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Evolution of Si-2N2Nb island configuration on NbN (0 0 1) surface: A first-principles calculation
Applied Surface Science 357 (2015) 1613–1618
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Evolution of Si-2N2Nb island configuration on NbN (0 0 1) surface: A first-principles calculation Yuan Ren ∗ , Qing Xia, Chao Zhang, Xuejie Liu, Zhi Li, Fucheng Zhang School of Mechanical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia 014010, PR China
a r t i c l e
i n f o
Article history: Received 1 July 2015 Received in revised form 7 September 2015 Accepted 6 October 2015 Available online 13 October 2015 Keywords: Si-2N2Nb island Island evolution Surface diffusion First-principles calculation
a b s t r a c t The separation and aggregation of Nb, Si, and N atoms around the NbN grain during the deposition of the Nb–Si–N nanocomposite film were discussed. The evolution behavior of the 2N2Nb island and the adsorption and diffusion energy of Nb, Si, and N atoms around the island on the NbN (0 0 1) surface were investigated using the first-principles method based on density functional theory. Results indicated that the most stable configuration of the Nb–Si–N island was the combination of Nb and N atoms to form the island and the possible aggregation of the Si atom to diagonal Nb atom outside the island. Substitution solid solution was eventually formed, in which the Nb atom of the 2N2Nb island was replaced by the Si atom during deposition. However, the Si atom was easily replaced by the Nb atom at the site with abundant Nb atoms. The diffusion energy of the evolution from Nb-2N1Nb1Si to Si-2N2Nb was 1.58 eV, and the total energy of the configuration decreased. Moreover, the interface of Si and NbN grains tended to separate. The highest energy adsorption sites for Nb, Si, N atoms adsorbed on the NbN (0 0 1) surface around the 2N2Nb island were P3, P1, and P2, respectively. The adsorption energies of Nb, Si, and N atoms on the NbN (0 0 1) surface around the 2N2Nb island were 7.3067, 5.3521, and 6.7113 eV, respectively, and their diffusion energies around the 2N2Nb island were 2.62, 1.35, and 5.094 eV, respectively. The low adsorption and diffusion energies of active Si atoms promoted the distribution of Nb and N atoms during deposition. Furthermore, the NbN grain was easily separated through Si atom diffusion into the 2N2Nb island. The grain was refined, and its growth was inhibited by the Si atom during deposition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The nanocomposite films of Me–X–N on surface coating areas are widely studied and applied (Me = Ti, Zr, Hf, V, Nb and Ta, X = Si, B, etc.); in these films, non-metallic elements were doped into high-hardness transition metal nitrides as the matrix material. The Ti–Si–N nanocomposite films were the first to be investigated. Veprek et al. [1] reported that the hardness of the Ti–Si–N nanocomposite films reaches 80–105 GPa and is twice higher than that of TiN. Several experiments have shown that the Me–X–N nanocomposite film exhibits high hardness and mechanical properties. The mechanical properties of Me–X–N are greater than those of transition metal nitride counterparts, such as Ti–Si–N, V–Si–N, Zr–Si–N, W–Si–N, and Nb–Si–N [2–5] nanocomposite films. The mechanical properties of NbN are also higher than those of TiN, and the Nb–Si–N nanocomposite films exhibit high hardness. Li et al. [6]
∗ Corresponding author at: Arding Str. No. 7, Baotou, Inner Mongolia 014010, PR China. E-mail address: renyuan [email protected] (Y. Ren). http://dx.doi.org/10.1016/j.apsusc.2015.10.045 0169-4332/© 2015 Elsevier B.V. All rights reserved.
performed deposition of Nb–Si–N nanocomposite films through reactive sputtering; they found that the high hardness (54 GPa) of Nb–Si–N caused by Si atoms dissolved in NbN creates crystal lattice defects, thereby increasing resistance against dislocation deformation and strengthening the thin film. Sandu et al. [7] revealed that the Nb–Si–N structure can be attributed to three models by Si atoms content (CSi ≤ 4 at.%, 7 at.% ≤ CSi ≤ 4 at.% and CSi ≥ 7 at.%). At low Si content (CSi ≤ 4 at.%), Si atoms replace Nb atoms in the NbN lattice. The nanocomposite film then grows and exceeds the solubility limit. The film is composed of NbN, which comprises Si nanocrystallites surrounded by an amorphous SiNy layer. Further increase in Si content (CSi ≥ 7 at.%) reduces the crystallite size, whereas the thickness of the SiNy layer on the crystallite surface remains constant (∼1.3 monolayer). Song et al. [8] and Wang et al. [9] investigated the microstructure and properties of superhard Nb–Si–N films by controlling N2 partial pressure during deposition. The results revealed the nanocomposite structure of the Nb–Si–N films with nano-sized NbN grains embedded in the amorphous SiNx phase. Microstructure, morphology of interfaces, and preparation conditions for deposition of the Nb–Si–N nanocomposite thin films have been experimentally investigated. In this study, the deposition
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Fig. 1. Local left view of the calculation slab of the 2N2Nb island on the NbN (0 0 1) surface [4 × 4 × (5 + 7)]: (a) side view, (b) top view, (c) Nb–Si–N island of Si Occupy N, (d) Si Occupy Nb, (e) Si neighbor N, (f) Si neighbor Nb, (g) Si diagonal N, and (h) Si diagonal Nb on NbN (0 0 1). Local right view of the calculation slab of the 2N2Nb island on the NbN (0 0 1) surface [6 × 6 × (5 + 7)]: (d) side view, (e) top view, and surface adsorption sites: Nb, Si, and N at P1, P2, P3, P4, P5, and P6 sites around the 2N2Nb island on the NbN (0 0 1) surface. Green, blue, and gray circles represent Nb, Si, and N atoms, respectively. The atoms of the islands and substrate were distinct as displayed by the semitransparent film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of grain and interface is theoretically simulated to investigate the composite structure and growth mechanism of the Nb–Si–N nanocomposite films. The influence of the Si atom on interface structure and grain growth is investigated during deposition of the Nb–Si–N nanocomposite film. The influence of the Si atom on the growth mechanism of the Nb–Si–N nanocomposite films was analyzed, and the evolution of 1Si-2N2Nb island configuration and atoms diffused into the 2N2Nb island was determined using the first-principles method. The adsorption energy of the island and the activation energy of atoms were determined during the evolution of island configurations. Saddle points (SP) and adsorption energy were assessed when Nb, Si, and N atoms diffused into the 2N2Nb island on the NbN (0 0 1) surface, from which the diffusion energy of the atoms was calculated. The separation and aggregation of Nb, Si, and N atoms around the NbN grain during deposition of the Nb–Si–N nanocomposite films were also discussed based on calculation results.
