Surface electronic structure, relaxations and thermodynamic energies of (100), (110) and (111) surfaces of Mg2Si: A first-principles theoretical study

Abstract

Mg2Si is an important semiconducting silicide with several promising applications in photovoltaics, thermoelectrics, and optoelectronics. In this article, we perform a comprehensive density functional study of surface electronic structure, formation of localized surface states and their influence on relaxation and thermodynamic energies of (100), (110) and (111) surfaces of Mg2Si. The Tran–Blaha (TB09) meta-GGA exchange-correlation (xc) functional is used in order to correctly describe the surface electronic structures and the band gaps. The band gap of bulk Mg2Si computed using TB09 xc-functional is found to be 0.71 eV in excellent agreement with reported experimental values of 0.65–0.74 eV. Mg2Si(100) surfaces are found to be semiconducting in contrast to previous studies wherein these surfaces were reported as metallic with zero band gaps computed using local density approximation (LDA). The surface band gap is found to be 0.32 eV for Mg-terminated (100)-(1 × 1) surface whereas it vanishes for Si-termination. However, reconstructed Si-terminated (100)-(2 × 1) surface is found to be semiconducting with band gap ∼0.42 eV. The band gap for (110) surface is computed to be 0.73 eV. For (111) orientation, three different terminations are considered and are found to be semiconducting. Localized surface states are formed near valence band maximum (VBM) extending in the band gap for both (100) and (110) surfaces. In addition, localized surface gap states are also formed in the gap at ∼7 eV below the VBM for Si-terminated (100) surfaces. These localized gap states are expected to have important implications for relaxations, reconstructions and thermodynamic energies of Mg2Si surfaces. In case of (100) surfaces, interlayer relaxation is found to be significantly large for Si termination as compared to that for Mg termination. The surface energy is found to be largest for Si-terminated (100)-(1 × 1) surface with magnitude ∼2.0 J/m2. The reconstructed Si-terminated (100)-(2 × 1) surface is found to be lower in energy by ∼0.2 J/m2 than that of (100)-(1 × 1) surface. The surface energy is found to be lowest at ∼0.7 J/m2 for (111) orientations.