G. A. Hirata, L. López and J. McKittrick*.

Centro de Ciencias de la Materia Condensada-UNAM. Apdo. Postal 2681, Ensenada, B.C. 22860, México.

*Dept. of AMES and Materials Science Program. University of California at San Diego. La Jolla, CA 92093-0411.

Ferroelectric thin-films of Ba_1-xSr_xTiO₃ or BST were deposited by pulsed laser ablation and RF-Sputtering on RuO₂/Ta/SiO₂/c-Si and Pt/Ti/SiO₂/c-Si substrates from BST targets fabricated via the combustion synthesis technique. The films were weakly crystalline in the as-deposited condition and full crystallization was induced by annealing the films in the range 550-750º C. The x-ray diffraction patterns of the films and the targets match exactly indicating that excellent stoichiometry preservation os attained by either PLD or RF-sputtering. The BST films deposited on Pt/Ti/SiO₂/c-Si substrates presented wide cracks that were promoted during the annealing process due to the thermal expansion mismatch between the BST films (a _BST=4 x10^{-6 0}C^-1)[1,2] and the Pt(a _Pt=9 x10^{-6 0}C^-1)[3]. Smoother films showing few cracked areas were obtained on RuO₂/Ta/SiO₂/c-Si substrates. The ruthenium oxide thermal expansion coefficient is a _RuO2=5.2 x 10^{-6 0}C^-1. A cross-sectional analysis at the ferroelectric/substrate inteface showed that for the lower annealing temperature (550º C) a mixed amorphous/nanocrystalline microstructure is formed. For temperatures above 600º C a randomly oriented polycrystalline material is obtained. However, an amorphous layer of 4-6 nm still remains on the substrate, as can be seen in figure 1 which was taken from a film annealed at 650º C.

The Curie temperature of bulk BST had a sharp peak at T_c=-25º C with a maxium dielectric constant of 9300 [3], while the BST films presented a broad peak at around - 31º C (see figure 2). This shows a reduction in temperature dependence but at the expense of maximum permittivity of BST films varied in the range of 180-340.

References

[1] S.B. Desu, Mater. Res. Soc. Symp. Proc., Vol. 200, (1990) 199.

[2] G.A. Hirata, J. McKittrick, S García, J. Portelles, D. Madrigal, J.M. Siqueiros and K.Ring,

Acta Microscopica., (submitted)

[3] Landolt-Bornstein Handbook, Springer-Verlag Seires, Vol. 16, New York, (1981)