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Electronic structure study of growth species adsorption and reaction on cluster models for the diamond surface using LDA method
Diamond and Related Materials 12 (2003) 2169–2174
Electronic structure study of growth species adsorption and reaction on cluster models for the diamond surface using LDA method Xizhong An, Guoquan Liu*, Fuzhong Wang, Shengxin Liu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Received 9 September 2002; received in revised form 9 July 2003; accepted 13 July 2003
Abstract Electronic structure calculation was carried out for the hydrogen-terminated carbon cluster designed to model the {111}oriented diamond surface, Cd (111). Using LDA method with local generalised gradient corrections, the subject of the calculation included the adsorption of single-carbon species and double-carbon species on an activated diamond surface. The results showed that: (1) Single-carbon species will more likely result in {111}-oriented CVD diamond film growth than double-carbon species, however, the role of double-carbon species should not be ignored; (2) The adsorption of single-carbon species on activated site makes the system more stable than that of double-carbon species. (3) Our computation result of CH3 adsorption on growth surface is in good agreement with that acquired both theoretically and experimentally. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Quantum mechanics; LDA method; Surface chemisorption
1. Introduction Even though quick progress has been made in the process of chemical vapor deposition (CVD) diamond film growth during the last decade, the growth mechanism of diamond film is not well understood yet. This may be due to following reasons: (1) various complex chemical reactions occurred on the substrate surface are very hard to ascertain; and (2) many kineticythermodynamic data are very difficult to measure accurately in situ. Therefore, in order to account for the growth mechanism of diamond film and acquire diamond film of high quality efficiently, researchers have proposed several models of diamond film growth and simulated the growth process by using Kinetic Monte Carlo (KMC) method w1–5x. However, the ability of KMC to model realistically three-dimensional deposition processes depends crucially on a detailed understanding of the appropriate growth mechanisms and a reliable database of the rates of the surface chemical reactions. Clearly, the key to resolve these types of issues is an *Corresponding author. Tel.: q86-10-62332179; fax: 86-1062327283. E-mail address: [email protected] (G. Liu).
understanding of the atomic-scale dynamics that lead to the CVD of diamond films. Electronic structure theory has played an important role in the development of a molecular-level understanding of diamond CVD. In some cases, small carbon clusters have been treated at extremely high levels of ab initio theory. For other cases, researchers have turned to semiempirical computational schemes w6,7x, molecular dynamics simulations of finite carbon clusters w8– 10x and quantum calculations of infinite, periodic carbon slabs w11,12x. These alternate approaches suffer from limitations as well w13x, for instance inability to identify reaction transition states, or inability to consider low symmetry environments. Recently, Brown et al. described an ab initio electronic structure theory study on abstraction of hydrogen from the diamond surface by atomic hydrogen on Cd (111) w14x. Then, they extended their work to include an ab initio quantum mechanical investigation of the adsorption of methyl radical at activated sites on Cd (111) w13x. They also admitted that acetylene can also be an important carbon source but it was not considered in their work. Brown et al. w13,14x can compute the energy of growth species adsorbed on the cluster, however, only the chemisorption of CH3 and abstraction of H were
0925-9635/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-9635(03)00254-1
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X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Table 1 The chemical reactions between growth species and diamond surface No.
