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Carbon clusters in amorphous hydrogenated carbon
Journal of Non-Crystalline Solids 227–230 Ž1998. 622–626
Carbon clusters in amorphous hydrogenated carbon S.G. Yastrebov ) , V.I. Ivanov-Omskii, V.I. Siklitsky, A.A. Sitnikova A.F. Ioffe Physico-Technical Institute, 194021, St. Petersburg, Russian Federation
Abstract Direct observation is reported of diamond cluster nucleation resulting from the thermalisation of accelerated carbon atoms Žions. during growth of hydrogenated amorphous carbon films by ion sputtering of graphite. Transmission electron microscopy ŽTEM. Ž100 keV., selected area electron diffraction ŽSAED. and optical absorption Ž1 to 6 eV. were used to study the prepared hydrogenated amorphous carbon films. The size-distribution function of diamond crystals derived from TEM images is considered in terms of the fluctuation theory. To investigate the graphite-like constituent of a-C:H films, the optical data are analyzed under the assumption that the fundamental absorption edge is due to nanometer building blocks constructed of a set of sp 2-bonded carbon atoms assembled together as six-membered units embedded in an amorphous matrix. A mechanism of diamond crystal nucleation is considered. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Diamond cluster; Transmission electron microscopy
1. Introduction Amorphous hydrogenated carbon Ža-C:H. has been shown to be a fertile medium for nucleation of diamond crystallites appearing as a result of self-assembly of carbon atoms and ions generated by ion sputtering of a graphite target w1x. In this paper the fundamental optical absorption edge was measured to determine the graphite-like constituent Žsp 2 bonded clusters. of the a-C:H matrix as a function of its growth parameters. Transmission electron microscopy ŽTEM. together with selected area electron diffraction ŽSAED. technique were applied to directly observe diamond crystals and derive their size distribution function. In this paper further develop-
)
Corresponding author. Fax: q7-812 247 1017; e-mail: [email protected].
ment of the phenomenology of diamond nucleation in a-C:H is presented.
2. Experimental procedures a-C:H films were grown by ion ŽDC magnetron. sputtering of a graphite target in argon–hydrogen Ž4:1. plasma onto KBr or fused quartz substrates. The gas pressure in the growth chamber was maintained between 5 and 13 mTorr; the substrate temperature was close to 500 K; the average magnetron power was varied in the range from 0.35 to 0.45 kW. The film thickness ranged from 0.1 to 2 m m. Optical absorption spectra Ž1–6 eV. of a-C:H films deposited onto fused quartz substrates were measured at room temperature with a spectrophotometer ŽHitachi-U3410.. A transmission electron microscope ŽPhilips 420. operating at 100 keV and equipped for SAED mea-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 4 1 - 0
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
623
surements was used for direct microstructural analysis of the a-C:H films. For this purpose, free-standing films approximately 100 nm thick were prepared by dissolving KBr substrates in water.
3. Results 3.1. Optical Studies Taking into account presence of sp 2-bonded building blocks Žclusters. embedded in an a-C:H matrix, one can obtain information about the structure of a-C:H from the optical absorption data. The following procedure was used. As a model of the cluster we used various hydrocarbons containing different numbers of p-electerons. Hydrocarbons with the number of p-electrons, N, given by: N s Ž 4 L q 2. ,
Ž 1.
where L is the number of aromatic rings in a hydrocarbon, are referred to as Huckel ones w2x. The experimental optical gaps of various hydrocarbons Žtaken from reference w3x. are plotted in Fig. 1 against the number of aromatic rings, L, in each
Fig. 2. TEM image Ždark field. of diamond nanocrystals in a-C:H. The frame is to set aside the region shown separately in Fig. 3b. ŽInsert. Electron diffraction pattern of selected area depicted in this figure.
hydrocarbon together with power-law fits. The solid curve is a power-law fit for Huckel hydrocarbons: Eg s 4 Ly0 .79 ,
Ž 2.
and the dashed curve represents a similar fit for Huckel and non-Huckel hydrocarbons together: Eg s 4.3 Ly0 .77 . A rather good correlation between experimental data and their power-law fits is seen in Fig. 1. Table 1 Data for diamond nanocrystals from SAED pattern in Fig. 2 Fig. 1. Optical gap Eg for hydrocarbons containing various number of six-membered rings L. Crosses stand for experimental data corresponding to Huckel hydrocarbons, circles stand for nonHuckel ones. Solid curve is the power-law fit for Huckel hydrocarbons Ž Eg s 4 Ly0 .79 ., dashed curve is the same for both series together Ž Eg s 4.3 Ly0 .77 ..
