S. Di Franco

Microscopic mechanisms of graphene electrolytic delamination from metal substrates

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.

Preferential oxidation of stacking faults in epitaxial off-axis (111) 3C-SiC films

The effect of thermal dry oxidation on an off-axis (111) 3C-SiC film have been studied in order to subsequently realize a metal-oxide-semiconductor structure. A morphological characterization of the SiO2 surface, grown at 1200 °C in an O2 flux, pointed out some defect-related effects as a consequence of the preferential oxidation of stacking faults over the (111) 3C-SiC surface. Scanning electron microscopy and atomic force microscopy confirmed such a hypothesis. Stacking faults are seen as promoters of a local polarity inversion in (111) 3C-SiC, from Si- to C-terminated surface, resulting in a higher oxidation rate as compared to defect-free zones.

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

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.

Reduction of the power dissipation in silicon carbide Schottky rectifiers by a dual-metal planar structure

A dual-metal-planar rectifier on 6H–SiC was fabricated using Ti and Ni2Si as Schottky metals. The forward current voltage (I–V) characteristic of the dual-metal devices was comparable with that of Ti diodes. On the other hand, under reverse bias, almost the same leakage current of the Ni2Si rectifiers was achieved, i.e., a factor 1000 lower than that of Ti diodes. The fabricated diodes allowed to obtain a power dissipation of 0.37 W/cm2, significantly reduced with respect to the Ti and Ni2Si diodes dissipation. Moreover, the breakdown voltage was the same as in the planar Ni2Si diode, thus indicating that the planar structure is very efficient in avoiding electric field crowding at the titanium stripes edges.

Potentialities of Nickel Oxide as Dielectric for GaN and SiC Devices

This paper reports on a structural and electrical analysis of nickel oxide (NiO) films grown both on AlGaN/GaN heterostructures and on 4H-SiC epilayers. The films were grown by metal organic chemical vapor deposition (MOCVD). The structural analysis showed epitaxially oriented films over the hexagonal substrates. The electrical characterization of simple devices onto AlGaN/GaN heterostructures enabled to demonstrate a dielectric constant of 11.7 and a reduction of the leakage current in insulated gate structures. On the other hand, epitaxial NiO films grown onto 4H-SiC epilayers exhibited the presence of an interfacial SiO2 layer and twinned NiO grains, and a lower dielectric constant.

Nanoscale electrical mapping of two-dimensional materials by conductive atomic force microscopy for transistors applications

Two-dimensional (2D) materials are currently object of many interests both from a basic and a technological standpoint. In particular, graphene (Gr) and the semiconducting transition metal dichalcogenides (including MoS2, WS2, MoSe2, WSe2) have been widely investigated for transistors applications. As a matter of fact, 2D materials present peculiar nanoscale structural and electrical inhomogeneities, related to the specific synthesis mechanisms and to the interaction with the substrate, which are ultimately reflected in the macroscopic electrical behaviour of electronic devices based on these systems. In this context, conductive atomic force microscopy (CAFM) is the method of choice to investigate the mechanisms of current injection between contacts and 2D materials and/or the lateral homogeneity of 2D materials electrical properties. This paper will discuss some case studies of CAFM applications to Gr and MoS2, to illustrate the potentiality of this characterization method for 2D materials investigation....

Advances in the Fabrication of Large-Area Back-Gated Graphene Field-Effect Transistors on Plastics: Platform for Flexible Electronics and Sensing

Graphene (Gr) is currently one of the most appealing materials as conductive transparent electrode for flexible electronics, thanks to its bendability/stretchability accompanied by small variations of the electrical properties after mechanical deformations. In addition, the field-effect tunable carrier density combined to a high mobility and saturation velocity make it an excellent channel material for field-effect transistors (FETs) even on flexible substrates. By proper design of the device structure (channel length, top- or back-gate configuration), Gr-FETs can be used for high-frequency (RF) electronics or for high-sensitivity chemical, biological, and environmental sensors exploiting transconductance variations in response to the chemi/physisorption of molecular species on Gr channel. In particular, miniaturized and flexible Gr-FET sensors can represent a strong advance with respect to current sensors technology and will be extremely useful for “in situ” applications. Here we report a wafer scale and semiconductor fab compatible processing strategy to fabricate arrays of Gr-FETs on a PEN substrate, adopting a local back-gate configuration, with a thin Al2O3 gate dielectric film deposited at low temperature (100 °C) by plasma-assisted Atomic Layer Deposition (ALD) and transfer of large-area Gr grown by chemical vapor deposition on copper foils. Electrical characterization of the fabricated devices is presented and their suitability for solid ion sensing FET (IS-FET) applications is discussed.

Probing at Nanoscale Underneath the Gate Oxides in 4H-SiC MOS-Based Devices Annealed in N2O and POCl3

In this paper a comparative study of the impact of N2O and POCl3 annealing on the SiO2/SiC system is presented, combining nanoscale electrical characterization of SiC surface doping by scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM) to the conventional capacitance-voltage (C-V) and current-voltage (I-V) measurements on MOS-based devices. A significant reduction of the interface states density (from 1.8×1012 to 5.7×1011 cm-2eV-1) and, correspondingly, an increase in the carrier mobility (from 19 to 108 cm2V-1s-1) was found moving from N2O to POCl3 annealing. Furthermore, SSRM measurements on bare p+-type SiC regions selectively exposed to N2O and POCl3 at high temperature provided the direct demonstration of the incorporation of N or P-related donors in the SiC surface, leading to a partial compensation of substrate acceptors during N2O treatment and to an overcompensation during POCl3 annealing. Finally, cross-sectional SCM profiles performed on epitaxial n-doped 4H-SiC with 45 nm SiO2 (subjected to post deposition annealing in the two ambients) allowed to quantify the active donors concentrations associated to P or N incorporation under the gate oxide, showing almost a factor of ten higher doping (4.5×1018cm-3 vs 5×1017cm-3) in the case of P related donors.

Morphological and Structural Characterization of Graphene Grown by Thermal Decomposition of 4H-SiC (0001) and by C Segregation on Ni

In this paper, we present a nanoscale morphological and structural characterization of few layers of graphene grown by thermal decomposition of off-axis 4H-SiC (0001) and by C segregation on Ni thin films from a solid carbon source. Transmission electron microscopy in different configurations, i.e. cross-section and plan view, was used to get information on the number of graphene layers as well as on the rotational order between the layers and with respect to the substrate. Atomic force microscopy was used to study the changes in the surface morphology produced by thermal annealing. In particular, the density and the height of peculiar corrugations (wrinkles) in the few layers of graphene, formed during the cool down in the thermal process, were investigated.

Evolution of Structural and Electrical Properties of Au/Ni Contacts onto P-GaN after Annealing

In this paper the structural and electrical evolution of Au/Ni contacts to p-type gallium nitride (GaN) upon annealing in different atmospheres was monitored. Rapid annealing of the contacts in an oxidizing atmosphere (N2/O2) resulted into a lower specific contact resistance (ρc) with respect to annealing in non-reacting ambient (Ar). The formation of a nickel oxide (NiO) layer was observed on the surface of the sample annealed in N2/O2 ,while was not present at the interface with p-GaN. The improvement of the contacts was associated with a reduction of the Schottky barrier height from 1.07 eV (Ar annealing) to 0.71 eV (N2/O2 annealing), as determined by the temperature dependence of the ρc. Local electrical measurements demonstrated the formation of inhomogeneous barriers. The electrical measurements were correlated with the different microstructure of the annealed contacts.