Alessandro Betti

Lateral graphene-hBCN heterostructures as a platform for fully two-dimensional transistors.

We propose that lateral heterostructures of single-atomic-layer graphene and hexagonal boron-carbon-nitrogen (hBCN) domains, can represent a powerful platform for the fabrication and the technological exploration of real two-dimensional field-effect transistors. Indeed, hBCN domains have an energy bandgap between 1 and 5 eV, and are lattice-matched with graphene; therefore they can be used in the channel of a FET to effectively inhibit charge transport when the transistor needs to be switched off. We show through ab initio and atomistic simulations that a FET with a graphene-hBCN-graphene heterostructure in the channel can exceed the requirements of the International Technology Roadmap for Semiconductors for logic transistors at the 10 and 7 nm technology nodes. Considering the main figures of merit for digital electronics, a FET with gate length of 7 nm at a supply voltage of 0.6 V exhibits I(on)/I(off) ratio larger than 10(4), intrinsic delay time of about 0.1 ps, and a power-delay-product close to 0.1 nJ/m. More complex graphene-hBCN heterostructures can allow the realization of different multifunctional devices, translating on a truly two-dimensional structure some of the device principles proposed during the first wave of nanoelectronics based on III-V heterostructures, as for example the resonant tunneling FET.

Simulation of hydrogenated graphene field-effect transistors through a multiscale approach

In this work, we present a performance analysis of field-effect transistors (FETs) based on recently fabricated 100% hydrogenated graphene (the so-called graphane) and theoretically predicted semihydrogenated graphene (i.e., graphone). The approach is based on accurate calculations of the energy bands by means of GW approximation, subsequently fitted with a three-nearest neighbor sp(3) tight-binding Hamiltonian, and finally used to compute ballistic transport in transistors based on functionalized graphene. Due to the large energy gap, the proposed devices have many of the advantages provided by one-dimensional graphene nanoribbon FETs, such as large I-on and I-on/I-off ratios, reduced band-to-band tunneling, without the corresponding disadvantages in terms of prohibitive lithography and patterning requirements for circuit integration.

Atomistic Investigation of Low-Field Mobility in Graphene Nanoribbons

We have investigated the main scattering mechanisms affecting the mobility in graphene nanoribbons using detailed atomistic simulations. We have considered carrier scattering due to acoustic and optical phonons, edge roughness, single defects, and ionized impurities, and we have defined a methodology based on simulations of statistically meaningful ensembles of nanoribbon segments. Edge disorder heavily affects the mobility at room temperature in narrower nanoribbons, whereas charged impurities and phonons are hardly the limiting factors. Results are favorably compared with the few experiments available in the literature.

Strong mobility degradation in ideal graphene nanoribbons due to phonon scattering

We investigate the low-field phonon-limited mobility in armchair graphene nanoribbons (GNRs) using full-band electron and phonon dispersion relations. We show that lateral confinement suppresses the intrinsic mobility of GNRs to values typical of common bulk semiconductors, and very far from the impressive experiments on two-dimensional graphene. 1 nm-wide suspended GNRs exhibit a mobility close to 500 cm2/V s at room temperature, whereas 1 nm-wide GNRs deposited on HfO2 exhibit a mobility of 60 cm2/V s due to surface phonons. We also show the occurrence of polaron formation, leading to band gap renormalization of ≈118 meV for 1-nm-wide armchair GNRs.

Shot Noise Suppression in Quasi-One-Dimensional Field-Effect Transistors

We present a novel method for the evaluation of shot noise in quasi-1-D field-effect transistors, such as those based on carbon nanotubes and silicon nanowires. The method is derived by using a statistical approach within the second quantization formalism and allows the inclusion of both the effects of Pauli exclusion and Coulomb repulsion among charge carriers. This way, it extends the Landauer-Buttiker approach by explicitly including the effect of Coulomb repulsion on noise. We implement the method through the self-consistent solution of the 3-D Poisson and transport equations within the nonequilibrium Greens function framework and a Monte Carlo procedure for populating injected electron states. We show that the combined effect of Pauli and Coulomb interactions reduces shot noise in strong inversion down to 23% of the full shot noise for a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion would lead to an overestimation of noise up to 180%.

Nanodevices in Flatland: Two-dimensional graphene-based transistors with high I on /I off ratio

We present a multi-scale investigation of graphene-based transistors with a hexagonal boron-carbon-nitride (h-BCN) barrier in the channel. Our approach exploits ab-initio calculations for an accurate extraction of energy bands and tight-binding simulations in order to compute charge transport. We show that the h-BCN barrier inhibits the ambipolar behavior of graphene transistors, leading to a large Ion/Ioff ratio, within the ITRS roadmap specifications for future semiconductor technology nodes.

Drift velocity peak and negative differential mobility in high field transport in graphene nanoribbons explained by numerical simulations

We present numerical simulations of high field transport in both suspended and deposited armchair graphene nanoribbon (A-GNR) on HfO2 substrate. Drift velocity in suspended GNR does not saturate at high electric field (F), but rather decreases, showing a maximum for F ≈ 10 kV/cm. Deposition on HfO2 strongly degrades the drift velocity by up to a factor ≈10 with respect to suspended GNRs in the low-field regime, whereas at high fields, drift velocity approaches the intrinsic value expected in suspended GNRs. Even in the assumption of perfect edges, the obtained mobility is far behind what expected in two-dimensional graphene, and is further reduced by surface optical phonons.

The principle of least cognitive action

By and large, the interpretation of learning as a computational process taking place in both humans and machines is primarily provided in the framework of statistics. In this paper, we propose a radically different perspective in which the emergence of learning is regarded as the outcome of laws of nature that govern the interactions of intelligent agents with their own environment. We introduce a natural learning theory based on the principle of least cognitive action, which is inspired to the related mechanical principle, and to the Hamiltonian framework for modeling the motion of particles. The introduction of the kinetic and of the potential energy leads to a surprisingly natural interpretation of learning as a dissipative process. The kinetic energy reflects the temporal variation of the synaptic connections, while the potential energy is a penalty that describes the degree of satisfaction of the environmental constraints. The theory gives a picture of learning in terms of the energy balancing mechanisms, where the novel notions of boundary and bartering energies are introduced. Finally, as an example of application of the theory, we show that the supervised machine learning scheme can be framed in the proposed theory and, in particular, we show that the Euler-Lagrange differential equations of learning collapse to the classic gradient algorithm on the supervised pairs.

Physical insights on graphene nanoribbon mobility through atomistic simulations

We present an investigation of the main mechanisms which limit mobility in GNR-FETs, by means of atomistic simulations based on the NEGF formalism. In particular, we focus on i) line edge roughness (LER), ii) single defects; iii) ionized impurities, iv) acoustic and optical phonons. Results show that the effect of ionized impurities is negligible, while phonons, LER and defects largely limits carrier mobility, especially for narrower GNRs.

Full band assessment of phonon-limited mobility in Graphene NanoRibbons

We present a full band investigation of electron-phonon interaction in Graphene NanoRibbons (GNRs) by exploiting a tight-binding description within the deformation potential approximation. We show that a full band approach is required to obtain accurate results: mobility as high as 800 cm2/Vs at room temperature can be achieved for 1 nm-wide ribbons, more than one order of magnitude higher than that obtainable in silicon nanowires, but still not enough to ensure ballistic transport in GNR-based devices.