## 2 Material Properties
### Structural Properties  
It is important to understand the behavior of GST materials around the phase transition within the context of its crystal structure. Upon crystallization from the amorphous state at around 150 \({}^{\circ}\)C, GST alloys form a poly-crystalline cubic (rock salt) state, with grain sizes of 10-20 nm. [PERSON] et al. proposed the crystal structure of GST at fcc state shown in Figure 7a. The 4(a) site is wholly occupied by only Te, and 4(b) site is randomly occupied by Ge, Sb atoms, with some vacancies with different probabilities depending on the composition of the materials [40; 41]. Besides, the Ge\({}_{2\times x}\)Sb\({}_{2}\)TeS and Ge\({}_{2}\)Sb\({}_{2+x}\)TeS was studied by [PERSON] et al. [41] and [PERSON] et al. [42], respectively. Their results indicate the excess Ge or Sb atoms does not occupy the vacancies, but rather accumulated at grain boundaries [2]. To get a closer look of GST at the atomic level, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) has been used to study the crystal structures and defects of crystalline chalcogenides [43; 44; 45; 46; 47]. The atomistic models and the images obtained by HAADF-STEM on rocksalt GST are shown in Figure 7b. The bright dots correspond to the contribution of the whole atomic columns along the viewing direction, and the brightness of each dot is roughly proportional to \(Z^{2}\), where \(Z\) is the average atomic number (Ge 32, Sb 51, Te 52, vacancy 0) of the atomic column [47]. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Label** & **Fabrication Method** & **Film Thickness (nm)** & **Reference** \\ \hline
1 & Thermal evaporation & 100–200 & [32] \\
2 & Sputtering & - & [33] \\
3 & Sputtering & 50 & [34] \\
4 & Sputtering & - & [35] \\
5 & Atomic Layer Deposition & 60 & [35] \\
6 & Sputtering & 98 & [36] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The fabrication method and thickness of the films in Figure 5.  
Figure 6: Programming window behavior as a function of Te at.%. Decreasing Te at.% results in a narrower resistance window. Adapted with permission from Reference [37]. ©2015 Elsevier. This phase-changing process of GST happens in a very short time range, as short as 50 ns. The reason for such a fast transition may because the fcc structure has a high-grade lattice symmetry which is similar to the amorphous state which has an isotropic atom distribution, indicating the composite atoms in the amorphous state do not need to travel long distances to be fixed in the fcc crystalline state [48; 49]. [PERSON] et al. performed the reverse Monte Carlo simulation with experimental data from synchrotron-radiation x-ray diffraction [50]. Their results indicated that amorphous GST is characterized by even-folded ring structures which is the key for the fast crystallization speed. This finding is schematically depicted in Figure 8. The X-ray diffraction (XRD) is a rapid analytical technique primarily used for phase identification of GST material. The typical XRD patterns of GST thin films at amorphous, fcc and hcp states are shown in Figure 9. The film is sputtered by sputter deposition on a glass slide. The structural information is given by the peak positions and its relative intensity. Due to the nature of the amorphous state, there is no obvious peak for the as-deposited film. The film annealed at 175\({}^{\circ}\) is crystallized into a face-centered cubic (fcc) state indicated by XRD peaks [111], [002], [022], and [024]. Further annealing at temperatures above 175\({}^{\circ}\) leads to the transition from fcc structure to the hexagonal closest packed (hcp) state shown by the XRD peaks [013], [110], [203], and [026]. Figure 8: Schematic presentation for the possible ring size transformation in crystal-liquid-amorphous phase change record and amorphous-crystal phase change erase in Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\). Stage I and II: recording process in optical storage; stage III: erasing process in optical storage. Adapted with permission from Reference [50].