Oxygen vacancy in monoclinic HfO2: A consistent interpretation of trap assisted conduction, direct electron injection, and optical absorption experiments

The authors calculate energy levels associated with the oxygen vacancy in monoclinic HfO2 using a hybrid density functional which accurately reproduces the experimental band gap. The most stable charge states are obtained for varying Fermi level in the HfO2 band gap. To compare with measured defect levels, they determine total energy differences specific to the considered experiment. Their results show that the oxygen vacancy can consistently account for the defect levels observed in (Poole-Frenkel-type) trap assisted conduction, direct electron injection, and optical absorption experiments. (c) 2006 American Institute of Physics.

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Peter Broqvist, Alfredo Pasquarello
2006
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https://infoscience.epfl.ch/record/141762?ln=fr

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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

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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

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Fermi-level pinning through defects at GaAs/oxide interfaces: A density functional study

Davide Colleoni, Giacomo Francesco Leonardo Miceli, Alfredo Pasquarello

Using density functional calculations, we study a set of candidate defects for Fermi-level pinning at GaAs/oxide interfaces. The set of considered defects comprises both bulklike and interfacial defects, including As antisites, Ga and As dangling bonds, the As-As dimer/dangling bond defect, and several defect complexes. The defects are generated within atomistic model structures representing the GaAs/Al2O3 interface. Formation energies of bulklike defects are obtained and compared with those of corresponding bulk defects, while interfacial defects are studied through their relative defect energies. Finite-size corrections to the defect energies are applied through a scheme that accounts for the interfacial geometry of our models. Defect levels are defined as thermodynamic transition levels between different charge states and are calculated for all considered defects. Through an alignment procedure based on hybrid functional calculations, the defect levels are then positioned within the calculated band gap of GaAs that reproduces the experimental one, thereby enabling direct comparisons with the experimental density of defect states. Our study shows that several As-related defects show a similar amphoteric bistability between an As-As dimer state and a configuration with two doubly occupied As dangling bonds. The associated charge transition levels generally lie in the midgap region, in accord with experimental observations. This mechanism is proposed as the origin of the observed Fermi-level pinning at GaAs/oxide interfaces.

American Physical Society2015

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Defect energy levels in density functional calculations: Alignment and band gap problem

Audrius Alkauskas, Peter Broqvist, Alfredo Pasquarello

For materials of varying band gap, we compare energy levels of atomically localized defects calculated within a semilocal and a hybrid density-functional scheme. Since the latter scheme partially relieves the band gap problem, our study describes how calculated defect levels shift when the band gap approaches the experimental value. When suitably aligned, defect levels obtained from total-energy differences correspond closely, showing average shifts of at most 0.2 eV irrespective of band gap. Systematic deviations from ideal alignment increase with the extent of the defect wave function. A guideline for comparing calculated and experimental defect levels is provided.

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A hybrid density functional study of lithium in ZnO: Stability, ionization levels, and diffusion

Audrius Alkauskas, Alfredo Pasquarello, Alexandra Sarabando de Carvalho, Nava Setter, Alexander Tagantsev

The properties of interstitial and substitutional Li in wurtzite ZnO are modeled using hybrid density functional calculations. We investigate the impact of the band-gap error on the formation energies of the two defects and their dependence on the Fermi level. It is found that within a local-density approximation, the acceptor level of LiZn is very close to the valence-band top but as the band gap is opened, the acceptor state becomes more localized and the respective level is shifted upward. Taking polaronic effects into account, we place the ionization level of LiZn between Ev+0.60 eV and Ev+1.1 eV. This deeper level explains the difficulty in realizing p-type ZnO using Li as monodopant. Further, the mobility of the defects was investigated. While interstitial Li is mobile at low temperatures, independent of the stoichiometry, the diffusion of LiZn depends on the concentrations of intrinsic defects. Our calculations show that in O-rich material, where the defect is more stable, the dominant diffusion process corresponds to a dissociative mechanism requiring a substantial activation energy.

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