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Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process
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Diamond & Related Materials 17 (2008) 415 – 418 www.elsevier.com/locate/diamond
Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process Y. Mokuno ⁎, A. Chayahara, H. Yamada Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 12 January 2008
Abstract A process of making a large, thick single crystal CVD diamond plates has been developed. This process consists of high rate homoepitaxial growth of CVD diamond and subsequent lift-off process using ion implantation. By using this process, single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm3 have been successfully fabricated. The crystallinity of the CVD diamond plates has been evaluated by X-ray topography, polarized light microscopy and high resolution X-ray diffraction. The results indicate the pretreatment of the seed substrate has strong effect on the crystallinity of the CVD diamond plates. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond; Microwave plasma CVD; High rate growth; Ion implantation; Lift-off; Self-standing plate
1. Introduction To realize a large-area, single crystal diamond wafer, large bulk diamonds and their slicing technique are required. Recent developments in chemical vapor deposition (CVD) process realize high rate growth of single crystal CVD diamond faster than 10 μm/h [1–3]. These diamonds are generally grown under the high power density plasma with the high methane concentration. Especially, by adding small amount of nitrogen for the purpose of promoting (100) growth, the growth temperature can be raised up to over 1100 °C and the growth rate exceeds 100 μm/h [1,2]. This very high rate growth technique has been successfully applied to grow very thick diamonds having 10 mm in thickness and a method of further enlarging the crystal size has been proposed [4]. However, slicing a bulk diamond is still difficult because diamond is the hardest material. When a conventional cutting technique, such as sawing or laser sawing, is used, a large amount of cutting loss (N0.3 mm in thickness) will be expected for slicing such a bulk diamond into wafers. One of the ⁎ Corresponding author. 1-8-31 Midorigaoka, Ikeda, Osaka, 563-8577, Japan. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (Y. Mokuno). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.058
promising solutions to minimize the cutting loss is to use a liftoff process using ion implantation. This process consists of ion implantation to a substrate to create a damaged layer under the surface, annealing of the substrate to convert the damaged layer into graphite and subsequent etching of the graphite layer and separation of a diamond film. This was first demonstrated by Parikh et al. [5] for removing thin diamond film from a diamond substrate. This was later used to separate homoepitaxially grown CVD diamond film grown on the ion implanted substrate [6,7]. However, the substrate size in these studies was a few mm square, and the film thickness was less than 0.1 mm which was not enough to use as a wafer. In this work, we tried to combine the high rate CVD homoepitaxial growth process and the improved lift-off process to make a further large (10 × 10 mm2) and thick (N0.2 mm) single crystal diamond plates. The crystallinity of these diamond plates was evaluated and the influence of surface pretreatment of the seed substrate prior to the CVD growth was investigated. 2. Experimental Single crystal HPHT Ib (100) diamonds were used as a seed substrate for the process of growing and separating CVD diamond plates. Before growing CVD diamond, the substrate was
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(XRD). The X-ray topograph was taken using Mo Kα X-rays with transmission (g = 220) diffraction geometry. The X-ray rocking curves of the (004) reflection were measured by an Xray diffractometer equipped with a Ge (440) four-crystal monochromator for Cu Kα X-rays. 3. Results and discussion
implanted with carbon ions at the energies of 3 MeV or 180 keV using a 1.5 MeV tandem accelerator (Nissin High Voltage NT1500) or a 200 kV ion implanter (ULVAC IKX-3500). The projectile ranges in diamond are 1.6 and 0.22 μm, respectively. After the ion implantation, single crystal diamond was grown on the ion implanted substrate by an ASTEX-type 5 kW CVD reactor (Seki Technotron AX-5250). The growth rate was enhanced by using a specially designed substrate holder and nitrogen addition [2,4]. The substrate temperature during the growth was 1130–1150 °C, which was measured by optical pyrometer (Chino IR-U). The pressure of the reactor was 24 kPa and the gas flow rate was 500 sccm for H2, 60 sccm for CH4 and 0.6 sccm for N2. The high growth temperature is enough to convert the damaged ion implanted layer into the graphite. After the growth, the graphite layer was etched by an etching process using a similar method proposed by Marchywka et al [7]. The surface morphology of the CVD diamond plate separated from the substrate was characterized by differential interferencecontrast microscopy (DICM) and atomic force microscopy (AFM). The crystallinity was evaluated by transmission X-ray topography, polarized light microscopy (PLM) and highresolution rocking curve measurement of X-ray diffraction
