- Email: [email protected]
An atomistic study of growth mode and microstructure evolution of amorphous carbon films by different incident carbon atoms
Applied Surface Science 314 (2014) 973–982
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An atomistic study of growth mode and microstructure evolution of amorphous carbon films by different incident carbon atoms Chen Xue a , Jianqiu Zhou a,b,∗ a b
Department of Mechanical Engineering, Nanjing Tech University, Nanjing, Jiangsu Province 210009, China Department of Mechanical Engineering, Wuhan Institute of Technology, Wuhan, Hubei Province 430070, China
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 20 June 2014 Accepted 20 June 2014 Available online 27 June 2014 Keywords: Molecular dynamics simulation Amorphous carbon films Growth mechanism Interfacial microstructure evolution
a b s t r a c t In this paper, molecular dynamics (MD) simulation has been performed to describe the growth and interfacial microstructure of amorphous carbon films. We focus on the film growth mode and surface morphology for diverse deposition process parameters mainly including incident energy and incident angle. To explore the relationship between the motion of deposition atoms and amorphous films growth, a series of snapshots for each deposition process has been taken for comparison. The snapshots show that the films growth modes are diverse at different incident parameters. In the next step, surface morphology, atom distribution along film growth direction and internal structure including vacancy defects evolution during deposition process are analyzed. The results reveal that incident energy on the horizontal plane dominates the surface roughness, and incident energy on the vertical plane dominates the compactness of the film. We conclude that a suitable incident parameter is not only beneficial to prepare amorphous films with compact and smooth or bump-like surface which will meet different needs, but also can avoid formation of defects during deposition. The simulation results are expected to provide useful guidance for improving amorphous carbon films quality. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ability of carbon to form sp, sp2 and sp3 hybridized electronic states gives rise to a vast variety of different phases which often show marvellous properties. Amorphous carbon with a considerable fraction of sp3 bonds is often known as diamond-like carbon (DLC) [1]. The combination of diamond-like properties and ultra smoothness is the main factor for the technological importance of amorphous carbon films. Hence, amorphous carbon films have widespread applications as high hardness drill tools and high performance wear-resistant materials [2]. Besides, owing to their great biocompatibility and chemical inertness, amorphous carbon films are promising in biomedical coatings such as hip implants and knee joint prostheses [3]. Amorphous carbon film was first prepared by ion beam deposition method. After decades of development, it is possible to prepare amorphous carbon films by a wide range of deposition methods including sputtering, plasma deposition and pulsed laser deposition. Due to the different deposition methods, the
∗ Corresponding author. Tel.: +86 25 83588706/13655192068; fax: +86 25 83374190. E-mail address: [email protected] (J. Zhou). http://dx.doi.org/10.1016/j.apsusc.2014.06.133 0169-4332/© 2014 Elsevier B.V. All rights reserved.
microstructure and properties of amorphous carbon films are varied [4]. Taking a typical deposition system as an example, such as ion beam deposition, carbon ions will be firstly produced by the plasma sputtering of a graphite cathode in an ion source. The carbon ions are then accelerated by a bias voltage or magnetic field in the high vacuum deposition chamber to obtain high kinetic energy, finally bombarding the substrate [5]. When the kinetic energy of the incident carbon ions is high enough, the ions can collide directly under the surface into the deposition film, resulting in local density increasing inside the film, which will be more conducive to form sp3 hybridization [6]. According to amorphous carbon films deposition mechanism mentioned above, we can see that deposition process requires a rigorous condition, such as incident energy, incident angle, substrate temperature, deposition pressure, deposition rate and so on [7,8]. It is generally accepted that film properties depend on the microscopic structure of the deposited layer. However there are no experimental methods for characterizing this microstructure from atomic scale viewpoint. In addition, the explanation for the growth mechanism of amorphous carbon films is still controversial. Thanks to computational simulation techniques, researcher can get a deeper insight into the microscopic structure and properties of amorphous carbon films, which allow us to explain the
974
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
deposition process in atomic scale. MD study shows that the density, sp3 fraction and compressive stress will change accordingly with the different incident energy. When the ion energy was 70 eV/atom, the content of the sp3 hybridized bond in the film and film density reach to their peaks. For those incident energy particles less than 70 eV/atom, they cannot overcome the potential barrier among substrate atoms, but can only be attached to the surface forming epitaxial sp2 hybridized structure, which will cause sp3 hybridized bond decrease. However, when the incident energy is much higher than 70 eV/atom, the density and sp3 hybrid phase content of amorphous carbon film decline on the contrary [9–11]. And relevant literature suggests that the impact-induced uphill transport of the surface atoms will be mitigated by adjusting incident angle [12]. And surface smoothness under normal or near-normal incidence condition has been investigated in the light of atomic displacement: ion-induced relaxation [13]. As for the movement mechanism of particles and interfacial microstructure evolution during deposition, they were not discussed in the previous work. In the present study, we took both incident energy and incident angle into consideration to research amorphous carbon films growth mode and surface morphology. Atom distribution along film growth direction and the interfacial microstructure evolution during deposition process were analyzed. Our studies demonstrated that there was extraordinary significance for the setting of deposition parameters, especially incident energy and incident angle.
2.1. Empirical potential function of carbon In order to simulate the deposition process of amorphous carbon films by molecular dynamics simulation, the atomic-scale calculations and simulations here were carried out by the Tersoff potential [14,15]. The earliest computational method used for amorphous carbon simulation was presented by combining the density functional theory (DFT) and molecular dynamics [16]. However, this method was found to be computationally inefficient. Tersoff was the first computationally efficient potential function to study different hybridization states of carbon such as graphite and diamond. The detailed information of the Tersoff potential, which has been described by equations as below: 1 Vij 2 i
j= / i
Vij = fc (rij ) fR (rij ) + bij fA (rij )
Incident particles
a
b
c
d
e
f
Incident angle (◦ ) Kinetic energy along X-axis (eV) Kinetic energy along Z-axis (eV)
– – 25
– – 50
45 25 25
35.3 25 50
54.7 50 25
– – 75
fc (r) =
⎧ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎩
r ≤R−D
1 1 1 − sin 2 2
2
−
r−R D
0
R−D
fR (r) = A exp(−1 r) fA (r) = −B exp(−2 r) Here Vij presents the bond energies between atom i and atom j. And E is the total energy of the system, which can be decomposed into Vij for convenience. rij is the distance from atom i to atom j. The repulsive and attractive potentials can be described by functions fR and fA , respectively. The cutoff function limiting the effective distance of each potentials equation described by func˚ The parameters of 1 tion fc . R equals 1.95 A˚ and D equals 0.15 A. and 2 are 3.2394 A˚ −1 and 1.3258 A˚ −1 , respectively. The parameters A and B are 1393.6 eV and 346.74 eV. 2.2. Schematic diagram and parameters of incident particles
2. Simulation details and method
E=
Table 1 Parameters of incident particles.
