Photoluminescence (PL) from a beryllium-doped Si{sub 0.92}Ge{sub 0.08} epilayer and three different beryllium-doped Si{sub 0.92}Ge{sub 0.08}/Si superlattices (SL{close_quote}s) commensurately grown on Si(100) substrates is examined at 9 K at ambient pressure and, for the epilayer and one SL, as a function of hydrostatic pressure. In each structure, excitons bind to the isoelectronic Be pairs in the strained Si{sub 0.92}Ge{sub 0.08} layers. The zero-phonon PL peaks of the epilayer and the {ital in situ} doped 50-{Angstrom} Si{sub 0.92}Ge{sub 0.08}/100-{Angstrom} Si SL shift linearly with pressure toward lower energy at the rate of 0.68{plus_minus}0.03 and 0.97{plus_minus}0.03 meV/kbar, respectively, which are near the 0.77-meV/kbar value for Si:Be. The PL energies at ambient and elevated pressure are analyzed by accounting for strain, quantum confinement, and exciton binding. A modified Hopfield-Thomas-Lynch model is used to model exciton binding to the Be pairs. This model, in which potential wells bind electrons to a site (that then trap holes), predicts a distribution of electron binding energies when an inhomogeneous distribution of potential-well depths is used. This accounts for the large PL linewidth and the decrease of linewidth with increasing pressure, among other observations. In SL{close_quote}s, the exciton binding energy is shown to depend on the width ofmore » the wells as well as the spatial distribution of Be dopants in the superlattice. Also, at and above 58 kbar a very unusual peak is observed in one of the SL{close_quote}s, which is associated with a free-exciton peak in Si, that shifts very fast with pressure ({minus}6.02{plus_minus}0.03 meV/kbar). {copyright} {ital 1996} {ital The American Physical Society}« less