Introduction. In the late 1990s, the success of SrTiO₃ epitaxial growth on Si by molecular beam epitaxy (MBE) opened a path for integrating complex oxides on Si-based platforms. In particular, ferroelectric perovskite oxides offer promising perspectives to improve or add functionalities on-chip like low power logic devices or integrated photonics [1]. BaTiO₃, the prototypical ferroelectric perovskite oxide, is a good candidate for these applications [2, 3]. However, regarding practical devices, integrating a perovskite oxide epitaxially on Si by MBE is still in its infancy. One key point is the control of the crystalline orientation, which determines the polarization orientation within the thin film. Playing with experimental parameters of growth and the composition of the Si _xGe_1-x semiconductor substrate are means to manipulate the competition between compressive stress from epitaxy and tensile stress from thermal expansion. In order to support MBE growth strategies, aberration-corrected scanning transmission electron microscopy has been investigated to determine both the strain and the chemical state of epitaxial BaTiO₃ thin films.

Experiment. BaTiO₃ was epitaxially grown on Si _xGe_1-x (x=1 and x = 0.20) substrates either using an SrTiO₃ buffer layer to reduce both thermal and lattice mismatches between BaTiO₃ and Si or without any buffer on Si_0;2Ge_0.8. The complex oxides were grown by MBE using Sr, Ba and Ti effusion cells. Details on the growth are given in refs [1,4]. BaTiO₃ was grown directly on strained Si_0.8Ge_0.2/Si substrates using a barium passivation. STEM-HAADF images were collected on a FEI Titan Low-Base 60-300 probe corrected microscope and the data treated using the geometric phase analysis (GPA). STEM-EELS data were also acquired with a special attention to Ba, Ti and O elements.

Main results. In a first part, we will first describe the crystalline structure and cationic composition studied at the nanoscale in BaTiO₃/SrTiO₃/Si heterostructures. The effect of oxygen pressure will be discussed. We show that the lattice parameter profile evolution within the thickness of the BaTiO₃ films is clearly associated with modifications of the cation stoichiometry within the thickness and that there is a clear impact of the oxygen pressure on both lattice parameter and composition profiles. In a second part, we will discuss the particular epitaxial state of BaTiO₃ grown directly on Ba-passivated strained Si_0.8Ge_0.2.

References.

[1] L. Mazet et al., A review of molecular beam epitaxy of ferroelectric BaTiO₃ films on Si, Ge and GaAs substrates and their applications, Science and Technology of Advanced Materials. 16, 036005 (2015)

[2] S. Salahuddin, S. Datta, Use of Negative Capacitance to Provide Voltage Amplification for Low Power Nanoscale Devices, Nano Letters. 8, 405–410 (2008)

[3] S. Abel et al., A strong electro-optically active lead-free ferroelectric integrated on silicon, Nature Communications, 4, 1671 (2013)

[4] L. Mazet et al., Structural study and ferroelectricity of epitaxial BaTiO₃ films on silicon grown by molecular beam epitaxy, Journal of Applied Physics. 116, 214102 (2014) 

Figures:

Fig. 1 Atomic structure image of an epitaxial BaTiO3 (BTO) crystal grown on a SrTiO3 (STO)-buffered Si substrate and corresponding lattice parameters maps determined by GPA analysis.

Fig. 2 Lattice parameters profiles obtained by integration of the maps shown in figure 1 and elemental profiles from a crystal of the same film.

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EMC Abstracts - https://emc-proceedings.com/abstract/epitaxial-batio3-on-si-and-sige-for-low-power-devices-nanoscale-characterization-of-the-film-and-its-interface-with-the-semiconductor-by-haadf-and-eels-in-stem/