Abstract:
A growth substrate having growth nuclei made of diamond and/or diamond-like carbon arranged on its growth surface, the orientation of over 50% of the growth nuclei deviating by less than 10° from the crystal orientation defined by the growth substrate corresponding to Miller&#39;s indices (h, k, l) and the nucleus density changes between the center of the growth substrate and a distance of maximally 15 mm from the center by maximally 80%. The method also relates to a process for the growth substrate&#39;s manufacture.

Description:
This application claims priority from U.S. Provisional Application Ser. No. 60/037,950, filed on Feb. 20, 1997. 
    
    
     BACKGROUND AND SUMMARY OF THE INVENTION 
     This application claims the priority of German Application No. 197 02 311.8, filed on Jan. 23, 1997, and U.S. Provisional Application Ser. No. 60/037,950, filed Feb. 20, 1997, the disclosures of both of which are expressly incorporated by reference herein. 
     The invention relates to a growth substrate having growth nuclei made of diamond and/or diamond-like carbon arranged on its growth surface as well as to a process for a uniform nucleation on the growth substrate. This invention is based on the article &#34;Nucleation of High Oriented Diamond on Silicon via an Alternating Current Substrate Bias&#34; by Wolter, et al. in Appl. Phys. Lett., Vol. 68, No. 25, Jun. 17, 1996, Page 3558-3560. 
     From the Wolter Article it is known to apply an alternating BIAS voltage of a plasma CVD process for nucleation on a growth substrate made of silicon and arranged in an evacuated reactor and pretreated, by growth nuclei made of diamond and/or a diamond-like carbon. As a result, a higher nucleus density of high-precision oriented growth nuclei in comparison to direct-current-type bias voltage is achieved on the growth substrate in otherwise similar conditions. High-precision orientation relates to growth nuclei whose orientation deviates by less than 10° from the crystal orientation defined by the growth substrate corresponding to Miller&#39;s indices (h, k, l). Despite the high nucleus density, the nucleation is highly inhomogeneous; that is, high lateral fluctuations occur in the density of high-precision oriented growth nuclei over the whole growth surface of the growth substrate. 
     From PCT patent application WO 94/08076, it is known for the nucleation on a Si growth substrate to pretreat the Si growth substrate and then provide the nucleation on it in a plasma CVD process from a gaseous phase. For the pretreatment, the growth substrate is installed in a reactor; the reactor is preferably evacuated to less than 10 -9  mbar; and the growth substrate is heated to more than 950° C. As an alternative or supplementation, the growth surface can also be subjected to a plasma treatment which preferably takes place by means of an almost 100% hydrogen plasma at substrate temperatures of between 300° C. and 1,100° C. In this case, it is advantageous to apply a bias voltage of between +50 and -300 v, particularly of approximately -50 v, to the growth substrate. After the cleaning, the nucleation can start in a known manner, the bias voltage advantageously being maintained. For example, CO and CH 4 , C 2  H 5  OH, acetylene and acetone or, for example, CF 4  and methane can be used as the process gases. A preferred combination is hydrogen and methane. 
     The nucleus density which can be achieved by means of this process is high but the nucleation is highly inhomogeneous because few to almost no high-precision oriented growth nuclei are formed in the center of the growth substrate. 
     It is an object of the invention to produce a growth substrate with a nucleus density which is more homogeneous over the surface than previously. 
     According to the invention, this object is achieved by a growth substrate having growth nuclei made of diamond and/or diamond like substances arranged on its surface with over 50% of the nuclei deviating less than 10° from the crystal orientation defined by the substrate according to Miller&#39;s indices. A manufacturing process is also defined. 
     Additional suitable embodiments include a nucleus density between the center of the growth substrate and a distance of maximally 15 mm from the center changing by maximally 50, 30 and 20%, a nucleus density in the center of the growth substrate of 1.10 8  nuclei per cm 2  and 3×10 8  nuclei per cm 2 , and a nucleus density between the center of the growth substrate and a distance maximally 15 mm from amounts to between 1×10 8  nuclei per cm 2  and 5×10 8  nuclei per cm 2  and 3.5×10 8  nucleii per cm 2  and 5.10 8  nuclei cm 2 . 
