U.S. patent number RE47,767 [Application Number 14/757,799] was granted by the patent office on 2019-12-17 for group iii-nitride layers with patterned surfaces.
This patent grant is currently assigned to NOKIA OF AMERICA CORPORATION. The grantee listed for this patent is Alcatel-Lucent. Invention is credited to Aref Chowdhury, Hock Ng, Richart Elliott Slusher.
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United States Patent |
RE47,767 |
Chowdhury , et al. |
December 17, 2019 |
Group III-nitride layers with patterned surfaces
Abstract
A fabrication method produces a mechanically patterned layer of
group III-nitride. The method includes providing a crystalline
substrate and forming a first layer of a first group III-nitride on
a planar surface of the substrate. The first layer has a single
polarity and also has a pattern of holes or trenches that expose a
portion of the substrate. The method includes then, epitaxially
growing a second layer of a second group III-nitride over the first
layer and the exposed portion of substrate. The first and second
group III-nitrides have different alloy compositions. The method
also includes subjecting the second layer to an aqueous solution of
base to mechanically pattern the second layer.
Inventors: |
Chowdhury; Aref (Berkeley
Heights, NJ), Ng; Hock (Westfield, NJ), Slusher; Richart
Elliott (Lebanon, NJ) |
Applicant: |
Name |
City |
State |
Country |
Type |
Alcatel-Lucent |
Boulogne Billancourt |
N/A |
FR |
|
|
Assignee: |
NOKIA OF AMERICA CORPORATION
(Murray Hill, NJ)
|
Family
ID: |
32869148 |
Appl.
No.: |
14/757,799 |
Filed: |
December 23, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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12315202 |
Dec 6, 2011 |
8070966 |
|
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|
11442032 |
Dec 23, 2008 |
7468578 |
|
|
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11180350 |
Aug 1, 2006 |
7084563 |
|
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|
10397799 |
Jan 17, 2006 |
6986693 |
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Reissue of: |
13248394 |
Sep 29, 2011 |
8613860 |
Dec 24, 2013 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J
3/022 (20130101); H01J 3/022 (20130101); B81B
1/00 (20130101); H01L 21/30612 (20130101); H01J
1/304 (20130101); B82Y 20/00 (20130101); G02B
6/1225 (20130101); B82Y 20/00 (20130101); H01J
9/025 (20130101); B81B 1/00 (20130101); H01J
9/025 (20130101); H01L 21/30612 (20130101); H01J
1/304 (20130101); G02B 6/1225 (20130101); H01L
21/0254 (20130101); G02B 2006/12035 (20130101); H01L
21/02639 (20130101); H01L 21/02639 (20130101) |
Current International
Class: |
B82Y
20/00 (20110101); H01L 21/02 (20060101); H01J
3/02 (20060101); H01L 21/306 (20060101); G02B
6/122 (20060101); H01J 1/304 (20060101); H01J
9/02 (20060101); B81B 1/00 (20060101) |
References Cited
[Referenced By]
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Mar 2004 |
|
WO |
|
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|
Primary Examiner: Lopez; Carlos N
Attorney, Agent or Firm: Mendelsohn Dunleavy, P.C. Gruzdkov;
Yuri
Parent Case Text
This application is a continuation of application Ser. No.
12/315,202, filed Dec. 1, 2008, now U.S. Pat. No. 8,070,966, which
is a divisional of application Ser. No. 11/442,032, filed May 27,
2006, now U.S. Pat. No. 7,468,578, which is a continuation of
application Ser. No. 11/180,350, filed Jul. 13, 2005, now U.S. Pat.
No. 7,084,563, which is a divisional of application Ser. No.
10/397,799, filed Mar. 26, 2003 now U.S. Pat. No. 6,986,693.
