Superconductivity of CoSi2/Si(100) Heterostructures
Shao-Pin Chiu1*, Chien-Jyun Chiou2, Yi-Chia Chou2, Juhn-Jong Lin1,2, Chang C. Tsuei3
1Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
2Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
3T. J. Watson Research Center, IBM, New York, USA
* presenting author:Shao-Pin Chiu,
Cobalt disilicide (CoSi2) has been used as interlinks in the ultra-large-scale integrated circuits. The extremely low lattice mismatch (1.22%) between CoSi2 and silicon ensures the feasibility to grow epitaxial CoSi2 films on silicon substrates. We have prepared CoSi2 films by annealing patterned cobalt films on Si(100) substrates at 800℃ for 30, 60, or 90 minutes in high vacuum. The as-grown films were ~100-nm thick and well crystalline to show characteristic peaks in the grazing incident x-ray diffraction spectra. All CoSi2 films revealed similar metallic behavior on their resistivity-temperature, ρ(T), curves. Their sharp superconducting (SC) transitions of ρ(T) happened at Tc ~ 1.53 - 1.54 K. However, the 30-minute and 60-minute annealed samples showed a tiny kink on the SC transition curves. It indicates that there are more than one SC phases inside the films. From the magneto-resistances (MR) of T < Tc, we can extract the relation of upper critical field Hc2 versus temperature in either in-plane or perpendicular magnetic fields (Hc2-in(T) and Hc2⊥(T)). The Hc2(T) of our 100-nm thick CoSi2 films exhibited the behavior of two-dimensional superconductors which can be qualitatively described by Tinkham’s model. The 90-minute annealed sample represented linear Hc2⊥(T) around Tc. However, the 30- and 60-minute annealed samples showed clear upturn curve of Hc2⊥(T) near the Tc. The upturn behavior is similar to the cases in the two-gap superconductors, and superconductor-normal metal multilayers. After analyzing Hc2⊥(T) with Werthamer-Helfand-Hohenberg theory, we found that there should be two SC phases, corresponding to different transition temperatures of Tc and Tc*. The difference between Tc and Tc* depends on annealing periods and vanishes in the 90-minute annealed samples. It implies that CoSi2 can achieve single SC phase by enough long annealing period. This result may relate to the atomic structures near the CoSi2/Si(100) interfaces.

Keywords: Cobalt disilicide, Superconductivity, magnetoresistance, upper critical field