9 Phase change material parameter Influence on PCM device performance Crystallization temperature & Data retention and archival lifetime thermal stability of the amorphous phase SET power Melting temperature RESET power Resistivity in amorphous and crystalline phases On/off ratio SET and RESET current Threshold voltage SET voltage and reading voltage Thermal conductivity in both phases SET and RESET power Crystallization speed SET pulse duration (and thus power) Data rate Melt-quenching speed RESET pulse duration (and thus power) TABLE I: Some phase change material parameters and the device performance characteristics they influence. rate. Here, the activation energy was found to fall from 2.34 eV for 190 nm diameter devices to 1.9 eV for 20 nm diameter devices, indicating a deterioration of data re- tention as the Ge$_{2}$Sb$_{2}$Te$_{5}$ nanowire diameter is reduced. However, Yu and co-workers did not observe a depen- dence of the crystallization temperature on the device diameter for PCM devices fabricated by contacting GeTe and Sb$_{2}$Te$_{3}$ nanowires using Cr/Au contacts[69]. Figure 6 shows phase change nanoparticles fabri- cated by a variety of techniques including electron beam lithography, solution-based chemistry, self-assembly- based lithography combined with sputter deposition, and self-assembly-based lithography combined with spin-on deposition of the phase change material. When the crystallization temperature of amorphous-as-fabricated nanoparticles was studied, it was found that larger phase change nanoparticles have a very similar crystallization temperature compared to bulk material[62, 70], whereas the smallest nanoparticles in the 10 nm range can show either decreased[71] or increased[72] crystallization tem- perature. In terms of size effects, ultra-thin films still can show crystallization down to thicknesses of only 1.3 nm[66], and nanoparticles as small as 2-5 nm synthe- sized by solution-based chemistry have been found to be crystalline[73]. This is very promising for the scalability of PCM technology to future device generations. Beyond crystallization temperature, the melting tem- perature is a parameter which can vary with composition and, at small dimensions, with size. In fact, reduction in the melting temperature of phase change materials has been observed for very thin films[74], nanowires[75] and nanoparticles[76]. This is advantageous for device per- formance, because a lower melting point implies a re- duction in the power (and current) required to RESET such a PCM cell. The electrical resistivity for thin films increases slightly for both phases when film thickness is reduced[65]. This is also beneficial for scaling because higher resistivities lead to higher voltage drop across the material and can thus reduce switching currents. The threshold voltage is a phenomenological parame- ter of PCM devices that describes the applied voltage (typically around 1V) required to induce an electrical However, a more accurate description of the underly- ing physical process calls for a threshold electric field, rather than a threshold voltage, that must be surpassed for the amorphous material to become highly conductive. Studies of phase change bridge devices (described in Sec- tion II D) have shown that the threshold voltage scales breakdown effect. Such a sudden increase in electrical conductivity allows the PCM device to rapidly and ef- ficiently attain a significantly lower dynamic resistance (typically 3–10 × lower than the room-temperature SET resistance), allowing efficient heating with moderate ap- plied voltages. Thus the presence of this electrical switch- ing effect is an important component of PCM technology. FIG. 6: (a) Phase change nanoparticles of Ge-Sb with 15 at. % Ge, fabricated by electron-beam lithography, diameter about 40 nm. Reprinted with permission from Reference [62] ( c ○ 2007 American Insti- tute of Physics). (b) GeTe nanoparticles synthesized by solution-based chemistry, diameter about 30 nm[73]. (c) Nanoparticles of Ge-Sb with 15 at. % Ge, fabricated by self-assembly based lithography and sput- ter deposition, diameter about 15 nm. Reprinted with permission from Reference [70] ( c ○ 2007 American Institute of Physics). (d) Nanopar- ticles of Ge-Sb-Se, fabricated by self-assembly based lithography and spin-on deposition, diameter about 30 nm. Reprinted with permission from Reference [71] ( c ○ 2007 Macmillan Publishers Ltd).