Claims
What we claim is:
1. A method of fabricating a structure, comprising: providing a
substrate with a planar surface; providing a crystalline layer of a
group III-nitride over the surface of the substrate; and subjecting
the layer to base to etch a top surface of the layer such that a
plurality of pyramids are formed from nitrogen-polarity first
lateral regions of the layer and pyramids are not formed in an
adjacent second lateral region of the layer, the second lateral
region being located between the first lateral regions; and wherein
the structure has a metallic electrode located on a flat surface of
the second lateral region of the layer.
2. The method of claim 1, wherein the subjecting forms the pyramids
by etching a portion of the top surface of the layer having
metal-polarity.
3. The method of claim 1, wherein the subjecting does not
significantly etch a portion of the top surface of the layer that
has metal-polarity.
4. The method of claim 3, wherein the subjecting forms the pyramids
by etching a portion of the top surface of the layer that has
metal-polarity.
5. The method of claim 1, wherein the group III-nitride comprises
gallium.
6. The method of claim 1, wherein the layer is an epitaxially grown
layer.
7. The method of claim 1, wherein the layer comprises a gallium
nitride layer.
8. The method of claim 1, wherein the pyramids spatially
overlap.
9. The method of claim 1, wherein tips of the pyramids have an
irregular spatial distribution.
10. The method of claim 1, wherein the pyramids are physically
separated from any electrode.
11. The method of claim 1, wherein the electrode is located on a
flat portion of the top surface.
12. The method of claim 1, wherein a portion of the layer of the
group III-nitride is located on the layer of another group
III-nitride alloy.
13. The method of claim 1, wherein the base is aqueous base.
14. The method of claim 13, wherein the subjecting does not
significantly etch a portion of the top surface of the layer that
has metal-polarity.
15. The method of claim 13, wherein the layer is an epitaxially
grown layer.
16. The method of claim 13, wherein the layer comprises gallium
nitride.
17. The method of claim 13, wherein the pyramids spatially
overlap.
18. The method of claim 13, wherein tips of the pyramids have an
irregular spatial distribution.
19. The method of claim 13, further comprising providing a metal
electrode on a portion of a surface of the layer.
20. The method of claim 13, wherein the pyramids are physically
separated from any electrode.
21. The method of claim 13, wherein the electrode is located on a
flat portion of the top surface.
.Iadd.22. An apparatus, comprising: a layer of group III-nitride
having a top surface, the top surface having a plurality of
pyramidal structures of the group III-nitride; a metallic electrode
located directly on a flat surface of the layer between portions of
the layer having the pyramidal structures, the pyramidal structures
of the top surface being physically separated from the electrode;
and a phosphor screen facing the top surface..Iaddend.
.Iadd.23. The apparatus of claim 22, wherein the pyramidal
structures are located on a part of the top surface having
nitrogen-polarity..Iaddend.
.Iadd.24. The apparatus of claim 22, wherein the group III-nitride
comprises gallium..Iaddend.
.Iadd.25. The apparatus of claim 22, wherein the pyramidal
structures spatially overlap..Iaddend.
.Iadd.26. The apparatus of claim 23, wherein the group III-nitride
comprises gallium..Iaddend.
.Iadd.27. The apparatus of claim 23, wherein the pyramidal
structures spatially overlap..Iaddend.
.Iadd.28. The apparatus of claim 22, wherein the pyramidal
structures have a random distribution..Iaddend.
Description
BACKGROUND
1. Field of the Invention
The invention relates to electrical and optical devices that
incorporate crystalline group III-nitrides.
2. Discussion of the Related Art
Crystalline group III-nitride semiconductors are used in both
electrical devices and optical devices.
With respect to electrical devices, group III-nitrides have been
used to make field-emitters. A field-emitter is a conductive
structure with a sharp tip. The sharp tip produces a high electric
field in response to being charged. The high electric field causes
electron emission from the tip. For this reason, an array of field
emitters can operate a phosphor image screen.
One prior art method has fabricated arrays of field-emitters from
group III-nitrides. Group III-nitrides have chemical and mechanical
stability due to the stability of the group III atom-nitrogen bond.
Such stability is very desirable in devices that use an array of
field-emitters.
