Diameter expansion of aluminum nitride crystal

Abstract

In various embodiments, the aluminum nitride single crystal rapidly expands during growth and has a large crystal enhancement parameter. The aluminum nitride single crystal can have a large ingot mass and volume. The aluminum nitride single crystal can be grown from the gas phase under an enhanced radial thermal gradient to achieve a high expansion rate.

Description

[0001] Related Applications

[0002] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62 / 887,033, filed on August 15, 2019, which is incorporated herein by reference in its entirety. Technical Field

[0003] In various embodiments, the present invention relates to the preparation of single crystal aluminum nitride (AlN). Background Art

[0004] Aluminum nitride (AlN) holds great promise as a semiconductor material for numerous applications, such as optoelectronic devices such as short-wavelength light-emitting diodes (LEDs) and lasers, dielectric layers of optical storage media, electronic substrates, and chip carriers requiring high thermal conductivity. In principle, the properties of AlN allow emission of light with wavelengths down to about 200 nanometers (nm). Recent work has shown excellent performance of ultraviolet (UV) LEDs fabricated on low-defect AlN substrates prepared from bulk AlN single crystals. The use of AlN substrates is also expected to improve the performance of high-power radio frequency (RF) devices made from nitride semiconductors due to their high thermal conductivity and low electrical conductivity. However, the commercial viability of AlN semiconductor devices is limited by the scarcity and high cost of low-defect AlN single crystals.

[0005] In order to more easily obtain cost-effective single-crystalline AlN substrates and make the devices fabricated thereon commercially viable, it is necessary to grow AlN bulk crystals at higher growth rates (>0.5 mm / hr) while maintaining crystal quality. The most effective method for growing AlN bulk single crystals is the "sublimation-condensation" method, which involves sublimating a lower quality (usually polycrystalline) AlN source material and recondensing the resulting vapor to form single crystal AlN. U.S. Patent 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) describe various aspects of AlN growth by seeded and unseeded sublimation-condensation methods, and the entirety of the above patents are incorporated into this application by reference.

[0006] While AlN substrates provide a platform for making UV light-emitting devices (such as LEDs) and electronic devices (such as high-speed transistors), the amount of high-quality bulk crystalline AlN material that can be provided is generally insufficient to meet the needs of widespread commercial application of these technologies. AlN sublimation-condensation crystal growth typically utilizes small-diameter, high-quality seed crystals as a platform for growing longer AlN ingots. However, over time, cost-effective AlN device production will require AlN substrates with larger diameters. To meet this demand, AlN crystal growth often involves "diameter expansion", which is changing the thermal field in the growth chamber to increase the lateral crystal growth rate (i.e., perpendicular to the "growth direction", the ingot increases in length away from the seed crystal along the "growth direction"). Conventional diameter expansion techniques can successfully expand the diameter of the growing crystal, but the expansion rate is limited due to adverse effects on the quality of the growing crystal. Specifically, excessive changes in the thermal field in conventional techniques can lead to high defects or even polycrystalline materials, especially at the edges of the growing crystal. Other defects such as small-angle grain boundaries and dislocations, uneven doping, and even crystal cracking may also result. As a result, many traditional expansion efforts can be both wasteful and frustrating, as the highly defective edge material is often unsuitable for device applications and must be removed from the ingot.

[0007] In view of the above circumstances, there is a need for a crystal growth technology capable of having a high diameter expansion rate while maintaining high crystalline quality of an AlN single crystal, and for a larger AlN single crystal that can be achieved by such a technology. Summary of the invention

[0008] In various embodiments of the present invention, high-quality, larger AlN single crystals are produced by techniques that achieve high expansion rates without affecting crystal quality. Exemplary growth techniques utilize baseline radial thermal gradients established at least in part by, for example, the arrangement and configuration of heat shields outside the growth chamber. These radial thermal gradients, and / or the lateral growth rate of the AlN crystals are subsequently enhanced by other techniques to more rapidly expand and grow larger, high-quality AlN single crystals. Despite the more rapid expansion of the crystals, the AlN single crystals formed according to embodiments of the present invention still maintain a high level of crystal quality, even at the edges of the crystals and even at higher expansion rates. Therefore, embodiments of the present invention provide techniques and AlN single crystals themselves that are more economical and more suitable for large-scale production of substrates and devices.

[0009] According to embodiments of the present invention, techniques for enhancing radial thermal gradients within a crystal growth chamber include the use of a heat shield disposed within the chamber and proximate to the growing crystal. According to various embodiments, such an internal heat shield more effectively affects the thermal field near the growing crystal and its lateral growth rate than an external heat shield located outside the growth crucible itself. For example, in various embodiments, the internal heat shield defines openings passing through it, and these openings accommodate the growth of the crystal through the heat shield while they affect the thermal field, thereby achieving rapid diameter expansion. According to embodiments of the present invention, other techniques for promoting an increase in the lateral crystal growth rate (and the accompanying diameter expansion) also include enhancing atomic nitrogen in the gas phase, preferably concentrated at the lateral edges of the crystal (e.g., by using plasma near the lateral edges of the crystal). These techniques promote lateral growth enhancement (i.e., high-speed diameter expansion) of AlN crystals while maintaining high crystalline quality.

[0010] Embodiments of the present invention enable and facilitate the growth of AlN single crystals with large crystal enhancement parameters (as defined below), mass and / or volume, which are currently not achievable with conventional crystal growth techniques. Therefore, according to embodiments of the present invention, AlN single crystals can serve as a cost-effective, high-quality platform for manufacturing electronic and optical devices. The techniques described in embodiments of the present invention are particularly suitable for growing AlN single crystals from seeds, and are not suitable for non-seed growth that relies on, for example, spontaneous nucleation of crystalline material and / or guided growth by the conical growth crucible itself. Such non-seed growth techniques are generally unable to produce AlN single crystals with large crystal enhancement parameters and uniform high crystal quality levels. Therefore, according to embodiments of the present invention, the AlN single crystal (or ingot) formed by seed growth generally has a planar surface whose size and shape substantially correspond to the seed (or exposed area of ​​the seed) used for crystal growth; this crystal shape is different from unseeded crystals, which generally taper to form smaller point-like areas because they are generally initially nucleated at a small point with limited volume (e.g., the tip of the conical portion of the crystal growth crucible).

[0011] In various embodiments, when the ingot is removed from the growth system after growth, all or part of the seed (e.g., at least the exposed area thereof) is enclosed in (i.e., part of) the ingot. That is, the ingot grown from the seed may contain at least part of the seed itself, and the interface between the seed and the ingot is further evidence of the growth of the seed. (In various embodiments, the portion of the seed that is not exposed for growth thereon may sublimate and disappear during the growth process and therefore not exist after growth.) The seed-ingot interface can generally be detected by one or more characterization techniques, including optical inspection (visible lines can be detected at the interface and are caused by differences in point defects and / or impurities incorporated into the seed and the ingot), luminescence contrast (e.g., due to differences in point defects and / or impurities incorporated, e.g., under 254 nm light, the seed may appear darker or brighter than the ingot), or measurement of UV absorption, which is different between the initial seed and the grown ingot.

[0012] According to an embodiment of the present invention, single crystal AlN can be manufactured by sublimation-condensation of polycrystalline AlN source material. As described in the '135 patent, the '346 patent, the '153 patent and the '612 patent, the sublimation-condensation growth process is preferably carried out under a steep axial (i.e., in the direction of crystal growth away from the seed crystal, if a seed crystal is present, and / or toward the sublimated source material) temperature gradient, while the radial temperature gradient can be used to control the diameter of the growing crystal and affect its crystalline quality. In different embodiments of the present invention, various different ways can be used to control the radial and / or axial thermal gradients in the crystal growth crucible to promote and control the growth of AlN material. For example, a single heating element arranged around the crucible can use different power levels (and therefore different temperatures) to establish a thermal gradient in the crucible. The above measures can also be supplemented or replaced by selectively arranging an insulating layer around the crucible, so that the insulation arranged around the area where the temperature is higher can be thinner and / or less. As described in detail in the '612 patent, heat shields may also be arranged around the crucible, e.g., above and / or below the crucible, in any of a number of different arrangements, in order to establish the desired baseline thermal gradients within the crucible. Once and / or while these baseline thermal gradients are established, one or more techniques for enhancing lateral crystal growth while maintaining crystal quality are used to enhance at least radial thermal gradients (i.e., thermal gradients perpendicular to the lateral growth direction away from the seed crystal and parallel to the diameter of the growing crystal (which may expand during all or part of the growth process)).

[0013] Embodiments of the present invention also enable AlN single crystal ingots to be rapidly expanded on seeds of arbitrary crystal orientation and polarity and within a single growth stage. For example, embodiments of the present invention do not need to utilize (but can utilize) Al polarity, c-plane seeds to rapidly expand and maintain high crystal quality without the need for multiple different growth stages, each starting on a larger seed, for example, as disclosed in U.S. Patent Application Serial No. 16 / 008,407 filed on June 14, 2018 (the '407 application), the entire contents of which are incorporated herein by reference. Therefore, embodiments of the present invention can utilize seeds having c-plane and N-polarity, c-plane and Al-polarity, m-plane, etc. In addition, in order to achieve high-quality crystal growth with rapid expansion, the seeds described in embodiments of the present invention do not need to have any specific diameter or minimum diameter. For the avoidance of doubt, the techniques detailed in the embodiments of the present invention enable growing AlN single crystals with higher diameter expansion ratios while maintaining crystal quality compared to the techniques detailed in the '407 application (thus, producing AlN single crystals with greater crystal enhancement parameters as detailed in this application).

[0014] The rapid expansion technique described in detail in this application can be combined with a technique for achieving high UV (especially deep UV wavelength) transparency of AlN single crystals. In various embodiments of the present invention, high UV transparency AlN single crystals are manufactured by vapor phase growth, impurity control, post-growth temperature control in the growth system, and isothermal or quasi-isothermal post-growth annealing techniques. Advantageously, the resulting single crystal AlN exhibits a low UV absorption coefficient (e.g., less than 10 cm) at wavelengths between 230 nm and 280 nm, or in various embodiments, at wavelengths between 210 nm and 280 nm. -1 , or even less than 8cm -1 Single crystal AlN may also desirably exhibit a substantially "flat" UV absorption spectrum at wavelengths between 210 nm and 280 nm, e.g., a substantially constant UV absorption coefficient within this wavelength range (or a portion thereof), e.g., constant within ±3 cm -1 、±2cm -1 or even ±1cm -1 Such a spectrum can facilitate the engineering and performance improvement of optical devices (e.g., light-emitting devices such as light-emitting diodes and lasers) because the optical properties of AlN single crystal substrates used for such devices will exhibit essentially constant optical properties in the deep ultraviolet wavelength range.

[0015] In addition, the AlN single crystals of the embodiments of the present invention exhibit a steep slope (i.e., a "dip") in their UV absorption spectra near the AlN band edge, for example, at wavelengths between about 210 nm and about 230 nm. This characteristic advantageously contributes to low UV absorption at deep UV wavelengths and contributes to more uniform optical properties of substrates made from AlN crystals and optical devices made thereon.

[0016] Furthermore, the annealing techniques of the present invention advantageously do not require the removal of carbon and oxygen from the single crystal AlN to unreasonably and impractically low levels. Specifically, the present invention successfully results in a reduction in the concentration of oxygen and / or carbon even at about 10 18 cm -3 Up to about 10 19 cm -3 The AlN crystal also has low UV absorption at deep UV wavelengths. In addition, the post-growth annealing techniques of embodiments of the present invention can be combined with high-speed cooling of the AlN crystal within the growth apparatus, for example, to avoid crystal cracking, even if such cooling techniques cause the AlN crystal to initially exhibit high levels of UV absorption at certain wavelengths.

[0017] The inventors have discovered that carbon impurities can cause high levels of UV absorption in AlN crystals. Carbon impurities cause UV absorption at a wavelength of approximately 265 nm, compromising the performance of UV light-emitting devices. In addition, oxygen impurities (or related point defects) typically cause UV absorption at a wavelength of approximately 310 nm. Therefore, while controlling oxygen contamination is desirable for UV transparency, it is not sufficient to achieve UV transparency at many UV wavelengths, particularly those in the deep UV portion of the spectrum. Embodiments of the present invention include techniques for improving UV absorption of AlN single crystals, even when oxygen and / or carbon impurity concentrations are controlled during the AlN manufacturing process.

[0018] As described above, the high radial and axial thermal gradients used during crystal growth necessarily result in the formation of crystals in a non-isothermal environment. While the thermal gradients enable the formation of high-quality large AlN crystals, the arrangement of heat shields, insulation, and related aspects of the growth system that result in the formation of thermal gradients during crystal growth also necessarily results in thermal gradients in the growth system during the cooling of the crystal after crystal growth. While various references suggest cooling the growing crystal within the growth apparatus at a fairly slow rate to control the formation of point defects, such slow cooling may result in cracking of the AlN crystal due to thermal expansion mismatch, particularly for larger AlN crystals (e.g., crystals with a diameter exceeding about 50 mm). Therefore, in contradiction to conventional wisdom, embodiments of the present invention include cooling the growing AlN crystal to approximately room temperature (e.g., approximately 25°C) within the growth chamber at a high cooling rate (e.g., in excess of 250°C / hour, 300°C / hour, 400°C / hour, or even 500°C / hour), despite the attendant deleterious effects on the UV transparency of the crystal. Cooling from the growth temperature may also be performed without any additional heat provided by the heating elements of the growth system (e.g., applied to reduce the cooling rate, referred to as "controlled cooling"). Cooling of the crystal can be performed at a rate limited only by, for example, the thermal mass of the growth system, and a number of steps can be taken to accelerate cooling of the crystal. For example, after growth, the AlN crystal can be moved or removed from the "hot zone" of the growth system (i.e., the portion of the growth system that is directly adjacent to and heated by the heating element or furnace) and / or a gas (e.g., nitrogen and / or an inert gas (e.g., argon)) can be flowed through the system (e.g., at a flow rate higher than any flow rate used during crystal growth) to increase the cooling rate.

