Quantum dot devices

From a recent study of the growth and the optical properties of quantum dots (QDs), we demonstrated that artificial atoms with sharp electronic shells can be fabricated with good control, using self-assembled QDs grown by molecular beam epitaxy. Size and shape engineering of the QDs during growth permits the tailoring of their intersublevel energy spacings. We demonstrate a much improved uniformity of the macroscopic ensembles of QDs, with well-resolved electronic shells. In addition to size and shape engineering of the QDs in the case of single-layer samples, we demonstrate significant improvements in the uniformity of the vertically self-aligned stacked QDs. State-filling spectroscopy of the zero-dimensional transitions between confined electrons and holes demonstrates that the energy levels are readily tunable. One to five confined levels, with an inter-level energy spacing between 25 and 90 meV, are obtained by adjusting the growth temperature or with post-growth annealings. Such QDs having well-defined excited-states have been grown in the active region of devices and results will be presented for lasers, detectors, or for structures displaying optical memory effects. For example, QD laser diodes with well-defined electronic shells are fabricated, and shape-engineered stacks of self-aligned QDs are used to increase the gain in the active region. Lasing is observed in the upper QD shells for small gain media, and progresses towards the QD ground states for longer cavity lengths. We obtained at 77K thresholds of J(th)= 15 A/cm(2) for a 2 mm cavity lasing in the first excited state (p-shell), and at 300K for a 5 mm cavity, J(th) is similar to 430A/cm(2) with lasing in the d-shell. For an increased QD density, J(th) is Smaller than 100A/cm(2) at room temperature. For inter-sublevel transitions, we demonstrated broadband normal incidence detection with responsivity approaching 1A/W at a detection wavelength of 5 microns. For interband detection, the photoluminescence decay time of a p-i-n diode can be changed from similar to 3nsec to similar to 0.3nsec (3Ghz) with a reverse bias. For QDs capped with less than similar to 10 nm, remarkable charge transfers between the QD and surface states lead to optical memory effects lasting over time-scales of several minutes.