The line shapes of electromodulation spectra exhibit extended Franz-Keldysh oscillations and interference beats in the presence of a uniform built-in electric field. For this work, we used photoreflectance and contactless electroreflectance to study the Franz-Keldysh oscillations in Si-delta-doped GaAs. The fast Fourier transformation taken to the photoreflectance and contactless electroreflectance spectra, produced more complicated results than were observed in previous studies, when the effect of the modulation field is nonnegligible. This indicates that the interference beats are not only due to different effective heavy-hole and light-hole mass but also to the modulation field. We propose that the Franz-Keldysh oscillations generally contain four frequencies, which correspond to the heavy-hole and light-hole splitting. A comparison between the experiments and some numerical simulations attest to the validity of our proposal. This line shape analysis could then be applied to estimate the strength of a modulation field in contactless electroreflectance and photoreflectance.