U.S. patent application number 16/991279 was filed with the patent office on 2021-02-18 for diameter expansion of aluminum nitride crystals during growth by physical vapor transport.
The applicant listed for this patent is Robert T. BONDOKOV, Jianfeng CHEN, Thomas MIEBACH, Leo J. SCHOWALTER, Takashi SUZUKI. Invention is credited to Robert T. BONDOKOV, Jianfeng CHEN, Thomas MIEBACH, Leo J. SCHOWALTER, Takashi SUZUKI.
Application Number | 20210047749 16/991279 |
Document ID | / |
Family ID | 1000005208620 |
Filed Date | 2021-02-18 |
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United States Patent
Application |
20210047749 |
Kind Code |
A1 |
BONDOKOV; Robert T. ; et
al. |
February 18, 2021 |
DIAMETER EXPANSION OF ALUMINUM NITRIDE CRYSTALS DURING GROWTH BY
PHYSICAL VAPOR TRANSPORT
Abstract
In various embodiments, aluminum nitride single crystals are
rapidly diameter-expanded during growth by physical vapor
transport. High rates of diameter expansion during growth may be
enabled by the use of internal thermal shields and directed
plasma-modification of the growth environment to augment radial
thermal gradients and increase radial growth rates.
Inventors: |
BONDOKOV; Robert T.;
(Latham, NY) ; MIEBACH; Thomas; (Malta, NY)
; CHEN; Jianfeng; (Clifton Park, NY) ; SUZUKI;
Takashi; (Fuji, JP) ; SCHOWALTER; Leo J.;
(Latham, NY) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
BONDOKOV; Robert T.
MIEBACH; Thomas
CHEN; Jianfeng
SUZUKI; Takashi
SCHOWALTER; Leo J. |
Latham
Malta
Clifton Park
Fuji
Latham |
NY
NY
NY
NY |
US
US
US
JP
US |
|
|
Family ID: |
1000005208620 |
Appl. No.: |
16/991279 |
Filed: |
August 12, 2020 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62887033 |
Aug 15, 2019 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C30B 29/40 20130101;
C30B 23/025 20130101; C30B 23/08 20130101 |
International
Class: |
C30B 23/08 20060101
C30B023/08; C30B 29/40 20060101 C30B029/40; C30B 23/02 20060101
C30B023/02 |
Claims
1. A method of forming single-crystal aluminum nitride (AlN), the
method comprising: providing within a growth chamber a seed crystal
having a growth face comprising AlN; establishing a radial thermal
gradient and an axial thermal gradient within the growth chamber;
condensing vapor comprising aluminum and nitrogen within the growth
chamber, thereby forming on the growth face of the seed crystal an
AlN single crystal that (a) increases in length along a growth
direction in response to the axial thermal gradient and (b) expands
in diameter along a radial direction substantially perpendicular to
the growth direction in response to the radial thermal gradient;
and thereduring, increasing a lateral growth rate of the AlN single
crystal to increase a rate of the diameter expansion of the AlN
single crystal.
2. The method of claim 1, wherein establishing the radial thermal
gradient and the axial thermal gradient within the growth chamber
comprises, at least in part, (i) heating the growth chamber and
(ii) configuring a plurality of thermal shields outside of the
growth chamber.
3. The method of claim 1, wherein increasing the lateral growth
rate of the AlN single crystal comprises enhancing the vapor with
atomic nitrogen proximate an edge portion of the AlN single
crystal.
4. The method of claim 3, wherein enhancing the vapor with atomic
nitrogen comprises (i) introducing nitrogen gas proximate the edge
portion of the AlN single crystal and (ii) generating a plasma
proximate the edge portion of the AlN single crystal with the
nitrogen gas.
5. The method of claim 1, wherein increasing the lateral growth
rate of the AlN single crystal comprises providing, within the
growth chamber, one or more internal thermal shields for directing
heat toward an edge of the AlN single crystal.
6. The method of claim 5, wherein at least one said internal
thermal shield is oriented substantially parallel to the radial
direction.
7. The method of claim 5, wherein at least one said internal
thermal shield is oriented substantially parallel to the growth
direction.
8. The method of claim 5, wherein at least one said internal
thermal shield is oriented at an inclination neither parallel nor
perpendicular to the radial direction.
9. The method of claim 5, wherein (i) the one or more internal
thermal shields comprises a plurality of internal thermal shields,
and (ii) thicknesses of at least two of the internal thermal
shields are different from each other.
10. The method of claim 5, wherein (i) the one or more internal
thermal shields comprises a plurality of internal thermal shields,
(ii) each internal thermal shield is annular and defines a central
opening therein, and (ii) sizes of the central openings of at least
two of the internal thermal shields are different from each
other.
11. The method of claim 5, wherein at least one said internal
thermal shield is annular and defines therein a central opening to
accommodate growth of the AlN single crystal therethrough.
12. The method of claim 1, further comprising separating from the
AlN single crystal a single-crystal AlN substrate having a diameter
of at least 25 mm.
13. The method of claim 12, further comprising fabricating a
light-emitting device over at least a portion of the AlN
substrate.
14. The method of claim 13, wherein the light-emitting device is
configured to emit ultraviolet light.
15. The method of claim 13, further comprising removing at least a
portion of the AlN substrate after or during fabrication of the
light-emitting device.
16. A light-emitting device formed in accordance with claim 13.
17. An AlN single crystal formed in accordance with claim 1.
18. A method of forming single-crystal aluminum nitride (AlN), the
method comprising: providing within a growth chamber a seed crystal
having a growth face comprising AlN; heating the growth chamber;
condensing vapor comprising aluminum and nitrogen within the growth
chamber during heating thereof, thereby forming an AlN single
crystal on the growth face of the seed crystal; and thereduring,
enhancing the vapor with atomic nitrogen proximate an edge portion
of the AlN single crystal.
19. The method of claim 18, wherein enhancing the vapor with atomic
nitrogen comprises (i) introducing nitrogen gas into the growth
chamber and (ii) generating a plasma proximate the edge portion of
the AlN single crystal with the nitrogen gas.
20. The method of claim 18, further comprising separating from the
AlN single crystal a single-crystal AlN substrate having a diameter
of at least 25 mm.
21.-60. (canceled)
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
Provisional Patent Application No. 62/887,033, filed Aug. 15, 2019,
the entire disclosure of which is hereby incorporated herein by
reference.
TECHNICAL FIELD
[0002] In various embodiments, the present invention relates to the
fabrication of semiconductor materials such as single-crystal
aluminum nitride (AlN) via physical vapor transport.
BACKGROUND
[0003] Aluminum nitride (AlN) holds great promise as a
semiconductor material for numerous applications, e.g.,
optoelectronic devices such as short-wavelength light-emitting
diodes (LEDs) and lasers, dielectric layers in optical storage
media, electronic substrates, and chip carriers where high thermal
conductivity is essential, among many others. In principle, the
properties of AlN may allow light emission at wavelengths down to
around 200 nanometers (nm) to be achieved. Recent work has
demonstrated that ultraviolet (UV) LEDs have superior performance
when fabricated on low-defect AlN substrates prepared from bulk AlN
single crystals. The use of AlN substrates is also expected to
improve high-power radio-frequency (RF) devices made with nitride
semiconductors due to the high thermal conductivity with low
electrical conductivity. However, the commercial feasibility of
AlN-based semiconductor devices is limited by the scarcity and high
cost of low-defect single crystals of AlN.
[0004] To make single-crystal AlN substrates more readily available
and cost-effective, and to make the devices built thereon
commercially feasible, it is desirable to grow AlN bulk crystals at
a high growth rate (>0.5 mm/hr) while preserving crystal
quality. The most effective method of growing AlN bulk single
crystals is the "sublimation-recondensation"method that involves
sublimation of lower-quality (typically polycrystalline) AlN source
material and recondensation of the resulting vapor to form the
single-crystal AlN. U.S. Pat. No. 6,770,135 (the '135 patent), U.S.
Pat. No. 7,638,346 (the '346 patent), U.S. Pat. No. 7,776,153 (the
'153 patent), and U.S. Pat. No. 9,028,612 (the '612 patent), the
entire disclosures of which are incorporated by reference herein,
describe various aspects of sublimation-recondensation growth of
AlN, both seeded and unseeded.
[0005] While AlN substrates are enabling platforms for the
fabrication of UV light-emitting devices such as LEDs and
electronic device such as high-speed transistors, high-quality bulk
crystalline AlN material is often unavailable in volumes necessary
for widespread commercial adoption of these technologies.
Sublimation-recondensation crystal growth of AlN often utilizes
small-diameter, high-quality seed crystals as platforms for the
growth of longer AlN crystalline boules. However, cost-effective
production of AlN devices will require larger diameter AlN
substrates over time. In order to address this need, crystal growth
of AlN frequently involves "diameter expansion," i.e., modification
of the thermal field in the growth chamber to increase the rate of
crystal growth in the lateral direction (i.e., perpendicular to the
"growth direction" along which the crystalline boule increases in
length away from the seed crystal). Conventional diameter-expansion
techniques may successfully enlarge the diameter of the growing
crystal, but the expansion rate is limited due to deleterious
impacts on the quality of the growing crystal. Specifically,
excessive modification of the thermal field in conventional
techniques can result in highly defective or even polycrystalline
material, particularly at the edges of the growing crystal. Other
defects such as low-angle grain boundaries and dislocations,
non-uniformities in doping, and even crystal fracture can also
result. Thus, many conventional diameter-expansion efforts can be
wasteful and self-defeating, as highly defective edge material is
often unsuitable for device applications and must be removed from
the crystalline boule.
[0006] In view of the foregoing, there is a need for crystal-growth
techniques capable of high rates of diameter expansion while
maintaining high crystalline quality of the resulting AlN single
crystals, as well as the large AlN single crystals enabled by such
techniques.
SUMMARY
[0007] In various embodiments of the present invention, large,
high-quality single crystals of AlN are produced via techniques
enabling high rates of diameter expansion without compromising
crystalline quality. Exemplary growth techniques utilize baseline
radial thermal gradients established at least in part via, for
example, the placement and configuration of thermal shielding
external to the growth chamber. These radial thermal gradients,
and/or the lateral growth rates of the AlN crystal, are
subsequently enhanced via additional techniques, thereby enabling
more rapid diameter expansion and the growth of large, high-quality
AlN single crystals. Despite the more rapid expansion of crystal
diameter, AlN single crystals formed in accordance with embodiments
of the present invention maintain high levels of crystal quality,
even at the edges of the crystal and even for high rates of
diameter expansion. Thus, embodiments of the present invention
provide techniques, and AlN single crystals themselves, which are
more economical and more suited for mass production of substrates
and devices.
[0008] In accordance with embodiments of the invention, techniques
for enhancing the radial thermal gradient in the crystal-growth
chamber include the use of thermal shields disposed internal to the
chamber and proximate the growing crystal. In accordance with
various embodiments, such internal thermal shields influence the
thermal field proximate the growing crystal, and the lateral growth
rate thereof, more effectively than external shields located
outside of the growth crucible itself. For example, in various
embodiments the internal thermal shields define openings
therethrough, and these openings accommodate the growth of the
crystal through the shields while they influence the thermal field
to enable rapid diameter expansion. Additional techniques in
accordance with embodiments of the invention for promoting
increased rates of lateral crystal growth (and thus concomitant
diameter expansion) also include enhancement of atomic nitrogen in
the vapor phase, preferentially concentrated at the lateral edge of
the crystal (e.g., via use of a plasma proximate the lateral edge
of the crystal). Such techniques promote enhanced lateral growth
(i.e., high-rate diameter expansion) of the AlN crystal while
preserving high crystalline quality.
[0009] Embodiments of the invention enable and facilitate the
growth of AlN single crystals having large crystal augmentation
parameters (as defined hereinbelow), masses, and/or volumes
heretofore unavailable via conventional crystal-growth techniques.
AlN single crystals in accordance with embodiments of the invention
may therefore be utilized as cost-effective, high-quality platforms
for the fabrication of electronic and optical devices. Techniques
in accordance with embodiments of the invention are particularly
suited for seeded growth of AlN single crystals from seed crystals,
rather than for unseeded growth relying upon, for example,
spontaneous nucleation of crystalline material and/or growth guided
by tapered growth crucibles themselves. Such unseeded growth
techniques are typically unable to produce AlN single crystals
having large crystal augmentation parameters and uniformly high
levels of crystalline quality. Thus, in accordance with embodiments
of the present invention, AlN single crystals (or boules), being
formed via seeded growth, typically have one planar surface having
the size and shape substantially corresponding to the seed crystal
(or exposed area of the seed crystal) utilized for growth of the
crystal; such crystal shapes are different from those of unseeded
crystals, which typically taper down to much smaller, point-like
areas, since they generally initially nucleate at a small,
limited-volume point (e.g., the sharp tip of a conical portion of a
crystal-growth crucible).
[0010] In various embodiments, all or a portion of the seed crystal
(e.g., at least the exposed area thereof) is incorporated into
(i.e., a portion of) the boule when it is removed from the growth
system after growth. That is, a boule grown by seeded growth may
incorporate at least a portion of the seed itself, and the
interface between the seed crystal and the boule is further
evidence of seeded growth. (In various embodiments, portions of the
seed not exposed for growth thereon may sublime away during the
growth and therefore not be present after the growth.) The
seed-boule interface is typically detectable by one or more
characterization techniques, including optical inspection (a
visible line may be detectable at the interface and result from
differential incorporation of point defects and/or impurities in
the seed and in the boule), luminescence contrast (e.g., under 254
nm light, the seed may appear darker or brighter than the boule due
to, for example, differential incorporation of point defects and/or
impurities), or measurement of UV absorption, which may vary
between the initial seed and the grown boule.
[0011] In accordance with embodiments of the present invention,
single-crystal AlN may be fabricated via sublimation-recondensation
from polycrystalline AlN source material. As described in the '135
patent, the '346 patent, the '153 patent, and the '612 patent, the
sublimation-recondensation growth process is desirably performed
under a steep axial (i.e., in the direction of crystal growth away
from a seed, if a seed is present, and/or toward sublimating source
material) temperature gradient, while radial temperature gradients
may be utilized to control the diameter of the growing crystal and
influence its crystalline quality. In various embodiments of the
present invention, the baseline radial and/or axial thermal
gradients within the crystal-growth crucible utilized to promote
and control the growth of the AlN material may be controlled in
various different manners. For example, individual heating elements
arranged around the crucible may be powered to different levels
(and thus different temperatures) to establish thermal gradients
within the crucible. In addition or instead, thermal insulation may
be selectively arranged around the crucible such that thinner
and/or less insulating insulation is positioned around areas of
higher desired temperature. As detailed in the '612 patent, thermal
shields may also be arranged around the crucible, e.g., above
and/or below the crucible, in any of a multitude of different
arrangements in order to establish desired baseline thermal
gradients within the crucible. Once and/or while these baseline
thermal gradients are established, at least the radial thermal
gradient (i.e., the thermal gradient perpendicular to the lateral
growth direction away from the seed crystal and parallel to the
diameter of the growing crystal (which may be expanding during all
or a portion of the growth process)) is enhanced via use of one or
more techniques that enhance lateral growth of the crystal while
maintaining crystal quality.
[0012] Embodiments of the present invention also enable rapid
diameter expansion of AlN single-crystal boules on seed crystals of
arbitrary crystalline orientation and polarity, as well as within
single growth stages. For example, embodiments of the invention
obviate the need to utilize (but may utilize) Al-polarity, c-face
seed crystals for rapid diameter expansion and preserve high
crystal quality without the need for multiple different growth
stages, each initiated on a larger seed crystal, for example as
disclosed in U.S. patent application Ser. No. 16/008,407, filed on
Jun. 14, 2018 (the '407 application), the entire disclosure of
which is incorporated by reference herein. Thus, embodiments of the
invention may utilize seed crystals having c-face and N-polarity,
c-face and Al-polarity, m-face, etc. In addition, seed crystals in
accordance with embodiments of the invention need not have any
particular diameter or minimum diameter to enable high-quality
crystal growth with rapid diameter expansion. For avoidance of
doubt, the techniques detailed herein in accordance with
embodiments of the invention enable higher rates of diameter
expansion of growing AlN single crystals, while preserving crystal
quality (and therefore, the production of AlN single crystals
having larger crystal augmentation parameters, as detailed herein),
than do techniques detailed in the '407 application.
