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Growth of diamond by laser ablation of graphite
g[PMOND RELATED MATERIALS
ELSEVIER
Diamond
Growth
and Related
of diamond
Materials 4 ( 1995) 780 7X3
by laser ablation
of graphite
M.C. Polo, J. Cifre, G. Stinchez, R. Aguiar, M. Varela, J. Esteve Dept. F’isicu Aplicuda
i Electr6Cu
b’niwrsitat
dr Barce/or~u, AI>. Diqorttrl
645 -647. E-OX028 Btrrwha,
.Sp~lin
Abstract Thin film growth by laser ablation deposition has been established as a useful method for obtaining films of novel materials on different substrates. The high energy of the emitted particles in the ablation process could be used to obtain diamond thin films at relatively low substrate temperatures. We tried this technique to grow carbon films with a high amount of diamond phases from a pyrolytic graphite target. The laser is an excimer laser (ArF, i,= 193 nm) focused at a fluence of 3 J cm-’ and operating at 5 Hz repetition rate. The films were grown on (100) silicon substrates at 450 ‘C. The films were deposited under (a) vacuum conditions, (b) 1 mbar of hydrogen or (c) I mbar of helium to determine the influence of a gas atmosphere in the growth process. Scanning electron microscopy demonstrated the growth of diamond on silicon substrates with crystals as big as 20 pm. Raman of the diamond structure. even for vacuum deposition spectroscopy studies of the films showed the peak at 1332 cn- ’ characteristic conditions. ~eq~~~rds: Laser-assisted
PVD; Diamond;
Raman
spectroscopy;
1. Introduction Laser ablation deposition has been demonstrated to be a powerful technique for high quality thin film deposition of a great number of materials [ 1- 31. This ability is mainly due to the high energy of the particles emitted from the ablated target that allows the production of non-equilibrium reactions during their condensation on the substrate. As regards diamond films produced by laser methods, few authors have confirmed the presence of diamond phases in the films. Several studies have reported the obtention of amorphous films with the characteristic properties of diamond-like carbon [4-61. Rengan et al. [7] have found particles with the morphology of diamond in their diamond-like films grown at 860 “C by laser ablation of graphite in a hydrogen discharge. However, neither Raman spectroscopy nor diffraction analysis corroborated the presence of diamond. More recent laser studies have reported the production of microcrystalline phases of diamond: nucleation of 0.01-0.15 ym diamond crystallites on self-biased (100) silicon substrates was achieved by Seth et al. [S] using the KrF excimer laser ablation of graphite combined with an argon r.f. plasma. Bourdon et al. [9] have obtained crystalline phases of diamond by laser ablation of graphite in a hydrogen environment, in an arrangement where the laser went through the fused silica substrates before reaching the target. In both studies 0925-9635’95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(94)05270-O
Scanning
electron
microscopy
electron diffraction was the only evidence of the presence of diamond whereas Raman spectra lacked the characteristic 1332 cm ’ line. In the present work we investigated the laser ablation of a graphite target in order to obtain diamond thin films on silicon substrates. The experiments were carried out in vacuum as well as hydrogen or helium atmospheres. In all cases the Raman spectra of the obtained films showed the 1332 cm ~’ peak. This feature demonstrates the likelihood of growing good quality diamond films by this technique.
2. Experimental
details
The films were deposited by laser ablation using an ArF excimer laser (i= 193 nm, Z= 23 ns, repetition rate 5 Hz). The laser beam was introduced into the vacuum chamber through a quartz window and was focused at a fluence of 3 J cm-’ onto a graphite target at an angle of 45”. The films were grown on (100) silicon substrates. Previous to the degrease in acetone and methanol ultrasonic baths, a portion of the substrate surface was mechanically scratched with 1 pm diamond powder in order to study the influence of this pretreatment on the nucleation process. The substrates were placed in front of the graphite target at a distance of 6 cm and maintained at a temperature of 450 ‘C during growth. The
781
M. C. Polo et ai.lDianiond and Reluted Materials 4 (19951 780-783
3. Results and discussion
the surface of the continuous film as the top view SEM images may seem to suggest, but are embedded in the continuous film as can be appreciated in the lateral view SEM image (Fig. 2(b)). Some single crystals larger than 20 urn stand out from the continuous film (Fig. 2(c)) but most of the crystals grow in groups (Fig. 2(d)) frequently located in the substrate scratches. Films grown in the presence of helium showed a surface morphology similar to that of films deposited in the hydrogen atmosphere, but there were some differences in the shape and size of the crystallites. Crystals smaller than 1 urn were cubic (Fig. 3(a)) whereas the shape of larger particles (up to 20 urn) was less geometric, as can be seen in Fig. 3(b).