In order to compare the movement mechanisms of particles and film growth modes under different incidence cases, the effects of the deposition energy were examined under three different incidence energies, E = 25 eV, 50 eV, and 75 eV, with four different incidence angles, 0◦ , 35.3◦ , 45◦ , and 54.7◦ . Fig. 1 lists six different kinds of incident carbon particles (a–e), more details about the incident atoms parameters are presented in Table 1. 2.3. Simulation model and details As shown in Fig. 2, atomistic model of deposition consists of diamond substrate and insert carbon atoms. The substrate consisted of 4800 carbon atoms with perfect diamond lattice structure arrangement, which constituted 24 monolayers with 200 atoms per layer. The dimensions of substrate were 3.556, 3.556, and 2.1 nm in the directions of X-axis, Y-axis, and Z-axis, respectively. Periodic boundary conditions are imposed on the surface plane along the directions of X-axis and Y-axis. There were three types of atoms in substrate: fixed atoms thermal-controlled atoms, and free atoms, and the thickness of each type atoms in the Z-axis were 0.35, 0.875 and 0.875 nm. To prevent the movement of substrate, the bottom
Fig. 1. Schematic diagram of the motion of carbon particles.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
975
performed using the large-scale atomic/molecular massively parallel simulator (LAMMPS) software [17]. The visualization and post analysis tool was OVITO [18]. 3. Results and discussion 3.1. Deposition process of amorphous carbon films
Fig. 2. Simulation diagram of high kinetic carbon atoms deposited on diamond surfaces.
four layers atoms were held fixed in their bulk lattice sites. Above the fixed layer, the middle ten layers were assigned as the thermal control layers where the velocities of all atoms were rescaled to room temperature (T = 293 K) by the Berendsen method, and these thermal controlled atoms were mainly used to bear the relaxation work for the whole system. The top ten layers of substrate were free atoms, which abide by the Newton’s second law. And the upper four layers of substrate could be entirely free to transmit energy with the incident atoms. The time interval between two sequential deposition processes was set to 5 ps, then the particles flux was approximately 1.58 × 1024 cm−2 s−1 . Fig. 3 shows the kinetic energy fluctuation of the system during depositing the first six incident carbon atoms. Due to the existence of the thermal controlled layer, the average kinetic energy of system atoms remained constant. The energy fluctuation indicates that 5 ps was enough for relaxing the deposited atoms, and the construction of the simulation model was also justified. The deposition process was simulated using constant microcanonical ensemble (NVE). Taking both computational efficiency and accuracy of the results into consideration, time step was set as 2 fs. All of the molecular dynamics simulations were
As we can see in Fig. 4, amorphous carbon films approximately 0.8–1 nm are formed on the diamond substrate. The color encodes the height of the particles. The arrow in the rightmost of Fig. 1 indicates the films growth direction in the XZ plane. The morphology evolution of amorphous carbon films during deposition process is correlated with the motion of incident atoms. The incident atoms carried with different angles and energies will make the deposition process and films growth modes tend to be diversified. Deposition behavior depends largely on both incident energy and incident angle. At normal incidence (Fig. 1a, b and f), the growth surface of films maintains smooth during the whole deposition process. Be different from the first two incident methods, at oblique incidence (Fig. 1c, d and e), due to the existence of the incident energy on the horizontal plane, the surface of films no longer maintains smooth, and the film growth seems to be relatively slow. Comparing Fig. 4a and c, or b and d, we can see that the chaotic degree of particles above the deposition surface change with the horizontal incident energy. The atomic motion mechanisms of incident atoms, including adsorption, rebound and implant, these three mechanisms lead to the structural rearrangements and surface diffusion during the whole deposition process, which dominate the resulting film morphology. As for the rebound mechanism, incident atoms collide with the substrate atoms by transferring energy and then rebound. Rebound atoms collide with the later incident atoms, which will produce a shadowing effect above the film growth surface. Because of the angle of incidence, rebound plays a dominant role in the motion mechanisms of particles. The existence of shadowing effects will reduce the incident energy of particles to the substrate surface, this will be helpful to form graphitization phase in the amorphous carbon films. By comparing Fig. 4a, c and e, we can draw the conclusion that the incident energy on the horizontal plane always increases the degree of atomic chaos, and the final energy deposited on amorphous films will reduce, which will lead to a looser structure with lots of defects in the uppermost layers of the film. If the incident angle is large, or even at grazing incidence, a bump-like surface structure emerged and led to rough surfaces. However the amorphous carbon films with a bumpy surface have large surface energy, which will have a great advantage in the application of energy conversion and storage devices such as supercapacitors or hydrogen storage devices [19–21]. 3.2. Surface morphology and atoms distribution
Fig. 3. Kinetic energetic variation during the deposition process when the incident energy is 75 eV/atom.
Figs. 5–10 show the surface structure fabricated by six different kinds of insert patterns. To explore how the incident patterns affect the film surface morphology, color gradient illustration is utilized to refer the atom distribution along the direction of Z-axis. The color denotes the height of the particles. The color changes from shallow to deep along the height position. The carbon atoms denoted by drab color indicates that they are in the low position, and the more diverse color of the carbon atoms, the rougher the surface morphology of films will be. It is obvious that, if the incident energy along Z-axis direction is smaller, for example Fig. 5a and b, Fig. 7a and b and Fig. 9a and b, the surface of the deposited film will be bumpier. Conversely, if the incident energy along Z-axis direction is higher, for example Fig. 6a and b, Fig. 8a and b and Fig. 10a and b,
976
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 4. Side view snapshots of six incident carbon atoms bombarded on diamond substrate. The film growth direction is indicated by the arrow in the rightmost. The shadowing effects caused by different incident atoms can be clearly displayed by the chaotic degree of particles above the deposition surface.
the surface morphology will be more compact and smoother. This is because the greater the energy in the horizontal direction, the diffusive events and atomic rearrangements above the deposition surface will become more intense, which have a strong impact on the resulting film morphology.
