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

除了用作光电器件的衬底外，如果在800°C以上的温度下用作压电传感器，单晶氮化铝显示出诱人的性能。这些特性提供了新的应用，如涡轮和发动机的共振压力和温度传感器。AlN中主要的共价键使得压电谐振器具有较高的热稳定性和较低的损耗。为了利用这些特性，需要高质量的块状晶体。目前，它们的可用性是有限的。初步调查清楚地表明，体声波共振的出现高达900°C，这是迄今为止在本研究中应用的最高温度。然而，根据共振模式和晶体取向的不同，可以观察到共振的扰动。然而，与压电氧化物晶体相比，AlN晶体表现出非凡的性能，这激发了该项目材料的选择。预期研究的主要目标是进一步改进氮化铝体晶体，从而产生大而几乎无缺陷的气泡。起点是晶体生长研究所（IKZ）的AlN晶体，由于其结构缺陷密度低，申请人认为其晶体质量处于世界领先水平。研究策略是基于在高达1300°C的温度下确定主要的原子输运过程及其与生长参数和声阻尼的相关性，以改进生长过程并确定操作极限。悬而未决的问题包括点缺陷和结构缺陷的影响和相互作用，以及高温下的机电性能。在应用方面，完整的压电、介电和弹性常数、电导率和AlN的热膨胀必须在1300°C的温度下确定。在项目的后期阶段，应研究在高温下的长期降解。本课题主要研究氮化铝的氧化、电触点的退化和机电性能。IKZ团队准备并优化了AlN块晶体作为高温压电换能器的使用。此外，它还提供了样品的化学、光学和结构特性的切割、抛光和表征。在Clausthal技术大学，重点是缺陷化学，原子输运，机电性能，以及这些性能在室温到1300°C之间的温度范围内的相关性。这种方法利用了申请人的互补能力。特别强调的是缺陷级别的建模。

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.