Optical grade vanadium-compensated 4H and 6H single crystals

Abstract

An optical device includes a vanadium-compensated high-resistivity 6H polytype or 4H polytype single crystal of silicon carbide (SiC), and the single crystal of silicon carbide (SiC) is configured to transmit light having a wavelength in the range of 420 nm to 4.5 μm. The device may include a window, a lens, a prism, or a waveguide. An optical transmission system includes 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 light, wherein the optical device includes a vanadium-compensated high-resistivity 6H polytype or 4H polytype single crystal of SiC.

Description

[0001] Cross - Reference to Related Applications

[0002] This application claims priority to U.S. Provisional Patent Application No. 62 / 984,177, filed on March 2, 2020. The entire disclosure of U.S. Provisional Patent Application No. 62 / 984,177 is incorporated herein by reference. Technical Field

[0003] This disclosure relates to optical devices and optical transmission systems. Background Art

[0004] This disclosure generally relates to optical devices for transmitting light energy or information, including but not limited to windows, lenses, prisms, and waveguides. This disclosure also generally relates to systems and methods for transmitting light energy or information using optical devices. Summary of the Invention

[0005] This disclosure relates to vanadium - compensated 4H hexagonal polytype and 6H hexagonal polytype single - crystal silicon carbide (SiC) having low light absorption at wavelengths within its 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 this 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.

[0006] This disclosure also relates to an 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 wavelengths in the range of 420 nm to 4.5 μm. According to one aspect of this disclosure, the optical device can include a window, a lens, a prism, or a waveguide for transmitting light having wavelengths in the range of 420 nm to 4.5 μm.

[0007] This disclosure also relates to an optical transmission system comprising: a light source for generating light having wavelengths 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. Brief Description of the Drawings

[0008] Figure 1 is a top view of an example of a silicon carbide (SiC) wafer constructed in accordance with this disclosure;

[0009] Figure 2 is Figure 1 a side view of the SiC wafer of

[0010] Figure 3Schematic diagram of an example of an optical transmission system constructed according to the present disclosure;

[0011] Figure 4 Is the transmission (T mes ) curve and reflection (R mes ) curve (T mes , R mes As a function of wavelength) of certain vanadium-compensated 4H-SiC wafers;

[0012] Figure 5 Is a plot of the transmission curve and reflection curve (T mes , R mes As a function of wavelength) of certain vanadium-compensated 6H-SiC wafers;

[0013] Figure 6 Is a plot showing the optical absorption curves (absorption coefficient as a function of wavelength) of two wafers;

[0014] Figure 7 Is a plot showing the transmission curves (transmittance as a function of wavelength) of six vanadium-compensated 6H-SiC wafers;

[0015] 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; and

[0016] 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.

[0017] The same reference numerals or other feature indicators are used in the drawings to indicate the same or similar features. Detailed Description

[0018] Figure 1 And 2 Illustrates an example of a silicon carbide (SiC) wafer 10 constructed according to 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 according to the present disclosure. The transmission system may include a light source 18 for generating light 20, an optical device 16 for receiving and transmitting light 20, and a target 22 to which light 20 is transmitted through the optical device 16.

[0019] 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.

[0020] Single-crystalline SiC is 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 (such as 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 (such as HEMTs). See U.S. Patent No. 9,484,284.

[0021] Large-size single-crystalline SiC 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 granules, can be placed in the high-temperature region of a graphite crucible. A SiC seed (such as a single-crystalline SiC plate or wafer) is positioned in the lower-temperature region of the crucible, for example, 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, thereby growing a SiC ingot (boule) of an appropriate size.

[0022] To meet certain requirements, dopants can be introduced into the growth system to change the electronic parameters of the grown SiC crystal, such as the conductivity type and resistivity. Nitrogen doping can be used to fabricate low-resistivity n-type 4H-SiC single crystals. 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.

[0023] The optical and spectral 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 cutoff in the visible to λ≈4.5 μm in the infrared, where the transparency is terminated by multi - phonon absorption bands (see, e.g., Singh's Figure 1 ).

[0024] 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 abrasion 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.

[0025] Hexagonal single - crystal 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 waveguide by ion implantation).

[0026] The optical absorption of sublimation-grown 4H-SiC single crystals and 6H-SiC single crystals has been studied in the visible (VIS) to infrared range. 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 are doped with N, B, and Al in amounts up to 1·10 18 cm -3 , and have poor optical transmission. The optical 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 include pure (not intentionally doped), N-doped, B-doped, VCSI, and HPSI. In all these studies, N-doped SiC crystals and B-doped SiC crystals show significant optical losses with increasing dopant concentration. Pure SiC crystals show smaller losses, especially in the visible range. The best IR optical transparency was measured on semi-insulating VCSI samples and HPSI samples.

