Si-delta-doped GaAs (N2D almost-equal-to 10(11) cm-2) samples grown by molecular-beam epitaxy are investigated by using photoreflectance spectroscopy. The oscillations observed above the GaAs fundamental band gap are attributed to the Franz-Keldysh effect in the region between the delta-doped layer and the crystal surface. This ascription is confirmed by detailed studies through varying the cap thickness (250-2500 angstrom), temperature (10-450 K), and laser pump power (0.05-7 mW/cm2). The surface potential deduced from the Franz-Keldysh oscillations is found to be temperature and laser pump power dependent, which is explained by taking the surface photovoltaic effect into account. The surface Fermi level has been measured by this method and is found to have the value 0.73+/-0.02 V.