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Epitaxie par jets moléculaires de l'oxyde BaTiO 3 sur Si et Si 1-x Gex : étude de la croissance, des propriétés structurales ou physico-chimiques et de la ferroélectricité -applications à des dispositifs à effet de champ

Lucie Mazet 1
1 INL - H&N - INL - Hétéroepitaxie et Nanostructures
INL - Institut des Nanotechnologies de Lyon
Abstract : Monolithic integration of ferroelectric oxides on semiconductor substrates could enable the implementation of new functionalities on nanoelectronic chips. In particular, low power devices could benefit from the use of a ferroelectric. However, their integration is challenging due to several scientific and technological issues that remain to be solved, such as the oxide/semiconductor interface control, the ferroelectric polarization instability in thin films or the compatibility of device fabrication methods with industrial processes. The objectives of my PhD work have been the following: the optimization of the MBE growth of epitaxial BaTiO3 on Si and Si1-xGex substrates in order to control the crystalline orientation and ferroelectric properties, the study of size effects on ferroelectricity and preliminary work on field-effect device fabrication using our epitaxial BaTiO3 films. Different conditions, such as the temperature and the oxygen pressure P(O2), were investigated regarding the growth on silicon substrates. Using X-ray diffraction (XRD) analysis combined with advanced transmission electron microscopy techniques (STEM-HAADF, GPA, EELS), we evidenced a correlation, at the local scale and throughout the thickness of the films, between the tetragonal lattice orientation and the cationic composition. The ferroelectricity of 16-20 nm thick c-axis films, grown under an oxygen partial pressures P(O2) of 1-5x10-7 Torr at 450-525°C and annealed post-deposition under oxygen, was evidenced by piezoresponse force microscopy (PFM). Size effects were then investigated. The ferroelectricity of ultra-thin films (1.6, 2.0, 2.8, 3.2 and 4.0 nm) was demonstrated using PFM and complementary measurements by contact-Kelvin probe microscopy (cKPFM). The cKPFM analyses precluded purely electrochemical mechanisms. For 4, 5, 7 and 8 monolayers, the amplitude of the polarization pointing toward the top interface (Pup) is found to be larger than that of the Pdown polarization. This is attributed to non-ferroelectric regions or to polar regions where the polarization is pinned at an interface. In order to develop new strategies of strain and chemical engineering, we have then studied the growth of BaTiO3 on Si1-xGex substrates; this approach is of particular interest to modulate strain, especially for future transistors. To understand the effect of the presence of Ge, we investigated the growth of BaTiO3 on strained Si0.8Ge0.2 on Si(001). The in-situ monitoring of the growth by X-ray photoelectron spectroscopy combined with crystalline structure and interface analyses by XRD and STEM-HAADF revealed the importance of the substrate surface preparation. Si0.8Ge0.2 passivation with Ba atoms promote the direct epitaxy of a (112) oriented BaTiO3 film, via the formation of an epitaxial interfacial layer, identified as the orthorhombic Ba2SiO4 silicate. This silicate is oriented along two directions in the Si0.8Ge0.2 plane, leading to the two <110> and <111> in-plane orientations observed for BaTiO3. Finally, in collaboration with IBM, a low temperature “gate-last” integration scheme was developed to integrate ferroelectric BaTiO3 thin films on silicon in field-effect devices (capacitors, transistors). BaTiO3 films were grown by MBE on pre-patterned substrates. A suitable process was developed for the TiN electrode deposition and for lithography/etch steps. Thanks to a detailed electrical characterization of the capacitors, we evidenced a ferroelectric behavior (Tc~105°C) in the structures composed of nano-grains embedded in an amorphous matrix. This first demonstration of ferroelectric “quasi-amorphous” BaTiO3-based capacitors on silicon, with a relatively moderate relative permittivity (κ~25) and with low leakage currents is of particular interest for low power logic and memory devices. The transistor fabrication following a damascene technology is being finalized.
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Lucie Mazet. Epitaxie par jets moléculaires de l'oxyde BaTiO 3 sur Si et Si 1-x Gex : étude de la croissance, des propriétés structurales ou physico-chimiques et de la ferroélectricité -applications à des dispositifs à effet de champ. Science des matériaux [cond-mat.mtrl-sci]. Ecole Centrale de Lyon, 2016. Français. ⟨tel-02081281⟩

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