High rate growth of CVD diamond was applied to the ion implanted substrate with the different ion energies. Fig. 1 shows the surface morphology of the single crystal CVD diamond films grown on the HPHT diamond substrates implanted with carbon ions at 3 MeV and 180 keV. The ion dose should be larger than a critical dose which is necessary to lead to graphitization in the next growth (or annealing) process, but at the same time, as low as possible to minimize surface damage. In the present case, the selected ion doses were 2 × 1016 ions/cm2 and 1 × 1016 ions/cm2, respectively. The film thicknesses were 0.29 and 0.38 mm after 6 h growth. Both films could be successfully separated from the substrate after the etching process. However, in case of 180 keV implantation, the grown film contains many hillocks on the surface. In contrast, using high-energy (3 MeV) ion implantation, the film with smooth surface morphology is obtained. This result suggests that low surface damage created by high-energy ion implantation is beneficial for the high rate growth process. Also, the thickness of diamond above the ion implanted layer increases with increasing the range of ions. This improve the process margin at the initial stages of the CVD process where the etching of diamond can not be negligible. The lift-off process using high-energy ion implantation was successfully applied to larger HPHT diamond substrates with the size of about 10 × 10 mm2. The substrate was implanted with 3 MeV carbon ions with the dose of 2 × 1016 ions/cm2. After the ion implantation, single crystal CVD diamond was grown on the substrate. For this purpose, a substrate holder with the upper surface diameter of 18 mm was used. Using this holder and the above-mentioned growth conditions, the typical growth rate of 30–50 μm/h was obtained over the entire (10 × 10 mm2) growth area. The etching of graphite layer took typically 1–2 days, and
Fig. 2. A 10 × 10 × 0.4 mm3 single crystal CVD diamond plate (right) separated from a HPHT diamond substrate (left) by combination of high rate growth and lift-off process using ion implantation.
Fig. 3. Large CVD diamond plates mass-produced by combination of high rate growth and lift-off process using ion implantation.
Fig. 1. DICM images of CVD diamond films grown on carbon ion implanted substrates at the energy of (a) 3 MeV and (b) 180 keV.
Y. Mokuno et al. / Diamond & Related Materials 17 (2008) 415–418
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Fig. 4. Crystallinity of a CVD diamond plate evaluated by (a) Transmission X-ray topograph and (b) PLM image.
as the result, a single crystal CVD diamond plate of similar size to the substrate could be separated. Fig. 2 shows an example of a separated CVD diamond plate with the thickness of 0.4 mm together with the HPHT substrate. Since the process is rapid and reliable, a lot of CVD diamond plates with the similar size can be produced as shown in Fig. 3. Crystallinity of the CVD diamond plates was evaluated by transmission X-ray topography and PLM. Fig. 4 shows the result for the CVD diamond plate with the size of about 5 mm in diameter and the thickness of 0.29 mm. In the X-ray topograph, many spot like and line shaped dislocation patterns were observed. Most of these patterns can be also observed in the
PLM image, because of the high contrast due to the increased thickness of the CVD diamond plate. The PLM image can be used as a rapid and easy evaluation method for monitoring dislocations in the CVD diamond plates. To improve the crystallinity of the CVD diamond plate, the influence of pretreatment on the crystallinity of CVD plates was investigated. Fig. 5 compares the PLM images of large (9 × 9 × 0.3–0.4 mm3) CVD diamond plates grown and separated from the same HPHT substrate with different pretreatments. The AFM images indicated below show the surface morphology of the separation plane (backside) of the each diamond plate which is almost identical to that of the initial substrate surface before
Fig. 5. PLM images of CVD diamond plates grown and separated from the same seed with different pretreatment. Lower images are AFM images of the separation plane (backside) of the each CVD plates reflecting the surface morphology of the initial substrate surface before the growth.
