arXiv subjects

José Coutinho

Publications and source records attributed to José Coutinho.

14 recordsLinked to original sources

arXiv2026

First principles modeling of group-III acceptors and their potential lifetime-limiting effects in n-type 4H-SiC

Accurate modeling of p-type dopants in 4H-SiC is essential for understanding the mechanisms governing doping efficiency and carrier transport. In this work, we revisit the electronic structure of boron- and aluminum-related acceptors using hybrid density functional methods. Besides defect formation energies and thermodynamic transition levels, we present a quantitative look into the carrier capture kinetics within the multi-phonon emission framework. Our results reveal striking differences between the two most relevant p-type dopants. While $\textrm{B}_{\textrm{Si}}$ and $\textrm{B}_{\textrm{C}}$ exhibit nearly identical formation energies, consistent with the occurrence of both defects, the formation energy of $\textrm{Al}_{\textrm{C}}$ under intrinsic conditions is approximately 6.5 eV higher than that of $\textrm{Al}_{\textrm{Si}}$, confirming previous findings that the former is unlikely to occur. We further find that $\textrm{B}_{\textrm{Si}}$ possesses large electron and hole capture cross sections, identifying it as a plausible source of minority-carrier lifetime degradation of n-type material contaminated with boron. In addition, we predict a previously unexplored donor transition for $\textrm{B}_{\textrm{C}}$. If experimentally confirmed, this defect would represent additional problems to both p-type doped and boron contaminated 4H-SiC, not only because of its ineffectiveness as an electric dopant, but also due to trapping of up to two free holes, reducing the free-hole concentration and increasing scattering effects.

cond-mat.mtrl-sci↗

arXiv2024

Hydrogen reactions with dopants and impurities in solar silicon from first principles

We present a theoretical account of some of the most likely hydrogen-related reactions with impurities in n-type and p-type solar-grade silicon. These include reactions with dopants and carbon, which are relevant in the context of life-time degradation of silicon solar cells, most notably of light and elevated temperature degradation (LeTID) of the cells. Among the problems addressed, we highlight a comparative study of acceptor-enhanced dissociation of hydrogen molecules in B- and Ga-doped material, their subsequent reaction steps toward formation of acceptor-hydrogen pairs, the proposal of mechanisms which explain the observed kinetics of photo-/carrier-induced dissociation of PH and CH pairs in n-type Si, analysis of reactions involving direct interactions between molecules with P and C, and the assignment of several electron and hole traps with detailed atomistic- and wavefunction-resolved models.

cond-mat.mtrl-sci↗

arXiv2023

Theory of reactions between hydrogen and group-III acceptors in silicon

The thermodynamics of several reactions involving atomic and molecular hydrogen with group-III acceptors is investigated. The results provide a first-principles-level account of thermally- and carrier-activated processes involving these species. Acceptor-hydrogen pairing is revisited as well. We present a refined physicochemical picture of long-range migration, compensation effects, and short-range reactions, leading to fully passivated $\equiv\textrm{Si-H}\cdots X\equiv$ structures, where $X$ is a group-III acceptor element. The formation and dissociation of acceptor-H and acceptor-H$_{2}$ complexes is considered in the context of Light and elevated Temperature Induced Degradation (LeTID) of silicon-based solar cells. Besides explaining observed trends and answering several fundamental questions regarding the properties of acceptor-hydrogen pairing, we find that the BH$_{2}$ complex is a by-product along the reaction of H$_{2}$ molecules with boron toward the formation of BH pairs (along with subtraction of free holes). The calculated changes in Helmholtz free energies upon the considered defect reactions, as well as activation barriers for BH$_{2}$ formation/dissociation (close to $\sim1$ eV) are compatible with the experimentally determined activation energies of degradation/recovery rates of Si:B-based cells during LeTID. Dihydrogenated acceptors heavier than boron are anticipated to be effective-mass-like shallow donors, and therefore, unlikely to show similar non-radiative recombination activity.

cond-mat.mtrl-sci↗

arXiv2023

Theory of shallow and deep boron defects in 4H-SiC

Abstract Despite advances toward improving the quality of $p$-type 4H-SiC substrates and layers, we still have no model capable of accounting for the multitude of boron-related optical, junction, and paramagnetic resonance experiments available in the literature. A conspicuous puzzle is the observation of two shallow boron defects with rather distinct axial orientations as found by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) data. This feature is not observed in material doped with other group-III elements. Another open issue involves conflicting conclusions from photoluminescence and EPR studies of a deeper boron center, which has been linked to rather distinct models, either based on substitutional or vacancy-related boron defects. We unlock these and other problems by means of first-principles calculations, where the temperature-dependent stability, the electronic activity, and the paramagnetic response of boron defects in 4H-SiC are investigated.

