GaMnAs layers were grown by MBE on GaAs (0 0 1) substrates. The structural properties of the epilayers were studied by atomic force microscopy, secondary ion mass spectroscopy, high-resolution X-ray diffraction, and Raman spectroscopy. Photoreflectance spectra at room temperature exhibited Franz-Keldysh oscillations; from an analysis of these oscillations we obtained the built-in internal electric field and the bandgap energy of the GaMnAs epilayers. We studied the variation of these parameters as a function of Mn in the epilayers. In addition the holes concentration in the samples was extracted from the bandgap narrowing value and compared with the carrier density obtained by the Hall measurements. (C) 2010 Elsevier B.V. All rights reserved.