Optical grade vanadium compensated 4H and 6H single crystals, as well as silicon carbide crystals and methods for production thereof

Abstract

The present application relates to optical grade vanadium compensated 4H single crystals and 6H single crystals, as well as silicon carbide crystals and methods for producing same, the SiC single crystals being used for transmitting light having a wavelength in the range of 420 nm to 4.5 [mu] m. The device may include a window, a lens, a prism, or a waveguide. A system includes a source for generating light having a wavelength in the range of 420 nm to 4.5 [mu] m and a device for receiving and transmitting the light, where the device includes a vanadium compensated high resistivity 6H polymorphic or 4H polymorphic SiC single crystal. The present disclosure also relates to crystals and methods for optical applications, the crystals comprising aluminum doped silicon carbide crystals having residual nitrogen and boron impurities, where the aluminum concentration is greater than the combined concentration of nitrogen and boron, and where the light absorption coefficient at wavelengths of about 400 nm to about 800 nm is less than about 0.4 cm-1.

Description

[0001] This application is a divisional application of the Chinese patent application titled "Optical Grade Vanadium-Compensated 4H and 6H Single Crystals, and Silicon Carbide Crystals and Methods for Producing the Same" with application number 202110232966.8. The patent application 202110232966.8 is an invention patent application filed with the Patent Office on March 2, 2021 in accordance with the Paris Convention.

[0002] Cross-reference to Related Applications

[0003] This application claims the priority of U.S. Provisional Patent Application No. 62 / 984,177 filed on March 2, 2020, and claims the benefit of U.S. Patent Application No. 17 / 029,746 filed on September 23, 2020. This application is a continuation of U.S. Patent Application No. 17 / 029,746. The entire disclosures of U.S. Provisional Patent Application No. 62 / 984,177 and U.S. Patent Application No. 17 / 029,746 are incorporated herein by reference in their entirety. Technical Field

[0004] The present disclosure relates to optical devices, optical transmission systems, compositions, and methods for preparing aluminum-doped silicon carbide crystals. Background Art

[0005] The present disclosure generally relates to optical devices for transmitting light energy or information, including but not limited to windows, lenses, prisms, and waveguides. The present disclosure also generally relates to systems and methods for transmitting light energy or information using optical devices.

[0006] The present disclosure also generally relates to silicon carbide crystals and methods for producing the same. More particularly but not exclusively, the present disclosure relates to silicon carbide crystals that can be suitable for optical applications and methods for producing the same. Summary of the Invention

[0007] The present disclosure relates to vanadium-compensated 4H and 6H polytypes of silicon carbide (SiC) single crystals that have low light absorption at wavelengths within their fundamental transparency range, particularly but not limited to wavelengths in the range of about 420 nm to about 4.5 μm. The SiC single crystals according to the present disclosure can be used in various optical applications, such as but not limited to optical windows, lenses, prisms, and waveguides operating in the visible and near-infrared (IR) spectral ranges.

[0008] The present disclosure also relates to an optical device, the optical device comprising: a vanadium-compensated high resistivity 6H polytype or 4H polytype SiC single crystal; wherein the SiC single crystal is configured to transmit light having a wavelength in the range of 420 nm to 4.5 μm. According to one aspect of the present disclosure, the optical device may include a window, a lens, a prism, or a waveguide for transmitting light having a wavelength in the range of 420 nm to 4.5 μm.

[0009] The present disclosure also relates to an optical transmission system, the optical transmission system including: a light source for generating light having a wavelength in the range of 420 nm to 4.5 μm; and an optical device for receiving and transmitting the light, wherein the optical device comprises a vanadium-compensated high resistivity 6H polytype or 4H polytype SiC single crystal.

[0010] In addition, in one embodiment, the composition comprises an aluminum-doped silicon carbide crystal having residual nitrogen and boron impurities. The silicon carbide crystal comprises aluminum in a concentration greater than the combined concentration of nitrogen and boron in the silicon carbide crystal, and the silicon carbide crystal has a light absorption coefficient of less than about 0.4 cm at wavelengths in the range of about 400 nm to about 800 nm. -1 。

[0011] In another embodiment, a method for preparing an aluminum-doped silicon carbide crystal includes providing a silicon carbide source material and a silicon carbide single crystal seed in a growth crucible. The method further includes providing a solid aluminum dopant source material containing a compound containing aluminum and oxygen in a container. The growth crucible is heated in such a manner that the container is positioned in the growth crucible: the manner is effective for generating silicon- and carbon-containing vapors from the silicon carbide source material in the growth crucible and aluminum-containing vapors from the solid aluminum dopant source material in the container, and effective for depositing the silicon- and carbon-containing vapors and the aluminum-containing vapors on the silicon carbide single crystal seed to grow the aluminum-doped silicon carbide crystal. The container comprises a first material resistant to damage by the aluminum dopant source and the aluminum-containing vapors, and a second material resistant to damage by the silicon- and carbon-containing vapors. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a top view of an example of a silicon carbide (SiC) wafer constructed in accordance with the present disclosure;

[0013] Figure 2 is Figure 1 a side view of the SiC wafer of

[0014] Figure 3 is a schematic diagram of an example of an optical transmission system constructed in accordance with the present disclosure;

[0015] Figure 4 is the transmission (T of certain vanadium-compensated 4H-SiC wafersmes ) Curve and reflection (R mes ) Curve (T mes 、 R mes as a function of wavelength);

[0016] Figure 5 is a plot of the transmission and reflection curves (T mes 、 R mes as a function of wavelength) for certain vanadium-compensated 6H-SiC wafers;

[0017] Figure 6 is a plot showing the optical absorption curves (absorption coefficient as a function of wavelength) for two wafers;

[0018] Figure 7 is a plot showing the transmission curves (transmittance as a function of wavelength) for six vanadium-compensated 6H-SiC wafers;

[0019] Figure 8 is a plot of the optical absorption (absorption coefficient as a function of wavelength) of a 4H-SiC wafer in the visible light range;

[0020] Figure 9 is a plot of the optical absorption (absorption coefficient as a function of wavelength) of a 6H-SiC wafer in the visible light range;

[0021] Figure 10 is a schematic diagram of a system for sublimation growth of aluminum-doped silicon carbide crystals;

[0022] Figure 11 is for Figure 10 a schematic diagram of a doping container in a system;

[0023] Figure 12 is a schematic diagram of the wavelength dispersion of the optical absorption (α, cm -1 ) measured for a 4H-SiC wafer; and

[0024] Figure 13 is a schematic diagram of the wavelength dispersion of the optical absorption (α, cm -1 ) measured for a 6H-SiC wafer.

[0025] In the drawings, the same reference numerals or other feature indicators are used to indicate the same or similar features. Detailed Description

[0026] Figure 1 and 2Shows an example of a silicon carbide (SiC) wafer 10 constructed in accordance with the present disclosure. The wafer 10 may have a face 12 and a cylindrical edge 14. As described in more detail below, the wafer 10 may be a vanadium-compensated, high-resistivity 4H hexagonal polytype or 6H hexagonal polytype SiC single crystal. Figure 3 Is a diagram of an example of an optical transmission system constructed in accordance with the present disclosure. The transmission system may include a light source 18 for generating light 20, an optical device 16 for receiving and transmitting the light 20, and a target 22 to which the light 20 is transmitted through the optical device 16.

[0027] The light 20 may have one or more wavelengths in the range of 420 nm to 4.5 μm. The optical device 16 may be fabricated from the wafer 10 and may be, for example, a window for transmitting the light 20 while providing a physical barrier between the light source 18 and the target 22, a lens for focusing or dispersing the light 20, a prism for separating spectral components of the light 20, or a waveguide for directing the light 20 to the target 22.

