Condensation-Driven Growth of Tellurene Nanosheets for Memristive Applications

The increasing demand for the development of memories with low operating voltage, compatible with CMOS technologies and simple structure, has recently targeted the class of two-dimensional (2D) materials [1]. Novel 2D materials such as transition metal dichalcogenides like MoS₂ and MoTe₂ have inspired ultra-scaled layouts of non-volatile random-access memory (RAM) devices based on a resistive switching or phase change mechanism [2,3]. In this respect, monoelemental 2D crystals, termed Xenes [4], may lead to a structural simplification with benefits in terms of energy consumption and performance. In this work, we report on the low temperature synthesis of tellurium nanosheets, known as tellurene, by condensation-driven vapor transport growth processing inside a chemical vapour deposition reactor towards the development of a tellurene-based memristive cell layout. First, we show that the process microscale tellurium structures like hexagonal pillars and nanotubes is scaled down to the 2D level by mastering the growth of tellurene sheet on SiO₂/Si substrates with lateral coverage on the order of tens of microns with a process temperature below 100°C, namely within BEOL compatible thermal regime. On the same line, we applied the same process to (111)-terminated metal templates like Au(111) and Ag(111) epifilm on mica as metallic substrates readily enabling the manufacture of a diode. For the case of tellurene growth on Au/mica substrate we can discriminate two different regimes depending on the substrate temperature T, i.e. a kinetically driven regime where tellurium grows as tellurene nanosheets with no interaction with Au with T&lt;350°C (condensation and nucleation) and a chemisorption regime where tellurene reacts with the Au surface to form AuTe₂ (tellurization) with T&gt;350°C. We characterized the two regimes by means of several probing techniques including Raman spectroscopy and atomic force microscopy, and we define the best condition for the growth by means of finite element simulation guidance.<br/>The tellurene nanosheet on Au(111)/mica stands out as an optimal configuration where to check a memristive behavior as reported in case of other 2D compounds like MoS₂ on metal substrates [5]. With this purpose in mind, we investigated the local current-voltage (I-V) response of tellurene nanosheet with thickness from 5 to 20 nm by means of conductive atomic force microscopy (C-AFM) with a sharp conductive diamond coated tip the Au substrate as top and bottom electrodes, respectively. A resistive switch behavior of tellurene was revealed by local C-AFM probing, i.e. by applying a linearly ramped voltage and measuring the current passing from the sample to the tip. A trap assisted space charge limited current conduction (SCLC) in the tellurene nanosheets was found to be consistent with the observed I-V behavior thus paving the way to store, read, and write a non-volatile information through tellurene. As such, the configurational details of the growth are accordingly developed for tellurene to be integrated as active layer in a memristor device, and thus transferred to a neuromorphic computing architecture.<br/>This work has been financially supported by the European Commission under the H2020 ERC-COG grant n. 772261 “XFab”.<br/><br/>[1] W. Huh, et al., Adv. Mater. (2020) 32, 2002092.<br/>[2] R. Ge, et al., Nano Letters (2018) 18, 434.<br/>[3] I. M. Datye, et al., Nano Letters (2020) 20, 1461.<br/>[4] “Xenes” Eds. A. Molle and C. Grazianetti, Woodhead Publ. (Elsevier), 2022.<br/>[5] S. M. Hus, et al., Nature Nanotechnol. (2021) 16, 58.