E. Philippot

Growth and dielectric characterization of large single crystals of GaAsO4, a novel piezoelectric material

AbstractGallium arsenate (GaAsO 4 )isanew α -quartz-type piezoelectric material. Large single crystals (8 mm along the c -direction) were grownfor thefirst timeby hydrothermal methods. The different crystal faces wereindexed by X-ray diffraction. The crystal quality was characterizedby infrared measurements. X -and Z -cut crystals were prepared in order to perform dielectric measurements. The dielectric constants ofGaAsO 4 ( e 11 =8 . 5, e 33 = 8 . 6) are the highest found among α -quartz type materials. A linear relationship has been established between astructural property, the tetrahedral tilt angle δ , and the dielectric constants for the well-known α -quartz homeotypes. These results indicatethat gallium arsenate should have the highest piezoelectric coupling coefficient of any material of this family.  2003 Editions scientifiques et medicales Elsevier SAS. All rights reserved. 1. IntroductionQuartz material is currently the most used piezoelectricmaterial. Nevertheless, its properties are limited for certainapplications due to its low electromechanical couplingcoefficient. GaAsO

Crystal growth and characterizations of quartz-like material: gallium phosphate (GaPO4)

This investigation concerns GaPO 4 crystal growth in phosphoric and sulphuric acid media by the slow heating and vertical reverse temperature gradient methods. Systematic study of growth parameters shows that the growth rate V x is generally much greater than the other two, V z and V y . The most interesting result is the ability to produce GaPO 4 epitaxy on large berlinite seeds in sulphuric acid and to use them for GaPO 4 crystal growth in phosphoric acid where the V x /V Z ratio can be close to 1. As for α-quartz and berlinite, V y growth rate being always very slow, the lengthening of these crystals in that direction is obtained by the use of several joined seeds accurately adjusted and spliced on a quartz plate. The epitaxial fit and the crystalline quality have been checked by X-ray topography. The OH content, determined by infrared and near infrared spectrometry, proves a lower OH concentration than in the case of berlinite when approximatively the same crystal growth conditions are used. Piezoelectric characterizations of resonators have been suitable to specify the AT-cut position, its very promising piezoelectric properties with a high thermal stability and a Q factor are already nearly sufficient for applications. The most interesting properties of these crystals, the piezoelectric properties, are compared with those of other known crystals, quartz and berlinite, and related to their crystal structure distortions. The observed evolution allows the prediction of some interesting unknown characteristics for other quartz-like materials.

Advances in crystal growth and characterizations of gallium orthophosphate, GaPO4

Abstract Gallium orthophosphate, GaPO 4 , is a piezoelectric material isostructural with α -quartz, SiO 2 , with better piezoelectric characteristics: higher coupling coefficient for its temperature compensated cut and wider thermal stability (up to 933°C). A new investigation of the solubility of GaPO 4 in different acids and their mixtures allows to determine the metastable zone of which the knowledge is primordial for a good growth restart. The comparison of experimental growth results obtained in horizontal glass vessels ( T c T c >170°C) leads to specify a necessary minimum value of the solute supply at the interface crystal/solution. On the other hand, natural large seeds being not available, the seed lengthening is carried out with the splicing technique followed by crossed crystal growths to decrease the density of structural defects. At the same time, characterizations of crystals have been undertaken to check: the OH content by infrared spectroscopy; the crystalline quality (specially crystals after splicing) and the growth restart by X-ray topography; the piezoelectric properties.

Crystal growth of GaPO4, a very promising material for manufacturing baw devices

GaPO4 cystals were obtained by hydrothermal solution crystal growth. The retrograd solubility of the material was investigated in different solvents. In static growth vessels, good quality crystals can be obtained only if the solute supply is higher than about 0.06M/L. Crystal characterization by infrared spectroscopy showed that dilute solvents at high temperature decrease the “-OH” group content. Seed lengthening by splicing along the Y-axis was designed. The AT cut angle and the C44 elastic constant were determined. Compared to quartz-type materials, the C0/C1 calculation shows the high ability of GaPO4 for manufacturing BAW devices.

Properties of AT cut gallium phosphate resonators

Abstract Gallium phosphate is a quartz analogue that possesses more intense piezoelectric properties. We report here a study performed to determine more accurately the properties of resonators made using crystal orientations situated near the main cut presenting a zero temperature coefficient at room temperature. Recent crystals having a much improved quality were used to cut plano-convexe resonators with five orientations situated in a range of nearly two degrees around the previously determined angular position of this cut. The temperature coefficients of the resonance frequencies of the electrically excited shear modes and the electrical properties of the resonators were determined. The vibration modes were studied using synchrotron radiation X-ray topography. For all the studied cuts, parabolic thermal variations of the resonance frequencies were observed. The temperature of the extremum varies somewhat with the rank of the mode and also, but slowly, with the cut angle. On the whole a very good thermal stability is obtained for the resonators. Q factors higher than those previously measured were obtained together with, as predicted, very interesting values of the equivalent electrical scheme. The observed vibration modes are generally close to those computed using the Tiersten theory. They present several very interesting properties but display several particularities, such as often possessing a second shear component with a noticeable amplitude, which make them somewhat different from the mode of the corresponding quartz resonators.

Spatial--OH impurity distribution in gallium phosphate crystals

Piezoelectric properties of quartz and quartz-like materials are strongly related to the impurity content in the crystals and, more especially, to their hydroxyl group (−OH) content. This work has been devoted to the determination of the spatial distribution of this impurity in as-grown crystals of gallium phosphate, GaPO4. The investigation was undertaken by infrared spectroscopy from eight samples with different growth conditions and completed by thermally stimulated current/relaxation map analysis techniques. The results allow the best growth parameters to be defined, leading to crystals with the lowest −OH impurity content.

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.