Investigation of Piezoelectric and Electrostrictive Properties of (Pb_(1-3y/2),La_y)(Zr_x,Ti_(1-x))O_3 Ferroelectric Thin Films Using a Doppler Laser Vibrometer

摘要

(Pb_(1-3y/2),La_y)(Zr_x,Ti_(1-x))O_3 material system offers interests for many applications as it presents strong ferroelectric, piezoelectric, electrostrictive, dielectric, electro-optic properties. Optimization and selection of a given property can be obtained by adjusting the composition (La content). <100> -oriented ferroelectric thin films of La doped PZT have been elaborated using RF multitarget sputtering on MgO substrates. The substrates were prepared by sputtering Pt in peculiar conditions in order to obtain <100> epitaxial relationships on MgO(100). An ultra thin TiO_2 upper layer is sputtered prior to PLZT deposition in order to promote in situ crystallization and oriented growth. The Zr/Ti ratio was 28/72, corresponding to the tetragonal ferroelectric phase for La free sample, anddifferent amounts of lanthanum were added to reach the paraelectric cubic phase. For all these thin films, we performed ferroelectric, dielectric and piezoelectric characterizations in order to determine exactly the effect of the amount of lanthanum on the electrical properties. In particular an optical interferometer (with resolution of 2 pm) is used to measure the displacement of the film under AC voltage from which the first and second harmonics of the displacement are obtained. This method enables to extract and to separate the piezoelectric contribution from the electrostrictive one. As a result, it appears that when lanthanum is added, the piezoelectric contribution decreases whereas the electrostrictive one increases.