[0019] After the AlN single crystal is formed and cooled from the growth temperature, the resulting crystal (or a portion thereof, such as a wafer or substrate separated from the ingot) can be placed in a high temperature annealing furnace and annealed under isothermal or quasi-isothermal conditions to ensure substantially uniform heating throughout the crystal. (As used herein, "quasi-isothermal" conditions within a furnace correspond to a temperature within the furnace (or its dedicated heating zone or "hot zone") being constant at ±5°C, ±2°C, ±1°C, or even ±0.5°C, and / or any temperature gradient in any direction within the furnace (or its dedicated heating zone or hot zone) being less than 5°C / cm, less than 2°C / cm, less than 1°C / cm, or even less than 0.5°C / cm; in various embodiments, the temperature gradient can be at least 0.05°C or at least 0.1°C). That is, the annealing conditions can be completely different from the initial growth and cooling conditions of the AlN crystal within the crystal growth crucible and growth system, which are suitably configured to produce axial and / or radial thermal gradients therein. For example, the crystal can be annealed in a resistively heated or radio frequency-heated furnace configured for isothermal annealing, rather than in the growth apparatus in which it was originally grown. After annealing, the annealed crystal is slowly and controllably cooled from the annealing temperature to at least a portion of the temperature range between the annealing temperature and room temperature to maintain the low UV absorption achieved during the annealing cycle. In various embodiments, during annealing and / or cooling, the crystal does not attach or adhere to any part of the furnace (e.g., unlike during crystal growth, where the crystal is attached to a crystal growth crucible, e.g., by a seed crystal).

[0020] Although the embodiments of the present invention use AlN as an exemplary crystal material prepared according to the method of the present application, the embodiments of the present invention are also applicable to other crystal materials, such as silicon carbide (SiC) and zinc oxide (ZnO); therefore, the AlN described in the present application can be replaced by SiC or ZnO in other embodiments. As used in this application, 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 has an irregular cross-section.

[0021] The "substrate" or "wafer" described in this application is a portion of the previously grown ingot, having an upper surface and an opposite lower surface, which are usually parallel surfaces. The substrate thickness is usually 200μm to 1mm, and can serve as a platform for epitaxial growth of semiconductor layers and for making semiconductor devices thereon (for example, light-emitting devices, such as lasers and light-emitting diodes, transistors, power devices, etc.). Once layers and / or devices are formed on the substrate, all or part of the substrate can be removed therefrom as part of subsequent processing; therefore, when such a structure is present, the thickness of the remaining "substrate" may be less than the above thickness. The "room temperature" described in this application is 25°C.

[0022] In one aspect, an embodiment of the present invention is characterized in that the diameter of the AlN single crystal increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal. The AlN single crystal has a crystal enhancement parameter (CAP) in millimeters greater than 20. CAP is defined as follows: A E , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the maximum diameter, d E is the maximum diameter of the AlN single crystal in mm, A S , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the minimum diameter, d S is the minimum diameter in mm, and L E It is the extension length of at least a portion of the AlN single crystal, in mm, along which the diameter increases from the minimum diameter to the maximum diameter.

[0023] Embodiments of the present invention may include one or more of the following combinations in any combination. The CAP may be greater than 40, greater than 50, greater than 90, greater than 100, greater than 250, greater than 500, greater than 750, or greater than 1000. The CAP may be less than 3000, less than 2500, less than 2000, or less than 1500. The ratio of the total length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.3 to about 0.6. The ratio of the total length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.35 to about 0.55. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.4. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.03. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.07 to about 0.3. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.02. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.08 to about 0.5. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.1 to about 0.3.

[0024] The first region of the AlN single crystal may be shaped as a truncated cone. The maximum diameter of the truncated cone may correspond to the maximum diameter of the AlN single crystal, and the minimum diameter of the truncated cone may correspond to the minimum diameter of the AlN single crystal. The second region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the first region. The maximum diameter of the dome or cone or the truncated cone may correspond to the maximum diameter of the AlN single crystal (and / or to the maximum diameter of the first region).

[0025] The first region of the AlN single crystal may be shaped as a truncated cone. The maximum diameter of the truncated cone may correspond to the maximum diameter of the AlN single crystal, and the minimum diameter of the truncated cone may correspond to the minimum diameter of the AlN single crystal. The second region of the AlN single crystal may be shaped as a cylinder extending from the first region, whose diameter corresponds to the maximum diameter of the AlN single crystal. The third region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the second region. The maximum diameter of the dome or cone or the truncated cone may correspond to the maximum diameter of the AlN single crystal (and / or corresponds to the maximum diameter of the first region and / or the diameter of the second region).

[0026] The threading edge dislocation density of AlN single crystal can be less than about 1×10 6 cm -2 , less than about 1×10 5 cm -2 , less than about 1×10 4 cm -2 , less than about 1×10 3 cm -2 , or less than about 1×10 2 cm -2 The threading screw dislocation density of AlN single crystal can be less than about 1000 cm -2 , less than about 100cm -2 , less than about 10cm -2 , or less than about 1 cm -2 The full width at half maximum of the X-ray rocking curve of the AlN single crystal is less than 200 arc seconds, less than 100 arc seconds, less than 75 arc seconds, less than 50 arc seconds, or less than 40 arc seconds. The carbon concentration in the AlN single crystal may be less than 5×10 18 cm -3 , less than 1×10 18 cm -3 , less than 5×10 17 cm -3 , less than 1×10 17 cm -3 , less than 5×10 16 cm -3 , or less than 1×10 16 cm-3 The thermal conductivity of the AlN single crystal measured according to the American Society for Testing and Materials (ASTM) standard E1461-13 may be greater than about 200 W / m·K, greater than about 250 W / m·K, greater than about 290 W / m·K, or greater than about 310 W / m·K.

[0027] The Urbach energy of the AlN single crystal in the incident photon energy range of 5.85 eV to 6.0 eV is about 0.2 eV to about 1.8 eV. U It can be defined as follows: Where α is the absorption coefficient of the AlN single crystal at the incident photon energy hν, and α0 is a constant corresponding to the absorption coefficient at zero photon energy. The Urbach energy range of the AlN single crystal is about 0.21 eV to about 1.0 eV. The ultraviolet (UV) absorption coefficient of the AlN single crystal in the entire wavelength range of 220 nm to 280 nm can be less than 10 cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 280nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 210nm to 220nm can be less than 30cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 210nm to 220nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 240nm to 280nm can be less than 8cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 240nm to 280nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 215nm to 220nm can be less than 20cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 5cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 10cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 220nm can be less than 20cm -1 The ultraviolet absorption coefficient at a wavelength of 220nm can be no less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 240nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 240nm may be not less than about 5cm -1The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 230nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 230nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 230nm can be less than 10cm -1 The ultraviolet absorption coefficient at a wavelength of 230nm can be no less than about 5cm -1 .

[0028] The minimum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, or at least about 100 mm. The maximum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, at least about 100 mm, at least about 125 mm, or at least about 150 mm.

[0029] In another aspect, an embodiment of the present invention features an AlN single crystal having a mass greater than 78 grams.

[0030] Embodiments of the present invention may include one or more groups of the following combinations in any of a variety of combinations. The mass may be greater than about 100 grams, greater than about 140 grams, greater than about 200 grams, greater than about 220 grams, greater than about 240 grams, greater than about 250 grams, greater than about 300 grams, greater than about 400 grams, greater than about 500 grams, greater than about 600 grams, greater than about 700 grams, greater than about 800 grams, greater than about 900 grams, or greater than about 1000 grams. The mass may be less than about 2000 grams, less than about 1500 grams, or less than about 1400 grams. The minimum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, or at least about 100 mm. The maximum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, at least about 100 mm, at least about 125 mm, or at least about 150 mm.

[0031] The threading edge dislocation density of AlN single crystal can be less than about 1×10 6 cm -2 , less than about 1×10 5 cm -2 , less than about 1×10 4 cm -2 , less than about 1×10 3 cm -2 , or less than about 1×10 2 cm -2 The threading screw dislocation density of AlN single crystal can be less than about 1000 cm -2, less than about 100cm -2 , less than about 10cm -2 , or less than about 1 cm -2 The full width at half maximum of the X-ray rocking curve of the AlN single crystal is less than 200 arc seconds, less than 100 arc seconds, less than 75 arc seconds, less than 50 arc seconds, or less than 40 arc seconds. The carbon concentration in the AlN single crystal may be less than 5×10 18 cm -3 , less than 1×10 18 cm -3 , less than 5×10 17 cm -3 , less than 1×10 17 cm -3 , less than 5×10 16 cm -3 , or less than 1×10 16 cm -3 The thermal conductivity of the AlN single crystal measured according to the American Society for Testing and Materials (ASTM) standard E1461-13 may be greater than about 200 W / m·K, greater than about 250 W / m·K, greater than about 290 W / m·K, or greater than about 310 W / m·K.

[0032] The Urbach energy of the AlN single crystal in the incident photon energy range of 5.85 eV to 6.0 eV is about 0.2 eV to about 1.8 eV. U It can be defined as follows: Where α is the absorption coefficient of the AlN single crystal at the incident photon energy hν, and α0 is a constant corresponding to the absorption coefficient at zero photon energy. The Urbach energy range of the AlN single crystal is about 0.21 eV to about 1.0 eV. The ultraviolet (UV) absorption coefficient of the AlN single crystal in the entire wavelength range of 220 nm to 280 nm can be less than 10 cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 280nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 210nm to 220nm can be less than 30cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 210nm to 220nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 240nm to 280nm can be less than 8cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 240nm to 280nm may be not less than about 5cm -1The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 215nm to 220nm can be less than 20cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 5cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 10cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 220nm can be less than 20cm -1 The ultraviolet absorption coefficient at a wavelength of 220nm can be no less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 240nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 240nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 230nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 230nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 230nm can be less than 10cm -1 The ultraviolet absorption coefficient at a wavelength of 230nm can be no less than about 5cm -1 .

[0033] The diameter of the AlN single crystal increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal. The AlN single crystal may have a crystal enhancement parameter (CAP) in millimeters greater than 20. The CAP may be defined as follows: Among them A E , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the maximum diameter, d E is the maximum diameter of the AlN single crystal in mm, A S , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the minimum diameter, d S is the minimum diameter in mm, and L Eis an extended length of at least a portion of the AlN single crystal, in mm, along which the diameter increases from a minimum diameter to a maximum diameter. The ratio of the total length of the AlN single crystal (in mm) to the maximum diameter of the AlN single crystal (in mm) may be about 0.3 to about 0.6. The ratio of the total length of the AlN single crystal (in mm) to the maximum diameter of the AlN single crystal (in mm) may be about 0.35 to about 0.55.

[0034] The diameter of the AlN single crystal increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal. The extended length of the AlN single crystal may correspond to the length of at least a portion of the AlN single crystal, along which the diameter increases from a minimum diameter to a maximum diameter. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.4. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.03. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.07 to about 0.3. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.02. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.08 to about 0.5. A ratio of an extended length (in mm) to a maximum diameter (in mm) of the AlN single crystal may be about 0.1 to about 0.3.

[0035] The first region of the AlN single crystal may be shaped as a truncated cone. The second region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the first region (whose diameter decreases in a direction away from the first region). The second region of the AlN single crystal may be shaped as a cylinder extending from the first region, whose diameter is substantially constant. The third region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the second region (whose diameter decreases in a direction away from the first and second regions).

[0036] On the other hand, an embodiment of the present invention is characterized in that the volume is greater than 24cm 3 AlN single crystal.

[0037] Embodiments of the present invention may include one or more of the following combinations in any combination. The volume may be greater than about 30 cm 3 , greater than about 40cm 3 , greater than about 70cm 3 , greater than about 75cm 3 , greater than about 80cm 3, greater than about 100cm 3 , greater than about 150cm 3 , greater than about 200cm 3 , greater than about 250cm 3 , greater than about 300cm 3 , greater than about 350cm 3 , or greater than about 400cm 3 The volume may be less than about 800cm 3 , or less than about 500cm 3 The minimum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, or at least about 100 mm. The maximum diameter of the AlN single crystal may be at least about 25 mm, at least about 50 mm, at least about 60 mm, at least about 75 mm, at least about 100 mm, at least about 125 mm, or at least about 150 mm.

[0038] The threading edge dislocation density of AlN single crystal can be less than about 1×10 6 cm -2 , less than about 1×10 5 cm -2 , less than about 1×10 4 cm -2 , less than about 1×10 3 cm -2 , or less than about 1×10 2 cm -2 The threading screw dislocation density of AlN single crystal can be less than about 1000 cm -2 , less than about 100cm -2 , less than about 10cm -2 , or less than about 1 cm -2 The full width at half maximum of the X-ray rocking curve of the AlN single crystal is less than 200 arc seconds, less than 100 arc seconds, less than 75 arc seconds, less than 50 arc seconds, or less than 40 arc seconds. The carbon concentration in the AlN single crystal may be less than 5×10 18 cm -3 , less than 1×10 18 cm -3 , less than 5×10 17 cm -3 , less than 1×10 17 cm -3 , less than 5×10 16 cm -3 , or less than 1×10 16 cm -3The thermal conductivity of the AlN single crystal measured according to the American Society for Testing and Materials (ASTM) standard E1461-13 may be greater than about 200 W / m·K, greater than about 250 W / m·K, greater than about 290 W / m·K, or greater than about 310 W / m·K.