2. Computational method and details First-principles calculations were performed using the VASP code based on density functional theory (DFT) [10–13]. The local density was described with the generalized gradient approximation using the Perdew–Wang 91 formula for exchange correlation energy. Calculation was performed with a cutoff energy of plane wave of 400 eV under periodic boundary condition. The precision values of the electron and ion relaxation convergence were 1.0 × 10−4 and 1.0 × 10−3 eV, respectively. The Brillouin zone was sampled with the Monkhorst–Pack k-point grid during selfconsistent calculation to identify the electronic ground state. A 5 × 5 × 1 k-point mesh was utilized for slab calculation. A lattice constant of 0.4442 nm, which was close to the experimental value of 0.4391 nm, was determined by optimizing the NbN crystalline prior to calculating the evolution of the particle behavior [14–18]. The free energies of Nb, N, and Si atoms were −2.623, −3.271, and −0.770 eV, respectively, as calculated through the spin-polarized method [18]. These values were found accurate by fitting them with the calculated cohesive energy. Island evolution and single atomistic process around the island on the NbN (0 0 1) surface were calculated using DFT. The optimized slab comprised five layers of substrate atoms (NbN) and seven layers of vacuum and deposit atoms. Each layer in the 4 × 4 × (5 + 7) model contained 16 atoms for a total of 80 atoms in the five layers
of the NbN substrate and a vacuum of 1.6 nm in the slab of island evolution. Atoms in the bottom two layers were fixed, whereas those in the three top layers were relaxed in three directions (x, y, and z). The surface energy of NbN (0 0 1) was 1.0104 J/m2 for completely relaxed structures. This result agreed with the reported value (1.13 J/m2 ) of Iskandarova et al. [19]. The 2N2Nb-island was built on the NbN (0 0 1) surface [4 × 4 × (5 + 7)]: (a) side view and (b) top view in Fig. 1. The Si-2N2Nb island consists of five atoms and deposited on the NbN (0 0 1) surface. Fig. 1 shows five kinds of island configurations, namely, Si Ocp N, where the Si atom replaces N atom in the 2N2Nb island (Fig. 1c); Si Ocp Nb, where Si replaces Nb in the 2N2Nb island (Fig. 1d); Si Neig N, where Si bonds with the neighbor N atom outside the 2N2Nb island (Fig. 1e); Si Neig Nb, where Si bonds with the neighbor Nb atom outside the 2N2Nb island (Fig. 1f); Si Diag N, where Si is located on the diagonal surface of the N atom outside the 2N2Nb island (Fig. 1g); and Si Diag Nb, where Si is located on the diagonal surface of the Nb atom outside the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1h). Atoms on the islands and substrate were distinct and semitransparent. The evolution of island configuration was investigated by calculating the diffusion energy of atoms on NbN (0 0 1) by using the nudged elastic band (NEB) method. The 2N2Nb island was deposited on the NbN (0 0 1) surface of a 6 × 6 × (5 + 7) slab, which comprised 5 N–Nb layers with 18 N and 18 Nb atoms per plane and seven vacuum layers (Fig. 1i). The adsorption and diffusion energies of a single atom around the island were calculated, with six highly symmetrical positions around the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1j). The adsorption energy of Nb, Si, and N at the P1, P2, P3, P4, P5, and P6 sites were extracted (Fig. 1j). In addition, the behavior of atoms diffused between the highly symmetrical positions on the NbN (0 0 1) surface around the island was calculated by the NEB method.
3. Results and discussion 3.1. Adsorption and evolution of island on the NbN (0 0 1) surface 3.1.1. Adsorption of the island on the NbN (0 0 1) surface The surface adsorption energy of the six types of the Si-2N2Nb islands was calculated to investigate the adsorption behavior of island configuration on the NbN (0 0 1) surface. The adsorption energy of the island determines the stability of adsorption and degree of evolution during the deposition of the Nb–Si–N
Y. Ren et al. / Applied Surface Science 357 (2015) 1613–1618
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Table 1 Adsorptive energy (Ead ) and configuration energy (Econfig ) of Nb–Si–N island of Si Occupy N, Si Occupy Nb, Si neighbor N, Si neighbor Nb, Si diagonal N, and Si diagonal Nb on the NbN (0 0 1) surface. [Eslab = −794.47343 eV, NbN (0 0 1)].
Econfig (eV) Ead (eV)
Si Ocp N
Si Ocp Nb
Si Neig N
Si Neig Nb
Si Diag N
Si Diag Nb
−837.15095 33.87
−836.76910 33.49
−836.26087 32.98
−836.95645 33.68
−836.11035 32.83
−837.34098 34.06
nanocomposite films [20,21]. The structure of the grain and interface was also discussed. The adsorption energy Ead is defined as Eq. (1). Ead = lENb + mESi + nEN + Eslab − Econfig
(1)
where Ead is the adsorption energy of island configuration adsorbed on the NbN (0 0 1) surface; Econfig is the total energy of the system that the Si-2N2Nb island and the NbN (0 0 1) surface; Eslab is the total energy of the relaxed slab (4 × 4 × (5 + 7)) without the adatoms or islands (Eslab = −794.47343 eV); l, m, and n represent the number of Nb, Si, and N atoms in the islands, respectively; and ENb , ESi , and EN represent the atomic energy of Nb, Si, and N atoms, respectively [22]. The total energy and adsorption energy of the six islands are listed in Table 1. The island configuration of Si Diag Nb, where the Si atom is located on the diagonal surface of Nb atom outside the 2N2Nb island, showed the lowest total energy and the highest adsorption energy. Thus, the Si Diag Nb island exhibited stable configuration. That is, the 2N2Nb island easily formed NbN grain and the Si atom outside the island formed the interface during Nb–Si–N film growth. Although the distance was 0.28 nm, in which the band length was between Si and Nb atoms of the 2N2Nb island, the Si atom distorted the 2N2Nb island. The combined band lengths of Nb N were 0.2036 and 0.1976 nm in the 2N2Nb island, in which the Nb of the island was oriented to the Si atom of the diagonal position. Si Diag N island exhibited the lowest adsorption energy, which indicated unstable configuration during Nb–Si–N film deposition. The adsorption energy of the island configuration, in which Si occupies the N atom (Si Ocp N) or the Nb atom (Si Ocp Nb) of 2N2Nb, was higher than that of Si beside the N atom (Si Neig N) or Nb atom (Si Neig Nb) of 2N2Nb. The N and Nb atoms of these islands were likely substituted by the Si atom when the latter was close to the island during deposition. Compared with Si Neig N and Si Neig Nb islands, those with Nb or N atoms replaced by Si were more stable. In the Si Ocp Nb configuration, Si N bond was formed between
the island atom Si and the substrate atom N. Similar phenomenon was observed in the Si Diag Nb configuration. The substitution solid solution of Si Ocp Nb was formed at poor-content Nb atoms during deposition, which is consistent with the findings reported by Sandu [6]. The structurally stable compounds of NbN grain and Si N interface were easily formed during the growth of the Nb–Si–N nanocomposite films. 3.1.2. Evolution of the island on the NbN (0 0 1) surface The evolution behavior of island configurations was determined using the NEB method to investigate the separation and aggregation of the grain and interface during deposition. The low-energy configuration (Si Ocp Nb) evolved to the lowest energy configuration (Si Diag Nb) on the NbN (0 0 1) surface. This process included two parts: first, the Si atom diffused to the diagonal position outside the island, then the Nb atom diffused into the island. The minimum energy pathway (MEP) is shown in Fig. 2. The diffusion energy of the Si atom diffused from the island to the diagonal position was 1.58 eV, which was the diffusion energy of the difference between configurations I and II (see Fig. 2). The diffusion process involved configuration I evolved to configuration V, with two SPs (configurations II and IV in Fig. 2) and one low energy position (configuration III in Fig. 2) in the pathway. The two Si N bonds between Si and N atoms of the island were opened, and the two Si Nb bonds between Si and Nb atoms of the substrate were formed in the first part of the Si diffusion pathway. The Si bond with two Nb and two N atoms of the substrate in configuration III was the stable structure (see Fig. 2). In the second part of the Si diffusion pathway, the two Si Nb bonds between Si and Nb atoms of the substrate were broken. The Si atom diffused to the top N of the substrate in configuration V. The opening of the Si N bonds in the first part of the pathway was complicated and required an energy amount of 1.58 eV. The energy needed by the Si atom across SP1 was the Si diffusion energy (see Fig. 2). However, the Si Nb bonds were easily opened in the second part of the pathway. The diffusion energy of the Si atom across SP2 was 0.09 eV. The diffusion energy
Fig. 2. MEP of the diffusion of the evolution of the Nb–Si–N island on the NbN (0 0 1) surface. The Nb–Si–N island configuration, in which Si Occupy the Nb island to Si diagonal Nb island on NbN (0 0 1). SPs represent the transition states during diffusion. The pathways of Si and Nb atoms involve diffusion from and into the island on the NbN (0 0 1) surface, respectively. Single atoms require diffusion energy to cross the saddle point in the pathway.
