p-GaN layer. The second substrate may be a silicon substrate. In
some cases, one or more intervening layers are formed prior to
providing the second substrate. The substrate adjacent to the AlN
layer may then be removed to expose the AlN layer.
[0101] In some embodiments, a first electrode is formed that is in
electrical communication with the n-GaN layer and a second
electrode is formed that is in electrical communication with the
p-GaN layer. In other embodiments, the first electrode, as formed,
is in contact with the n-GaN layer and the second electrode, as
formed, is in contact with the second substrate (adjacent to the
p-GaN layer). The first electrode may include one or more elemental
metals such as titanium, aluminum, nickel, platinum, gold, silver,
rhodium, copper, chromium, or combinations thereof. The second
electrode may include one or more elemental metals such as
aluminum, titanium, chromium, platinum, nickel, gold, rhodium,
silver, or combinations, thereof.
[0102] The light emitting device formed according to the method 300
may have reduced strain at room temperature. In some cases, the
formation of the buffer layer, per operations 305-325, provides a
compressive strain that balances the tensile strain in the buffer
layer, thereby reducing bowing and in some cases crack formation in
the buffer layer and/or the device stack at room temperature.
[0103] FIG. 4 schematically illustrates the strain and accumulated
stress on a light emitting device at various stages of growth of a
buffer layer over a silicon substrate of a light emitting device,
in accordance with an embodiment. The y-axis schematically
illustrates the strain and accumulative stress in the buffer layer
at various stages of growth of the buffer layer. The shaded
rectangles (top) show the relative strain in each layer, and the
layer schematics (bottom) show the degree of bowing of the buffer
layer at various stages of growth. The x-axis shows film thickness.
The buffer layer, which is formed on a silicon substrate, includes
an aluminum nitride (AlN) layer adjacent to the silicon substrate,
a first aluminum gallium nitride (Al.sub.xGa.sub.1-xN) layer
adjacent to the AlN layer, a second aluminum gallium nitride
(Al.sub.yGa.sub.1-yN) layer adjacent to the first aluminum gallium
nitride layer, and a gallium nitride layer adjacent to the second
aluminum gallium nitride layer. Upon the formation of each layer,
the buffer layer of the light emitting device is strained by
selecting one or more process parameters to effect strain in the
layer--that is, each layer is formed to have a predetermined level
of strain.
[0104] In some embodiments, the AlN is provided to aid in the
formation of the gallium-containing layers. AlN may minimize or
eliminate the formation of a gallium-silicon alloy adjacent to the
silicon substrate.
[0105] In some cases, the buffer layer is formed at a growth
temperature. In other cases, the various layers of the buffer layer
are formed at the same growth temperature or different growth
temperatures.
[0106] With continued reference to FIG. 4, the AlN layer is formed
such that the buffer layer is under tensile strain. The light
emitting device following the formation of the AlN layer bows (or
is concave). The Al.sub.xGa.sub.1-xN layer is formed on the AlN
layer under process conditions selected such that the tensile
strain in the buffer layer is balanced by compressive strain in the
Al.sub.xGa.sub.1-xN layer. The light emitting device in such a case
is under minimal strain at the growth temperature. The
Al.sub.yGa.sub.1-yN layer is formed on the Al.sub.xGa.sub.1-xN
layer under process conditions selected such that the
Al.sub.yGa.sub.1-yN layer is under compressive strain. The light
emitting device is under compressive strain. The light emitting
device in such a case is convex--the compressive strain in the
buffer layer is greater than the tensile strain. The GaN layer is
formed on the Al.sub.xGa.sub.1-xN layer under process conditions
selected such that the GaN layer is under compressive strain. In
some embodiments, each layer of the buffer layer is formed to have
a defect density between about 1.times.10.sup.8 cm .sup.-2 and
2.times.10.sup.10 cm.sup.-2.
[0107] Following the formation of the GaN layer, a light emitting
diode device stack ("LED device stack") is formed. The LED device
stack is configured to generate light upon the recombination of
electrons and holes. The device stack comprises an n-GaN layer, a
p-GaN layer and an active layer between the n-GaN layer and the
p-GaN. The device stack in some cases is formed to have a defect
density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.9 cm.sup.-2.
[0108] During the formation of the AlN layer, the buffer layer has
a negative strain. During the formation of subsequent layers, the
strain in the buffer layer increases. The slope of the plot of FIG.
4 (strain divided by thickness) is nearly or substantially
constant. In some embodiments, the strain of the buffer layer at
various stages of growth, when divided by thickness, is nearly or
substantially constant.
