Search

Ministry of Science and Technology of China (grant nos. 2019YFE0124200 and 2018YFE0100800), National Natural Science Foundation of China (grant no. 61874075), Baseline funding scheme of the King Abdullah University of Science and Technology

The development of the internet-of-things requires cheap, light, small and reliable true random number generator (TRNG) circuits to encrypt the data—generated by objects or humans—before transmitting them. However, all current solutions consume too much power and require a relatively large battery, hindering the integration of TRNG circuits on most objects. Here we fabricated a TRNG circuit by exploiting stable random telegraph noise (RTN) current signals produced by memristors made of two-dimensional (2D) multi-layered hexagonal boron nitride (h-BN) grown by chemical vapor deposition and coupled with inkjet-printed Ag electrodes. When biased at small constant voltages (≤70 mV), the Ag/h-BN/Ag memristors exhibit RTN signals with very low power consumption (∼5.25 nW) and a relatively high current on/off ratio (∼2) for long periods (>1 hour). We constructed TRNG circuits connecting an h-BN memristor to a small, light and cheap commercial microcontroller, producing a highly-stochastic, high-throughput signal (up to 7.8 Mbit s−1) even if the RTN at the input gets interrupted for long times up to 20 s, and if the stochasticity of the RTN signal is reduced. Our study presents the first full hardware implementation of 2Dmaterial- based TRNGs, enabled by the unique stability and figures of merit of the RTN signals in h-BN based memristors., Ministry of Science and Technology, China 2019YFE0124200 2018YFE0100800, National Natural Science Foundation of China (NSFC) 61874075, Collaborative Innovation Centre of Suzhou Nano Science and Technology, Priority Academic Program Development of Jiangsu Higher Education Institutions, 111 Project from the State Administration of Foreign Experts Affairs of China, King Abdullah University of Science & Technology, Ministerio de Ciencia, Tecnologia e Innovacion (MINCyT) PICT 2016/0579 PME 2015-0196 PICTE 2018-0192 UTN-FRBA CCUTIBA4764TC MATUNBA4936 CCUTNBA5182 CCUTNBA6615

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.2c22617, We present a new methodology to quantify the variability of resistive switching memories. Instead of statistically analyzing few data points extracted from current versus voltage (I− V) plots, such as switching voltages or state resistances, we take into account the whole I−V curve measured in each RS cycle. This means going from a one-dimensional data set to a two-dimensional data set, in which every point of each I−V curve measured is included in the variability calculation. We introduce a new coefficient (named two-dimensional variability coefficient, 2DVC) that reveals additional variability information to which traditional one-dimensional analytical methods (such as the coefficient of variation) are blind. This novel approach provides a holistic variability metric for a better understanding of the functioning of resistive switching memories, Consejería de Conocimiento, Investigación y Universidad, Junta de Andalucía (Spain), FEDER: B-TIC-624-UGR20, PID2020-113961GB-I00, A-FQM-66-UGR20, FQM-307, IMAG María de Maeztu CEX2020-001105-M/AEI/10.13039/501100011033, King Abdullah University of Science and Technology

Summary Stacking perovskite solar cells onto crystalline silicon bottom cells in a monolithic tandem configuration enables power-conversion efficiencies (PCEs) well above those of their single-junction counterparts. However, state-of-the-art wide-band-gap perovskite films suffer from phase stability issues. Here, we show how carbazole as an additive to the perovskite precursor solution can not only reduce nonradiative recombination losses but, perhaps more importantly, also can suppress phase segregation under exposure to moisture and light illumination. This enables a stabilized PCE of 28.6% (independently certified at 28.2%) for a monolithic perovskite/silicon tandem solar cell over ∼1 cm2 and 27.1% over 3.8 cm2, built from a textured silicon heterojunction solar cell. The modified tandem devices retain ∼93% of their performance over 43 days in a hot and humid outdoor environment of almost 100% relative humidity over 250 h under continuous 1-sun illumination and about 87% during a 85/85 damp-heat test for 500 h, demonstrating the improved phase stability.

