Jacquie Schwartzel

Crystal growth of berlinite, AlPO4: 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 H3PO4 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 cm2. 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 H2O 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.

Evaluation of high quality berlinite crystals grown in sulphuric acid medium

Abstract This investigation concerns berlinite crystal growth in sulphuric acid solvent through the reverse temperature gradient method and its consequences on crystalline quality and piezoelectric characteristics. Systematic study of growth parameters shows a high sensitivity of growth rates to crystal growth temperature, temperature gradient and acid concentration. The choice of good crystal growth conditions allows us to obtain crystals with a very good crystalline quality checked by X-ray topography. The -OH content, followed by infrared spectrometry, has been lowered to very small values and then electrical measurements of Y and Y-rotated resonators lead to high Q factors, first and second order temperature coefficients lower than those of quartz and a favourable coupling coefficients. For extensional bars, the existence of compensated cuts is also demonstrated.