Electronic and optical properties of SbSI: Application of density functional theory

In this dissertation, the electronic and optical properties of ferroelectric-semiconductor SbSI crystal have been investigated by using the density functional theory (DFT) and ab-initio pseudopotential method within the local density approximation (LDA) in a wide temperature region include paraelectric and ferroelectric phases. It has been shown that SbSI crystal has an indirect band gap in both phases (1.45 eV and 1.49 eV in the paraelectric and ferroelectric phases, respectively) and that the smallest direct band gap (1.56 eV and 1.58 eV in the paraelectric and ferroelectric phases, respectively) is at the S high symmetry point of the Brillouin zone. Furthermore, it is shown that the phese transition, from paraelectric phase to ferroelectric phase (the transition temperature is about 22 ), does not change the nature of the band gap. The total density of states (DOS) of SbSI crystal have been calculated in the paraelectric and ferroelectric phases. The linear photon-energy dependent dielectric functions and some optical constants such as the absorption coefficient, extinction coefficient, refractive index, energy-loss function, reflectivity and optical conductivity are calculated in both phases. The effective number of valence electrons and the effective optical dielectric constant are also calculated for both phases. Moreover, some components of second-order photon-energy dependent susceptibility tensor of SbSI crystal are calculated in the ferroelectric phase. Co Key Words: SbSI, ferroelectric-semiconductor, electronic structure, optical properties, density functional theory.