We studied MOCVD processing for direct growth of BN on 2” sapphire substrates as a template for heterostructures with two dimensional (2D) and three dimensional (3D) materials. The combined experimental evidence points to three growth modes for BN: self-terminating, 3D random, and layer-by-layer, all of which are dependent on V/III ratio, temperature, pressure, and substrate surface modification via nitridation. At moderate temperature (950-1050°C), BN growth using high V/III ratio is self-terminating, resulting in c-oriented films aligned in-plane with respect to the orientation of the sapphire substrate. BN films grown under low V/III ratios are 3D, randomly oriented, and nano-crystalline. At higher temperature (1100°C), self-terminating growth transitions to a continuous layer-by-layer growth mode. When BN growth is self-terminating, films exhibit atomically smooth surface morphology and highly uniform thickness over a 2” sapphire wafer. Using these BN/sapphire templates we studied the growth of 2D and 2D/3D heterostructures. To study direct growth of 2D on 2D layered material we deposited graphene on BN in a continued process within the same MOCVD system. Furthermore, we explore the growth and nucleation of 3D materials (GaN and AlN) on BN. AlGaN/GaN based high electron mobility transistor (HEMT) structures grown on BN/sapphire exhibited two-dimensional electron gas characteristics at the AlGaN/GaN heterointerface, with room-temperature electron mobility and sheet electron density about 1900cm2/Vs and 1x1013cm-2, respectively.