Using a fully-coupled, three-dimensional electro-thermal device simulator, we study the mechanism of efficiency degradation at high current operation in planar GaN-based light emitting

X-ray topographic and chemical etching examination of Si:Ge single crystals containing 1.2 at% and 3.0 at% Ge, together with precise lattice parameter measurements, was performed. 

Diffraction contrasts in the form of concentric `quasi-circles' (striations), probably due to the non-uniform distribution of Ge atoms, were observed in projection topographs.

The etching patterns revealed bands corresponding to striations and dislocations as etch pits. The central `core' crystal region (striation-free) exhibited a crystal lattice strongly disturbed by microdefects, as was concluded from the section topography analysis.

 The lattice parameter measurements have shown the non-uniformity of the distribution of Ge atoms across the samples.

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