The prior art method grows the field emitters from group
III-nitrides. The growth method includes epitaxially growing a
gallium nitride (GaN) layer on a sapphire substrate, forming a
SiO.sub.2 mask on the GaN layer, and epitaxially growing pyramidal
GaN field-emitters in circular windows of the mask. While the
growth method produces field-emitters of uniform size, the field
emitters do not have very sharp tips. Sharper tips are desirable to
produce higher electron emission rates and lower turn on
voltages.
With respect to optical devices, group III-nitrides have high
refractive indices. Materials with high refractive indices are
desirable in the manufacture of photonic bandgap structures. For a
fixed photonic bandgap, such materials enable making a photonic
bandgap structure with larger feature dimensions than would be
possible if the structure was made from a lower refractive index
material.
One method for making a planar photonic bandgap structure involves
dry etching a smooth layer of group III-nitride. Unfortunately, the
chemical stability of group III-nitrides causes dry etchants to
have a low selectivity for the group III-nitride over mask
material. For that reason, a dry etch does not produce a deep
surface relief in a layer of group III-nitride. Consequently, the
thy-etch method only produces thin planar photonic bandgap
structures from group III-nitrides.
Unfortunately, light does not efficiently edge couple to thin
planar structures. For this reason, it is desirable to have a
method capable of fabricating a photonic bandgap structure with a
higher surface relief from a group III-nitride.
BRIEF SUMMARY
Herein a mechanically patterned surface has an airily of
deformations therein, e.g., an array of holes, trenches, or
physically rough regions.
Various embodiments provide methods for fabricating layers of group
III-nitride with mechanically patterned surfaces. The patterned
surfaces provide functionalities to the resulting structures. The
fabrication methods exploit the susceptibility of nitrogen-polar
(N-polar) group III-nitride layers to attack by strong bases. The
methods use basic solutions to wet etch a layer of group
III-nitride in a manner that produces a patterned surface.
Exemplary patterned surfaces provide photonic bandgap structures
and field-emitter arrays.
In a first aspect, the invention features a fabrication method. The
method includes providing a crystalline substrate and forming a
first layer of a first group III-nitride on a planar surface of the
substrate. The first layer has a single polarity and also has a
pattern of holes or trenches that expose a portion of the
substrate. The method includes epitaxially growing a second layer
of a second group III-nitride over both the first layer and the
exposed portion of substrate. The first and second group
III-nitrides have different alloy compositions. The method includes
subjecting the second layer to an aqueous solution of base to
mechanically pattern the second layer.
In a second aspect, the invention features an apparatus with a
mechanically patterned surface. The apparatus includes a
crystalline substrate with a planar surface and a plurality of
pyramidal field-emitters located over a portion of the surface. The
apparatus includes a layer of a first group-III nitride, which is
located on another portion of the surface, and a layer of a second
group III-nitride, which is located over the layer of the first
group III-nitride. The layer of the second group III-nitride is
free of pyramidal surface structures. The field-emitters include
the second group III-nitride. The first and second group
III-nitrides have different alloy compositions.
In a third aspect, the invention features an apparatus that
includes a crystalline substrate and a mechanically patterned layer
of a first group III-nitride duct is located on a planar surface of
the substrate. The apparatus also includes a layer of a second
group III-nitride that is located on the mechanically patterned
layer of the first group III-nitride. The layer of second group
III-nitride has a pattern of columnar holes or trenches therein.
The first and second group III-nitrides have different alloy
compositions.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a cross-sectional view of a planar structure for a
field-emitter array of group III-nitride;
FIG. 1B is a cross-sectional view of another planar structure for a
field-emitter array of group III-nitride field-emitters;
FIG. 1C shows a flat panel image display that incorporates the
field-emission array of FIG. 1A or 1B;
FIG. 2 is a cross-sectional view of a structure that incorporates a
layer of group III-nitride that is mechanically periodically
patterned with holes or trenches;
FIG. 2A is top view of one embodiment of photonic bandgap device
that incorporates a structure represented by the structure of FIG.