[0039] The Urbach energy of the AlN single crystal in the incident photon energy range of 5.85 eV to 6.0 eV is about 0.2 eV to about 1.8 eV. U It can be defined as follows: Where α is the absorption coefficient of the AlN single crystal at the incident photon energy hν, and α0 is a constant corresponding to the absorption coefficient at zero photon energy. The Urbach energy range of the AlN single crystal is about 0.21 eV to about 1.0 eV. The ultraviolet (UV) absorption coefficient of the AlN single crystal in the entire wavelength range of 220 nm to 280 nm can be less than 10 cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 280nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 210nm to 220nm can be less than 30cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 210nm to 220nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 240nm to 280nm can be less than 8cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 240nm to 280nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 215nm to 220nm can be less than 20cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 5cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 215nm to 220nm may be not less than about 10cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 220nm can be less than 20cm -1 The ultraviolet absorption coefficient at a wavelength of 220nm can be no less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 240nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 240nm may be not less than about 5cm -1The ultraviolet (UV) absorption coefficient of AlN single crystal in the entire wavelength range of 220nm to 230nm can be less than 15cm -1 The ultraviolet absorption coefficient in the entire wavelength range of 220nm to 230nm may be not less than about 5cm -1 The ultraviolet (UV) absorption coefficient of AlN single crystal at a wavelength of 230nm can be less than 10cm -1 The ultraviolet absorption coefficient at a wavelength of 230nm can be no less than about 5cm -1 .

[0040] The diameter of the AlN single crystal increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal. The AlN single crystal may have a crystal enhancement parameter (CAP) in millimeters greater than 20. The CAP may be defined as follows: Among them A E , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the maximum diameter, d E is the maximum diameter of the AlN single crystal in mm, A S , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the minimum diameter, d S is the minimum diameter in mm, and L E is an extended length of at least a portion of the AlN single crystal, in mm, along which the diameter increases from a minimum diameter to a maximum diameter. The ratio of the total length of the AlN single crystal (in mm) to the maximum diameter of the AlN single crystal (in mm) may be about 0.3 to about 0.6. The ratio of the total length of the AlN single crystal (in mm) to the maximum diameter of the AlN single crystal (in mm) may be about 0.35 to about 0.55.

[0041] The diameter of the AlN single crystal increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal. The extended length of the AlN single crystal may correspond to the length of at least a portion of the AlN single crystal, along which the diameter increases from a minimum diameter to a maximum diameter. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.4. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.03. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.07 to about 0.3. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.002 to about 0.02. The ratio of the extended length (in mm) to the maximum diameter (in mm) of the AlN single crystal may be about 0.08 to about 0.5. A ratio of an extended length (in mm) to a maximum diameter (in mm) of the AlN single crystal may be about 0.1 to about 0.3.

[0042] The first region of the AlN single crystal may be shaped as a truncated cone. The second region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the first region (whose diameter decreases in a direction away from the first region). The second region of the AlN single crystal may be shaped as a cylinder extending from the first region, whose diameter is substantially constant. The third region of the AlN single crystal may be shaped as a dome or a cone or a truncated cone extending from the second region (whose diameter decreases in a direction away from the first and second regions).

[0043] On the other hand, an embodiment of the present invention is characterized by a method for forming single crystal aluminum nitride (AlN). A seed crystal having a growth surface including, consisting essentially of, or consisting of AlN is provided in a growth chamber. A radial thermal gradient and an axial thermal gradient are established in the growth chamber. Vapor including, consisting essentially of, or consisting of aluminum and nitrogen is condensed in the growth chamber, thereby forming an AlN single crystal on the growth surface of the seed crystal, the single crystal (a) increasing in length along the growth direction in response to the axial thermal gradient, and (b) expanding in diameter in a radial direction substantially perpendicular to the growth direction in response to the radial thermal gradient. In the process of forming the AlN single crystal, the lateral growth rate of the AlN single crystal is increased to increase the diameter expansion rate of the AlN single crystal.

[0044] Embodiments of the present invention may include one or more of the following combinations in any of a variety of combinations. Establishing a radial thermal gradient and an axial thermal gradient within a growth chamber may at least partially include, consist essentially of, or be composed of: (i) heating the growth chamber and (ii) configuring a plurality of heat shields outside the growth chamber. Increasing the lateral growth rate of an AlN single crystal may include, consist essentially of, or be composed of atomic nitrogen enhanced vapor near an edge portion of the AlN single crystal. The atomic nitrogen enhanced vapor includes, consists essentially of, or is composed of: (i) introducing nitrogen (and / or nitrogen-containing) gas near an edge portion of the AlN single crystal and (ii) generating a plasma near an edge portion of the AlN single crystal using nitrogen (and / or nitrogen-containing gas).

[0045] Increasing the lateral growth rate of the AlN single crystal may include, consist essentially of, or consist of providing one or more internal heat shields in the growth chamber that direct heat to the edge of the AlN single crystal. At least one (or even all) of the internal heat shields may be substantially parallel to the radial direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be substantially parallel to the growth direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be tilted at an angle that is neither parallel nor perpendicular to the radial direction. At least one (or even all) of the internal heat shields may be annular and define a central opening therein to accommodate the growth of the AlN single crystal therethrough. One or more internal heat shields may include, consist essentially of, or consist of a plurality of internal heat shields. The thickness of at least two (or even all) of the internal heat shields may be different from each other. The density of at least two of the internal heat shields may be different from each other. Each internal heat shield may be annular and define a central opening therein. The sizes of at least two (or even all) of the inner heat shield central openings may differ from each other.

[0046] A single crystal AlN substrate may be separated from the AlN single crystal. The diameter of the single crystal AlN substrate may be at least 25 mm, at least 50 mm, at least 75 mm, or at least 100 mm. A light emitting device may be fabricated on at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After or during fabrication of the light emitting device, at least a portion of the AlN substrate may be removed from the light emitting device.

[0047] In another aspect, an embodiment of the invention features a method of forming single crystal aluminum nitride (AlN). A seed crystal having a growth face comprising, consisting essentially of, or consisting of AlN is provided in a growth chamber. The growth chamber is heated. Vapor comprising, consisting essentially of, or consisting of aluminum and nitrogen condenses in the growth chamber during the heating of the vapor, thereby forming an AlN single crystal on the growth face of the seed crystal. During the formation of the AlN single crystal, atomic nitrogen near an edge portion of the AlN single crystal enhances the vapor.

[0048] Embodiments of the present invention may include one or more of the following combinations in any of a variety of combinations. The atomic nitrogen enhanced vapor includes, consists essentially of, or is composed of: (i) introducing nitrogen (and / or nitrogen-containing) gas into the growth chamber and (ii) generating a plasma near an edge portion of the AlN single crystal with nitrogen (and / or nitrogen-containing gas). A single crystal AlN substrate may be separated from the AlN single crystal. The diameter of the single crystal AlN substrate may be at least 25 mm, at least 50 mm, at least 75 mm, or at least 100 mm. A light emitting device may be manufactured on at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After or during the manufacture of the light emitting device, at least a portion of the AlN substrate may be removed from the light emitting device.

[0049] In another aspect, an embodiment of the present invention is characterized by a method for forming single crystal aluminum nitride (AlN). A seed crystal having a growth face including, consisting essentially of, or consisting of AlN is provided in a growth chamber. The growth chamber is heated. Vapor including, consisting essentially of, or consisting of aluminum and nitrogen condenses in the growth chamber during its heating, thereby forming an AlN single crystal on the growth face of the seed crystal. The AlN single crystal extends from the seed crystal in an axial direction perpendicular to the growth face. During the formation of the AlN single crystal, heat is directed to edge portions of the AlN single crystal due to one or more internal heat shields disposed in the growth chamber.

[0050] Embodiments of the present invention may include one or more of the following combinations in any of a variety of combinations. One or more or several external heat shields may be disposed outside the growth chamber. During heating of the growth chamber, one or more thermal gradients may be established within the growth chamber. The one or more thermal gradients may be established at least in part by different furnace heating and / or different insulation outside the growth chamber. The one or more thermal gradients may be established at least in part by one or more heat shields disposed outside the growth chamber. The vapor may be enhanced with atomic nitrogen near an edge portion of the AlN single crystal. The atomic nitrogen enhanced vapor includes, consists essentially of, or is composed of: (i) introducing nitrogen (and / or nitrogen-containing) gas into the growth chamber and (ii) generating a plasma near an edge portion of the AlN single crystal with nitrogen (and / or nitrogen-containing gas).

[0051] At least one (or even all) of the internal heat shields may be substantially parallel to the axial direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be substantially perpendicular to the axial direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be tilted at an angle, neither parallel nor perpendicular to the axial direction. At least one (or even all) of the internal heat shields may be annular and define a central opening therein to accommodate the growth of the AlN single crystal therethrough. One or more internal heat shields may include, consist essentially of, or consist of a plurality of internal heat shields. The thickness of at least two (or even all) of the internal heat shields may be different from each other. The density of at least two of the internal heat shields may be different from each other. Each internal heat shield may be annular and define a central opening therein. The size of the central opening of at least two (or even all) of the internal heat shields may be different from each other.

[0052] A single crystal AlN substrate may be separated from the AlN single crystal. The diameter of the single crystal AlN substrate may be at least 25 mm, at least 50 mm, at least 75 mm, or at least 100 mm. A light emitting device may be fabricated on at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After or during fabrication of the light emitting device, at least a portion of the AlN substrate may be removed from the light emitting device.

[0053] In another aspect, an embodiment of the present invention features a method of forming single crystal aluminum nitride (AlN). A seed crystal having a growth face comprising, consisting essentially of, or consisting of AlN is provided in a growth chamber. An internal support is provided in the growth chamber. The internal support defines an opening for accommodating the growth of the AlN single crystal therethrough. One or more internal heat shields are provided in the growth chamber. Each internal heat shield is at least partially supported by the internal support. The growth chamber is heated. Vapor comprising, consisting essentially of, or consisting of aluminum and nitrogen condenses in the growth chamber during its heating, thereby forming an AlN single crystal on the growth face of the seed crystal. The AlN single crystal extends from the seed crystal in an axial direction perpendicular to the growth face.

[0054] Embodiments of the invention may include one or more of the following combinations in any of a variety of combinations. At least a portion of the internal support may be truncated conical. The vapor may be enhanced with atomic nitrogen near an edge portion of the AlN single crystal. The atomic nitrogen enhanced vapor comprises, consists essentially of, or consists of: (i) introducing nitrogen (and / or nitrogen-containing) gas into the growth chamber and (ii) generating a plasma near an edge portion of the AlN single crystal with nitrogen (and / or nitrogen-containing gas).

[0055] At least one (or even all) of the internal heat shields may be substantially parallel to the axial direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be substantially perpendicular to the axial direction (e.g., ±5°, ±4°, ±3°, ±2°, ±1°, or ±0.5°). At least one (or even all) of the internal heat shields may be tilted at an angle, neither parallel nor perpendicular to the axial direction. At least one (or even all) of the internal heat shields may be annular and define a central opening therein to accommodate the growth of the AlN single crystal therethrough. One or more internal heat shields may include, consist essentially of, or consist of a plurality of internal heat shields. The thickness of at least two (or even all) of the internal heat shields may be different from each other. The density of at least two of the internal heat shields may be different from each other. Each internal heat shield may be annular and define a central opening therein. The size of the central opening of at least two (or even all) of the internal heat shields may be different from each other.

[0056] A single crystal AlN substrate may be separated from the AlN single crystal. The diameter of the single crystal AlN substrate may be at least 25 mm, at least 50 mm, at least 75 mm, or at least 100 mm. A light emitting device may be fabricated on at least a portion of the AlN substrate. The light emitting device may be configured to emit ultraviolet light. After or during fabrication of the light emitting device, at least a portion of the AlN substrate may be removed from the light emitting device.

[0057] Embodiments of the present invention may include an AlN ingot, a wafer and / or a light emitting device formed or formable according to any of the above methods.

