S. Di Franco

Bio: S. Di Franco is an academic researcher. The author has contributed to research in topics: Schottky diode & Schottky barrier. The author has an hindex of 12, co-authored 35 publications receiving 442 citations.

Temperature dependence of the specific resistance in Ti∕Al∕Ni∕Au contacts on n-type GaN

Abstract: The temperature dependence of the specific resistance ρc in annealed Ti∕Al∕Ni∕Au contacts on n-type GaN was monitored, obtaining information on the current transport mechanisms. After annealing at 600°C, the contacts exhibited a rectifying behavior and became Ohmic only after high temperature processes (>700°C), with ρc in the low 10−5Ωcm2 range. The results demonstrated that the current transport is ruled by two different mechanisms: thermoionic field emission occurs in the contacts annealed at 600°C, whereas field emission dominates after higher temperature annealing. The significant physical parameters related to the current transport, i.e., the Schottky barrier height and the carrier concentration under the contact, could be determined. In particular, a reduction of the Schottky barrier from 1.21eV after annealing at 600°Cto0.81eV at 800°C was determined, accompanied by a strong increase of the carrier concentration, i.e., from 2×1018cm−3 in the as-prepared sample to 4.6×1019cm−3 in the annealed conta...

Microscopic mechanisms of graphene electrolytic delamination from metal substrates

Abstract: In this paper, hydrogen bubbling delamination of graphene (Gr) from copper using a strong electrolyte (KOH) water solution was performed, focusing on the effect of the KOH concentration (CKOH) on the Gr delamination rate. A factor of ∼10 decrease in the time required for the complete Gr delamination from Cu cathodes with the same geometry was found increasing CKOH from ∼0.05 M to ∼0.60 M. After transfer of the separated Gr membranes to SiO2 substrates by a highly reproducible thermo-compression printing method, an accurate atomic force microscopy investigation of the changes in Gr morphology as a function of CKOH was performed. Supported by these analyses, a microscopic model of the delamination process has been proposed, where a key role is played by graphene wrinkles acting as nucleation sites for H2 bubbles at the cathode perimeter. With this approach, the H2 supersaturation generated at the electrode for different electrolyte concentrations was estimated and the inverse dependence of td on CKOH was quantitatively explained. Although developed in the case of Cu, this analysis is generally valid and can be applied to describe the electrolytic delamination of graphene from several metal substrates.

Effect of temperature–bias annealing on the hysteresis and subthreshold behavior of multilayer MoS2 transistors

Abstract: Schematic representation of a back-gated multilayer MoS2 field effect transistor (left) and transfer characteristics (right) measured at 25 °C on an as-prepared device and after the temperature–bias annealing at 200 °C under a positive gate bias ramp from 0 V to +20 V. The transfer characteristics (ID–VG) of multilayers MoS2 transistors with a SiO2/Si backgate and Ni source/drain contacts have been measured on as-prepared devices and after annealing at different temperatures (Tann from 150 °C to 200 °C) under a positive bias ramp (VG from 0 V to +20 V). Larger Tann resulted in a reduced hysteresis of the ID–VG curves (from ∼11 V in the as-prepared sample to ∼2.5 V after Tann at 200 °C). The field effect mobility (∼30 cm2 V–1 s–1) remained almost unchanged after the annealing. On the contrary, the subthreshold characteristics changed from the common n-type behaviour in the as-prepared device to the appearance of a low current hole inversion branch after annealing. This latter effect indicates a modification of the Ni/MoS2 contact that can be explained by the formation of a low density of regions with reduced Schottky barrier height (SBH) for holes embedded in a background with low SBH for electrons. Furthermore, a temperature dependent analysis of the subthreshold characteristics revealed a reduction of the interface traps density from ∼9 × 1011 eV–1cm–2in the as-prepared device to ∼2 × 1011 eV–1cm–2after the 200 °C temperature–bias annealing, which is consistent with the observed hysteresis reduction.

Photocurrent gain in 4H-SiC interdigit Schottky UV detectors with a thermally grown oxide layer

Abstract: A large photocurrent increase in 4H-SiC interdigit Schottky UV detectors was observed in the presence of a thermally grown silicon oxide layer. In particular, internal quantum efficiency higher than unity indicated the presence of an internal gain strictly correlated with the presence of the superficial oxide on SiC. Moreover, a long recovery time, in the range of 10–19s, was evaluated by fall-time photocurrent measurements due to the detrapping of charges in the oxide after the irradiation switching off. The photoresponse of the device was analytically described considering the lowering of the surface potential barrier due to charges trapped at the oxide/semiconductor interface.

