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Spiky diamond field emitters
Diamond and Related Materials 12 (2003) 1681–1684
Spiky diamond field emitters Yutaka Andoa,*, Yoshiki Nishibayashia, Hiroshi Furutaa, Koji Kobashia, Takashi Hiraob, Kenjiro Ourab a
FCT ProjectyJFCC, Center for Advanced Research Projects, 6F, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan b Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract An array of very sharp emitter tips, whose top curvatures were less than 10 nm, was fabricated at the surfaces of boron (B)doped high-pressure high-temperature synthetic diamond by reactive ion etching. Electron emission from the emitter arrays was measured using a planar-diode-type I–V measurement system that can also observe a 2D mapping of emission sites by scanning an anode with a 20-mmf pinhole. The shape of each emitter was carefully observed by scanning electron microscopy. It was found that electrons were intensely emitted from the areas where sharp emitters were successfully fabricated. In certain areas, however, electrons were not at all emitted even though very sharp emitters were formed. It was inferred that the emitters were fabricated on growth sectors that had high resistivity. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Field emission; Microstructure; Reactive ion etching; Single crystals; Diamond
1. Introduction Diamond is considered to be a good cold cathode material because of its unique properties such as negative electron affinity (NEA), extremely high thermal conductivity, high hardness and chemical inertness. So far, considerable efforts have been made to fabricate diamond field emitters w1–10x. However, most of these emitters were made using polycrystalline diamond films and diamond films that included significant amount of non-diamond carbons. Thus, it is considered that several kinds of factors such as grain boundaries, surface morphology and lattice defects are complicatedly involved in the field emission process. It is thus very difficult to estimate how each effect influences the field emission of electrons and how product specifications can be met using such materials. By contrast, the electron filed emitters using single-crystal diamonds, which are presented in this work, have many advantages over polycrystalline or highly defective diamond films because of its uniformity and the NEA property at hydrogenated *Corresponding author. Department of Electrical Engineering and Electronics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Huchinobe, Sagamihara, Kanagawa 229–8558, Japan. Tel.: q81-42-759-6252; fax: q81-42-759-6522. E-mail address: [email protected] (Y. Ando).
surfaces. Several works about the field emitter using single-crystal diamonds have been reported w11–14x. For instance: (i) A diode-type electron emitter using a homoepitaxial diamond layer was fabricated w12x. The emission efficiency (defined as the emission current divided by the injection current) of this emitter was reached to 100%. (ii) An emitter tip array using singlecrystal diamond has been made by an oriented crystal growth technique, in which a specific crystal surface of diamond can be selected to maximize the electron emission efficiency w13x. (iii) A low threshold electron emission was observed from a smooth surface of Ndoped homoepitaxial diamond film w14x. Recently, we have developed a microfabrication technique of diamond and successfully fabricated microcylindrical arrays with high aspect ratios (the heighty diameter was ;8 for standard array structures and ;25 for unique cases) on smooth basal surfaces (Ra-0.4 nm) w15x. Using this technique, single-crystal diamond emitters with very sharp tips, whose top curvatures were less than 10 nm, were fabricated w16x. In the present work, we will report the observed results of electron emission from the array with sharp emitters fabricated at a single-crystal diamond surface, and discuss the relationships between the emission sites and the emitter structures.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-9635(03)00257-7
Y. Ando et al. / Diamond and Related Materials 12 (2003) 1681–1684
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(SEM) were used to observe the growth sector boundaries of the HP-HT diamond substrate and the tip structures of the emitter array. The field emission from the diamond emitter array was measured using a planardiode-type I–V measurement system (Tokyo Cathode Lab., Cathode Emission Profiler CEPS-OFE). Also, a 2D mapping of electron emission sites was observed by the same system by scanning the anode with a 20-mmf pinhole over the specimen. The distance between the specimen and the anode was set at 100 mm for I–V measurements and emission mappings. 3. Results and discussion
Fig. 1. SEM micrograph of an array of single-crystal diamond emitters with very sharp tips fabricated by RIE. The inset is a magnified view of an emitter.
