Study On Optical, Electric And Gas Sensing Properties Of Epitaxial LaBaCo₂O_5+? Films

Transition metal perovskite oxides have been the object of numerous studies due to their rich magnetic and electric transport properties. LaBaCo2O5+? (LBCO) is one of interesting strongly correlated electron oxides and is considered promising for a number of important technologies such as solid oxide fuel cells, batteries, surface catalysts, chemical sensors, and thin-film functional devices related to giant magnetoresistance and spintronics. In this dissertation, highly-epitaxial LBCO films were deposited at optimized conditions using a pulsed-laser deposition (PLD) method, and then the electric and gas sensing properties of the LBCO films were investigated. In addition, optimized fabrication of highly-epitaxial LBCO films were explored using a magnetron sputtering method and the optical properties were studied. The main achievements in this work are summarized as follows:(1) Because electric transport coefficients such as carrier type, density, and mobility are significant for fabrication of widely used components like metal-semiconductor contacts and p-n junctions, Hall measurement was used to characterize highly-epitaxial LBCO films grown on MgO (001) substrates using a PLD method. In order to study the effects of oxygen deficiency, the LBCO films were annealed in 1 atm of O2, N2, Ar, and H2 at 350 ?. The electric transport behaviors and "p-to-n" transition were found rather unusual, and thus were studied for the first time by plotting the carrier density and mobility as a function of electric conductivity with comparison to the results calculated by the theory for mixed conduction. In these LBCO films, the carrier mobility was found maintaining nearly constant at-0.85 and -40 cm2/Vs for holes and electrons, respectively, and the density of p-type carriers was strongly dependent on the oxygen content in the films. Solid evidence was given demonstrating that the oxygen deficiency cannot make LBCO materials changed from p-type to n-type though some samples in Hall measurement exhibited n-type conduction due to deficiency in oxygen. The observed n-type conduction was assigned to a counterfeit phenomenon caused by the deficiency in Hall measurement, rather than a realistic transition induced by oxygen deficiency. In addition, the temperature-differential conductivity was found more sensitive than the film conductivity in detecting magnetic transition in LBCO materials with variation in temperature.(2) As the electric transport properties were strongly dependent on the annealing process that the LBCO films underwent, difference in the microstructures was further studied for the PLD-grown LBCO films annealed in O2, N2, Ar, and H2 at 350 ?. The LBCO film annealed in H2 ambient was found special in structure because a new structural ordering was observed for the first time in the x-ray diffraction (XRD) pattern. With comparison to the calculated XRD patterns of ordered and disordered LBCO phases by taking into consideration oxygen-vacancy arrangement, the new structural ordering was assigned to the ordered oxygen vacancies created in an ordered LBCO phase, strongly demonstrating that oxygen vacancy ordering could be built up at a temperature as low as 350 ? for an ordered LBCO film in H2 ambient. By determining the strains in the LBCO films annealed in ambient atmospheres, different concentration of oxygen vacancies was suggested to be one of important reasons responsible for the formation of the ordered oxygen vacancies in LBCO films.(3) In the study of LnBCO (Ln= lanthanoid) films in response to O2/H2 switching, a phenomenon special interesting is the abnormal behavior of the sensors when they were operated at temperatures below-400 ?, at which the sensors exhibited time-dependent resistances with a sharp maximum after switching gas from O2 to H2 as well as from H2 to O2. Therefore, a gas sensor was fabricated using an epitaxial LBCO film grown on MgO (001) substrate by a PLD method, and then, the gas sensing properties were comparatively studied in O2,4% H2+96%N2, N2, and Ar at temperatures ranging from 300 to 800 ?. The gas sensor was proved to have extremely high sensitivity in response to H2, and the abnormal behaviors of the sensor in response to O2/H2 switching were ascribed to a LBCO-catalyzed H-O reaction in mixed O2+H2 gas. With solid evidence from relevant experiments, the H-O reaction was proved taking place on the surface of the LBCO film; and then, the high sensitivity of LBCO films in detection of H2 or other reducing gases was suggested to be in association with a successive injection of electrons into the films maintained by the H-O reaction. Such a physical model might be important for design and improvement of other gas sensors in sensitivity either.(4) For the purpose of cost-effective and large-scale applications, optimized growth strategy for deposition of epitaxial LBCO films was explored using a radio-frequency magnetron sputtering method. The epitaxial window of LBCO films was found rather narrow on MgO (001) substrate and strongly dependent on the substrate temperature and deposition rate. In this work, epitaxial LBCO films were successfully deposited on MgO (001) substrate with the relationship of LBCO (001)//MgO (001) and LBCO [100]//MgO [100] under optimized conditions of 87 W,830 ?, and 7 Pa by radio-frequency sputtering of a-5 mm thick LBCO ceramic target. Furthermore, a high-temperature annealing process was employed for improving the crystallinity of LBCO films by rehabbing the damage of MgO (001) substrates produced during manufacture and milling. Using the rehabbed MgO substrates annealed in a clean atmosphere of air ambient at 1200,1350, and 1400 ? for 3 h, highly-epitaxial LBCO films with single-crystal feature were obtained for the first time using radio-frequency magnetron sputtering method.(5) Because of unavailable optical constants of LBCO materials in literature, the optical properties were studied in detail for the first time using the epitaxial LBCO films grown on MgO (001) substrates by magnetron sputtering method. The LBCO film was found different from a typical metal or a typical semiconductor because the dielectric functions, either real and imaginary parts, are positive and no obvious band-gap absorption is found in absorbance spectrum in the spectral range of ultraviolet-to-infrared light; and thus the LBCO film was assigned to be semi-metallic. In the spectral range from 4.5 to 0.1 eV, the absorption coefficient almost linearly decreases from 3.7×105 to 6.7×103 cm-1 and an allowed direct UV transition was recognized at photon energy of -3.20 eV in the absorption spectrum. The optical conductivity of the LBCO film could vary from ?150 to ?1500 S/cm with the increase in photon energy in the spectral range of 245?1700 nm, larger than its DC conductivity, suggesting that it has potential for applications of photo-detectors and solar cells.