Large-size ultraviolet transparent aluminum nitride single crystal and preparation method thereof
阅读说明:本技术 大尺寸紫外透明氮化铝单晶及其制备方法 (Large-size ultraviolet transparent aluminum nitride single crystal and preparation method thereof ) 是由 R·T·伯恩多考夫 陈贱峰 山岡慶祐 王时超 S·P·拉奥 鈴木崇志 L·J·邵瓦尔特 于 2018-11-09 设计创作,主要内容包括:在不同的实施例中,形成具有较高紫外光透明度和低缺陷密度的单晶氮化铝晶锭及衬底。所述单晶氮化铝可作为制作发光器件,如发光二极管和激光器的平台。(In various embodiments, single crystal aluminum nitride ingots and substrates having high ultraviolet transparency and low defect density are formed. The single crystal aluminum nitride can be used as a platform for manufacturing light emitting devices, such as light emitting diodes and lasers.)
1. A method of forming single crystal aluminum nitride (AlN), the method comprising:
providing a bulk polycrystalline AlN ceramic;
placing at least a portion of the AlN ceramic in a first crucible;
annealing and densifying the at least a portion of the AlN ceramic in the first crucible to form a polycrystalline AlN source material, the annealing and densifying comprising:
(i) heating the at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours; and (ii) then heating the at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or
(i) Heating the at least a portion of the AlN ceramic to a third temperature ranging from 1900 ℃ to 2250 ℃ over a third time ranging from 5 hours to 25 hours during the exotherm; and (ii) then heating the at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours;
cooling the AlN source material to near room temperature;
placing a second crucible in the furnace, the second crucible comprising an AlN source material and a seed crystal comprising single crystal AlN;
furnace heating the second crucible to a growth temperature of at least 2000 ℃;
maintaining the second crucible at the growth temperature for a holding time of 1 to 10 hours;
after the holding time while the second crucible is maintained at the growth temperature, (i) condensing aluminum and nitrogen containing vapor on the seed crystal to form a monocrystalline AlN ingot epitaxial from the seed crystal, and (ii) moving the second crucible relative to the furnace, the AlN ingot growing at a rate approximately equal to the rate of relative motion between the second crucible and the furnace;
the AlN ingot is then cooled through a cooling cycle comprising:
(i) cooling the AlN ingot from the growth temperature to a fifth temperature ranging from 1450 ℃ to 2150 ℃ over a fifth time ranging from 10 minutes to 90 minutes, and (ii) then cooling the AlN ingot from the fifth temperature to a sixth temperature ranging from 1000 ℃ to 1650 ℃ over a sixth time ranging from 10 seconds to 10 minutes, or
(i) Cooling the AlN ingot from the growth temperature to a seventh temperature ranging from 1450 ℃ to 2150 ℃ over a seventh time ranging from 10 seconds to 10 minutes, and (ii) then cooling the AlN ingot from the seventh temperature to an eighth temperature ranging from 1000 ℃ to 1650 ℃ over an eighth time ranging from 10 minutes to 90 minutes, and
the AlN ingot was then cooled to near room temperature.
2. The method of claim 1, wherein the growth temperature is about 2300 ℃ or less.
3. The method of claim 1, wherein the diameter of the at least a portion of the AlN boule is at least about 50 mm.
4. The method of claim 1, wherein the diameter of the at least a portion of the AlN boule is about 50 mm.
5. The method of claim 1, further comprising introducing oxygen into the second crucible during epitaxial formation of the single crystal AlN ingot from the seed crystal.
6. The method of claim 1, further comprising crushing the AlN ceramic into pieces having a width greater than about 0.5cm and less than about 2cm per piece, prior to placing the at least a portion of the AlN ceramic into the first crucible, such that the at least a portion of the AlN ceramic includes at least some of the pieces.
7. The method of claim 1, wherein the crystalline orientation of the seed crystal is substantially parallel to the c-axis.
8. The method of claim 1, wherein the first crucible and second crucible are the same crucible.
9. The method of claim 1, wherein the first crucible and second crucible are different crucibles.
10. The method of claim 1, wherein the growth rate is at least 0.5 mm/hour.
11. The method of claim 1, wherein the incubation time is about 5 hours.
12. The method of claim 1, wherein the seed crystal is about 35mm in diameter or greater.
13. The method of claim 1, further comprising slicing the AlN ingot to form a single crystal AlN substrate having a diameter of at least 50 mm.
14. The method of claim 13, further comprising fabricating a light emitting device on at least a portion of the AlN substrate.
15. The method of claim 14, wherein the light emitting device is configured to emit ultraviolet light.
16. The method of claim 14, further comprising removing at least a portion of the AlN substrate from the light-emitting device after forming at least a portion of the light-emitting device.
17. The method of claim 14, wherein the light emitting device comprises a light emitting diode or a laser.
18. The method of claim 1, further comprising gettering oxygen and/or carbon during formation of the single-crystal AlN ingot.
19. The method of claim 18, wherein the oxygen and/or carbon may be gettered with a getter material added to the second crucible and/or furnace prior to and/or during formation of the single crystal AlN ingot.
20. A method according to claim 19, wherein the melting point of the getter material is greater than the growth temperature and/or the eutectic melting point thereof with AlN is greater than the growth temperature.
21. The method of claim 19, wherein the getter material comprises at least one of boron, iridium, niobium, molybdenum, tantalum, or rhenium.
22. The method of claim 1, further comprising slicing the AlN ingot to form a single crystal AlN substrate having a diameter of approximately 50 mm.
23. The method of claim 1, wherein the bulk polycrystalline AlN ceramic contains less than approximately 1% excess Al.
24. The method of claim 1, wherein the concentration of oxygen in the bulk polycrystalline AlN ceramic is less than 2 × 1019cm-3。
25. An AlN ingot formed in accordance with claim 1.
26. A single crystal AlN substrate having (i) a diameter of at least about 50mm and (ii) an Ultraviolet (UV) transparency measure of about 5cm at a wavelength of interest of 265nm3To about 5000cm3The ultraviolet transparency is measured in units of cm3Is defined as:
where d is the diameter of the AlN substrate in mm, FWHM is the full width at half maximum of the X-ray diffraction curve of the AlN substrate in radians, and α is the absorption coefficient of the AlN substrate at the wavelength of interest.
27. The AlN substrate according to claim 26, wherein the diameter of the AlN substrate is approximately 50 mm.
28. The AlN substrate according to claim 26, wherein the AlN substrate has a diameter of no more than about 150 mm.
29. The AlN substrate according to claim 26, wherein the AlN substrate has a thermal conductivity of about 290W/m-K or greater at room temperature.
30. The AlN substrate of claim 26, wherein the AlN substrate has a crystallographic orientation that is substantially parallel to the c-axis.
31. The AlN substrate of claim 26, wherein the AlN substrate has a crystallographic orientation that is at least about 10 ° from the c-axis.
32. The AlN substrate of claim 26, further including a light emitting device thereon.
33. The AlN substrate according to claim 32, wherein the light-emitting device is configured to emit ultraviolet light.
34. The AlN substrate of claim 32, wherein the light-emitting device includes a light-emitting diode or a laser.
35. The AlN substrate of claim 26, wherein the AlN substrate has a threading edge dislocation density of less than 5 × 103cm-2。
36. The AlN substrate of claim 26, wherein the AlN substrate has a threading edge dislocation density of less than 10cm-2。
37. The AlN substrate of claim 26, wherein the concentration of silicon in the AlN substrate is less than 1 × 1017cm-3。
38. The AlN substrate of claim 26, wherein the concentration of oxygen in the AlN substrate is less than 1 × 1017cm-3。
39. The AlN substrate of claim 26, wherein the concentration of carbon in the AlN substrate is less than 3 × 1017cm-3。
40. The AlN substrate according to claim 26, wherein a ratio of a carbon concentration of the AlN substrate to an oxygen concentration of the AlN substrate is less than 0.5.
41. A single crystal AlN substrate having an Ultraviolet (UV) transparency, measured at a wavelength of interest of 265nm, of about 20cm3To about 5000cm3The ultraviolet transparency is measured in units of cm3Is defined as:
where d is the diameter of the AlN substrate in mm, FWHM is the full width at half maximum of the X-ray diffraction curve of the AlN substrate in radians, and α is the absorption coefficient of the AlN substrate at the wavelength of interest.
42. The AlN substrate of claim 41, wherein the AlN substrate has a diameter of about 50mm or more.
43. The AlN substrate of claim 41, wherein the AlN substrate has a diameter of about 50 mm.
44. The AlN substrate of claim 41, wherein the AlN substrate has a diameter of no more than about 150 mm.
45. The AlN substrate of claim 41, wherein the AlN substrate has a thermal conductivity of about 290W/m-K or more at room temperature.
46. The AlN substrate of claim 41, wherein the AlN substrate has a crystallographic orientation that is substantially parallel to the c-axis.
47. The AlN substrate of claim 41, wherein the AlN substrate has a crystallographic orientation of at least about 10 ° with respect to the c-axis.
48. The AlN substrate of claim 41, further comprising a light-emitting device thereon.
49. The AlN substrate of claim 48, wherein the light-emitting device is configured to emit ultraviolet light.
50. The AlN substrate according to claim 48, wherein the light-emitting device comprises a light-emitting diode or a laser.
51. The AlN substrate of claim 41, wherein the AlN substrate has a threading edge dislocation density of less than 5 × 103cm-2。
52. The AlN substrate of claim 41, wherein the AlN substrate has a threading edge dislocation density of less than 10cm-2。
53. The AlN substrate of claim 41, wherein the concentration of silicon in the AlN substrate is less than 1 × 1017cm-3。
54. The AlN substrate of claim 41, wherein the concentration of oxygen in the AlN substrate is less than 1 × 1017cm-3。
55. The AlN substrate of claim 41, wherein the concentration of carbon in the AlN substrate is less than 3 × 1017cm-3。
56. The AlN substrate of claim 41, wherein the ratio of the carbon concentration to the oxygen concentration in the AlN substrate is less than 0.5.
57. A single-crystal ingot of AlN having a diameter of about 50mm or more and a length of about 15mm or more, wherein:
the AlN ingot has an Ultraviolet (UV) transparency of less than 60cm at a wavelength of about 220nm to about 480nm-1;
The AlN ingot has an oxygen concentration of less than 4 × 1017cm-3;
The AlN ingot has a carbon concentration of less than 4 × 1017cm-3(ii) a And
the AlN ingot has a ratio of carbon concentration to oxygen concentration of less than 1.0.