2;
FIG. 2B is top view of another embodiment of a photonic bandgap
device that incorporates a structure represented by the structure
of FIG. 2;
FIG. 2C is top view of another embodiment of a photonic bandgap
device that incorporates a structure represented by the structure
of FIG. 2;
FIG. 3 is a flow chart illustrating a method for fabricating
structures with patterned layers of group III-nitride as shown in
FIGS. 1A-1B, 2, 2A, and 2B;
FIG. 4 is oblique view scanning electron micrograph (SEM) of a
structure made by an embodiment of the method of FIG. 3 in which
the wet etch time is short;
FIG. 5 is top view SEM of a structure made by an embodiment of the
method of FIG. 3 in which the wet etch time is of intermediate
length;
FIG. 6 is a top view SEM of a structure made by an embodiment of
method of FIG. 3 in which the wet etch time is long; and
FIG. 7 is a flow chart for specific method of fabricating GaN
structures with patterned layers as shown in FIGS. 1A-1B, 2, 2A,
and 2B.
In the Figures and text like reference numbers refer to similar
elements.
DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The chemical stability of the bond between group III metals and
nitrogen causes group III-nitride semiconductors to be chemically
resistant to many etchants. Nevertheless, aqueous solutions of
strong bases will etch a nitrogen-polar surface of a layer of group
III-nitride. Such wet etchants are able to mechanically pattern a
group III-nitride layer that is already been polarity patterned.
Exemplary patterned surfaces produce field-emitter arrays, as shown
in FIGS. 1A-1B, and photonic bandgap structures, as shown in FIGS.
2, 2A, and 2B.
FIG. 1A shows a field-emitter array 10A. The field emitter array
10A includes a substrate 12 and a regular pattern of laterally
inter-dispersed columnar first and second regions 14, 15. The first
and second regions 14, 15 cover a planar surface 17 of the
substrate 12. The substrate 12 is a crystalline material such as
silicon carbide (SiC) or (0 0 0 1)-plane sapphire. The first and
second regions 15, 16 include respective (0 0 0 1)-polarity and (0
0 0 1)-polarity forms of a group III-nitride. The group III-nitride
of the regions 14, 15 has a wide lattice mismatch with the planar
surface 17 of the substrate 12.
Herein, the (0 0 0 1)-polarity and (0 0 0 1)-polarity forms of a
group III-nitrides are referred to as the N-polar and metal-polar
forms. The N-polar and metal-polar forms have opposite intrinsic
polarizations. The free surfaces of flat N-polar and metal-polar
layers terminate with layers of nitrogen atoms and group III-metal
atoms, respectively.
The first and second columnar regions 14, 15 have physically
different surfaces and thus, form a mechanically patterned layer of
group III-nitride on the substrate 12. The first regions 14 include
one or more hexagonal pyramids 16 of the group III-nitride. Thus,
the first regions 14 have non-flat exposed surfaces, which include
sharp tips 20. The second regions 15 include smooth layers of the
group nitride and are devoid of pyramidal structures. Thus, exposed
surfaces 22 of the second regions 15 are smooth and flat. The
surfaces 22 of the second regions 15 are also farther above the
planar surface 17 of the substrate 12 than the highest tips 20 in
the first regions 14.
The hexagonal pyramids 16 of the first regions 14 have sharp apical
points 20 and thus, can function as field emitters. The apical tips
20 have diameters of less than 100 nanometers (nm). In embodiments
where the group III-nitride is GaN, the pyramids 16 have tips 20
with diameters of less than about 20 nm-30 nm and have faces that
make angles of about 56.degree.-58.degree. with the planar surface
17. The pyramids 16 have six faces that are (1 0 1 1) facets.
Within a single first region 14, the distribution of the hexagonal
pyramids 16 and apical rips 90 is random. Various first regions 14
may have different numbers of the hexagonal pyramids 16. The sizes
of the first regions 14 are constant, because the inter-dispersed
second regions 15 have the same size and regular lateral
distribution.
