L. Raymond

Ballistic transport in Si, Ge, and GaAs nanowire MOSFETs

The influence of various channel materials and crystallographic orientations on the performance of nanowire MOSFETs operating in pure ballistic regime is investigated using 3D quantum-mechanical simulations. We consider three different materials (Si, Ge, GaAs) in nanowire transistors fabricated on a <010>-wafer with an arbitrary channel orientation and provide a better understanding of the transport phenomena that may occur in each device configuration

Single donor induced negative differential resistance in silicon n-type nanowire metal-oxide-semiconductor transistors

This work presents a theoretical study of the influence of a single donor on the transport properties of silicon nanowire transistors. Using a three-dimensional self-consistent nonequilibrium Green’s function approach we find that the donor states induce transitions from resonant to antiresonant Breit–Wigner interferences when increasing the gate or drain voltages. Numerical and analytical calculations demonstrate that these interferences strongly degrade the transistor performances but can also generate a remarkable negative differential resistance behavior. The robustness of this phenomenon with respect to a change of the defect position in the channel is an opportunity to develop novel device properties.

Structure factor of 1D systems (superlattices) based on two-letter substitution rules. I. delta (Bragg) peaks

The recent generalization to the case of arbitrary tile lengths and arbitrary scattering factors of the calculation of the structure factor of 1D substitutional systems is studied in detail. This method makes a easy to find all the peaks in the diffraction spectrum of a system. The well known periodic and quasiperiodic spectra with delta peaks at integer multiples of a single number and integer linear combinations of two incommensurate frequencies, respectively. were found to be the l=0 subsets of two more general types of spectra, infinite-periodic (or limit-periodic) and infinite-quasiperiodic (or limit-quasiperiodic) characterized by rational numbers of the type m/nl, l=0, ..., infinity in place of the above integers. Substitution rules that produce quasicrystalline quasiperiodic and infinite-quasiperiodic spectra give the same type of spectrum for all values of the ratio rho = rho a/ rho b of the two tile lengths rho a and rho b. This is not the case for the other rules. Thus the same substitution rule (such as the copper-mean rule) can give an infinite-periodic spectrum for a single rational ratio rho = rho a/ rho b of the two tile lengths rho a and rho b, a periodic-like spectrum for other rational rho , and a spectrum in some aspects similar to that of a random system when rho is an irrational number. On the other hand, a Thue-Morse system diffracts as a periodic crystal when p not=1 but has no non-trivial delta peaks when rho =1. Other Thue-Morse-like systems can have infinite-periodic spectra for all rho .

Three-dimensional k · p real-space quantum transport simulations of p-type nanowire transistors: Influence of ionized impurities

We present a three-dimensional quantum transport simulator for p-type nanowire transistors. This self-consistent model expresses a six-band k · p Hamiltonian within the non-equilibrium Green’s function formalism. Transport properties are analyzed with and without the presence of ionized impurities in the channel. We observe that inter-subband coupling generates a rich structure of peaks in the transmission coefficients even in the intrinsic situation. A single donor leads to a current decrease whereas its acceptor counterpart induces complicated resonant and anti-resonant features. Unlike n-type devices, our conclusions pinpoint that the p-type nanowire transistors exhibit intricate transmission variations that can potentially generate larger variability and whose modeling requires a multi-band based simulator.

Mechanisms of magnetoelectricity in manganese-doped incipient ferroelectrics

We report magnetization measurements and magnetic resonance data for SrTiO3 doped by manganese. We show that the recently reported coexistent spin and dipole glass (multiglass) behaviours are strongly affected by the distribution of Mn ions between the Sr and Ti sites. Motivated by this finding we calculate the magnetic interactions between Mn impurities of different kinds. Both LSDA+U and many-body perturbation theory evidence that magnetic and magnetoelectric interactions are mediated by MnB4+ ions substituting for Ti. We propose two microscopic magnetoelectric coupling mechanisms, which can be involved in all magnetoelectric systems based on incipient ferroelectrics. In the first one, the electric field modifies the spin susceptibility via spin-strain coupling of MnB4+. The second mechanism concerns Mn pairs coupled by the position-dependent exchange interaction.

