The laser-induced chemical etching mechanism of Cr-O doped GaAs (100) substrate in 40% HF-H2O solution is explained by the generation of e-h pairs through defect states in the presence of sub-bandgap photon illumination by using a Nd:YAG Laser (lambda similar to 1.06 mum). The central feature of the laser etching technique is pits initiation by surface defects. The etched GaAs samples are characterised by photoluminescence (PL) spectroscopy. The PL spectrum shows the formation of GaAs nanostructures and chemical reaction products. The shape of the PL band (1.3-1.8 eV) obtained from the top surface depends on the penetration depth of excitation wavelength. An estimate of the nanocrystallites sizes present in the laser etched layer is obtained by quantum confinement model. (C) 2004 Elsevier B.V. All rights reserved.