Observation and characterization of titanium-like nano-filament in TiO2 memristor using superconducting electrode(s) and Andreev spectroscopy

Author:

Moško Martin12ORCID,Koscelanská Mária12,Mošková Antónia2ORCID,Vidiš Marek1ORCID,Volkov Serhii1,Gregor Maroš1ORCID,Poláčková Magdaléna1ORCID,Roch Tomáš1ORCID,Grančič Branislav1ORCID,Satrapinskyy Leonid1ORCID,Kúš Peter1ORCID,Plecenik Andrej1,Plecenik Tomáš1ORCID

Affiliation:

1. Centre for Nanotechnology and Advanced Materials, Faculty of Mathematics, Physics and Informatics, Comenius University Bratislava 1 , Mlynská dolina, 84248 Bratislava, Slovakia

2. Slovak Academy of Sciences, Institute of Electrical Engineering 2 , Dúbravská cesta 9, 84104 Bratislava, Slovakia

Abstract

A thin TiO2 semiconductor film embedded between two metal electrodes works as a memristor after being formed by soft breakdown. The forming creates a nano-filament that penetrates through the poorly conducting TiO2 film and connects the electrodes conductively. While previous works characterized the nano-filament properties (shape, composition, and resistivity) by electron microscopy techniques, we present a characterization by electrical measurements. In a typical memristor, both electrodes are made of normal metals. We study the metal/TiO2/metal memristors with a bottom electrode made of a superconducting NbN layer and a top electrode made of a normal (Pt) or superconducting (Nb) metal. The nano-filament connecting the electrodes touches the bottom electrode as a point contact, thus allowing us to perform point-contact Andreev reflection spectroscopy of the NbN superconductor. The spectra, measured below the critical temperature (15 K) of NbN, are analyzed theoretically. The analysis reveals the presence of one nano-filament and determines the nano-filament resistance, Sharvin resistance of the point contact, and Maxwell resistance of the electrodes. Moreover, it shows that the nano-filament is a conical-shaped Ti-like metal point contact with a tip diameter of ∼3–5 nm, Fermi velocity of 2×106m/s, and low-temperature resistivity of ∼10−8–10−7Ωm. Thus, the nano-filament in our device is not the Ti4O7 phase observed in previous works. Remarkably, the point contact spectrum of the superconducting NbN layer shows the Andreev peak typical for ballistic transport. This is because the point contact probes the NbN layer through a thin Al layer that mimics superconductivity of NbN via the proximity effect and eliminates the effects of tunneling and disorder.

Funder

Agentúra na Podporu Výskumu a Vývoja

European Regional Development Fund

Vedecká Grantová Agentúra MŠVVaŠ SR a SAV

Publisher

AIP Publishing

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