As shown in Fig. 5c to Fig. 10c, the incident particles and free particles are marked blue and red respectively to make a detailed explanation of the injected growth mechanism. An incident particle with low energy will not have enough kinetic energy to penetrate the surface, so it will just stick to the upper surface, and remain
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
977
Fig. 5. Top view (a) and side view (b) of amorphous film prepared by 25 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 5c.
in its lowest energy state finally forming sp2 hybridization. If the kinetic energy of the incident atom was higher than the penetration threshold, it had a chance to penetrate the surface, and enter a subsurface interstitial site. This will cause local density increasing. The local bonding will reform around that atom. When the solid became amorphous, the incident atom and the target atoms were equivalent. We assume that in the highly energetic conditions of particle bombardment existing during film growth, atomic hybridization will adjust easily to change in the local density, and became more sp3 if the density was high [22], or more sp2 if the density was low, the MD simulation results of surface structure deposited by low energy carbon particles will be 1-D chain or 2-D ring [23,24]. However, when the energy of the particles exceeds a certain range, the
sputtering reaction will be more apparent [25], which make films dense but reduce its thickness. Here statistics about the distribution of particles along film growth direction are analyzed. Figs. 11 and 12 are statistical charts of incident particles and free particles distributed along the Z-axis direction. These statistics can make a better reflection about the mechanism of amorphous carbon films deposition. By comparing six different atoms distributions, we can see that the free carbon particles distributions along the Z-axis direction show downward ˚ the light green, trend. As we can see in the chart, when Z < 21 A, blue and red dotted line are always located above the orange, dark green and purple dotted line, and this situation reverse in ˚ This can be the position of the outermost layer where Z = 21 A.
Fig. 6. Top view (a) and side view (b) of amorphous film prepared by 50 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 6c.
978
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 7. Top view (a) and side view (b) of amorphous film prepared by 50 eV kinetic energy and 45◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 7c.
explained that the original substrate has been damaged by incident atoms, and a sputtering effect emerged at the same time. And when the incidence energy in the vertical direction is larger, just like incident case b, d and f which represented individually by dark green, orange and purple dotted line in the chart, the sputtering effect of free atoms on the substrate become more intense. In other incident cases a, c and e, since the kinetic energy of the incident in the vertical direction is small, the sputtering effect is not so obvious by contrasting the three incident cases mentioned above. The tendencies of insert particles concentration along the Z-axis direction increase firstly then decrease as shown in Fig. 11. Particle distribution seems disorderly and unsystematic, but after careful observation we can sum up some conclusions. As for incident case
a, because of the low vertically incident energy, adsorption is the predominant mechanism among three atomic motion mechanisms. And the deposited atoms are absorbed in the low energetic position of the substrate. That is why when Z > 22 A˚ the light green dotted line always located above the other dotted lines. As for incident case c and e, due to the existence of horizontal incident energy, rebound mechanism takes a leading role. On one hand, the existence of horizontal energy promotes the intensity of atom diffusion above the deposition surface. On the other hand, the collision with sputtered particles will transfer large amounts of energy, which is disadvantageous to form the sp3 bonding configuration. Implant mechanism has a leading role when the vertically incident energy is high. Implant atoms can be embedded into a deeper position of substrate. Just as the chart shown, when Z < 22 A˚ the incident atoms
Fig. 8. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 35.3◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 8c.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
979
Fig. 9. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 54.7◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 9c.
number of incident case b, d and f is larger than that of incident case a, c and e. 3.3. RDF of amorphous carbon films Both the radial distribution functions (RDF) of amorphous carbon films and radial distribution functions of crystal diamond are illustrated in Fig. 13 and Fig. 14 for comparison. First of all, as Fig. 11 shown the RDF of amorphous carbon films reveals the short-range ordered and long-range disordered arrangement, and the first and second peaks with narrow peak width and large peak value are ˚ which are the similar observed at the location of 1.52 A˚ and 2.54 A,
positions comparing with those of diamond [26]. This suggests that amorphous carbon films are similar in structure with diamond. But except the first two amorphous peaks, other peaks toward larger interatomic distance were also observed at the position of approx˚ implying stronger degree of order. At the position of imate 2.9 A, 2.1 A˚ shown in Fig. 13, there are small sharp peaks caused by the cut-off radius of the Tersoff potential, termed as “false peak” [27]. 3.4. Interfacial microstructure evolution Fig. 15 shows us the cross-sectional view of amorphous region, ˚ which is also the interface of the substrate and the (Z = 20–22 A)
Fig. 10. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 10c.
980
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 14. RDF of diamond. Fig. 11. Distribution of free particles along the Z-axis direction.
Fig. 12. Distribution of insert particles along the Z-axis direction.
thin film. The diamond lattice arranged carbon atoms are bombed by incident atoms then escape from the original location, finally the vacancy defects formed which represented by rectangular shaded area. And the undamaged diamond structure is represented by
rhombus frame. As we can see in each figure, when the deposition time is 800 ps, the diamond surface have been damaged by insert atoms and some vacancies are formed, but the substrate still remains relatively unbroken lattice structure. As the deposition prolongs to 2000 ps, substrate surface is gradually covered by the incident particles and sputtered particles, the original lattice structure become increasingly blurred. Comparing incident case a, c and e, the kinetic energy of each incident case in vertical direction is the same, the degree of surface damage and horizontal kinetic energy display a proportional relationship. The scale and number of vacancy defects are increased with the kinetic energy in the horizontal direction. In other words, the evolution of defects reflects the degree of diffusive events of the amorphous region, and the diffusive events are primarily determined by the kinetic energy in the horizontal direction. With the further development of the deposition process just as the third part of each incident case, vacancy defects would gradually be filled by upper particles, which are accumulated by the upper squeezing action.
3.5. MSD of amorphous region The mean square displacement (MSD) of the amorphous region ˚ which is related to the atomic diffusion coefficient, (Z = 20–22 A), defined by
MSD =
n 2 2 2 (dx) + (dy) + (dz) i=1
Fig. 13. RDF of amorphous region generated by six different kinds of insert carbon atoms.
