Kyung-Woo Yi

One-dimensional heat conduction model for an electrical phase change random access memory device with an 8F2 memory cell (F=0.15 μm)

A one-dimensional heat conduction model is developed for a phase change random access memory device with an 8F2 memory cell structure (F=0.15 μm). The required current level for a reset operation, which corresponds to the phase switching from a crystalline (“1” state) to an amorphous phase (“0” state) of Ge2Sb2Te5, was investigated by calculating one-dimensional temperature profiles for the memory cell structure. It is revealed that a reset operation is not achieved at the current level (2 mA) reported for existing devices with a subquarter micron plug size when only TiN is used as a resistive heater. However, it is possible when an additional heating layer of 5 nm thickness is inserted between the TiN and Ge2Sb2Te5 layers, for which the electrical resistivity ρelec is higher than 105 μΩ cm, and the thermal conductivity κ and specific heat c are as low as those of Ge2Sb2Te5. In addition, it is shown that a reset operation at a low current level of 1 mA can be realized in this memory cell when amorphous ca...

Electric-Field-Induced Mass Movement of Ge2Sb2Te5 in Bottleneck Geometry Line Structures

We report an electric-field-induced directional mass movement of Ge 2 Sb 2 Te 5 in bottleneck geometry. Under high-electric-stress circumstances (>10 6 A cm -2 ), a mass of Ge 2 Sb 2 Te 5 tends to move toward the cathode (-) by the remaining mass depletion at the anode (+). The high electric stress induces an asymmetric compositional separation such that Sb is distributed toward the cathode (-) whereas Te is distributed toward the anode (+). Ionicity in Ge 2 Sb 2 Te 5 at high temperature and high electric stress can be one of the origins of the asymmetric behavior. The electric-field-induced mass movement may provide insight on the device reliability of phase-change random access memory.

The morphology of Al−Ti−O complex oxide inclusions formed in an ultra low-carbon steel melt during the RH process

Internal morphologies of bulk shape oxides in a melt of Ti bearing low-carbon steel were analyzed. The outer shapes of nearly spherical inclusions containing Ti were quite different from the shape of alumina inclusions that typically have a cluster form. A cross-section investigation using a polishing method and the FIB method revealed that the TiOx−Al2O3 complex oxide covers alumina clusters and that the interfaces of the alumina and complex oxide are modified as time passes.

The effects of pulling rates on the shape of crystal/melt interface in Si single crystal growth by the Czochralski method

Abstract The temperature profile and the shape of the crystal/melt interface in a Czochralski (CZ) furnace for large Si single crystal growth was simulated taking into consideration the fluid flow and surface radiational heat transfer. The view factors of the surface elements were calculated for radiation heat transfer. Two phases (crystal and melt) were treated as a continuous phase by assigning artificially large viscosity to the solid phase and the latent heat was accounted for by iterative heat evolution method, This simulation model of a CZ system was applied to the growth process of a 6 inch Si single crystal to study the effects of pulling rates on the interface shape. It was found that the interface becomes more concave to the melt as the crystal grows or as the pulling speeds become higher, and that the interface height is linearly dependent upon the pulling rate.

Change of Damage Mechanism by the Frequency of Applied Pulsed DC in the Ge2Sb2Te5 Line

We investigated the damage on the Ge 2 Sb 2 Te 5 line structure by pulsed-dc stressing with various frequencies. The line immediately burnt out due to Joule heating under constant dc stress 2.5 MA/cm 2 . However, when pulsed dc 2.5 MA/cm 2 was stressed at the frequency of 5 MHz, failure due to thermal fatigue damage was observed. At higher frequency such as 10 MHz, no noticeable damage was observed, yet the compositional change in constitutive elements by electromigration was detected. The change of damage mechanism by varying of frequency is explained by the difference in thermal cycling extent in response to the pulsed-current operation at various frequencies, which is computed using a finite-difference method.

