Faculté des sciences

Probing nanoscale ferroelectricity by photoelectron diffraction

Despont, Laurent ; Aebi, Philippe (Dir.)

Thèse de doctorat : Université de Neuchâtel, 2007 ; 1942.

This thesis is based on research carried out at the Institut de Physique, Université de Neuchâtel, Switzerland during the period 2003 - 2007, in collaboration with the DPMC, University of Geneva, where the samples have been grown. A ferroelectric is generally defined as a material with two or more stable states of different non-zero intrinsic lattice polarization which can be reversed through... Plus

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    Summary
    This thesis is based on research carried out at the Institut de Physique, Université de Neuchâtel, Switzerland during the period 2003 - 2007, in collaboration with the DPMC, University of Geneva, where the samples have been grown. A ferroelectric is generally defined as a material with two or more stable states of different non-zero intrinsic lattice polarization which can be reversed through the application of an external electric field. The information storage in ferroelectric random access memories (FeRAM) is a quite evident and interesting application for such materials. Despite the evident potential of ferroelectric materials and most specifically of thin ferroelectric films, some questions concerning their limitations remain open. One of them consists in whether or not ferroelectricity can be maintained at reduced dimensions and represents the main topic of this thesis. This thesis consist of six different chapter shortly discussed below. The first chapter gives an introduction to the ferroelectricity. It explains shortly the concept of phonon softening which is at the origin of the ferroelectric instability but also why this instability can be suppressed in very thin films. The second chapter is dedicated to the photoemission techniques used in this thesis, namely the x-ray photoelectron spectroscopy, as a probe of the electronic structure, and the x-ray photoelectron diffraction, that provides the local atomic structure around a given atom. The third chapter deals with the theoretical approach of the photoelectron diffraction. Here a new code taking into account multiple scattering events is described and the main parameters used in the multiple scattering program are discussed. The first part of this chapter has been arranged as a tool for further multiple scattering investigations. In the second part, differences and similarities between single and multiple scattering are discussed for the Cu(111) surface for two different photoelectron kinetic energies. Then the 1T-TaS2 surface termination has been investigated using the multiple scattering approach and the application of this method to other similar systems is discussed. Finally, using single scattering calculations have been used in order to determine the polarity of an InN(0001) surface. The aim of all these examples is to show, despite some limitations, the utilization domain of scattering calculations. In the fourth chapter we report an analysis of the Nb-SrTiO3 which has been mainly employed as substrate for the ferroelectric films studied in the fifth chapter. A multiple scattering based structure optimization of the substrate suggests that, contrary to long-established beliefs, its surface is not paraelectric but exhibits a polar distortion analogous to the one observed in ferroelectric films. In this chapter, we also emphasize some aspects regarding the surface reconstruction and the chemistry at the Nb-SrTiO3 surface according to different cleaning procedures. The fifth chapter consists of a wide investigation of ferroelectric PbTiO3 films. The ferroelectricity is typically regarded as a collective phenomenon and is consequently expected to be strongly influenced by finite-size and surface effects. Both of these aspects are investigated in this chapter. First the intra-cell polar atomic distortion and the tetragonality associated with the ferroelectricity have been explored leading to information about the non-centro-symmetry in films a few unit cell thick, the preservation of the ferroelectricity in films as thin as 3 unit cell and the influence of the surface relaxation on the ferroelectric distortion switchability. Second, the environment of the PbTiO3 films has been modified, either by changing the interface/substrate (La0.67Sr0.33MnO3 and SrRuO3), the surface boundary conditions using plasma based cleaning procedures for instance, or even both using a superlattice configuration made of alternated layers of PbTiO3 and SrTiO3. Finally, we have suggested a way to understand an unexpected behavior of XPS line positions as a function of the film thickness, combining XPS and XPD measurements together with an electrostatic model. This approach could be used to determine the effective screening length characterizing the screening efficiency of electrode materials.