Surface chemical reactions
1 2 3 4 5 6 7 8 9 10 11
Cd(s)qH(g)mCd –H(s) Cd(s)qC(g)mCd –C(s) Cd(s)qCH(g)mCd –CH(s) Cd(s)qCH2(g)mCd –CH2(s) Cd(s)qCH3(g)mCd –CH3(s) Cd(s)qC2(g)mCd –C2(s) Cd(s)qC2H(g)mCd –C2H(g) Cd(s)qC2H2(g)mCd –C2H2(s) Cd(s)qC2H3(g)mCd –C2H3(s) Cd(s)qC2H4(g)mCd –C2H4(s) Cd(s)qC2H5(g)mCd –C2H5(s)
included in their model. In the real process of CVD diamond film growth, the growth species cannot be limited to only CH3 and H. Therefore, Brown et al.’s work cannot resolve the mechanism of CVD diamond in more detail, and the method they used has the long computation time and low efficiency. In this paper, we compute the energy of many species (including singlecarbon and double-carbon species) adsorbed on {111}oriented cluster by using local density approximation (LDA) method. In order to avoid the energy overestimation appeared in conventional LDA computation w15– 17x, we adopted local generalised gradient corrections in our computation, which proves to be an effective method for the investigations of the atomic geometry of CVD diamond film growth surfaces w12,18–21x. Thus, our computational results will allow the proper understanding on the growth mechanism of {111}-oriented CVD diamond film. 2. Cluster model and computational methods In order to study the energy variation of the species adsorbed on the surface, the chemical reaction model is provided in Table 1, where the Cd(s) represents an activated site on diamond surface, Cd –CHm (ms0, 1, 2, 3) and Cd –C2Hn (ns0, 1, 2, 3, 4, 5) mean the chemisorption of CHm and C2Hn on the activated site of CVD diamond film growth surface, respectively. In addition, s in each equation represents surface site and g is the symbol of vapor phase. It is well known that increasing the cluster size will definitely decrease the accuracy of computational results and increase computation time, i.e. there is a trade-off between cluster size and theoretical accuracy. Our strategy is to take as larger cluster as possible, but the accuracy must be assured beforehand. Therefore, the cluster containing 22 carbon atoms was used in our computation (as shown in Fig. 1), which is also very convenient for us to compare directly with Brown’s computation results w13x. In Fig. 1, the large circles indicate diamond atoms and the small ones represent
hydrogen atoms. This cluster was constrained to have the highest possible symmetries, so as to make the calculations as efficient as possible. The chemisorption of growth species begins on the carbon atom labeled ‘1’ in the cluster model, because the effect of all the first and second neighbor atoms are all considered in this model, and other atoms are unlikely to have a large effect upon the reaction energy, so this makes the cluster a more realistic model of the infinite diamond lattice. The following atoms of the {111}-oriented cluster were allowed to relax: the radical carbon labeled with ‘1’, its 3 nearest carbon neighbors and 9 carbon atoms of second neighbor, and all associated hydrogen atoms. In the whole process, we employ the local density approximation with the generalized gradient corrections implemented in the code DMol, the MSI software package w22–24x. The system can be in more stable state through the optimization of code DMol on the LDA cluster. Also, one of its best advantages is that it can compute the interaction between growth species and diamond surface and dynamics from atomic-scale in relatively high accuracy and speed. 3. Results and discussions In our computation, the whole energy E of the system can be divided into four parts. The relationship among them can be expressed as follows: EsEbqEaqEtqEv
(1)
Where Eb is bond energy, Ea is angle energy, Et is the energy of bond torsion and Ev is the van der Waals force between adsorption radicals and diamond growth surface. The value of total energy E determines the
Fig. 1. Cluster model of {111}-oriented CVD diamond film.
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Fig. 2. Variation of the total energy according to the adsorption of all species on activated sites.
