Photoflectance (PR) spectroscopy has been applied to the investigation of Si delta -doped CaAs, Al0.35Ga0.65As and AlAs layers grown by metal-organic vapor phase epitaxy (MOVPE) on GaAs substrates. The observation of Franz-Keldysh oscillations (FKO) and the application of fast Fourier transform (FFT) has allowed us to determine the internal electric field and, hence, the potential barrier between surface and delta -doped region of the layer. The FFT of the photoreflectance spectra has exhibited two separate heavy and light hole frequencies showing that the FKO in the PR signal are always the superposition of these two components.