Microstructure and Chemistry of Nonstoichiometric (Ba,Sr)TiO 3 Thin Films Deposited by Metalorganic Chemical Vapor Depos
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Richard D. Leapman Bioengineering and Physical Science Program, Office of Research Services, National Institutes of Health, Bethesda, Maryland 20892
Debra L. Kaiser Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 27 December 1999; accepted 3 April 2000)
The microstructure and chemistry of (Ba,Sr)TiO3 thin films deposited on Pt/SiO2/Si substrates by metalorganic chemical vapor deposition were studied using highresolution transmission electron microscopy and quantitative spectrum imaging in electron energy loss spectroscopy. The grain boundaries in all films with overall Ti content ranging from 50.7% to 53.4% exhibit a significant increase in Ti/Ba ratio as compared to the grain interiors. The results suggest that the deviations of Ti/(Ba + Sr) ratio from the stoichiometric value of unity are accommodated by the creation of Ba/Sr vacancies, which segregate to the grain boundary regions. The films with Ti contents equal to or greater than 52% additionally contained an amorphous Ti-rich phase at some grain boundaries and multiple grain junctions; the amount of this phase increases with increasing overall Ti content. The analysis indicates that the amorphous phase can only partially account for the significant drop in dielectric permittivity accompanying increases in the Ti/(Ba + Sr) ratio. Recently, much attention has been focused on the fabrication of polycrystalline thin films of barium strontium titanate (BST).1–7 This material can offer a high dielectric permittivity at a relatively large film thickness in dynamic random-access memories,6–8 and has properties of interest for tunable microwave devices.7 Deposition of the BST films with an excess of Ti has been found to be necessary to achieve acceptable leakage currents and dielectric lifetimes,9 and to obtain reproducible, stable, and smooth film morphology. However, deviation of the Ti/ (Ba + Sr) ratio from the stoichiometric value of unity has an adverse effect on dielectric permittivity of the BST films.10 For example, the dielectric constant of 40-nmthick films decreases by about 60% when the Ti content is increased from 51 to 53.4 at.%.10 This effect is even more pronounced for larger film thicknesses. Despite this dramatic effect of stoichiometry on the dielectric properties, the exact mechanism for accommodation of excess Ti in these films still has not been determined. Recently, structural imaging in a high-resolution transmission electron microscope (HRTEM) and high-spatial resolution electron energy loss spectroscopy (EELS) were applied to study the microstructure and chemistry of BST films deposited on Pt/SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD).11,12 Stemmer et al.11 reported a systematic increase in the Ti/O ratio at the grain boundaries of a film containing 53.4% Ti, while J. Mater. Res., Vol. 15, No. 7, Jul 2000
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no such increase was detected for the specimen with 50.7% Ti. Their analysis of the energy loss n
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