Precipitation of gold into metastable gold silicide in silicon

We report a detailed investigation of the precipitation behavior of gold in float-zone silicon from a highly supersaturated solution. Nucleation, morphology, and crystallography as well as the decomposition of the solution were examined using high-resolution electron microscopy, selected area diffraction combined with tilting experiments, Hall-effect measurements, and energy dispersive x-ray spectroscopy. After in-diffusion of gold at 1275 °C annealing experiments were performed at 850 °C for durations ranging from 5 min up to 35 d. It is shown that gold precipitates in small spherical particles (diameter: 10–20 nm) consisting of a metastable gold silicide. By means of selected area diffraction combined with a special tilting procedure, the unit cell is proved to be orthorhombic with lattice parameters a=0.971 nm, b=0.768 nm, and c=0.703 nm. Systematic absence of reflections in several precipitate zone-axis patterns reveals the space group of the silicide to be Pnma or Pn$2_1$a. According to Hall-effect measurements the concentration of substitutional gold decreases to a few percent within 5 min annealing at 850 °C. Only a part of it has precipitated in gold silicide particles, which are found at small extrinsic stacking faults. The stacking faults represent a density of self-interstitials ${\mathrm{Si}}_{\mathit{i}}$ of about ${10}^{18}$ ${\mathrm{cm}}^{\mathrm{-}3}$, which according to control experiments is about a factor of 50 above the equilibrium concentration of ${\mathrm{Si}}_{\mathit{i}}$ at 1274 °C. As annealing proceeds the stacking faults disappear, and gold is finally found in spherical particles embedded stress-free into the silicon matrix.