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Table 2 Adsorptive energy (Ead ) and configuration energy (Econfig ) of Nb, Si, and N at P1, P2, P3, P4, P5, and P6 sites around the 2N2Nb-island on the NbN (0 0 1) surface. [Eslab = −1825.6499 eV, NbN (0 0 1)]. Nb
P1 P2 P3 P4 P5 P6
Si
N
Econfig (eV)
Ead (eV)
Econfig (eV)
Ead (eV)
Econfig (eV)
Ead (eV)
−1834.3035 −1832.1121 −1835.5671 −1832.5845 −1832.9483 −1835.5671
6.0431 3.8517 7.3067 4.3241 4.6879 7.3067
−1831.7588 −1830.3456 −1831.3861 −1828.4683 −1830.6641 −1831.3861
5.3521 4.6957 4.9794 2.0616 4.2574 4.9794
−1834.0283 −1835.6259 −1833.3660 −1833.3737 −1826.2921 −1833.3660
5.1137 6.7113 4.4514 4.4591 −2.6225 4.4514
The underlined were the highest adsorption energy values of atoms on NbN (0 0 1) around the island.
of the Si atom across SP1 was higher than that across SP2. Thus, the diffusion energy of the Si atom outside the island was 1.58 eV. The bond lengths between the diffusion atom Si and Nb and nearest atom (B Si-N and B Nb-N) were shown in Fig. 2 in the process of evolution. The migration energy of the Nb atom was 0.88 eV upon entering after Si deposited outside the island. The two Nb N bonds of the initial configuration (Nb bond with one N of the substrate and one N atom of the island) evolved to three Nb N bonds with stable configuration (Nb bond with one N of the substrate and two N atoms of the island). The Nb atom around the N atom of the island with unbroken bond formed the 2N2Nb island. The diffusion energy of the Nb atom across SP3required the Nb N bond between Nb and N of the substrate to be opened. However, the diffusion energy of Nb atom outside the island was 1.85 eV higher than that of Si outside the island (see Fig. 2). The Nb atom outside the island was difficult to maintain compared with the Si atom. As such, substitution solid solution was formed, in which the Nb atom of the 2N2Nb island was replaced by the Si atom at the poor-quality Nb atoms during deposition. However, the Si atom was easily replaced by the Nb atom at site with abundant Nb atoms. The interface of Si and grain of NbN tended to separate.
symmetrical to the N N diagonal of the 2N2Nb island (Fig. 1j), the adsorption energy of Nb, Si, and N atoms around the 2N2Nb island was higher than that of Nb, Si, and N atoms on the clean NbN (0 0 1) surface [22]. After NbN grain nucleation during deposition, Nb, Si, and N atoms would be easily adsorbed around the grain. The Si atom was the active element with low adsorption and could promote the diffusion of Nb and N atoms. Gratifyingly, the nucleation and growth of the NbN grain were uniform. The stable Nb N covalent bond was formed with the Nb atom at P3 and P1 with high adsorption energy to promote NbN grain growth on the NbN (0 0 1) surface. The stability of the bond between deposited Nb atoms and substrate Nb atoms was low at P2, P4, and P5 sites. The bond strength between Si and Nb atoms of the substrate and the island was weaker than that of Si deposited at P2 with low adsorption energy. The dimmer of N2 was adsorbed on the NbN (0 0 1) surface, in which deposited N atoms and island N atoms combined at P3, P4, and P6. The adsorption energy of the N atom at P5 far from the island was the lowest, and the complete bonds of substrate N atoms did not combine with deposited N atoms. The N atom could adsorb and grow beside the island during deposition to form N2 molecule desorption from the NbN (0 0 1) surface when N atom was deposited far from the island.
3.2. Adsorption and diffusion of single atom around the island
3.2.2. Diffusion of single atom around the island Adsorption around the island was analyzed. Nb, Si, and N atoms exhibited different types of activity at the six symmetrical positions. The effect of deposited atoms on the growth of the NbN grain was determined through the atom that diffused between symmetrical positions. The behavior of deposited atoms around the 2N2Nb island was calculated on the NbN (0 0 1) surface. The pathway of Nb and Si atoms around the island were from P3 to P5 because the total energy of Nb and Si atoms at P2 were high. Nb and Si allowed P3 to diffuse to P5 with adequate activation energy compared with that diffused around the island. The N atom from the high adsorption energy P2 diffused to the lower adsorption energy P5, which represented the desorption trend of the 2N2Nb island. A high potential state can be obtained by increasing the slope of the Nb atom from the diffusion at P3 to P5 outside the island. The diffusion energy of the Nb atom gradually increased in the pathway. An SP was observed in the pathway, and the diffusion energy of the Nb atom across SP was 2.619 eV, as illustrated in Fig. 3a. The bond between deposited Nb atoms and island N atoms was opened, and Nb bonds with substrate N atoms at P5. The SP was the bridge between the two Nb atoms of the substrate. The Si atom from the lowest energy position diffused to the lower energy position across the SP, and the diffusion energy from P3 to P5 was 1.349 eV, as depicted in Fig. 3b. Compared with diffused Nb atoms, the total energy decreased across the SP in the Si diffusion pathway because that the Si N covalent bond strength was stronger than that of Nb N. The total energy of Si at SP was higher than that of Si at the initial and final positions. Different phenomena were observed during migration of N atoms, and SP did not exist from P2 to P5 but existed in the basin.