cond-mat.mtrl-sci↗

arXiv2022

Crucial role of vibrational entropy in the Si(111)-7$\times$7 surface structure stability

We investigate the relative thermodynamic stability of the $3\times3$, $5\times5$, $7\times7$, $9\times9$ and infinitely large structures related to the dimers-adatoms-stacking faults family of Si$(111)$ surface reconstructions by means of first-principles calculations. Upon accounting for the vibrational contribution to the surface free energy, we find that the $5\times5$ structure is more stable than the $7\times7$ at low temperatures. While a phase transition is anticipated to occur at around room temperature, the $7\times7\rightarrow5\times5$ transformation upon cooling is hindered by the limited mobility of Si atoms. The results not only flag a crucial role of vibrational entropy in the formation of the $7\times7$ structure at elevated temperatures, but also point for its metastable nature below room temperature.

cond-mat.mtrl-sci↗

arXiv2022

Dynamics of hydrogen in silicon at finite temperatures from first-principles

Hydrogen point defects in silicon still hold unsolved problems, whose disclosure is fundamental for future advances in Si technologies. Among the open issues is the mechanism for the condensation of atomic hydrogen into molecules in Si quenched from above $T\sim700\,^{\circ}$C to room temperature. Based on first-principles calculations, we investigated the thermodynamics of hydrogen monomers and dimers at finite temperatures within the harmonic approximation. The free energies of formation indicate that the population of H$^{-}$ cannot be neglected when compared that of H$^{+}$ at high temperatures. The results allow us to propose that the formation of molecules occurs during cooling processes, in the temperature window $T\sim700\textrm{-}500\,$K, above which the molecules collide with Si-Si bonds and dissociate, and below which the fraction of H$^{-}$ becomes negligible. The formation of H$^{-}$ and most notably of a fast-diffusing neutral species could also provide an explanation for the apparent \emph{accelerated} diffusivity of atomic hydrogen at elevated temperatures in comparison to the figures extrapolated from measurements carried out at cryogenic temperatures. We finally show that the observed diffusivity of the molecules is better described upon the assumption that they are nearly free rotors, all along the minimum energy path, including at the transition state.

cond-mat.mtrl-sci↗

arXiv2020

Silicon carbide diodes for neutron detection

In the last two decades we have assisted to a rush towards finding a He3-replacing technology capable of detecting neutrons emitted from fissile isotopes. The demand stems from applications like nuclear war-head screening or preventing illicit traffic of radiological materials. Semiconductor detectors stand among the stronger contenders, particularly those based on materials possessing a wide band gap like silicon carbide. We review the workings of SiC-based neutron detectors, along with several issues related to material properties, device fabrication and testing. The paper summarizes the experimental and theoretical work carried out within the E-SiCure project, co-funded by the NATO SPS Programme. Among the achievements, we have the development of successful Schottky barrier based detectors and the identification of the main carrier life-time-limiting defects in the SiC active areas, either already present in pristine devices or introduced upon exposure to radiation fields. The physical processes involved in neutron detection are described. Material properties as well as issues related to epitaxial growth and device fabrication are addressed. The presence of defects in as-grown material, as well as those introduced by ionizing radiation are reported. We finally describe several experiments carried out at the Jozef Stefan Institute TRIGA Mark II reactor (Ljubljana, Slovenia), where a set of SiC-based neutron detectors were tested, some of which being equipped with a thermal neutron converter layer. We show that despite the existence of large room for improvement, Schottky barrier diodes based on state-of-the-art 4H-SiC are closing the gap regarding the sensitivity offered by gas-based and that of semiconductor detectors.

physics.ins-det↗

arXiv2020

Characterisation of negative-U defects in semiconductors

This review aims at providing a retrospective, as well as a description of the state-of-the-art and future prospects regarding the theoretical and experimental characterisation of negative-U defects in semiconductors. This is done by complementing the account with a description of the work that resulted in some of the most detailed, and yet more complex defect models in semiconductors. The essential physics underlying the negative-U behaviour is presented, including electronic correlation, electron-phonon coupling, disproportionation, defect transition levels and rates. Techniques for the analysis of the experimental data and modelling are also introduced, namely defect statistics, kinetics of carrier capture and emission, defect transformation, configuration coordinate diagrams and other tools. We finally include a showcase of several works that led to the identification of some of the most impacting negative-U defects in group-IV and compound semiconductors.

