Giant piezoelectricity in PMN-PT thin films: Beyond PZT
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Introduction Microelectromechanical systems (MEMS) devices are being continually pushed in the direction of smaller size and increased integration density with a faster, larger range of motion and more powerful actuating elements. Currently, this is accomplished by incorporating passive mechanical elements with nanoscale dimensions within larger MEMS devices driven electrostatically. In this scheme, silicon is only a structural material driven by forces that are external to it. Incorporation of active materials directly into the nanoelectromechanical systems (NEMS) structure enables direct coupling of the mechanical deformation to the internal electric fields for nanomechanical actuation and for generation of electrical signals in response to ultrasmall mechanical displacements.1–6 In this case, the sensing and actuating can be combined into the same system using piezoelectric materials. Piezoelectricity is an intrinsic, electromechanical coupling phenomenon arising from the non-centrosymmetric arrangement of atoms within the unit cell. Piezoelectric materials develop surface charges in response to an applied pressure (direct effect) and create mechanical displacements in response to an applied electric field (converse effect), which are useful for sensors and actuators, respectively. These active materials are highly desirable for a number of reasons. An integrated
piezoelectric spring element, for example, cantilever-type devices, can generate a large force with a small applied voltage in a linear fashion. This decreases the complexity, retains the integration density, and reduces the voltage burden on the integrated control electronics. In addition, robustness comes from the fact that high electric fields are confined inside solidstate materials. In MEMS actuators, a piezoelectric drive is always beneficial at low voltages compared to an electrostatic approach, as the force per unit area is linear in an electric field (V/thickness) for the piezoelectric drive and quadratic for the electrostatic approach.3–5 Applications of piezoelectric materials in MEMS structures typically take advantage of particular piezoelectric response modes, identified by numerical subscripts indicating the axis of the applied field and the axis of the strain or stress response.1,3 For instance, a strain induced in the same direction as an applied electric field is labeled as “33,” and a strain induced in a direction transverse to the applied electric field is labeled “31.” These are quantified with piezoelectric coefficients relating the strain to the applied electric field (d coefficients) and stress to the applied electric field (e coefficients). Thin films are also constrained by the boundary conditions imposed by the substrate. These effects are captured by effective piezoelectric coefficients for thin films, e31, f relating in-plane stress to out-of-plane electric
Seung-Hyub Baek, Electronic Materials Research Center, Korea Institute of Science and Technology, Seoul; [email protected] Mark S. Rzchowski, Physics Department, University of Wisconsin–M
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