[0013] The techniques detailed herein for enablement of rapid
diameter expansion may be combined with techniques for enabling
high UV transparency of the AlN single crystals, particularly at
deep-UV wavelengths. In various embodiments of the present
invention, production of highly UV-transparent single crystals of
AlN is enabled via vapor-phase growth, impurity control,
post-growth temperature control within the growth system, and
post-growth annealing techniques that are isothermal or
quasi-isothermal. The resulting single-crystal AlN advantageously
exhibits a low UV absorption coefficient (e.g., below 10 cm.sup.-1,
or even below 8 cm.sup.-1) for wavelengths between 230 nm and 280
nm, or, in various embodiments, for wavelengths between 210 nm and
280 nm. The single-crystal AlN may also desirably exhibit a
substantially "flat" UV absorption spectrum for wavelengths between
210 nm and 280 nm, e.g., a UV absorption coefficient that is
substantially constant within that wavelength range (or a portion
thereof), e.g., constant within .+-.3.+-.2 cm.sup.-1, or even .+-.1
cm.sup.-1. Such a spectrum may facilitate the engineering and
improved performance of optical devices (e.g., light-emitting
devices such as light-emitting diodes and lasers), as the optical
performance of the AlN single-crystal substrate for such devices
will exhibit substantially constant optical properties over the
deep-UV wavelength range.
[0014] Moreover, AlN single crystals in accordance with embodiments
of the invention exhibit steep slopes (i.e., "drop-offs") in their
UV absorption spectra near the band edge of AlN, e.g., for
wavelengths between approximately 210 nm and approximately 230 nm.
This property advantageously contributes to the low UV absorption
at deep-UV wavelengths and contributes to more uniform optical
performance of substrates fabricated from the AlN crystals, as well
as optical devices fabricated thereon.
[0015] Furthermore, annealing techniques in accordance with
embodiments of the present invention advantageously do not require
the elimination of carbon and oxygen from the single-crystal AlN to
unreasonably low, impractical levels. Specifically, embodiments of
the invention successfully result in low UV absorption at deep-UV
wavelengths even for AlN crystals having oxygen and/or carbon
concentrations ranging from approximately 10.sup.18 cm.sup.-3 to
approximately 10.sup.19 cm.sup.-3. In addition, post-growth
annealing techniques in accordance with embodiments of the
invention may be coupled with high-rate cooling of the AlN crystal
within the growth apparatus to, for example, avoid cracking of the
crystal, even when such cooling techniques result in the AlN
crystal initially exhibiting high levels of UV absorption at
certain wavelengths.
[0016] The present inventors have found that the presence of carbon
impurities can lead to high levels of UV absorption in AlN
crystals. Carbon incorporation leads to UV absorption at
wavelengths around 265 nm, which can hinder the performance of UV
light-emitting devices. In addition, oxygen impurities (or related
point defects) typically result in UV absorption at wavelengths
around 310 nm. Thus, while control of oxygen contamination is
desirable for UV transparency, it is not sufficient to enable UV
transparency at many UV wavelengths, particularly those in the
deep-UV portion of the optical spectrum. Embodiments of the present
invention include techniques for the improvement of UV absorption
in AlN single crystals even when oxygen and/or carbon impurity
concentrations have been controlled during the AlN fabrication
process.
[0017] The high radial and axial thermal gradients utilized during
crystal growth, as described above, necessarily result in the
crystal being formed in a non-isothermal environment. While the
thermal gradients enable the formation of large, high-quality AlN
crystals, the arrangements of thermal shields, insulation, and
related aspects of the growth system responsible for the formation
of the thermal gradients during crystal growth also necessarily
result in thermal gradients in the growth system during cool-down
of the crystal after crystal growth. While various references
recommend cooling the as-grown crystal within the growth apparatus
at a fairly slow rate in order to control point-defect formation,
such slow cooling may result in cracking of the AlN crystal due to
thermal-expansion mismatch, particularly for larger AlN crystals
(e.g., crystals exceeding approximately 50 mm in diameter). Thus,
embodiments of the present invention include cooling the as-grown
AlN crystal to approximately room temperature (e.g., approximately
25.degree. C.) within the growth chamber at a high cooling rate
(e.g., exceeding 250.degree. C./hour, 300.degree. C./hour,
400.degree. C./hour, or even 500.degree. C./hour) in contradiction
of the conventional wisdom and despite concomitant deleterious
effects on the UV transparency of the crystal. The cooling from the
growth temperature may also be performed without any additional
applied heat from the heating elements of the growth system (e.g.,
applied to decrease the cooling rate, known as "controlled
cooling"). The cooling of the crystal may be performed at a rate
limited only by, e.g., the thermal mass of the growth system, and
steps may be taken to accelerate the cooling of the crystal. For
example, after growth the AlN crystal may be moved away from or out
of the "hot zone" of the growth system (i.e., the portion of the
growth system directly proximate and heated by the heating elements
or furnace), and/or gas (e.g., nitrogen gas and/or an inert gas
such as argon) may be flowed (e.g., at a flow rate higher than any
flow rate utilized during crystal growth) within the system to
increase the cooling rate.
[0018] After formation of the AlN single crystal and cooling from
the growth temperature, the resulting crystal (or a portion
thereof, e.g., a wafer or substrate separated from a crystalline
boule) may be placed within a high-temperature annealing furnace
and annealed under isothermal, or quasi-isothermal, conditions to
ensure substantially even heating of the entire crystal. (As
utilized herein, "quasi-isothermal" conditions within a furnace
correspond to the temperature within the furnace (or a dedicated
heating area or "hot zone" thereof) being constant within
.+-.5.degree. C., .+-.2.degree. C., .+-.1.degree. C., or even
.+-.0.5.degree. C., and/or to any temperature gradient in any
direction within the furnace (or a dedicated heating area or hot
zone thereof) being less than 5.degree. C./cm, less than 2.degree.
C./cm, less than 1.degree. C./cm, or even less than 0.5.degree.
C./cm; such temperature gradients may be at least 0.05.degree. C.
or at least 0.1.degree. C. in various embodiments). That is, the
annealing conditions may be quite different from those under which
the AlN crystal is initially grown and cooled within the
crystal-growth crucible and growth system, which are desirably
configured to create axial and/or radial thermal gradients
therewithin. For example, the crystal may be annealed within a
resistively heated or RF-heated furnace configured for isothermal
annealing, rather than within the growth apparatus in which it was
initially grown. After annealing, the annealed crystal is slowly
and controllably cooled from the annealing temperature, for at
least a portion of the temperature range between the annealing
temperature and room temperature, in order to maintain the low UV
absorption achieved within the annealing cycle. In various
embodiments, the crystal is not attached or adhered to any part of
the furnace during annealing and/or cooling (e.g., unlike during
crystal growth, during which the crystal is attached to the
crystal-growth crucible, for example, via a seed crystal). Although
embodiments of the invention have been presented herein utilizing
AlN as the exemplary crystalline material fabricated in accordance
therewith, embodiments of the invention may also be applied to
other crystalline materials such as silicon carbide (SiC) and zinc
oxide (ZnO); thus, herein, all references to AlN herein may be
replaced, in other embodiments, by SiC or ZnO. As utilized herein,
the term "diameter" refers to a lateral dimension (e.g., the
largest lateral dimension) of a crystal, growth chamber, or other
object, even if the crystal, growth chamber, or other object is not
circular and/or is irregular in cross-section.
[0019] As utilized herein, a "substrate" or a "wafer" is a portion
of a previously grown crystalline boule having top and bottom
opposed, generally parallel surfaces. Substrates typically have
thicknesses ranging between 200 .mu.m and 1 mm and may be utilized
as platforms for the epitaxial growth of semiconductor layers and
the fabrication of semiconductor devices (e.g., light-emitting
devices such as lasers and light-emitting diodes, transistors,
power devices, etc.) thereon. Once layers and/or devices have been
formed on a substrate, all or a portion of the substrate may be
removed therefrom as part of subsequent processing; thus, when such
structures are present, remnant "substrates" may have thicknesses
less than those mentioned above. As utilized herein, "room
temperature" is 25.degree. C.
[0020] In an aspect, embodiments of the invention feature an AlN
single crystal having a diameter that increases, along at least a
portion of a length of the AlN single crystal, from a minimum
diameter to a maximum diameter. The AlN single crystal has a
crystal augmentation parameter (CAP), in mm, greater than 20. The
CAP is defined by:
CAP = A E - A S L E = .pi. 4 .times. L E ( d E 2 - d S 2 ) .
##EQU00001##
A.sub.E, in mm.sup.2, is the cross-sectional area of the AlN single
crystal at the maximum diameter, d.sub.E is the maximum diameter of
the AlN single crystal in mm, A.sub.S, in mm.sup.2, is the
cross-sectional area of the AlN single crystal at the minimum
diameter, d.sub.S is the minimum diameter in mm, and LE is an
expansion length, in mm, of the at least a portion of the AlN
single crystal along which the diameter increases from the minimum
diameter to the maximum diameter.
[0021] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The CAP may be
greater than 40, greater than 50, greater than 90, greater than
100, greater than 250, greater than 500, greater than 750, or
greater than 1000. The CAP may be less than 3000, less than 2500,
less than 2000, or less than 1500. A ratio of a total length of the
AlN single crystal, in mm, to the maximum diameter, in mm, may
range from approximately 0.3 to approximately 0.6. A ratio of a
total length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.35 to approximately
0.55. A ratio of the expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.002 to approximately 0.4. A ratio of the expansion length of the
AlN single crystal, in mm, to the maximum diameter, in mm, may
range from approximately 0.002 to approximately 0.03. A ratio of
the expansion length of the AlN single crystal, in mm, to the
maximum diameter, in mm, may range from approximately 0.07 to
approximately 0.3. A ratio of the expansion length of the AlN
single crystal, in mm, to the maximum diameter, in mm, may range
from approximately 0.002 to approximately 0.02. A ratio of the
expansion length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.08 to approximately
0.5. A ratio of the expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.1 to approximately 0.3.
[0022] A first region of the AlN single crystal may be shaped as a
frustum. A maximum diameter of the frustum may correspond to the
maximum diameter of the AlN single crystal and the minimum diameter
of the frustum may correspond to the minimum diameter of the AlN
single crystal. A second region of the AlN single crystal may be
shaped as a dome or cone or frustum extending from the first
region. A maximum diameter of the dome or cone or frustum may
correspond to the maximum diameter of the AlN single crystal
(and/or to the maximum diameter of the first region).
[0023] A first region of the AlN single crystal may be shaped as a
frustum. A maximum diameter of the frustum may correspond to the
maximum diameter of the AlN single crystal and the minimum diameter
of the frustum may correspond to the minimum diameter of the AlN
single crystal. A second region of the AlN single crystal may be
shaped as a cylinder extending from the first region and having a
diameter corresponding to the maximum diameter of the AlN single
crystal. A third region of the AlN single crystal may be shaped as
a dome or cone or frustum extending from the second region. A
maximum diameter of the dome or cone or frustum may correspond to
the maximum diameter of the AlN single crystal (and/or to the
maximum diameter of the first region and/or to the diameter of the
second region).
[0024] A density of threading edge dislocations in the AlN single
crystal may be less than approximately 1.times.10.sup.6 cm.sup.-2,
less than approximately 1.times.10.sup.5 cm.sup.-2, less than
approximately 1.times.10.sup.4 cm.sup.-2, less than approximately
1.times.10.sup.3 cm.sup.-2, or less than approximately
1.times.10.sup.2 cm'. A density of threading screw dislocations in
the AlN single crystal may be less than approximately 1000
cm.sup.-2, less than approximately 100 cm.sup.-2, less than
approximately 10 cm.sup.-2, or less than approximately 1 cm'. The
AlN single crystal may exhibit an x-ray rocking curve having a full
width at half maximum value less than 200 arcsec, less than 100
arcsec, less than 75 arcsec, less than 50 arcsec, or less than 40
arcsec. A carbon concentration within the AlN single crystal may be
less than 5.times.10.sup.18 cm.sup.-3, less than 1.times.10.sup.18
cm.sup.-3, less than 5.times.10.sup.17 cm.sup.-3, less than
1.times.10.sup.17 cm.sup.-3, less than 5.times.10.sup.16 cm.sup.-3,
or less than 1.times.10.sup.16 cm.sup.-3. A thermal conductivity of
the AlN single crystal, as measured in accordance with the American
Society for Testing and Materials (ASTM) Standard E1461-13, may be
greater than approximately 200 W/mK, greater than approximately 250
W/mK, greater than approximately 290 W/mK, or greater than
approximately 310 W/mK.
[0025] The AlN single crystal may have an Urbach energy ranging
from approximately 0.2 eV to approximately 1.8 eV within an
incident photon energy range of 5.85 eV to 6.0 eV. The Urbach
energy E.sub.U may be defined by:
ln .alpha. = ln .alpha. 0 + ( hv E U ) , ##EQU00002##
where .alpha. is an absorption coefficient of the AlN single
crystal at an incident photon energy hv, and .alpha..sub.0 is a
constant corresponding to the absorption coefficient at zero photon
energy. The Urbach energy of the AlN single crystal may range from
approximately 0.21 eV to approximately 1.0 eV. The AlN single
crystal may have an ultraviolet (UV) absorption coefficient of less
than 10 cm.sup.-1 for an entire wavelength range of 220 nm to 280
nm. The UV absorption coefficient may be no less than approximately
5 cm.sup.-1 for the entire wavelength range of 220 nm to 280 nm.
The AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 30 cm.sup.-1 for an entire wavelength
range of 210 nm to 220 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 210 nm to 220 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 8 cm.sup.-1 for an entire
wavelength range of 240 nm to 280 nm. The UV absorption coefficient
may be no less than approximately 5 cm.sup.-1 for the entire
wavelength range of 240 nm to 280 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 20
cm.sup.-1 for an entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 10
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 20 cm.sup.-1 for a wavelength of 220 nm.
The UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the wavelength of 220 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 15
cm.sup.-1 for an entire wavelength range of 220 nm to 240 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 220 nm to 240 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 15 cm.sup.-1 for an entire wavelength
range of 220 nm to 230 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 220 nm to 230 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 10 cm.sup.-1 for a
wavelength of 230 nm. The UV absorption coefficient may be no less
than approximately 5 cm.sup.-1 for the wavelength of 230 nm.
[0026] The minimum diameter of the AlN single crystal may be at
least approximately 25 mm, at least approximately 50 mm, at least
approximately 60 mm, at least approximately 75 mm, or at least
approximately 100 mm. The maximum diameter of the AlN single
crystal may be at least approximately 25 mm, at least approximately
50 mm, at least approximately 60 mm, at least approximately 75 mm,
at least approximately 100 mm, at least approximately 125 mm, or at
least approximately 150 mm.
[0027] In another aspect, embodiments of the invention feature an
AlN single crystal having a mass greater than 78 grams.
[0028] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The mass may be
greater than approximately 100 grams, greater than approximately
140 grams, greater than approximately 200 grams, greater than
approximately 220 grams, greater than approximately 240 grams,
greater than approximately 250 grams, greater than approximately
300 grams, greater than approximately 400 grams, greater than
approximately 500 grams, greater than approximately 600 grams,
greater than approximately 700 grams, greater than approximately
800 grams, greater than approximately 900 grams, or greater than
approximately 1000 grams. The mass may be less than approximately
2000 grams, less than approximately 1500 grams, or less than
approximately 1400 grams. The minimum diameter of the AlN single
crystal may be at least approximately 25 mm, at least approximately
50 mm, at least approximately 60 mm, at least approximately 75 mm,
or at least approximately 100 mm. The maximum diameter of the AlN
single crystal may be at least approximately 25 mm, at least
approximately 50 mm, at least approximately 60 mm, at least
approximately 75 mm, at least approximately 100 mm, at least
approximately 125 mm, or at least approximately 150 mm.