[0058] By reference to the following description, drawings and claims, these and other purposes of the present invention disclosed in the present application, as well as advantages and features will become more apparent. In addition, it should be understood that the features of the various embodiments described in the present application are not mutually exclusive, and may exist in different combinations and arrangements. The words "approximately", "about" and "substantially" used in the present application refer to ±10%, and in some embodiments, ±5%. Unless otherwise specified, all numerical ranges specified in the present application include their endpoints. Unless otherwise defined in the present application, the word "essentially consisting of..." refers to not including other materials that have an effect on the function. Nevertheless, the other materials may exist, and their total content or separate content is a trace amount. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] In the drawings, similar reference characters generally refer to the same parts in different views. In addition, the drawings are not necessarily drawn to scale, instead emphasis is generally placed on illustrating the principles of the invention. In the following description, various embodiments of the invention are described with reference to the following drawings, in which:

[0060] FIG. 1A is a schematic diagram of a seed crystal according to various embodiments of the present invention;

[0061] 1B-1D are schematic diagrams of expanded diameter bulk crystals according to various embodiments of the present invention;

[0062] 1E and 1F are schematic diagrams of bulk crystals and their associated parameters according to various embodiments of the present invention;

[0063] Figure 2 is a schematic diagram of an apparatus for growing single crystal AlN according to various embodiments of the present invention;

[0064] Figure 3 Schematic diagram of various parts of single crystal AlN growth equipment according to various embodiments of the present invention;

[0065] Figure 4 Schematic diagram of various parts of single crystal AlN growth equipment according to various embodiments of the present invention;

[0066] Figure 5 is an exemplary AlN single crystal image prepared according to various embodiments of the present invention;

[0067] Figures 6A and 6B They are respectively graphs of the mass and volume distribution of the AlN single crystal ingots prepared according to the embodiments of the present invention;

[0068] FIG7A is a schematic diagram of a light emitting device prepared according to various embodiments of the present invention;

[0069] 7B is a plan view photograph of the light emitting device of FIG. 7A during light emission at a peak wavelength of about 230 nm according to various embodiments of the present invention;

[0070] Figure 8 is a comparison diagram of the relationship between UV absorption coefficient and wavelength of conventional single crystal AlN and single crystal AlN grown and annealed according to various embodiments of the present invention;

[0071] Fig. 9 For estimating the Figure 8 Urbach energy of AlN sample;

[0072] Fig.10 is a schematic diagram of various light components for determining UV absorption spectra and Urbach energy according to various embodiments of the present invention;

[0073] 11 is a graph comparing the emission intensity and wavelength of a simulated LED emitting light having a peak wavelength of about 217 nm according to various embodiments of the present invention; and

[0074] Fig.12 11 is a comparison of the emission spectra, in which the relative intensities of the LEDs have been independently normalized to the same value to show that the intensity peaks of the devices of various embodiments of the present invention are narrower. DETAILED DESCRIPTION

[0075] Embodiments of the present invention can produce high-quality single-crystalline AlN bulk crystals (ie, ingots and / or substrates) that undergo significant diameter expansion during crystal growth. Figures 1A-1C 1A shows an exemplary seed crystal 100 having a diameter 102, a front surface 104, and a rear surface 106. Although the seed crystal 100 is shown as being cylindrical with a circular surface, the seed crystal 100 is not limited to these shapes. Therefore, the diameter 102 generally refers to the maximum lateral dimension of the seed crystal 100, and thus, for example, for a seed crystal 100 having a non-circular shape, may correspond to a "width" or "maximum width". In various embodiments of the present invention, the thickness of the seed crystal 100 is about 0.1 mm to about 3 mm. Typically, the front surface 104 is exposed to input steam for crystal growth, and the resulting crystal extends from the front surface 104. The seed crystal 100 can be mounted in a growth device through the rear surface 106 (see, for example Figure 2 ). Depending on the seeding installation procedure, the exposed growth surface of the seed 100 may be equal to or less than the area of ​​the front surface 104 (i.e., a portion of the front surface 104 may be covered or otherwise blocked from receiving input steam). As used herein, "seed diameter" or "seed diameter" refers to the diameter of the actual area exposed for the seed 100 to grow thereon (i.e., the "seed area" or "seed area"), even if that area is less than the total area of ​​the front surface 104. Additionally, the shape of the seed diameter or seed diameter may be different than the actual surface 104 of the seed 100 itself, due to, for example, masking or otherwise obstructing a portion of the surface 104. For example, the seed diameter may be circular while the actual surface 104 is non-circular, or vice versa.

[0076] FIG. 1B is a schematic diagram of a crystal (or "crystallized ingot" or "ingot") 108 produced by crystal growth (e.g., by a vapor transport technique such as sublimation-condensation) on a seed crystal 100. It is noteworthy that, due to the initiation of growth on the seed crystal 100, the crystal 108 does not terminate at a tip, but has a relatively flat surface. The crystal 108 has an initial seed diameter 110 (i.e., the diameter of the seed region of the crystal, which may correspond to the diameter of the initial seed crystal or a portion thereof), and due to the expansion during growth, can be described as a geometric combination of a truncated cone 112 and a dome 114, the truncated cone 112 being produced by the expansion during growth, and the dome 114 being produced at least in part by the shape of the thermal field in the growth chamber. The truncated cone 112 may (but need not be) for example be square, circular, and conical. The dome 114 may (but need not be) for example be a spherical cap or a spherical segment. In various embodiments, dome 114 may be in the form of a cone (e.g., having a rounded tip) or a frustum of a cone (e.g., a frustum that tapers in a direction opposite to frustum 112). As shown, the diameter (or other lateral dimension) of the crystal may increase as a result of the expansion to a maximum crystal diameter 116. The curvature of dome 114 may increase as the radial thermal gradient used during crystal growth increases. Thus, a crystal 108 having a small (or even substantially non-existent) dome 114 may result from the use of a small radial thermal gradient during crystal growth. That is, according to embodiments of the present invention, the radial thermal gradient may be adjusted (e.g., during growth) to reduce the size of dome 114 or actually eliminate dome 114 altogether. Notably, because crystals 108 are grown from seed crystals 100, they are larger than similar crystals grown without seeds and contain more usable high-quality volume (e.g., for the production of single crystal AlN wafers). (Unseeded growth typically relies on spontaneous nucleation, which can introduce excessive defects and / or non-uniform crystal orientation.) As disclosed herein, the crystal 108 produced by seeded growth may also include at least a portion of the seed crystal 100 itself therein.

[0077] FIG. 1C is a cross-sectional view of an exemplary crystal 108. As shown, the crystal 108 has a total length 118 including the truncated cone portion and the dome portion of the crystal. The total length 118 includes an expanded length 120 (i.e., the length of the expanded volume of the crystal in the growth direction, for example, the length perpendicular to the surface of the seed crystal 100) and a dome length 122. In various embodiments of the present invention, the crystal 108 may include a portion 124 whose diameter is not expanded (for example, due to intentional modification of the radial thermal gradient and / or expansion sufficient to reach the inner wall of the growth device), and the portion 124 may have a length 126, which is a portion of the total length 118. The portion 124 may be cylindrical, for example, or may have one or more flat surfaces (for example, may have the shape of a hexagonal prism (for example, with sides parallel to the m-plane {1-100})). In embodiments of the present invention, the portion 124 may be present but is not required to be present. As shown in FIG. 1C, when present, the diameter of the portion 124 may be substantially equal to the maximum or expanded diameter 116. The frustum 112 also has an expanded height or slope height 128 measured along the surface of the crystal diameter expanded volume. As can be readily seen from Figure 1C, in the absence of diameter expansion, the expanded height 128 and the expanded length are equal.

[0078] Figure 1D 1 is a schematic diagram of another exemplary crystal 108 prepared according to an embodiment of the present invention. As shown in the figure, Figure 1D 1C, but the "straight" portion 124 of substantially constant diameter is longer than the expansion length 120, which is caused by the rapid initial expansion of the crystal (e.g., due to the use of one or more techniques described in detail in embodiments of the present invention). Figure 1D The illustrated crystal 108 may advantageously provide a large crystal volume from which many wafers of substantially the same diameter may be produced.

[0079] In various exemplary embodiments, the extended length 120 may be from about 1%, 2%, 3%, 5%, or 10% to about 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of the total length 118, and the length 126 may be from about 0%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3%, 5%, or 10% to about 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% of the total length 118. 75%, 80%, 85%, 90%, 95% or 98%, and the dome length 122 can be approximately 0%, 0.1%, 0.2%, 0.5%, 1%, 2%, 3% or 5% to approximately 20%, 25%, 30%, 35%, 40% or 45% of the total length 118 (whereas as shown in Figures 1C and 1D, the sum of the extended length 120, length 126 and dome length 122 equals 100% of the total length 118).

[0080] 1E and 1F are other schematic diagrams of various crystals and their associated parameters associated with embodiments of the present invention. FIG. 1E schematically depicts how wafers are cut from a crystal 108 described in various embodiments of the present invention. As shown, wafers 130-1 and 130-2 with larger diameters can be cut from a crystal 108 that is substantially parallel to the seed crystal 100. In other embodiments, wafers can be cut from the crystal 108 in other directions, even in a direction substantially perpendicular to the plane of the seed crystal 100. Wafers (e.g., wafer 130-2) cut from the crystal 108 with a diameter greater than that of the seed crystal 100 can then be used as seeds for growing crystals with larger diameters, such as disclosed in U.S. patent application Ser. No. 16 / 008,407 filed on June 14, 2018 ('407 application), the entire contents of which are incorporated herein by reference. As shown in FIGS. 1C and 1D, wafers cut from a portion 124 of the crystal 108 can have substantially the same diameter as each other.

[0081] FIG. 1F is a schematic cross-sectional view of crystal 108, depicting (indicated by shading) an expansion region 132, i.e., a cross-sectional area of ​​crystal 108 that exceeds seed region 110 due to the expansion. FIG. 1F also depicts an exemplary expansion angle 134, which corresponds to the angle between the normal of the seed and the plane of expansion height 128. In various embodiments, expansion angle 134 can be, but is not necessarily, substantially constant throughout the growth of crystal 108. That is, the plane of expansion height 128 need not be linear. When wafers are cut from crystal 108, expansion region 132 thus corresponds to an annular region extending inward from the outer edge of the wafer, while the seed region corresponds to a central region of the wafer, which is substantially the same size and shape as the seed 100 used to grow crystal 108. Thus, at least within the frustum 112 of the expanded crystal 108, a wafer cut from a portion of the ingot further from the seed crystal 100 will have a larger expansion region 132 than a wafer cut from a portion of the ingot closer to the seed crystal 100, while the seed regions of the two wafers may be approximately the same size and shape. On the other hand, multiple wafers cut from the portion 124 of the crystal 108 will have approximately the same expansion region 132 and approximately the same seed region.

[0082] The orientation of the wafer or seed crystal can be selected from the ingot or other crystal during slicing by, for example, X-ray diffraction measurements and / or other material characterization methods capable of identifying crystal orientation; these techniques are well known to those skilled in the art and do not require much experimentation to perform. In accordance with embodiments of the present invention, the newly cut wafer or seed crystal can be polished to reduce surface roughness and remove cutting artifacts and / or damage. The polarity of the wafer or seed crystal can also be identified and selected by chemical methods. For example, the polarity can be identified and selected by exposing the wafer or seed crystal to an alkaline or acidic solution, which will roughen the N-polar face while leaving the Al-polar face smooth, as described in detail in the '407 application.

[0083] Figure 2 A crystal growth apparatus 200 suitable for growing single crystal AlN according to various embodiments of the present invention is described. As shown, the apparatus 200 includes a crucible 205 located on a crucible holder 210 within a receptacle 215. The crucible 205 and the receptacle 215 can have any suitable geometry, for example, cylindrical. During a typical growth process, an AlN ingot 220 (e.g., crystal 108) is formed by condensation of a vapor 225, the vapor 225 including, consisting essentially of, or consisting of elemental precursors of the AlN ingot 220, i.e., Al atoms and N atoms and / or N2 molecules. In a typical embodiment, a source material 230 is sublimated to form the vapor 225, which may include, consist essentially of, or consist of the polycrystalline AlN source material described above. The AlN ingot 220 can be formed on and epitaxially grown from a seed crystal 235. (Alternatively, the AlN ingot 220 may be nucleated on and epitaxially grown from a portion of the crucible 205 itself.) The seed crystal 235 may be a single crystal (e.g., a polished wafer) comprising, consisting essentially of, or consisting of AlN. In various embodiments, the diameter (or width or other lateral dimension) of the seed crystal 235 is at least about 10 mm, at least about 25 mm, at least about 35 mm, at least about 40 mm, or even at least about 50 mm. In various embodiments, the diameter (or width or other lateral dimension) of the seed crystal 235 is about 50 mm or less, about 100 mm or less, about 150 mm or less, and / or the diameter (or width or other lateral dimension) of the single crystal AlN grown thereon is about 150 mm or less. In various embodiments, the crystallographic orientation (i.e., the normal to the exposed plane (e.g., the c-plane)) of the seed crystal 235 is substantially parallel to the c-axis. In other embodiments, the crystal orientation of the seed crystal 235 is at least about 5°, or even at least about 10°, from the c-axis; the seed crystal 235 may be oriented in a non-polar direction. In various embodiments, the crystal orientation of the seed crystal 235 is no more than about 30°, or no more than about 20°, from the c-axis.

[0084] Crucible 205 may include, consist essentially of, or consist of one or more refractory materials, such as tungsten, rhenium, tantalum carbide, and / or tantalum nitride. As described in the '135 patent and the '153 patent, crucible 205 has one or more surfaces (e.g., walls) configured to selectively allow nitrogen to diffuse therethrough and selectively prevent aluminum from diffusing therethrough.

[0085] like Figure 2 As shown, during the formation of the AlN ingot 220, polycrystalline material 240 may (but need not) form at one or more locations within the crucible 205 that are not covered by the seed crystal 235. However, during the formation of the AlN ingot 220, the diameter (or other radial dimension) of the AlN ingot 220 may expand, i.e., increase, thereby sealing off regions of polycrystalline material 240 (if any) from intimate contact with the vapor 225, substantially limiting or even eliminating its growth. Figure 2 As shown, the diameter of the AlN ingot 220 may be expanded (or even, in embodiments, started with a larger seed crystal 235 ) to be substantially equal to the inner diameter of the crucible 205 (at which point no further lateral expansion of the AlN ingot 220 may occur).

[0086] Since a relatively large axial thermal gradient (eg, from about 5°C / cm to about 100°C / cm) is formed in the crucible 205, the AlN ingot 220 generally grows along the growth direction 245. Figure 2 The susceptor 215 (and therefore the crucible 205) is heated to a high temperature, typically about 1800° C. to about 2300° C., by a heating element (not shown), such as an RF heater, one or more heating coils, and / or other heating elements or furnaces. Before growth begins, the crucible 205 and its contents (e.g., the seed crystal 235 and the source material 230) may be maintained at a temperature approximately equal to the desired growth temperature for a predetermined time (e.g., about 1 hour to about 10 hours). In various embodiments, maintaining this temperature stabilizes the thermal field within the crucible 205, promotes efficient nucleation on the seed crystal 235, and promotes high-quality transition from nucleation to bulk growth of single crystal AlN.

