Du-Yeong Lee

Effects of Pt capping layer on perpendicular magnet anisotropy in pseudo-spin valves of Ta/CoFeB/MgO/CoFeB/Pt magnetic-tunneling junctions

We investigated how the Pt capping layer affected perpendicular magnet anisotropy in magnetic-tunnel-junctions fabricated with a Ta electrode, a lower CoFeB layer, an MgO barrier, an upper CoFeB layer, and a Pt capping electrode, which was estimated by using an anisotropy constant multiplied by the upper CoFeB layer thickness (Ku * t). The maximum Ku * t was found at an annealing temperature of 300 °C for an magnetic tunnel junction with an upper CoFeB layer thickness of 0.9 nm, indicating a highly textured MgO (100) barrier of 1.0 nm with none of the remaining Pt inter-diffused in the upper CoFeB layer.

Irradiation-induced shrinkage and expansion mechanisms of SiO2 circle membrane nanopores.

20 nm diameter SiO(2) nanopore arrays on gradient-thickness membranes were formed by a focused electron beam with in situ transmission electron microscopy (TEM). Nanopore shrinkage was seen in nanopores on thicker membranes, with the rate of diameter change remaining constant during the shrinkage process. In contrast, pore expansion was observed in thinner membranes, with the expansion rate being constant at the initial stage but with a slight increase at the later stage. The geometry model of shrinkage and expansion of the nanopores in relation to the electron irradiation time was investigated by utilizing the TEM tilting method.

Correlation of the structural properties of a Pt seed layer with the perpendicular magnetic anisotropy features of full Heusler-based Co2FeAl/MgO/Co2Fe6B2 junctions via a 12-inch scale Si wafer process

We elucidated the interfacial-perpendicular magnetic anisotropy (i-PMA) features of full Heusler-based Co2FeAl/MgO/Co2Fe6B2 magnetic-tunnel-junctions as functions of the structural properties of the Pt seed layer including its thickness and ex situ annealing temperature. All of the samples were prepared in a 12-inch silicon wafer process for real industry applications. The observations of the M-H loops emphasize that a thinner Pt seed layer and a high ex situ annealing temperature enhance the surface roughness of the seed layer, providing better i-PMA characteristics. HR-TEM images of the samples were evaluated to understand the structural effects of thin and thick Pt seed layers.

Effects of the radio-frequency sputtering power of an MgO tunneling barrier on the tunneling magneto-resistance ratio for Co2Fe6B2/MgO-based perpendicular-magnetic tunnel junctions

For Co2Fe6B2–MgO based p-MTJ spin valves with [Co/Pt]n–SyAF layers ex situ annealed at 350 °C and 30 kOe for 30 min, the tunneling magneto-resistance (TMR) ratio strongly depended on the radio-frequency (RF) sputtering power in a 0.65–1.15 nm thick MgO tunneling barrier, achieving a TMR ratio of 168% at 300 W. The TMR ratio rapidly and linearly increased with a decrease in the RF sputtering power between 300 and 500 W and then abruptly decreased at 250 W since the face-centered-cubic crystallinity of the tunneling barrier improved with a decrease in the RF sputtering power between 300 and 500 W and then abruptly degraded at 250 W. Optical properties measured by spectroscopic ellipsometry, such as the defect state density and energy band gap of a ∼1.0 nm thick tunneling barrier layer, indicate that the RF sputtering power needed to obtain a larger poly grain size for the barrier tends to enhance the barriers face-centered-cubic crystallinity.

Dependency of Tunneling-Magnetoresistance Ratio on Nanoscale Spacer Thickness and Material for Double MgO Based Perpendicular-Magnetic-Tunneling-Junction

It was found that in double MgO based perpendicular magnetic tunneling junction spin-valves ex-situ annealed at 400 °C, the tunneling magnetoresistance ratio was extremely sensitive to the material and thickness of the nanoscale spacer: it peaked at a specific thickness (0.40~0.53 nm), and the TMR ratio for W spacers (~134%) was higher than that for Ta spacers (~98%). This dependency on the spacer material and thickness was associated with the (100) body-centered-cubic crystallinity of the MgO layers: the strain enhanced diffusion length in the MgO layers of W atoms (~1.40 nm) was much shorter than that of Ta atoms (~2.85 nm) and the shorter diffusion length led to the MgO layers having better (100) body-centered-cubic crystallinity.

