Chong seung Yoon

Nonvolatile memory cell effect in multilayered Ni1−xFex self-assembled nanoparticle arrays in polyimide

Transmission electron microscopy images showed that self-assembled Ni1−xFex nanoparticle arrays were periodically inserted in the polyimide (PI) layers. Capacitance-voltage (C-V) measurements on Al/PI/multiple-stacked Ni1−xFex nanoparticle arrays/PI/p-Si (100) structures at 300K showed a metal-insulator-semiconductor capacitor behavior with different flatband voltage shifts, which depended on the value of the sweep voltage, due to the variations of the charged electron density in the multiple-stacked Ni1−xFex nanoparticle arrays. Conductance-voltage (G-V) measurements showed that the conductance peak related to the interface trap disappeared, and that the positions of the C-V and the G-V hystereses at the sweep voltage were different.

Structural, optical, and magnetic properties of As-doped (Zn0.93Mn0.07)O thin films

The As-doped (Zn0.93Mn0.07)O thin film prepared by As+ ion implantation showed a clear peak of (A0,X) having acceptor binding energy of 181meV. The sample showed high TC ferromagnetism persisting up to 285K. The contribution of magnetization from Mn ion at 280K was determined to be 0.13μB∕Mn. The improved ferromagnetism is expected to be originated from hole-induced ferromagnetism and enhanced magnetic anisotropy because crystallographically improved sample showed p-type conductivity with hole concentration of 4.8×1018cm−3 and hole mobility of 11.8cm2V−1s−1. These results suggest that high TC ferromagnetism can be realized by codoping the acceptor dopant and improving the magnetic anisotropy.

Microstructure of femtosecond laser-induced grating in amorphous silicon

The femtosecond laser-induced grating (FLIG) formation and crystallization were investigated in amorphous silicon (a-Si) films, prepared on glass by plasma-enhanced chemical-vapor deposition. Probe-beam diffraction, micro-Raman spectroscopy, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy were employed to characterize the diffraction properties and the microstructures of FLIGs. It was found that i) the FLIG can be regarded as a pattern of alternating a-Si and microcrystalline-silicon (microc-Si) lines with a period of about 2microm, and ii) efficient grating formation and crystallization were achieved by high-intensity recording with a short writing period.

Arbitrary surface structuring of amorphous silicon films based on femtosecond-laser-induced crystallization

The arbitrary surface structuring of amorphous silicon (a-Si) films was performed by applying the Fourier-transform (FT) method to the femtosecond-laser-induced crystallization. In order to realize the arbitrary structuring, the logo q-Psi was produced in the a-Si film by the FT of a computer-generated hologram. The crystallization of a-Si was performed using the near-infrared femtosecond-laser pulses. By micro-Raman spectroscopy, scanning-electron microscopy, and transmission-electron microscopy, it was found that the femtosecond-laser pulses induced a localized phase transformation from the amorphous to the crystalline phase, and the spatially selected crystallization of the a-Si was responsible for the formation of the two-dimensional pattern.

Synthesis of low thermal expansion ceramics based on CaZr4(PO4)6-Li2O system

Abstract Ultra-low thermal expansion ceramics based upon CaZr 4 (PO 4 ) 6 –Li 2 O system were synthesized through solid state sintering. Lithium oxide was introduced to the CaZr 4 (PO 4 ) 6 lattice to promote liquid phase sintering to form CaZr 4 (PO 4 ) 6 with strongly negative thermal expansion. A small addition of Li 2 O has improved the densification rate, but also caused an excessive grain growth, which resulted in extensive microcracking. The microcracking lowered the bulk thermal expansion to −6×10 −6 °C −1 . Excessive amount of Li 2 O content also led to formation of the secondary phase, Li 2 Zr(PO 4 ) 2 , and also caused the swelling of the CaZr 4 (PO 4 ) 6 phase to lower the sintered density. An addition of over 5% of lithium oxide, the glass phase surrounding the CaZr 4 (PO 4 ) 6 grain, together with the secondary phase increased the thermal expansion. At 2.5% addition, there was an indication that the viscous glassy phase has partially relieved the stresses associated with the thermal expansion anisotropy during cooling from the processing temperature. It was demonstrated that the microstructure of the CaZr 4 (PO 4 ) 6 could be altered by adding varying amount of lithium oxide to tailor the thermophysical properties of the material.

