Sang Su Kim

Effects of niobium doping on microstructures and ferroelectric properties of bismuth titanate ferroelectric thin films

Abstract Bi 4 Ti 3 O 12 thin films doped with 3 mol.% niobium were prepared on Pt/Ti/SiO 2 /Si substrates by sol–gel method. The niobium-doped Bi 4 Ti 3 O 12 (Nb–BiTiO) films annealed at 700 °C for 30 min in oxygen showed randomly oriented layered perovskite structures and was composed of plate-like and rod-like grains with no crack. The remanent polarization (2 P r ) and coercive field (2 E c ) of the Nb–BiTiO film annealed at 700 °C were 28 μC/cm 2 and 110 kV/cm, respectively. In addition, the film showed good switching endurance under bipolar pulse at least up to 4.5×10 10 cycles. Substitution of Nb in BiTiO thin films was effective for reducing the oxygen vacancies and generating good ferroelectric properties.

Ferroelectric properties of tungsten-doped bismuth titanate thin film prepared by sol–gel route

Abstract Ferroelectric properties of W-doped Bi 4 Ti 3 O 12 (W-BIT) thin film were investigated in comparison with those of undoped Bi 4 Ti 3 O 12 (BIT), which were prepared by sol–gel method and spin-coating technique. X-ray diffraction (XRD) measurements showed layered perovskite structures with a single phase in both films. The W-BIT film appeared to have superior ferroelectric properties to the undoped film prepared under the same conditions. The remanent polarization (2 P r ) and the coercive field (2 E c ) of the W-BIT film were 20 μC/cm 2 and 90 kV/cm, respectively, with a maximum applied field of 170 kV/cm. In addition, the W-BIT film showed a fatigue-free behavior up to 4.5×10 10 read/write cycles.

Low crystallization temperature and unusual switching properties of ferroelectric Nb-doped Bi 4 Ti 3 O 12 thin films prepared by rapid thermal annealing

Nb-doped Bi 4 Ti 3 O 1 2 (Nb-BIT) ferroelectric thin films were prepared in the presence of a nonionic surfactant (pluronic P123) added as an additive to the sol solution and by rapid thermal annealing (RTA). The film annealed at the relatively low temperature of 600 °C was well crystallized and showed good ferroelectricity. The switching charge of capacitors with polarization reversal rapidly increased with a large amplitude and low frequency of the applied pulse, and gradually decreased with a small amplitude and high frequency. The remanent polarization (2P r ) after subjecting the Nb-BIT capacitors to 10 8 read/write cycles was 46 μC/cm 2 , which is remarkably higher than 20 μC/cm 2 observed in the initial state. These phenomena seem to appear by the presence of space charges trapped after heat treatment by the RTA process.

Effects of Excess Bismuth Content in Precursor Solutions on Ferroelectric Properties of BiFeO3 Thin Films Prepared by a Chemical Solution Deposition

Cr-substituted BiFeO3 (BFCr) thin films prepared from precursor solutions with stoichiometric composition and various excess Bi contents ranged from 5 to 20 mol% were fabricated on Pt/TiO2/SiO2/Si(100) substrates by a chemical solution deposition method, and the effects of excess Bi content in precursor solutions on the ferroelectric properties of the as-deposited BFCr thin films were studied. It was found that the BFCr thin film prepared from precursor solution with excess Bi content of 5 mol% exhibited the best dielectric constant–frequency and polarization–electric field characteristics. In detail, its dielectric constant is 158 at frequency of 100 kHz and remnant polarization (P r ) value is 49 μ C/cm2 at electric field of 600 kV/cm.