stability of the whole system. The smaller the value of E, the more stable the whole system. If the total energy E of two systems is in the same value, then the system with larger Eb and smaller Ea or Et is more desirable. The total energy E of all species adsorbed on activated sites is shown in Fig. 2 (where DB is dangling bond). When considering the energy difference of the same species with different morphologies caused by the steric repulse effects, two cases for the adsorption of C2H2 and C2H4 on activated sites are listed in Figs. 3 and 4, respectively. Fig. 2 indicates that the adsorption of single-carbon cluster is much easier than that of doublecarbon cluster. Meanwhile, the adsorption of clusters with smaller size will make the system more stable, which has been proved by the experimental work of D’Evelyn et al. w25x. Through computation, we acquired the total energy E of CH3 adsorption on the activated
2171
site, which is compared with other researchers’ results listed in Table 2. It is concluded from Table 2 that the present calculated adsorption energy of CH3 is in good accordance with the results of previous studies and our results of hydrocarbon adsorption energy are also in the extent of 78– 112 kcalymol, which is described by Lu et al. w30x. Furthermore, our computational H adsorption energy is 62 kcalymol, which is nearly the same with Foord et al.’s simulation result (58.5 kcal/mol) of their experiment w31x. Variation of the angle energy Ea is shown in Fig. 5. In our computation, the value of Ea reflects the deviation degree of bond between adsorbed species and LDA cluster i.e. larger Ea will result in more serious deviation from normal diamond lattice, which will not be fit for CVD diamond film growth. Fig. 5 shows that doublecarbon species is much easier to deviate from normal site than single-carbon species and the larger the species size, the more serious the deviation. Fig. 6 is the variation of torsion energy Et according to the adsorption of all species on activated sites. Unlike Ea, Et determines the rotation degree of the adsorbed species on the activated site. The larger Et will also results in the mismatch of chemisorption species with the original diamond lattice. The variation of Et in Fig. 6 can obviously make us draw a conclusion that the adsorbed double-carbon cluster on {111}-oriented CVD diamond film surface can rotate more easily than singlecarbon cluster. Also, the smaller the species size, the less possibility of rotation. The van der Waals forces of all species adsorbed on the activated sites are listed in Fig. 7. Since the van der Waals force is the interaction molecular level, the size of molecule will definitely determine the value of the force. Fig. 7 indicates that the variation of van der
Fig. 3. Two cases for C2H2 adsorbed on the activated sites.
2172
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Fig. 4. Two cases for C2H4 adsorbed on the activated sites. Table 2 Energy data of CH3 adsorption on diamond (111) surface Adsorption energy of CH3 (kcalymol)
References
80 75 81.5 84.0
w25–27x w2,28,29x Computational result in this study w13x
Waals force is not so different for both single-carbon species and double-carbon species. On the contrary, atom H and DB express the lower value, because compared with the carbon-containing species, their size is much smaller. Variation of the minimum energy for all species to adsorb on the activated sites is shown in Fig. 8. This figure also indicates that it is more difficult for the
adsorption of double-carbon species than that of singlecarbon species and larger the species size, the larger energy for them to adsorb on the growth surface. Therefore, from Figs. 2, 5–8, we can draw the following conclusions: the growth of {111}-oriented CVD diamond film is more likely from the adsorption of singlecarbon species (especially CH3), but this cannot
Fig. 5. Variation of the angle energy according to the adsorption of all species on activated sites.
Fig. 6. Variation of the torsion energy according to the adsorption of all species on activated sites.
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
2173
2. Even though single-carbon species has the advantageous for CVD diamond film growth, however, we cannot neglect the role of double-carbon species, only their adsorption possibility is relatively small. 3. Our computation result of CH3 and H adsorption on the activated surface is in good agreement with that of others’, which proved the reliability of our LDA method. Also, our computation results will feed the information into the Kinetic Monte Carlo (KMC) model and this is very helpful for further understanding of the growth mechanism from atomic-scale.
Acknowledgments
Fig. 7. Variation of the van der Waals forces according to the adsorption of all species on activated sites.
The support of the National Natural Science Funds of China (No. 59872003) is gratefully acknowledged. References
predicate that the adsorption of double-carbon species (mainly C2H2) will not result in the growth of CVD diamond film, only somewhat difficult, which has been supported by both theoretical w32–34x and experimental w25,30x studies. 4. Conclusions From the computation of all species adsorbed on the {111}-oriented CVD diamond film by using LDA method with local generalised gradient corrections from electronic-scale, we can draw conclusions as following: 1. Single-carbon species can adsorb on the activated site more easily than double-carbon species and the former will result in a more stable state than the latter, which is very helpful for the acquisition of CVD diamond film with good quality.