Enhanced Curie temperature of InMnP:Zn—TC∼300K

P-type bulk InP was prepared by the liquid encapsulated Czochralski method and subsequently diffused with Mn by heat treatment after the evaporation of Mn on top of InP:Zn using a molecular beam epitaxy system. The characteristics of Mn-diffused InMnP:Zn were investigated by an energy dispersive x-ray spectroscopy, photoluminescence, and a superconducting quantum interference device magnetometer measurements. The samples were characterized by transmission electron microscopy and no evidence of secondary phase formation of InMnP:Zn was found. The results of energy dispersive x-ray peak displayed injected concentration of Mn near 3%. The results of photoluminescence measurement showed that optical broad transitions related to Mn appeared around 1.2eV and it was confirmed that the transitions around 1.2eV were Mn-related band by the diffusion of Mn into InP:Zn. Clear ferromagnetic hysteresis loops were observed at 10 and 300K and the temperature-dependent magnetization showed ferromagnetic behavior around 30...

Clarification of Mn–Zn interaction for InMnP:Zn epilayer by photoluminescence and x-ray photoelectron spectroscopy

Transition related to the Mn–Zn interaction was observed in photoluminescence (PL) study of the InMnP:Zn epilayer and the peak position blueshifted with increasing Mn concentration. X-ray photoelectron spectroscopy was used to clarify the blueshift of the PL peak. The binding energy shifts of Mn 2p and Zn 2p core levels indicative of the interaction between Mn and Zn were observed. This mutual interaction between Mn 2p and Zn 2p agrees with the result that the Mn-related transition in InMnP:Zn codoped with Zn is shifted to the higher energy region in comparison with InMnP without additional doping of Zn.

One-dimensional magnetic grating structure made easy

A simple and easy method of the fabrication of one-dimensional magnetic grating structure is developed. By using the interference pattern of two femtosecond laser beams, a selective-area annealing of as-deposited Co2MnSi film was achieved and one-dimensional magnetic grating structures were fabricated. The as-deposited films exhibit no magnetic response at room temperature. Several microscopies were applied to confirm the periodic crystalline and magnetic structures. The Faraday-like rotation of the polarization of the zeroth- and first-order diffracted beams were measured, and that of the first-order diffracted beam is nearly six times larger than that of the zeroth-order beam.

Spatially periodic magnetic structure produced by femtosecond laser-interference crystallization of amorphous Co2MnSi thin film

Femtosecond laser-interference crystallization (FLIC) was used to form a spatially periodic magnetic structure by selectively crystallizing a paramagnetic amorphous Co2MnSi thin film. Regularly spaced alternating lines of polycrystalline and microcrystalline regions with a periodicity of 2μm were produced by FLIC. The crystalline region composed of ∼100-nm-sized grains contained a nonequilibrium ferromagnetic phase intermixed with β-Mn. The areas between the crystallized lines also received sufficient energy, crystallizing into a microcrystalline state with its grain size ranging from 1 to 5 nm. The magnetic force microscopy of the samples clearly revealed the one-dimensional periodic magnetic domains resulting from the modulated microstructure.

Ferromagnetic formation of two phases due to MnP and InMn3 from InMnP:Zn implanted with Mn (10at.%)

InMnP:Zn samples implanted with Mn (10at.%) were annealed at 350°C for 60s and at 450°C for 30s. Using transmittance electron microscopy, both single crystalline and polycrystalline structures containing MnP and InMn3 sized ∼20nm were observed depending on the annealing condition. These samples exhibited two different Curie temperatures: TC1 at 291K and another well above 291K. The high temperature-ferromagnetic behavior up to TC1 and above TC2 is believed to have originated from two magnetic MnP and InMn3 phases, respectively.