Spectroscopic ellipsometry and raman spectroscopy of Bi1-xSbxTeI solid solutions with x≤0.1

Abstract

Spectroscopic ellipsometry supported by reflectivity measurements and Raman spectroscopy are applied at room temperature to n-type Rashba semiconductors BiTeI and Bi$_{1-x}$Sb$_{x}$TeI with nominal compositions x = 0.05 and 0.1. Complementary four probe Hall measurements are made using standard ac technique and the concentration of free carriers is determined. The Raman spectra, including TO-LO resonances are found to occupy a frequency range below 250 cm$^{-1}$ in the studied materials. The pseudodielectric function, retrieved at different angles of incidence is analyzed focusing on the free carrier absorption (intraband transitions) and the optical transitions between the Rashba-split branches of the conduction band (intersubband vertical transitions). The former transitions with screened plasma frequency anchored to the zero-crossing point of the real part of the pseudodielectric function are described within a simple Drude model and the important parameters such as electron effective mass, electron mobility and high frequency dielectric constant are obtained. Anchored to the Rashba energy, the intersubband transitions obtained for each material by refining the pseudodielectric function from Drude contribution, appear in the imaginary part as a broad peak in the photon energy range between 0.15 and 0.4 eV. A noticeable red shift of this peak for Bi$_{0.9}$Sb$_{0.1}$TeI as compared to BiTeI is proposed to be a manifestation of the reduction of the spin splitting after part of Bi atoms is replaced by lighter Sb atoms.