Microwave microscope unmasks, measures surface acoustic waves in ferroelectrics

Using a customized microwave impedance microscope (MIM), a team of U.S., German, and Chinese scientists has demonstrated how the increasingly versatile analytical tool can measure, with nanoscale resolution, properties of surface acoustic waves (SAWs) in a ferroelectric material, even capturing SAW behavior within individual domains of electric polarization. Reported in the Journal of Applied Physics, the research introduces the prospect of using one instrument for rapidly patterning and testing SAW devices.

Measurements on the piezoelectric lithium tantalate were performed in the lab of Stanford physicist Zhi-Xun Shen, where much of the early development of MIM has been carried out. Fitted to an atomic force microscope, the MIM consists of a shielded tip that emits microwaves as it scans the sample surface and collects reflected microwaves. Localized interactions between probe and sample yield data on conductivity and permittivity, used to make nanoscale images of the sample’s electrical characteristics.

The researchers set out to measure the conductivity of the ferroelectric’s domain walls, a recently discovered phenomenon eyed for creating one-dimensional conduction paths in wide band-gap devices. However, according to lead author and Stanford applied physics graduate student Scott Johnston, the team encountered unexpected changes in microwave loss, so they turned their attention to understanding the cause.

Gathering measurements across tip-generated microwave frequencies up to 1 gigahertz, the researchers found compelling evidence that SAWs accounted for the power loss at each probed position. Multiple measurements indicated a constant velocity of about 6 km/s, in good agreement with SAW modes in the material. The article advises that studies on domain-wall conduction recognize the potential influence of nearby SAW effects on measurements.

Source: “Measurement of surface acoustic wave resonances in ferroelectric domains by microwave microscopy,” by Scott R. Johnston, Yongliang Yang, Yong-Tao Cui, Eric Yu Ma, Thomas Kämpfe, Lukas M. Eng, Jian Zhou, Yan-Feng Chen, Minghui Lu, and Zhi-Xun Shen, Journal of Applied Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4997474 .