Density functional theory investigations of MnSb(0001)/GaAs(111)B.

Heteroepitaxial growth of magnetic materials (e.g. ferromagnets) onto III-V semiconductors (e.g. GaAs and InP) based on spin injection methods has received great attention due to their potential applications in spintronics. However, the formation of distinct surface states, highly mismatched heterointerfaces, and the consequent lower spin polarisation of the ferromagnets have been regarded as major challenges. Therefore, comprehensive understanding the surface and interface properties to select the better magnetic materials for growth are of great importance. In recent studies, MnSb thin films have been successfully grown on GaAs substrates by molecular beam epitaxy (MBE). During the thin film growth, significant surface variations including surface reconstruction, atomic diffusion, and the formation of secondary phases [cubic MnSb (c-MnSb)] have been observed. Hence, the formation of the surface defects (atomic segregation/surface reconstruction) and secondary phases for the niccolite MnSb(n-MnSb)/GaAs heterostructures are presented in this thesis using first-principles calculations. Experimentally, a number of surface reconstructions of n-MnSb(0001) have been found after the growth of MnSb thin films on GaAs(111)B substrates, wherein the (2 × 2) surface reconstruction is prevalent. Therefore, quantitative surface structure of MnSb(0001) - (2 × 2) determination is presented, using density functional theory (DFT) and low energy electron diffraction (LEED) data fitting methods. Atomic out-diffusion and surface segregation of Ga has been found in the MnSb(0001)/GaAs(111) system by X-ray photoemission spectroscopy (XPS) and medium energy ion scattering (MEIS) measurements. DFT calculations are used to explain these results. In addition, an optimal concentration of the diffused Ga atoms has been shown by comparing the formation energy/surface energy change. Modifications in the structural and electronic properties [atomic bond length, atomic bond angle, charge density difference, and density of states (DOS)] of the Ga-in-MnSb(0001) system are discussed. Finally, the formation mechanism of c-MnSb polymorph through thermodynamic phase transition caused by Ga segregation in MnSb(0001)/GaAs(111)B has been presented. Atomic relaxation of the model structures driven by Ga substitution into Mn sites at the subsurface layers provides critical insights for the evolution of secondary c-MnSb polymorph. Further theoretical prediction of the new system (MnSb/GaSb) with the cubic polymorph is also provided. All the theoretical models, calculations, and results presented give a crucial guideline for epitaxial growth of half-metallic c-MnSb for the next generation of magnetic applications