Numerical simulation of transport and noise in low-dimensional devices for nanoelectronics

This thesis presents the results obtained in the field of the numerical simulation of conduction in low-dimensional carrier systems on different materials. Motivated by the significant interest aroused by the potential of graphene for the development of an alternative technology with margin for improvement larger than the current CMOS, we have taken into account some important issues related to transport in monolayer and bilayer graphene. In the case of monolayer graphene, an analysis of transport in large area samples has been carried out within the envelope function approximation. An efficient numerical method has been developed for the simulation of conduction in the presence of a quite general electrostatic potential. The method provides an alternative to the atomistic approaches, that in the case of the considered do- main would require an enormous computational burden. Some peculiarities of transport in monolayer graphene have been studied in detail, by inscribing them into the framework of the recent theory of non Hermitian Hamiltonians and the of the spontaneous breaking of the PT symmetry. The simulations of transport on bilayer graphene have been oriented to the explanation of scanning gate spectroscopy measurements performed in the presence of the integer quantum Hall effect. We started form a semiclassical model, initially proposed for ordinary two-dimensional systems, and we have developed a numerical method able to reproduce with good accuracy the experimental results and to explain them. The last subject we deal with is the suppression of the shot noise power spectral density in mesoscopic semiconductor devices. We simulated transport in quantum wires defined in GaAs/AlGaAs hetherostructures in the presence of one-dimensional and two-dimensional disorder, investigating the possibility that a completely diffusive transport regime be established in real samples, and thus a 1/3 suppression of the shot noise power spectral density measured. The obtained results clearly indicate that it is unlikely to measure such a suppression and therefore contribute to explain the exisisting disagreement between theory and experiments

High-performance solution of the transport problem in a graphene armchair structure with a generic potential

We propose an efficient numerical method to study the transport properties of armchair graphene ribbons in the presence of a generic external potential. The method is based on a continuum envelope-function description with physical boundary conditions. The envelope functions are computed in the reciprocal space, and the transmission is then obtained with a recursive scattering matrix approach. This allows a significant reduction of the computational time with respect to finite difference simulations.Comment: 12 pages, 6 eps figures. Final published version. Comments and comparisons adde

Armchair graphene nanoribbons: PT-symmetry breaking and exceptional points without dissipation

We consider a single layer graphene nanoribbon with armchair edges in a longitudinally constant external potential and point out that its transport properties can be described by means of an effective non-Hermitian Hamiltonian. We show that this system has some features typical of dissipative systems, namely the presence of exceptional points and of PT-symmetry breaking, although it is not dissipative.Comment: 5 pages, 2 eps figure

Lateral NbS2_2/MoS2_2/NbS2_2 transistors: physical modeling and performance assessment

Reducing the contact resistance of field-effect transistors based on two-dimensional materials is one of the key improvements required to to enable the integration of such transistors in an industrially relevant process. Suitably designed lateral heterojunctions provide an opportunity to independently tailor the contact and channel properties and to mitigate the problem of high contact resistance. Inspired by the recent experimental demonstration of a two-dimensional $p$-type Schottky barrier, here we use quantum transport simulations to estimate the performance of $p$-type transistors in which the channel consists of a lateral heterostructure of NbS$_2$/MoS$_2$/NbS$_2$ (semimetal-semiconductor-semimetal). We find that the gate alignment with the channel is a critical design parameter, strongly influencing the capability of the gate to modulate the Schottky barrier at the MoS$_2$/NbS$_2$ interfaces. This effect is also found to significantly affect the scaling behavior of the device.Comment: 6 pages, 5 figure

Impact of momentum mismatch on 2D van der Waals tunnel field-effect transistors

We numerically investigate electron quantum transport in 2D van der Waals tunnel field-effect-transistors in the presence of lateral momentum mismatch induced by lattice mismatch or rotational misalignment between the two-dimensional layers. We show that a small momentum mismatch induces a threshold voltage shift without altering the subthreshold swing. On the contrary, a large momentum mismatch produces significant potential variations and ON-current reduction. Short-range scattering, such as that due to phonons or system edges, enables momentum variations, thus enhancing interlayer tunneling. The coupling of electrons with acoustic phonons is shown to increase the ON current without affecting the subthreshold swing. In the case of optical phonons, the ON-current increase is accompanied by a subthreshold swing degradation due to the inelastic nature of the scattering