N
When the amorphous region is in a solid state or equilibrium, the MSD has an upper limit. If the MSD increases linearly with simulation time, then the amorphous region is in a liquid state. Moreover, the intensity of atom diffusion in amorphous region depends on the kinetic energy in the horizontal direction. Fig. 16 shows that the MSD is a function of the deposition and relaxation time. The MSD curves of first two and the last incident cases are approximately straight lines during the whole deposition and relaxation process, which indicates that amorphous carbon films have been in a solid state, and the kinetic energy in the vertical direction do not contribute to the MSD. Another three MSD curves show an increasing trend, which indicates that the amorphous region is in a liquid state. This is because the incident particle kinetic energy in the horizontal direction is still not dissipated.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
˚ during deposition processes. Fig. 15. Cross-sectional views of amorphous films (Z = 20–22 A)
981
982
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
References
˚ as a function of time. Fig. 16. MSD of amorphous region (Z = 20–22 A)
4. Conclusion In this work, amorphous carbon films approximately 0.8–1 nm are prepared by six different incident carbon atoms using molecular dynamic simulation. By comparing the deposition process surface morphology, atom distribution along film growth direction and internal structure including vacancy defects change during deposition process, several conclusions can be obtained: – The incident atoms with different angles and energy will diversify the films growth mode. The incident energy on the horizontal plane always increases the degree of atomic chaos above the substrate, which will lead to a rougher surface. And the vertical incident energy makes the films more compact and surface morphology smoother. – When the vertically energy of an incident atom is low, adsorption is the predominant mechanism among three atomic motion mechanisms. The existence of horizontal incident energy makes rebound the predominant atomic motion mechanism. And implant mechanism has a leading role when the vertically incident energy is high. – A proper incident parameter is not only beneficial to prepare amorphous films with great surface properties, but also can avoid the formation of defects during deposition process. This analysis suggests a strategy to optimize the deposition parameters and provide a theoretical tool to prepare amorphous film with superior performance. Acknowledgments This work was supported by Key Project of Chinese Ministry of Education (211061), National Natural Science Foundation of China (10502025, 10872087, 11272143), Program for Chinese New Century Excellent Talents in university (NCET-12-0712)
[1] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281. [2] B. Krzan, N.F. Franz, V. Joze, Tribological behavior of tungsten-doped DLC coating under oil lubrication, Tribol. Int. 42 (2009) 229–235. [3] C. Casiraghi, J. Robertson, F.C. Andrea, Diamond-like carbon for data and beer storage, Mater. Today 10 (2007) 44–53. [4] N.A. Marks, Thin film deposition of tetrahedral amorphous carbon: a molecular dynamics study, Diam. Relat. Mater. 14 (2005) 1223–1231. [5] X.Q. Liu, Y. Jun, J.Y. Hao, J.Y. Zheng, Q.Y. Gong, W.M. Liu, Microstructure, mechanical and tribological properties of Si and Al co-doped hydrogenated amorphous carbon films deposited at various bias voltages, Surf. Coat. Technol. 206 (2012) 4119–4125. [6] A. Savvatimskiy, Measurements of the melting point of graphite and the properties of liquid carbon, Carbon 43 (2005) 1115. [7] C.J. Chu, T.C. Chen, Surface properties of film deposition using molecular dynamics simulation, Surf. Coat. Technol. 201 (2006) 1796–1804. [8] D.A. Bonis, J.V. Rau, A. Santagata, R. Teghil, Diamond-like carbon thin films produced by femtosecond pulsed laser deposition of fullerite, Surf. Coat. Technol. 205 (2011) 3747–3753. [9] S. Lee, C. Lee, S. Lee, K. Lee, Structural properties of amorphous carbon films by molecular dynamics simulation, Surf. Coat. Technol. 812 (2004) 177–178. [10] T. Ma, Y.Z. Hu, H. Wang, X. Li, Microstructural and stress properties of ultrathin diamond-like carbon films during growth: molecular dynamics simulations, Phys. Rev. B 75 (2007) 1–8. [11] X.W. Li, P.L. Ke, H. Zhang, A.Y. Wang, Structural properties and growth evolution of diamond-like carbon films with different incident energies: a molecular dynamics study, Appl. Surf. Sci. 273 (2013) 670–675. [12] M.W. Joe, M.W. Moon, K.H. Lee, K.R. Lee, Molecular dynamics simulation study of the growth of a rough amorphous carbon film by the grazing incidence of energetic carbon atoms, Carbon 50 (2012) 404–410. [13] M. Moseler, P. Gumbsch, C. Casiraghi, A.C. Ferrari, J. Robertson, The ultrasmoothness of diamond-like carbon surface, Science 309 (2005) 1545–1548. [14] J. Tersoff, New empirical model for the structural properties of silicon, Phys. Rev. Lett. 61 (1998) 2879–2882. [15] L. Li, et al., The effect of empirical potential functions on modeling of amorphous carbon using molecular dynamics method, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc,2013.09.073. [16] R. Car, M. Parrinello, Unified approach for molecular dynamics and density functional theory, Phys. Rev. Lett. 56 (1986) 632–635. [17] S.J. Plimpton, Fast parallel algorithms for short-range molecular dynamics, J. Comp. phys. 117 (1995) 1–19. [18] A. Stukowski, Visualization and analysis of atomistic simulation data with OVITO-the open Visualization Tool Modelling Simul, Mater. Sci. Eng. 18 (2010) 12–15. [19] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, J. Electrochem. Soc. 150 (2003) 324–327. [20] G. Yushi, R. Dash, J. Jagiello, J.E. Fischer, Y. Gogotsi, Carbide derived carbons: effect of pore size on hydrogen uptake and heat of adsorption, Adv. Funct. Mater. 16 (2006) 2283–2293. [21] J.P. Lu, R.M. James, Structure and hydrogen adsorption properties of low density nanoporous carbons from simulations, Carbon 50 (2012) 1394–1406. [22] B. Zheng, W.T. Zheng, S.S. Yu, H.W. Tian, F.L. Meng, Y.M. Wang, J.Q. Zhu, S.H. Meng, X.D. He, J.C. Han, Growth of tetrahedral amorphous carbon film: Tightbinding molecular dynamics study, Carbon 43 (2005) 1976–1983. [23] T.B. Ma, Y.Z. Hu, H. Wang, Formation of liner carbon chains during the initial stage of nanostructured carbon film growth, J. Appl. Phys. 104 (2008) 1–5. [24] T.B. Ma, Y.Z. Hu, H. Wang, Formation and coalescence of linear chains in growth of nanostructured sp-sp2 amorphous carbon films, Chem. Phys. Lett. 462 (2008) 104–108. [25] S. Zhang, H.L. Wang, Y.B. Li, D. Sun, Bias effect on microstructure and mechanical properties of magnetron sputtered nanocrystalline titanium carbide thin films, Thin. Solid. Films. 516 (2008) 5419–5423. [26] A. Claye, J.E. Fischer, Short-range order in disorder carbons: where does the Li go? Electrochim. Acta 45 (1999) 107–120. [27] T.B. Ma, Y.Z. Hu, H. Wang, Molecular dynamics simulation of the growth and structural properties of ultra-thin diamond-like carbon films, Acta. Phys. Sin. 55 (2006) 2922–2927.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
An atomistic study of growth mode and microstructure evolution of amorphous carbon films by different incident carbon atoms Chen Xue a , Jianqiu Zhou a,b,∗ a b
Department of Mechanical Engineering, Nanjing Tech University, Nanjing, Jiangsu Province 210009, China Department of Mechanical Engineering, Wuhan Institute of Technology, Wuhan, Hubei Province 430070, China
a r t i c l e
i n f o
Article history: Received 4 April 2014 Received in revised form 20 June 2014 Accepted 20 June 2014 Available online 27 June 2014 Keywords: Molecular dynamics simulation Amorphous carbon films Growth mechanism Interfacial microstructure evolution
a b s t r a c t In this paper, molecular dynamics (MD) simulation has been performed to describe the growth and interfacial microstructure of amorphous carbon films. We focus on the film growth mode and surface morphology for diverse deposition process parameters mainly including incident energy and incident angle. To explore the relationship between the motion of deposition atoms and amorphous films growth, a series of snapshots for each deposition process has been taken for comparison. The snapshots show that the films growth modes are diverse at different incident parameters. In the next step, surface morphology, atom distribution along film growth direction and internal structure including vacancy defects evolution during deposition process are analyzed. The results reveal that incident energy on the horizontal plane dominates the surface roughness, and incident energy on the vertical plane dominates the compactness of the film. We conclude that a suitable incident parameter is not only beneficial to prepare amorphous films with compact and smooth or bump-like surface which will meet different needs, but also can avoid formation of defects during deposition. The simulation results are expected to provide useful guidance for improving amorphous carbon films quality. © 2014 Elsevier B.V. All rights reserved.