Optimization of laser parameters for the maximum efficiency in the generation of water-window radiation using a liquid nitrogen jet

A laser-produced plasma is a suitable compact x-ray source that can be of broad band or quasimonochromatic with a proper choice of material and filter. To address the maximum conversion efficiency for an efficient, quasimonochromatic source at 2.88 nm (NVI1s2‐1s2p transition) using liquid nitrogen jet for soft x-ray microscopy, the radiation characteristics such as absolute intensity, spectra, and angular distribution have been investigated for different laser pulse durations (picosecond and femtosecond pulses) and laser energies The comparison of conversion efficiencies between picosecond [120 ps full width at half maximum (FWHM)] and femtosecond (40–500fs FWHM) lasers indicates that the picosecond laser would provide better conversion efficiency, which is 1.6% at 2×1013W∕cm2. The investigation shows that the laser intensity for the maximum conversion efficiency scales as Im∝1∕τα, where α=0.9±0.15. This empirical formula is useful to choose the laser parameters properly for a given pulse width.

Existence and atomic arrangement of microtwins in hexagonal MnAs ferromagnetic epilayers grown on GaAs (100) substrates with monolayer InAs Buffers

Selected-area electron-diffraction pattern (SADP) and high-resolution transmission electron microscopy (HRTEM) measurements were carried out to investigate the existence and the atomic arrangement of microtwins in hexagonal MnAs ferromagnetic epilayers grown on GaAs (100) substrates with monolayer InAs buffer layers by using molecular beam epitaxy. The magnetization curve as a function of the magnetic field at 5 K showed that the MnAs thin films were ferromagnetic, and the magnetization curve as a function of the temperature revealed that the ferromagnetic transition temperature was as high as 325 K. The SADP and HRTEM results showed that an epitaxial relationship was formed among the hexagonal MnAs layer, the InAs layer, and the zincblende GaAs substrate. Microtwins existed between the MnAs grain boundaries on top of the InAs layers. Based on the SADP and HRTEM results, we present a possible atomic arrangement of the microtwins for the MnAs-InAs-GaAs heterostructure.

Microstructural properties and atomic arrangements in GaN/sapphire and AlxGa1−xN∕AlN∕GaN∕sapphire heterostructures

Microstructural and atomic structure studies of GaN/sapphire and AlxGa1−xN∕AlN∕GaN heterointerfaces have been performed by using bright-field transmission electron microscopy (TEM) and selected area electron diffraction pattern (SADP) measurements. TEM and SADP results from the GaN/sapphire and the Al0.4Ga0.6N∕AlN∕GaN∕sapphire heterointerfaces showed that GaN epilayers and Al0.4Ga0.6N, AlN, and GaN active layers, respectively, had been grown on the sapphire substrates. The values of the strain and the stress in the Al0.4Ga0.6N, the AlN, and the GaN layers were determined. Possible schematic diagrams of the two heterostructures, as well as diagrams of the [21¯1¯0] projections of the GaN epilayer and the Al0.4Ga0.6N∕AlN∕GaN heterostructure, are presented based on the TEM and SADP results. These results can help improve the understanding of the microstructural properties of the strained AlxGa1−xN∕AlN∕GaN∕sapphire heterostructures for applications in the high-speed and high-power electronic devices.

Dielectric function of Si1−xGex films grown on silicon-on-insulator substrates

The dielectric functions of undoped and P-doped Si1−xGex (SiGe) films with a compressive strain on silicon-on-insulator (SOI) substrates are obtained by using spectroscopic ellipsometry. The respective Kato–Adachi and Tauc–Lorentz models are best fitted to the undoped and P-doped SiGe films to obtain their complex dielectric functions. The undoped SiGe films are characterized by multimodal peaks in the dielectric function, whereas the P-doped SiGe films exhibit only a broad peak. Further, the E0 and E1 critical points (CPs) of the undoped SiGe films are strongly dependent on the Ge concentration, whereas the E2 CPs are independent of concentration. The E0 and E2 CPs in the undoped SiGe films on an SOI substrate are lower than those of SiGe on a bulk-Si substrate owing to the higher strain. For P doping in SiGe, its dose causes non-monotonic variations in Eg and E0.

Dependency of tunneling magnetoresistance ratio on Pt seed-layer thickness for double MgO perpendicular magnetic tunneling junction spin-valves with a top Co2Fe6B2 free layer ex-situ annealed at 400 °C

For the double MgO based perpendicular magnetic tunneling junction (p-MTJ) spin-valves with a top Co2Fe6B2 free layer ex situ annealed at 400 °C, the tunneling-magnetoresistance ratio (TMR) strongly depended on the platinum (Pt) seed layer thickness (t Pt): it peaked (∼134%) at a specific t Pt (3.3 nm). The TMR ratio was initially and slightly increased from 113%-134% by the enhancement of the magnetic moment of the Co2Fe6B2 pinned layer when t Pt increased from 2.0-3.3 nm, and then rapidly decreased from 134%-38.6% by the degrading face-centered-cubic crystallinity of the MgO tunneling barrier when t Pt increased from 3.3-14.3 nm.