[0028] SiC single crystals are used in certain semiconductor devices, including high-power and high-frequency diodes and transistors, ultrafast semiconductor optical switches, and detectors operating in harsh environments. Hexagonal SiC crystals can be used as substrates for the epitaxial growth of SiC or gallium nitride (GaN) epitaxial layers. Nitrogen-doped n-type 4H-SiC crystals are used as substrates for epitaxial 4H-SiC power conversion diodes and transistors (e.g., MOSFETs). See U.S. Patent No. 8,507,986. High-resistivity (semi-insulating) 4H-SiC crystals and 6H-SiC crystals are used as substrates for epitaxial GaN-based high-frequency transistors (e.g., HEMTs). See U.S. Patent No. 9,484,284.

[0029] Large-size SiC single crystals can be grown from the gas phase by sublimation. See U.S. Patent No. 5,746,827. When preparing for growth, a SiC source, which may be in the form of SiC powder or grains, can be provided in the high-temperature region of a graphite crucible. A SiC seed, such as a single-crystal SiC plate or wafer, is positioned in the lower-temperature region of the crucible, e.g., attached to the crucible lid. The crucible is heated to sublimate the SiC source and fill the crucible with the sublimated gaseous product. The resulting vapor migrates to the cooler SiC seed and deposits on the seed to grow a SiC boule of appropriate size.

[0030] To meet certain requirements, dopants can be introduced into the growth system to change the electronic parameters of the grown SiC crystal, such as conductivity type and resistivity. Nitrogen doping can be used to fabricate n-type 4H-SiC single crystals with low resistivity. There are at least two types of compensated high-resistivity SiC single crystals, namely, vanadium-compensated semi-insulating (VCSI) SiC crystals fabricated using vanadium doping (see U.S. Patent No. 5,611,955), and high-purity semi-insulating (HPSI) SiC crystals (see U.S. Patent No. 7,601,441). The latter can be fabricated without doping and is compensated by introducing deep-level point defects.

[0031] The optical and spectroscopic properties of various crystalline SiC forms have been studied in the ultraviolet (UV), visible, and IR ranges. See Singh et al., Nonlinear Optical Properties of Hexagonal Silicon Carbide, Appl. Phys. Lett, Vol. 19, 2 (1971) 53-56. The region of fundamental optical transmission for 4H-SiC and 6H-SiC extends from the band edge cut-off in the visible to λ≈4.5 μm in the infrared, where the transparency is terminated by multi-phonon absorption bands (see, for example, Singh's Figure 1 ).

[0032] The mechanical, chemical, and thermal properties of SiC can be attractive for various optical applications. Desirable properties can include low density, high strength and hardness, high wear resistance, high thermal shock resistance, high thermal conductivity, and chemical stability. Polycrystalline 3C-SiC grown by chemical vapor deposition (CVD) has been studied as a potential material for mid-IR windows and domes operating in harsh environments. See Goela et al., Transparent SiC for mid-IR windows and domes, SPIE Vol. 2286 (1994) 46-59.

[0033] Hexagonal single-crystalline SiC has been explored as a material for optical waveguides. See Luan et al., Optical ridgewaveguides in 4H-SiC single crystal produced by combination of carbon ion irradiation and femtosecond laser ablation, Optical Materials Express, Vol. 4, No. 6 (2014) 1166-1171; Japanese Patent Document No. 6002106 (Silicon Carbide Optical Waveguide Element); and Chinese Patent Document No. 103472533 (A method for preparing erbium-doped silicon carbide optical waveguides by ion implantation).

[0034] The optical absorption of sublimation-grown 4H-SiC single crystals and 6H-SiC single crystals in the visible light (VIS) to infrared range has been studied. See Wellmann et al., Optical Quantitative Determination of Doping Levels and Their Distribution in SiC, Mat. Sci. Eng. B91-92 (2002) 75-78. These crystals were doped with N, B, and Al in amounts up to 1·10 18 cm -3 and had poor light transmission. The light transmission and reflectivity of 4H-SiC crystals and 6H-SiC crystals in the infrared have been studied. Cuia et al., Infrared Transmission and Reflectivity Measurements of 4H- and 6H-SiC Single Crystals, Mat. Sci. For. Volumes 821 to 823, Pages 265 to 268 (2015). The samples under study included pure (not intentionally doped), N-doped, B-doped, VCSI, and HPSI. In all these studies, N-doped SiC crystals and B-doped SiC crystals showed significant optical losses with increasing dopant concentration. Pure SiC crystals showed smaller losses, especially in the visible light range. The best IR optical transparency was measured on semi-insulating VCSI samples and HPSI samples.

[0035] 4H-SiC and 6H-SiC belong to the 6mm space group and are positive uniaxial crystals. The refractive index dependencies on wavelength, polarization (ordinary vs. extraordinary), and temperature of semi-insulating 4H-SiC single crystals and 6H-SiC single crystals grown by sublimation have been studied. See Xu et al., Temperature Dependence of Refractive Indices for 4H and 6H, J. Appl. Phys. 115, 113501 (2014) 1-4. For these measurements, prisms with low apex angles were fabricated from single crystals, where the prism edges were parallel to the c-axis. It was found that the refractive index values and their dispersions for 4H-SiC and 6H-SiC are virtually the same.

[0036] Although large-size semi-insulating SiC single crystals (both VCSI and HPSI) are commercially available, there are no known industrial applications of such crystals in the field of transmission optics. This may be due to the residual optical losses in hexagonal SiC, which have not been well understood or controlled yet. Various mechanisms that can cause light absorption within the fundamental transparency range of SiC have been discussed in the literature. These mechanisms include doping-induced bandgap narrowing, transitions involving dopants, impurities, or defects (see, for example, Atabaev et al., Spectral Dependence of Optical Absorption of 4H-SiC Doped with Boron and Aluminum, J. of Spectroscopy (2018) Article ID 8705658 Figure 1 ), intra-band transitions, and absorption by free carriers.

[0037] Figures 1 to 3 The wafer 10 and the optical device 16 shown as examples in can include vanadium-compensated 6H polytype or 4H polytype SiC single crystals, which have excellent light transmission in the wavelength range from approximately 420 nm in the visible light to approximately 4.5 μm in the near infrared. The SiC single crystals can be used as optical materials for applications in the field of transmission optics (such as, but not limited to, optical windows, lenses, prisms, and waveguides).

[0038] The SiC crystals described herein can be vanadium-compensated 4H-SiC crystals and 6H-SiC crystals grown by sublimation. During growth, as measured by secondary ion mass spectrometry (SIMS), the crystals can be doped (compensated) with vanadium to a level of 9·10 16 cm -3 to 1.5·10 17 cm -3 The concentration of shallow impurities (such as N, B, and Al) can be controlled to not exceed 3·1016 cm -3 Techniques for vanadium doping and shallow impurity control are described in U.S. Patent Nos. 7,608,524, 8,216,369, 8,361,227, 8,858,709, 9,017,629, and 9,090,989.

[0039] The SiC crystal forming the wafer 10 or the optical device 16 may have a high structural quality without dimensional crystal defects (such as inclusions or sub-grains), and a total dislocation density of less than 1·10 4 cm -2 . The overall crystal quality can be evaluated using X-ray rocking curve techniques. An X-ray reflection with an FWHM below 25 arcseconds can be a typical indication of high crystal quality.

[0040] The wafer 10 formed of a 4H-SiC single crystal or a 6H-SiC single crystal can be chemically mechanically polished (CMP), and has a diameter of 150 mm and a thickness of 0.5 mm. The wafer 10 can be oriented "coaxial", i.e., where its face 12 is perpendicular to the hexagonal c-axis. Slicing, grinding, and polishing of the wafer 10 can be carried out according to known manufacturing techniques.

