Stress-induced depolarization of (Pb, La)TiO3 ferroelectric thin films by nanoindentation

M. Algueró, A. J. Bushby, M. J. Reece, R. Poyato, J. Ricote, M. L. Calzada, L. Pardo
2001 Applied Physics Letters  
Electrical depolarization has been observed in lanthanum-modified lead titanate ferroelectric thin films stressed by nanoindentation. A spherical metallic indenter was used as a top electrode to locally pole the films and then to measure the depolarization current intensity. The current intensity had distinctive maxima at given indentation forces. These are related to the stress thresholds for the depolarization mechanism, which is probably 90°domain wall movements. Knowledge of the
more » ... of the depolarization stresses is necessary for the design of microelectromechanical systems that include a ferroelectric layer. Microelectromechanical systems ͑MEMS͒ sensors and actuators can use a range of mechanical to electrical, and vice versa, transduction mechanisms, among which the piezoelectric effect is especially suitable at high frequencies. 1 Examples of MEMS that use piezoelectric active layers are the force sensor for atomic force microscopy 2 and the ultrasonic micromotor. 3 Ferroelectric oxides with the perovskite structure, such as lead zirconate titanate 4 or lanthanummodified lead titanate ͑PTL͒, 5 are the preferred materials when high piezoelectric coefficients are required. Films of these compositions often need to be poled to obtain a significant and reproducible electrical polarization and then useful piezoelectric coefficients. 6 The stability of this electrical polarization during the device operation is a key factor in determining the reliability of the microdevice. Stresses induce depolarization by ferroelastic, non-180°, domain wall movements in ceramics, 7 but this phenomenon has not been studied for films. The stability of the polarization of a film under stress depends on whether there are ferroelastic domains in the film, and if so on the mobility of their domain walls. It has been shown that ferroelastic domains exist in ferroelectric thin films. 8 The mobility of the walls during polarization switching at electric fields above the coercive field has been questioned. 9 The walls are thought to be clamped by the planar, biaxial stress exerted by the substrate. Recent experimental studies using piezoresponse force microscopy showed that ferroelastic domain walls did not move during polarization switching for PZT epitaxial films 10 and polycrystalline lead titanate ͑PT͒ films. 11 We report here results showing that stress-induced depolarization occurs for fine grained ͑50-100 nm͒ PTL films. The experiments were accomplished with a UMIS 2000 nanoindentation system, which has been modified to allow a 100 m radius, WC-Co spherical indenter to be used as a top electrode to locally pole the films and then to measure the electrical current intensity produced by depolarization during a subsequent indentation test at the same location. 12 The results are presented for two single phase PTL films of Pb 0.88 La 0.08 TiO 3 composition, as monitored by Rutherford backscattering spectroscopy, 13 and 250 and 700 nm thickness. They were prepared by a diol-based sol-gel process with a rapid thermal annealing on Pt/TiO 2 /Si substrates. The tetragonal perovskite structure was tailored to show a strong mixed ͗001͘, ͗100͘ preferred orientation, which fixed the orientation of the 90°domain walls relative to the indentation axis to ϳ45°. Quantitative texture analysis of x-ray pole figures was used to characterize this orientation. 14 A texture index of 7.9 and 9.6 m.r.d. ͑multiple of a random distribu-tion͒ was obtained for the films of 250 and 700 nm thickness, respectively. The relative contribution to the global texture of the ͗100͘ and ͗001͘ components was similar, ϳ50%, for both films. The films were tested without being poled, and small depolarization current intensity transients were recorded ͑2-4 pC͒. Therefore, a spontaneous electrical polarization existed in the films. This is in agreement with the spontaneous piezo-and pyroelectric activity found in the films. 14 The sign of the current intensity indicated that the polarization vector pointed to the surface. Films were poled by using the spherical indenter as a top electrode. The poling was accomplished during an indentation contact with a 10 mN s Ϫ1 force rate, 500 mN maximum load, and 60 s dwell at maximum load before unloading. The electric field was switched on before mechanical contact, and maintained throughout the contact. The poling field was a train of square pulses with a nominal height of 150 kV cm Ϫ1 and a frequency of 200 Hz. This field was sufficient to saturate the induced polarization for the 700 nm film ͑E c ϭ50 kV cm Ϫ1 , P r ϭ16.6 C cm Ϫ2 ͒, but was not sufficient to do the same for the 250 nm film ͑E c ϭ136 kV cm Ϫ1 , P r ϭ16.6 C cm Ϫ2 ͒. 14 The effects of positive, parallel to the initial polarization, and negative, antiparallel, electric fields were investigated. The depolarization current intensity was measured dur-a͒ Electronic
doi:10.1063/1.1418258 fatcat:nupq34ezkvd5rml7bzbke67z5m