Epitaxial thin films of the wide-band-gap chalcopyrite semiconductor CuGaS2 have directly been grown on single crystalline Si substrates of (1 1 1) orientation. The CuGaS2 films crystallize exclusively in the chalcopyrite structure of bulk CuGaS2 material without considerable strain. Photoluminescence (PL) measurements reveal excitonic emissions from 5 to 300 K and defect-related optical transitions at low temperatures. Based on the identification of two free-to-bound and the two related donor-acceptor transitions, we propose a model for intrinsic defect levels, which includes one shallow donor and two acceptor states. (C) 2003 Elsevier B.V. All rights reserved.