2. Experimental Diamond used in the present work was a 1.5=2.0 mm2 B-doped high-pressure high-temperature (HP-HT) synthetic diamond with (1 0 0) surface. As the masks, an array of Al dots of 1.5 mm in diameter and 0.5 mm in thickness was formed on the diamond surface by conventional photolithography. For the etching of diamond, a reactive ion etching (RIE) system (Anelva L201D-L) with 13.56 MHz, 300 W RF power was used. The base pressure was less than 9.8=10y5 Pa. The etching conditions, i.e. the CF4 yO2 ratio, the total gas pressure and the RF power, were 2%, 2 Pa and 200 W, respectively. The etching time was 75 min. The sample holder was water-cooled, but the temperature of the diamond specimen, measured using an optical pyrometer, was 470–570 K during the plasma etching. The substrate was cleaned with HF after the etching. An optical microscopy and a scanning electron microscopy
Fig. 1 shows an array of single-crystal diamond emitters with very sharp tips fabricated on a B-doped HP-HT synthetic diamond by RIE. The inset in Fig. 1 shows a magnified view of one of the emitters. The spacing between adjacent emitters is 10 mm, and the minimum value of the tip radius rt is less than 10 nm although rt depended on the size of Al mask and the etching time. The details of the microfabrication processes to make the sharp tips were presented in our previous work w16x. Fig. 2a shows the sample observed by optical microscopy. The dark areas in Fig. 2a except for the edge region are where Al masks were successfully patterned and thus the sharp tips were supposed to be fabricated. Unfortunately, the Al masks were not perfectly uniform over the whole sample surface, because the patterning technique on such a small diamond surface was not yet optimized for this sample. Fig. 2b shows a 2D mapping of electron emission sites observed using a planar-diode-type I–V measurement system by scanning an anode with a 20-mmf pinhole over the sample surface. From the results shown in Fig. 2, it was concluded that the electron emission sites approximately correspond to the areas where sharp tips were successfully fabricated. However, a closer exami-
Fig. 2. (a) Sample surface observed by optical microscopy. (b) 2D mapping of electron emission sites.
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Fig. 3. (a) SEM micrograph of an array of single-crystal diamond emitter tips used for electron emission measurement. (b–f) Typical shapes of five kinds of emitter sites.
nation of the area indicated that the correspondence was not precise. For instance, Fig. 3 shows an array of tips at the diamond surface observed by SEM. There were approximately 5000 patterned Al masks at the substrate surface, but only 400 sharp tips were successfully fabricated. We classified all the tips in five categories on the basis of the sharpness, as shown in Fig. 3b–f, i.e. (b) well-sharpened, (c) sharpened, (d) columnar, (e) collapsed and (f) flat, and made a map of sharpness as shown in Fig. 4. Note that Fig. 4 is not a map of emission sites, but indicates the sharpness at the emitter sites. The white dots in Fig. 4 indicate the sites where well-sharpened tips existed. By comparing the sharp emitter area of Fig. 4 with the emission sites of Fig. 2b, it was found that they were in fairly good agreement. That is, it was concluded that electrons were mostly emitted from sharp tips. In Fig. 5, the areas surrounded
Fig. 4. 2D mapping on tip sharpness.
by the curves are electron emission sites and the white dots are sharp emitter sites. One may, however, notice that there are areas where very sharp tips existed (see the areas indicated by ‘a’ in Fig. 5), but electrons were not at all emitted. Fig. 6 shows the sample surface observed by optical microscopy, where the dotted lines indicate the growth sector boundary of the B-doped single-crystal diamond surface. It is generally known that for HP-HT synthetic diamonds, the impurity concentration (in the present case, B concentration) is
Fig. 5. Relationship between electron emission and sharp emitter sites. The areas surrounded by the curves and the white dots indicate the electron emission sites and the sharp emitter sites, respectively.
