Electronic and optical properties of bulk crystals of semiconducting orthorhombic BaSi2 prepared by the vertical Bridgman method

BaSi2 crystals were grown from a near-stoichiometric melt containing mixtures of Ba (99.9%) and Si (99.99999%) with Ba:Si at.% ratios of 36:64 (Ba-rich) and 30:70 (Si-rich) by the vertical Bridgman growth method. X-ray diffraction analysis revealed that the grown crystals were semiconducting orthorhombic BaSi2. The samples produced were polycrystalline consisting of single crystal grains. Optical absorption measurements were carried out to identify the different types of optical transition at room temperature. All the bulk orthorhombic BaSi2 grown exhibited an indirect transition with an indirect band-gap energy (Egindir) of 1.13 eV. Emissions from photoluminescence measurements at 15 K, that seem to be due to indirect transitions, revealed that there were impurities and crystal defects. To understand the experimental results, we calculated the electronic-band structure from first principles using a local density approximation (LDA) in the all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculation is consistent with the experimental indirect band gap results giving an indirect band gap of 0.72 eV and a direct gap of 0.81 eV. The band gap arises from the Si 3p orbitals, while Ba 5d orbitals are strongly hybridized with the Si 3p orbitals in the conduction band.