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Fig. 6. PLM images of (a) HPHT substrate in Fig. 2 (b) 1st CVD diamond plate grown and separated from the substrate (c) 2nd CVD diamond plate produced from the same substrate reused without polishing the surface.
the growth. The substrate in Fig. 5(a) was an as-received one with a rough surface and the substrate was used again in Fig. 5(b) after polishing the surface. The surface roughness (Ra) measured for the 10 × 10 μm2 area is 2.3 nm for the as-received substrate and 0.84 nm after polishing. The PLM image in Fig. 5(a) indicates the presence of many dislocations in the CVD diamond plate. On the other hand, the PLM image in Fig. 5(b) only shows smaller number of spot like patterns presumably associated with a dislocation bundle which is frequently observed in CVD diamond. From the PLM observation, most of these patterns seems to start at near the separation plane of the CVD diamond plate. These results clearly indicate the importance of surface finishing of the substrate. The change in crystallinity was also confirmed by high resolution rocking curve measurement of XRD. The FWHM was 38 arcsec for the sample in Fig. 5(a) and 7.6 arcsec for the sample in Fig. 5(b). The FWHM for the latter sample is comparable to the typical value for HPHT Ib diamond (6–20 arcsec) [8], indicating the crystallinity of the CVD diamond plate is comparable to that of the HPHT Ib diamond. In the present process, the substrate could be used as a seed for more than three times without polishing the surface. Fig. 6 shows PLM images of the HPHT substrate in Fig. 2 and two CVD plates obtained by repeating the lift-off process without polishing the substrate surface. The plate thickness was 0.4 mm. Despite no dislocation patterns are observed in the substrate, many dislocation patterns are appeared in the 1st CVD diamond plate. But the number of the dislocation patterns is clearly decreased in the 2nd CVD diamond plate. Similar improvement in crystalline quality was observed in the previous study [9]. However, since the growth condition is the same for these plates, this improvement may be attributed to the removal of surface damage on the substrate. By the lift-off process, the upper part of the seed above the ion implanted layer is removed with a CVD diamond plate. Since the range of ions is long (1.6 μm) for high-energy ion implantation, this would lead to
the removal of the surface damage formed by mechanical polishing process. 4. Summary Large, thick single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm 3 has been successfully synthesized by combining the high rate homoepitaxial growth of CVD diamond and the lift-off process using high-energy (MeV) ion implantation. The PLM images and the result of high resolution XRD rocking curve measurements indicate the surface pretreatment of the substrate has strong effects on the crystallinity of the CVD diamond plates. Also, the substrate can be reused without polishing the surface. The quality of the CVD diamond plate is improved by repeating the lift-off process due to the possible removal of the surface damage of the substrate. References [1] C.S. Yan, Y.K. Vohra, H.K. Mao, R.J. Hemley, Proc. Natl. Acad. Sci. U. S.A. 99 (2002) 12523. [2] A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, N. Fujimori, Diamond Relat. Mater. 13 (2004) 1954. [3] A. Tallaire, J. Achard, F. Silva, R.S. Sussmann, A. Gicquel, Diamond Relat. Mater. 14 (2005) 249. [4] Y. Mokuno, A. Chayahara, Y. Soda, Y. Horino, N. Fujimori, Diamond Relat. Mater. 14 (2005) 1743–1746. [5] N.R. Parikh, J.D. Hunn, E. McGucken, M.L. Swanson, C.W. White, R.A. Rudder, D.P. Malta, J.B. Posthill, R.J. Markunas, Appl. Phys. Lett. 61 (1992) 3124. [6] Y. Tzeng, J. Wei, J.T. Woo, W. Lanford, Appl. Phys. Lett. 63 (1993) 2216. [7] M. Marchywka, P.E. Pehrsson, D.J. Vestyck Jr., D. Moses, Appl. Phys. Lett. 63 (1993) 3521. [8] H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Cryst. Growth 178 (1997) 485. [9] R. Locher, D. Behr, H. Gullich, N. Herres, P. Koidl, R. Samlenski, R. Brenn, Diamond Relat. Mater. 6 (1997) 654.