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existed was evaluated to be ;100 mAycm2 at 2000 V (y100 mm). 4. Conclusion
Fig. 6. Sample surface observed by optical microscopy. The dotted lines indicate the growth sector boundary of the HP-HT diamond used.
strongly dependent on growth sectors w17x. The substrate we used was a B-doped conducting HP-HT diamond, and it was inferred that the conductivity was extremely low in the growth sectors that included the area ‘a’ of Fig. 5, and this can be the reason that there was no electron emission from the area ‘a’. From I–V characteristic measurements (Fig. 7), the threshold voltage of field emission for 1.0=10y8 A was approximately 1000 V (y100 mm), and the emission current density was approximately 1 mAycm2 at 2000 V (y100 mm), if the emission current was divided by the whole area (0.03 cm2) of sample. In the present work, we found that the electron was mainly emitted from the areas where sharp tips were fabricated, except for the area ‘a’. Thus, the emission current divided by the areas (10 mm=10 mm=400 tipss4=10y4 cm2) where sharp emitters
Fig. 7. I–V characteristic of single-crystal diamond emitters with very sharp tips fabricated by RIE.
An array of single-crystal diamond emitters with very sharp tips, whose radii of curvatures at the top were less than 10 nm, was fabricated on a B-doped HP-HT synthetic diamond by RIE. A 2D mapping of electron emission sites was obtained for the emitter array. It was found that electrons were mainly emitted from the areas where sharp tips were fabricated, except for the area ‘a’. The emission current divided by the area where the sharp emitters existed was evaluated to be ;100 mAy cm2 at 2000 V (y100 mm). It is expected that further optimization of the emitter shape, the emitter density and the device structure will improve the threshold voltage and the emission current to a level enabling practical applications. Acknowledgments This work was supported by the FCT Project, which was consigned to JFCC by NEDO. References w1x K.M. Song, J.Y. Shim, H.K. Baik, Diamond Relat. Mater. 11 (2002) 185–190. w2x A. Hatta, T. Sumitomo, H. Inomoto, A. Hiraki, New Diamond Front. Carbon Technol. 11 (5) (2001) 307–312. w3x A. Karabutov, V. Ralchenko, I. Vlasov, et al., New Diamond Front. Carbon Technol. 11 (5) (2001) 355–364. w4x V.D. Frolov, A.V. Karabutov, S.M. Pimenov, V.I. Konov, Diamond Relat. Mater. 9 (2000) 1196–1200. w5x C.F. Chen, H.C. Hsief, Diamond Relat. Mater. 9 (2000) 1257–1262. w6x M.V. Ugarov, V.P. Ageev, A.V. Karabutov, et al., J. Appl. Phys. 85 (12) (1999) 8436–8440. w7x K. Okano, T. Yamada, A. Sawabe, et al., Appl. Surf. Sci. 146 (1999) 274–279. w8x H. Ito, Y. Show, M. Iwase, T. Izumi, Diamond Films Technol. 9 (2) (1999) 147–150. w9x Y. Show, F. Matsuoka, M. Hayashi, H. Ito, T. Izumi, M. Iwase, Diamond Films Technol. 8 (6) (1998) 451–456. w10x K. Kuriyama, S. Kawasaki, T. Sugino, Diamond Films Technol. 8 (6) (1998) 441–450. w11x C. Kimura, K. Kuriyama, S. Koizumi, M. Kamo, T. Sugino, Appl. Surf. Sci. 146 (1999) 295–298. w12x M. Nishimura, K. Ogawa, A. Hatta, T. Ito, Diamond Relat. Mater. 7 (1999) 754–758. w13x Y. Nishibayashi, H. Saito, T. Imai, N. Fujimori, Diamond Relat. Mater. 9 (2000) 290–294. w14x T. Yamada, A. Sawabe, S. Koizumi, T. Kamio, K. Okano, Diamond Relat. Mater. 11 (2002) 257–261. w15x Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, K. Oura, Diamond Relat. Mater. 11 (2002) 824–827. w16x Y. Ando, Y. Nishibayashi, H. Furuta, K. Kobashi, T. Hirao, K. Oura, New Diamond Front. Carbon Technol. 12 (3) (2002) 137–140. w17x R.C. Burns, V. Cvetkovic, C.N. Dodge, et al., J. Cryst. Growth 104 (1990) 257.
