We employ density-functional theory (DFT) within the local-density approximation (LDA) and generalized-gradient approximation (GGA) to study structural and electronic properties of clean $\mathrm{GaN}\left(0001\right)$ surfaces. The pseudopotential method is used to investigate surfaces with $(1\times 1)$, $(2\times 2)$, and $(\sqrt{3}\times \sqrt{3})R30°$ reconstructions. We also report calculations for the ${\mathrm{N}}_2$ molecule and for the bulk phases of Ga and $\mathrm{GaN}$. We find that GGA give better results than LDA for the cohesive energies, but not for the structural properties. Bulk band structures are found to be very similar for both exchange-correlation potentials. Examining the clean $\mathrm{GaN}\left(0001\right)$ surfaces we conclude that both potentials give very similar relaxations and an almost identical dispersion for the surface states. We also report results for ionization energies, electron affinities, and work function for the $\mathrm{GaN}\left(0001\right)$ surfaces. As a general trend the ionization energy decreases monotonically with the increasing of the Ga-coverage.

• Received 17 August 2005

DOI:https://doi.org/10.1103/PhysRevB.73.205346