Jurae Kim

Critical thickness of ultrathin ferroelectric BaTiO3 films

To investigate the critical thickness of ferroelectric BaTiO3 (BTO) films, we fabricated fully strained SrRuO3∕BTO∕SrRuO3 heterostructures on SrTiO3 substrates by pulsed laser deposition with in situ reflection high-energy electron diffraction. We varied the BTO layer thickness from 3to30nm. By fabricating 10×10μm2 capacitors, we could observe polarization versus electric-field hysteresis loops, which demonstrate the existence of ferroelectricity in BTO layers thicker than 5nm. This observation provides an experimental upper bound of 5nm for the critical thickness. The BTO thickness-dependent scaling of the remanent polarization agrees with the predictions of recent first-principle simulations [J. Junquera and P. Ghosez, Nature 422, 506 (2003)].

Chemical sensors for sensing gas adsorbed on the inner surface of carbon nanotube channels

The authors have developed gas sensors that operate when gas flowing through carbon nanotube (CNT) channels is adsorbed on the inner surface of the channels. CNTs fabricated on anodic alumina membranes were used. The CNTs are well ordered and connected in parallel, forming parallel channels. The sensors are highly responsive to NH3 and NO2 molecules and the response times are relatively short. They are completely recovered within 10min when a dc voltage of 10V is applied for 2min. The fabrication processes are relatively simple and do not require special techniques such as e-beam lithography.

Coexistence of metallic and insulating phases in epitaxial CaRuO3 thin films observed by scanning microwave microscopy

Using a scanning microwave microscope, we investigated the local electrical properties of epitaxial CaRuO3 thin films. The films showed a metal–insulator transition depending on the growth temperature and their thickness. We observed spatially separated highly conducting and poorly conducting regions in the films grown at a high temperature of 800 °C, which showed insulating behavior. The conduction in the CaRuO3 thin films with insulating behavior is percolative through the highly conducting regions and is closely related to this two-phase behavior.

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 Charge Retention Loss in Ferroelectric Pt/Pb(Zr,Ti)O3/Pt Capacitors and Its Relation to Fatigue and Imprint

Charge retention loss was investigated for Pt/Pb(Zr0.4Ti0.6)O3/Pt capacitors in short- (t < 1 s) and long-time (t > 1 s) regimes. The short-time retention loss behaviors of fatigued capacitors were well described by a power-law function and analyzed in terms of a superposition of polarization relaxation with a distribution of relaxation time. We showed that depolarization field governs the retention loss in the short-time regime. Fatigue-induced defects, probably the interfacial passive layers, seemed to increase the depolarization field. In the long-time regime, the retention loss is shown to be related to the internal field developed parallel to the polarization direction. Particularly in “opposite-state” retention, the internal field caused polarization backswitching resulting in a severe degradation of retention characteristics. The internal field developed during retention was estimated by using Merz equations of polarization switching. Defects dynamics under electric field was discussed in conjunction with the recovery process of imprint.