A Comprehensive Review of Semiconductor Ultraviolet Photodetectors: From Thin Film to One-Dimensional Nanostructures

Abstract: Ultraviolet (UV) photodetectors have drawn extensive attention owing to their applications in industrial, environmental and even biological fields. Compared to UV-enhanced Si photodetectors, a new generation of wide bandgap semiconductors, such as (Al, In) GaN, diamond, and SiC, have the advantages of high responsivity, high thermal stability, robust radiation hardness and high response speed. On the other hand, one-dimensional (1D) nanostructure semiconductors with a wide bandgap, such as β-Ga2O3, GaN, ZnO, or other metal-oxide nanostructures, also show their potential for high-efficiency UV photodetection. In some cases such as flame detection, high-temperature thermally stable detectors with high performance are required. This article provides a comprehensive review on the state-of-the-art research activities in the UV photodetection field, including not only semiconductor thin films, but also 1D nanostructured materials, which are attracting more and more attention in the detection field. A special focus is given on the thermal stability of the developed devices, which is one of the key characteristics for the real applications.

Flexible Graphene-Based Wearable Gas and Chemical Sensors

Abstract: Wearable electronics is expected to be one of the most active research areas in the next decade; therefore, nanomaterials possessing high carrier mobility, optical transparency, mechanical robustness and flexibility, lightweight, and environmental stability will be in immense demand. Graphene is one of the nanomaterials that fulfill all these requirements, along with other inherently unique properties and convenience to fabricate into different morphological nanostructures, from atomically thin single layers to nanoribbons. Graphene-based materials have also been investigated in sensor technologies, from chemical sensing to detection of cancer biomarkers. The progress of graphene-based flexible gas and chemical sensors in terms of material preparation, sensor fabrication, and their performance are reviewed here. The article provides a brief introduction to graphene-based materials and their potential applications in flexible and stretchable wearable electronic devices. The role of graphene in fabricating ...

Surface chemistry and catalysis confined under two-dimensional materials

Abstract: Two-dimensional (2D) materials are characterised by their strong intraplanar bonding but weak interplanar interaction. Interfaces between neighboring 2D layers or between 2D overlayers and substrate surfaces provide intriguing confined spaces for chemical processes, which have stimulated a new area of “chemistry under 2D cover”. In particular, well-defined 2D material overlayers such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have been deposited on solid surfaces, which can be used as model systems to understand the new chemistry. In the present review, we first show that many atoms and molecules can intercalate ultrathin 2D materials supported on solid surfaces and the space under the 2D overlayers has been regarded as a 2D nanocontainer. Moreover, chemical reactions such as catalytic reactions, surface adlayer growth, chemical vapor deposition, and electrochemical reactions occur in the 2D confined spaces, which further act as 2D nanoreactors. It has been demonstrated that surface chemistry and catalysis are strongly modulated by the 2D covers, resulting in weakened molecule adsorption and enhanced surface reactions. Finally, we conclude that the confinement effect of the 2D cover leads to new chemistry in a small space, such as “catalysis under cover” and “electrochemistry under cover”. These new concepts enable us to design advanced nanocatalysts encapsulated with 2D material shells which may present improved performance in many important processes of heterogeneous catalysis, electrochemistry, and energy conversion.