[0041] Light transmission and reflection associated with the optical device 16 can be measured in the VIS-IR range using a Cary 7000 Universal Measurement Spectrophotometer (UMS). The mathematical forms for calculating the optical parameters according to the present disclosure can be as described below in connection with Equations (A1a) to (A7):

[0042] 4H-SiC and 6H-SiC are hexagonal uniaxial crystals. In such crystals, the optical axis coincides with the crystallographic hexagonal c-axis. When a light beam is incident perpendicular to the c-plane of such a crystal, its polarization is always perpendicular to the optical axis. That is, such a light beam is an ordinary beam, and its propagation is controlled by the ordinary refractive index n0. Using Equation (3) and Table 1 for n0 from Xu (cited above) and substituting T = 300 K, an expression for n0(λ) at room temperature can be obtained:

[0043] For 4H-SiC:

[0044] For 6H-SiC:

[0045] For a light beam incident perpendicular to the c-plane of a hexagonal uniaxial crystal, the reflectivity (R) is expressed as:

[0046]

[0047] The light transmission and reflection of a plate with parallel surfaces are calculated by considering multiple reflections at the front and back interfaces of the plate. This approximation yields the following equations for the measured transmission (T mes ) and reflection (R mes ). See Pankove J., Optical Processes in Semiconductors, Dover Publ. NY 1971, page 93; F. Soler, Multiple Reflections in an Approximately Parallel Plate, Opt. Comm. 139 (1997) 165 - 169.

[0048]

[0049] In equations (A3) and (A4), α is the absorption coefficient in cm -1 , and d is the plate thickness in cm. If the value of the reflectivity R is known, equation (A3) can be solved for α as follows:

[0050]

[0051] The transmission and reflection in the extreme case of a completely transparent plate can be obtained by substituting α = 0 into equations (A3) and (A4) as follows:

[0052]

[0053] The optical absorption coefficient of single - crystal SiC is calculated from the measured transmission (T mes ) and reflectivity (R) using equation (A5). The R value is calculated using equation (A2) from the refractive index dispersions (A1a) and (A1b) of 4H and 6H determined by Xu.

[0054] In addition to light transmission and reflection, the resistivity of single - crystal SiC is measured at room temperature using a non - contact instrument COREMA - W with a sensitivity of 1·10 5 Ωcm to 1·10 12 Ωcm. The temperature dependence of the resistivity in the temperature range from 25 °C to 400 °C is measured using a temperature - variable non - contact resistivity meter COREMA - VT, and the value of the activation energy of the conductivity (E A ) is calculated. In the case where the wafer resistivity exceeds 1·10 12 Ωcm, the room - temperature resistivity is evaluated by extrapolating the resistivity to T = 300 K using the E A value. The resistivity of all SiC crystals in this study is 1·10 6Ω cm to 1·10 14 Ω cm.

[0055] Transmission curves 50 (T mes @0° AOI) and reflection curves 52 (R mes @6° AOI) measured in the visible range of 0.35 μm to 0.80 μm on several high-resistivity vanadium-compensated 4H-SiC wafers are shown in Figure 4 In. Transmission curves 54 (T mes @0° AOI) and reflection curves 56 (R mes @6° AOI) measured in the visible range of 0.35 μm to 0.80 μm on several high-resistivity vanadium-compensated 6H-SiC wafers are shown in Figure 5 In.

[0056] Examples of the absorption spectra α(λ) calculated using Equation (A5) in the visible range for two wafers (one high-resistivity 4H-SiC (line 60) and one high-resistivity 6H-SiC (line 58)) are shown in Figure 6 In. These absorption curves 60, 58 show two different absorption regions: a steep rise in absorption at wavelengths below about 0.40 μm for the 4H crystal and a steep rise in absorption at wavelengths below about 0.44 μm for the 6H crystal. This increased absorption is thought to be due to the fundamental cutoff, i.e., the transition from the valence band to the conduction band. Due to the indirect nature of the hexagonal SiC bandgap and phonon-assisted electron transitions, its slope is proportional to λ -2 See Sridhara et al., Absorption Coefficient of 4H Silicon Carbide from 3900 to 3250 Å, J. Appl. Phys. Vol. 84, No. 5 (1998) 2963 - 2964.

[0057] As Figure 6 shown in, there is a near-band-edge absorption “shoulder” between 0.45 μm and 0.7 μm, where its amplitude is about 0.01 cm at λ ≈ 0.7 μm -1 to about 1 cm at λ ≈ 0.45 μm -1 . This residual absorption is typically observed in wide-bandgap semiconductors and is usually attributed to electronic transitions involving unspecified shallow energy levels such as crystal defects or impurities. The optical absorption at wavelengths beyond λ ≈ 0.7 μm is very low, below 0.1 cm -1 .

[0058] Although all vanadium-compensated SiC crystals were optically transmissive in this study, their transmittances showed significant variations. An example of such a variation is shown in Figure 7 In,Figure 7 shows the optical transmission in the VIS-NIR range measured on six different vanadium-compensated 6H-SiC wafers. The two curves 66 with the lowest transmission were measured on two Nu-type 6H wafers. (The Nu-type refers to the type of compensated SiC crystal in which the shallow donor (nitrogen) predominates over the shallow acceptor (Al + B).) The resistivity of the Nu-type wafers is in the range of 9·10 9 Ωcm to 2·10 11 Ωcm.

[0059] Figure 7 The two curves 62 with the highest transmission in 6 show two Pi-type wafers. (The Pi-type means the type of compensated SiC crystal in which the shallow acceptor (in this case Al) predominates over the shallow donor (N).) The resistivity of these wafers is 1·10 7 Ωcm to 1·10

[0060] Figure 7 Ωcm. 12 The two curves 64 in 14 show the optical transmission of two Pi-type 6H-SiC wafers containing boron (shallow acceptor) at a concentration exceeding that of N (shallow donor). The resistivity of these wafers is 1·10

[0061] The optical absorption in the visible range calculated using Equation (A5) for several 4H-SiC wafers and 6H-SiC wafers with different doping and resistivity is shown in Figure 8 and Figure 9 .

[0062] The strongest absorption, shown by the lines 68 and 76 in Figure 8 and Figure 9 , is that of the Nu-type wafers with nitrogen predominating in the shallow impurity background. The resistivity of the 4H wafers is 1·10 11 Ωcm, while that of the 6H wafers is 1·10 9 Ωcm. For these wafers, the optical absorption is not only the highest in the visible range but also the highest in the infrared.

[0063] The slightly lower optical absorption shown by the lines 70 and 77 is also that of the Nu-type wafers. However, these wafers have a higher resistivity of 5·10 12 Ωcm for 4H and 5·10 10 Ωcm for 6H.

[0064] Figure 8 and Figure 9The lines 72, 74, and 78 in 13 show the optical absorption measured on Pi-type wafers with a boron preponderance in a shallow impurity background. The resistivity of the 4H wafers is 1·10 14 Ω cm (line 72) and 1·10 11 Ω cm (line 78). The resistivity of the 6H wafers is 1·10

[0065] Ω cm (line 78). The absorption in these wafers is confined to the band-edge shoulders and does not extend into the infrared. Figure 9 The lowest optical absorption is shown by the curves 80 and 82 in 5 which were measured on two Pi-type 6H-SiC wafers with an Al preponderance in a shallow impurity background. The wafers have relatively low resistivities of 1·10 7 Ω cm and 1·10

[0066] The following Table 1 gives the relationship between the optical absorption, its type, doping, and resistivity of vanadium-compensated SiC crystals. The optical absorption coefficient determined in the infrared light at λ = 2.5 μm is added in the last column.

[0067] Table 1

[0068]

[0069]

[0070] The data obtained can be used as a guide for selecting certain types of vanadium-compensated high-resistivity SiC single crystals for applications in the field of transmission optics for the visible and near-infrared spectral ranges.

[0071] Both vanadium-compensated high-resistivity 6H polytype and 4H polytype SiC single crystals are suitable for demanding applications in the field of transmission optics in the wavelength range from 420 nm in the visible to approximately 4.5 μm in the infrared, especially if their doping and resistivity have been optimized.

[0072] Due to the wider bandgap, vanadium-compensated 4H-SiC single crystals are preferred for optical applications at shorter wavelengths below 450 nm.

[0073] If the resistivity of vanadium-compensated Nu-type SiC single crystals is below 1·10 10 Ω cm for 6H and below 1·10 12 Ω cm for 4H, they cannot be used for demanding optical applications. The optical losses of such crystals in the visible range will be close to 1 cm -1 , and the optical losses in the infrared will be as high as 0.1 cm -1 .