1. Introduction The ability of carbon to form sp, sp2 and sp3 hybridized electronic states gives rise to a vast variety of different phases which often show marvellous properties. Amorphous carbon with a considerable fraction of sp3 bonds is often known as diamond-like carbon (DLC) [1]. The combination of diamond-like properties and ultra smoothness is the main factor for the technological importance of amorphous carbon films. Hence, amorphous carbon films have widespread applications as high hardness drill tools and high performance wear-resistant materials [2]. Besides, owing to their great biocompatibility and chemical inertness, amorphous carbon films are promising in biomedical coatings such as hip implants and knee joint prostheses [3]. Amorphous carbon film was first prepared by ion beam deposition method. After decades of development, it is possible to prepare amorphous carbon films by a wide range of deposition methods including sputtering, plasma deposition and pulsed laser deposition. Due to the different deposition methods, the
∗ Corresponding author. Tel.: +86 25 83588706/13655192068; fax: +86 25 83374190. E-mail address: [email protected] (J. Zhou). http://dx.doi.org/10.1016/j.apsusc.2014.06.133 0169-4332/© 2014 Elsevier B.V. All rights reserved.
microstructure and properties of amorphous carbon films are varied [4]. Taking a typical deposition system as an example, such as ion beam deposition, carbon ions will be firstly produced by the plasma sputtering of a graphite cathode in an ion source. The carbon ions are then accelerated by a bias voltage or magnetic field in the high vacuum deposition chamber to obtain high kinetic energy, finally bombarding the substrate [5]. When the kinetic energy of the incident carbon ions is high enough, the ions can collide directly under the surface into the deposition film, resulting in local density increasing inside the film, which will be more conducive to form sp3 hybridization [6]. According to amorphous carbon films deposition mechanism mentioned above, we can see that deposition process requires a rigorous condition, such as incident energy, incident angle, substrate temperature, deposition pressure, deposition rate and so on [7,8]. It is generally accepted that film properties depend on the microscopic structure of the deposited layer. However there are no experimental methods for characterizing this microstructure from atomic scale viewpoint. In addition, the explanation for the growth mechanism of amorphous carbon films is still controversial. Thanks to computational simulation techniques, researcher can get a deeper insight into the microscopic structure and properties of amorphous carbon films, which allow us to explain the
974
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
deposition process in atomic scale. MD study shows that the density, sp3 fraction and compressive stress will change accordingly with the different incident energy. When the ion energy was 70 eV/atom, the content of the sp3 hybridized bond in the film and film density reach to their peaks. For those incident energy particles less than 70 eV/atom, they cannot overcome the potential barrier among substrate atoms, but can only be attached to the surface forming epitaxial sp2 hybridized structure, which will cause sp3 hybridized bond decrease. However, when the incident energy is much higher than 70 eV/atom, the density and sp3 hybrid phase content of amorphous carbon film decline on the contrary [9–11]. And relevant literature suggests that the impact-induced uphill transport of the surface atoms will be mitigated by adjusting incident angle [12]. And surface smoothness under normal or near-normal incidence condition has been investigated in the light of atomic displacement: ion-induced relaxation [13]. As for the movement mechanism of particles and interfacial microstructure evolution during deposition, they were not discussed in the previous work. In the present study, we took both incident energy and incident angle into consideration to research amorphous carbon films growth mode and surface morphology. Atom distribution along film growth direction and the interfacial microstructure evolution during deposition process were analyzed. Our studies demonstrated that there was extraordinary significance for the setting of deposition parameters, especially incident energy and incident angle.
2.1. Empirical potential function of carbon In order to simulate the deposition process of amorphous carbon films by molecular dynamics simulation, the atomic-scale calculations and simulations here were carried out by the Tersoff potential [14,15]. The earliest computational method used for amorphous carbon simulation was presented by combining the density functional theory (DFT) and molecular dynamics [16]. However, this method was found to be computationally inefficient. Tersoff was the first computationally efficient potential function to study different hybridization states of carbon such as graphite and diamond. The detailed information of the Tersoff potential, which has been described by equations as below: 1 Vij 2 i
j= / i
Vij = fc (rij ) fR (rij ) + bij fA (rij )
Incident particles
a
b
c
d
e
f
Incident angle (◦ ) Kinetic energy along X-axis (eV) Kinetic energy along Z-axis (eV)
– – 25
– – 50
45 25 25
35.3 25 50
54.7 50 25
– – 75
fc (r) =
⎧ ⎪ ⎪ ⎪ ⎪ ⎨ ⎪ ⎪ ⎪ ⎪ ⎩
r ≤R−D
1 1 1 − sin 2 2
2
−
r−R D
0
R−D
fR (r) = A exp(−1 r) fA (r) = −B exp(−2 r) Here Vij presents the bond energies between atom i and atom j. And E is the total energy of the system, which can be decomposed into Vij for convenience. rij is the distance from atom i to atom j. The repulsive and attractive potentials can be described by functions fR and fA , respectively. The cutoff function limiting the effective distance of each potentials equation described by func˚ The parameters of 1 tion fc . R equals 1.95 A˚ and D equals 0.15 A. and 2 are 3.2394 A˚ −1 and 1.3258 A˚ −1 , respectively. The parameters A and B are 1393.6 eV and 346.74 eV. 2.2. Schematic diagram and parameters of incident particles
2. Simulation details and method
E=
Table 1 Parameters of incident particles.
