Resistive Switching Mechanisms on TaO_x and SrRuO₃ Thin-Film Surfaces Probed by Scanning Tunneling Microscopy

Abstract

The local electronic properties of tantalum oxide (TaO x , 2 ≤ x ≤ 2.5) and strontium ruthenate (SrRuO 3 ) thin-film surfaces were studied under the influence of electric fields induced by a scanning tunneling microscope (STM) tip. The switching between different redox states in both oxides is achieved without the need for physical electrical contact by controlling the magnitude and polarity of the applied voltage between the STM tip and the sample surface. We demonstrate for TaO x films that two switching mechanisms operate. Reduced tantalum oxide shows resistive switching due to the formation of metallic Ta, but partial oxidation of the samples changes the switching mechanism to one mediated mainly by oxygen vacancies. For SrRuO 3 , we found that the switching mechanism depends on the polarity of the applied voltage and involves formation, annihilation, and migration of oxygen vacancies. Although TaO x and SrRuO 3 differ significantly in their electronic and structural properties, the resistive switching mechanisms could be elaborated based on STM measurements, proving the general capability of this method for studying resistive switching phenomena in different classes of transition metal oxides. KEYWORDS: resistive switching, strontium ruthenate, tantalum oxide, scanning tunneling microscopy, electric field effect R edox-based resistance switching random access memories (ReRAMs) are considered as the next-generation memory devices to replace the present flash-based technology.1,2 ReRAMs have a simple metal−solid electrolyte− metal architecture, storing binary code information using the change in the resistance induced by filament formation and rupture, defining the low-resistive ON state (also denoted as LRS) and the high-resistive OFF state (or HRS), respectively. High scalability, CMOS compatibility, switching times in the subnanosecond range, excellent endurance and retention, and low power consumption are key but otherwise difficult-toduplicate features of ReRAM devices.

Sub-10 nm Ta Channel Responsible for Superior Performance of a HfO2 Memristor

“…As a result, motion of Ag or Cu ions is more preferable than O anions in ECM devices. The transition from VCM- to ECM- type switching behaviors in typical VCM material system (Ta/TaO x /Pt) has been achieved by inserting a thin amorphous carbon layer at Ta/TaO x interface19 or using highly reduced TaO x to suppress the role of O anions41.…”

Probing the Critical Region of Conductive Filament in Nanoscale HfO2 Resistive-Switching Device by Random Telegraph Signals

“…3-D tomography with conductive atomic force microscopy (scalpel-SPM) provides a 3D image of the conductive filament of a HfO 2 device at LRS [13,14] but does not have the required sensitivity to probe the CF at HRS due to the reduced conductivity. Scanning tunneling microscopy (STM) provides the ultimate lateral resolution, but is limited to probing the surface of bare materials [17,18]. In situ transmission electron microcopy (TEM) [15,16] has linked sub-10 nm conductive channels to the local migration of oxygen vacancies, however, it is destructive, time consuming and statistically unfriendly.…”

Recommended Methods to Study Resistive Switching Devices

“…[1] RS is called filamentary if the atomic rearrangements inducing the switching take place in the form of small (<100 nm 2 ) spots within the RS medium. [1,27,28] Distributed RS has the advantage of a lower power consumption (as no CF is completely formed/disrupted, in each state transition the currents in LRS cannot be so high), but the I LRS /I HRS ratios and switching speeds are not as competitive as in filamentary RS devices. However, the high currents in LRS may increase the power consumption, plus the complexity of controlling the set/reset transition (due to their stochastic nature) results in a high cycle-to-cycle and cell-to-cell variability.…”