     Suitable embodiments of the process claims include employing bias voltages between -30 and -130 volts, frequencies between 1 and 100 Hz, 1 and 10 Hz, and a frequency at 5 Hz. Process embodiments also include varying active (t on ) and currentless (t off ) voltage values and using growth substrates of silicon and β-silicon carbide. 
     Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a full-wave-rectified sinusoidal voltage as the bias voltage; 
     FIG. 2 is a view of a growth surface of a Si growth substrate which is provided with nucleation by means of a bias voltage according to FIG. 1; 
     FIG. 3 is a view of a sinusoidal voltage as the bias voltage; 
     FIG. 4 is a view of a growth surface of a Si growth substrate provided with nucleation by means of a bias voltage according to FIG. 3; 
     FIG. 5 is a view of a half-wave-rectified sinusoidal voltage as the bias voltage; 
     FIG. 6 is a view of a growth surface of a Si growth substrate provided with nucleation by means of a bias voltage according to FIG. 1; 
     FIG. 7 is a view of the center of a growth surface with diamond nucleation (Welcher detection) in the case of a half-wave-rectified bias voltage of 0.5 Hertz; 
     FIG. 8 is a view of the growth surface of the growth substrate according to FIG. 7 at a distance of approximately 10 mm from the center; 
     FIG. 9 is a view of the center of a growth surface with diamond nucleation (Welcher detection) in the case of a half-wave-rectified bias voltage of 5 Hertz; 
     FIG. 10 is a view of the growth surface of the growth substrate according to FIG. 9 at a distance of approximately 10 mm from the center; 
     FIG. 11 is a view of the center of a growth surface with diamond nucleation (Welcher detection) in the case of a half-wave-rectified bias voltage of 50 Hertz; 
     FIG. 12 is a view of the growth surface of the growth substrate according to FIG. 11 at a distance of approximately 10 mm from the center; 
     FIG. 13 is a view of the center of a growth surface with diamond nucleation (Welcher detection) in the case of a half-wave-rectified bias voltage of 500 Hertz; 
     FIG. 14 is a view of the growth surface of the growth substrate according to FIG. 13 at a distance of approximately 10 mm from the center; 
     FIG. 15 is a view of the center of a growth surface with diamond nucleation (Welcher detection) in the case of a half-wave-rectified bias voltage of 10 Hertz; 
     FIG. 16 is a view of the growth surface of the growth substrate according to FIG. 15 at a distance of approximately 10 mm from the center; 
     FIG. 17 is a view of the course of the nucleus density as a function of the distance from the center in the case of the growth substrate according to FIG. 7; 
     FIG. 18 is a view of the course of the nucleus density as a function of the distance from the center in the case of the growth substrate according to FIG. 9; 
     FIG. 19 is a view of the course of the nucleus density as a function of the distance from the center in the case of the growth substrate according to FIG. 11; and 
     FIG. 20 is a view of the course of the nucleus density as a function of the distance from the center in the case of the growth substrate according to FIG. 13; 
     FIG. 21 is a view of the course of the nucleus density as a function of the distance from the center in the case of the growth substrate according to FIG. 15. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The nucleation on the growth substrates illustrated in the following took place at largely the same CVD process conditions in a reactor which can be evacuated. The only difference was the frequency of the AC-bias voltage. 
     The basic process course consists of plasma etching for cleaning the growth surface. For this purpose, the reactor was pumped down to approximately 10 -8  mbar and the plasma etching was carried out in an H 2  plasma without any bias voltage at a substrate temperature of 810° C. 
     Plasma etching is followed by bias pretreatment. A periodic alternating voltage of between 0.5 and 500 Hz with a maximal amplitude in the amount of 230 v is applied to the substrate. H 2 , CH 4 , N 2  and O 2  are used as process gases. The growth substrate, which is particularly formed of silicon, is heated to a temperature of between 870° C. and 890° C. The bias pretreatment is concluded when a sufficient nucleation by diamond is detected on the growth surface of the Si growth substrate by the rise of the amount of the median bias current. 
     The bias pretreatment may be followed by the growth of the diamond layer, in which case the growth parameter α is adjusted by means of the formula ##EQU1## wherein v 110  : rate of growth in the &lt;100&gt;-direction, and V 111  : rate of growth in the &lt;111&gt;-direction, in which case, 
