About the STM Group
The research activities in S. Trolier-McKinstry’s group revolve around the processing and characterization of bulk and thin film electroceramics. The group works both to probe the fundamental mechanisms that control the magnitude of the achievable properties and to tackle the integration issues associated with incorporating new materials into devices.
In the area of dielectric thin films, the group has worked on the development of new tunable dielectric materials, where the permittivity can be changed by the application of a dc bias field. The group was the first to demonstrate that Bi1.5ZnxNb1.5O6+x (where x = 0.5 to 1) provide reasonable tunability (~50%) coupled with excellent temperature stability and low losses. Subsequently, it was shown that pulsed laser crystallization can be used to decrease the processing temperatures to 400°C, enabling integration onto polyimide. In the area of high permittivity thin films, the group, in conjunction with Clive Randall and Kemet Electronics has demonstrated that microcontact printing can be use to pattern BaTiO3 thin films for multilayer capacitors. This offers a potential means of preparing high layer count, thin layer (<0.2 µm) multilayer capacitors economically.
Thin film piezoelectric materials offer a number of advantages in microelectromechanical systems (MEMS), due to the large motions that can be generated, often with low hysteresis, the high available energy densities, as well as high sensitivity sensors with wide dynamic ranges, and low power requirements. The Trolier-McKinstry group has explored a wide range of perovskite thin films for these applications, and has developed new characterization tools for measurement of thin film piezoelectric properties. Non-180° domain wall motion is typically heavily clamped in perovskite thin films. One consequence of this is that 180° domain wall motion governs the nonlinearity in both the dielectric and piezoelectric coefficients. It has been demonstrated that the highest achievable piezoelectric responses can be observed in {001} oriented rhombohedrally-distorted perovskites. A wide variety of piezoelectric MEMS devices have been fabricated, including accelerometers, switches, pump actuators, medical ultrasound transducers, and adaptive optics systems (with the groups of Tom Jackson, Srinivas Tadigadapa and numerous external collaborators).
Recent work on fundamentals in the behavior of ferroelectric materials includes the observation of fatigue anisotropy in perovskite ferroelectrics with a relaxor end member (with Clive Randall) and investigation into relaxor ferroelectricity in strained SrTiO3 thin films (with Darrell Schlom). Fatigue is the process by which the switchable polarization decreases as a function of the number of times that a ferroelectric has been cycled through its hysteresis loop. It was demonstrated that rhombohedrally distorted perovskite single crystals and epitaxial films with a relaxor end member show much less fatigue along the [001] than [111] orientation. This was attributed to the existence of a domain substructure in [001] crystals that provided a high concentration of nucleation sites for domain reorientation, coupled with fast polarization kinetics. In epitaxial SrTiO3 films, it was shown that very high crystalline quality films (rocking curve widths of 7 arc seconds) can be grown on single crystal scandate substrates. The resulting films are relaxor ferroelectrics, with Tmax values near 250°C. The induction of ferroelectricity is attributed to the in-plane strain associated with the lattice mismatch. In addition, it was observed that for films grown on orthorhombic DyScO3, the anisotropy in the in-plane strain resulted in successive ferroelectric transitions.
Another focus for the group is the use of spectroscopic ellipsometry as a non-destructive characterization technique for surfaces and thin films. Spectroscopic ellipsometry entails reflecting a weak beam of light from a sample surface and measuring the polarization state change of the light. The resulting data can be used to characterize the depth profile of the optical frequency dielectric function of materials, frequently with Ångstrom scale depth resolution. This technique is used to map oxidation state as a function of depth, profile buried interfaces, and determine density depth profiles in thin films.
Finally, in the area of bulk piezoelectric materials, the Messing and Trolier-McKinstry groups have worked to develop the process of templated grain growth to develop high levels of crystallographic texture. Because many piezoelectric materials are strongly anisotropic in either their polarization or their polarizability, use of appropriately oriented single crystals can maximize the achievable piezoelectric response. Unfortunately, growth of large single crystals in multicomponent oxides is often difficult, and the resulting materials are too expensive for use in some applications. Oriented polycrystalline ceramics offer a useful compromise, as they can be prepared using ceramic processing approaches, and offer properties intermediate between those of single crystals and randomly oriented polycrystalline samples.