arXiv subjects

Sanjoy Kumar Nandi

Publications and source records attributed to Sanjoy Kumar Nandi.

10 recordsLinked to original sources

arXiv2026

Anomalous Platinum and Oxygen Transport during Electroforming of NbOx Memristors

Electroforming of metal-oxide-metal memristors is generally attributed to the creation of oxygen-vacancy filaments within the oxide, with noble metal electrodes such as Pt and Au remaining chemically inert. Here, we demonstrate that electroforming and subsequent operation of Pt/NbOx/Nb2O5/Pt devices can induce an unexpected and highly correlated redistribution of both oxygen and platinum. Time-of-flight secondary ion mass spectrometry reveals a filamentary pathway characterized by micrometer-scale oxygen enrichment extending from the Nb2O5 layer through NbOx and deep into the Pt top electrode. Surprisingly, this is accompanied by the formation of a Pt-rich filament penetrating the oxide stack along the same filamentary path. Finite-element and lumped-element modelling show that current-controlled negative-differential-resistance operation produces localized Joule heating and high-frequency thermal cycling, which strongly enhances oxygen migration and enables thermally assisted Pt diffusion along vacancy-rich pathways. These findings reveal a previously unrecognized metal-ion transport mechanism in NbOx memristors and highlight the critical role of post-forming electrical dynamics in determining filament chemistry, stability, and device reliability.

cond-mat.mtrl-sci↗

arXiv2026

High-Mobility Indium Native Oxide Transistors via Liquid-Metal Printing in Air

Oxide semiconductors have emerged as common channel materials in transistors and hold promise for next-generation electronics, yet achieving high mobility typically requires costly vacuum-based techniques. Here, ultrathin (5-nm) indium native oxide (InOx) prepared by ambient-air liquid-metal printing (LMP) at low temperature (250 °C), is applied as semiconducting channel in field-effect transistor (FET). The resulting InOx is found to be polycrystalline with large lateral grains that extend vertically throughout the film thickness. InOx FETs in a transfer length method (TLM) configuration demonstrate a high conductivity mobility (uCON) of 125 cm2 V-1 s-1, with systematic analysis of contact resistance confirming potential for channel length scaling. Integration with atomic-layer-deposited (ALD) gate dielectrics further reveals excellent compatibility, for instance, InOx FET integrated with HfO2 exhibits a high field-effect mobility (uFE) of 107 cm2 V-1 s-1, an on/off current ratio (ION/IOFF) of >107, a subthreshold swing (SS) of 204 mV dec-1, a gate leakage of <10-6 A cm-2, while maintaining stable performance over 104 endurance cycles without degradation. Post-fabrication oxygen-plasma treatment is applied to achieve enhancement-mode operation and a depletion-load inverter is demonstrated, exhibiting a voltage gain of 69.8 V/V. These results demonstrate the great potential of LMP InOx as semiconducting channel in high-performance and power-efficient transistors for next-generation oxide electronics.

cond-mat.mtrl-sci↗

arXiv2023

A balanced Memristor-CMOS ternary logic family and its application

The design of balanced ternary digital logic circuits based on memristors and conventional CMOS devices is proposed. First, balanced ternary minimum gate TMIN, maximum gate TMAX and ternary inverters are systematically designed and verified by simulation, and then logic circuits such as ternary encoders, decoders and multiplexers are designed on this basis. Two different schemes are then used to realize the design of functional combinational logic circuits such as a balanced ternary half adder, multiplier, and numerical comparator. Finally, we report a series of comparisons and analyses of the two design schemes, which provide a reference for subsequent research and development of three-valued logic circuits.

eess.SP↗

arXiv2020

Understanding modes of negative differential resistance in amorphous and polycrystalline vanadium oxides

Metal-oxide-metal devices based on amorphous VOx are shown to exhibit one of two distinct negative differential resistance (NDR) characteristics depending on the maximum current employed for electroforming. For low compliance currents they exhibit a smooth S-type characteristic and have a temperature-dependent device resistance characterised by an activation energy of 0.25 eV, consistent with conduction in polycrystalline VO2, while for high-compliance currents they exhibit an abrupt snap-back characteristic and a resistance characterised by an activation energy of 0.025 eV, consistent with conduction in oxygen deficient VOx. In both cases, the temperature dependence of the switching voltage implies that the conductivity change is due to the insulator-metal transition in VO2. From this analysis it is concluded that electroforming at low currents creates a conductive filament comprised largely of polycrystalline VO2, while electroforming at high currents creates a composite structure comprised of VO2 and a conductive halo of oxygen deficient VOx. The effect of electroforming on the NDR mode is then explained with reference to a lumped element model of filamentary conduction that includes the effect of a parallel resistance created by the halo. These results provide new insight into the NDR response of vanadium-oxide-based devices and a basis for designing devices with specific characteristics.

physics.app-ph↗

arXiv2020

Electric field and current induced electroforming modes in NbOx

Electroforming is used to initiate the memristive response in metal/oxide/metal devices by creating a filamentary conduction path in the oxide film. Here we use a simple photoresist-based detection technique to map the spatial distribution of conductive filaments formed in Nb/NbOx/Pt devices, and correlate these with current-voltage characteristics and in-situ thermoreflectance measurements to identify distinct modes of electroforming in low and high conductivity NbOx films. In low conductivity films the filaments are randomly distributed within the oxide film, consistent with a field-induced weakest-link mechanism, while in high conductivity films they are concentrated in the center of the film. In the latter case the current-voltage characteristics and in-situ thermoreflectance imaging show that electroforming is associated with current bifurcation into regions of low and high current density. This is supported by finite element modelling of the current distribution and shown to be consistent with predictions of a simple core-shell model of the current distribution. These results clearly demonstrate two distinct modes of electroforming in the same materials system and show that the dominant mode depends on the conductivity of the film, with field-induced electroforming dominant in low conductivity films and current-bifurcation induced electroforming dominant in high conductivity films. Finally, we demonstrate S-type and snap-back negative differential resistance in the high conductivity films and explain this behavior in terms of two-zone model.

