Ge nanocrystals in lanthanide-based Lu_2O_3 high-k dielectric for nonvolatile memory applications

摘要

Ge nanocrystals embedded in lanthanide-based high-k dielectric (amorphous Lu_2O_3 in this work) were formed using pulsed laser deposition followed by rapid thermal annealing in N_2 ambient. The formation and evolution of the Ge nanocrystals have been studied using transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) in conjunction with depth profiling, and secondary ion mass spectroscopy (SIMS) analysis. Plan-view TEM images indicated that the formation of nanocrystals was first initiated during the deposition process. The annealing treatment significantly enhanced the nucleation of Ge nanocrystals, resulting in a high areal density of 7 × 10~(11) cm~(-2) Ge nanocrystals with a mean size of about 6 nm in diameter in the amorphous Lu_2O_3 matrix. XPS depth profile analysis revealed that Ge nanocrystals were predominantly formed from the precipitation of Ge nuclei from the oxide phase. A low annealing temperature of 400 ℃ was sufficient to dissociate the GeO_2 and GeO_x leading to the formation of Ge nanocrystals. An accumulation of Ge species close to the upper Ge/Lu_2O_3 interface was observed from XPS and SIMS depth profile analysis. Different charge storage behaviors observed from the memory capacitor devices before and after annealing could be correlated to the changes in structure and composition of the film. The memory capacitor device fabricated from the annealed sample showed efficient charge storage effect under a low operation voltage without significant initial charge decay.