arXiv subjects

Alfonso Sanchez-Soares

Publications and source records attributed to Alfonso Sanchez-Soares.

7 recordsLinked to original sources

arXiv2021

Top-Gated Carbon Nanotube FETs from Quantum Simulations: Comparison with Experiments

We present quantum simulations of carbon nanotube field-effect transistors (CNT-FETs) based on top-gated architectures and compare to electrical characterization on devices with 15 nm channel lengths. A non-equilibrium Green's function (NEGF) quantum transport method coupled with a $\vec{k} \cdot \vec{p}$ description of the electronic structure is demonstrated to achieve excellent agreement with the reported experimental data. Factors influencing the electrostatic control of the channel are investigated and reveal that detailed modeling of the electrostatics and the electronic band structure of the CNT is required to achieve quantitative agreement with experiment.

cond-mat.mes-hall↗

arXiv2021

Epitaxial Stabilisation of ${\bf \mathrm{Ge_{1-x}Sn_x}}$ Alloys

The thermodynamic stability of germanium tin $\mathrm{Ge_{1-x}Sn_x}$ alloys is investigated across the composition range $0 \le x \le 1$ by applying density functional theory (DFT) together with the cluster expansion formalism (CE). It is known that GeSn alloys are immiscible and that non-equilibrium growth techniques are required to produce metastable films and nanostructures. Insight into the driving forces behind component segregation is gained by investigating the equilibrium thermodynamics of GeSn systems. The alloy free energy of mixing is computed by combining enthalpies from CE with entropy terms for configurational and vibrational degrees of freedom. Volume deformations due to the large mismatch in ionic radii are readily found to be the key driving force for immiscibility at all temperatures of relevance. This leads to a study of epitaxial stabilisation by employing latticed matched substrates to favour growth of alloys with fractional compositions of $\mathrm{x=0}$, approximately $\mathrm{x=0.5}$ and $\mathrm{x=1}$. Reduction of the free energy of mixing due to epitaxial strain in thin films is quantified for each substrate leading to indicators for growth of kinetically stable films.

cond-mat.mtrl-sci↗

arXiv2020

Impact of stoichiometry and strain on Ge$_{1-x}$Sn$_{x}$ alloys from first principles calculations

We calculate the electronic structure of germanium-tin (Ge$_{1-x}$Sn$_{x}$) binary alloys for $0 \leq x \leq 1$ using density functional theory (DFT). Relaxed alloys with semiconducting or semimetallic behaviour as a function of Sn composition $x$ are identified, and the impact of epitaxial strain is included by constraining supercell lattice constants perpendicular to the [001] growth direction to the lattice constants of Ge, zinc telluride (ZnTe), or cadmium telluride (CdTe) substrates. It is found that application of 1% tensile strain reduces the Sn composition required to bring the (positive) direct band gap to zero by approximately 5% compared to a relaxed Ge$_{1-x}$Sn$_{x}$ alloy having the same gap at $Γ$. On the other hand, compressive strain has comparatively less impact on the alloy band gap at $Γ$. Using DFT calculated alloy lattice and elastic constants, the critical thickness for Ge$_{1-x}$Sn$_{x}$ thin films as a function of $x$ and substrate lattice constant is estimated, and validated against supercell DFT calculations. The analysis correctly predicts the Sn composition range at which it becomes energetically favourable for Ge$_{1-x}$Sn$_{x}$/Ge to become amorphous. The influence of stoichiometry and strain is examined in relation to reducing the magnitude of the inverted (``negative'') $Γ_{7}^{-}$-$Γ_{8}^{+}$ band gap, which is characteristic of semimetallic alloy electronic structure. Based on our findings, strategies for engineering the semimetal-to-semiconductor transition via strain and quantum confinement in Ge$_{1-x}$Sn$_{x}$ nanostructures are proposed.