Microstructure and electrical properties of cosubstituted BiFeO3 thin films prepared by a chemical solution deposition

5 mol % La 3 + , Nd 3 + or Y 3 + -cosubstituted BiFe 0.97 Cr 0.03 O 3 thin films (denoted by BLaFCr, BNdFCr, and BYFCr, respectively) have been successfully deposited on Pt (200)/TiO 2 /SiO 2 /Si substrates by a chemical solution deposition method. Well-saturated ferroelectric hysteresis loops and low leakage current densities have been observed in BLaFCr, BNdFCr, and BYFCr thin films with single-phase structure. The values of remanent polarization (P r ) and coercive field (E c ) were 64 μ C/cm 2 and 275 kV/cm, 61 μC/cm 2 and 290 kV/cm, and 31 μ C/cm 2 and 315 kV/cm for BLaFCr, BNdFCr and BYFCr thin films at the maximum electric field of 712 kV/cm, respectively. In the present study, the differences in values of P r seem to be related to the doped ionic radii.

Influences of Cr Doping on the Electrical Properties in BiFeO3 Thin Films

Cr-doped BiFeO3 (BFO) thin films were deposited on Pt(200)/TiO2/SiO2/Si(100) substrates by a chemical solution deposition method. The dielectric constant and dissipation factor of the BFO thin films decrease from 165 and 0.054 (undoped) to 100 and 0.02 (3 mol% Cr-doped) at a frequency of 10 kHz as the Cr content increases. The leakage current and ferroelectric properties were improved significantly by means of Cr doping. The leakage current density of 4.1×10−6 A/cm2 for the 3 mol% Cr-doped BFO thin film is about four orders of magnitude lower than that of undoped BFO thin film at an external electric field of 100 kV/cm. The 3 mol% Cr-doped BFO thin film exhibited a well-saturated hysteresis loop with a large remanent polarization (P r) of 61 μC/cm2 at room temperature. The reason for the improved leakage current and ferroelectric properties in Cr-doped BFO thin films can be attributed to the reduced oxygen vacancies in the films by Cr doping. Communicated by Dr. George W. Taylor

Effects of Annealing Temperature on the Electrical Properties of Cr-Substituted BiFeO3 Thin Films

Cr-substituted BiFeO 3 (BFCr-3 mol%) thin films were deposited on Pt(200)/TiO 2 /SiO 2 /Si substrates by a chemical solution deposition. The coated films were annealed in the temperature range of 500°C∼ 600°C for 30 min under nitrogen atmosphere. The electrical properties of BFCr thin films were found to be sensitive to the annealing temperature. It was found that BFCr thin films exhibited good ferroelectric properties, such as an improved leakage-current density and P−E hysteresis characteristics. The BFCr thin film annealed at 525°C exhibits a low leakage current density of 9.2 × 10− 7 A/cm 2 at an applied electric field of 100 kV/cm. The films exhibited a well-saturated hysteresis loop with a remanent polarization (P r ) of 47 μC/cm 2 at room temperature. The reason for the improved leakage current and ferroelectric properties in BFCr thin films can be attributed to the effects of increased grain size and oxygen vacancies reduced by Cr doping.

DIELECTRIC PROPERTIES OF ASYMMETRICALLY ANNEALED FERROELECTRIC (Ba,Sr)TiO3:MGO THICK FILMS

ABSTRACT A 20 wt.% of MgO added (Ba0.6Sr0.4)TiO3 (BST:MgO) thick film have been fabricated by a tape casting and firing method for tunable microwave applications. In order to improve ferroelectric properties, the composite BST:MgO thick films have been asymmetrically annealed by a focused beam method. Dielectric constants of BST:MgO films are changed from 1050 to 1300 at 100 kHz after 150 s annealing by the focused beam. Even though it was not observed any prominent changes from the thick films before-and after-annealing in terms of chemical composition and surface morphology, it is clear that the average particle size of the thick films calculated by Scherrers formula were increased by annealing. Furthermore, a strong correlation between particle size and dielectric constant of the BST:MgO has been observed; dielectric constant increases with increased particle size. This has been attributed to the increased volume of ferroelectric domain due to increased particle sizes.