ZnS epilayers were grown on (100) semi-insulating GaAs substrates using an atmospheric pressure metal-organic chemical vapor deposition (CVD) technique under the atomic-layer epitaxy (ALE) mode. Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to find the effect of the II-VI ratio of the 30-nm thick ZnS epilayers and to investigate the thickness-dependent characteristics of ZnS epilayers with the thicknesses of 30 and 100 nm. While the II-VI ratio-dependent ZnS quality is consistent regardless of the measurement, the thickness-dependent epilayer quality is quite contrary depending on the measurement. This difference demonstrates the non-uniform distribution of the strain-relaxation in the ZnS epilayer along the depth. (c) 2009 Elsevier B.V. All rights reserved.