Tunnelling of Direct and Indirect Excitons in Slightly Asymmetric Double Quantum Wells

We report the first studies of luminescence spectra from asymmetric double quantum wells of GaAs with widths around 100 Å differing by only one (2.8 Å) or two monolayers. Studies of the position and intensity of the direct and indirect exciton lines suggest the existence of acoustic phonon-assisted tunnelling between exciton states separated by a few meV. At temperatures 10-20 K the electron tunnelling process is fast enough to maintain thermal equilibrium between these direct and indirect excitons which are connected with holes in the same quantum well.