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 by Krenner et al (Phys. Rev. Lett. 94 057402, 2005) 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 applied where 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 used 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 wavefunction 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.
Muhammad Usman graduated from Electrical & Computer Engineering Department, Purdue University in August, 2010. He is currently working as a researcher in Tyndall National Institute. His area of research is theory, modeling, simulation, and computation of III-V semiconductor materials, alloys, hetero-structures, and optoelectronic devices. He is closely affiliated with NCN and nanoHub.org. He is a junior member of IEEE, American Physical Society (APS), and Material Research Society (MRS). Further details about his work can be found at: http://web.ics.purdue.edu/~usman
Computational resources from Rosen Center for Advanced Computing (RCAC) and nanohub.org are acknowledged. This work was done in Prof. Klimeck's research group. Yui-Hong Matthias Tan also significantly contributed to this work.
Muhammad Usman's work was funded by the USA Department of States through Fulbright Fellowship.
Muhammad Usman, Yui-Hong Matthais Tan, Hoon Ryu, Shaikh Ahmed, Hubert Krenner, Timothy B. Boykin, and Gerhard Klimeck, Nanotechnology, 22, 315709, (2011); DOI: http://dx.doi.org/10.1088/0957-4484/22/31/315709