58. The AlN ingot of claim 57, wherein the ultraviolet transparency is less than 30cm at wavelengths of about 220nm to about 480nm-1。
59. The AlN ingot of claim 57, wherein the concentration of oxygen in the AlN ingot is less than 1 × 1017cm-3。
60. The AlN ingot of claim 57, wherein the concentration of carbon in the AlN ingot is less than 3 × 1017cm-3。
61. The AlN ingot of claim 57, wherein the ratio of the carbon concentration of the AlN ingot to the oxygen concentration of the AlN ingot is less than 0.5.
62. The AlN ingot of claim 57, wherein the length of the ingot is about 20mm or more.
63. The AlN ingot of claim 57, wherein the length of the ingot is about 35mm or more.
64. The AlN ingot of claim 57, wherein the diameter of the ingot is about 50 mm.
65. The AlN ingot of claim 57, wherein the AlN ingot has a thermal conductivity of about 290W/m-K or greater at room temperature.
66. The AlN ingot of claim 57, wherein the AlN ingot has an X-ray diffraction profile with a Full Width Half Maximum (FWHM) of less than 50 arc seconds.
67. The AlN ingot of claim 57, wherein the AlN ingot has a threading edge dislocation density of less than 5 × 103-2
cm-2。
68. The AlN ingot of claim 57, wherein the AlN ingot has a threading edge dislocation density of less than 10cm-2。
69. The AlN ingot of claim 57, wherein the concentration of silicon in the AlN ingot is less than 1 × 1017cm-3。
70. The AlN ingot of claim 57, wherein the ultraviolet transparency is less than 10cm at wavelengths of about 350nm to about 480nm-1。
71. A method of improving Ultraviolet (UV) transparency of a single-crystal AlN bulk crystal, the method comprising:
heating the AlN bulk crystal to an annealing temperature of at least 2000 ℃;
the AlN bulk crystal is then cooled over a cooling period that includes:
(i) cooling the AlN bulk crystal from an annealing temperature to a first temperature ranging from 1450 ℃ to 2150 ℃ over a first time period ranging from 10 minutes to 90 minutes, and (ii) then cooling the AlN bulk crystal from the first temperature to a second temperature ranging from 1000 ℃ to 1650 ℃ over a second time period ranging from 10 seconds to 10 minutes, or
(i) Cooling the AlN bulk crystal from the annealing temperature to a third temperature ranging from 1450 ℃ to 2150 ℃ over a third time ranging from 10 seconds to 10 minutes, and (ii) then cooling the AlN bulk crystal from the third temperature to a fourth temperature ranging from 1000 ℃ to 1650 ℃ over a fourth time ranging from 10 minutes to 90 minutes; and
the AlN bulk crystal is then cooled to near room temperature.
72. The method of claim 71, further comprising holding the AlN bulk crystal at the annealing temperature for a soak time of 1 hour to 10 hours after heating the AlN bulk crystal to the annealing temperature and before cooling the AlN bulk crystal by the cooling period.
73. The method of claim 71, wherein the diameter of the AlN bulk crystal is about 50mm or greater.
74. The method of claim 71, wherein the diameter of the AlN bulk crystal is about 50 mm.
75. The method of claim 71, further comprising fabricating a light emitting device on at least a portion of the AlN bulk crystal.
76. The method of claim 75, wherein the light emitting device is configured to emit ultraviolet light.
77. The method of claim 75, further comprising removing at least a portion of the AlN bulk crystal from the light-emitting device after forming at least a portion of the light-emitting device.
78. The method of claim 75, wherein the light emitting device comprises a light emitting diode or a laser.
79. An AlN bulk crystal formed according to the method of claim 71.
80. A method of improving Ultraviolet (UV) transparency of a single-crystal AlN bulk crystal, the method comprising:
heating the AlN bulk crystal to an annealing temperature of at least 2000 ℃;
the AlN bulk crystal is then cooled over a cooling period that includes:
cooling the AlN bulk crystal from an annealing temperature to a first temperature ranging from 1000 ℃ to 1650 ℃ over 1 hour to 10 hours; and
the AlN bulk crystal is then cooled to near room temperature.
81. The method of claim 80, further comprising holding the AlN bulk crystal at an annealing temperature for a soak time of 1 hour to 10 hours after heating the AlN bulk crystal to the annealing temperature and before cooling the AlN bulk crystal through a cooling period.
82. The method of claim 80, wherein the diameter of the AlN bulk crystal is about 50mm or greater.
83. The method of claim 80, wherein the diameter of the AlN bulk crystal is about 50 mm.
84. The method of claim 80, further comprising fabricating a light emitting device on at least a portion of the AlN bulk crystal.
85. The method of claim 84, wherein the light emitting device is configured to emit ultraviolet light.
86. The method of claim 84, further comprising removing at least a portion of the AlN bulk crystal from the light-emitting device after forming at least a portion of the light-emitting device.
87. The method of claim 84, wherein the light emitting device comprises a light emitting diode or a laser.
88. An AlN bulk crystal formed according to the method of claim 80.
89. A method of forming a polycrystalline AlN source material, the method comprising;
providing a bulk polycrystalline AlN ceramic;
placing at least a portion of the AlN ceramic in a crucible;
annealing and densifying the at least a portion of the AlN ceramic in the crucible to form a polycrystalline AlN source material, the annealing and densifying comprising:
(i) heating the at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours; and (ii) then heating the at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or
(i) Heating the at least a portion of the AlN ceramic to a third temperature ranging from 1900 ℃ to 2250 ℃ over a third time ranging from 5 hours to 25 hours during the exotherm; and (ii) then heating the at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours; and
the AlN source material was cooled to near room temperature.
90. The method of claim 89, further comprising, prior to placing the at least a portion of the AlN ceramic into the crucible, crushing the AlN ceramic into pieces having a width of greater than about 0.5cm and less than about 2cm, such that the at least a portion of the AlN ceramic includes at least some of the pieces.
91. The method of claim 89, wherein the bulk polycrystalline AlN ceramic contains less than about 1% excess Al.
92. The method of claim 89, wherein the concentration of oxygen in the bulk polycrystalline AlN ceramic is less than 2 × 1019cm-3。
93. A polycrystalline AlN source material formed according to claim 89.
94. A method of forming single crystal aluminum nitride (AlN), the method comprising:
providing a bulk polycrystalline AlN ceramic;
placing at least a portion of the AlN ceramic in a first crucible;
annealing and densifying the at least a portion of the AlN ceramic in the first crucible to form a polycrystalline AlN source material, the annealing and densifying comprising:
(i) heating the at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours; and (ii) then heating the at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or
(i) Heating the at least a portion of the AlN ceramic to a third temperature ranging from 1900 ℃ to 2250 ℃ over a third time ranging from 5 hours to 25 hours during the exotherm; and (ii) then heating the at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours;
cooling the AlN source material to near room temperature;
placing a second crucible in the furnace, the second crucible comprising an AlN source material and a seed crystal comprising single crystal AlN;
furnace heating the second crucible to a growth temperature of at least 2000 ℃;
maintaining the second crucible at the growth temperature for a holding time of 1 to 10 hours;
after the holding time while the second crucible is at the growth temperature, (i) condensing aluminum and nitrogen containing vapor on the seed crystal to form a monocrystalline AlN ingot epitaxial from the seed crystal, and (ii) moving the second crucible relative to the furnace, the AlN ingot growing at a rate approximately equal to the rate of relative motion between the second crucible and the furnace;
the AlN ingot is then cooled through a cooling cycle comprising:
cooling the AlN ingot from a growth temperature to a fifth temperature ranging from 1000 ℃ to 1650 ℃ over a fifth time ranging from 1 hour to 10 hours; and
the AlN ingot was then cooled to near room temperature.
95. The method of claim 94, wherein said growth temperature is about 2300 ℃ or less.
96. The method of claim 94, wherein the diameter of the at least a portion of the AlN ingot is at least about 50 mm.
97. The method of claim 94, wherein the diameter of the at least a portion of the AlN boule is about 50 mm.
98. The method of claim 94, further comprising introducing oxygen into the second crucible during epitaxial formation of the single crystal AlN ingot from the seed crystal.
99. The method of claim 94, further comprising, prior to placing the at least a portion of the AlN ceramic into the first crucible, crushing the AlN ceramic into pieces having a width of greater than about 0.5cm and less than about 2cm, such that the at least a portion of the AlN ceramic includes at least some of the pieces.
100. The method of claim 94, wherein the crystalline orientation of the seed crystal is substantially parallel to the c-axis.
101. The method of claim 94, wherein the first crucible and second crucible are the same crucible.
102. The method of claim 94, wherein the first crucible and second crucible are different crucibles.
103. The method of claim 94, wherein said growth rate is at least 0.5 mm/hour.
104. The method of claim 94, wherein the incubation time is about 5 hours.
105. The method of claim 94, wherein the seed crystal is about 35mm in diameter or greater.
106. The method of claim 94, further comprising slicing the AlN ingot to form a single crystal AlN substrate having a diameter of at least 50 mm.
107. The method of claim 106, further comprising fabricating a light emitting device on at least a portion of the AlN substrate.
108. The method of claim 107, wherein the light emitting device is configured to emit ultraviolet light.
109. The method of claim 107, further comprising removing at least a portion of the AlN substrate from the light-emitting device after forming at least a portion of the light-emitting device.
110. The method of claim 107, wherein the light emitting device comprises a light emitting diode or a laser.
111. The method of claim 94, further comprising gettering oxygen and/or carbon during formation of the single-crystal AlN ingot.
112. The method of claim 111, wherein the oxygen and/or carbon may be gettered with a getter material added to the second crucible and/or furnace prior to and/or during formation of the single crystal AlN ingot.
113. A method according to claim 112, wherein the melting point of the getter material is greater than the growth temperature and/or the eutectic melting point thereof with AlN is greater than the growth temperature.
114. The method of claim 112, wherein the getter material comprises at least one of boron, iridium, niobium, molybdenum, tantalum, or rhenium.
