  We demonstrate an efficient method to engineer the quantum confinement in a
system of two quantum dots grown in a vertical stack. We achieve this by using
materials with a different lattice constant for the growth of the outer and
inner barriers. We monitor the resulting dot morphology with transmission
electron microscopy studies and correlate the results with ensemble quantum dot
photoluminescence. Furthermore, we embed the double quantum dots into diode
structures and study photoluminescence as a function of bias voltage. We show
that in properly engineered structures, it is possible to achieve a resonance
of the hole states by tuning the energy levels with electric field. At the
resonance, we observe signatures of a formation of a molecular state,
hybridized over the two dots.
