U.S. patent application number 09/836780 was filed with the patent office on 2001-09-20 for method and apparatus for single crystal gallium nitride (gan) bulk synthesis.
Invention is credited to Cho, Hak Dong, Kang, Sang Kyu.
Application Number | 20010022154 09/836780 |
Document ID | / |
Family ID | 26812927 |
Filed Date | 2001-09-20 |
United States Patent
Application |
20010022154 |
Kind Code |
A1 |
Cho, Hak Dong ; et
al. |
September 20, 2001 |
Method and apparatus for single crystal gallium nitride (GAN) bulk
synthesis
Abstract
A method and apparatus for homoepitaxial growth of freestanding,
single bulk crystal Gallium Nitride (GaN) are provided, wherein a
step of nucleating GaN in a reactor results in a GaN nucleation
layer having a thickness of a few monolayers. The nucleation layer
is stabilized, and a single bulk crystal GaN is grown from gas
phase reactants on the GaN nucleation layer. The reactor is formed
from ultra low oxygen stainless steel.
Inventors: |
Cho, Hak Dong; (Cupertino,
CA) ; Kang, Sang Kyu; (Cupertino, CA) |
Correspondence
Address: |
WILSON SONSINI GOODRICH & ROSATI
650 PAGE MILL ROAD
PALO ALTO
CA
943041050
|
Family ID: |
26812927 |
Appl. No.: |
09/836780 |
Filed: |
April 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09836780 |
Apr 16, 2001 |
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09478954 |
Jan 7, 2000 |
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60115177 |
Jan 8, 1999 |
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Current U.S.
Class: |
117/2 ;
257/E21.108 |
Current CPC
Class: |
C30B 25/00 20130101;
H01L 21/02458 20130101; C30B 29/64 20130101; H01L 33/0075 20130101;
H01L 21/0254 20130101; C30B 25/02 20130101; H01L 21/02389 20130101;
H01L 21/0262 20130101; C30B 29/60 20130101; C30B 29/406
20130101 |
Class at
Publication: |
117/2 |
International
Class: |
C30B 001/00 |
Claims
What is claimed is:
1. A method for growing a single freestanding Gallium Nitride (GaN)
crystal, comprising: generating a GaN substrate structure by
growing a GaN nucleation layer on a susceptor, wherein a thickness
of the nucleation layer is at least one monolayer; stabilizing the
GaN substrate structure; and growing a GaN layer on at least one
surface of the GaN substrate structure using a plurality of gas
phase reactants.
2. The method of claim 1, further comprising: providing a reactor
chamber formed from ultra low oxygen stainless steel; cleaning the
susceptor; setting the susceptor in the reactor chamber; rotating
at least one of the reactor chamber and at least one heating
element; and initializing and stabilizing an environment of the
reactor chamber.
3. The method of claim 2, wherein generation of the GaN substrate
structure is performed when the environment of the reactor chamber
is stabilized and controlled within a first set of environmental
parameters.
4. The method of claim 3, wherein the first set of environmental
parameters includes a pressure selected from a range of 10.sup.-3
Torr and 10.sup.-6 Torr and a temperature selected from a range of
300 degrees Celsius and 800 degrees Celsius, wherein the selected
temperature is maintained within plus or minus 1 degree
Celsius.
5. The method of claim 2, wherein growth of the GaN layer is
performed when the environment of the reactor chamber is stabilized
and controlled within a second set of environmental parameters.
6. The method of claim 5, wherein the second set of environmental
parameters includes a pressure selected from a range of 10.sup.-3
Torr and atmosphere and a temperature selected from a range of 450
degrees Celsius and 1250 degrees Celsius, wherein the selected
temperature is maintained within plus or minus 2 degrees
Celsius.
7. The method of claim 2, wherein stabilizing the GaN substrate
structure comprises changing the environment of the reactor chamber
from a first se of environmental parameters to a second set of
environmental parameters.
8. The method of claim 1, wherein the plurality of gas phase
reactants comprise gases selected from a group consisting of
Nitrogen, Hydrogen, Ammonia, Gallium, Aluminum, and Indium.
9. The method of claim 1, wherein growth of the GaN nucleation
layer comprises exposing a Pyro-Boron-Nitride (PBN) susceptor to a
first gas mixture comprising hydrogen gas, nitrogen gas, ammonia
gas, and a second gas mixture comprising at least one group III-V
metal alloy.