[0074] If the resistivity of vanadium-compensated Nu-type SiC single crystals is greater than 1·10 11 Ω cm for 6H and greater than 5·10 12 Ω cm for 4H, then it can be used for infrared optical applications. For such crystals, the optical absorption in the infrared will be lower than 0.01 cm -1 .

[0075] If the resistivity of vanadium-compensated Pi-type SiC single crystals under the predominance of boron in a shallow impurity background is greater than 1·10 11 Ω cm for 6H and greater than 1·10 13 Ω cm for 4H, then it can be used for infrared optical applications. For such crystals, the optical absorption in the infrared will be lower than 0.01 cm -1 .

[0076] If the resistivity of vanadium-compensated Pi-type SiC single crystals under the predominance of boron in a shallow impurity background is greater than 1·10 13 Ω cm for 4H and greater than 5·10 11 Ω cm for 6H, then it can be used for optical applications in the visible range. For such crystals, the optical absorption should be lower than 0.8 cm -1 at λ = 450 nm and lower than 0.01 cm -1 at λ = 750 nm.

[0077] If the resistivity of vanadium-compensated Pi-type 6H-SiC single crystals under the predominance of aluminum in a shallow impurity background is from 1·10 5 Ω cm to 1·10 8 Ω cm, then it can be used for demanding optical applications. Such crystals can provide optimal transmittance in the visible range. Their optical absorption should be lower than 0.8 cm -1 at λ = 450 nm and lower than 0.01 cm -1 at λ = 750 nm. Compared with all other crystal types, their near-band-edge absorption shoulders are lower and narrower.

[0078] In addition, large, industrially sized single crystals of silicon carbide (SiC) can be grown from the gas phase by a sublimation technique commonly known as physical vapor transport (PVT). In this technique, a source of silicon carbide, which can be in the form of silicon carbide powder or grains, can be provided in the high-temperature region of a crucible. A silicon carbide seed, such as a 4H polytype or 6H polytype single crystal silicon carbide plate or wafer, can be positioned in the lower-temperature region of the crucible. The crucible can be heated to sublimate the silicon carbide source and fill the crucible with the sublimated gaseous products. The resulting vapor migrates to the cooler silicon carbide seed and deposits on the seed to grow a silicon carbide pear-shaped crystal of the desired polytype, diameter, and thickness.

[0079] During sublimation growth, silicon carbide crystals can be exposed to various impurities present in the growth system, and such exposure can lead to the formation of an impurity background in the crystal. Background impurities found in silicon carbide crystals include boron and nitrogen, which in some cases may be present at levels as high as n·10 16 cm -3 . In silicon carbide crystals grown by sublimation, graphite can be a source of boron and nitrogen background impurities.

[0080] The optical properties of bulk silicon carbide single crystals grown by sublimation have been studied. The overall optical absorption properties of pure, undoped or otherwise not deliberately doped hexagonal silicon carbide crystals are described in Singh et al., “Nonlinear optical properties of hexagonal silicon carbide”, Appl. Phys. Lett, Vol. 19, 2 (1971) 53 - 56. This description shows that the fundamental transparency region of silicon carbide crystals of this nature extends from the band edge cut-off value (about 380 nm for 4H and about 410 nm for 6H) to λ≈4 μm in the infrared, where it terminates with a multi-phonon absorption band. The magnitude of the near-band edge absorption shoulder is about 1 cm -1 , which is typical for undoped 4H-SiC and 6H-SiC crystals.

[0081] As discussed in Scajev et al., “Application of a Time-Resolved Four-Wave Mixing Technique for the Determination of Thermal Properties of 4H–SiC Crystals.” J. Phys. D Appl. Phys. 42(5):055413, Feb. 2009 and Tarekegne et al., “Investigation of the Absorption Mechanisms of SiC for Lighting Applications”, 6th International Workshop on Wideband Semiconductor Materials & Devices, Fujian, China, 2018, silicon carbide crystals grown by sublimation that have been heavily doped with boron, nitrogen, and aluminum to 10 18 cm -3Optical properties of 4H and 6H silicon carbide single crystals or higher. Each of these impurities produces a specific near-band-edge absorption band in the visible light range. The peak of the optical absorption band related to boron is at about 430 nm to 480 nm for 4H-SiC and at about 450 nm to 510 nm for 6H-SiC. The absorption shoulder related to aluminum is at about 410 nm to 420 nm and is very close to the band-edge cut-off value. The absorption band related to nitrogen is at about 460 nm to 470 nm in 4H and at about 620 nm to 630 nm in 6H..

[0082] Although these crystals have been observed to have some optical transparency, it has been found that their optical applications are very limited, mainly due to optical absorption that may be caused by residual impurities. One way to minimize near-band-edge absorption in sublimation-grown 4H polytype silicon carbide crystals and 6H polytype silicon carbide crystals is to reduce the presence of unwanted background impurities. However, considering the high temperature of the PVT growth process and the use of graphite as the crucible material, reducing the background impurities of boron and nitrogen in PVT-grown silicon carbide single crystals to below 1·10 15 cm -3 level can be considered at least challenging, if not impractical.

[0083] According to the present disclosure, 4H polytype or 6H polytype silicon carbide crystals can have high optical transparency in the visible spectral range. Silicon carbide crystals exhibiting these properties or similar properties can be used in various optical applications, such as but not limited to optical windows, lenses, and optical waveguides operating at wavelengths from about 410 nm to about 750 nm. The sublimation physical vapor transport (PVT) technique can be used to grow optically transparent 4H silicon carbide crystals and 6H silicon carbide crystals. In one form, the silicon carbide single crystal is doped with aluminum and contains residual boron and nitrogen impurities. On the one hand, the aluminum dopant is present at a concentration exceeding the combined concentration of the residual nitrogen and boron impurities, otherwise below the level that may cause crystal growth instability. Although not intended to be bound by any particular theory, it is believed that the aluminum dopant reduces the magnitude of the near-band-edge optical absorption related to the presence of boron and nitrogen impurities. Similarly, maintaining the lowest possible concentration of background boron and nitrogen impurities can also reduce optical absorption.

[0084] As described above, sublimation techniques can be used to grow single crystals of silicon carbide, including 4H polytype and 6H polytype single crystals of silicon carbide. On the one hand, these techniques can be designed to reduce or remove boron and nitrogen impurities in or from the growth system. For example, the presence of background boron can be reduced by using halogen-purified graphite components (including but not limited to growth crucibles). In one form, the halogen purification of the graphite component can include heating the graphite component in a furnace chamber having an atmosphere containing chlorine, fluorine, or both chlorine and fluorine. At high temperatures, boron bound to carbon in the graphite reacts with gaseous halogens such as chlorine and fluorine and forms volatile halides such as BCl3 and BF3. The volatile halide can be removed from the furnace chamber by an inert gas flow through the furnace chamber. After this purification, the graphite component may still contain residual boron at a concentration of about 10 weight ppb to about 100 weight ppb. Similarly, silicon carbide crystals grown by sublimation using these purified graphite components may still contain about 10 15 cm -3 to 10 16 cm -3 of residual boron impurities.

[0085] Reducing or removing nitrogen impurities in or from the growth system can include pre-growth high-temperature baking of the crystal growth hot zone in a vacuum. Additionally, sublimation crystal growth can be carried out under a flow of ultra-high purity argon. Silicon carbide crystals grown under these conditions may still contain a concentration of about 10 15 cm -3 to 10 16 cm -3 of residual nitrogen impurities.

