Yun-Sok Shin

Direct observation of conducting filaments on resistive switching of NiO thin films

The Hg∕NiO∕Pt capacitor with a Hg top electrode diameter of about 35μm showed the typical bistable resistive switching characteristic. After the removal of the Hg top electrode, we directly observed the formation and removal of filaments for a high resistive state (Roff) and a low resistive state (Ron) by conducting atomic force microscope (CAFM). CAFM images for Roff and Ron states directly exhibit evidence of the formation and removal of filaments on the surface, which supports well the filament model as a switching mechanism of resistive random access memory.

A Nonvolatile Memory Device Made of a Ferroelectric Polymer Gate Nanodot and a Single-Walled Carbon Nanotube

We demonstrate a field-effect nonvolatile memory device made of a ferroelectric copolymer gate nanodot and a single-walled carbon nanotube (SW-CNT). A position-controlled dip-pen nanolithography was performed to deposit a poly(vinylidene fluoride-ran-trifluoroethylene) (PVDF-TrFE) nanodot onto the SW-CNT channel with both a source and drain for field-effect transistor (FET) function. PVDF-TrFE was chosen as a gate dielectric nanodot in order to efficiently exploit its bipolar chemical nature. A piezoelectric force microscopy study confirmed the canonical ferroelectric responses of the PVDF-TrFE nanodot fabricated at the center of the SW-CNT channel. The two distinct ferroelectric polarization states with the stable current retention and fatigue-resistant characteristics make the present PVDF-TrFE-based FET suitable for nonvolatile memory applications.

High-mobility graphene nanoribbons prepared using polystyrene dip-pen nanolithography.

Graphene nanoribbons (GNRs) are fabricated by dip-pen nanolithography and polystyrene etching techniques on a SrTiO(3)/Nb-doped SrTiO(3) substrate. A GNR field-effect transistor (FET) shows bipolar FET behavior with a high mobility and low operation voltage at room temperature because of the atomically flat surface and the large dielectric constant of the insulating SrTiO(3) layer, respectively.

Epitaxial BiAlO3 thin film as a lead-free ferroelectric material

We fabricated high quality epitaxial BiAlO3 thin films that exhibited a relatively high c∕a ratio of about 1.05 with a pseudotetragonal structure. On the atomic force microscope morphology of the BiAlO3 thin film, we observed large terraces with a width of about 1000A and terrace heights of nearly the same to one lattice constant. This indicates that the BiAlO3 thin film has an ideal layer-by-layer growth mode. The BiAlO3 thin film also showed a good ferroelectric property with a high remanent polarization of about 29μC∕cm2.

Multiferroic BiMnO3 thin films with double SrTiO3 buffer layers

High quality BiMnO3 thin films with double SrTiO3 buffer layers were fabricated on Pt∕Ti∕SiO2∕Si and SrTiO3 substrates, in which SrTiO3 buffer layers were used to reduce leakage current in BiMnO3 thin films. The SrTiO3 buffer layers had a thickness of about 5nm obtained from the fitting of ellipsometer data, which gave the remarkable enhancement in leakage current. BiMnO3 thin films exhibited the ferromagnetic transition with the Curie temperature of about 105K. The Pt∕SrTiO3∕BiMnO3∕SrTiO3∕Pt and SrRuO3∕SrTiO3∕BiMnO3∕SrTiO3∕SrRuO3 capacitors showed good ferroelectric properties with the remanent polarization of about 9 and 16μC∕cm2, respectively.

Coherent manipulation of individual electron spin in a double quantum dot integrated with a micromagnet

We report the coherent manipulation of electron spins in a double quantum dot integrated with a micro-magnet. We performed electric dipole spin resonance experiments in the continuous wave (CW) and pump-and-probe modes. We observed two resonant CW peaks and two Rabi oscillations of the quantum dot current by sweeping an external magnetic field at a fixed frequency. Two peaks and oscillations are measured at different resonant magnetic field, which reflects the fact that the local magnetic fields at each quantum dot are modulated by the stray field of a micro-magnet. As predicted with a density matrix approach, the CW current is quadratic with respect to microwave (MW) voltage while the Rabi frequency (\nu_Rabi) is linear. The difference between the \nu_Rabi values of two Rabi oscillations directly reflects the MW electric field across the two dots. These results show that the spins on each dot can be manipulated coherently at will by tuning the micro-magnet alignment and MW electric field.

Bistable resistive states of amorphous SrRuO3 thin films

We fabricated amorphous SrRuO3 thin films which exhibited the electronic transport behavior of an insulator that showed a three-dimensional hopping transport. Depending on the polarity of a sweep bias, bistable resistive states were observed in the capacitor consisted of an amorphous SrRuO3 thin film and Pt electrodes, which gives the opportunity for nonvolatile memory applications. From electric transport and optical conductivity data, we indirectly confirmed a probability of the mixed phase of SrO and RuO2 in the amorphous SrRuO3 thin film. This supports the applicability of a filament model as a mechanism for the bistable resistive states.

Spin relaxation in mesoscopic superconducting Al wires

We studied the diffusion and the relaxation of the polarized quasiparticle spins in superconductors. To that end, quasiparticles of polarized spins were injected through an interface of a mesoscopic superconducting Al wire in proximity contact with an overlaid ferromagnetic Co wire in the single-domain state. The superconductivity was observed to be suppressed near the spin-injecting interface, as evidenced by the occurrence of a finite voltage for a bias current below the onset of the superconducting transition. The spin-diffusion length, estimated from finite voltages over a certain length of Al wire near the interface, was almost temperature independent in the temperature range sufficiently below the superconducting transition but grew as the transition temperature was approached. This temperature dependence suggests that the relaxation of the spin polarization in the superconducting state is governed by the condensation of quasiparticles to the paired state. The spin relaxation in the superconducting state turned out to be more effective than in the normal state.

Writing ferroelectric domain bits on the PbZr0.48Ti0.52O3 thin film

PbZr0.48Ti0.52O3 thin films were fabricated on (111) Pt/TiO2/SiO2/Si substrate by eclipse pulsed laser deposition with the very low deposition rate of 0.02 A/pulse. High crystalline PbZr0.48Ti0.52O3 thin films showed a good ferroelectric property with the high remanent polarization of 48 μC/cm2 as well as an enhanced fatigue behavior. The small roughness of PZT thin films makes a negligible influence on electron force microscope images, which gave the enhanced resolution for the observation of a minimum ferroelectric bit size. We demonstrated the minimum ferroelectric bits below 9 nm, which were clearly observed.

Local and Nonlocal Fraunhofer-like Pattern from an Edge-Stepped Topological Surface Josephson Current Distribution

We report a surface-dominant Josephson effect in superconductor-topological insulator-superconductor (S-TI-S) devices, where a Bi1.5Sb0.5Te1.7Se1.3 (BSTS) crystal flake was adopted as an intervening TI between Al superconducting electrodes. We observed a Fraunhofer-type critical current modulation in a perpendicular magnetic field in an Al-TI-Al junction for both local and nonlocal current biasing. Fraunhofer-type modulation of the differential resistance was also observed in a neighboring Au-TI-Au normal junction when it was nonlocally biased by the Al-TI-Al junction. In all cases, the Fraunhofer-like signal was highly robust to the magnetic field up to the critical field of the Al electrodes, corresponding to the edge-stepped nonuniform supercurrent density arising from the top and rough side surfaces of the BSTS flake, which strongly suggests that the Josephson coupling in a TI is established through the surface conducting channels that are topologically protected.