In the second regions 15, the layer of first group III-nitride
rests on a much thinner base layer 18 that is made of a second
group III-nitride. The second group III nitride has a wide lattice
mismatch with the planar surface 17 of the substrate 12. More
importantly, the second group III-nitride grows with metal-polarity
on the surface 17 of the substrate 12.
FIG. 1B shows an alternate embodiment of a field-emitter array 10B.
The field emitter array 10B includes a substrate 12 and a regular
pattern of inter-dispersed columnar first and second regions 14, 15
as already described with respect to FIG. 1A. In the field-emitter
array 10B, the second regions 14 include overlapping hexagonal
pyramids 16 rather than isolated pyramids 16 as in the
field-emitter array 10A. Also, the pyramids 16 have a range of
sizes and rest on a thick layer 19 of N-polar group III-nitride
rather than directly on the planar surface 17 as in the
field-emitter array 10A of FIG. 1A. In the field-emitter array 10B,
the hexagonal pyramids 16 still have sharp apical points 20 and
thus, can still function effectively as field-emitters. The apical
tips 20 are still lower than die exposed top surfaces 22 of the
second regions 15.
FIG. 1C shows art embodiment of a flat panel image display 24. The
display 24 incorporates a field-emitter array 10, e.g., array 10A
or 10B from FIGS. 1A and 1B. The display 24 also includes metallic
electrodes 26 and a phosphor screen 28. The metallic electrodes 26
are supported by the flat top surface 22 of the field emitter
array's second regions 15. The top surfaces 22 support the metallic
electrodes 26 along a plane that is nearer to the phosphor screen
28 than are the tips 20 themselves. For that reason, the metallic
electrodes 26 are able to control emission of electrons from the
field-emitter array. The metallic electrodes 26 function as control
gates for field-emitters in adjacent first regions 14. Supporting
the metallic electrodes 26 on the second regions 22 conveniently
avoids a need to self-align the electrodes on individual tips 20.
Such an alignment process would be complex, because the positions
of the tips 20 are random in individual first regions 14.
FIG. 2 shows another structure 30 that has a mechanically patterned
layer 32 of a first group III-nitride. The layer 32 is located on a
planar surface 17 of crystalline substrate 12 e.g., the (0 0 0
1)-plane of a sapphire substrate. The layer 32 includes a regular
array of identical columnar holes or trenches 34. The holes or
trenches 34 have substantially rectangular cross sections and
traverse the entire thickness of layer 32. The layer 32 rests on a
mechanically patterned base layer 18. The base layer 13 is a second
crystalline group III-nitride with a different alloy composition
than the first group III-nitride. The base layer 18 aligns
epitaxially on the planar surface 17 to be group III
metal-polar.
For the pair of layers 18 and 32, exemplary pairs of second and
first group III-nitride semiconductors are: the pair AlN and GaN or
the pair AlN and AlGaN.
The layer 32 has thickness that is typically 100-10,000 times than
the thickness of the base layer 18. An exemplary GaN layer 32 has a
thickness of 30 .mu.m or more, and an exemplary AlN base layer 18
has a thickness of only about 20 nm-30 nm. The base layer 18 only
has to be thick enough to align the polarization of another layer
located on the base layer 18.
In optical devices, the layer 32 usually functions as an optical
core of a planar waveguide. The waveguide receives input light 36
via an edge 37 and transmits output light 38 via an opposite edge
39. Such edge coupling of the layer 32 to optical fibers and other
optical waveguides is more efficient for embodiments in which the
layer 32 is thicker. It is thus, advantageous that the layer 32 can
be relatively thick, i.e., 30 .mu.m or more, because such a thicker
layer 32 enables efficient end coupling to standard optical fibers
and waveguides.
The patterned thick layer 32 can, e.g., be a thick photonic bandgap
structure. Thick photonic bandgap structures provide more efficient
optical edge coupling than thinner photonic bandgap structures that
can be made by dry etching.