Fig. 8. Variation of the minimum energy needed for the adsorption of all species on activated sites.
w1x S.J. Harris, D.G. Goodwin, J. Phys. Chem. 97 (1993) 23. w2x E.J. Dawnkaski, D. Srivastava, B.J. Garrison, J. Chem. Phys. 104 (1996) 5997. w3x C.C. Battaile, D.J. Srolovitz, J.E. Butler, J. Cryst. Growth 194 (1998) 353. w4x M. Grujicic, S.G. Lai, J. Mater. Sci. 34 (1999) 7. w5x X.Z. An, Y. Zhang, G.Q. Liu, X.G. Qin, F.Z. Wang, S.X. Liu, Chin. Phys. Lett. 19 (7) (2002) 1019. w6x D. Huang, M. Frenklach, J. Phys. Chem. 96 (1992) 1868. w7x S. Skokov, B. Weiner, M. Frenklach, J. Phys. Chem. 98 (1994) 7073. w8x S. Skokov, C.S. Carmer, B. Weiner, M. Frenklach, Phys. Rev. B 49 (1994) 5662. w9x K. Song, P. DeSainteClaire, W.L. Hase, K.C. Hass, Phys. Rev. B 52 (1995) 2949. w10x D. Alfonso, D. Drabold, S. Ulloa, J. Phys.: Condens. Mater. 8 (1996) 641. w11x B.N. Davidson, W.E. Pickett, Phys. Rev. B 49 (1994) 11 253. w12x C. Kress, M. Fiedler, W.G. Schmidt, F. Bechstedt, Surf. Sci. 331–333 (1995) 1152. w13x R.C. Brown, C.J. Cramer, J.T. Roberts, Diamond Relat. Mater. 10 (2001) 39. w14x R.C. Brown, C.J. Cramer, J.T. Roberts, J. Phys. Chem. B 101 (1997) 9574. w15x M.R. Pederson, K.A. Jackson, W.E. Pickett, Phys. Rev. B 44 (8) (1991) 3891. w16x J.P. Perdew, Y. Wang, Phys. Rev. B33 (1986) 8800. w17x E. Clementi, C. Chakravorty, J. Chem. Phys. 23 (1990) 2591. w18x D. Vanderbilt, S.G. Louie, Phys. Rev. B 29 (1984) 7099. w19x W.G. Schmidt, A. Scholze, F. Bechstedt, Surf. Sci. 351 (1996) 183. w20x G. Kern, J. Hafner, G. Kresse, Surf. Sci. 366 (1996) 445. w21x Z. Zhang, M. Wensell, J. Bernholc, Phys. Rev. B 51 (1995) 5291. w22x B. Delley, J. Chem. Phys. 92 (1990) 508. w23x R.D. Groot, P.B. Warren, J. Chem. Phys. 107 (1997) 4423. w24x I.I. Oleinik, D.G. Pettifor, A.P. Sutton, J.E. Butler, Diamond Relat. Mater. 9 (2000) 241. w25x M.P. D’Evelyn, J.D. Graham, L.R. Martin, J. Crys. Growth 231 (2001) 506. w26x M. Frenklach, H. Wang, Phys. Rev. B 43 (1991) 1520.
2174 w27x w28x w29x w30x w31x
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174 M. Frenklach, S. Skokov, J. Phys. Chem. B 101 (1997) 3025. S.J. Harris, D.G. Goodwin, J. Phys. Chem. 97 (1993) 23. M.E. Coltrin, D.S. Dandy, J. Appl. Phys. 74 (1993) 5803. H.F. Lu, Y.C. Sun, Diamond Relat. Mater. 11 (2002) 1560. J.S. Foord, K.P. Loh, R.B. Jackman, Surf. Sci. 399 (1998) 1.
w32x C.C. Battaile, D.J. Srolovitz, J.E. Butler, Diamond Relat. Mater. 6 (1997) 1198. w33x M. Frenklach, K. Spear, J. Mater. Res. 3 (1988) 133. w34x M. Frenklach, J. Chem. Phys. 97J (1992) 5794.
