Lida Ansari

Simulation of junctionless Si nanowire transistors with 3 nm gate length

Inspired by recent experimental realizations and theoretical simulations of thin silicon nanowire-based devices, we perform proof-of-concept simulations of junctionless gated Si nanowire transistors. Based on first-principles, our primary predictions are that Si-based transistors are physically possible without major changes in design philosophy at scales of ∼1 nm wire diameter and ∼3 nm gate length, and that the junctionless transistor avoids potentially serious difficulties affecting junctioned channels at these length scales. We also present investigations into atomic-level design factors such as dopant positioning and concentration.Inspired by recent experimental realizations and theoretical simulations of thin silicon nanowirebased devices, we perform predictive first-principles simulations of junctionless gated Si nanowire transistors. Our primary predictions are that Si-based transistors are physically possible without major changes in design philosophy at scales of ∼1 nm wire diameter and ∼3 nm gate length, and that the junctionless transistor [1, 2] may be the only physically sensible design at these length scales. We also present investigations into atomic-level design factors such as dopant positioning and concentration.

A proposed confinement modulated gap nanowire transistor based on a metal (tin).

Energy bandgaps are observed to increase with decreasing diameter due to quantum confinement in quasi-one-dimensional semiconductor nanostructures or nanowires. A similar effect is observed in semimetal nanowires for sufficiently small wire diameters: A bandgap is induced, and the semimetal nanowire becomes a semiconductor. We demonstrate that on the length scale on which the semimetal-semiconductor transition occurs, this enables the use of bandgap engineering to form a field-effect transistor near atomic dimensions and eliminates the need for doping in the transistors source, channel, or drain. By removing the requirement to supply free carriers by introducing dopant impurities, quantum confinement allows for a materials engineering to overcome the primary obstacle to fabricating sub-5 nm transistors, enabling aggressive scaling to near atomic limits.

Transport properties and electrical device characteristics with the TiMeS computational platform: Application in silicon nanowires

Nanoelectronics requires the development of a priori technology evaluation for materials and device design that takes into account quantum physical effects and the explicit chemical nature at the atomic scale. Here, we present a cross-platform quantum transport computation tool. Using first-principles electronic structure, it allows for flexible and efficient calculations of materials transport properties and realistic device simulations to extract current-voltage and transfer characteristics. We apply this computational method to the calculation of the mean free path in silicon nanowires with dopant and surface oxygen impurities. The dependence of transport on basis set is established, with the optimized double zeta polarized basis giving a reasonable compromise between converged results and efficiency. The current-voltage characteristics of ultrascaled (3 nm length) nanowire-based transistors with p-i-p and p-n-p doping profiles are also investigated. It is found that charge self-consistency affects the device characteristics more significantly than the choice of the basis set. These devices yield source-drain tunneling currents in the range of 0.5 nA (p-n-p junction) to 2 nA (p-i-p junction), implying that junctioned transistor designs at these length scales would likely fail to keep carriers out of the channel in the off-state.

Strain induced effects on electronic structure of semi-metallic and semiconducting tin nanowires

Semimetal nanowires are known to undergo a semimetal to semiconductor transition as a consequence of quantum confinement as their diameters are decreased. Using density functional theory calculations, the electronic structure of tin nanowires (SnNWs) under uniaxial strain within a range of −4% to +4% is investigated. It is demonstrated that a [110]-oriented semi-metallic SnNW with a diameter of ∼4.2 nm can be made either more metallic or semiconducting by the application of tensile or compressive strain, respectively. On the contrary, a [100]-oriented semi-metallic SnNW with a slightly larger diameter of ∼4.5 nm remains semiconducting with the application of either compressive or tensile strain. Carrier effective masses are calculated from the band structures; it is shown that for semimetal SnNW along [110] orientation the conduction and valence bands display near linear dispersion under both compressive and tensile strains (<3%) which leads to very small effective masses of ∼0.007m0. We also show that st...

First Principle-Based Analysis of Single-Walled Carbon Nanotube and Silicon Nanowire Junctionless Transistors

Junctionless transistors made of silicon have previously been demonstrated experimentally and by simulations. Junctionless devices do not require fabricating an abrupt source-drain junction, and thus, can be easier to implement in aggressive geometries. In this paper, we explore a similar architecture for aggressively scaled devices with the channel consisting of doped carbon nanotubes (CNTs). Gate all around field effect transistor (FET) structures are investigated for n- and p- type doping. Current-voltage characteristics and subthreshold characteristics for a CNT-based junctionless FET is compared with a junctionless silicon nanowire FET with comparable dimensions. Despite the higher on-current of the CNT channels, the device characteristics are poorer compared to the silicon devices due to the smaller CNT bandgap.

