Current Localization and Redistribution as the Basis of Discontinuous Current Controlled Negative Differential Resistance in NbOx

• Published in: Advanced functional materials Vol. 29; no. 50
• Main Authors: Nandi, Sanjoy Kumar, Nath, Shimul Kanti, El‐Helou, Assaad E., Li, Shuai, Liu, Xinjun, Raad, Peter E., Elliman, Robert G.
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 12-12-2019
• Subjects: brain‐inspired computing; Current voltage characteristics; Electrical resistivity; Electronic devices; Localization; Materials science; negative differential resistance; Neural networks; niobium oxide; Oxide coatings; Temperature dependence; Thermal imaging; thermoreflectance imaging; transition metal oxides
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201906731

Devices exploiting negative differential resistance (NDR) are of particular interest for analog computing applications, including oscillator‐based neural networks. These devices typically exploit the continuous S‐shaped current–voltage characteristic produced by materials with a strong temperature‐dependent electrical conductivity, but recent studies have also highlighted the existence of a second, discontinuous (snap‐back) characteristic that has the potential to provide additional functionality. The development of devices based on this characteristic is currently limited by uncertainty over the underlying physical mechanism, which remains the subject of active debate. In situ thermoreflectance imaging and a simple model are used to finally resolve this issue. Specifically, it is shown that the snap‐back response is a direct consequence of current localization and redistribution within the oxide film, and that material and device dependencies are consistent with model predictions. These results conclusively demonstrate that the snap‐back characteristic is a generic response of materials with a strong temperature‐dependent conductivity and therefore has the same physical origin as the S‐type characteristic. This is a significant outcome that resolves a long‐standing controversy and provides a solid foundation for engineering functional devices with specific NDR characteristics.
In situ thermoreflectance imaging is used to show that discontinuous current‐controlled negative differential resistance in metal–oxide–metal devices is a generic response that arises from current localization and redistribution within the oxide film. Its dependence on material and device parameters is further shown to be consistent with simple model predictions thereby providing a basis for engineering functional devices with specific device characteristics.