FK 06 Fakultät für angewandte Naturwissenschaften und Mechatronik
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Ferroelectric field- effect-transistors, based on Zirconia or Hafnia, have the potential to play an important roll in future non-volatile electronic memory devices. Artificial stabilization of the ferroelectric phase is necessary, since the ferroelectric phase is not the natural occurring phase of those materials. The intension of this work is to investigate the influence of II- and IV-valent dopants on the phase stability of Zirconia, using Ab Initio calculations based on density functional theory. Experimental results show ferroelectricity for dopant concentrations in the percent margins. Therefore the necessity to use large 96 atomic supercells arises, in which individual atoms can be removed or replaced with a dopant atom. The calculations were performed on the ICE1-segment of the Leibniz Rechenzentrum in Munich, using the program Abinit. Parallelization occurred, using up to 120 cores. The used exchange correlation method was PW92 at a cut off energy of 30 Ha, the atomic positions were optimized using the Broyden-Fletcher-Goldfarb-Shanno-Minimization-Method. Several different external constraints to optimize lattice parameters were investigated. The results show, that for the chosen dopant concentration the monoclinic phase is always favored. That implies that other effects like thermal energy or surface tension might also play a roll in stabilizing the ferroelectric phase. None the less dopants have a big impact on the total energy of the different phases, especially the tetragonal phase. Since in contrast to experimental results the dopants did not stabilize the ferroelectric phase in Zirconia, calculations including III-valent dopants as well as thermal and surface energies for both Zirconia and Hafnia are necessary.