Motoharu Imai

1. High pressure is a powerful tool in materials science because the crystal structure, which determines the physical properties of a compound, can be manipulated by an external field. During the s...

Barium disilicide (BaSi ₂ ) is a Zintl compound attracting growing interest as a non-toxic, Earth-abundant material for thin-film solar cells. However, its photovoltaic performance is strongly influenced by intrinsic and extrinsic (mainly oxygen-based) defects. This review summarizes recent advances in the characterization of these defects using photoluminescence (PL), electron paramagnetic resonance (EPR), and density functional theory (DFT). These methods reveal how different growth conditions (Si-rich versus Ba-rich) dictate the dominant defect types and their spectroscopic signatures. They also reveal the complex interplay between complex oxygen-containing defects, H passivation, and B or As doping. Overall, PL, EPR, and DFT can be used in synergy, providing a powerful framework to identify and model detrimental defects in BaSi ₂ and characterize their passivation, paving the way for improved device performance in sustainable photovoltaics.

The electronic states of Mg ₂ Si, BaSi ₂ , and β -FeSi ₂ , considering chemical bonding, are analyzed based on the results of first-principles calculation of their density of states, crystal orbital bond indices, and Löwdin charges. Band gap ( E _g ) formation depends on the material: In Mg ₂ Si, the E _g forms between the Si 3p and Mg 3s states; in BaSi ₂ , the E _g forms between the Si 3p–Si 3p bonding states and a combination of the Si 3p–Si 3p antibonding and Ba 5d and 6s states; and in β -FeSi ₂ , the E _g forms between the Fe 3d nonbonding and Si 3p–Fe 3d antibonding states.

We thoroughly investigated the Raman peaks of Sb- or Bi-doped Mg2Si single crystals grown by the vertical Bridgman techniques and clarified the relationship between the Raman peak shift and lattice strain using first-principles calculations. Rietveld analysis of powder x-ray diffraction revealed that the lattice constant increased monotonically with Sb and Bi doping amounts. Raman spectroscopy demonstrated that the F2g mode of Mg2Si (near 258 cm−1) shifted to lower wavenumbers with increasing doping amounts. This peak shift was mainly attributed to the lattice strain, which was revealed by the first-principles calculations. These findings provide the usefulness of Raman spectroscopy for Mg2Si substrates.

The electronic properties and chemical bonding of semiconducting 3d transition-metal (TM) silicides—CrSi₂, Mn₄Si₇, and β-FeSi₂—were investigated using first-principles calculations. The density of states (DOS), orbital-projected DOS, and crystal orbital bond indices (COBIs) revealed that the electronic states of these silicides consisted of Si 3s states, bonding states of Si 3p and TM 3d orbitals, nonbonding states of TM 3d orbitals, antibonding states of Si 3p and TM 3d orbitals, and a bandgap (E _g) formed between the TM 3d nonbonding states and Si 3p-TM 3d antibonding states. The Löwdin charges and integrated COBI values indicate minimal charge transfer between the TM and Si atoms and suggest that the TM–Si interaction has a delocalized nature, similar to metallic bonding. Additionally, the E _g calculated using the generalized gradient approximation is comparable to the E _g determined experimentally for these silicides.