Spiky diamond field emitters Yutaka Andoa,*, Yoshiki Nishibayashia, Hiroshi Furutaa, Koji Kobashia, Takashi Hiraob, Kenjiro Ourab a
FCT ProjectyJFCC, Center for Advanced Research Projects, 6F, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan b Faculty of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
Abstract An array of very sharp emitter tips, whose top curvatures were less than 10 nm, was fabricated at the surfaces of boron (B)doped high-pressure high-temperature synthetic diamond by reactive ion etching. Electron emission from the emitter arrays was measured using a planar-diode-type I–V measurement system that can also observe a 2D mapping of emission sites by scanning an anode with a 20-mmf pinhole. The shape of each emitter was carefully observed by scanning electron microscopy. It was found that electrons were intensely emitted from the areas where sharp emitters were successfully fabricated. In certain areas, however, electrons were not at all emitted even though very sharp emitters were formed. It was inferred that the emitters were fabricated on growth sectors that had high resistivity. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Field emission; Microstructure; Reactive ion etching; Single crystals; Diamond
1. Introduction Diamond is considered to be a good cold cathode material because of its unique properties such as negative electron affinity (NEA), extremely high thermal conductivity, high hardness and chemical inertness. So far, considerable efforts have been made to fabricate diamond field emitters w1–10x. However, most of these emitters were made using polycrystalline diamond films and diamond films that included significant amount of non-diamond carbons. Thus, it is considered that several kinds of factors such as grain boundaries, surface morphology and lattice defects are complicatedly involved in the field emission process. It is thus very difficult to estimate how each effect influences the field emission of electrons and how product specifications can be met using such materials. By contrast, the electron filed emitters using single-crystal diamonds, which are presented in this work, have many advantages over polycrystalline or highly defective diamond films because of its uniformity and the NEA property at hydrogenated *Corresponding author. Department of Electrical Engineering and Electronics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Huchinobe, Sagamihara, Kanagawa 229–8558, Japan. Tel.: q81-42-759-6252; fax: q81-42-759-6522. E-mail address: [email protected] (Y. Ando).
surfaces. Several works about the field emitter using single-crystal diamonds have been reported w11–14x. For instance: (i) A diode-type electron emitter using a homoepitaxial diamond layer was fabricated w12x. The emission efficiency (defined as the emission current divided by the injection current) of this emitter was reached to 100%. (ii) An emitter tip array using singlecrystal diamond has been made by an oriented crystal growth technique, in which a specific crystal surface of diamond can be selected to maximize the electron emission efficiency w13x. (iii) A low threshold electron emission was observed from a smooth surface of Ndoped homoepitaxial diamond film w14x. Recently, we have developed a microfabrication technique of diamond and successfully fabricated microcylindrical arrays with high aspect ratios (the heighty diameter was ;8 for standard array structures and ;25 for unique cases) on smooth basal surfaces (Ra-0.4 nm) w15x. Using this technique, single-crystal diamond emitters with very sharp tips, whose top curvatures were less than 10 nm, were fabricated w16x. In the present work, we will report the observed results of electron emission from the array with sharp emitters fabricated at a single-crystal diamond surface, and discuss the relationships between the emission sites and the emitter structures.
0925-9635/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-9635(03)00257-7
Y. Ando et al. / Diamond and Related Materials 12 (2003) 1681–1684
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(SEM) were used to observe the growth sector boundaries of the HP-HT diamond substrate and the tip structures of the emitter array. The field emission from the diamond emitter array was measured using a planardiode-type I–V measurement system (Tokyo Cathode Lab., Cathode Emission Profiler CEPS-OFE). Also, a 2D mapping of electron emission sites was observed by the same system by scanning the anode with a 20-mmf pinhole over the specimen. The distance between the specimen and the anode was set at 100 mm for I–V measurements and emission mappings. 3. Results and discussion
Fig. 1. SEM micrograph of an array of single-crystal diamond emitters with very sharp tips fabricated by RIE. The inset is a magnified view of an emitter.
