Germanium segregation in the Co/SiGe/Si(001) thin film system
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Germanium segregation in the Co/SiGe/Si(001) thin film system Peter T. Goeller, Boyan I. Boyanov, Dale E. Sayers, and Robert J. Nemanicha) Department of Physics and Department of Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695
Alline F. Myers and Eric B. Steel Microanalysis & Surface Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 25 May 1999; accepted 9 August 1999)
Cobalt disilicide contacts to silicon–germanium alloys were formed by direct deposition of pure cobalt metal onto silicon–germanium films on Si(001) substrates. Segregation of germanium was observed during the reaction of the cobalt with the silicon–germanium alloy. The nature of the Ge segregation was studied by transmission electron microscopy, energy dispersive spectroscopy, and x-ray diffraction. In the case of cobalt films deposited onto strained silicon–germanium films, the Ge segregation was discovered to be in the form of Ge-enriched Si1−xGex regions found at the surface of the film surrounding CoSi and CoSi2 grains. In the case of cobalt films deposited onto relaxed silicon–germanium films, the Ge segregation was dependent on formation of CoSi2. In samples annealed below 800 °C, where CoSi was the dominant silicide phase, the Ge segregation was similar in form to the strained Si1−xGex case. In samples annealed above 800 °C, where CoSi2 was the dominant silicide phase, the Ge segregation was also in the form of tetrahedron-shaped, Ge-enriched, silicon–germanium precipitates, which formed at the substrate/silicon– germanium film interface and grew into the Si substrate. A possible mechanism for the formation of these precipitates is presented based on vacancy generation during the silicidation reaction coupled with an increased driving force for Ge diffusion due to silicon depletion in the alloy layer.
I. INTRODUCTION
Silicide/Si1−xGex /Si(001) heterostructures are promising for use in devices such as the heterojunction bipolar transistor and are interesting candidates for devices such as improved infrared detectors with high cutoff wavelengths.1 Perhaps even more significant, Si1−xGex /Si heterostructures are being considered for raised source and drain structures in integrated circuits.2–5 Many of these structures may require electrical contacts to the Si1−xGex layer. There have been several efforts to evaluate a number of metal silicide contact systems to Si1−xGex alloys by using metals such as Co,6–14 Ni,15 Pd,16,17 Pt,16,18 Ti,8–10,19–21 W,22 and Zr.23 Due to its high crystalline quality and structural perfection of the interface on silicon, as well as its low electrical resistivity and thermal stability, CoSi2 is one of the more promising candidates for contact to Si and by a)
Address all correspondence to this author. e-mail: Robert [email protected]
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J. Mater. Res., Vol. 14, No. 11, Nov 1999 Downloaded: 17 Mar 2015
extension to SiGe layers. Direct deposi
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