[0086] Considering that even after steps are taken to reduce or remove impurities, some residual boron and nitrogen impurities may remain, and near-band-edge optical absorption may still exist due to their presence. However, as described above, the addition of an aluminum dopant can be used to reduce the near-band-edge optical absorption caused by the presence of boron and nitrogen impurities. Similar to boron, aluminum is a shallow acceptor in 4H-SiC and 6H-SiC. In the SiC interstitial, the energy level of aluminum is located approximately 0.2 eV above the valence band maximum (see, for example, Atabaev et al., “Spectral Dependence of Optical Absorption of 4H-SiC Doped with Boron and Aluminum”, J. Spectroscopy (2018) Article ID 8705658 and the online NSM Archive - Silicon Carbide (SiC) - Impurities and Defects). The lowest energy level of the boron acceptor is approximately 0.35 eV above the valence band edge and is higher than the energy level of aluminum. As an acceptor, aluminum can combine with boron that compensates for nitrogen (a donor). If aluminum is introduced at a sufficient concentration such that N Al +N B >N N , then all electrons will be stripped from the nitrogen energy level and thus there will be no electrons at the bottom of the conduction band. Therefore, nitrogen-related optical absorption can be reduced or eliminated.

[0087] Doping with aluminum causes a redistribution of charge in the silicon carbide crystal, which will be manifested by a shift in the position of the Fermi level closer to the aluminum energy level. If aluminum is introduced at a sufficient concentration such that N Al >N B +N N , then the Fermi level will move to a position below the lowest energy level of boron, thereby reducing or eliminating the population of electrons present at the boron energy level. Therefore, boron-related optical absorption can be reduced or eliminated.

[0088] Considering the above, in order to “reduce” both boron acceptors and nitrogen donors from electrons, as shown in Equation (1), the concentration of the aluminum dopant (N Al ) exceeds the combined concentration of the residual boron (N B ) and nitrogen (N N ):

[0089] N Al >N B +N N (1)

[0090] Doping with aluminum may cause aluminum-related optical absorption, but any such absorption band will be very close to the band-edge cut-off value and will actually merge with it.

[0091] In aluminum-doped 4H and 6H polytypes of silicon carbide, aluminum substitutes for silicon. The covalent radius of aluminum is while the covalent radius of silicon is Considering the larger radius of aluminum, the dissolution of aluminum in the silicon carbide crystal may cause local expansion and stress in the lattice. At a specific concentration of aluminum, crystal defects, misoriented grains, and foreign polytypes may occur during the growth of the silicon carbide crystal. By maintaining the aluminum concentration below a specific level (e.g., about 5·10 17 cm -3 ), these problems can be avoided, although variations are possible.

[0092] Although aluminum-doped silicon carbide single crystals can be grown by directly adding aluminum carbide (Al4C3) to a silicon carbide solid source, Al4C3 is unstable at high temperatures and may undergo decomposition into solid carbon and Al+C liquid through a peritectic reaction, where the partial pressure of gaseous aluminum over the liquid exceeds 100 torr. Thus, there may be an initial peak in the aluminum concentration in the crystal followed by rapid consumption of the aluminum source.

[0093] U.S. Patent No. 8,216,369, the contents of which are incorporated herein by reference in their entirety, relates to more spatially uniform doping in a PVT process. For example, it discloses a growth method that includes a dopant-release container loaded with a dopant and housed inside a growth crucible. Referring to Figure 10 of the present application, a system 110 for the sublimation growth of an aluminum-doped silicon carbide crystal having a more spatially uniform aluminum concentration throughout the crystal is schematically shown. System 110 includes a chamber 112 in which a growth crucible 114 is positioned.

[0094] Crucible 114 is sealed by a lid 116 and surrounded by a thermal insulator 118. A heating element 120 is positioned around chamber 112 and configured to provide heat to crucible 114. Heating element 120 can be an RF coil or a resistive heating element, although other variations are possible.

[0095] Within crucible 114, SiC source 122 and SiC seed 124 are arranged in a spatial relationship suitable for PVT crystal growth. More specifically, SiC seed 124 may be positioned near the top of crucible 114 and may be attached or coupled to lid 116, for example, and SiC source 122 may be positioned below SiC seed 124. SiC source 122 is contained within crucible 126 supported by support structure 128. Support structure 128 includes first member 130 spaced apart from second member 132, and first member 130 and second member 132 extend from the lower portion of crucible 114 toward lid 116. In this arrangement, crucible 126, and thus SiC source 122 positioned within crucible 126, is positioned above the lower portion of crucible 114 and free space 134 is positioned therebetween. First member 130 includes aperture 136 and second member 132 includes aperture 138. Apertures 136, 138 provide fluid communication between free space 134 and the space positioned between crucible 126 and crucible 114, as Figure 10 indicated by direction arrow A.

[0096] When crucible 114 is heated by heating element 120 during sublimation growth of the crystal, SiC source 122 vaporizes and fills the interior of crucible 114 with Si- and C-containing vapor that flows toward SiC seed 124 as indicated by direction arrow B. The Si- and C-containing vapor may include volatile molecular species such as Si, Si2C, and SiC2. A vertical temperature gradient may be established across crucible 114, where the temperature of the lower portion of crucible 114 is higher than the temperature of the upper portion of crucible 114 (e.g., near lid 116). A vertical temperature gradient of this nature creates a driving force for vapor transport as indicated by direction arrow B. This vapor transport conveys vapor nutrients from SiC source 122 to SiC seed 124. Once reaching SiC seed 124, the supersaturated vapor deposits on SiC seed 124, thereby causing growth of SiC single crystal 140 on SiC seed 124.

[0097] The slow-release doping container 142 that can be used for aluminum doping is positioned in the free space 134 in the crucible 114. The doping container 142 includes an internal space or chamber for receiving and holding the dopant source 144. In one form, the dopant source 144 is an aluminum dopant source and can be in the form of a solid aluminum compound having a low decomposition pressure at the SiC sublimation growth temperature. For example, in one form, the decomposition pressure can be less than about 1 torr, although other variations are possible. Non-limiting examples of solid aluminum compounds that can be used include aluminum compounds containing oxygen, such as but not limited to aluminum carbonate, aluminum silicate, and aluminum oxide. In a specific but non-limiting form, the solid aluminum compound is aluminum oxide (Al2O3), which is also known as alumina. The aluminum oxide can be provided in the form of pre-melted alumina "cracks", such as pure, undoped sapphire wafers. Generally, the amount of the dopant source 144 contained in the doping container 142 can be sufficient to support aluminum doping throughout the crystal growth cycle.

[0098] During the crystal growth cycle, the crucible 114 is heated and as the temperature of the crucible 114 rises up to about 2000 °C, the aluminum compound containing oxygen undergoes a chemical transformation that ultimately produces solid aluminum oxide. As the temperature of the crucible 114 further rises and reaches and exceeds the melting point of aluminum oxide of about 2040 °C, the aluminum oxide melts and fills the interior of the doping container 142 with vapor that is a product of the evaporation and decomposition of the aluminum oxide melt. The vapor can contain molecules of Al2O3, Al2O, AlO, Al, and O. At the melting point of aluminum oxide, the total vapor pressure above the melt is about 0.01 torr, while at 2200 °C, it is less than 0.1 torr. Similarly, if the temperature of the SiC sublimation growth process does not exceed about 2400 °C, the vapor pressure of the molten aluminum oxide should not exceed about 1 torr.

[0099] The doping container 142 includes capillary channels, further details of which will be discussed below in conjunction with Figure 11 The aluminum- and oxygen-containing vapor escapes from the doping container 142 through the capillary channels and migrates within the crucible 114, as shown by the direction arrow C in Figure 10 During the migration process, these vapors can contact and react with the carbon of the graphite to form elemental aluminum vapor and carbon monoxide gas. Eventually, the aluminum vapor can reach the silicon carbide crystal growth interface and adsorb on the interface, thereby causing doping of the growing silicon carbide crystal with aluminum.

[0100] The doping container 142 can be formed of one or more such materials that provide stability in the high-temperature environment of SiC sublimation growth and resist the erosion of molten aluminum oxide and highly corrosive silicon- and carbon-containing vapors. For example, referring to Figure 11, in a non - limiting form, the doping container 142 is formed by an external component 146 and an internal component 148, with the external component 146 formed on or around the internal component 148. The external component 146 can be at least partially formed of a first material, and the internal component 148 can be at least partially formed of a second different material.