[0087] The apparatus 200 features one or more sets of top heat shields 250, and / or one or more sets of bottom axial heat shields 255, which are arranged to establish a larger axial thermal gradient (by, for example, better insulating the bottom end of the crucible 205 and the source material 230 from heat loss than the top end of the crucible 205 and the growing AlN ingot 220). During the growth process, the susceptor 215 (and therefore the crucible 205) can be translated within the heating zone established by the heating apparatus by the drive mechanism 260 to maintain the axial thermal gradient near the surface of the growing AlN ingot 220. One or more pyrometers 265 (or other characterization devices and / or sensors) can be used to monitor the temperature of one or more locations within the susceptor 215. The top heat shield 250 and / or the bottom heat shield 255 can include, consist essentially of, or consist of one or more refractory materials (e.g., tungsten), and can be relatively thin (e.g., about 0.125 mm to 0.5 mm thick). As described in detail in the '612 patent, the top heat shield 250 and / or the bottom heat shield 255 can be arranged in different configurations and / or have different characteristics (i.e., different numbers of heat shields, different spacing between heat shields, different thicknesses, different sized holes defined therethrough, different dimensions, etc.) to produce various different axial and radial thermal gradients within the crucible 205 and, therefore, different AlN ingot 220 growth (e.g., growth rate, radial expansion during growth, if any, etc.).

[0088] In various embodiments, the crucible 205 has a lid 270 that is sufficiently radiation transparent to at least partially control the thermal profile within the crucible 205 by disposing a top heat shield 250. Additionally, in embodiments featuring a seed crystal 235, the seed crystal 235 is typically placed on the lid 270 prior to the growth of the AlN ingot 220. The lid 270 is typically mechanically stable at growth temperatures (e.g., up to about 2300° C.) and can substantially prevent Al-containing vapors from diffusing therethrough. The lid 270 typically includes, consists essentially of, or consists of one or more refractory materials (e.g., tungsten, rhenium, and / or tantalum nitride), and can be relatively thin (e.g., less than about 0.5 mm thick).

[0089] like Figure 2 As shown, each top heat shield typically has an opening 275. The opening 275 typically mimics the geometry and / or symmetry of the crucible 205 (e.g., the opening 275 of a cylindrical crucible 205 may be substantially circular). The size of each opening 275 may vary, typically ranging from a minimum of 10 mm less than the diameter of the crucible 205 to a maximum of about 5 mm (or even 2 mm) less than the diameter of the crucible 205.

[0090] For example, in one embodiment, five heat shields 250 are used, each having a diameter of 68.5 mm and an opening size (diameter) of 45 mm. The thickness of each heat shield 250 is 0.125 mm, and the heat shields 250 are spaced approximately 7 mm apart from each other. At a typical growth temperature of 2065°C, this arrangement of heat shields results in a radial thermal gradient (measured from the center of the semiconductor crystal to the inner edge of the crucible) of 27°C / cm. Of course, this value is exemplary only, and those skilled in the art can arrange heat shields to achieve a series of different radial thermal gradients without conducting excessive experiments.

[0091] Embodiments of the present invention provide a higher expansion rate for the AlN crystal by providing a radial thermal gradient enhancement generated by the arrangement of the heat shield 250. For the avoidance of doubt, the techniques described in detail in the embodiments of the present invention enable the grown AlN single crystal to have a higher expansion rate while maintaining crystal quality (thereby, as described in detail in this application, producing an AlN single crystal with a greater crystal enhancement parameter). Generally speaking, the techniques described in the embodiments of the present invention enhance the lateral growth of the crystal by increasing the radial thermal gradient by customized heating of the edge of the growing crystal and / or changing the condensing vapor. In conventional technology, it is generally conventional to suppress the radial thermal gradient so as to, for example, minimize the curvature of the leading edge of the growing crystal. Conventional practices in the art also tend to emphasize maintaining a substantially uniform radial temperature during crystal growth. In order to further enhance the expansion (e.g., beyond the expansion that can be achieved by simply providing an external heat shield, or even in combination with different external heating and insulation techniques), while maintaining the high crystalline quality of the resulting bulk crystal, the embodiments of the present invention contradict this conventional practice.

[0092] Figure 3 A technique for enhancing radial thermal gradient according to an embodiment of the present invention is shown. Figure 3In an embodiment of the present invention, one or more internal heat shields (or baffles) 300 are arranged in the crucible 205 near the edge of the growing AlN ingot 220. According to various embodiments, the internal heat shields 300 transfer heat from the wall of the crucible 205 to the edge of the AlN ingot 220, increasing its temperature, and the internal heat shields 300 also retain heat near the edge of the AlN ingot 220. In this way, the internal heat shields 300 enhance or increase the radial thermal gradient within the crucible 205, resulting in enhanced lateral crystal growth and increased diameter expansion of the AlN ingot 220. In various embodiments of the present invention, the number of internal heat shields 300 disposed in the crucible 205 is 1 to 10, or even 1 to 15. The present inventors have discovered that the use of an internal heat shield 300 within the crucible 205 allows the AlN ingot 220 to expand more rapidly and thereby form a larger ingot with more available volume for substrate fabrication compared to conventional growth techniques, despite the conventional wisdom that other objects disposed within the growth crucible tend to deleteriously disrupt crystal growth and / or act as extraneous nucleation centers for parasitic growth of polycrystalline or other unwanted material.

[0093] In an embodiment of the present invention, the internal heat shield 300 includes, consists essentially of, or consists of one or more refractory materials (e.g., tungsten and / or TaC), and can be relatively thin (e.g., a thickness of about 0.125 mm to 0.5 mm). In other embodiments, the one or more heat shields can have a greater thickness, for example, from about 1 mm to about 3 mm. In various embodiments, the density of the one or more internal heat shields 300 (and the accompanying effects on the thermal field near the heat shield) can vary. For example, the density of the one or more internal heat shields 300 can be from about 10% full density to about 100% full density (e.g., 100% full density of tungsten is about 19.3 g / cm 3 ). Thin refractory foils of varying densities and / or thicknesses are commercially available and can be provided without undue experimentation. In various embodiments, a thicker and / or denser inner heat shield can transfer more heat than a thinner and / or denser heat shield, thereby increasing the radial thermal gradient more.

[0094] like Figure 3As shown, the outer boundary of the internal heat shield 300 can substantially conform to the shape and size of the inner wall of the crucible 205, and the size of the central opening in the heat shield 300 can vary to accommodate the expansion (e.g., expected expansion) of the AlN ingot 220. For example, the central opening of the internal heat shield 300 can increase as the distance of the heat shield away from the seed crystal 235 increases, at least when the internal heat shield 300 is positioned where the AlN ingot 220 is expected or expected to undergo expansion. In addition, the density and / or thickness of a single internal heat shield 300 can vary (e.g., increase) as the distance away from the seed crystal 235 changes, at least when the internal heat shield 300 is positioned where the AlN ingot 220 is expected or expected to undergo expansion. Such an increase can compensate for the volume loss of the internal heat shield 300 with a larger central opening. In various embodiments, the central opening size, density, and / or thickness of the internal heat shield 300 can be substantially constant at locations where the AlN ingot 220 is expected or expected to not undergo expansion.

[0095] Similarly, in areas where more rapid expansion is desired, the spacing between internal heat shields 300 may be reduced compared to areas where expansion is not desired or expected (e.g., to the same extent). Example spacing between internal heat shields 300 may be about 1 mm to about 50 mm, or about 5 mm to about 10 mm.

[0096] According to embodiments of the present invention, internal heat shields 300 can be mounted within crucible 205 by a variety of different methods. For example, internal heat shields 300 can be held or secured at their outer edges to the inner surface of crucible 205. Internal heat shields 300 can each be placed on a platform or pedestal within crucible 205 (e.g., extending from an inner wall thereof), or internal heat shields 300 can be placed at their central openings on internal supports extending from the top surface of the crucible near seed crystal 235. (For clarity, internal supports are shown in FIG. 1 ). Figure 3300, but it may correspond to the outer shape of the crystal and be in contact with it or spaced apart from it; the inner edge of the internal heat shield 300 may be in contact with the internal support and thus supported by it. The internal support, which may include, consist essentially of, or consist of one or more of the same materials as the internal heat shield 300, may have a truncated cone shape, having a first inner diameter at its upper end (i.e., the end near the seed crystal 235) that is approximately equal to or even smaller than the diameter of the seed crystal 235, and a larger second inner diameter at its lower end. In various embodiments, the rate of increase of the inner diameter of the internal support may be substantially equal to or greater than (e.g., desired or expected) the expansion rate and / or expansion angle of the AlN ingot 220. That is, in embodiments of the present invention, the AlN ingot 220 may not contact (at least not completely contact except at one or more discrete points) the inner surface of the internal support (i.e., the internal support may not be in circumferential contact with and tightly fit the AlN ingot 220). In other embodiments, the inner diameter increase rate of the internal support can be less than the expected expansion angle of the AlN ingot 220 (i.e., the expansion angle under given growth parameters in the absence of the internal support), and therefore, the internal support can limit the expansion angle and expansion rate of the AlN ingot 220 to the desired values ​​defined by the geometry of the support.

[0097] In various embodiments, all or a portion of the inner support may be conical (i.e., increasing in diameter in a direction away from the seed crystal 235), e.g., where the AlN ingot 220 is expected or anticipated to expand in diameter. For example, all or a portion of the inner support may have the shape of a frustum of a cone having a top opening of a smaller diameter to accommodate the seed crystal 235, and which flares outward to accommodate the AlN ingot 220 of increasing diameter. In various embodiments, all or a portion of the inner support may be cylindrical (i.e., having a substantially constant diameter with distance away from the seed crystal 235), e.g., where the AlN ingot 220 is expected or anticipated not to experience expansion in diameter. In one example, the inner support may be partially conical and partially cylindrical, echoing the change in diameter of portions 112 and 124 of the crystal 108 shown in FIG. 1C .

[0098] exist Figure 3, the internal heat shields 300 are depicted as extending around the AlN ingot 220, generally parallel to the plane of the seed crystal 235 (i.e., generally perpendicular to the lateral growth direction) and / or to the top or bottom surface 205 of the crucible, but in various embodiments, the internal heat shields 300 are at other angles. For example, one or more internal heat shields 300 may extend generally perpendicular to the plane of the seed crystal 235 or at an angle oblique thereto (e.g., about 5° to about 85° to the plane of the seed crystal) and / or to the top and / or bottom surface of the crucible. In an exemplary embodiment, one or more internal heat shields 300 may be generally perpendicular to a plane defined by the extended height of the AlN ingot 220 (i.e., generally perpendicular to the edge of the crystal frustum)—for example, one or more internal heat shields 300 may extend generally perpendicularly from the hypotenuse of the internal support. In various embodiments, the angle of one or more internal heat shields 300 may be different from the angle of one or more other internal heat shields 300. In various embodiments, the internal heat shield 300 only affects the thermal field within the crucible 205 (eg, increases radial thermal gradients at one or more points and / or regions) and does not contact the AlN ingot 220 itself during growth of the AlN ingot 220 .

[0099] In various embodiments of the present invention, atmospheric plasma is used to enrich the source gas phase within the crucible 205 with nitrogen atoms and preferentially concentrate these atoms at the lateral edges of the growing crystal. The excess nitrogen produced by the plasma process promotes increased lateral growth of the AlN crystal at a rate that exceeds that achieved by the introduction of nitrogen (or nitrogen-containing gas) alone, even at superatmospheric growth pressures. Figure 4 As shown, nitrogen (and / or nitrogen-containing gas) may be introduced into the crucible 205 through one or more nozzles 400 to provide excess nitrogen near the edge of the growing AlN ingot 220. In addition, one or more plasma electrodes 410 are disposed near (e.g., about 0.3 cm to about 1 cm from) the edge of the AlN ingot 220 (e.g., the edge expected to be reached or located during growth) and / or near the internal support (if present). As shown, the electrode 410 may be arranged at a certain angle to accommodate the diameter expansion of the AlN ingot 220, but in other embodiments, the electrode 410 may be arranged in other configurations (e.g., parallel to the crucible wall or at a certain angle to the crucible wall, then parallel to the crucible wall). AC or DC current may be applied to the electrode 410 by a high frequency current source (which may be combined with an RF source for crystal growth), and a pulsed arc may be generated by a high voltage discharge at the electrode 410. Nitrogen from the nozzle 400 may be close to or flow through the electrode 410 and converted into a plasma surrounding all or part of the edge of the AlN ingot 220. This nitrogen plasma significantly and preferentially increases the amount of nitrogen within the extended portion of the AlN ingot 220, increasing its lateral growth rate (and thus enhancing the diameter expansion of the AlN ingot 220).

[0100] In various embodiments, the electrode 410 may be operated during most of the growth of the AlN ingot 220 and the plasma may be uniformly formed. In other embodiments, the electrode 410 may be operated only during one, two, or more intervals during the growth, and there may be no plasma between these intervals. In another embodiment, the current applied to the electrode 410 may be changed one or more times during the growth, thereby increasing or decreasing the amount of plasma generated at a particular point in the growth process. In this way, the expansion rate of the AlN ingot 220 and its resulting shape may be affected by the presence or absence of plasma and / or the power level provided to the electrode 410.

[0101] Embodiments of the present invention can grow AlN single crystals with greater mass, volume, and / or expansion rate than conventional techniques. For example, embodiments of the present invention can form AlN single crystals with greater crystal enhancement parameters (CAP), where CAP in mm is defined as:

[0102]

[0103] Among them A E is the extended area (i.e., the cross-sectional area of ​​the portion of the crystal having the largest diameter 116 in FIGS. 1B and 1C ), in mm 2 , d E is the expanded diameter (i.e., the maximum diameter 116 in Figures 1B and 1C), in mm, A S is the seed crystal area (i.e., the cross-sectional area of ​​the crystal portion having the seed crystal diameter 110 in FIGS. 1B and 1C ), in mm 2 , d S is the seed diameter (i.e., the seed diameter 110 in FIGS. 1B and 1C , which may correspond to the minimum diameter of the crystal), in mm, and L E is the expansion length of the crystal (i.e., length 120 in Figures 1C and 1F), in mm. According to embodiments of the present invention, the CAP value provides a better measure of expansion (normalized to the crystal length) than the expansion angle 134 (see Figure 1F), because the expansion angle can vary during crystal growth and / or is difficult to measure.