2. Experimental Diamond used in the present work was a 1.5=2.0 mm2 B-doped high-pressure high-temperature (HP-HT) synthetic diamond with (1 0 0) surface. As the masks, an array of Al dots of 1.5 mm in diameter and 0.5 mm in thickness was formed on the diamond surface by conventional photolithography. For the etching of diamond, a reactive ion etching (RIE) system (Anelva L201D-L) with 13.56 MHz, 300 W RF power was used. The base pressure was less than 9.8=10y5 Pa. The etching conditions, i.e. the CF4 yO2 ratio, the total gas pressure and the RF power, were 2%, 2 Pa and 200 W, respectively. The etching time was 75 min. The sample holder was water-cooled, but the temperature of the diamond specimen, measured using an optical pyrometer, was 470–570 K during the plasma etching. The substrate was cleaned with HF after the etching. An optical microscopy and a scanning electron microscopy
Fig. 1 shows an array of single-crystal diamond emitters with very sharp tips fabricated on a B-doped HP-HT synthetic diamond by RIE. The inset in Fig. 1 shows a magnified view of one of the emitters. The spacing between adjacent emitters is 10 mm, and the minimum value of the tip radius rt is less than 10 nm although rt depended on the size of Al mask and the etching time. The details of the microfabrication processes to make the sharp tips were presented in our previous work w16x. Fig. 2a shows the sample observed by optical microscopy. The dark areas in Fig. 2a except for the edge region are where Al masks were successfully patterned and thus the sharp tips were supposed to be fabricated. Unfortunately, the Al masks were not perfectly uniform over the whole sample surface, because the patterning technique on such a small diamond surface was not yet optimized for this sample. Fig. 2b shows a 2D mapping of electron emission sites observed using a planar-diode-type I–V measurement system by scanning an anode with a 20-mmf pinhole over the sample surface. From the results shown in Fig. 2, it was concluded that the electron emission sites approximately correspond to the areas where sharp tips were successfully fabricated. However, a closer exami-
Fig. 2. (a) Sample surface observed by optical microscopy. (b) 2D mapping of electron emission sites.
Y. Ando et al. / Diamond and Related Materials 12 (2003) 1681–1684
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Fig. 3. (a) SEM micrograph of an array of single-crystal diamond emitter tips used for electron emission measurement. (b–f) Typical shapes of five kinds of emitter sites.
nation of the area indicated that the correspondence was not precise. For instance, Fig. 3 shows an array of tips at the diamond surface observed by SEM. There were approximately 5000 patterned Al masks at the substrate surface, but only 400 sharp tips were successfully fabricated. We classified all the tips in five categories on the basis of the sharpness, as shown in Fig. 3b–f, i.e. (b) well-sharpened, (c) sharpened, (d) columnar, (e) collapsed and (f) flat, and made a map of sharpness as shown in Fig. 4. Note that Fig. 4 is not a map of emission sites, but indicates the sharpness at the emitter sites. The white dots in Fig. 4 indicate the sites where well-sharpened tips existed. By comparing the sharp emitter area of Fig. 4 with the emission sites of Fig. 2b, it was found that they were in fairly good agreement. That is, it was concluded that electrons were mostly emitted from sharp tips. In Fig. 5, the areas surrounded
Fig. 4. 2D mapping on tip sharpness.
by the curves are electron emission sites and the white dots are sharp emitter sites. One may, however, notice that there are areas where very sharp tips existed (see the areas indicated by ‘a’ in Fig. 5), but electrons were not at all emitted. Fig. 6 shows the sample surface observed by optical microscopy, where the dotted lines indicate the growth sector boundary of the B-doped single-crystal diamond surface. It is generally known that for HP-HT synthetic diamonds, the impurity concentration (in the present case, B concentration) is
Fig. 5. Relationship between electron emission and sharp emitter sites. The areas surrounded by the curves and the white dots indicate the electron emission sites and the sharp emitter sites, respectively.