Atomic scale simulation of a junctionless silicon nanowire transistor

We have simulated silicon nanowire junctionless transistors with a 3 nm gate length within a Density Functional Theory (DFT) framework. We explored the response of transistors to source-drain bias, VDS, and gate voltage, Vg. Also, the effect of bulk and surface adatom in the wire cross section was evaluated.

Rhenium-doped MoS2 films

Tailoring the electrical properties of transition metal dichalcogenides by doping is one of the biggest challenges for the application of 2D materials in future electronic devices. Here, we report on a straightforward approach to the n-type doping of molybdenum disulfide (MoS2) films with rhenium (Re). High-Resolution Scanning Transmission Electron Microscopy and Energy-Dispersive X-ray spectroscopy are used to identify Re in interstitial and lattice sites of the MoS2 structure. Hall-effect measurements confirm the electron donating influence of Re in MoS2, while the nominally undoped films exhibit a net p-type doping. Density functional theory (DFT) modelling indicates that Re on Mo sites is the origin of the n-type doping, whereas S-vacancies have a p-type nature, providing an explanation for the p-type behaviour of nominally undoped MoS2 films.

Reinventing solid state electronics: harnessing quantum confinement in bismuth thin films

Solid state electronics relies on the intentional introduction of impurity atoms or dopants into a semiconductor crystal and/or the formation of junctions between different materials (heterojunctions) to create rectifiers, potential barriers, and conducting pathways. With these building blocks, switching and amplification of electrical currents and voltages is achieved. As miniaturization continues to ultra-scaled transistors with critical dimensions on the order of ten atomic lengths, the concept of doping to form rectifying junctions fails and heterojunction formation becomes extremely difficult. Here it is shown there is no need to introduce dopant atoms nor is the formation of a heterojunction required to achieve the fundamental electronic function of current rectification. Ideal diode behavior or rectification is achieved for the first time solely by manipulation of quantum confinement in approximately 2 nanometer thick films consisting of a single atomic element, the semimetal bismuth. Crucially for nanoelectronics, this new quantum approach enables room temperature operation.

Electronic and structural properties of rhombohedral [1 1 1] and [1 1 0] oriented ultra-thin bismuth nanowires

Structures and electronic properties of rhombohedral [1 1 1] and [1 1 0] bismuth nanowires are calculated with the use of density functional theory. The formation of an energy band gap from quantum confinement is studied and to improve estimates for the band gap the GW approximation is applied. The [1 1 1] oriented nanowires require surface bonds to be chemically saturated to avoid formation of metallic surface states, whereas the surfaces of the [1 1 0] nanowires do not support metallic surface states. It is found that the onset of quantum confinement in the surface passivated [1 1 1] nanowires occurs at larger critical dimensions than for the [1 1 0] nanowires. For the [1 1 1] oriented nanowires it is predicted that a band gap of ~0.5 eV can be formed at a diameter of approximately 6 nm, whereas for the [1 1 0] oriented nanowires a diameter of approximately 3 nm is required to achieve a similar band gap energy. The GW correction is also applied to estimates of the electron affinity, ionisation potentials and work functions for both orientations of the nanowires for various diameters below 5 nm. The magnitude of the energy band gaps that arise in bismuth at critical dimensions of a few nanometers are of the same order as for conventional bulk semiconductors.

Modeling the effects of DLTs and carrier transport on the turn-on delay, steady-state time and wavelength chirp of SCH-QW lasers

In this paper we drive a numerical model for a separate confinement heterostructure quantum well (SCH-QW) laser, as one of the key elements of photonic communications systems and networks. The physical effects of Deep Level Traps (DLTs) and carrier transport on the threshold current, turn-on delay, steady state time, transient response and wavelength chirp of a SCH-QW laser are modeled by means of finite difference method. This numerical model calculates the time-dependent variations of optical output phase, photon density and carrier densities in the SCH layer and QW region, self-consistently. These equations are also coupled with the trap density equation. The exact carrier dependence of the linewidth enhancement factor and the gain function for the QW laser are included.