Transformation of N-polar inversion domains from AlN buffer layers during the growth of AlGaN layers

Atomic force microscopy and dynamic secondary ion mass spectrometry were used to study the structural properties and crystal perfection of Al$_x$Ga$_{1-x}$N/AlN/Al$_2$O$_3$ heterostructures grown by ammonia molecular beam epitaxy with a high silicon concentration in Al$_x$Ga$_{1-x}$N : Si layers. It is shown that if AlN buffer layers of metallic polarity contain inversion domains of nitrogen polarity, during the subsequent growth of Al$_x$Ga$_{1-x}$N : Si layers, inversion domains do not grow to the surface, but change nitrogen polarity to metallic. In place of the inversion domains AlN, broadening columns Al$_x$Ga$_{1-x}$N of metallic polarity grow, which coalesce with the metallic polarity matrix surrounding them into an even uniform film. The thickness at which complete intergrowth occurs increases with increasing Al content in the layers.