Light atom impurities in polycrystalline AlN thin films deposited by atomic layer epitaxy on glass substrates have been analysed by combining two high-energy ion beam techniques, namely time-of-flight elastic recoil detection analysis and nuclear resonance broadening techniques. The advantages of their combined use in depth profiling of light elements, H, C, O and Cl impurity traces in the present study, are demonstrated in the framework of optimizing the production of pure AlN thin films from AlCl3 and Al(CH3)(3) aluminium sources.