Ring
˚. Lattice spacing d ŽA 1
2
3
4
Experimental value Standard ŽASTM. value Ž hkl . plane
2.06 2.06 Ž111.
1.46 1.783 Ž200.
1.32 1.26 Ž220.
1.04 1.075 Ž311.
624
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
Fig. 3. Ža. High magnification TEM view of the region inside the frame in Fig. 2 with an isolated poorly resolved object. Žb. Contour plot of the object in Ža..
3.2. TEM studies A black-field TEM image of the a-C:H films grown at 10 mTorr and magnetron potential of 360 V is shown in Fig. 2. Parameters of the SAED pattern Žinsert in Fig. 2. from the cloud visible in this figure are listed in Table 1. It is seen that the lattice spacing corresponding to each diffraction ring provides a reasonable match to the ASTM standard spacing for cubic polycrystalline diamond w4x. The appearance of a smaller forbidden reflex 200 originates most probably from strongly distorted sections of diamond crystals. This source is consistent with the deviation of the spacing from the standard value Ž20%.. High magnification of the area confined within the frame in Fig. 2 reveals a poorly resolved object ŽFig. 3a., whose contour plot ŽFig. 3b. resembles that of diamond particles nucleated in graphite fragments observed by Li et al. w5x.
Tauc rule w6,7x. We found that in our case the Tauc optical gap, Eg , ranging from 1.6 to 2.3 eV depends on the growth conditions w6x. We employ these data to describe the a-C:H matrix. Using Eqs. Ž1. and Ž2. we calculated the N, for the graphite-like constituent of a-C:H. The calculated
4. Discussion 4.1. Tauc edge of fundamental optical absorption The energy dependence of absorption coefficients of the fabricated films can be approximated using the
Fig. 4. Number p electrons N contained in the graphite-like constituent of a-C:H matrix vs. pressure for two magnetron voltages: Triangles stand for 360 V and squares stand for 400 V. Data were obtained from the experimental Tauc optical gaps of a-C:H using Eqs. Ž1. and Ž2.. Solid lines are guide to eye.
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
625
presented in Fig. 2, is plotted in Fig. 5. The result of fitting of Eq. Ž3. to experimental points is also shown in Fig. 5. The fitting parameter is R av s 2.8 nm. The Gaussian shape of the size-distribution function indicates that the nucleation stage of crystal formation occurs under the thermodynamic conditions w9x resulting from the fluctuation mechanism of self-assembly of carbon atoms appearing at the surface of a growing film. As a result, the carbon atoms confined within the correlation radius of the fluctuation in the presence of hydrogen reassemble themselves into the diamond lattice.
5. Conclusions Fig. 5. The size variation of diamond nanocrystals. The points with bars are experimental values. The solid line is a fit of Gaussian distribution function.
data are presented in Fig. 4. This figure can be used together with Eq. Ž1. to estimate sizes of the sp 2bonded clusters, taking into account the approximate size of a single graphite ring Ž d 0 f 0.3 nm.. The estimations suggest that a magnetron potential of 360 V N and the average size, d, of the graphite-like fragments are practically independent of pressure, with d f 0.6 nm Ž L f 2.. At larger magnetron potentials the N and, therefore, the d become a stronger function of pressure and d ranges from d f 0.6 nm Ž L f 3. in our experiment. A correlation between sizes of graphite-like fragments obtained from the Tauc edge of a-C:H films and from their TEM images was also found and studied by us in more detail in w6x. 4.2. Phenomenology of diamond nucleation in a-C:H The fluctuation theory yields an expression for the size distribution function of nucleating particles in the following explicit form w8x:
ž
W Ž R . ; Exp y
Ž R y R av . 2 2 R av
2
/
Ž 3.
where R is the particle radius and R av is the average radius. The size distribution of diamond crystals, found by visual analysis of the TEM micrograph
It was shown that conditions exist when diamond nucleation in the a-C:H matrix becomes possible during a-C:H film formation. The shape of the size-distribution function of diamond crystals indicates the early Žnucleation. stage of their formation. The spectral position of the fundamental optical absorption edge of a-C:H, mainly controlled by the presence of a graphite-like constituent in a-C:H structure.
