Formation of Ge Nanocrystals in Lu2O3 High-k Dielectric and its Application in Non-Volatile Memory Device

Abstract

A simple technique for the formation of Ge nanocrystals embedded in amorphous Lu 2 O 3 high-k dielectric was demonstrated by pulsed laser ablation followed by rapid thermal annealing in N 2 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 Lu 2 O 3 matrix with a mean size of about 6nm in diameter and a high areal density of 7 x 10 11 cm -2 . The nanocrystals are well-isolated by the amorphous Lu 2 O 3 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 GeO 2 and Ge suboxides. A spontaneous reduction of GeO 2 and GeO x was obtained after the annealing treatment, which provides Ge nuclei for nanocrystal formation. It is found that a low annealing temperature of 400 o C is sufficient to dissociate the GeO 2 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 Al 2 O 3 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.

A Review on Ge Nanocrystals Embedded in SiO2 and High‐k Dielectrics

“…Ge‐rich Lu 2 O 3 films were deposited by PLD . The samples were annealed at 400 °C to activate the phase separation process, which leads to the formation of Ge nanocrystals within an amorphous matrix . It is worth to note that the crystallization temperature of the Ge nanoparticles is much smaller in Lu 2 O 3 than in other high‐k matrices.…”

“…Ge nanocrystals in Lu 2 O 3 were investigated for the use in non‐volatile memory devices and for optical applications . The reported memory window of 1.2 V is relatively small .…”

“…Ge nanocrystals in Lu 2 O 3 were investigated for the use in non‐volatile memory devices and for optical applications . The reported memory window of 1.2 V is relatively small . A PL band can be observed in the visible range at around 2.5 eV (pure Ge nanocrystals) or 2.7 eV (Ge‐GeO x core‐shell nanocrystals), which is attributed to interface defects …”