Diamond & Related Materials 17 (2008) 415 – 418 www.elsevier.com/locate/diamond
Synthesis of large single crystal diamond plates by high rate homoepitaxial growth using microwave plasma CVD and lift-off process Y. Mokuno ⁎, A. Chayahara, H. Yamada Diamond Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Available online 12 January 2008
Abstract A process of making a large, thick single crystal CVD diamond plates has been developed. This process consists of high rate homoepitaxial growth of CVD diamond and subsequent lift-off process using ion implantation. By using this process, single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm3 have been successfully fabricated. The crystallinity of the CVD diamond plates has been evaluated by X-ray topography, polarized light microscopy and high resolution X-ray diffraction. The results indicate the pretreatment of the seed substrate has strong effect on the crystallinity of the CVD diamond plates. © 2008 Elsevier B.V. All rights reserved. Keywords: Diamond; Microwave plasma CVD; High rate growth; Ion implantation; Lift-off; Self-standing plate
1. Introduction To realize a large-area, single crystal diamond wafer, large bulk diamonds and their slicing technique are required. Recent developments in chemical vapor deposition (CVD) process realize high rate growth of single crystal CVD diamond faster than 10 μm/h [1–3]. These diamonds are generally grown under the high power density plasma with the high methane concentration. Especially, by adding small amount of nitrogen for the purpose of promoting (100) growth, the growth temperature can be raised up to over 1100 °C and the growth rate exceeds 100 μm/h [1,2]. This very high rate growth technique has been successfully applied to grow very thick diamonds having 10 mm in thickness and a method of further enlarging the crystal size has been proposed [4]. However, slicing a bulk diamond is still difficult because diamond is the hardest material. When a conventional cutting technique, such as sawing or laser sawing, is used, a large amount of cutting loss (N0.3 mm in thickness) will be expected for slicing such a bulk diamond into wafers. One of the ⁎ Corresponding author. 1-8-31 Midorigaoka, Ikeda, Osaka, 563-8577, Japan. Tel.: +81 72 751 9531; fax: +81 72 751 9631. E-mail address: [email protected] (Y. Mokuno). 0925-9635/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2007.12.058
promising solutions to minimize the cutting loss is to use a liftoff process using ion implantation. This process consists of ion implantation to a substrate to create a damaged layer under the surface, annealing of the substrate to convert the damaged layer into graphite and subsequent etching of the graphite layer and separation of a diamond film. This was first demonstrated by Parikh et al. [5] for removing thin diamond film from a diamond substrate. This was later used to separate homoepitaxially grown CVD diamond film grown on the ion implanted substrate [6,7]. However, the substrate size in these studies was a few mm square, and the film thickness was less than 0.1 mm which was not enough to use as a wafer. In this work, we tried to combine the high rate CVD homoepitaxial growth process and the improved lift-off process to make a further large (10 × 10 mm2) and thick (N0.2 mm) single crystal diamond plates. The crystallinity of these diamond plates was evaluated and the influence of surface pretreatment of the seed substrate prior to the CVD growth was investigated. 2. Experimental Single crystal HPHT Ib (100) diamonds were used as a seed substrate for the process of growing and separating CVD diamond plates. Before growing CVD diamond, the substrate was
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(XRD). The X-ray topograph was taken using Mo Kα X-rays with transmission (g = 220) diffraction geometry. The X-ray rocking curves of the (004) reflection were measured by an Xray diffractometer equipped with a Ge (440) four-crystal monochromator for Cu Kα X-rays. 3. Results and discussion
implanted with carbon ions at the energies of 3 MeV or 180 keV using a 1.5 MeV tandem accelerator (Nissin High Voltage NT1500) or a 200 kV ion implanter (ULVAC IKX-3500). The projectile ranges in diamond are 1.6 and 0.22 μm, respectively. After the ion implantation, single crystal diamond was grown on the ion implanted substrate by an ASTEX-type 5 kW CVD reactor (Seki Technotron AX-5250). The growth rate was enhanced by using a specially designed substrate holder and nitrogen addition [2,4]. The substrate temperature during the growth was 1130–1150 °C, which was measured by optical pyrometer (Chino IR-U). The pressure of the reactor was 24 kPa and the gas flow rate was 500 sccm for H2, 60 sccm for CH4 and 0.6 sccm for N2. The high growth temperature is enough to convert the damaged ion implanted layer into the graphite. After the growth, the graphite layer was etched by an etching process using a similar method proposed by Marchywka et al [7]. The surface morphology of the CVD diamond plate separated from the substrate was characterized by differential interferencecontrast microscopy (DICM) and atomic force microscopy (AFM). The crystallinity was evaluated by transmission X-ray topography, polarized light microscopy (PLM) and highresolution rocking curve measurement of X-ray diffraction
