46 research outputs found
Nonequilibrium electron charging in carbon-nanotube-based molecular bridges
We evidence the importance of electron charging under nonequilibrium
conditions for carbon-nanotube-based molecular bridges, using a self-consistent
Green's function method with an extended Huckel Hamiltonian and a
three-dimensional Poisson solver. Our analysis demonstrates that such feature
is highly dependent on the chirality of the carbon nanotube as well as on the
type of the contact metal, conditioning in a nongeneralized way the system's
conduction mechanism. Based on its impact on transport, we argue that
self-consistency is essential for the current-voltage calculations of
semiconducting nanotubes, whereas less significant in the case of metallic
ones.Comment: 4 pages, 4 figure
Theoretical study of the role of metallic contacts in probing transport features of pure and defected graphene nanoribbons
Understanding the roles of disorder and metal/graphene interface on the electronic and transport properties of graphene-based systems is crucial for a consistent analysis of the data deriving from experimental measurements. The present work is devoted to the detailed study of graphene nanoribbon systems by means of self-consistent quantum transport calculations. The computational formalism is based on a coupled Schrödinger/Poisson approach that respects both chemistry and electrostatics, applied to pure/defected graphene nanoribbons (ideally or end-contacted by various fcc metals). We theoretically characterize the formation of metal-graphene junctions as well as the effects of backscattering due to the presence of vacancies and impurities. Our results evidence that disorder can infer significant alterations on the conduction process, giving rise to mobility gaps in the conductance distribution. Moreover, we show the importance of metal-graphene coupling that gives rise to doping-related phenomena and a degradation of conductance quantization characteristics
Local Coordination Modulates the Reflectivity of Liquefied Si-Ge Alloys
The properties of liquid Si-Ge binary systems at melting conditions deviate
from those expected by the ideal alloy approximation. Particularly, a
non-linear dependence of the dielectric functions occurs with the reflectivity
of liquid Si-Ge being 10\% higher at intermediate Ge content than in pure Si or
Ge. Using \textit{ab initio} methodologies, we revealed a direct correlation
between reflectivity and atomic coordination, discovering that Si-Ge's higher
local coordination drives the aforementioned optical behavior. These findings
extend the physical understanding of liquefied semiconductors and hold the
promise of further generalization
Phonon Driven Nonlinear Electrical Behavior in Molecular Devices
Electronic transport in a model molecular device coupled to local phonon
modes is theoretically analyzed. The method allows for obtaining an accurate
approximation of the system's quantum state irrespective of the electron and
phonon energy scales. Nonlinear electrical features emerge from the calculated
current-voltage characteristics. The quantum corrections with respect to the
adiabatic limit characterize the transport scenario, and the polaronic
reduction of the effective device-lead coupling plays a fundamental role in the
unusual electrical features.Comment: 14 pages, 4 figure
Comparing different solutions for testing resistive defects in low-power SRAMs
Low-power SRAM architectures are especially sensitive to many types of defects that may occur during manufacturing. Among these, resistive defects can appear. This paper analyzes some types of such defects that may impair the device functionalities in subtle ways, depending on the defect characteristics, and that may not be directly or easily detectable by traditional test methods, such as March algorithms. We analyze different methods to test such defects and discuss them in terms of complexity and test time
Genesis and evolution of extended defects: The role of evolving interface instabilities in cubic SiC
Emerging wide bandgap semiconductor devices such as the ones built with SiC have the potential to revolutionize the power electronics industry through faster switching speeds, lower losses, and higher blocking voltages, which are superior to standard silicon-based devices. The current epitaxial technology enables more controllable and less defective large area substrate growth for the hexagonal polymorph of SiC (4H-SiC) with respect to the cubic counterpart (3C-SiC). However, the cubic polymorph exhibits superior physical properties in comparison to its hexagonal counterpart, such as a narrower bandgap (2.3 eV), possibility to be grown on a silicon substrate, a reduced density of states at the SiC/SiO2 interface, and a higher channel mobility, characteristics that are ideal for its incorporation in metal oxide semiconductor field effect transistors. The most critical issue that hinders the use of 3C-SiC for electronic devices is the high number of defects in bulk and epilayers, respectively. Their origin and evolution are not understood in the literature to date. In this manuscript, we combine ab initio calibrated Kinetic Monte Carlo calculations with transmission electron microscopy characterization to evaluate the evolution of extended defects in 3C-SiC. Our study pinpoints the atomistic mechanisms responsible for extended defect generation and evolution, and establishes that the antiphase boundary is the critical source of other extended defects such as single stacking faults with different symmetries and sequences. This paper showcases that the eventual reduction of these antiphase boundaries is particularly important to achieve good quality crystals, which can then be incorporated in electronic devices
First Experimental Evidence of Amorphous Tin Oxide Formation in Lead-Free Perovskites by Spectroscopic Ellipsometry
The most promising lead-free options for producing perovskite solar cells are tin halide perovskite materials. Herein, while in situ monitoring the optical evolution of the material in humid air, spectroscopic ellipsometry is used to investigate the dielectric function of FASnI3 layers (with and without additives) within the range of 1–5 eV. According to calculations based on the density functional theory that shows oxygen diffusion on FASnI3 surfaces, the steady decrease in absorption coefficient in the band gap region (1.47 eV) and simultaneous increase in absorption in the 3–4.5 eV region suggest the production of amorphous tin oxide. Concurrently, X-ray diffraction reveals a clear degradation of FASnI3. With the addition of sodium borohydride and dipropylammonium iodide, the optically active area of about 1.47 eV is preserved for a longer period while SnO2 production is prevented. Last but not least, FASnI3's stability is investigated in dry N2 environment and shown that it is optically durable for thermal operations up to 100 °C, particularly when additives are used
Two-step MAPbI3 deposition by low-vacuum proximity-space-effusion for high-efficiency inverted semitransparent perovskite solar cells
The innovative two-step Low Vacuum-Proximity Space Effusion (LV-PSE) method exploits the conversion of a textured PbI2 layer into MAPbI3 by adsorption–incorporation–migration of energetic MAI molecules, thus enabling a best efficiency of 17.5% in 150 nm thick layers
Atomistic insights into ultrafast SiGe nanoprocessing
Controlling ultrafast material transformations with atomic precision is
essential for future nanotechnology. Pulsed laser annealing (LA), inducing
extremely rapid and localized phase transitions, is a powerful way to achieve
this, but it requires careful optimization together with the appropriate system
design. We present a multiscale LA computational framework able to simulate
atom-by-atom the highly out-of-equilibrium kinetics of a material as it
interacts with the laser, including effects of structural disorder. By
seamlessly coupling a macroscale continuum solver to a nanoscale super-lattice
Kinetic Monte Carlo code, this method overcomes the limits of state-of-the-art
continuum-based tools. We exploit it to investigate nontrivial changes in
composition, morphology and quality of laser-annealed SiGe alloys. Validations
against experiments and phase-field simulations, as well as advanced
applications to strained, defected, nanostructured and confined SiGe are
presented, highlighting the importance of a multiscale atomistic-continuum
approach. Current applicability and potential generalization routes are finally
discussed