The electronic structures of two Mg- Si codoped configurations in wurtzite GaN were calculated by an ab initio "mixed-basis 1 norm conserving nonlocal pseudopotential" method. The results show that the charge densities redistribute and concentrate in the N-Mg bonds for the configuration with a local polarized field component that strengthens the polarizability. The tops of the valence bands are thus split widely and shifted up towards the conduction band. The N atoms bonding to the Mg atom are found to be as important as the Mg atom for the formation of the tops of the valence band. These electronic structure shifts can enhance the hole concentration about 103 times higher than that of Mg-doped GaN.

The properties of layer structures in AlxGa12xN/AlN interfaces were studied by employing first-principles total-energy calculations with density-functional theory. A structural transformation of AlxGa12xN overlayers from normal wurtzite structure to zinc blende structure was found when the compressive in-plane strain was between two critical values.

The electronic structures of the substitutional O on N site and C on Ga site in wurtzite GaN have been studied by employing ab initio [1]mixed-basis +norm conserving non-local pseudo-potential method and a 32-atom wurtzite su percell with and without lattice relaxations. Present calculations indicate that the host Ga atoms bonding to O impurity relax outward slightlywhile one of them draws along the c-axis toward another. The charge densitydistribution appears distinctlylower with lattice relaxations near the host Ga atoms bonding to the O impurity. These results suggest that the substitutional O with cation-cation-bond configuration is likelyto act as the DX center in wurtzite GaN with heavyO dopants. On the other hand, the host N atoms bonding to the substitutional C relax inward largely which is accompanied byone of them turning toward another. The charge densitydistributions around the substitutional C are distinctlyhigher with lattice relaxations. The results of the energyband structure suggest that the substitutional C acts as a deep electron trap that is expected to offer electrons under light excitation.

Photoluminescence (PL) was investigated in undoped GaN from 4.8 K to room temperature. The 4.8 K spectra exhibited recombinations of free exciton, donor-acceptor pair (DAP), blue and yellow bands (Ybs). The blue band (BB) was also identified to be a DAP recombination. The YB was assigned to a recombination from deep levels. The energy-dispersive X-ray spectroscopy show that C and O are the main residual impurities in undoped GaN and that C concentration is lower in the epilayers with the stronger BB. The electronic structures of native defects, C and O impurities, and their complexes were calculated using ab initio local-density-functional (LDF) methods with linear muffin-tin-orbital and 72-atomic supercell. The theoretical analyses suggest that the electron transitions from ON states to CN and to VGa states are responsible for DAP and the BB, respectively, and the electron transitions between the inner levels of the CN-ON complex may be responsible for the YB in our samples.