Polycrystalline (100) and amorphous A1N thin films have been synthesized by plasma-enhanced chemical vapour deposition (PECVD) at low (35 and 440 kHz) and high (13.56 MHz) frequency on silicon single crystal wafers. High resolution transmission electron microscopy (HRTEM) has been used to confirm the presence of crystallites oriented (100) perpendicularly to the (100) silicon surface. Three different types of structure are identified from silicon to A1N bulk. The first zone is a weak amorphous interlayer on which has grown a polycrystalline layer with small misoriented crystallites and finally, the bulk which displays larger and well-oriented crystallites. The chemical composition of the A1N/Si interlayer is investigated by Auger electron spectroscopy from the effect of an in situ silicon surface plasma-cleaning nitrogen.