115. The method of claim 94, further comprising slicing the AlN ingot to form a single crystal AlN substrate having a diameter of about 50 mm.
116. The method of claim 94, wherein the bulk polycrystalline AlN ceramic contains less than approximately 1% excess Al.
117. The method of claim 94, wherein the concentration of oxygen in the bulk polycrystalline AlN ceramic is less than 2 × 1019cm-3。
118. An AlN ingot formed according to claim 94.
Technical Field
In various embodiments, the present disclosure relates to the preparation of single crystal aluminum nitride (AlN).
Background
Aluminum nitride (AlN) has great promise as a semiconductor material for a number of applications, such as optoelectronic devices (e.g., short wavelength Light Emitting Diodes (LEDs)) and lasers, dielectric layers for optical storage media, electronic substrates, and chip carriers that require high thermal conductivity. In principle, the properties of AlN allow emission of light with wavelengths as low as about 200 nanometers (nm). Recent work has shown that Ultraviolet (UV) LEDs have superior performance when fabricated using low defect AlN substrates fabricated from bulk AlN single crystals. With AlN substrates, due to high thermal conductivity and low electrical conductivity, it is expected that the performance of high power Radio Frequency (RF) devices made from nitride semiconductors will also be improved. However, commercial viability of AlN semiconductor devices is limited by the large size, rarity of low defect AlN single crystals, and high cost.
To more easily obtain a large-diameter AlN substrate at low cost and to make commercially viable devices fabricated using large-diameter AlN substrates, it is desirable to grow large-diameter (>25mm) AlN bulk crystals at higher growth rates (>0.5mm/hr) while maintaining crystal quality. The most efficient method of growing bulk single crystals of AlN is the "sublimation-condensation" method, which involves subliming a relatively low quality (typically polycrystalline) source material of AlN and recondensing the resulting vapor to form single-crystal AlN. Various aspects of seeded and unseeded sublimation-condensation growth of AlN are described in U.S. Pat. nos. 6,770,135 (the '135 patent), 7,638,346 (the' 346 patent), 7,776,153 (the '153 patent), and 9,028,612 (the' 612 patent), which are incorporated herein by reference in their entirety.
While AlN substrates enable platforms to fabricate UV-light emitting devices, such as LEDs, their performance in these applications is often limited by their transparency to ultraviolet light (i.e., "UV transparency") or lack of UV transparency. It is often difficult to prepare AlN substrates with higher uv transparency because uv transparency can be compromised by contamination and/or point defects introduced during the AlN growth process. Such problems have been limited to solutions using the methods disclosed in U.S. patent nos. 8,012,257, 9,034,103, and 9,447,519, which are incorporated herein by reference in their entirety. In particular, these patents disclose methods for controlling the introduction of oxygen impurities during polycrystalline AlN source-material preparation and during the growth of single crystal AlN sublimation-condensation processes. Although these methods are reported to produce bulk AlN crystals with low absorption coefficients and thus high uv transparency, the present inventors have found that these methods fail to achieve high uv transparency when seeded with AlN bulk crystals having diameters in excess of 25mm (e.g., 30mm to 75mm diameter crystals, e.g., about 50mm diameter) at high growth rates (e.g., at least 0.5 to 0.8mm/hr) while employing the large axial and radial thermal gradients necessary for such growth.
For example, fig. 6 of U.S. patent No.9,447,519 (the' 519 patent) describes the absorption spectrum of AlN crystals prepared using the method of oxygen control and controlled post-growth cooling described in the 519 patent. As shown, the absorption coefficient of AlN crystals at wavelengths of 300nm to 350nm is less than about 10cm-1. However, the crystals were prepared using unseeded growth, with a maximum diameter of less than 25 mm. The inventors tried to reproduce this high uv transparency using the same technique, but with the difference that crystals of about 50mm diameter were grown using substantially the same growth process with seed crystals added. Unfortunately, even with the technique of the' 519 patent, the resulting crystals are not practically transparent, i.e., have an absorption coefficient at one or more ultraviolet wavelengths of greater than 100-200cm-1And/or at about 265The absorption coefficient at nm and/or 310nm shows a large peak. Various wafers were sliced from 50mm diameter ingots prepared using the technique of the' 519 patent, and examples of absorption coefficients are shown in FIG. 1A. As shown, each wafer sliced from such an ingot is substantially opaque at different ultraviolet wavelengths. The uv transparency of similar crystals is particularly poor for crystals prepared using on-axis AlN seeds. For economic reasons, on-axis growth is preferred; the off-axis AlN boule must be cut at an angle to obtain the on-axis substrate, and therefore, the number of substrates that can be produced from the off-axis boule must be less than the number of substrates that can be produced from the on-axis boule. In view of these results, the present inventors recognized a need for a new and improved method to achieve higher uv transparency levels for large-size AlN bulk crystals.
Disclosure of Invention
In various embodiments of the present invention, large-size, high-quality, high-uv-transparency AlN single crystals can be grown by employing techniques that limit the introduction of contaminants and defects that compromise uv-transparency. The technique is particularly advantageous for producing AlN bulk crystals having a diameter greater than 25mm (e.g., about 50mm or greater) at high growth rates (e.g., growth rates of at least 0.5mm/hr) using a seed crystal growth program-a substrate preparation system in which higher uv transparency and substrate quality cannot be achieved using conventional methods. Embodiments of the present invention enable the growth of large-sized AlN single crystals with high ultraviolet transparency and high crystal quality (e.g., low crystal defect density such as dislocations) even when the crystal diameter is greater than 25mm or about 50mm or greater.
The inventors have discovered that carbon impurities can cause high levels of uv absorption in the bulk crystal of AlN. Carbon impurities cause uv absorption at a wavelength of about 265nm, impairing the performance of the uv light emitting device. (in the' 519 patent, control of carbon impurities is not expressly contemplated, as carbon is suggested as a possible dopant for AlN production and crucible material.) in contrast, oxygen impurities (or related point defects) typically result in uv absorption at wavelengths of about 310 nm. Thus, while controlling oxygen contamination is desirable for uv transparency, it is not sufficient to achieve uv transparency at many uv wavelengths. Embodiments of the present invention include controlling and limiting carbon and oxygen contamination during AlN growth procedures by physical vapor transport methods, such as sublimation-condensation methods. Such methods include reducing or substantially eliminating carbon and oxygen impurities in polycrystalline AlN source material, controlling various growth parameters (e.g., growth pressure) during fabrication, and actively controlling cooling of the growing AlN crystal from high growth temperatures, reducing contamination, while also avoiding material cracking.
In various embodiments of the present invention, the growth of AlN material may be promoted and controlled in a variety of different ways by controlling the radial and/or axial thermal gradients in the crystal growth crucible. For example, individual heating elements arranged around the crucible may employ different power levels (and thus different temperatures) to establish a thermal gradient within the crucible. The above measures can also be supplemented or replaced by arranging a heat insulating layer around the crucible, so that the insulation arranged around the region where the higher temperature is required can be thinner and/or less. As described in detail in the' 612 patent, a thermal shield layer may also be disposed around the crucible in any of a number of different arrangements, for example, above and/or below the crucible, in order to establish a desired thermal gradient within the crucible.
Although embodiments of the present invention employ AlN as the exemplary crystalline material prepared in accordance with the methods herein, embodiments of the present invention are also applicable to other crystalline materials, such as silicon carbide (SiC) and zinc oxide (ZnO); thus, AlN as described herein, in other embodiments, may be replaced with SiC or ZnO. As used herein, the term "diameter" refers to the lateral dimension (e.g., the maximum lateral dimension) of a crystal, growth chamber, or other object, even if the crystal, growth chamber, or other object is not circular and/or irregular in cross-section.
The "substrate" or "wafer" is herein a portion of a previously grown ingot having an upper surface and an opposing lower surface, typically parallel surfaces. The substrate is typically 200 μm to 1mm thick and can serve as a platform for epitaxial growth of semiconductor layers and fabrication of semiconductor devices (e.g., light emitting devices such as lasers and light emitting diodes, transistors, power devices, etc.) thereon. As used herein, "room temperature" is 25 ℃.
In one aspect, embodiments of the invention feature a method of forming single crystal aluminum nitride (AlN). A bulk polycrystalline AlN ceramic is provided. At least a portion of the AlN ceramic is placed in the first crucible. The at least a portion of the AlN ceramic is annealed and densified within a first crucible to form a polycrystalline AlN source material. Annealing and densifying comprises, consists essentially of, or consists of (i) heating at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or (i) heating at least a portion of the AlN ceramic at a third temperature ranging from 1900 ℃ to 2250 ℃ for a third time ranging from 5 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours. The AlN source material was cooled to near room temperature. The second crucible was placed in the furnace. The second crucible contains an AlN source material and a seed crystal comprising, consisting of, or consisting essentially of single-crystal AlN. The second crucible is furnace heated to a growth temperature of at least 2000 ℃. The second crucible is held at the growth temperature for a holding time of 1 hour to 10 hours. After the holding time while the second crucible is at the growth temperature, (i) a vapor phase comprising, consisting essentially of, or consisting of aluminum and nitrogen condenses on the seed crystal to epitaxially form a single crystal AlN ingot from the seed crystal, and (ii) the second crucible is moved relative to the furnace. At least a portion of the AlN ingot grows at a rate approximately equal to the rate of relative motion between the second crucible and the furnace. The AlN ingot is then cooled through a cooling cycle. The cooling period comprises, consists essentially of, or consists of (i) cooling the AlN ingot from the growth temperature to a fifth temperature ranging from 1450 ℃ to 2150 ℃ over a fifth time period ranging from 10 minutes to 90 minutes, and (ii) then cooling the AlN ingot from the fifth temperature to a sixth temperature ranging from 1000 ℃ to 1650 ℃ over a sixth time period ranging from 10 seconds to 10 minutes, or (i) cooling the AlN ingot from the growth temperature to a seventh temperature ranging from 1450 ℃ to 2150 ℃ over a seventh time period ranging from 10 seconds to 10 minutes, and (ii) then cooling the AlN ingot from the seventh temperature to an eighth temperature ranging from 1000 ℃ to 1650 ℃ over an eighth time period ranging from 10 minutes to 90 minutes. Then, the AlN ingot was cooled to near room temperature.
