Formation of Ge Nanocrystals in Lu2O3 High-k Dielectric and its Application in NonVolatile Memory Device
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Formation of Ge Nanocrystals in Lu2O3 High-k Dielectric and its Application in NonVolatile Memory Device Mei Yin Chan1,2, Pooi See Lee1, and Vincent Ho2 1 School of Materials Science and Engineering, Nanyang Tehcnological University, Block N4.1, Nanyang Avenue, Singapore, 639798, Singapore 2 Chartered Semiconductor Manufacturing Ltd, 60 Woodlands Ind. Park D, Street 2, Singapore, 738406, Singapore
ABSTRACT 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 Lu2O3 matrix with a mean size of about 6nm in diameter and a high areal density of 7 x 1011cm-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 GeO2 and Ge suboxides. A spontaneous reduction of GeO2 and GeOx was obtained after the annealing treatment, which provides Ge nuclei for nanocrystal formation. It is found that a low annealing temperature of 400oC is sufficient to dissociate the GeO2 and GeOx 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. INTRODUCTION Nanocrystals embedded in dielectric matrix have been extensively investigated due to their interesting physical phenomena and potential applications in optoelectronic and nanoelectronic devices. Recently, metal-oxide-semiconductor field-effect transistor (MOSFET) memory structures utilizing semiconductor nanocrystals as discrete charge storage nodes have attracted much attention as a potential candidate to replace the conventional floating-gate memory devices for better scaling capabilities. Although semiconductor nanocrystal-based memory devices have been shown to exhibit promising device characteristics, even the state-of-the-art devices with SiO2 tunneling barrier suffer from the trade-off between the programming efficiency and data retention characteristics [1,2]. Hence recent efforts have focused on the utilization of high-k dielectrics in nanocrystal memory devices to achieve both good retention
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