Exploring Electro-Thermal Localizations and Metal Insulator Transitions in Micron to Sub-Micron Thin Film Devices

Two separate mechanisms, namely electrically induced current density and temperature localization (electro-thermal localization) and temperature driven metal–insulator-transition (MIT), give rise to volatile electrical responses, such as negative differential resistance (NDR) and threshold switching. [1] Devices exhibiting such electrical responses have recently garnered significant interest due to their potential applications in hardware based neuromorphic computing. [2, 3] Within microscale devices, both phenomena are characterized by spatial inhomogeneity, leading to the formation of channels with high current density due to increased temperature or the emergence of a higher conductivity high temperature phase in the material.<br/>In transition metal oxide materials like VO₂, the emergence of electro-thermal localizations and MIT has been observed to occur sequentially, [4, 5] with the current induced MIT believed to occur within the previously established steady state localized channel. Nevertheless, it remains unclear under what conditions localization behavior precedes MITs, and whether these spatial inhomogeneities disappear as device dimensions are reduced below a certain critical size, along with the impact this may have on the device’s electrical response. The objective of this study is to investigate the interactions between electro-thermal localizations and MIT with the goal of providing insight into how localization behaviors affect the onset and properties of the MIT and how these behaviors vary with variations in device dimensions.<br/>Here, the interplay between spatially inhomogeneous electro-thermal localizations and phase transitions is studied in 3D finite element models of two terminal lateral thin film devices with varying lengths (0.7 to 6 µm) and widths (0.7 to 10 µm) using COMSOL Multiphysics. The Joule heating Multiphysics interface combining the electrical and heat transfer modules is used throughout the simulations. The device model contains four domains including a thin film domain, an underlying dielectric substrate domain, separated from the thin film domain by an interfacial thermal conductance, and two electrode domains that span the width of the device. Simulations were conducted by applying a constant electrical current to one electrode and then allowing the system to evolve over time to reach steady state thermal and electrical solutions.<br/>We demonstrate that the dynamical localization of current density and temperature precedes the onset of the MIT, determining the location of the emerging high temperature phase (metallic phase) in micron scale devices. Furthermore, we observe that as device dimensions are reduced to sub micron scales, distinguishing the emergence of spatial phase and thermal inhomogeneities becomes challenging. These results highlight the significant role that electrothermal localizations play on the onset of MIT, offering insight into electrically induced behaviors and corresponding responses of thin films with variable dimensions. Understanding this behavior suggests a pathway for designing desired electrical responses in non linear oscillator type neuromorphic devices.<br/><br/><b>References</b><br/>1. Chae, B.-G., et al., <i>Abrupt metal–insulator transition observed in VO2 thin films induced by a switching voltage pulse.</i> Physica B: Condensed Matter, 2005. <b>369</b>(1): p. 76-80.<br/>2. Goodwill, J.M., et al., <i>Spontaneous current constriction in threshold switching devices.</i> Nature communications, 2019. <b>10</b>(1): p. 1628.<br/>3. Parija, A., et al., <i>Metal-Insulator transitions in β′-CuxV2O5 mediated by polaron oscillation and cation shuttling.</i> Matter, 2020. <b>2</b>(5): p. 1166-1186.<br/>4. Brown, T.D., et al., <i>ElectroThermal Characterization of Dynamical VO2 Memristors via Local Activity Modeling.</i> Advanced Materials, 2022: p. 2205451.<br/>5. Das, S.K., et al., <i>Physical Origin of Negative Differential Resistance in V3O5 and Its Application as a Solid State Oscillator.</i> Advanced Materials, 2023. <b>35</b>(8): p. 2208477.