10. The method of claim 9, wherein the PBN susceptor has a
thickness of approximately 4 millimeters and a diameter of
approximately 5 inches, wherein the PBN susceptor holds 3 GaN
semiconductor wafers.
11. The method of claim 1, wherein the nucleation layer comprises 5
to 30 monolayers, wherein the nucleation layer has a thickness
dimension approximately in a range of 10 to 70 Angstroms.
12. The method of claim 1, wherein the GaN layer is grown at a rate
between 20 and 100 micrometers per hour, wherein the lattice
structure is wurtzite.
13. A nitride semiconductor device comprising: a GaN substrate
structure formed by growing a GaN nucleation layer on a susceptor,
wherein a thickness of the nucleation layer is at least one
monolayer, wherein the GaN substrate structure is stabilized; and a
GaN layer, wherein the GaN layer is grown on at least one surface
of the GaN substrate structure using a plurality of gas phase
reactants.
14. The nitride semiconductor device of claim 13, wherein the GaN
substrate structure and the GaN layer are grown in a reactor
chamber formed from ultra low oxygen stainless steel.
15. The nitride semiconductor device of claim 14, wherein
generation of the GaN substrate structure is performed when the
environment of the reactor chamber is stabilized and controlled
within a first set of environmental parameters, wherein the first
set of environmental parameters includes a pressure selected from a
range of 10.sup.-3 Torr and 10.sup.-6 Torr and a temperature
selected from a range of 300 degrees Celsius and 800 degrees
Celsius, wherein the selected temperature is maintained within plus
or minus 1 degree Celsius.
16. The nitride semiconductor device of claim 14, wherein growth of
the GaN layer is performed when the environment of the reactor
chamber is stabilized and controlled within a second set of
environmental parameters, wherein the second set of environmental
parameters includes a pressure selected from a range of 10.sup.-3
Torr and atmosphere and a temperature selected from a range of 450
degrees Celsius and 1250 degrees Celsius, wherein the selected
temperature is maintained within plus or minus 2 degrees
Celsius.
17. The nitride semiconductor device of claim 13, wherein the
plurality of gas phase reactants comprise gases selected from a
group consisting of Nitrogen, Hydrogen, Ammonia, Gallium, Aluminum,
and Indium.
18. The nitride semiconductor device of claim 13, wherein growth of
the GaN nucleation layer comprises exposing a Pyro-Boron-Nitride
(PBN) susceptor to a first gas mixture comprising hydrogen gas,
nitrogen gas, ammonia gas, and a second gas mixture comprising at
least one group III-V metal alloy.
19. The nitride semiconductor device of claim 13, wherein the
nucleation layer comprises 5 to 30 monolayers, wherein the
nucleation layer has a thickness dimension approximately in a range
of 10 to 70 Angstroms, wherein the GaN layer is grown at a rate
between 20 and 100 micrometers per hour, wherein the lattice
structure is wurtzite.
20. The nitride semiconductor device of claim 13, wherein the
nitride semiconductor device is used in at least one device
selected from a group consisting of light-emitting diodes and laser
diodes.
21. A light emitting device comprising a nitride semiconductor
device, the nitride semiconductor device comprising: a GaN
substrate structure formed by growing a GaN nucleation layer on a
susceptor, wherein a thickness of the nucleation layer is at least
one monolayer, wherein the GaN substrate structure is stabilized;
and a GaN layer, wherein the GaN layer is grown on at least one
surface of the GaN substrate structure using a plurality of gas
phase reactants.
22. The light emitting device of claim 21, wherein the GaN
substrate structure and the GaN layer are grown in a reactor
chamber formed from ultra low oxygen stainless steel.
23. The light emitting device of claim 22, wherein generation of
the GaN substrate structure is performed when the environment of
the reactor chamber is stabilized and controlled within a first set
of environmental parameters, wherein the first set of environmental
parameters includes a pressure selected from a range of 10.sup.-3
Torr and 10.sup.-6 Torr and a temperature selected from a range of
300 degrees Celsius and 800 degrees Celsius, wherein the selected
temperature is maintained within plus or minus 1 degree
Celsius.
24. The light emitting device of claim 22, wherein growth of the
GaN layer is performed when the environment of the reactor chamber
is stabilized and controlled within a second set of environmental
parameters, wherein the second set of environmental parameters
includes a pressure selected from a range of 10.sup.-3 Torr and
atmosphere and a temperature selected from a range of 450 degrees
Celsius and 1250 degrees Celsius, wherein the selected temperature
is maintained within plus or minus 2 degrees Celsius.