[0104] Embodiments of the present invention are capable of growing AlN single crystals with CAPs that are unattainable with conventional techniques, at least in part due to faster diameter expansion during crystal growth. Despite the faster diameter expansion during crystal growth, embodiments of the present invention also maintain high crystal quality. Figure 5is a graph of an exemplary AlN single crystal 500 grown according to various embodiments of the present invention. The CAP value of the AlN single crystal described in the embodiments of the present invention is greater than 20, greater than 40, greater than 60, greater than 80, greater than 90, greater than 100, greater than 150, greater than 500, greater than 1000, or even greater than 1500 (herein, unless otherwise stated, all CAP values ​​are in mm), while the CAP calculated value of the AlN crystal prepared by conventional techniques and the AlN crystal reported in the literature is less than 20 (e.g., 10 to 15, or even less). Conventional growth techniques that cannot rapidly expand the diameter require longer growth (and the resulting greater expansion length and smaller CAP) to achieve a larger expansion area of ​​the AlN single crystal. Therefore, embodiments of the present invention facilitate faster and more economical preparation of large, high-quality AlN crystals (e.g., single crystal AlN wafers) from small seed crystals. For example, Figure 5 The CAP of the crystal 500 is 45, which shows that the embodiments of the present invention are superior to conventional technologies. According to various embodiments, the CAP of the AlN single crystal may be not greater than about 1600, or not greater than about 1700, or not greater than about 2000.

[0105] Table 1 below reports various CAP values ​​for various different crystals prepared by the present inventors, as well as various dimensional parameter ratios of the crystals (in %), as shown in Figures 1B-1D. In Table 1, crystals #1-#4 and #10-#15 have the shape of the crystal 108 shown in Figure 1B (i.e., without the "flat" portion 124), while crystal #5 has the shape of the crystal 108 shown in Figure 1C (i.e., with a longer frustum 112 and a corresponding extended length), and crystals #6-#9 have Figure 1D The shape of the crystal 108 is shown (ie, having a shorter extended length and a longer straight portion 124).

[0106]

[0107]

[0108] Table 1

[0109] Embodiments of the present invention can also prepare AlN single crystals with unusually large mass and / or volume compared to conventional AlN crystals. For example, the mass of the AlN single crystal ingot grown according to embodiments of the present invention may be greater than about 78g, greater than about 100g, greater than about 120g, or greater than about 140g, greater than about 220g, or even greater than about 240g. According to various embodiments, the mass may be less than about 350g, or less than about 300g. When a larger seed crystal is used, the AlN single crystal ingot grown according to embodiments of the present invention may have an even greater mass, such as greater than about 300g, greater than about 500g, greater than about 800g, greater than about 1000g, or even greater than about 1200g. According to various embodiments, the mass may be less than about 1500g, or less than about 1400g. Therefore, the mass range of the exemplary ingots described in embodiments of the present invention includes, but is not limited to, about 78g-about 1300g, about 78g-about 300g, and about 380g-about 1300g.

[0110] Accordingly (and assuming that the ingot density of AlN is constant at 3.255 g / cm 3 ), the volume of the AlN single crystal ingot grown according to the embodiment of the present invention is greater than about 24 cm 3 , greater than about 30cm 3 , greater than about 50cm 3 , greater than about 70cm 3 , greater than about 75cm 3 , or greater than approximately 80cm 3 According to various embodiments, the volume may be less than about 100 cm 3 , or less than about 90cm 3 When a larger seed crystal is used, the AlN single crystal ingot grown according to embodiments of the present invention may have an even larger volume, for example, greater than about 100 cm 3 , greater than about 200cm 3 , greater than about 300cm 3 , or even greater than about 350 cm 3 According to various embodiments, the volume may be less than about 500 cm 3 , or less than about 400cm 3 Therefore, the exemplary ingot volume range described in the embodiments of the present invention includes but is not limited to approximately 24 cm 3 – Approximately 400cm 3 , about 24cm 3 – Approximately 80cm 3 , and about 120cm 3 – Approximately 400cm 3 .

[0111] FIG6A shows the mass distribution (and standard deviation) of more than 1200 different AlN single crystal ingots grown according to embodiments of the present invention using seeds of 52 mm diameter or less. As shown, using these seeds, the mass of the ingots ranged from about 70 g to over about 250 g. FIG6B shows the calculated ingot volume distribution (and standard deviation) of more than 1200 different AlN single crystal ingots. In these example embodiments, the volume range is about 20 cm 3 Up to about 80cm 3 As described above, the mass and / or volume of the AlN single crystal ingot prepared according to the embodiment of the present invention is larger than that prepared using conventional techniques. When a larger seed crystal is used for growth according to the embodiment of the present invention, Figures 6A and 6B The values ​​reported in are scaled accordingly, so that Figures 6A and 6B The values ​​reported in should not be construed as limiting the embodiments of the present invention. As described in detail above, the present inventors achieved higher quality and volume ingots using larger seed crystals.

[0112] In various embodiments (and as shown, for example, in Table 1 above), the ratio of the ingot length (i.e., the total length 118 in FIGS. 1C and 1F) to the maximum diameter (i.e., the maximum crystal diameter 116 in FIGS. 1B and 1C) of the AlN single crystal ingots grown according to embodiments of the present invention is about 0.3 to about 0.7, or about 0.35 to about 0.66. In various embodiments, the ratio of the extended length (i.e., the extended length 120 in FIGS. 1C and 1F) to the maximum diameter (i.e., the maximum crystal diameter 116 in FIGS. 1B and 1C) of the AlN single crystal ingots grown according to embodiments of the present invention falls within one of two different ranges depending on the speed of the diameter expansion. For example, an AlN single crystal ingot having a smaller extended length (e.g., as shown in FIG. 1C ) grown according to embodiments of the present invention may have a smaller extended length (e.g., as shown in FIG. 1B ). Figure 1D For example, the ratio of the extended length to the total length is about 0.5% to about 5%, or about 1% to about 4% of the ingot) has a ratio of the extended length to the maximum diameter of about 0.002 to about 0.02, or about 0.003 to about 0.02, or about 0.003 to about 0.01. In another example, the AlN single crystal ingot with a larger extended length grown according to an embodiment of the present invention (for example, as shown in FIGS. 1B and 1C; for example, the ratio of the extended length to the total length is about 15% to about 80%, or about 30% to about 70% of the ingot) has a ratio of the extended length to the maximum diameter of about 0.08 to about 0.5, or about 0.1 to about 0.3, or about 0.15 to about 0.25.

[0113] The ratio of both ranges is lower than that previously obtained in the art and demonstrates the superiority of the AlN single crystal ingots grown according to embodiments of the present invention compared to those prepared using conventional techniques. For example, the ingots described in embodiments of the present invention are capable of producing a greater number of large diameter AlN single crystal wafers per total ingot length, i.e., the single crystal AlN is more advantageously distributed within the ingot, at least from the perspective of large wafer production. Therefore, compared to conventional crystals and their production techniques, crystals produced according to embodiments of the present invention are more economical and larger wafers can be produced therefrom.

[0114] According to embodiments of the present invention, the seed crystal diameter may be about 5 mm to about 100 mm, about 5 mm to about 52 mm, or about 52 mm to about 100 mm. The total length of the ingot may be about 18 mm to about 50 mm, about 18 mm to about 35 mm, or about 30 mm to about 50 mm. The maximum crystal diameter may be about 17 mm to about 120 mm, about 17 mm to about 65 mm, or about 65 mm to about 120 mm. These values ​​are exemplary and should not be construed as limiting embodiments of the present invention.

[0115] In addition, the single crystal AlN ingots prepared according to embodiments of the present invention have high crystal quality despite the high diameter expansion ratio used in their formation. For example, the threading dislocation density of the ingots prepared according to embodiments of the present invention is less than 10 5 cm -2 , or even less than 3×10 4 cm -2 , as confirmed by X-ray topography. Moreover, this low defect density is roughly the same in the peripheral extended region of the ingot as in the central part of the ingot.

[0116] After crystal growth, one or more substrates (or "wafers") may be separated from the AlN ingot 220 using, for example, a diamond ring saw or a wire saw. In one embodiment, the crystal orientation of the substrates 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 wafers have an Al-polar surface or an N-polar surface and may 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 substrates may be oriented within about 2° of the m-plane or a-plane direction (thus having a non-polar orientation) or may have a semi-polar orientation if the AlN ingot 220 is cut in different directions. The surfaces of these wafers are also processed as described in U.S. Patent 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 the properties of the AlN ingot 220 described herein. After the substrate is cut from the AlN ingot 220, one or more epitaxial semiconductor layers and / or one or more light emitting devices, such as ultraviolet light emitting diodes or lasers, may be fabricated on the substrate, for example, as described in U.S. Patent Nos. 8,080,833 and 9,437,430, both of which are incorporated herein by reference in their entirety.

[0117] The measured etch pit density (i.e., an etch measurement indicating defects such as threading dislocations intersecting the crystal surface) of an AlN bulk crystal (e.g., an ingot and / or wafer) prepared according to an embodiment of the present invention is approximately 5×10 3 cm -2 To about 1×10 4 cm -2 The threading edge dislocation density of the AlN crystal of the present invention is about 1×10 3 cm -2 To about 1×10 4 cm -2 , the threading screw dislocation density is about 1cm -2 Up to about 10cm -2 , for example, the total threading dislocation density is less than about 10 4 cm -2When measured by x-ray diffraction, the full width at half maximum (FWHM) value of the x-ray rocking curve (e.g., along (0002) and / or (10-12)) of the AlN crystal according to the embodiment of the present invention is less than 50 arc seconds (e.g., about 30 arc seconds to about 50 arc seconds, or about 40 arc seconds to about 50 arc seconds), or even less than 40 arc seconds (e.g., about 20 arc seconds to about 40 arc seconds, about 30 arc seconds to about 40 arc seconds, or about 20 arc seconds to about 35 arc seconds). As measured by secondary ion mass spectrometry (SIMS), the carbon concentration of the AlN single crystal of the embodiment of the present invention is about 1.8×10 16 cm -3 -5×10 17 cm -3 , and the oxygen concentration is about 1×10 17 cm -3 -7.9×10 17 cm -3 In various embodiments, the carbon concentration may be approximately 1.8×10 16 cm -3 Up to about 5×10 16 cm -3 The thermal conductivity of the AlN single crystal of the present embodiment can be greater than about 290 Watts per meter-Kelvin (W / m·K), which is provided by commercial suppliers such as NETZSCH Inc. of Exton, Pennsylvania. The thermal conductivity is measured according to the American Society for Testing and Materials (ASTM) standard E1461-13 (Standard Test Method for Thermal Diffusivity by Flash Method), the entire disclosure of which is incorporated herein by reference.

[0118] FIG7A is a schematic diagram of a UV LED 700 fabricated on an AlN substrate according to an embodiment of the present invention. As shown, the UV LED 700 has a set of layers epitaxially grown on an AlN substrate 705 and two top-side metal contacts 710, 715. Specifically, directly on the substrate is a 500 nm layer 720 of undoped (i.e., not intentionally doped) AlN, on which is a 500 nm thick n-doped (Si concentration of 2×10 18 cm -3 )Al 0.83 Ga 0.17 The bottom contact layer 725 is a multi-quantum well (MQW) layer 730 having five sets of 2 nm thick Al 0.78 Ga 0.22 N quantum well and 6nm thick Al 0.85 Ga 0.15 N barrier layer, all of which are undoped. Above the MQW layer 730 is a 10 nm thick layer of undoped Al 0.95 Ga0.05 The electron blocking layer is formed by N. On the electron blocking layer is an undoped graded layer 735, which has a thickness of 30nm and is composed of Al 0.95 Ga 0.05 N is gradually changed to GaN. Finally, a 10 nm thick p-doped layer (Mg concentration is 1×10 19 cm -3 ) GaN cap layer 740. A p-metal layer 710 is formed on the cap layer 740, and an n-metal layer 715 is formed on the bottom contact layer 725 (exposed by, for example, etching away the overlying structure). FIG. 7B is a plan view photograph of the UV LED 700 of FIG. 7A when emitting light at approximately 230 nm. Figures 7A and 7B The output power of the device under continuous wave (CW) operation at room temperature and a current of 20 mA is 20 μW to 500 μW. This output power indicates an external quantum efficiency of 0.02% to 0.5% in the wavelength range of 228 nm to 238 nm.

[0119] After forming the electrodes (e.g., contacts 710, 715), the resulting light emitting device can be electrically connected to a package, for example, as described in detail in U.S. Patent No. 9,293,670 (the '670 patent), filed on April 6, 2015, which is hereby incorporated by reference in its entirety. A lens can also be mounted on the device to transmit (and, in various embodiments, form) light emitted by the device. For example, a hard lens can be placed on the device, as described in the '670 patent or in U.S. Patent No. 8,962,359, filed on July 19, 2012, or in U.S. Patent No. 9,935,247, filed on July 23, 2015, which are hereby incorporated by reference in their entirety. After encapsulation, any remaining portion of the substrate can be removed.

