ZnSe/GaAs heteroepitaxial layers were grown by metal-organic chemical vapor deposition and investigated by micro-Raman spectroscopy. The layer thickness (h) was varied by controlling the growth period in the range 0.5-3.0 h. The Raman peaks were attributed to the LO and TO phonon modes of GaAs and ZnSe. As for the LO peak of the ZnSe layers, the thickness-dependent: full widths at half-maximum of micro-Raman spectra was accounted for by assuming uncorrelated misfit dislocations which were due to in-plane strain in the layers. The strain (epsilon) was estimated from the LO-peak shift. The plots of epsilon versus layer thickness were best fitted to the epsilon-h(-1) curve, which gave the critical layer thickness h(c) = 0.14 mum. X-ray diffraction indicated that strain relaxation occurred on thick ZnSe layers. (C) 2002 Elsevier Science B.V. All rights reserved.