Influence of ultrathin AlN interlayer on the microstructure and the electrical transport properties of AlxGa1−xN/GaN heterostructures

The microstructure and electrical properties of Al0.25Ga0.75N/GaN heterostructures with various AlN thicknesses have been investigated. An optimum thickness of AlN interlayer can remarkably improve the microstructure of Al0.25Ga0.75N barrier with the most uniform strain and the lowest density of threading dislocations, leading to the highest Hall mobility of the two-dimensional electron gas in the heterostructures. Transmission electron microscopy images show that the AlN interlayer with an optimum thickness can make the threading dislocations bend and be annihilated with each other in the vicinity of the heterointerface due to the larger mismatch strain between AlN interlayer and GaN. We believe that such behavior reduces the local strain and improves the uniformity of the strain in the AlxGa1−xN barrier, and thus depresses the scattering induced by the fluctuations of the piezoelectric interface charge owing to the nonuniformity of piezoelectric polarization field in AlxGa1−xN/GaN heterostructures.