J. K. 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.

PHASE DEVELOPMENTS, MICROSTRUCTURES, AND FERROELECTRIC PROPERTIES OF BiFeO3 THIN FILMS PREPARED BY A SOLUTION

ABSTRACT The formation of phase-pure BiFeO3 (BFO) thin films using a chemical solution deposition on platinium electrode has been investigated systematically by changing the annealing condition of the films. From the X-ray diffraction results, it was found that the formation of the phase-pure BFO was favorable under lower oxygen partial pressure, slower heating rate and Pt(200)/Ti/SiO2/Si(100) substrate. The remanent polarization (P r ) and coercive field (E c ) values of the phase-pure BFO thin film annealed at 600°C in nitrogen atmosphere by conventional furnace annealing method are approximately 0.9 μC/cm2 and 20 kV/cm, respectively.

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.

ENHANCED TUNABILITY AND ELECTRICAL PROPERTIES OF Ba0.7Sr0.3TiO3 THIN FILMS BY CU DOPING

ABSTRACT Cu-doped Ba0.7Sr0.3TiO3 (BST) thin films were prepared on Pt(111)/Ti/SiO2/Si(100) substrates by a sol-gel spin coating process. The coated thin films were annealed at 750°C for 3 min in oxygen, air and nitrogen atmospheres by a rapid thermal annealing process. All of the Cu-doped BST thin films were consisted of a single phase perovskite structure and exhibited smooth crack-free surfaces. We investigated the effects of the annealing atmosphere on the microstructure and the electrical properties of the Cu-doped BST films. The Cu-doped BST film by annealing in oxygen exhibits a dielectric constant of 923 with electric field dependent tunability of 48.6% at electric field 250 kV/cm and a leakage current density of 1.82 × 10− 5A/cm2 at electric field of 150 kV/cm.

STRUCTURAL AND ELECTRICAL PROPERTIES OF (BaxSr1-x)TiO3 THIN FILMS PREPARED BY A SOL-GEL METHOD

ABSTRACT BaxSr1-xTiO3(BST) thin films with x = 0.5, 0.6, 0.7 and 0.8 were fabricated on p-type Si(100) substrates by a sol-gel spin coating method. All samples show the perovskite polycrystalline structures without the pyrochlore phase and have dense surface texture and crack-free uniform microstructures. The C-V characteristics of the BST thin film/p-type Si(100) capacitors exhibit clockwise hysteresis loops and also clearly show regions of accumulation, depletion and inversion corresponding to those of metal-ferroelectric-semiconductor structures. Furthermore, a constant memory window has been observed through the entire frequency ranges from 1 kHz to 10 MHz.

PROPERTIES OF SOL-GEL DERIVED BISMUTH-BASED PYROCHLORE THIN FILMS

ABSTRACT A new type of bismuth-based relaxor, such as Bi2(Y1/2Nb1/2)2O7 and Bi2(Mg1/3Nb2/3)2O7 thin films were fabricated on Pt/Ti/SiO2/Si substrate by a sol-gel spin coating process. The films exhibit characteristic diffused phase transition. The temperature dependent reciprocal dielectric constant characteristics of the films deviates from the Curie-Weiss law in the temperature range over the dielectric constant maximum temperature (T m ). The diffuseness of the phase transition of the thin films was estimated by calculating the difference between T m and T cw (starting temperature following the Curie-Weiss law). For the thin films annealed at 750°C, the temperature T m , the Curie-Weiss temperature Θ, the Curie-Weiss constant c and the temperature T cw were measured at frequency of 103 Hz, respectively.