cond-mat.mtrl-sci↗

arXiv2019

Metal-oxide interface reactions and their effect on integrated resistive/threshold switching in NbOx

Reactive metal electrodes (Nb, Ti, Cr, Ta, and Hf) are shown to play an important role in controlling the volatile switching characteristics of metal/Nb2O5/Pt devices. In particular, devices are shown to exhibit stable threshold switching under negative bias but to have a response under positive bias that depends on the choice of metal. Three distinct responses are highlighted: Devices with Nb and Ti top electrodes are shown to exhibit stable threshold switching with symmetric characteristics for both positive and negative polarities; devices with Cr top electrodes are shown to exhibit stable threshold switching but with asymmetric hysteresis windows under positive and negative polarities; and devices with Ta and Hf electrodes are shown to exhibit an integrated threshold-memory (1S1M) response. Based on thermodynamic data and lumped element modelling these effects are attributed to the formation of a metal-oxide interlayer and its response to field-induced oxygen exchange. These results provide important insight into the physical origin of the switching response and pathways for engineering devices with reliable switching characteristics.

physics.app-ph↗

arXiv2019

Schottky barrier induced asymmetry in the negative differential resistance response of Nb/NbOx/Pt cross-point devices

The negative differential resistance (NDR) response of Nb/NbOx/Pt cross-point devices is shown to have a polarity dependence due to the effect of the metal/oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snap-back NDR, and combined S-type and snap-back NDR, depending on the stoichiometry of the oxide film and device area. In-situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localisation, thereby justifying the use of a previously developed two-zone, core shell thermal model of the device. The observed polarity dependent NDR responses, and their dependence on stoichiometry and area are then explained by extending this model to include the effect of the polarity dependent contact resistance. This study provides an improved understanding of the NDR response of metal/oxide/metal structures and informs the engineering of devices for neuromorphic computing and non-volatile memory applications.

physics.app-ph↗

arXiv2019

Current localisation and redistribution as the basis of discontinuous current controlled negative differential resistance in NbOx

In-situ thermo-reflectance imaging is used to show that the discontinuous, snap-back mode of current-controlled negative differential resistance (CC-NDR) in NbOx-based devices is a direct consequence of current localization and redistribution. Current localisation is shown to result from the creation of a conductive filament either during electroforming or from current bifurcation due to the super-linear temperature dependence of the film conductivity. The snap-back response then arises from current redistribution between regions of low and high current-density due to the rapid increase in conductivity created within the high current density region. This redistribution is further shown to depend on the relative resistance of the low current-density region with the characteristics of NbOx cross-point devices transitioning between continuous and discontinuous snap-back modes at critical values of film conductivity, area, thickness and temperature, as predicted. These results clearly demonstrate that snap-back is a generic response that arises from current localization and redistribution within the oxide film rather than a material-specific phase transition, thus resolving a long-standing controversy.

physics.app-ph↗

arXiv2019

Origin of current-controlled negative differential resistance modes and the emergence of composite characteristics with high complexity

Current-controlled negative differential resistance has significant potential as a fundamental building block in brain-inspired neuromorphic computing. However, achieving desired negative differential resistance characteristics, which is crucial for practical implementation, remains challenging due to little consensus on the underlying mechanism and unclear design criteria. Here, we report a material-independent model of current-controlled negative differential resistance to explain a broad range of characteristics, including the origin of the discontinuous snap-back response observed in many transition metal oxides. This is achieved by explicitly accounting for a non-uniform current distribution in the oxide film and its impact on the effective circuit of the device, rather than a material-specific phase transition. The predictions of the model are then compared with experimental observations to show that the continuous S-type and discontinuous snap-back characteristics serve as fundamental building blocks for composite behaviour with higher complexity. Finally, we demonstrate the potential of our approach for predicting and engineering unconventional compound behaviour with novel functionality for emerging electronic and neuromorphic computing applications.

physics.app-ph↗

arXiv2018

Detection and spatial mapping of conductive filaments in metal/oxide/metal cross-point devices

A simple means of detecting and spatially mapping volatile and nonvolatile conductive filaments in metal/oxide/metal cross-point devices is introduced and its application demonstrated. The technique is based on thermal discolouration of a thin photoresist layer deposited on the top electrode of the cross-point device and relies on the increase in temperature produced by local Joule heating of an underlying conductive filament. Finite element modelling of the temperature distribution and its dependencies shows that the maximum temperature at the top-electrode/photoresist interface is particularly sensitive to the top-electrode thickness. The technique is demonstrated on NbOx based metal-oxide-metal cross-point devices with a 25 nm thick top (Pt) electrode, where it is used to undertake a statistical analysis of the filament location as a function of device area. This shows that filament formation is heterogeneous. The majority of filaments form preferentially along the top-electrode edge and the fraction of these increases with decreasing device area. Transmission electron microscopy of the top and bottom electrode edges is used to explain this observation and suggests that it is due to a reduction in the effective oxide thickness in this region.

physics.app-ph↗