  Atomistic electronic structure calculations are performed to study the
coherent inter-dot couplings of the electronic states in a single InGaAs
quantum dot molecule. The experimentally observed excitonic spectrum [12] is
quantitatively reproduced, and the correct energy states are identified based
on a previously validated atomistic tight binding model. The extended devices
are represented explicitly in space with 15 million atom structures. An excited
state spectroscopy technique is presented in which the externally applied
electric field is swept to probe the ladder of the electronic energy levels
(electron or hole) of one quantum dot through anti-crossings with the energy
levels of the other quantum dot in a two quantum dot molecule. This technique
can be applied to estimate the spatial electron-hole spacing inside the quantum
dot molecule as well as to reverse engineer quantum dot geometry parameters
such as the quantum dot separation. Crystal deformation induced piezoelectric
effects have been discussed in the literature as minor perturbations lifting
degeneracies of the electron excited (P and D) states, thus affecting
polarization alignment of wave function lobes for III-V Heterostructures such
as single InAs/GaAs quantum dots. In contrast this work demonstrates the
crucial importance of piezoelectricity to resolve the symmetries and energies
of the excited states through matching the experimentally measured spectrum in
an InGaAs quantum dot molecule under the influence of an electric field. Both
linear and quadratic piezoelectric effects are studied for the first time for a
quantum dot molecule and demonstrated to be indeed important. The net
piezoelectric contribution is found to be critical in determining the correct
energy spectrum, which is in contrast to recent studies reporting vanishing net
piezoelectric contributions.