[0027] 4H-SiC and 6H-SiC belong to the 6mm space group and are positive uniaxial crystals. The dependence of the refractive index of semi-insulating 4H-SiC single crystals and 6H-SiC single crystals grown by sublimation on wavelength, polarization (ordinary vs. extraordinary), and temperature has 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 are parallel to the c-axis. It was found that the refractive index values and their dispersion of 4H-SiC and 6H-SiC are actually the same.

[0028] Although large-sized 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 residual optical losses in hexagonal SiC, which have not been well understood or controlled. 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, Figure 1 ) in Atabaev et al., Spectral Dependence of Optical Absorption of 4H-SiC Doped with Boron and Aluminum, J. of Spectroscopy (2018) Article ID 8705658, intra-band transitions, and absorption by free carriers.

[0029] Figures 1 to 3 The wafer 10 and the optical device 16 shown as examples in

[0030] may comprise vanadium-compensated 6H polytype or 4H polytype SiC single crystals that have excellent light transmission in the wavelength range from about 420 nm in the visible light to about 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). 16 cm -3 to 1.5·10 17 cm -3 level of vanadium doping (compensation). As measured by secondary ion mass spectrometry (SIMS), the concentration of shallow impurities (such as N, B, and Al) can be controlled to not exceed 3·10 16 cm -3 . The 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.

[0031] The SiC crystals constituting the wafer 10 or the optical device 16 can have a high structural quality without dimensional crystal defects (such as inclusions or sub-grains), and less than 1·10 4 cm -2The total dislocation density. The overall crystal quality can be evaluated using X-ray rocking curve techniques. X-ray reflections with an FWHM below 25 arcseconds can be a typical indication of high crystal quality.

[0032] 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., wherein 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.

[0033] 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):

[0034] 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 light beam, and its propagation is controlled by the ordinary refractive index n0. Using Equation (3) for n0 from Xu (cited above) and Table 1 and substituting T = 300 K, an expression for n0(λ) at room temperature can be obtained:

[0035] For 4H-SiC:

[0036] For 6H-SiC:

[0037] For a light beam incident perpendicular to the c-plane of a hexagonal uniaxial crystal, the reflectivity (R) is expressed as:

[0038]

[0039] 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 equations for the measured transmission (T mes ) and reflection (R mesThe following equations. 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.

[0040]

[0041]

[0042] 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:

[0043]

[0044] The transmission and reflection in the extreme case of a completely transparent plate can be obtained as follows by substituting α = 0 into equations (A3) and (A4):

[0045]

[0046]

[0047] The optical absorption coefficient of the SiC single crystal is calculated from equation (A5) using the measured transmission (T mes ) and reflectivity (R). The value of R is calculated from equations (A2) based on the refractive index dispersions (A1a) and (A1b) of 4H and 6H determined by Xu.

[0048] In addition to optical transmission and reflection, a non - contact instrument COREMA - W with a sensitivity of 1·10 5 Ωcm to 1·10 12 Ωcm is used to measure the resistivity of the SiC single crystal at room temperature. A temperature - variable non - contact resistivity meter COREMA - VT is used to measure the temperature dependence of the resistivity in the temperature range from 25 °C to 400 °C, 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 from 1·10 6 Ωcm to 1·10 14Ωcm.

[0049] 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.

[0050] 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 distinct absorption regions: a steep rise in absorption at wavelengths below approximately 0.40 μm for the 4H crystal and a steep rise in absorption at wavelengths below approximately 0.44 μm for the 6H crystal. This increase in 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 3250A, J. Appl. Phys. Vol. 84, No. 5 (1998) 2963 - 2964

[0051] 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 approximately 0.01 cm at λ ≈ 0.7 μm -1 to approximately 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 .

[0052] Although all vanadium-compensated SiC crystals in this study are optically transmissive, their transmittances show significant variations. An example of such a variation is shown in Figure 7 In, Figure 7Shows 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 Ω cm to 2·10 11 Ω cm.

[0053] Figure 7 The two curves 62 with the highest transmission in 6 (one solid line and one dashed line) represent 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 Ω cm to 1·10

[0054] Figure 7 Ω cm. 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

[0055] 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 .

[0056] 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 Ω cm, while that of the 6H wafers is 1·10 9 Ω cm. For these wafers, the optical absorption is not only the highest in the visible range but also the highest in the infrared.

[0057] The slightly lower optical absorption shown by lines 70 and 77 is also that of the Nu-type wafers. However, these wafers have a higher resistivity of 5·10 12 Ω cm for 4H and 5·10 10 Ω cm for 6H.