Acknowledgements This work was partly supported by the Russian Foundation for Basic Research ŽGrant nos. 97-0218110 and 97-03-32273. and the US Department of defense. The Russian Program ‘Physics of Solid State Nanostructures’ is acknowledged for informational service. One of us ŽV.S.. was supported by the St.Petresburg Government.
References w1x V.I. Ivanov-Omskii, A.A. Sitnikova, A. A Suvorova, S.G. Yastrebov, T.K. Zvonareva, Mol. Mat. 8 Ž1996. 99. w2x M.J.S. Dewar, The Molecular Orbital Theory of Organic Chemistry, McGraw Hill, New York, 1969. w3x F. Gutman, L.E. Lyons, Organic Semiconductors. ŽWiley, 1967.. w4x L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nording, J. Phys. Chem. Solids 30 Ž1969. 1835.
626
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
w5x Z. Li, L. Wang, T. Suzuki, A. Argoitia, P. Pirouz, J.C. Angus, J. Appl. Phys. 73 Ž2. Ž1993. 711. w6x V.I. Ivanov-Omskii, A.B. Lodygin, A.A. Sitnikova, A.A. Suvorova, S.G. Yastrebov, A.V. Tolmatchev, J. Chem. Vapor Deposition 5 Ž1997. 198.
w7x V.I. Ivanov-Omskii, A.V. Tolmatchev, S.G. Yastrebov, Phil. Mag. B 73 Ž4. Ž1996. 715. w8x L.D. Landau, E.M. Lifshitz, Physical Kinetics, Course of Theoretical Physics, Pergamon, New York, 1981. w9x F.P. Bundy, J. Geophys. Res. 85 Ž1971. 6930.
Carbon clusters in amorphous hydrogenated carbon S.G. Yastrebov ) , V.I. Ivanov-Omskii, V.I. Siklitsky, A.A. Sitnikova A.F. Ioffe Physico-Technical Institute, 194021, St. Petersburg, Russian Federation
Abstract Direct observation is reported of diamond cluster nucleation resulting from the thermalisation of accelerated carbon atoms Žions. during growth of hydrogenated amorphous carbon films by ion sputtering of graphite. Transmission electron microscopy ŽTEM. Ž100 keV., selected area electron diffraction ŽSAED. and optical absorption Ž1 to 6 eV. were used to study the prepared hydrogenated amorphous carbon films. The size-distribution function of diamond crystals derived from TEM images is considered in terms of the fluctuation theory. To investigate the graphite-like constituent of a-C:H films, the optical data are analyzed under the assumption that the fundamental absorption edge is due to nanometer building blocks constructed of a set of sp 2-bonded carbon atoms assembled together as six-membered units embedded in an amorphous matrix. A mechanism of diamond crystal nucleation is considered. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Amorphous hydrogenated carbon; Diamond cluster; Transmission electron microscopy
1. Introduction Amorphous hydrogenated carbon Ža-C:H. has been shown to be a fertile medium for nucleation of diamond crystallites appearing as a result of self-assembly of carbon atoms and ions generated by ion sputtering of a graphite target w1x. In this paper the fundamental optical absorption edge was measured to determine the graphite-like constituent Žsp 2 bonded clusters. of the a-C:H matrix as a function of its growth parameters. Transmission electron microscopy ŽTEM. together with selected area electron diffraction ŽSAED. technique were applied to directly observe diamond crystals and derive their size distribution function. In this paper further develop-
)
Corresponding author. Fax: q7-812 247 1017; e-mail: [email protected].
ment of the phenomenology of diamond nucleation in a-C:H is presented.