[0029] A density of threading edge dislocations in the AlN single
crystal may be less than approximately 1.times.10.sup.6 cm.sup.-2,
less than approximately 1.times.10.sup.5 cm.sup.-2, less than
approximately 1.times.10.sup.4 cm.sup.-2, less than approximately
1.times.10.sup.3 cm.sup.-2, or less than approximately
1.times.10.sup.2 cm.sup.-2. A density of threading screw
dislocations in the AlN single crystal may be less than
approximately 1000 cm.sup.-2, less than approximately 100
cm.sup.-2, less than approximately 10 cm.sup.-2, or less than
approximately 1 cm.sup.-2. The AlN single crystal may exhibit an
x-ray rocking curve having a full width at half maximum value less
than 200 arcsec, less than 100 arcsec, less than 75 arcsec, less
than 50 arcsec, or less than 40 arcsec. A carbon concentration
within the AlN single crystal may be less than 5.times.10.sup.18
cm.sup.-3, less than 1.times.10.sup.18 cm.sup.-3, less than
5.times.10.sup.17 cm.sup.-3, less than 1.times.10.sup.17 cm.sup.-3,
less than 5.times.10.sup.16 cm.sup.-3, or less than
1.times.10.sup.16 cm.sup.-3. A thermal conductivity of the AlN
single crystal, as measured in accordance with the American Society
for Testing and Materials (ASTM) Standard E1461-13, may be greater
than approximately 200 W/mK, greater than approximately 250 W/mK,
greater than approximately 290 W/mK, or greater than approximately
310 W/mK.
[0030] The AlN single crystal may have an Urbach energy ranging
from approximately 0.2 eV to approximately 1.8 eV within an
incident photon energy range of 5.85 eV to 6.0 eV. The Urbach
energy E.sub.U may be defined by: ln
.alpha. = ln .alpha. 0 + ( hv E U ) , ##EQU00003##
where .alpha. is an absorption coefficient of the AlN single
crystal at an incident photon energy hv, and .alpha..sub.0 is a
constant corresponding to the absorption coefficient at zero photon
energy. The Urbach energy of the AlN single crystal may range from
approximately 0.21 eV to approximately 1.0 eV. The AlN single
crystal may have an ultraviolet (UV) absorption coefficient of less
than 10 cm.sup.-1 for an entire wavelength range of 220 nm to 280
nm. The UV absorption coefficient may be no less than approximately
5 cm.sup.-1 for the entire wavelength range of 220 nm to 280 nm.
The AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 30 cm.sup.-1 for an entire wavelength
range of 210 nm to 220 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 210 nm to 220 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 8 cm.sup.-1 for an entire
wavelength range of 240 nm to 280 nm. The UV absorption coefficient
may be no less than approximately 5 cm.sup.-1 for the entire
wavelength range of 240 nm to 280 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 20
cm.sup.-1 for an entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 10
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 20 cm.sup.-1 for a wavelength of 220 nm.
The UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the wavelength of 220 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 15
cm.sup.-1 for an entire wavelength range of 220 nm to 240 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 220 nm to 240 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 15 cm.sup.-1 for an entire wavelength
range of 220 nm to 230 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 220 nm to 230 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 10 cm.sup.-1 for a
wavelength of 230 nm. The UV absorption coefficient may be no less
than approximately 5 cm.sup.-1 for the wavelength of 230 nm.
[0031] The AlN single crystal may have a diameter that increases,
along at least a portion of a length of the AlN single crystal,
from a minimum diameter to a maximum diameter. The AlN single
crystal may have a crystal augmentation parameter (CAP), in mm,
greater than 20. The CAP may be defined by:
CAP = A E - A S L E = .pi. 4 .times. L E ( d E 2 - d S 2 ) ,
##EQU00004##
where A.sub.E, in mm.sup.2, is the cross-sectional area of the AlN
single crystal at the maximum diameter, d.sub.E is the maximum
diameter of the AlN single crystal in mm, A.sub.S, in mm.sup.2, is
the cross-sectional area of the AlN single crystal at the minimum
diameter, d.sub.S is the minimum diameter in mm, and L.sub.E is an
expansion length, in mm, of the at least a portion of the AlN
single crystal along which the diameter increases from the minimum
diameter to the maximum diameter. A ratio of a total length of the
AlN single crystal, in mm, to a maximum diameter of the AlN single
crystal, in mm, may range from approximately 0.3 to approximately
0.6. A ratio of a total length of the AlN single crystal, in mm, to
a maximum diameter of the AlN single crystal, in mm, may range from
approximately 0.35 to approximately 0.55.
[0032] The AlN single crystal may have a diameter that increases,
along at least a portion of a length of the AlN single crystal,
from a minimum diameter to a maximum diameter. The AlN single
crystal may have an expansion length corresponding to a length of
the at least a portion of the AlN single crystal along which the
diameter increases from the minimum diameter to the maximum
diameter. A ratio of expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.002 to approximately 0.4. A ratio of expansion length of the AlN
single crystal, in mm, to the maximum diameter, in mm, may range
from approximately 0.002 to approximately 0.03. A ratio of
expansion length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.07 to approximately
0.3. A ratio of the expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.002 to approximately 0.02. A ratio of the expansion length of the
AlN single crystal, in mm, to the maximum diameter, in mm, may
range from approximately 0.08 to approximately 0.5. A ratio of the
expansion length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.1 to approximately
0.3.
[0033] A first region of the AlN single crystal may be shaped as a
frustum. A second region of the AlN single crystal may be shaped as
a dome or cone or frustum extending from the first region (and
having a diameter that decreases in a direction away from the first
region). A second region of the AlN single crystal may be shaped as
a cylinder extending from the first region and having a
substantially constant diameter. A third region of the AlN single
crystal is shaped as a dome or cone or frustum extending from the
second region (and having a diameter that decreases in a direction
away from the first and second regions).
[0034] In yet another aspect, embodiments of the invention feature
an AlN single crystal having a volume greater than 24 cm.sup.3.
[0035] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The volume may be
greater than approximately 30 cm.sup.3, greater than approximately
40 cm.sup.3, greater than approximately 70 cm.sup.3, greater than
approximately 75 cm.sup.3, greater than approximately 80 cm.sup.3,
greater than approximately 100 cm.sup.3, greater than approximately
150 cm.sup.3, greater than approximately 200 cm.sup.3, greater than
approximately 250 cm.sup.3, greater than approximately 300
cm.sup.3, greater than approximately 350 cm.sup.3, or greater than
approximately 400 cm.sup.3. The volume may be less than
approximately 800 cm.sup.3, or less than approximately 500
cm.sup.3. The minimum diameter of the AlN single crystal may be at
least approximately 25 mm, at least approximately 50 mm, at least
approximately 60 mm, at least approximately 75 mm, or at least
approximately 100 mm. The maximum diameter of the AlN single
crystal may be at least approximately 25 mm, at least approximately
50 mm, at least approximately 60 mm, at least approximately 75 mm,
at least approximately 100 mm, at least approximately 125 mm, or at
least approximately 150 mm.
[0036] A density of threading edge dislocations in the AlN single
crystal may be less than approximately 1.times.10.sup.6 cm.sup.-2,
less than approximately 1.times.10.sup.5 cm.sup.-2, less than
approximately 1.times.10.sup.4 cm.sup.-2, less than approximately
1.times.10.sup.3 cm.sup.-2, or less than approximately
1.times.10.sup.2 cm.sup.-2. A density of threading screw
dislocations in the AlN single crystal may be less than
approximately 1000 cm.sup.-2, less than approximately 100
cm.sup.-2, less than approximately 10 cm.sup.-2, or less than
approximately 1 cm.sup.-2. The AlN single crystal may exhibit an
x-ray rocking curve having a full width at half maximum value less
than 200 arcsec, less than 100 arcsec, less than 75 arcsec, less
than 50 arcsec, or less than 40 arcsec. A carbon concentration
within the AlN single crystal may be less than 5.times.10.sup.18
cm.sup.-3, less than 1.times.10.sup.18 cm.sup.-3, less than
5.times.10.sup.17 cm.sup.-3, less than 1.times.10.sup.17 cm.sup.-3,
less than 5.times.10.sup.16 cm.sup.-3, or less than
1.times.10.sup.16 cm.sup.-3. A thermal conductivity of the AlN
single crystal, as measured in accordance with the American Society
for Testing and Materials (ASTM) Standard E1461-13, may be greater
than approximately 200 W/mK, greater than approximately 250 W/mK,
greater than approximately 290 W/mK, or greater than approximately
310 W/mK.
[0037] The AlN single crystal may have an Urbach energy ranging
from approximately 0.2 eV to approximately 1.8 eV within an
incident photon energy range of 5.85 eV to 6.0 eV. The Urbach
energy E.sub.U may be defined by: ln
.alpha. = ln .alpha. 0 + ( hv E U ) , ##EQU00005##
where .alpha. is an absorption coefficient of the AlN single
crystal at an incident photon energy hv, and .alpha..sub.0 is a
constant corresponding to the absorption coefficient at zero photon
energy. The Urbach energy of the AlN single crystal may range from
approximately 0.21 eV to approximately 1.0 eV. The AlN single
crystal may have an ultraviolet (UV) absorption coefficient of less
than 10 cm.sup.-1 for an entire wavelength range of 220 nm to 280
nm. The UV absorption coefficient may be no less than approximately
5 cm.sup.-1 for the entire wavelength range of 220 nm to 280 nm.
The AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 30 cm.sup.-1 for an entire wavelength
range of 210 nm to 220 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 210 nm to 220 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 8 cm.sup.-1 for an entire
wavelength range of 240 nm to 280 nm. The UV absorption coefficient
may be no less than approximately 5 cm.sup.-1 for the entire
wavelength range of 240 nm to 280 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 20
cm.sup.-1 for an entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
UV absorption coefficient may be no less than approximately 10
cm.sup.-1 for the entire wavelength range of 215 nm to 220 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 20 cm.sup.-1 for a wavelength of 220 nm.
The UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the wavelength of 220 nm. The AlN single crystal may
have an ultraviolet (UV) absorption coefficient of less than 15
cm.sup.-1 for an entire wavelength range of 220 nm to 240 nm. The
UV absorption coefficient may be no less than approximately 5
cm.sup.-1 for the entire wavelength range of 220 nm to 240 nm. The
AlN single crystal may have an ultraviolet (UV) absorption
coefficient of less than 15 cm.sup.-1 for an entire wavelength
range of 220 nm to 230 nm. The UV absorption coefficient may be no
less than approximately 5 cm.sup.-1 for the entire wavelength range
of 220 nm to 230 nm. The AlN single crystal may have an ultraviolet
(UV) absorption coefficient of less than 10 cm.sup.-1 for a
wavelength of 230 nm. The UV absorption coefficient may be no less
than approximately 5 cm.sup.-1 for the wavelength of 230 nm.
[0038] The AlN single crystal may have a diameter that increases,
along at least a portion of a length of the AlN single crystal,
from a minimum diameter to a maximum diameter. The AlN single
crystal may have a crystal augmentation parameter (CAP), in mm,
greater than 20. The CAP may be defined by:
CAP = A E - A S L E = .pi. 4 .times. L E ( d E 2 - d S 2 ) ,
##EQU00006##
where A.sub.E, in mm.sup.2, is the cross-sectional area of the AlN
single crystal at the maximum diameter, d.sub.E is the maximum
diameter of the AlN single crystal in mm, A.sub.S, in mm.sup.2, is
the cross-sectional area of the AlN single crystal at the minimum
diameter, d.sub.S is the minimum diameter in mm, and LE is an
expansion length, in mm, of the at least a portion of the AlN
single crystal along which the diameter increases from the minimum
diameter to the maximum diameter. A ratio of a total length of the
AlN single crystal, in mm, to a maximum diameter of the AlN single
crystal, in mm, may range from approximately 0.3 to approximately
0.6. A ratio of a total length of the AlN single crystal, in mm, to
a maximum diameter of the AlN single crystal, in mm, may range from
approximately 0.35 to approximately 0.55.
[0039] The AlN single crystal may have a diameter that increases,
along at least a portion of a length of the AlN single crystal,
from a minimum diameter to a maximum diameter. The AlN single
crystal may have an expansion length corresponding to a length of
the at least a portion of the AlN single crystal along which the
diameter increases from the minimum diameter to the maximum
diameter. A ratio of expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.002 to approximately 0.4. A ratio of expansion length of the AlN
single crystal, in mm, to the maximum diameter, in mm, may range
from approximately 0.002 to approximately 0.03. A ratio of
expansion length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.07 to approximately
0.3. A ratio of the expansion length of the AlN single crystal, in
mm, to the maximum diameter, in mm, may range from approximately
0.002 to approximately 0.02. A ratio of the expansion length of the
AlN single crystal, in mm, to the maximum diameter, in mm, may
range from approximately 0.08 to approximately 0.5. A ratio of the
expansion length of the AlN single crystal, in mm, to the maximum
diameter, in mm, may range from approximately 0.1 to approximately
0.3.
[0040] A first region of the AlN single crystal may be shaped as a
frustum. A second region of the AlN single crystal may be shaped as
a dome or cone or frustum extending from the first region (and
having a diameter that decreases in a direction away from the first
region). A second region of the AlN single crystal may be shaped as
a cylinder extending from the first region and having a
substantially constant diameter. A third region of the AlN single
crystal is shaped as a dome or cone or frustum extending from the
second region (and having a diameter that decreases in a direction
away from the first and second regions).
[0041] In another aspect, embodiments of the invention feature a
method of forming single-crystal aluminum nitride (AlN). A seed
crystal having a growth face that includes, consists essentially
of, or consists of AlN is provided within a growth chamber. A
radial thermal gradient and an axial thermal gradient are
established within the growth chamber. Vapor including, consisting
essentially of, or consisting of aluminum and nitrogen is condensed
within the growth chamber, thereby forming on the growth face of
the seed crystal an AlN single crystal that (a) increases in length
along a growth direction in response to the axial thermal gradient
and (b) expands in diameter along a radial direction substantially
perpendicular to the growth direction in response to the radial
thermal gradient. During formation of the AlN single crystal, a
lateral growth rate of the AlN single crystal is increased to
increase a rate of the diameter expansion of the AlN single
crystal.
[0042] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. Establishing the
radial thermal gradient and the axial thermal gradient within the
growth chamber may include, consist essentially of, or consist of,
at least in part, (i) heating the growth chamber and (ii)
configuring a plurality of thermal shields outside of the growth
chamber. Increasing the lateral growth rate of the AlN single
crystal may include, consist essentially of, or consist of
enhancing the vapor with atomic nitrogen proximate an edge portion
of the AlN single crystal. Enhancing the vapor with atomic nitrogen
include, consist essentially of, or consist of (i) introducing
nitrogen (and/or nitrogen-containing) gas proximate an edge portion
of the AlN single crystal and (ii) generating a plasma proximate
the edge portion of the AlN single crystal with the nitrogen
(and/or nitrogen-containing) gas.
[0043] Increasing the lateral growth rate of the AlN single crystal
may include, consist essentially of, or consist of providing,
within the growth chamber, one or more internal thermal shields for
directing heat toward an edge of the AlN single crystal. At least
one (or even all) said internal thermal shield may be oriented
substantially parallel (e.g., .+-.5.degree., .+-.4.degree.,
.+-.3.degree., .+-.2.degree., .+-.1.degree., or .+-.0.5.degree.) to
the radial direction. At least one (or even all) said internal
thermal shield may be oriented substantially parallel (e.g.,
.+-.5.degree., .+-.4.degree., .+-.3.degree., .+-.2.degree.,
.+-.1.degree., or) .+-.0.5.degree. to the growth direction. At
least one (or even all) said internal thermal shield may be
oriented at an inclination neither parallel nor perpendicular to
the radial direction. At least one (or even all) said internal
thermal shield may be annular and define therein a central opening
to accommodate growth of the AlN single crystal therethrough. The
one or more internal thermal shields may include, consist
essentially of, or consist of a plurality of internal thermal
shields. Thicknesses of at least two (or even all) of the internal
thermal shields may be different from each other. Densities of at
least two of the internal thermal shields may be different from
each other. Each internal thermal shield may be annular and define
a central opening therein. Sizes of the central openings of at
least two (or even all) of the internal thermal shields may be
different from each other.
