Electrolyte Incorporation in Anodic Memristors
Authors: Zrinski Ivana , Mardare Andrei Ionut , Mardare Cezarina Cela , Hassel Achim Walter 
Published: July 2021

In the field of semiconductor electronics, the metal-oxide- semiconductor technology (CMOS) has attracted a lot of scientific attention as one of the main representatives for non-volatile memories. The development of memory systems is crucial for a fast data processing for which purpose namely microprocessors are used, following the miniaturization trends at the same time. However, the limits of CMOS technology are already exhausted in terms of power consumptions, leakage current and processing speed. Surely, novel non-volatile elements and materials are necessary to fulfill such demands and memristors are in the front as the most promising candidates.
Valve metals and their oxides are frequently used as the main constitutes of metal-insulator-metal (MIM) memristive structures. Recently, Hf and Ta became the most studied valve metals playing an important role in the next-generation of thin film micro- and nano-electronic devices primarily due to applications in redox-based resistive switching memories (ReRAMs), logic circuits, sensors and artificial neural networks. The operating mechanism of memristors is based on the electric field assisted conductive filaments (CFs) formation due to the ionic drift inside the insulating oxide layer. The understanding of CFs formation is a key point to ensure excellent switching and memory characteristics of memristive devices. In accordance with the reported studies, the regulated movement of oxygen vacancies or other mobile species through the CFs can allow their pining at the constant positions and consequently high switching reproducibility of a device. Therefore, the immediate findings on the oxygen enriched media that will ensure the controlled CFs formation is of high importance. With this regard, the selection of a proper electrolyte for the fabrication of the oxide layer sandwiched between two metallic electrodes may play a significant role. It can be assumed that incorporation of electrolyte species can increase the amount of oxygen vacancies leading to a possible manipulation with their movement.
Tantalum and Hf were sputtered onto Si wafers to produce thin films. Both metals were electrochemically anodized to produce oxides responsible for the memristive properties. The aim was to replace expensive fabrication methods for oxides such as sputtering or laser deposition with cost-efficient, simple and rapid anodization. The bottom electrode was either Ta or Hf thin films, Ta2O5 or HfO2 were anodic oxide layers and Pt was patterned as the top electrode, that can be seen in the inserted Figure. Two groups of these samples were anodized in the same conditions, in three electrolytes (phosphate buffer (PB), borate buffer (BB), citrate buffer (CB)) to produce three different anodic oxide layers with the thickness in the range of 20 nm. For all cases, memristive switching between high and low resistance states was demonstrated and the corresponding I-U switching curves were compared. Additionally, writing (switching) and reading tests were done for high numbers of cycles and the endurance and retention of devices were analyzed. The same tests were performed when samples were thermally treated resulting in improved or worsened switching properties, depending on the solid electrolyte placed between the metallic electrodes. Unquestionably, the use of PB, CB and BB for anodization has differently influenced the electrical properties of the memristors likely due to the trapped electrolyte species inside the oxide layer. The incorporation was confirmed by the X-ray photoelectron spectroscopy (XPS) and the presence of CFs by transmission electron microscopy (TEM). Interestingly, Ta memristors formed in PB showed the presence of Ta oxyphosphate which has induced the increase of O vacancies and thus improved switching features. However, the presence of Hf-O-P bonds found in Hf memristors that was anodized in PB did not show any improvement of electrical properties.
In summary, the current study offers a reliable, controllable and cost-efficient approach for the production of ReRAMs based on Hf and Ta anodic memristors as potential candidates for the future industrial implementation.

Figure 1