Electronic structure study of growth species adsorption and reaction on cluster models for the diamond surface using LDA method Xizhong An, Guoquan Liu*, Fuzhong Wang, Shengxin Liu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Received 9 September 2002; received in revised form 9 July 2003; accepted 13 July 2003
Abstract Electronic structure calculation was carried out for the hydrogen-terminated carbon cluster designed to model the {111}oriented diamond surface, Cd (111). Using LDA method with local generalised gradient corrections, the subject of the calculation included the adsorption of single-carbon species and double-carbon species on an activated diamond surface. The results showed that: (1) Single-carbon species will more likely result in {111}-oriented CVD diamond film growth than double-carbon species, however, the role of double-carbon species should not be ignored; (2) The adsorption of single-carbon species on activated site makes the system more stable than that of double-carbon species. (3) Our computation result of CH3 adsorption on growth surface is in good agreement with that acquired both theoretically and experimentally. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: CVD diamond; Quantum mechanics; LDA method; Surface chemisorption
1. Introduction Even though quick progress has been made in the process of chemical vapor deposition (CVD) diamond film growth during the last decade, the growth mechanism of diamond film is not well understood yet. This may be due to following reasons: (1) various complex chemical reactions occurred on the substrate surface are very hard to ascertain; and (2) many kineticythermodynamic data are very difficult to measure accurately in situ. Therefore, in order to account for the growth mechanism of diamond film and acquire diamond film of high quality efficiently, researchers have proposed several models of diamond film growth and simulated the growth process by using Kinetic Monte Carlo (KMC) method w1–5x. However, the ability of KMC to model realistically three-dimensional deposition processes depends crucially on a detailed understanding of the appropriate growth mechanisms and a reliable database of the rates of the surface chemical reactions. Clearly, the key to resolve these types of issues is an *Corresponding author. Tel.: q86-10-62332179; fax: 86-1062327283. E-mail address: [email protected] (G. Liu).
understanding of the atomic-scale dynamics that lead to the CVD of diamond films. Electronic structure theory has played an important role in the development of a molecular-level understanding of diamond CVD. In some cases, small carbon clusters have been treated at extremely high levels of ab initio theory. For other cases, researchers have turned to semiempirical computational schemes w6,7x, molecular dynamics simulations of finite carbon clusters w8– 10x and quantum calculations of infinite, periodic carbon slabs w11,12x. These alternate approaches suffer from limitations as well w13x, for instance inability to identify reaction transition states, or inability to consider low symmetry environments. Recently, Brown et al. described an ab initio electronic structure theory study on abstraction of hydrogen from the diamond surface by atomic hydrogen on Cd (111) w14x. Then, they extended their work to include an ab initio quantum mechanical investigation of the adsorption of methyl radical at activated sites on Cd (111) w13x. They also admitted that acetylene can also be an important carbon source but it was not considered in their work. Brown et al. w13,14x can compute the energy of growth species adsorbed on the cluster, however, only the chemisorption of CH3 and abstraction of H were
0925-9635/03/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0925-9635(03)00254-1
2170
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Table 1 The chemical reactions between growth species and diamond surface No.
