Abstract
The adsorption of a single Ag atom on both clear Si(111)7 × 7 and 19 hydrogen terminated Si(111)7 × 7 (hereafter referred as 19HSi(111)7 × 7) surfaces has been investigated using firstprinciples calculations. The results indicated that the preadsorbed H on Si surface altered the surface electronic properties of Si and influenced the adsorption properties of Ag atom on the H terminated Si surface (e.g., adsorption site and bonding properties). Difference charge density data indicated that covalent bond is formed between adsorbed Ag and H atoms on 19HSi(111)7 × 7 surface, which increases the adsorption energy of Ag atom on Si surface.
Keywords:
Si(111); H adsorption; Ag adsorption; Firstprinciples calculationsIntroduction
Due to both scientific and technological interest, the metal/semiconductor (M/S) interfaces have attracted much attention in order to further advance semiconductor devices and technologies. The current success of the micro and nanoelectronics is made possible by the improvements in the controlled growth of thin layers of semiconductors, metals and dielectrics. The further development of micro and nanoelectronic device technology requires detailed knowledge of the M/S contact formation and thus places new demands on the M/S interfaces. The development of smaller and more complex devices is based on the ability to control these structures down to the atomic level. In this sense, the understanding of the dynamical processes and the local stability of atomic structures on semiconductor surfaces have a significant importance. Among these M/S interfaces, Ag/Si interface has been extensively investigated due to the important applications of Si in the field of semiconductor technology. Moreover, (1) thin Ag film can be used as a model system in the study of twodimensional (2D) electrical transport phenomena; (2) the Ag/Si system is an example of an abrupt interface with very limited interdiffusion of the two elements and is thus a “prototypical nonreactive” system; and (3) the Ag/Si interface is widely used for contacts and metallization of electronic devices [13]. There is a wide range of Si(111) reconstruction surfaces, such as 1 × 1, 2 × 2, 5 × 5 and 7 × 7 as well. Because of the high stability and large unit cell, the adsorption of various metal atoms on Si(111)7 × 7 surfaces has been extensively studied, for example Au [4,5], Ge [6], Pd [7], Cu [8], Co [9], In [10], and Zn [11]. Diverse surface science techniques have been applied to study these interfaces, e.g., scanning tunnelling microscopy [1215], electron energy loss spectroscopy [16], infrared reflecting adsorption spectroscopy [17], photoelectron emission spectroscopy [18] and temperatureprogrammed desorption [19]. In order to better understand the physical properties of the Ag/Si interfaces, firstprinciples calculations have also been employed to study these systems [20]. The changes in the atomic and electronic structures of the Si(111)√3 × √3Ag surface, Ag nanocluster formation on the Hterminated Si(111)1 × 1 surfaces and diffusion of Ag on the Hterminated Si(111)1 × 1 and clear Si(111)1 × 1 surfaces have been studied experimentally and theoretically [2025]. In present work, we take Ag as an example to investigate the influences of H on the adsorption of metal on the Si(111)7 × 7 surface using firstprinciples calculations. H is the main surfactant during the heteroepitaxy of the metals on Si surfaces. When H interacts with Si surfacedangling bonds, this will cause the relaxation of the surface bond strain and reduce the surface free energy [26,27]. The preadsorption of H on Si(111) 7 × 7 will alter the growth mode and morphology of the metal overlayers on the surface [2830]. It is expected that ideal Hterminated Si single crystal surfaces are generally considered rather unreactive, which will lead to the different surface kinetics and energetics between clean and Hterminated Si(111)7 × 7 surface.
Calculation Method and Substrate Structures
Firstprinciples calculations within the framework of density functional theory (DFT) were applied to study the influences of H on the adsorption of Ag on the Si(111)7 × 7 surface using the Vienna ab initio simulation package (VASP) [31]. Ab initio density functional calculations of surfaces and interfaces play a critical role in providing a nanoscopic understanding of the chemical bonding in these systems in the determination of the atomic geometry and electronic structure. A planewave method with the Vanderbilt ultrasoft pseudopotentials [32] was used within the spinindependent generalized gradient approximation (GGA) [33] for the exchangecorrelation energy. The planewave cutoff energy was 200 eV, and the surface Brillouin zone was sampled at the Γ point for the total energy calculations and geometry optimizations. The Si(111)7 × 7 and 19HSi(111)7 × 7 substrate structures were built based on the dimeradatomstacking fault (DAS) model [34]. On the 19HSi surface, the 19 Si surface dangling bonds (DBs) per unit cell are saturated by H atoms, corresponding to 12 adatoms, six rest atoms and a corner hole of the DAS. The top and side views of these models are shown in Fig. 1. The unit cell contains a slab of five Si layers (200 Si atoms) and a ~12 Å vacuum layer. The bottom of the slab has a bulklike structure with each Si atom saturated by an H atom. All atoms except for the H and Si atoms at the bottom were fully relaxed to optimize the surface total energy. In this work, the faulted half unit cell (FHUC) was deliberately selected for study because there is little difference in electronic properties between FHUC and unfaulted half unit cell (UHUC) [35,36] on the Si(111)7 × 7 surface.