Inkjet-printed h-BN memristors exhibit multiple stochastic phenomena that are very attractive for use as entropy sources in electronic circuits for data encryption. The high variability can be exploited to create unique and unpredictable patterns.

Two-dimensional (2D) materials could potentially be used to develop advanced monolithic integrated circuits. However, despite impressive demonstrations of single devices and simple circuits—in some cases with performance superior to those of silicon-based circuits—reports on the fabrication of integrated circuits using 2D materials are limited and the creation of large-scale circuits remains in its infancy. Here we examine the development of integrated circuits based on 2D layered materials. We assess the most advanced circuits fabricated so far and explore the key challenges that need to be addressed to deliver highly scaled circuits. We also propose a roadmap for the future development of integrated circuits based on 2D layered materials. This Perspective examines the development of integrated circuits based on layered two-dimensional materials, exploring where they are likely to first find commercial use and considers the challenges than need to be addressed to create highly scaled circuits.

Resistive switching (RS) devices are being studied exhaustively for multiple applications such as information storage and bio-inspired computing, but some reliability problems are still hindering their massive use. Among them, random telegraph noise (RTN) is one of the phenomena that still needs to be understood. In this work, we develop a general model to describe RTN in RS devices by demonstrating the existence of coupling effects between multiple traps, and their effect in the current amplitude deviation. We accurately describe anomalous RTN signals observed in high resistive state (HRS) and low-resistance state (LRS) of RS devices based on TiO2 and hexagonal boron nitride (h-BN).

Advanced data encryption requires the use of true random number generators (TRNGs) to produce unpredictable sequences of bits. TRNG circuits with high degree of randomness and low power consumption may be fabricated by using the random telegraph noise (RTN) current signals produced by polarized metal/insulator/metal (MIM) devices as entropy source. However, the RTN signals produced by MIM devices made of traditional insulators, i.e., transition metal oxides like HfO and AlO, are not stable enough due to the formation and lateral expansion of defect clusters, resulting in undesired current fluctuations and the disappearance of the RTN effect. Here, the fabrication of highly stable TRNG circuits with low power consumption, high degree of randomness (even for a long string of 2 − 1 bits), and high throughput of 1 Mbit s by using MIM devices made of multilayer hexagonal boron nitride (h-BN) is shown. Their application is also demonstrated to produce one-time passwords, which is ideal for the internet-of-everything. The superior stability of the h-BN-based TRNG is related to the presence of few-atoms-wide defects embedded within the layered and crystalline structure of the h-BN stack, which produces a confinement effect that avoids their lateral expansion and results in stable operation., M.L. acknowledges generous support from the King Abdullah University of Science and Technology. This work was supported by the Ministry of Science and Technology of China (grants no. 2018YFE0100800, 2019YFE0124200), the National Natural Science Foundation of China (grants no. 61874075), the Collaborative Innovation Centre of Suzhou Nano Science & Technology, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the 111 Project from the State Administration of Foreign Experts Affairs of China. A.A. and S.R. acknowledge the project: ModElling Charge and Heat trANsport in 2D-materIals based Composites—MECHANIC reference number: PCI2018-093120 funded by Ministerio de Ciencia, Innovación y Universidades. ICN2 is funded by the CERCA Programme/Generalitat de Catalunya and is supported by the Severo Ochoa program from Spanish MINECO (Grant No. SEV-2017-0706). Y.S. acknowledges support from the European Union (Marie Sklodowska-Curie actions (grant no. 894840). The authors acknowledge technical advice from H.-S. Philip Wong from Stanford University and Xiaoming Xie from Chinese Academy of Sciences.

Two-dimensional (2D) materials have many outstanding properties that make them attractive for the fabrication of electronic devices, such as high conductivity, flexibility, and transparency. However, integrating 2D materials in commercial devices and circuits is challenging because their structure and properties can be damaged during the fabrication process. Recent studies have demonstrated that standard metal deposition techniques (like electron beam evaporation and sputtering) significantly damage the atomic structure of 2D materials. Here we show that the deposition of metal via inkjet printing technology does not produce any observable damage in the atomic structure of ultra-thin 2D materials, and that it can keep a sharp interface. These conclusions are supported by abundant data obtained via atomistic simulations, transmission electron microscopy, nano-chemical metrology, and device characterization in a probe station. Our results are important for the understanding of inkjet printing technology applied to 2D materials, and they could contribute to the better design and optimization of electronic devices and circuits. This article is protected by copyright. All rights reserved.

