Ju-Young Cho

Observation of polyamorphism in the phase change alloy Ge1Sb2Te4

A high-pressure synchrotron x-ray diffraction study of the phase change alloy Ge1Sb2Te4 demonstrates the existence of a polyamorphic phase transition between the “as deposited” low density amorphous (LDA) phase and a high density amorphous (HDA) phase at ∼10 GPa. The entropy of the HDA phase is expected to be higher than that of the LDA phase resulting in a negative Clapeyron slope for this transition. These phase relations may enable the polyamorphic transition to play a role in the memory and data storage applications.

Inhibition of the Electrostatic Force-Induced Atomic Migration in Ge2Sb2Te5 by Nitrogen Doping

The effects of nitrogen doping on the electromigration in molten Ge 2 Sb 2 Te 5 (GST) were studied using an isolated-line structure. A change in composition and morphology in the GST line was observed with an increased duration of applied electrical stress. The diffusion rate of the constituent atoms in GST was decreased by N doping. The ratios of the product of diffusion coefficient and effective charge (DZ * ) in N-doped GST to that of undoped GST were 0.412, 0.622, and 0.624 for Ge, Sb, and Te, respectively. In addition, the catastrophic failure of the GST line by void formation was retarded in N-doped GST.

Growth Mechanism of Strain-Dependent Morphological Change in PEDOT:PSS Films

Understanding the mechanism of the strain-dependent conductivity change in polymers in stretched conditions is important. We observed a strain-induced growth of the conductive regions of PEDOT:PSS films, induced by a coalescence of conductive PEDOT-rich cores. This growth due to coalescence leads to a gradual decrease in the electrical resistivity up to 95%, independent of the thickness of the PEDOT:PSS films. The primary mechanism for the evolution of the PEDOT-rich cores proceeds by the cores growing larger as they consuming relatively smaller cores. This process is caused by a strain-induced local rearrangement of PEDOT segments in the vicinity of PSS shells around the cores and also changes the chemical environment in PEDOT, induced by the electron-withdrawing effects around the PEDOT chains. The strain-induced growth mechanism is beneficial to understanding the phenomenon of polymeric chain rearrangement in mechanical deformation and to modulating the electrical conductivity for practical applications.

New pathway for the formation of metallic cubic phase Ge-Sb-Te compounds induced by an electric current.

The novel discovery of a current-induced transition from insulator to metal in the crystalline phase of Ge2Sb2Te5 and GeSb4Te7 have been studied by means of a model using line-patterned samples. The resistivity of cubic phase Ge-Sb-Te compound was reduced by an electrical current (~1 MA/cm2), and the final resistivity was determined based on the stress current density, regardless of the initial resistivity and temperature, which indicates that the conductivity of Ge-Sb-Te compound can be modulated by an electrical current. The minimum resistivity of Ge-Sb-Te materials can be achieved at high kinetic rates by applying an electrical current, and the material properties change from insulating to metallic behavior without a phase transition. The current-induced metal transition is more effective in GeSb4Te7 than Ge2Sb2Te5, which depends on the intrinsic vacancy of materials. Electromigration, which is the migration of atoms induced by a momentum transfer from charge carriers, can easily promote the rearrangement of vacancies in the cubic phase of Ge-Sb-Te compound. This behavior differs significantly from thermal annealing, which accompanies a phase transition to the hexagonal phase. This result suggests a new pathway for modulating the electrical conductivity and material properties of chalcogenide materials by applying an electrical current.

Thermomechanical Analysis on the Phase Stability of Nitrogen-Doped Amorphous Ge2Sb2Te5 Films

