Influence of structure and chemistry on piezoelectric properties of lead zirconate titanate in a microelectromechanical
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Lead zirconate titanate (PZT) films between 1 and 3 m thick were grown using solution deposition techniques to study the effects of crystal structure and orientation on the direct piezoelectric output of these films on platinized Si membranes. By varying the chemistry of the film from Zr-rich to Ti-rich, the {100}/(111) relative intensity increased for films grown on randomly oriented Pt films. The 40:60 PZT had a tetragonal crystal structure and produced greater electrical output at a given strain than the rhombohedral film (Zr:Ti concentrations less than 50:50), while having a similar e31 constant, between 4.8 and 6.3 C/m2. Orientation and voltage output at a given strain were not strongly influenced by thickness in the ranges investigated. Defects in internal PZT/PZT crystallization interfaces were identified and include porosity on the order of tens of nm, with a corresponding depletion in Pb and accumulation of O at these interfaces. The {100} texture of rhombohedral films deposited upon (111) textured Pt films is significantly greater than the {100} texture of tetragonal films, which show both a {100} and {111} orientation on the same Pt film.
I. INTRODUCTION
Lead zirconate titanate (PbZrxTi1−xO3), a piezoelectric oxide, is a preferred material in many microelectromechanical systems applications due to its high piezoelectric and electromechanical coupling coefficients.1–5 One such application transforms mechanical energy into electrical energy for power generation by flexing a PZT membrane.6 The sealed cavity of the microengine contains a saturated two-phase fluid. With the addition of heat from an external source, the fluid forms a vapor and causes volume expansion and similar pressure impulse, which deflects the piezoelectric membrane. This applied strain is then used to generate an electrical charge to create electrical power. This device generates relatively high membrane strains (beyond 0.05%), and so determining the relationships between structure and properties at these conditions is required. For the proposed application, the power output is proportional to the voltage squared. Therefore, the most direct method to improve performance of the microengine is to maximize the output voltage. This can be achieved through increasing the thickness of the PZT films or improving its piezoelectric properties. The texture of the PZT has been found to contribute to the electrical properties of the film and may be controlled by heat treatments,1,7–12 PZT thickness,13 J. Mater. Res., Vol. 18, No. 9, Sep 2003
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chemistry,14,15 growth interface chemistry (including bottom electrode and internal PZT interfaces),16,17 and deposition technique of the bottom electrode.4,18,19 The interface on which the PZT nucleated is expected to play the most significant role in texture formation and PZT morphology.4,20–22 Heterogeneous nucleation at the interface leads to columnar grains, which have been shown to exhibit better electrical fatigue and polarization than do equiaxe
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