[0109] With continued reference to FIG. 4, in some situations,
process conditions are selected such that the thickness of various
layers of the buffer layer and the light emitting device are within
a predetermined limit. In some embodiments, during the formation of
the light emitting diode, process conditions are selected such that
the light emitting diode, as formed, has a thickness that is less
than or equal to about 5 .mu.m, or less than or equal to about 4
.mu.m, or less than or equal to about 3 micrometers (".mu.m"). In
some embodiments, during the formation of the AlN layer, process
conditions are selected such that a thickness of the AlN layer, as
formed, is less than or equal to about 1 .mu.m. In some
embodiments, the thickness of the AlN layer is less than or equal
to about 0.5 .mu.m, while in other embodiments the thickness of the
AlN layer is than or equal to about 0.3 .mu.m. In some embodiments,
during the formation of the Al.sub.xGa.sub.1-xN and
Al.sub.yGa.sub.1-yN layers, process conditions are selected such
that a combined thickness of the Al.sub.xGa.sub.1-xN and
Al.sub.yGa.sub.1-yN layers, as formed, is less than or equal to
about 1 .mu.m. In other embodiments, the combined thickness is less
than or equal to about 0.8 .mu.m, while in other embodiments the
combined thickness is less than or equal to about 0.7 .mu.m. In
some embodiments, during the formation of the GaN layer, process
conditions are selected such that a thickness of the GaN layer is
less than or equal to about 4 .mu.m. In other embodiments, the
thickness of the GaN layer is less than or equal to about 3 .mu.m,
while in other embodiments the thickness of the GaN layer is less
than or equal to about 2.5 .mu.m. In some embodiments, during the
formation of the buffer layer, process conditions are selected such
that a thickness of the buffer layer, as formed, is less than or
equal to about 5 .mu.m. In other embodiments, the thickness of the
buffer layer is less than or equal to about 4 .mu.m, while in other
embodiments the thickness of the buffer layer is less than or equal
to about 3 .mu.m. Process conditions, which are used to control
these thicknesses, include one or more of growth temperature,
precursor flow rate, carrier gas (e.g., H.sub.2 gas) flow rate,
reaction chamber pressure, growth rate and susceptor (or platten)
rotation rate.
[0110] With continued reference to FIG. 4, each layer may have a
different amount of strain. In some cases, however, during the
formation of an individual layer, the strain in the individual
layer as a function of the thickness of the individual layer is
constant.
[0111] FIG. 5 shows a method for forming a buffer layer, in
accordance with an embodiment. The buffer layer is part of a light
emitting device, which may be a nascent light emitting device.
Initially, an AlN layer is formed on a substrate under process
conditions selected such that the AlN layer, as formed, has a
predetermined level of strain. The strain in some cases is tensile
strain. In an embodiment, the AlN layer is formed to have a defect
density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2. The AlN layer in such a case is under
tensile strain at the growth temperature; the nascent light
emitting device, comprising the AlN layer and the substrate, bows
(or is concave). In some situations, the substrate is a
silicon-containing substrate, such as a substrate having a
predominantly silicon content (e.g., Si(111)).
[0112] Next, an aluminum gallium nitride layer is formed on the AN
layer under process conditions selected such that the aluminum
gallium nitride layer, as formed, has a compressive strain that
balances the tensile strain in the nascent light emitting device.
In some cases, the aluminum gallium nitride layer is formed to have
a defect density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2. At the growth temperature, the nascent
light emitting in such a case does not bow and is thus neither
concave nor convex.
[0113] Next, a GaN layer is formed on the aluminum gallium nitride
layer under process conditions selected such that the GaN layer, as
formed, has a compressive strain. The nascent light emitting device
in such a case has a net compressive strain at the growth
temperature. In some cases, the GaN layer is formed to have a
defect density ranging between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2. The light emitting device in such a
case is convex. Following cool-down to room temperature, the
nascent light emitting device has little to no net strain (i.e.,
the compressive strain balances the tensile strain).
[0114] At the growth temperature, additional layers may be formed
on the buffer layer. In an example, a light emitting stack is
formed on the GaN layer, the light emitting stack having an n-GaN
layer, a p-GaN layer and an active layer between the n-GaN layer
and the p-GaN layer.
[0115] In some embodiments, during the formation of various device
layers, the substrate is exposed to two or more precursor
simultaneously. In other situations, during the formation of
various device layers, the substrate is exposed to the various
precursors an alternating and sequential fashion. In an example, a
gallium nitride layer is formed by exposing a substrate to a
gallium precursor (e.g., TMG) and followed by a nitrogen precursor
(e.g., NH.sub.3), with an intervening purging or evacuation
operation. Generally, if a plurality of precursor are required to
form a device layer, the precursor may be supplied into the
reaction chamber simultaneously or in an alternating and sequential
fashion.