25. The light emitting device of claim 21, wherein the plurality of
gas phase reactants comprise gases selected from a group consisting
of Nitrogen, Hydrogen, Ammonia, Gallium, Aluminum, and Indium.
26. The light emitting device of claim 21, wherein growth of the
GaN nucleation layer comprises exposing a Pyro-Boron-Nitride (PBN)
susceptor to a first gas mixture comprising hydrogen gas, nitrogen
gas, ammonia gas, and a second gas mixture comprising at least one
group III-V metal alloy.
27. The light emitting device of claim 21, wherein the nucleation
layer comprises 5 to 30 monolayers, wherein the nucleation layer
has a thickness dimension approximately in a range of 10 to 70
Angstroms, wherein the GaN layer is grown at a rate between 20 and
100 micrometers per hour, wherein the lattice structure is
wurtzite.
28. The light emitting device of claim 21, wherein the light
emitting device comprises at least one device selected from a group
consisting of light-emitting diodes and laser diodes.
29. A composition of matter for a nitride semiconductor device
comprising: a GaN substrate structure including a GaN nucleation
layer on a susceptor, the GaN substrate structure being stabilized,
and wherein a thickness of the nucleation layer is at least one
monolayer; and a GaN layer adjacent the substrate structure,
wherein the GaN layer is grown on at least one surface of the GaN
substrate structure.
30. The composition of matter of claim 29, wherein the nucleation
layer comprises 5 to 30 monolayers.
31. The composition of matter of claim 29, wherein the nucleation
layer has a thickness approximately in the range of 10 to 70
Angstroms.
32. The composition of matter of claim 29, wherein at least one of
the nucleation layer or the GaN substrate structure has a wurtzite
lattice structure.
33. The composition of matter of claim 29, wherein one of a defect
density, a dislocation defect density, or an optical defect density
is less than 10.sup.8/cm.sup.2.
34. The composition of matter of claim 29, wherein one of a defect
density, a dislocation defect density, or an optical defect density
is less than 10.sup.7/cm.sup.2.
35. The composition of matter of claim 29, wherein one of a defect
density, a dislocation defect density, or an optical defect density
is less than 10.sup.6/cm.sup.2.
36. The composition of matter of claim 29, wherein one of a defect
density, a dislocation defect density, or an optical defect density
is less than 10.sup.5/cm.sup.2.
37. The composition of matter of claim 29, wherein an amount of
nitrogen vacancies is less than 10.sup.19/cm.sup.3.
38. The composition of matter of claim 29, wherein an amount of
nitrogen vacancies is less than 10.sup.18/cm.sup.3.
39. The composition of matter of claim 29, wherein an amount of
nitrogen vacancies is less than 10.sup.17/cm.sup.3.
40. The composition of matter of claim 29, wherein the GaN layer
has a full-width half maximum intensity of less than 100 arc
seconds as measured using an .omega.-scan measurement.
41. The composition of matter of claim 29, further comprising an
impurity.
42. The composition of matter of claim 41, wherein the GaN layer is
doped with the impurity.
43. The composition of matter of claim 41, wherein the impurity is
a dopant.
44. The composition of matter of claim 41, wherein the impurity is
an n-type dopant.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to the field of materials science and
more particularly to the growth of semiconductor crystals.
[0003] 2. Description of the Related Art
[0004] There is currently a demand in the computer and
telecommunication industries for multicolor light emitting displays
and improved data density in communication and recording. As a
result of this demand, there is a strong desire for a semiconductor
light emitting element capable of emitting light having shorter
wavelengths ranging from a blue light wavelength to an ultraviolet
wavelength.
[0005] The III-V nitrides, as a consequence of their electronic and
optical properties and heterostructure character, are highly
advantageous for use in the fabrication of a wide range of
microelectronic structures. In addition to their wide band gaps,
the III-V nitrides also have direct band gaps and are able to form
alloys which permit fabrication of well lattice-matched
heterostructures. Consequently, devices made from the III-V
nitrides can operate at high temperatures, with high power
capabilities, and can efficiently emit light in the blue and
ultraviolet regions of the electromagnetic spectrum. Devices
fabricated from III-V nitrides have applications in full color
displays, super-luminescent light-emitting diodes (LEDs), high
density optical storage systems, and excitation sources for
spectroscopic analysis applications. Furthermore, high temperature
applications are found in automotive and aeronautical
electronics.
