Jonghyun Song

Polarity-tunable magnetic tunnel junctions based on ferromagnetism at oxide heterointerfaces

Complex oxide systems have attracted considerable attention because of their fascinating properties, including the magnetic ordering at the conducting interface between two band insulators, such as LaAlO3 and SrTiO3. However, the manipulation of the spin degree of freedom at the LaAlO3/SrTiO3 heterointerface has remained elusive. Here, we have fabricated hybrid magnetic tunnel junctions consisting of Co and LaAlO3/SrTiO3 ferromagnets with the insertion of a Ti layer in between, which clearly exhibit magnetic switching and the tunnelling magnetoresistance effect below 10 K. The magnitude and sign of the tunnelling magnetoresistance are strongly dependent on the direction of the rotational magnetic field parallel to the LaAlO3/SrTiO3 plane, which is attributed to a strong Rashba-type spin-orbit coupling in the LaAlO3/SrTiO3 heterostructure. Our study provides a further support for the existence of the macroscopic ferromagnetism at LaAlO3/SrTiO3 heterointerfaces and opens a novel route to realize interfacial spintronics devices.

Quantum Electrical Transport in Mesoscopic LaAlO3/SrTiO3 Heterostructures

We report the fabrication and electrical measurements for LaAlO3/SrTiO3 (LAO/STO) heterostructures having various lateral sizes. When the sample size is reduced to the sub-micrometer range, the magnetoconductance (MC) curve deviates from classical parabolic behavior. Instead, a negative MC behavior appears with superimposed conductance fluctuations, which is attributed to weak antilocalization and universal conductance fluctuations. Using the Maekawa–Fukuyama model, the spin–orbit scattering and phase coherence lengths are estimated from the MC data, which are dependent on the gate voltage and temperature. Our comprehensive study shows that LAO/STO heterointerfaces provide a useful platform to explore quantum coherent transport of strongly correlated electrons.

Simple Fabrication of a Three-Dimensional CMOS Inverter Using p-Type Poly-Si and n-Type Amorphous Ga–In–Zn–O Thin-Film Transistors

We have successfully fabricated a novel 3-D vertically stacked complementary metal-oxide-semiconductor hybrid inverter using a lower p-type poly-Si thin-film transistor (TFT) and an upper n-type amorphous Ga-In-Zn-O (a-GIZO) TFT. The device was fabricated step by step, starting from the fabrication of the lower poly-Si TFT via a high-temperature process to the fabrication of the upper a-GIZO TFT via a low-temperature process; this was done to prevent damage to the TFTs and to optimize their characteristics. The entire process was further simplified by the coplane gate structure that allows successive formation of the a-GIZO TFT on the poly-Si TFT. The transfer characteristics show clear inverter actions, indicating high performances of the TFTs.

New Pixel Circuit Design Employing an Additional Pixel Line Insertion in AMOLED Displays Composed by Excimer Laser-Crystallized TFTs

Active matrix organic light-emitting diode (AMOLED) displays are fabricated from polycrystalline silicon, which is formed in the single and double (overlap) scanned area during the excimer laser annealing (ELA) process. A redundant pixel line (RPL) design is proposed to remove the overlapping mura and, as a result, a 5-in AMOLED display is successfully fabricated without any non-uniform line image on the overlapping scanned area. This result indicates that the fabrication of a large-sized AMOLED panel is possible using ELA crystallization through an RPL design.

Effects of Excimer Laser Annealing Process on the Ni-Sputtered Amorphous Silicon Film

This article conducted a comparative analysis on thin film transistors (TFTs) fabricated by using the excimer laser annealed (ELA) polycrystalline silicon (poly-Si) and those fabricated by using the Ni-sputtered ELA poly-Si. The grain size of the Ni-sputtered ELA poly-Si is 3 times greater than that of the ELA poly-Si. The Ni-sputtered ELA poly-Si TFTs showed a higher drain current, lower threshold voltage, smaller subthreshold swing, and higher electron mobility than the ELA poly-Si TFTs. This improvement is attributed mainly to the reduction of the defect density rendered by the large grain size.

Growth of Bi2Sr2CaCu2O8+δ thin films with enhanced superconducting properties by incorporating CaIrO3 nanoparticles

We synthesized Bi2Sr2CaCu2O8+δ (Bi-2212) thin films with incorporated CaIrO3 (Ca-iridate) nanoparticles, by using pulsed laser deposition and post-growth ex situ annealing. We found that the density of incorporated Ca-iridate strongly affected the superconducting properties and microstructure of the Bi-2212 thin films. Incorporating a low density of Ca-iridate (450 laser pulses) in the Bi-2212 improved its superconducting properties (Tc-onset = 97 K, Tc-zero = 84 K) over those of pure Bi-2212 (Tc-onset = 94 K, Tc-zero = 80 K). However, incorporating a higher density (1800 pulses) significantly reduced Tc-zero to ≈ 57 K. Incorporating a low density of Ca-iridate also decreased the c-axis lattice constant. Films with incorporated Ca-iridate exhibited greater critical current density, Jc(B), than the pure Bi-2212 film. These results indicate that incorporating low densities of Ca-iridate nanoparticles into Bi-2212 can improve its superconducting properties.

Multiple relaxations of the cluster surface diffusion in a homoepitaxial SrTiO3 layer

We examine the surface diffusion process of adatomic clusters on a (001)-oriented SrTiO3 single crystal using reflection high energy electron diffraction (RHEED). We find that the recovery curve of the RHEED intensity acquired after a homoepitaxial half-layer growth can be accurately fit into a double exponential function, indicating the existence of two dominant relaxation mechanisms. The characteristic relaxation times at selected growth temperatures are investigated to determine the diffusion activation barriers of 0.67 eV and 0.91 eV, respectively. The Monte Carlo simulation of the cluster hopping model suggests that the decrease in the number of dimeric and trimeric clusters during surface diffusion is the origin of the observed relaxation phenomena.

Measurement of Rat Magnetocardiograms by Using a High-

We have measured biomagnetic signals in rat by using a high-TC SQUID magnetometer system employing a small magnetically shielded box. For the rat cardiac magnetic field measurements, a healthy rat with body weight of about 350 g was anesthetized and fixed on a plastic xyz-stage in supine position inside the magnetically shielded box. We could obtain strong R-peak amplitude up to 56 pT with well defined P-, QRS-, T-wave, and ST-segment separating T-wave from QRS complex. We measured and analysed rat MCG waveforms for 5 × 5 grid measurements covering 60 mm × 60 mm area and 12 circular measurements centered at the heart position. This work demonstrates that the potential advantages of high-TC SQUID magnetometer system in evaluating the rat MCG measurement for animal models of cardiac electrical activities.