High-temperature electromechanical properties and atomistic transport processes in AlN bulk crystals

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Apart from the use as substrate for optoelectronic devices, single crystalline aluminum nitride shows attractive properties if used as piezoelectric sensors at temperatures above 800 °C. These properties offer new applications like resonant pressure and temperature sensors for turbines and engines. The predominant covalent bonds of the atoms in AlN result in high thermal stability and low losses of piezoelectric resonators. To take advantage of these properties, high quality bulk crystals are required. Presently, their availability is limited.Preliminary investigations clearly demonstrate the appearance of bulk acoustic wave resonances up to 900 °C which is the maximum temperature applied so far in this study. However, disturbances of the resonances are observed depending on the resonance mode and the crystal orientation. Nevertheless, the AlN crystals show extraordinary properties in comparison to piezoelectric oxide crystals which motivate the materials choice for this project.The primary objective of the anticipated research is the further improvement of AlN bulk crystals, thereby producing large and nearly defect free boules. The starting point is given by AlN crystals of the Institute for Crystal Growth (IKZ), which show in the applicants opinion a worldwide leading crystal quality due to their low structural defect density. The research strategy is based on the identification of prevailing atomistic transport processes at temperatures up to 1300 °C and its correlation with the growth parameters and the acoustic damping in order to improve the growth process and to determine the operation limits. Open questions include the effect and the interaction of point and structural defects as well as the electromechanical properties at high temperatures. With respect to applications, the complete set of piezoelectric, dielectric, and elastic constants, the conductivity, and the thermal expansion of AlN must be determined up to temperatures of 1300 °C. In an advanced stage of the project the long-term degradation should be investigated at high temperatures. This task is focused on the oxidation of AlN, the degradation of electrical contacts and the electromechanical properties.The IKZ team prepares and optimizes AlN bulk crystals with respect to their use as high-temperature piezoelectric transducers. Further, it provides for cutting, polishing, and characterization of chemical, optical, and structural properties of the samples. At the Technical University of Clausthal the focus lies on defect chemistry, atomistic transport, electromechanical properties, and the correlation of these properties in the temperature range between room temperature and 1300 °C. The approach takes advantage of the complementary competencies of the applicants. Special emphasis is put on the modeling of defect levels.