cond-mat.mtrl-sci↗

arXiv2019

Sub-Band-Gap Absorption Mechanisms Involving Oxygen Vacancies in Hydroxyapatite

Hydroxyapatite (HAp) is a widely used biomaterial for the preparation of bone and dental implants. Despite the relevance of HAp in medicine, exciting applications involving this material as a bio-compatible photocatalyst, depend on how well we understand its fundamental properties. Experimental evidence suggests that oxygen vacancies play a critical role in the production of surface radicals upon exposure of HAp to ultra-violet (UV) light. However, very little is known about the underlying physical and chemical details. We present a hybrid density-functional study of the structural and electronic properties of oxygen vacancies in large HAp supercells within the plane-wave formalism. We find that under equilibrium conditions, vacancies occur either as a simple vacant oxygen site (in the neutral charge state), or as extended structures replacing several crystalline moieties (in the double plus charge state). Large atomic relaxations upon ionization make the oxygen vacancy a negative-$U$ defect, where the single plus charge state is metastable, being only accessible under UV excitation. From inspection of the transition levels, we find that electron promotion from the valence band top to the donor state of the vacancy, involves a zero-phonon transition of 3.6-3.9 eV. This mechanism is the most likely explanation to the 3.4-4.0 eV absorption onset for the observation of photocatalysis using HAp under persistent UV illumination.

cond-mat.mtrl-sci↗

arXiv2019

Acceptor levels of the carbon vacancy in $4H$-SiC: combining Laplace deep level transient spectroscopy with density functional modeling

We provide direct evidence that the broad Z$_{1/2}$ peak, commonly observed by conventional DLTS in as-grown and at high concentrations in radiation damaged $4H$-SiC, has two components, namely Z$_{1}$ and Z$_{2}$, with activation energies for electron emission of 0.59 and 0.67~eV, respectively. We assign these components to $\mathrm{Z}_{1/2}^{=}\rightarrow\mathrm{Z}_{1/2}^{-}+e^{-}\rightarrow\mathrm{Z}_{1/2}^{0}+2e^{-}$ transition sequences from negative-$U$ ordered acceptor levels of carbon vacancy (V$_{\mathrm{C}}$) defects at hexagonal/pseudo-cubic sites, respectively. By employing short filling pulses at lower temperatures, we were able to characterize the first acceptor level of V$_{\mathrm{C}}$ on both sub-lattice sites. Activation energies for electron emission of 0.48 and 0.41~eV were determined for $\mathrm{Z}_{1}(-/0)$ and $\mathrm{Z}_{2}(-/0)$ transitions, respectively. Based on trap filling kinetics and capture barrier calculations, we investigated the two-step transitions from neutral to doubly negatively charged Z$_{1}$ and Z$_{2}$. Positions of the first and second acceptor levels of V$_{\mathrm{C}}$ at both lattice sites, as well as $(=\!/0)$ occupancy levels were derived from the analysis of the emission and capture data.

cond-mat.mtrl-sci↗

arXiv2018

Atomic and electronic structure of the Si(331)-(12$\times$1) surface

We report on the investigation of the atomic and electronic structures of a clean Si(331)-(12$\times$1) surface using a first-principles approach with both plane wave and strictly localized basis sets. Starting from the surface structure proposed by Zhachuk and Teys [Phys. Rev. B 95, 041412(R) (2017)], we develop significant improvements to the atomic model and localized basis set which are critical for the correct description of the observed bias dependence of scanning tunneling microscopy (STM) images. The size mismatch between the Si pentamers from the surface model and those seen by STM is explained within the context of the Tersoff-Hamann model. The energy barriers that separate different Si(331) buckled configurations were estimated, showing that the surface structure is prone to dynamic buckling at room temperature. It is found that empty electronic states on Si(331) are essentially localized on the pentamers with interstitials and under-coordinated Si sp$^2$-like atoms between them, while filled electronic states are localized on under-coordinated Si sp$^3$-like atoms and dimers on trenches. The calculated electronic density of states exhibits two broad peaks in the fundamental bandgap of Si: one near the valence band top and the other near the conduction band bottom. The resulting surface bandgap of 0.58 eV is in an excellent agreement with spectroscopy studies.

