(Invited) Understanding the Structure and Electrical Response of Combined Threshold and Memristive Switching Devices Obtained from Amorphous Nb2O5 Layers
Authors: Hoffmann-Eifert Susanne , Funck Carsten , Menzel Stephan , Aslam Nabeel , Blanquart Timothee , Niinistö Jaakko , Ritala Mikko , Leskelä Markku , Du Hongchu , Bornhöfft Manuel , Mayer Joachim , Waser Rainer 
Published: February 2020

Redox-based resistive switching cells (ReRAM) are intensively studied for next generation non-volatile memory due to simple device structure, low power consumption, and good endurance.[1] High density integration demands for technologies like three dimensional (3D) stacking and 4F
2 crossbar array architectures.[2] However, passive ReRAM arrays require the integration of bidirectional selector devices with high selectivity to suppress sneak-path currents.[3] A promising concept is based on threshold-type devices as obtained from NbO2, which typically show a negative differential resistance (NDR) characteristic.[4] This volatile resistance change can be utilized for the realization of a highly non-linear selector element.
The presentation comprises two studies. The first one addresses the fabrication and nano-scale characterization of combined threshold and memory cells (TS-ReRAM) with a potential for future high-density 3D integration. The cells are built from a 10 nm thick amorphous Nb2O5 layer grown by atomic layer deposition (ALD) and integrated into Pt/Nb2O5/Ti/Pt nano-crossbar structures. After electroforming, the devices combine the functions of volatile threshold and non-volatile memresistive switching. The structural changes induced by the electroforming step are analyzed by means of electron nano-diffraction. The results confirm the formation of crystalline NbO2 within a small regime localized inside the Nb2O5-x based switching cell.[5] This finding supports the proposal of an Nb2O5-x/NbO2 filament being the central element of the TS-ReRAM cell.
The second study aims upon a deeper understanding of the threshold switching (TS) phenomenon in NbO2-based TS-ReRAM devices. A widely accepted model for TS in NbO2 is based on a combination of Joule heating and an insulator to metal transition (IMT) at TIMT

, NbO2 ≈ 1080 K.[6] This model, however, leads to fundamental contradictions like a too low discontinuity in the I-V characteristics. Recently it has been shown that an increase in temperature of about 200 K is sufficient to suppress the TS-effect.[7] Therefore we set-up a two-dimensional axial symmetric simulation of the TS-behavior of the Nb2O5-x / NbO2 based cells.[8] The continuum model includes a filamentary switching, divided into a threshold and a memristive switching region. The fully coupled heat transfer and continuity equation system is solved by Newton-Raphson iteration. The conduction mechanism for the threshold regime is based on a Poole Frenkel (PF) like barrier lowering and exponentially coupled to the electric field. The interplay of Joule heating and an increase of conducting charge carriers causes an electric field controlled thermal runaway process, which is interrupted by the resistance of the memory in low resistance state limiting further Joule heating. The proposed model reproduces the experimental results fairly well. [8]
The understanding of the structural modifications associated with the electroforming and resistive switching process in NbO2/Nb2O5-x based TS-ReRAM cells makes the fabrication of such devices by means of the ALD technique feasible. In addition, our proposed two dimensional simulation based on an electric field triggered thermal runaway (FTTR) mechanism provides a deeper understanding of the switching limitations in the TS-ReRAM structures. Additionally, the realization of Boolean logic functionality using the TS-ReRAM devices has been demonstrated.[9]
This work was supported in part by the Deutsche Forschungsgemeinschaft (SFB917), and by FP7 under grant ENHANCE-238409.
References:
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