In this paper, we studied the phase-separation phenomenon of Hf0.5 Si0.5 O2 film deposited on Si O2 or sandwiched by Si O2, by x-ray photoelectron spectroscopy and transmission electron microscopy. The Hf0.5 Si0.5 O2 film underwent phase separation to form a doublet-phase Hf O2 - Hfx Si1-x O2 (x<0.5) film, and was used as a trapping layer in a metal-blocking oxide-silicon nitride-tunnel oxide-silicon-type memory structure, where the dual-phase Hf O2 - Hfx Si1-x O2 (DPHSO) film replaces the conventional silicon nitride (Si3 N4) trapping layer. The charge storage properties of the DPHSO film were investigated and compared with Hf O2 and Si3 N4. It was found that for a given electric field applied to the tunnel oxide, the programming speed of memory devices using a DPHSO or Hf O2 film as a trapping layer is faster than that using Si3 N4. This indicates the higher electron-capture efficiency of the DPHSO and Hf O2 films. In addition, the double-phase microstructure of the DPHSO film also provided better retention property than pure Hf O2.