Diffusion of Sr, Bi, and Ta in amorphous SiO_2

摘要

Diffusion of Sr, Bi, and Ta in SiO_2 was studied to determine diffusion coefficients and diffusion mechanisms of these elements in the amorphous material. Technological motivation is the introduction of strontium bismuth tantalate (SBT) as ferroelectric material for nonvolatile memories. Diffusion from an SBT layer into a thin layer of SiO_2 was promoted by annealing at 800 ℃ for times ranging from 1 to 24 h. Concentration profiles of the diffusing elements in the SiO_2 layer were recorded by secondary ion mass spectroscopy (SIMS). The measured profiles were compared to profiles derived from calculations for different diffusion mechanisms. The theoretical profile best matching the form of the measured profile was fitted to the experimental data, yielding information about the diffusion mechanism and the diffusion coefficient. The diffusion coefficient of Sr is in the range of 10~(-15) cm~2/s at 800 ℃ and 10~(-14) cm~2/s for Bi. It was found out that Sr and Bi react with vacancies in the SiO_2, forming fast-moving complexes. Tantalum shows no measurable diffusion in this time and temperature range. No information is available about the exact nature of the different Sr and Bi species, since SIMS can only detect elements but not their binding situation. Only indirect indications lead to the theory that vacancies play a role in the transformation from slow- to fast-moving species. Theoretical calculations of the behavior of the elements in SiO_2 may help to solve this problem.