Uncovering Two Competing Switching Mechanisms for Epitaxial and Ultrathin Strontium Titanate-Based Resistive Switching Bits

Abstract

Resistive switches based on anionic electronic conducting oxides are promising devices to replace transistor-based memories due to their excellent scalability and low power consumption. In this study, we create a model switching system by manufacturing resistive switches based on ultrathin 5 nm, epitaxial, and grain boundary-free strontium titanate thin films with subnanometer surface roughness. For our model devices, we unveil two competing nonvolatile resistive switching processes being of different polarities: one switching in clockwise and the other in counterclockwise direction. They can be activated selectively with respect to the effective switching voltage and time applied to the device. Combined analysis of both processes with electrical DC-methods and electrochemical impedance spectroscopy reveals that the first resistive switching process is filament-based and exhibits counterclockwise bipolar resistive switching. The R(OFF)/R(ON) resistance ratio of this process is extremely stable and can be tuned in the range 5-25 depending on the switching voltage and time. Excitingly, at high electric field strength a second bipolar resistive switching process was found. This process is clockwise and, therefore, reveals the opposite polarity switching direction when compared to the first one. Both processes do not obstruct each other, consequently, stable 1, 2, or even 3 crossover current-voltage (I-V) characteristics can be addressed for the memory bits. Equivalent circuit model analysis and fitting of impedance characteristics unequivocally show for the created grain boundary free switches that the oxide's defects and its carrier distribution close to the electrode interface contribute to the resistive switching mechanism. The addressability of two sets of resistive ON and OFF states in one device through electric field strength and switching time offers exciting new operation schemes for memory devices.

Effect of write voltage and frequency on the reliability aspects of memristor-based RRAM

“…This large electric field produces nonlinear drifting of vacancies near the boundary interfaces, which further results in high nonlinearity in the I-V characteristics. The increase in the loop area with an increase in the bias is in good agreement with the experimental result reported in the literature for different active layer materials and device structures [24,25].…”

Pavlovian conditioning demonstrated with neuromorphic memristive devices

“…Electronic synapses are regarded as the physical building block for a hardware based artificial neural network with the function of brain-like computing 2 . Quite encouragingly, memristive devices 3–6 have been reported to efficiently emulate several neuromorphic and cognitive properties, such as all-or-nothing spiking of an action potential in neuristors 7 , synaptic plasticity (e.g. spike-timing-dependent plasticity (STDP) and metaplasticity) 8–16 , pattern learning 17, 18 and Pavlovian conditioning 19–27 .…”

Investigation of the Eightwise Switching Mechanism and Its Suppression in SrTiO3 Modulated by Humidity and Interchanged Top and Bottom Platinum and LaNiO3 Electrode Contacts

“…[20,21] Resistive switching devices were demonstrated on donor (i.e., Nb) doped, [22][23][24] acceptor (i.e., Fe) doped [25,26] and nominally undoped SrTiO 3 . [19,[27][28][29] Nb is a shallow donor dopant that creates a strong n-type conductivity with a very low concentration of oxygen vacancies [30] while Fe forms an acceptor level in the SrTiO 3 band gap. [31] In Fe doped SrTiO 3 oxygen vacancies were suggested to be responsible for the resistive switching [25] while in Nb doped SrTiO 3 it was both reoxidation through the top Pt electrode [23] and electronic charge trapping at the metal-oxide interface.…”

“…[22] The switching polarity has been consistently different with both dopants, Nb exhibiting eightwise polarity [22,23] and Fe counter-eightwise or both competing polarities. [25,29,32,33] Competing switching polarities dependent on the electrical field are possible in a single switching bit [25,29] and a debate in the literature has been ongoing about the switching mechanism in devices with the eightwise polarity. [19,34] Recently, Cooper et al proposed oxygen evolution and reincorporation into the high work function platinum electrode as the mechanism in undoped SrTiO 3 .…”