33 gradual cell degradation associated with stuck-SET is strongly correlated to the melting inherent in each RE- SET operation. A number of groups have been using techniques such as Energy Dispersion Spectroscopy (EDS), Secondary Ion Mass Spectrometry (SIMS), and Energy Dispersive X- ray (EDX) spectrometry to perform elemental analysis on failed cells[54, 213–224]. Measurements on mushroom cells built from Ge$_{2}$Sb$_{2}$Te$_{5}$ material[213, 214] tend to show agglomeration of Antimony (Sb) at the bottom elec- trode at the expense of Tellurium (Te). (The tendency of Germanium (Ge) is not clear — Reference [214] shows it clearly depleting from the repeatedly molten mushroom cap over the heater, while Reference [213] indicates no motion). Sarkar et. al. used these results to explain why their Ge$_{2}$Sb$_{2}$Te$_{5}$ material actually improves slightly, in characteristics such as resistance contrast and RESET current, over the first 10,000 SET-RESET cycles[225]. They surmise that as the phase-change material evolves in composition through cycling, the volume melted by each RESET pulse increases (increasing RESET resis- tance) while the inherent lower resistivity of the more Sb-rich GST material provides a lower SET resistance. Eventually, however, it would appear that such compo- sitional changes driven by cycling will steadily decrease the dynamic resistance of the active region, shifting the required RESET current to larger values. This leads to stuck-SET failure if the RESET pulse is not adaptively increased, or to stuck-RESET failure if the RESET pulse energy is increased to compensate. A number of recent measurements of phase segrega- tion, in various types of phase change bridge devices, have forced a re-interpretation of the failure analysis re- sults just described within the context of two previously unknown bias-polarity-dependent effects. D. Polarity issues FIG. 30: Top-down TEM images of large phase change bridge devices ( L =740nm, W =300nm bridges of 20nm thick Ge-doped SbTe material), showing a ∼ 100nm polarity-dependent shift of the amorphous plug to- wards the anode (+). Reprinted with permission from Reference [104] ( c ○ 2007 IEEE). side), there was a “bad” polarity of operation (large- area electrode negative and small-area electrode positive) for which subsequent SET operations were unable to re- turn the device to low resistance. In symmetric bridge devices, other researchers have reported that the most reliable SET operations can only be produced by alter- nating the polarity between SET and RESET, with little dependence on the absolute sign of the bias polarity[226]. Tellingly, these results were only observed for bridges fab- ricated from GST, and not for ultra-thin bridges fabri- cated from doped–GeSb[42]. Similarly, other researchers have reported bias- dependent operation of pore devices where only the “good” choice of polarity (positive on large-area elec- trode) can be used to produce low SET resistances, while the “bad” polarity is associated with “hard-to-SET” operation[227]. In these experiments, only a narrow volt- age window could be used for the SET operation, which also required longer pulses and which never produced the same low SET resistances as the “good” polarity[227]. Fortunately, this “good” polarity corresponds exactly to the typical operation of integrated PCM devices where the positive voltage is applied to top of the PCM device built over the underlying transistor[138]. In fact, oper- ation of such integrated devices in the “bad” polarity is difficult to study, since in that configuration the gate-to- Tio Castro et. al. showed convincing top-down TEMs of bridge devices in the RESET state (Figure 30) that demonstrate that the amorphous plugs in their devices were shifted by the polarity of the applied bias, by as much as 100nm[104]. They attributed this effect to the thermo-electric Thomson effect, in which the overlap of temperature gradients with electrical current can lead to additional heat generation or absorption. Because the hot spot in the center of a phase-change device is surrounded by temperature gradients of opposite signs but the current flow is uni-directional, the Thomson ef- fect acts to shift the centroid of the hot spot depending on the polarity of the applied voltage. Tio Castro et. al. estimated from their observations that the Thom- son coefficient in their material might be in the range of -100 µ V/K[104]. In those same experiments, it was observed that for intentionally-asymmetric “dog-bone” bridge devices (somewhat like a high-aspect-ratio pore device on its