Isabelle Gélard

Oxides heterostructures for nanoelectronics

We summarise in this paper the work of two groups focusing on the synthesis and characterisation of functional oxide for nanoelectronic applications. In the first section, we discuss the growth by liquid-injection MOCVD of oxides heterostructures. Interface engineering for the minimisation of silicate formation during the growth of polycrystalline SrTiO3 on Si is first presented. It is realised via the change of reactant flow or chemical nature at the Si surface. We then report on the epitaxy on oxide substrates of manganites films and superlattices and on their magnetic and electrical properties. La0.7Sr0.3MnO3 and La0.8MnO3-δ as well as multiferroic hexagonal ReMnO3 manganites are considered. We show that the film thickness and related strain may be used to tune the properties. Finally, we demonstrate the growth of MgO nanowires by CVD at a moderate temperature of 600°C, using gold as a catalyst. In the second section, we discuss the growth of epitaxial oxide heterostructures by MBE. First, the direct epitaxy of SrTiO3 on Si is considered. Issues and control of the SrTiO3/Si interface are discussed. An abrupt interface is achieved. We show that SrTiO3 on Si can be used as a buffer layer for the epitaxy of various perovskite oxides such as LaAlO3 or La0.7Sr0.3MnO3. La0.7Sr0.3MnO3 films are ferromagnetic and metallic at room temperature. The epitaxial growth of complex oxides on Si wafers opens up the route to the integration of a wide variety of functionalities in nanoelectronics. Finally, we discuss the monolithic integration of III-V compounds such as InP on Si using epitaxial SrTiO3 buffer layers for the future integration of optics on Si.

Magnetic frustration on a Kagomé lattice in R3Ga5SiO14 langasites with R = Nd, Pr

In the R3Ga5SiO14 compounds, the network R of rare earth cations forms well separated planes of corner sharing triangles topologically equivalent to a Kagome lattice. Powder samples and single crystals with R = Nd and Pr were prepared and magnetostatic measurements were performed under magnetic field up to 10 T in the temperature range from 1.6 to 400 K. Analysis of the magnetic susceptibility at the high temperatures where only the quadrupolar term of the crystal electric field prevails suggests that the Nd and Pr magnetic moments can be modelled as coplanar elliptic rotators perpendicular to the threefold axis of the crystal structure that interact antiferromagnetically. Nonetheless, a disordered phase that can be ascribed to geometric frustration persists down to the lowest temperature, which is about 25 times smaller than the energy scale for the exchange interactions.

Neutron diffraction study of hexagonal manganite YMnO3, HoMnO3, and ErMnO3 epitaxial films

The antiferromagnetic phase transition in epitaxial RMnO3 (R=Y,Ho,Er) films of thickness 450–500nm was investigated by neutron diffraction by recording the intensity variation of magnetic reflections as a function of temperature. For YMnO3 and ErMnO3, the Neel temperature for the Mn3+ ions ordering is of 66 and 68K, respectively. For HoMnO3, the magnetic 011 reflection, which normally appears in bulk, is very weak and thus the onset of the Mn3+ ordering is not clearly observed; at 50K there is a magnetic phase transition, which temperature corresponds to the reorientation temperature in bulk HoMnO3.

Structure and defect characterization of multiferroic ReMnO(3) films and multilayers by TEM.

Epitaxial rare earth manganite thin films (ReMnO(3); Re = Tb, Ho, Er, and Y) and multilayers were grown by liquid injection metal organic chemical vapor deposition (MOCVD) on YSZ(111) and the same systems were grown c-oriented on Pt(111) buffered Si substrates. They have been structurally investigated by electron diffraction (ED) and high resolution transmission electron microscopy (HRTEM). Nanodomains of secondary orientation are observed in the hexagonal YMnO(3) films. They are related to a YSZ(111) and Pt(111) misorientation. The epitaxial film thickness has an influence on the defect formation. TbO(2) and Er(2)O(3) inclusions are observed in the TbMnO(3) and ErMnO(3) films respectively. The structure and orientation of these inclusions are correlated to the resembling symmetry and structure of film and substrate. The type of defect formed in the YMnO(3)/HoMnO(3) and YMnO(3)/ErMnO(3) multilayers is also influenced by the type of substrate they are grown on. In our work, atomic growth models for the interface between the film/substrate are proposed and verified by comparison with observed and computer simulated images.

Thin films and superlattices of multiferroic hexagonal rare earth manganites

Hexagonal YMnO3 and HoMnO3 as well as (YMnO3/HoMnO3)15 superlattices were grown on (111) ZrO2 (Y2O3) and (111) Pt/TiO2/SiO2/p-type (100)Si substrates. Epitaxial stabilization was used on the same substrates to grow DyMnO3 or TbMnO3 (which normally crystallize in an orthorhombic, perovskite-type, structure). All heterostructures were obtained oriented, with the c-axis of the hexagonal cell perpendicular to the substrate plane. This orientation is desired since it is the direction of the ferroelectric polarization in hexagonal manganites. The strain state of the films grown on conductive Pt electrodes, tracked by the evolution of the c parameter as a function of film thickness, was found to be completely different than the one obtained on YSZ. Such a study is important for future strain engineering in multilayers. High-resolution transmission electron microscopy on superlattices grown on (111)Pt indicates a high crystalline quality along the c-axis and sharp interfaces.