[0116] Device layers may be formed using various deposition
techniques. In some embodiments, device layers are formed using
chemical vapor deposition (CVD), atomic layer deposition (ALD),
plasma enhanced CVD (PECVD), plasma enhanced ALD (PEALD), metal
organic CVD (MOCVD), hot wire CVD (HWCVD), initiated CVD (iCVD),
modified CVD (MCVD), vapor axial deposition (VAD), outside vapor
deposition (OVD), physical vapor deposition (e.g., sputter
deposition, evaporative deposition).
[0117] While methods and structures provided herein have been
described in the context of light emitting devices having Group
III-V semiconductor materials, such as, for example, gallium
nitride, such methods and structures may be applied to other types
of semiconductor materials. Methods and structures provided herein
may be used with light emitting devices formed at least in part of
gallium nitride (GaN), gallium arsenide (GaAs), aluminum gallium
arsenide (AlGaAs), gallium arsenide phosphide (GaAsP), aluminum
gallium indium phosphide (AlGaInP), gallium phosphide (GaP), indium
gallium nitride (InGaN), aluminum gallium phosphide (AlGaP), zinc
selenide (ZnSe), aluminum nitride (AlN), aluminum gallium nitride
(AlGaN), and aluminum gallium indium nitride (AlGaInN).
Systems Configured to Form Light Emitting Devices
[0118] In another aspect of the invention, a system for forming a
light emitting device comprises a reaction chamber for holding a
substrate, a pumping system in fluid communication with the
reaction chamber, the pumping system configured to purge or
evacuate the reaction chamber, and a computer system having a
processor for executing machine readable code implementing a method
for forming the light emitting device. The code may implement any
of the methods provided herein. In an embodiment, the code
implements a method comprising forming a plurality of layers
adjacent to a silicon substrate, the plurality of layers including
i) an aluminum nitride layer adjacent to the silicon substrate, ii)
an aluminum gallium nitride layer adjacent to the aluminum nitride
layer and iii) a gallium nitride layer adjacent to the aluminum
gallium nitride layer. During the formation of each of the
plurality of layers, one or more process parameters are selected
such that an individual layer of the plurality of layers has a
tensile strain or compressive strain that is nonzero with
increasing thickness of the individual layer. In another
embodiment, the code implements a method comprising (a) providing a
substrate in a reaction chamber, (b) forming an aluminum nitride
(AlN) layer adjacent to the substrate under processing conditions
selected to generate defects (e.g., dislocations) in the AlN layer,
(c) forming an aluminum gallium nitride layer adjacent to the AlN
layer under processing conditions selected to generate (or form)
defects in the aluminum gallium nitride layer, and (d) forming a
gallium nitride (GaN) layer adjacent to the aluminum gallium
nitride layer under processing conditions selected to generate
defects in the GaN layer. The defects induce strain (i.e.,
compressive strain or tensile strain) in each of the layers. In
some embodiments, processing conditions are selected to generate
and maintain a predetermined density of defects, such as, e.g., a
defect density between about 1.times.10.sup.8 cm.sup.-2 and
2.times.10.sup.10 cm.sup.-2.
[0119] FIG. 6 shows a system 600 for forming a light emitting
device, in accordance with an embodiment. The system 600 includes a
reaction chamber 605 having a susceptor (or substrate holder) 610
configured to hold a substrate that is used to form the light
emitting device. The system comprises a first precursor storage
vessel (or tank) 615, a second precursor storage vessel 620, and a
carrier gas storage tank 625. The first precursor storage vessel
615 may be for holding a Group III precursor (e.g., TMG) and the
second precursor storage vessel 620 may be for holding a Group V
precursor (e.g., NH.sub.3). The carrier gas storage tank 625 is for
holding a carrier gas (e.g., H.sub.2). The system 600 may include
other storage tanks or vessels, such as for holding additional
precursors and carrier gases. The system 600 includes valves
between the storage vessels and the reaction chamber 605 for
fluidically isolating the reaction chamber 605 from each of the
storage vessels.
[0120] The system 600 further includes a vacuum system 630 for
providing a vacuum to the reaction chamber 605. The vacuum system
630 is in fluid communication with the reaction chamber 605. In
some cases, the vacuum system 630 is configured to be isolated from
the reaction pace 605 with the aid of a valve, such as a gate
valve.
[0121] A controller (or control system) 635 of the system 600
facilitates a method for forming a light emitting device in the
reaction chamber 605, such as forming one or more layers of the
light emitting device. The controller 635 is communicatively
coupled to a valve of each of the first precursor storage vessel
615, the second precursor storage vessel 620, the carrier gas
storage tank 625 and the vacuum system 630. The controller 635 is
operatively coupled to the susceptor 610 for regulating the
temperature of the susceptor and a substrate on the susceptor, and
the vacuum system 630 for regulating the pressure in the reaction
chamber 605.