3.2.1. Adsorption of single atom around the island The results of island evolution calculation show that entrance of the Si atom into the NbN grain or substitution solid solution formation was difficult. The NbN grain and Si N interface were formed during deposition. The effect of Nb, Si, and N atoms on the grain and interface was investigated. The adsorption and diffusion energy of single atom around the 2N2Nb island was calculated on the NbN (0 0 1) surface. Symmetrical positions included P1, P2, P3, P4, P5, and P6 around the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1j). The stable and diffusion behavior was determined using the adsorption energy of single atom at the six symmetrical positions around the island. The adsorption energy was obtained using Eq. (1). The results of adsorption energy of single atom around the island showed that the lowest energy position differed for Nb, Si, and N atoms. The values of calculated are reported in Table 2. The highest adsorption energy position of the Si atom was P1 around the island, which is consistent with island configuration evolution (Section 3.1.1) results, in which the stable configuration was Si atom diagonal to the Nb atom outside the 2N2Nb island. High adsorption energy was observed in the configuration with Nb atom at P3 around the island in six symmetrical positions. Similar phenomenon was observed in the result of island evolution, in which the Nb atom outside the island Nb atom was located beside and bond with the N atom of the island. The N atom outside the island easily bond with the Nb atom of the island. Thus, the N atom exhibited a stable state at P2 around the island in six symmetrical positions. The adsorption energy of Nb, Si and N atoms was consistent at P3 and P6, as reported in Table 2. As P3 and P6 were
Y. Ren et al. / Applied Surface Science 357 (2015) 1613–1618
(a) Nb atom Diffusion P3 toP5 SP
(b) Si atom Diffusion P3 toP5
2.5
P5
2.0 1.5 1.0 0.5 0.0
P3
-0.05 0.00
0.05
0.10
0.15
0.20
0.25
SP
1.4
D i f f u si o n E n e r g y ( e V )
D iffu sio n E ne rg y (eV )
3.0
0.30
1.2 1.0 0.8 0.6
P5
0.4 0.2
P3
0.0
-0.2 -0.05
0.35
0.00
0.05
Pathway(nm)
0.10
0.15
0.20
0.25
0.30
0.35
Pathway (nm)
(c) N atom Diffusion P2 to P5
9.0
Diffusion Energy (eV)
1617
P5
7.5 6.0 4.5
P2
3.0 1.5
DB
0.0 0.00
0.05
0.10
0.15
0.20
0.25
Pathway (nm) Fig. 3. x- and y-axes represent the pathway and diffusion energy of a single atom, respectively. The diffusion pathways of Nb, Si, and N from P3 to P5 or P2 to P5 around the 2N2Nb island on the NbN (0 0 1) surface. Nb, Si, and N atoms require diffusion energy to traverse the saddle point in the pathway.
The diffusion energy of the N atom from the basin to P5 was 5.094 eV, as illustrated in Fig. 3c. As the N atom was bound to the Nb atoms of the island and substrate at P2, the structure was stable with low total energy. The combination of long and short bonds between N and Nb atoms were formed in the lattice disorder of NbN when N diffused to the basin position. The declined migration of the N atom at the beginning was similar to Si atom diffusion on the clean NbN (0 0 1) surface [22]. The high diffusion energy of the N atom around the island required high deposition temperature. However, N atom could be desorbed from the island and substrate at high deposit temperatures, which resulted in decreased rate of deposition and increased number of defects. The diffusion of deposited atoms differed around the 2N2Nb island. The Nb atom diffused from the foot up to the peak plane of the mountain. The diffusion of Si atom was trailed through the ridge between cols. The N atom glided down to the basin and climbed to the peak plane of the mountain (see Fig. 3). Compared with the diffusion of Nb and N atoms, the diffusion of the Si atom occurred in the minimum energy pathway around the island on the NbN (0 0 1) surface. The effect of the 2N2Nb island and substrate on the behavior of deposited atoms around the island was regarded as a composite factor. If deposited atoms were located closed to the 2N2Nb island, the island would prevailed. If deposited atoms were located away from the island, the substrate would dominate. If the Nb atom obtained more than 2.6189 eV energy, it will freely rotate around the island. By contrast, the Si atom requires a diffusion energy of 1.3493 eV to rotate around the island but received more than 8 eV energy for the N atom around the island. The N atom was withdrawn from the substrate through N atom diffusion, which is similar to N diffusion on the clean NbN (0 0 1) surface [22]. Compared with the difficulty encountered by Nb and N atoms around the island, the NbN grain
was easily separated by Si atom diffusion around the 2N2Nb island. The grain was refined to inhibit the growth of the Si atom during deposition. 4. Conclusions The evolution of the Si-2N2Nb island and the behavior of atoms around the 2N2Nb island on the Nb–Si–N nanocomposite films were studied using the first-principles method. The structure and properties of the nanocomposite films were determined with the stability of the Si-2N2Nb island and the mechanism of evolution and growth on NbN (0 0 1) during deposition. The calculation results showed that NbN grain was formed and easily nucleated, and the Si atom of substitution solid solution was extruded from the grain and formed the interface phase in the films. The composite structure of the Nb–Si–N nano films was determined by the combination of the NbN grain and the Si interface phase. The diffusion of Nb, Si, and N atoms around the island of the NbN grain presented three types. The active Si atom could diffuse around the easily nucleated NbN grain. The growth expansion of the NbN grain was inhibited by Si atoms, and the grain was refined during deposition. As an active element, the Si atom could improve migration of Nb and N, form poor-quality defects, and compacted the nanocomposite films. The composite structure of the Nb–Si–N nano films was based on the interface with the Si atom. Deposition temperature was determined to the active energy of the Si atom. Temperature was accurately controlled during deposition to control the diffusion active energy of Si. The Si atom could not successfully diffuse around the island at low deposit temperatures, thereby facilitating the growth of the NbN grain. The N atom would desorb from the surface in the form of N2 at high deposit temperatures, which causes decline in deposition rate. The adsorption
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and diffusion energy of Nb, Si, and N atoms around the 2N2Nb island on NbN (0 0 1) was higher than that of Nb, Si, and N atoms on the clean NbN (0 0 1) surface. As the energy of adsorption and diffusion differed from that of nucleation, accurate application of deposition temperature during nucleation and growth may be ineffective. Acknowledgments This research was supported by the National Natural Science Foundation of China (Grant No. 51562031), National Natural Science Foundation of Inner Mongolia Autonomous Region (Grant No. 2014MS0516), and Innovation Foundation of Inner Mongolia University of Science and Technology (Grant No. 2012NCL050). References [1] S. Veprek, A. Niederhofer, K. Moto, Composition, nanostructure and origin of the ultrahardness in nc-TiN/a-Si3 N4 /a- and nc-TiSi2 nanocomposites with Hv = 80 to ≥105 GPa, Surf. Coat. Technol. 133–134 (133) (2000) 152–160. [2] S. Veprek, Conventional and new approach towards the design of novel superhard materials, Surf. Coat. Technol. 97 (1–3) (1997) 15–22. [3] Y. Liu, Y.S. Dong, J.Z. Huang, L.Z. Zhang, G.Y. Li, Microstructures and mechanical properties of Zr–Si–N films prepared by reactive sputtering, Chin. J. Vac. Sci. Technol. 26 (3) (2006) 200–203. [4] T. Fu, Z.F. Zhou, K.Y. Li, Y.G. Shen, Characterization of sputter deposited W–Si–N coatings based on alpha-W structure, Mater. Lett. 59 (6) (2005) 618–623. [5] X.J. Liu, Y. Ren, X. Tan, S.Y. Sun, E. Westkämper, The structure of Ti–Si–N superhard nanocomposite coatings: ab initio study, Thin Solid Films 520 (2) (2011) 876–880. [6] Y. Liu, Y.S. Dong, J.W. Dai, G.Y. Li, The microstructure and mechanical properties of Nb–Si–N films prepared by reactive sputtering, J. Shanghai Jiaotong Univ. 40 (10) (2006) 1763–1766. [7] C.S. Sandu, M. Benkahoul, R. Sanjinés, F. Lévy, Model for the evolution of Nb–Si–N thin films as a function of Si content relating the nanostructure to electrical and mechanical properties, Surf. Coat. Technol. 201 (6) (2006) 2897–2903.
