Defect levels of carbon-related defects at the SiC/SiO2 interface from hybrid functionals

We investigate carbon single-atom and pair defects at the SiC/SiO2 interface as candidate defects for the density of defect states in the SiC band gap. In order to accurately describe the electronic defect levels with respect to the SiC band edges, we use a hybrid density functional which reproduces the experimental band gap of SiC. The carbon pair defect consisting of two neighboring sp(2) hybridized carbon atoms is modeled in various configurations within a SiC/SiO2 model interface showing good structural parameters and an oxide density typical of amorphous SiO2. The carbon pair defect is found to contribute to the density of defect states not only in the lower and/or mid band gap of SiC, but also in the upper band gap, in contrast with previous studies. The carbonpair defect is also investigated via molecular models to achieve insight into the energy range spanned by its defect levels when the relative orientation of its sp(2) hybridization planes and the chemical nature of its neighbors are varied. Carbon single-atom defects on the oxide side of the interface are also modeled and found to contribute to the defect density in the band gap in a similar way as carbonpair defects. Comparison to the experimental defect density suggests that defects involving one or two carbon atoms cannot account for the high defect density observed in the vicinity of the SiC conduction band.

Charge transition levels of carbon-, oxygen-, and hydrogen-related defects at the SiC/SiO2 interface through hybrid functionals

Audrius Alkauskas, Peter Broqvist, Fabien Devynck, Alfredo Pasquarello

We calculate charge transition levels of various defects at the SiC/SiO2 interface within a scheme based on hybrid density functionals, which accurately reproduce the involved band gaps and band offsets. The defect levels are first evaluated in bulk models of the interface components and then aligned with respect to the interface band diagram through the use of a model interface showing good structural and electronic properties. Interface-specific polarization effects are evaluated separately through classical electrostatics. We considered carbon-related defects involving single atoms and dimers in both the crystalline SiC substrate and the amorphous SiO2 oxide. Our investigation also comprises oxygen-and hydrogen-related defects, including the Si-Si bond (O vacancy), the Si-2-C-O structure, the peroxy linkage, and the hydrogen bridge (Si-H-Si). Among the defects studied, the Si-2-C-O structure represents the best candidate for the high defect density measured near the conduction band of SiC.

2011

Extrinsic Defects in Amorphous Oxides: Hydrogen, Carbon, and Nitrogen Impurities in Alumina

Francesco Ambrosio, Zhendong Guo, Alfredo Pasquarello

We present a procedure for addressing extrinsic defects in amorphous oxides, in which the most stable defect configurations are identified through ab initio molecular dynamics in various charge states and studied through hybrid functional calculations. The protocol is further complemented with an electron counting scheme based on maximally localized Wannier functions, which allows one to identify the nominal charge state and the composition of the defect core unit. Here, we apply this approach to the study of hydrogen, carbon and nitrogen impurities in amorphous alumina (alpha-Al2O3), a highly relevant material for technological applications. Hydrogen is found to be amphoteric with a thermodynamic +1/-1 charge transition level lying at similar to 4.6 eV above the valence band maximum, in qualitative agreement with results obtained with crystalline models, but at a substantially different energy level. Hydroxyl groups are further shown to lead to the same defect states as observed for hydrogen. Application of our procedure to carbon and nitrogen impurities leads to structural configurations that are found to depend on the total charge set in the simulation cell. Through the adopted electron counting rule, we assess that carbon and nitrogen impurities are only found in neutral and in singly positive charge states, respectively. This indicates that neither carbon nor nitrogen give charge transition levels in the band gap, in strong contrast with results achieved for crystalline models. In addition, the defect core units are shown to incorporate a varying number of oxygen atoms, by which their formation energy depends on the oxygen chemical potential mu O. In oxygen-poor conditions, both the carbon and the nitrogen impurities favor bonding to Al atoms, while they tend to form single or double bonds with oxygen atoms as mu(0) increases. Based on the band alignment of alpha-Al2O3 with three technologically relevant semiconductors (GaAs, GaN and alpha-Fe2O3), we discuss the possible role of point defects in degrading the performance of electronic devices and in favoring hole transport across the oxide in water-splitting setups.

2019

Hybrid-functional calculations with plane-wave basis sets: Effect of singularity correction on total energies, energy eigenvalues, and defect energy levels

Audrius Alkauskas, Peter Broqvist, Alfredo Pasquarello

When described through a plane-wave basis set, the inclusion of exact nonlocal exchange in hybrid functionals gives rise to a singularity, which slows down the convergence with the density of sampled k points in reciprocal space. In this work, we investigate to what extent the treatment of the singularity through the use of an auxiliary function is effective for k-point samplings of limited density, in comparison to analogous calculations performed with semilocal density functionals. Our analysis applies, for instance, to calculations in which the Brillouin zone is sampled at the sole Gamma point, as often occurs in the study of surfaces, interfaces, and defects or in molecular-dynamics simulations. In the adopted formulation, the treatment of the singularity results in the addition of a correction term to the total energy. The energy eigenvalue spectrum is affected by a downwards shift in the energy eigenvalues of the occupied states, while those of the unoccupied states remain unaffected. Analogous corrections also speed up the convergence of screened exchange interactions despite the absence of a proper singularity. Focusing first on neutral systems, both finite and extended, we show that the account of the singularity corrections bears convergence properties which are quantitatively similar to those observed with semilocal density functionals. We emphasize that this is not the case for uncorrected energies, particularly for elongated simulation cells for which qualitatively different trends are found. We then consider differences between total energies of systems differing by their charge state. For systems involving localized electron states, such as ionization potentials and electron affinities of molecular systems or charge transition levels of point defects, the proper account of the singularity correction yields convergence properties which are similar to those of neutral systems. In the case of extended systems, such energy differences provide an alternative way to determine the band edges, but are found to converge more slowly with simulation cells than in corresponding semilocal functionals because of the exchange self-interaction associated to the extra charge.

2009