[0044] A single-crystal AlN substrate may be separated from the AlN
single crystal. The single-crystal AlN substrate may have a
diameter of at least 25 mm, at least 50 mm, at least 75 mm, or at
least 100 mm. A light-emitting device may be fabricated over at
least a portion of the AlN substrate. The light-emitting device may
be configured to emit ultraviolet light. At least a portion off the
AlN substrate may be removed from the light-emitting device after
or during fabrication of the light-emitting device.
[0045] In yet another aspect, embodiments of the invention feature
a method of forming single-crystal aluminum nitride (AlN). A seed
crystal having a growth face that includes, consists essentially
of, or consists of AlN is provided within a growth chamber. The
growth chamber is heated. Vapor including, consisting essentially
of, or consisting of aluminum and nitrogen is condensed within the
growth chamber during heating thereof, thereby forming an AlN
single crystal on the growth face of the seed crystal. During
formation of the AlN single crystal, the vapor is enhanced with
atomic nitrogen proximate an edge portion of the AlN single
crystal.
[0046] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. Enhancing the vapor
with atomic nitrogen may include, consist essentially of, or
consist of (i) introducing nitrogen (and/or nitrogen-containing)
gas into the growth chamber and (ii) generating a plasma proximate
the edge portion of the AlN single crystal with the nitrogen
(and/or nitrogen-containing) gas. A single-crystal AlN substrate
may be separated from the AlN single crystal. The single-crystal
AlN substrate may have a diameter of at least 25 mm, at least 50
mm, at least 75 mm, or at least 100 mm. A light-emitting device may
be fabricated over at least a portion of the AlN substrate. The
light-emitting device may be configured to emit ultraviolet light.
At least a portion off the AlN substrate may be removed from the
light-emitting device after or during fabrication of the
light-emitting device.
[0047] In another aspect, embodiments of the invention feature
embodiments of the invention feature a method of forming
single-crystal aluminum nitride (AlN). A seed crystal having a
growth face that includes, consists essentially of, or consists of
AlN is provided within a growth chamber. The growth chamber is
heated. Vapor including, consisting essentially of, or consisting
of aluminum and nitrogen is condensed within the growth chamber
during heating thereof, thereby forming an AlN single crystal on
the growth face of the seed crystal. The AlN single crystal extends
from the seed crystal in an axial direction perpendicular to the
growth face. During formation of the AlN single crystal, heat is
directed toward an edge portion of the AlN single crystal with one
or more internal thermal shields disposed within the growth
chamber.
[0048] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. One or more, or a
plurality of external thermal shields may be disposed outside of
the growth chamber. During heating of the growth chamber, one or
more thermal gradients may be established within the growth
chamber. The one or more thermal gradients may be established, at
least in part, via differential furnace heating and/or differential
insulation outside the growth chamber. The one or more thermal
gradients may be established, at least in part, via a configuration
of one or more external thermal shields disposed outside of the
growth chamber. The vapor may be enhanced with atomic nitrogen
proximate the edge portion of the AlN single crystal. Enhancing the
vapor with atomic nitrogen may include, consist essentially of, or
consist of (i) introducing nitrogen (and/or nitrogen-containing)
gas into the growth chamber and (ii) generating a plasma proximate
the edge portion of the AlN single crystal with the nitrogen
(and/or nitrogen-containing) gas.
[0049] At least one (or even all) said internal thermal shield may
be oriented substantially parallel (e.g., .+-.5.degree.,
.+-.4.degree., .+-.3.degree., .+-.2.degree., .+-.1.degree., or
.+-.0.5.degree.) to the axial direction. At least one (or even all)
said internal thermal shield may be oriented substantially
perpendicular (e.g., .+-.5.degree., .+-.4.degree., .+-.3.degree.,
.+-.2.degree., .+-.1.degree., or .+-.0.5.degree.) to the axial
direction. At least one (or even all) said internal thermal shield
may be oriented at an inclination neither parallel nor
perpendicular to the axial direction. At least one (or even all)
said internal thermal shield may be annular and define therein a
central opening to accommodate growth of the AlN single crystal
therethrough. The one or more internal thermal shields may include,
consist essentially of, or consist of a plurality of internal
thermal shields. Thicknesses of at least two (or even all) of the
internal thermal shields may be different from each other.
Densities of at least two of the internal thermal shields may be
different from each other. Each internal thermal shield may be
annular and define a central opening therein. Sizes of the central
openings of at least two (or even all) of the internal thermal
shields may be different from each other.
[0050] A single-crystal AlN substrate may be separated from the AlN
single crystal. The single-crystal AlN substrate may have a
diameter of at least 25 mm, at least 50 mm, at least 75 mm, or at
least 100 mm. A light-emitting device may be fabricated over at
least a portion of the AlN substrate. The light-emitting device may
be configured to emit ultraviolet light. At least a portion off the
AlN substrate may be removed from the light-emitting device after
or during fabrication of the light-emitting device.
[0051] In yet another aspect, embodiments of the invention feature
a method of forming single-crystal aluminum nitride (AlN). A seed
crystal having a growth face that includes, consists essentially
of, or consists of AlN is provided within a growth chamber. In
internal support is provided within the growth chamber. The
internal support defines an opening to accommodate growth of the
AlN single crystal therethrough. One or more internal thermal
shields are provided within the growth chamber. Each internal
thermal shield is at least partially supported by the internal
support. The growth chamber is heated. Vapor including, consisting
essentially of, or consisting of aluminum and nitrogen is condensed
within the growth chamber during heating thereof, thereby forming
an AlN single crystal on the growth face of the seed crystal. The
AlN single crystal extends from the seed crystal in an axial
direction perpendicular to the growth face.
[0052] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. At least a portion
of the internal support may be frusto-conical. The vapor may be
enhanced with atomic nitrogen proximate the edge portion of the AlN
single crystal. Enhancing the vapor with atomic nitrogen may
include, consist essentially of, or consist of (i) introducing
nitrogen (and/or nitrogen-containing) gas into the growth chamber
and (ii) generating a plasma proximate the edge portion of the AlN
single crystal with the nitrogen (and/or nitrogen-containing)
gas.
[0053] At least one (or even all) said internal thermal shield may
be oriented substantially parallel (e.g., .+-.5.degree.,
.+-.4.degree., .+-.3.degree., .+-.2.degree., .+-.1.degree., or
.+-.0.5.degree.) to the axial direction. At least one (or even all)
said internal thermal shield may be oriented substantially
perpendicular (e.g., .+-.5.degree., .+-.4.degree., .+-.3.degree.,
.+-.2.degree., .+-.1.degree., or .+-.0.5.degree.) to the axial
direction. At least one (or even all) said internal thermal shield
may be oriented at an inclination neither parallel nor
perpendicular to the axial direction. At least one (or even all)
said internal thermal shield may be annular and define therein a
central opening to accommodate growth of the AlN single crystal
therethrough. The one or more internal thermal shields may include,
consist essentially of, or consist of a plurality of internal
thermal shields. Thicknesses of at least two (or even all) of the
internal thermal shields may be different from each other.
Densities of at least two of the internal thermal shields may be
different from each other. Each internal thermal shield may be
annular and define a central opening therein. Sizes of the central
openings of at least two (or even all) of the internal thermal
shields may be different from each other.
[0054] A single-crystal AlN substrate may be separated from the AlN
single crystal. The single-crystal AlN substrate may have a
diameter of at least 25 mm, at least 50 mm, at least 75 mm, or at
least 100 mm. A light-emitting device may be fabricated over at
least a portion of the AlN substrate. The light-emitting device may
be configured to emit ultraviolet light. At least a portion off the
AlN substrate may be removed from the light-emitting device after
or during fabrication of the light-emitting device.
[0055] Embodiments of the invention may include AlN boules, wafers,
and/or light-emitting devices formed or formable in accordance with
any of the above methods.
[0056] These and other objects, along with advantages and features
of the present invention herein disclosed, will become more
apparent through reference to the following description, the
accompanying drawings, and the claims. Furthermore, it is to be
understood that the features of the various embodiments described
herein are not mutually exclusive and may exist in various
combinations and permutations. As used herein, the terms
"approximately," "about," and "substantially" mean.+-.10%, and in
some embodiments, .+-.5%. All numerical ranges specified herein are
inclusive of their endpoints unless otherwise specified. The term
"consists essentially of" means excluding other materials that
contribute to function, unless otherwise defined herein.
Nonetheless, such other materials may be present, collectively or
individually, in trace amounts.
BRIEF DESCRIPTION OF THE DRAWINGS
[0057] In the drawings, like reference characters generally refer
to the same parts throughout the different views. Also, the
drawings are not necessarily to scale, emphasis instead generally
being placed upon illustrating the principles of the invention. In
the following description, various embodiments of the present
invention are described with reference to the following drawings,
in which:
[0058] FIG. 1A is a schematic diagram of a seed crystal in
accordance with various embodiments of the invention;
[0059] FIGS. 1B-1D are schematic diagrams of diameter-expanded bulk
crystals in accordance with various embodiments of the
invention;
[0060] FIGS. 1E and 1F are schematic illustrations of bulk
crystals, and associated parameters thereof, in accordance with
various embodiments of the invention;
[0061] FIG. 2 is a schematic diagram of an apparatus for the growth
of single-crystal AlN in accordance with various embodiments of the
invention;
[0062] FIG. 3 is a schematic diagram depicting portions of an
apparatus for the growth of single-crystal AlN in accordance with
various embodiments of the invention;
[0063] FIG. 4 is a schematic diagram depicting portions of an
apparatus for the growth of single-crystal AlN in accordance with
various embodiments of the invention;
[0064] FIG. 5 is a picture of an exemplary AlN single-crystal boule
produced in accordance with various embodiments of the
invention;
[0065] FIGS. 6A and 6B are graphs of distribution of boule mass and
volume, respectively, of AlN single-crystal boules produced in
accordance with various embodiments of the invention;
[0066] FIG. 7A is a schematic diagraph of a light-emitting device
fabricated in accordance with various embodiments of the
invention;
[0067] FIG. 7B is a plan view photograph of the light-emitting
device of FIG. 7A during emission of light having a peak wavelength
of approximately 230 nm in accordance with various embodiments of
the invention;
[0068] FIG. 8 is a comparative graph of UV absorption coefficients,
as functions of wavelength, of conventional single-crystal AlN and
single-crystal AlN grown and annealed in accordance with various
embodiments of the invention;
[0069] FIG. 9 is a graph utilized to estimate Urbach energies of
the AlN samples of FIG. 8 in accordance with various embodiments of
the invention;
[0070] FIG. 10 is a schematic diagram of various components of
light utilized to determine UV absorption spectra and Urbach
energies in accordance with various embodiments of the
invention;
[0071] FIG. 11 is a comparative graph of emission intensity as a
function of wavelength for simulated LEDs emitting light having a
peak wavelength at about 217 nm in accordance with various
embodiments of the invention; and
[0072] FIG. 12 is a comparative graph of the emission spectra of
FIG. 11 in which the relative intensities of the LEDs have been
independently normalized to the same value in order to demonstrate
the narrower intensity peak of the device in accordance with
embodiments of the present invention.
DETAILED DESCRIPTION
[0073] Embodiments of the present invention enable the fabrication
of high-quality single-crystal AlN bulk crystals (i.e., boules
and/or substrates) that undergo significant diameter expansion
during crystal growth. FIGS. 1A-1C are schematic illustrations of
various crystals, and associated parameters thereof, relevant to
embodiments of the present invention. FIG. 1A depicts an exemplary
seed crystal 100 having a diameter 102, a front surface 104, and a
back surface 106. While the seed crystal 100 is depicted as
cylindrical with circular surfaces, the seed crystal 100 is not
limited to such shapes. As such, diameter 102 generally refers to
the largest lateral dimension of the seed crystal 100, and may
therefore correspond to a "width" or "maximum width," e.g., for
seed crystals 100 having non-circular shapes. In various
embodiments of the invention, the seed crystal 100 has a thickness
ranging from approximately 0.1 mm to approximately 3 mm. Typically,
the front surface 104 is exposed to the incoming vapor utilized for
crystal growth, and the resulting crystal extends from the front
surface 104. The seed crystal 100 may be mounted within the growth
apparatus via the back surface 106 (see, e.g., FIG. 2). Depending
upon the seed-mounting procedure, the seed crystal 100 may have an
exposed growth surface that is equal to or less than the area of
the front surface 104 (i.e., a portion of the front surface 104 may
be covered or otherwise prevented from receiving the incoming
vapor). Herein, references to "seed diameter" or "seeded diameter"
refer to the diameter of actual area of the seed crystal 100
exposed for growth thereon (i.e., the "seed area" or "seeded
area"), even if that area is less than the total area of front
surface 104. In addition, the seed diameter or seeded diameter may
have a shape different from that of the actual surface 104 of the
seed crystal 100 itself, resulting from, e.g., the masking or
otherwise occlusion of a portion of surface 104. For example, the
seeded diameter may be circular while the actual surface 104 is
non-circular or vice versa.
[0074] FIG. 1B is a schematic depiction of a crystal (or
"crystalline boule" or "boule") 108 resulting from crystal growth
on seed crystal 100 (e.g., via a vapor-phase transport technique
such as sublimation-recondensation). Note that the crystal 108 does
not terminate at a pointed tip, but rather has a relatively planar
surface due to the initiation of growth on the seed crystal 100.
The crystal 108 has an initial seeded diameter 110 (i.e., the
diameter of the seeded area of the crystal, which may correspond to
the diameter of the initial seed crystal or a portion thereof) and,
due to diameter expansion during growth, may be described as a
geometrical combination of a frustum 112 and a dome 114, the
frustum 112 resulting from diameter expansion during growth and the
dome 114 resulting from, at least in part, the shape of the thermal
field within the growth chamber. The frustum 112 may (but need not)
be, for example, right, circular, and conical. The dome 114 may
(but need not) be, e.g., a spherical cap or a spherical segment. In
various embodiments, the dome 114 may have the form of a cone
(e.g., with a rounded tip) or a truncated cone (e.g., a frustum
tapering in the opposite direction from that of the frustum 112).
As shown, the diameter (or other lateral dimension) of the crystal
may increase, due to diameter expansion to a maximum crystal
diameter 116. The curvature of the dome 114 may increase as the
radial thermal gradient utilized during crystal growth increases.
As such, crystals 108 having small (or even substantially
non-existent) domes 114 may result from the use of small radial
thermal gradients during crystal growth. That is, in accordance
with embodiments of the invention, the radial thermal gradient may
be adjusted (e.g., during growth) to decrease the size of dome 114
or to virtually eliminate dome 114 entirely. Note that, since
crystals 108 are grown from seed crystals 100, they are larger, and
contain more usable, high-quality volume (e.g., for the production
of single-crystal AlN wafers) than similar crystals grown by
unseeded growth. (Unseeded growth is typically reliant upon
spontaneous nucleation, which can introduce excessive numbers of
defects and/or non-uniformity in crystalline orientation.) As
disclosed herein, crystals 108, being produced by seeded growth,
may also incorporate at least a portion of the seed crystal 100
itself therein.
[0075] FIG. 1C is a cross-sectional view of an exemplary crystal
108. As shown, the crystal 108 has a total length 118 that
encompasses both the frustum and dome sections of the crystal. The
total length 118 includes both an expansion length 120 (i.e., the
length of the diameter-expanded volume of the crystal in the growth
direction, e.g., perpendicular from the surface of the seed crystal
100) and a dome length 122. In various embodiments of the
invention, the crystal 108 may include a portion 124 in which the
diameter is not expanded (due to, for example, deliberate
modification of the radial thermal gradient and/or diameter
expansion sufficient to reach the interior wall of the growth
apparatus), and portion 124 may have a length 126 that contributes
to total length 118. Portion 124 may be, e.g., cylindrical, or may
have one or more flat surfaces (e.g., may have the shape of a
hexagonal prism (for example, having sides parallel to the m-planes
{1-100})). Portion 124 may be present but is not necessarily
present in embodiments of the present invention. When present,
portion 124 may have a diameter that is substantially equal to the
maximum, or expanded, diameter 116, as shown in FIG. 1C. The
frustum 112 also has an expansion height, or slant height, 128,
which is measured along the surface of the diameter-expanded volume
of the crystal. It is readily apparent from FIG. 1C that, in the
absence of diameter expansion, the expansion height 128 and the
expansion length are equivalent.
