By expanding a gas mixture into vacuum through a supersonic nozzle, a heavy "'seed'' species in alight diluent gas can be aerodynamically accelerated to suprathermal translational energies. Such beams are intense, directional, easily tuneable in energy, and narrowly;distributed in energy. They thereby offer the means of selectively promoting activated gas-surface reactions. We report the use of 10% NH3 in He seeded beams to grow GaN and AlN epitaxially on 6H-SiC(0001) and to grow GaN on ALN buffer layers deposited on SiC(0001). The III-N films were grown under a variety of incident energies and angles of the NH3 beam, with the III metal species supplied from an effusive evaporator source. Film thickness and morphology were characterized, ex situ with Rutherford backscattering spectroscopy, Auger spectroscopy, transmission electron microscopy, and atomic force microscopy. Of particular relevance to the III-N growth are the following results: (1) Selected energy epitaxial growth was observed, evidently via a direct reaction channel over a barrier of 0.25+/-0.1 eV. A comparison of films grown at 0 degrees (normal) and 30 degrees angles of NH3 incidence indicated total energy scaling of this chemisorption process. (2) A low energy reaction channel (<0.10 eV) was explicitly confirmed. The mechanism by which this might occur is discussed.