3.1. Morphology
3.2. Structure
films were deposited either under vacuum conditions ( 10 ~’ mbar), or in a gas atmosphere: 1 mbar of hydrogen or 1 mbar of helium. Films with thicknesses of 200 nm, measured with a surface profilometer, were obtained under vacuum conditions after 2.5 x lo4 laser pulses, whereas films with thicknesses of about 50 nm were obtained after 4 x lo4 pulses when deposited in both gas atmospheres. The morphology of the films was studied by scanning electron microscopy and their nature was determined by Raman spectroscopy.
of thejlms
Films deposited under vacuum conditions were very compact. When viewed in SEM, with high magnification (Fig. l), the surface morphology of the films grown onto untreated silicon showed very fine grains (Fig. l(a)). Films grown on scratched substrates presented larger grains, but did not show any crystalline appearance (Fig. l(b)). When a gas was present (H, or He) during the ablation process, the film deposition rates was divided by six and the film surface showed no granular morphology and became very smooth. This was observed regardless of the substrate pretreatment. However, a great number of well-faceted crystals appeared embedded in these continuous films. The crystals appeared in both the scratched and non-scratched substrates, but the nucleation density was much higher in the scratched ones. Fig. 2 shows the SEM images of the crystals present on the surface of the films obtained in a hydrogen atmosphere. These crystals exhibited cubic or rectangular shapes (Fig. 2(a)). The crystals are not located on
(4 Fig. 1. SEM micrographs
of thefilms
The results of the morphological study led us to perform two different Raman analysis in the films obtained both under vacuum and in the presence of a gas. The first series of Raman spectra were acquired from the continuous films (Fig. 4) and the second series were carried out on the crystallites embedded in the background films (Fig. 5). Spectrum a in Fig. 4 corresponds to the film grown under vacuum conditions. This spectrum shows the two rather narrow peaks at 1355 cm-r and 1580 cm-’ characteristic of microcrystalline graphite. Spectra b and c in Fig. 4 represent films obtained in helium and hydrogen atmospheres respectively, and longer data acquisition times were necessary because these films were thinner than those obtained in vacuum. Both spectra showed the second order line of silicon (940 cm-‘) and two broad bands centred at about 1340cm-’ and 1600 cm-‘, which were attributed to disordered or amorphous sp2 carbon [lo]. The lower intensity of the silicon peak in spectrum c indicates that the film grown
(b) of the surface
of the film deposited
under vacuum
on (a) untreated
and (b) scratched
(100) silicon substrates.
M. C. Polo et ui.iDirtmorld and Rrlutrd Materiuis 4 ( 1995) 780-783
782
(h)
(4
(4 Fig. 2. SEM images showing
the well-defined
morphology
of the diamond
crystals
grown
under
1 mbar hydrogen
atmosphere.
(b)
(4 Fig. 3. SEM micrographs
of the particles
present
in the surface
under hydrogen is thicker than that grown under helium atmosphere. The sharp, intense peak at 1332 cm-’ present in the Raman spectra shown in Fig. 5 is clear evidence of the
of the films deposited
under
1 mhar helium atmosphere.
existence of diamond. Spectra b and c were obtained on the crystals present in the films deposited in helium and hydrogen environments respectively. In both cases, the full width at half-maximum of the diamond peak had a
A4. C. Polo et al. JDiamond and Related Materials
4 (1995) 780-783
783
Conclusions
b 1200
1
Raman
1500
shift
1
800
(cm-‘)
Fig.4. Raman spectra of the background films deposited under (a) vacuum, (b) helium and (c) hydrogen. Data acquisition times were 60 s in (a) and 20 min in (b) and (c).
We grew diamond crystals on silicon substrates by laser ablation of graphite at relatively low substrate temperature (450 “C). The high quality of these crystals was confirmed by Raman spectroscopy, having spectra very similar to those of natural diamond. When hydrogen was used as ambient gas, reactions with either excited carbon atoms or carbon cluster ions might have taken place in the laser plume. This possibility, together with the etching role of amorphous phases attributed to hydrogen in CVD processes [ 121 could be the main reason for the growth of more perfect large diamond crystals. However, the feasibility of growing diamond even in vacuum or in helium atmosphere has demonstrated that the presence of hydrogen is not essential. Further studies are necessary to discuss the growth mechanisms of diamond films under different gas atmospheres as a function of the composition of the laser ablation plume; and more technological conditions should be assayed to attempt to obtain continuous crystalline films.
