Large atomic displacements associated with the nitrogen antisite in GaN

Theoretical study of point defects in GaN and AlN; lattice relaxations and pressure effects

Native defects and some common dopants (Mg, Zn, and C) in cubic GaN and AlN are examined by means of ab initio theoretical calculations using two methods: i) the Green's function technique based on the linear muffin-tin orbital method in the atomic-spheres approximation; ii) a supercell approach in connection with the full-potential linear muffin-tin-orbital method. We apply the first method to look mainly at the energetic positions of the defect and impurity states in different charge states and their dependence on hydrostatic pressure. The second method allows us to study lattice relaxations. Whereas small relaxations are found near vacancies and substitutional Mg and Zn, the calculations predict large atomic displacements around antisite defects and the substitutional carbon impurity on the cation site.

Observation of Midgap States in GaN with Optical-Isothermal Capacitance Transient Spectroscopy

Optical-isothermal capacitance transient spectroscopy (O-ICTS) was used to distinguish the deep levels which occur in unintentionally doped n-type GaN by means of their characteristic optical cross section. GaN grown by metalorganic vapor phase epitaxy (MOVPE) and hydride vapor phase epitaxy (HVPE) were compared. Correspondence between optical and thermal emission characteristics of previously discovered levels, E2 (∼Ec-0.55 eV) and E4 (∼EC-1.0 eV), were clearly determined by observing their sequential appearance in the ICTS spectra. Whether by thermal or optical stimulation, the emission from E4 was found to be broad in nature; it is consequently believed to involve a defect. The total measured concentration of deep levels, including a prominent level which photoionizes in the range 2.5 to 3.0 eV below the conduction band, is greater in the GaN grown by MOVPE than by HVPE that was tested.

Theoretical Study of Native Point Defects in Aln and InN

We have studied native point defects in AlN and InN using density-functional calculations employing both the local-density and generalized gradient approximations for the exchange-correlation functional. For both materials we find that the nitrogen vacancy acts as a compensating center in p-type material. For AIN in the zinc-blende structure, the aluminum interstitial has an equally low formation energy as the nitrogen vacancy. For n-type material the aluminum vacancy is the dominant compensating center in AlN. For n-type InN, all defect formation energies are high.

Vacancies in wurtzite GaN and AlN

Vacancies in wurtzite GaN and AlN are studied using a computational method which is based on the density functional theory (DFT) and takes into account the errors arising from use of finite-sized supercells and the DFT band gap underestimation. Negatively charged N vacancies in GaN and AlN are found to be stable, with formation energies similar to and higher than those of Ga and Al vacancies in n-type material under Ga- and Al-rich growth conditions, respectively. The localization and energies of the defect levels close to the computational conduction band edge are considered in detail.

Nitrogen vacancies as major point defects in gallium nitride

We present results of ab initio calculations for vacancies and divacancies in GaN. Particular attention is paid to nitrogen vacancies and mixed Ga-N divacancies in negatively charged states, which in n-type GaN are found to be energetically comparable with gallium vacancies. We also demonstrate that the activation energy for self-diffusion over the nitrogen sublattice is lower than over the gallium one for all Fermi-level positions, which implies the nitrogen vacancies are major defects in samples annealed at high temperatures. Possibilities for direct observations of nitrogen vacancies through positron annihilation experiments are discussed.