High rate growth of CVD diamond was applied to the ion implanted substrate with the different ion energies. Fig. 1 shows the surface morphology of the single crystal CVD diamond films grown on the HPHT diamond substrates implanted with carbon ions at 3 MeV and 180 keV. The ion dose should be larger than a critical dose which is necessary to lead to graphitization in the next growth (or annealing) process, but at the same time, as low as possible to minimize surface damage. In the present case, the selected ion doses were 2 × 1016 ions/cm2 and 1 × 1016 ions/cm2, respectively. The film thicknesses were 0.29 and 0.38 mm after 6 h growth. Both films could be successfully separated from the substrate after the etching process. However, in case of 180 keV implantation, the grown film contains many hillocks on the surface. In contrast, using high-energy (3 MeV) ion implantation, the film with smooth surface morphology is obtained. This result suggests that low surface damage created by high-energy ion implantation is beneficial for the high rate growth process. Also, the thickness of diamond above the ion implanted layer increases with increasing the range of ions. This improve the process margin at the initial stages of the CVD process where the etching of diamond can not be negligible. The lift-off process using high-energy ion implantation was successfully applied to larger HPHT diamond substrates with the size of about 10 × 10 mm2. The substrate was implanted with 3 MeV carbon ions with the dose of 2 × 1016 ions/cm2. After the ion implantation, single crystal CVD diamond was grown on the substrate. For this purpose, a substrate holder with the upper surface diameter of 18 mm was used. Using this holder and the above-mentioned growth conditions, the typical growth rate of 30–50 μm/h was obtained over the entire (10 × 10 mm2) growth area. The etching of graphite layer took typically 1–2 days, and
Fig. 2. A 10 × 10 × 0.4 mm3 single crystal CVD diamond plate (right) separated from a HPHT diamond substrate (left) by combination of high rate growth and lift-off process using ion implantation.
Fig. 3. Large CVD diamond plates mass-produced by combination of high rate growth and lift-off process using ion implantation.
Fig. 1. DICM images of CVD diamond films grown on carbon ion implanted substrates at the energy of (a) 3 MeV and (b) 180 keV.
Y. Mokuno et al. / Diamond & Related Materials 17 (2008) 415–418
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Fig. 4. Crystallinity of a CVD diamond plate evaluated by (a) Transmission X-ray topograph and (b) PLM image.
as the result, a single crystal CVD diamond plate of similar size to the substrate could be separated. Fig. 2 shows an example of a separated CVD diamond plate with the thickness of 0.4 mm together with the HPHT substrate. Since the process is rapid and reliable, a lot of CVD diamond plates with the similar size can be produced as shown in Fig. 3. Crystallinity of the CVD diamond plates was evaluated by transmission X-ray topography and PLM. Fig. 4 shows the result for the CVD diamond plate with the size of about 5 mm in diameter and the thickness of 0.29 mm. In the X-ray topograph, many spot like and line shaped dislocation patterns were observed. Most of these patterns can be also observed in the
PLM image, because of the high contrast due to the increased thickness of the CVD diamond plate. The PLM image can be used as a rapid and easy evaluation method for monitoring dislocations in the CVD diamond plates. To improve the crystallinity of the CVD diamond plate, the influence of pretreatment on the crystallinity of CVD plates was investigated. Fig. 5 compares the PLM images of large (9 × 9 × 0.3–0.4 mm3) CVD diamond plates grown and separated from the same HPHT substrate with different pretreatments. The AFM images indicated below show the surface morphology of the separation plane (backside) of the each diamond plate which is almost identical to that of the initial substrate surface before
Fig. 5. PLM images of CVD diamond plates grown and separated from the same seed with different pretreatment. Lower images are AFM images of the separation plane (backside) of the each CVD plates reflecting the surface morphology of the initial substrate surface before the growth.
