Guangda Hu

Effects of Gd substitution on structure and ferroelectric properties of BiFeO3 thin films prepared using metal organic decomposition

BiFeO3 and Bi1−xGdxFeO3 (BGFO) (x=0.03, 0.05, 0.07, 0.10, and 0.15) thin films were deposited on Pt∕Ti∕SiO2∕Si substrates using a metal organic decomposition process. X-ray diffraction results show that a gradual phase transition from rhombohedral to pseudotetragonal structure may occur in BGFO films with the increase of Gd content. Due to the lower leakage currents resulting from the smaller grain sizes and smoother surfaces, well saturated P-E hysteresis loops, which show weak dependence of frequency in the range of 3–50kHz, can be observed in all BGFO films at room temperature. The remanent polarization for BGFOx=0.03 film is about 79μC∕cm2. In addition, no evident fatigue can be observed after 109 switching cycles.

Reduced leakage current, enhanced ferroelectric and dielectric properties in (Ce,Fe)-codoped Na0.5Bi0.5TiO3 film

Na0.5Bi0.5TiO3 (NBT), Ce-doped NBT (NBTCe), Fe-doped NBT (NBTFe), and (Ce,Fe)-codoped NBT (NBTCeFe) thin films were fabricated on LaNiO3(100)/Si substrates by metal organic decomposition. The leakage current density of NBTCeFe at 500 kV/cm is reduced by approximately two orders of magnitude by reducing the density of oxygen vacancies and forming the defect complexes, compared with NBT film. Enhanced ferroelectricity is achieved in NBTCeFe with a large remanent polarization of 24 μC/cm2 due to the reduced leakage current, extra A-site vacancies, and lattice distortion. The NBTCeFe also exhibits a dielectric constant of 585 and dielectric loss of 0.05 at 10 kHz.

Enhanced Piezoelectric Properties of Epitaxial W-Doped BiFeO3 Thin Films

BiFeO3 and BiFe1-xWxO3 (x = 0.005, 0.01, 0.02) films were epitaxially deposited on LaNiO3(100)/Si substrates using a metal organic decomposition process. Well-saturated polarization–electric field hysteresis loops can be observed in all BiFe1-xWxO3 films owing to their leakage currents being lower than that of the BiFeO3 film. Additionally, the domains in BiFe0.995W0.005O3 and BiFe0.99W0.01O3 films can be uniformly switched using a piezoelectric-mode atomic force microscope (AFM) due to the efficient deaging effect of W6+. More importantly, the BiFe0.99W0.01O3 film exhibits the highest piezoelectric coefficient (~132 pm/V) among all the films, making it promising for applications in high-temperature piezoelectric devices and AFM-tip-based data storage.

Enhanced ferroelectric properties of predominantly (100)-oriented CaBi4Ti4O15 thin films on Pt∕Ti∕SiO2∕Si substrates

Predominantly (100)-oriented CaBi4Ti4O15 (CBTi) films were fabricated on Pt (111)∕Ti∕SiO2∕Si substrates using a metal organic decomposition method at annealing temperatures ranging from 600to800°C. The growth mode of the predominantly (100)-oriented CBTi films fabricated by the sequential layer annealing method was discussed based on the structure evolution with the annealing temperature. The remanent polarization and coercive field of the CBTi film annealed at 750°C are 38.1μC∕cm2 and 216kV∕cm, respectively. No fatigue can be observed after 109 switching cycles. The remanent polarization of the purely a-axis-oriented CBTi film should be higher than 50μC∕cm2.

Effects of Bi2Ti2O7 buffer layer on memory properties of BiFe0.95Mn0.05O3 thin film

BiFe0.95Mn0.05O3 (BFMO) thin films with and without Bi2Ti2O7 (BTO) buffer layer were fabricated on p-type Si (111) substrates by metal organic decomposition. The maximum memory window of BFMO/Si is only 0.3 V due to the severe charge injection. In contrast, the larger memory windows are 0.8 and 2.4 V, respectively, for BFMO deposited on as-deposited BTO/Si and annealed BTO/Si. More importantly, the memory window of BFMO/annealed BTO/Si is not affected by changing voltage ramp rate and frequency at ±6 V. The BFMO also shows much reduced leakage current by using an annealed BTO buffer layer.

