Davide Mencarelli

A New 3-D Transmission Line Matrix Scheme for the Combined SchrÖdinger–Maxwell Problem in the Electronic/Electromagnetic Characterization of Nanodevices

This paper introduces a novel technique in which Maxwell equations, discretized by the transmission line matrix method in a 3-D domain, are coupled to the Schrodinger equation and simultaneously solved. The aim is to develop a method that accounts for deterministic electromagnetic field dynamics together with the quantum phenomena, which are typical of nanodevices.

Broadband Microwave Attenuator Based on Few Layer Graphene Flakes

This paper presents the design and fabrication of a broadband microstrip attenuator, operating at 1-20 GHz, based on few layer graphene flakes. The RF performance of the attenuator has been analyzed in depth. In particular, the use of graphene as a variable resistor is discussed and experimentally characterized at microwave frequencies. The structure of the graphene-based attenuator integrates a micrometric layer of graphene flakes deposited on an air gap in a microstrip line. As highlighted in the experiments, the graphene film can range from being a discrete conductor to a highly resistive material, depending on the externally applied voltage. As experimental evidence, it is verified that the application of a proper voltage through two bias tees changes the surface resistivity of graphene, and induces a significant change of insertion loss of the microstrip attenuator.

Calibration Protocol for Broadband Near-Field Microwave Microscopy

In this paper, we describe how to define and build a set of known loads to be used in near-field microwave microscopy. Such loads are necessary to set up a microwave calibration kit, enabling local quantitative measurements by the microscope. The proposed protocol is validated through the microscopy system we have recently developed that combines a scanning tunneling microscope and a 70-GHz vector network analyzer.

A tunable microwave slot antenna based on graphene

The paper presents the experimental and modeling results of a microwave slot antenna in a coplanar configuration based on graphene. The antennas are fabricated on a 4 in. high-resistivity Si wafer, with a ∼300 nm SiO2 layer grown through thermal oxidation. A CVD grown graphene layer is transferred on the SiO2. The paper shows that the reflection parameter of the antenna can be tuned by a DC voltage. 2D radiation patterns at various frequencies in the X band (8–12 GHz) are then presented using as antenna backside a microwave absorbent and a metalized surface. Although the radiation efficiency is lower than a metallic antenna, the graphene antenna is a wideband antenna while the metal antennas with the same geometry and working at the same frequencies are narrowband.

Boundary Immittance Operators for the SchrÖdinger–Maxwell Problem of Carrier Dynamics in Nanodevices

We have recently introduced a novel algorithm in which Maxwell equations, discretized by the transmission line matrix method, are coupled to the Schrodinger equation and simultaneously solved. The goal of this study is to develop a method that accounts for deterministic electromagnetic field dynamics together with the quantum coherent transport in the nanoscale environment. We present exact boundary conditions for the Schrodinger equation that rigorously model absorption and injection of charge at the terminal planes. As a nontrivial application of the above concept, we show the dynamics of a charge wavepacket from source to drain electrodes in a carbon nanotube transistor environment. We then compare computed characteristics with experimental ones reported in the literature.

Scattering matrix approach to multichannel transport in many lead graphene nanoribbons

Multichannel analysis of graphene nanoribbons (GNR), often required for describing applications to practical devices, constitutes a heavy computational task, even in a simplified framework like that provided by discrete or nearest neighbour models. Scattering (S) matrix techniques, widely used for quantum transport in low dimensional systems and for the computation of electromagnetic fields, is shown here to provide a powerful formal platform for the analysis, and, in principle, the synthesis, of GNR multiport circuits. Periodic modes, solutions of GNR waveguides, are demonstrated to obey charge conservation and reciprocity constraints corresponding to unitary and symmetry properties of the S-matrix, under proper normalization conditions. We propose a systematic use of this approach to deal with problems such as scattering by lattice defects, the presence of external applied fields, crossing GNRs and T-junctions.

High Resolution Scanning Microwave Microscopy for Applications in Liquid Environment

In this work, we demonstrate a hybrid scanning tunneling microscope/near field scanning microwave microscope featuring nanometric resolution in liquid environment. The system performs an ultra-broadband microwave analysis, since the frequency is swept (up to 50 GHz) for each spatial point of the sample under measurement. A conversion in time-domain allows to disentangle near-field and far-field probe-sample interactions. Results are reported for highly oriented pyrolitic graphite immersed in water, and demonstrate microwave nanometric resolution in spite of the presence of the losses induced by the aqueous environment.

Broadband Scanning Microwave Microscopy investigation of graphene

In this work we describe the application of a dual-channel scanning probe microscope performing simultaneously Scanning Tunneling Microscopy (STM) and wide-band Near Field Scanning Microwave Microscopy (wide-band SMM) - developed by ourselves- to a graphene flake. In our system we introduce a conversion in Time-Domain to discriminate the desired information, achieving high quality microwave images with nanometric resolution. The graphene sample is deposited on a substrate of SiO2 with an additional deposition of gold (a contact finger). The preliminary measurements seem to show evidence of localized change of impedance near the edge of the flake.

A multichannel model for the self-consistent analysis of coherent transport in graphene nanoribbons.

In this contribution, we analyze the multichannel coherent transport in graphene nanoribbons (GNRs) by a scattering matrix approach. We consider the transport properties of GNR devices of a very general form, involving multiple bands and multiple leads. The 2D quantum transport over the whole GNR surface, described by the Schrödinger equation, is strongly nonlinear as it implies calculation of self-generated and externally applied electrostatic potentials, solutions of the 3D Poisson equation. The surface charge density is computed as a balance of carriers traveling through the channel at all of the allowed energies. Moreover, formation of bound charges corresponding to a discrete modal spectrum is observed and included in the model. We provide simulation examples by considering GNR configurations typical for transistor devices and GNR protrusions that find an interesting application as cold cathodes for X-ray generation. With reference to the latter case, a unified model is required in order to couple charge transport and charge emission. However, to a first approximation, these could be considered as independent problems, as in the example.

Vertically aligned CNT-Cu nano-composite material for stacked through-silicon-via interconnects

For future miniaturization of electronic systems using 3D chip stacking, new fine-pitch materials for through-silicon-via (TSV) applications are likely required. In this paper, we propose a novel carbon nanotube (CNT)/copper nanocomposite material consisting of high aspect ratio, vertically aligned CNT bundles coated with copper. These bundles, consisting of hundreds of tiny CNTs, were uniformly coated by copper through electroplating, and aspect ratios as high as 300:1 were obtained. The resistivity of this nanomaterial was found to be as low as ∼10(-8) Ω m, which is of the same order of magnitude as the resistivity of copper, and its temperature coefficient was found to be only half of that of pure copper. The main advantage of the composite TSV nanomaterial is that its coefficient of thermal expansion (CTE) is similar to that of silicon, a key reliability factor. A finite element model was set up to demonstrate the reliability of this composite material and thermal cycle simulations predicted very promising results. In conclusion, this composite nanomaterial appears to be a very promising material for future 3D TSV applications offering both a low resistivity and a low CTE similar to that of silicon.