Studies of Si Surface Chemistry and Epitaxy Using Scanning Tunneling Microscopy and Spectroscopy
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STUDIES OF Si SURFACE CHEMISTRY AND EPITAXY USING SCANNING TUNNELING MICROSCOPY AND SPECTROSCOPY PHAEDON AVOURIS AND ROBERT WOLKOW IBM Research Division, T. J. Watson Research Center, Yorktown Heights, NY 10598.
ABSTRACT We apply scanning tunneling microscopy (STM) and spectroscopy (STS) to study the reaction of NH 3 with Si(111)-(7x7), and the epitaxial growth of CaF 2 on Si(1l). By a combination of topographs and atom-resolved spectra we can follow the spatial distribution of the reaction and changes in electronic structure with atomic resolution. We find that there are strong site-selectivities for the NH 3 reaction on the 7x7 surface. We also observe the initial stages of the CaF 2 deposition and even are able to image insulating multi-layer CaF 2 films.
INTRODUCTION - WHY THE STM IS NEEDED IN SURFACE CHEMISTRY. Surface chemistry is a local phenomenon. Reaction can take place at different sites of a crystal surface or at defect sites. Different sites may interact, in the sense that reaction at one site may influence reaction at a neighboring site. Moreover, understanding chemistry implies that one can make a correlation between local electronic structure and reactivity. From the above it is clear that in studying surface chemistry ideally we need an experimental technique (or techniques) that will allow us to follow the spatial distribution of a reaction as well as the local electronic structure with atomic resolution. Conventional topographical and electronic structure techniques, however, average over an area defined by the probe (light, electron, . . .) beam. This is usually larger than -1011 atomic sites. In the following, we will try to demonstrate that STM [1] and STS can, under the appropriate conditions, be used to achieve the above goals. Specifically, we will present results on the application of STM/STS in : (A) the study of surface electronic structure and reactivity using the reaction of Si( 1I )-(7x7) with NH 3 as an example, and (B) in the study of epitaxy and thin-film growth processes using the epitaxy of CaF 2 on Si( 111) as an example.
THE STRUCTURE OF THE CLEAN Si(1 1 )-(7x7) SURFACE.
The nature of the 7x7 reconstruction has been the subject of intense study for about 30 years. Currently, the model proposed by Takayanagi et al. [2] is generally accepted. This model for the 7x7 unit cell is shown in Fig. 1 (top). There are two triangular subunits, each surrounded by nine Si dimers. In addition, there is a stacking-fault in the left triangle. On the surface there are six triply-coordinated Si atoms, (labeled A and B in Fig. 1), so-called restatoms. The top layer is composed of twelve Si adatoms (black circles), and finally, at the corners of the unit cell there are vacancies usually referred to as corner-holes. We further separate the adatoms in two groups: the ones located next to a
Mat. Res. Soc. Symp. Proc. Vol. 131. v1989 Materials Research Society
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corner-hole are termed corner-adatoms while the other six are called center-adatoms. The most important chemical effect of the reco
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