Figure 1. a The top and side views of dimmeradatomstacking (DAS) fault Si(111)7 × 7 structure. The blue balls are the Si adatoms, and the pink balls are the Si rest atoms. The positions of H_{3}, B_{2} and S sites are indicated in the top view within a “basin”, b the top view of 19HSi(111)7 × 7 model surface. The small yellow balls on the Si atoms with dangling bond are H atoms
Results and Discussion
To understand the influences of H on the Ag adsorption at a Si(111)7 × 7 surface, we first calculate the adsorption energies of Ag atom at the high coordination sites on the clear and 19HSi(111)7 × 7 surfaces, because all the previous data have confirmed that the high coordination sites on the Si surface are the most favorable adsorption sites for different metal atoms (including Ag) [20,37]. On account of the symmetry of the three equivalent “basins” in a FHUC, only the adsorption energies at three different high coordination Si surface sites (H_{3}, B_{2} and S) on a “basin” were considered [38]. We derived the adsorption energies from calculating the total energy of the system including full relaxation of all Si atoms and H atoms (except for the bottom hydrogenated Si atoms) and the Ag adatom. The adsorption energies (E_{ad}) are defined as,
where E_{sys} is the system energy combining the bonding energy of the Ag adatom on the surface and the surface relaxation energy; E_{sur} is the energy of either Si(111)7 × 7 or 19HSi(111)7 × 7 surfaces, which is −1,197.073 or −1,278.822 eV, respectively; E_{atom} is the binding energy of one bulk Ag atom , i.e −0.012 eV, and this value is very close to the experimental result [39]. The calculation results show that the most stable site for a single Ag atom adsorption is the S site for clear Si surface, and H_{3} site for the 19HSi(111)7 × 7 surface. The adsorption energies for Ag atom at the H_{3}, B_{2} and S places on different surfaces are listed in Table 1. The locations of the different sites are indicated in Fig. 1, the S site is almost at the middle between the H_{3} and B_{2} sites.
Table 1. The system energy (E_{sys}) and adsorption energy (E_{ad}) of a single Ag atom adsorption on different high coordination sites (H_{3}, B_{2} and S) at Si(111)7 × 7 and 19HSi(111)7 × 7 surfaces
The change of the adsorption site of Ag atom because of the preadsorption of H on Si(111)7 × 7 may be due to the reconstruction of Si surface electronic structures induced by H. To depict the charge redistribution associated with the H adsorption on Si(111)7 × 7 surface in real space, we first calculate the difference charge density after H saturating the 19 surface DBs on the Si(111)7 × 7 substrate by subtracting the charge densities of the separate Si substrate and H atoms from that of 19HSi(111)7 × 7. To verify the differences, the charge densities of the clean Si substrate, 19HSi(111) 7 × 7 and isolated H atoms are calculated with the same lattice parameters and atomic positions as the relaxed Ag adsorbed 19HSi(111)7 × 7 surface. This allows the charge densities to be easily subtracted point by point in the real space, even for Ag adsorbed surfaces. Figure 2 presents the calculated total valence charge density plots of (a) clean Si substrate, (b) isolated H atoms, (c) Hterminated Si surface in FHUC, and (d) the difference charge density plot. The plot in Fig. 2d is generated by subtracting Fig. 2a, 2b from 2c in the plan determined by H atoms, Si adatom and the rest atom in FHUC along the solid line shown in Fig. 1b. In Fig. 2d, the positive contours (solid lines) represent the charge accumulation, whereas the negative contours (dashed lines) represent the charge depletion. The charge density depletes around the H atom and transfer toward the Si adatom when the H sits on the Si adatom. There is a strong covalent bond between the H and the Si rest atom when the H locates on the Si rest atom. These results indicate that due to the strong charge transfer from adsorbed H to the Si adatom, a local positive surface dipole will then form at the Si adatom (H^{+}Si^{−}). This means that H adsorbed on Si adatom has different electronic properties from one adsorbed on the Si rest atom. The calculations also show that the surface atomic charge distribution is much more uniform once all 19 surface DBs have been saturated by H, which is consistent with the previous results reported by Stauffer and Minot [40]. The more uniformity of the surface charge distribution may decrease the Ag diffusion barrier on Hterminated Si(111) surface [20].
