An amorphous interlayer between the AlN buffer layer and crystalline BN is observed in all samples. This interlayer was also found in previous studies of BN thin films grown on sapphire substrates in CVD at similar growth temperatures. Fig. : Cross-section HRTEM micrograph for the triangular pyramidal BN growth observed in Fig. for the film grown on aplane sapphire using TEB as the B precursor. An enlarged image on the right, for the selected section in this growth structure, displays the stacking sequence of the BN basal planes in detail. We observe from the ABAB… stacking of the basal planes that the h-BN polytype nucleates and then undergoes a polytype transition into r-BN with the ABCA…-stacking sequence. The bottom right enlarged section also shows the occurrence of the first r-BN stacking basal plane on top of the h-BN growth.