In order to compare the movement mechanisms of particles and film growth modes under different incidence cases, the effects of the deposition energy were examined under three different incidence energies, E = 25 eV, 50 eV, and 75 eV, with four different incidence angles, 0◦ , 35.3◦ , 45◦ , and 54.7◦ . Fig. 1 lists six different kinds of incident carbon particles (a–e), more details about the incident atoms parameters are presented in Table 1. 2.3. Simulation model and details As shown in Fig. 2, atomistic model of deposition consists of diamond substrate and insert carbon atoms. The substrate consisted of 4800 carbon atoms with perfect diamond lattice structure arrangement, which constituted 24 monolayers with 200 atoms per layer. The dimensions of substrate were 3.556, 3.556, and 2.1 nm in the directions of X-axis, Y-axis, and Z-axis, respectively. Periodic boundary conditions are imposed on the surface plane along the directions of X-axis and Y-axis. There were three types of atoms in substrate: fixed atoms thermal-controlled atoms, and free atoms, and the thickness of each type atoms in the Z-axis were 0.35, 0.875 and 0.875 nm. To prevent the movement of substrate, the bottom
Fig. 1. Schematic diagram of the motion of carbon particles.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
975
performed using the large-scale atomic/molecular massively parallel simulator (LAMMPS) software [17]. The visualization and post analysis tool was OVITO [18]. 3. Results and discussion 3.1. Deposition process of amorphous carbon films
Fig. 2. Simulation diagram of high kinetic carbon atoms deposited on diamond surfaces.
four layers atoms were held fixed in their bulk lattice sites. Above the fixed layer, the middle ten layers were assigned as the thermal control layers where the velocities of all atoms were rescaled to room temperature (T = 293 K) by the Berendsen method, and these thermal controlled atoms were mainly used to bear the relaxation work for the whole system. The top ten layers of substrate were free atoms, which abide by the Newton’s second law. And the upper four layers of substrate could be entirely free to transmit energy with the incident atoms. The time interval between two sequential deposition processes was set to 5 ps, then the particles flux was approximately 1.58 × 1024 cm−2 s−1 . Fig. 3 shows the kinetic energy fluctuation of the system during depositing the first six incident carbon atoms. Due to the existence of the thermal controlled layer, the average kinetic energy of system atoms remained constant. The energy fluctuation indicates that 5 ps was enough for relaxing the deposited atoms, and the construction of the simulation model was also justified. The deposition process was simulated using constant microcanonical ensemble (NVE). Taking both computational efficiency and accuracy of the results into consideration, time step was set as 2 fs. All of the molecular dynamics simulations were
As we can see in Fig. 4, amorphous carbon films approximately 0.8–1 nm are formed on the diamond substrate. The color encodes the height of the particles. The arrow in the rightmost of Fig. 1 indicates the films growth direction in the XZ plane. The morphology evolution of amorphous carbon films during deposition process is correlated with the motion of incident atoms. The incident atoms carried with different angles and energies will make the deposition process and films growth modes tend to be diversified. Deposition behavior depends largely on both incident energy and incident angle. At normal incidence (Fig. 1a, b and f), the growth surface of films maintains smooth during the whole deposition process. Be different from the first two incident methods, at oblique incidence (Fig. 1c, d and e), due to the existence of the incident energy on the horizontal plane, the surface of films no longer maintains smooth, and the film growth seems to be relatively slow. Comparing Fig. 4a and c, or b and d, we can see that the chaotic degree of particles above the deposition surface change with the horizontal incident energy. The atomic motion mechanisms of incident atoms, including adsorption, rebound and implant, these three mechanisms lead to the structural rearrangements and surface diffusion during the whole deposition process, which dominate the resulting film morphology. As for the rebound mechanism, incident atoms collide with the substrate atoms by transferring energy and then rebound. Rebound atoms collide with the later incident atoms, which will produce a shadowing effect above the film growth surface. Because of the angle of incidence, rebound plays a dominant role in the motion mechanisms of particles. The existence of shadowing effects will reduce the incident energy of particles to the substrate surface, this will be helpful to form graphitization phase in the amorphous carbon films. By comparing Fig. 4a, c and e, we can draw the conclusion that the incident energy on the horizontal plane always increases the degree of atomic chaos, and the final energy deposited on amorphous films will reduce, which will lead to a looser structure with lots of defects in the uppermost layers of the film. If the incident angle is large, or even at grazing incidence, a bump-like surface structure emerged and led to rough surfaces. However the amorphous carbon films with a bumpy surface have large surface energy, which will have a great advantage in the application of energy conversion and storage devices such as supercapacitors or hydrogen storage devices [19–21]. 3.2. Surface morphology and atoms distribution
Fig. 3. Kinetic energetic variation during the deposition process when the incident energy is 75 eV/atom.
Figs. 5–10 show the surface structure fabricated by six different kinds of insert patterns. To explore how the incident patterns affect the film surface morphology, color gradient illustration is utilized to refer the atom distribution along the direction of Z-axis. The color denotes the height of the particles. The color changes from shallow to deep along the height position. The carbon atoms denoted by drab color indicates that they are in the low position, and the more diverse color of the carbon atoms, the rougher the surface morphology of films will be. It is obvious that, if the incident energy along Z-axis direction is smaller, for example Fig. 5a and b, Fig. 7a and b and Fig. 9a and b, the surface of the deposited film will be bumpier. Conversely, if the incident energy along Z-axis direction is higher, for example Fig. 6a and b, Fig. 8a and b and Fig. 10a and b,
976
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 4. Side view snapshots of six incident carbon atoms bombarded on diamond substrate. The film growth direction is indicated by the arrow in the rightmost. The shadowing effects caused by different incident atoms can be clearly displayed by the chaotic degree of particles above the deposition surface.
the surface morphology will be more compact and smoother. This is because the greater the energy in the horizontal direction, the diffusive events and atomic rearrangements above the deposition surface will become more intense, which have a strong impact on the resulting film morphology.