[0076] FIG. 1D is a schematic diagram of another exemplary crystal
108 produced in accordance with embodiments of the present
invention. As shown, the crystal 108 of FIG. 1D is similar to the
crystal 108 of FIG. 1C, except that the "straight" portion 124
having a substantially constant diameter is longer than the
expansion length 120, due to rapid initial expansion of the crystal
(resulting from, for example, use of one or more of the techniques
in accordance with embodiments of the invention detailed herein).
Crystals 108 as shown in FIG. 1D may beneficially provide a large
crystalline volume from which many wafers having substantially the
same diameter may be produced.
[0077] In various exemplary embodiments, the expansion length 120
may range from approximately 1%, 2%, 3%, 5%, or 10% to
approximately 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of
total length 118, while the length 126 may range from approximately
0%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 5%, or 10% to approximately 60%,
65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of total length 118, and
the dome length 122 may range from approximately 0%, 0.1%, 0.2%,
0.5%, 1%, 2%, 3%, or 5% to approximately 20%, 25%, 30%, 35%, 40%,
or 45% of total length 118 (while, as shown in FIGS. 1C and 1D, the
sum of the expansion length 120, length 126, and dome length 122 is
equal to 100% of the total length 118).
[0078] FIGS. 1E and 1F are additional schematic illustrations of
various crystals, and associated parameters thereof, relevant to
embodiments of the present invention. FIG. 1E schematically depicts
how wafers may be sliced from the crystal 108 in accordance with
various embodiments of the invention. As shown, a wafer 130-1 and a
wafer 130-2 having a larger diameter may be sliced from the crystal
108 substantially parallel to the seed crystal 100. In other
embodiments, wafers may be sliced from the crystal 108 along other
directions, even substantially perpendicular to the plane of the
seed crystal 100. Wafers sliced from crystal 108 having a diameter
greater than that of the seed crystal 100 (e.g., wafer 130-2) may
be subsequently utilized as seed crystals for additional growth of
larger-diameter crystals, for example as disclosed in U.S. patent
application Ser. No. 16/008,407, filed on Jun. 14, 2018 (the '407
application), the entire disclosure of which is incorporated by
reference herein. Wafers sliced from the portion 124 of a crystal
108 as shown in FIGS. 1C and 1D may have substantially the same
diameter as each other.
[0079] FIG. 1F is a schematic sectional view of the crystal 108
depicting (in shading) expanded area 132, i.e., the cross-sectional
area of the crystal 108 exceeding the seeded area 110 and resulting
from diameter expansion. FIG. 1F also depicts an exemplary
expansion angle 134, which corresponds to the angle between the
normal of the seed crystal and the plane of the expansion height
128. In various embodiments, the expansion angle 134 may be, but is
not necessarily, substantially constant during the entire growth of
the crystal 108. That is, the plane of the expansion height 128
need not be linear. When wafers are cut from the crystal 108, the
expanded area 132 therefore corresponds to an annular region
extending inward from the outer edge of the wafer, and the seeded
area corresponds to a central area of the wafer having
approximately the size and shape of that of the seed crystal 100
utilized to grow the crystal 108. Thus, at least within the frustum
112 of a diameter-expanded crystal 108, a wafer separated from a
portion of the boule farther away from the seed crystal 100 will
have a larger expanded area 132 than one cut from the boule closer
to the seed crystal 100, while the seeded areas of both wafers may
be approximately the same in size and shape. On the other hand,
multiple wafers separated from a portion 124 of a crystal 108 will
have approximately the same expanded areas 132 and approximately
the same seeded areas.
[0080] The orientation of a wafer or seed crystal may be selected
from a boule or other crystal during slicing via, for example,
x-ray diffraction measurements and/or other materials
characterization enabling identification of the orientation of the
crystal; such techniques are known to those of skill in the art and
may be performed without undue experimentation. In accordance with
embodiments of the invention, a newly sliced wafer or seed may be
polished to reduce surface roughness and remove cutting artifacts
and/or damage. The polarity of a wafer or seed crystal may also be
identified and selected chemically. For example, the polarity may
be identified and selected via exposure of the wafer or seed to a
basic or acidic solution, which will roughen an N-polarity face
while leaving an Al-polarity face smooth, as detailed in the '407
application.
[0081] FIG. 2 depicts a crystal-growth apparatus 200 suitable for
the growth of single-crystal AlN in accordance with various
embodiments of the present invention. As shown, apparatus 200
includes a crucible 205 positioned on top of a crucible stand 210
within a susceptor 215. Both the crucible 205 and the susceptor 215
may have any suitable geometric shape, e.g., cylindrical. During a
typical growth process, an AlN boule 220 (e.g., a crystal 108) is
formed by condensation of a vapor 225 that includes, consists
essentially of, or consists of the elemental precursors of the AlN
boule 220, i.e., Al and N atoms and/or N.sub.2 molecules. In
typical embodiments, the vapor 225 arises from the sublimation of a
source material 230, which may include, consist essentially of, or
consist of the polycrystalline AlN source material described above.
The AlN boule 220 may form on and extend from a seed crystal 235.
(Alternatively, the AlN boule 220 may nucleate upon and extend from
a portion of the crucible 205 itself.) The seed crystal 235 may be
a single crystal (e.g., a polished wafer) including, consisting
essentially of, or consisting of AlN. In various embodiments, the
seed crystal 235 has a diameter (or width or other lateral
dimension) of at least approximately 10 mm, at least approximately
25 mm, at least approximately 35 mm, at least approximately 40 mm,
or even at least approximately 50 mm. In various embodiments, the
seed crystal 235 has a diameter (or width or other lateral
dimension) of approximately 50 mm or less, approximately 100 mm or
less, or approximately 150 mm or less, and/or single-crystal AlN
grown therefrom has a diameter (or width or other lateral
dimension) of approximately 150 mm or less. In various embodiments,
the crystalline orientation (i.e., the normal to the exposed plane
(e.g., c-plane)) of the seed crystal 235 is substantially parallel
to the c-axis. In other embodiments, the crystalline orientation of
the seed crystal 235 is at least approximately 5.degree., or even
at least approximately 10.degree. away from the c-axis; the
orientation of the seed crystal 235 may be toward a non-polar
direction. In various embodiments, the crystalline orientation of
the seed crystal 235 may be no more than approximately 30.degree.,
or no more than approximately 20.degree., away from the c-axis.
[0082] The crucible 205 may include, consist essentially of, or
consist of one or more refractory materials, such as tungsten,
rhenium, tantalum carbide, and/or tantalum nitride. As described in
the '135 patent and the '153 patent, the crucible 205 may have one
or more surfaces (e.g., walls) configured to selectively permit the
diffusion of nitrogen therethrough and selectively prevent the
diffusion of aluminum therethrough.
[0083] As shown in FIG. 2, during formation of the AlN boule 220, a
polycrystalline material 240 may (but does not necessarily) form at
one or more locations within the crucible 205 not covered by the
seed crystal 235. However, the diameter (or other radial dimension)
of the AlN boule 220 may expand, i.e., increase, during formation
of the AlN boule 220, thereby occluding the regions of
polycrystalline material 240 (if present) from impinging vapor 225
and substantially limiting or even eliminating their growth. As
shown in FIG. 2, the diameter of the AlN boule 220 may expand to
(or even start out at, in embodiments utilizing larger seed
crystals 235) be substantially equal to the inner diameter of the
crucible 205 (in which case no further lateral expansion of the AlN
boule 220 may occur).
[0084] The growth of the AlN boule 220 along a growth direction 245
typically proceeds due to a relatively large axial thermal gradient
(e.g., ranging from approximately 5.degree. C./cm to approximately
100.degree. C./cm) formed within the crucible 205. A heating
apparatus (not shown in FIG. 2 for clarity), e.g., an RF heater,
one or more heating coils, and/or other heating elements or
furnaces, heats the susceptor 215 (and hence the crucible 205) to
an elevated temperature typically ranging between approximately
1800.degree. C. and approximately 2300.degree. C. Prior to the
onset of growth, the crucible 205 and its contents (e.g., seed
crystal 235 and source material 230) may be held at a temperature
approximately equal to the desired growth temperature for a
predetermined soak time (e.g., between approximately 1 hour and
approximately 10 hours). In various embodiments, this soak at
temperature stabilizes the thermal field within the crucible 205,
promotes effective nucleation on the seed crystal 235, and promotes
high-quality transition from nucleation to bulk growth of the
single-crystalline AlN.
[0085] The apparatus 200 may feature one or more sets of top
thermal shields 250, and/or one or more sets of bottom axial
thermal shields 255, arranged to create the large axial thermal
gradient (by, e.g., better insulating the bottom end of crucible
205 and the source material 230 from heat loss than the top end of
crucible 205 and the growing AlN boule 220). During the growth
process, the susceptor 215 (and hence the crucible 205) may be
translated within the heating zone created by the heating apparatus
via a drive mechanism 260 in order to maintain the axial thermal
gradient near the surface of the growing AlN boule 220. One or more
pyrometers 265 (or other characterization devices and/or sensors)
may be utilized to monitor the temperature at one or more locations
within susceptor 215. The top thermal shields 250 and/or the bottom
thermal shields 255 may include, consist essentially of, or consist
of one or more refractory materials (e.g., tungsten), and may be
quite thin (e.g., between approximately 0.125 mm and 0.5 mm thick).
As detailed in the '612 patent, the top thermal shields 250 and/or
the bottom thermal shields 255 may be arranged in various
configurations and/or have various characteristics (i.e., different
numbers of shields, different spacings between shields, different
thicknesses, different sized apertures defined therethrough,
different sizes, etc.) in order to form a variety of different
axial and radial thermal gradients within the crucible 205 and
thus, the growth of the AlN boule 220 (e.g., the growth rate, the
degree of radial expansion during growth, if any, etc.).
[0086] In various embodiments, the crucible 205 has a lid 270 with
sufficient radiation transparency to enable at least partial
control of the thermal profile within the crucible 205 via the
arrangement of the top thermal shields 250. Furthermore, in
embodiments featuring a seed crystal 235, the seed crystal 235 is
typically mounted on the lid 270 prior to the growth of AlN boule
220. The lid 270 is typically mechanically stable at the growth
temperature (e.g., up to approximately 2300.degree. C.) and may
substantially prevent diffusion of Al-containing vapor
therethrough. Lid 270 generally includes, consists essentially of,
or consists of one or more refractory materials (e.g., tungsten,
rhenium, and/or tantalum nitride), and may be fairly thin (e.g.,
less than approximately 0.5 mm thick).
[0087] As shown in FIG. 2, each of the top thermal shields may have
an opening 275 therethrough. The opening 275 normally echoes the
geometry and/or symmetry of the crucible 205 (e.g., the opening 275
may be substantially circular for a cylindrical crucible 205). The
size of each opening 275 may be varied; typically, the size(s)
range from a minimum of 10 mm to a maximum of approximately 5 mm
(or even 2 mm) less than the diameter of the crucible 205.
[0088] For example, in an embodiment, five thermal shields 250,
each having a diameter of 68.5 mm and an opening size (diameter) of
45 mm, are used. The thickness of each of the thermal shields 250
is 0.125 mm, and the thermal shields 250 are spaced approximately 7
mm from each other. At a typical growth temperature of 2065.degree.
C., this shield arrangement results in a radial thermal gradient
(measured from the center of the semiconductor crystal to the inner
edge of the crucible) of 27.degree. C./cm. Of course, this value is
merely exemplary, and those of skill in the art may arrange thermal
shields to achieve a range of different radial thermal gradients
without undue experimentation.
[0089] Embodiments of the present invention enable even higher
rates of diameter expansion of the AlN crystal via augmentation of
the radial thermal gradient resulting from the arrangement of
thermal shields 250. (For avoidance of doubt, the techniques
detailed herein in accordance with embodiments of the invention
enable higher rates of diameter expansion of growing AlN single
crystals, while preserving crystal quality (and therefore, the
production of AlN single crystals having larger crystal
augmentation parameters, as detailed herein), than do techniques
detailed in the '612 patent.) In general, techniques in accordance
with embodiments of the invention increase the radial thermal
gradient via tailored heating of the edges of the growing crystal
and/or altering the condensing vapor to enhance lateral growth of
the crystal. In conventional techniques, often the conventional
wisdom is the suppression of the radial thermal gradient in order
to, e.g., minimize the curvature of the leading edge of the growing
crystal. The conventional wisdom in the art also tends to emphasize
the maintenance of a substantially uniform temperature in the
radial direction during crystal growth. Embodiments of the
invention contradict such conventional wisdom in order to further
enhance diameter expansion (for example, beyond that achievable
merely by the arrangement of external thermal shields, even in
combination with external differential heating and insulation
techniques) while maintaining high crystalline quality of the
resulting bulk crystal.
[0090] FIG. 3 illustrates one technique for enhancing the radial
thermal gradient in accordance with embodiments of the invention.
In the embodiment of FIG. 3, one or more internal shields (or
baffles) 300 are disposed within the crucible 205 proximate the
edge of the growing AlN boule 220. In accordance with various
embodiments, the internal shields 300 transfer heat from the walls
of the crucible 205 toward the edge of AlN boule 220, raising the
temperature thereof, and the internal shields 300 also retain the
heat in proximity to the edge of the AlN boule 220. In this manner,
the internal shields 300 augment, or increase, the radial thermal
gradient within the crucible 205, resulting in enhanced lateral
crystal growth and increased diameter expansion of AlN boule 220.
In various embodiments of the invention, the number of internal
shields 300 disposed within the crucible 205 ranges from 1 to 10,
or even 1 to 15. The present inventors have found that the use of
internal shields 300 within the crucible 205 enables more rapid
diameter expansion of the AlN boule 220 and, relatedly, the
formation of larger boules, with more usable volume for the
fabrication of substrates therefrom, than is possible with
conventional growth techniques, despite the conventional wisdom
that additional objects disposed within the growth crucible tend to
deleteriously disrupt crystal growth and/or act as nucleation
centers for extraneous, parasitic growth of polycrystalline or
otherwise unusable material.
[0091] In various embodiments of the invention, the internal
shields 300 include, consist essentially of, or consist of one or
more refractory materials (e.g., tungsten and/or TaC), and may be
quite thin (e.g., between approximately 0.125 mm and 0.5 mm thick).
In other embodiments, one or more of the thermal shields may have a
greater thickness, e.g., ranging from approximately 1 mm to
approximately 3 mm. In various embodiments, the density (and
concomitant impact on the thermal field proximate the shield) of
one or more of the internal shields 300 may vary. For example, one
or more of the internal shields 300 may have a density ranging from
approximately 10% full density to approximately 100% full density
(as an example, the 100% full density of tungsten is approximately
19.3 g/cm.sup.3). Thin foils of refractory materials having
different densities and/or thicknesses are commercially available
and may be provided without undue experimentation. In various
embodiments, an internal shield having a larger thickness and/or a
larger density may transfer more heat, and therefore increase the
radial thermal gradient, more than such shields having smaller
thicknesses and/or smaller densities.
[0092] As shown in FIG. 3, the outer boundary of the internal
shields 300 may conform substantially to the shape and size of the
interior wall of the crucible 205, and the size of the central
openings in the internal shields 300 may vary to accommodate
diameter expansion (e.g., expected diameter expansion) of the AlN
boule 220. For example, the central openings of the internal
shields 300 may increase as the distance of the individual shields
away from the seed crystal 235 increases, at least when the
internal shields 300 are positioned where it is desired or expected
for the AlN boule 220 to undergo diameter expansion. In addition,
the density and/or thickness of the individual internal shields 300
may vary (e.g., increase) as a function of distance away from the
seed crystal 235, at least when the internal shields 300 are
positioned where it is desired or expected for the AlN boule 220 to
undergo diameter expansion. Such increases may compensate for the
loss of volume of the shields 300 having larger central openings.
In various embodiments, at positions where it is desired or
expected for the AlN boule 220 to not undergo diameter expansion,
the central opening size, density, and/or thickness of the internal
shields 300 may be substantially constant.
[0093] Similarly, in regions in which more rapid diameter expansion
is desired, the spacing between the internal shields 300 may be
decreased, compared to regions in which diameter expansion is not
desired or expected (e.g., to as large a degree). Example spacings
between the internal shields 300 may range from approximately 1 mm
to approximately 50 mm, or from approximately 5 mm to approximately
10 mm.