     &lt;abc&gt; corresponds to Miller&#39;s indices in crystallography, 
     to a value of between 2 and 3. 
     FIG. 1 illustrates a time sequence of an alternating-current-type (AC) bias-voltage as applied during the nucleation on a silicon growth substrate according to FIG. 2. For implementing this bias voltage, a sinusoidal voltage of a frequency of 50 Hertz (s-1) was full-wave-rectified. The maximal voltage applied to the growth substrate was 0 volts, and the lowest voltage was -200 volts. The largest amount of the electric voltage was therefore 200 volts. 
     FIG. 2 is a cutout of the growth surface provided with the nucleation by means of a bias voltage according to FIG. 1 at a distance of approximately 15 mm from the center of the growth substrate enlarged 10,000 times. 
     The quality of nucleation is relatively poor because only a small percentage of the forming growth nuclei deviate well, that is, by less than 10° with respect to the crystal axes deviating from the crystal axes defined by the growth substrate. After the nucleation, the samples were coated for one hour under growth conditions α=2 . . . 3. The nucleus densities were then determined by means of scanning electron microscope pictures (REM) by a manual counting of the nuclei which have a faulty in-plane orientation of &lt;10%. The number of the oriented nuclei is &gt;80%. Nuclei size depends on the coating time. 
     FIG. 3 illustrates a time sequence of an alternating-voltage-type (AC) bias voltage as it was applied during the nucleation on a silicon growth substrate according FIG. 4. A normal sinusoidal voltage of a frequency of 50 Hertz (Hz) was used as the bias voltage. The respective maximal voltage applied to the growth substrate was 200 volts and the lowest voltage was -200 volts. The largest amount of the electrical voltage was therefore 200 volts. 
     FIG. 4 is a cutout of the growth surface provided with the nucleation by means of a bias voltage according to FIG. 3 at a distance of approximately 10 mm from the center of the growth substrate enlarged 10,000 times. The quality of the nucleation is better than the embodiment of FIG. 2 because the percentage of forming highly oriented growth nuclei is higher. 
     FIG. 5 illustrates a time sequence of an alternating-voltage-type (AC) bias voltage as it was applied during the nucleation on a silicon growth substrate according FIG. 6. For implementing this bias voltage, a sinusoidal voltage of a frequency of 50 Hertz (Hz) was half-wave-rectified so that only the negative half-wave is used. The maximal voltage applied to the growth substrate was 0 volts and the lowest voltage was -200 volts. The largest amount of the electrical voltage was therefore 200 volts. 
     It is advantageous in the case of this preferred wave form of the bias voltage to adjust the wave form such that its negative peak voltage is lower than -30 V, particularly lower than -50 V; that, within a period, the bias voltage is in each case applied for a predeterminable active time t on  with a value lower than -30 V; and that within the same period, the bias voltage for a defined currentless time t off  in an interval is higher than -30 V. 
     FIG. 6 is a cutout of the growth surface provided with the nucleation by means of a bias voltage according to FIG. 5 at a distance of approximately 10 mm from the center of the growth substrate enlarged 10,000 times. The quality of the nucleation is the best that has been reached. The percentage of the forming growth nuclei is the highest. 
     In the following, five embodiments (Sample 1 to Sample 5) are illustrated which were provided with a nucleation at identical wave forms of the bias voltage but at different frequencies. In Sample 1, the frequency was 0.5 Hz; in Sample 2, the frequency was 5 Hz; Sample 3, the frequency was 50 Hz; and Sample 4, the frequency was 500 Hz. In all samples 1 to 4, the voltage of the bias voltage was between 0 V and -200 V, during the currentless time t off , the voltage amounting to 0 V. Simultaneously, the wave form of the bias voltage was adjusted such that the currentless time t off  corresponded approximately to the duration of the active time t on . 
     FIGS. 7 and 8 both show a cutout of the growth surface at different distances from the center of the growth surface of Sample 1. For the purpose of nucleation, a bias voltage of 0.5 Hz was applied to Sample 1, which voltage also meets the above-mentioned criteria and with respect to its type approximately corresponds to that of FIG. 5. FIGS. 7 and 8 are enlarged 10,000 times. 
     The conditions during the nucleation on Sample 1 by means of plasma CVD are listed in the following. 
     
