Effects of gradient and non-linear core corrections on structural and electronic properties of GaN bulk and (001) surfaces

We have investigated the effects of density gradient and non-linear core co rrections, within the first-principles pseudopotential method, on structura l and electronic properties of GaN bulk and (0 0 1) surfaces. We find that the combined use of the generalized gradient approximation and non-linear c ore correction for exchange and correlation (NGGA) produces important chang es in structural properties. The calculated bulk valence band electronic st ructure shows much better agreement with experiment when the NGGA scheme is used than when the Ga 3d electrons are considered explicitly as a part of the valence shell. We have discussed the atomic structure and chemical bond ing on the gallium terminated (1 x 1), (2 x 2), c(2 x 2) and (1 x 4) cubic- GaN(0 0 1) surface reconstructions, and find that the most stable of these, viz. the (1 x 4) structure, is characterised by a linear Ga tetramer with an energy gain of 0.29 eV per (1 x 1) cell over the unreconstructed (1 x 1) structure in agreement with previous results by Neugebauer and coworkers. (C) 2000 Elsevier Science B.V. All rights reserved.