[0006] Effective use of these advantages of the III-V nitrides,
however, requires that such materials have device quality and
structure accommodating abrupt heterostructure interfaces. As such,
the III-V nitrides must be of single crystal character and
substantially free of defects that are electrically or optically
active.
[0007] Gallium nitride (or GaN) is a particularly advantageous
III-V nitride and attention has recently focused on gallium nitride
related compound semiconductors (In(x)Ga(y)Al(1-x-yN) (0.ltoreq.x,
y; x+y.ltoreq.1) as materials for emitting blue light. This nitride
species can be used to provide optically efficient, high
temperature, wide band gap heterostructure semiconductor systems
having a convenient, closely matched heterostructure character. The
direct transition type band structure of GaN permits highly
efficient emission of light. Moreover, GaN emits light of shorter
wavelength ranging from a blue light wavelength to an ultraviolet
wavelength, due to a great band gap at room temperature of about
3.4 eV.
[0008] As no GaN substrates are currently found in the art, growth
of these compounds must initially take place heteroepitaxially, for
example GaN on silicon. However, heteroepitaxial growth has several
technical drawbacks. In particular, two types of defects arise as a
result of heteroepitaxial growth: (i) dislocation defects due to
lattice mismatch: and (ii) dislocation defects due to different
thermal coefficients between the substrate and the epitaxial
layer.
[0009] The first type of defect includes dislocations due to the
lattice mismatch between the GaN layer and the substrate. One
typical substrate is sapphire. In the case where a gallium nitride
related compound semiconductor crystal is grown on a sapphire
substrate, a lattice mismatch up to approximately 16% is found
between the GaN and the substrate. SiC is a closer lattice match,
at ant approximate lattice mismatch of 3%, but the mismatch is
still quite large. Many other substrates have been used, but all of
them have large lattice mismatches and result in a high density of
defects in the grown layers.
[0010] The second type of defect includes dislocations generated
during cool-down after growth. This defect is a result of different
thermal coefficients of expansion of the substrate and epitaxial
layer.
[0011] There are two typical methods in use for growing GaN
compound semiconductor crystals. However, both suffer from
deficiencies and/or limitations adversely affecting the quality of
the GaN product. A first method uses a single crystalline sapphire
as a substrate. A buffer layer is grown on the substrate for the
purpose of relaxation of lattice mismatching between the sapphire
substrate and the GaN compound semiconductor crystal. The buffer
layer may be a AlN buffer layer or a GaAlN buffer layer. A GaN
compound semiconductor crystal is grown in the buffer layer. While
the buffer layers improve the crystallinity and surface morphology
of the GaN compound semiconductor crystal, the crystal remains in a
distorted state because of the lattice mismatch with the sapphire
substrate. This distorted state results in dislocation defects
described herein.
[0012] A second method attempts to reduce the lattice mismatch by
providing a single crystal material as a substrate having a crystal
structure and lattice constant that closely matches that of the GaN
compound semiconductor crystal. One embodiment of this method uses
aluminum garnet or gallium garnet as a substrate, but the lattice
match using these compounds is not sufficient to provide much
improvement. Another embodiment of this method uses substrate
materials including MnO, ZnO, MgO, and CaO. While these oxides
provide a better lattice match with the substrate, the oxides
undergo thermal decomposition at the high temperatures required for
the growth of the GaN compound semiconductor. Thermal decomposition
of the substrate adversely affects the quality of the
semiconductors obtained using this method.
[0013] As a result of these problems, typical GaN semiconductor
devices suffer from poor device characteristics, short life span,
and high cost. Full utilization of the properties of GaN
semiconductors cannot be realized until a suitable substrate is
available that allows for growth of high quality homoepitaxial
layers. This requires development of processes for growth of the
substrate material. For device applications, therefore, it would be
highly advantageous to provide substrates of GaN, for epitaxial
growth thereon of a GaN crystal layer. Thus, it would be a
significant advance in materials science to provide GaN in bulk
single crystal form, suitable for use as a substrate body for the
fabrication of microelectronic structures.
SUMMARY OF THE INVENTION
[0014] A method and apparatus for homoepitaxial growth of
freestanding, single bulk crystal Gallium Nitride (GaN) are
provided. The fabrication method includes a step of nucleating GaN
in a reactor at a temperature less than approximately 800 degrees
Celsius and a pressure substantially in the range of 10.sup.-3 Torr
to 10.sup.-6 Torr. This nucleation phase results in a first GaN
structure, or GaN nucleation layer, having a thickness of a few
monolayers. The nucleation layer is stabilized, and a single bulk
crystal GaN is grown from gas phase reactants on the GaN nucleation
layer in the reactor at a temperature substantially in the range of
450 degrees Celsius to 1250 degrees Celsius and a pressure
substantially in the range of 10.sup.-3 Torr to atmosphere. The
reactor is formed from ultra low oxygen stainless steel.
