Formation of SiP₂ Nanocrystals Embedded in SiO₂ from Phosphorus-Rich SiO1.5 Thin Films

We investigate the structural, vibrational, and optical properties of phosphorus-rich SiO1.5 thin films annealed at 1100 °C. For phosphorus (P) contents larger than 3 atom %, high-resolution transmission electron microscopy characterizations reveal the presence of both spherical-shaped SiP2 nanoparticles crystallizing in an orthorhombic structure and bumps in epitaxy with the underlying Si substrate. Energy-dispersive spectroscopy measurements confirm the SiP2 stoichiometry. Moreover, electron energy loss spectroscopy characterizations allow us to determine the exact location of P and Si atoms. Apart from SiP2 nanoparticles, P atoms are found to be located in the bumps and in the Si substrate to a level of 1 atom %, which is explained by P diffusion during annealing. The vibrational properties determined by Raman spectroscopy are found to be in excellent agreement with density functional theory calculations of the vibration modes for the SiP2 alloy. Finally, the quenching of photoluminescence with an increasing P content is explained on the basis of structural data.