[0094] In accordance with embodiments of the invention, the
internal shields 300 may be mounted within the crucible 205 via a
variety of different approaches. For example, the internal shields
300 may be held by or affixed to the interior surface of the
crucible 205 at their outer edges. The internal shields 300 may
each be rested on a platform or pedestal within the crucible 205
(e.g., extending from the inner wall thereof), or the internal
shields 300 may rest at their central openings on an internal
support extending from the top surface of the crucible proximate
the seed crystal 235. (The internal support is not depicted in FIG.
3 for clarity, but may echo the outer shape of the crystal and
either be in contact therewith or spaced away therefrom; the inner
edges of the internal shields 300 may contact the internal support
and therefore be supported thereby.) The internal support, which
may include, consist essentially of, or consist of one or more of
the same materials as internal shields 300, may have the shape of a
truncated cone having a first inner diameter at its upper end
(i.e., the end proximate the seed crystal 235) approximately equal
to, or even less than the diameter of seed crystal 235, and a
second, larger, inner diameter at its lower end. In various
embodiments, the inner diameter of the internal support may
increase at a rate substantially equal to or greater than the
(e.g., desired or expected) expansion rate and/or angle of the AlN
boule 220. That is, in embodiments of the invention, the AlN boule
220 may not contact (at least, not fully contact except at one or
more discrete points) the interior surface of the internal support
(i.e., the internal support may not contact and fit snugly around
the AlN boule 220). In other embodiments, the inner diameter of the
internal support may increase at a rate smaller than the expected
expansion angle of the AlN boule 220 (i.e., the expansion angle
that would otherwise occur given the growth parameters in the
absence of the internal support), and thus the internal support may
restrict the expansion angle and rate of expansion of the AlN boule
220 to desired values defined by the geometry of the support.
[0095] In various embodiments, all or a portion of the internal
support may be conical (i.e., have a diameter that increases in a
direction away from the seed crystal 235), e.g., at positions where
it is desired or expected for the AlN boule 220 to undergo diameter
expansion. For example, all or a portion of the internal support
may have the shape of a frustum having a smaller-diameter top
opening to accommodate the seed crystal 235, and which flares out
to accommodate the diameter-expanded AlN boule 220. In various
embodiments, all or a portion of the internal support may be
cylindrical (i.e., have a diameter than is substantially constant
as a function of distance away from the seed crystal 235), e.g., at
positions where it is desired or expected for the AlN boule 220 to
not undergo diameter expansion. In one example, the internal
support may be partially conical and partially cylindrical, echoing
the diameter change of portions 112 and 124 of the crystal 108
shown in FIG. 1C.
[0096] In FIG. 3, the interior shields 300 are depicted as
extending around the AlN boule 220 approximately parallel to the
plane of the seed crystal 235 (i.e., approximately perpendicular to
the lateral growth direction) and/or to the top or bottom surface
of the crucible 205, but in various embodiments the internal
shields 300 are oriented at other angles. For example, one or more
of the interior shields 300 may extend approximately perpendicular
to the plane of the seed crystal 235 or at an inclined angle
thereto (e.g., at an angle ranging from approximately 5.degree. to
approximately 85.degree. with respect to the plane of the seed
crystal and/or to the top and/or bottom surface of the crucible).
In an example embodiment, one or more of the interior shields 300
may be oriented at an angle approximately perpendicular to the
plane defined by the expansion height of the AlN boule 220 (i.e.,
approximately perpendicular to the edge of the frustum of the
crystal)--for example, one or more of the internal shields 300 may
extend approximately perpendicular from the inclined edge of the
internal support. In various embodiments, one or more of the
interior shields 300 may be oriented at angles different from those
at while one or more of the other interior shields 300 are
oriented. In various embodiments, the interior shields 300 merely
influence the thermal field within the crucible 205 (e.g., increase
the radial thermal gradient at one or more points and/or regions)
and do not contact the AlN boule 220 itself during growth
thereof.
[0097] In various embodiments of the invention, atmospheric plasma
is utilized to enrich the source vapor phase within the crucible
205 with nitrogen atoms and concentrate such atoms preferentially
at the lateral edge of the growing crystal. The excess nitrogen
produced by the plasma process promotes increased lateral growth of
the AlN crystal at rates exceeding those enabled by the mere
introduction of nitrogen gas (or a nitrogen-containing gas) itself,
even at super-atmospheric growth pressures. As shown in FIG. 4,
nitrogen gas (and/or a nitrogen-containing gas) may be introduced
into the crucible 205 via one or more nozzles 400 in order to
provide excess nitrogen proximate the edge of the growing AlN boule
220. In addition, one or more plasma electrodes 410 are disposed
proximate to (e.g., at a distance ranging from approximately 0.3 cm
to approximately 1 cm away from) the edge of the AlN boule 220
(e.g., where the edge is expected to reach or be during growth)
and/or to the internal support (if present). As shown, the
electrodes 410 may be arranged at an angle to accommodate diameter
expansion of the AlN boule 220, but in other embodiments the
electrodes 410 may be arranged in other configurations (e.g.,
parallel to the crucible walls or at an angle and then parallel to
the crucible walls). AC or DC current may be applied to the
electrodes 410 via a high-frequency current source (which may be
incorporated with the RF source utilized for crystal growth), and a
pulsed electric arc may be generated via high-voltage discharge at
the electrodes 410. The nitrogen gas from nozzles 400 may flow
proximate or through the electrodes 410 and be converted into a
plasma that envelops all or a portion of the edge of the AlN boule
220. This nitrogen plasma significantly and preferentially
increases the amount of nitrogen within the expanded portion of the
AlN boule 220, increasing its lateral growth rate (and therefore
enhancing the diameter expansion of AlN boule 220).
[0098] In various embodiments, the electrodes 410 may be operated,
and the resulting plasma formed, uniformly during most of
significantly all of the growth of the AlN boule 220. In other
embodiments, the electrodes 410 may be operated only during one,
two, or more intervals during the growth, and the plasma may not be
present between such intervals. In yet other embodiments, the
current applied to the electrodes 410 may be varied one or more
times during the growth to increase or decrease the amount of
plasma produced during particular points of the growth process. In
this manner, the rate of diameter expansion of AlN boule 220, and
the resulting shape thereof, may be influenced by the presence or
absence of the plasma, and/or of the level of power supplied to the
electrodes 410.
[0099] Embodiments of the present invention enable the growth of
AlN single crystals having masses, volumes, and/or rates of
diameter expansion greater than those enabled by conventional
techniques. For example, embodiments of the invention enable the
formation of AlN single crystals having large crystal augmentation
parameters (CAPs), where the CAP, in mm, is defined as:
CAP = A E - A S L E = .pi. 4 .times. L E ( d E 2 - d S 2 )
##EQU00007##
where A.sub.E is the expanded area (i.e., the cross-sectional area
of the portion of the crystal having the maximum diameter 116 in
FIGS. 1B and 1C) in mm.sup.2, d.sub.E is the expansion diameter
(i.e., the maximum diameter 116 in FIGS. 1B and 1C) in mm, A.sub.S
is the seeded area (i.e., the cross-sectional area of the portion
of the crystal having the seeded diameter 110 in FIGS. 1B and 1C)
in mm.sup.2, d.sub.S is the seed diameter (i.e., the seed diameter
110 in FIGS. 1B and 1C, which may correspond to the minimum
diameter of the crystal) in mm, and L.sub.E is the expansion length
(i.e., length 120 in FIGS. 1C and 1F) of the crystal in mm. In
accordance with embodiments of the invention, the CAP value
provides a superior measure of diameter expansion, normalized to
crystal length, than the expansion angle 134 (see FIG. 1F), as the
expansion angle may vary during growth of the crystal and/or may be
difficult to measure.
[0100] Embodiments of the present invention enable the growth of
AlN single crystals having CAPs unattainable utilizing conventional
techniques, due at least in part to faster diameter expansion
during crystal growth. Embodiments of the invention also maintain
high crystal quality, notwithstanding the faster diameter expansion
during crystal growth. FIG. 5 is a photograph of an exemplary AlN
single crystal 500 grown in accordance with embodiments of the
invention. AlN single crystals in accordance with embodiments of
the invention may have CAP values greater than 20, greater than 40,
greater than 60, greater than 80, greater than 90, greater than
100, greater than 150, greater than 500, greater than 1000, or even
greater than 1500 (herein, all CAP values are in units of mm unless
otherwise indicated), while AlN crystals produced by conventional
techniques and AlN crystals reported in the literature have
computed CAP values less than 20 (e.g., between 10 and 15, or even
less). Conventional growth techniques incapable of fast diameter
expansion require much longer growths (and concomitantly larger
expansion lengths and smaller CAPs) to achieve large expanded areas
of the resulting AlN single crystals. Thus, embodiments of the
invention facilitate the faster, more economical production of
large, high-quality AlN crystals (e.g., single-crystal AlN wafers)
from small seed crystals. For example, the crystal 500 of FIG. 5
has a CAP of 45, thereby illustrating the superiority of
embodiments of the present invention over conventional techniques.
In accordance with various embodiments, the CAP of AlN single
crystals may be no greater than approximately 1600, or no greater
than approximately 1700, or no greater than approximately 2000.
[0101] Table 1 below reports various CAP values for a variety of
different crystals produced by the present inventors, as well as
the ratios (in %) of various dimensional parameters for the
crystals as shown in FIGS. 1B-1D. In Table 1, crystals #1-#4 and
#10-#15 had the shape of crystal 108 depicted in FIG. 1B (i.e.,
with no "straight" portion 124), while crystal #5 had the shape of
crystal 108 depicted in FIG. 1C (i.e., with a longer frustum 112
and corresponding expansion length) and crystals #6-#9 had the
shape of crystal 108 depicted in FIG. 1D (i.e., with a short
expansion length and longer straight portion 124).
TABLE-US-00001 TABLE 1 Ratios, % Boule Expansion Straight Expansion
Length/ Length/ Height/ Length/ Expanded Total Total Expanded
Crystal CAP Diameter Length Length Diameter # (mm) (118/116)
(120/118) (126/118) (120/116) 1 80 33 44 0 14.55 2 92 35 64 0 22.58
3 99 44 54 0 23.44 4 132 54 34 0 18.46 5 75 42 19 37 8.08 6 1570 38
1 18 0.38 7 1059 52 2 61 0.93 8 777 48 2 49 0.96 9 314 54 4 73 1.92
10 122 35 50 0 17.65 11 91 47 50 0 23.53 12 110 27 50 0 13.64 13
138 42 50 0 20.83 14 28 66 72 0 47.87 15 110 44 42 0 18.51
[0102] Embodiments of the invention also enable the fabrication of
AlN single crystals having unusually large masses and/or volumes
compared to conventional AlN crystals. For example, AlN
single-crystal boules grown in accordance with embodiments of the
present invention may have a mass greater than approximately 78 g,
greater than approximately 100 g, greater than approximately 120 g,
or greater than approximately 140 g, greater than approximately 220
g, or even greater than approximately 240 g. In accordance with
various embodiments, the mass may be less than approximately 350 g,
or less than approximately 300 g. When larger seeds are utilized,
AlN single-crystal boules grown in accordance with embodiments of
the present invention may have even larger masses, e.g., greater
than approximately 300 g, greater than approximately 500 g, greater
than approximately 800 g, greater than approximately 1000 g, or
even greater than approximately 1200 g. In accordance with various
embodiments, the mass may be less than approximately 1500 g, or
less than approximately 1400 g. Thus, exemplary ranges of boule
mass in accordance with embodiments of the present invention
include, but are not limited to, approximately 78 g-approximately
1300 g, approximately 78 g-approximately 300 g, and approximately
380 g-approximately 1300 g.
[0103] Correspondingly (and assuming a constant boule density of
3.255 g/cm.sup.3 for AlN), AlN single-crystal boules grown in
accordance with embodiments of the present invention may have a
volume greater than approximately 24 cm.sup.3, greater than
approximately 30 cm.sup.3, greater than approximately 50 cm.sup.3,
greater than approximately 70 cm.sup.3, greater than approximately
75 cm.sup.3, or greater than approximately 80 cm.sup.3. In
accordance with various embodiments, the volume may be less than
approximately 100 cm.sup.3, or less than approximately 90 cm.sup.3.
When larger seeds are utilized, AlN single-crystal boules grown in
accordance with embodiments of the present invention may have even
larger volumes, e.g., greater than approximately 100 cm.sup.3,
greater than approximately 200 cm.sup.3, greater than approximately
300 cm.sup.3, or even greater than approximately 350 cm.sup.3. In
accordance with various embodiments, the volume may be less than
approximately 500 cm.sup.3, or less than approximately 400
cm.sup.3. Thus, exemplary ranges of boule volume in accordance with
embodiments of the present invention include, but are not limited
to, approximately 24 cm.sup.3-approximately 400 cm.sup.3,
approximately 24 cm.sup.3-approximately 80 cm.sup.3, and
approximately 120 cm.sup.3-approximately 400 cm.sup.3.
[0104] FIG. 6A is a plot showing the distribution of mass (and
standard deviation thereof) of over 1200 different AlN
single-crystal boules grown in accordance with embodiments of the
present invention with seed crystals having diameters of
approximately 52 mm or less. Using such seed crystals, as shown,
the mass of the boules ranges from approximately 70 g to over
approximately 250 g. FIG. 6B is a plot showing the distribution
(and standard deviation) of the calculated boule volumes of the
over 1200 different AlN single-crystal boules. Such volumes, in
these example embodiments, range from approximately 20 cm.sup.3 to
approximately 80 cm.sup.3. As demonstrated, AlN single-crystal
boules grown in accordance with embodiments of the present
invention have larger mass and/or volume than those produced using
conventional techniques. The values reported in FIGS. 6A and 6B
scale up accordingly when larger seed crystals are utilized for
growth in accordance with embodiments of the present invention, and
thus the values reported in FIGS. 6A and 6B should not be
interpreted as limiting embodiments of the present invention. The
present inventors have achieved boules having larger masses and
volumes, as detailed above, using larger seed crystals.
[0105] In various embodiments (and as demonstrated by, e.g., Table
1 above), AlN single-crystal boules grown in accordance with
embodiments of the invention have ratios of boule length (i.e.,
total length 118 in FIGS. 1C and 1F) to maximum diameter (i.e.,
maximum crystal diameter 116 in FIGS. 1B and 1C) ranging from
approximately 0.3 to approximately 0.7, or ranging from
approximately 0.35 to approximately 0.66. In various embodiments
AlN single-crystal boules grown in accordance with embodiments of
the invention have ratios of expansion length (i.e., expansion
length 120 in FIGS. 1C and 1F) to maximum diameter (i.e., maximum
crystal diameter 116 in FIGS. 1B and 1C) falling into one of two
different ranges, depending upon the rapidity of the diameter
expansion. For example, AlN single-crystal boules grown in
accordance with embodiments of the invention having small expansion
lengths (e.g., as shown in FIG. 1D; for example, boules having
ratios of expansion length to total length ranging from
approximately 0.5% to approximately 5%, or approximately 1% to
approximately 4%) may have ratios of expansion length to maximum
diameter ranging from approximately 0.002 to approximately 0.02, or
from approximately 0.003 to approximately 0.02, or from
approximately 0.003 to approximately 0.01. In another example, AlN
single-crystal boules grown in accordance with embodiments of the
invention having larger expansion lengths (e.g., as shown in FIGS.
1B and 1C; for example, boules having ratios of expansion length to
total length ranging from approximately 15% to approximately 80%,
or from approximately 30% to approximately 70%) may have ratios of
expansion length to maximum diameter ranging from approximately
0.08 to approximately 0.5, or from approximately 0.1 to
approximately 0.3, or from approximately 0.15 to approximately
0.25.
[0106] The values of both ratios are lower than those previously
achieved in the art and demonstrate the superiority of AlN
single-crystal boules grown in accordance with embodiments of the
present invention compared to those produced using conventional
techniques. For example, boules in accordance with embodiments of
the present invention enable the fabrication of greater numbers of
large-diameter AlN single-crystal wafers per total boule length,
i.e., the single-crystal AlN is more beneficially distributed
within the boule, at least from the standpoint of large wafer
production. The crystals produced in accordance with embodiments of
the invention are therefore more economical, and enable production
of larger wafers therefrom, when compared to conventional crystals
and production techniques therefor.