[0015] The descriptions provided herein are exemplary and
explanatory and are provided as examples of the claimed
invention.
BRIEF DESCRIPTION OF THE FIGURES
[0016] The accompanying drawings illustrate embodiments of the
claimed invention. In the drawings:
[0017] FIG. 1 is a flow chart of a method of growing freestanding,
single bulk crystal Gallium Nitride (GaN) by homoepitaxy in an
embodiment.
[0018] FIG. 2 is a side elevation diagram of a single bulk crystal
GaN of an embodiment.
[0019] FIG. 3 is a diagram of a reactor chamber in which the GaN
semiconductor crystal of an embodiment is grown.
DETAILED DESCRIPTION
[0020] FIG. 1 is a flow chart of a method of growing freestanding,
single bulk crystal Gallium Nitride (GaN) by homoepitaxy in an
embodiment. FIG. 2 is a side elevation diagram of a single bulk
crystal GaN of an embodiment The fabrication begins at step 102,
the step of nucleating GaN on a susceptor 202 in a reactor at a
temperature less than approximately 800 degrees Celsius and a
pressure substantially in the range of 10.sup.-3 Torr to 10.sup.-6
Torr. In various embodiments, the temperature can be in the range
of 300 to 800.degree. C., with a preferred range of 350 to
750.degree. C. and specific embodiments of 400, 500 and 600.degree.
C. Similarly, the pressure can be in the range of 10.sup.-3 Torr to
10.sup.-5 Torr, with a preferred embodiment of 10.sup.-5 Torr. The
nucleation step 102 results in a first GaN structure, or GaN
nucleation layer 204, with a thickness of a few monolayers having a
dimension thickness in the range of 10 to 70 Angstroms. In a
preferred embodiment, nucleation layer 204 comprises ten monolayers
with a thickness of 25 Angstroms.
[0021] The GaN nucleation layer 204 is stabilized, at step 104, and
a single bulk crystal GaN 206 is grown from gas phase reactants on
the GaN nucleation layer 204 in the reactor at a temperature
substantially in the range of 450 degrees Celsius to 1250 degrees
Celsius and a pressure substantially in the range of 10.sup.-3 Torr
to atmospheric pressure, at step 106. The single bulk crystal GaN
structure 204 and 206 is removed from the susceptor 202, at step
108. The reactor of an embodiment is formed from ultra low oxygen
stainless steel.
[0022] By eliminating the need for a base substrate of different
material, or non-GaN material, the method and apparatus described
herein achieves markedly superior results. Dislocation defect
densities are several orders of magnitude less than typical
methods. Furthermore, this method permits the growth of crystals to
thickness greater than 100 micrometers, several orders of magnitude
greater than typical methods.
[0023] The freestanding single bulk GaN semiconductors produced
using the technologies described herein can be used in a variety of
optoelectronic devices, including blue and green light-emitting
diodes (LEDs) and laser diodes (LDs). These semiconductors and the
devices in which they are components are used in providing an
enabling technology for a wide variety of consumer, computer,
business, telecommunications and industrial products including, but
not limited to, digital video disk (DVD) devices, audio compact
disk (CD) devices, computer CD-ROM drives, optical data storage
devices, laser printers, rewritable optical storage drives, barcode
scanners, computer-to-plate digital printing presses, detectors,
lasers for optical fiber communication, full color electronic
outdoor displays, and flat panel displays. Furthermore, as three
primary colors can be generated using GaN semiconductor materials,
white light sources with adjustable mood coloring will become
available using the freestanding single bulk GaN semiconductor
devices described herein.
[0024] In an embodiment, a GaN semiconductor crystal is grown
without a typical non-GaN base substrate. A three-step process is
provided including a first step of creating a nucleation layer, a
second step of an interconnection process, and a third step of
growing a single crystal GaN layer on the nucleation layer.
[0025] FIG. 3 is a diagram of a reactor chamber 300 in which the
GaN semiconductor crystal of an embodiment is grown. The reactor
chamber 300 of an embodiment is a double walled chamber having an
inside diameter of approximately 14 inches. The reactor chamber 300
includes a cooling system in the main body and the inlet gas body.