cond-mat.mtrl-sci↗

arXiv2016

Electronic structure tuning via surface modification in semimetallic nanowires

Electronic structure properties of nanowires (NW) with diameters of 1.5 nm and 3 nm based on semimetallic $α$-Sn are investigated by employing density functional theory and perturbative $GW$ methods. We explore the dependence of electron affinity, band structure and band gap values with crystallographic orientation, NW cross-sectional size and surface passivants of varying electronegativity. We consider four chemical terminations in our study: methyl ($\rm{CH_3}$), hydrogen ($\rm{H}$), hydroxyl ($\rm{OH}$), and fluorine ($\rm{F}$). Results suggest a high degree of elasticity of Sn-Sn bonds within the SnNWs' cores with no significant structural variations for nanowires with different surface passivants. Direct band gaps at Brillouin zone centers are found for most studied structures with quasi-particle corrected band gap magnitudes ranging from 0.25 eV to 3.54 eV in 1.5 nm diameter structures indicating an exceptional range of properties for semimetal NWs below the semimetal-to-semiconductor transition. Band gap variations induced by changes in surface passivants indicate the possibility of realizing semimetal-semiconductor interfaces in NWs with constant cross-section and crystallographic orientation allowing the design of novel dopant-free NW-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2016

Effect of strain, thickness, and local surface environment on electron transport properties of oxygen-terminated copper thin films

Electron transport is studied in surface oxidized single-crystal copper thin films with a thickness of up to 5.6 nm by applying density functional theory and density functional tight binding methods to determine electron transport properties within the ballistic regime. The variation of the electron transmission as a function of film thickness as well as the different contributions to the overall electron transmission as a function of depth into the the films is examined. Transmission at the oxidized copper film surfaces is found to be universally low. Films with thickness greater than 2.7 nm exhibit a similar behavior in local transmission per unit area with depth from the film surface; transmission per unit area initially increases rapidly and then plateaus at a depth of approximately 0.35-0.5 nm away from the surface, dependent on surface facet. Unstrained films tend to exhibit a higher transmission per unit area than corresponding films under tensile strain.

cond-mat.mes-hall↗

arXiv2016

Electron transport properties of sub-3-nm diameter copper nanowires

Density functional theory and density functional tight-binding are applied to model electron transport in copper nanowires of approximately 1 nm and 3 nm diameters with varying crystal orientation and surface termination. The copper nanowires studied are found to be metallic irrespective of diameter, crystal orientation and/or surface termination. Electron transmission is highly dependent on crystal orientation and surface termination. Nanowires oriented along the [110] crystallographic axis consistently exhibit the highest electron transmission while surface oxidized nanowires show significantly reduced electron transmission compared to unterminated nanowires. Transmission per unit area is calculated in each case, for a given crystal orientation we find that this value decreases with diameter for unterminated nanowires but is largely unaffected by diameter in surface oxidized nanowires for the size regime considered. Transmission pathway plots show that transmission is larger at the surface of unterminated nanowires than inside the nanowire and that transmission at the nanowire surface is significantly reduced by surface oxidation. Finally, we present a simple model which explains the transport per unit area dependence on diameter based on transmission pathways results.

cond-mat.mes-hall↗

arXiv2016

A semimetal nanowire rectifier: balancing quantum confinement and surface electronegativity

For semimetal nanowires with diameters smaller than a few tens of nanometers, a semimetal-to-semiconductor transition is observed as the emergence of an energy band gap resulting from quantum confinement. Quantum confinement in a semimetal results in either lifting of the degeneracy of the conduction and valence bands in a zero gap semimetal, or shifting of bands with a negative energy overlap to form conduction and valence bands. For semimetal nanowires with diameters below 10 nanometer, the magnitude of the band gap can become significantly larger than the thermal energy at room temperature resulting in a new class of semiconductors relevant for nanoelectronics with critical dimensions on the order of a few atomic lengths. The smaller a nanowire's diameter, the larger its surface-to-volume ratio thus leading to an increasing impact of surface chemistry on its electronic structure. Energy level shifts to states in the vicinity of the Fermi level due to the electronegativity of surface terminating species are shown to be comparable in magnitude to quantum confinement effects at nanowire diameters of a few nanometer; these two effects can be used to counteract one another leading to semimetallic behavior for nanowire cross sections at which the quantum confinement effect would otherwise dominate. Abruptly changing the surface terminating species along the length of a nanowire leads to an abrupt change in the surface electronegativity. This can result in the formation of a semimetal-semiconductor junction within a monomaterial nanowire, without the need for impurity doping nor requiring the formation of a heterojunction.

cond-mat.mes-hall↗