Lattice Defects and Plastic Deformation of CoSi2
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LATTICE DEFECTS AND PLASTIC DEFORMATION OF COS12 M. YAMAGUCHI, Y. SHIRAI and H. INUI Department of Metal Science and Technology, Kyoto University Kyoto 606, Japan ABSTRACT Current knowledge of the properties of vacancies and the slip behavior and mechanisms of CoSi2 is reviewed based on the results of our recent studies on CoSi2. The concentration of thermal vacancies in CoSi2 is much higher than that in ordinary metals and alloys with melting points comparable with that of CoSi2. Vacancy defects are easily retained in CoSi2 even after air cooling from high temperatures. An annealing stage observed at around 310 K after electron irradiation is concluded to occur by the migration of vacancies to form secondary defects. Peculiar phenomena recently reported on CoSi2 such as an anisotropy of electrical resistivity and the climbing of dislocations at room temperature can be understood on the basis of the current knowledge of defect properties. Slip in COSi2 occurs along on {001} at low temperatures. The selection of {001} as the primary slip system in CoSi2 can be interpreted in terms of high covalency of Co-Si bonding. a dislocations have a strong tcndcncy to align along their edge orientation and are dissociated into two a / 2 partial dislocations separated by a stacking fault on {001}. {001} slip is augmented by {111} and {110} slip at high temperatures. Thermal activation analysis of deformation indicates that while deformation at low temperatures is controlled by the Peicrls mechanism, the greatly increased concentration of thermal vacancies influence the mobility of dislocations at high temperatures. INTRODUCTION AND STATUS CoSi2 is one of the transition metal disilicides which have recently received considerable interest as materials for ohmic contacts and gate electrodes to Si in VLSI devices because of their low electrical resistivity and chemical stability. In particular, CoSi2 crystallizes in a cubic (Cl type) structure resembling the structure of Si (A4 type) and is lattice matched (within 1.2%) with Si. Thus, single crystalline thin films of CoSi2 can be formed epitaxially on a silicon wafer. CoSi2 is attractive not only for such microelectronics applications but also as a high temperature structural material. Although the melting point of CoSi2 is not high (Tm=1599 K) in comparison with the disilicides of transition metals of groups IV-VI, it has low density (4.95 g/cm3 ) and excellent oxidation resistance [1]. Furthermore, CoSi2 possibly provides a sufficient number of equivalent slip systems and, hence, some ductility even at low temperatures because of its cubic structure. The first report of the deformation behavior of CoSi2 appeared in 1968. Sauer and Freise [2] examined deformation modes of single crystal and coarse-grained polycrystalline cobalt silicides, CoSi2, CoSi and Co2Si using hardness indentations and concluded that the cubic CoSi2 deformed primarily by slip on {001} planes at room temperature. More recently, there has been an increase in research activity on the mechanical properties of CoS
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