(Invited) Impact of Surface Roughness of the Electrodes on the Resistive Switching in ReRam Devices
Authors: Al-Mamun Mohammad , Chakraborty Amrita , Li Yanlong , Orlowski Marius K 
Published: May 2020

Cu/TaOx/Pt and Cu/TaOx/Ru devices have been manufactured in the same process flow except for the separate deposition at identical deposition rate of the Pt and Ru electrodes. The details of sample manufacturing are given in [1]. The identical processing after the deposition of the bottom electrode assures that the thickness and composition of the TaOx solid electrolyte is exactly the same. The two devices have similar I-V switching characteristics, albeit with some important differences: the mean forming voltage (when the conductive filament (CF) is formed for the very first time) of the Ru device (Vform,m(Ru)=7.3V) is 2.7V higher than the mean forming voltage for the Pt device (Vform,m(Pt)=4.6V). The mean set voltage of the Ru device (Vset,m(Ru)=4.4V) is also higher by 1.6V than the mean set voltage of the Pt device (Vset,m(Pt)=2.8V). The set voltage is the voltage needed to rebuild the CF after it has been ruptured in a preceding reset operation. Thus, in the set operation only a small segment of the filament has to be rebuilt. It is assumed that the gap of ruptured filament is small (a few nm) and is located close to the tip of the cone-shaped filament near the active Cu electrode. Here we attempt to account for the additional 1.1V in Vform,m. One part of these differentials between Vform and Vset of 2.7V and 1.6V, respectively, can be explained by the work function difference between the inert electrodes Pt (φ=6.3eV) and Ru (φ=4.7eV) being Δφ=1.6eV. Since the work function for Pt can be as low as φ=5.6eV, the value 1.6V is considered an upper boundary for Δφ. The same Δφ contributes also to the difference between the forming voltages. To account for the remaining 1.1V difference voltage we have measured the surface roughness in Atomic Force Microscope (AFM) of the Pt and Ru electrodes before the deposition of TaOx. AFM analysis shows that Pt displays significantly higher surface roughness than Ru. The root mean square (rms) roughness for Pt is 2.30nm versus 1.20nm for Ru, thus almost 100% higher. The AFM images reveal also a different texture of the surface roughness for Ru and Pt. The surface protrusions of Ru are finer and smaller than those of Pt with a maximum height of 3.5nm while the protrusions of Pt are bulkier (surface coarser) and much higher with a maximum of 7.9nm. It appears that the Ru surface roughness is due to single atoms spatial variation while the Pt surface appears to consist of stacked clusters of Pt atoms. We have also taken AFM images of the Cu surface roughness. The copper surface appears coarse as if consisting of clumped Cu atoms with a rms of 1.17nm, i.e. comparable to the surface roughness of Pt. Electrically, higher surface roughness of Pt generates higher local electric fields thus allowing filament formation at lower applied voltages. The differential in set voltage between Pt and Ru of ΔVset=1.6V is accounted solely (or close to it) by Δφ(Pt-Ru). This is further confirmed by the observation that since for the set operation the surface roughness of the inert electrode is irrelevant as the gap in the filament near the Cu electrode is independent of the surface roughness. The built-in field in case of Pt electrode is the work function difference between Pt and Cu divided by the thickness of the TaOx dielectric, which in case of Pt devices comes to a quite high field of 6.8×105V/cm. This field favors Cu+ ion transport toward the Pt electrode. The corresponding built-in field in case of Ru devices is 6.0×104V/cm, i.e. one order of magnitude lower than for the Pt device. The field responsible for the formation of the Cu filament in its final stages has been estimated to be (1-2)×106V/cm [2]. The calculation of the electric fields at Vset for Pt and Ru including the built-in potential, are, indeed, in this range and approximately the same: (Δφ(Pt,Cu)/q+Vset,m(Pt))V/25nm≈(Δφ(Ru,Cu)/q+Vset,m(Ru))V/25nm≈1.8×106V/cm. The present analysis demonstrates that surface roughness is an important parameter for resistive ReRAM switching. It offers a new way to lower Vform by increasing the surface roughness (by e.g. argon ion bombardment or RIE etching) of the metal electrodes. In some cases, we observe spontaneous setting operation for the Pt but not for the Ru devices. The spontaneous formation of a filament is the result of the high electric fields resulting from built-in voltage Δφ along with high surface roughness of Pt electrode.
[1] Y.Fan, M.Al-Mamun, B.Conlon, S.W.King, M.Orlowski, ECS Transactions 75(32), 13-23(2017)
[2] R.Ali, Y.Fan, S.W.King, M.Orlowski, APL Materials 6, 066101(2018)

Figure 1