Effects of Ge-related storage centers formation in Al2O3 enhancing the performance of floating gate memories

Abstract

In this paper, we report studies on Al2O3/Ge/Al2O3 trilayer memory structures deposited by magnetron sputtering at room temperature on p-Si substrates coated with 3 nm SiO2. The changes of the structure, morphology and memory properties induced by rapid thermal annealing (RTA) in a broad temperature range 550–900 °C have been carefully investigated.High resolution transmission electron microscopy (HRTEM) revealed the existence of distinct RTA effects for different temperature ranges, in correlation with memory properties measured on Al/Al2O3/Ge/Al2O3/SiO2/p-Si/Al devices. Thus, at temperatures smaller than 650 °C, Ge diffuses into adjacent Al2O3, the layers remaining amorphous. The memory window increases from as-deposited samples to those annealed at 600 °C reaching the maximum of 5.4 V. After RTA at 700 °C, Ge nanocrystals (NCs) in intermediate Ge layer and Ge-rich amorphous nanoparticles in Al2O3 tunnel oxide are formed. Increasing RTA temperature to 800 and 900 °C, Ge NCs are no longer formed due to Ge strong diffusion. Instead, Ge-rich mixed GeAl oxide NCs of unknown crystalline structure are evidenced by HRTEM. The memory window continuously decreases with annealing temperature in the range 650–900 °C. The ON (OFF) charge loss of only 11% (9.8%) was found by extrapolation to 10 years.