Three dimensional analysis of the composition in solid alloys by variable probe in scanning transmission electron microscopy.

This paper reports on a novel approach to quantitatively reconstruct the column by column composition and the 3D distribution of guest atoms inside a host matrix by scanning transmission electron microscopy high angle annular dark field technique. We propose a new mathematical framework that allows to jointly analyze the information from a set of experiments with variable beam convergence and/or defocus. Our scheme allows to reconstruct the atomic distribution along the imaged columns from the measured intensity, for any dependence of the probe intensity on the depth. It is therefore well suited to incorporate channeling effects that are usually neglected in other approaches. As a case study, we focus here on the systematic variation of the beam convergence that permits to set the maximum of the channeling oscillations at different depths. We aim here to define the reliability and the limitation of this technique by the application of the method to accurate dynamic simulations in the case of the InGaN alloy.

Blocking growth by an electrically active subsurface layer: the effect of Si as an antisurfactant in the growth of GaN.

Combining aberration corrected high resolution transmission electron microscopy and density functional theory calculations we propose an explanation of the antisurfactant effect of Si in GaN growth. We identify the atomic structure of a Si delta-doped layer (commonly called SiN(x) mask) as a SiGaN(3) monolayer that resembles a √3×√3 R30° surface reconstruction containing one Si atom, one Ga atom, and a Ga vacancy (V(Ga)) in its unit cell. Our density functional theory calculations show that GaN growth on top of this SiGaN(3) layer is inhibited by forming an energetically unfavorable electrical dipole moment that increases with layer thickness and that is caused by charge transfer between cation dangling bonds at the surface to V(Ga) bound at subsurface sites.

X-ray diffraction from nonperiodic layered structures with correlations: analytical calculation and experiment on mixed Aurivillius films.

X-ray diffraction from films consisting of layers with different thicknesses, structures and chemical contents is analysed. The disorder is described by probabilities for different sequences of layers. Closed analytical expressions for the diffracted X-ray intensity are obtained when the layers form a stationary Markov chain. The proposed model is applied to the diffraction data from epitaxial sodium bismuth titanate thin films with Aurivillius structure possessing such one-dimensional disorder. In this case, the disorder is caused by a random stacking of three and four perovskite units separated by bismuth oxide interlayers. The results of analytical calculations are in good agreement with the experimental data and indicate that the incorporation of sodium in the Bi(4)Ti(3)O(12) phase causes the formation of a fourth perovskite unit.

Compositional correlation and anticorrelation in quaternary alloys: competition between bulk thermodynamics and surface kinetics.

We analyze the atomistic mechanisms driving the compositional correlation of In and N in the quaternary Inx Ga1-xAs1-yNy alloys combining atomic scale chemical analysis in transmission electron microscopy and density-functional theory calculations. Our results show that for typical growth conditions surface kinetics prevail over bulk thermodynamics resulting in a hitherto unexpected compositional anticorrelation between In and N.

Strain induced deep electronic states around threading dislocations in GaN.

Combining through-focus high-resolution transmission electron microscopy and hierarchical multiscale simulations consisting of density-functional theory, analytical empirical potentials, and continuum elastic theory we demonstrate the existence of a new dislocation type in GaN. In contrast with all previously identified or suggested dislocation structures in GaN, all core atoms are fully coordinated; i.e., no broken bonds occur, implying that the dislocation should be electrically inactive. However, as we show, the giant local strain-field around the dislocation core, in combination with the small lattice constant of GaN, causes deep defect states and thus electrically active edge dislocations independent on the specific core structure.

Interstitial chemotherapy of experimental gliomas.

Two new approaches of interstitial (intratumoral) chemotherapy of gliomas are presented. Using s.c.-transplanted rat gliomas (G616) the therapeutic activity of biologically degradable polylactide rods as carriers for methotrexate was investigated. Carrier-mediated intratumoral chemotherapy was superior both to a systemic treatment and an intratumoral treatment with the free drug. The activity of the alkyllysophospholipids Et-18-OCH3 and BM 41.440 and of the alkylphosphocholine He-PC was investigated in a human glioma xenograft (T 406). All three compounds were highly active following intratumoral administration.