Surface chemical reactions
1 2 3 4 5 6 7 8 9 10 11
Cd(s)qH(g)mCd –H(s) Cd(s)qC(g)mCd –C(s) Cd(s)qCH(g)mCd –CH(s) Cd(s)qCH2(g)mCd –CH2(s) Cd(s)qCH3(g)mCd –CH3(s) Cd(s)qC2(g)mCd –C2(s) Cd(s)qC2H(g)mCd –C2H(g) Cd(s)qC2H2(g)mCd –C2H2(s) Cd(s)qC2H3(g)mCd –C2H3(s) Cd(s)qC2H4(g)mCd –C2H4(s) Cd(s)qC2H5(g)mCd –C2H5(s)
included in their model. In the real process of CVD diamond film growth, the growth species cannot be limited to only CH3 and H. Therefore, Brown et al.’s work cannot resolve the mechanism of CVD diamond in more detail, and the method they used has the long computation time and low efficiency. In this paper, we compute the energy of many species (including singlecarbon and double-carbon species) adsorbed on {111}oriented cluster by using local density approximation (LDA) method. In order to avoid the energy overestimation appeared in conventional LDA computation w15– 17x, we adopted local generalised gradient corrections in our computation, which proves to be an effective method for the investigations of the atomic geometry of CVD diamond film growth surfaces w12,18–21x. Thus, our computational results will allow the proper understanding on the growth mechanism of {111}-oriented CVD diamond film. 2. Cluster model and computational methods In order to study the energy variation of the species adsorbed on the surface, the chemical reaction model is provided in Table 1, where the Cd(s) represents an activated site on diamond surface, Cd –CHm (ms0, 1, 2, 3) and Cd –C2Hn (ns0, 1, 2, 3, 4, 5) mean the chemisorption of CHm and C2Hn on the activated site of CVD diamond film growth surface, respectively. In addition, s in each equation represents surface site and g is the symbol of vapor phase. It is well known that increasing the cluster size will definitely decrease the accuracy of computational results and increase computation time, i.e. there is a trade-off between cluster size and theoretical accuracy. Our strategy is to take as larger cluster as possible, but the accuracy must be assured beforehand. Therefore, the cluster containing 22 carbon atoms was used in our computation (as shown in Fig. 1), which is also very convenient for us to compare directly with Brown’s computation results w13x. In Fig. 1, the large circles indicate diamond atoms and the small ones represent
hydrogen atoms. This cluster was constrained to have the highest possible symmetries, so as to make the calculations as efficient as possible. The chemisorption of growth species begins on the carbon atom labeled ‘1’ in the cluster model, because the effect of all the first and second neighbor atoms are all considered in this model, and other atoms are unlikely to have a large effect upon the reaction energy, so this makes the cluster a more realistic model of the infinite diamond lattice. The following atoms of the {111}-oriented cluster were allowed to relax: the radical carbon labeled with ‘1’, its 3 nearest carbon neighbors and 9 carbon atoms of second neighbor, and all associated hydrogen atoms. In the whole process, we employ the local density approximation with the generalized gradient corrections implemented in the code DMol, the MSI software package w22–24x. The system can be in more stable state through the optimization of code DMol on the LDA cluster. Also, one of its best advantages is that it can compute the interaction between growth species and diamond surface and dynamics from atomic-scale in relatively high accuracy and speed. 3. Results and discussions In our computation, the whole energy E of the system can be divided into four parts. The relationship among them can be expressed as follows: EsEbqEaqEtqEv
(1)
Where Eb is bond energy, Ea is angle energy, Et is the energy of bond torsion and Ev is the van der Waals force between adsorption radicals and diamond growth surface. The value of total energy E determines the
Fig. 1. Cluster model of {111}-oriented CVD diamond film.
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Fig. 2. Variation of the total energy according to the adsorption of all species on activated sites.