Figure 2. Calculated total valence charge density plots of a clean Si substrate, b isolated H atoms, c9HSi(111)7 × 7 and d the difference charge density plot by subtracting Fig. 2a and 2b from 2c. The area is 11.5 × 8 Å; the contours interval is 0.1e Å^{−3} for Fig. 2a, 2b and 2c and 0.5e Å^{−3} for Fig. 2d. Positive contours are shown as solid lines, negative contours as dashed lines and zero contours have been omitted. A is for Si adatom and R for Si rest atom, respectively
By using the same calculation methods, we also obtain the charge distribution associated with the most stable adsorption of Ag at H_{3} sites on 19HSi(111)7 × 7 surface (in Fig. 3) and the H_{3} and S sites on Si(111)7 × 7 surface (in Fig. 4). Figure 3 shows the total valence charge density plots of (a) the Ag reacted 19HSi(111)7 × 7 surface with Ag at the H_{3} site in FHUC, (b) isolated Ag atom, and (c) the difference charge density plot. The plot in Fig. 3c is calculated by subtracting Figs. 2c and 3b from Fig. 3a in the plan determined by H atoms, absorbed Ag atom, Si corner adatom and the rest atom. Figure 3c reveals that the charge depletion and accumulation mainly occur between the Ag atom and near H atoms, but no obvious charge difference happens around the close Si atoms. This suggests that after the H passivation, the direct interaction between Ag and Si atoms becomes weak. However, it is interesting to note that the obvious charge accumulation takes place around the third Si atom bonding with Ag at the second layer (not in the plane of Fig. 3c), which has not been adsorbed by H. The charge around the H atom at the Si adatom removes toward the adsorbed Ag atom and forms a covalentlike AgH bond. Due to the charge transfer from the H to the Si adatom on the 19HSi(111)7 × 7 surface, the H atom is expected to be positively charged. When Ag adsorbs on the surface, charges are much easier to transfer from Ag to this H and form strong covalent bonds. No strong bonding was found between Ag and the H at the Si rest atom.
Figure 3. Calculated total valence charge density plots of: a Ag reacted 19HSi(111)7 × 7 surface with Ag at the H_{3} site, b isolated Ag atom and c the charge density difference plot by subtracting Figs. 2c and 3b from Fig. 3a. The area is 11.5 × 8 Å, the contours interval is 0.1e Å^{−3} for Fig. 3a and 3b, and 0.5e Å^{−3} for Fig. 3c
Figure 4. The calculated total valence charge density plots of a Ag reacted Si(111)7 × 7 surface with Ag at the H_{3} site, b isolated Ag atom, c the difference charge density plot by subtracting Figs. 2a and 4b from Fig. 4a, and 4d the difference charge density plot with Ag at the S site on Si(111)7 × 7. The area is 11.5 × 8 Å, and the contours interval is 0.1e Å^{−3} for Fig. 4a and 4b, and 0.02e Å^{−3} for Fig. 4c and 4d
Figure 4 shows the calculated total valence charge density plots of (a) Ag reacted Si(111)7 × 7 surface with Ag at the H_{3} site in FHUC, (b) isolated Ag atom, (c) the difference charge density plot which is obtained by subtracting Figs. 2a and 4b from Fig. 4a and 4(d) the difference charge density plot with Ag adsorption at S sites. Without the H atoms on the Si surface, we observe that the charge accumulates around the Ag atom, and strongly depletes around the Si adatom, rest atom and the third adjacent Si atom at the second layer (not in the plane) when Ag adsorbs at H_{3} sites on Si(111)7 × 7 (see in Fig. 4c). These Ag–Si bonds caused by nearly absolute charge diversion are considered as an electrovalentlike bond. However, when Ag adsorbs on the most stable site (S), the charge depletes around Ag atom and transfer toward the Si rest atom and the Si atom at the second layer. It is surprising to find that there is no influence on the charge density around the Si adatom (see in Fig. 4d). Brommer et al. [41] predicted from their principles calculations of a clean Si(111)7 × 7 surface that nucleophilic species (e.g., Ag), relative to a Si atom, should react with Sidangling bonds in the order of adatoms, corner holes, and rest atoms. Our results do not support this conclusion.
From above results, one can see that the adsorption behaviors of Ag atom on the Si(111)7 × 7 and 19HSi(111)7 × 7 surfaces are quite different. After passivating the Si surface by H atoms, the adsorbed Ag will form covalent bonds with H atoms at the Si adatom, and consequently, the interaction between the Ag and the Si atoms become much weaker. Jeong et al. [20] have calculated the diffusion barriers for Ag atom inside the HUCs on the Si(111) and Hterminated Si(111) surfaces, which are 0.14 and 0.27 eV, respectively. The smaller diffusion barrier for Ag atom on the Hterminated Si surface is probably due to the uniformity of the surface atomic charge distribution because of the saturation of the surface Si DBs by H atoms. They further concluded that due to the lower diffusion barrier, three dimension Ag islands would be easily grown on the Hterminated Si(111) surface because all the Si dangling bonds are saturated by H atoms.
Conclusions
The adsorption of a single Ag atom on clear Si(111)7 × 7 and 19HSi(111)7 × 7 surfaces was investigated using firstprinciples calculations. The results indicated that the adsorption of H atoms at DBs on Si(111)7 × 7 surface will uniform the surface charge distribution and consequently alter the surface electronic structures. A local surface positive dipole (H^{+}Si^{−}) may form due to the strong charge transfer from H to the Si adatom. When Ag adsorbs at H_{3} site on the 19HSi(111)7 × 7 surface, a strong covalent bond with the H at the Si adatom was found. The present results provide a theoretic framework for the understanding of the Ag bonding properties on Si(111) and Hterminated Si(111) surfaces.
Acknowledgments
This work was financially supported by National Natural Science Foundation of China (20603028).
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