Crystallization of the Amorphous Dome in Ge-Rich GeSbTe-Based Phase Change Memory Cells

Ge-rich GeSbTe (GGST) alloys have been developed by the industry to fulfill the high-temperature data retention requirements for embedded applications¹. Due to the off-stoichiometry nature of the as-deposited amorphous GGST, it undergoes chemical phase separation during thermal annealing and forms multiple crystalline phases at different crystallization stages².<br/><br/>In phase change memory (PCM) cells, the GGST alloy is confined within a nanometer-scale volume. An amorphous dome is created by melt-quench and is in contact with grains of different crystalline phases.Crystallization of this dome can occur either by applying electrical pulses or under thermal stress. Since crystallization is involved during both cell programming and data retention, it is important to understand the crystallization behavior of GGST material in the real PCM device environment.<br/><br/>In this work, a variety of (scanning) transmission electron microscopy ((S)TEM) techniques have been used to study the microstructural and chemical evolution of the amorphous dome in GGST cells during crystallization. The studied devices are 1R analytical cells with the wall architecture. Electrical tests were first performed to prepare the cells to different resistance states. Extremely thin (&lt; 30 nm) TEM lamellas were prepared using the focused-ion beam (FIB) technique. Multiple (S)TEM-based techniques were applied to study the GGST material: dark-field (DF) and high-resolution (HR) imaging for crystallographic analyses and high-angle annular dark-field (HAADF) imaging and electron-energy loss spectroscopy (EELS) for chemical mapping.<br/><br/>In the RESET state, the GGST cell contains an amorphous dome of a few tens of nanometers. The amorphous region is chemically homogenous, showing a Ge-rich composition. The dome is surrounded by a specific polycrystalline environment, including two crystalline "walls" where Ge grains have accumulated and a "ceiling" made of cubic Sb-poor GST grains.<br/><br/>We first show the crystallization of the dome during thermal annealing by combining<i> in-situ</i> TEM heating and <i>ex-situ</i> analyses. Progressive crystallization of the dome is observed during annealing from 250 to 330 °C. The growth of the GST crystals from the upper crystalline-to-amorphous interface is found to be the mechanism dominating the overall crystallization. During thermal recrystallization, chemical phase separation occurs inside the dome: Sb segregates and forms pure Sb₂ grains, while the remaining amorphous material becomes more enriched in Ge. Such Ge-rich amorphous compositions slow down the crystallization process, ensuring high RESET retention performances.<br/><br/>We then show the crystallization of the amorphous dome by electrical programming through <i>ex-situ</i> TEM analyses. By applying a proper partial-SET pulse, GGST cells show an intermediate resistance state, resulting from partial crystallization of the dome. The growth of GST grains from the upper crystalline periphery is again found to be an important process when crystallizing the dome by applying electrical pulses.<br/><br/>This study provides direct experimental observations of the crystallization behavior in GGST cells. The results apply to devices directly used for applications and offer important information for understanding the microscopic origin enabling the functionality and the high thermal stability of GGST-based PCM.<br/><br/>Reference:<br/>1. F. Arnaud, <i>et al. 2020 IEDM </i>(2020)<i>.</i><br/>2. E. Rahier, <i>et al. Phys. </i><i>Status Solidi – Rapid Res. Lett. </i>(2023).