Dinesh Kumar Venkatachalam

Reduced Threshold Current in NbO 2 Selector by Engineering Device Structure

The leakage current scaling issues for NbO2 selector devices are investigated. By introducing a rough Pt bottom electrode (BE) (RMS roughness ~2.5 nm) and inserting a 20-nm-thick dielectric layer (Nb2O5 and HfO2) between the BE and NbO2 layer, we show that the threshold current for the insulator-metal-transition in microscale devices (~150 μm) can be reduced to ~20 μA, close to that realized in nanoscale (~10 nm) 3-D vertical ReRAM. This could be attributed to a thermal confinement effect caused by the presence of a permanent conductive filament in dielectric layer. The experimental results are supported by finite element simulation.

Threshold current reduction for the metal–insulator transition in NbO2−x-selector devices: the effect of ReRAM integration

The threshold current for inducing the metal–insulator transition in a NbO2−x selector element is shown to be affected by the properties of an adjacent memory element when integrated into a hybrid selector-memory device structure. Experimental results are reported for homogeneous NbO2−x/Nb2O5−y and heterogeneous NbO2−x/HfO2 device structures, and show that the threshold current is lower in both hybrid structures than in the selector element alone, and is lower in the heterogeneous structure than in the homogeneous structure. Finite element modeling of the selector-memory structure shows that this results primarily from current confinement produced by the filamentary conduction path in the resistive-switching memory layer (i.e. Nb2O5−y or HfO2), an observation that further implies a smaller diameter filament in HfO2 than in Nb2O5−y. The thermal and electrical conductivities of the memory layer are also shown to influence the threshold current, but to a lesser extent.

Nanomechanical properties of sputter-deposited HfO2 and HfxSi1-xO2 thin films

The authors wish to acknowledge the Australian Research Council (ARC) for financial support. J.E.B. wishes to further acknowledge the ARC for QEII Fellowship.

High-endurance megahertz electrical self-oscillation in Ti/NbO x bilayer structures

Electrical self-oscillation is reported for a Ti/NbOx negative differential resistance device incorporated in a simple electric circuit configuration. Measurements confirm stable operation of the oscillator at source voltages as low as 1.06 V, and demonstrate frequency control in the range from 2.5 to 20.5 MHz for voltage changes as small as ∼1 V. Device operation is reported for >6.5 × 1010 cycles, during which the operating frequency and peak-to-peak device current decreased by ∼25%. The low operating voltage, large frequency range, and high endurance of these devices makes them particularly interesting for applications such as neuromorphic computing.

Low-temperature, site selective graphitization of SiC via ion implantation and pulsed laser annealing

A technique is presented to selectively graphitize regions of SiC by ion implantation and pulsed laser annealing (PLA). Nanoscale features are patterned over large areas by multi-ion beam lithography and subsequently converted to few-layer graphene via PLA in air. Graphitization occurs only where ions have been implanted and without elevating the temperature of the surrounding substrate. Samples were characterized using Raman spectroscopy, ion scattering/channeling, SEM, and AFM, from which the degree of graphitization was determined to vary with implantation species, damage and dose, laser fluence, and pulsing. Contrasting growth regimes and graphitization mechanisms during PLA are discussed.

Threshold switching and electrical self-oscillation in niobium oxide films

Electrical self-sustained oscillations have been observed in a broad range of two-terminal systems and are of interest as possible building blocks for bio-inspired neuromorphic computing. In this work, we experimentally explore voltage-controlled oscillations in NbOx devices with a particular focus on understanding how the frequency and waveform are influenced by circuit parameters. We also introduce a finite element model of the device based on a Joule-heating induced insulator-metal transition. The electroformed device structure is represented by a cylindrical conductive channel (filament) comprised of NbO/NbO2 zones and surrounded by an Nb2O5−x matrix. The model is shown to reproduce the current-controlled negative differential resistance observed in measured current-voltage curves, and is combined with circuit elements to simulate the waveforms and dynamics of an isolated Pearson–Anson oscillator. Such modeling is shown to provide considerable insight into the relationship between the material respons...

A high-energy electron scattering study of the electronic structure and elemental composition of O-implanted Ta films used for the fabrication of memristor devices

This research was made possible by funding of the Australian Research Council. Oxygen-implanted Tantalum films were provided by Dr. S. Ruffell and Dr. J. England of Varian Semiconductor Equipment, a Division of Applied Materials, as part of a broader collaboration funded by an Australian Research Council Linkage Project Grant. The stay of P.L.G. at the ANU was made possible by a Grant No. 10209/12-3 from CAPES (Brazil). S.K.N. gratefully acknowledges RSAA for his Ph.D. scholarship.

Rapid, substrate-independent thickness determination of large area graphene layers

Phase-shifting interferometric imaging is shown to be a powerful analytical tool for studying graphene films, providing quantitative analysis of large area samples with an optical thickness resolution of ≤0.05 nm. The technique is readily able to identify single sheets of graphene and to quantitatively distinguish between layers composed of multiple graphene sheets. The thickness resolution of the technique is shown to result from the phase shift produced by a graphene film as incident and reflected light pass through it, rather than from path-length differences produced by surface height variations. This is enhanced by the high refractive index of graphene, estimated in this work to be nG = 2.99 ± 0.18.

Determination of thickness and composition of high-k dielectrics using high-energy electrons

We demonstrate the application of high-energy elastic electron backscattering to the analysis of thin (2–20 nm) HfO2 overlayers on oxidized Si substrates. The film composition and thickness are determined directly from elastic scattering peaks characteristic of each element. The stoichiometry of the films is determined with an accuracy of 5%–10%. The experimental results are corroborated by medium energy ions scattering and Rutherford backscattering spectrometry measurements, and clearly demonstrate the applicability of the technique for thin-film analysis. Significantly, the presented technique opens new possibilities for nm depth profiling with high spatial resolution in scanning electron microscopes.

Temperature dependence of threshold switching in NbO x thin films

The threshold switching characteristics of amorphous NbOx thin films is investigated with particular emphasis on temperature dependence of the switching characteristics. Threshold switching in this material is believed to result from a thermally-induced insulator-metal-transition (IMT) induced along a filamentary path by local Joule heating. Increasing the operating temperature is shown to lead to a reduction in the resistance of the high-resistance state and to a reduction in the threshold switching voltage. These results are discussed in relation to the transport properties of NbOx and the IMT switching model.