Defects and nucleation of GaN layers on (0001) sapphire

The morphology and microstructural evolution of a nucleation layer are analysed using high-resolution transmission electron microscopy. Low-temperature nucleation of GaN on (0001) sapphire is investigated. Depositions were made for 20, 40, 60, 120 and 180 s at 560degreesC by metal-organic chemical vapour deposition. It is shown that the shortest deposition times give rise to the formation of cubic islands. Subsequently, the density and the size of the nucleated islands increase and they start to transform into wurtzite from the interface with the substrate. From the start, the nuclei contain misfit dislocations. At these early growth stages. the relaxation state changes from one island to another this probably underlies the subsequent mosaic growth of the high-temperature-active GaN layers.