Thickness-dependent ferroelectric behavior of predominantly (117)-oriented Bi3.15Nd0.85Ti3O12 thin-film capacitors

Ferroelectric hysteresis loops, remanent polarization, coercive field, and leakage current characteristics were investigated by increasing the film thickness (from 103 nm to 401 nm) of predominantly (117)-oriented Bi3.15Nd0.85Ti3O12 films. The thickness dependence of the coercive fields shows log coercive field vs. log thickness with a slope of approximately −0.65, which is characteristics of the Kay–Dunn scaling law. The films exhibited very low leakage current on the order of 10−7A/cm2 at an applied electric field of 100 kV/cm. Without a discernible thickness-dependence behavior, the leakage current characteristics, including both Schottky emission and space-charge-limited conduction, were well described by a modified Schottky contact model.

Elimination of domain backswitching in BiFe0.95Mn0.05O3 thin films by lowering the layer thickness

BiFe0.95Mn0.05O3 thin films with different layer thicknesses annealed at temperatures ranging from 500 to 600 °C were fabricated on indium tin oxide/glass substrates using a metal organic decomposition process. The ferroelectric hysteresis loops reveal that the remanent polarization (Pr) in the films of 42, 31, and 20 nm per layer can be improved dramatically by increasing the annealing temperature, while the Pr in the films of 25 nm/l shows weak dependence on the annealing temperature. For a given temperature, the Pr in the films of 25 nm/l is much larger than those in the films of 20, 31, and 42 nm/l, especially at temperatures below 575 °C. These phenomena can be ascribed to the lower content of defect complexes resulting from the columnar structure formed by decreasing layer thickness, which in turn results in the elimination of domain backswitching. The results also indicate that reducing the layer thickness moderately is an effective method for lowering the annealing temperature.

Enhanced multiferroic properties of (1 1 0)-oriented BiFeO3 film deposited on Bi3.5Nd0.5Ti3O12-buffered indium tin oxide/Si substrate

BiFeO3 (BFO) films with and without a Bi3.5Nd0.5Ti3O12 (BNT) buffer layer were fabricated on indium tin oxide (ITO)/Si substrates using a metal organic decomposition process. X-ray diffraction measurements reveal that a BNT buffer layer can favour the growth of (1 1 0)-oriented grains in the BFO film. BFO film with a BNT buffer layer exhibits well saturated P–E hysteresis loops with good rectangularity as well as large remanent polarization (~70.2 µC cm−2) owing to its higher volume fraction of (1 1 0)-oriented grains, lower leakage current and lower coercive field in comparison with those of the BFO film deposited directly on the ITO/Si substrate. In addition, the magnetization of the (1 1 0)-oriented BFO film is more easily saturated compared with that of the polycrystalline BFO film without a BNT buffer layer.

Na0.5Bi0.5)0.87Pb0.13TiO3 thin films on different substrates for ferroelectric memory applications

(Na0.5Bi0.5)0.87Pb0.13TiO3 thin films have been prepared on Pt∕Ti∕SiO2∕Si and Bi2Ti2O7∕Si substrates using a metal organic decomposition method. The electrical measurements were conducted on metal-ferroelectric-metal or metal-ferroelectric-insulator-semiconductor capacitors. As Bi2Ti2O7 is an effective diffusion barrier, the leakage current and memory properties of (Na0.5Bi0.5)0.87Pb0.13TiO3∕Bi2Ti2O7∕Si are relatively improved. The polarization-electric field and capacitance-voltage measurements of (Na0.5Bi0.5)0.87Pb0.13TiO3 on Pt show good ferroelectric properties. The three phase transitions in (Na0.5Bi0.5)0.87Pb0.13TiO3 can be recognized by the change of capacitance with temperature.

PREPARATION OF (100)- AND (110)-ORIENTED LaNiO3 THIN FILMS BY CHEMICAL SOLUTION DEPOSITION

LaNiO3(LNO) thin films were prepared on Si(100) substrates by a chemical solution deposition method. The orientation of LNO films was controlled by changing the acetic acid amount, concentration of the precursor solutions and pre-annealing time. The highly (100)- and (110)-oriented LNO films were obtained by optimizing these processing conditions. The orientation factors of the optimized (100)- and (110)-oriented LNO films were more than 0.99 and 0.93. Their room-temperature resistivities were 5.1 × 10-4Ωcm and 5.4 × 10-4Ωcm, respectively. The growth mechanisms of the (110)- and (100)-oriented LNO films were discussed.