Phase change random access memory (PRAM) technology is based on electrically triggered, reversible amorphousto-crystalline phase transformations in phase change materials. Ge2Sb2Te5 (GST) is the most widely studied phase change material, but its application is restricted because of its insufficient write/erase speed and data retention time. Write/erase speed is related to the relatively slow transition to the crystalline phase, and date retention is caused by the spontaneous crystallization of the amorphous phase. These limitations are related to the phase stability of GST, especially the amorphous phase stability because the PRAM uses the phase transition between the crystalline and amorphous states, and the meta-stable nature of the amorphous phase leads to the changes of properties over time. Therefore, phase stability of amorphous GST is one of the most significant properties that determines the performance and reliability of the PRAM. The phase stability of amorphous GST has already been studied by monitoring the electrical, 1) optical, 2) and thermal 3) characteristics of GST by investigating the crystallization behavior. It has been also suggested that the phase stability of amorphous GST can be monitored by measuring the stress change. Stress relaxation behavior occurs because the amorphous GST tends to change its structure to reach a more stable state by relieving the stress in the film and can be explained by changes of viscosity in glass. Because viscosity is related to the motion of atoms, it is strongly related to the resistance to the structural changes in amorphous GST. The glass forming ability can also be determined by the evolution of stress accompanied by the glass transition and crystallization behaviors. The glass transition temperature, in particular, has been able to be observed by the detection of stress change. The viscosity and the glass forming ability of amorphous GST was investigated by using thermomechanical measurements in this study. 4)

Electrical current-induced gradual failure of crystalline Ge2Sb2Te5 for phase-change memory

Electrical failure in crystalline Ge2Sb2Te5 was observed under a direct current bias, which induces a steady degradation of the electrical conductivity. This failure is induced by electromigration because alternating current bias stressing does not trigger this behavior. Nano-scaled voids were generated during current stressing, which explains the gradual increase in the quantitative resistance. Each nano-void previously comprised a molten phase that was induced by localized melting, which produced compositional variation during the solidification process. The phase-change memory can be damaged by electrical stressing in the non-active regions.

Effects of dopings on the electric-field-induced atomic migration and void formation in Ge 2 Sb 2 Te 5

Electromigration in molten and crystalline Ge2Sb2Te5 (GST) was characterized using pulsed DC stress to an isolated line structure. In the electomigration of molten GST, the effects of N-and Bi-doping on the electromigration were aslo studied to find the solution for inhibiting the electromigration. When a single pulse (∼10−3 s) was applied to the lines, both undoped and doped GST lines were melted by Joule heating, and Ge and Sb atoms migrate to the cathode, whereas Te atoms migrate to the anode. This elemental separation in the molten GST was caused by an electrostatic force-induced electromigration. The migration rate of the constituent atoms in the undoped GST was similar to that in Bi-doped GST, but was decreased by N-doping. Under applying a 10 MHz pulsed DC, the melting by Joule heating was inhibited, and electromigration in the crystalline state was detected. All constituent elements migrated to the cathode, which is originated from the electromigration by hole-windforce. This study provide the basic understanding about the degradation phenomena in phase change memory, and suggest the method for inhibiting the endurance failures.

Electromigration-limited reliability of advanced metallization for memory devices

As the design rule for memory devices shrinks, the reliability issue of electromigration (EM) is emerged due 10 the increase of high current density, therefore, the reliability for memory devices can be limited by EM failure of metal lines (Al. Cu. W). But EM reliability with respect to structures of interconnects is still underestimated even though EM behavior for each material has been reported for decades. Therefore, we investigated the kinetics of EM in various metal line and via in memory devices under direct current (DC) stressing because failure of metal interconnects depends not only on metal materials but also on structures of interconnects. Under EM tests, mean time failure of Al with W via was shorter than that of Cu with W via. These results came from abrupt failure behavior due to void nucleation and growth at Al with W via and gradual failure behavior at Cu with W via due to void generation and growth as well as conduction in Ta/TaN. Additionally. Cu with W via showed different behavior compared to Cu with Cu via. It can be explained that the joule heating between W and Cu interface caused lateral void expansion and resistance increases rapidly. And it was observed that W line had the longest lifetime of EM failure but the high resistivity of W should be considered for memory chip design. As the results, we conclude that Al has the weakest reliable property for EM reliability among Al. W and Cu metal lines and W via can affect the degradation of EM reliability. These results mean that reliability of Al and W interconnects beyond nanometer-scale should be improved to guarantee reliability in memory chip. This study could provide the guideline for the optimal materials for interconnects in highly-reliable memory chips.