Thickness-Dependent Optical Conductivity of CoSi Thin Film Studied by Infrared Spectroscopy

Chiral topological semimetal CoSi has attracted lots of attentions for its clean band structure with multifold fermionic excitation near the Femi level and Fermi arc surface states. For electrical transport, unconventional decrease of the resistivity of CoSi as the shrinkage in reduced dimensions, in contrast to conventional metals, has been theoretically proposed to arise from the topological surface states, offering remarkable potential for its interconnect application in silicon-based semiconductor industry. For an experimental investigation, we report here the thickness-dependent optical conductivity of highly oriented CoSi thin films using Fourier transform infrared spectroscopy. We observe clear thickness- and temperature-dependent (10 ~ 60 nm and 77 ~ 300 K) evolution of the CoSi thin films from the reflectance measurements at 5 meV ~ 2 eV. The extracted optical conductivity of the thin films exhibits a clear red shift of the optical spectral weight with an increase in thickness, particularly, higher electron density and longer scattering time for thick samples. Spectral red shifts of the interband transition with the thickness is discussed through the substrate-induced strain and/or a defect-induced shift in the Fermi level. Detailed analysis aimed to resolve the surface versus bulk contributions will be given in the presentation.