References
I
.200
1300
Raman
1400
1500
shift
1600
1' 0
(cm-‘)
Fig. 5. Raman spectra of: (a) samples deposited under vacuum, (b) and (c) diamond crystals deposited under helium and hydrogen respectively. All data acquisition times were 60 s.
value of about 3.5 cm-l, slightly higher than that of natural diamond (2 cm-‘) [ 111. Despite the absence of crystals in the film grown under vacuum conditions, Raman analysis showed a small peak revealing the presence of diamond phases as can be seen in spectrum a.
T. Venkatesan, X.D. Wu, S. Shaheen, N. Jisrawi, Cl1 D. Dijkkamp, Y.H. Min-Lee, W.L. McLean and M. Croft, Appl. Phys. Left., 51 (1987) 619. PI J.S. Horwitz, KS. Grabowski, D.B. Chrisey and R.E. Leuchtner, Appl. Phys. Lett., 59 (1991) 1565. 131 F. SBnchez, M. Varela, X. Queralt, R. Aguiar and J.L. Morenza, Appl. Phys. Len., 61 (1992) 2228. Appl. Phys. A, c41 T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, 45 (1988) 355. CSI E.B.D. Bourdon and R.H. Prince, Appl. Surf. Sci., 48/49(1991) 50. Ccl J.A. Martin, L. Vazquez, P. Bernard, F. Comin and S. Ferrer, Appl. Phys. Lett., 57 (1990) 1742. Mater. Res. Sot. Proc., c71 A. Rengan, N. Biunno and J. Narayan, 162 (1990) 162. D.H. Rasmussen and S.V. Babu, Appi. PI J. Seth, R. Padiyath, Phys Lett., 63 (4) (1993) 473. c91 E.B.D. Bourdon, Peter Kovarik and R.H. Prince, Diamond Relat. Mater., 2 (1993) 425. L. Bergman, Y.M. LeGrice and R.E. Shroder, Cl01 R.J. Nemanich, Mater. Rex Sot. Proc. (1990) 741. Cl11 S.A. Solin and A.K. Ramdas, Phys. Rev. B, 1 (1970) 1687. Cl21 A.R. Badzian and R.C. DeVries, Mater. Res. Bull., 23 (1988) 385.
ELSEVIER
Diamond
Growth
and Related
of diamond
Materials 4 ( 1995) 780 7X3
by laser ablation
of graphite
M.C. Polo, J. Cifre, G. Stinchez, R. Aguiar, M. Varela, J. Esteve Dept. F’isicu Aplicuda
i Electr6Cu
b’niwrsitat
dr Barce/or~u, AI>. Diqorttrl
645 -647. E-OX028 Btrrwha,
.Sp~lin
Abstract Thin film growth by laser ablation deposition has been established as a useful method for obtaining films of novel materials on different substrates. The high energy of the emitted particles in the ablation process could be used to obtain diamond thin films at relatively low substrate temperatures. We tried this technique to grow carbon films with a high amount of diamond phases from a pyrolytic graphite target. The laser is an excimer laser (ArF, i,= 193 nm) focused at a fluence of 3 J cm-’ and operating at 5 Hz repetition rate. The films were grown on (100) silicon substrates at 450 ‘C. The films were deposited under (a) vacuum conditions, (b) 1 mbar of hydrogen or (c) I mbar of helium to determine the influence of a gas atmosphere in the growth process. Scanning electron microscopy demonstrated the growth of diamond on silicon substrates with crystals as big as 20 pm. Raman of the diamond structure. even for vacuum deposition spectroscopy studies of the films showed the peak at 1332 cn- ’ characteristic conditions. ~eq~~~rds: Laser-assisted
PVD; Diamond;
Raman
spectroscopy;
1. Introduction Laser ablation deposition has been demonstrated to be a powerful technique for high quality thin film deposition of a great number of materials [ 1- 31. This ability is mainly due to the high energy of the particles emitted from the ablated target that allows the production of non-equilibrium reactions during their condensation on the substrate. As regards diamond films produced by laser methods, few authors have confirmed the presence of diamond phases in the films. Several studies have reported the obtention of amorphous films with the characteristic properties of diamond-like carbon [4-61. Rengan et al. [7] have found particles with the morphology of diamond in their diamond-like films grown at 860 “C by laser ablation of graphite in a hydrogen discharge. However, neither Raman spectroscopy nor diffraction analysis corroborated the presence of diamond. More recent laser studies have reported the production of microcrystalline phases of diamond: nucleation of 0.01-0.15 ym diamond crystallites on self-biased (100) silicon substrates was achieved by Seth et al. [S] using the KrF excimer laser ablation of graphite combined with an argon r.f. plasma. Bourdon et al. [9] have obtained crystalline phases of diamond by laser ablation of graphite in a hydrogen environment, in an arrangement where the laser went through the fused silica substrates before reaching the target. In both studies 0925-9635’95/$09.50 0 1995 Elsevier Science S.A. All rights reserved SSDI 0925-9635(94)05270-O
Scanning
electron
microscopy
electron diffraction was the only evidence of the presence of diamond whereas Raman spectra lacked the characteristic 1332 cm ’ line. In the present work we investigated the laser ablation of a graphite target in order to obtain diamond thin films on silicon substrates. The experiments were carried out in vacuum as well as hydrogen or helium atmospheres. In all cases the Raman spectra of the obtained films showed the 1332 cm ~’ peak. This feature demonstrates the likelihood of growing good quality diamond films by this technique.
