Low temperature sapphire nitridation: A clue to optimize GaN layers grown by molecular beam epitaxy

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Abstract
The sapphire nitridation temperature is investigated as a possible parameter to improve the properties of GaN epilayers grown by molecular beam epitaxy using a radio frequency plasma source. It is found out that lowering the nitridation temperature to values as low as 200 °C allows us to drastically improve the GaN structural and optical properties. Careful examination of the interface by transmission electron microscopy reveals that, in this case, the interface between the nitridated sapphire and the AlN buffer consists of an ordered array of pure edge dislocations. In contrast, high nitridation temperatures result in a perturbed interface with the occurrence of cubic crystallites in the AlN buffer. These results, complemented by a thorough reflection high-energy electron diffraction analysis of the nitridation procedure and a secondary ion mass spectrometry investigation, are interpreted in the framework of a model whereby a higher oxygen concentration is extracted from the substrate at high nitridation temperature, leading to the formation of cubic grains with a smaller lattice parameter than the surrounding matrix and to the concomitant occurrence of defects within the buffer.