A. Goiffon

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.

Comparative crystal habit study of quartz and MPO4 isomorphous compounds (M = Al,Ga)

Abstract Comparative investigation of the crystal habit of α-quartz and two quartz-like materials (AlPO 4 and GaPO 4 ) has been undertaken on the basis of the Bravais-Friedel-Donnay-Harker (BFDH) law and the Hartman-Perdock (PBCs) theory. Unlike to these predictions, very strong morphological differences are observed. The crystal habit changes can be expressed in terms of crystal structure distortions and extrinsic parameters such as: temperature, pressure, solvent and impurities of the crystal growth experiments. All these parameters are not independent from one another, so only a qualitative explanation can be given.

Solubilités des phases MIIIXVO4 (M = Al, Ga; X = P, As) dans les solutions d'acide H3XO4 sous conditions hydrothermales: Application à la cristallogénèse de l'arséniate d'aluminium AlAsO4

Abstract The solubilities of AlAsO 4 , GaXO 4 (X = P, As) in the corresponding acid solutions and of AlPO 4 in H 3 AsO 4 acid solution were studied as a function of temperature and solvent concentration. Our results were applied to the crystal growth of aluminum arsenate by the slow increasing temperature technique using seeds of aluminum phosphate.

New approach of crystal growth and characterization of a quartz and berlinite isomorph: GaPO/sub 4/

GaPO/sub 4/ crystal growth in sulfuric and phosphoric acid media through the slow heating and vertical reverse temperature gradient methods is discussed. The systematic study of growth parameters shows a V/sub chi / growth rate always much greater than V/sub z/ and V/sub y/. It is possible to epitaxially grow GaPO/sub 4/ on large berlinite seeds in sulfuric acid and to use them for GaPO/sub 4/ crystal growth in phosphoric acid. The epitaxial process and crystalline quality are analyzed by X-ray topography. Infrared spectrometry proves a lower OH concentration than in the berlinite case for approximately the same crystal growth conditions. First piezoelectric characterizations for resonators near the AT-cut show GaPO/sub 4/ to be a very promising piezoelectric material with a large coupling coefficient near 16%, a quartzlike thermal stability, and a Q factor nearly sufficient for the applications.<<ETX>>

Controlled crystal dissolution applied to quartz

Quartz chemical lapping of AT and SC cuts was performed in NaOH·xH2O medium and this controlled dissolution resulted in reasonable quality of surface texture. Activation energy was calculated from dissolution rates measured against temperature. The initial surface texture of the samples influenced roughness parameter evolution but not the final roughness value. The electrical response of piezoelectric devices made by chemical lapping has been compared against others obtained by the IBE process. The comparison has shown that the controlled dissolution process is at least as good as IBE for producing piezoelectric devices.

CHEMICAL ETCHING OF BERLINITE IN SULPHURIC ACID SOLUTIONS

The aim of this work is to chemically etch AT wafers of berlinite AIPO4 in sulphuric acid solutions in order to reach a thickness previously chosen (manufacturing of high frequency plate resonators). The kinetics of etching is studied: the decrease of thickness is followed by frequency measurement. The influence of temperature and acid concentration is measured and the evolution of surface texture is checked both by roughness measurements, resonance frequency and scanning electron microscopy methods.

Surface state improvement of ground AT and SC quartz cuts by controlled dissolution in NaOH, KOH, LiOH

After the studies which allowed us to perfect the chemical thinning down, or controlled dissolution, of both AT and SC quartz plates in NaOH and KOH media, respectively, we have investigated mixtures of these with another basic compound (LiOH) in order to discover a common solvent for both AT and SC cuts. The results show that crystallographic orientation and solvent nature have a great influence on controlled dissolution especially on the surface texture. It appears that the basic compound which has the smallest cation imposes its properties and consequently, the chemical polishing of both AT and SC cuts cannot be made in good conditions with the same solvent.