Structural Model For Si Research Articles

Synchrotron photoemission studies on reconstructed strained surfaces

Recently, based on scanning tunneling microscopy studies of the reconstructed Si(5 5 12)−2×1 surface, it has been suggested that its unit cell simply consists of four kinds of one-dimensional (1D) structures: π-bonded (π) chain, honeycomb (H) chain, tetramer (T) row, and dimer-adatom (D-A) row. In the present study, by angle-resolved ultraviolet photoelectron spectroscopy, it has been found out that the Si(5 5 12)−2×1 surface has two kinds of surface states, one with a negligible dispersion originating from row structures (T/D-A) and the other with a strong dispersion originating from chain structures (π/H). Also, the Si 2p core-level spectrum shows at least two kinds of surface components, one with 0.23 eV higher binding energy originating from upward-relaxed surface atoms and subsurface atoms, and the other with 0.52 eV lower binding energy originating from downward-relaxed surface atoms. It can be realized that these spectroscopic results quantitively match with the structural model of Si(5 5 12)−2×1 having four kinds of 1D structures.

Structural models for Si(553)–Au atomic chain reconstruction

Recent photoemission experiments on Si(553)–Au reconstruction show a one-dimensionalband with a peculiar filling. This band could provide an opportunity for observing large spin–charge separationif electron–electron interactions could be increased. To this end, it is necessary tounderstand in detail the origin of this surface band. A first step is the determination of thestructure of the reconstruction. We present here a study of several structural modelsusing first-principles density functional calculations. Our models are based on aplausible analogy with the similar and better known Si(557)–Au surface, andcompared against the sole structure proposed to date for the Si(553)–Au system(Crain et al 2004 Phys. Rev. B 69 125401). Results for the energetics and theband structures are given. Avenues for future investigation are also outlined.

Morphology, Atomic and Electronic Structure of 6H-SiC(0001) Surfaces

Recent findings concerning primarily the √3×√3 and 6√3×6√3 reconstructed surfaces of 6H-SiC(0001) are reviewed. First, the morphology of some different types of 6H-SiC crystals is discussed. The scanning tunneling microscopy (STM) and atomic force microscopy (AFM) results presented show that surfaces with a morphology suitable for surface investigations can be prepared using sublimation- or hydrogen-etching. Then results obtained concerning the atomic and electronic structure for the reconstructed surfaces, prepared using an ex situ method for oxide removal and in situ heating, are presented. For the √3×√3 reconstruction, recent STM and photoelectron spectroscopy (PES) data are discussed in view of available theoretical results. The STM images presented are shown to be consistent with a structural model of Si or C adatoms in threefold symmetric sites. The theoretical results favor Si adatoms in T4 sites as the optimal configuration for this reconstruction. However, the surface shifted components extracted in studies of the C 1s and Si 2p core levels and the location of a surface state band mapped out in angle resolved experiments cannot be explained using this structural model. At present, there is no structural model that satisfactorily can explain all experimental findings for the √3×√3 reconstruction. A monocrystalline graphite overlayer on top of bulk-terminated or √3×√3-reconstructed SiC has previously been proposed to explain the 6√3×6√3-reconstructed surface. However, STM and PES results are presented that unambiguously show that there is no graphite on the surface when a well developed 6√3×6√3 low-energy electron diffraction (LEED) pattern is observed. The STM images recorded during the gradual development of the 6√3×6√3 surface show growing fractions of pseudo-periodic 6×6 and 5×5 reconstructions. These reconstructed regions dominate on the surface, but small √3×√3-reconstructed regions are still present when a well developed 6√3×6√3 LEED pattern is observed. It is shown that the 6√3×6√3 LEED pattern can be fully explained by scattering from surfaces with a mixture of 6×6, 5×5 and √3×√3 reconstructions. Due to the complexity of the STM data, no structural model is proposed for the 6×6 and 5×5 reconstructions. STM and PES results are presented showing that graphitization of the surface is obtained only after heating at higher temperatures than that required for observing a well developed 6√3×6√3 LEED pattern. The STM images then show that the graphite appears as a monocrystalline overlayer on top of the 6×6 reconstruction and not on bulk-terminated or √3×√3-reconstructed SiC(0001).