[0120] According to the embodiments of the present invention, other methods of partially or completely removing the substrate may be used if necessary. For example, an etching method may be used, such as the electrochemical etching method described in U.S. Patent Application No. 16 / 161,320 filed on October 16, 2018, which is incorporated herein by reference in its entirety. In other embodiments, techniques such as those used in U.S. Patent Application No. 15 / 977,031 filed on May 11, 2018 may be used.

[0121] According to embodiments of the present invention, AlN crystals and wafers made therefrom can advantageously exhibit high levels of UV transparency even at deep UV wavelengths, such as described in U.S. Patent Application Serial No. 16 / 444,147 filed on June 18, 2019 (the '147 Application), the entire disclosure of which is incorporated herein by reference. For example, as described in detail below, embodiments of the present invention include techniques for controlling and reducing carbon content in source materials for growing AlN single crystals and enhancing UV-transparency through thermal treatment.

[0122] In various embodiments, polycrystalline AlN ceramics may be manufactured according to the technique described in U.S. Pat. No. 9,447,519 (the '519 patent), i.e., a "pellet-drop" technique, which uses high purity Al particles melted in the presence of nitrogen to form AlN polycrystalline ceramic material, the entire disclosure of which is incorporated herein by reference. In various embodiments, the ceramic is broken into fragments to facilitate removal of a majority of carbon therefrom. The ceramic may be broken, for example, by applying mechanical force. The inventors surprisingly found that most of the carbon present in the polycrystalline AlN ceramic is retained on smaller fragments and / or dust (e.g., particles having a large aggregate surface area and / or a diameter less than about 2 mm) formed by the crushing process, while larger fragments (e.g., fragments having a width, diameter, or other lateral dimension of 0.5 cm to 2 cm) have a smaller carbon concentration. In various embodiments, one or more screens may be used to separate the AlN ceramic fragments according to size, and / or compressed air or another fluid (e.g., nitrogen or an inert gas such as argon) may be applied to the fragments to minimize or reduce the content of dust or other particles thereon. For example, as reported in the '147 application, the entire disclosure of which is incorporated herein by reference, after crushing and separation, the carbon concentration of the larger fragments was about 5 ppm to about 60 ppm, with an average carbon concentration of about 26 ppm. In sharp contrast, the carbon concentration of the resulting powder and smaller fragments was about 108 ppm to about 1800 ppm, with an average carbon concentration of about 823 ppm.

[0123] Thus, according to various embodiments of the present invention, one or more larger fragments of the AlN polycrystalline ceramic, once separated from the smaller fragments and powder, can be used directly as source material for forming single crystal AlN (as described in detail above). In other embodiments, one or more (typically multiple) fragments are collected and placed in a crucible (e.g., a tungsten (W) container) for subsequent heat treatment. (While in preferred embodiments only the larger fragments of the polycrystalline AlN ceramic are heat treated, embodiments of the present invention do include heat treatment of the intact, unbroken ceramic.)

[0124] In various embodiments, an optional subsequent preparation stage includes annealing and densification of at least a portion of the polycrystalline AlN ceramic (e.g., one or more larger fragments thereof) to form a high-quality polycrystalline AlN source material. According to various embodiments of the present invention, the AlN ceramic (or a portion thereof) can be heated to a first temperature T1 of 1100°C to 2000°C and maintained at temperature T1 for a period of time t1, for example, 2 hours to 25 hours. Then, the ceramic (or a portion thereof) can be heated to a higher second temperature T2 (e.g., a temperature of 2000°C to 2250°C) and maintained at temperature T2 for a period of time t2, for example, 3 hours to 15 hours. During the thermal treatment, the ceramic (or a portion thereof) is annealed and densified to form a polycrystalline AlN source material, which can be used to subsequently form a single crystal AlN bulk crystal. Since the polycrystalline AlN source material is generally AlN that is approximately stoichiometric and has a low impurity concentration, it can be used to form AlN bulk crystals without further processing (e.g., no intermediate sublimation-condensation step is required).

[0125] In an alternative heat treatment according to an embodiment of the present invention, a longer temperature rise temperature T2 is used to replace the first annealing step at temperature T1. According to various embodiments of the present invention, the AlN ceramic (or a portion thereof) can be heated to T2 (e.g., a temperature of 2000°C to 2250°C) for a period of time t1 (e.g., 5 hours to 25 hours). Then, the ceramic (or a portion thereof) can be maintained at temperature T2 for a period of time t2, for example, 3 hours to 25 hours. During the thermal treatment, the ceramic (or a portion thereof) is annealed and densified to form a polycrystalline AlN source material, which can be used to subsequently form high-quality single-crystalline AlN bulk crystals. Since the polycrystalline AlN source material is generally AlN that is approximately stoichiometric and has a low impurity concentration, it can be used to form AlN bulk crystals without further processing (e.g., no intermediate sublimation-condensation step is required).

[0126] In various embodiments, the carbon concentration of the polycrystalline AlN source material (as measured by instrumental gas analysis (IGA)) is about 3.0×10 18 cm -3 to about 1.8×10 19 cm -3 , about 3.8×10 18 cm -3 To about 1.2×10 19 cm -3 , or even about 3.0×10 18 cm -3 To about 9.0×10 18 cm -3After the optional densification heat treatment, the density of the polycrystalline AlN source material (measured by pyrometry at room temperature) is approximately equal to the density of single-crystalline AlN, i.e., about 3.25 g / cm 3 Up to 3.26g / cm 3 In various embodiments, the measured density of the AlN ceramic without densification heat treatment may be lower, for example, about 2.95 g / cm 3 Up to about 3.20g / cm 3 In various embodiments, after the optional densification heat treatment, the polycrystalline AlN source material is generally amber in color and consists of relatively large (ie, average grain size of about 0.1 mm to about 5 mm) grains.

[0127] Back to Figure 2 According to an embodiment of the present invention, one or more internal components of the crystal growth apparatus 200 (e.g., the crucible 205, the susceptor 215, and / or the crucible holder 210) may be annealed prior to crystal growth and formation of the AlN ingot 220, which may advantageously reduce the concentration of carbon in the AlN ingot 220. In various embodiments, one or more internal components of the crystal growth apparatus 200 are annealed at a temperature, for example, about 1000° C. to about 1800° C. for about 5 hours to about 50 hours.

[0128] In various embodiments of the invention, the concentration of carbon in the AlN ingot 220 may be reduced by introducing one or more getter materials into the crucible 205 prior to or during the growth of the AlN ingot 220. The getter materials may be introduced as part of or all of one or more components of the crystal growth apparatus 200 (e.g., the crucible 205, a liner located within the crucible 205 and near its inner surface or walls, the susceptor 215, and / or the crucible holder 210), and / or the getter materials may be introduced as separate materials within the crystal growth apparatus 200. The getter materials may be located between the source material 230 and the growing AlN ingot 220 to, for example, absorb or adsorb contaminants, such as carbon, in vapor flowing toward the AlN ingot 220 (e.g., toward the seed crystal 235). In various embodiments, the getter materials are stable, have a melting point greater than the growth temperature (e.g., greater than about 2000° C.), and have a low vapor pressure to prevent the getter materials themselves from contaminating the growing AlN ingot 220. In various embodiments, the getter material has a eutectic melting point with AlN that is greater than the growth temperature (e.g., greater than about 2000° C.). According to embodiments of the present invention, examples of getter materials include boron (melting point about 2300° C.), iridium (melting point about 2410° C.), niobium (melting point about 2468° C.), molybdenum (melting point about 2617° C.), tantalum (melting point about 2996° C.), rhenium (melting point about 3180° C.), and / or tungsten (melting point about 3410° C.). In various embodiments, the getter material (or a component or portion thereof of the device 200) may include, consist essentially of, or consist of one or more non-tungsten materials having a melting temperature of at least about 2300° C.

[0129] After the AlN ingot 220 is grown, the AlN ingot 220 may be cooled to near room temperature for subsequent removal from the crystal growth apparatus 200. For example, the AlN ingot 220 may be cooled in a two-stage process as described in the '519 patent. However, in various embodiments of the present invention, because the thermal treatment detailed below eliminates the need for the two-stage process of the '519 patent, the AlN ingot 220 may simply be cooled down from the growth temperature in a single stage at any rate. In fact, in various embodiments of the present invention, the AlN ingot 220 is cooled from the growth temperature to near room temperature at a high rate (e.g., greater than 70°C / hour, greater than 80°C / hour, greater than 100°C / hour, greater than 150°C / hour, greater than 200°C / hour, greater than 250°C / hour, greater than 300°C / hour, greater than 400°C / hour, or even greater than 500°C / hour; in various embodiments, the rate may not exceed 2000°C / hour, 1500°C / hour, or 1000°C / hour) without any "controlled cooling" achieved by powering the heating elements of the crystal growth apparatus 200. In various embodiments of the present invention, a gas (e.g., nitrogen and / or an inert gas) is flowed within the crystal growth apparatus 200 at a high rate (e.g., approximately equal to or higher than any gas flow rate used during crystal growth) to cool the AlN ingot 220. For example, the gas flow rate used during crystal growth may be about 4 slm or less, about 3 slm or less, about 2 slm or less, or about 1 slm or less. The gas flow rate used during crystal growth may be about 0.1 slm or more, about 0.5 slm or more, about 1 slm or more, or about 2 slm or more. In various embodiments, the gas flow rate used during cooling may be about 5 slm or more, about 10 slm or more, about 15 slm or more, about 20 slm or more, or about 25 slm or more. The gas flow rate used during cooling may be about 30 slm or less, about 25 slm or less, about 20 slm or less, about 15 slm or less, or about 10 slm or less. In addition, in embodiments of the present invention, the crucible 205 (and therefore the AlN ingot 220 therein) may be moved to the edge of the hot zone formed by the heating element of the crystal growth apparatus 200, or above the hot zone, thereby cooling the AlN ingot 220 more quickly.

[0130] Advantageously, high cooling rates of the AlN ingot 220 minimize or eliminate the formation of cracks within the AlN ingot 220, particularly when the diameter of the AlN ingot 220 is about 50 mm or greater. However, as described in the '147 application, high cooling rates may also result in detrimentally high UV absorption of the AlN ingot 220 at one or more wavelengths (e.g., wavelengths around about 310 nm). Figure 3A shows the UV absorption spectrum of an exemplary AlN ingot 220 rapidly cooled from the growth temperature detailed in this application. For example, the UV absorption spectrum of the exemplary AlN ingot 220 rapidly cooled from the growth temperature may show an elevated peak at about 310 nm, which may impair the UV transparency of the crystal over a wide wavelength range, and a UV absorption coefficient greater than 20 cm in the entire wavelength range of 210 nm to 400 nm. -1 The UV absorption coefficient in the wavelength range of 210 nm to 380 nm may also be greater than about 30 cm -1 Therefore, according to various embodiments of the present invention, controlling various impurity concentrations (e.g., carbon) during the growth process and within the resulting AlN crystal may not be sufficient to achieve low UV absorption coefficients, especially at deep UV wavelengths (e.g., at 210 nm to 280 nm, 230 nm to 280 nm, or 210 nm to 250 nm).

[0131] After cooling to room temperature, the AlN ingot 220 or a portion thereof may be heat treated to further improve its UV transparency, particularly deep UV wavelength transparency. For example, as described in detail herein, one or more wafers may be separated from the AlN ingot 220, and the one or more wafers may be heat treated to improve UV transparency. The subsequent description refers to the heat treatment of the AlN ingot 220, but it should be understood that only one or more portions of the ingot (e.g., one or more wafers) may be heat treated, rather than the entire ingot. In addition, the heat treatment described in detail herein may be performed on a variety of different AlN crystals (e.g., AlN single crystals), even if they were not initially grown and cooled in the manner detailed herein, to improve UV absorption.

[0132] In various embodiments of the present invention, the AlN ingot 220 is annealed in a heating device (e.g., a furnace such as a resistance furnace or a radio frequency (RF) furnace) configured for substantially isothermal or quasi-isothermal heating. The interior of the furnace (at least in the heating zone or "hot" zone), as well as any hardware in the furnace (e.g., a platform or other support), may include, consist essentially of, or consist of one or more refractory materials (e.g., W or other refractory metals) having a melting point in excess of about 2800°C or even in excess of about 3000°C. In various embodiments, the interior of the furnace (at least in the heating zone or "hot" zone) and the hardware in the furnace (e.g., a platform or other support) may be carbon-free, carbon-based or carbon-containing materials, graphite, quartz, alumina, and / or molybdenum. Prior to placing the AlN ingot 220 in the furnace, the furnace may be baked at an elevated temperature to reduce or minimize the presence of any contaminants therein. For example, the furnace may be heated to about 2600°C under vacuum and maintained for a period of time, such as about 0.5 hours to about 2 hours. After the furnace cools, the AlN ingot 220 may be placed in the furnace, which may then be filled with nitrogen at a pressure of, for example, about 1 bar to about 2 bar. The AlN ingot 220 may be placed "loosely" (i.e., not attached, adhered, or fixed) on a platform within the furnace, which may include, consist essentially of, or consist of W or other refractory metals. In various embodiments, the loose placement of the AlN ingot 220 reduces or substantially eliminates stress caused by any thermal expansion differences between the AlN ingot 220 and the platform.

