  Most self-assembled quantum dot molecules are intrinsically asymmetric with
inequivalent dots resulting from imperfect control of crystal growth. We have
grown vertically-aligned pairs of InAs/GaAs quantum dots by molecular beam
epitaxy, introducing intentional asymmetry that limits the influence of
intrinsic growth fluctuations and allows selective tunneling of electrons or
holes. We present a systemic investigation of tunneling energies over a wide
range of interdot barrier thickness. The concepts discussed here provide an
important tool for the systematic design and characterization of more
complicated quantum dot nanostructures.