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Fig. 6. PLM images of (a) HPHT substrate in Fig. 2 (b) 1st CVD diamond plate grown and separated from the substrate (c) 2nd CVD diamond plate produced from the same substrate reused without polishing the surface.
the growth. The substrate in Fig. 5(a) was an as-received one with a rough surface and the substrate was used again in Fig. 5(b) after polishing the surface. The surface roughness (Ra) measured for the 10 × 10 μm2 area is 2.3 nm for the as-received substrate and 0.84 nm after polishing. The PLM image in Fig. 5(a) indicates the presence of many dislocations in the CVD diamond plate. On the other hand, the PLM image in Fig. 5(b) only shows smaller number of spot like patterns presumably associated with a dislocation bundle which is frequently observed in CVD diamond. From the PLM observation, most of these patterns seems to start at near the separation plane of the CVD diamond plate. These results clearly indicate the importance of surface finishing of the substrate. The change in crystallinity was also confirmed by high resolution rocking curve measurement of XRD. The FWHM was 38 arcsec for the sample in Fig. 5(a) and 7.6 arcsec for the sample in Fig. 5(b). The FWHM for the latter sample is comparable to the typical value for HPHT Ib diamond (6–20 arcsec) [8], indicating the crystallinity of the CVD diamond plate is comparable to that of the HPHT Ib diamond. In the present process, the substrate could be used as a seed for more than three times without polishing the surface. Fig. 6 shows PLM images of the HPHT substrate in Fig. 2 and two CVD plates obtained by repeating the lift-off process without polishing the substrate surface. The plate thickness was 0.4 mm. Despite no dislocation patterns are observed in the substrate, many dislocation patterns are appeared in the 1st CVD diamond plate. But the number of the dislocation patterns is clearly decreased in the 2nd CVD diamond plate. Similar improvement in crystalline quality was observed in the previous study [9]. However, since the growth condition is the same for these plates, this improvement may be attributed to the removal of surface damage on the substrate. By the lift-off process, the upper part of the seed above the ion implanted layer is removed with a CVD diamond plate. Since the range of ions is long (1.6 μm) for high-energy ion implantation, this would lead to
the removal of the surface damage formed by mechanical polishing process. 4. Summary Large, thick single crystal CVD diamond plates with the size of about 10 × 10 × 0.2–0.45 mm 3 has been successfully synthesized by combining the high rate homoepitaxial growth of CVD diamond and the lift-off process using high-energy (MeV) ion implantation. The PLM images and the result of high resolution XRD rocking curve measurements indicate the surface pretreatment of the substrate has strong effects on the crystallinity of the CVD diamond plates. Also, the substrate can be reused without polishing the surface. The quality of the CVD diamond plate is improved by repeating the lift-off process due to the possible removal of the surface damage of the substrate. References [1] C.S. Yan, Y.K. Vohra, H.K. Mao, R.J. Hemley, Proc. Natl. Acad. Sci. U. S.A. 99 (2002) 12523. [2] A. Chayahara, Y. Mokuno, Y. Horino, Y. Takasu, H. Kato, N. Fujimori, Diamond Relat. Mater. 13 (2004) 1954. [3] A. Tallaire, J. Achard, F. Silva, R.S. Sussmann, A. Gicquel, Diamond Relat. Mater. 14 (2005) 249. [4] Y. Mokuno, A. Chayahara, Y. Soda, Y. Horino, N. Fujimori, Diamond Relat. Mater. 14 (2005) 1743–1746. [5] N.R. Parikh, J.D. Hunn, E. McGucken, M.L. Swanson, C.W. White, R.A. Rudder, D.P. Malta, J.B. Posthill, R.J. Markunas, Appl. Phys. Lett. 61 (1992) 3124. [6] Y. Tzeng, J. Wei, J.T. Woo, W. Lanford, Appl. Phys. Lett. 63 (1993) 2216. [7] M. Marchywka, P.E. Pehrsson, D.J. Vestyck Jr., D. Moses, Appl. Phys. Lett. 63 (1993) 3521. [8] H. Sumiya, N. Toda, Y. Nishibayashi, S. Satoh, J. Cryst. Growth 178 (1997) 485. [9] R. Locher, D. Behr, H. Gullich, N. Herres, P. Koidl, R. Samlenski, R. Brenn, Diamond Relat. Mater. 6 (1997) 654.