2. Experimental
details
The films were deposited by laser ablation using an ArF excimer laser (i= 193 nm, Z= 23 ns, repetition rate 5 Hz). The laser beam was introduced into the vacuum chamber through a quartz window and was focused at a fluence of 3 J cm-’ onto a graphite target at an angle of 45”. The films were grown on (100) silicon substrates. Previous to the degrease in acetone and methanol ultrasonic baths, a portion of the substrate surface was mechanically scratched with 1 pm diamond powder in order to study the influence of this pretreatment on the nucleation process. The substrates were placed in front of the graphite target at a distance of 6 cm and maintained at a temperature of 450 ‘C during growth. The
781
M. C. Polo et ai.lDianiond and Reluted Materials 4 (19951 780-783
3. Results and discussion
the surface of the continuous film as the top view SEM images may seem to suggest, but are embedded in the continuous film as can be appreciated in the lateral view SEM image (Fig. 2(b)). Some single crystals larger than 20 urn stand out from the continuous film (Fig. 2(c)) but most of the crystals grow in groups (Fig. 2(d)) frequently located in the substrate scratches. Films grown in the presence of helium showed a surface morphology similar to that of films deposited in the hydrogen atmosphere, but there were some differences in the shape and size of the crystallites. Crystals smaller than 1 urn were cubic (Fig. 3(a)) whereas the shape of larger particles (up to 20 urn) was less geometric, as can be seen in Fig. 3(b).
3.1. Morphology
3.2. Structure
films were deposited either under vacuum conditions ( 10 ~’ mbar), or in a gas atmosphere: 1 mbar of hydrogen or 1 mbar of helium. Films with thicknesses of 200 nm, measured with a surface profilometer, were obtained under vacuum conditions after 2.5 x lo4 laser pulses, whereas films with thicknesses of about 50 nm were obtained after 4 x lo4 pulses when deposited in both gas atmospheres. The morphology of the films was studied by scanning electron microscopy and their nature was determined by Raman spectroscopy.
of thejlms
Films deposited under vacuum conditions were very compact. When viewed in SEM, with high magnification (Fig. l), the surface morphology of the films grown onto untreated silicon showed very fine grains (Fig. l(a)). Films grown on scratched substrates presented larger grains, but did not show any crystalline appearance (Fig. l(b)). When a gas was present (H, or He) during the ablation process, the film deposition rates was divided by six and the film surface showed no granular morphology and became very smooth. This was observed regardless of the substrate pretreatment. However, a great number of well-faceted crystals appeared embedded in these continuous films. The crystals appeared in both the scratched and non-scratched substrates, but the nucleation density was much higher in the scratched ones. Fig. 2 shows the SEM images of the crystals present on the surface of the films obtained in a hydrogen atmosphere. These crystals exhibited cubic or rectangular shapes (Fig. 2(a)). The crystals are not located on
(4 Fig. 1. SEM micrographs
of thefilms
The results of the morphological study led us to perform two different Raman analysis in the films obtained both under vacuum and in the presence of a gas. The first series of Raman spectra were acquired from the continuous films (Fig. 4) and the second series were carried out on the crystallites embedded in the background films (Fig. 5). Spectrum a in Fig. 4 corresponds to the film grown under vacuum conditions. This spectrum shows the two rather narrow peaks at 1355 cm-r and 1580 cm-’ characteristic of microcrystalline graphite. Spectra b and c in Fig. 4 represent films obtained in helium and hydrogen atmospheres respectively, and longer data acquisition times were necessary because these films were thinner than those obtained in vacuum. Both spectra showed the second order line of silicon (940 cm-‘) and two broad bands centred at about 1340cm-’ and 1600 cm-‘, which were attributed to disordered or amorphous sp2 carbon [lo]. The lower intensity of the silicon peak in spectrum c indicates that the film grown
(b) of the surface
of the film deposited
under vacuum
on (a) untreated
and (b) scratched
(100) silicon substrates.