Embodiments of the invention may include one or more of any of the following combinations. The growth temperature may be about 2300 ℃ or less, about 2200 ℃ or less, or about 2100 ℃ or less. At least a portion (or even all) of the AlN boule may be at least about 35mm, at least about 50mm, at least about 75mm, or at least about 100mm in diameter. At least a portion (or even all) of the AlN boule may have a diameter of up to about 150mm, up to about 125mm, up to about 100mm, or up to about 75 mm. At least a portion (or even all) of the AlN boule may be about 50mm in diameter. Oxygen (e.g., oxygen or an oxygen-containing gas) may be introduced into the second crucible during epitaxial formation of the single-crystal AlN ingot from the seed crystal. The AlN ceramic may be broken into pieces prior to placing at least a portion of the AlN ceramic in the first crucible. The width (or diameter, or length, or other dimension) of one or more (or even each) of the fragments is greater than about 0.1cm, greater than about 0.2cm, greater than about 0.3cm, greater than about 0.4cm, greater than about 0.5cm, greater than about 0.7cm, or greater than about 1 cm. The width (or diameter, or length, or other dimension) of one or more (or even each) of the fragments is less than about 5cm, less than about 4cm, less than about 3cm, less than about 2cm, less than about 1.5cm, or less than about 1 cm. At least a portion of the AlN ceramic may include, consist essentially of, or consist of one or more pieces.
The crystalline orientation of the seed may be substantially parallel to the c-axis. The first crucible and the second crucible may be the same crucible or different crucibles. The growth rate may be at least 0.1 mm/hr, at least 0.2 mm/hr, at least 0.3 mm/hr, at least 0.4 mm/hr, at least 0.5mm/hr, at least 0.7 mm/hr, or at least 1 mm/hr. The growth rate may be at most 3 mm/hour, at most 2.5 mm/hour, at most 2 mm/hour, at most 1.5 mm/hour or at most 1 mm/hour. The incubation time may be at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, or at least about 6 hours. The incubation time may be up to about 9 hours, up to about 8 hours, up to about 7 hours, up to about 6 hours, or up to about 5 hours. The incubation time may be about 5 hours. The diameter of the seed may be about 25mm or greater, about 30mm or greater, about 35mm or greater, about 40mm or greater, about 45mm or greater, or about 50mm or greater. The diameter of the seed may be about 150mm or less, about 100mm or less, about 75mm or less, or about 50mm or less.
The AlN ingot may be sliced to form single crystal AlN substrates having a diameter of at least 25mm, at least 30mm, at least 35mm, at least 40mm, at least 45mm, or at least 50 mm. The diameter of the single-crystal AlN substrate may be about 150mm or less, about 100mm or less, about 75mm or less, or about 50mm or less. The diameter of the single crystal AlN substrate may be about 50 mm. The light emitting device may be fabricated using at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After forming at least a portion of the light emitting device (e.g., all or part of the epitaxial light emitting layer structure), at least a portion (or even all) of the AlN substrate of the light emitting device may be removed. The light emitting device may comprise, consist essentially of, or consist of a light emitting diode and/or a laser.
The oxygen and/or carbon may be gettered during formation of the single crystal AlN ingot the oxygen and/or carbon may be gettered prior to and/or during formation of the single crystal AlN ingot with a getter material added to the second crucible and/or furnace, the melting point of the getter material is greater than the growth temperature and/or the eutectic melting point thereof with AlN is greater than the growth temperature19cm-3. Embodiments of the invention may include an AlN ingot and/or substrate formed according to any of the methods described above.
In another aspect, embodiments of the invention feature a single-crystal AlN substrate having (i) a diameter of at least about 50mm and (ii) an Ultraviolet (UV) transparency of about 5cm at a wavelength of interest of 265nm3To about 5000cm3. UV transparency is defined as cm3Such as
Where d is the diameter of the AlN substrate in mm, FWHM is the full width at half maximum of the X-ray diffraction curve of the AlN substrate in radians, and α is the absorption coefficient of the AlN substrate at the wavelength of interest.Embodiments of the invention may include one or more of any of the following combinations. The diameter of the AlN substrate may be about 50 mm. The AlN substrate may be at least about 60mm, at least about 65mm, at least about 70mm, at least about 75mm, at least about 80mm, at least about 85mm, at least about 90mm, at least about 95mm, or at least about 100mm in diameter. The AlN substrate may be no greater than approximately 150mm, no greater than approximately 125mm, no greater than approximately 110mm, or no greater than approximately 100mm in diameter. The AlN substrate may have a room temperature thermal conductivity of about 250W/m.K or more, about 270W/m.K or more, about 290W/m.K or more, about 300W/m.K or more, or about 320W/m.K or more. The AlN substrate may have a room temperature thermal conductivity of about 400W/m.K or less, about 350W/m.K or less, or about 300W/m.K or less. The crystal orientation of the AlN substrate is substantially parallel to the c-axis. The AlN substrate may have a crystallographic orientation of at least about 10 °, at least about 12 °, at least about 15 °, or at least about 20 ° with respect to the c-axis. The AlN substrate may have a crystallographic orientation with respect to the c-axis of at most about 30 °, at most about 25 °, at most about 20 °, or at most about 15 °.
The light emitting device may include, consist essentially of, or consist of a light emitting diode and/or a laser, the through-edge dislocation density of the AlN substrate may be less than 5 × 105cm-2Less than 1 × 105cm-2Less than 5 × 104cm-2Less than 1 × 104cm-2Less than 5 × 103cm-2Or less than 1 × 103cm-2. The AlN substrate may have a threading edge dislocation density of more than 10cm-2Greater than 100cm-2Greater than 500cm-2Or greater than 1000cm-2. The threading dislocation density of the AlN substrate may be less than 100cm-2Is less than 50cm-2Is less than 10cm-2Is less than 5cm-2Or less than 1cm-2. The threading dislocation density of the AlN substrate may be more than 0.1cm-2Greater than 0.5cm-2Greater than 1cm-2Greater than 2cm-2Or greater than 5cm-2。
The silicon concentration of the AlN substrate may be less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have a silicon concentration greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The AlN substrate may have an oxygen concentration of less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have an oxygen concentration of more than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The carbon concentration of the AlN substrate may be less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have a carbon concentration greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3. The AlN substrate may have a ratio of carbon concentration to oxygen concentration of less than 1, less than 0.9, less than 0.8, less than 0.7, less than 0.6, less than 0.5, less than 0.4, or less than 0.3. The AlN substrate may have a ratio of carbon concentration to oxygen concentration greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4.
In another aspect, embodiments of the invention feature a single crystal AlN substrate having an Ultraviolet (UV) transparency measure (transmittance measure) of about 20cm at a wavelength of interest of 265nm3To about 5000cm3. The ultraviolet transparency measure is defined as (in cm)3) Such asWhere d is the diameter of the AlN substrate in mm, FWHM is the full width at half maximum of the X-ray diffraction curve of the AlN substrate in radians, and α is the absorption coefficient of the AlN substrate at the wavelength of interest.
Embodiments of the invention may include one or more of any of the following combinations. The diameter of the AlN substrate may be about 50 mm. The AlN substrate may be at least about 60mm, at least about 65mm, at least about 70mm, at least about 75mm, at least about 80mm, at least about 85mm, at least about 90mm, at least about 95mm, or at least about 100mm in diameter. The AlN substrate may be no greater than approximately 150mm, no greater than approximately 125mm, no greater than approximately 110mm, or no greater than approximately 100mm in diameter. The AlN substrate may have a room temperature thermal conductivity of about 250W/m.K or more, about 270W/m.K or more, about 290W/m.K or more, about 300W/m.K or more, or about 320W/m.K or more. The AlN substrate may have a room temperature thermal conductivity of about 400W/m.K or less, about 350W/m.K or less, or about 300W/m.K or less. The crystal orientation of the AlN substrate is substantially parallel to the c-axis. The AlN substrate may have a crystallographic orientation of at least about 10 °, at least about 12 °, at least about 15 °, or at least about 20 ° with respect to the c-axis. The AlN substrate may have a crystallographic orientation with respect to the c-axis of at most about 30 °, at most about 25 °, at most about 20 °, or at most about 15 °.
The light emitting device may include, consist essentially of, or consist of a light emitting diode and/or a laser, the through-edge dislocation density of the AlN substrate may be less than 5 × 105cm-2Less than 1 × 105cm-2Less than 5 × 104cm-2Less than 1 × 104cm-2Less than 5 × 103cm-2Or less than 1 × 103cm-2. The AlN substrate may have a threading edge dislocation density of more than 10cm-2Greater than 100cm-2Greater than 500cm-2Or greater than 1000cm-2. The threading dislocation density of the AlN substrate may be less than 100cm-2Is less than 50cm-2Is less than 10cm-2Is less than 5cm-2Or less than 1cm-2. The threading dislocation density of the AlN substrate may be more than 0.1cm-2Greater than 0.5cm-2Greater than 1cm-2Greater than 2cm-2Or greater than 5cm-2。
The silicon concentration of the AlN substrate may be less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have a silicon concentration greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The AlN substrate may have an oxygen concentration of less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have an oxygen concentration of more than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The carbon concentration of the AlN substrate may be less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN substrate may have a carbon concentration greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3. The AlN substrate may have a ratio of carbon concentration to oxygen concentration of less than 1, less than 0.9, less than 0.8, less than 0.7, less than 0.6, less than 0.5, less than 0.4, or less than 0.3. The AlN substrate may have a ratio of carbon concentration to oxygen concentration greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4.
In another aspect, embodiments of the invention feature a single-crystal AlN ingot having a diameter of about 50mm or more and a length of about 15mm or more. The AlN ingot has an ultraviolet transparency of less than 60cm at a wavelength of about 220nm to about 480nm-1The AlN ingot has an oxygen concentration of less than 4 × 1017cm-3And/or the AlN ingot has a carbon concentration of less than 4 × 1017cm-3. The AlN ingot has a ratio of the carbon concentration to the oxygen concentration of less than 1.0.