[0058] 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 wafer is 1·10 14 Ω cm (line 72) and 1·10 11 Ω cm (line 78). The resistivity of the 6H wafer is 1·10

[0059] The lowest optical absorption is shown by the curves 80 and 82 in Figure 9 which were measured on two Pi-type 6H-SiC wafers with an aluminum preponderance in a shallow impurity background. The wafers have relatively low resistivities of 1·10 5 Ω cm and 1·10 7 Ω cm.

[0060] Table 1 below 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.

[0061] Table 1

[0062]

[0063] 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.

[0064] 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 light to about 4.5 μm in the infrared, especially if their doping and resistivity have been optimized.

[0065] Due to the wider bandgap, vanadium-compensated 4H-SiC single crystals are preferably used for optical applications at shorter wavelengths below 450 nm.

[0066] 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 loss of such crystals in the visible light range will be close to 1 cm -1 and the optical loss in the infrared will be as high as 0.1 cm -1 .

[0067] If the resistivity of vanadium-compensated Nu-type SiC single crystals is greater than 1·10 11Ω cm, for 4H greater than 5·10 12 Ω cm, then it can be used for infrared optical applications. For such a crystal, the light absorption in the infrared will be less than 0.01 cm -1 .

[0068] If the resistivity of a vanadium-compensated Pi-type SiC single crystal with boron predominating 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 a crystal, the light absorption in the infrared will be less than 0.01 cm -1 .

[0069] If the resistivity of a vanadium-compensated Pi-type SiC single crystal with boron predominating 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 a crystal, the light absorption should be less than 0.8 cm at λ = 450 nm -1 and less than 0.01 cm at λ = 750 nm -1 .

[0070] If the resistivity of a vanadium-compensated Pi-type 6H-SiC single crystal with aluminum predominating in a shallow impurity background is 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 light absorption should be less than 0.8 cm at λ = 450 nm -1 and less than 0.01 cm at λ = 750 nm -1 . Compared with all other crystal types, their near-band-edge absorption shoulders are lower and narrower.

[0071] The above are examples. This disclosure is intended to cover variations, modifications, and changes to the subject matter described herein that fall within the scope of this 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 disclosure or claims recite an indefinite article "a," "an," "a first," or "another" element or its equivalent, it may be construed to include one or more than one such element, neither requiring nor precluding two or more such elements.

Claims

1. A composition for use in the field of optical transmission, comprising: A vanadium-compensated resistive 6H polytype or 4H polytype single crystal of silicon carbide; wherein the silicon carbide single crystal is used as an optical material for applications in the field of optical transmission and is configured to transmit light with a wavelength in the range of 420 nm to 4.5 μm, the silicon carbide single crystal has a resistivity of at least 1·10 5 Ωcm, the concentrations of shallow impurities N, B, and Al in the silicon carbide single crystal do not exceed 3·10 16 cm -3 , and wherein (ii) When the silicon carbide single crystal is of the 4H polytype Nu type, the resistivity is greater than or equal to 1·10 12 Ω·cm; (iii) When the silicon carbide single crystal is of the 6H polytype Nu type, the resistivity is greater than or equal to 1·10 10 Ω·cm.

2. The composition according to claim 1, wherein the single crystal of silicon carbide is a vanadium-compensated 4H polytype single crystal of silicon carbide, and wherein the wavelength of the light is less than 450 nm.

3. The composition according to claim 1, wherein the single crystal of silicon carbide is a vanadium-compensated 6H polytype Nu single crystal of silicon carbide, and has a resistivity greater than 1·10 11 Ωcm, and wherein the wavelength of the light is infrared.

4. The composition according to claim 1, wherein the single-crystal silicon carbide is a vanadium-compensated 4H polytype Nu single-crystal silicon carbide, and has a resistivity greater than 5·10 12 Ω cm, and wherein the wavelength of the light is infrared.

5. The composition according to claim 1, wherein the single crystal of silicon carbide is a 6H polytype Pi single crystal of silicon carbide compensated with vanadium under the predominance of boron 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.

6. The composition according to claim 1, wherein the single crystal of silicon carbide is a 4H polytype Pi single crystal of silicon carbide compensated with vanadium under the predominance of boron 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.

7. The composition according to claim 1, wherein the single-crystal silicon carbide is a 4H polytype Pi single-crystal silicon carbide compensated with vanadium under the predominance of boron in a shallow impurity background, and has a resistivity greater than 1·10 13 Ω cm.

8. The composition according to claim 1, wherein the single crystal of silicon carbide is a 6H polytype Pi single crystal of silicon carbide compensated with vanadium under the predominance of boron in a shallow impurity background, and has a resistivity greater than 5·10 11 Ω cm.

9. The composition according to claim 1, wherein the single-crystalline silicon carbide is a vanadium-compensated Pi-type 6H-SiC single crystal with aluminum predominance in a shallow impurity background, and has a resistivity of 1·10 5 Ω·cm to 1·10 8 Ω·cm.

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