Seed Layers of the titania - lead oxide system
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Seed Layers of the titania - lead oxide system Paul Muralt, Stephane Hiboux and Marco Cantoni Ceramics Laboratory, Swiss Federal Institute of Technology EPFL, CH-1015 Lausanne, SwitzerlandABSTRACT Seed layers of (TiO2)x(PbO)y for the growth Pb(Z,Ti)O3 (PZT) thin films have been studied as a function of thickness and composition. The seed layers have been deposited by reactive in-situ sputter deposition at 530°C from two metal targets in a dynamic sputtering mode. The compositional variation was achieved by varying the relative fluxes. The PZT process was run with a limited lead excess resulting in pyrochlore nucleation on bare Pt electrodes. When the electrode was covered with a dense seed layer, perovskite was obtained everywhere. Ti rich seed layers yielded (111)-textured PZT even for very thin seed layers. Pb rich seed layers nucleated as PbTiO3{100} and gave rise to (100)-textured PZT. In an intermediate zone, the major perovskite orientations (100), (110) and (111) have been found together with pyrochlore for a small seed layer thickness. In this zone, the seed layer nucleates in islands leaving bare Pt spots. The intermediate region is a transition region between (111) seeds and (100) seeds. The latter exhibit diverging critical sizes at a given critical flux ratio, leading to large nuclei and bare Pt. Small perovskite(111) seed grains in the 10 to 100 nm range are found at equal lead oxide and titania flux. INTRODUCTION The perovskite material Pb(Zr,Ti)O3 is sufficiently complex to exhibit a nucleation controlled growth, as observed in the early days of sol-gel process development [1]. With suitable substrates, the activation energy for nucleation can therefore be reduced. It was for instance shown that the growth temperature of PZT can be reduced using PbTiO3 template layers [2]. Quite a number of atoms have to be gathered correctly to define a nucleus of a given orientation. When a film is grown in-situ, or nucleation is stimulated by the substrate, the first atoms involved in nucleation are those on the substrate. It is evident that low index planes are defined by less atoms, and thus form more easily as their correct gathering is more likely [3]. For this reason, thin film grains generally exhibit a low index plane orientation with respect to the substrate, even on non-ordered or differently ordered surfaces. In case of PZT one usually observes (100), (110) or (111) orientation. It is of interest to grow textured films, because properties are generally optimal at a given orientation only. A good starting point for textured PZT growth is a well textured Pt(111) electrode. Although geometry and atom distance on this trigonal surface matches fairly well with PZT(111), the (111)-orientation nucleates not much better than the (100)-orientation. With sol-gel processing, the resulting texture is much dependent on lead excess, pyrolysis temperature and ramping speed during the crystallization anneal [4]. Better results are obtained by adding a seeding layer that lowers the nucleation activation energy for
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