The sapphire substrates were 10x10 mm 2 cut from 2" 330 µm-thick on-axis wafers of either cplane or a-plane orientation, both from The Roditi International Corp. Ltd. Before immersion into the CVD reactor, substrate cleaning was done by ultrasonication in first acetone, then ethanol at 80 °C for 3 minutes each, followed by standard clean 1 (SC1, NH3:H2O2:H2O with relative concentrations 1:1:26 at 80 °C) and standard clean 2 (SC2, NH3:H2O2:H2O with relative concentrations 1:1:22 at 80 °C). The substrates were placed in the elliptically shaped susceptor coated with tantalum-carbide (TaC). The two substrates were placed next to each other in the middle of the susceptor for each CVD process. An AlN buffer has been shown to improve crystallinity in the CVD of BN films, this AlN buffer was formed in the CVD reactor by introducing the NH3 after temperature stabilization at 1100 °C. The temperature was then ramped up to the respective growth temperatures for the TEB and TMB processes. The NH3 flow was maintained for 10 minutes at 1400°C before introducing TMB and 8 minutes at 1500°C before introducing TEB to compensate for the difference in temperature ramp-up times.