Optical absorption spectra of modulation-doped n-type semiconductor quantum-well structures are studied theoretically. It is shown that effective Coulomb interaction of a photo-excited hole with the sea of electrons in n=1 conduction-subband via 2D hydrogenic exciton states associated with the unoccupied higher subbands yields an appearance of new subband interference structure on the continuous tail of the Mahan exciton. Numerical results with use of material parameters for GaAs exhibit clearly such effects on the calculated absorption spectra.