Abstract
Niobium dioxide can exhibit negative differential resistance (NDR) in metal-insulator-metal (MIM) devices, which has recently attracted significant interest for its potential applications as a highly non-linear selector element in emerging nonvolatile memory (NVM) and as a locally-active element in neuromorphic circuits. In order to further understand the processing of this material system, we studied the effect of thermal annealing on a 15 nm thick NbO2 thin film sandwiched inside a nanoscale MIM device and compared it with 180 nm thick blanket NbOx (x = 2 and 2.5) films deposited on a silicon dioxide surface as references. A systematic transmission electron microscope (TEM) study revealed a similar structural transition from amorphous to a distorted rutile structure in both cases, with a transition temperature of 700 °C for the NbO2 inside the MIM device and a slightly higher transition temperature of 750 °C for the reference NbO2 film. Quantitative composition analysis from electron energy loss spectroscopy (EELS) showed the stoichiometry of the nominal 15 nm NbO2 layer in the as-fabricated MIM device deviated from the target 1:2 ratio because of an interaction with the electrode materials, which was more prominent at elevated annealing temperature.
Highlights
Microscopy (TEM) combined with atom probe tomography showed local crystallization in amorphous NbO2 accompanied the threshold switching (TS) behavior[15]
Two-dimensional STEM/energy loss spectroscopy (EELS) mapping were collected from the entire niobium oxide layer in the NbO2 and Nb2O5 specimens, as illustrated by the cross-sectional STEM dark field (DF) image in the inset
Each spectrum in the spectrum image (SI, i.e., the two-dimensional STEM/EELS maps with EELS spectra in the 3rd dimension corresponding to each pixel) was quantified based on integration of the Nb-M and O-K edges, with a standard power-law background subtraction and cross section calculation using the Hartree-Slater model[16,17]
Summary
It was observed that the NbO2/TiOx/TiN interfaces were not as sharp as in the as-prepared device (Fig. 3), indicating some degree of intermixing between the layers occurred upon annealing at high temperature. Crystallization in the 15 nm NbO2 film within the nanoscale MIM devices started at 700 °C, with significant Ti mixed into the Nb oxide layer when annealed at 700–800 °C These results suggest that crystallization and intermixing with TiN electrodes may occur in nano-scaled Nb oxide layers during operation of NbO2-based MIM switching devices, during the high local temperature that typically occur during electrically induced forming step. Because Joule heating underpins the physical mechanism of the NDR behavior[8], local structural and compositional changes at elevated temperatures should be considered when designing MIM structures for selector applications
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