stability of the whole system. The smaller the value of E, the more stable the whole system. If the total energy E of two systems is in the same value, then the system with larger Eb and smaller Ea or Et is more desirable. The total energy E of all species adsorbed on activated sites is shown in Fig. 2 (where DB is dangling bond). When considering the energy difference of the same species with different morphologies caused by the steric repulse effects, two cases for the adsorption of C2H2 and C2H4 on activated sites are listed in Figs. 3 and 4, respectively. Fig. 2 indicates that the adsorption of single-carbon cluster is much easier than that of doublecarbon cluster. Meanwhile, the adsorption of clusters with smaller size will make the system more stable, which has been proved by the experimental work of D’Evelyn et al. w25x. Through computation, we acquired the total energy E of CH3 adsorption on the activated
2171
site, which is compared with other researchers’ results listed in Table 2. It is concluded from Table 2 that the present calculated adsorption energy of CH3 is in good accordance with the results of previous studies and our results of hydrocarbon adsorption energy are also in the extent of 78– 112 kcalymol, which is described by Lu et al. w30x. Furthermore, our computational H adsorption energy is 62 kcalymol, which is nearly the same with Foord et al.’s simulation result (58.5 kcal/mol) of their experiment w31x. Variation of the angle energy Ea is shown in Fig. 5. In our computation, the value of Ea reflects the deviation degree of bond between adsorbed species and LDA cluster i.e. larger Ea will result in more serious deviation from normal diamond lattice, which will not be fit for CVD diamond film growth. Fig. 5 shows that doublecarbon species is much easier to deviate from normal site than single-carbon species and the larger the species size, the more serious the deviation. Fig. 6 is the variation of torsion energy Et according to the adsorption of all species on activated sites. Unlike Ea, Et determines the rotation degree of the adsorbed species on the activated site. The larger Et will also results in the mismatch of chemisorption species with the original diamond lattice. The variation of Et in Fig. 6 can obviously make us draw a conclusion that the adsorbed double-carbon cluster on {111}-oriented CVD diamond film surface can rotate more easily than singlecarbon cluster. Also, the smaller the species size, the less possibility of rotation. The van der Waals forces of all species adsorbed on the activated sites are listed in Fig. 7. Since the van der Waals force is the interaction molecular level, the size of molecule will definitely determine the value of the force. Fig. 7 indicates that the variation of van der
Fig. 3. Two cases for C2H2 adsorbed on the activated sites.
2172
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
Fig. 4. Two cases for C2H4 adsorbed on the activated sites. Table 2 Energy data of CH3 adsorption on diamond (111) surface Adsorption energy of CH3 (kcalymol)
References
80 75 81.5 84.0
w25–27x w2,28,29x Computational result in this study w13x
Waals force is not so different for both single-carbon species and double-carbon species. On the contrary, atom H and DB express the lower value, because compared with the carbon-containing species, their size is much smaller. Variation of the minimum energy for all species to adsorb on the activated sites is shown in Fig. 8. This figure also indicates that it is more difficult for the
adsorption of double-carbon species than that of singlecarbon species and larger the species size, the larger energy for them to adsorb on the growth surface. Therefore, from Figs. 2, 5–8, we can draw the following conclusions: the growth of {111}-oriented CVD diamond film is more likely from the adsorption of singlecarbon species (especially CH3), but this cannot
Fig. 5. Variation of the angle energy according to the adsorption of all species on activated sites.
Fig. 6. Variation of the torsion energy according to the adsorption of all species on activated sites.
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174
2173
2. Even though single-carbon species has the advantageous for CVD diamond film growth, however, we cannot neglect the role of double-carbon species, only their adsorption possibility is relatively small. 3. Our computation result of CH3 and H adsorption on the activated surface is in good agreement with that of others’, which proved the reliability of our LDA method. Also, our computation results will feed the information into the Kinetic Monte Carlo (KMC) model and this is very helpful for further understanding of the growth mechanism from atomic-scale.
Acknowledgments
Fig. 7. Variation of the van der Waals forces according to the adsorption of all species on activated sites.
The support of the National Natural Science Funds of China (No. 59872003) is gratefully acknowledged. References
predicate that the adsorption of double-carbon species (mainly C2H2) will not result in the growth of CVD diamond film, only somewhat difficult, which has been supported by both theoretical w32–34x and experimental w25,30x studies. 4. Conclusions From the computation of all species adsorbed on the {111}-oriented CVD diamond film by using LDA method with local generalised gradient corrections from electronic-scale, we can draw conclusions as following: 1. Single-carbon species can adsorb on the activated site more easily than double-carbon species and the former will result in a more stable state than the latter, which is very helpful for the acquisition of CVD diamond film with good quality.
