Seung Pil Ko

Effect of external magnetic field on anisotropy of Co∕Cu multilayer nanowires

We have investigated the effect of external magnetic field exercised during the fabrication of the Co and Co∕Cu multilayer nanowires in anodic aluminum oxide (AAO) templates using pulse electrodeposition. It is found that the effect becomes significant when the pore size is small. Deterioration occurs in the magnetic properties for the Co nanowires studied, whereas a distinct enhancement in the magnetic properties for the Cu∕Co multilayer nanowires is achieved by applying an external magnetic field during nanowire growth using the AAO template comprising pores of 50nm diameter.

Bias voltage dependence of magnetic tunnel junctions comprising double barriers and amorphous NiFeSiB layers

A double-barrier magnetic tunnel junction (DMTJ) comprising an amorphous ferromagnetic NiFeSiB was investigated to reduce bias voltage dependence of tunneling magnetoresistance (TMR) ratio. The typical DMTJ structures were Ta45∕Ru9.5∕IrMn10∕CoFe7∕AlOx∕free layer7∕AlOx∕CoFe7∕IrMn10∕Ru60 (in nanometers). Various free layers such as CoFe 7, NiFeSiB 7, CoFe3.5∕NiFeSiB3.5, and NiFeSiB3.5∕CoFe3.5 were prepared and compared. The NiFeSiB-used DMTJ shows a low root-mean-square surface roughness of 0.17 nm, a resistance of about 860Ω, a Vh (voltage where the TMR ratio becomes half of its nonbiased value) of 1.1 V, and a high junction breakdown voltage of 2.0 V. The DMTJ with an amorphous NiFeSiB free layer offers smooth surface roughness resulting in reduced interlayer coupling field and bias voltage dependence.

Current fluctuation of electron and hole carriers in multilayer WSe2 field effect transistors

Two-dimensional materials have outstanding scalability due to their structural and electrical properties for the logic devices. Here, we report the current fluctuation in multilayer WSe2 field effect transistors (FETs). In order to demonstrate the impact on carrier types, n-type and p-type WSe2 FETs are fabricated with different work function metals. Each device has similar electrical characteristics except for the threshold voltage. In the low frequency noise analysis, drain current power spectral density (SI) is inversely proportional to frequency, indicating typical 1/f noise behaviors. The curves of the normalized drain current power spectral density (NSI) as a function of drain current at the 10 Hz of frequency indicate that our devices follow the carrier number fluctuation with correlated mobility fluctuation model. This means that current fluctuation depends on the trapping-detrapping motion of the charge carriers near the channel interface. No significant difference is observed in the current fluc...

Control of magnetic behavior in Fe/sub 3/O/sub 4/ nanostructures

Hexagonally well-ordered nanoporous anodized aluminum oxide (AAO) templates were synthesized by electrochemical method. Then, magnetite (Fe/sub 3/O/sub 4/) nanoparticles of 8-15-nm diameter were filled in the pores of the AAO template, and then the morphology and magnetic properties were characterized. Vibrating sample magnetometer measurement indicated that the nanoparticles inside AAO showed superparamagnetic behavior at as-filled state. But after annealing at 600/spl deg/C for 2 hr, the nanostructure (i.e., an assembly of nanoparticles) exhibited coercivity of 80 Oe. Superconducting quantum interference measurement device data showed that after annealing the blocking temperature was above 300 K. Thus, the annealed magnetite nanostructures possessed ferrimagnetism.

Capacitance-voltage analysis of electrical properties for WSe2 field effect transistors with high-k encapsulation layer

Doping effects in devices based on two-dimensional (2D) materials have been widely studied. However, detailed analysis and the mechanism of the doping effect caused by encapsulation layers has not been sufficiently explored. In this work, we present experimental studies on the n-doping effect in WSe2 field effect transistors (FETs) with a high-k encapsulation layer (Al2O3) grown by atomic layer deposition. In addition, we demonstrate the mechanism and origin of the doping effect. After encapsulation of the Al2O3 layer, the threshold voltage of the WSe2 FET negatively shifted with the increase of the on-current. The capacitance-voltage measurements of the metal insulator semiconductor (MIS) structure proved the presence of the positive fixed charges within the Al2O3 layer. The flat-band voltage of the MIS structure of Au/Al2O3/SiO2/Si was shifted toward the negative direction on account of the positive fixed charges in the Al2O3 layer. Our results clearly revealed that the fixed charges in the Al2O3 encapsulation layer modulated the Fermi energy level via the field effect. Moreover, these results possibly provide fundamental ideas and guidelines to design 2D materials FETs with high-performance and reliability.

Low frequency noise reduction in multilayer WSe2 field effect transistors

We report that noise level was reduced by passivating WSe2 field effect transistors (FETs) with high-k dielectric material. Low frequency noise and I-V characteristics of the device were measured from WSe2 FETs before and after the passivation with ZrO2 to confirm the effect of passivation by high-k dielectric material. As a result, there was no significant change in the I-V characteristics. In the low frequency noise analysis, our device showed 1/f noise behaviors, in agreement with the carrier number fluctuation (CNF) model. The passivation process contributed to the reduction of trap sites at the top surface, leading to the decrease of noise level at the low current regime. Extracted volume trap densities were reduced from 2.0 × 1020 cm-3eV-1 to 8.7 × 1019 cm-3eV-1.

Finite-size effect and magnetic properties of ultra-small magnetite nanocrystals for biomedical applications

The authors synthesized ultra-small magnetite (Fe3O4) nanoparticles using aqueous coprecipitation. The crystal structure of the samples was analyzed by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The magnetic properties of the magnetite nanocrystals were investigated using a SQUID magnetometer.

Magnetic properties of Fe/sub 3/O/sub 4/ nanostructures

In this research, nano-dot array with magnetite (Fe/sub 3/O/sub 4/) nanoparticles using an anodized aluminum oxide (AAO) nanotemplate is produced. The hexagonally ordered nanotemplates are created using a two-step anodization method. After a pore-widening process of the nanotemplates, the 10 nm nanoparticles are inserted by magnetic stirring method. Annealing of the nanostructures at 500/spl deg/C for 1 hr is done. Surface and cross-section of the template filled with nanoparticles are observed by field emission scanning electron microscopy (FESEM). At as-filled state, the nanostructure shows low coercivity of 20 Oe but after annealing, it increased to 85 Oe. The magnetic properties of nano wires consisting of nanoparticles are improved due to recrystallization and grain growth of each magnetic particles and their structure may be changed due to agglomeration.