In the race of fabricating solid-state nano/microelectronic devices using 2D layered materials (LMs), achieving high yield and low device-to-device variability are the two main challenges. Electronic devices that drive currents in-plane and homogeneously along the 2D-LMs (i.e., transistors, memtransistors) are strongly affected by local defects (i.e., grain boundaries, wrinkles, thickness fluctuations, polymer residues), as they create inhomogeneities and increase the device-to-device variability, resulting in a poor performance at the circuit level. Here, it is shown that memristors are insensitive to most types of defects in 2D-LMs, even when fabricated in academic laboratories that do not meet industrial standards. The reason is that the currents produced in these devices, which flow out-of-plane across the 2D-LM, are always driven locally by the most conductive locations. Consequently, it is concluded that it is much easier to fabricate 2D-LMs-based solid-state nano/microelectronic circuits using memristors than using transistors or memtransistors, not only due to the inherent simpler fabrication process (i.e., less lithography steps) but also because the local defects do not degrade the yield and variability of memristors considerably.

Memristors have shown an extraordinary potential to emulate the plastic and dynamic electrical behaviors of biological synapses and have been already used to construct neuromorphic systems with in-memory computing and unsupervised learning capabilities; moreover, the small size and simple fabrication process of memristors make them ideal candidates for ultradense configurations. So far, the properties of memristive electronic synapses (i.e., potentiation/depression, relaxation, linearity) have been extensively analyzed by several groups. However, the dynamics of electroforming in memristive devices, which defines the position, size, shape, and chemical composition of the conductive nanofilaments across the device, has not been analyzed in depth. By applying ramped voltage stress (RVS), constant voltage stress (CVS), and pulsed voltage stress (PVS), we found that electroforming is highly affected by the biasing methods applied. We also found that the technique used to deposit the oxide, the chemical composition of the adjacent metal electrodes, and the polarity of the electrical stimuli applied have important effects on the dynamics of the electroforming process and in subsequent post-electroforming bipolar resistive switching. This work should be of interest to designers of memristive neuromorphic systems and could open the door for the implementation of new bioinspired functionalities into memristive neuromorphic systems.

Two-dimensional (2D) material-based memristors have shown several properties that are not shown by traditional ones, such as high transparency, robust mechanical strength and flexibility, superb chemical stability, enhanced thermal heat dissipation, ultralow power consumption, coexistence of bipolar and threshold resistive switching, and ultrastable relaxation when used as electronic synapse (among others). However, several electrical performances often required in memristive applications, such as the generation of multiple stable resistive states for high-density information storage, still have never been demonstrated. Here, we present the first 2D material-based memristors that exhibit three stable and well-distinguishable resistive states. By using a multilayer hexagonal boron nitride (h-BN) stack sandwiched by multilayer graphene (G) electrodes, we fabricate 5 μm × 5 μm cross-point Au/Ti/G/h-BN/G/Au memristors that can switch between each two or three resistive states, depending on the current limitation (CL) and reset voltage used. The use of graphene electrodes plus a small cross-point structure are key elements to observe the tristate operation, which has not been observed in larger (100 μm × 100 μm) devices with an identical Au/Ti/G/h-BN/G/Au structure nor in similar small (5 μm × 5 μm) devices without graphene interfacial layers (i.e., Au/Ti/h-BN/Au). Basically, we generate an intermediate state between the high resistive state and the low resistive state (LRS), named soft-LRS (S-LRS), which may be related to the formation of a narrower conductive nanofilament across the h-BN because of the ability of graphene to limit metal penetration (at low CLs). All the 2D materials have been fabricated using the scalable chemical vapor deposition approach, which is an immediate advantage compared to other works using mechanical exfoliated 2D materials.

