Papers by Keyword: Ge₂Sb₂Te₅

Abstract: The Te-Te van der Waals-type secondary bond in stable mGeTe·nSb₂Te₃ (GST) pseudo-binary chalcogenides was investigated by means of ab initio calculations. The generalized gradient approximation of Perdew-Burke-Ernzerhof pseudopotential describes the Te-Te secondary bond very well. Whereas the local density approximation pseudopotential over-estimates the bond energy by underestimating the repulsive forces between the adjacent Te atoms. The electron localization function illustrates the chemical bonding nature of the Te-Te secondary bond. Our present results will shed insights on the secondary bonds in GST phase change materials.

Abstract: Phase change materials are the basis for next generation memory devices, but the fundamental mechanism of the phase transitions has not been elucidated clearly. In this paper, the microstructure treated by isothermal and the laser radiance was compared by Raman spectroscopy. It was found that the peak ~105 cm^-1 originated from GeTe₄ tetrahedral structure and the intensity of this peak became weak with the enhancement of laser pulse energy and extension of annealing time. Moreover, we also ascribed the Raman peaks in specific positions to the GeTe_4-nGe_n (n=0, 1, 2, 3) tetrahedral structures.

Abstract: Phase transformation and crystal growth behavior of Ge2Sb2Te5 were investigated systematically by means of in situ heating (from room temperature to 500 oC) of amorphous Ge2Sb2Te5 alloy in a high voltage electron microscope with real-time monitoring. Large-scale crystallization occurred to amorphous Ge2Sb2Te5 around 200 oC. Large crystal growth developed on heating from 200 oC to 400 oC, and single crystalline grains grew up to 150 nm. Eventually the onset of partial melting of thin Ge2Sb2Te5 foil was at 500 oC and liquid Ge2Sb2Te5 was observed for the first time by high-resolution transmission electron microscopy. Hexagonal Ge2Sb2Te5 phase remains after a subsequent cooling.

Abstract: Characteristic 60° dislocations occurred in hexagonal phase of Ge2Sb2Te5 thin foil cooled from 500°C to room temperature in a high voltage transmission electron microscope. The Burgers vector of dislocation was identified as 1/ 24 < 9902 > which is the edge component of 1 3 < 2110 > projected on the (1120) lattice plane. The dislocation resulted from the cooling-induced stress/strain in the Ge2Sb2Te5 alloy.

Abstract: Thermal stress model considering the effect of phase transformation is proposed for Phase-Change Random Access Memory (PRAM). The results of simulation show that the high level of stress is generated on the junction where Ge2Sb2Te5(GST), TiN and SiO2 meet together. The high level of stress can also be observed in the interface between TiN and SiO2. From simulation results, it can be predictable that delamination between GST and TiN can occur during operation of PRAM. It is expected that the simulation model, which has been developed in this research, is very useful tool for PRAM device design.

Abstract: Ge2Sb2Te5 (GST) has been widely studied for PRAM as reversible phase change material. GST is expected to reduce RESET (crystalline → amorphous) operation power, which is one of important issues for PRAM technology. In order to investigate the effect of nitrogen doping on electrical switching characteristics, we fabricated two kinds of PRAM cells with nitrogen-doped (N-doped) and un-doped GST, which were different bottom electrode contact size (0.80~1.00
). N-doped GST PRAM cells have higher dynamic resistance with small sized bottom electrode contact and lower RESET voltage (about 1.2 V, 50 ns) than un-doped GST PRAM cells (about 1.6 V, 50 ns). The resistance switching ratio (RRESET to RSET) was about 100. The results of this study indicate that nitrogen doping into GST film and smaller size of bottom electrode contact reduce RESET power for PRAM operation.