arXiv subjects

J. Coutinho

Publications and source records attributed to J. Coutinho.

12 recordsLinked to original sources

arXiv2025

Dynamic buckling of Si tetramers on the Si(111)-7x7

The atomic structure of Si tetramers that form on the Si$(111)\textrm{-}7\times7$ surface during homoepitaxy, is investigated by means of first principles calculations with the currently available atomistic model as starting point. It is demonstrated that the rectangular shape of the Si tetramer is unstable against buckling. Comparison of calculated results with available scanning tunnelling microscopy (STM) data provides a new understanding of the problem, indicating that the recorded STM images are influenced by dynamic buckling.

cond-mat.mtrl-sci↗

arXiv2023

Impact of strain and surface reconstruction on long-range diffusion of Ge atoms on Ge(111) surface

We investigate the effect of surface reconstruction and strain on diffusion of adsorbed Ge atoms on Ge$(111)\textrm{-}5\times5$ and Ge$(111)\textrm{-}7\times7$ surfaces by means of first principles calculations. Stable adsorption sites, their energies, diffusion paths, and corresponding activation barriers are reported. We demonstrate that the decisive migration path is located near the corner holes of surface structures, and they are associated with formation of weak bonds between the adsorbed Ge atom and surface dimers (within the $5\times5$ or $7\times7$ structures). The results show that Ge diffusion rates on $5\times5$ and $7\times7$ reconstructed Ge$(111)$ surfaces should be similar. Conversely, the diffusion barrier on a compressively strained Ge$(111)$ surface is considerably higher than that on a strain-free surface, thus explaining previous experimental results. Comparable diffusion rates on $5\times5$ and $7\times7$ reconstructed surfaces are explained by the identical local atomic arrangements of these structures. The increase of the migration barrier on a strained surface is explained by dimer bond strengthening upon surface compression, along with a weakening of bonds between the adsorbed Ge and dimer atoms.

cond-mat.mtrl-sci↗

arXiv2021

The M-center in 4H-SiC is a carbon self-interstitial

The list of semiconductor materials with spectroscopically fingerprinted self-interstitials is very short. The M-center in 4H-SiC, a bistable defect responsible for a family of electron traps, has been deprived of a model which could unveil its real importance for almost two decades. Using advanced first-principles calculations and junction spectroscopy, we demonstrate that the properties of M, including bistability, annealing, reconfiguration kinetics, and electronic levels, match those of the carbon self-interstitial.

cond-mat.mtrl-sci↗

arXiv2020

Formation and dissociation reactions of complexes involving interstitial carbon and oxygen defects in silicon

We present a detailed first-principles study which explores the configurational space along the relevant reactions and migration paths involving the formation and dissociation of interstitial carbon-oxygen complexes, $\mathrm{C_{i}O_{i}}$ and $\mathrm{C_{i}O_{2i}}$, in silicon. The formation/dissociation mechanisms of $\mathrm{C_{i}O_{i}}$ and $\mathrm{C_{i}O_{2i}}$ are found as occurring via capture/emission of mobile $\mathrm{C_{i}}$ impurities by/from O-complexes anchored to the lattice. The lowest activation energies for dissociation of $\mathrm{C_{i}O_{i}}$ and $\mathrm{C_{i}O_{2i}}$ into smaller moieties are 2.3 eV and 3.1 eV, respectively. The first is compatible with the observed annealing temperature of $\mathrm{C_{i}O_{i}}$ , which occurs at around 400 $^{\circ}$C, and below the threshold for $\mathrm{O_{i}}$ diffusion. The latter exceeds significantly the measured activation energy for the annealing of $\mathrm{C_{i}O_{2i}}$ ($E_{\mathrm{a}}=2.55$ eV). We propose that instead of dissociation, the actual annealing mechanism involves the capture of interstitial oxygen by $\mathrm{C_{i}O_{2i}}$, thus being governed by the migration barrier of $\mathrm{O_{i}}$ ($E_{\mathrm{m}}=2.53$ eV). The study is also accompanied by measurements of hole capture cross sections and capture barriers of $\mathrm{C_{i}O_{i}}$ and $\mathrm{C_{i}O_{2i}}$. In combination with previously reported data, we find thermodynamic donor transitions which are directly comparable to the first-principles results. The two levels exhibit close features, conforming to a model where the electronic character of $\mathrm{C_{i}O_{2i}}$ can be described by that of $\mathrm{C_{i}O_{i}}$ perturbed by a nearby O atom.

cond-mat.mtrl-sci↗

arXiv2019

Can we rely on hybrid-DFT energies of solid-state problems with local-DFT geometries?