[8] Z.X. Song, Y. Wang, C.J.F. Wang, C.L. Liu, K.W. Xu, The effect of N2 partial pressure on the properties of Nb–Si–N films by RF reactive magnetron sputtering, Surf. Coat. Technol. 201 (9–11) (2007) 5412–5415. [9] J.F. Wang, Z.X. Song, K.W. Xu, Influence of sputtering bias on the microstructure and properties of Nb–Si–N films, Surf. Coat. Technol. 201 (9–11) (2007) 4931–4934. [10] G. Henkelman, H. Jónsson, Long time scale kinetic Monte Carlo simulations without lattice approximation and predefined event table, J. Chem. Phys. 115 (2001) 9657–9666. [11] G. Kresse, J. Furthmuller, Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set, Comput. Mater. Sci. 6 (1) (1996) 15–50. [12] S. Sanna, R. Hölscher, W.G. Schmidt, Temperature dependent LiNbO3 (0 0 0 1): surface reconstruction and surface charge, Appl. Surf. Sci. 301 (2014) 70–78. [13] C. Kittel, Introduction to Solid State Physics, 8th ed., John Wiley & Sons, Inc., 2005, p. 19; 59. [14] P. Villars, L.D. Calvert, W.B. Pearson, Pearson’s Handbook of Crystallographic Data for Intermetallic Phases, American Society for Metals, Metals Park, OH, 1985. [15] R. Riedel, Handbook of Ceramic Hard Materials, vol. 7, John Wiley & Sons, Inc., 2000, pp. 202. [16] K. Kobayashi, First-principles study of the electronic properties of transition metal nitride surfaces, Surf. Sci. 493 (1–3) (2001) 665–670. [17] M.G. Brik, C.G. Ma, First-principles studies of the electronic and elastic properties of metal nitrides XN (X = Sc, Ti, V, Cr, Zr, Nb), Comput. Mater. Sci. 51 (1) (2012) 380–388. [18] P. Ojha, M. Aynyas, S.P. Sanyal, Pressure-induced structural phase transformation and elastic properties of transition metal mononitrides, J. Phys. Chem. Solids 68 (2) (2007) 148–152. [19] I.M. Iskandarova, A.A. Knizhnik, B.V. Potapkin, A.A. Safonov, A.A. Bagatur’yants, L.R.C. Fonseca, First-principles investigation of the electronic properties of niobium and molybdenum mononitride surfaces, Surf. Sci. 583 (1) (2005) 69–79. [20] Y. Ren, X.J. Liu, X. Tan, E. Westkämper, Adsorption and pathways of single atomistic processes on TiN (1 1 1) surfaces: a first principle study, Comput. Mater. Sci. 77 (2013) 102–107. [21] X.J. Liu, F. Lu, S. Wu, Y. Ren, Effects of different nitrogen-to-titanium atomic ratios on the evolution of Ti–Si–N islands on TiN (0 0 1) surfaces: first-principle studies, J. Alloys Compd. 586 (2014) 431–435. [22] Y. Ren, X.J. Liu, X. Tan, S.Y. Sun, H. Wei, F. Lu, Adsorption and pathways of single atomistic processes on NbN (0 0 1) and (1 1 1) surfaces: a first-principle study, Appl. Surf. Sci. 298 (2014) 236–242.
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Evolution of Si-2N2Nb island configuration on NbN (0 0 1) surface: A first-principles calculation Yuan Ren ∗ , Qing Xia, Chao Zhang, Xuejie Liu, Zhi Li, Fucheng Zhang School of Mechanical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia 014010, PR China
a r t i c l e
i n f o
Article history: Received 1 July 2015 Received in revised form 7 September 2015 Accepted 6 October 2015 Available online 13 October 2015 Keywords: Si-2N2Nb island Island evolution Surface diffusion First-principles calculation
a b s t r a c t The separation and aggregation of Nb, Si, and N atoms around the NbN grain during the deposition of the Nb–Si–N nanocomposite film were discussed. The evolution behavior of the 2N2Nb island and the adsorption and diffusion energy of Nb, Si, and N atoms around the island on the NbN (0 0 1) surface were investigated using the first-principles method based on density functional theory. Results indicated that the most stable configuration of the Nb–Si–N island was the combination of Nb and N atoms to form the island and the possible aggregation of the Si atom to diagonal Nb atom outside the island. Substitution solid solution was eventually formed, in which the Nb atom of the 2N2Nb island was replaced by the Si atom during deposition. However, the Si atom was easily replaced by the Nb atom at the site with abundant Nb atoms. The diffusion energy of the evolution from Nb-2N1Nb1Si to Si-2N2Nb was 1.58 eV, and the total energy of the configuration decreased. Moreover, the interface of Si and NbN grains tended to separate. The highest energy adsorption sites for Nb, Si, N atoms adsorbed on the NbN (0 0 1) surface around the 2N2Nb island were P3, P1, and P2, respectively. The adsorption energies of Nb, Si, and N atoms on the NbN (0 0 1) surface around the 2N2Nb island were 7.3067, 5.3521, and 6.7113 eV, respectively, and their diffusion energies around the 2N2Nb island were 2.62, 1.35, and 5.094 eV, respectively. The low adsorption and diffusion energies of active Si atoms promoted the distribution of Nb and N atoms during deposition. Furthermore, the NbN grain was easily separated through Si atom diffusion into the 2N2Nb island. The grain was refined, and its growth was inhibited by the Si atom during deposition. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The nanocomposite films of Me–X–N on surface coating areas are widely studied and applied (Me = Ti, Zr, Hf, V, Nb and Ta, X = Si, B, etc.); in these films, non-metallic elements were doped into high-hardness transition metal nitrides as the matrix material. The Ti–Si–N nanocomposite films were the first to be investigated. Veprek et al. [1] reported that the hardness of the Ti–Si–N nanocomposite films reaches 80–105 GPa and is twice higher than that of TiN. Several experiments have shown that the Me–X–N nanocomposite film exhibits high hardness and mechanical properties. The mechanical properties of Me–X–N are greater than those of transition metal nitride counterparts, such as Ti–Si–N, V–Si–N, Zr–Si–N, W–Si–N, and Nb–Si–N [2–5] nanocomposite films. The mechanical properties of NbN are also higher than those of TiN, and the Nb–Si–N nanocomposite films exhibit high hardness. Li et al. [6]
∗ Corresponding author at: Arding Str. No. 7, Baotou, Inner Mongolia 014010, PR China. E-mail address: renyuan [email protected] (Y. Ren). http://dx.doi.org/10.1016/j.apsusc.2015.10.045 0169-4332/© 2015 Elsevier B.V. All rights reserved.