[0101] In Figure 11 the form shown, the external component 146 is in the form of a crucible 150 including a lid 152. Since the crucible 150 and the lid 152 can be positioned in the free space 134 and thus can be exposed to the silicon - and carbon - containing vapors released by the SiC source 122, the crucible 150 and its lid 152 can be at least partially formed of a material that is stable against corrosion by the silicon - and carbon - containing vapors that may be present in the crucible 114. For example, the outer surface that is at least likely to be in contact with the silicon - and carbon - containing vapors can be formed of a material that is stable against corrosion by the silicon - and carbon - containing vapors. Non - limiting examples of materials that can provide such stability include graphite, carbides of refractory metals such as tantalum carbide (TaC) or niobium carbide (NbC), and graphite coated with or laminated with refractory carbides. In a specific but non - limiting form, the crucible 150 and its lid 152 can be formed of dense fine - grained graphite.

[0102] In Figure 11 the form shown, the internal component 148 is in the form of a crucible 154 including a lid 156. For example, the lid 152 of the crucible 150 can be removed to facilitate the positioning of the crucible 154 and its lid 156 within the crucible 150, and the lid 152 can be re - engaged with the crucible 150 to fix the crucible 154 and the lid 156 within the crucible 150. Since the crucible 154 and the lid 156 include an internal space or chamber for receiving and holding the dopant source 144 (e.g., which can be an aluminum dopant source) and can thus be exposed to molten aluminum oxide as well as to aluminum - containing vapors released by the dopant source 144, the crucible 154 and its lid 156 can be at least partially formed of a material that is stable against corrosion or damage by molten aluminum oxide or aluminum - containing vapors. For example, at least the inner surfaces of the crucible 154 and the lid 156 that face the dopant source 144 can be formed of a material that is stable or resistant to corrosion or damage by molten aluminum oxide or aluminum - containing vapors. Non - limiting examples of materials that can provide such stability include refractory metals or metal alloys such as, but not limited to, tantalum, molybdenum, tungsten, rhenium, or their alloys. In a specific but non - limiting form, the crucible 154 and its lid 156 can be formed of tungsten.

[0103] Although the doping container 142 has been described as being formed from separate internal and external components made of different materials, it should be understood that the doping container 142 can also be made of a single component having a composite structure formed of different materials. For example, different materials can be present in separate layers, where a layer of a first material is deposited on a second layer formed of a separate material. For example, in the case of the crucible 154 and the lid 156, these components can be formed of a first material and then a second material can be coated or fixed on their outer surfaces to provide a structure similar to the Figure 11 structure shown, although other variations are possible.

[0104] As described above, the doping container 142 includes capillary channels 158 through which vapors containing aluminum and oxygen escape from the doping container 142 when the aluminum oxide melts. The capillary channels 158 are formed by a hole 160 through the lid 152 and a hole 162 through the lid 156. The rate at which the vapors containing aluminum and oxygen can escape or be released from the doping container 142 can be controlled at least in part by the temperature at which the aluminum oxide melts and the sizes of the holes 160, 162. For example, when the temperature of the molten aluminum oxide is kept high and the sizes of the holes 160, 162 are large, a higher rate of escape or release of the vapors containing aluminum and oxygen from the doping container 142 can occur. In some forms, the sizes of the holes 160, 162 can be the same or different, with the diameter of one or both being in the range of about 0.1 mm to about 1.5 mm.

[0105] Using the doping container 142 can, for example, facilitate the production of single-crystalline silicon carbide containing an aluminum dopant with a spatially uniform concentration throughout the crystal by sublimation growth. The above techniques can also similarly avoid a situation where a high aluminum concentration exists in the initially grown pear-shaped crystal portion and then rapidly depletes the aluminum source. Additionally, the aluminum dopant concentration can be controlled such that it exceeds the combined concentration of residual boron and nitrogen while remaining below the level that causes crystal defects to occur (e.g., about 5·10 17 cm -3 ).

[0106] Example 1: Four aluminum-doped SiC single crystals, namely two 4H polytypes and two 6H polytypes, are grown by sublimation. For comparison purposes, two additional SiC pear-shaped crystals are grown by sublimation without aluminum doping, one being a 4H polytype and one being a 6H polytype. The following experimental conditions for sublimation growth are provided with reference to the above-described system 110.

[0107] Prepare similar to Figure 11The doping container of the doping container 142. The doping container includes an inner crucible and a lid, and the inner crucible and the lid correspond to or are similar to the crucible 154 and the lid 156, and are made of pure tungsten obtainable from Plansee USALLC, 115 Constitution Boulevard, Franklin, MA 02038. The inner crucible has the following dimensions: 25 mm OD × 15 mm ID × 25 mm high. The lid is 5 mm thick and has a single through-hole with a diameter of 1.0 mm drilled in its center. The outer crucible and the lid corresponding to the crucible 150 and the lid 152 are made of halogen-purified dense isostatic graphite grade IG-11 obtainable from Toyo Tanso USA Inc., 2575 NW Graham Cir, Troutdale, OR 97060. The outer crucible has the following dimensions: 32 mm OD × 25 mm ID × 30 mm high. The graphite lid is 5 mm thick and has a hole with a diameter of 1.0 mm drilled in its center. The inner crucible is tightly fitted inside the outer graphite crucible.

[0108] Load 7 grams in total weight of pure pre-melted sapphire crack wafers into the inner crucible of the slow-release container. The graphite growth crucible and the lid similar to or corresponding to the crucible 114 and the lid 116 are made of halogen-purified dense isostatic graphite grade IG-11 obtainable from Toyo Tanso USA Inc., 2575 NW Graham Cir, Troutdale, OR 97060. Before its use for crystal growth, the crucible and the lid are halogen-purified to an ash grade of less than 5 ppm by weight. By performing impurity analysis with a glow discharge mass spectrometer (EAG LLC, 4747 Executive Drive, Suite 700, San Diego, CA 92121), the boron content in the purified graphite is determined to be 30 weight ppb.

[0109] Load the doping container at the bottom of the graphite growth crucible, and load the source crucible containing the SiC source corresponding to or similar to the crucible 126 into the graphite growth crucible. Place the source crucible on a support so that a free space is formed to accommodate the doping container. Attach the SiC seed to the lid of the graphite growth crucible and seal the graphite growth crucible with the lid. The SiC seed is a 4H-SiC and 6H-SiC wafer with a diameter of 150 mm. The 4H-SiC seed is oriented with its growth surface facing the <000-1> crystallization direction. The 6H-SiC seed is oriented with its growth surface facing the <0001> crystallization direction.

[0110] Place the loaded graphite growth crucible into a furnace chamber where it is surrounded by thermal insulation. Evacuate the chamber and energize the RF coil to heat the graphite growth crucible to an initial temperature of 1400 °C. At this temperature and with continuous pumping, immerse the growth system for 24 hours to achieve the deepest possible outgassing and reach a residual pressure of 1·10 -6 Torr. Fill the chamber with UHP argon containing less than 10 ppb of residual N2 until the pressure is 5 Torr and establish a UHP argon gas flow of 300 sccm throughout the chamber.

[0111] Then heat the graphite growth crucible to the sublimation growth temperature. More specifically, heat the graphite growth crucible to reach a temperature of 2180 °C at the top of the crucible and 2210 °C at the bottom of the crucible. Measure and monitor the top and bottom temperatures using an optical pyrometer. Immerse the graphite growth crucible at the above temperature for a predetermined period of time and then cool it to room temperature.

[0112] After completing the growth run, recover the grown SiC crystal pears and fabricate them into wafers with a diameter of 150 mm and a thickness of 500 microns according to SEMI standards. First, mechanically polish the wafers and then polish them using a final CMP polish.

