Reconciliation between atomic and electronic structure periodicity in Si(111)-7×7

By Andy Massey on July 13, 2017

In their recent work published in Journal of Physics: Condensed Matter, Sheverdyaeva et al. investigate the electronic structure of an Si(111)-7×7 solid crystal, in order to look for evidence of corresponding intensity modulation of a surface state. Read more about their research in the authors’ own words below:

The surface of a solid crystal is created by breaking interatomic bonds within the 3-D periodicity, which leaves these bonds dangling into the vacuum. In order to minimize the number of dangling bonds, a wide range of surfaces undergo a reconstruction, which consists in a rearrangement of the surface atoms with respect to the bulk structure and can affect one or more layers at the surface. As an example, the 7×7-reconstructed Si(111) surface is among the most fascinating. Its atomic structure indeed is described by the so-called Dimer-Adatom-Stacking-fault (DAS) model, where the surface unit cell is 7 times larger than the surface unit cell of the unreconstructed structure and involves 4 atomic layers (the expression 7×7 gives account of the two main crystallographic directions in the (111) plane).

As for studies concerning electronic band structure, in addition to the bands relative to the bulk, a number of surface states had been discovered and attributed. Nonetheless, one point still remained elusive. If in the real space the surface unit cell is 7 times larger with respect to the unreconstructed surface, in the reciprocal space the surface unit cell should be 7 times smaller. That is, the 7×7 surface periodicity of the atomic structure (see the sharp LEED pattern of Fig. 1a demonstrating the 7×7 reconstruction), should be reflected in the electronic structure as well, but with a period of 1/7. In other words, evidence of an intensity modulation of a surface state must be observed in correspondence with each 7×7 small grey hexagons of Fig. 1c, representing the 7×7 reciprocal unit cells.

Figure 1. (a) Low Energy Electron Diffraction (LEED) pattern of the Si(111) 7×7 reconstruction. (b) Photoemission intensity map of the surface state associated with the adatoms. (c) Two-dimensional photoemission intensity map taken at a binding energy correponding to the bottom of the surface state (white dashed line of Fig. 1b). Taken from J. Phys.: Condens. Matter 29 215001 © IOP Publishing 2017.

In our research we probed the electronic structure of 7×7 reconstruction by means of angle resolved photoemission spectroscopy (ARPES). We focussed on the adatoms’ surface state (Figure 1b) and studied its periodicity using high angular and energy resolution measurements, complemented by advanced data analysis. Fig. 1c shows a two-dimensional photoemission intensity map taken at a fixed binding energy corresponding to the bottom of the surface state (dashed white line of Fig. 1b). We can clearly observe several intense spot-like features located at exactly the center of the small grey hexagons. This picture demonstrates the first clear evidence of a 7×7 intensity modulation of the Si(111) electronic band structure.

Our work aims at pushing forward the studies about this prototypical system, which still remains challenging both from the point of view of the experiment and of the theory. Even the mere presence of only one adatom’s surface state is not ascertained and still not explained by the existing band structure calculations. The complex surface unit cell consisting of 364 atoms makes the electronic band structure calculations highly demanding. However, we hope that novel computational methods will help in the study of this prototypical system, which is still a source of great interest more than 60 years after the first observation.

About the authors

Polina Sheverdyaeva and Paolo Moras work at CNR-ISM in Trieste, Italy. They perform photoemission studies at the VUV-Photoemission beamline at Elettra synchrotron. The research activity of the group covers a wide range of metal and semiconductor systems, with a focus on reduced dimensionality.

Sanjoy Kr Mahatha, former member of the above group, currently works at Aahrus University, Denmark.

Mauro Satta works at CNR-ISMN and Department of Chemistry of Rome “Sapienza” University. His interests are mainly concentrated upon the use of molecular dynamics and ab-initio methods for the study of the interaction between molecules and surfaces.

Fabio Ronci, Stefano Colonna and Roberto Flammini work at CNR-ISM in Rome, Italy. Their work is mainly focussed on low dimensional systems, studied by means of spectroscopy (photoemission and adsorption) as well as microscopy (STM/STS) techniques.

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