China & India: Bayer – polycarbonate compounds & colour competence centres

Abstract

GaAsO4 crystals were grown by a hydrothermal epitaxial process. Microcrystalline GaAsO4 powder was first synthesized by hydrothermal methods by using GaAs as a starting material in a sulfuric acid solvent under oxidizing conditions. Experimental conditions were optimized and the yield of the nutrient synthesis reached 88%. GaAsO4 powder was then used as the nutrient for growing GaAsO4 single crystals by epitaxy on (21¯0) oriented GaPO4, AlPO4 and GaAsO4 plates. Crystal growth was performed in a horizontal PTFE-lined autoclave divided into two parts using sulfuric acid as the solvent under a slow heating gradient. Hydrothermal conditions were optimized in order to improve the crystal quality. The influence of the nature and orientation of the seeds on the growth rate was studied. From the solubility curve, the solute supply ΔS as a function of temperature during the growth process was calculated. Transparent crystals were obtained with ΔS between 0.002 and 0.009 mol/L. Large crystals (3.9×1.8×1.4 cm3) of gallium arsenate were obtained. These crystals were cut into (21¯0), (010) and (001) plates. Mapping using Raman spectroscopy was performed in order to study the growth recovery on the seed. GaAsO4 crystals were characterized by infrared spectroscopy in order to quantify the amount of OH groups in the crystal structure. A low value of the absorption coefficient α at 3300 cm−1 was measured 0.067 cm−1 indicating a good crystal quality. Optical measurements were performed on GaAsO4 crystals by UV–vis–NIR spectrophotometry. Gallium arsenate exhibits the highest value of birefringence in the α-quartz group with Δn=0.033. The transmission in the spectral region above 250 nm yields a bandgap of 5.85 eV. Piezoelectric properties were measured on resonators. An electromechanical coupling coefficient of 20% was measured which is the highest in the α-quartz group. A quality factor QF of 3.2×1010 was obtained on a Y-cut plate. The C66 elastic constant was found to be 20 GPa.