| dc.description.abstract | As conventional memory technologies approach their limit of scalability, there is a quest to find new technologies to replace existing memories. Of the emerging switching phenomena, ferroelectric switching and resistive switching have been considered for this work. Ferroelectricity is a property by which a material develops a spontaneous polarization that can be reversed by and external electric field. Resistive switching, the basis for the novel “memristor” devices, is a property that enables a device switch to a low or high resistance state depending on the magnitude and polarity of the applied voltage. In this work, various nanostructures have been explored to achieve property enhancement in functional oxides. For example, vertically aligned nanocomposite structures consist of two different materials that are simultaneously deposited onto a single substrate, and grow as two distinct phases. Vertically aligned nanocomposite structures offer the advantage of strain tuning through the vertical interfaces between phases.
First, to improve the ferroelectric properties of BaTiO3, a conventional ferroelectric material, epitaxial vertically aligned nanocomposite BaTiO3-CeO2 films have been deposited on SrTiO3 substrates. These films exhibit a columnar structure with high epitaxial quality. The films show a similar ferroelectric response as that of pure thin film BaTiO3, but with an improved Curie temperature, despite the incorporation of CeO2. These nanocomposite structures have been replicated on Si substrates using a double buffer layer of SrTiO3/TiN to achieve the eventual integration of these films on Si. No reduction in ferroelectric properties has been observed, but the films again showed an improvement in the Curie temperature.
Second, a simple resistive switching device has been demonstrated by the in situ partial oxidation of a TiN film under three different oxidation time periods. The oxidized region consists of near stoichiometric TiO2, and serves as the oxide layer, while the unoxidized TiN serves as the bottom electrode. All films exhibit bipolar resistive switching and all films are forming-free. The forming-free property is attributed to an oxygen deficient TiO2-x layer at the interface between the oxide and nitride regions.
Third, ZnO, a piezoelectric, has been selected as another complementary second phase material for BaTiO3. Epitaxial and highly textured vertically aligned BaTiO3-ZnO composite films have been deposited on SrTiO3 substrates and SrTiO3/TiN buffered Si substrates, respectively. Electrical characterization shows that the films grown on both substrates are ferroelectric at room temperature and exhibit similar properties. Composition analysis shows that both the laser fluence and the oxygen partial pressure can modulate the Ba/Ti cation stoichiometry which, in turn, impacts the ferroelectric properties. This is the first demonstration of the vertically aligned nanocomposite of BaTiO3 and ZnO and its silicon based integration.
Finally, based on the excellent buffer layer and diffusion barrier properties of TiN for integrating functional oxides on Si, TiN has been applied as a protective layer on metal surfaces. A 500 nm thick TiN layer has been demonstrated to serve as an excellent diffusion barrier in extreme environments. | en |