The influence of ammonia pre-heating to InGaN films grown by TPIS-MOCVD

Abstract The InGaN films on GaN layers using a thermally pre-cracked ion-supplied metalorganic chemical vapor deposition (TPIS-MOCVD) system were investigated by a high-resolution X-ray diffraction and the thermodynamic analysis was performed for an NH3 pre-heater by using the computational fluid-dynamic simulations. As the flow rates of NH3 increased, the In composition and the thickness of the InGaN films increased, which meant that the relative indium incorporation efficiency was dependent on the NH3 flow rate and affected by the growth rate. In a low NH3 flow condition, indium metal droplets appeared on the surface of the InGaN layer in a conventional MOCVD system and decreased the indium incorporation efficiency of the InGaN films. The thermodynamic model of the ammonia dissociation did not follow equilibrium conditions and adduct-driven species seemed the actual growth precursors in nitride. The catalyst effect of the NH3 pre-heater plays an important role in the mixing region and growing surface, resulting in the higher indium incorporation and droplet-free surface in a TPIS-MOCVD system.

A numerical simulation of the thickness of molten mold flux film in continuous casting

The lubricative quality of the liquid flux film formed between a solidified shell and a mold in continuous casting is investigated in this study. Specifically, the effects of the thickness of the liquid flux film and the difference in the thickness between the top and the bottom of the film on the pressure force involved in compensating the ferrous static pressure of cast steel are investigated using a numerical simulation of the fluid flow in the film. The experimentally observed relationship between the operational conditions and the mold flux consumption can be explained using calculated results. The analysis also shows that the shape of the flux film is not uniquely determined by the force balance between the static pressure of the cast steel and the internal pressure of the film.

Development of a numerical model to predict areas of plume eye of ladle furnace process

The effects of operating parameters on the behavior of plume eye area were investigated with a numerical simulation model and water model experiments for ladle furnace process. The concept of total energy equivalence at the interface of slag and melt was adopted in the model for predicting the position of the slag/melt boundary of plume eye. The predicted results of plume eye area were well matched with water model experiments of wide range of gas flow (0.5 l/min~10 l/min) and other researchers’ results with various design of ladle of water model experiments and field operations. Numerical simulation and water model experiments also showed that fluid flow in the ladle was changed by increasing gas flow rates and the shape of flow patterns was strongly affected by the size of plume eye. By parametric study using simulation results and water model experiments, it was found that plume eye area is proportional to the square root of gas flow rate and is inversely proportional to the square root of initial slag height when the other parameters were keep constants.

Fluid flow and mixing behavior in gas stirred ladle with submerged lance

A numerical study for analyzing both fluid flow and mixing behavior in gas-stirred ladles with a submerged lance was performed. At first, the shape and volume of the plumes generated by the submerged nozzle were calculated. Subsequently, the fluid flow driven by these plumes, and the mass transfer in the ladle were calculated by a three dimensional turbulent simulation program. Water model experiments for the velocity measurements were performed to verify the accuracy of the calculation results. It was shown that as the gas flow rate increases, the downward velocity at the ladle wall increases and mixing time decreases. Mixing time is sensitive to the alloy addition position especially at a lower gas flow rate. The result suggested that the alloy should be added at the plume zone close to the center at the melt surface

Analysis of the origin of periodic oscillatory flow in the continuous casting mold

It is very important to understand flow patterns within the continuous casting mold because they have a significant impact on product quality. Water model experiment and particle image velocimetry were conducted to identify the fluid flow pattern in the steel slab continuous casting mold. The fluid flow pattern in the mold is not steady but instead an oscillatory flow with a specific oscillation frequencies. Many studies have been reported about oscillatory flow within the mold. However, these studies do not provide a clear explanation of physical origin of oscillatory flow. We identified the physical origins of various specific oscillation frequencies, and confirmed through experimentation and simulation that each frequency is related to the cross flow and injection stream oscillation. Moreover, the degree of oscillation at each frequency appears differently depending on the location within the mold, and is shown to have a effect near the mold wall. These results provide a better understanding of complex oscillatory flow patterns within the mold.