## 3 Applications  
### Phase-change Memory (PCM)  
Nonvolatile memory devices have been the key elements for a wide variety of charge storage memories, such as flash memories, hard disk drives, and solid state drives. Even though the flash memories are widely used in the electronic products as the primary storage devices, new architectures are necessary to allow its scalability below the 16 nm technology node, due to the size limitation of flash memories [62]. Phase-change memory (PCM) has become an application of interest for non-volatile memories over the past few decades, given the simplicity of fabrication as well as the small footprint. The concept of phase-change memory originated in 1960s [63; 64]. With the improvements in fabrication capability and material engineering, chalcogenide compounds, in particular, the Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) (GST), \begin{table}
\begin{tabular}{c c} \hline \hline
**Composition** & **Contrast of Optical Constant** \\ \hline GeTe & — \\ GeSb\({}_{2}\)Te\({}_{11}\) & \(-1.48\) + i1.35 \\ GeSb\({}_{2}\)Te\({}_{9}\) & \(-1.45\) + i1.27 \\ Ge\({}_{4}\)Sb\({}_{2}\)Te\({}_{7}\) & \(-1.26\) + i1.09 \\ Ge\({}_{2}\)Sb\({}_{2}\)Te\({}_{5}\) & \(-1.20\) + i1.05 \\ Sb\({}_{2}\)Te\({}_{3}\) & — \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The stoichiometry effect on difference between index (n) and extinction coefficient (k) of GST at amorphous and crystalline state at 405 nm wavelength [8]. All these compounds are on the pseudobinary line connecting GeTe and Sb\({}_{2}\)Te\({}_{3}\). Figure 11: Reflection and transmission spectra of 350 nm GST films on fused silica annealed at (a) 145 C, and (b) 175 C. Adapted with permission from Reference [15]. ©2018 IEEE. have become the material of choice for the non-volatile phase transition memories [65; 66]. Essentially, the change in electrical resistance that accompanies the change in phase is utilized as a mean of binary data representation. One of the benefits of PCM is that it offers a much faster-switching speed. Flash memories work by modulating the storage of charge. Changing the bit's state requires removing the accumulated charge, which demands a relatively large voltage to manipulate charges. As this voltage burst requires time to build up power, the general write-times for common Flash devices is in the order of 100 \(\upmu\)s. In PCM devices, the memory element can be switched ON and OFF more quickly (in nanoseconds), therefore, PCM can provide much higher operation speed. Another advantage of PCM is its higher data storage stability since the information is stored in the form of structural difference instead of the electric charge in flash memories. Depending on the size of the PCM devices, the typical pulse for the SET operation (refer to 1) has a time duration around 100-500 ns and the current is around 0.2 mA, while typical RESET (reset to 0) time is 50-100 ns with a higher current about 0.5 mA [67; 68]. One of the major difficulties of the integration of PCM is that the high-density electronics requires high programming currents, especially for the RESET operation to switch the cell from crystalline to amorphous state. In fact, as shown in Figure 12a, in order to scale down the size of a PCM cell and integrate within consumer electronics, the writing current for each cell needs to be reduced down from \(m\)A to \(\upmu\)A [68]. The simulation results from [PERSON] et al. [68] showed that a confined cell structure is effective in writing current reduction, footprint reduction, and contributes to a reduction in the degradation of the PCM cells over time. As shown in Figure 12b, the resistivity of the set (crystalline) and reset (amorphous) state have large contrast below the threshold switching voltage (\(V\)th). The largest power consumption happens at the reset programming stage since it needs to reach the melting temperature of the material. In the following sections, we will discuss the power consumption reduction by modifying the device structures (Section 3.1.1), and the phase change material itself (Section 4).