         ______________________________________Sample 1      Etching  Bias Treatment                               Growth______________________________________Bias Voltage  --       sinudoidal, half-                               --    wave rectified  Bias Voltage Value -- 0 V to -200 V --  Bias Frequency 0 0.5 Hz 0  H.sub.2 in sccm 1,500 1,500 1,500  CH.sub.4 in % -- 1.0 1.7  N.sub.2 in ppm 5 50 60  O.sub.2 in ppm 5 25 --  Microwave power in W 2,500 2,500 2,500  Temperature in ° C. 810 879 830  pressure in mbar 21.8 32.3 21.0  Time in min 5 20 60______________________________________ 
    
     Nucleation in the area of the center of the growth surface of Sample 1--as illustrated in FIG. 7--is limited to small islands. The nucleation on the growth surface at a distance of 10 mm from the center is illustrated in FIG. 8 and is improved with respect to the nucleation in the center. 
     FIG. 17 illustrates the nucleus density of oriented nuclei on the growth surface of Sample 1 as a function of the distance from the center, in which case the nucleus density is entered as 10 8  nuclei per cm 2 , and the distance from the center of the growth surface of the growth substrate of Sample 1 is entered in mm. 
     The diagram according to FIG. 17 shows that the nucleus density of Sample 1 improves steadily with an increasing distance from the center and, at a distance of approximately 14-15 mm from the center, has a maximum with a nucleus density of approximately 2.2×10 8  nuclei per cm 2 . Then the nucleus density decreases again but, at a distance of approximately 20 mm, still has a nucleus density of approximately 1.1×10 8  nuclei per cm 2 . In the given item, the nucleus density in the center of Sample 1 is not worth mentioning. 
     On the whole, it should be stated that the homogeneity of the nucleus density over the growth surface of Sample 1 has a good value only at a distance of between 10 and 20 mm from the center and, in the area from the center to a distance of 10 mm, is poor. 
     For establishing the diagram according to FIG. 17 and also according to FIGS. 18 to 21, the nucleus density was determined by analysis of REM. For this purpose, all those crystals were manually marked as being oriented whose azimuthal orientation was tilted or rotated by less than 10° with respect to the growth substrate. 
     FIGS. 9 and 10 each show a cutout of the growth surface at different distances from the center of the growth surface of Sample 2. In contrast to Sample 1, the nucleation was carried out here at a frequency of the bias voltage of 5 Hz. Here also, FIGS. 9 and 10 are enlarged 10,000 times. 
     The conditions during the nucleation on Sample 2 by means of plasma CVD are: 
     
         ______________________________________Sample 2      Etching  Bias Treatment                               Growth______________________________________Bias Voltage  --       sinudoidal, half-                               --    wave rectified  Bias Voltage Value -- 0 V to -200 V --  Bias Frequency 0 5 Hz 0  H.sub.2 in sccm 1,500 1,500 1,500  CH.sub.4 in % -- 1.0 1.7  N.sub.2 in ppm 5 50 60  O.sub.2 in ppm 5 25 --  Microwave power in W 2,5OO 2,500 2,500  Temperature in ° C. 810 880 830  Pressure in mbar 21.8 32.8 21.6  Time in min 5 20 60______________________________________ 
    
     Nucleation in the area of the center of the growth surface of Sample 2--as illustrated in FIG. 9--is good. Nucleation on the growth surface at a distance of 10 mm from the center is illustrated in FIG. 8 and is still improved with respect to the nucleation in the center. 
     FIG. 18 illustrates the nucleus density of oriented nuclei on the growth surface of Sample 2 as a function of the distance from the center, where the nucleus density is entered as 10 8  nuclei per cm 2  and the distance from the center of the growth surface of the growth substrate of Sample 2 is entered in mm. 
     As illustrated in the diagram of FIG. 18, the nucleus density of Sample 2 has a value of approximately 3.6×10 8  nuclei per cm 2  in the center. Here also, the nucleus density increases steadily with an increasing distance form the center and, at a distance of approximately 14-15 mm from the center, has a maximum with a nucleus density of approximately 4.8×10 8  nuclei per cm 2 . Then the nucleus density decreases rapidly and, at a distance of approximately 20 mm from the center, only has a nucleus density of approximately 0.2×10 8  nuclei per cm 2 . The difference between nucleus density in the center and the maximal nucleus density in Sample 2 amounts to approximately 1.2×10 8  nuclei per cm 2 , approximately 25%. On the whole, it should be stated that the homogeneity of the nucleus density over the growth surface of Sample 2 has the best result of Samples 1-4. 
     FIGS. 11 and 12 show a cutout of the growth surface at different distances from the center of the growth surface of Sample 3. In contrast to Samples 1 and 2, the nucleation on Sample 3 was carried out at a frequency of the bias voltage of 50 Hz. FIGS. 11 and 12 also are enlarged 10,000 times. 
     The conditions during the nucleation of Sample 3 by means of plasma CVD are: 
     
