The Atomic Structure of the Si/CoSi 2 Interface
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THE ATOMIC STRUCTURE OF THE Si/CoSi2 INTERFACE. J.H. Mazur and P. Grodzinski, Materials Science Department, University of Southern California, Los Angeles, CA 90089-0241. ABSTRACT. High resolution electron microscopy lattice images were calculated for six atomic arrangements at the CoSi 2 - (111)Si interface i.e. type A and type B, each type with Co atoms at the interface 8-, 7- and 5- fold coordinated. The results suggest that the new 8-fold coordinated variant should be easily distinguishable from the 5-fold coordinated variant, while 8- and 5-fold coordinated variants should be discernable from the 7-fold coordinated variant by the observation of a relative rigid body translation of Si and CoSi2 lattices. Unit cell size variation was found not to affect the contrast in the computed images of the interface provided that (i) the unit cells were chosen correctly i.e. to assure continuity of the periodic extension and (ii) the sampling point density was sufficient to prevent aliasing. INTRODUCTION. The atomic structure of CoSi 2 - (1ll)Si interface is of interest because the small 1.2% mismatch makes this interface a model structure for studies of properties of Schottky barriers and the origin of Fermi level pinning. The Si(l1l)/CoSi 2 interface can be found in two different orientation relationships: type A - (l1l)CoSi 2 // (111)Si, [112]CoSi 2 // [ii2]Si and, type B - (111)CoSi 2 // (111)Si, [ii2]CoSi 2 // [112]Si. In addition, two bonding configurations across the interface might exist: Si Type A and B Si and Co - Si with varying Co coordination numbers [1]. orientation-relationships can each have three different configurations of atoms across the interface: (I) Co atoms 8-fold coordinated, Si-Co bonding across the interface, (II) - Co atoms 5-fold coordinated, Si-Co bonding across the interface, (III) - Co atoms 7-fold coordinated, Si-Si bonding across the interface. Six models of the interface atomic structure considered here are shown in Fig. la. The type A orientation-relationship has been rarely found, when CoSi 2 has been grown on (111) Si, and then mostly in the form of grains [2]. However, it has been more frequently observed, when Si was grown on CoSi 2 [3,4]. Type B, in which the {f11} planes of CoSi 2 inclined to the interface are in a twinned orientation-relationship with respect to {111} Si planes inclined to the interface, has been found to be the dominant orientation-relationship during the growth of the CoSi 2 films on Si [3,4,5]. Previous discussions of the HREM contrast [4,5,6] accounted only for 5- (II) and 7-fold (III) coordination of Co at the interface i.e. type A-I, type A-III and type B-II, type B- III. In this work we have performed computer simulations of the high resolution lattice images for these cases as well as for the model involving an 8-fold coordinated Co at the interface [1]. The new atomic model for the interface (variant I) with 8-fold coordinated Co atom results in a lower number of dangling bonds than the model with 5-fold coordinated Co atoms at the interface (variant II).
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