Two types of threading dislocations with edge components were investigated by a high-resolution transmission electron microscope in undoped GaN epilayers grown on Al2O3 substrates. One is a fully filled core with regular contraction and stretch of bright dots, and the other is incompletely filled with one bright dot less and irregular contraction and stretch of bright dots. The bright dots were distorted and degenerated into bright line segments at cores in areas with smaller local dislocation intervals. The calculated results suggested that the distorted bright regions are attributable to the glide and/or climb caused by nearby dislocation interactions.

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.

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.

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.

Heavily Ge-doped GaAs single crystals were grown by the liquid-encapsulated Czochralski technique in the absence and the presence of a magnetic field of 4000 G. Three photoluminescence emission bands A, B, and C were observed in the as-grown crystals. Bands A and B were explained by a deep acceptor and self-activated center emissions, respectively. The central energy and half width of band C increased with increasing Ge-doped concentration. The photoluminescence excitation spectrum of band C was observed to be broad. The temperature dependence of the full width at half maximum of band C was analyzed. The results were considered to be caused by distributed states with different energy positions and electron-lattice couplings due to the impurity concentration effect on the unknown Ge-related complexes responsible for band C. The emission intensity of band C was markedly weaker in the magnetic-field growth crystals where both donor and acceptor concentrations were higher than those in traditional growth crystals, which showed that the formation of the unknown Ge-related complex can be reduced and the Ge doping efficiency can be improved during crystal growth by applying a magnetic field to suppress the temperature fluctuations in molten GaAs.

Dislocations around precipitates in undoped AlGaN were investigated by transmission electron microscopy. The dislocation images were taken under different diffraction conditions. The dislocations are classified into two types, a pure edge dislocation loop and a close-coiled helical dislocation. Both types of dislocations were found to depend on the shape and size of the precipitate sources. It is suggested that the pure edge dislocation loop results from homogeneous shear stress and the close-coiled helical dislocation is caused by spherically symmetrical stress concentration at round ends of the precipitates and chemical force due to defect concentration change.

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.

GaN epilayers nitridated initially for different times have been investigated by light scattering tomography (LST) and Raman scattering. In the LST images of the plan-view epilayers, the light scattering defects mainly distribute in <112I0> directions. The density of the defects is lower in epilayer nitridated initially for a longer time. The defects are considered to be the straight threading edge dislocations on {1100} planes. The Raman shift of E2 mode is larger in the sample initially nitridated for a longer time. Our results show that the misfit between the GaN epilayer and the Al2O3 substrate is more unfavorably accommodated by the threading edge dislocations in the epilayers initially nitridated for a longer time.

High resolution Laplace defect spectroscopy was used to study the fine structure of the electron emission process of DXSn! centers in AlxGa12xAs (x50.26,0.53). Two groups of peaks in the spectra of electron emission rates were found to correspond to two DX-like centers observed by deep level transient spectroscopy. The line splitting in both groups derives from the alloy disorder effect attributed to the different local configurations of Al and Ga atoms around two DX-like centers. Experimental evidence for the microscopic nature of two DX-like centers in Sn-doped AlGaAs is provided.