As shown in Fig. 5c to Fig. 10c, the incident particles and free particles are marked blue and red respectively to make a detailed explanation of the injected growth mechanism. An incident particle with low energy will not have enough kinetic energy to penetrate the surface, so it will just stick to the upper surface, and remain
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
977
Fig. 5. Top view (a) and side view (b) of amorphous film prepared by 25 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 5c.
in its lowest energy state finally forming sp2 hybridization. If the kinetic energy of the incident atom was higher than the penetration threshold, it had a chance to penetrate the surface, and enter a subsurface interstitial site. This will cause local density increasing. The local bonding will reform around that atom. When the solid became amorphous, the incident atom and the target atoms were equivalent. We assume that in the highly energetic conditions of particle bombardment existing during film growth, atomic hybridization will adjust easily to change in the local density, and became more sp3 if the density was high [22], or more sp2 if the density was low, the MD simulation results of surface structure deposited by low energy carbon particles will be 1-D chain or 2-D ring [23,24]. However, when the energy of the particles exceeds a certain range, the
sputtering reaction will be more apparent [25], which make films dense but reduce its thickness. Here statistics about the distribution of particles along film growth direction are analyzed. Figs. 11 and 12 are statistical charts of incident particles and free particles distributed along the Z-axis direction. These statistics can make a better reflection about the mechanism of amorphous carbon films deposition. By comparing six different atoms distributions, we can see that the free carbon particles distributions along the Z-axis direction show downward ˚ the light green, trend. As we can see in the chart, when Z < 21 A, blue and red dotted line are always located above the orange, dark green and purple dotted line, and this situation reverse in ˚ This can be the position of the outermost layer where Z = 21 A.
Fig. 6. Top view (a) and side view (b) of amorphous film prepared by 50 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 6c.
978
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 7. Top view (a) and side view (b) of amorphous film prepared by 50 eV kinetic energy and 45◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 7c.
explained that the original substrate has been damaged by incident atoms, and a sputtering effect emerged at the same time. And when the incidence energy in the vertical direction is larger, just like incident case b, d and f which represented individually by dark green, orange and purple dotted line in the chart, the sputtering effect of free atoms on the substrate become more intense. In other incident cases a, c and e, since the kinetic energy of the incident in the vertical direction is small, the sputtering effect is not so obvious by contrasting the three incident cases mentioned above. The tendencies of insert particles concentration along the Z-axis direction increase firstly then decrease as shown in Fig. 11. Particle distribution seems disorderly and unsystematic, but after careful observation we can sum up some conclusions. As for incident case
a, because of the low vertically incident energy, adsorption is the predominant mechanism among three atomic motion mechanisms. And the deposited atoms are absorbed in the low energetic position of the substrate. That is why when Z > 22 A˚ the light green dotted line always located above the other dotted lines. As for incident case c and e, due to the existence of horizontal incident energy, rebound mechanism takes a leading role. On one hand, the existence of horizontal energy promotes the intensity of atom diffusion above the deposition surface. On the other hand, the collision with sputtered particles will transfer large amounts of energy, which is disadvantageous to form the sp3 bonding configuration. Implant mechanism has a leading role when the vertically incident energy is high. Implant atoms can be embedded into a deeper position of substrate. Just as the chart shown, when Z < 22 A˚ the incident atoms
Fig. 8. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 35.3◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 8c.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
979
Fig. 9. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 54.7◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 9c.
number of incident case b, d and f is larger than that of incident case a, c and e. 3.3. RDF of amorphous carbon films Both the radial distribution functions (RDF) of amorphous carbon films and radial distribution functions of crystal diamond are illustrated in Fig. 13 and Fig. 14 for comparison. First of all, as Fig. 11 shown the RDF of amorphous carbon films reveals the short-range ordered and long-range disordered arrangement, and the first and second peaks with narrow peak width and large peak value are ˚ which are the similar observed at the location of 1.52 A˚ and 2.54 A,
positions comparing with those of diamond [26]. This suggests that amorphous carbon films are similar in structure with diamond. But except the first two amorphous peaks, other peaks toward larger interatomic distance were also observed at the position of approx˚ implying stronger degree of order. At the position of imate 2.9 A, 2.1 A˚ shown in Fig. 13, there are small sharp peaks caused by the cut-off radius of the Tersoff potential, termed as “false peak” [27]. 3.4. Interfacial microstructure evolution Fig. 15 shows us the cross-sectional view of amorphous region, ˚ which is also the interface of the substrate and the (Z = 20–22 A)
Fig. 10. Top view (a) and side view (b) of amorphous film prepared by 75 eV kinetic energy and 0◦ incidence angle. The color denotes the height of the particles. Insert atoms and free atoms are marked by red and blue respectively in Fig. 10c.
980
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
Fig. 14. RDF of diamond. Fig. 11. Distribution of free particles along the Z-axis direction.
Fig. 12. Distribution of insert particles along the Z-axis direction.
thin film. The diamond lattice arranged carbon atoms are bombed by incident atoms then escape from the original location, finally the vacancy defects formed which represented by rectangular shaded area. And the undamaged diamond structure is represented by
rhombus frame. As we can see in each figure, when the deposition time is 800 ps, the diamond surface have been damaged by insert atoms and some vacancies are formed, but the substrate still remains relatively unbroken lattice structure. As the deposition prolongs to 2000 ps, substrate surface is gradually covered by the incident particles and sputtered particles, the original lattice structure become increasingly blurred. Comparing incident case a, c and e, the kinetic energy of each incident case in vertical direction is the same, the degree of surface damage and horizontal kinetic energy display a proportional relationship. The scale and number of vacancy defects are increased with the kinetic energy in the horizontal direction. In other words, the evolution of defects reflects the degree of diffusive events of the amorphous region, and the diffusive events are primarily determined by the kinetic energy in the horizontal direction. With the further development of the deposition process just as the third part of each incident case, vacancy defects would gradually be filled by upper particles, which are accumulated by the upper squeezing action.
3.5. MSD of amorphous region The mean square displacement (MSD) of the amorphous region ˚ which is related to the atomic diffusion coefficient, (Z = 20–22 A), defined by
MSD =
n 2 2 2 (dx) + (dy) + (dz) i=1
Fig. 13. RDF of amorphous region generated by six different kinds of insert carbon atoms.
N
When the amorphous region is in a solid state or equilibrium, the MSD has an upper limit. If the MSD increases linearly with simulation time, then the amorphous region is in a liquid state. Moreover, the intensity of atom diffusion in amorphous region depends on the kinetic energy in the horizontal direction. Fig. 16 shows that the MSD is a function of the deposition and relaxation time. The MSD curves of first two and the last incident cases are approximately straight lines during the whole deposition and relaxation process, which indicates that amorphous carbon films have been in a solid state, and the kinetic energy in the vertical direction do not contribute to the MSD. Another three MSD curves show an increasing trend, which indicates that the amorphous region is in a liquid state. This is because the incident particle kinetic energy in the horizontal direction is still not dissipated.