[0122] In some situations, the vacuum system 630 includes one or
more of a turbomolecular ("turbo") pump, a diffusion pump and a
mechanical pump. In some cases, the vacuum system 630 includes a
turbo pump, diffusion pump and/or mechanical pump. A pump may
include one or more backing pumps. For example, a turbo pump may be
backed by a mechanical pump.
[0123] In some embodiments, the controller 635 is configured to
regulate one or more processing parameters, such as the substrate
temperature, precursor flow rates, growth rate, carrier gas flow
rate and reaction chamber pressure. The controller 635, in some
cases, is in communication with valves between the storage vessels
and the reaction chamber 605, which aids in terminating (or
regulating) the flow of a precursor to the reaction chamber 605.
The controller 635 includes a processor configured to aid in
executing machine-executable code that is configured to implement
the methods provided herein. The machine-executable code is stored
on a physical storage medium, such as flash memory, a hard disk, or
other physical storage medium configured to store
computer-executable code.
[0124] In some embodiments, the controller 635 is configured to
regulate one or more process parameters. In some situations, the
controller 635 regulates the growth temperature, carrier gas flow
rate, precursor flow rate, growth rate and/or growth pressure (or
reaction chamber pressure).
[0125] In some situations, the controller 635 is configured to
regulate process parameters such that one or more layers of a light
emitting device are strained. For instance, the controller 635
regulates one or more of the growth temperature, the precursor flow
rate the carrier gas flow rate, reaction chamber pressure, and
growth rate to generate a predetermined level of strain in one or
more layers of a buffer layer of a nascent or completed light
emitting device.
[0126] In some embodiments, the system 600 includes various surface
or bulk analytical instruments (spectroscopies) for qualitatively
and/or quantitatively analyzing a substrate and various layers
formed over the substrate. In some cases, the system includes a
deflectometer for measuring the curvature of the substrate or a
thin film formed on the substrate. The curvature in some cases is
related to the stress in the substrate or the thin film (e.g., a
thin film under stress is concave or convex).
EXAMPLE
[0127] A silicon substrate is provided on a susceptor in a reaction
chamber and a dislocation density maintaining buffer layer is
formed on the silicon substrate. The dislocation density
maintaining buffer layer includes an aluminum nitride layer, an
aluminum gallium nitride adjacent to the AlN layer, and a gallium
nitride layer adjacent to the aluminum gallium nitride layer.
[0128] With the susceptor at a temperature of about 850.degree. C.,
the buffer layer is formed by exposing the silicon substrate to TMA
and NH.sub.3 to form the AlN layer on the silicon substrate. The
AlN layer has a thickness of about 0.4 micrometer (".mu.m"). Next,
with the susceptor at a temperature of about 850.degree. C., the
AlN layer is exposed to TMA, TMG and NH.sub.3 to form an aluminum
gallium nitride layer on the AlN layer. The aluminum gallium
nitride has a thickness of about 0.7 .mu.m. Next, with the
susceptor at a temperature of about 850.degree. C., the aluminum
gallium nitride layer is exposed to TMG and NH.sub.3 to form a GaN
layer at a thickness of about 2.5 .mu.m. At the growth temperature,
the substrate has a radius of curvature (absolute value) of about 5
m. Upon cool down to room temperature, the substrate has a radius
of curvature (absolute value) greater than 50 m.
[0129] Unless the context clearly requires otherwise, throughout
the description and the claims, words using the singular or plural
number also include the plural or singular number respectively.
Additionally, the words `herein,` `hereunder,` `above,` `below,`
and words of similar import refer to this application as a whole
and not to any particular portions of this application. When the
word `or` is used in reference to a list of two or more items, that
word covers all of the following interpretations of the word: any
of the items in the list, all of the items in the list and any
combination of the items in the list.
[0130] It should be understood from the foregoing that, while
particular implementations have been illustrated and described,
various modifications may be made thereto and are contemplated
herein. It is also not intended that the invention be limited by
the specific examples provided within the specification. While the
invention has been described with reference to the aforementioned
specification, the descriptions and illustrations of embodiments of
the invention herein are not meant to be construed in a limiting
sense. Furthermore, it shall be understood that all aspects of the
invention are not limited to the specific depictions,
configurations or relative proportions set forth herein which
depend upon a variety of conditions and variables. Various
modifications in form and detail of the embodiments of the
invention will be apparent to a person skilled in the art. It is
therefore contemplated that the invention shall also cover any such
modifications, variations and equivalents.
* * * * *