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Croissance épitaxiale d'oxydes "high-κ" sur silicium pour CMOS avancé : LaAlO3, Gd2O3, γ-Al2O3

Abstract : Metal oxide semiconductor field effect transistors (MOSFETs) are at the very heart of modern integrated circuits (IC's). A fundamental limit of the downscaling of these devices is in view, concerning the exponential increase in the tunneling current with further decreasing the film thickness of the SiO2 gate oxide. The reduction of the leakage current while maintaining the same gate capacitance requires therefore a thicker film with a higher dielectric constant.
While amorphous oxides will likely be employed in the first phase, the semiconductor industry requires solutions for crystalline oxides with an epitaxial interface to the silicon substrate. The use of monocrystalline oxides would allow to reach the properties of bulk oxides and to obtain abrupt interfaces without presence of interfacial layers. However the material choice is limited by the lattice mismatch of the oxide with the substrate and also by the thermodynamic stability of oxides with respect to silicon. The oxides explored in this thesis were LaAlO3 and Gd2O3, selected for their electronic properties (permittivity and band offsets), and γ-Al2O3 chosen for its thermodynamic properties with respect to Si. The oxide heterostructures have been elaborated using molecular beam epitaxy (MBE).
Attempts to epitaxially grow LAO on Si have been hindered by unfavorable thermodynamic growth conditions leading to the formation of silicates at the interface. Consequently, interface engineering appears to be indispensable to grow LAO films with good crystallinity on Si(001). Different approaches have been considered in this study. The first approach consists in using a thin buffer layer made of strontium oxide (SrO) or strontium titanate (SrTiO3). Atomically flat LaAlO3 epitaxial films were grown on these buffers but this approach has some limitations due to the low thermal stability of the (SrO,SrTiO3)/Si interfaces.
The second studied oxide was gadolinium oxide (Gd2O3). If the growth appeared to be monodomain and of high quality on Si(111), we observed a bidomain growth on (001)-oriented Si substrates. The origin of this behavior comes from the alignment of the (110) planes of the oxide on the (001) planes of Si, with a 90° rotation at each silicon step. We then showed that the use of a vicinal Si(001) substrate allows to obtain a monodomain growth of Gd2O3(110). In spite of its limitations (interfacial silicate formation at high temperatures) the Gd2O3/Si system is considered now as one of most interesting crystalline high-κ oxide for integration in CMOS technologies.
Optimal growth conditions leading to the formation of high quality 2D γ-Al2O3 films have been defined. For alumina films thicker than 1.6 nm on Si(001), TEM and RHEED studies have allowed to detect a surface transition from (001)-oriented γ-Al2O3, with a cubic surface symmetry, to (111)-oriented γ-Al2O3 , with a hexagonal surface symmetry. This result shows that cubic γ-Al2O3(001) can be grown pseudomorphically cube-on-cube on Si(001) (mismatch of -2.9%) up to 1.6 nm with an abrupt coherent interface.
Finally, the high potentiality of γ-Al2O3(001) to be used as a buffer layer for further growth of amorphous or crystalline high-κ oxides on Si(001) was demonstrated. This combination allows obtaining robust heterostructures on silicon, avoiding silicate or SiO2 formation at the interface despite high temperature and high oxygen pressure growth conditions.
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Clément Merckling. Croissance épitaxiale d'oxydes "high-κ" sur silicium pour CMOS avancé : LaAlO3, Gd2O3, γ-Al2O3. Matière Condensée [cond-mat]. Ecole Centrale de Lyon, 2007. Français. ⟨tel-00201791⟩

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