Production and processing of graphene and related materials

Abstract: © 2020 The Author(s). We present an overview of the main techniques for production and processing of graphene and related materials (GRMs), as well as the key characterization procedures. We adopt a 'hands-on' approach, providing practical details and procedures as derived from literature as well as from the authors' experience, in order to enable the reader to reproduce the results. Section I is devoted to 'bottom up' approaches, whereby individual constituents are pieced together into more complex structures. We consider graphene nanoribbons (GNRs) produced either by solution processing or by on-surface synthesis in ultra high vacuum (UHV), as well carbon nanomembranes (CNM). Production of a variety of GNRs with tailored band gaps and edge shapes is now possible. CNMs can be tuned in terms of porosity, crystallinity and electronic behaviour. Section II covers 'top down' techniques. These rely on breaking down of a layered precursor, in the graphene case usually natural crystals like graphite or artificially synthesized materials, such as highly oriented pyrolythic graphite, monolayers or few layers (FL) flakes. The main focus of this section is on various exfoliation techniques in a liquid media, either intercalation or liquid phase exfoliation (LPE). The choice of precursor, exfoliation method, medium as well as the control of parameters such as time or temperature are crucial. A definite choice of parameters and conditions yields a particular material with specific properties that makes it more suitable for a targeted application. We cover protocols for the graphitic precursors to graphene oxide (GO). This is an important material for a range of applications in biomedicine, energy storage, nanocomposites, etc. Hummers' and modified Hummers' methods are used to make GO that subsequently can be reduced to obtain reduced graphene oxide (RGO) with a variety of strategies. GO flakes are also employed to prepare three-dimensional (3d) low density structures, such as sponges, foams, hydro- or aerogels. The assembly of flakes into 3d structures can provide improved mechanical properties. Aerogels with a highly open structure, with interconnected hierarchical pores, can enhance the accessibility to the whole surface area, as relevant for a number of applications, such as energy storage. The main recipes to yield graphite intercalation compounds (GICs) are also discussed. GICs are suitable precursors for covalent functionalization of graphene, but can also be used for the synthesis of uncharged graphene in solution. Degradation of the molecules intercalated in GICs can be triggered by high temperature treatment or microwave irradiation, creating a gas pressure surge in graphite and exfoliation. Electrochemical exfoliation by applying a voltage in an electrolyte to a graphite electrode can be tuned by varying precursors, electrolytes and potential. Graphite electrodes can be either negatively or positively intercalated to obtain GICs that are subsequently exfoliated. We also discuss the materials that can be amenable to exfoliation, by employing a theoretical data-mining approach. The exfoliation of LMs usually results in a heterogeneous dispersion of flakes with different lateral size and thickness. This is a critical bottleneck for applications, and hinders the full exploitation of GRMs produced by solution processing. The establishment of procedures to control the morphological properties of exfoliated GRMs, which also need to be industrially scalable, is one of the key needs. Section III deals with the processing of flakes. (Ultra)centrifugation techniques have thus far been the most investigated to sort GRMs following ultrasonication, shear mixing, ball milling, microfluidization, and wet-jet milling. It allows sorting by size and thickness. Inks formulated from GRM dispersions can be printed using a number of processes, from inkjet to screen printing. Each technique has specific rheological requirements, as well as geometrical constraints. The solvent choice is critical, not only for the GRM stability, but also in terms of optimizing printing on different substrates, such as glass, Si, plastic, paper, etc, all with different surface energies. Chemical modifications of such substrates is also a key step. Sections IV-VII are devoted to the growth of GRMs on various substrates and their processing after growth to place them on the surface of choice for specific applications. The substrate for graphene growth is a key determinant of the nature and quality of the resultant film. The lattice mismatch between graphene and substrate influences the resulting crystallinity. Growth on insulators, such as SiO2, typically results in films with small crystallites, whereas growth on the close-packed surfaces of metals yields highly crystalline films. Section IV outlines the growth of graphene on SiC substrates. This satisfies the requirements for electronic applications, with well-defined graphene-substrate interface, low trapped impurities and no need for transfer. It also allows graphene structures and devices to be measured directly on the growth substrate. The flatness of the substrate results in graphene with minimal strain and ripples on large areas, allowing spectroscopies and surface science to be performed. We also discuss the surface engineering by intercalation of the resulting graphene, its integration with Si-wafers and the production of nanostructures with the desired shape, with no need for patterning. Section V deals with chemical vapour deposition (CVD) onto various transition metals and on insulators. Growth on Ni results in graphitized polycrystalline films. While the thickness of these films can be optimized by controlling the deposition parameters, such as the type of hydrocarbon precursor and temperature, it is difficult to attain single layer graphene (SLG) across large areas, owing to the simultaneous nucleation/growth and solution/precipitation mechanisms. The differing characteristics of polycrystalline Ni films facilitate the growth of graphitic layers at different rates, resulting in regions with differing numbers of graphitic layers. High-quality films can be grown on Cu. Cu is available in a variety of shapes and forms, such as foils, bulks, foams, thin films on other materials and powders, making it attractive for industrial production of large area graphene films. The push to use CVD graphene in applications has also triggered a research line for the direct growth on insulators. The quality of the resulting films is lower than possible to date on metals, but enough, in terms of transmittance and resistivity, for many applications as described in section V. Transfer technologies are the focus of section VI. CVD synthesis of graphene on metals and bottom up molecular approaches require SLG to be transferred to the final target substrates. To have technological impact, the advances in production of high-quality large-area CVD graphene must be commensurate with those on transfer and placement on the final substrates. This is a prerequisite for most applications, such as touch panels, anticorrosion coatings, transparent electrodes and gas sensors etc. New strategies have improved the transferred graphene quality, making CVD graphene a feasible option for CMOS foundries. Methods based on complete etching of the metal substrate in suitable etchants, typically iron chloride, ammonium persulfate, or hydrogen chloride although reliable, are time- and resourceconsuming, with damage to graphene and production of metal and etchant residues. Electrochemical delamination in a low-concentration aqueous solution is an alternative. In this case metallic substrates can be reused. Dry transfer is less detrimental for the SLG quality, enabling a deterministic transfer. There is a large range of layered materials (LMs) beyond graphite. Only few of them have been already exfoliated and fully characterized. Section VII deals with the growth of some of these materials. Amongst them, h-BN, transition metal tri- and di-chalcogenides are of paramount importance. The growth of h-BN is at present considered essential for the development of graphene in (opto) electronic applications, as h-BN is ideal as capping layer or substrate. The interesting optical and electronic properties of TMDs also require the development of scalable methods for their production. Large scale growth using chemical/physical vapour deposition or thermal assisted conversion has been thus far limited to a small set, such as h-BN or some TMDs. Heterostructures could also be directly grown.