Complex oxides exhibit a wide range of electrical, magnetic, optical, mechanical properties, which may even be coupled. In the past fifteen years, tremendous progresses have been achieved in the growth of complex oxides on oxide substrates, resulting in unit-cell scale control (~0.4 nm for a perovskite). Concurrently, breakthroughs in characterization techniques and modeling have resulted in a better understanding of the effects of composition, size, strain and boundaries conditions on their properties. If complex oxides could be integrated as an add-on to semiconductor technologies in a seamless process, their extraordinary wealth of physical properties could offer new functionalities in devices in order to address societal needs related to health, energy or information & communication technologies.

In this presentation, I will review some aspects on the synthesis, characterization and integration of oxide heterostructures on semiconductors. The scientific and technological challenges for achieving monolithic integration will be presented. Molecular beam epitaxy provides unique advantages to precisely construct, almost atom by atom, the oxide/semiconductor interface to achieve epitaxy. With the case of the ferroelectric BaTiO₃ perovskite on Si and SiGe, we will exemplify the various strategies developed to engineer the interface with the semiconductor and to control the oxide crystalline structure and properties.

Regarding SiGe substrates, we will show that a direct growth of BaTiO₃ can be achieved on strained Si_0.8Ge_0.2/Si. The passivation of the starting surface will be discussed. From the X-ray diffraction analysis, the films are found to be oriented with <112> directions perpendicular to the substrate's surface and show two distinct in-plane orientations. An orthorhombic epitaxial silicate is formed at the interface with Si_0.8Ge_0.2 which gives rise to the 112 epitaxial growth of BaTiO₃.The cationic and oxygen compositions of the film and interface will be discussed.

Regarding the growth on Si substrates, we will emphasize the role of size effect and of electrical /mechanical/chemical boundary conditions on the resulting ferroelectric properties. Ferroelectricity down to 4 monolayers of BaTiO3 (1.6 nm) is evidenced with two distinct switchable polarization states.

Finally, we will present current and potential realizations towards complex oxide nanoelectronics with reduced power consumption and towards integrated photonics. The integration of amorphous/nanocrystallites BaTiO₃ is a promising approach to reach ferroelectricity in a metal-oxide-semiconductor device. Such composite material exhibits the advantageous properties of an amorphous dielectric of medium relative permittivity with a low leakage current density (˜ 10^-7 A/cm²at -1V) while being ferroelectric at room temperature. It is particularly suitable for negative capacitance field-effect devices as well as for memristive devices.