Langasite and langatate nonuniform material properties correlated to the performance of SAW devices

Material properties of Langasite (LGS) and its homologous compounds, e.g., Langatate (LGT) have been investigated employing lattice constants measurements revealing that the micro structured growth striations affect the a and c-parameter of the unit cell. Thus deviations off the congruent melt composition of the crystal are assumed. Transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDX) exhibit amorphous inclusions having a nearly identical chemical composition as the surrounding crystal. X-ray topography (XRT) has been applied to correlate the crystal quality of 3" LGS and LGT wafers including defects and transverse growth banding (TGB) with SAW frequency shifts. Evaluation of the RF-signal of SAW test devices showed frequency deviations of about 1000 ppm even within one wafer. The pattern of the frequency shifts is related to the orientation of the TGB. Local accumulations of defects, e.g., dislocations, however, did not cause observable interference with the SAW frequency