[0133] The temperature in the furnace can then be raised to the desired annealing temperature at a ramp rate of, for example, about 1°C / min to about 50°C / min. In various embodiments, the annealing temperature is about 2100°C to about 2500°C, for example, about 2400°C. In various embodiments, the annealing temperature is about 2150°C to about 2400°C. The inventors have found that lower annealing temperatures (e.g., about 2000°C) are generally insufficient to increase the ultraviolet transparency of the AlN ingot 220 at deep ultraviolet wavelengths to the desired level. Once the desired annealing temperature is reached, the AlN ingot 220 is annealed at the temperature for, for example, about 0.5 hours to about 100 hours, about 0.5 hours to about 5 hours, or about 1 hour. After annealing, the furnace temperature is slowly decreased to an intermediate temperature (e.g., about 800°C to about 1200°C, for example, about 1000°C) at a rate of about 60°C / hour to about 120°C / hour. For example, the furnace temperature can be cooled from an exemplary annealing temperature of 2200° C. to 1000° C. in 15 hours. This slow cooling can be achieved by controlled heating of the furnace (e.g., at a low power level). Thereafter, the furnace can be shut down and the furnace and the AlN ingot 220 can be allowed to cool to room temperature. Thus, in various embodiments of the present invention, the entire annealing cycle of the AlN ingot 220, including cooling, is performed under substantially isothermal or quasi-isothermal conditions.

[0134] Figure 8 8 is a graph comparing the conventional UV absorption spectrum 800 reported in the '519 patent and the UV absorption spectrum 810 of an AlN single crystal fabricated and annealed according to an embodiment of the present invention. As shown, the crystal of the embodiment of the present invention exhibits a low absorption coefficient over the entire wavelength range, and the spectrum is substantially constant (or "flat") between wavelengths of 210 nm and 280 nm. At about 230 nm, the absorption coefficient of the crystal of the embodiment of the present invention is less than 10 cm -1 (In the example shown, approximately 7 cm -1 –8cm -1 ), which is significantly lower than the result achieved by the '519 patent. In addition, the slope of the absorption coefficient as a function of near-band-edge wavelength is much steeper, as described in more detail below.

[0135] As described above, embodiments of the present invention include and are capable of producing single crystal AlN with a sharp drop in absorption coefficient near the band edge, i.e., AlN with a low Urbach energy. The "Urbach band tail" is the exponential portion of the absorption coefficient curve near the band edge, which is related to crystal disorder and localized electronic states extending into the band gap. The spectral dependence of the absorption coefficient (α) and photon energy (hν) is called the Urbach rule of thumb and is given by the following equation:

[0136]

[0137] (See Franz Urbach, "The Long-Wavelength Edge of Photographic Sensitivity and of the Electronic Absorption of Solids," Phys. Rev. 92 (1953) 1324, the entire disclosure of which is incorporated herein by reference.) α0 is a constant, E U is the Urbach energy, i.e. the energy with tail. The above formula can be rewritten as:

[0138]

[0139] The Urbach energy can be determined from the slope of the line when ln(α) is plotted as a function of the incident photon energy hν; on this plot, ln(α0) is the y-intercept of the line and therefore corresponds to ln(α) at ​​theoretical zero photon energy. Specifically, the Urbach energy is the inverse of the slope.

[0140] Fig. 9 Used to determine whether the '519 patent has Figure 8 The Urbach energy of the sample shown in the absorption spectrum 800 and the embodiment of the present invention has Figure 8 The Urbach energy of the sample having the absorption spectrum 810 shown. As shown in the figure, the slope of the curve 900 obtained for the sample of the present invention is much steeper (the slope is about 4.7 / eV), resulting in an Urbach energy of about 0.21eV in the photon energy range of 5.85eV to 6.00eV. In sharp contrast, the slope of the curve 910 of the sample corresponding to the absorption spectrum 800 is about 0.5 / eV, resulting in an Urbach energy of about 2.0eV. According to an embodiment of the present invention, the inventors have produced samples with an Urbach energy of about 0.2eV to about 1.8eV, for example, about 0.21eV to about 1.0eV, which is significantly lower than traditional samples and samples reported in the literature.

[0141] In general, the UV absorption spectrum (and the Urbach energy derived therefrom) can be determined by measuring the reflection of incident light on a sample using a spectrometer. For example, the UV absorption spectra of the samples of the embodiments of the present invention were measured using a V-670 (Class I) spectrometer and an XY stage from Jasco. 52 points of each sample were measured using a dual-axis stage controller from Chuo Precision Industrial Co., Ltd. The wavelength range of the measurement was 200nm to 800nm, but measurements up to 2000nm wavelength can be measured using this device. The absorption spectrum of a sample with a thickness of L is estimated based on the light incident on the sample and the light transmitted by the sample, taking into account the light reflected back from the two surfaces of the sample toward the light emission. The thickness L can be measured using, for example, a gauge (e.g., ACANTO, CERTO, METRO, or SPECTO length measuring instruments, and associated GAGE-CHEK evaluation electronics, available from Heidenhain Corporation of Schaumburg, IL) or an optical system (e.g., ULTRA-MAP 100B or ULTRA-MAP C200, available from MicroSense LLC of Lowell, MA). Fig.10 Summarizing this calculation, the absorption coefficient α at a specific wavelength λ of incident light can be calculated using the following formula:

[0142]

[0143] Among them I T is the intensity of the transmitted light, and I0 is the intensity of the incident light. The reflectivity R can be determined as follows:

[0144]

[0145] Among them, the refractive index n can be determined by the dispersion formula:

[0146]

[0147] The dispersion formula is provided by J. Pastrňák and L. Roskovcová, “Refraction index measurements on AlN single crystals,” Phys. Stat. Sol. 14, K5-K8 (1966), the entire contents of which are incorporated herein by reference.

[0148] The improved UV absorption spectra of embodiments of the present invention can enhance the performance of light emitting devices (e.g., lasers and light emitting diodes (LEDs)) fabricated on AlN substrates with improved spectra, particularly at short wavelengths. FIG. 11 is a graph of the emission intensity of an LED device versus wavelength for a simulated LED emitting at about 217 nm. The upper curve 1100 is a graph of the absorption spectra of an AlN substrate with improved spectra of embodiments of the present invention—in this example, having Figure 8 The emission intensity of an LED fabricated on a substrate having a UV absorption spectrum 810 as a function of wavelength is shown. The lower curve 1110 corresponds to the emission intensity of an LED fabricated on a substrate having a UV absorption spectrum 810 as a function of wavelength. Figure 8 The same LED structure fabricated on a substrate having an absorption spectrum 800 is shown. As shown in FIG. 11 , the emission intensity of the embodiment of the present invention is increased by nearly two times at the peak emission wavelength of about 217 nm and is higher across the entire wavelength range. Fig.12 is a graph of the same spectrum over a smaller wavelength range, where the relative intensity of the LEDs has been independently normalized to the same value to show that the intensity peaks of the devices of the various embodiments of the present invention are narrower. This narrower peak enables excellent LED performance. The device simulations shown in Figures 11 and 12 show that when a substrate thickness of 0.55 mm is used in the simulation, the emission power of the device of the embodiment of the present invention will increase by at least 1.6 times. This advantage will be greater for greater substrate thicknesses due to improved UV absorption. In addition, when a reflector is used to reflect the light emitted by the device in a preferred direction, the power of the device will increase each time the reflected light passes through the substrate. For example, when the reflected light passes through a substrate with an improved absorption spectrum three times, the improvement in the device emission power achieved by the embodiments of the present invention can be approximated as 2×(1.6) 3 , or nearly 8 times.

[0149] The present application describes the growth of bulk single crystals, primarily by a process commonly referred to as "sublimation" or "sublimation-condensation", in which a source vapor for preparing AlN, an AlN crystalline solid containing AlN, or other solid or liquid is generated at least in part when Al or N is preferably sublimated. However, the source vapor can be generated in whole or in part by injecting a source gas or a similar process that some people refer to as "high temperature CVD". In addition, other words are sometimes used to describe these contents and methods of growing bulk AlN single crystals according to embodiments of the present invention. Therefore, the words "deposition", "growth", "depositing gas phase components" and similar words used in this application generally cover those methods of growing crystals according to embodiments of the present invention.

[0150] The words and expressions used in this application are used as words of description rather than limitation, and when used, they do not exclude any equivalents of the features shown or described or parts thereof, but it should be recognized that various modifications are possible and still fall within the scope of the invention.

Claims

1. An AlN single crystal, the diameter of which increases from a minimum diameter of 50 mm to 100 mm to a maximum diameter along at least a portion of the length of the AlN single crystal, the AlN single crystal having a crystal enhancement parameter in mm greater than 100, the crystal enhancement parameter being defined as: Among them A E , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the maximum diameter; d E is the maximum diameter of the AlN single crystal, in mm; A S , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the minimum diameter; d S is the minimum diameter in mm; and L E It is the extension length of at least a portion of the AlN single crystal, in mm, along which the diameter increases from the minimum diameter to the maximum diameter. 2 . The AlN single crystal according to claim 1 , wherein the crystal enhancement parameter is greater than 500. The AlN single crystal according to claim 1 , wherein the crystal enhancement parameter is less than 2000. 4 . The AlN single crystal according to claim 1 , wherein a ratio of a total length in mm to a maximum diameter in mm of the AlN single crystal is 0.3 to 0.

6. 5 . The AlN single crystal according to claim 1 , wherein a ratio of an extended length in mm to a maximum diameter in mm of the AlN single crystal is 0.002 to 0.

02. 6 . The AlN single crystal according to claim 1 , wherein a ratio of an extended length in mm to a maximum diameter in mm of the AlN single crystal is 0.08 to 0.

5.

7. The AlN single crystal of claim 1, wherein (a) the first region of the AlN single crystal is shaped as a truncated cone, the maximum diameter of the truncated cone corresponds to the maximum diameter of the AlN single crystal, and the minimum diameter of the truncated cone corresponds to the minimum diameter of the AlN single crystal, and (b) the second region of the AlN single crystal is shaped as a dome or a cone extending from the first region, the maximum diameter of the dome or the cone corresponds to the maximum diameter of the aluminum nitride single crystal.

8. The AlN single crystal of claim 1, wherein (a) the first region of the AlN single crystal is shaped as a truncated cone, the maximum diameter of the truncated cone corresponds to the maximum diameter of the AlN single crystal, and the minimum diameter of the truncated cone corresponds to the minimum diameter of the AlN single crystal, (b) the second region of the AlN single crystal is shaped as a cylinder extending from the first region, the diameter of which corresponds to the maximum diameter of the AlN single crystal, and (c) the third region of the AlN single crystal is shaped as a dome or a cone extending from the second region, the maximum diameter of the dome or the cone corresponds to the maximum diameter of the aluminum nitride single crystal.

9. The AlN single crystal according to claim 1, wherein (i) the threading edge dislocation density of the AlN single crystal is less than 1×10 4 cm -2 (ii) the threading screw dislocation density of the AlN single crystal is less than 10 cm -2 and (iii) the full width at half maximum of the X-ray rocking curve of the AlN single crystal is less than 50 arc seconds.

10. The AlN single crystal according to claim 1, wherein the Urbach energy of the AlN single crystal in the incident photon energy range of 5.85 eV to 6.0 eV is 0.2 eV to 1.8 eV, and the Urbach energy E U The definition is as follows: where α is the absorption coefficient of the AlN single crystal at incident photon energy hv, and α0 is a constant corresponding to the absorption coefficient at zero photon energy. The AlN single crystal according to claim 10 , wherein the Urbach energy of the AlN single crystal is in the range of 0.21 eV to 1.0 eV.

12. The AlN single crystal according to claim 1, wherein the ultraviolet absorption coefficient of the AlN single crystal in the entire wavelength range of 220 nm to 280 nm is less than 10 cm -1 .

13. The AlN single crystal according to claim 12, wherein the ultraviolet absorption coefficient in the entire wavelength range of 220 nm to 280 nm is not less than 5 cm -1 .

14. An AlN single crystal having a diameter that increases from a minimum diameter to a maximum diameter along at least a portion of the length of the AlN single crystal, the AlN single crystal having (i) a crystal enhancement parameter in mm greater than 100, and (ii) a ratio of the extended length in mm to the maximum diameter in mm of the AlN single crystal of 0.002 to 0.03, the crystal enhancement parameter being defined as: Among them A E , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the maximum diameter; d E is the maximum diameter of the AlN single crystal, in mm; A S , in mm 2 The unit is the cross-sectional area of ​​the AlN single crystal at the minimum diameter; d S is the minimum diameter in mm; and L E It is the extension length of at least a portion of the AlN single crystal, in mm, along which the diameter increases from the minimum diameter to the maximum diameter. The AlN single crystal according to claim 14 , wherein the crystal enhancement parameter is greater than 500. The AlN single crystal according to claim 14 , wherein the crystal enhancement parameter is greater than 1000. The AlN single crystal according to claim 14 , wherein the crystal enhancement parameter is less than 2000.

18. The AlN single crystal of claim 14, wherein (a) the first region of the AlN single crystal is shaped as a truncated cone, the maximum diameter of the truncated cone corresponds to the maximum diameter of the AlN single crystal, and the minimum diameter of the truncated cone corresponds to the minimum diameter of the AlN single crystal, (b) the second region of the AlN single crystal is shaped as a cylinder extending from the first region, the diameter of which corresponds to the maximum diameter of the AlN single crystal, and (c) the third region of the AlN single crystal is shaped as a dome or a cone extending from the second region, the maximum diameter of the dome or the cone corresponds to the maximum diameter of the aluminum nitride single crystal.

19. The AlN single crystal according to claim 14, wherein the Urbach energy of the AlN single crystal in the incident photon energy range of 5.85 eV to 6.0 eV is 0.2 eV to 1.8 eV, and the Urbach energy E U The definition is as follows: where α is the absorption coefficient of the AlN single crystal at incident photon energy hv, and α0 is a constant corresponding to the absorption coefficient at zero photon energy.

20. The AlN single crystal according to claim 19, wherein the ultraviolet absorption coefficient of the AlN single crystal in the entire wavelength range of 220 nm to 280 nm is less than 10 cm -1 .

21. The AlN single crystal of claim 14, wherein a minimum diameter of the AlN single crystal is at least 50 mm.

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