The cooling system uses water, but is not so limited. The reactor
chamber 300 includes at least one port 302 for viewing, loading,
and unloading, and at least one pumping port 304. In one embodiment
the chamber includes two or more pumping ports 304.
[0026] The reactor chamber 300 of an embodiment is formed from
ultra low oxygen stainless steel, for example grade 316L or 30316L
or S31603 stainless steel or other 316 stainless steel known in the
art, in order to reduce or eliminate introduction of impurities
during the crystal growing process. The welds used in forming the
reactor chamber 300 are performed so that oxygen contamination is
prevented in the area of the welds. Furthermore, ultra low oxygen
copper gaskets are used in sealing the reactor chamber access
ports. In one embodiment, the copper gaskets are used with 316L
stainless steel Conflat.RTM. flanges and flange components.
[0027] The reactor chamber supports pressures as low as 10.sup.-12
Torr, but is not so limited. A staged vacuum system and scrubber
305 are used, wherein rotary pumps 306 generate and/or support
pressures as low as approximately 10.sup.-3 Torr. The rotary pumps
306 are rated for 700 liters per minute. Reactor chamber pressures
between approximately 10.sup.-3 Torr and 10.sup.-2 Torr are
provided using at least one turbomolecular pump 308 and another
rotary pump 310. The turbomolecular pump 308 is rated for 1,000
liters per second, and the other rotary pump 310 is rated for 450
liters per minute.
[0028] The reactor chamber is coupled to a number of gas sources
through a number of valves or regulators 312. The gas sources are
contained in a gas source control cabinet 314.
[0029] The reactor chamber comprises at least one heating unit
capable of providing a reactor chamber environment having a
temperature of at least 2500 degrees Celsius. In an embodiment,
reactor chamber temperature disparity is minimized using a three
zone heating unit, but is not so limited. Furthermore, the heating
unit is rotatable independently of the reactor chamber up to a
speed of approximately 1500 revolutions per minute (RPM), but is
not so limited. The heating unit height can be raised or lowered
through a range of approximately 0.50 inches to 0.75 inches. The
heating elements of the heating unit comprise graphite elements
epitaxially coated with silicon carbide or pyro-boron nitride. The
pyro-boron nitride material can be commercially obtained.
[0030] The first step in growing a GaN semiconductor crystal
without a typical base substrate includes growing a nucleation
layer. In an embodiment, a Pyro-Boron-Nitride (PBN) susceptor is
rinsed with an organic solvent and set in the reactor chamber. The
susceptor has a thickness of approximately 3.5 to 4.5 millimeters
and a diameter of approximately 5 inches, and is capable of holding
three GaN semiconductor wafers, but is not so limited. In a
preferred embodiment, the susceptor thickness is 4 mm. The
parameters of the reactor chamber environment are then set and
stabilized, parameters including but not limited to pressure,
temperature, and rotational velocity.
[0031] In setting the parameters of the reactor chamber, the
reactor chamber environment is controlled to maintain a selected
pressure of between 10.sup.-3 Torr and 10.sup.-6 Torr. In an
embodiment, the selected pressure is 10.sup.-5 Torr, but is not so
limited. In an embodiment, the selected pressures of the particular
process phases, including the nucleation phase and the GaN crystal
growth phase, are maintained through the use of the pumps described
herein. The reactor chamber environment is heated to a selected
temperature in the range of 300 degrees Celsius to 800 degrees
Celsius. The reactor chamber temperature is controlled to maintain
the selected temperature plus or minus 1 degree Celsius. The
reactor chamber is then controllably rotated at 700 RPM plus or
minus 50 RPM.
[0032] The reactor chamber includes receptacles for receiving a
number of gases into the reactor chamber. The gases received in the
reactor chamber of an embodiment include Nitrogen (N.sub.2),
Hydrogen (H.sub.2), Ammonia (NH.sub.3), Gallium (Ga), Aluminum
(Al), and Indium (In), each having a purity of at least
99.99999%.
[0033] Upon stabilizing the reactor chamber environment, the
surface of the susceptor is cleaned by introducing Nitrogen gas
into the reactor chamber. Following this cleaning, the gases used
in the nucleation phase are simultaneously introduced into the
reactor chamber. The flow rates of the gases are adjusted for the
nucleation phase, and the following flow rates are used in an
embodiment: Nitrogen is provided at a flow rate of 5 to 10 cubic
centimeters per minute; Ammonia is provided at a flow rate of 0.1
to 0.25 liters per minute; Gallium is provided at a flow rate of
0.001 to 0.002 liters per minute; Aluminum is provided at a flow
rate of 0.001 to 0.002 liters per minute; and, Indium is provided
at a flow rate of 0.001 to 0.002 liters per minute.
