Liquid-solid phase transition of Ge-Sb-Te alloy observed by in-situ transmission electron microscopy

Phase Stability and Anisotropic Sublimation of Cubic Ge–Sb–Te Alloy Observed by In Situ Transmission Electron Microscopy

“…An amorphous Ge 1 Sb 2 Te 4 (GST124) film was deposited at room temperature on a cleaned single crystalline silicon (111) wafer in an ultrahigh vacuum chamber using effusion cells for the individual elements. Details about the deposition process can be found elsewhere. , The thickness of the as-grown film as determined by cross-sectional transmission electron microscopy (TEM) amounts to 55 nm and furthermore, the chemical composition was examined by electron energy-loss spectroscopy in the TEM …”

“…A bright-field (BF) scanning (S)­TEM overview image of the as-deposited amorphous GST film prepared in the lamella geometry is shown in Figure b. In order to generate epitaxially aligned GST the amorphous film was first heated to 625 °C followed by a quick cool down within 3 s. At the high temperature, sublimation instead of melting has occurred because the film was not encapsulated . The BF-STEM image at 625 °C in Figure c shows that the GST thin film decomposes leading to an overall thinning of the sample, leaving faceted grains and holes behind.…”

“…24,25 The thickness of the as-grown film as determined by cross-sectional transmission electron microscopy (TEM) amounts to 55 nm and furthermore, the chemical composition was examined by electron energy-loss spectroscopy in the TEM. 10 The TEM data were collected in a JEOL 2100F operating at 200 kV and equipped with an UltraScan 4000 (model 895) CCD camera. The in situ videos were recorded using the continuous mode for a (1k × 1k) image in Digital Micrograph software from Gatan giving us a temporal resolution of 1.15 s. A microelectro-mechanical system (MEMS)-based double-tilt sample holder from DENSsolutions (model D6+) is used to carry out the low-drift in situ heating measurements.…”

“…In order to generate epitaxially aligned GST the amorphous film was first heated to 625 °C followed by a quick cool down within 3 s. At the high temperature, sublimation instead of melting has occurred because the film was not encapsulated. 10 The BF-STEM image at 625 °C in Figure 1c shows that the GST thin film decomposes leading to an overall thinning of the sample, leaving faceted grains and holes behind. Simultaneously, reorientations of crystal grains with cubic structure proceeds resulting in an epitaxial alignment to the crystalline Si(111) substrate (cf.…”

“…The ternary Ge–Sb–Te (GST) alloy has attracted great interest due toits broad application as a phase-change material in nonvolatile storage devices utilizing the fast and reversible-phase transformation between its amorphous and crystalline cubic phase. − Highly intensive local heating as well as extremely fast heating and cooling rates are required to switch with great speed between the phases whereby material degradation has to be avoided at the relevant operating temperatures. Generally, Ge–Sb–Te alloys crystallize from the metastable amorphous into the cubic phase between 130 and 150 °C and undergo a second phase transition into the trigonal phase at a temperature between 300 and 350 °C depending on the actual chemical composition. − While these solid–solid phase transitions as well as the transition into the molten state have been studied in detail, − only very few experiments address problems related to the phase stability. For example, volume shrinkage during recrystallization can lead to void formation in encapsulated GST material after high temperature processes, and thus, sublimation can occur at the crystal–void interface.…”