[0107] In accordance with embodiments of the invention, the seed
diameter may range from approximately 5 mm to approximately 100 mm,
approximately 5 mm to approximately 52 mm, or approximately 52 mm
to approximately 100 mm. The total boule length may range from
approximately 18 mm to approximately 50 mm, approximately 18 mm to
approximately 35 mm, or approximately 30 mm to approximately 50 mm.
The maximum crystal diameter may range from approximately 17 mm to
approximately 120 mm, approximately 17 mm to approximately 65 mm,
or approximately 65 mm to approximately 120 mm. These values are
exemplary and should not be interpreted as limiting embodiments of
the present invention.
[0108] Moreover, single-crystal AlN boules fabricated in accordance
with embodiments of the invention exhibit high crystal quality,
notwithstanding the high rates of diameter expansion utilized
during their formation. For example, boules fabricated in
accordance with embodiments of the invention exhibit threading
dislocation densities less than 10.sup.5 cm.sup.-2, or even less
than 3.times.10.sup.4 cm.sup.-2, as confirmed by x-ray topography
measurements. Moreover, such low defect densities are approximately
the same in peripheral, expanded regions of the boules and the
central portions of the boules.
[0109] One or more substrates (or "wafers") may be separated from
AlN boule 220 by the use of, e.g., a diamond annular saw or a wire
saw, after crystal growth. In an embodiment, a crystalline
orientation of a substrate thus formed may be within approximately
2.degree. (or even within approximately 1.degree., or within
approximately 0.5.degree.) of the (0001) face (i.e., the c-face).
Such c-face wafers may have an Al-polarity surface or an N-polarity
surface, and may subsequently be prepared as described in U.S. Pat.
No. 7,037,838, the entire disclosure of which is hereby
incorporated by reference. In other embodiments, the substrate may
be oriented within approximately 2.degree. of an m-face or a-face
orientation (thus having a non-polar orientation) or may have a
semi-polar orientation if AlN boule 220 is cut along a different
direction. The surfaces of these wafers may also be prepared as
described in U.S. Pat. No. 7,037,838. The substrate may have a
roughly circular cross-sectional area with a diameter of greater
than approximately 50 mm. The substrate may have a thickness that
is greater than approximately 100 .mu.m, greater than approximately
200 .mu.m, or even greater than approximately 2 mm. The substrate
typically has the properties of AlN boule 220, as described herein.
After the substrate has been cut from the AlN boule 220, one or
more epitaxial semiconductor layers and/or one or more
light-emitting devices, e.g., UV-emitting light-emitting diodes or
lasers, may be fabricated over the substrate, for example as
described in U.S. Pat. Nos. 8,080,833 and 9,437,430, the entire
disclosure of each of which is hereby incorporated by
reference.
[0110] AlN bulk crystals (e.g., boules and/or wafers) produced in
accordance with embodiments of the present invention may have etch
pit density measurements (i.e., etching measurements that reveal
defects such as threading dislocations intersecting the surface of
the crystal) ranging from approximately 5.times.10.sup.3 cm.sup.-2
to approximately 1.times.10.sup.4 cm.sup.-2. AlN crystals in
accordance with embodiments of the present invention may have a
density of threading edge dislocations ranging from approximately
1.times.10.sup.3 cm.sup.-2 to approximately 1.times.10.sup.4
cm.sup.-2 and a density of threading screw dislocations ranging
from approximately 1 cm.sup.-2 to approximately 10 cm.sup.-2, e.g.,
a total threading dislocation density less than approximately
10.sup.4 cm.sup.-2. When measured via x-ray diffraction, x-ray
rocking curves (e.g., along (0002) and/or (10-12)) of AlN crystals
in accordance with embodiments of the invention may have full width
at half maximum (FWHM) values less than 50 arcsec (e.g., ranging
from approximately 30 arcsec to approximately 50 arcsec, or from
approximately 40 arcsec to approximately 50 arcsec), or even less
than 40 arcsec (e.g., ranging from approximately 20 arcsec to
approximately 40 arcsec, approximately 30 arcsec to approximately
40 arcsec, or approximately 20 arcsec to approximately 35 arcsec).
As measured by secondary ion mass spectroscopy (SIMS), AlN single
crystals in accordance with embodiments of the invention may have
carbon concentrations of approximately 1.8.times.10.sup.16
cm.sup.-3-5.times.10.sup.17 cm.sup.-3, as well as oxygen
concentrations of approximately 1.times.10.sup.17
cm.sup.-3-7.9.times.10.sup.17 cm.sup.-3. In various embodiments,
the carbon concentration may range from approximately
1.8.times.10.sup.16 cm.sup.-3 to approximately 5.times.10.sup.16
cm.sup.-3. The thermal conductivity of AlN single crystals in
accordance with embodiments of the invention may be greater than
approximately 290 Watts per meter-Kelvin (W/mK), as measured by the
American Society for Testing and Materials (ASTM) Standard E1461-13
(Standard Test Method for Thermal Diffusivity by the Flash Method),
the entire disclosure of which is incorporated by reference herein,
and provided by a commercial vendor such as NETZSCH Inc. of Exton,
Pa.
[0111] FIG. 7A is a schematic view of a UV LED 700 fabricated on an
AlN substrate in accordance with embodiments of the present
invention. As shown, the UV LED 700 features a set of layers
epitaxially grown over an AlN substrate 705 and two top-side metal
contacts 710, 715. Specifically, immediately above the substrate is
a 500 nm layer 720 of undoped (i.e., unintentionally doped) AlN,
topped with a bottom contact layer 725 of n-doped (with Si at a
concentration of 2.times.10.sup.18 cm.sup.-3)
Al.sub.0.83Ga.sub.0.17N that is 500 nm thick. Above the bottom
contact layer 725 is a multiple-quantum-well (MQW) layer 730
featuring five sets of a 2 nm thick Al.sub.0.78Ga.sub.0.22N quantum
well and a 6 nm thick Al.sub.0.85Ga.sub.0.15N barrier, all of which
are undoped. Above the MQW layer 730 is a 10 nm thick
electron-blocking layer formed of undoped Al.sub.0.95Ga.sub.0.05N.
Above the electron-blocking layer is an undoped graded layer 735
graded from Al.sub.0.95Ga.sub.0.05N to GaN over a thickness of 30
nm. Finally, over the graded layer 735 is a 10 nm thick p-doped
(with Mg at a concentration of 1.times.10.sup.19 cm.sup.-3) GaN cap
layer 740. The p-metal layer 710 is formed over the cap layer 740,
while the n-metal layer 715 is formed over the bottom contact layer
725 (revealed by etching away the overlying structure, for
example). FIG. 7B is a plan-view photograph of the UV LED 700 of
FIG. 7A when emitting light at approximately 230 nm. Devices such
as that depicted in FIGS. 7A and 7B exhibited output powers between
20 .mu.W and 500 .mu.W at currents of 20 mA and at room
temperature, continuous wave (CW) operation. Such output powers are
indicative of external quantum efficiencies ranging from 0.02% to
0.5% in the wavelength range of 228 nm to 238 nm.
[0112] After formation of the electrodes (e.g., contacts 710, 715),
the resulting light-emitting device may be electrically connected
to a package, for example as detailed in U.S. Pat. No. 9,293,670,
filed on Apr. 6, 2015 (the '670 patent), the entire disclosure of
which is incorporated by reference herein. A lens may also be
positioned on the device to transmit (and, in various embodiments,
shape) the light emitted by the device. For example, a rigid lens
may be disposed over the device as described in the '670 patent or
in U.S. Pat. No. 8,962,359, filed on Jul. 19, 2012, or in U.S. Pat.
No. 9,935,247, filed on Jul. 23, 2015, the entire disclosure of
each of which is incorporated by reference herein. After packaging,
any remaining portion of the substrate may be removed.
[0113] In accordance with embodiments of the invention, various
techniques for partial or complete substrate removal may be
utilized if desired. For example, etching techniques, such as
electrochemical etching techniques described in U.S. patent
application Ser. No. 16/161,320, filed on Oct. 16, 2018, the entire
disclosure of which is incorporated by reference herein, may be
utilized. In other embodiments, techniques like those utilized in
U.S. patent application Ser. No. 15/977,031, filed on May 11, 2018,
may be utilized.
[0114] AlN crystals, and wafers produced therefrom, in accordance
with embodiments of the present invention may also advantageously
exhibit high levels of UV transparency, even at deep-UV
wavelengths, for example as described in U.S. patent application
Ser. No. 16/444,147, filed on Jun. 18, 2019 (the '147 application),
the entire disclosure of which is incorporated by reference herein.
For example, embodiments of the invention include techniques for
the control and reduction of carbon content in the source material
utilized to grow the AlN single crystal and UV-transparency
enhancement via thermal treatments, as detailed below.
[0115] In various embodiments, the polycrystalline AlN ceramic may
be fabricated in accordance with the techniques described in U.S.
Pat. No. 9,447,519 (the '519 patent), the entire disclosure of
which is incorporated by reference herein, i.e., a "pellet-drop"
technique using high-purity Al pellets melted in the presence of
nitrogen to form AlN polycrystalline ceramic material. In various
embodiments, the ceramic is broken up into fragments to facilitate
removal of much of the carbon therefrom. The ceramic may be
fragmented by, e.g., application of mechanical force. The present
inventors have found that, surprisingly, much of the carbon present
in the polycrystalline AlN ceramic remains on smaller fragments
and/or dust (e.g., particles having large aggregate surface area
and/or having diameters less than about 2 mm) resulting from the
fragmentation process, while larger fragments (e.g., ones having
widths, diameters, or other lateral dimensions ranging from 0.5 cm
to 2 cm) exhibit smaller carbon concentrations. In various
embodiments, the fragments of the AlN ceramic may be separated on
the basis of size using one or more sieves, and/or compressed air
or another fluid (e.g., nitrogen or an inert gas such as argon) may
be applied to the fragments to minimize or reduce the amount of
dust or other particles thereon. For example, as reported in the
'147 application, the entire disclosure of which is incorporated by
reference herein, after fragmentation and separation, the larger
fragments have carbon concentrations that range from approximately
5 ppm to approximately 60 ppm, with an average carbon concentration
of approximately 26 ppm. In stark contrast, the resulting powder
and smaller fragments have carbon concentrations that range from
approximately 108 ppm to approximately 1800 ppm, with an average
carbon concentration of approximately 823 ppm.
[0116] Thus, in accordance with various embodiments of the
invention, one or more of the larger fragments of the AlN
polycrystalline ceramic, once separated from the smaller fragments
and powder, may be utilized directly as the source material for
formation of single-crystal AlN (as detailed above). In other
embodiments, one or more (typically more) of the fragments are
collected and placed into a crucible (e.g., a tungsten (W) vessel)
for subsequent heat treatment. (While in preferred embodiments only
the larger fragments of the polycrystalline AlN ceramic are heat
treated, embodiments of the invention do encompass heat treatment
of the entire, unfragmented ceramic.)
[0117] In various embodiments, the optional subsequent preparation
stage involves an annealing and densification treatment of at least
a portion of the polycrystalline AlN ceramic (e.g., one or more
larger fragments thereof) to form high-quality polycrystalline AlN
source material. In accordance with various embodiments of the
invention, the AlN ceramic (or portion thereof) may be heated to a
first temperature T1 ranging from 1100.degree. C. to 2000.degree.
C. and held at temperature T1 for a time period t1 of, for example,
2 hours to 25 hours. Thereafter, the ceramic (or portion thereof)
may be heated to a higher second temperature T2 (e.g., a
temperature ranging from 2000.degree. C. to 2250.degree. C.) and
held at temperature T2 for a time period t2 of, for example, 3
hours to 15 hours. During the heat treatment, the ceramic (or
portion thereof) is annealed and densified to form a
polycrystalline AlN source material that may be utilized in the
subsequent formation of single-crystal AlN bulk crystals. Because
the polycrystalline AlN source material is generally approximately
stoichiometric AlN with low concentrations of impurities, it may be
used to form an AlN bulk crystal without further preparation (e.g.,
without intermediate sublimation-recondensation steps).
[0118] In an alternative heat treatment in accordance with
embodiments of the invention, a longer ramp to temperature T2 is
utilized in place of the first annealing step at temperature T1. In
accordance with various embodiments of the invention, the AlN
ceramic (or portion thereof) may be ramped to temperature T2 (e.g.,
a temperature ranging from 2000.degree. C. to 2250.degree. C.) over
a time period t1 ranging from, for example, 5 hours to 25 hours.
Thereafter, the ceramic (or portion thereof) may be held at
temperature T2 for a time period t2 of, for example, 3 hours to 25
hours. During the heat treatment, the ceramic (or portion thereof)
is annealed and densified to form a polycrystalline AlN source
material that may be utilized in the subsequent formation of
high-quality single-crystal AlN bulk crystals. Because the
polycrystalline AlN source material is generally approximately
stoichiometric AlN with low concentrations of impurities, it may be
used to form an AlN bulk crystal without further preparation (e.g.,
without intermediate sublimation-recondensation steps).
[0119] In various embodiments, the carbon concentration of the
polycrystalline AlN source material, as measured by instrumental
gas analysis (IGA), ranges from approximately 3.0.times.10.sup.18
cm.sup.-3 to approximately 1.8.times.10.sup.19 cm.sup.-3,
approximately 3.8.times.10.sup.18 cm.sup.-3 to approximately
1.2.times.10.sup.19 cm.sup.-3, or even from approximately
3.0.times.10.sup.18 cm.sup.-3 to approximately 9.0.times.10.sup.18
cm.sup.-3. After the optional densification heat treatment, the
density of the polycrystalline AlN source material, as measured by
pycnometry at room temperature, may be approximately equal to that
of single-crystal AlN, i.e., approximately 3.25 g/cm.sup.3 to 3.26
g/cm.sup.3. In various embodiments, the measured density of the AlN
ceramic without the densification heat treatment may be lower,
e.g., approximately 2.95 g/cm.sup.3 to approximately 3.20
g/cm.sup.3. In various embodiments, after the optional
densification heat treatment, the polycrystalline AlN source
material typically has an amber color and is composed of fairly
large grains (e.g., average grain diameter ranging from
approximately 0.1 mm to approximately 5 mm).
[0120] Referring back to FIG. 2, in accordance with embodiments of
the invention, one or more internal parts of the crystal-growth
apparatus 200 (e.g., the crucible 205, the susceptor 215, and/or
the crucible stand 210) may be annealed before crystal growth and
formation of AlN boule 220, and such annealing may advantageously
decrease the carbon concentration in the AlN boule 220. In various
embodiments, the one or more internal parts of the crystal-growth
apparatus 200 may be annealed at, for example, a temperature
ranging from approximately 1000.degree. C. to approximately
1800.degree. C. for a time period of approximately 5 hours to
approximately 50 hours.
[0121] In various embodiments of the invention, the concentration
of carbon within the AlN boule 220 may be decreased via the
introduction of one or more gettering materials within the crucible
205 prior to and during growth of the AlN boule 220. The gettering
materials may be introduced as a portion or all of one or more of
the components of the crystal-growth apparatus 200 (e.g., the
crucible 205, a liner situated within the crucible 205 and
proximate an interior surface or wall thereof, the susceptor 215,
and/or the crucible stand 210), and/or the gettering materials may
be introduced as discrete masses of material within the
crystal-growth apparatus 200. The gettering materials may be
disposed between the source material 230 and the growing AlN boule
220 in order to, e.g., getter or absorb contaminants such as carbon
from the vapor flowing toward the AlN boule 220 (i.e., toward the
seed crystal 235). In various embodiments, the gettering materials
are stable at and have melting points greater than the growth
temperature (e.g., greater than approximately 2000.degree. C.) and
have low vapor pressures to prevent contamination of the growing
AlN boule 220 with the gettering materials themselves. In various
embodiments, a gettering material has a eutectic melting point with
AlN that is greater than the growth temperature (e.g., greater than
approximately 2000.degree. C.). Examples of gettering materials in
accordance with embodiments of the present invention include boron
(melting point of approximately 2300.degree. C.), iridium (melting
point of approximately 2410.degree. C.), niobium (melting point of
approximately 2468.degree. C.), molybdenum (melting point of
approximately 2617.degree. C.), tantalum (melting point of
approximately 2996.degree. C.), rhenium (melting point of
approximately 3180.degree. C.), and/or tungsten (melting point of
approximately 3410.degree. C.). In various embodiments, the
gettering material (or the component of the apparatus 200 or
portion thereof) may include, consist essentially of, or consist of
one or more non-tungsten materials having melting temperatures of
at least approximately 2300.degree. C.