FIGS. 2A and 2B show two exemplary planar photonic bandgap
structures 30A, 30D. Cross-sectional views through the structures
30A and 30B are faithfully represented in FIG. 2. The structures
30A and 30B include a layer 32 of a metal-polar group III-nitride,
i.e., (0 0 0 1)-plane group III-nitride. The layer 32 is located on
the top surface of the crystalline substrate 12 shown in FIG. 2.
The layer 32 is mechanically patterned by an array of substantially
identical columnar features 34A, 34B. The columnar features 34A,
34B are holes and trenches in the structures 30A and 30B,
respectively.
The holes 34A and trenches 34B form regular arrays that have one
and two discrete lattice symmetries, respectively. For this reason,
the holes 34A and trenches 34D produce respective 2-dimensional and
1-dimensional periodic modulations of the refractive index of the
layer 32. The refractive index modulations produce a photonic
bandgap structure for selected lattice lengths in the arrays.
Lattice lengths that are odd integral multiples of 1/4 times the
effective wavelength of input light in the medium will produce
photonic bandgap structures.
FIG. 2C shows a photonic bandgap structure 30C similar to the
photonic bandgap structure 30A of FIG. 2A except that the holes and
the group III-nitride material layer are exchanged. In the
structure 30C, the layer 32C of group nitride is a two-dimensional
array of isolated pillars. Between the group III-nitride pillars is
an interconnected two-dimensional pattern of trenches 34C. The
trenches 34C isolate the pillars from each other.
FIG. 3 illustrates a method 40 for fabricating a structure with a
mechanically patterned layer of group III-nitride, e.g., as shown
in FIG. 1A-1B, 2, 2A, or 2B.
The method 40 includes forming a metal-polarity first layer of a
first group III-nitride on a selected planar surface of a
crystalline substrate (step 42). Forming the layer includes
performing an epitaxial growth of a first group III-nitride, and
mechanically patterning the layer lithographically. The composition
of the first group III-nitride is selected to insure that the
epitaxial growth produces a metal-polarity layer. The mechanical
patterning produces a regular pattern of identical holes or
trenches that expose a portion of the substrate through the
layer.
Next, the method 40 includes epitaxially growing a thicker second
layer of a second group III-nitride over the first layer and the
exposed portion of the substrate (step 44). Over the first layer,
the second layer grows with metal-polarity. Over the exposed
portion of the substrate, the second layer glows with N-polarity.
The first and second group III-nitrides have different alloy
compositions, e.g., AlN and GaN, and have a wide lattice-mismatch
with the substrate.
Finally, the method 40 also includes subjecting the second layer to
an aqueous solution of a strong base such as potassium hydroxide
(KOH) or sodium hydroxide (NaOH) (step 46). The aqueous solution
mechanically patterns the second layer by selectively etching
N-polar surfaces. Aqueous solutions of strong bases do not
significantly etch metal-polar surfaces of group III-nitrides. The
form of the mechanical patterning qualitatively depends on the
etching time and the concentration of the etchant.
FIG. 4 is a scanning electron Micrograph (SEM) of a polarity
striped GaN layer 50 that has been wet etched with a 2 molar
aqueous solution of KOH for 45 minutes. The GaN layer was
maintained at a temperature of about 90.degree. C. during the wet
etch.
The etched GaN layer 50 has N-polar GaN stripes 52 and Ga-polar GaN
stripes 54. The relatively short etch has removed significant
material from the N polar GaN stripes 52 without removing
significant material from the Ga-polar GaN stripes 54. In the
N-polar stripes 52, the etch produces a surface formed of densely
packed hexagonal GaN pyramids, e.g., as shown in FIG. 1B. The
pyramids have various sizes and sharp apical tips with diameters of
about 20-30 nm or less.
Measurements indicate that the pyramid density, .rho..sub..DELTA.,
varies with etching temperature, T, as:
[.rho..sub..DELTA.].sup.-1=[.rho..sub..DELTA.0].sup.-1exp(-E.sub.a/k.sub.-
BT) where k.sub.B is Boltzmann's constant. For a 2 molar solution
of KOH, a 15 minute etch, and temperatures between 25.degree. C.
and 100.degree. C., measurements show that the activation energy E,
equal to about 0.537 eV.