Embodiments of the invention may include one or more of any of the following combinations. The UV transparency at a wavelength of about 220nm to about 480nm may be less than 55cm-1Is less than 50cm-1Is less than 45cm-1Less than 40cm-1Less than 35cm-1Or less than 30cm-1. The UV transparency may be greater than 10cm in the wavelength range of about 220nm to about 480nm-1Greater than 15cm-1Greater than 20cm-1Or greater than 25cm-1The silicon concentration of the AlN ingot may be less than 1 × 1019cm-3Less than 5 × 1018cm-3Less than 1 × 1018cm-3Less than 5 × 1017cm-3Less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The silicon concentration of the AlN ingot may be greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The AlN ingot may have an oxygen concentration of less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The AlN ingot may have an oxygen concentration of greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3The carbon concentration of the AlN ingot may be less than 3 × 1017cm-3Less than 1 × 1017cm-3Less than 5 × 1016cm-3Or less than 1 × 1016cm-3The carbon concentration of the AlN ingot may be greater than 1 × 1014cm-3Greater than 5 × 1014cm-3Greater than 1 × 1015cm-3Greater than 5 × 1015cm-3Greater than 1 × 1016cm-3Greater than 5 × 1016cm-3Or greater than 1 × 1017cm-3. The AlN ingot may have a ratio of carbon concentration to oxygen concentration of less than 0.9, less than 0.8, less than 0.7, less than 0.6, less than 0.5, less than 0.4, or less than 0.3. The ratio of the carbon concentration to the oxygen concentration of the AlN ingot may be greater than 0.1, greater than 0.2, greater than 0.3, or greater than 0.4.
The length of the ingot may be about 17mm or greater, about 20mm or greater, about 25mm or greater, about 30mm or greater, or about 35mm or greater. The length of the ingot may be about 50mm or less, about 45mm or less, about 40mm or less, or about 35mm or less. The diameter of the AlN ingot may be about 50 mm. The diameter of the AlN boule may be at least about 60mm, at least about 65mm, at least about 70mm, at least about 75mm, at least about 80mm, at least about 85mm, at least about 90mm, at least about 95mm, or at least about 100 mm. The diameter of the AlN ingot may be no greater than about 150mm, no greater than about 125mm, no greater than about 110mm, or no greater than about 100 mm. The AlN ingot may have a room temperature thermal conductivity of about 250W/m.K or more, about 270W/m.K or more, about 290W/m.K or more, about 300W/m.K or more, or about 320W/m.K or more. The AlN ingot may have a room temperature thermal conductivity of about 400W/m.K or less, about 350W/m.K or less, or about 300W/m.K or less.
The full width at half maximum (FWHM) of the X-ray diffraction curve of the AlN ingot may be less than 85 angular seconds, less than 80 angular seconds, less than 75 angular seconds, less than 70 angular seconds, less than 65 angular seconds, less than 60 angular seconds, less than 55 angular seconds, less than 50 angular seconds, less than 45 angular seconds, or less than 40 angular seconds the full width at half maximum (FWHM) of the X-ray diffraction curve of the AlN ingot may be greater than 10 angular seconds, greater than 15 angular seconds, greater than 20 angular seconds, greater than 25 angular seconds, greater than 30 angular seconds, greater than 35 angular seconds, greater than 40 angular seconds, or greater than 45 angular seconds the threading edge-like dislocation density of the AlN ingot may be less than 5 × 10 angular seconds5cm-2Less than 1 × 105cm-2Less than 5 × 104cm-2Less than 1 × 104cm-2Less than 5 × 103cm-2Or less than 1 × 103cm-2. The AlN ingot may have a threading edge dislocation density of more than 10cm-2Greater than 100cm-2Greater than 500cm-2Or greater than 1000cm-2. The threading dislocation density of the AlN ingot may be less than 100cm-2Is less than 50cm-2Is less than 10cm-2Is less than 5cm-2Or less than 1cm-2. The AlN ingot may have a threading dislocation density of more than 0.1cm-2Greater than 0.5cm-2Greater than 1cm-2Greater than 2cm-2Or greater than 5cm-2. The ultraviolet transparency of the ingot in the wavelength range of about 350nm to about 480nm may be less than 25cm-1Is less than 20cm-1Is less than 15cm-1Is less than 10cm-1Is less than 8cm-1Or less than 5cm-1. The UV transparency may be greater than 1cm at a wavelength ranging from about 350nm to about 480nm-1Greater than 3cm-1Greater than 5cm-1Or greater than 8cm-1。
In another aspect, embodiments of the invention feature a method of improving Ultraviolet (UV) transparency of a single-crystal AlN bulk crystal. The AlN bulk crystal is heated to an annealing temperature of at least 1800 ℃, at least 1900 ℃, at least 1950 ℃, or at least 2000 ℃. The AlN bulk crystal is then cooled through a cooling cycle. The cooling period comprises, consists essentially of, or consists of (i) cooling the AlN bulk crystal from the annealing temperature to a first temperature ranging from 1450 ℃ to 2150 ℃ over a first time period ranging from 10 minutes to 90 minutes, and (ii) then cooling the AlN bulk crystal from the first temperature to a second temperature ranging from 1000 ℃ to 1650 ℃ over a second time period ranging from 10 seconds to 10 minutes, or (i) cooling the AlN bulk crystal from the annealing temperature to a third temperature ranging from 1450 ℃ to 2150 ℃ over a third time period ranging from 10 seconds to 10 minutes, and (ii) then cooling the AlN bulk crystal from the third temperature to a fourth temperature ranging from 1000 ℃ to 1650 ℃ over a fourth time period ranging from 10 minutes to 90 minutes. The AlN bulk crystal is then cooled to near room temperature.
Embodiments of the invention may include one or more of any of the following combinations. After heating the AlN bulk crystal to the annealing temperature and before cooling the AlN bulk crystal through the cooling cycle, the AlN bulk crystal may be held at the annealing temperature for a holding time. The incubation time may be at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, or at least about 6 hours. The incubation time may be up to about 10 hours, up to about 9 hours, up to about 8 hours, up to about 7 hours, up to about 6 hours, or up to about 5 hours. The incubation time may be about 5 hours. At least a portion (or even all) of the AlN bulk crystal may be at least about 35mm, at least about 50mm, at least about 75mm, or at least about 100mm in diameter. At least a portion (or even all) of the AlN bulk crystal may be up to about 150mm, up to about 125mm, up to about 100mm, or up to about 75mm in diameter. At least a portion (or even all) of the AlN bulk crystal may be about 50mm in diameter.
A light emitting device may be fabricated on at least a portion of the AlN bulk crystal. The light emitting device may be configured to emit ultraviolet light. After forming at least a portion of the light emitting device (e.g., all or part of the epitaxial light emitting layer structure), at least a portion (or even all) of the AlN bulk crystal of the light emitting device may be removed. The light emitting device may comprise, consist essentially of, or consist of a light emitting diode and/or a laser. Embodiments of the invention may include AlN bulk crystals formed according to any of the methods described above.
In another aspect, embodiments of the invention feature a method of improving Ultraviolet (UV) transparency of a single-crystal AlN bulk crystal. The AlN bulk crystal is heated to an annealing temperature of at least 1800 ℃, at least 1900 ℃, at least 1950 ℃, or at least 2000 ℃. The AlN bulk crystal is then cooled through a cooling cycle. The cooling period comprises, consists essentially of, or consists of cooling the AlN bulk crystal from the annealing temperature to a first temperature ranging from 1000 ℃ to 1650 ℃ over a period of 1 hour to 10 hours, and then cooling the AlN bulk crystal to near room temperature.
Embodiments of the invention may include one or more of any of the following combinations. After heating the AlN bulk crystal to the annealing temperature and before cooling the AlN bulk crystal through the cooling cycle, the AlN bulk crystal may be held at the annealing temperature for a holding time. The incubation time may be at least about 1 hour, at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, or at least about 6 hours. The incubation time may be up to about 10 hours, up to about 9 hours, up to about 8 hours, up to about 7 hours, up to about 6 hours, or up to about 5 hours. The incubation time may be about 5 hours. At least a portion (or even all) of the AlN bulk crystal may be at least about 35mm, at least about 50mm, at least about 75mm, or at least about 100mm in diameter. At least a portion (or even all) of the AlN bulk crystal may be up to about 150mm, up to about 125mm, up to about 100mm, or up to about 75mm in diameter. At least a portion (or even all) of the AlN bulk crystal may be about 50mm in diameter.
A light emitting device may be fabricated on at least a portion of the AlN bulk crystal. The light emitting device may be configured to emit ultraviolet light. After forming at least a portion of the light emitting device (e.g., all or part of the epitaxial light emitting layer structure), at least a portion (or even all) of the AlN bulk crystal of the light emitting device may be removed. The light emitting device may comprise, consist essentially of, or consist of a light emitting diode and/or a laser. Embodiments of the invention may include AlN bulk crystals formed according to any of the methods described above.
In another aspect, embodiments of the invention feature a method of forming a polycrystalline aluminum nitride (AlN) source material. A bulk polycrystalline AlN ceramic is provided. At least a portion of the AlN ceramic is placed in a crucible. The at least a portion of the AlN ceramic is annealed and densified within a first crucible to form a polycrystalline AlN source material. Annealing and densifying comprises, consists essentially of, or consists of (i) heating at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or (i) heating at least a portion of the AlN ceramic at a third temperature ranging from 1900 ℃ to 2250 ℃ for a third time ranging from 5 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours. The AlN source material was cooled to near room temperature.
Embodiments of the invention may include one or more combinations of any combination of one or more fragments of the AlN ceramic having a width (or diameter, or length, or other dimension) greater than about 0.1cm, greater than about 0.2cm, greater than about 0.3cm, greater than about 0.4cm, greater than about 0.5cm, greater than about 0.7cm, or greater than about 1 cm. and one or more fragments of a width (or diameter, or length, or other dimension) less than about 5cm, less than about 4cm, less than about 3cm, less than about 2cm, less than about 1.5cm, or less than about 1 cm. at least a portion of the AlN ceramic may include, consist essentially of, or consist of one or more fragments19cm-3. Embodiments of the invention may include polycrystalline AlN sources formed according to any of the methods described above.
