Physical changes occurring in TiN/TaO2.0±0.2/TiN resistive random-access memory devices after prolonged cycling have been analyzed by two scanning transmission electron microscopy modalities: high angle annular dark field and x-ray energy dispersive spectroscopy. In just formed devices, filaments had a shape of a 10 nm diameter Ta-enriched column with the O-rich gap next to electrodes, which was positively biased during electroformation. Devices that failed by stuck-in-high resistance state mode exhibited Ta depletion and oxygen interdiffusion at interfaces with both electrodes akin to effects observed in complementary switching devices. Initially narrow Ta-rich filaments broadened into ∼50 nm diameter columns showing speckled contrast due to phase separation. In devices that failed by stuck-in-low resistance state mode, we have observed a strong Ta-enriched sub-filament bridging the gap. The amount of oxygen in the TiN anode in the vicinity of the filament has not changed significantly between as-formed and failed devices, thus indicating that oxygen is not lost during switching. All devices at the end of endurance exhibited interdiffusion of O into TiN and Ti and N into TaOx.

Thermal gradients have been predicted to play a large role in compositional segregation leading to failure in phase change memories. We have developed a methodology for isolating thermal-gradient driven segregation effects without interference from the electric field. In Ge2Sb2Te5 functional layers, Sb and Te move along the temperature gradient, while Ge segregates in the opposite direction. The direction of segregation is consistent for devices that were repeatedly melted, as well as for devices that were never melted and remained in the polycrystalline state. The results have implications for the reliability of phase change memories.

Electromigration in Ge2Sb2Te5 and N-doped Ge2Sb2Te5 line cell structures has been studied by mapping out electric field/current-induced composition changes using x-ray energy dispersive spectroscopy. Both materials exhibit pronounced segregation in a molten state, with Te moving toward the anode and Ge and Sb toward the cathode. The width of the transition region from a composition of over 90% Te to over 90% Ge–Sb was 500 nm for an electric field of 1.1 × 107 V/m. In the Ge–Sb-rich end of the cell, Ge precipitates out of the melt, forming almost pure Ge inclusions with a size up to 100 nm. The Ge–Sb–Te segregation and precipitation do not appear to be affected by doping with nitrogen.

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