Morphology and microstructure evolution of AlxGa1-xN epilayers grown on GaN/sapphire templates with AlN interlayers observed by transmission electron microscopy

Morphology and microstructure evolution of Al_0.3Ga_0.7N epilayers grown on GaN/sapphire templates with low-temperature (LT) AlN interlayers (ILs) by means of metal organic chemical vapor deposition have been investigated by transmission electron microscopy and atomic force microscopy. It is found that the IL improves the surface morphology, and suppresses edge-type threading dislocations (TDs). When the IL thickness is 20 nm, there is the lowest density of the edge-type TD with 8.7×10⁸ cm^-2. But the edge-type TD density increases somewhat as IL thickness increases to 40 nm. It is believed that two mechanisms determine the microstructure evolution of the Al_xGa_1-xN epilayers. One is the TDs suppression effect of LT-AlN ILs that ILs can provide an interface for edge-type TDs termination. Another is the TDs introduction effect of ILs that new edge-type TDs are produced. Due to the lattice mismatch between AlN, GaN and Al_xGa_1-xN, the strain in Al_xGa_1-xN epilayers is modified by inserting the AlN IL, and thus changes the formation of the edge-type TDs.