In-situ tem study of filament evolution in metal filament-based resistive switching devices
Authors
Pandey, Saurabh
Issue Date
2023-12
Type
Electronic thesis
Thesis
Thesis
Language
en_US
Keywords
Materials engineering
Alternative Title
Abstract
The growing requirement of higher speed and lower energy computation for ever increasing data workloads have impressed the need for development of neuromorphic computing hardware like artificial synapses that exhibit fast, low power, and reliable switching into multiple resistance states. A good potential candidate for these artificial synapses is metal filament based resistive switching devices which switch between different resistance states via modulation of metallic filament(s) present in the insulator of its metal/insulator/metal structure. However, the observed variability in the state resistance, which is linked to the stochastic nature of the filament evolution process, poses a challenge to its artificial synapse application. Therefore, it is important to fundamentally understand the filament evolution process in these devices and determine the role of factors that influence it. Due to the dynamic nature of the nanoscale filament evolution process during device operation, in-situ transmission electron microscope (TEM) electrical testing can play a big role in the development of the understanding about the filament evolution process. The process broadly consists of three sub processes: i) Redox reactions at the electrodes to generate metal cations, ii) Metal cation injection into the insulator and transport under high electric field, and iii) Reduction of metal cations to form precipitates that grow into a filament spanning the insulator between the electrodes. In this thesis the effect of four different factors: electrode material, testing temperature, TEM imaging current density and ambient vacuum on these sub processes and how it affected the filament evolution is explored.
Lateral metal/SiO2(~ 100 nm)/metal devices (metal: Cu, Ag, Co) compatible to in-situ TEM electrical testing in an electrical biasing and heating holder were designed and fabricated. It was found by in-situ TEM electrical testing of Cu/SiO2/Cu devices at room temperature, that the metal filament initial forms near the anode and grows as a collection of discontinuous precipitates towards the cathode. The filament was confirmed to be in the bulk of SiO2 film rather than the exposed surface. Image processing methods were developed to quantify the information about filament evolution from the contrast evolution in the in-situ TEM data. This was used to compare the effect of different factors on the filament evolution process.
It was found that for Cu/SiO2/Cu devices, the metal filament evolution occurred in air or in TEM vacuum with imaging electron irradiation but not in TEM vacuum alone. It was also shown that the imaging electron current density had a positive correlation with the amount of Cu injected into SiO2. A mechanism for filament evolution in a vacuum ambient for SiO2 based metal filament resistive switching devices was proposed wherein Cu oxidation at the anode is enabled by the reduction of oxygen vacancies (V_O^(2+)+2e^-→ V_O) generated in SiO2 by high energy imaging electrons, at the cathode.
Filament formation in Cu/SiO2/Cu devices for 3 temperatures (298 K, 498 K and 698 K) was investigated and found that the filament in all three cases originates from the anode. Increase in temperature results in a larger dispersion of sizes which was explained by increase in redox reaction rate at the electrodes with temperature. It was concluded that the increase in temperature influences the filament evolution primarily through increase in redox reaction rates at the electrodes with lesser impact on ionic mobility.
The effect of change of electrode material on the filament evolution process was also studied by performing in-situ TEM electrical testing of lateral metal/SiO2/metal devices with Cu, Ag, and Co electrodes at 298 K. Qualitatively, the filament evolution process was similar in all three metals, but the amount of metal injected into the SiO2 followed the trend of the reduction potential (Ag>Cu>Co) of the respective ions. This was deduced from the largest amount of total projected area fraction of SiO2 covered by metal precipitates for Ag, followed by Cu and then Co.
In summary, we studied the role of 4 different factors on the filament evolution in metal filament resistive switching devices. We found that filament evolution for in-situ TEM conditions with ambient vacuum and electron irradiation is different than that for air ambient. We proposed, that a vacancy reduction based cathodic reaction for in-situ TEM replaces the standard water reduction cathodic reaction in the air ambient. We also showed that the precipitates comprising the filament become larger with decreasing reduction potential of the electrode metal or by increase in temperature during filament formation, while the filament originates near anode for all cases. This led to the conclusion that the redox processes dominate filament formation in SiO2.
Description
December 2023
Full Citation
Publisher
Rensselaer Polytechnic Institute, Troy, NY