The microstructure, local indium composition and photoluminescence in green-emitting InGaN/GaN quantum wells.

In this work, we analyse the microstructure and local chemical composition of green-emitting Inx Ga1-x N/GaN quantum well (QW) heterostructures in correlation with their emission properties. Two samples of high structural quality grown by metalorganic vapour phase epitaxy (MOVPE) with a nominal composition of x = 0.15 and 0.18 indium are discussed. The local indium composition is quantitatively evaluated by comparing scanning transmission electron microscopy (STEM) images to simulations and the local indium concentration is extracted from intensity measurements. The calculations point out that the measured indium fluctuations may be correlated to the large width and intensity decrease of the PL emission peak.

Compositional characterization of GaAs/GaAsSb nanowires by quantitative HAADF-STEM.

The Sb concentration in axial GaAs(1-x)Sb(x) inserts of otherwise pure GaAs nanowires has been investigated with quantitative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The Sb concentration was quantified by comparing the experimental image intensities normalized to the incident beam intensity with intensities simulated with a frozen lattice multislice approach. Including static atomic displacements in the simulations was found to be crucial for correct compositional analysis of GaAs(1-x)Sb(x). HAADF intensities of individual nanowires were analysed both across the nanowires, exploiting their hexagonal cross-sectional shape, and along the evenly thick central part of the nanowires. From the cross-sectional intensity profiles, a decrease in the Sb concentration towards the nanowire outer surfaces was found. The longitudinal intensity profiles revealed a gradual build-up of Sb in the insert. The decrease of the Sb concentration towards the upper interface was either gradual or abrupt, depending on the growth routine chosen. The compositional analysis with quantitative HAADF-STEM was verified by energy dispersive X-ray spectroscopy.

Scattering amplitudes and static atomic correction factors for the composition-sensitive 002 reflection in sphalerite ternary III-V and II-VI semiconductors.

Modified atomic scattering amplitudes (MASAs), taking into account the redistribution of charge due to bonds, and the respective correction factors considering the effect of static atomic displacements were computed for the chemically sensitive 002 reflection for ternary III-V and II-VI semiconductors. MASAs were derived from computations within the density functional theory formalism. Binary eight-atom unit cells were strained according to each strain state s (thin, intermediate, thick and fully relaxed electron microscopic specimen) and each concentration (x = 0, …, 1 in 0.01 steps), where the lattice parameters for composition x in strain state s were calculated using continuum elasticity theory. The concentration dependence was derived by computing MASAs for each of these binary cells. Correction factors for static atomic displacements were computed from relaxed atom positions by generating 50 × 50 × 50 supercells using the lattice parameter of the eight-atom unit cells. Atoms were randomly distributed according to the required composition. Polynomials were fitted to the composition dependence of the MASAs and the correction factors for the different strain states. Fit parameters are given in the paper.

Light-emitting diode based on mask- and catalyst-free grown N-polar GaN nanorods.

We report on the fabrication of a light-emitting diode based on GaN nanorods containing InGaN quantum wells. The unique system consists of tilted N-polar nanorods of high crystalline quality. Photoluminescence, electroluminescence, and spatially resolved cathodoluminescence investigations consistently show quantum well emission around 2.6 eV. Scanning transmission electron microscopy and energy-dispersive x-ray spectroscopy measurements reveal a truncated shape of the quantum wells with In contents of (15 ± 5)%.

A structural investigation of highly ordered catalyst- and mask-free GaN nanorods.

GaN nanorods were grown on r-plane sapphire substrates by a two-step approach. Nucleation sites for the nanorods were provided by the formation of AlN islands during nitridation in a metal organic vapor phase system. These islands are a-plane oriented as expected for nitride growth on r-plane sapphire. The nanorods themselves were grown by plasma assisted molecular beam epitaxy. The nanorods show an inclination towards the surface normal of 28.3° and are highly ordered. Studies with high resolution x-ray diffraction polar plots reveal the epitaxial relationship between the substrate and nanorods as a c-direction growth on inclined m-plane facets of the nitridated islands. The determined lattice constants show nanorods which are strain free. The growth direction of the nanorods has been confirmed in a transmission electron microscope by convergent beam electron diffraction patterns to be in the N-polar [Formula: see text] direction.

Temperature-dependent Debye-Waller factors for semiconductors with the wurtzite-type structure.

We computed Debye-Waller factors in the temperature range from 0.1 to 1000 K for AlN, GaN, InN, ZnO and CdO with the wurtzite-type structure. The Debye-Waller factors were derived from phonon densities of states obtained from Hellmann-Feynman forces computed within the density-functional-theory formalism. The temperature dependences of the Debye-Waller factors were fitted and fit parameters are given.

Computation and parametrization of the temperature dependence of Debye-Waller factors for group IV, III-V and II-VI semiconductors.

We calculated the temperature dependence of the Debye-Waller factors for a variety of group IV, III-V and II-VI semiconductors from 0.1 to 1000 K. The approach used to fit the temperature dependence is described and resulting fit parameters are tabulated for each material. The Debye-Waller factors are deduced from generalized phonon densities of states which were derived from first principles using the WIEN2k and the ABINIT codes.

Determination of the mean inner potential in III-V semiconductors, Si and Ge by density functional theory and electron holography.

The mean inner potentials of various III-V semiconductors, Si and Ge have been calculated by density functional theory methods. For that purpose, the Coulomb potential of slabs consisting of a crystal and vacuum region has been computed and averaged inside the crystal region. The computed values are in agreement with experimental values obtained by electron holography for Si and GaAs. For the other semiconductors, the deviations are smaller than 0.8 V. The results from density functional theory are approximately 10% smaller than the values derived from atomic scattering factors computed by Hartree Fock calculations.