Suppression of Ni Silicide Formation by Se Passivation of Si(001)
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Suppression of Ni Silicide Formation by Se Passivation of Si(001) Janadass Shanmugam, Michael Coviello, Darshak Udeshi, Wiley P. Kirk, Meng Tao NanoFAB Center and Department of Electrical Engineering, University of Texas at Arlington, Arlington, TX 76019 USA Abstract Valence mending of a semiconductor surface, such as the Se-passivated Si(001) surface, improves the chemical and thermal stability of the surface as compared to the bare Si(001) surface. In this paper, we report the suppression of Ni silicide formation between Ni and Si(001) through monolayer passivation of Si(001) by Se. Ni was deposited on both Se-passivated and bare Si(001) surfaces. The samples were annealed at temperatures from 400°C to 700°C. Cross-sectional TEM (Transmission Electron Microscopy) revealed that Ni on bare samples reacted with Si at 400°C and formed silicide, whereas Ni on Se-passivated samples did not react with Si at 500°C. Surface composition analysis by XPS (X-Ray Photoelectron Spectroscopy) showed pure Ni surface on Se-passivated samples annealed at 400°C and 500°C, but silicide surface on bare samples annealed at the same temperatures. Hence, Se passivation suppresses the formation of Ni silicide on the Si(001) surface by over 100°C as compared to the bare Si(001) surface. These results may have important implications in source/drain engineering in sub-100 nm Si CMOS (Complementary Metal Oxide Semiconductor) devices. 1. Introduction Dangling bonds on the Si(001) surface react readily, making the surface chemically unstable. As shown in Fig. 1(a), each surface atom on Si(001) has a dangling bond and shares a dimer bond with an adjacent atom. When Se is deposited on this surface, the dangling bonds are eliminated and a surface structure similar to the bulk Si crystal structure is formed on Si(001). This concept of eliminating dangling bonds on Si(001) is called valence mending and is proposed by Kaxiras [1]. The resultant surface structure with Se passivation is shown in Fig. 1(b), which is thought to be more stable than the bare Si(001) surface [2]. In this paper, we report our findings on interfacial reactions between Ni and bare Si(001), and between Ni and Se passivated Si(001). XPS was used to study the surface composition of Ni silicide formation on both Se passivated Si(001) and bare Si(001). TEM was used to examine the interface between Ni and Se passivated Si(001), and [011]
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Fig. 1. Atomic structures of (a) reconstructed Si(001) surface and (b) Se passivated Si(001) surface.
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between Ni and bare Si(001). Both p-type and n-type Si(001) wafers were investigated. It was observed that Se passivation of Si(001) suppresses Ni silicide formation at above 500°C, while the formation of Ni silicide on bare Si(001) is well known [3]. At 200°C, Ni2Si begins to form at the interface. Between 350°C and 750°C, the stable phase between Ni and Si is NiSi. At higher temperatures, NiSi2 forms at the interface. Our findings may have significant implications in source/drain engineering in sub-100 nm Si CMOS
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