performed deposition of Nb–Si–N nanocomposite films through reactive sputtering; they found that the high hardness (54 GPa) of Nb–Si–N caused by Si atoms dissolved in NbN creates crystal lattice defects, thereby increasing resistance against dislocation deformation and strengthening the thin film. Sandu et al. [7] revealed that the Nb–Si–N structure can be attributed to three models by Si atoms content (CSi ≤ 4 at.%, 7 at.% ≤ CSi ≤ 4 at.% and CSi ≥ 7 at.%). At low Si content (CSi ≤ 4 at.%), Si atoms replace Nb atoms in the NbN lattice. The nanocomposite film then grows and exceeds the solubility limit. The film is composed of NbN, which comprises Si nanocrystallites surrounded by an amorphous SiNy layer. Further increase in Si content (CSi ≥ 7 at.%) reduces the crystallite size, whereas the thickness of the SiNy layer on the crystallite surface remains constant (∼1.3 monolayer). Song et al. [8] and Wang et al. [9] investigated the microstructure and properties of superhard Nb–Si–N films by controlling N2 partial pressure during deposition. The results revealed the nanocomposite structure of the Nb–Si–N films with nano-sized NbN grains embedded in the amorphous SiNx phase. Microstructure, morphology of interfaces, and preparation conditions for deposition of the Nb–Si–N nanocomposite thin films have been experimentally investigated. In this study, the deposition
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Fig. 1. Local left view of the calculation slab of the 2N2Nb island on the NbN (0 0 1) surface [4 × 4 × (5 + 7)]: (a) side view, (b) top view, (c) Nb–Si–N island of Si Occupy N, (d) Si Occupy Nb, (e) Si neighbor N, (f) Si neighbor Nb, (g) Si diagonal N, and (h) Si diagonal Nb on NbN (0 0 1). Local right view of the calculation slab of the 2N2Nb island on the NbN (0 0 1) surface [6 × 6 × (5 + 7)]: (d) side view, (e) top view, and surface adsorption sites: Nb, Si, and N at P1, P2, P3, P4, P5, and P6 sites around the 2N2Nb island on the NbN (0 0 1) surface. Green, blue, and gray circles represent Nb, Si, and N atoms, respectively. The atoms of the islands and substrate were distinct as displayed by the semitransparent film. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
of grain and interface is theoretically simulated to investigate the composite structure and growth mechanism of the Nb–Si–N nanocomposite films. The influence of the Si atom on interface structure and grain growth is investigated during deposition of the Nb–Si–N nanocomposite film. The influence of the Si atom on the growth mechanism of the Nb–Si–N nanocomposite films was analyzed, and the evolution of 1Si-2N2Nb island configuration and atoms diffused into the 2N2Nb island was determined using the first-principles method. The adsorption energy of the island and the activation energy of atoms were determined during the evolution of island configurations. Saddle points (SP) and adsorption energy were assessed when Nb, Si, and N atoms diffused into the 2N2Nb island on the NbN (0 0 1) surface, from which the diffusion energy of the atoms was calculated. The separation and aggregation of Nb, Si, and N atoms around the NbN grain during deposition of the Nb–Si–N nanocomposite films were also discussed based on calculation results.
2. Computational method and details First-principles calculations were performed using the VASP code based on density functional theory (DFT) [10–13]. The local density was described with the generalized gradient approximation using the Perdew–Wang 91 formula for exchange correlation energy. Calculation was performed with a cutoff energy of plane wave of 400 eV under periodic boundary condition. The precision values of the electron and ion relaxation convergence were 1.0 × 10−4 and 1.0 × 10−3 eV, respectively. The Brillouin zone was sampled with the Monkhorst–Pack k-point grid during selfconsistent calculation to identify the electronic ground state. A 5 × 5 × 1 k-point mesh was utilized for slab calculation. A lattice constant of 0.4442 nm, which was close to the experimental value of 0.4391 nm, was determined by optimizing the NbN crystalline prior to calculating the evolution of the particle behavior [14–18]. The free energies of Nb, N, and Si atoms were −2.623, −3.271, and −0.770 eV, respectively, as calculated through the spin-polarized method [18]. These values were found accurate by fitting them with the calculated cohesive energy. Island evolution and single atomistic process around the island on the NbN (0 0 1) surface were calculated using DFT. The optimized slab comprised five layers of substrate atoms (NbN) and seven layers of vacuum and deposit atoms. Each layer in the 4 × 4 × (5 + 7) model contained 16 atoms for a total of 80 atoms in the five layers
of the NbN substrate and a vacuum of 1.6 nm in the slab of island evolution. Atoms in the bottom two layers were fixed, whereas those in the three top layers were relaxed in three directions (x, y, and z). The surface energy of NbN (0 0 1) was 1.0104 J/m2 for completely relaxed structures. This result agreed with the reported value (1.13 J/m2 ) of Iskandarova et al. [19]. The 2N2Nb-island was built on the NbN (0 0 1) surface [4 × 4 × (5 + 7)]: (a) side view and (b) top view in Fig. 1. The Si-2N2Nb island consists of five atoms and deposited on the NbN (0 0 1) surface. Fig. 1 shows five kinds of island configurations, namely, Si Ocp N, where the Si atom replaces N atom in the 2N2Nb island (Fig. 1c); Si Ocp Nb, where Si replaces Nb in the 2N2Nb island (Fig. 1d); Si Neig N, where Si bonds with the neighbor N atom outside the 2N2Nb island (Fig. 1e); Si Neig Nb, where Si bonds with the neighbor Nb atom outside the 2N2Nb island (Fig. 1f); Si Diag N, where Si is located on the diagonal surface of the N atom outside the 2N2Nb island (Fig. 1g); and Si Diag Nb, where Si is located on the diagonal surface of the Nb atom outside the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1h). Atoms on the islands and substrate were distinct and semitransparent. The evolution of island configuration was investigated by calculating the diffusion energy of atoms on NbN (0 0 1) by using the nudged elastic band (NEB) method. The 2N2Nb island was deposited on the NbN (0 0 1) surface of a 6 × 6 × (5 + 7) slab, which comprised 5 N–Nb layers with 18 N and 18 Nb atoms per plane and seven vacuum layers (Fig. 1i). The adsorption and diffusion energies of a single atom around the island were calculated, with six highly symmetrical positions around the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1j). The adsorption energy of Nb, Si, and N at the P1, P2, P3, P4, P5, and P6 sites were extracted (Fig. 1j). In addition, the behavior of atoms diffused between the highly symmetrical positions on the NbN (0 0 1) surface around the island was calculated by the NEB method.
3. Results and discussion 3.1. Adsorption and evolution of island on the NbN (0 0 1) surface 3.1.1. Adsorption of the island on the NbN (0 0 1) surface The surface adsorption energy of the six types of the Si-2N2Nb islands was calculated to investigate the adsorption behavior of island configuration on the NbN (0 0 1) surface. The adsorption energy of the island determines the stability of adsorption and degree of evolution during the deposition of the Nb–Si–N
Y. Ren et al. / Applied Surface Science 357 (2015) 1613–1618
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Table 1 Adsorptive energy (Ead ) and configuration energy (Econfig ) of Nb–Si–N island of Si Occupy N, Si Occupy Nb, Si neighbor N, Si neighbor Nb, Si diagonal N, and Si diagonal Nb on the NbN (0 0 1) surface. [Eslab = −794.47343 eV, NbN (0 0 1)].
