An old controversy concerning the origin of the short-period oscillations in the photoreflectance (PR) spectra of GaAs high electron mobility transistor (HEMT) structures is solved. Combining experimental and theoretical PR studies, it is demonstrated that these oscillations are not generated in the channel region, but are due to an ionized acceptor plane at the buffer/substrate interface which causes a weak homogeneous electric field in the buffer layer. The interface sheet charge density is obtained from the PR oscillation period. The utilization of PR spectroscopy for measuring charge densities at interfaces is of interest beyond the scope of this work.