Infoscience

Thesis

Defect Insensitive Light Emission in Blue Diode Materials Investigated by Electron Microscopy and Transmission Cathodoluminescence

We study in this thesis the structural origin of defect insensitive light emission in indium containing group-III nitride ternary alloys grown on sapphire, which are the active materials for the commercially available high brightness blue diodes. These structures are known to be efficient light emitters despite the large number of dislocations they contain. Structural analysis was performed using scanning electron microscopes and (scanning) transmission electron microscopes, using various techniques, among them: bright field/dark field imaging, weak beam dark field imaging, high resolution transmission electron microscopy, energy dispersive x-ray spectroscopy, and high angle annular scanning transmission electron microscopy for elemental contrast (z-contrast). We identify structurally several types of compositional fluctuation around pure-edge, pure screw, and mixed dislocations in AlInN layers. We show that the elemental environment of an edge type dislocation is asymmetric, with an indium rich side and an indium poor side. We also observe spiral-like In-rich ring patterns around pure screw and mixed dislocations in AlInN . Using the obtained information from AlInN , we could identify the same disorder patterns in InGaN quantum wells by cathodoluminescence in a scanning transmission electron microscope. And we finally observe that stress induced disorder confines or shields charge carriers from dislocation cores in InGaN quantum wells, hence explaining defect insensitivity. Furthermore, we come to the interesting conclusion that, at least around mixed dislocations, elemental disorder engendered by the stress field of dislocations causes not only dislocation screening from carriers, but also increases luminescence by localizing charges on the In-rich sides of dislocations. We provide evidence of spatial confinement of carriers in these In-rich areas.

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