Resistive switching devices based on transition metal oxides require formation of a conductive filament in order for the device to be able to switch. Such filaments have been proposed to form by the reduction of oxide due to the application of the electric field, but this report seeks to rebut that interpretation. Frequently reported physical changes during electro-formation include delamination of electrodes, crystallization of functional oxide, intermixing of electrode and oxide materials, and extensive loss of oxygen presumably to the ambient. Here, we show that most of these effects are not inherent to the formation and switching processes and instead are due to an experimental artifact: the discharge of parasitic capacitances in the forming circuit. Discharge of typical BNC cables can raise the temperature of the filament to between 2000 and 5000 K resulting in extensive physical changes. Discharge and associated effects mentioned above can be eliminated using an on-chip load element without affecting the ability to switch.

The valence change model describes the resistive switching in metal oxide-based devices as due to electroreduction of the oxide and subsequent electromigration of oxygen vacancies. Here, we present cross-sectional X-ray energy-dispersive spectroscopy elemental maps of Ta, O, N, and Ti in electroformed TiN/TaO2.0/TiN structures. O, N, and Ti were exchanged between the anode and the functional oxide in devices formed at high power (∼1 mW), but the exchange was below the detection limit at low power (<0.5 mW). All structures exhibit a similar Ta-enriched and O-depleted filament formed by the elemental segregation in the functional oxide by the temperature gradient. The elemental interchange is interpreted as due to Fick’s diffusion caused by high temperatures in the gap of the filament and is not an essential part of electroformation.

The density variations in reactively sputtered amorphous TaOx thin films deposited on planar and patterned substrates have been quantified by high-angle annular dark-field scanning transmission electron microscopy. The experiments have been performed both in plan-view and cross-sectional geometries. The planar films exhibit a cellular structure consisting of high-density cells with low-density boundaries. Laterally, cell sizes varied from 5 to 20 nm as the deposition temperature was changed from 298 K to 573 K. The corresponding density ratio of the cell boundary over the cell interior varied between 0.98 and 0.93 corresponding to 2%–7% free volume in the functional layer. The film microstructure is consistent with the self-shadowing effect of surface roughness. Similar low-density areas were found at the locations of steps of the patterned substrates with a local density decrease of up to 9%. The influence of the low-density region in the resistive random-access memory device is discussed.

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.

The density, crystallinity, and microstructure of reactively sputtered amorphous Ge2Sb2Te5 thin films have been assessed as a function of deposition temperature. The continuous density increase was observed with increasing substrate temperature between room temperature and 200 °C. The films deposited at room temperature are amorphous and exhibit a columnar structure with a lateral size of cells in the 10–15 nm range. Cells consist of high-density interior with boundaries with the density lower by ∼9% due to incorporation of pores. The pores and the columnar microstructure can be eliminated by deposition at 80 °C while still preserving the amorphous phase. The density of pore- and stress-free amorphous Ge2Sb2Te5 is 6.16 g/cm3 and is only 1.5% lower than the crystalline Ge2Sb2Te5 with NaCl structure.