Econfig (eV) Ead (eV)
Si Ocp N
Si Ocp Nb
Si Neig N
Si Neig Nb
Si Diag N
Si Diag Nb
−837.15095 33.87
−836.76910 33.49
−836.26087 32.98
−836.95645 33.68
−836.11035 32.83
−837.34098 34.06
nanocomposite films [20,21]. The structure of the grain and interface was also discussed. The adsorption energy Ead is defined as Eq. (1). Ead = lENb + mESi + nEN + Eslab − Econfig
(1)
where Ead is the adsorption energy of island configuration adsorbed on the NbN (0 0 1) surface; Econfig is the total energy of the system that the Si-2N2Nb island and the NbN (0 0 1) surface; Eslab is the total energy of the relaxed slab (4 × 4 × (5 + 7)) without the adatoms or islands (Eslab = −794.47343 eV); l, m, and n represent the number of Nb, Si, and N atoms in the islands, respectively; and ENb , ESi , and EN represent the atomic energy of Nb, Si, and N atoms, respectively [22]. The total energy and adsorption energy of the six islands are listed in Table 1. The island configuration of Si Diag Nb, where the Si atom is located on the diagonal surface of Nb atom outside the 2N2Nb island, showed the lowest total energy and the highest adsorption energy. Thus, the Si Diag Nb island exhibited stable configuration. That is, the 2N2Nb island easily formed NbN grain and the Si atom outside the island formed the interface during Nb–Si–N film growth. Although the distance was 0.28 nm, in which the band length was between Si and Nb atoms of the 2N2Nb island, the Si atom distorted the 2N2Nb island. The combined band lengths of Nb N were 0.2036 and 0.1976 nm in the 2N2Nb island, in which the Nb of the island was oriented to the Si atom of the diagonal position. Si Diag N island exhibited the lowest adsorption energy, which indicated unstable configuration during Nb–Si–N film deposition. The adsorption energy of the island configuration, in which Si occupies the N atom (Si Ocp N) or the Nb atom (Si Ocp Nb) of 2N2Nb, was higher than that of Si beside the N atom (Si Neig N) or Nb atom (Si Neig Nb) of 2N2Nb. The N and Nb atoms of these islands were likely substituted by the Si atom when the latter was close to the island during deposition. Compared with Si Neig N and Si Neig Nb islands, those with Nb or N atoms replaced by Si were more stable. In the Si Ocp Nb configuration, Si N bond was formed between
the island atom Si and the substrate atom N. Similar phenomenon was observed in the Si Diag Nb configuration. The substitution solid solution of Si Ocp Nb was formed at poor-content Nb atoms during deposition, which is consistent with the findings reported by Sandu [6]. The structurally stable compounds of NbN grain and Si N interface were easily formed during the growth of the Nb–Si–N nanocomposite films. 3.1.2. Evolution of the island on the NbN (0 0 1) surface The evolution behavior of island configurations was determined using the NEB method to investigate the separation and aggregation of the grain and interface during deposition. The low-energy configuration (Si Ocp Nb) evolved to the lowest energy configuration (Si Diag Nb) on the NbN (0 0 1) surface. This process included two parts: first, the Si atom diffused to the diagonal position outside the island, then the Nb atom diffused into the island. The minimum energy pathway (MEP) is shown in Fig. 2. The diffusion energy of the Si atom diffused from the island to the diagonal position was 1.58 eV, which was the diffusion energy of the difference between configurations I and II (see Fig. 2). The diffusion process involved configuration I evolved to configuration V, with two SPs (configurations II and IV in Fig. 2) and one low energy position (configuration III in Fig. 2) in the pathway. The two Si N bonds between Si and N atoms of the island were opened, and the two Si Nb bonds between Si and Nb atoms of the substrate were formed in the first part of the Si diffusion pathway. The Si bond with two Nb and two N atoms of the substrate in configuration III was the stable structure (see Fig. 2). In the second part of the Si diffusion pathway, the two Si Nb bonds between Si and Nb atoms of the substrate were broken. The Si atom diffused to the top N of the substrate in configuration V. The opening of the Si N bonds in the first part of the pathway was complicated and required an energy amount of 1.58 eV. The energy needed by the Si atom across SP1 was the Si diffusion energy (see Fig. 2). However, the Si Nb bonds were easily opened in the second part of the pathway. The diffusion energy of the Si atom across SP2 was 0.09 eV. The diffusion energy
Fig. 2. MEP of the diffusion of the evolution of the Nb–Si–N island on the NbN (0 0 1) surface. The Nb–Si–N island configuration, in which Si Occupy the Nb island to Si diagonal Nb island on NbN (0 0 1). SPs represent the transition states during diffusion. The pathways of Si and Nb atoms involve diffusion from and into the island on the NbN (0 0 1) surface, respectively. Single atoms require diffusion energy to cross the saddle point in the pathway.
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Table 2 Adsorptive energy (Ead ) and configuration energy (Econfig ) of Nb, Si, and N at P1, P2, P3, P4, P5, and P6 sites around the 2N2Nb-island on the NbN (0 0 1) surface. [Eslab = −1825.6499 eV, NbN (0 0 1)]. Nb
P1 P2 P3 P4 P5 P6
Si
N
Econfig (eV)
Ead (eV)
Econfig (eV)
Ead (eV)
Econfig (eV)
Ead (eV)
−1834.3035 −1832.1121 −1835.5671 −1832.5845 −1832.9483 −1835.5671
6.0431 3.8517 7.3067 4.3241 4.6879 7.3067
−1831.7588 −1830.3456 −1831.3861 −1828.4683 −1830.6641 −1831.3861
5.3521 4.6957 4.9794 2.0616 4.2574 4.9794
−1834.0283 −1835.6259 −1833.3660 −1833.3737 −1826.2921 −1833.3660
5.1137 6.7113 4.4514 4.4591 −2.6225 4.4514
The underlined were the highest adsorption energy values of atoms on NbN (0 0 1) around the island.
of the Si atom across SP1 was higher than that across SP2. Thus, the diffusion energy of the Si atom outside the island was 1.58 eV. The bond lengths between the diffusion atom Si and Nb and nearest atom (B Si-N and B Nb-N) were shown in Fig. 2 in the process of evolution. The migration energy of the Nb atom was 0.88 eV upon entering after Si deposited outside the island. The two Nb N bonds of the initial configuration (Nb bond with one N of the substrate and one N atom of the island) evolved to three Nb N bonds with stable configuration (Nb bond with one N of the substrate and two N atoms of the island). The Nb atom around the N atom of the island with unbroken bond formed the 2N2Nb island. The diffusion energy of the Nb atom across SP3required the Nb N bond between Nb and N of the substrate to be opened. However, the diffusion energy of Nb atom outside the island was 1.85 eV higher than that of Si outside the island (see Fig. 2). The Nb atom outside the island was difficult to maintain compared with the Si atom. As such, substitution solid solution was formed, in which the Nb atom of the 2N2Nb island was replaced by the Si atom at the poor-quality Nb atoms during deposition. However, the Si atom was easily replaced by the Nb atom at site with abundant Nb atoms. The interface of Si and grain of NbN tended to separate.
symmetrical to the N N diagonal of the 2N2Nb island (Fig. 1j), the adsorption energy of Nb, Si, and N atoms around the 2N2Nb island was higher than that of Nb, Si, and N atoms on the clean NbN (0 0 1) surface [22]. After NbN grain nucleation during deposition, Nb, Si, and N atoms would be easily adsorbed around the grain. The Si atom was the active element with low adsorption and could promote the diffusion of Nb and N atoms. Gratifyingly, the nucleation and growth of the NbN grain were uniform. The stable Nb N covalent bond was formed with the Nb atom at P3 and P1 with high adsorption energy to promote NbN grain growth on the NbN (0 0 1) surface. The stability of the bond between deposited Nb atoms and substrate Nb atoms was low at P2, P4, and P5 sites. The bond strength between Si and Nb atoms of the substrate and the island was weaker than that of Si deposited at P2 with low adsorption energy. The dimmer of N2 was adsorbed on the NbN (0 0 1) surface, in which deposited N atoms and island N atoms combined at P3, P4, and P6. The adsorption energy of the N atom at P5 far from the island was the lowest, and the complete bonds of substrate N atoms did not combine with deposited N atoms. The N atom could adsorb and grow beside the island during deposition to form N2 molecule desorption from the NbN (0 0 1) surface when N atom was deposited far from the island.
