Jae-Bok Seol

Novel approach for observing the asymmetrical evolution and the compositional nonuniformity of laser pulsed atom probe tomography of a single ZnO nanowire

The characterization of ZnO nanowires is crucial for developing nanostructured devices together with related compounds and alloys with an atomic-scale regime. This study describes the effects of laser energy on the atom probe tomography analysis of a single ZnO nanowire with a high aspect ratio, diameters of 80?100 nm and lengths of 10 µm. We observed both an asymmetrical evolution in the apex morphology and the compositional nonuniformities of Zn and O ions with respect to the laser energies ranging from 5 to 40 nJ. When the higher laser illumination exposed to the ZnO nanowires, non-uniform field strength becomes noticeable especially at the laser incident side of the samples. Moreover, we measured the charge state ratios of Zn+ and Zn2+ ions as a function of the applied laser energies. Our results proved important for accurate quantitative characterization and better interpretation for the laser-pulsed atom probe tomography of ZnO-based devices.

Nanometer‐Scale Phase Transformation Determines Threshold and Memory Switching Mechanism

Creation of nanometer-scale conductive filaments in resistive switching devices makes them appealing for advanced electrical applications. While in situ electrical probing transmission electron microscopy promotes fundamental investigations of how the conductive filament comes into existence, it does not provide proof-of-principle observations for the filament growth. Here, using advanced microscopy techniques, electrical, 3D compositional, and structural information of the switching-induced conductive filament are described. It is found that during in situ probing microscopy of a Ag/TiO2 /Pt device showing both memory- and threshold-switching characteristics, a crystalline Ag-doped TiO2 forms at vacant sites on the device surface and acts as the conductive filament. More importantly, change in filament morphology varying with applied compliance currents determines the underlying switching mechanisms that govern either memory or threshold response. When focusing more on threshold switching features, it is demonstrated that the structural disappearance of the filament arises at the end of the constricted region and leads to the spontaneous phase transformation from crystalline conductive state into an initial amorphous insulator. Use of the proposed method enables a new pathway for observing nanosized features in a variety of devices at the atomic scale in three dimensions.

Atomic-scale quantification of interdiffusion and dopant localization in GeSbTe-based memory devices

Fabrication of phase-change memory devices at modest or ambient temperatures leads to nanoscale compositional variations in phase-transition layers, where amorphous-polycrystalline phase change takes place via electrical switching, and can alter the devices performances. Here, by transmission electron microscopy and atom probe tomography, we address that thermal annealing at 400 °C for 20 min induces an elemental interdiffusion in the devices consisting of TiN (top electrode), carbon-doped GeSbTe (phase-transition layer), and TiSiN (bottom heater). With respect to the employed annealing process, the Ge atoms of GeSbTe layer have diffused into TiSiN layer at a given sample volume, while the Ti atoms of TiSiN layer into GeSbTe layer. Furthermore, non-random nature of dopant distribution in the GeSbTe materials leads to a Ti-localization including dopants at the GeSbTe/TiSiN interfaces. Our findings have two important implications: First, the annealing-driven interdiffusion of Ge and Ti is a predominant mec...

Three-dimensional indium distribution in electron-beam irradiated multiple quantum wells of blue-emitting InGaN/GaN devices

The compositional distribution of In atoms in InGaN/GaN multiple quantum wells is considered as one of the candidates for carrier localization center, which enhances the efficiency of the light-emitting diodes. However, two challenging issues exist in this research area. First, an inhomogeneous In distribution is initially formed by spinodal decomposition during device fabrication as revealed by transmission electron microscopy. Second, electron-beam irradiation during microscopy causes the compositional inhomogeneity of In to appear as a damage contrast. Here, a systematic approach was proposed in this study: Electron-beam with current density ranging from 0 to 20.9 A/cm2 was initially exposed to the surface regions during microscopy. Then, the electron-beam irradiated regions at the tip surface were further removed, and finally, atom probe tomography was performed to run the samples without beam-induced damage and to evaluate the existence of local inhomegenity of In atoms. We proved that after eliminating the electron-beam induced damage regions, no evidence of In clustering was observed in the blue-emitting InGaN/GaN devices. In addition, it is concluded that the electron-beam induced localization of In atoms is a surface-related phenomenon, and hence spinodal decomposition, which is typically responsible for such In clustering, is negligible for biaxially strained blue-emitting InGaN/GaN devices.

Grain Boundary Segregation and Core/Shell Structured Nanofeatures in Oxide-Dispersion Strengthened Fe-Cr Alloys

Since the oxygen-rich nanofeatures (NFs) in oxide-dispersion-strengthened (ODS) ferritic steels is a key factor for improving their whole mechanical properties [1-3], atom-probe tomography (APT) and transmission electron microscopy (TEM) have been widely employed to identify the oxygen-rich NFs in the materials [2-5]. The current work presents three challenges to deeply understand the chemical origin of the nano-sized precipitates containing Y, O, and Ti. First, the segregation of alloying elements at grain boundaries is observed. Secondly, we compare the concentrations of nano-sized precipitates with a size of ~3 nm to those of 10-nm precipitates. Finally, the existence of NF structure with the Y-rich cores and Tiand Cr-enriched shells is confirmed as previously reported by APT. The nominal composition of the materials studied here is 14Cr, 3.0W, 0.4Ti in wt.%. Analyses were performed using a LEAP-4000HR microscope in voltage-pulsing mode at 200 kHz pulse repetition rate, 0.005 atom/pulse detection rate, 20% pulse fraction and 70 K.