Dielectric resonances in disordered media

Abstract:Binary disordered systems are usually obtained by mixing two ingredients in variable proportions: conductor and insulator, or conductor and super-conductor. They present very specific properties, in particular the second-order percolation phase transition, with its fractal geometry and the multi-fractal properties of the current moments. These systems are naturally modeled by regular bi-dimensional or tri-dimensional lattices, on which sites or bonds are chosen randomly with given probabilities. The two significant parameters are the ratio h = σ1/σ of the complex conductances, σ and σ1, of the two components, and their relative abundances p (or, respectively, 1 - p). In this article, we calculate the impedance of the composite by two independent methods: the so-called spectral method, which diagonalises Kirchhoffs Laws via a Green function formalism, and the Exact Numerical Renormalization method (ENR). These methods are applied to mixtures of resistors and capacitors (R-C systems), simulating e.g. ionic conductor-insulator systems, and to composites constituted of resistive inductances and capacitors (LR-C systems), representing metal inclusions in a dielectric bulk. The frequency dependent impedances of the latter composites present very intricate structures in the vicinity of the percolation threshold. In this paper, we analyse the LR-C behavior of compounds formed by the inclusion of small conducting clusters (“n-legged animals”) in a dielectric medium. We investigate in particular their absorption spectra who present a pattern of sharp lines at very specific frequencies of the incident electromagnetic field, the goal being to identify the signature of each animal. This enables us to make suggestions of how to build compounds with specific absorption or transmission properties in a given frequency domain.

Anomalous quantum Hall effect induced by disorder in topological insulators

We investigate a transition between a two-dimensional topological insulator conduction state, characterized by a conductance G = 2 (in fundamental units e 2 /h) and a Chern insulator with G = 1, induced by polarized magnetic impurities. Two kinds of coupling, ferromagnetic and antiferromagnetic, are considered with the electron and hole subbands. We demonstrate that for strong disorder, a phase G = 1 exists even for ferromagnetic order, in contrast with the prediction of the mean field approximation. This result is supported by direct numerical computations using Landauer transport formula, and by analytical calculations of the chemical potential and mass renormalization as a function of the disorder strength, in the self-consistent Born approximation. The transition is related to the suppression of one of the spin conduction channels, for strong enough disorder, by selective spin scattering and localization.

Theoretical comparison of Si, Ge, and GaAs ultrathin p-type double-gate metal oxide semiconductor transistors

Based on a self-consistent multi-band quantum transport code including hole-phonon scattering, we compare current characteristics of Si, Ge, and GaAs p-type double-gate transistors. Electronic properties are analyzed as a function of (i) transport orientation, (ii) channel material, and (iii) gate length. We first show that ⟨100⟩-oriented devices offer better characteristics than their ⟨110⟩-counterparts independently of the material choice. Our results also point out that the weaker impact of scattering in Ge produces better electrical performances in long devices, while the moderate tunneling effect makes Si more advantageous in ultimately scaled transistors. Moreover, GaAs-based devices are less advantageous for shorter lengths and do not offer a high enough ON current for longer gate lengths. According to our simulations, the performance switching between Si and Ge occurs for a gate length of 12 nm. The conclusions of the study invite then to consider ⟨100⟩-oriented double-gate devices with Si for gat...

Carrier injection engineering in nanowire transistors via dopant and shape monitoring of the access regions

This work theoretically studies the influence of both the geometry and the discrete nature of dopants of the access regions in ultra-scaled nanowire transistors. By means of self-consistent quantum transport simulations, we show that discrete dopants induce quasi-localized states which govern carrier injection into the channel. Carrier injection can be enhanced by taking advantage of the dielectric confinement occurring in these access regions. We demonstrate that the optimization of access resistance can be obtained by a careful control of shape and dopant position. These results pave the way for contact resistance engineering in forthcoming device generations.

Original shaped nanowire metal-oxide-semiconductor field-effect transistor with enhanced current characteristics based on three-dimensional modeling

This work presents original nanowire transistor architectures leading to device performance improvement. The influence of channel geometrical patterns on nanowire metal-oxide-semiconductor-field-effect-transistor characteristics is investigated using three-dimensional real-space quantum-mechanical simulations. Our study shows that indented channel improves the on-off current ratio (Ion/Ioff) by 32%. This remarkable result is induced by both quasiresonances in the on-regime and more significantly by the presence of a thicker channel potential barrier in the subthreshold domain. We then demonstrate that an optimized indented channel represents a manufacturable opportunity to have a much better control of short channel effects in nanowire transistors.