Conduction Mechanisms and Thermoelectric Properties of Semimetallic CaSi and CaSi2 Films on Si(100) and Si(111) Substrates

Abstract

Nanocrystalline CaSi films with thicknesses from 80 to 130 nm were grown on high-resistance silicon substrates with orientations (111) and (100) by the methods of low-temperature (190–330°C) molecular-beam epitaxy and low-temperature (330°C) solid-phase epitaxy, for which the microstructure, phase composition, and crystal structures were studied. It is found that the polycrystalline, nanocrystalline (NC), and amorphous CaSi and CaSi₂ films are characterized by preferential contribution of holes in the range 1.4–300 K. In magnetic fields 1–4 T and at temperatures 40–100 K, a giant linear magnetoresistive effect (MRE) (to 500%) was observed for the first time in CaSi films with the contribution of another CaSi₂ phase. In CaSi₂ film containing another phase (CaSi), peaks are detected on the temperature dependences of the resistivity and the Hall coefficient that correspond to a phase transition. In addition, in this film, the transition from the positive MRE to negative MRE is observed at Т = 120–200 K. This effect is not observed in the single-phase CaSi₂ film, which corresponds to a certain reconstruction of carrier flows in a magnetic field only in the two-phase system. The study of the thermoelectric properties of CaSi and CaSi₂ films shows that the semimetallic type of the conduction in them leads to the independence of the positive Seebeck coefficient Т = 330–450 K. It is found that the maximum contribution to the Seebeck coefficient and the power factor are observed in the amorphous CaSi film in the case of the presence of some fraction of NC Ca₂Si phase. In the single-phase CaSi₂ films, the Seebeck coefficient and the power factor are halved due to an increase in the hole concentration as compared to the CaSi films.