Filamentary Threshold Switching In Niobium Oxides

Abstract

Two-terminal metal/oxide/metal (MOM) structures exhibit characteristic resistance changes, including non-volatile memory and volatile threshold switching responses when subjected to electrical stress (i.e., voltage or current stimuli), which are of interest as active elements in non-volatile memory arrays and neuromorphic computing.

Recently, the threshold switching response in MOM devices based on vanadium oxides and niobium oxides have attracted particular attention due to their simple structure and reliability. Interestingly, specific phases of these oxides (e.g., VO2, NbO2 etc.) exhibit a metal-insulator transition (MIT) which causes dramatic changes in their intrinsic properties, including electrical and thermal conductivities, and often arguably reported as the dominant cause of the observed threshold switching response. While this response has been extensively studied for VO2, but the low transition temperature (~ 340K) limits their use only to low temperature microelectronics applications. In contrast, NbO2 has a much higher transition temperature ~ 1070 K, and NbO2 and other NbOx phases have drawn recent attention due to their reliable threshold switching characteristics.

The resistance changes in MOM structures are often initiated by a one-step electroforming process that forms a filamentary conduction path. Knowledge about the structure, composition and spatial distribution of these filaments is essential for a full understanding of filamentary resistive/threshold-switching and for effective modelling and optimisation of associated devices. Additionally, NbOx-based devices exhibit a wide range of resistive and threshold switching responses that critically depend on operating condition, composition and device geometry. Thus, a proper understanding of these factors is important for achieving reliable switching with desired characteristics.

This thesis focuses on understanding the electroforming process and subsequent threshold switching responses in NbOx by employing different techniques, including electrical testing, and thermo-reflectance imaging. At first, a simple means of detecting and spatially mapping conductive filaments in metal/oxide/metal cross-point devices is introduced and the utility of this technique is demonstrated to identify distinct modes of electroforming in low- and high-conductivity NbOx films. After that, the role of metal/oxide interface reactions on the post-forming characteristics of reactive-metal/Nb2O5/Pt devices is demonstrated. Specifically, devices are shown to exhibit stable threshold switching under negative bias but the response under positive bias depends on the choice of metal. Then, the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 are compared. In particular it is shown that doping offers an effective means of engineering the device response. Based on temperature dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature. Finally, the physical origin of the discontinuous 'snapback' NDR is investigated. Specifically, it is shown that the snapback response is a direct consequence of current localisation and redistribution within the oxide film. Furthermore, it is demonstrated that material and device dependencies are consistent with predictions of a two-zone parallel memristor model of NDR which is based on a non-uniform current distribution after electroforming.

These results advance the current understanding of threshold switching response in amorphous NbOx films, and provide a strong basis for engineering devices with specific NDR characteristics. Significantly, these results also resolve a long-standing controversy about the origin of the snapback response which has been a subject of considerable debate. Show more