Electric-field-induced oxygen vacancy migration and electronic property modulation in epitaxial Ca-substituted BiFeO3 thin films

Abstract

Transition metal oxides are a group of substances that form a solid system by the combination of metal ions and oxygen ions, and have various physical properties depending on the kind or configuration of metal ions bound to oxygens. Oxygen vacancies are omnipresent in the transition metal oxides, and their physical properties vary depending on their concentration and configuration. The oxygen vacancies play a role of electron donors to the pristine material and modify its lattice parameter, electronic band structure, spin structure, and electronic conduction. From these versatile effects, many researches have been performed to modulate the characteristics of transition metal oxides by controlling the concentration of oxygen vacancies. There are many industrial applications such as resistive switching memories, solid oxide fuel cells (SOFCs), photocatalysts and electrochromic devices. The dynamic motion of oxygen vacancies and the induced properties are needed to clarify in order to understand the unexpected physical phenomena and the functionalities for further applications. In this dissertation, we explore the migration of oxygen vacancies and the modulation of electronic properties in calcium substituted bismuth ferrite (BCFO). BCFO is one of the promising model oxides for oxygen vacancy motion, because it contains a lot of oxygen vacancies spontaneously generated in proportion to the amount of Ca ions (N$_{Vac}$ = N$_{Ca}$/2}) to stabilize the oxidation number of Fe$^{3+}$. Furthermore, even if it has a large amount of oxygen vacancies, it is structurally stable. We deposit epitaxial BCFO films on SrTiO$_3$ substrates using the pulsed laser deposition method. Oxygen vacancies are regarded as positively charged particles, thus, they can migrate under application of an external electric field at elevated temperatures. They are accumulated on the negatively biased electrode, thereby transforming most regions to an electrically-formed state of which the electrical conductivity becomes 10$^5$ greater than that of the as-grown state. Free hole carriers in the electrically-formed region absorb visible lights, thus showing a dark contrast. By analyzing the oxygen 1$s$-binding energy spectrum of both as-grown and electrically-formed areas, we find that the oxygen stoichiometry of the electrically-formed region approaches ‘3’ for all Ca doping ratios. By using time-dependent X-ray diffraction in a spatial resolved way, we not only check the uniformity of electrically-formed region, but also detect the first order structural transition alongside an abrupt decrease of $c$-axis lattice parameter. Disappearance of the superlattice peaks right after establishing the electrically-formed state implies that oxygen vacancy migration is related to the oxygen vacancy ordering. While the as-grown BCFO films are highly insulating states, electrically-formed BCFO regions still show a semiconducting behavior near room temperature. When temperature is lower, the electrical conductivity decreases exponentially with an exponent of temperature to the power of -1/2. It is understood in the context of the Coulomb glass transport. In addition, we observe the electronic band structure using X-ray absorption spectroscopy-photoemission electron microscopy (XAS-PEEM) and X-ray photoelectron spectroscopy (XPS). Oxygen K-edge from XAS-PEEM represents the conduction band, while valence band is obtained from XPS measurement. In the conduction band, an emerging band appears below the conduction band. This band originates from polaronic hole carriers as a consequence of a strong electron-lattice interaction. This interaction splits the valence band above the chemical potential and makes the polaronic band. This electronic band structure can also be supported through simulation of a Holstein-Hubbard model that describes the electron-lattice interaction as well as the electron correlation. We use an optical microscope to trace electric-field-induced propagation of a boundary between two different phases based on optical contrast. A clear color contrast boundary, interface between oxygen-rich and -poor region, gives hints on a trajectory of collective oxygen-vacancy migration. We quantitatively obtain a drift velocity of the order of 100 $\mu$m$\cdot$s$^{-1}$. We also observe an activation energy of 0.79 eV with an exceptionally large ionic mobility 2 $\times$ 10$^{-6}$ cm$^2\cdot$s$^{-1}\cdot$V$^{-1}$ at low temperature of 390 $^\circ$C in BCFO films at $x$=0.3. We investigate the activation energy ($E_A$) for different Ca substitution ratio ($x$= 0.1 $\sim$ 0.6). Oxygen vacancy diffusivity ($D$) depends on temperature, according to $D_0exp(-E_A/k_BT)$. At a low Ca substitution ratio of 10\%, the activation energy becomes 0.9 eV. The activation energy decreases as Ca substitution ratio increases, and steeply decreases even when Ca exceeds 40\%. At Ca substitution of 45\%, the activation energy records the lowest value of 0.43 eV. The prefactor of diffusivity ($D_0$) exhibits a similar trend to the activation energy that indicates the Ca 0.45 substituted BiFeO$_3$ thin film has the lowest hopping frequency. We perform the structural analysis for all BCFO films and confirm that strain effect on the BCFO film of $x$=0.45 lies at the boundary between compressive and tensile misfit strains on the SrTiO$_3$ substrate. From these studies, we present that the BCFO system is promising for oxygen ionic conductor and electrochromism.