Growth and properties of doped silicon nanocrystals

International audienceSi nanocrystals (Si-NCs) embedded in dielectric matrices have been extensively studied in the last decades owing to their electrical and optical properties. Provide their size is lower than the Bohr radius of the exciton in bulk silicon, quantum confinement of carriers produces strong changes of their electronic band structure and then of their properties. In addition, associated with optically active dopants in their vicinity, coupling interactions can induce in efficient optical emission. However, to promote their implementation in the fabrication of optoelectronic or electronic devices, a deep understanding of the electrical doping of Si-NCs is mandatory. Both theoretical and experimental studies have shown the possibility to incorporate impurities such as boron or phosphorus (P) in Si-NCs. However, several fundamental issues are still under debate. In particular, because of the strong experimental difficulty to analyze the presence of dopants in nanocrystals embedded in an insulating matrix, the exact location of the impurities with respect to the core and the surface need to be analyzed in depth. Moreover the ability of the incorporated impurities to produce free carrier is still an open question. In this work, we investigate the incorporation of phosphorus atoms in Si-NCs embedded in a silica matrix. Doped SiO1.5 thin films and SiO/SiO2 multilayers were prepared by evaporation under high vacuum with a wide range of concentration. The films were prepared by co-evaporation of both SiO and SiO2 from e-beam guns while phosphorus was supplied by a GaP decomposition source. Samples were annealed at 1100 °C to promote the formation of Si-NCs. The structural properties were studied by means of X-ray diffraction, Raman and Fourier transform infrared absorption. The investigation at the nanoscale was carried out by scanning transmission electron microscopy and atom probe tomography. Optical properties were analyzed by photoluminescence spectroscopy. The structural and optical properties will be discussed as a function of the P concentration. For the high P content, we provide clear experimental evidence of the formation of SiP2 nanoparticles crystallizing in an orthorhombic structure. For P concentration below around 1 at.%, optically active Si-NCs are obtained. Their optical emission at around 800 nm, induced by the quantum confinement, is a decreasing function of the P content. Another contribution located around 1200 nm is discussed in relation with the presence of active P dopants in Si-NC