2. Experimental procedures a-C:H films were grown by ion ŽDC magnetron. sputtering of a graphite target in argon–hydrogen Ž4:1. plasma onto KBr or fused quartz substrates. The gas pressure in the growth chamber was maintained between 5 and 13 mTorr; the substrate temperature was close to 500 K; the average magnetron power was varied in the range from 0.35 to 0.45 kW. The film thickness ranged from 0.1 to 2 m m. Optical absorption spectra Ž1–6 eV. of a-C:H films deposited onto fused quartz substrates were measured at room temperature with a spectrophotometer ŽHitachi-U3410.. A transmission electron microscope ŽPhilips 420. operating at 100 keV and equipped for SAED mea-
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 4 1 - 0
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
623
surements was used for direct microstructural analysis of the a-C:H films. For this purpose, free-standing films approximately 100 nm thick were prepared by dissolving KBr substrates in water.
3. Results 3.1. Optical Studies Taking into account presence of sp 2-bonded building blocks Žclusters. embedded in an a-C:H matrix, one can obtain information about the structure of a-C:H from the optical absorption data. The following procedure was used. As a model of the cluster we used various hydrocarbons containing different numbers of p-electerons. Hydrocarbons with the number of p-electrons, N, given by: N s Ž 4 L q 2. ,
Ž 1.
where L is the number of aromatic rings in a hydrocarbon, are referred to as Huckel ones w2x. The experimental optical gaps of various hydrocarbons Žtaken from reference w3x. are plotted in Fig. 1 against the number of aromatic rings, L, in each
Fig. 2. TEM image Ždark field. of diamond nanocrystals in a-C:H. The frame is to set aside the region shown separately in Fig. 3b. ŽInsert. Electron diffraction pattern of selected area depicted in this figure.
hydrocarbon together with power-law fits. The solid curve is a power-law fit for Huckel hydrocarbons: Eg s 4 Ly0 .79 ,
Ž 2.
and the dashed curve represents a similar fit for Huckel and non-Huckel hydrocarbons together: Eg s 4.3 Ly0 .77 . A rather good correlation between experimental data and their power-law fits is seen in Fig. 1. Table 1 Data for diamond nanocrystals from SAED pattern in Fig. 2 Fig. 1. Optical gap Eg for hydrocarbons containing various number of six-membered rings L. Crosses stand for experimental data corresponding to Huckel hydrocarbons, circles stand for nonHuckel ones. Solid curve is the power-law fit for Huckel hydrocarbons Ž Eg s 4 Ly0 .79 ., dashed curve is the same for both series together Ž Eg s 4.3 Ly0 .77 ..
Ring
˚. Lattice spacing d ŽA 1
2
3
4
Experimental value Standard ŽASTM. value Ž hkl . plane
2.06 2.06 Ž111.
1.46 1.783 Ž200.
1.32 1.26 Ž220.
1.04 1.075 Ž311.
624
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
Fig. 3. Ža. High magnification TEM view of the region inside the frame in Fig. 2 with an isolated poorly resolved object. Žb. Contour plot of the object in Ža..
3.2. TEM studies A black-field TEM image of the a-C:H films grown at 10 mTorr and magnetron potential of 360 V is shown in Fig. 2. Parameters of the SAED pattern Žinsert in Fig. 2. from the cloud visible in this figure are listed in Table 1. It is seen that the lattice spacing corresponding to each diffraction ring provides a reasonable match to the ASTM standard spacing for cubic polycrystalline diamond w4x. The appearance of a smaller forbidden reflex 200 originates most probably from strongly distorted sections of diamond crystals. This source is consistent with the deviation of the spacing from the standard value Ž20%.. High magnification of the area confined within the frame in Fig. 2 reveals a poorly resolved object ŽFig. 3a., whose contour plot ŽFig. 3b. resembles that of diamond particles nucleated in graphite fragments observed by Li et al. w5x.
Tauc rule w6,7x. We found that in our case the Tauc optical gap, Eg , ranging from 1.6 to 2.3 eV depends on the growth conditions w6x. We employ these data to describe the a-C:H matrix. Using Eqs. Ž1. and Ž2. we calculated the N, for the graphite-like constituent of a-C:H. The calculated
4. Discussion 4.1. Tauc edge of fundamental optical absorption The energy dependence of absorption coefficients of the fabricated films can be approximated using the
Fig. 4. Number p electrons N contained in the graphite-like constituent of a-C:H matrix vs. pressure for two magnetron voltages: Triangles stand for 360 V and squares stand for 400 V. Data were obtained from the experimental Tauc optical gaps of a-C:H using Eqs. Ž1. and Ž2.. Solid lines are guide to eye.