The inventors believe that the wet KOH etch produces a distribution
of packed hexagonal GaN pyramids, in part, due to the Ga-polar GaN
stripes 54 that are not etched. In particular, the Ga-polar stripes
laterally confine the N-polar stripes 52 so that the etchant
attacks top surface of the N-polar stripes 52 rather than side
surfaces thereof. The KOH wet etch produces a dense-packing of
sharp tipped hexagonal GaN pyramids when the N-polar GaN stripes 52
have widths of about 7 microns (.mu.m). It is believed that a dense
packing of hexagonal pyramids will also result from a wet KOH etch
of GaN surfaces in which N-polar GaN stripes have widths of about
100 .mu.m or less. It is not however, believed that a wet KOH etch
of an unconfined planar surface N-polar GaN will produce a dense
packing of sharp tipped, hexagonal GaN pyramids.
FIG. 5 is an SEM image showing a polarity-striped GaN layer 50 that
has been wet etched with a 4 molar aqueous solution of KOH for 60
minutes. Again, the GaN layer was maintained at a temperature of
about 90.degree. C. during the wet etch.
The more intense etch has removed all material from the N-polar GaN
stripes 52 except for isolated hexagonal GaN pyramids 56. The wet
etch stopped an the underlying crystalline sapphire substrate. This
intermediate length etch produces patterning like that of FIG. 1A,
at least, within individual N-polar GaN stripes 52. Within these
regions, the hexagonal GaN pyramids 56 have a random
distribution.
FIG. 6 is an SEM image of a polarity-striped GaN layer 50 that has
been etched with a 4 molar aqueous solution of KOH for more than 60
minutes. Again, the GaN layer is maintained at a temperature of
25.degree. C.-125.degree. C. and preferably of about 90.degree. C.
during the wet etch.
This longer etch has completely removed the original N-polar GaN
stripes 52. As a result, substantially vertical trenches separate
the unetched Ga-polar stripes 51. The sidewalls of the Ga-polar
stripes 54 are not completely vertical, because the wet etchant
slowly attacks sidewalls of Ga-polar GaN layers. Aqueous solutions
with higher concentrations of KOH than 4 molar tend to erode
exposed side and end surfaces of Ga-polar stripes 54. The resulting
structure has a patterned Ga-polar layer of group III-nitride like
structures 30, 30A, and 30B of FIGS. 2, 2A, and 2B.
FIG. 7 illustrates a method 60 for fabricating GaN structures that
are mechanically patterned as in FIGS. 1A, 1B, 2, 2A, and 2B. The
method 60 includes preparing a planar sapphire growth substrate
(step 62), growing and patterning a Ga-polarity aligning layer on
the substrate (step 64), and epitaxially growing a
polarity-patterned GaN layer over the aligning layer (step 66). The
method 60 also includes wet etching the GaN layer to produce
mechanical patterning by selectively removing GaN in the N-phase
regions (step 68).
In step 62, preparing the sapphire growth subsume includes cleaning
a (0 0 0 1)-plane surface of a crystalline sapphire substrate. The
cleaning includes washing the surface for 1 minute in an aqueous
cleaning solution. Mixing a first aqueous solution having about 96
weight % H.sub.2SO.sub.4 with a second aqueous solution having
about 30 weight % H.sub.2O.sub.2 produces the aqueous cleaning
solution. During the mixing, about 10 volume parts of the first
solution are combined with one volume part of the second solution.
The cleaning also includes rinsing the washed surface with
de-ionized water and then spin-drying the sapphire growth
substrate.
In step 62, preparing the growth substrate also includes degassing
the sapphire substrate in the buffer chamber of a molecular beam
epitaxy (MBE) system at about 200.degree. C. The degassing
continues until the chamber pressure is below about
5.times.10.sup.-9 Torr. After the degassing, the sapphire substrate
is transferred to the growth chamber of the plasma-assisted MBE
system.
