Ab-initio study of the properties of the 3D SrS-based mono-doped and co-doped compounds. A comparative study of the 2D and 3D mono-doped compounds.

Abstract

In this dissertation, we aim to predict the evolution of the various physical properties of the binary semiconductor SrS after its doping with iron (Fe) and sp-type metals. In this context of study, we consider ternary compounds (SrS mono-doped with iron) and quaternary compounds (SrS co-doped with iron and sp-type alkali metals), some of which are studied in volume (3D) and others in monolayer form (2D). Properties calculations are based on the Full Potential Linearized Augmented Plane Wave (FP-LAPW) method and on the PBE and PBE + mBJ approximations implemented in the WIEN2K simulation program. The work involves assessing the impact of elements added to the SrS semiconductor host matrix on the properties of final compounds produced by doping and co-doping. Initially, we investigated the structural, mechanical, electronic and magnetic properties of single-doped Sr1-xFexS compounds in the rock-salt structure, considering the following Fe concentrations: 0%, 12.5%, 25%, 50% and 75%. The study is carried out at 3D and the main objective is to investigate new potential half-metallic ferromagnets (HMF) for spintronic applications. The results obtained show that the compounds containing 12.5%, 25% and 50% iron are half-metallic ferromagnetswith a total magnetic moment value equal to 4µB, and are thermodynamically and mechanically stable; nevertheless, the iron-rich compound (75% Fe) is metallic. We then turned our attention to co-doping the SrS matrix to produce quaternary compounds. The latter are realized by doping the ternary compound (12.5% Fe) with alkali metals (Li, Na and K). The aim is to see the effect of the sp metals on the properties of the ternary compounds. The study of these new compounds, also in 3D, is based on density functional theory (DFT) and semi-classical Boltzmann theory (BT), and covers structural, electronic, magnetic, optical and thermoelectric properties. The results obtained show that the incorporation of alkali metals into the SrS: 12.5% Fe compound renders them half-semiconducting (HSC) with narrower energy gap values than those of HMF ternary compound (SrS: Fe). The Curie temperature (Tc) values obtained are higherthan room temperature. In addition, the magnetic moment of quaternary compounds is higher than that of ternary compounds, having a value of 5µB. Examination of the optical properties revealed a shift of the spectra towards the visible region, accompanied by a broadening of the compounds' absorption band. Investigation of the thermoelectric properties of p-type compounds showed that their figure of merit (ZT) values are greater than unity at 1200K, underlining their exceptional transport efficiency and making them highly promising candidates for optoelectronics and high-temperature thermoelectric applications. The 2D study focused on the structural, electronic and magnetic properties of ternary compounds. The calculations are based on a plane-wave pseudopotential (PAW) method using the PBE and PBE+HSE06 functionals, integrated into the VASP simulation software. The effect of dimensionality reduction was mainly observed at the level of electronic structures, where the character of ternary compounds becomes HSC instead of HMF observed in their 3D counterparts. This result has a positive impact on the optical and transport properties of ternary compounds. Under these conditions, the total magnetic moment resulting from p-d hybridization is 4µB per unit cell.