The GaN/AlGaN active regions for terahertz (THz) quantum cascade lasers were grown by metal organic chemical vapor deposition (MOCVD). The surface of the sample was characterized by atomic force microscopy (AFM). The structure of this sample was evaluated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD pattern and cross-sectional TEM images showed that a well-controlled quantum cascade GaN/AlGaN layers could be prepared. Optical properties of the active region of a terahertz GaN/AlGaN have been investigated by Fourier transform infrared (FTIR) spectrometer. It was found that the frequency of E-1(LO) phonon decreased in quantum cascade GaN/AlGaN structures. The phonon frequency shift could be attributed to the effect of phonon zone folding. (c) 2006 Elsevier B.V. All rights reserved.