Simple formula for exciton binding energy in quantum wells with zero band offsets

Band offsets in II-VI semiconductor quantum wells can be such as to show small or zero confinement in the conduction band while having considerable confinement in the valence band, or vice versa. By assuming purely two-dimensional motion and a simple variational exciton function, we calculate an explicit analytical expression for the 1s-exciton binding energy in this case and evaluate it for a range of electron- to hole-mass ratios.