34 source voltage changes dynamically during each pulse as the PCM device resistance changes. Thus it is not sur- prising that such effects have not been widely reported for PCM devices integrated together with access transis- tors. The Thomson effect would seem to be inadequate by it- self to explain all of these bias-polarity effects. However, a few groups have been performing bias-dependent fail- ure analysis experiments on various types of phase change bridge devices. Early versions of these experiments were affected by lingering uncertainties related to the role of metallic electrodes at the failure point[220], and to the difficulty in understanding material desegregation during long-term cycling by studying the aftermath of a single- pulse “blown-fuse” failure[54, 220, 221]. However, the most recent experiments have investigated the cycling failure of tapered bridge structures located far from any metal electrodes[222], and the controlled fast melting of large symmetric bridge devices[223, 224]. The results of these experiments, together with the ear- lier failure analysis data on mushroom devices[213, 214], sketch out a convincingly consistent story for Ge$_{2}$Sb$_{2}$Te$_{5}$ devices: Te moves towards the positive electrode (an- ode), while Sb moves toward the negative electrode (cathode)[54, 213, 214, 220–224]. This motion is at- tributed to the higher electronegativity (5.49eV) of Te compared to Ge and Sb (4.6eV and 4.85eV)[224]. Most of the data seem to indicate that Ge moves together with the Sb towards the cathode, but as mentioned earlier, there are some data which indicate otherwise. In con- necting these interpretations of bridge and mushroom devices, we assume that the mushroom cycling was per- formed in the “good” polarity direction, with positive voltage on the large-area top electrode. One particularly interesting study was performed by Yang et. al. on very large symmetric devices (20 µ m long by 2 µ m wide bridges of 300nm-thick Ge$_{2}$Sb$_{2}$Te$_{5}$), where both high-amplitude millisecond-long pulses and day- long exposure to low-amplitude 10Mhz pulsed DC were used to explore elemental segregation through Wave- length Dispersive Spectroscopy (WDS)[224]. They were able to show that the material segregation is very rapid in the molten state, observing nearly complete desegre- gation along a 10 µ m length of bridge after a 1.5ms pulse, as shown in Figure 31. This works out to an effective dif- fusion coefficient of 1-2 × 10 − 5 cm $^{2}$/s[224], which roughly corresponds to a field- or current-driven migration of one nanometer every nanosecond. In contrast, their long- term measurements of bias-induced elemental desegrega- tion through the crystalline state seemed to suggest dif- fusion coefficients nine orders of magnitude lower[224], implying that the drift of elements through that same one nanometer would take one second. While it is not yet clear how these observed effects (the Thomson effect and polarity-dependent elemental segregation via electromigration) can be combined into a unified theory that quantitatively explains PCM polarity and cycling endurance, it is already quite clear that such bias-polarity effects and cycling endurance are intimately related. It has already been independently observed that switching from one cycling polarity to the other can be used to continue cycling of bridges after a stuck-SET failure[212]. In fact, as shown in Figure 32, even as few as 10 aggressive pulses applied in the opposite polarity direction can allow cycling to not only resume after a stuck-SET failure, but to continue for another 10 5 cycles with the original RESET pulse conditions[228, 229]. Continued understanding of what differentiates the “good” and “bad” polarity should allow researchers to continue to improve PCM cycling endurance, through a combination of creative use of the “bad” polarity[212, 228, 229], improved cell design, and new materials that show greater resistance to elemental segregation. As with Section III D, most of the detailed data available is limited to GST because of its ubiquity. However, it has been observed that the phase change material GeSb phase-segregates quite readily[230], implying that simply reducing the number of atomic species involved is not FIG. 31: WDS profiles of elemental concentration (Te, Sb, and Ge) along the length of a 20 µ -long Ge$_{2}$Sb$_{2}$Te$_{5}$ bridge at a) 0.17ms and b) 1.27ms after melting was initiated by a voltage-pulse, showing rapid de- segregation of elements in the molten state. Reprinted with permission from Reference [224] ( c ○ 2009 American Institute of Physics)