Formation of Ge nanocrystals in Lu203 high-k dielectric and its application in non-nolatile memory device

A simple technique for the formation of Ge nanocrystals embedded in amorphous Lu₂O₃ high-k dielectric was demonstrated by pulsed laser ablation followed by rapid thermal annealing in N₂ ambient. The structure and composition of the Ge nanocrystals in the oxide matrix have been studied by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) analysis. A significant change in the structure and chemical composition of the film was obtained upon annealing. Cross-sectional and plan-view TEM images confirmed the formation of small Ge nanocrystals in amorphous Lu₂2O₃ matrix with a mean size of about 6nm in diameter and a high areal density of 7 × 10 ¹¹cm^-2. The nanocrystals are well-isolated by the amorphous Lu₂O₃ in between, with almost spherical shape which are favorable for non-volatile memory (NVM) application due to an effective charge confinement XPS measurements on the as-deposited sample indicate the existence of Ge in its oxidized state, consisting of GeOz and Ge suboxides. A spontaneous reduction of GeCb and GeO, was obtained after the annealing treatment, which provides Ge nuclei for nanocrystal formation. It is found that a low annealing temperature of 400°C is sufficient to dissociate the GeO₂ and GeO_x leading to the formation of Ge nanocrystals. The application of the nanocrystals in NVM devices was demonstrated by C-V characterization of the memory capacitor devices fabricated with Al₂O₃ control oxide layer. C-V results show a significant effect of the structure and composition of the film on the electrical performance of the device. The annealed device exhibits good memory behavior with a large memory window of 1.2V achieved with a low operation voltage.