Yanfeng Ji

Boron nitride as two dimensional dielectric: Reliability and dielectric breakdown

Boron Nitride (BN) is a two dimensional insulator with excellent chemical, thermal, mechanical, and optical properties, which make it especially attractive for logic device applications. Nevertheless, its insulating properties and reliability as a dielectric material have never been analyzed in-depth. Here, we present the first thorough characterization of BN as dielectric film using nanoscale and device level experiments complementing with theoretical study. Our results reveal that BN is extremely stable against voltage stress, and it does not show the reliability problems related to conventional dielectrics like HfO2, such as charge trapping and detrapping, stress induced leakage current, and untimely dielectric breakdown. Moreover, we observe a unique layer-by-layer dielectric breakdown, both at the nanoscale and device level. These findings may be of interest for many materials scientists and could open a new pathway towards two dimensional logic device applications.

Nanoscale characterization of PM2.5 airborne pollutants reveals high adhesiveness and aggregation capability of soot particles

In 2012 air pollutants were responsible of seven million human death worldwide, and among them particulate matter with an aerodynamic diameter of 2.5 micrometers or less (PM2.5) are the most hazardous because they are small enough to invade even the smallest airways and penetrate to the lungs. During the last decade the size, shape, composition, sources and effect of these particles on human health have been studied. However, the noxiousness of these particles not only relies on their chemical toxicity, but particle morphology and mechanical properties affect their thermodynamic behavior, which has notable impact on their biological activity. Therefore, correlating the physical, mechanical and chemical properties of PM2.5 airborne pollutants should be the first step to characterize their interaction with other bodies but, unfortunately, such analysis has never been reported before. In this work, we present the first nanomechanical characterization of the most abundant and universal groups of PM2.5 airborne pollutants and, by means of atomic force microscope (AFM) combined with other characterization tools, we observe that fluffy soot aggregates are the most sticky and unstable. Our experiments demonstrate that such particles show strong adhesiveness and aggregation, leading to a more diverse composition and compiling all possible toxic chemicals.

Ageing mechanisms and reliability of graphene-based electrodes

The development of flexible transparent electrodes for next generation devices has been appointed as the major topic in carbon electronics research for the next five years. Among all candidate materials tested to date, graphene and graphene based nanocomposites have shown the highest performance. Although some incipient anti-oxidation tests have been reported, in-deep ageing studies to assess the reliability of carbon-based electrodes have never been performed before. In this work, we present a disruptive methodology to assess the ageing mechanisms of graphene electrodes, which is also extensible to other carbonbased and two-dimensional materials. After performing accelerated oxidative tests, we exhaustively analyze the yield of the electrodes combining nanoscale and device level experiments with Weibull probabilistic analyses and tunneling current simulation, based on the Fowler-Nordheim/Direct-Tunneling models. Our experiments and calculations reveal that an ultra-thin oxide layer can be formed on the pristine surface of graphene. We quantitatively analyze the consequences of this layer on the properties of the electrodes, and observed a change in the conduction mode at the interface (from Ohmic to Schottky), an effect that should be considered in the design of future graphene-based devices. Future mass production of carbon-based devices should include similar reliability studies, and the methodologies presented here (including the accelerated tests, characterization and modeling) may help other scientists to move from lab prototypes towards industrial device production.

A Future Way of Storing Information: Resistive Random Access Memory.

Electronic information storage has become one of the major needs of modern societies, and it represents a market of more than US

Characterization of the photocurrents generated by the laser of atomic force microscopes

5 billion [1]. Among all of the existing technologies, flash memory is the most widespread because of its simple structure, high integration, and fast speed [2]. The core cell of this device is based on the charge and discharge of a capacitor using a transistor as a tiny switch [3], but, as the devices are scaled down, this configuration presents some physical limitations [4]. Therefore, new ways for information storage are required, and, among all existing nonvolatile memories, one that has raised major expectations in recent years is resistive random access memory (RRAM) [5]. In this article, we present the working principle and functioning of the most promising RRAM devices for future information storage.

Note: Fabrication of a fast-response and user-friendly environmental chamber for atomic force microscopes

The conductive atomic force microscope (CAFM) has become an essential tool for the nanoscale electronic characterization of many materials and devices. When studying photoactive samples, the laser used by the CAFM to detect the deflection of the cantilever can generate photocurrents that perturb the current signals collected, leading to unreliable characterization. In metal-coated semiconductor samples, this problem is further aggravated, and large currents above the nanometer range can be observed even without the application of any bias. Here we present the first characterization of the photocurrents introduced by the laser of the CAFM, and we quantify the amount of light arriving to the surface of the sample. The mechanisms for current collection when placing the CAFM tip on metal-coated photoactive samples are also analyzed in-depth. Finally, we successfully avoided the laser-induced perturbations using a two pass technique: the first scan collects the topography (laser ON) and the second collects the current (laser OFF). We also demonstrate that CAFMs without a laser (using a tuning fork for detecting the deflection of the tip) do not have this problem.

Nanoscale homogeneity and degradation process of two dimensional atomically thin hexagonal boron nitride dielectric stacks

The atomic force microscope is one of the most widespread tools in science, but many suppliers do not provide a competitive solution to make experiments in controlled atmospheres. Here, we provide a solution to this problem by fabricating a fast-response and user-friendly environmental chamber. We corroborate the correct functioning of the chamber by studying the formation of local anodic oxidation on a silicon sample (biased under opposite polarities), an effect that can be suppressed by measuring in a dry nitrogen atmosphere. The usefulness of this chamber goes beyond the example here presented, and it could be used in many other fields of science, including physics, mechanics, microelectronics, nanotechnology, medicine, and biology.

On the ageing mechanisms of graphene-on-metal electrodes

In this paper we analyze the reliability of atomically thin hexagonal boron nitride (A-BN) dielectric stacks subjected to electrical stresses. The 2D insulating stacks are grown by chemical vapor deposition, meaning that (unlike exfoliated nanosheets) they can cover large areas and are suitable for the fabrication of scalable devices using photolithography tools. By comparing HfO2 and A-BN stacks with similar equivalent oxide thickness we find that the 2D dielectric shows a striking stable conduction when subjected to sequences of ramped voltage stresses, indicating that it is much more stable versus electrical-field-induced defects. These results point A-BN as superb dielectric for electronic devices.

New insights on the origin of Resistive switching in HfO 2 thin films: The role of local mechanical strength

Graphene electrodes are being massively introduced in a wide range of electronic devices. On the contrary, exhaustive ageing studies, which are necessary prior to device commercialization, have never been performed before. Here we present the first complete reliability study of a carbon-based electrode, and the main ageing mechanisms are discussed by means of accelerated tests, nanoscale and device level experiments, as well as Weibull statistical analyses and tunneling current simulations. Our results indicate that the formation of an ultra-thin oxide layer on pristine graphene and tall oxide hillocks at graphene point defects are the main two ageing mechanisms, and they differently affect the electron transfer in the graphene sheets.

On the use of two dimensional hexagonal boron nitride as dielectric

In the Resistive Random Access Memory (RRAM) devices, switching between high and low resistive states is controlled by the processes of disruption and restoration of a conductive filament, which could be formed through the dielectric film. In this study, we demonstrate that RS is strongly linked to the mechanical properties of the insulator that should be considered in the design of flexible memories, which are usually subjected to significant mechanical strains.