Dinghua Bao

Band-gap energies of sol-gel-derived SrTiO3 thin films

Band-gap energies of sol-gel-derived SrTiO3 thin films were studied in terms of annealing temperature and film thickness. The band-gap energies of highly crystallized films were comparable to those of single crystals reported, whereas for poor-crystallized films, their band-gap energy values were much larger than those of single crystals. The larger band-gap energy shift was believed to be mainly due to both quantum size effect and existence of amorphous phase in thin films. The band-gap energies also showed a strong dependence on film thickness. There was a critical film thickness (∼200 nm), above which the films had band-gap energies close to those of crystals or bulks, but below that, the values shifted largely, which can be attributed to the influence of crystallinity of thin films. Such a thickness effect of band-gap energy should be of high interest in optical device applications.

Structural, dielectric, and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films with conductive LaNiO3 bottom electrodes

We report structural, dielectric, and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films on LaNiO3/SiO2/Si substrates, where all the films were prepared by the sol-gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of film layer. Upgraded film exhibited (100) preferentially oriented growth, whereas downgraded film showed a randomly oriented growth, where the films with La content increasing or decreasing gradually along film thickness from the substrate to the top surface are called “upgraded” or “downgraded” films, respectively. The dielectric constants, for upgraded and downgraded films annealed at 650 °C for 60 min, were found to be 659 and 641, respectively. The thin films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of the offsets displayed a power law dependence on the electric field, and the direction of the offsets depended on t...

Ferroelectric properties of sandwich structured (Bi, La)4T3O12/Pb(Zr, Ti)O3/ (Bi, La)4Ti3O12 thin films on Pt/Ti/SiO2/Si substrates

Sandwich structured (Bi, La)4Ti3O12/Pb(Zr, Ti)O3/(Bi, La)4Ti3O12 thin films were fabricated on Pt/Ti/SiO2/Si substrates, with the intention of simultaneously utilizing the advantages of both (Bi, La)4Ti3O12 (BLT) and Pb(Zr, Ti)O3 (PZT) thin films such as non-fatigue behaviours of BLT and good ferroelectric properties of PZT. Both BLT and PZT layers were prepared by a chemical solution deposition technique. The experiments demonstrated that the sandwich structure showed fatigue-free characteristics at least up to 1010 switching bipolar pulse cycles under 8 V and excellent retention properties. The sandwich structured thin films also exhibited well-defined hysteresis loops with a remanent polarization (2Pr) of 8.8 µC cm-2 and a coercive field (Ec) of 47 kV cm-1. The room-temperature dielectric constant and dissipation factor were 210 and 0.031, respectively, at a frequency of 100 kHz. These results suggest that this sandwich structure is a promising material combination for ferroelectric memory applications.

Structural and electrical characteristics of chemical-solution-derived (Bi,La)4Ti3O12 thin films with various Bi2O3 template layers

Ferroelectric (Bi,La)4Ti3O12 (BLT) thin films with different Bi2O3 template layers were prepared on Pt/Ti/SiO2/Si substrates by a chemical-solution deposition method. The BLT films with a thin Bi2O3 bottom layer and those with a thin Bi2O3 intermediate layer had a (117) preferentially oriented growth after annealing at 750 °C, while those with a thin Bi2O3 upper layer and those without a Bi2O3 template layer exhibited a high c-axis orientation. The surface morphologies changed with different preferential orientations. The electrical measurements showed that the use of Bi2O3 template layers improved significantly the P–E hysteresis loops of BLT thin films. The remanent polarization (2Pr) and coercive field (Ec) values of BLT films without a Bi2O3 template layer, with a Bi2O3 upper layer, with a Bi2O3 bottom layer, and with a Bi2O3 intermediate layer annealed at 750 °C were 10.8, 29.12, 26.17, and 19.67 μC/cm2; 79.0, 74.5, 75.5, and 76.3 kV/cm, respectively, at an applied electric field of 350 kV/cm. The di...

Improved electrical properties of (Pb, La)TiO3 thin films using compositionally and structurally compatible LaNiO3 thin films as bottom electrodes

Homogeneous LaNiO3 thin films were grown on thermally oxidized silicon (SiO2/Si) substrates by a sol-gel technique, for the subsequent sol-gel deposition of (Pb, La)TiO3 thin films, under the assumption that the structural and compositional compatibility between ferroelectric films and bottom electrodes could lead to enhanced electrical properties of ferroelectric thin films. In this work, the LaNiO3 films served three functions: the first was used as bottom electrodes for the fabrication of integrated ferroelectric devices on Si due to their low resistivity; the second was used as a seeding layer, promoting perovskite phase formation due to their structural compatibility with ferroelectric films; and the third was to suppress the composition diffusion between ferroelectric films and bottom electrodes due to their composition compatibility. The experimental results demonstrated that (Pb, La)TiO3 films prepared on the LaNiO3/SiO2/Si substrates had excellent electrical properties. The dielectric constant an...

