Raman spectra of Si nanocrystals under high pressure: Metallization and solid state amorphization

Abstract

We have observed and identified two crystalline peaks at ∼270 cm−1 and ∼400 cm−1 in nanocrystalline silicon during both compression and decompression. We attribute the first peak to the orthorhombic Imma phase (Si-XI) and the second one to the TO mode of the silicon metallic β-Sn phase (Si-II). Also, in the process of decompression, we observed a first-order-like transition from a highly coordinated metallic crystalline β-Sn phase to the normal tetrahedral LDA phase of the amorphous semiconductor. When the pressure is completely relieved, the powder regains its initial crystalline structure, which suggests that the amorphous phase has retained some structural features of the parent crystalline phase.