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existed was evaluated to be ;100 mAycm2 at 2000 V (y100 mm). 4. Conclusion
Fig. 6. Sample surface observed by optical microscopy. The dotted lines indicate the growth sector boundary of the HP-HT diamond used.
strongly dependent on growth sectors w17x. The substrate we used was a B-doped conducting HP-HT diamond, and it was inferred that the conductivity was extremely low in the growth sectors that included the area ‘a’ of Fig. 5, and this can be the reason that there was no electron emission from the area ‘a’. From I–V characteristic measurements (Fig. 7), the threshold voltage of field emission for 1.0=10y8 A was approximately 1000 V (y100 mm), and the emission current density was approximately 1 mAycm2 at 2000 V (y100 mm), if the emission current was divided by the whole area (0.03 cm2) of sample. In the present work, we found that the electron was mainly emitted from the areas where sharp tips were fabricated, except for the area ‘a’. Thus, the emission current divided by the areas (10 mm=10 mm=400 tipss4=10y4 cm2) where sharp emitters
Fig. 7. I–V characteristic of single-crystal diamond emitters with very sharp tips fabricated by RIE.
An array of single-crystal diamond emitters with very sharp tips, whose radii of curvatures at the top were less than 10 nm, was fabricated on a B-doped HP-HT synthetic diamond by RIE. A 2D mapping of electron emission sites was obtained for the emitter array. It was found that electrons were mainly emitted from the areas where sharp tips were fabricated, except for the area ‘a’. The emission current divided by the area where the sharp emitters existed was evaluated to be ;100 mAy cm2 at 2000 V (y100 mm). It is expected that further optimization of the emitter shape, the emitter density and the device structure will improve the threshold voltage and the emission current to a level enabling practical applications. Acknowledgments This work was supported by the FCT Project, which was consigned to JFCC by NEDO. References w1x K.M. Song, J.Y. Shim, H.K. Baik, Diamond Relat. Mater. 11 (2002) 185–190. w2x A. Hatta, T. Sumitomo, H. Inomoto, A. Hiraki, New Diamond Front. Carbon Technol. 11 (5) (2001) 307–312. w3x A. Karabutov, V. Ralchenko, I. Vlasov, et al., New Diamond Front. Carbon Technol. 11 (5) (2001) 355–364. w4x V.D. Frolov, A.V. Karabutov, S.M. Pimenov, V.I. Konov, Diamond Relat. Mater. 9 (2000) 1196–1200. w5x C.F. Chen, H.C. Hsief, Diamond Relat. Mater. 9 (2000) 1257–1262. w6x M.V. Ugarov, V.P. Ageev, A.V. Karabutov, et al., J. Appl. Phys. 85 (12) (1999) 8436–8440. w7x K. Okano, T. Yamada, A. Sawabe, et al., Appl. Surf. Sci. 146 (1999) 274–279. w8x H. Ito, Y. Show, M. Iwase, T. Izumi, Diamond Films Technol. 9 (2) (1999) 147–150. w9x Y. Show, F. Matsuoka, M. Hayashi, H. Ito, T. Izumi, M. Iwase, Diamond Films Technol. 8 (6) (1998) 451–456. w10x K. Kuriyama, S. Kawasaki, T. Sugino, Diamond Films Technol. 8 (6) (1998) 441–450. w11x C. Kimura, K. Kuriyama, S. Koizumi, M. Kamo, T. Sugino, Appl. Surf. Sci. 146 (1999) 295–298. w12x M. Nishimura, K. Ogawa, A. Hatta, T. Ito, Diamond Relat. Mater. 7 (1999) 754–758. w13x Y. Nishibayashi, H. Saito, T. Imai, N. Fujimori, Diamond Relat. Mater. 9 (2000) 290–294. w14x T. Yamada, A. Sawabe, S. Koizumi, T. Kamio, K. Okano, Diamond Relat. Mater. 11 (2002) 257–261. w15x Y. Ando, Y. Nishibayashi, K. Kobashi, T. Hirao, K. Oura, Diamond Relat. Mater. 11 (2002) 824–827. w16x Y. Ando, Y. Nishibayashi, H. Furuta, K. Kobashi, T. Hirao, K. Oura, New Diamond Front. Carbon Technol. 12 (3) (2002) 137–140. w17x R.C. Burns, V. Cvetkovic, C.N. Dodge, et al., J. Cryst. Growth 104 (1990) 257.