Investigating the origins of high multilevel resistive switching in forming free Ti/TiO2−x-based memory devices through experiments and simulations

Abstract

Although multilevel capability is probably the most important property of resistive random access memory (RRAM) technology, it is vulnerable to reliability issues due to the stochastic nature of conducting filament (CF) creation. As a result, the various resistance states cannot be clearly distinguished, which leads to memory capacity failure. In this work, due to the gradual resistance switching pattern of TiO2−x-based RRAM devices, we demonstrate at least six resistance states with distinct memory margin and promising temporal variability. It is shown that the formation of small CFs with high density of oxygen vacancies enhances the uniformity of the switching characteristics in spite of the random nature of the switching effect. Insight into the origin of the gradual resistance modulation mechanisms is gained by the application of a trap-assisted-tunneling model together with numerical simulations of the filament formation physical processes.

Ultra‐Low Power Multilevel Switching with Enhanced Uniformity in Forming Free TiO2−x‐Based RRAM with Embedded Pt Nanocrystals

“…However, the deviation from the ohmic character especially at low I cc , indicates that the presence of space charges, such as oxygen vacancies, imposes a different conduction mechanism. On the other hand, HRS is fitted quite well by TAT mechanism (see Figure (d)), which has been presented in our previous work . The fitting of the various states can be performed by increasing only the ionic component of the TAT mechanism, while the electronic component is not seriously affected.…”

“…The various switching patterns can be interpreted by calculating the electrical conductivity from the oxygen vacancy density ( n d ), using the following equation: where σ 0 is a pre‐exponential factor, E α ( n d ) is the activation energy, k B is the Boltzmann constant, and T is the absolute temperature. The results that are presented in Figure (a) indicate a threshold value of oxygen vacancy density which determines the boundary between TAT and ohmic conduction mechanism, while an almost linear function was selected for E α ( n d ) . As it can be seen from Figure (b), n d value is of great importance, since small variations in n d < n TAT region can cause large variability in the various desired multilevel states, due to the non‐linear dependence of the electrical conduction pattern.…”

“…In our case, however, the gradual transitions and the elevated n d are capable to ensure reliable multilevel performance despite the fact that when the device is operating at lower I cc , it seems to be in the non‐linear region. The resistance map of the LRS was calculated by considering cylindrical CFs (see Figure (c)), while the calculation details can be found elsewhere . n TAT value is considered to be 0.5 × 10 22 cm −3 , which yields in distance below 1 nm among oxygen vacancies .…”

Tolerance against conducting filament formation in nanosheet-derived titania thin films

“…In addition, these films show poor switching as shown in figure 3(c), where the incomplete switching of the initial plot may be attributed to the incomplete movement of oxygen vacancies due to the short duration of the switching voltage application. On the contrary, Au-contacted titania thin films formed by other methods have been reported to show filamentary resistive switching with high switching ratios [29][30][31][32][33][34]. A plausible explanation of this difference is a lower expected density of oxygen vacancies in nanosheet-derived films.…”

Chemically addressed switching measurements in graphene electrode memristive devices using in situ XPS

“…Electron spectroscopy techniques such as X-ray photoelectron spectroscopy (XPS) have been implemented in order to chemically address memristive characteristics. 13,17,18 XPS has been predominantly used to investigate the composition of memristive devices or to carry out ex situ measurements where devices are excited prior to XPS measurements. Similar to this effort, a recent study reported in situ measurements using X-ray photoelectron emission microscopy (PEEM) to detect valence change in TaO x devices during switching.…”