Two dimensional (2D) materials have been used in memristors to improve and stabilize resistive switching (RS) behavior; however, most reports used large device area $(\geq 10^{4} \mu \text{m}^{2})$ , in which currents in each resistive state may be driven by largely defective regions that are not applicable in smaller devices. Here we present the first fabrication of cross-point memristors with sizes of $5 \mu \text{m} \times 5 \mu \text{m}$ using multilayer heterogeneous van der Waals structure. We have constructed graphene/hexagonal boron nitride/graphene (G/h-BN/G) devices using Ti and Au electrodes. The devices can show two-state or tristate operation depending on the current limitation (CL) used. Similar memristors with larger size of $100 \mu \text{m} \times 100 \mu \text{m}$ do not show this behavior, indicating that miniaturization of 2D materials based memristors is key to achieve this performance.

The Collaborative Innovation Center of Suzhou Nano Science & Technology, the Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, and the Priority Academic Program Development of Jiangsu Higher Education Institutions are also acknowledged. Stanford co-authors acknowledge support from the Non-volatile Memory Technology Research Initiative (NMTRI). An Chen from IBM is acknowledged for revision of section 5. Alok Ranjan from Singapore University of Technology and Design is acknowledged for useful discussion on section 3.5., Resistive switching (RS) is an interesting property shown by some materials systems that, especially during the last decade, has gained a lot of interest for the fabrication of electronic devices, being electronic non-volatile memories those that have received most attention. The presence and quality of the RS phenomenon in a materials system can be studied using different prototype cells, performing different experiments, displaying different figures of merit, and developing different computational analyses. Therefore, the real usefulness and impact of the findings presented in each study for the RS technology will be also different. In this manuscript we describe the most recommendable methodologies for the fabrication, characterization and simulation of RS devices, as well as the proper methods to display the data obtained. The idea is to help the scientific community to evaluate the real usefulness and impact of an RS study for the development of RS technology., This work has been supported by the Young 1000 Global Talent Recruitment Program of the Ministry of Education of China, the National Natural Science Foundation of China (grants no. 61502326, 41550110223, 11661131002), the Jiangsu Government (grant no. BK20150343), and the Ministry of Finance of China (grant no. SX21400213).

Chemical vapor deposition (CVD) is one of the most common techniques to grow large-area hexagonal boron nitride (h-BN). However, the substrate on which the h-BN is grown plays an important role in the electrical properties of this insulating material. The high temperature used during the CVD process produces the polycrystallization of the metallic substrates, which may modify the electrical properties of the h-BN/metal from one grain to another. In this work, we compare the electrical properties of CVD-grown multilayer h-BN on three different metallic substrates: Pt, Cu and Fe. This study reveals that the properties of h-BN based devices variate remarkably from one grain to another. On the contrary, this behavior is not relevant for when using Cu and Fe. On the other hand, we have also studied the resistive switching behavior in Au/Ti/h-BN/Pt, Au/Ti/h-BN/Cu, and Au/Ag/h-BN/Fe devices, and demonstrated low variability when they are grown on the same metallic grain.

Hexagonal boron nitride (h-BN) is an attractive insulating material for nanoelectronic devices due to its high reliability as dielectric and excellent compatibility with other two dimensional (2D) materials (e.g. graphene, MoS2). Multilayer h-BN stacks have been readily grown on Cu and Pt substrates via chemical vapor deposition (CVD) approach, confirming its potential for wafer scale integration. However, the growth of h-BN on other substrates needs to be also achieved in order to expand the use of this material. Recently, the CVD growth of monolayer h-BN on Fe substrates was reported, but it just focused on material structure characterization. Here we present the first fabrication of electronic devices using multilayer h-BN dielectric stacks grown on Fe foils. We fabricate and characterize resistive switching (RS) devices based on Au/Ag/h-BN/Fe nanojunctions, and observe the coexistence of both volatile and non-volatile RS depending on the electrode to which the bias is applied. The characteristics measured agree well with those simulated via SIM2RRAM software and QPC modeling, and the cycle-to-cycle variability is slightly lower than that of transition metal oxide based RS devices.