Energetics of native point defects in GaN: A density-functional study

We study the formation energies of native point defects in GaN through density-functional theory. In our first-principles scheme, the band edges are positioned in accord with hybrid density functional calculations, thus yielding a band-gap in agreement with experiment. With respect to previous semilocal calculations, the calculated formation energies and charge transition levels are found to be significantly different in quantitative terms, while the overall qualitative trend remains similar. In Ga-rich conditions, the nitrogen vacancy corresponds to the most stable defect for all Fermi energies in the band gap, but its formation energy is too high to account for autodoping. Our calculations also indicate that the gallium vacancy does not play any compensating role in n-type GaN. (C) 2015 Elsevier B.V. All rights reserved.

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Giacomo Francesco Leonardo Miceli, Alfredo Pasquarello
Elsevier, 2015
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https://infoscience.epfl.ch/record/213986?ln=fr

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Théorie de la fonctionnelle de la densité

La théorie de la fonctionnelle de la densité (DFT, sigle pour Density Functional Theory) est une méthode de calcul quantique permettant l'étude de la structure électronique, en principe de manière ex

Bande interdite

redresse=.9|vignette|Bandes d'un semiconducteur. La bande interdite d'un matériau, ou gap, est l'intervalle, situé entre la bande de valence et la bande de conduction, dans lequel la densité d'états

Fonctionnelle hybride

Une fonctionnelle hybride est un élément d'une classe d'approximations à la fonctionnelle d'échange-corrélation, introduite en 1993 par Axel Becke, utilisé au sein de la théorie de la fonctionnelle de

Effect of improved band-gap description in density functional theory on defect energy levels in alpha-quartz

Audrius Alkauskas, Alfredo Pasquarello

We compare defect energy levels of point defects in alpha-quartz using density functionals of different accuracy: a standard generalized-gradient-approximation functional and a hybrid functional. The latter is known to improve the description of band gaps through the incorporation of a fraction of Hartree-Fock exchange. The separations between the defect levels are found to be preserved when going from the standard to the more accurate hybrid functional, indicating that they are already well described with the standard functional. We also find that the defect wave functions obtained with the two functionals are very similar. Consequently, the defect energy levels pertaining to the hybrid functional can be obtained through the perturbational method. (C) 2007 Elsevier B.V. All rights reserved.

2007

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Band-edge problem in the theoretical determination of defect energy levels: The O vacancy in ZnO as a benchmark case

Audrius Alkauskas, Alfredo Pasquarello

Calculations of formation energies and charge transition levels of defects routinely rely on density functional theory (DFT) for describing the electronic structure. Since bulk band gaps of semiconductors and insulators are not well described in semilocal approximations to DFT, band-gap correction schemes or advanced theoretical models, which properly describe band gaps, need to be employed. However, it has become apparent that different methods that reproduce the experimental band gap can yield substantially different results regarding charge transition levels of point defects. We investigate this problem in the case of the (+2/0) charge transition level of the O vacancy in ZnO, which has attracted considerable attention as a benchmark case. For this purpose, we first perform calculations based on nonscreened hybrid density functionals, and then compare our results with those of other methods. While our results agree very well with those obtained with screened hybrid functionals, they are strikingly different compared to those obtained with other band-gap-corrected schemes. Nevertheless, we show that all the different methods agree well with each other and with our calculations when a suitable alignment procedure is adopted. The proposed procedure consists in aligning the electron band structure through an external potential, such as the vacuum level. When the electron densities are well reproduced, this procedure is equivalent to an alignment through the average electrostatic potential in a calculation subject to periodic boundary conditions. We stress that, in order to give accurate defect levels, a theoretical scheme is required to yield not only band gaps in agreement with experiment, but also band edges correctly positioned with respect to such a reference potential.

2011

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Identification of defect levels at InxGa1-xAs/oxide interfaces through hybrid functionals

Hannu-Pekka Komsa, Alfredo Pasquarello

Hybrid density functional calculations and atomistic models are used to study point defects in III-V compounds. We first study dangling-bonds in InxGa1-xAs. In GaAs, the cation dangling-bond levels are found in the upper part of the band gap, but they are in resonance with the conduction band in In0.53Ga0.47 As and InAs. The As dangling-bond level falls within the band gap, at about 0.1 eV from the valence band maximum. Dangling bonds are thus identified as possible origin for the experimentally observed defect states near the band edges at InxGa1-xAs/oxide interfaces. To investigate the origin of the experimental defect density at mid gap, we then focus on GaAs and consider other native defects, such as the As antisite, the Ga vacancy, and the As vacancy. The assignment to the As antisite is favored by the correspondence of calculated and measured defect levels, its low formation energy, and the observed dependence of the defect density on As surface concentration. (C) 2011 Elsevier B.V. All rights reserved.

Elsevier2011

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