[0034] A GaN nucleation layer is grown during the nucleation phase
for a period of 10 minutes following introduction of the gas
mixtures to the reactor chamber. The nucleation layer obtained in
an embodiment includes 5 to 30 monolayers having a thickness
substantially in the range of 10 Angstroms to 70 Angstroms plus or
minus 10 Angstroms. The nominal nucleation layer includes 10
monolayers having a thickness of approximately 25 Angstroms, but is
not so limited.
[0035] The second step in growing a GaN semiconductor crystal
without a typical base substrate includes an interconnection
process between the generation of a nucleation layer and the GaN
layer growth. The interconnection process is used to stabilize the
GaN nucleation layer during a change in the environmental
conditions of the reactor chamber.
[0036] During the interconnection process, the temperature of the
reactor chamber environment is changed at a constant rate of 3
degrees Celsius per minute to a second selected temperature that is
appropriate for growth of the GaN layer. This second selected
temperature is in the range of 450 degrees Celsius to 1250 degrees
Celsius. Upon reaching the second selected temperature, the reactor
chamber environment temperature is controlled to maintain the
second selected temperature plus or minus 2 degrees Celsius. The
reactor chamber environment is controlled to a selected pressure
between 10.sup.-3 Torr and atmospheric pressure. The reactor
chamber continues to be controllably rotated at 700 RPM plus or
minus 50 RPM. The gases continue to be provided using the flow
rates of the nucleation phase.
[0037] Measurements may be taken of the nucleation layer during the
growth of the nucleation layer and the interconnection process, but
the embodiment is not so limited. Specific measurements that can be
made include thickness and composition using elipsometric methods
and instrumentation known in the art. Also, temperature measurement
can by made using a pyrometer or other thermal instrumentation
known in the art.
[0038] Following completion of the interconnection process, the gas
flow rates into the reactor chamber are adjusted for the process of
growing the GaN layer on the nucleation layer, or the bulk phase.
The following flow rates are used in the bulk phase of an
embodiment: Nitrogen is provided at a flow rate of 2 to 3 liters
per minute; Hydrogen is provided at a flow rate of 2 to 3 liters
per minute; Ammonia is provided at a flow rate of 1 to 2 liters per
minute; Gallium is provided at a flow rate of 0.2 to 0.5 liters per
minute; Aluminum is provided at a flow rate of 0.2 to 0.5 liters
per minute; and, Indium is provided at a flow rate of 0.2 to 0.5
liters per minute.
[0039] The third step in growing a GaN semiconductor crystal
without a typical base substrate includes growing a GaN layer on
the nucleation layer. A growth rate of between 20 and 100
micrometers per hour can be achieved, with a nominal growth rate of
100 micrometers per hour. The resultant single bulk crystals
produced have dimensions of approximately 2 inches in diameter and
a thickness of between 100 to 350 .mu.m, but are not so
limited.
[0040] The specifications of the GaN layer obtained as disclosed
herein provide many advantages over the typical GaN layer grown on
a non-GaN substrate. The lattice structure of the GaN layer is a
wurtzite structure. The orientation of the GaN layer is (0001). The
thickness of the GaN layer of an embodiment can be greater than 100
micrometers with a thickness uniformity of +/-5 percent, where the
best thickness typically found in the prior art is only a few
micrometers with a thickness uniformity of +/-10 percent. The
dislocation density of an embodiment averages less than 10.sup.5
per square centimeter. The dislocation density of the prior art
averages approximately 10.sup.9 per square centimeter. The
full-width half-maximum intensity of an embodiment as measured
using .omega.-scan measurement is less than 100 arc seconds, where
that of the prior art is approximately 200 arc seconds.
[0041] The GaN semiconductor of an embodiment can easily
accommodate uniform n-type doping as a result of the GaN substrate.
This GaN substrate makes uniform doping possible prior to
fabrication of GaN-based optoelectronic devices because the dopant
can be applied directly to the GaN substrate. In an embodiment the
GaN substrate is doped with an impurity such as an n-type dopant
known in the art. The impurity or dopant can be applied to one or
both sides of the GaN substrate. This provides the advantage of
allowing for the production of semiconductor devices, LEDS or other
optoelectronic devices with doping on one both sides of the device.
