Influences of H on the Adsorption of a Single Ag Atom on Si(111)-7 × 7 Surface
© to the authors 2009
Received: 20 July 2009
Accepted: 26 September 2009
Published: 13 October 2009
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 19H-Si(111)-7 × 7) surfaces has been investigated using first-principles calculations. The results indicated that the pre-adsorbed 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 19H-Si(111)-7 × 7 surface, which increases the adsorption energy of Ag atom on Si surface.
KeywordsSi(111) H adsorption Ag adsorption First-principles calculations
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 nano-electronics is made possible by the improvements in the controlled growth of thin layers of semiconductors, metals and dielectrics. The further development of micro- and nano-electronic 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 two-dimensional (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 [1–3]. 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 , Pd , Cu , Co , In , and Zn . Diverse surface science techniques have been applied to study these interfaces, e.g., scanning tunnelling microscopy [12–15], electron energy loss spectroscopy , infrared reflecting adsorption spectroscopy , photoelectron emission spectroscopy  and temperature-programmed desorption . In order to better understand the physical properties of the Ag/Si interfaces, first-principles calculations have also been employed to study these systems . The changes in the atomic and electronic structures of the Si(111)-√3 × √3-Ag surface, Ag nanocluster formation on the H-terminated Si(111)-1 × 1 surfaces and diffusion of Ag on the H-terminated Si(111)-1 × 1 and clear Si(111)-1 × 1 surfaces have been studied experimentally and theoretically [20–25]. 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 first-principles calculations. H is the main surfactant during the heteroepitaxy of the metals on Si surfaces. When H interacts with Si surface-dangling bonds, this will cause the relaxation of the surface bond strain and reduce the surface free energy [26, 27]. The pre-adsorption of H on Si(111)- 7 × 7 will alter the growth mode and morphology of the metal overlayers on the surface [28–30]. It is expected that ideal H-terminated Si single crystal surfaces are generally considered rather unreactive, which will lead to the different surface kinetics and energetics between clean and H-terminated Si(111)-7 × 7 surface.
Calculation Method and Substrate Structures
Results and Discussion
The system energy (E sys) and adsorption energy (E ad) of a single Ag atom adsorption on different high coordination sites (H3, B2 and S) at Si(111)-7 × 7 and 19H-Si(111)-7 × 7 surfaces
Si(111)-7 × 7
19H- Si(111)-7 × 7
Figure 4 shows the calculated total valence charge density plots of (a) Ag reacted Si(111)-7 × 7 surface with Ag at the H3 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 H3 sites on Si(111)-7 × 7 (see in Fig. 4c). These Ag–Si bonds caused by nearly absolute charge diversion are considered as an electrovalent-like 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.  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 Si-dangling 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 19H-Si(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.  have calculated the diffusion barriers for Ag atom inside the HUCs on the Si(111) and H-terminated Si(111) surfaces, which are 0.14 and 0.27 eV, respectively. The smaller diffusion barrier for Ag atom on the H-terminated 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 H-terminated Si(111) surface because all the Si dangling bonds are saturated by H atoms.
The adsorption of a single Ag atom on clear Si(111)-7 × 7 and 19H-Si(111)-7 × 7 surfaces was investigated using first-principles 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 H3 site on the 19H-Si(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 H-terminated Si(111) surfaces.
This work was financially supported by National Natural Science Foundation of China (20603028).
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