Study of structural defects in Ga2-xFexO3 thin layers
Résumé
The aim of the study is to unveil the polarization switching mechanisms in multiferroic Ga2-xFexO3 (1≤x≤1.4), (GFO) thin films. GFO crystallizes in the noncentrosymmetric orthorhombic space group Pc21n [1], its polarization has been determine by calculation [2] and the polarization vector is along the [010] direction of the GFO cell and points from the Ga2 Octahedral site towards the Ga1 tetrahedral site as shown on figure 1. GFO thin films have been deposited by pulsed laser deposition and radio frequency sputtering on Yttrium stabilized Zirconia (YSZ) substrates. The structural studies performed by Resonant X Rays Scattering and Transmission Electron Microscopy allowed to determine the electric polarization state of the layers. It has been shown that the two different growth methods lead to different polarization states. In the case of GFO thin layer elaborated by PLD, the polarization direction points towards the surface, whereas it points towards the substrate when the GFO layer is elaborated by radio frequency sputtering [3]. In order to study specifically the polarization reversal mechanisms, a GFO layer was first grown by radio frequency sputtering on a YSZ substrate, then covered by a GFO layer grown by PLD (Figure 2). The polarization domains boundaries of the bilayer have been observed by High Angle Annular Dark Field Scanning Transmission Electron Microscopy (HAADFSTEM) coupled with Electron Energy Loss Spectroscopy map at the atomic scale in order to study their structural and chemical nature. Observations were performed on a JEOL ARM 200F Cold FEG microscope, operating at 80kV, equipped with a spherical aberration corrector on the condenser lens, allowing to have a probe size of 0.15 nm. The EELS mapping were acquired thanks to a Gatan GIF Quantum ER. As observed in the case of the GFO elaborated by sputtering, the growth starts with few Fe atomic layers followed by the expected composition of GFO. In the PLD part of the sample, different configurations of polarization have been observed, head to head or head to tail. Figure 3 presents a typical 2D defect at the interface between head to head polarization domains. Such polarization domains boundary is associated to cations stacking fault whereas in the case of head to tail domains boundaries, only a shift in the unit cell was observed from one domain to the other, similar to antiphase boundaries. As a perspective to this study and in order to propose a polarization reversal mechanism, it will be helpful to investigate at the atomic scale, the structural change of the GFO layer next to the domain boundaries by in-situ biasing TEM.