Hybrid functionals often improve considerably the accuracy of density-functional calculations, in particular of quantities resulting from the band structure. In plane-wave (PW) calculations this benefit comes at the cost of an increase in computation time by several orders of magnitude. For this reason, large-scale problems addressed within the PW formalism have to rely on pre-relaxed atomistic geometries, obtained with cheaper local or semi-local exchange-correlation functionals. We investigate how suitable these geometries are when plugged into single-point hybrid-DFT calculations. Based on several case studies, we find two important sources of error originating from (i) bond strain and (ii) over-mixing between defect and crystalline states. The first arises from the mismatch between the pre-relaxed geometry and that obtained after a subsequent hybrid-DFT-level relaxation. The second occurs when defect states edging an underestimated band gap artificially mix with crystalline states, affecting the local bonding character of the defect, and therefore leading the spurious hybrid-DFT energies. Due to cancelation effects, the lingering strain contributes little ($\lesssim10$ meV) to the error bar of quantities based on energy differences of pre-relaxed structures. The error from state over-mixing does not benefit from cancelation effects and has to be monitored with caution.

cond-mat.mtrl-sci↗

arXiv2017

A comment on "Different STM Images of the Superstructure on a Clean Si(133)-6x2 Surface" (JETP Letters 105 (8), 477-483 (2017))

In the recent paper by Teys [JETP Letters 105 (8), 477-483 (2017)], an atomic model for the Si(331) reconstructed surface (hereby referred to as T-model) was proposed on the basis of high-resolution scanning tunneling microscopy (STM) images. While detailing the virtues against previous and abandoned models, the author avoids any reference to the rather distinct 8P-model advocated few weeks earlier by Zhachuk and Teys [R. Zhachuk, S. Teys, Phys. Rev. B 95, 041412 (2017)], casting doubts to his own work. Formulated that way, findings from Ref. [JETP Letters 105 (8), 477-483 (2017)] leave readers of JETP Letters with a partial and confusing view of the problem, and above all, leaves the observations open to ambiguous interpretation. The 8P-model is also based on STM measurements, and unlike the T-model, passed through the scrutiny of first-principles calculations. The present comment reconciles Ref. [JETP Letters 105 (8), 477-483 (2017)] with the literature by supplementing the discussion with a missing and critical account on the stability and electronic structure of the T- versus 8P-models of Si(331).

cond-mat.mtrl-sci↗

arXiv2017

Si(111) strained layers on Ge(111): evidence for c(2x4) domains

The tensile strained Si(111) layers grown on top of Ge(111) substrates are studied by combining scanning tunneling microscopy, low energy electron diffraction and first-principles calculations. It is shown that the layers exhibit c(2x4) domains, which are separated by domain walls along <-110> directions. A model structure for the c(2x4) domains is proposed, which shows low formation energy and good agreement with the experimental data. The results of our calculations suggest that Ge atoms are likely to replace Si atoms with dangling bonds on the surface (rest-atoms and adatoms), thus significantly lowering the surface energy and inducing the formation of domain walls. The experiments and calculations demonstrate that when surface strain changes from compressive to tensile, the (111) reconstruction converts from dimer-adatom-stacking fault-based to adatom-based structures.

cond-mat.mtrl-sci↗

arXiv2016

A first-principles model of copper-boron interactions in Si: implications for the light-induced degradation of solar Si