Structural model of Si(100)-c(4 x 4).

The $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstruction of Si(100) clean surface has been obtained by suitable thermal annealing within the temperature range of 580-630\ifmmode^\circ\else\textdegree\fi{}C. The structure transition between $c(4\ifmmode\times\else\texttimes\fi{}4)$ and (2\ifmmode\times\else\texttimes\fi{}1) has been observed. From the experimental result of hydrogen adsorption that the $c(4\ifmmode\times\else\texttimes\fi{}4)$ structure could not convert into (2\ifmmode\times\else\texttimes\fi{}1) or (1\ifmmode\times\else\texttimes\fi{}1) after hydrogen exposure, the buckled dimer arrangement seems unlikely to form such $c(4\ifmmode\times\else\texttimes\fi{}4)$ periodicity. We propose that Pandey's $\ensuremath{\pi}$-bonded defect model with small modification might be a possible model of $c(4\ifmmode\times\else\texttimes\fi{}4)$ reconstruction in explaining all the observed evidences in our experiment.

Structural analysis of Si(111)-7×7 by UHV-transmission electron diffraction and microscopy

Structural analysis of the surface reconstructions investigated by ultrahigh vacuum (UHV) transmission electron microscopy (TEM) and diffraction (TED) is shown. By TED intensity analysis a new structural model of Si(111)-7×7 is derived. The model basically consists of 12 adatoms arranged locally in the 2×2 structure, nine dimers on the sides of the triangular subunits of the 7×7 unit cell and a stacking fault layer. UHV–HREM of Si (111)-7×7 surface is commented.

Structural models for Si(111)-(7×7)

A class of structural models is studied for the Si(111)-(7×7) surface which is consistent with scanning tunneling microscopy (STM) results. The hills observed with STM are identified with trimers that are formed by bond flipping. The valleys correspond to boundaries between differently stacked triangular areas. Atoms with broken bonds are removed along these boundaries. A modified Keating strain calculation is used to determine the orientation of the trimers and to obtain accurate atomic positions. The trimer models exhibit large atomic displacements perpendicular to the surface accompanied with small parallel displacements in qualitative agreement with Rutherford backscattering results. The structural changes upon hydrogenation of Si(111)-(7×7) are explained by a reversal of the bond flip whereby the stacking is not altered.

Atomic structure of Si{001}2×1

New experiments on reconstructed Si{001}2\ifmmode\times\else\texttimes\fi{}1 and their analysis by low-energy electron diffraction (LEED) are reported. A new structural model containing asymmetric and buckled dimers and strains extending to three and four atomic layers is presented, which fits the LEED data substantially better than any other model tested. The new structure involves a dimer length of 2.54 \AA{}, an average contraction of first-layer spacing of 8%, and three asymmetric displacements of the dimer atoms. A detailed discussion of the needed intensity-averaging procedures among eight equivalent domains is presented. Two theoretical structural models for Si{001}2\ifmmode\times\else\texttimes\fi{}1 developed by others on the basis of minimization of total energy are shown to fail the LEED test.

Structural model for Si(111)-(7×7)

A new structure is proposed for the Si(111)-(7\ifmmode\times\else\texttimes\fi{}7) surface which is consistent with recent tunneling microscopy and ion-scattering data. It consists of 12 trimers which are located near the points (1,1), (3,1), (1,3), (5,1), (1,5), (3,3), (4,4), (6,2), (2,6), (6,4), (4,6), (6,6) of a (7\ifmmode\times\else\texttimes\fi{}7) unit cell. The bonding of the trimers is similar to the $\ensuremath{\pi}$-bonded chain model for Si(111)-(2\ifmmode\times\else\texttimes\fi{}1) but only half as many bond flips are required to reconstruct the surface. Broken bonds are eliminated by removing atoms along the boundaries between eclipsed and staggered structures.