How Silver Grows on the Silicon (001) Surface: A Theoretical and Experimental Investigation

Abstract

When a silver film is grown on the Si(001) surface at room temperature, it forms a monolayer density film with a (2 × 2) periodicity. Previous models of this (2 × 2) surface, simulated by density functional theory (DFT), are found to be inconsistent with experimental observations. The DFT simulations provide evidence that a new model, the square tetramer model, describes the structure of the observed (2 × 2)-Ag phase and is fully consistent with scanning tunneling microscopy data. Theoretical evidence of a covalent bond shared between the Ag and Si is found that matches previous experiments. Interestingly, the simulations also show that the stoichiometry between Si and Ag changes with metal coverage as adsorbate−adsorbate and adsorbate−substrate interactions balance out. At low coverages, when individual Ag adatoms interact solely with Si, a two-to-one Si−Ag−Si interaction scheme is energetically preferred. At 1 monolayer, when Ag−Ag interactions must be considered, a one-to-one Si− Ag interaction scheme is preferred, as it maximizes Ag−Ag interactions.

An ab initio approach to anisotropic alloying into the Si(001) surface

“…17,19,28,45,112 Additionally in the KMC model we include more complex interactions between Ag and Si at a higher coverage that leads to the appearance of Ag aggregates in silver dimer and tetramer forms. 19,43,45,46,82–84,107 Using first-principles calculations we found that two individual Ag atoms have a strong tendency to arrange in a tilted dimer configuration parallel to substrate dimer bonds that is slightly more energetically preferential with respect to the horizontal geometry with a binding energy of 2.31 eV. 45,46 We also examined tetramer silver atom arrangements and observed that they also adopt buckled geometry in contrast to the recently reported four atom square and diamond shape configurations.…”

“…The cave site positioned between two up silicon atoms forming neighbouring dimers (C site in Fig. 1) is found to be the preferred adsorption site over considered for a single Ag atom, 82,84,107 while the M site is the most favorable adsorption position for the In atom. 12,85,86 It should also be mentioned that indium atoms are known to grow in one-dimensional dimer chains perpendicularly to the underlying SDRs, which, in turn, is more energetically stable than the attachment of a single atom.…”

“…The results of first-principles simulations supported by STM measurements of the very initial stages of Ag adsorption suggest that individual Ag adatoms on the silicon surface preferably sit at the sites centering between neighboring silicon dimers attributed to adjacent SDRs and tend to spontaneously form silver aggregates, such as silver dimers and tetramers. 19,45,46,67,[82][83][84] Meanwhile, for In adatoms the energetically preferential lattice sites are located between the SDRs with the offset by half a period along the SDR direction. To address this complex behavior, in Fig.…”

Adsorption and Migration of Silver on Group IV Semiconductor (001) Surfaces by Density Functional Theory

“…In work as early in 1990, for example, Hashizume et al reported the spontaneous formation of various nanostructures of sliver on Si(001) upon adsorption and the subsequent structural evolution via adatom migration. In our recent series of work, − the same Ag/Si(001) system was examined by cryogenic scanning tunneling microscopy (STM), revealing the atomistic details of silver aggregates (e.g., Ag 2 and Ag 4 ), either in isolation (0D), in a chain (1D), or in an extended film (2D). The observed migration and assembly of silver were interpreted by density functional theory (DFT), with the computed heats of adsorption increasing with the dimension of nanostructures.…”

Solving a Long-Standing Problem Regarding Atomic Structure of Si(100)2×3-Ag

“…The Ag/Si(100) system can serve as an example. Though onset of the extensive study of the system dates back to the mid-1990s, − atomic arrangement of its surface reconstructions still remains unknown with the only fresh exception; namely, the atomic structure of the Si(100)­2×2-Ag reconstruction forming at room temperature (RT) has very recently been determined. , The advanced key idea that has allowed the authors to get a success was to adopt an Ag tetramer as a reconstruction building block instead of a more habitual Ag dimer.…”