The BN polytype was determined using HRTEM of region I as shown in Fig. . The BN film initially nucleates as the h-BN polytype with the ABA…-stacking sequence. However, after ~4 nm h-BN, the growth undergoes a polytype transition to r-BN with the ABCA…-stacking sequence, that persists throughout the epitaxial BN film growth. The BN film in Figs and is grown from TEB on a-plane sapphire; a similar result from a film grown using TMB is available in the supporting information (Fig. ).