[0113] Example 2: Evaluate the optical properties of the wafers produced in Example 1. This evaluation includes the measurement of their light transmission and reflection. Measure unpolarized light in the wavelength range of 350 nm to 850 nm using a spectrophotometer (Agilent Cary 7000UMS). During the measurement of transmission, the angle of incidence (AOI) is 0°. During the measurement of reflection, the AOI is 6°. As outlined in Optical Processes in Semiconductors, J.I. Pankove, Dover Publ, NY, 1971, pages 93 to 94, calculate the optical absorption coefficient (α, cm -1 -1) from the data on transmission and reflection using the approximation of a plate with parallel surfaces, thickness d, and an infinite number of internal reflections.

[0114] The wavelength dispersion of the optical absorption (α, cm -1 -1) measured for some 4H-SiC wafers is shown in Figure 12 , and the wavelength dispersion of the optical absorption (α, cm -1 -1) measured for some 6H-SiC wafers is shown in Figure 13 . Figure 12 And Figure 13The curves show the absorption coefficients of aluminum-doped (lower curve) and undoped (upper curve) wafers. After the optical measurements, some of the measured wafers were sliced into small pieces and the fragments were sent for N, B, and Al impurity analysis by secondary ion mass spectrometry (SIMS) (EAG LLC, 4747 Executive Drive, Suite 700, San Diego, CA 92121). The results of the SIMS impurity analysis are shown in Table 2. Measures aimed at reducing background B and N contaminants included the use of halogen-purified graphite and extended hot zone vacuum baking, such that the levels of B and N in the grown SiC crystals were below 1·10 16 cm -3 . The results of the optical absorption coefficient measurements are also shown in Table 2. The SiC crystals in the last two rows of Table 2 were grown without deliberate aluminum doping and are shown in the table for comparison. The results obtained show that by aluminum doping, a significant reduction in light transmission in the visible spectral range is achieved.

[0115] Table 2

[0116]

[0117]

[0118] The foregoing are examples. The present disclosure is intended to cover modifications, variations, and changes to the subject matter described herein that fall within the scope of the present application including the appended claims. As used herein, the term "comprising" means including but not limited to. The term "based on" means at least partially based on. Additionally, where the present disclosure or the claims recite an indefinite article "a", "an", "a first", or "another" element or the equivalent thereof, it may be construed to include one or more than one such element, neither requiring nor precluding two or more such elements.

[0119] The present invention relates to the following technical solutions:

[0120] Solution 1. An optical device, comprising:

[0121] A vanadium-compensated high-resistivity 6H polytype or 4H polytype silicon carbide (SiC) single crystal;

[0122] wherein the SiC single crystal is configured to transmit light having wavelengths in the range of 420 nm to 4.5 μm; and

[0123] wherein the optical device includes a window, a lens, a prism, or a waveguide for transmitting the light having wavelengths in the range of 420 nm to 4.5 μm.

[0124] Solution 2. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 4H-SiC single crystal, and wherein the wavelength of the light is less than 450 nm.

[0125] Solution 3. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 6H polytype Nu-SiC single crystal, and has a resistivity greater than or equal to 1·10 10 Ω·cm.

[0126] Solution 4. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 4H polytype Nu-SiC single crystal, and has a resistivity greater than or equal to 1·10 12 Ω·cm.

[0127] Solution 5. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 6H polytype Nu-SiC single crystal, and has a resistivity greater than 1·10 11 Ω·cm, and wherein the wavelength of the light is infrared.

[0128] Solution 6. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 4H polytype Nu-SiC single crystal, and has a resistivity greater than 5·10 12 Ω·cm, and wherein the wavelength of the light is infrared.

[0129] Solution 7. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 6H polytype Pi-SiC single crystal with boron predominating in a shallow impurity background, and has a resistivity greater than 1·10 11 Ω·cm, and wherein the wavelength of the light is infrared.

[0130] Solution 8. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 4H polytype Pi-SiC single crystal with boron predominating in a shallow impurity background, and has a resistivity greater than 1·10 13 Ω·cm, and wherein the wavelength of the light is infrared.

[0131] Solution 9. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 4H polytype Pi-SiC single crystal with boron predominating in a shallow impurity background, and has a resistivity greater than 1·10 13 Ω·cm.

[0132] Solution 10. The optical device according to Solution 1, wherein the SiC single crystal is a vanadium-compensated 6H polytype Pi-SiC single crystal with boron predominating in a shallow impurity background, and has a resistivity greater than 5·10 11 Ω·cm.

[0133] Embodiment 11. The optical device according to Embodiment 1, wherein the SiC single crystal is a vanadium-compensated Pi-type 6H-SiC single crystal with aluminum predominating in a shallow impurity background, and having a resistivity of 1·10 5 Ωcm to 1·10 8 Ωcm.

[0134] Embodiment 12. An optical transmission system, comprising:

[0135] a light source for generating light having a wavelength in the range of 420 nm to 4.5 μm; and

[0136] an optical device for receiving and transmitting the light, wherein the optical device comprises a vanadium-compensated high-resistivity 6H polytype or 4H polytype silicon carbide (SiC) single crystal.

[0137] Embodiment 13. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 4H-SiC single crystal, and wherein the wavelength of the light is less than 450 nm.

[0138] Embodiment 14. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 6H polytype Nu-type SiC single crystal, and having a resistivity greater than or equal to 1·10 10 Ωcm.

[0139] Embodiment 15. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 4H polytype Nu-type SiC single crystal, and having a resistivity greater than or equal to 1·10 12 Ωcm.

[0140] Embodiment 16. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 6H polytype Nu-type SiC single crystal, and having a resistivity greater than 1·10 11 Ωcm, and wherein the wavelength of the light is infrared.

[0141] Embodiment 17. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 4H polytype Nu-type SiC single crystal, and having a resistivity greater than 5·10 12 Ωcm, and wherein the wavelength of the light is infrared.

[0142] Embodiment 18. The optical transmission system according to Embodiment 12, wherein the SiC single crystal is a vanadium-compensated 6H polytype Pi-type SiC single crystal with boron predominating in a shallow impurity background, and having a resistivity greater than 1·10 11 Ωcm, and wherein the wavelength of the light is infrared.

[0143] Embodiment 19. The optical transmission system according to Embodiment 12, wherein the single-crystal SiC is a vanadium-compensated 4H polytype Pi-type single-crystal SiC with boron predominating in a shallow impurity background, and has a resistivity greater than 1·10 13 Ω·cm, and wherein the wavelength of the light is infrared.

[0144] Embodiment 20. The optical transmission system according to Embodiment 12, wherein the single-crystal SiC is a vanadium-compensated 4H polytype Pi-type single-crystal SiC with boron predominating in a shallow impurity background, and has a resistivity greater than 1·10 13 Ω·cm.

[0145] Embodiment 21. The optical transmission system according to Embodiment 12, wherein the single-crystal SiC is a vanadium-compensated 6H polytype Pi-type single-crystal SiC with boron predominating in a shallow impurity background, and has a resistivity greater than 5·10 11 Ω·cm.

[0146] Embodiment 22. The optical transmission system according to Embodiment 12, wherein the single-crystal SiC is a vanadium-compensated Pi-type 6H-SiC with aluminum predominating in a shallow impurity background, and has a resistivity of 1·10 5 Ω·cm to 1·10 8 Ω·cm.

[0147] Embodiment 23. A composition comprising an aluminum-doped silicon carbide crystal, the aluminum-doped silicon carbide crystal having residual nitrogen and boron impurities, wherein the silicon carbide crystal contains aluminum at a concentration greater than the combined concentration of nitrogen and boron in the silicon carbide crystal, and the silicon carbide crystal has a light absorption coefficient of less than about 0.4 cm -1 .

[0148] Embodiment 24. The composition according to Embodiment 23, wherein the concentration of boron in the silicon carbide crystal is less than or equal to about 1·10 16 cm -3 .

[0149] Embodiment 25. The composition according to Embodiment 23, wherein the concentration of nitrogen in the silicon carbide crystal is less than or equal to about 1·10 16 cm -3 .

[0150] Embodiment 26. The composition according to Embodiment 23, wherein the concentration of aluminum in the silicon carbide crystal is less than about 5·10 17 cm -3 .

[0151] Aspect 27. The composition according to Aspect 23, wherein the silicon carbide crystal is substantially free of defects caused by aluminum.

