Al-Flux-Grown Reticular-Merohedral Twinned Diamond Cubic Silicon with Pseudohexagonal Symmetry and Its Optical Properties

Abstract

Silicon is a key engineering material widely used in electronics due to its excellent semiconducting properties and technological versatility. Flux-grown single-crystalline silicon exhibiting reticular-merohedral twinning was investigated using X-ray diffraction (XRD), Raman spectroscopy, differential thermal analysis (DTA), X-ray photoelectron spectroscopy (XPS), and optical absorption measurements. Reconstructed precession images displayed reduced intensity of the {h, k, 4n – 2} reflections, indicative of twinning. The diffraction data are consistent with reticular-merohedral twinning on the (111) plane involving a 60° rotation about [111], corresponding to Σ3 coincidence site lattice boundaries as confirmed by HRTEM, with a volumetric domain ratio of approximately 70:30. Powder XRD of a single twinned grain confirmed a pseudohexagonal c lattice parameter equal to 3·d111 of cubic Si. DTA revealed a latent heat of fusion consistent with literature values for monocrystalline silicon, while the melting onset was shifted by 66 °C to lower temperatures, suggesting localized melting induced by twin-related heterogeneity. Raman spectra exhibited a weak downshift of the optical phonon peak to 518 cm–1, attributed to tensile strain associated with ordered twins. XPS confirmed chemically pure elemental Si with no secondary bonding. Optical absorption spectra showed a band gap of 1.1 eV, identical to that of crystalline Si, indicating preservation of its intrinsic semiconducting properties. This finding is consistent with density functional theory calculations, which indicate that although the introduction of twinning planes into cubic silicon does not significantly alter its band structure, it leads to XRD patterns that resemble those of a hexagonal phase.