Structure Determination of the Si(111)-CaF 2 Interface
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STRUCTURE DETERMINATION OF THE Si(111)-CaF
2
INTERFACE
R.M. Tromp, W. Krakow and F.K. LeGoues IBM Research Division Thomas J. Watson Research Center P.O. Box 218 Yorktown Heights, NY 10598 USA
ABSTRACT
The structure of the Si(111)-CaF 2 interface has been determined with Medium Energy Ion Scattering and High Resolution Transmission Electron Microscopy. Methods to determine this interface structure with HRTEM are discussed.
INTRODUCTION
The Si(111)-CaF 2 interface has received much attention. CaF 2 is an insulator with a bandgap of 12.1 eV which can be grown epitaxially on Si (lattice mismatch 0.6 percent). CaF 2 is therefore an attractive dielectric in fully epitaxial three dimensional devices in Si. In a recent paper Tromp and Reuter have determined the detailed atomic structure of this interface using Medium Energy Ion Scattering (MEIS) combined with the effects of channeling and blocking [1]. The interface structure determined for a single monolayer (ML, 1ML = 7.83x10 14) atoms/cm 2 of CaF 2 adsorbed on Si(111) is shown in figure 1. We mention the following important features: 1. There is only one F atom per Ca atom at the interface. 2. Ca bonds to Si and is adsorbed on the filled threefold, or T4 site above a second layer Si atom. The second layer Si atom is fivefold coordinated. 3. There are no unpaired electrons at the interface. Ca atoms bond to Si, removing the Si dangling bond, and use their second valence electron to form a polar bond with the capping F layer. The Ca atoms are in oxidation state 1 . Mat. Res. Soc. Symp. Proc. Vol. 139. 11989 Materials Research Society
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4. The spacing of the outer Si-Si double layer is increased by 0.125 A in order to obtain a reasonable bondlength between Ca and the second layer Si atoms. The resulting bondlengths, shown in figure 1, are identical to the bondlenghts found in bulk CaSi 2 , to within experimental uncertainty. In CaSi 2 Ca atoms are sandwiched between Si double layers [2]. Each Ca atom occupies a T4 site on one double layer and a H3 (open threefold hollow) site on the other. T (top) sites do not occur in either CaSi 2 or in the structure shown in figure 1.
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Co
FIGURE CAPTIONS Figure 1 Structure of 1 monolayer of CaF 2 adsorbed on Si(111). 3One unit cell is outlined. The adsorption site and bondlengths
. Si
were determined using Medium Energy Ion Scattering (ref.1)
The object of this study is to determine if the structure of the interface between Si(111) and a much thicker CaF 2 layer is the same as shown in figure 1 for a single monolayer. We have obtained high resolution images of crosssectional samples using a Phillips 430 microscope operated at 300 kV. We found CaF 2 to be very radiation sensitive. Through focus series could therefore not be obtained. However, we were able to obtain high quality single snapshot images of the interface near Scherzer defocus, with white dots corresponding to the crystal channels. A high resolution micrograph of the Si(111)-CaF 2 interface (Fourier filtered for noise removal and contrast stretched) is sho
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