[0152] Aspect 28. The composition according to Aspect 23, wherein the silicon carbide crystal is a 4H polytype or a 6H polytype.

[0153] Aspect 29. The composition according to Aspect 23, wherein the silicon carbide crystal exhibits an absorption coefficient of less than about 0.4 cm -1 at a wavelength of about 450 nm.

[0154] Aspect 30. The composition according to Aspect 23, wherein the silicon carbide crystal exhibits an absorption coefficient of less than about 0.05 cm -1 at a wavelength of about 550 nm.

[0155] Aspect 31. The composition according to Aspect 23, wherein the silicon carbide crystal exhibits an absorption coefficient of less than about 0.01 cm -1 at a wavelength of 650 nm.

[0156] Aspect 32. A method for preparing an aluminum-doped silicon carbide crystal, comprising:

[0157] providing a silicon carbide source material and a silicon carbide single crystal seed in a growth crucible;

[0158] providing a solid aluminum dopant source material containing a compound containing aluminum and oxygen in a container; and

[0159] heating the growth crucible in such a manner that the growth crucible is positioned in the container, the manner being effective to generate silicon- and carbon-containing vapors from the silicon carbide source material in the growth crucible and aluminum-containing vapors from the solid aluminum dopant source material in the container, and effective to deposit the silicon- and carbon-containing vapors and the aluminum-containing vapors on the silicon carbide single crystal seed to grow the aluminum-doped silicon carbide crystal;

[0160] wherein the container comprises a first material resistant to damage by the aluminum dopant source and the aluminum-containing vapors, and a second material resistant to damage by the silicon- and carbon-containing vapors.

[0161] Aspect 33. The method according to Aspect 32, wherein the heating is further effective to generate solid aluminum oxide in the container and subsequently melt the solid aluminum oxide.

[0162] Aspect 34. The method according to Aspect 32, wherein the container comprises an internal component formed at least partially of the first material, and an external component disposed around the internal component and formed at least partially of the second material.

[0163] Aspect 35. The method according to Aspect 32, wherein the container comprises a first layer formed at least in part of the first material and a second layer formed at least in part of the second material.

[0164] Aspect 36. The method according to Aspect 32, wherein the first material comprises a refractory metal selected from tantalum, molybdenum, tungsten, rhenium, and alloys thereof.

[0165] Aspect 37. The method according to Aspect 32, wherein the second material is a refractory metal carbide.

[0166] Aspect 38. The method according to Aspect 37, wherein the refractory metal carbide is tantalum carbide or niobium carbide.

[0167] Aspect 39. The method according to Aspect 32, wherein the second material is graphite.

[0168] Aspect 40. The method according to Aspect 32, wherein the container further comprises a layer of refractory carbide.

[0169] Aspect 41. The method according to Aspect 32, wherein the container further comprises a capillary in communication with the aluminum dopant source material gas.

[0170] Aspect 42. The method according to Aspect 32, wherein the aluminum dopant source material comprises aluminum oxide.

Claims

1. A composition comprising an aluminum-doped silicon carbide crystal, the aluminum-doped silicon carbide crystal having impurities comprising residual nitrogen and residual boron, wherein (a) the silicon carbide crystal comprises aluminum at a concentration greater than the combined concentration of nitrogen and boron in the silicon carbide crystal, (b) The optical absorption coefficient of the silicon carbide crystal at wavelengths in the range of 400 nm to 800 nm is less than 0.4 cm -1 , and (c)(i) The concentration of boron in the silicon carbide crystal is less than or equal to 1·10 16 cm -3 , and / or (ii) The concentration of aluminum in the silicon carbide crystal is less than 5·10 17 cm -3 .

2. The composition according to claim 1, wherein the concentration of nitrogen in the silicon carbide crystal is less than or equal to 1·10 16 cm -3 .

3. The composition according to claim 1, wherein the silicon carbide crystal is substantially free of defects caused by aluminum.

4. The composition according to claim 1, wherein the silicon carbide crystal is a 4H polytype or a 6H polytype.

5. The composition according to claim 1, wherein the silicon carbide crystal exhibits an absorption coefficient of less than 0.4 cm -1 at a wavelength of 450 nm.

6. The composition according to claim 1, wherein the silicon carbide crystal exhibits an absorption coefficient of less than 0.05 cm -1 at a wavelength of 550 nm.

7. The composition according to claim 1, wherein the silicon carbide crystal exhibits an absorption coefficient of less than 0.01 cm -1 at a wavelength of 650 nm.

8. A method for preparing an aluminum-doped silicon carbide crystal according to any one of claims 1 to 7, comprising: providing a silicon carbide source material and a silicon carbide single crystal seed in a growth crucible; providing a solid aluminum dopant source material containing a compound containing aluminum and oxygen in a container; and heating the growth crucible in such a manner that the growth crucible is positioned in the container, the manner being effective to produce silicon- and carbon-containing vapors from the silicon carbide source material in the growth crucible and aluminum-containing vapors from the solid aluminum dopant source material in the container, and effective to deposit the silicon- and carbon-containing vapors and the aluminum-containing vapors on the silicon carbide single crystal seed to grow the aluminum-doped silicon carbide crystal; wherein the container comprises a first material resistant to damage by the aluminum dopant source and the aluminum-containing vapors, and a second material resistant to damage by the silicon- and carbon-containing vapors, the first material being different from the second material, and wherein the container includes an internal component formed at least in part of the first material, and an external component disposed around the internal component and formed at least in part of the second material, wherein the external component is in the form of an external crucible, and wherein the internal component is in the form of an internal crucible positioned in the external crucible.

9. The method according to claim 8, wherein the heating is also effective to produce solid aluminum oxide in the container and subsequently melt the solid aluminum oxide.

10. The method according to claim 8, wherein the first material comprises a refractory metal selected from tantalum, molybdenum, tungsten, rhenium, and alloys thereof.

11. The method according to claim 8, wherein the second material is a refractory metal carbide.

12. The method according to claim 11, wherein the refractory metal carbide is tantalum carbide or niobium carbide.

13. The method according to claim 8, wherein the second material is graphite.

14. The method according to claim 8, wherein the container further comprises a layer of refractory carbide.

15. The method according to claim 8, wherein the container further comprises a capillary in gas communication with the aluminum dopant source material.

16. The method according to claim 8, wherein the aluminum dopant source material comprises aluminum oxide.

17. A method for preparing an aluminum-doped silicon carbide crystal according to any one of claims 1 to 7, comprising: providing a silicon carbide source material and a silicon carbide single crystal seed in a growth crucible; providing a solid aluminum dopant source material containing a compound containing aluminum and oxygen in a container; and With the container positioned in the growth crucible, heat the growth crucible in a manner effective to produce silicon- and carbon-containing vapor from the silicon carbide source material in the growth crucible and aluminum-containing vapor from the solid aluminum dopant source material in the container, and effective to deposit the silicon- and carbon-containing vapor and the aluminum-containing vapor on the silicon carbide single crystal seed to grow the aluminum-doped silicon carbide crystal; wherein the container comprises a first material resistant to damage by the aluminum dopant source and the aluminum-containing vapor, and a second material resistant to damage by the silicon- and carbon-containing vapor, the first material being different from the second material, and wherein the container includes a first layer formed at least in part of the first material and a second layer formed at least in part of the second material.

18. The method according to claim 8, wherein the outer crucible includes a lid, wherein the inner crucible includes a lid, and wherein the lid of the outer crucible is removable to position the inner crucible in the outer crucible.

19. The method according to claim 18, further comprising providing a capillary passage through the lid of the outer crucible and the lid of the inner crucible, and releasing the aluminum-containing vapor from the container through the capillary passage.

20. The method according to claim 18, wherein the outer crucible comprises the second material such that the outer surface of the outer crucible is resistant to damage by the silicon- and carbon-containing vapor, and wherein the inner crucible comprises the first material such that the inner surface of the inner crucible is resistant to damage by the aluminum dopant source and the aluminum-containing vapor.

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