Microstructure-dependent DC set switching behaviors of Ge–Sb–Te-based phase-change random access memory devices accessed by in situ TEM

Abstract

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 high-density 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 polarity-dependent 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. Using in situ electron microscopy, researchers in Korea have discovered why phase-change memory devices can fail during switching operations. Phase-change materials, such as germanium–antimony–tellurium (Ge–Sb–Te) alloys, rapidly switch between crystalline and amorphous phases on electrical heating. Researchers have used this to make memory chips that are hundreds of times faster than flash devices, but inconsistent performance currently limits practical, high-density applications. Sang Ho Oh from Pohang University of Science and Technology and colleagues observed the phase changes of Ge–Sb–Te memory cells in real time by applying direct-current voltages through a transmission electron microscope. They found that the device lifetime critically depended on the atomic microstructure around the active switching site. Ironically, higher degree of crystallinity of this passive zone can cause more excessive heat dissipation, leading to eventual melting and evaporation of cells. Applying in situ TEM techniques to GST-based vertical PCRAM cells, we directly observed the DC set switching process of real devices for the first time. The results show that the microstructure of crystalline GST matrix is an important structural parameter determining the local temperature distribution. In the case of highly crystallized GST matrix, the device failure occurred via two-step void formation due to the polarity-dependent electromigration.