In another aspect, embodiments of the invention feature a method of forming single crystal aluminum nitride (AlN). A bulk polycrystalline AlN ceramic is provided. At least a portion of the AlN ceramic is placed in the first crucible. The at least a portion of the AlN ceramic is annealed and densified within a first crucible to form a polycrystalline AlN source material. Annealing and densifying comprises, consists essentially of, or consists of (i) heating at least a portion of the AlN ceramic at a first temperature ranging from 1100 ℃ to 1900 ℃ for a first time ranging from 2 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a second temperature ranging from 1900 ℃ to 2250 ℃ for a second time ranging from 3 hours to 15 hours, or (i) heating at least a portion of the AlN ceramic at a third temperature ranging from 1900 ℃ to 2250 ℃ for a third time ranging from 5 hours to 25 hours, and (ii) then heating at least a portion of the AlN ceramic at a fourth temperature ranging from 1900 ℃ to 2250 ℃ for a fourth time ranging from 3 hours to 25 hours. The AlN source material was cooled to near room temperature. The second crucible was placed in the furnace. The second crucible contains an AlN source material and a seed crystal comprising, consisting of, or consisting essentially of single-crystal AlN. The second crucible is furnace heated to a growth temperature of at least 2000 ℃. The second crucible is held at the growth temperature for a holding time of 1 hour to 10 hours. After the holding time while the second crucible is at the growth temperature, (i) a vapor phase comprising, consisting essentially of, or consisting of aluminum and nitrogen condenses on the seed crystal to epitaxially form a single crystal AlN ingot from the seed crystal, and (ii) the second crucible is moved relative to the furnace. At least a portion of the AlN ingot grows at a rate approximately equal to the rate of relative motion between the second crucible and the furnace. Then, the AlN ingot was cooled by a cooling cycle. The cooling cycle comprises, consists essentially of, or consists of cooling the AlN ingot from the growth temperature to a fifth temperature ranging from 1000 ℃ to 1650 ℃ over a fifth time period ranging from 1 hour to 10 hours, and then cooling the AlN ingot to near room temperature.
Embodiments of the invention may include one or more of any of the following combinations. The growth temperature may be about 2300 ℃ or less, about 2200 ℃ or less, or about 2100 ℃ or less. At least a portion (or even all) of the AlN boule may be at least about 35mm, at least about 50mm, at least about 75mm, or at least about 100mm in diameter. At least a portion (or even all) of the AlN boule may have a diameter of up to about 150mm, up to about 125mm, up to about 100mm, or up to about 75 mm. At least a portion (or even all) of the AlN boule may be about 50mm in diameter. Oxygen (e.g., oxygen or an oxygen-containing gas) may be introduced into the second crucible during epitaxial formation of the single-crystal AlN ingot from the seed crystal. The AlN ceramic may be broken into pieces prior to placing at least a portion of the AlN ceramic in the first crucible. The width (or diameter, or length, or other dimension) of one or more (or even each) of the fragments is greater than about 0.1cm, greater than about 0.2cm, greater than about 0.3cm, greater than about 0.4cm, greater than about 0.5cm, greater than about 0.7cm, or greater than about 1 cm. The width (or diameter, or length, or other dimension) of one or more (or even each) of the fragments is less than about 5cm, less than about 4cm, less than about 3cm, less than about 2cm, less than about 1.5cm, or less than about 1 cm. At least a portion of the AlN ceramic may include, consist essentially of, or consist of one or more pieces.
The crystalline orientation of the seed may be substantially parallel to the c-axis. The first crucible and the second crucible may be the same crucible or different crucibles. The growth rate may be at least 0.1 mm/hr, at least 0.2 mm/hr, at least 0.3 mm/hr, at least 0.4 mm/hr, at least 0.5mm/hr, at least 0.7 mm/hr, or at least 1 mm/hr. The growth rate may be at most 3 mm/hour, at most 2.5 mm/hour, at most 2 mm/hour, at most 1.5 mm/hour or at most 1 mm/hour. The incubation time may be at least about 2 hours, at least about 3 hours, at least about 4 hours, at least about 5 hours, or at least about 6 hours. The incubation time may be up to about 9 hours, up to about 8 hours, up to about 7 hours, up to about 6 hours, or up to about 5 hours. The incubation time may be about 5 hours. The diameter of the seed may be about 25mm or greater, about 30mm or greater, about 35mm or greater, about 40mm or greater, about 45mm or greater, or about 50mm or greater. The diameter of the seed may be about 150mm or less, about 100mm or less, about 75mm or less, or about 50mm or less.
The AlN ingot may be sliced to form single crystal AlN substrates having a diameter of at least 25mm, at least 30mm, at least 35mm, at least 40mm, at least 45mm, or at least 50 mm. The diameter of the single-crystal AlN substrate may be about 150mm or less, about 100mm or less, about 75mm or less, or about 50mm or less. The diameter of the single crystal AlN substrate may be about 50 mm. The light emitting device may be fabricated using at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After forming at least a portion of the light emitting device (e.g., all or part of the epitaxial light emitting layer structure), at least a portion (or even all) of the AlN substrate of the light emitting device may be removed. The light emitting device may comprise, consist essentially of, or consist of a light emitting diode and/or a laser.
Oxygen and/or carbon may be gettered during formation of the single-crystal AlN ingot. The oxygen and/or carbon may be eliminated prior to and/or during formation of the single crystal AlN ingot using a getter material added to the second crucible and/or furnace. What is needed isThe getter material may include, consist essentially of, or consist of boron, iridium, niobium, molybdenum, tantalum, tungsten, and/or rhenium, the bulk polycrystalline AlN ceramic may have less than about 1% excess Al and/or an oxygen concentration of less than 2 × 1019cm-3. Embodiments of the invention may include an AlN ingot and/or substrate formed according to any of the methods described above.
These and other objects, advantages and features of the invention disclosed herein will become more apparent by reference to the following description, drawings and claims. Furthermore, it is to be understood that the features of the various embodiments described herein are not mutually exclusive and can exist in various combinations and permutations. The terms "approximately," "about," and "substantially" as used herein refer to ± 10%, and in some embodiments, ± 5%. Unless otherwise specified, all numerical ranges specified herein include their endpoints. Unless otherwise defined herein, the term "consisting essentially of … …" means that no other materials that contribute to the function are included. Nevertheless, such other materials may be present, with the total or individual content being trace amounts.
Drawings
In the drawings, like reference characters generally refer to the same parts throughout the different views. Furthermore, 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:
fig. 1A is a graph of uv absorption coefficient versus wavelength for AlN wafers prepared using conventional growth methods.
FIGS. 1B-1D are schematic diagrams of reactors for forming polycrystalline source material according to various embodiments of the invention;
FIGS. 1E and 1F are schematic illustrations of crucibles used to form polycrystalline source materials according to embodiments of the invention;
FIGS. 1G and 1H are diagrams of example thermal processes for forming polycrystalline source material according to various embodiments of the invention;
FIG. 2 is a schematic illustration of an apparatus for single crystal AlN growth according to various embodiments of the invention;
FIG. 3 is a diagram of an example cooling cycle for single crystal AlN according to various embodiments of the invention;
FIG. 4A is a graph of UV absorption coefficient versus wavelength for single crystal AlN grown according to various embodiments of the present invention;
FIG. 4B is an etch pit density determination micrograph of single crystal AlN grown according to various embodiments of the invention;
fig. 4C is an x-ray topographic map of single crystal AlN grown in accordance with various embodiments of the present invention;
FIG. 4D is an x-ray rocking graph of single crystal AlN grown according to various embodiments of the present invention;
fig. 4F is a graph of thermal conductivity of single crystal AlN grown according to various embodiments of the present invention;
fig. 5A is a series of micrographs of a single-crystal AlN wafer grown in accordance with various embodiments of the present invention; each wafer shows a normal illumination pattern and a PL (photoluminescence) pattern;
FIG. 5B is a table of data for Si, O and C concentrations and UV absorption coefficients, measured at the points indicated by the circles in FIG. 5A;
FIGS. 6A and 6B are plots of C concentration, C/O ratio, and UV absorption coefficient for single crystal AlN wafers grown according to embodiments of the present invention;
FIG. 7 is a graph of UV LED output power versus UV transparency metric for LED substrates according to various embodiments of the present invention;
fig. 8A and 8B are schematic cross-sectional views of light-emitting devices of embodiments of the invention;
fig. 9A and 9B are schematic cross-sectional views of light emitting devices after substrate removal according to various embodiments of the present invention.
Detailed Description
Embodiments of the present invention enable the production of high quality, high uv transparent, single crystal AlN bulk crystals (i.e., boules and/or substrates). In various embodiments, the AlN bulk crystal is prepared by first starting with a two-stage process that prepares a highly stoichiometric polycrystalline AlN source material, which may be used in a vapor-phase transport growth process (e.g., a sublimation-condensation process) for forming an AlN bulk crystal. The formation of AlN source material in each example minimizes the concentration of impurities, such as carbon (C) and oxygen (O), that compromise ultraviolet transparency.
In various embodiments, the first stage of AlN source material preparation is the formation of polycrystalline AlN ceramic from high purity Al particles, as described in detail in the' 519 patent. For example, referring to fig. 1B-1F,
In various embodiments, the
As shown,
The
To form the
The
Then, a reaction cycle is carried out to form
After the reaction cycle is complete, the crucible 130 (and polycrystalline ceramic 195) is cooled to about room temperature under a positive pressure of nitrogen (e.g., within about 1 hour.) the polycrystalline ceramic 195 so formed comprises, consists essentially of, or consists of high purity AlN in one embodiment, the concentration of oxygen (and/or other impurities, such as boron or transition metals) in the
Polycrystalline ceramic 195 comprises, consists essentially of, or consists of AlN in approximately stoichiometric proportions, i.e., AlN with an excess Al content of less than about 1%, less than about 0.5%, or even less than about 0.1%. After formation, the polycrystalline ceramic 195 may be stored in an inert atmosphere in preparation for preparing a high quality polycrystalline ceramic AlN source material.
