Date of Award

2009

Level of Access

Campus-Only Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics

Advisor

Mauricio Pereira da Cunha

Second Committee Member

Robert J. Lad

Third Committee Member

David Frankel

Abstract

Langatate (LGT), a synthetic piezoelectric crystal with chemical composition La3Ga5.5Ta0.5O14, has recently received significant interest in the sensor and frequency control communities as a possible alternative to quartz owing to its higher piezoelectric coupling, structural stability up to 1400 °C and presence of temperature compensated acoustic wave (AW) orientations. With these exciting properties, LGT is expected to find applications in AW sensor, timing, and frequency control. This thesis focuses on the characterization of the acoustic wave material properties of LGT up to 120 °C. Such a characterization is critical for the design and fabrication of LGT acoustic wave devices. The elastic and piezoelectric constants were determined through measurements of bulk acoustic wave phase velocities by two independent methods, the pulse echo overlap technique and a combined resonance technique. The extracted constants and temperature coefficients enabled the identification of a range of particularly interesting LGT surface acoustic wave (SAW) orientations with Euler angles (90°, 23°, 118-124°) that exhibits predicted electromechanical coupling up to 0.7% and reduced or zero temperature coefficient of delay (TCD). The consistency of the determined constants and temperature coefficients was established using SAW measurements of seven crystallographic orientations at temperatures ranging up to 120 °C. Measured SAW phase velocities and TCDs were found to be in agreement with predictions based on the determined constants. Two of the seven SAW orientations exhibited temperature compensation within 40 °C of room temperature, agreeing with predictions. Deposition of SiAlON films on top of LGT SAW devices for surface protection in chemically and mechanically harsh environments was also investigated. SiAlON films deposited by reactive RF magnetron co-sputtering of Al and Si targets were controlled to within a few percent for film thickness and composition. SiAlON thin film elastic constants were extracted using differential SAW delay line methods and were found to be: C11,s= 160 ± 30 GPa and C44,s= 55 ± 5 GPa. SiAlON films up to 800 nm in thickness were shown to have no measurable effect on the TCD of LGT SAW delay lines.

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