Crystal growth of berlinite, AlPO 4: Physical characterization and comparison with quartz

Abstract

The feasibility of large crystals of aluminum phosphate has already been demonstrated, so as the interest of this material for obtaining BAW and SAW devices with attractive properties. Our purpose in this study was: (a) to synthetize crystals with low acoustical losses; (b) to specify the useful crystal orientations more accurately; (c) to design devices that take in account all the specificities and advantages of this material. With a view to obtain high perfection and high Q crystals, three hydrothermal crystal growth methods were investigated in H 3PO 4 solvent, compared and improved with the assistance of crystal characterization techniques and BAW device measurements. Experimental conditions used most frequently with the slow heating, the reverse vertical gradient and the horizontal gradient methods are reported together with specific features of each method as applied to grow berlinite. X-ray topography has shown that the best crystals have a dislocation density of 10 to 100 dislocations per cm 2. X-ray topography examination above room temperature (25–150°C) has revealed temperature dependent quasi-reversible localized variations of strain in water-containing crystals. Transmission electron microscopy was used after high temperature precipitation of water to assess the H 2O content of crystals as a function of some growth parameters and to determine the distribution of this impurity within crystals. Thickness shear resonators of several Y rotated cuts were measured to compare the growth methods. Some experiments with recently obtained crystals have demonstrated the feasibility of very high Q crystals and the possibility of obtaining superior thermal behaviour from this material. Experiments concerning the AT cut has demonstrated the possibility to obtain band-pass of filter of shift of oscillators twice that of quartz. Specificity of energy trapping in this material is then discussed. We conclude that waterless berlinite is of major interest for device applications.