This paper reports the synthesis and characterization of GaN thin films deposited on Si (1 1 1) substrate, using electrochemical deposition technique below room temperature for different durations. The effects of deposition duration on the morphology, structure and optical properties of GaN thin films are reported. The SEM images showed different surface morphologies of the grown structures on Si substrate for different duration times. The shape and the dimensions of the structures were also found to be dependent on the growth time. From XRD analysis the lattice constants, the average size of h-GaN crystals and the in-plane (along a-axis) and out of plane (along c-axis) strains were calculated. The effects of deposition duration on crystal parameters from XRD and Raman modes were investigated. Photoluminescence spectrum showed that the energy gaps of h-GaN and c-GaN were about 3.39 and 3.2 eV, respectively, at room temperature and the PL-intensity of h-GaN increased with deposition duration. The growth mechanism is also proposed. (C) 2011 Elsevier B.V. All rights reserved.