The effect of the atomic relaxation around defects on the electronic structure and optical properties of β-SiC

Abstract

The electronic structure and the optical properties associated with antisite defects in cubic SiC have been computed by means of the LMTO (linear muffin-tin orbital) method and the supercell approach. The orbital-dependent LDA + U potential (LDA ≡ local density approximation) used in the present work gives rise to an improved description both of the electronic structure near the energy gap and of the optical functions. Attention has been mainly focused on the effects caused by the local lattice relaxation around the defects. For compositions that deviate from the stoichiometric SiC towards higher content of carbon atoms, the small reduction of the energy gap which is observed experimentally can be explained only if the lattice relaxation is taken into account. The local electronic structure of antisite defects is characterized by s-and p-like resonance states in the valence band. Strong resonances occur also in the conduction band (especially for C Si ). The Si C (C Si ) antisite has more (fewer) valence electrons localized in the atomic sphere than the official Si (C) atom, but this difference is considerably reduced by the lattice relaxation. The results of the calculations show how the presence of point defects modifies the shape of the optical functions of the perfect SiC crystal and how the lattice relaxation has a strong effect on the fine structure of the optical functions. Different kinds of defect lead to different shapes of the optical functions.

Ab-initiostudy of exchange constants and electronic structure in diluted magnetic group-IV semiconductors

“…Due to the failures of bare GGA in predicting the correct semiconducting behavior for pure Ge, we used the LDA + U (or GGA + U ) approach [22] and added a Hubbard U on Ge p states (forcing the density matrix to be fixed so as to fully occupy the p-bands). In fact, although the GGA + U approach is generally used for systems with strongly correlated electrons, it was successfully applied to SCs [23] to partially correct the deficiencies of a bare GGA treatment even in SCs. A value of U = 2 eV was found to reproduce the correct experimental band gap in pure Ge (E gap ∼ 0.65 eV) and was consistently applied in all the simulations (i.e.…”

Topologicalα-Sn surface states versus film thickness and strain

“…The calculations are however not straightforward. In order to obtain a correct energetic ordering of the bulk bands near an empirical on-site Coulomb repulsion U = 3.5 eV [16] has to be added to the LDA XC functional for p states [14]. A modified Becke-Johnson (MBJ) potential together with the XC potential from LDA [17,18] also describes the correct ordering of the bulk bands and their energetic distances.…”

Study of intrinsic defects in 3C-SiC using first-principles calculation with a hybrid functional

“…16,17 A variety of methods have been developed and utilized to overcome the band gap problem, including the self-interaction correction method (SIC), 18 quasi-particle approximation (GW approximation), 19,20 Hubbard-U correction (DFT+U) 21,22 with negative U values, [23][24][25] and application of the Hartree-Fock hybrid exchange-correlation functional. 26 Regarding SiC, several studies have been performed using the SIC method, 12,27 GW approximation, [28][29][30][31][32] DFT+U, 33,34 and hybrid functionals. [35][36][37] In these results, not only the band gap description but also the nature of defects, such as transition levels of the defect charge state and the location of defect levels, were also largely improved.…”