We have characterized GaN films and their interfaces with sapphire (0 0 0 1) substrates prepared by pulsed laser deposition (PLD) using coaxial impact-collision ion scattering spectroscopy (CAICISS). Grazing incidence X-ray reflection (GIXR) measurements have revealed the absence of the interfacial layer. The abruptness of the heterointerface between the films and the substrate is estimated to be 0.7nm. CAICISS analysis has revealed that the GaN films without and with an AlN buffer layer have the polarity of (0 0 0 (1) over bar) (N face; -c) and (0 0 0 1) (Ga face; + c), respectively, which indicates that the insertion of AlN causes a change in the crystal polarity. (C) 2002 Elsevier Science B.V. All rights reserved.