GaN(0001) surface states: Experimental and theoretical fingerprints to identify surface reconstructions

The surface electronic structure of GaN(0001) surfaces is characterized by photoelectron spectroscopy. Depending on the surface preparation conditions, such as cooldown in nitrogen plasma after growth and additional vacuum annealing or Ga deposition, different surface states are observed at the valence-band edge and inside the band gap of GaN, while the surface Fermi-level position E-F was found to be independent at 2.9-3.0 eV, indicative of unoccupied surface states that pin E-F. The experimental results are combined with band structures of different 2 x 2 reconstructed surfaces calculated by density-functional theory. Comparing the experimental results with the theoretical density of surface states allows an identification of the microscopic origin of these states and an assignment of the related surface structure. The presence of a 2 x 2 nitrogen adatom structure after growth is found that can be identified by its fingerprint surface states approximate to 0.9 eV above and approximate to 0.6 eV below the valence-band maximum (VBM). For Ga vacancy and adatom structures a similar agreement is found, revealing surface states approximate to 0.3 and approximate to 1.4 or approximate to 1.6 eV above the VBM, respectively.