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An analytical TEM study on Ge-Sb-Te based chalcogenide superlattices for phase-change memory

 The interfacial phase-change memory fabricated using a superlattice, a cross-stack of GeTe and Sb2Te3, can implement a phase transition from high-resistance to low-resistance without going through an amorphous state. This new device exhibits superior characteristics, such as faster phase-change rate, lower reset current, and higher cyclability compared to conventional phase-change memory. However, there are various opinions on the atomic arrangement of the high and low resistance states and the lattice structure model of the superlattice phase change memory, and there are no clearly proven observations. In this study, an in-situ transmission electron microscopy is used to reveal the structure of the superlattice as well as to investigate the phase transition mechanism.

In addition, research on selectors to improve noise problems between cells as well as cells themselves is very important. The switching mechanism of the selector is being investigated using in-situ transmission electron microscopy.

Analysis of oxide ReRAM with analytical electron microscopy

The ReRAM(resistive switching random access memory) is a memory device that stores information using a phenomenon that causes a change in the resistance through the formation and rupture of a conductive filament(CF) with a width of several nm. ReRAM, with its outstanding performance and similarity to the synaptic connection of neurons, is drawing attention as a new next-generation memory. Our group is testing a new material system for high-performance ReRAM and is analyzing devices using an in-situ TEM technique to elucidate the mechanism of resistance change and failure.


In-situ electron microscopy and environmental electron microscopy

 In-situ electron microscopy (real-time observation technique) can provide dynamic information for understanding the relationship between the properties of the material and the microstructure by observing the changes in the microstructure in response to various stimuli (heat, electricity, and stress, etc.). This real-time observation technique uses a specially made holder for various purposes. For instance, the phase change and recrystallization can be observed at high temperatures using an in-situ heating holder, and changes caused by an electrical stimulus can be observed using an in-situ probing holder.

 Another area of ​​in-situ electron microscopy is environmental electron microscopy, which is the technique of observing changes occurring in complex dynamic environments by introducing liquid/gas into the high-vacuum electron microscope. Synthesis of nanomaterials, electrochemical and catalytic reactions are mostly liquid/gas involved reactions, and it is very important to observe the microstructure change in liquid/gas atmosphere in real-time to understand the fundamental of the reaction mechanism, and to investigate the kinetics of nucleation, growth, and thermal/chemical stability of nanostructures.

Direct observation of  the reaction mechanism of Ni / 6H-SiC at low temperature by using in situ TEM

The reaction of Ni/SiC has been attracting attention because it has excellent properties for various device applications. However, the research on the initial reaction of Ni/SiC and exact mechanism is currently insufficient. To control appropriate variables for using device, it is necessary to understand the mechanism of initial reaction state. Therefore, we investigated the reaction of Ni films on a 6H-SiC substrate and the behavior of carbon at 550 °C by direct observation by in situ transmission electron microscopy (TEM).


Control of oxidation behavior in high vacuum transmission electron microscopy

Transmission electron microscopy (TEM) is a critical tool for observing and understanding nanoscale phenomena that occur in nanomaterials. Although TEM is a high-vacuum instrument, due to the presence of molecules remaining under the equilibrium gas partial pressure, unintended reactions can be thermodynamically driven by various factors during TEM observation. In this work, an oxidation reaction caused by the electron beam irradiation and heating in a microscope was studied using pristine copper nanowires (Cu NWs), with a high oxygen affinity. Also, this study presents a method (i.e., graphene encapsulation) for preventing the unintended oxidation reaction of a TEM specimen.

Computer aided image process ; Statistical calculation, machine learning

As TEM hardware technology reaches the cutting edge, we have seen a remarkable improvement in microscope performance. However, there is still room for improvement in image and spectral information. Statistical/arithmetic analysis, interpretation, and computer processing of these data can yield information that can outperform the equipment. In addition, it is possible to obtain an ideal image by removing artifacts caused by equipment.

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