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

Gouveia, José D.; Coutinho, J.

doi:10.1088/2516-1075/aafc4b

Cited by 23 publications

(17 citation statements)

References 47 publications

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“…The HSE06 functional is widely accepted as capable of accurately capturing the 3C-and 4H-SiC electronic structure, yielding calculated band gaps of 2.24 eV and 3.17 eV at T = 0 K, respectively, close to the experimental values of 2.4 eV [11] and 3.27 eV [47]. The use of screened hybrid DFT energies of PBE-relaxed structures was recently tested for solid-state problems, including defects in SiC [48]. This approach allows us (i) to employ large enough supercells to accommodate the strain fields produced by the vacancy and at the same time (ii) to avoid artificial hybridization between vacancy and crystalline states.…”

Section: Computational Detailsmentioning

confidence: 89%

Anisotropic and plane-selective migration of the carbon vacancy in SiC: Theory and experiment

et al. 2019

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We investigate the migration mechanism of the carbon vacancy (V C) in silicon carbide (SiC) using a combination of theoretical and experimental methodologies. The V C , commonly present even in state-of-the-art epitaxial SiC material, is known to be a carrier lifetime killer and therefore strongly detrimental to device performance. The desire for V C removal has prompted extensive investigations involving its stability and reactivity. Despite suggestions from theory that V C migrates exclusively on the C sublattice via vacancy-atom exchange, experimental support for such a picture is still unavailable. Moreover, the existence of two inequivalent locations for the vacancy in 4H-SiC [hexagonal, V C (h), and pseudocubic, V C (k)] and their consequences for V C migration have not been considered so far. The first part of the paper presents a theoretical study of V C migration in 3C-and 4H-SiC. We employ a combination of nudged elastic band (NEB) and dimer methods to identify the migration mechanisms, transition state geometries, and respective energy barriers for V C migration. In 3C-SiC, V C is found to migrate with an activation energy of E A = 4.0 eV. In 4H-SiC, on the other hand, we anticipate that V C migration is both anisotropic and basal-plane selective. The consequence of these effects is a slower diffusivity along the axial direction, with a predicted activation energy of E A = 4.2 eV, and a striking preference for basal migration within the h plane with a barrier of E A = 3.7 eV, to the detriment of the k-basal plane. Both effects are rationalized in terms of coordination and bond angle changes near the transition state. In the second part, we provide experimental data that corroborates the above theoretical picture. Anisotropic migration of V C in 4H-SiC is demonstrated by deep level transient spectroscopy (DLTS) depth profiling of the Z 1/2 electron trap in annealed samples that were subject to ion implantation. Activation energies of E A = (4.4 ± 0.3) eV and E A = (3.6 ± 0.3) eV were found for V C migration along the c and a directions, respectively, in excellent agreement with the analogous theoretical values. The corresponding prefactors of D 0 = 0.54 cm 2 /s and 0.017 cm 2 /s are in line with a simple jump process, as expected for a primary vacancy point defect.

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Section: Computational Detailsmentioning

confidence: 89%

Anisotropic and plane-selective migration of the carbon vacancy in SiC: Theory and experiment

et al. 2019

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“…For the calculation of forces and structural relaxation (GGA-level), the Brillouin zone (BZ) was sampled on a Γ-centered 2 × 2 × 2 k-point grid (Γ−2 3 ). On the other hand, for total energy calculations (HSE06-level), the sampling was done at the Γ-point (see [47,55] for details).…”

Section: Theoretical Methodsmentioning

confidence: 99%

“…With the above specifications, quantities derived from the energy of (local) minimum structures (e.g., formation energies, transition levels) have an estimated numerical accuracy of about 10 meV. On the other hand, transition state calculations (activation energies for migration and structural transformation) have a numerical accuracy of 0.1 eV [47,55].…”

Section: Theoretical Methodsmentioning

confidence: 99%

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

Coutinho

2021

Crystals

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This paper presents a theoretical study of the electronic and dynamic properties of silicon vacancies and self-interstitials in 4H–SiC using hybrid density functional methods. Several pending issues, mostly related to the thermal stability of this defect, are addressed. The silicon site vacancy and the carbon-related antisite-vacancy (CAV) pair are interpreted as a unique and bistable defect. It possesses a metastable negative-U neutral state, which “disproportionates” into VSi+ or VSi−, depending on the location of the Fermi level. The vacancy introduces a (−/+) transition, calculated at Ec−1.25 eV, which determines a temperature threshold for the annealing of VSi into CAV in n-type material due to a Fermi level crossing effect. Analysis of a configuration coordinate diagram allows us to conclude that VSi anneals out in two stages—at low temperatures (T≲600 °C) via capture of a mobile species (e.g., self-interstitials) and at higher temperatures (T≳1200 °C) via dissociation into VC and CSi defects. The Si interstitial (Sii) is also a negative-U defect, with metastable q=+1 and q=+3 states. These are the only paramagnetic states of the defect, and maybe that explains why it escaped detection, even in p-type material where the migration barriers are at least 2.7 eV high.

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“…On a second step the total energy was calculated on hybrid-DFT level for obtained structures. It was shown [40][41][42] that relative errors in energies obtained within this methodology usually have an order of 10 meV.…”

Section: First-principles Defect Energeticsmentioning

confidence: 99%

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

et al. 2021

Self Cite

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Iron-doped hydroxyapatite (Fe-HAp) is regarded as a promising magnetic material with innate biocompatibility. Despite the many studies reported in the literature, a detailed theoretical description of Fe inclusions is still missing. There is even no consensual view on what kind of Fe defects take place in Fe-HAp—iron interstitial or calcium substitutions? In order to address these questions, we employ modern first-principles methodologies, including hybrid density functional theory, to find the geometry, electronic, magnetic and thermodynamic properties of iron impurities in Fe-HAp. We consider a total of 26 defect configurations, including substitutional (phosphorus and calcium sites) and interstitial defects. Formation energies are estimated considering the boundaries of chemical potentials in stable hydroxyapatite. We show that the most probable defect configurations are: Fe3+ and Fe2+ substitutions of Ca(I) and Ca(II) sites under Ca-poor conditions. Conversely, Fe interstitials near the edge of the hydroxyl channel are favored in Ca-rich material. Substitutional Fe on the P site is also a probable defect, and unlike the other forms of Fe, it adopts a low-spin state. The analysis of Fe K-XANES spectra available in the literature shows that Fe-HAp usually contains iron in different configurations.

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

Cited by 23 publications

References 47 publications

Anisotropic and plane-selective migration of the carbon vacancy in SiC: Theory and experiment

Anisotropic and plane-selective migration of the carbon vacancy in SiC: Theory and experiment

Theory of the Thermal Stability of Silicon Vacancies and Interstitials in 4H–SiC

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

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