Coulomb correlation effects in zinc monochalcogenides

Abstract

Electronic structure and band characteristics for zinc monochalcogenides with zinc-blende-and wurtzite-type structures are studied by first-principles density-functional-theory calculations with different approximations. It is shown that the local-density approximation underestimates the band gap and energy splitting between the states at the top of the valence band, misplaces the energy levels of the Zn-3d states, and overestimates the crystal-field-splitting energy. The spin-orbit-coupling energy is found to be overestimated for both variants of ZnO, underestimated for ZnS with wurtzitetype structure, and more or less correct for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at the top of the valence band is found to be anomalous for both variants of ZnO, but is normal for the other zinc monochalcogenides considered. It is shown that the Zn-3d electrons and their interference with the O-2p electrons are responsible for the anomalous order. The effective masses of the electrons at the conduction-band minimum and of the holes at the valence-band maximum have been calculated and show that the holes are much heavier than the conduction-band electrons in agreement with experimental findings. The calculations, moreover, indicate that the effective masses of the holes are much more anisotropic than the electrons. The typical errors in the calculated band gaps and related parameters for ZnO originate from strong Coulomb correlations, which are found to be highly significant for this compound. The local-density-approximation with multiorbital mean-field Hubbard potential approach is found to correct the strong correlation of the Zn-3d electrons, and thus to improve the agreement between the experimentally established location of the Zn-3d levels and that derived from pure LDA calculations.

Electronic structure and optical properties ofZnX(X=O,S, Se, Te): A density functional study

“…21,22 These findings were ascribed 21,22 to strong Coulomb correlation effects. DFT calculations within LDA plus self-interaction correction (LDA+SIC) and LDA+U are found 21,22,26 to rectify the errors related to ∆ CF and ∆ SO , order of states at the top VB, width and location of the Zn 3d band, as well as effective masses. In other semiconductors, in which the Coulomb correlation is not sufficiently strong, the ∆ CF and ∆ SO values derived from DFT calculations within LDA are found to be quite accurate.…”

“…21,22,26,62,63,64 For the DFT calculations within LDA+U explicit values of the parameters U and J are required as input. In previous papers 21,22 we have estimated the values of the U and J parameters within the constrained DFT theory 65 and in a semiempirical way by performing the calculations for different values of U and forcing match to the experimentally established 66 location of the Zn-3d bands. Based on the results 21,22 the values of the parameters U and J listed in Table I are chosen to study optical spectra.…”

“…This was demonstrated for diamond-like group IV, z-type group III-V, II-VI, and I-VII semiconductors, 28 w-type AlN, GaN, and InN 29 using the LAPW and VASP-PAW, the wtype CdS and CdSe, 27 z-type ZnSe, CdTe, HgTe, 30 using the ab initio LMTO-ASA, z-and w-type ZnSe and ZnTe 21,22 as well as z-type CdTe 31 using the VASP-PAW and FP LMTO methods. Although the SO splitting at the top of VB is known to play an important role in electronic structure, and chemical bonding of semiconductors 21,22,26,28,29,30,32,33 there is no systematic study of the role of the SO coupling in optical properties of these materials.…”

“…Not only the band gap (E g ), but also the crystal-field (CF) and spin-orbit (SO) splitting energies (∆ CF and ∆ SO ), the order of states at the top of the valence band (VB), the location of the Zn-3d band and its width, and the band dispersion are found 21,22,26,27 to be incorrect for ZnO-w by the ab initio full potential (FP) and atomic-sphere-approximation (ASA) lin- ear muffin-tin orbital (LMTO) methods within the pure LDA, 26,27 and by the projector-augmented wave (PAW) method within LDA and GGA. 21,22 These findings were ascribed 21,22 to strong Coulomb correlation effects. DFT calculations within LDA plus self-interaction correction (LDA+SIC) and LDA+U are found 21,22,26 to rectify the errors related to ∆ CF and ∆ SO , order of states at the top VB, width and location of the Zn 3d band, as well as effective masses.…”

“…21,22,23,24,25 This is due to the mean-field character of the Kohn-Sham equations and the poor description of the strong Coulomb correlation and exchange interaction between electrons in narrow d bands (viz. the potential U ).…”