Influence of Laser-Pulse Energy on Field Evaporation of LaAlO 3 in Atom Probe Tomography Analysis

As followed by the design concept of metal-oxide-semiconductor (MOS) devices, the development of dielectric materials with high-k property becomes a crucial for reducing the equivalent oxide thickness. In order to develop the novel dielectric materials, understanding of correlations between electrical properties and chemical nature at metal-oxide interfaces has been an essential topic [1]. To identify this subject, atomic-scale investigation of bulk oxide materials through laser-pulsed atom probe tomography (APT) has been widely performed [2-7]. From the fundamental point of view, we present the influence of laser-pulse energy on APT results of lanthanum aluminum oxide (LaAlO3), termed LAO, especially mass-resolving power (MRP). The LAO is one of the potential candidates for substituting the conventional SiO2, which exhibit many advantages such as a high dielectric constant and large band-gap energy with ranging between 5.8 and 6.6 eV. Analysis were performed using a LAWATAP microscope in laser-pulsing mode of pulse energy varying with systematically 0.03~0.21 μJ at ~100 kHz pulse repetition rate, 0.002 atom/pulse detection rate, and 30 K.

2-D & 3-D Observations on the Microstructure of Super Bainite TRIP Steels using Total Analysis System

It has been widely reported that carbide-free bainitic steels or super-bainite TRIP (SB-TRIP) steels for the automotive industry are a new family of steels offering a unique combination of high strength and ductility. Hence, it is important to exactly evaluate the volume fraction of RA and to identify the 3-D morphology of constituent phases, because it plays a crucial role in mechanical properties. Recently, as electron back-scattered diffraction (EBSD) equipped with focused ion beam (FIB) has been developed, 3-D EBSD technique for materials science are used to these steels. Moreover, newly developed atom probe tomography (APT) technique can provide the exact distribution and chemical concentration of alloying elements in a sub-nm scale. The APT analysis results indicate exactly the distribution and composition of alloying elements in the austenite and bainite phases of SB-TRIP steels with the atomic-scale resolution. And thus, no partitioning of aluminum and manganese atoms was showed between the austenite containing at% C and the bainitic ferrite associated with at% C in SB-TRIP steel.

A new method for mapping the three-dimensional atomic distribution within nanoparticles by atom probe tomography (APT)

We present a new method of preparing needle-shaped specimens for atom probe tomography from freestanding Pd and C-supported Pt nanoparticles. The method consists of two steps, namely electrophoresis of nanoparticles on a flat Cu substrate followed by electrodeposition of a Ni film acting as an embedding matrix for the nanoparticles. Atom probe specimen preparation can be subsequently carried out by means of focused-ion-beam milling. Using this approach, we have been able to perform correlative atom probe tomography and transmission electron microscopy analyses on both nanoparticle systems. Reliable mass spectra and three-dimensional atom maps could be obtained for Pd nanoparticle specimens. In contrast, atom probe samples prepared from C-supported Pt nanoparticles showed uneven field evaporation and hence artifacts in the reconstructed atom maps. Our developed method is a viable means of mapping the three-dimensional atomic distribution within nanoparticles and is expected to contribute to an improved understanding of the structure-composition-property relationships of various nanoparticle systems.

Correlation of Controllable Aggregation with Light-Emitting Property in Polymer Blend Optoelectronic Devices

The control of solution-processed emitting layers in organic-based optoelectronic devices enables cost-effective processing and highly efficient properties. However, a solution-based protocol for emitter fabrication is highly complex, and the link between the device performance and internal nanoscale features as well as three associated fabricating parameters (e.g., the employed solvents, annealing temperatures, and molecular concentration) needs to be understood. Here, this study investigates the influence of the solution-processing parameters on the nanostructure-property relationship in light emitters that consist of iridium complexes doped in polymer. The boiling points and evaporation rates of the selected solvents govern the nanomorphology of molecular aggregation in the as-processed state, and the aggregation is either needle-like, spherical, or even a mixture of needles and spheres. Furthermore, a direct observation via in situ heating microscopy indicates that annealing of emitters containing a needle-type aggregation promotes the associated molecular transport, leading to a substantial reduction in the surface roughness. Consequently, a nearly threefold increase in the current efficiency of the device is induced. These findings have important implications for the tuning of the aggregation of iridium complexes for emitters used in the new evolution of high-performance organic-based optoelectronic devices.

Direct In situ Observation of Tempering-induced Austenite Decomposition and Atom Probe Analyses of k -Carbide Precipitates in Lightweight Fe-Mn-Al-C Steels

Tempering processing at temperatures of 500~600°C leads to the decomposition of retained austenite into Al-partitioned carbides (termed k-carbide) and Al-depleted ferrite, profoundly deteriorating the overall mechanical property of ferrite-based lightweight steels containing high-Al contents [1,2]. To evaluate the thermal stability of retained austenite, direct observation of austenite decomposition by in situ heating transmission electron microscopy (TEM) at 500°C is a promising topic. In addition, atom probe tomography (APT) can offer a pathway to three-dimensionally display the redistribution of microalloying elements in the products formed from austenite decomposition [1,2]. In particular, the current work pays attention to determine the concentrations of Al and C atoms in the products found in tempered steels through clipping them in conjunction with separate mass-spectra. Analyses were performed using a LEAP-4000HR microscope in voltage-pulsing mode at 200 kHz pulse repetition rate, 0.005 atom/pulse detection rate, 20% pulse fraction and 70 K.