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
˚ during deposition processes. Fig. 15. Cross-sectional views of amorphous films (Z = 20–22 A)
981
982
C. Xue, J. Zhou / Applied Surface Science 314 (2014) 973–982
References
˚ as a function of time. Fig. 16. MSD of amorphous region (Z = 20–22 A)
4. Conclusion In this work, amorphous carbon films approximately 0.8–1 nm are prepared by six different incident carbon atoms using molecular dynamic simulation. By comparing the deposition process surface morphology, atom distribution along film growth direction and internal structure including vacancy defects change during deposition process, several conclusions can be obtained: – The incident atoms with different angles and energy will diversify the films growth mode. The incident energy on the horizontal plane always increases the degree of atomic chaos above the substrate, which will lead to a rougher surface. And the vertical incident energy makes the films more compact and surface morphology smoother. – When the vertically energy of an incident atom is low, adsorption is the predominant mechanism among three atomic motion mechanisms. The existence of horizontal incident energy makes rebound the predominant atomic motion mechanism. And implant mechanism has a leading role when the vertically incident energy is high. – A proper incident parameter is not only beneficial to prepare amorphous films with great surface properties, but also can avoid the formation of defects during deposition process. This analysis suggests a strategy to optimize the deposition parameters and provide a theoretical tool to prepare amorphous film with superior performance. Acknowledgments This work was supported by Key Project of Chinese Ministry of Education (211061), National Natural Science Foundation of China (10502025, 10872087, 11272143), Program for Chinese New Century Excellent Talents in university (NCET-12-0712)
[1] J. Robertson, Diamond-like amorphous carbon, Mater. Sci. Eng. R 37 (2002) 129–281. [2] B. Krzan, N.F. Franz, V. Joze, Tribological behavior of tungsten-doped DLC coating under oil lubrication, Tribol. Int. 42 (2009) 229–235. [3] C. Casiraghi, J. Robertson, F.C. Andrea, Diamond-like carbon for data and beer storage, Mater. Today 10 (2007) 44–53. [4] N.A. Marks, Thin film deposition of tetrahedral amorphous carbon: a molecular dynamics study, Diam. Relat. Mater. 14 (2005) 1223–1231. [5] X.Q. Liu, Y. Jun, J.Y. Hao, J.Y. Zheng, Q.Y. Gong, W.M. Liu, Microstructure, mechanical and tribological properties of Si and Al co-doped hydrogenated amorphous carbon films deposited at various bias voltages, Surf. Coat. Technol. 206 (2012) 4119–4125. [6] A. Savvatimskiy, Measurements of the melting point of graphite and the properties of liquid carbon, Carbon 43 (2005) 1115. [7] C.J. Chu, T.C. Chen, Surface properties of film deposition using molecular dynamics simulation, Surf. Coat. Technol. 201 (2006) 1796–1804. [8] D.A. Bonis, J.V. Rau, A. Santagata, R. Teghil, Diamond-like carbon thin films produced by femtosecond pulsed laser deposition of fullerite, Surf. Coat. Technol. 205 (2011) 3747–3753. [9] S. Lee, C. Lee, S. Lee, K. Lee, Structural properties of amorphous carbon films by molecular dynamics simulation, Surf. Coat. Technol. 812 (2004) 177–178. [10] T. Ma, Y.Z. Hu, H. Wang, X. Li, Microstructural and stress properties of ultrathin diamond-like carbon films during growth: molecular dynamics simulations, Phys. Rev. B 75 (2007) 1–8. [11] X.W. Li, P.L. Ke, H. Zhang, A.Y. Wang, Structural properties and growth evolution of diamond-like carbon films with different incident energies: a molecular dynamics study, Appl. Surf. Sci. 273 (2013) 670–675. [12] M.W. Joe, M.W. Moon, K.H. Lee, K.R. Lee, Molecular dynamics simulation study of the growth of a rough amorphous carbon film by the grazing incidence of energetic carbon atoms, Carbon 50 (2012) 404–410. [13] M. Moseler, P. Gumbsch, C. Casiraghi, A.C. Ferrari, J. Robertson, The ultrasmoothness of diamond-like carbon surface, Science 309 (2005) 1545–1548. [14] J. Tersoff, New empirical model for the structural properties of silicon, Phys. Rev. Lett. 61 (1998) 2879–2882. [15] L. Li, et al., The effect of empirical potential functions on modeling of amorphous carbon using molecular dynamics method, Appl. Surf. Sci. (2013), http://dx.doi.org/10.1016/j.apsusc,2013.09.073. [16] R. Car, M. Parrinello, Unified approach for molecular dynamics and density functional theory, Phys. Rev. Lett. 56 (1986) 632–635. [17] S.J. Plimpton, Fast parallel algorithms for short-range molecular dynamics, J. Comp. phys. 117 (1995) 1–19. [18] A. Stukowski, Visualization and analysis of atomistic simulation data with OVITO-the open Visualization Tool Modelling Simul, Mater. Sci. Eng. 18 (2010) 12–15. [19] P. Simon, Y. Gogotsi, Materials for electrochemical capacitors, J. Electrochem. Soc. 150 (2003) 324–327. [20] G. Yushi, R. Dash, J. Jagiello, J.E. Fischer, Y. Gogotsi, Carbide derived carbons: effect of pore size on hydrogen uptake and heat of adsorption, Adv. Funct. Mater. 16 (2006) 2283–2293. [21] J.P. Lu, R.M. James, Structure and hydrogen adsorption properties of low density nanoporous carbons from simulations, Carbon 50 (2012) 1394–1406. [22] B. Zheng, W.T. Zheng, S.S. Yu, H.W. Tian, F.L. Meng, Y.M. Wang, J.Q. Zhu, S.H. Meng, X.D. He, J.C. Han, Growth of tetrahedral amorphous carbon film: Tightbinding molecular dynamics study, Carbon 43 (2005) 1976–1983. [23] T.B. Ma, Y.Z. Hu, H. Wang, Formation of liner carbon chains during the initial stage of nanostructured carbon film growth, J. Appl. Phys. 104 (2008) 1–5. [24] T.B. Ma, Y.Z. Hu, H. Wang, Formation and coalescence of linear chains in growth of nanostructured sp-sp2 amorphous carbon films, Chem. Phys. Lett. 462 (2008) 104–108. [25] S. Zhang, H.L. Wang, Y.B. Li, D. Sun, Bias effect on microstructure and mechanical properties of magnetron sputtered nanocrystalline titanium carbide thin films, Thin. Solid. Films. 516 (2008) 5419–5423. [26] A. Claye, J.E. Fischer, Short-range order in disorder carbons: where does the Li go? Electrochim. Acta 45 (1999) 107–120. [27] T.B. Ma, Y.Z. Hu, H. Wang, Molecular dynamics simulation of the growth and structural properties of ultra-thin diamond-like carbon films, Acta. Phys. Sin. 55 (2006) 2922–2927.