Optical Spectroscopy of Laterally Spaced InAs/GaAs Quantum Dot Molecules
University of Delaware
Over the past decade, potential device applications have fueled an extensive effort to fabricate lateral arrays of quantum dots (QDs) with specific dot densities, spacings and size distributions. An essential element for the further development of QD devices with new functionalities is the introduction of controllable quantum coupling between two or more QDs in such an array. Tunable quantum coupling between vertically stacked InAs QDs has been demonstrated, with the coupling mediated by coherent tunneling and tunable with a static electric field. While these studies have revealed that the spatial arrangement of QDs can lead to remarkable effects, it will be impossible to scale vertical coupling to a large number of QDs. Investigations of lateral quantum coupling have been slower to develop, however, because special growth protocols are required and the ability to independently tune the luminescence energy of separate QDs is lost. Spectroscopy of single pairs of laterally separated QDs is required to resolve the signatures of quantum coupling from inhomogeneously broadened ensemble spectra. Laterally-coupled quantum dots require modified sample preparation methods to isolate single pairs of QDs and apply electric fields that tune the relative energies of the two dots. The electric field must be applied along the surface of the sample, perpendicular to the growth direction. To apply this lateral field, we use interdigitated electrodes patterned onto the sample surface with photolithography and metal deposition. Electrical connections are made to each of the interdigitated top contacts as well as to an ohmic back contact. This three-terminal arrangement makes it possible to independently control both the relative energies of the dots and the charging of the QDs. We present photoluminescence spectra of laterally coupled QDs whose coupling and charge states are tuned with this three-terminal arrangement. We further discuss the implications and opportunities for control of quantum coupling in laterally-scalable architectures.