Non-local screened-exchange calculations for defects in semiconductors: vacancy in silicon

Abstract

The microscopic structure of a silicon vacancy is studied theoretically using first-principles supercell calculations. Both the standard Kohn-Sham local-density approximation (LDA) scheme and the generalized Kohn-Sham screened-exchange local-density approximation (sX-LDA) scheme are used. The latter approximation is expected to improve the description of electronic levels in the gap region substantially, while providing accurate total energies and bond lengths. The present LDA calculations are in line with the earlier corresponding calculations of the silicon vacancy,predicting an inward relaxation of the nearest neighbours of the vacant site. The LDA calculations also predict the Jahn-Teller distortions and negative effective-U effects for charged vacancies, qualitatively in agreement with the experimental results and the Watkins model. In contrast to LDA results, the present sX-LDA calculations predict an outward relaxation and sp 2 type hybridization for the ions surrounding the vacancy. This somewhat surprising result is explained by the removal of the systematic overbinding associated with LDA.

Sampling the diffusion paths of a neutral vacancy in silicon with quantum mechanical calculations

“…The LDA results appear to be at odds with recent screened-exchange LDA calculations on partially relaxed neutral Si vacancy, it indicates that the relaxation could be outward 33 . More work remains to establish if LDA really fails here.…”

Band structure of functional oxides by screened exchange and the weighted density approximation

“…sX has the formal advantage that it is a true energy functional, which could be used in an energy minimisation, for example for defect structures [40]. However it has given an incorrect relaxation for the Si vacancy for as yet unknown reasons [41]. Screened exchange has also recently been implemented in the 3.0 and 4.0 versions of the planewave pseudopotential code, CASTEP [42,43], and it is this which is used here.…”

Alignment of defect levels and band edges through hybrid functionals: Effect of screening in the exchange term

“…3,6-10 However, their ability [11][12][13][14][15] of yielding band gaps and excitation energies in generally better agreement with experiment has often been one major motivation for adopting this new paradigm. Indeed, electronicstructure research involving the positioning of defect levels with respect to relevant band edges [16][17][18][19][20][21][22][23][24][25][26][27][28] and the band alignment at interfaces 29,30 have already been taking advantage of such hybrid functional schemes.…”