Substitutional p-type doping in NbS2 -MoS2 lateral heterostructures grown by MOCVD

Monolayer MoS2 has attracted significant attention owing to its excellent performance as an n-type semiconductor from the transition metal dichalcogenide (TMDC) family. It is however strongly desired to develop controllable synthesis methods for 2D p-type MoS2, which is crucial for complementary logic applications but remains difficult. In this work, high-quality NbS2-MoS2 lateral heterostructures are synthesized by one-step metal-organic chemical vapor deposition (MOCVD) together with monolayer MoS2 substitutionally doped by Nb, resulting in a p-type doped behavior. The heterojunction shows a p-type transfer characteristic with a high on/off current ratio of approximate to 10(4), exceeding previously reported values. The band structure through the NbS2-MoS2 heterojunction is investigated by density functional theory (DFT) and quantum transport simulations. This work provides a scalable approach to synthesize substitutionally doped TMDC materials and provides an insight into the interface between 2D metals and semiconductors in lateral heterostructures, which is imperative for the development of next-generation nanoelectronics and highly integrated devices

Studio di fattibilita' e analisi delle proprieta' di dispositivi basati su cavita' mesoscopiche in eterostrutture GaAs/AlGaAs

Fin dalla realizzazione della prima eterostruttura (1979) i sistemi di portatori a dimensionalità ridotta hanno ricevuto considerevoli attenzioni da parte della comunità scientifica. Un interesse motivato, in parte, dalla conseguente possibilità di realizzare dispositivi elettronici di nuova generazione, da aggiungere al novero dei cosiddetti "dispositivi emergenti", candidati al subentro ai transistori MOS, in parte dall'opportunità che questi sistemi offrono di indagare le proprietà microscopiche della materia. In presenza di confinamenti così forti divengono infatti evidenti e rilevanti gli aspetti quantistici del processo di conduzione; cosicché, da un lato, è possibile basare su nuovi effetti il funzionamento di un dispositivo, dall'altro estrarre importanti informazioni sulla dinamica dei portatori, le caratteristiche microscopiche del mezzo e le relazioni che intercorrono tra le peculiarità di un sistema descritto classicamente e le sue proprietà nella corrispondente descrizione quantistica. In questo lavoro di tesi, nel quale si fa riferimento esclusivamente a dispositivi mesoscopici realizzati su eterostrutture di tipo AlGaAs/GaAs, verranno esplorate entrambe le possibilità: tramite simulazioni numeriche si procederà a una valutazione della realizzabilità di un nanosensore di campo magnetico e, successivamente, a un'analisi della soppressione del rumore shot in alcune strutture mesoscopiche, con l'obiettivo di investigare alcune caratteristiche di particolare interesse del trasporto

Efficient numerical method to study the transport behavior of a graphene armchair nanoribbon in the presence of a generical potential using an envelope function approach

We describe the numerical method we have developed to efficiently study transport in armchair graphene nanoribbons in the presence of an external potential using an approach based on the solution of the Dirac equation. The original problem with the exact boundary conditions is recasted in an equivalent problem with periodic boundary conditions and a solution in the Fourier space. Applying this technique inside each slice with a longitudinally constant potential and then using a scattering matrix approach, we can obtain the transmission of the device in the presence of a generical potential

Numerical simulation of shot noise in disordered graphene

Following the intriguing results analytically obtained for the shot noise suppression in wide and short graphene samples with doped contacts, efforts were made to achieve an experimental verification, and, while one experiment yielded a clear confirmation of the theory, another one provided data with no clear dependence of the Fano factor on gate voltage. This was attributed to the presence of a disordered potential. Here we perform a numerical study, based on an envelope function analysis, of disordered graphene samples with different aspect ratios, focusing in particular on the dependence of shot noise suppression on gate voltage. We conclude that such a dependence should survive, unless disorder with an unrealistically large amplitude is considered