Nikhil Shukla

A unified model for insulator selection to form ultra-low resistivity metal-insulator-semiconductor contacts to n-Si, n-Ge, and n-InGaAs

A comprehensive, physics-based unified model is developed for study of low resistivity metal-insulator-semiconductor (M-I-S) ohmic contact. Reduction in metal-induced gap state density and Fermi unpinning in semiconductor as a function of insulator thickness is coupled with electron transport including tunnel resistance through the metal-insulator-semiconductor (M-I-S) system to calculate specific contact resistivity at each insulator thickness for n-Si, n-Ge, and n-InGaAs. Low conduction band offset results in ∼1×10−9 Ω−cm2 contact resistivity with TiO2 insulator on n-Si, ∼7×10−9 Ω−cm2 can be achieved using TiO2 and ZnO on n-Ge, and ∼6×10−9 Ω−cm2 can be achieved with CdO insulator on n-InGaAs, which meet the sub-22nm CMOS requirements.

Pairwise coupled hybrid vanadium dioxide-MOSFET (HVFET) oscillators for non-boolean associative computing

Information processing applications related to associative computing like image / pattern recognition consume excessive computational resources in the Boolean processing framework. This motivates the exploration of a non-Boolean computing approach for such applications. In this work, we demonstrate, (i) novel hybrid set of pair-wise coupled oscillators comprising of vanadium dioxide (VO2) metal-insulator-transition (MIT) system integrated with MOSFET; (ii) degree of synchronization between oscillators based on input analog voltage difference; (iii) implementation of hardware platform for fast and efficient evaluation of Lk fractional distance norm (k<;1); (iv) improved quality of image processing and ~20X lower power consumption of the coupled oscillators over a CMOS accelerator.

Intrinsic electronic switching time in ultrathin epitaxial vanadium dioxide thin film

This letter investigates the intrinsic electronic switching time associated with the insulator-to-metal phase transition in epitaxial single crystal vanadium dioxide (VO2) thin films using impedance spectroscopy and ac conductivity measurements. The existence of insulating and metallic phase coexistence, intrinsic to the epitaxial (001) oriented VO2 thin film grown on a (001) rutile TiO2 substrate, results in a finite capacitance being associated with the VO2 films in their insulating phase that limits the electronic switching speed. Insights into the switching characteristics and their correlation to the transport mechanism in the light of phase coexistence are obtained by performing a detailed scaling study on VO2 two-terminal devices.

Transport properties of ultra-thin VO2 films on (001) TiO2 grown by reactive molecular-beam epitaxy

We report the growth of (001)-oriented VO2 films as thin as 1.5 nm with abrupt and reproducible metal-insulator transitions (MIT) without a capping layer. Limitations to the growth of thinner films with sharp MITs are discussed, including the Volmer-Weber type growth mode due to the high energy of the (001) VO2 surface. Another key limitation is interdiffusion with the (001) TiO2 substrate, which we quantify using low angle annular dark field scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy. We find that controlling island coalescence on the (001) surface and minimization of cation interdiffusion by using a low growth temperature followed by a brief anneal at higher temperature are crucial for realizing ultrathin VO2 films with abrupt MIT behavior.

Neuro Inspired Computing with Coupled Relaxation Oscillators

Harnessing the computational capabilities of dynamical systems has attracted the attention of scientists and engineers form varied technical disciplines over decades. The time evolution of coupled, non-linear synchronous oscillatory systems has led to active research in understanding their dynamical properties and exploring their applications in brain-inspired, neuromorphic computational models. In this paper we present the realization of coupled and scalable relaxation-oscillators utilizing the metal-insulator-metal transition of vanadium-dioxide (VO2) thin films. We demonstrate the potential use of such a system in pattern recognition, as one possible computational model using such a system.

Synchronization of pairwise-coupled, identical, relaxation oscillators based on metal-insulator phase transition devices: A model study

Computing with networks of synchronous oscillators has attracted wide-spread attention as novel materials and device topologies have enabled realization of compact, scalable and low-power coupled oscillatory systems. Of particular interest are compact and low-power relaxation oscillators that have been recently demonstrated using MIT (metal-insulator-transition) devices using properties of correlated oxides. Further the computational capability of pairwise coupled relaxation oscillators has also been shown to outperform traditional Boolean digital logic circuits. This paper presents an analysis of the dynamics and synchronization of a system of two such identical coupled relaxation oscillators implemented with MIT devices. We focus on two implementations of the oscillator: (a) a D-D configuration where complementary MIT devices (D) are connected in series to provide oscillations and (b) a D-R configuration where it is composed of a resistor (R) in series with a voltage-triggered state changing MIT device ...

Nanoscale structural evolution of electrically driven insulator to metal transition in vanadium dioxide

The structural evolution of tensile strained vanadium dioxide thin films was examined across the electrically driven insulator-to-metal transition by nanoscale hard X-ray diffraction. A metallic filament with rutile (R) structure was found to be the dominant conduction pathway for an electrically driven transition, while the majority of the channel area remained in the monoclinic M1 phase. The filament dimensions were estimated using simultaneous electrical probing and nanoscale X-ray diffraction. Analysis revealed that the width of the conducting channel can be tuned externally using resistive loads in series, enabling the M1/R phase ratio in the phase coexistence regime to be tuned.

Joule Heating-Induced Metal–Insulator Transition in Epitaxial VO2/TiO2 Devices

DC and pulse voltage-induced metal-insulator transition (MIT) in epitaxial VO2 two terminal devices were measured at various stage temperatures. The power needed to switch the device to the ON-state decrease linearly with increasing stage temperature, which can be explained by the Joule heating effect. During transient voltage induced MIT measurement, the incubation time varied across 6 orders of magnitude. Both DC I-V characteristic and incubation times calculated from the electrothermal simulations show good agreement with measured values, indicating Joule heating effect is the cause of MIT with no evidence of electronic effects. The width of the metallic filament in the ON-state of the device was extracted and simulated within the thermal model.

Field Effect and Strongly Localized Carriers in the Metal-Insulator Transition Material VO(2).

The intrinsic field effect, the change in surface conductance with an applied transverse electric field, of prototypal strongly correlated VO(2) has remained elusive. Here we report its measurement enabled by epitaxial VO(2) and atomic layer deposited high-κ dielectrics. Oxygen migration, joule heating, and the linked field-induced phase transition are precluded. The field effect can be understood in terms of field-induced carriers with densities up to ∼5×10(13)  cm(-2) which are trongly localized, as shown by their low, thermally activated mobility (∼1×10(-3)  cm(2)/V s at 300 K). These carriers show behavior consistent with that of Holstein polarons and strongly impact the (opto)electronics of VO(2).

Exploiting Synchronization Properties of Correlated Electron Devices in a Non-Boolean Computing Fabric for Template Matching

As complementary metal-oxide-semiconductor (CMOS) scaling continues to offer insurmountable challenges, questions about the performance capabilities of Boolean, digital machine based on Von-Neumann architecture, when operated within a power budget, have also surfaced. Research has started in earnest to identify alternative computing paradigms that provide orders of magnitude improvement in power-performance for specific tasks such as graph traversal, image recognition, template matching, and so on. Further, post-CMOS device technologies have emerged that realize computing elements which are neither CMOS replacements nor suited to work as a binary switch. In this paper, we present the realization of coupled and scalable relaxation-oscillators utilizing the metal-insulator-metal transition of vanadium-dioxide (VO2) thin films. We demonstrate the potential use of such a system in a non-Boolean computing paradigm and demonstrate pattern recognition, as one possible application using such a system.