Theory of the “Honeycomb Chain-Channel” Reconstruction ofM/Si(111)-(3×1)

Abstract

First-principles electronic-structure methods are used to study a structural model for Ag/Si(111)3×1 recently proposed on the basis of transmission electron diffraction data. The fully relaxed geometry for this model is far more energetically favorable than any previously proposed, partly due to the unusual formation of a Si double bond in the surface layer. The calculated electronic properties of this model are in complete agreement with data from angle-resolved photoemission and scanning tunneling microscopy.The surfaces of silicon reconstruct in strikingly diverse ways. This diversity provides a rich proving ground for simple, physically intuitive ideas about the stability of semiconductor surfaces-ideas which are invaluable for understanding more complex dynamical phenomena such as growth, etching, and reactivity. Two such simple concepts-elimination of surface dangling bonds and relief of surface stress-explain the frequent appearance of elementary "building blocks" in silicon reconstructions. For example, dimers appear on Si (001) One reconstruction that remains controversial is the metal-induced M /Si(111)3×1 (where M =Li, Na, K, Ag, Mg), which is widely believed to have a single common structure. Starting from known building blocks, we and others have recently proposed two models for this reconstruction, both with low dangling-bond density and low surface stress [2][3][4][5][6]. First-principles total-energy calculations showed these models to be stable relative to previously proposed ones [6]. The more stable of the two, the extended Pandey chain model, was also consistent with scanning tunneling microscopy (STM) images, but the calculated surface-state band structure of both models was in serious disagreement with angle-resolved photoemission (ARPES) data [7].In this Letter, we examine theoretically a new model for the M :3×1 surface proposed very recently by CollazoDavila, Grozea, and Marks (CGM) on the basis of direct phasing of transmission diffraction data [8], and independently by Lottermoser et al. from surface x-ray diffraction and total-energy calculations [9]. First, starting from coordinates obtained by CGM, we further relax the atomic positions so as to minimize the calculated total energy. The resulting model is by far the most stable of any proposed to date. Second, we show how a nearly perfect "surface symmetry" of this reconstruction neatly resolves a long-standing puzzle regarding ARPES data for Li:3×1 (and may also explain the apparent insulating nature of Mg:3×1, which has an odd number of electrons per unit cell). Third, we show that this model derives its remarkable stability from the formation of a true Si double bond-a "building block" not seen on any other surface of silicon. Finally, we show that this model completely accounts for the appearance of the existing STM images (including the differences between M =Li and Ag).The model proposed by CGM was based on data from Ag:3×1 [8]. Previous experimental results from lowenergy electron diffraction [10], STM [11], and core level spe...

Systematic investigation of the structure of the Si(553)-Au surface from first principles

“…In the case of reconstructions induced by alkalis, the metal atoms donate electrons to the HC, which becomes a closed-shell structure and, thus, contribute to the stabilization of the surface. 39 In the case of gold, which has a stronger electron affinity, the situation is different. Gold is likely to take electrons away from the silicon structure.…”

“…5,7 The HC structure was initially proposed to explain the low coverage reconstructions induced by some alkali metals ͑Li, Na, and K͒, Mg, and Ag on Si͑111͒. 39 We see that the HC structure involves two unit cells of the unreconstructed Si͑111͒ surface, with one atom removed from the top Si layer. This flattens the surface and removes surface stress.…”

“…͑iii͒ the positions of gold in the middle of terraces are favored over step-edge decoration; and, ͑iv͒ the frequent appearance of the so-called honeycomb-chain 39 structure. Figure 1͑a͒ shows a schematic view of an unreconstructed Si͑553͒ surface.…”

“…The population of the different bands can roughly be understood by taking into account the larger electron affinity of gold and the HC structure 39 as compared to other atoms in the surface. The population of the dangling-bond band is thus depleted in favor of the other surface bands.…”

“…The most stable structures were those in which the step edge is formed by a honeycomb chain ͑HC͒. 39 In the present paper, we perform a more comprehensive search of the structure of the Si͑553͒-Au reconstruction. We have considered models in which the topmost bilayer contains eight atoms per 5 ϫ 1 unit cell, including one gold atom.…”