Monocrystalline PZT thin films

Abstract

More with has been the motto behind the hardware miniaturization trend in the microelectronics industry since the 1970s. Active research in the growth of oxide films, including ferroelectrics, which started soon after, followed the same trend. Meanwhile, in ferroelectrics, domain walls have arisen recently as elements having new, potentially useful functional properties. The contribution of this thesis work to the global miniaturization trend is focused at exploring strain engineering techniques to control and possibly redefine limits for imaginable domain patterns and their periodicity in lead zirconate titanate (PZT) monocrystalline thin films. PZT is one of the most heavily used ferroelectrics in microelectronics and piezoelectric applications. In its monocrystalline formit has been much less studied. Here, epitaxial thin films were made and investigated in both, the highly rich titanium region (highly tetragonal with a high spontaneous polarization in bulk PZT ceramics) and compositions near the morphotropic boundary (tetragonal and rhombohedral side, both highly active piezoelectrically in bulk PZT ceramics). In films with rhombohedral bulk compositions, it was found that, compressive misfit strain of 1.5 % result in (001) PZT 60/40 films with tetragonal symmetry. On the other hand, the application of 1 % tensile misfit strain through a PZT 80/20 buffer layer resulted in (001) PZT 60/40 films with rhombohedral symmetry having random domain patterns. All films were grown on PrScO3 (PSO) substrates with SrRuO3 (SRO) bottomelectrodes by the pulsed laser deposition technique (PLD). Re-ordering of these domains in to stripes of 71 ° domain walls was possible through poling with an AFMtip. Monitoring the stability of these patterns revealed that the stripe domain structures relaxed in to herringbone patterns consisting of 180 ° domain walls in amatrix of 109 ° domain walls. This was interpreted in terms of electrostatic effects. Tetragonal PZT films offered the opportunity to study both single domain and polydomain films. In single domain films, the investigation turned to the source of the built-in field that dictates the polarization orientation. The upward oriented c-domain of as-grown 30 nm (001) PZT 50/50 thin films grown on PSO with SRO bottomelectrodes, were made to switch downwards by using (i) substrates with polar surface termination, (ii) 25 % excess PbO in the PLD targets and (iii) doping. While interfacial mechanisms seem to get weaker with film thickness, bulk work function modification through Nb doping appeared to play a significant role in films as thick as 150 nm. Theoretical calculations were conducted in order to predict the stability of c-domains as a function of temperature andmisfit strain in (001) films. The effect of elastic compliances and higher order Landau coefficients on the phase diagram of single domain epitaxial (001) PZT 50/50 thin films was studied by using a single domain theory. It is shown that while values of elastic compliances affect the positions of the phase boundaries including phase transition temperature of the cubic phase; higher order terms can potentially introduce an a1a2-phase previously predicted in the pure PbTiO3 phase diagram. In the literature, it has been demonstrated that, under special treatment conditions, dense domain patterns can be realized in single crystals through poling along non-polar orientations. Triggered by this knowledge, we studied multidomain ...