Epitaxial growth of piezoelectric oxide films

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The preparation and characterization of oxide and thus oxidation-resistant electrodes is performed by using single crystalline piezoelectric LGS substrates (La3Ga5SiO14). The main object of investigation are epitaxially deposited LGS films. Due to doping, they exhibit increased electrical conductivity with respect to the substrate and thus act as electrodes for e. g. piezoelectric resonators. In general, a large difference in conductivity of electrode and substrate is required to form a homogeneous electric field between the electrodes. The difference in conductivity can be increased even more by using piezoelectric substrates with lower conductivity. Since new piezoelectric crystals of the LGS family, namely CTGS crystals (Ca3TaGa3Si2O14), are available in sufficient size and quality at the beginning of the project, these crystals are also used as substrates and included in the investigations. These LGS isomorphs exhibit only slightly different lattice constants. The incorporation of CTGS substrates represents an extension of the work program.Overall, it can be stated that monolithic or almost monolithic electrodes are operational at 1000 °C and result in a moderate improvement of the quality factor at high temperatures. However, the unexpected temperature response of the frequency is of particular importance as it opens new research perspectives. Therefore, the focus of the project is shifted from film development to acoustic properties in relation to electrode conductivity. Thereby, temperature-compensated resonators represent an important application relevant aspect. In order to understand the underlying mechanisms, the modes of vibration must be determined. Further, the hypothesis must be validated that a superposition of transverse and lateral vibrations can be adjusted by the conductivity of the electrodes. The question is, whether the position and the width of the maximum of the temperature-dependent resonant frequency can be adjusted so that tailored resonators with a flat temperature profile for certain temperature ranges can be applied. In order to take advantage of a wide range of electrode conductivity, non-epitaxial oxide films other than LGS should be included.The tasks to be solved include the determination of the vibration modes by measuring the lateral and transversal displacements of the resonators. It is a challenging aspect since displacements in the (sub)nanometer range have to be expected at high temperatures. Furthermore, a detailed structural, chemical, electrical and electromechanical characterization of the system of layer and substrate is required.