         ______________________________________Sample 3      Etching  Bias Treatment                               Growth______________________________________Bias Voltage  --       sinudoidal, half-                               --    wave rectified  Bias Voltage Value -- 0 V to -200 V --  Bias Frequency 0 50 Hz 0  H.sub.2 in sccm 1,500 1,500 1,500  CH.sub.4 in % -- 1.0 1.7  N.sub.2 in ppm 5 50 60  O.sub.2 in ppm 5 25 --  Microwave power in W 2,500 2,500 2,500  Temperature in ° C. 810 879 830  pressure in mbar 21.6 32.3 21.7  Time in min 5 20 60______________________________________ 
    
     Nucleation in the area of the center of the growth surface of Sample 3--as illustrated in FIG. 11--is better than in the case of Sample 1 but poorer than in the case of Sample 2. The nucleation on the growth surface at a distance of 10 mm from the center is illustrated in FIG. 12 and is improved with respect to nucleation in the center. 
     FIG. 19 illustrates nucleus density of oriented nuclei on the growth surface of Sample 3 as a function of the distance from the center, in which case the nucleus density is entered as 10 8  nuclei per cm 2  and the distance from the center of the growth surface of the growth substrate of Sample 3 is entered in mm. 
     As illustrated in the diagram of FIG. 19, the nucleus density of Sample 3 has a value of approximately 1×10 8  nuclei per cm 2  in the center. The nucleus density increases steadily and significantly with an increasing distance from the center and, at a distance of approximately 14-15 mm from the center, has a maximum with a nucleus density of approximately 4.4×10 8  nuclei per cm 2 . Then, the nucleus density decreases rapidly and, at a distance of approximately 20 mm, still has a nucleus density of approximately 2.2×10 8  nuclei per cm 2 . The drop of the nucleus density after the maximum is less than in the case of that of Sample 2. On the whole, it should be stated that, although the homogeneity of the nucleus density is poorer than in Sample 2, it can nevertheless be considered good particularly in a distance range between approximately 0.5 mm and 20 mm from the center. 
     FIGS. 13 and 14 each show a cutout of the growth surface at different distances from the center of the growth surface of Sample 4. In contrast to Samples 1 to 3, the nucleation on Sample 4 was carried out at a frequency of the bias voltage of 500 Hz. FIGS. 13 and 14 have been enlarged 10,000 times. 
     The conditions during nucleation on Sample 4 by means of plasma CVD are: 
     
         ______________________________________Sample 4      Etching  Bias Treatment                               Growth______________________________________Bias Voltage  --       sinudoidal, half-                               --    wave rectified  Bias Voltage Value -- 0 V to -200 V --  Bias Frequency 0 500 Hz 0  H.sub.2 in sccm 1,500 1,500 1,500  CH.sub.4 in % -- 1.0 1.7  N.sub.2 in ppm 5 50 60  O.sub.2 in ppm 5 25 --  Microwave power in W 2,500 2,500 2,500  Temperature in ° C. 810 879 830  pressure in mbar 21.7 32.2 21.2  Time in min 5 20 60______________________________________ 
    