The recent discovery that Cu contamination of Si combined with light exposure has a significant detrimental impact on carrier life-time has drawn much concern within the solar-Si community. The effect, known as the copper-related light-induced degradation (Cu-LID) of Si solar cells, has been connected to the release of Cu interstitials within the bulk [Solar Energy Materials & Solar Cells, 147:115-126, 2016]. In this paper, we describe a comprehensive analysis of the formation/dissociation process of the CuB pair in Si by means of first-principles modelling, as well as the interaction of CuB defects with photo-excited minority carriers. We confirm that the long-range interaction between the Cu cation and the B anion has a Coulomb-like behaviour, in line with the trapping-limited diffusivity of Cu observed by transient ion drift measurements. On the other hand, the short-range interaction between the d-electrons of Cu and the excess of negative charge on B produces a repulsive effect, thereby decreasing the binding energy of the pair when compared to the ideal point-charge Coulomb model. We also find that metastable CuB pairs produce acceptor states just below the conduction band minimum, which arise from the Cu level emptied by the B acceptor. Based on these results, we argue that photo-generated minority carriers trapped by the metastable pairs can switch off the Coulomb interaction that holds the pairs together, enhancing the release of Cu interstitials, and acting as a catalyst for Cu-LID.

cond-mat.mtrl-sci↗

arXiv2016

Mössbauer parameters of Fe-related defects in group-IV semiconductors: first principles calculations

We employ a combination of pseudopotential and all-electron density functional calculations, to relate the structure of defects in supercells to the isomer shifts and quadrupole splittings observed in Mössbauer spectroscopy experiments. The methodology is comprehensively reviewed and applied to the technologically relevant case of iron-related defects in silicon, and to other group-IV hosts to a lesser degree. Investigated defects include interstitial and substitutional iron, iron-boron pairs, iron-vacancy and iron-divacancy. We find that in general, agreement between the calculations and Mössbauer data is within a 10% error bar. Nonetheless, we show that the methodology can be used to make accurate assignments, including to separate peaks of similar defects in slightly different environments.

cond-mat.mtrl-sci↗

arXiv2013

Electronic structure modification of Si-nanocrystals with F4 -TCNQ

We use first-principles models to demonstrate how an organic oxidizing agent, F4 -TCNQ (7,7,8,8- tetracyano-2,3,5,6-tetrafluoroquinodimethane), modifies the electronic structure of silicon nanocrys- tals, suggesting it may enhance p-type carrier density and mobility. The proximity of the lowest unoccupied level of F4 -TCNQ to the highest occupied level of the Si-nanocrystals promotes the formation of an empty hybrid state overlapping both nanocrystal and molecule, reducing the exci- tation energy to about 0.8-1 eV in vacuum. Hence, it is suggested that F4 -TCNQ can serve both as a surface oxidant and a mediator for hole hopping between adjacent nanocrystals.

cond-mat.mtrl-sci↗

arXiv2013

Strain-induced structure transformations on Si(111) and Ge(111) surfaces: a combined density-functional and scannning tunnneling microscopy report

Si(111) and Ge(111) surface formation energies were calculated using density functional theory for various biaxial strain states ranging from -0.04 to 0.04, and for a wide set of experimentally observed surface reconstructions: 3x3, 5x5, 7x7 dimer-adatom-stacking fault reconstructions and c(2x8), 2x2 and \sqrt{3}x\sqrt{3} adatoms based surfaces. The calculations are compared with scanning tunneling microscopy data obtained on stepped Si(111) surfaces and on Ge islands grown on a Si(111) substrate. It is shown that the surface structure transformations observed in these strained systems are accounted for by a phase diagram that relates the equilibrium surface structure to the applied strain. The calculated formation energy of the unstrained Si(111)-9x9 dimer-adatom-stacking fault surface is reported for the first time and it is higher than corresponding energies of Si(111)-5x5 and Si(111)-7x7 dimer-adatom-stacking fault surfaces as expected. We predict that the Si(111) surface should adopt a c(2x8) reconstruction when tensile strain is above 0.03.

cond-mat.mtrl-sci↗

arXiv2008

Inducing energy gaps in graphene monolayer and bilayer

In this paper we propose a mechanism for the induction of energy gaps in the spectrum of graphene and its bilayer, when both these materials are covered with water and ammonia molecules. The energy gaps obtained are within the range 20-30 meV, values compatible to those found in experimental studies of graphene bilayer. We further show that the binding energies are large enough for the adsorption of the molecules to be maintained even at room temperature.

cond-mat.mtrl-sci↗