In step 64, growing and patterning a Ga-polarity aligning layer
includes performing an MBE growth of an AlN layer on the sapphire
substrate (substep 64a). To perform the MBE growth, the temperature
of the growth chamber is raised at a rate of about 8.degree. C. per
minute to a final temperature of about 720.degree. C. The sapphire
substrate is maintained at a uniform temperature with the aid of a
300 nm thick layer of titanium deposited on the substrate's back
surface.
The MBE system grows the AlN layer to a thickness of about 20 nm to
30 nm. This thin AlN layer is sufficiently thick to cover the
entire exposed surface of the sapphire substrate. In the model 32P
Molecular Beam Epitaxy system made by Riber Corporation of 133
boulevard National, Boite Postale 231, 92503 Rueil Malmaison
France, the growth conditions are: Al effusion cell temperature of
about 1050.degree. C., nitrogen flow rate of about 2 seem, and RF
power of about 500 watts (W).
In step 64, forming the patterned AlN layer 12 includes performing
an MBE growth of about 50 nm of protective GaN on the already grown
AlN layer (substep 64b). The GaN layer protects the underlying AlN
from oxidation during subsequent removal of the substrate from the
MBE growth chamber. Growth conditions for the GaN layer are similar
to those for the MBE growth of the AlN layer except that the
temperature is raised in the Ga effusion cell rather than in die Al
effusion cell. During this growth, the Ga effusion cell has a
temperature of about 1000.degree. C. to about 1020.degree. C.
After cooling the sapphire substrate to about 200.degree. C., the
GaN/AlN layer is lithographically patterned with a regular army of
windows that expose selected portions of the sapphire substrate
(substep 64c). The patterning step includes forming a photoresist
mask on the GaN layer and then, performing a conventional
chlorine-based plasma etch to remove unmasked portions of the
GaN/AlN layer. Exemplary conditions for the plasma etch are: RF
source power of about 300-500 watts, source bias of 100 volts to
200 volts, chlorine-argon flow rate of about 10-25 seem (20% to 50%
of the flow being argon), and a gas pressure of about 1 millitorr
to about 10 millitorr. The plasma etch produces a preselected
pattern of GaN topped AlN regions.
After the plasma etch, the sapphire substrate with a pattern of GaN
topped AlN regions is cleaned in an aqueous solution of HCl, rinsed
in de-ionized water, and blown dry with nitrogen. This aqueous
cleaning solution includes between about 36.5 weight % HCl and
about 48 weight % HCl. Then, the above-described steps are again
used to reintroduce the sapphire substrate into the MBE system.
In step 66, epitaxially growing a GaN layer includes performing a
plasma enhanced MBE growth of a GaN layer to a thickness of about 2
.mu.m or more. During the MBE growth, the system conditions, are:
Ga effusion cell temperature of about 1000.degree. C. to about
1020.degree. C., nitrogen flow rate of about 2 seem, and RF power
of about 500 watts (W). During, this growth, the GaN topped AlN
regions initiate growth of Ga-polar GaN, and the exposed regions of
the sapphire substrate 10 initiate growth of N-polar GaN.
In step 68, the anisotropic wet etching includes immersing the GaN
layer and substrate in an aqueous solution of KOH. Exemplary wet
etches use 1 to 4 molar aqueous solutions of KOH and etch periods
of about 15 minutes to 60 minutes at temperatures of 100.degree. C.
The concentration of KOH and etch time determines the qualitative
form of the resulting mechanical patterning as illustrated in FIGS.
4-6. The wet etch selectively removes GaN with N-polarity.
Nevertheless, wet etches with more basic aqueous solutions than 4
molar KOH can erode end faces of Ga-polar portions of the original
GaN layer.
From the disclosure, drawings, and claims, other embodiments of the
invention will be apparent to those skilled in the art.
* * * * *