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
625
presented in Fig. 2, is plotted in Fig. 5. The result of fitting of Eq. Ž3. to experimental points is also shown in Fig. 5. The fitting parameter is R av s 2.8 nm. The Gaussian shape of the size-distribution function indicates that the nucleation stage of crystal formation occurs under the thermodynamic conditions w9x resulting from the fluctuation mechanism of self-assembly of carbon atoms appearing at the surface of a growing film. As a result, the carbon atoms confined within the correlation radius of the fluctuation in the presence of hydrogen reassemble themselves into the diamond lattice.
5. Conclusions Fig. 5. The size variation of diamond nanocrystals. The points with bars are experimental values. The solid line is a fit of Gaussian distribution function.
data are presented in Fig. 4. This figure can be used together with Eq. Ž1. to estimate sizes of the sp 2bonded clusters, taking into account the approximate size of a single graphite ring Ž d 0 f 0.3 nm.. The estimations suggest that a magnetron potential of 360 V N and the average size, d, of the graphite-like fragments are practically independent of pressure, with d f 0.6 nm Ž L f 2.. At larger magnetron potentials the N and, therefore, the d become a stronger function of pressure and d ranges from d f 0.6 nm Ž L f 3. in our experiment. A correlation between sizes of graphite-like fragments obtained from the Tauc edge of a-C:H films and from their TEM images was also found and studied by us in more detail in w6x. 4.2. Phenomenology of diamond nucleation in a-C:H The fluctuation theory yields an expression for the size distribution function of nucleating particles in the following explicit form w8x:
ž
W Ž R . ; Exp y
Ž R y R av . 2 2 R av
2
/
Ž 3.
where R is the particle radius and R av is the average radius. The size distribution of diamond crystals, found by visual analysis of the TEM micrograph
It was shown that conditions exist when diamond nucleation in the a-C:H matrix becomes possible during a-C:H film formation. The shape of the size-distribution function of diamond crystals indicates the early Žnucleation. stage of their formation. The spectral position of the fundamental optical absorption edge of a-C:H, mainly controlled by the presence of a graphite-like constituent in a-C:H structure.
Acknowledgements This work was partly supported by the Russian Foundation for Basic Research ŽGrant nos. 97-0218110 and 97-03-32273. and the US Department of defense. The Russian Program ‘Physics of Solid State Nanostructures’ is acknowledged for informational service. One of us ŽV.S.. was supported by the St.Petresburg Government.
References w1x V.I. Ivanov-Omskii, A.A. Sitnikova, A. A Suvorova, S.G. Yastrebov, T.K. Zvonareva, Mol. Mat. 8 Ž1996. 99. w2x M.J.S. Dewar, The Molecular Orbital Theory of Organic Chemistry, McGraw Hill, New York, 1969. w3x F. Gutman, L.E. Lyons, Organic Semiconductors. ŽWiley, 1967.. w4x L. Ramqvist, K. Hamrin, G. Johansson, A. Fahlman, C. Nording, J. Phys. Chem. Solids 30 Ž1969. 1835.
626
S.G. YastreboÕ et al.r Journal of Non-Crystalline Solids 227–230 (1998) 622–626
w5x Z. Li, L. Wang, T. Suzuki, A. Argoitia, P. Pirouz, J.C. Angus, J. Appl. Phys. 73 Ž2. Ž1993. 711. w6x V.I. Ivanov-Omskii, A.B. Lodygin, A.A. Sitnikova, A.A. Suvorova, S.G. Yastrebov, A.V. Tolmatchev, J. Chem. Vapor Deposition 5 Ž1997. 198.
w7x V.I. Ivanov-Omskii, A.V. Tolmatchev, S.G. Yastrebov, Phil. Mag. B 73 Ž4. Ž1996. 715. w8x L.D. Landau, E.M. Lifshitz, Physical Kinetics, Course of Theoretical Physics, Pergamon, New York, 1981. w9x F.P. Bundy, J. Geophys. Res. 85 Ž1971. 6930.