[0122] After growth of the AlN boule 220, the AlN boule 220 may be
cooled down to approximately room temperature for subsequent
removal from the crystal-growth apparatus 200. For example, the AlN
boule 220 may be cooled in a two-stage process as described in the
'519 patent. However, in various embodiments of the invention, the
AlN boule 220 may simply be cooled down from the growth temperature
in a single stage, at an arbitrary rate, as the heat treatment
detailed below obviates the need for the two-stage process of the
'519 patent. In fact, in various embodiments of the present
invention, the AlN boule 220 is cooled down from the growth
temperature to approximately room temperature at a high rate (e.g.,
greater than 70.degree. C./hour, greater than 80.degree. C./hour,
greater than 100.degree. C./hour, greater than 150.degree. C./hour,
greater than 200.degree. C./hour, greater than 250.degree. C./hour,
greater than 300.degree. C./hour, greater than 400.degree. C./hour,
or even greater than 500.degree. C./hour; in various embodiments,
the rate may be no more than 2000.degree. C./hour, 1500.degree.
C./hour, or 1000.degree. C.) without any "controlled cooling"
achieved via application of power to the heating elements of
crystal-growth apparatus 200. In various embodiments of the
invention, gas (e.g., nitrogen and/or an inert gas) is flowed
within the crystal-growth apparatus 200 at a high rate (e.g., a
rate approximately equal to or higher than any gas-flow rate
utilized during crystal growth) in order to cool the AlN boule 220.
For example, the gas-flow rate utilized during crystal growth may
be approximately 4 slm or less, approximately 3 slm or less,
approximately 2 slm or less, or approximately 1 slm or less. The
gas-flow rate utilized during crystal growth may be approximately
0.1 slm or more, approximately 0.5 slm or more, approximately 1 slm
or more, or approximately 2 slm or more. In various embodiments,
the gas-flow rate utilized during cooling may be approximately 5
slm or more, approximately 10 slm or more, approximately 15 slm or
more, approximately 20 slm or more, or approximately 25 slm or
more. The gas-flow rate utilized during cooling may be
approximately 30 slm or less, approximately 25 slm or less,
approximately 20 slm or less, approximately 15 slm or less, or
approximately 10 slm or less. In addition, in embodiments of the
invention, the crucible 205 (and thus the AlN boule 220
therewithin) may be moved to an edge of the hot zone, or above the
hot zone, formed by the heating elements of the crystal-growth
apparatus 200 in order to more rapidly cool the AlN boule 220.
[0123] Advantageously, the high-rate cooling of AlN boule 220
minimizes or eliminates the formation of cracks within the AlN
boule 220, particularly when the AlN boule 220 has a diameter of
approximately 50 mm or greater. However, the high cooling rate may
also result in deleteriously high UV absorption within the AlN
boule 220 at one or more wavelengths (e.g., wavelengths around
approximately 310 nm), as described in the '147 application. FIG.
3A depicts the UV absorption spectrum for an exemplary AlN boule
220 cooled quickly from the growth temperature as detailed herein.
For example, the UV absorption spectrum of an exemplary AlN boule
220 cooled quickly from the growth temperature may exhibit an
elevated peak at approximately 310 nm that impairs the UV
transparency of the crystal over a wide range of wavelengths, and
the UV absorption coefficient may be greater than 20 cm.sup.-1 over
the entire wavelength range of 210 nm to 400 nm. The UV absorption
coefficient may also be greater than 30 cm.sup.-1 over the
wavelength range of 210 nm to 380 nm. Thus, in accordance with
various embodiments of the present invention, control of various
impurity concentrations such as carbon during the growth of and
within the resulting AlN crystal may be insufficient to achieve low
UV absorption coefficients, particularly at deep-UV wavelengths
(e.g., between 210 nm and 280 nm, between 230 nm and 280 nm, or
between 210 nm and 250 nm).
[0124] After cooling to room temperature, the AlN boule 220, or a
portion thereof, may be heat treated to further improve its UV
transparency, particularly at deep-UV wavelengths. For example, one
or more wafers may be separated from AlN boule 220, as detailed
herein, and one or more of the wafers may be heat treated for
improvement of UV transparency. The ensuing description refers to
the heat treatment of the AlN boule 220, but it should be
understood that only one or more portions of the boule (e.g., one
or more wafers) may be heat treated, rather than the entire boule.
In addition, the heat treatments detailed herein may be performed
on various different AlN crystals (e.g., AlN single crystals), even
if not initially grown and cooled as detailed herein, in order to
improve UV absorption.
[0125] In various embodiments of the invention, the AlN boule 220
is annealed in a heating apparatus (e.g., a furnace such as a
resistive furnace or a radio-frequency (RF) furnace) configured for
substantially isothermal or quasi-isothermal heating. The interior
of the furnace (at least in the heated, or "hot" zone), as well as
any hardware (e.g., a platform or other support) within the
furnace, may include, consist essentially of, or consist of one or
more refractory materials (e.g., W or another refractory metal)
having a melting point exceeding about 2800.degree. C., or even
exceeding about 3000.degree. C. In various embodiments, the
interior of the furnace (at least in the heated, or "hot" zone),
and the hardware (e.g., a platform or other support) within the
furnace, may be free of carbon, carbon-based or carbon-containing
materials, graphite, quartz, alumina, and/or molybdenum. Before the
AlN boule 220 is placed within the furnace, the furnace may undergo
a bake-out run at high temperature to reduce or minimize the
presence of any contaminants therewithin. For example, the furnace
may be heated to about 2600.degree. C. under vacuum for a time
period of, e.g., approximately 0.5 hours to approximately 2 hours.
After the furnace has cooled, the AlN boule 220 may be placed
within the furnace, which may then be filled with nitrogen gas at a
pressure of, e.g., approximately 1 bar to approximately 2 bars. The
AlN boule 220 may be placed "loosely" (i.e., not attached, adhered,
or fastened to) on a platform within the furnace that may include,
consist essentially of, or consist of W or another refractory
metal. In various embodiments, the loose placement of the AlN boule
220 reduces or substantially eliminates stresses due to any
differential thermal expansion between AlN boule 220 and the
platform.
[0126] The temperature within the furnace may then be ramped to the
desired annealing temperature at a ramp rate of, e.g.,
approximately 1.degree. C./min to approximately 50.degree. C./min.
In various embodiments, the annealing temperature is between
approximately 2100.degree. C. and approximately 2500.degree. C.,
e.g., approximately 2400.degree. C. In various embodiments, the
annealing temperature is between approximately 2150.degree. C. and
approximately 2400.degree. C. The present inventors have found that
lower annealing temperatures (e.g., about 2000.degree. C.) are
generally insufficient to improve the UV transparency of AlN boule
220 at deep-UV wavelengths to the desired level. Once the desired
annealing temperature has been achieved, the AlN boule 220 is
annealed at that temperature for a time period of, for example,
approximately 0.5 hour to approximately 100 hours, approximately
0.5 hour to approximately 5 hours, or approximately 1 hour. After
annealing, the temperature of the furnace is slowly ramped down to
an intermediate temperature (for example, between approximately
800.degree. C. and approximately 1200.degree. C., e.g.,
approximately 1000.degree. C.) at a rate ranging between
approximately 60.degree. C./hour and approximately 120.degree.
C./hour. For example, the furnace may be cooled from an exemplary
annealing temperature of 2200.degree. C. to 1000.degree. C. over a
time period of 15 hours. Such slow cooling may be achieved via
controlled application of heat with the furnace (e.g., at low power
levels). Thereafter, the furnace may be turned off, and the furnace
and the AlN boule 220 may be allowed to cool to room temperature.
Thus, in various embodiments of the invention, the entire annealing
cycle, including the cool-down therefrom, of the AlN boule 220 is
performed in substantially isothermal or quasi-isothermal
conditions.
[0127] FIG. 8 is a graphical comparison of a UV absorption spectrum
800, corresponding to a conventional UV absorption spectrum
reported in the '519 patent and a UV absorption spectrum 810 of an
AlN single crystal fabricated and annealed in accordance with
embodiments of the present invention. As shown, over the entire
range of wavelengths, the crystal in accordance with embodiments of
the invention exhibits a lower absorption coefficient, and the
spectrum is substantially constant (or "flat") for wavelengths
between 210 nm and 280 nm. At about 230 nm, the crystal in
accordance with embodiments of the invention has an absorption
coefficient of less than 10 cm.sup.-1 (in the depicted example,
approximately 7 cm.sup.-1-8 cm.sup.-1), which is dramatically lower
than the results achieved in the '519 patent. In addition, the
slope of the absorption coefficient as a function of wavelength
near the band edge is much steeper, as described in more detail
below.
[0128] As mentioned above, embodiments of the present invention
include and enable the production of single-crystal AlN having a
steep drop-off in the absorption coefficient near the band edge,
i.e., AlN having a low Urbach energy. The "Urbach tail" is the
exponential part of the absorption coefficient curve near the
optical band edge, and is related to crystalline disorder and
localized electronic states extending into the band gap. The
spectral dependence of the absorption coefficient (a) and photon
energy (hv) is known as Urbach empirical rule, which is given by
the following equation:
.alpha. = .alpha. 0 exp ( hv E U ) ##EQU00008##
(see Franz Urbach, "The Long-Wavelength Edge of Photographic
Sensitivity and of the Electronic Absorption of Solids," Phys. Rev.
92 (1953) 1324, the entire disclosure of which is incorporated by
reference herein). .alpha..sub.0 is a constant, and E.sub.U is the
Urbach energy, i.e., the energy of the band tail. The above
equation may be rewritten as:
ln .alpha. = ln .alpha. 0 + ( hv E U ) ##EQU00009##
and the Urbach energy may be determined from the slope of the line
when ln (.alpha.) is plotted as a function of the incident photon
energy hv; on such a plot, ln (.alpha..sub.0) is the y-intercept of
the line and thus corresponds to ln (.alpha.) at a theoretical zero
photon energy. Specifically, the Urbach energy is the inverse of
the slope.
[0129] FIG. 9 is a plot used to determine the Urbach energy of the
sample from the '519 patent having the absorption spectrum 800
presented in FIG. 8 as well as the Urbach energy of the sample in
accordance with embodiments of the invention having the absorption
spectrum 810 presented in FIG. 8. As shown, the slope of the
resulting curve 900 for the inventive sample is much steeper (the
slope is approximately 4.7/eV) and results in an Urbach energy of
approximately 0.21 eV in the range of photon energies of 5.85 eV to
6.00 eV. In stark contrast, the curve 910 corresponding to the
sample having the absorption spectrum 800 exhibits a slope of
approximately 0.5/eV, which results in an Urbach energy of
approximately 2.0 eV. In accordance with embodiments of the
invention, the present inventors have fabricated samples having
Urbach energies ranging from approximately 0.2 eV to approximately
1.8 eV, e.g., from approximately 0.21 eV to approximately 1.0 eV,
which are significantly lower than those of conventional samples
and samples reported in the literature.
[0130] In general, UV absorption spectra (and Urbach energies
derived therefrom) may be determined by measuring reflections of
incident light on a sample using a spectrometer. For example, the
UV absorption spectra of samples in accordance with embodiments of
the invention were measured using a V-670 (Class I) spectrometer
and X-Y stage from Jasco Corporation. 52 points per sample were
measuring utilizing a two-axis stage controller from Chuo Precision
Industrial Co., Ltd. Wavelengths from 200 nm to 800 nm were
measured, but measurements up to wavelengths of 2000 nm may be
acquired utilizing this set-up. The absorption spectrum of a sample
having a thickness L is estimated based on the light incident on
the sample and the light transmitted by the sample, taking into
account the light reflected back toward the light emission from
both surfaces of the sample. The thickness L may be measured using,
for example, a gauge (e.g., ACANTO, CERTO, METRO, or SPECTO length
gauges, and associated GAGE-CHEK evaluation electronics, available
from Heidenhain Corp. of Schaumburg, Ill.) or an optical system
such as the ULTRA-MAP 100B or ULTRA-MAP C200, available from
MicroSense, LLC of Lowell, Mass. FIG. 10 summarizes this
calculation, and absorption coefficient .alpha. at a particular
wavelength of incident light .lamda. may be calculated from:
I T I 0 = ( 1 - R ) 2 e - .alpha. L ##EQU00010##
where I.sub.T is the intensity of the transmitted light and I.sub.0
is the intensity of the incident light. The reflectance R may be
determined from:
R = ( n - 1 n + 1 ) 2 ##EQU00011##
where the refractive index n may be determined from the dispersion
formula:
n 2 - 1 = 2.1399 + 1.3786 .lamda. 2 .lamda. 2 - 0.1715 2 + 3.861
.lamda. 2 .lamda. 2 - 15.03 2 ##EQU00012##
and where dispersion formula is provided from J. Pastrn ak and L.
Roskovcova, "Refraction index measurements on AlN single crystals,"
Phys. Stat. Sol. 14, K5-K8 (1966), the entire disclosure of which
is incorporated by reference herein.
[0131] The improved UV absorption spectra of embodiments of the
present invention enable enhanced performance of light-emitting
devices (e.g., lasers and light-emitting diodes (LEDs)) fabricated
on AlN substrates having the improved spectra, particularly at
short wavelengths. FIG. 11 is a graph of LED device emission
intensity as a function of wavelength for simulated LEDs emitting
at about 217 nm. The top curve 1100 is the emission intensity as a
function of wavelength for an LED fabricated on a substrate having
the improved absorption spectrum enabled by embodiments of the
present invention--in this example, the UV absorption spectrum 810
depicted in FIG. 8. The bottom curve 1110 corresponds to the same
LED structure fabricated on a substrate having the absorption
spectrum 800 of FIG. 8). As shown in FIG. 11, the emission
intensity enabled by embodiments of the present invention is
increased by nearly a factor of two at the peak emission wavelength
of about 217 nm and is higher over the entire wavelength range.
FIG. 12 is a graph of the same spectra over a smaller wavelength
range, in which the relative intensities of the LEDs have been
independently normalized to the same value in order to demonstrate
the narrower intensity peak of the device in accordance with
embodiments of the present invention. This narrower peak enables
superior LED performance. The simulations for the devices depicted
in FIGS. 11 and 12 indicate that the emission power for the device
in accordance with embodiments of the invention will be increased
by at least 1.6.times. for the substrate thickness of 0.55 mm
utilized in the simulations. Due to the improved UV absorption,
this advantage will be larger for larger substrate thicknesses. In
addition, when reflectors are utilized to reflect light emitted by
the device into a preferred direction, the power of the device will
increase for each pass through the substrate traveled by the
reflected light. For example, the improvement in device emission
power enabled by embodiments of the present invention may be
approximated as 2.times.(1.6).sup.3, or approximately 8.times.,
when reflected light traverses the substrate having the improved
absorption spectrum three times.
[0132] The growth of bulk single crystals has been described herein
primarily as being implemented by what is commonly referred to as a
"sublimation" or "sublimation-recondensation" technique wherein the
source vapor is produced at least in part when, for production of
AlN, crystalline solids of AlN or other solids or liquids
containing AlN, Al or N sublime preferentially. However, the source
vapor may be achieved in whole or in part by the injection of
source gases or the like techniques that some would refer to as
"high-temperature CVD." Also, other terms are sometimes used to
describe these and techniques that are used to grow bulk single AlN
crystals in accordance with embodiments of the invention.
Therefore, the terms "depositing," "growing," "depositing vapor
species," and like terms are used herein to generally cover those
techniques by which the crystal may be grown pursuant to
embodiments of this invention.
[0133] The terms and expressions employed herein are used as terms
of description and not of limitation, and there is no intention, in
the use of such terms and expressions, of excluding any equivalents
of the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
* * * * *