New Bonding Configuration on Si(111) and Ge(111) Surfaces Induced by the Adsorption of Alkali Metals

Abstract

The structure of the ͑3 3 1͒ reconstructions of the Si(111) and Ge(111) surfaces induced by adsorption of alkali metals has been determined on the basis of surface x-ray diffraction and low-energy electron diffraction measurements and density functional theory. The ͑3 3 1͒ surface results primarily from the substrate reconstruction and shows a new bonding configuration consisting of consecutive fivefold and sixfold Si (Ge) rings in ͗110͘ projection separated by channels containing the alkali metal atoms. [S0031-9007(98)05973-0] PACS numbers: 61.10.Eq, 61.14. Hg, 68.35.Bs, 71.15.Mb Over the last decade there has been a large effort to understand the structure and properties of reconstructions on elemental semiconductor surfaces. The main driving force behind these reconstructions is the reduction of the number of dangling bonds without introducing too much strain in the surface region. Three structural elements meeting this principle have emerged so far. Particularly important are adatoms which saturate three dangling bonds on (111) surfaces while creating only one unsaturated bond. Adatoms stabilize the clean Ge(111)-c͑2 3 8͒ surface [1] and are also a major stabilizing factor in the dimer-adatom-stacking-fault model of the clean Si͑111͒-͑7 3 7͒ surface [2]. The second structural element that effectively reduces the number of dangling bonds is the dimer frequently found on the (001) surfaces [3]. The third structural element is the p-bonded chain which was first proposed for the clean Si͑111͒-͑2 3 1͒ reconstruction [4]. Yet this simple principle has been of no utility in predicting surface structures as demonstrated by the metal induced ͑3 3 1͒ reconstructions on the (111) surfaces of Si and Ge. Despite the small unit cell, the atomic geometry is still unknown and has been heavily debated over the last ten years [5][6][7][8][9][10][11][12][13]. The mere observation of the symmetry-breaking ͑3 3 1͒ unit cell calls for a unidirectional structural motif and it was, therefore, appealing to introduce p-bonded chains to explain the 3 3 1 periodicity. At present, there are two promising models for the ͑3 3 1͒ reconstruction that have been proposed on the basis of scanning tunneling microscopy (STM) [8], electronic properties [9], and total-energy calculations [10]. The Seiwatz model [see Fig. 1(a)] [7,11,12] consists of parallel p-bonded chains formed by fivefold rings of Si (Ge) atoms in ͗110͘ projection, separated by empty channels, with a top-site adsorbate saturating the surface dangling bonds. The second model is the extended Pandey model [10,13] [see Fig. 1(b)] which consists of a sevenfold ring carrying the p-bonded chain alternating with a five and six-member ring of Si. It is intuitive to describe the ring sequences from these models with the notation 567567 (extended Pandey) and 500500 (Seiwatz model). Unfortunately, neither of these structures is able to explain our surface x-ray diffraction data (SXRD) or low-energy electron diffraction (LEED) data.We determined the structure by using a multipletechnique ...

Identification of the basic structure of the Ag/Si(111)-(6×1)surface: Observation of a low-temperaturec(12×2)phase

“…9,12 Recently, a model that is energetically more stable than the former ones was reported independently by several groups. [13][14][15] This model, called the honeycomb-chainchannel ͑HCC͒ model, [13][14][15] is fully compatible with the SCLS results and the STM images.…”

“…In the Si 2p core-level spectrum of the Ag/ Si͑111͒-(ͱ3ϫͱ3) surface, 28 the surface component of Si atoms bonded directly to Ag atoms shows a shift of 250 meV to the higher binding energy side of the bulk component. Comparing the density of Ag and the number of surrounding Si atoms of the Ag/Si͑111͒-(ͱ3ϫͱ3) surface 29 with that of the HCC model, [13][14][15] the charge transfer to Ag from the surrounding Si atoms should be smaller on the Ag/Si͑111͒-(6 ϫ1) and/or Ag/Si͑111͒-c(12ϫ2) surfaces. Since a smaller charge transfer leads to a smaller energy shift for the surface component, we assign the S2 component to Si atoms bonded directly to the Ag atoms.…”

“…Further, comparing the intensity ratio of the S1 and S2 components, the number of Si atoms that contributes to S1 and S2 should be the same for the HCC model. Since two Si atoms of the top layer are bonded directly to Ag and two are not in the HCC model, [13][14][15] we assign S1 to the Si atoms in the top layer not bonded to Ag. Moreover, these Si atoms are suggested to rehybridize into a sp 2 and p z orbitals 13,15 and form bonds.…”

Ab initio pseudopotentials for electronic structure calculations of poly-atomic systems using density-functional theory

“…Questions addressed with pseudopotentials provided by this code, or its earlier version, range from phase transitions [10,11], defects in semiconductors [12][13][14], the structure of and diffusion on surfaces of semiconductors [15][16][17], simple metals [18], and transition metals [19][20][21], up to surface reactions [22,23], including molecules [24,25] of first-row species.…”

“…Part 1 (program psgen) serves to generate the pseudopotentials using the schemes by Hamann [33] or by Troullier and Martins [34]. This combination provides efficient pseudopotentials for "canonical" applications like group IV and III-V semiconductors [9,10,[15][16][17]22,23], as well as for systems where first-row, transition or noble metal elements are present [11,[19][20][21]24,25], or where "semicore" d-states must be treated as valence states, like in GaN [41] or InN [14]. In such cases the strongly localized 2p and 3, 4, 5d valence states are readily handled with the Troullier-Martins scheme.…”