Homogeneity of Langasite and Langatate wafers for acoustic wave applications

Variations in the concentration of the chemical constituents of Langasite (La₃Ga₅SiO₁₄) and its homeotype Langatate (La₃Ta_0.5Ga_5.5O₁₄) have been found by the means of X-ray methods and selective crystal etching. To determine their influence on the acoustic properties SAW resonators have been designed and processed on 3" wafers from Langasite and Langatate crystal boules of different suppliers. Compositional changes on a short range scale according to growth striations and on a long range scale as well could be distinguished leading to variations of the SAW velocity up to 1000 ppm within a wafer. If a device covers several growth striations a superposition of different propagation characteristics occurs leading to a multimode behavior and thereby degrading the effective Q-value of the SAW resonators. Wafers from recently grown boules, however, reveal frequency shifts with a standard deviation of 50 ppm only and maximum Q-values of up to 15000, thus demonstrating the progress in crystal growing.