Fig. 8. Variation of the minimum energy needed for the adsorption of all species on activated sites.
w1x S.J. Harris, D.G. Goodwin, J. Phys. Chem. 97 (1993) 23. w2x E.J. Dawnkaski, D. Srivastava, B.J. Garrison, J. Chem. Phys. 104 (1996) 5997. w3x C.C. Battaile, D.J. Srolovitz, J.E. Butler, J. Cryst. Growth 194 (1998) 353. w4x M. Grujicic, S.G. Lai, J. Mater. Sci. 34 (1999) 7. w5x X.Z. An, Y. Zhang, G.Q. Liu, X.G. Qin, F.Z. Wang, S.X. Liu, Chin. Phys. Lett. 19 (7) (2002) 1019. w6x D. Huang, M. Frenklach, J. Phys. Chem. 96 (1992) 1868. w7x S. Skokov, B. Weiner, M. Frenklach, J. Phys. Chem. 98 (1994) 7073. w8x S. Skokov, C.S. Carmer, B. Weiner, M. Frenklach, Phys. Rev. B 49 (1994) 5662. w9x K. Song, P. DeSainteClaire, W.L. Hase, K.C. Hass, Phys. Rev. B 52 (1995) 2949. w10x D. Alfonso, D. Drabold, S. Ulloa, J. Phys.: Condens. Mater. 8 (1996) 641. w11x B.N. Davidson, W.E. Pickett, Phys. Rev. B 49 (1994) 11 253. w12x C. Kress, M. Fiedler, W.G. Schmidt, F. Bechstedt, Surf. Sci. 331–333 (1995) 1152. w13x R.C. Brown, C.J. Cramer, J.T. Roberts, Diamond Relat. Mater. 10 (2001) 39. w14x R.C. Brown, C.J. Cramer, J.T. Roberts, J. Phys. Chem. B 101 (1997) 9574. w15x M.R. Pederson, K.A. Jackson, W.E. Pickett, Phys. Rev. B 44 (8) (1991) 3891. w16x J.P. Perdew, Y. Wang, Phys. Rev. B33 (1986) 8800. w17x E. Clementi, C. Chakravorty, J. Chem. Phys. 23 (1990) 2591. w18x D. Vanderbilt, S.G. Louie, Phys. Rev. B 29 (1984) 7099. w19x W.G. Schmidt, A. Scholze, F. Bechstedt, Surf. Sci. 351 (1996) 183. w20x G. Kern, J. Hafner, G. Kresse, Surf. Sci. 366 (1996) 445. w21x Z. Zhang, M. Wensell, J. Bernholc, Phys. Rev. B 51 (1995) 5291. w22x B. Delley, J. Chem. Phys. 92 (1990) 508. w23x R.D. Groot, P.B. Warren, J. Chem. Phys. 107 (1997) 4423. w24x I.I. Oleinik, D.G. Pettifor, A.P. Sutton, J.E. Butler, Diamond Relat. Mater. 9 (2000) 241. w25x M.P. D’Evelyn, J.D. Graham, L.R. Martin, J. Crys. Growth 231 (2001) 506. w26x M. Frenklach, H. Wang, Phys. Rev. B 43 (1991) 1520.
2174 w27x w28x w29x w30x w31x
X. An et al. / Diamond and Related Materials 12 (2003) 2169–2174 M. Frenklach, S. Skokov, J. Phys. Chem. B 101 (1997) 3025. S.J. Harris, D.G. Goodwin, J. Phys. Chem. 97 (1993) 23. M.E. Coltrin, D.S. Dandy, J. Appl. Phys. 74 (1993) 5803. H.F. Lu, Y.C. Sun, Diamond Relat. Mater. 11 (2002) 1560. J.S. Foord, K.P. Loh, R.B. Jackman, Surf. Sci. 399 (1998) 1.
w32x C.C. Battaile, D.J. Srolovitz, J.E. Butler, Diamond Relat. Mater. 6 (1997) 1198. w33x M. Frenklach, K. Spear, J. Mater. Res. 3 (1988) 133. w34x M. Frenklach, J. Chem. Phys. 97J (1992) 5794.