Microstructural effect of gadolinium oxide nanocrystals upon annealing on electrical properties of memory devices

The microstructure evolution of sputtered gadolinium oxide nanocrystal (NC) memory devices upon annealing has been characterized in detail by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). TEM results indicate that the as-deposited film is composed of metallic Gd clusters embedded in an amorphous Gd _xO _y matrix. The Gd clusters undergo phase transformation to oxide NCs upon annealing, reaching a maximum density of 7.9-9.1 × 10 ¹¹ cm ^{- 2} at 850°C, which is consistent with the largest memory window width. Upon annealing at even higher temperature, TEM diffraction patterns and XPS composition profiles indicate apparent Si diffusion into the NC layer, probably from the SiO ₂ tunneling oxide or the Si substrate, leading to the formation of gadolinium silicate NCs. The presence of silicate NCs gradually deteriorates the device performance due to the reduction of barrier confinement for stored charges, although the dot density is only marginally decreased. The results suggest that the optimum memory device performance is dominated by not only the most considered size and density of NCs, but also the composition and phase inside.