Void formation and migration that drive the device failure of Ge2Sb2Te5 (GST)-based practical devices were revealed via in situ TEM.

Phase-change random access memory (PCRAM) is one of the most promising nonvolatile memory devices. However, inability to secure consistent and reliable switching operations in nanometer-scale programing volumes limits its practical use for highdensity applications. Here, we report in situ transmission electron microscopy investigation of the DC set switching of Ge-Sb-Te (GST)-based vertical PCRAM cells. We demonstrate that the microstructure of GST, particularly the passive component surrounding the dome-shaped active switching volume, plays a critical role in determining the local temperature distribution and is therefore responsible for inconsistent cell-to-cell switching behaviors. As demonstrated by a PCRAM cell with a highly crystallized GST matrix, the excessive Joule heat can cause melting and evaporation of the switching volume, resulting in device failure. The failure occurred via two-step void formation due to accelerated phase separation in the molten GST by the polaritydependent atomic migration of constituent elements. The presented real-time observations contribute to the understanding of inconsistent switching and premature failure of GST-based PCRAM cells and can guide future design of reliable PCRAM. INTRODUCTIONPhase-change random access memory (PCRAM) devices store and erase information by utilizing a large resistivity difference between the crystalline and amorphous states of chalcogenide materials. 1 Among various chalcogenide materials, the Ge-Sb-Te (GST) alloy system is currently considered the most promising candidate material for PCRAM owing to its fast and reversible phase-transition capability, in both the amorphous-to-crystalline (set) and the reverse crystallineto-amorphous (reset) switching. 2,3 Moreover, the set switching of GST occurs through an abrupt increase in the current density at a critical electric field, which is known as threshold switching. 1,4 The threshold switching provides local Joule heat and thereby facilitates the crystallization process at a relatively low electric field, ranging from 30 to 50 V μm − 1 . 5,6 Many modern PCRAM cell designs adopt a nanometer-scale hemispherical or cylindrical shape for programing volume to increase the cell density and reduce the operation power. 7 In such device structures, only a portion of the programing volume makes direct contact with a heater electrode, and the other sides are surrounded by passive GST that remains in the crystalline state and acts as an electrical conductivity path during the set and reset switching. One of the critical reliability issues related to these cell structures is the compositional change, or phase separation, caused by electric field-

The precipitation behavior in an Al-Cu alloy isothermally annealed at 373K～493K was studied, using the Vickers microhardness tests, Differential scanning calorimetry(DSC) and Transmission electron microscope(TEM) observations. An additional endothermic peak was found in DSC measurements which may be attributed to the θ″-phase independent from the formations of the G.P.(Ⅱ) and the θ′-phase at the aging temperatures. High resolution transmission electron microscopy (HRTEM) revealed that the G.P.(Ⅰ) is formed at a very early stage of isothermal aging at 463K. Comparing the Vickers microhardness and TEM images, it was concluded that G.P.(Ⅱ) is mainly responsible for the peak hardness.

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