Phase-Change Behavior of Carbon-Doped Ge2Sb2Te5 Investigated by In Situ Electrical Biasing Transmission Electron Microscopy

The phase change behavior of a carbon-doped Ge2Sb2Te5 (C-GST) material under direct current (DC) biasing (forward and reverse bias) was investigated via in situ electrical biasing transmission electron microscopy (TEM). Threshold switching was governed primarily by the electrical transition mechanism, as evidenced by the absence of nuclei (during current) from the high-resolution TEM images and the corresponding fast Fourier transform (FFT). The chemical inhomogeneity was clearly observed under voltage polarity. For example, under reverse biasing, voids were formed at the interface between the C-GST and bottom electrode but were absent in the case of forward biasing; this absence resulted from the difference in the Joule heat and the current density. In addition, Ge and Sb atoms migrated to the cathode, whereas Te atoms migrated to the anode, in a similar manner to the migration that occurs during the amorphous-to-crystalline phase change. The lack of a stable Ge-C compound resulted in the segregation of carbon atoms at the amorphous region, regardless of the polarity of the voltage. The determination of a suitable direction of voltage that prevents atomic migration and void formation is therefore essential to the realization of stable C-GST materials.