Although polycrystalline AlN ceramic 195 may advantageously contain low concentrations of oxygen, embodiments of the present invention feature a second stage of processing that reduces or minimizes the concentration of other impurities (e.g., carbon). In various embodiments, the second stage involves an annealing and densification process of at least a portion of polycrystalline ceramic 195 to form a high quality polycrystalline AlN source material. In various embodiments, the ceramic 195 is broken into pieces prior to the annealing and densification processes. The ceramic 195 may be fragmented by, for example, applying mechanical force, and one or more (typically multiple) fragments collected and placed in a crucible (which may resemble the
As shown in fig. 1G, according to various embodiments of the invention, the ceramic 195 (or a portion thereof) may be heated to a first temperature T1 of 1100 ℃ to 1900 ℃ and held at a temperature T1 for a period of time T1, e.g., 2 hours to 25 hours. The ceramic 195 (or a portion thereof) may then be heated to a second, higher temperature T2 (e.g., a temperature of 1900 ℃ to 2250 ℃) and held at the temperature T2 for a period of time T2, e.g., 3 hours to 15 hours. During the heat treatment, ceramic 195 (or a portion thereof) is annealed and densified to form a polycrystalline AlN source material, which may be used to subsequently form high-quality, single-crystal AlN bulk crystals. Because polycrystalline AlN source material is typically approximately stoichiometric AlN, with low impurity concentrations, it can be used to form AlN bulk crystals without further processing (e.g., without an intermediate sublimation-condensation step).
FIG. 1H schematically illustrates the heat treatment alternative of FIG. 1G, wherein the first annealing step at temperature T1 is replaced with a longer ramp-up temperature T2. As shown in fig. 1H, according to various embodiments of the invention, the ceramic 195 (or a portion thereof) may be warmed to a T2 (e.g., a temperature of 1900 ℃ to 2250 ℃) over a period of time T1 (e.g., 5 hours to 25 hours). The ceramic 195 (or a portion thereof) may then be held at the temperature T2 for a period of time T2, e.g., 3 hours to 25 hours. During the heat treatment, ceramic 195 (or a portion thereof) is annealed and densified to form a polycrystalline AlN source material, which may be used to subsequently form high-quality, single-crystal AlN bulk crystals. Because polycrystalline AlN source material is typically approximately stoichiometric AlN, with low impurity concentrations, it can be used to form AlN bulk crystals without further processing (e.g., without an intermediate sublimation-condensation step).
In various embodiments, the concentration of oxygen in the resulting polycrystalline AlN source material (as determined by Instrument Gas Analysis (IGA)) is about 1.0 × 1019cm-3To about 3.0 × 1019cm-3. In various embodimentsThe concentration of carbon in the polycrystalline AlN source material (as determined by Instrument Gas Analysis (IGA)) was about 3.8 × 1018cm-3To about 1.8 × 1019cm-3. The density of the polycrystalline AlN source material (as measured by hydrometallurgy at room temperature) is approximately equal to that of single-crystal AlN, i.e., about 3.25g/cm3To 3.26g/cm3. In contrast, AlN ceramic 195 has a measured density lower than that of the polycrystalline AlN source material, e.g., about 2.95g/cm3To about 3.20g/cm3. Furthermore, polycrystalline AlN source material is typically amber in color and composed of relatively large grains (i.e., average grain size of about 0.1mm to about 5 mm).
Fig. 2 depicts a single
The crucible 205 may include, consist essentially of, or consist of one or more refractory materials, such as tungsten nitride, rhenium nitride, and/or tantalum nitride. As described in the '135 patent and the' 153 patent, the crucible 205 has one or more surfaces (e.g., walls) configured to selectively allow nitrogen gas to diffuse therethrough and to selectively prevent aluminum from diffusing therethrough.
In accordance with an embodiment of the present invention, one or more internals of crystal growth apparatus 200 (e.g., crucible 205, susceptor 215, and/or crucible holder 210) may be annealed prior to crystal growth and formation of AlN ingot 220, which may advantageously reduce the concentration of carbon (and/or the concentration of oxygen) in AlN ingot 220. In various embodiments, one or more internal pieces of the
In various embodiments of the present invention, the concentration of oxygen and/or the concentration of carbon in AlN ingot 220 may be reduced by introducing one or more gettering materials within crucible 205 prior to or during growth of AlN ingot 220. The getter material may be introduced as part or all of one or more components of crystal growing apparatus 200 (e.g., crucible 205, the inner liner and adjacent inner surfaces or walls within crucible 205, susceptor 215, and/or crucible holder 210), and/or the getter material may be introduced as a separate bulk material within
As shown in fig. 2, polycrystalline material 240 may be formed at one or more locations within crucible 205 not covered by seed crystal 235 during formation of AlN boule 220. However, during formation of AlN boule 220, the diameter (or other radial dimension) of AlN boule 220 may expand, i.e., increase, thereby enclosing a region of polycrystalline material 240 from intimate contact with vapor 225, substantially limiting or even eliminating growth thereof. As shown in fig. 2, AlN boule 220 may expand in diameter (or even, starting with a larger seed 235 in an embodiment) to be substantially equal to the inner diameter of crucible 205 (at which point AlN boule 220 may not expand further laterally).
Typically, AlN ingot 220 is grown in growth direction 245 due to the relatively large axial thermal gradient (e.g., from about 5 ℃/cm to about 100 ℃/cm) formed within crucible 205. Heating equipment (not shown in FIG. 2 for clarity), such as an RF heater, one or more heating coils, and/or other heating elements or furnaces, heats susceptor 215 (and thus crucible 205) to an elevated temperature, typically about 1800℃ to about 2300℃. Before growth begins, the crucible 205 and the contents therein (i.e., the seed crystal 235, if present, and the source material 230) may be held at a temperature approximately equal to the desired growth temperature for a predetermined soak time (e.g., about 1 hour to about 10 hours). In various embodiments, the incubation at such temperatures stabilizes the thermal field within crucible 205, promotes efficient nucleation on seed crystal 235, and promotes a high quality transition of single crystal AlN from nucleation to bulk growth.
Maximum mass transfer of source material 230 and/or vapor 225 (and thus the growth rate of AlN ingot 220) is typically achieved by maximizing the axial thermal gradient within crucible 205 (i.e., maximizing the temperature differential between source material 230 and growing crystal 220, thereby resulting in greater supersaturation of growing crystal 220). In various embodiments, the onset of crystal-quality degradation (e.g., increased dislocation density, formation of grain boundaries, and/or polycrystalline growth) determines an approximate upper limit for supersaturation at a given growth temperature. For typical growth temperatures (e.g., about 2125 ℃ to about 2275 ℃), such an axial temperature gradient upper limit is typically close to 100 ℃/cm (although such a maximum may depend at least in part on the size and/or shape of the growth chamber, and thus some systems may be larger). However, as the cross-sectional area of AlN boule 220 increases (and/or for larger areas of seed 235), the likelihood of parasitic nucleation (on seed 235 or elsewhere) increases. Each parasitic nucleation event may result in the formation of additional growth centers, resulting in the formation of grains or sub-grains (thereby resulting in a low quality and/or polycrystalline material). Lateral growth is promoted by providing a non-zero radial thermal gradient in a direction substantially perpendicular to the growth direction 245, thereby minimizing the likelihood of parasitic nucleation. The formation of a radial thermal gradient also enables the growth of larger high quality crystals at higher growth rates.
In various embodiments, crucible 205 has a lid 270, and lid 270 has sufficient radiation transparency to at least partially control the thermal profile within crucible 205 by disposing a top insulating
As shown in fig. 2, each top insulation layer typically has one opening 275. The apertures 275 generally mimic the geometry and/or symmetry of the crucible 205 (e.g., the apertures 275 of a cylindrical crucible 205 may be substantially circular). The size of each aperture 275 may vary, typically ranging from a minimum of 10mm less than the diameter of the crucible 205 to a maximum of about 5mm (or even 2mm) less than the diameter of the crucible 205.
For example, in one embodiment, five
As shown in fig. 2, the openings 275 of the
Also, while the thickness of each
Controlled cooling may be employed to maintain, and in many embodiments even enhance, the ultraviolet transparency of AlN ingot 220 as AlN ingot 220 is cooled to room temperature after growth. FIG. 3 schematically illustrates two different cooling cycles that may be utilized by embodiments of the present invention. As shown, in a first embodiment ("
In addition, as shown in fig. 3, in the second embodiment ("path 2"), AlN ingot 220 undergoes a fast-cooling phase first, and then a slow-cooling phase. In particular, AlN ingot 220 may first be provided at a time t in the range of about 10 seconds to about 10 minutesA2Internal cooling from growth temperature to intermediate temperature TA. Reaches the temperature TAThereafter, AlN ingot 220 may be grown for a time t of about 10 minutes to about 90 minutesB2Internal cooling to a second intermediate temperature TB. Reaches the temperature TBThereafter, AlN ingot 220 may be cooled to near room temperature (e.g., about 25 ℃) at an uncontrolled rate, i.e., a rate that depends only on the cooling rate of the growth system (i.e., the ambient environment of AlN ingot 220), with no power applied to its heating elements. After cooling to a temperature TBAt any point thereafter, AlN ingot 220 and crucible 205 may be withdrawn from the growth system and allowed to cool to near room temperature in the ambient environment.
Cooling to an intermediate temperature T in AlN ingot 220AAnd TBDuring this time, the temperature of the growth system surrounding crucible 205 can be controlled to achieve the desired temperature change over the desired time. For example, the power of the heating element (e.g., RF coil) may be reduced for a desired time to adjust the temperature of AlN ingot 220, and/or the temperature may be directly controlled by feedback initiated by, for example, the temperature measured by a pyrometer or other temperature sensor in the growth system. Although the temperature changes of
In different embodiments,
In various embodiments, the two-step cooling periods described above (i.e.,
In various embodiments of the present invention, a two-step or single-step cooling cycle described with reference to fig. 3 may be employed after the annealing cycle to enhance the ultraviolet transparency of a previously grown AlN bulk crystal (e.g., an ingot or substrate separated therefrom). For example, in various embodiments, AlN bulk crystals (grown according to embodiments of the invention or other methods) may be annealed at temperatures above 2000 deg.C (e.g., about 2000 deg.C to about 2400 deg.C). The AlN bulk crystal may then be cooled to room temperature through either
After AlN ingot 220 is formed, one or more substrates (or "wafers") may be separated from AlN ingot 220 using, for example, a diamond ring saw or wire saw. In one embodiment, the crystallographic orientation of the substrate so formed may be within about 2 ° (or even within about 1 °, or within about 0.5 °) of the (0001) plane (i.e., c-plane). The c-plane wafer has an Al-polar surface or an N-polar surface and can then be prepared as described in U.S. patent No.7,037,838, which is incorporated herein by reference in its entirety. In other embodiments, the substrate may be oriented within about 2 ° of the m-plane or a-plane direction (and thus have a non-polar orientation) or may have a semi-polar orientation if AlN boule 220 is cut in a different direction. The surfaces of these wafers are also processed as described in U.S. Pat. No.7,037,838. The substrate has an approximately circular cross-section with a diameter greater than about 50 mm. The thickness of the substrate is greater than about 100 μm, greater than about 200 μm, or even greater than about 2 mm. The substrate generally has all of the properties of AlN boule 220 described herein. After the substrate is cut from AlN boule 220, one or more epitaxial semiconductor layers and/or one or more light emitting devices, e.g., ultraviolet light emitting diodes or lasers, may be fabricated on the substrate, e.g., as described in U.S. patents 8,080,833 and 9,437,430, both of which are incorporated herein by reference in their entirety.
