Electrical transport properties of CoSi thin films on silicon

Shao-Pin Chiu^1*, Shi-Wei Chen¹, Cheng-Tai Chen¹, Ruey-Tay Wang¹, Juhn-Jong Lin^1,2

¹Institute of Physics, National Chiao Tung University, Hsinchu, Taiwan
²Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan

* Presenter:Shao-Pin Chiu, email:fluentbb@gmail.com

Cobalt monosilicide (CoSi) has been studied since the 1960s as a thermoelectric material. Recently, its topological nontrivial band structure has been unveiled by theoretical calculation and angle-resolved photoemission spectroscopy. Thus CoSi is classified as a new kind of topological semimetals, similar to Dirac and Weyl semimetals. Here we study how to grow epitaxial CoSi thin films and report the electrical transport properties.
After depositing patterned Co films of different thicknesses on Si substrates, a series of CoSi thin films (thickness t = 8 - 400 nm) were grown with high-vacuum annealing. X-ray diffraction (XRD), transmission electron microscopy (TEM) and temperature-dependent resistivity ρ(T) are utilized to check if the films are pure CoSi phase. Magnetoresistance (MR) and Hall effect were measured down to 1.4 K or 0.25 K with ⁴He or ³He cryostats, respectively.
Suitable annealing temperature (375 – 400°C) and sustaining time are chosen for avoiding Co residue and minimizing the formation of high-temperature phase CoSi₂. The behavior of ρ(T) is strongly thickness (t) -dependent. Thinner films (t < 50 nm) showed higher residual resistivities ρ0 and insulating ρ(T), while thicker films (t > 50 nm) exhibited metallic ρ(T) and their ρ₀ were close to bulk values (ρ₀ ~ 100 μΩ cm). The MR was positive as magnetic-field orientation perpendicular to the current, while MR changed to negative as magnetic-field orientation parallel to the current at T < 2 K. This effect is likely the manifestation of chiral anomaly in the topological semimetals. Besides, MR shows hysteresis at T < 2 K, while diamagnetism of bulk CoSi was confirmed in literature.
We have successfully grown epitaxial CoSi thin films on silicon and measured their electrical transport properties. The negative and hysteretic MR could reflect topological properties of CoSi band structure which deserve more investigation. Our result could be useful for making CoSi-related functional devices in the future.

Keywords: Topological semimetal, Dirac and Weyl semimetals, Cobalt monosilicide