Jong-Gul Yoon

Origin of metal–insulator transition temperature enhancement in underdoped lanthanum manganite films

The possibility of controlling transport properties of colossal magnetoresistance manganite films using substrate-induced strain has attracted great interest. We have investigated transport properties of La0.9Ca0.1MnO3, La0.92Ba0.08MnO3, La0.8Ba0.2MnO3, and LaMnO3 films. When the films were post-annealed at proper conditions, all of them showed metal–insulator transitions. Their transition temperatures TMI were much higher than the corresponding bulk values, irrespective of the type of substrate-induced biaxial strain. This surprising fact demonstrated that strain could not be the main origin of the TMI enhancement observed in the underdoped (dopant concentration x<0.3) manganite films. We suggested that TMI enhancements should be attributed mostly to the cationic vacancies in the post-annealed films.

Mechanisms for retention loss in ferroelectric Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors

The switching current profiles of ferroelectric Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors were investigated after high-temperature baking to elucidate the mechanisms for retention loss. In the same-state retention, a decrease in the peak value of switching current and increase in the switching time were observed. These changes in the switching characteristics were attributed to the growth of an internal field. By comparing with the switching characteristics of a virgin capacitor and using the Merz equations, we estimated quantitatively the magnitude of the internal field. In the opposite-state retention, backswitching of polarization, triggered by the internal field, was found to be the main cause of the retention loss.

Polarization dynamics and retention loss in fatigued PbZr0.4Ti0.6O3 ferroelectric capacitors

Short-time retention loss behaviors were investigated for fatigued Pt/PbZr0.4Ti0.6O3/Pt capacitors. In the short-time regime of t<1 s, fatigued capacitors showed a significant loss in retained polarization, which could be well described by a power-law function. This behavior was interpreted in terms of a superposition of polarization relaxations with a relaxation time distribution. The effects of the pulse voltage on the relaxation time distribution suggested that the retention loss should be activated by a depolarization field. As the fatigue stress was applied, the retention loss became worse. This effect can be explained in terms of the increase of the depolarization field, possibly due to the growth of an interfacial passive layer.

Retention Characteristics of Bi3.25La0.75Ti3O12 Thin Films.

Polarization retention characteristics of ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films, fabricated on Pt/TiO2/SiO2/Si substrates by pulsed laser deposition, have been investigated. The retained polarization showed a power-law-like decay within one second of writing and the normalized retained polarization was well scaled with respect to the initial polarization regardless of the write/read pulse fields. Thermally accelerated retention failure tests, performed at 120°C for 3.6 ×105 s, showed that the BLT films had quite good retention characteristics, retaining 85% of the value measured at t=1 s. It was also found that there was an accompanying imprint phenomenon during the retention test. However, the retention loss cannot be explained solely by imprint but polarization charge compensation by redistribution of defect charges should also be considered.

Hydrogen-induced degradation in ferroelectric Bi3.25La0.75Ti3O12

Effects of forming gas annealing (FGA) were investigated on ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films fabricated by a chemical solution deposition. With the FGA up to 350 °C, the Pt/BLT/Pt capacitors showed good ferroelectric characteristics without significant degradation. As the FGA temperature was increased, a decomposition of the BLT powder sample was observed by using thermogravimetric analysis. By comparing the time-dependent weight loss of BLT with that of Bi4Ti3O12 during FGA, La doping was found to significantly impede the decomposition rate. The decomposition, especially in the (Bi2O2)2+ layers, was discussed as a hydrogen-induced degradation mechanism in the Bi-layered perovskite ferroelectrics.

Mechanism of low temperature hydrogen-annealing-induced degradation in Pb(Zr0.4Ti0.6)O3 capacitors

Changes in the polarization–field hysteresis loop were systematically investigated for Pb(Zr0.4Ti0.6)O3 capacitors after forming gas annealing at 200 °C. Voltage shift in hysteresis was strongly dependent on the polarization states and ascribed to an asymmetric distribution of defect charges and pinned defect dipoles. Field recovery of the imprinted capacitors and increase in coercive field after the recovery were discussed in conjunction with reversible defect dipoles. From the relaxation of the voltage shift with an activation energy of 0.21 eV, it is inferred that charge trapping may be the main cause of the voltage shift and the subsequent degradation of the capacitors by pinning the polarization and defect dipoles.

Precursor dependent properties of Ba/sub 1-x/Sr/sub x/TiO/sub 3/ thin films fabricated by sol-gel method

Thin films of barium strontium titanate, Ba/sub 1-x/Sr/sub x/TiO/sub 3/, were deposited on Si and ITO/glass substrates with 200/spl sim/300 nm thickness by sol-gel method. The precursor solution of BST was prepared by mixing the 0.2 M precursor solutions of BaTiO/sub 3/ and SrTiO/sub 3/ with proper molar ratio. The X-ray diffraction (XRD) patterns show characteristic peaks with weak and broad features indicating that the polycrystalline BST film has poor crystallinity. The grain size and the surface morphology of the films were investigated by atomic force microscope (AFM). Especially, dependence of the film structures on the treatments of precursor solution such as hydrolysis and modification with acetic acid was investigated. The I-V (current-voltage) characteristics of the films was also dependent on the precursor structure. The C-V (capacitance-voltage) behavior of the films with MIS (metal-insulator-semiconductor) and MIM (metal-insulator-metal) structures were studied. Also, dielectric permittivity which was very strongly dependent on the crystallinity of the films was discussed in conjunction with precursor structures.

Hydrogen-Induced Degradation Mechanisms in Ferroelectric PbZr0.4Ti0.6O3 and Bi3.25La0.75Ti3O12 Thin Films

Hydrogen-induced degradation in PbZr0.4Ti0.6O3 and Bi3.25La0.75Ti3O12 (BLT) capacitors was investigated after forming gas annealing (FGA). By comparison, the BLT capacitors were found to have a high resistance against hydrogen-induced degradation at the low temperature FGA below 350°C. The BLT capacitors showed only a small reduction of remanent polarization and no deformation of hysteresis loops while the PZT showed a large polarization degradation after the same FGA process. The degradation of PZT at the low temperature FGA was investigated systematically by measuring the voltage shifts of hysteresis loop, and current–voltage and capacitance–voltage characteristics. Different degradation mechanisms are suggested for PZT and BLT by comparing experimental results including those of thermogravimetric analysis. For PZT films, pinning of domains and defect dipoles due to charged defects is inferred to dominate the initial stage of the degradation. On the other hand, for BLT films, hydrogen-induced decomposition seems to govern the degradation resulting in the increase of leakage current density and production of non-ferroelectric oxide(s).

Orientation dependent ferroelectric properties of SrBi/sub 2/Ta/sub 2/O/sub 9/ ferroelectric thin films

SrBi/sub 2/Ta/sub 2/O/sub 9/ (SBT) thin films With various orientations: [001] oriented, partially [115]&[001] oriented, partially [115]&[200] oriented, and [116] oriented films were grown by rf magnetron sputtering deposition method. The orientations of the films were controlled varying the substrate orientation and the deposition temperature. Their structural properties were investigated by the X-ray diffraction /spl theta/-2/spl theta/ scan. Surface morphologies were examined by atomic force microscope. Longer grains were observed in partially oriented and [116] oriented films than those in [001] oriented film. The remanent polarizations were about 3-5 and 10 /spl mu/C/cm/sup 2/ in partially oriented films and [116] oriented films, respectively. The activation fields were calculated by measuring the switching currents of the samples. The remanent polarizations and the activation fields of various films were explained in conjunction with the inclination of SBT grains to the film surface.