Sung Jin Kang

Resistance switching in epitaxial SrCoOx thin films

We observed bipolar switching behavior from an epitaxial strontium cobaltite film grown on a SrTiO3 (001) substrate. The crystal structure of strontium cobaltite has been known to undergo topotactic phase transformation between two distinct phases: insulating brownmillerite (SrCoO2.5) and conducting perovskite (SrCoO3−δ) depending on the oxygen content. The current–voltage characteristics of the strontium cobaltite film showed that it could have a reversible insulator-to-metal transition triggered by electrical bias voltage. We propose that the resistance switching in the SrCoOx thin film could be related to the topotactic phase transformation and the peculiar structure of SrCoO2.5.

Interface Control of Ferroelectricity in an SrRuO3/BaTiO3/SrRuO3 Capacitor and its Critical Thickness

The atomic-scale synthesis of artificial oxide heterostructures offers new opportunities to create novel states that do not occur in nature. The main challenge related to synthesizing these structures is obtaining atomically sharp interfaces with designed termination sequences. In this study, it is demonstrated that the oxygen pressure (PO2) during growth plays an important role in controlling the interfacial terminations of SrRuO3 /BaTiO3 /SrRuO3 (SRO/BTO/SRO) ferroelectric (FE) capacitors. The SRO/BTO/SRO heterostructures are grown by a pulsed laser deposition method. The top SRO/BTO interface, grown at high PO2 (around 150 mTorr), usually exhibits a mixture of RuO2 -BaO and SrO-TiO2 terminations. By reducing PO2, the authors obtain atomically sharp SRO/BTO top interfaces with uniform SrO-TiO2 termination. Using capacitor devices with symmetric and uniform interfacial termination, it is demonstrated for the first time that the FE critical thickness can reach the theoretical limit of 3.5 unit cells.

Origin of Leakage Paths Driven by Electric Fields in Al‐Doped TiO2 Films

The growth of leakage current paths in Al-doped TiO2 (ATO) films is observed by in situ TEM under negative bias stress. Through systematic HAADF-STEM, STEM-EDS, and STEM-EELS studies, it is confirmed that the electric field-induced growth of the Ru-doped TiO2 phase is the main reason for the ATO films negative leakage.

Strain-induced non-linear optical characteristics of pyroelectric PbVO 3 epitaxial thin films

Highly strained PbVO3 films have potential applications in pyroelectric sensors and optoelectronic devices. To understand how strain influences the nonlinear optical characteristic of PbVO3, highly distorted tetragonal PbVO3 epitaxial thin films were grown on LaAlO3 and SrTiO3 substrates. Using Raman scattering spectroscopy, theoretical and experimental studies on the phonon spectra yield insights on the strained structures of V-O apical bondings. Effects of applied strain on the linear optical properties of PbVO3 thin films were experimentally characterized by spectroscopic ellipsometry. Second-order nonlinear susceptibility tensor components of PbVO3/SrTiO3 thin films were determined to be almost twice as large as those of PbVO3/LaAlO3 thin films.