     Nucleation in the area of the center of the growth surface of Sample 4--as illustrated in FIG. 13--is limited to individual islands which on the edge side are partly connected with one another. In principle, the nucleation looks like the one of Sample 1 (see FIG. 7) but is nevertheless better because a higher nucleus density exists. The nucleation on the growth surface at a distance of 10 mm from the center is illustrated in FIG. 14 and is improved with respect to the nucleation in the center. 
     FIG. 20 illustrates nucleus density of oriented nuclei on the growth surface of Sample 4 as a function of the distance from the center, in which case the nucleus density is entered as  10   8  nuclei per cm 2  and the distance from the center of the growth surface of the growth substrate of Sample 4 is entered in mm. 
     As illustrated in the diagram according to FIG. 20, the nucleus density of Sample 4 has a value in the center which is not worth mentioning in the entered item of 1×10 8  nuclei per cm 2 . The nucleus density steadily and very significantly increases with an increasing distance from the center and, at a distance of approximately 14-15 mm from the center, has a maximum with a nucleus density of approximately 3.4×10 8  nuclei per cm 2 . Subsequently, the nucleus density decreases rapidly and, at a distance of approximately 20 mm still has a nucleus density of approximately 0.8×10 8  nuclei per cm 2 . On the whole, it should be stated that, although the homogeneity of the nucleus density is poorer than in Samples 1 and 2, it can nevertheless be considered acceptable in a distance range between approximately 0.5 mm and 20 mm from the center. In general, the homogeneity of the nucleus density of Sample 4 is better than Sample 1. 
     FIGS. 15 and 16 each show a cutout of the growth surface at different distances from the center of the growth surface of Sample 5. The nucleation was carried out at a frequency of the bias voltage of 10 Hz. FIGS. 15 and 16 are enlarged 10,000 times. 
     The conditions during nucleation on Sample 5 by means of plasma CVD are: 
     
         ______________________________________Sample 5      Etching  Bias Treatment                               Growth______________________________________Bias Voltage  --       sinudoidal, half-                               --    wave rectified  Bias Voltage Value -- 0 V to -230 V --  Bias Frequency 0 10 Hz 0  H.sub.2 in sccm 1,500 1,500 1,500  CH.sub.4 in % -- 0.7 1.7  N.sub.2 in ppm 5 50 51  O.sub.2 in ppm 5 25  Microwave power in W 2,500 2,500 2,500  Temperature in ° C. 810 872 750  pressure in mbar 25 35 19  Time in min 5 20 300______________________________________ 
    
     As illustrated above the results of Sample 5 cannot be compared directly with the results of Samples 1 to 4 because the plasma geometry was slightly different and the sample was also subsequently coated for 5 hours after the actual nucleation. 
     Nevertheless, FIG. 21 also very clearly shows a very good radial homogeneity of nucleus density of oriented nuclei. Compared to previous examples, the absolute values of the nucleus densities are shifted toward slightly lower values which is a result of the large layer thicknesses. 
     Nucleation in the area of the center of the growth surface of Sample 5 is good--as illustrated in FIG. 15. The nucleation on the growth surface at a distance of 10 mm from the center is also good, as illustrated in FIG. 21. 
     FIG. 21 illustrates the nucleus density on the growth surface of Sample 5 as a function of the distance from the center, in which case the nucleus density is entered as 10 8  nuclei per cm 2  and the distance from the center of the growth surface of the growth substrate of Sample 5 is entered in mm. This diagram also shows the total nucleus density (by points) and also the nucleus density at the highly oriented nuclei (square points). 
     As illustrated in the diagram according to FIG. 21, the nucleus density of oriented nuclei of Sample 5 has a value of approximately 2.4×10 8  nuclei per cm 2  in the center, while total nucleus density amounts to approximately 3.4×10 8  nuclei per cm 2 . In contrast to Samples 1 to 4, no maximum of the nucleus density is formed here. On the contrary, the nucleus density increases toward the outside. Thus, the oriented nucleus density at a distance of 20 mm from the center amounts to approximately 3.8×10 8  nuclei per cm 2 , and the total nucleus density amounts to approximately 5.8×10 8  nuclei per cm 2 . On the whole, it should be stated that the homogeneity of the nucleus density over the growth surface of Sample 5 is good, in which case, starting at a distance of between 10 and 15 mm from the center, the total nucleus density increases more than the oriented nucleus density. 
     Concerning Samples 1 to 5, the homogeneity of the nucleus density is a function of the frequency; with an increasing frequency of up to approximately 500 Hz, is improved, and has the best result in a frequency range around 5 Hz. 
     Instead of a growth substrate made of crystalline Si, a growth substrate made of crystalline β silicon carbide (β SiC) can be selected. 
     Although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example, and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.