Uniform doping of typical GaN semiconductors having a non-GaN
substrate is extremely difficult because the dopant has to be
applied to the non-GaN substrate material.
[0042] The background donor concentration (Nd--Na) of the GaN layer
of an embodiment is less than 10.sup.16 per cubic centimeter. In
other embodiments, the background donor concentration can be less
than 10.sup.15, 10.sup.14 or 10.sup.13 per cubic centimeter
(background donor concentrations can be measured using Hall
methods). The best background donor concentrations typically found
in the prior art are approximately 10.sup.18 per cubic centimeter.
The reduction of the background donor concentration of the GaN
layer of an embodiment is a significant improvement because it
provides a higher quality GaN crystal and hence improved
performance of semiconductors, LEDs or other optoelectronic devices
made from GaN material of an embodiment.
[0043] Although the claimed invention is described in terms of
specific embodiments, it will be understood that numerous
variations and modifications may be made without departing from the
spirit and scope of the claimed invention as described herein and
as set forth in the accompanying claims.
EXPERIMENTAL EXAMPLE
[0044] An experimental example of the production of the GaN single
crystal includes a single crystal GaN grown by a three step process
of creating a nucleation layer on a Pyro-Boron-Nitride susceptor, a
second interconnection or stabilization step, and a third step of
growing a GaN bulk layer on the nucleation layer. The
Pyro-Boron-Nitride susceptor had a thickness of 3.5 to 4.5 mm, and
it was cleaned with an organic solvent. The reactor chamber
described herein was then evacuated to a pressure of 10.sup.-9
Torr. The temperature was then raised to a temperature in the range
300 to 800 degrees Celsius. The susceptor was rotated relative to
the reactor chamber using a rotational velocity of 700 rpm. Chamber
conditions were stabilized for ten minutes. The surface of the
susceptor was cleaned by introducing 99.9999% pure Nitrogen
(N.sub.2) gas at a pressure of 10.sup.-3 Torr. Nitrogen (N.sub.2)
gas was provided at a flow rate of 5 to 10 cubic centimeters per
minute; Ammonia (NH.sub.3) gas was provided at a flow rate of 0.1
to 0.25 liters per minute; Gallium (Ga) gas was provided at a flow
rate of 0.001 to 0.002 liters per minute; Aluminum (Al) gas was
provided at a flow rate of 0.001 to 0.002 liters per minute; and,
Indium (In) gas was provided at a flow rate of 0.001 to 0.002
liters per minute. A Gan nucleation layer was then grown for a
period of ten minutes. The Gan nucleation layer had consisted of 5
to 30 monolayers having a total thickness of ten to 70 Angstroms.
Measurements of the nucleation layer were made using an
elipsometer. Next, during the stabilization step the chamber was
raised to a temperature of 450 degrees Celsius at rate of 3 degrees
per minute. The susceptor continued to be rotated relative to the
reactor chamber at a rate of 700 rpm. The gas flow rates into the
reactor chamber were adjusted for the bulk growth phase. The
following flow rates were used in the bulk phase of an embodiment:
Nitrogen (N.sub.2) gas was provided at a flow rate of 2 to 3 liters
per minute; Hydrogen (H.sub.2) gas was provided at a flow rate of 2
to 3 liters per minute; Ammonia (NH.sub.3) gas was provided at a
flow rate of 1 to 2 liters per minute; Gallium (Ga) gas was
provided at a flow rate of 0.2 to 0.5 liters per minute; Aluminum
(Al) gas was provided at a flow rate of 0.2 to 0.5 liters per
minute; and, Indium (In) gas was provided at a flow rate of 0.2 to
0.5 liters per minute. Next, during the bulk growth step, a GaN
bulk layer was grown on the nucleation layer at a rate of 20 to 100
micrometers per hour. The final dimension of the bulk crystal was
100 to 350 micrometers in thickness. The resulting film had a
(0001) orientation. There was little GaN material in another
orientation as determined by X-ray crystallography. The dislocation
density was 10.sup.5 per square centimeter as measured by SEM. The
Hall mobility was 300 vs/cm.sup.2. The background donor
concentration (Nd--Na) of the layer was <10.sup.16 per cm.sup.3.
The luminescence of the GAN bulk crystal was 3.42 electron volts
(eV) at room temperature.
* * * * *