All substrates used are sized at 10×10 mm 2 except for the m-cut substrates which were 20×20 mm 2 . The c-cut, a-cut, r-cut substates are 2", 330 µm-thick on axis substrates manufactured at The Roditi International Corp. Ltd. While the m-cut substrate is 2'', 430 µm thick substrate manufactured at MTI Corporation (Materials Tech. Intl.). They are all cleaned with the same procedure which includes 3 minutes in an ultrasonic bath with acetone at 80 °C, 3 minutes in an ultrasonic bath with ethanol at 80 °C, 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 are then placed in the heating part of the reaction cell(susceptor), this is a elliptically shaped and this susceptor is coated with tantalum-carbide (TaC). The depositions on the different substrates are done separately as they are always placed in the same position in the susceptor for each process. Prior to BN deposition, the α-Al2O3 substrates were heated to 1100 °C for 5 min in H2 gas, after which NH3 was introduced and the temperature is ramped to the selected growth temperatures and maintained for 10 min to form an in situ aluminum nitride buffer layer as previously reported.