Dielectric and ferroelectric properties of compositionally graded (Pb,La)TiO3 thin films on Pt/Ti/SiO2/Si substrates

Compositionally graded (Pb,La)TiO3 thin films were prepared on platinum-coated silicon substrates by a sol–gel technique. The crystalline orientation and surface morphology of the graded films were closely related to the deposition sequence of the film layer. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 765 and 374, respectively. The compositionally graded films had large polarization offsets in hysteresis loops when driven by an alternating electric field. The magnitude of polarization offsets displayed a power-law dependence on the electric field, and the direction of the offsets depended on the direction of the composition gradient with respect to the substrate. The offset, 250 μC/cm2 at the driving electric field of 250 kV/cm, was obtained. These results showed that the sol–gel technique was a very promising route for the realization of compositionally graded ferroelectric thin films and the compositionally-graded (Pb,La)TiO3 thin films ...

Abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films with LaNiO3 bottom electrodes

We report abnormal ferroelectric properties of compositionally graded Pb(Zr,Ti)O3 thin films on LaNiO3-coated SiO2/Si substrates, where Pb(Zr,Ti)O3 and LaNiO3 films were prepared by a metalorganic decomposition technique and a sol–gel technique, respectively. It was found that the hysteresis loops of the Pb(Zr,Ti)O3 graded films measured by the conventional Sawyer–Tower method shifted along the polarization axis, i.e., they showed polarization offsets when applied by an alternating electric field. The polarization offsets were 82.5 μC/cm2 at 270 kV/cm and 62.5 μC/cm2 at 185 kV/cm for the up-graded film and the down-graded film, respectively. The absolute magnitude of the polarization offsets was closely related to the magnitude of the driving electric field, and the direction of the polarization offsets depended on the direction of the composition gradient with respect to the substrate. Analysis indicated that the polarization offsets did not originate from asymmetric contact effects, oxygen vacancies, al...

Preparation of conductive LaNiO3 film electrodes by a simple chemical solution deposition technique for integrated ferroelectric thin film devices

Well-crystallized LaNiO3 (LNO) thin films were grown on thermally oxidized silicon (SiO2/Si) and SrTiO3 substrates by a simple chemical solution deposition (CSD) technique. The LNO thin films obtained had pseudocubic phase without the existence of impurity phase. The LNO films on SiO2/Si substrates were polycrystalline, dense, and randomly oriented with a uniform surface and cross-section, whereas those on SrTiO3 substrates were (100) highly oriented with uniform grain size. The room temperature resistivity of the films on SiO2/Si substrates annealed at 750°C was about 0.54 mΩ cm, which is much lower than that of the films derived by the water-based solution technique. The subsequent deposition and electrical measurements of PbTiO3, (Pb,La)TiO3, Pb(Zr,Ti)O3 thin films on the CSD derived LNO/SiO2/Si confirmed the LNO films to be promising electrode materials for ferroelectric thin films.

Crystallization and optical properties of sol–gel-derived PbTiO3 thin films

PbTiO3 (PT) thin films were prepared on ZrO2, SrTiO3, thermally oxidized silicon, and fused silica substrates by a simple sol–gel technique. The crystallization and optical properties of the thin films were investigated. The PT films on ZrO2 and SrTiO3 substrates have a c-axis preferential orientation, whereas the films on thermally oxidized silicon and fused silica substrates are slightly a-axis oriented. The thin films exhibited good optical transmittivity, and had optical direct transitions. The band gap energy of the film annealed at 650°C was 3.60 eV, which is comparable to those of PT polycrystalline ceramic bulk and films. The thin films on Pt/Ti/SiO2/Si substrates exhibited a ferroelectric hysteresis loop with a remanent polarization of 17.1 μC cm−2 and a coercive field of 132.1 kV cm−1.

Composition gradient optimization and electrical characterization of (Pb, Ca)TiO3 thin films

Compositionally graded (Pb, Ca)TiO3 thin films were prepared by a monoethanolamine-modified sol–gel technique on platinum-coated silicon substrates at the annealing temperature of 600 °C. The composition gradient of the films was greatly improved by a modified annealing method. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 469 and 355, respectively. Both were larger than those reported for conventional (Pb, Ca)TiO3 thin films. The compositionally graded films had large polarization offsets in hysteresis loops when excited by an alternating electric field. The more smooth the composition gradient of the graded film, the larger the polarization offset. This was consistent with a theoretical model reported previously by Mantese and coworkers [Appl. Phys. Lett. 71, 2047 (1997)]. The magnitude of polarization offset displayed a power-law dependence on the electric field, and the direction of the offset depended on the direction of the composition gradient with respect to the substrate. Both up-graded and down-graded films had good leakage current characteristics.Compositionally graded (Pb, Ca)TiO3 thin films were prepared by a monoethanolamine-modified sol–gel technique on platinum-coated silicon substrates at the annealing temperature of 600 °C. The composition gradient of the films was greatly improved by a modified annealing method. The dielectric constants, for up-graded and down-graded films annealed at 600 °C for 60 min, were found to be 469 and 355, respectively. Both were larger than those reported for conventional (Pb, Ca)TiO3 thin films. The compositionally graded films had large polarization offsets in hysteresis loops when excited by an alternating electric field. The more smooth the composition gradient of the graded film, the larger the polarization offset. This was consistent with a theoretical model reported previously by Mantese and coworkers [Appl. Phys. Lett. 71, 2047 (1997)]. The magnitude of polarization offset displayed a power-law dependence on the electric field, and the direction of the offset depended on the direction of the composition ...