M. C. Polo et ui.iDirtmorld and Rrlutrd Materiuis 4 ( 1995) 780-783
782
(h)
(4
(4 Fig. 2. SEM images showing
the well-defined
morphology
of the diamond
crystals
grown
under
1 mbar hydrogen
atmosphere.
(b)
(4 Fig. 3. SEM micrographs
of the particles
present
in the surface
under hydrogen is thicker than that grown under helium atmosphere. The sharp, intense peak at 1332 cm-’ present in the Raman spectra shown in Fig. 5 is clear evidence of the
of the films deposited
under
1 mhar helium atmosphere.
existence of diamond. Spectra b and c were obtained on the crystals present in the films deposited in helium and hydrogen environments respectively. In both cases, the full width at half-maximum of the diamond peak had a
A4. C. Polo et al. JDiamond and Related Materials
4 (1995) 780-783
783
Conclusions
b 1200
1
Raman
1500
shift
1
800
(cm-‘)
Fig.4. Raman spectra of the background films deposited under (a) vacuum, (b) helium and (c) hydrogen. Data acquisition times were 60 s in (a) and 20 min in (b) and (c).
We grew diamond crystals on silicon substrates by laser ablation of graphite at relatively low substrate temperature (450 “C). The high quality of these crystals was confirmed by Raman spectroscopy, having spectra very similar to those of natural diamond. When hydrogen was used as ambient gas, reactions with either excited carbon atoms or carbon cluster ions might have taken place in the laser plume. This possibility, together with the etching role of amorphous phases attributed to hydrogen in CVD processes [ 121 could be the main reason for the growth of more perfect large diamond crystals. However, the feasibility of growing diamond even in vacuum or in helium atmosphere has demonstrated that the presence of hydrogen is not essential. Further studies are necessary to discuss the growth mechanisms of diamond films under different gas atmospheres as a function of the composition of the laser ablation plume; and more technological conditions should be assayed to attempt to obtain continuous crystalline films.
References
I
.200
1300
Raman
1400
1500
shift
1600
1' 0
(cm-‘)
Fig. 5. Raman spectra of: (a) samples deposited under vacuum, (b) and (c) diamond crystals deposited under helium and hydrogen respectively. All data acquisition times were 60 s.
value of about 3.5 cm-l, slightly higher than that of natural diamond (2 cm-‘) [ 111. Despite the absence of crystals in the film grown under vacuum conditions, Raman analysis showed a small peak revealing the presence of diamond phases as can be seen in spectrum a.
T. Venkatesan, X.D. Wu, S. Shaheen, N. Jisrawi, Cl1 D. Dijkkamp, Y.H. Min-Lee, W.L. McLean and M. Croft, Appl. Phys. Left., 51 (1987) 619. PI J.S. Horwitz, KS. Grabowski, D.B. Chrisey and R.E. Leuchtner, Appl. Phys. Lett., 59 (1991) 1565. 131 F. SBnchez, M. Varela, X. Queralt, R. Aguiar and J.L. Morenza, Appl. Phys. Len., 61 (1992) 2228. Appl. Phys. A, c41 T. Sato, S. Furuno, S. Iguchi and M. Hanabusa, 45 (1988) 355. CSI E.B.D. Bourdon and R.H. Prince, Appl. Surf. Sci., 48/49(1991) 50. Ccl J.A. Martin, L. Vazquez, P. Bernard, F. Comin and S. Ferrer, Appl. Phys. Lett., 57 (1990) 1742. Mater. Res. Sot. Proc., c71 A. Rengan, N. Biunno and J. Narayan, 162 (1990) 162. D.H. Rasmussen and S.V. Babu, Appi. PI J. Seth, R. Padiyath, Phys Lett., 63 (4) (1993) 473. c91 E.B.D. Bourdon, Peter Kovarik and R.H. Prince, Diamond Relat. Mater., 2 (1993) 425. L. Bergman, Y.M. LeGrice and R.E. Shroder, Cl01 R.J. Nemanich, Mater. Rex Sot. Proc. (1990) 741. Cl11 S.A. Solin and A.K. Ramdas, Phys. Rev. B, 1 (1970) 1687. Cl21 A.R. Badzian and R.C. DeVries, Mater. Res. Bull., 23 (1988) 385.