Epitaxial ferroelectric thin films on si(001)strain tuning of batio3 and stabilization of polar phase in hf0.5zr0.5o2

Resumen

Ferroelectrics are widely used materials for nonvolatile memories and other various devices due to their promising characteristics such as low power consumption, high switching speeds and non-volatility. However, the Pb-containing in most commercial devices of ferroelectrics is a major concern. Also, cell size of the commercial ferroelectric memories cannot be scaled down. Therefore, alternative ferroelectric material are needed. In this thesis, we investigated two different lead free ferroelectric materials − barium titanate (BaTiO3) and hafnium zirconium oxide (Hf0.5Zr0.5O2), aiming to grow epitaxial ferroelectric films integrated with Silicon. We have developed a new strain engineering for integrating epitaxial BaTiO3 films on Si using LaNiO3/CeO2/YSZ buffer layers by pulsed laser deposition. The strategy is to control the deposition parameters (substrate temperature, oxygen pressure and growth rate) which can affect the balance between thermodynamics and kinetics during the growth of BaTiO3 films. We show that epitaxial growth of the BaTiO3 films with selectable polar axis orientation can be achieved in a broad deposition parameter window with a large impact on the lattice strain of the BaTiO3 polar axis (exceeding 2% in films thicker than 100 nm) and ferroelectric properties. This flexible strain method overcomes the primary limitations of conventional substrate-based strain engineering: it can be applied to films on specific substrates including Si(001) and perovskites, and it is not restricted to ultrathin films. Then, we investigated the role of electrode-interfaces in a series of BaTiO3 capacitors using different electrodes (perovskite metallic oxides La2/3Sr1/3MnO3 and LaNiO3 and noble metal Pt). The impact of the electrodes on the electrical properties such as ferroelectric polarization, leakage and imprint field of epitaxial BaTiO3 films was determined. We also present a detailed study of epitaxial growth of Hf0.5Zr0.5O2 on SrTiO3 substrate buffered with La2/3Sr1/3MnO3 electrode by pulsed laser deposition. The growth window of deposition temperature, oxygen pressure and film thickness is mapped and the polar orthorhombic phase with (111) orientation can be stabilized in the Hf0.5Zr0.5O2 films. We find that the growth parameters and thickness determine the relative amount of orthorhombic phase and its lattice strain, permitting the control of the structural and functional properties. Through selection of deposition parameters and film thickness, the electrical properties can be tailored with low leakage and high remnant polarization up to 24 μC/cm2 without the need of wake-up. The coercive field−thickness-2/3 scaling, often observed in ferroelectric perovskites, is reported for the first time for ferroelectric hafnia films. Then, we investigated the epitaxial integration of Hf0.5Zr0.5O2 film in a capacitor heterostructure on Si(001). High quality (111)-oriented Hf0.5Zr0.5O2 films have been successfully integrated on Si with La2/3Sr1/3MnO3/LaNiO3/CeO2/YSZ or simpler La2/3Sr1/3MnO3/SrTiO3 buffer layers, presenting robust ferroelectric properties, including the remnant polarization above 30 μC/cm2. The notable endurance (above 1010 cycles) and long retention (over 10 years) either on SrTiO3(001) or Si(001) are also reported.