cond-mat.mtrl-sci↗

arXiv2018

First-principles calculations of iron-hydrogen reactions in silicon

Controlling the contamination of silicon materials by iron, especially dissolved interstitial iron (Fe$_{\mathrm{i}}$), is a longstanding problem with recent developments and several open issues. Among these we have the question whether hydrogen can assist iron diffusion, or if significant amounts of substitutional iron (Fe$_{\mathrm{s}}$) can be created. Using density functional calculations we explore the structure, formation energies, binding energies, migration, and electronic levels of several FeH complexes in Si. We find that a weakly bound Fe$_{\mathrm{i}}$H pair has a migration barrier close to that of isolated Fe$_{\mathrm{i}}$ and a donor level at $E_{\mathrm{v}}+0.5$~eV. Conversely, Fe$_{\mathrm{i}}$H$_{2}(0/+)$ is estimated at $E_{\mathrm{v}}+0.33$~eV. These findings suggest that the hole trap at $E_{\mathrm{v}}+0.32$~eV measured by capacitance measurements should be assigned to Fe$_{\mathrm{i}}$H$_{2}$ . Fe$_{\mathrm{s}}$H-related complexes show only deep acceptor activity and are expected to have little effect on minority carrier life-time in $p$-type Si. The opposite conclusion can be drawn for $n$-type Si. We find that while in H-free material Fe$_{\mathrm{i}}$ defects have lower formation energy than Fe$_{\mathrm{s}}$, in hydrogenated samples Fe$_{\mathrm{s}}$-related defects become considerably more stable. This would explain the observation of an EPR signal attributed to a Fe$_{\mathrm{s}}$H-related complex in hydrogenated Si, which was quenched from above 1000$^{\circ}$C to iced-water temperature.

cond-mat.mtrl-sci↗

arXiv2018

Optoelectronics and defect levels in hydroxyapatite by first-principles

Hydroxyapatite (HAp) is an important component of mammal bones and teeth, being widely used in prosthetic implants. Despite the importance of HAp in medicine, several promising applications involving this material e.g. in photo-catalysis), depend on how well we understand its fundamental properties. Among the ones that are either unknown or not known accurately we have the electronic band structure and all that relates to it, including the band gap width. We employ state-of-the-art methodologies, including density hybrid-functional theory and many-body perturbation theory within the GW approximation, to look at the optoelectronic properties of HAp. These methods are also applied to the calculation of defect levels. We find that the use of a mix of (semi-)local and exact exchange in the exchange-correlation functional, brings a drastic improvement to the band structure. Important side-effects include improvements in the description of dielectric and optical properties, not only involving conduction band (excited) states, but also the valence. We find that the highly dispersive conduction band bottom of HAp originates from anti-bonding $σ^{*}$ states along the $\cdots\textrm{OH-OH-}\cdots$ infinite chain, suggesting the formation of a conductive 1D-ice phase. The choice of the exchange-correlation treatment to the calculation of defect levels was also investigated by using the OH-vacancy as testing-model. We find that donor and acceptor transitions obtained within semi-local DFT differ from those of hybrid-DFT by almost 2 eV. Such a large discrepancy emphasizes the importance of using a high-quality description of the electron-electron interactions in the calculation of electronic and optical transitions of defects in HAp.

cond-mat.mtrl-sci↗

arXiv2017

Theory of the carbon vacancy in $4H$-SiC: crystal field and pseudo Jahn-Teller effects

The carbon vacancy in $4H$-SiC is a powerful minority carrier recombination center and a major cause of degradation of SiC-based devices. Despite the extensiveness of the literature regarding the characterization and modeling of the defect, many fundamental questions persist. Among them we have the shaky connection of the EPR data to the electrical measurements, the physical origin of the pseudo-Jahn-Teller (pJT) effect, the reasoning for the observed sub-lattice dependence of the paramagnetic states, and the severe temperature-dependence of some hyperfine signals which cannot be accounted for by a thermally-activated dynamic averaging between equivalent JT-distorted structures. In this work we address these problems by means of semi-local and hybrid density functional calculations. We start by inventorying a total of four vacancy structures. Diamagnetic states have well defined low-energy structures, whereas paramagnetic states display metastability. This rich structural variety is traced back to the filling of electronic states which are shaped by a crystal-field-dependent pJT effect. From calculated minimum energy paths for defect rotation and transformation mechanisms, combined with the calculated formation energies and electrical levels, we arrived at a configuration-coordinate diagram of the defect. The diagram provides us with a detailed first-principles picture of the defect when subject to optical and thermal excitations. The calculated acceptor and donor transitions agree well with Z$_{1/2}$ and EH$_{6/7}$ trap energies, respectively. From comparison of calculated and measured $U$-values, and correlating the site-dependent formation energies with the relative intensity of the DLTS peaks in as-grown material, we assign Z$_{1}$ (EH$_{6}$) and Z$_{2}$ (EH$_{7}$) signals to acceptor (donor) transitions of carbon vacancies located on the $h$ and $k$ sub-lattice sites, respectively.

cond-mat.mtrl-sci↗