Growth of Piezoelectric <Li_2> <B_4> <O_7> Single Crystal and its Applications to Ultra High Frequency Surface Acoustic Wave Devices

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Lithium tetraborate ( <Li_2> <B_4> <O_7> ) has attracted much attention as a promising candidate for the new surface acoustic wave (SAW) and also as bulk acoustic wave (BAW) materials because of its very low temperature coefficient of frequency (TCF) and high coupling. The object of this project is growth of good quality <Li_2> <B_4> <O_7> (LBO) crystal and its applications. The abstract is as follows:1. Transparent, core-free and good quality single crystals of <Li_2> <B_4> <O_7> , up to 23mm in diameter and 80mm in length, have been successfully grown from a congruent melt at rotation rates around 40rpm and growth rate of 1.7mm/h by an automatic diameter control (ADC) of the Czochralski method. It has been pointed out that the growth rate will become higher by making the rotation rate higher and the temperature profile steeper at the solid-melt boundary.2. All the dielectric, elastic and piezoelectric constants and their first- and second-order temperature coefficients have been determined in detail by measuring resonant and antiresonant frequencies.3. The existance of orientations of lithium tetraborate with zero temperature coefficient of delay or frequency have been predicted on the basis of the material constants determined by the present authors, and confirmed experimentally for the Rayleigh and leaky SAW and BAW. There are useful substrate orientations for SAW devices with zero TCD around (0゜,78゜,90゜), (0゜,90゜,78゜) and (45゜,90゜,90゜), where <k_s^2> is approximately 0.8-1%. However, among these substrates, the (45゜,90゜,90゜) is the most suitable substrate, Further, it has been confirmed that the temperature compensated cuts for rotated Y-cut bulk wave devices exist at <theta> of 32゜ for the quasi-longitudinal mode and <theta> of 53゜ for the quasi-shear mode.4. The resonater with Q of 3400 have been successfully fabricated.