Conductive CaSi2 transparent in the near infra-red range

Abstract

The methods of heteroepitaxial growth of Si/CaSi2/Si(111) double heterostructures (DHS) at 500 °C have been developed. Thin CaSi2 layers with the thicknesses of 14-40 nm have been successfully embedded in the silicon matrix. The hR6-CaSi2(001)||Si(111) with hR6-CaSi2[100]||Si[110] epitaxial relationship has been conserved for the embedded CaSi2 layer regardless of its thickness and the Si overgrowth mode (molecular beam epitaxy or solid phase epitaxy). The embedded CaSi2 layers are characterized by the lattice parameter distortion of about ±4% due to the difference in the thermal expansion coefficients of the silicide and silicon. Two types of Si overgrowth atop CaSi2(001) planes have been observed: (i) {111}-twinned Si crystals were found onto the CaSi2(001) surface in the DHS with CaSi2 thickness of 32-40 nm, which have preserved the {111} planes parallel to the Si(111) ones of the substrate; (ii) a polycrystalline twinned Si capping layer with a variable thickness has been formed in the samples with the smallest CaSi2 thickness (14-16 nm). Experimentally determined optical functions for the CaSi2 layer embedded in the silicon matrix have shown the presence of degenerate semiconducting properties with strong absorbance at the photon energies higher than 2.3 eV and small contribution from the free carrier absorption at 0.4-1.2 eV. Ab initio calculations within the generalized gradient approximation and screened hybrid functional of the hR-6 CaSi2 bulk with and without lattice distortion (by ±3%) have demonstrated the metal or gapless semiconductor energy band structure, because the Fermi level crosses several bands also assuming a huge free carrier concentration. The low-temperature Hall measurements and magnetoresistance measurements have proved that CaSi2 films on silicon are a gapless semiconductor with two types of carrier “pockets” (holes and electrons) that determine the resulting conductivity, concentration and mobility as a function of the Fermi level shift with the temperature increase. Mechanisms of the experimentally observed optical transparency of CaSi2 in the infra-red range are discussed.