3.2. Adsorption and diffusion of single atom around the island
3.2.2. Diffusion of single atom around the island Adsorption around the island was analyzed. Nb, Si, and N atoms exhibited different types of activity at the six symmetrical positions. The effect of deposited atoms on the growth of the NbN grain was determined through the atom that diffused between symmetrical positions. The behavior of deposited atoms around the 2N2Nb island was calculated on the NbN (0 0 1) surface. The pathway of Nb and Si atoms around the island were from P3 to P5 because the total energy of Nb and Si atoms at P2 were high. Nb and Si allowed P3 to diffuse to P5 with adequate activation energy compared with that diffused around the island. The N atom from the high adsorption energy P2 diffused to the lower adsorption energy P5, which represented the desorption trend of the 2N2Nb island. A high potential state can be obtained by increasing the slope of the Nb atom from the diffusion at P3 to P5 outside the island. The diffusion energy of the Nb atom gradually increased in the pathway. An SP was observed in the pathway, and the diffusion energy of the Nb atom across SP was 2.619 eV, as illustrated in Fig. 3a. The bond between deposited Nb atoms and island N atoms was opened, and Nb bonds with substrate N atoms at P5. The SP was the bridge between the two Nb atoms of the substrate. The Si atom from the lowest energy position diffused to the lower energy position across the SP, and the diffusion energy from P3 to P5 was 1.349 eV, as depicted in Fig. 3b. Compared with diffused Nb atoms, the total energy decreased across the SP in the Si diffusion pathway because that the Si N covalent bond strength was stronger than that of Nb N. The total energy of Si at SP was higher than that of Si at the initial and final positions. Different phenomena were observed during migration of N atoms, and SP did not exist from P2 to P5 but existed in the basin.
3.2.1. Adsorption of single atom around the island The results of island evolution calculation show that entrance of the Si atom into the NbN grain or substitution solid solution formation was difficult. The NbN grain and Si N interface were formed during deposition. The effect of Nb, Si, and N atoms on the grain and interface was investigated. The adsorption and diffusion energy of single atom around the 2N2Nb island was calculated on the NbN (0 0 1) surface. Symmetrical positions included P1, P2, P3, P4, P5, and P6 around the 2N2Nb island on the NbN (0 0 1) surface (Fig. 1j). The stable and diffusion behavior was determined using the adsorption energy of single atom at the six symmetrical positions around the island. The adsorption energy was obtained using Eq. (1). The results of adsorption energy of single atom around the island showed that the lowest energy position differed for Nb, Si, and N atoms. The values of calculated are reported in Table 2. The highest adsorption energy position of the Si atom was P1 around the island, which is consistent with island configuration evolution (Section 3.1.1) results, in which the stable configuration was Si atom diagonal to the Nb atom outside the 2N2Nb island. High adsorption energy was observed in the configuration with Nb atom at P3 around the island in six symmetrical positions. Similar phenomenon was observed in the result of island evolution, in which the Nb atom outside the island Nb atom was located beside and bond with the N atom of the island. The N atom outside the island easily bond with the Nb atom of the island. Thus, the N atom exhibited a stable state at P2 around the island in six symmetrical positions. The adsorption energy of Nb, Si and N atoms was consistent at P3 and P6, as reported in Table 2. As P3 and P6 were
Y. Ren et al. / Applied Surface Science 357 (2015) 1613–1618
(a) Nb atom Diffusion P3 toP5 SP
(b) Si atom Diffusion P3 toP5
2.5
P5
2.0 1.5 1.0 0.5 0.0
P3
-0.05 0.00
0.05
0.10
0.15
0.20
0.25
SP
1.4
D i f f u si o n E n e r g y ( e V )
D iffu sio n E ne rg y (eV )
3.0
0.30
1.2 1.0 0.8 0.6
P5
0.4 0.2
P3
0.0
-0.2 -0.05
0.35
0.00
0.05
Pathway(nm)
0.10
0.15
0.20
0.25
0.30
0.35
Pathway (nm)
(c) N atom Diffusion P2 to P5
9.0
Diffusion Energy (eV)
1617
P5
7.5 6.0 4.5
P2
3.0 1.5
DB
0.0 0.00
0.05
0.10
0.15
0.20
0.25
Pathway (nm) Fig. 3. x- and y-axes represent the pathway and diffusion energy of a single atom, respectively. The diffusion pathways of Nb, Si, and N from P3 to P5 or P2 to P5 around the 2N2Nb island on the NbN (0 0 1) surface. Nb, Si, and N atoms require diffusion energy to traverse the saddle point in the pathway.
The diffusion energy of the N atom from the basin to P5 was 5.094 eV, as illustrated in Fig. 3c. As the N atom was bound to the Nb atoms of the island and substrate at P2, the structure was stable with low total energy. The combination of long and short bonds between N and Nb atoms were formed in the lattice disorder of NbN when N diffused to the basin position. The declined migration of the N atom at the beginning was similar to Si atom diffusion on the clean NbN (0 0 1) surface [22]. The high diffusion energy of the N atom around the island required high deposition temperature. However, N atom could be desorbed from the island and substrate at high deposit temperatures, which resulted in decreased rate of deposition and increased number of defects. The diffusion of deposited atoms differed around the 2N2Nb island. The Nb atom diffused from the foot up to the peak plane of the mountain. The diffusion of Si atom was trailed through the ridge between cols. The N atom glided down to the basin and climbed to the peak plane of the mountain (see Fig. 3). Compared with the diffusion of Nb and N atoms, the diffusion of the Si atom occurred in the minimum energy pathway around the island on the NbN (0 0 1) surface. The effect of the 2N2Nb island and substrate on the behavior of deposited atoms around the island was regarded as a composite factor. If deposited atoms were located closed to the 2N2Nb island, the island would prevailed. If deposited atoms were located away from the island, the substrate would dominate. If the Nb atom obtained more than 2.6189 eV energy, it will freely rotate around the island. By contrast, the Si atom requires a diffusion energy of 1.3493 eV to rotate around the island but received more than 8 eV energy for the N atom around the island. The N atom was withdrawn from the substrate through N atom diffusion, which is similar to N diffusion on the clean NbN (0 0 1) surface [22]. Compared with the difficulty encountered by Nb and N atoms around the island, the NbN grain
was easily separated by Si atom diffusion around the 2N2Nb island. The grain was refined to inhibit the growth of the Si atom during deposition. 4. Conclusions The evolution of the Si-2N2Nb island and the behavior of atoms around the 2N2Nb island on the Nb–Si–N nanocomposite films were studied using the first-principles method. The structure and properties of the nanocomposite films were determined with the stability of the Si-2N2Nb island and the mechanism of evolution and growth on NbN (0 0 1) during deposition. The calculation results showed that NbN grain was formed and easily nucleated, and the Si atom of substitution solid solution was extruded from the grain and formed the interface phase in the films. The composite structure of the Nb–Si–N nano films was determined by the combination of the NbN grain and the Si interface phase. The diffusion of Nb, Si, and N atoms around the island of the NbN grain presented three types. The active Si atom could diffuse around the easily nucleated NbN grain. The growth expansion of the NbN grain was inhibited by Si atoms, and the grain was refined during deposition. As an active element, the Si atom could improve migration of Nb and N, form poor-quality defects, and compacted the nanocomposite films. The composite structure of the Nb–Si–N nano films was based on the interface with the Si atom. Deposition temperature was determined to the active energy of the Si atom. Temperature was accurately controlled during deposition to control the diffusion active energy of Si. The Si atom could not successfully diffuse around the island at low deposit temperatures, thereby facilitating the growth of the NbN grain. The N atom would desorb from the surface in the form of N2 at high deposit temperatures, which causes decline in deposition rate. The adsorption
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