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heats the material above its crystallization temperature for SET, or above the melting point for RESET. The advantage of optical testing is that large-area, thin-film samples of new phase change materials can be quickly prepared, without the need for any patterning or other steps required for fabrication of full PCM devices. Then a wide range of powers (e.g., temperatures) and times can be tested out rapidly. 

As expected from the above discussion, it has been ob- served that the recrystallization time of a melt-quenched area in a crystalline matrix is typically orders of mag- nitude faster than the first-crystallization time of as- deposited amorphous films[97]. Because device opera- tion hinges on the repeated cycling between the two phases, the relevant parameter to use for assessing the viability of a new material for PCM is the recrystalliza- tion time. However, since it is quite difficult to prepare isolated regions of melt-quenched material without in- troducing new interface effects, amorphous-as-deposited films remain the best way to study the physics of the nucleation process, to avoid the difficulty of decon- volving the entangled roles of nucleation and growth once a crystalline-amorphous boundary is present. For growth-dominated materials, it is the presence of these crystalline-amorphous boundaries which make the recrys- tallization speed so much faster than the initial nucle- ation from the amorphous-as-deposited state. 

Figure 7 shows the change in reflectivity of a Ge-Sb thin film with 15 at. % Ge, as a function of laser power and duration measured by a static laser tester[62]. The film was first crystallized by annealing it in a furnace for 5 min at 300 $^{◦}$C, which is above this material’s crystalliza- tion temperature[62]. A two pulse experiment was then performed. The first pulse, of fixed time and power (100 ns for 50 mW), created a small region of melt-quenched amorphous material at a previously unused spot in the crystalline film. Then a second laser spot of variable power and duration, applied a few seconds later at the same location, was used to recrystallize the amorphous spot. Figure 7(b) plots the normalized change in re- flectivity caused by this second pulse. Since the crys- talline phase has a higher reflectivity than the amor- phous, the increase in reflectivity observed for all pulses longer than 5-10 ns, independent of applied power, indi- cates extremely fast recrystallization. Because this mate- rial is crystallizing the amorphous spot predominantly by growth from the surrounding crystalline-amorphous bor- der, this time scales with the size of the melt-quenched amorphous spot[97]. In contrast, Figure 7(c) shows the much slower initial crystallization from the amorphous- as-deposited phase of Ge-Sb measured in a single-pulse experiment[35]. 

Several factors contribute to nucleation and growth ki- netics: temperature[98], composition[77, 99, 100], ma- terial interfaces[101], device geometry[77, 102], device size[33, 103], material thickness[42, 66], polarity[104], and device history[96, 97]. Of these, the two most important factors governing switching speeds are tem- 

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perature and local composition. In fact, most of the macroscopically-observable nucleation effects associated with geometry, size, thickness, device history, polarity and even material interfaces can be understood in terms of the effects of varying local composition on the delicate balance between surface and volume energies that drive crystallization. 

For any given composition, the crystallization prop- erties of a phase change material tend to be a strong function of temperature. As shown in Figure 8, the 

