The modulation spectrum of semiconductor superlattices is shown to be a sensitive function of photon wavelength and local energy band-bending in the superlattice structure. The added dependence leads directly to the concept of a modulation spectroscopy surface for heterostructures as opposed to a simple plot in two dimensions of amplitude as a function of photon energy. The implications of these results for the use of modulation spectroscopy methods such as contactless photoreflectance spectroscopy, where the band bending cannot be controlled, are discussed. ©1997 American Institute of Physics.