Fig. 4A-4E depict various features of uncracked AlN bulk crystal, approximately 50mm in diameter, prepared in accordance with an embodiment of the present invention. As shown in FIG. 4A, the crystal has high ultraviolet transparency, for example, an ultraviolet absorption coefficient of less than 30cm at an ultraviolet wavelength of about 220nm to about 480nm-1An ultraviolet absorption coefficient of less than 20cm at an ultraviolet wavelength of about 250nm to about 480nm-1FIG. 4B is about 7 × 103cm-2FIG. 4C is an x-ray topographic map of an AlN crystal grown according to an embodiment of the present invention, showing a threading edge dislocation density of about 3 × 103cm-2The threading screw dislocation density was about 10cm-2I.e., total threading dislocation density is less than about 104cm-2FIGS. 4D and 4E depict x-ray rocking curves (along (0002) and (10-12), respectively) with a full width at half maximum (FWHM) of less than 50 arc seconds, FIGS. 4A-4E depict samples with a carbon concentration of less than 3 × 10, as determined by Secondary Ion Mass Spectrometry (SIMS)17cm-3-4×1017cm-3Oxygen concentration of less than 1 × 1017cm-3-4×1017cm-3The concentration of silicon is less than 1 × 1017cm-3The ratio of carbon/oxygen concentration is less than 0.5.
In various embodiments of the present invention, oxygen may be intentionally added to an AlN crystal during and/or after growth to maintain the carbon/oxygen concentration ratio in the AlN crystal at a level less than 0.5 As described in detail herein, various measures may be taken to minimize the carbon concentration in the AlN crystal (e.g., at a level of about 3 × 10)17cm-3Or lower) and therefore, without supplemental oxygen, the ratio of carbon/oxygen concentration within the crystal may be greater than 0.5. Thus, oxygen is introduced into the growth crucible during at least a portion of the growth of the AlN crystal, and/or the AlN crystal may be annealed in an oxygen-containing environment after growth. In various embodiments, the polycrystalline AlN source material may be exposed to oxygen (e.g., during a high temperature annealing cycle), so that oxygen absorbed into the source material may be released into the gas phase during growth. Although the inventors do not wish to be bound by any particular theory of the operation of supplemental oxygen, the introduction of supplemental oxygen into the AlN crystal has one or more beneficial effects, resulting in increased ultraviolet transparency. For example, oxygen may react with any carbon in the gas phase to form CO and/or CO2Due to the low vapor pressure of carbon, it is common to attach itself to another substance primarily or to be vaporized with high fluxIs delivered by being rushed on the crystal. Supplemental oxygen may also introduce point defects (e.g., vacancies and/or complexes) within the AlN crystal, reducing ultraviolet light absorption centers (e.g., by vacancy annihilation) caused by carbon impurities.
In various embodiments, the concentration of carbon in an AlN single crystal having a diameter of at least 50mm, as determined by SIMS, is about 0.6 × 1017cm-3-6.2×1017cm-3The oxygen concentration is about 1 × 1017cm-3-7.9×1017cm-3The thermal conductivity of AlN boule 220 and/or a substrate derived therefrom is greater than about 290 watts/meter-kelvin (W/m-K), as determined in accordance with American Society for Testing and Materials (ASTM) standard E1461-13 (standard test method for thermal diffusivity by flash), which is incorporated herein by reference in its entirety and provided by commercial suppliers, e.g., NETZSCH inc. of Exton, Pennsylvania, the thermal conductivity of a non-cracking AlN bulk crystal having a diameter of about 50mm prepared in accordance with embodiments of the present invention is about 293W/m-K, slope-1.55, as shown in fig. 4F, the concentration of oxygen and the concentration of carbon in this sample are separately identified by SIMS as 3.5 × 1017cm-3And 1.2 × 1017cm-3。
The inventors have also found that the visible Photoluminescence (PL) color is strongly correlated with the carbon concentration in the single-crystal AlN prepared according to the example of the present invention. In particular, in various embodiments, AlN single crystals may emit light by a mercury light source having a wavelength of 254 nm. The luminescence can be observed by the naked eye or captured by an imaging device, such as a digital camera. If the emission is bright blue, it corresponds to a high carbon concentration and a high UV absorption, while dark blue, black or dark green emission corresponds to a low carbon concentration. Fig. 5A is a micrograph of a series of individual single crystal AlN wafers emitting light under normal lighting conditions, and the PL emission of the samples when emitted as described above. The regions on the substrate that exhibit bright blue emission were also found to have higher carbon content and correspond to higher levels of uv absorption, particularly around the wavelength 265 nm. The corresponding impurity concentrations in the sampling area (indicated by small circles) of the sample of fig. 5A, and the uv absorption coefficient at 265nm are given in the table of fig. 5B, where sample (1) corresponds to the top sample of fig. 5A, and the sample numbers increase downward in numerical order. As shown in fig. 5A and 5B, not only is PL luminescence closely related to ultraviolet absorption and impurity concentration, but the ultraviolet absorption coefficient itself is closely related to the concentration of carbon and the carbon/oxygen ratio in the sample.
FIGS. 6A and 6B illustrate the relationship between the ultraviolet absorption coefficient at a wavelength of 265nm and the ratios of the carbon concentration (FIG. 6A) and the carbon concentration/oxygen concentration (measured by SIMS) (FIG. 6B) of a 50mm diameter AlN single crystal. As shown in the figure, the ultraviolet absorption coefficient is closely related to the concentration of carbon and the carbon-oxygen ratio, and is less than 50cm-1Having a carbon concentration of less than about 3 × 1017cm-3The ratio of carbon-to-oxygen concentration is less than about 0.5.
AlN bulk crystals prepared according to embodiments of the invention advantageously have a relatively large Ultraviolet (UV) transparency measure, where the UV transparency measure is defined as (in cm)3):
Where d is the diameter of the AlN crystal in mm, FWHM is the full width at half maximum of the X-ray diffraction curve of the AlN substrate in radians, and α is the absorption coefficient of the AlN crystal at the wavelength of interest3To about 5000cm3Or even an ultraviolet transparency at 265nm of about 30cm3To about 5000cm3. The AlN crystal is at least about 50mm in diameter (or width or other lateral dimension). The following table illustrates the relationship between the ultraviolet transparency measure and the crystal diameter, FWHM and absorption coefficient.
The following table illustrates the range of ultraviolet transparency measurements for AlN single crystals prepared in accordance with examples of the present invention, having a FWHM of 25 arc-seconds, wavelength dependent, and a diameter of at least 50 mm.
According to embodiments of the present invention, the output power of an ultraviolet light emitting device, such as an LED, advantageously increases as the ultraviolet transparency metric of the underlying substrate increases. In one example, 6 AlN single crystal substrates were cut from ingots prepared as described herein, but not following the controlled cooling cycle described in detail with reference to fig. 3. Further, 6 AlN single-crystal substrates were cut from an ingot prepared in a similar manner and using the controlled cooling period. All substrates are about 50mm in diameter. An ultraviolet LED emitting at 265nm was prepared using all 12 substrates, and the power of the device at an operating power of 100mA was measured without thinning or cleaning the underlying AlN substrate. The UV absorption at 265nm was measured for each substrate, and the measurements at 52 different locations were averaged for each substrate. The FWHM of the x-ray rocking curve for each substrate is the average of 13 measurements at different locations. Fig. 7 is a graph of output power versus ultraviolet transparency, as shown, the output power strongly depends on the ultraviolet transparency measure. Thus, the ultraviolet transparency metrics defined herein provide a suitable tool for evaluating the performance of light emitting devices fabricated substantially on single crystal substrates prepared in accordance with embodiments of the present invention.
Single crystal AlN, and wafers formed therefrom, may be used to fabricate electronic and optoelectronic devices thereon. For example, portions of an AlN single crystal grown in accordance with embodiments of the present invention described in detail herein may be used as a substrate for subsequent epitaxial growth and processing to form LEDs and/or lasers that emit light in the ultraviolet wavelength range.
Fig. 8A schematically illustrates a light emitting
The surface of
The various layers of
In accordance with embodiments of the present invention, a
In various embodiments, the
In an exemplary embodiment, the
In various embodiments, the
In various embodiments, the
The
The thickness of the
The
In various embodiments of the present invention, an
As shown in fig. 8A,
The
In various embodiments, a
As described above, embodiments of the invention feature a graded
In various embodiments of the present invention, one or more (or even all) of the layers of the
In accordance with embodiments of the present invention, light (e.g., laser light) and/or heat may be used to separate all or a portion of
In various embodiments, at least a portion of the
As described above, after all or a portion of the
After forming
Other methods of partially or completely removing the substrate may be used in accordance with embodiments of the present invention. For example, an etching process, such as the electrochemical etching process described in U.S. patent application serial No.16/161,320, filed 2018, 10, 16, incorporated herein by reference in its entirety, may be used.
Bulk single crystal growth is described herein and is primarily carried out using what is commonly referred to as a "sublimation" or "sublimation-condensation" process, wherein Al or N is preferably sublimated, at least in part, to produce AlN, AlN-containing crystalline solid, or other solid or liquid. However, the source vapor may be generated in whole or in part by the injection of source gases or a similar process known to some as "high temperature CVD". Moreover, other words are sometimes used to describe these contents and methods of growing bulk AlN single crystals in accordance with embodiments of the present invention. Thus, as used herein, the terms "depositing," "growing," "depositing a vapor phase component," and the like generally encompass those methods of growing crystals in accordance with embodiments of the present invention.
The words and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention, in the use of such words 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.
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