Quantum dot structures and devices with sharp adjustable electronic shells

Abstract

Quantum-dot (QD) heterostructures with up to five well-defined electronic shells have been fabricated using self-assembled QDs grown by molecular beam epitaxy (MBE). Shape-engineered stacks of self-aligned QDs with improved uniformity have been used to increase the gain in the active region of laser diodes. Lasing is observed in the upper QD shells for small gain media, and progresses towards the QD ground states for longer cavity lengths. At 77 K thresholds of J th =15 A/cm 2 were obtained for a 2 mm cavity lasing in the first excited state (p-shell), and J th =125 A/cm 2 for a 1 mm cavity lasing in n=3 (d-shell). At 300 K for a 1 mm cavity, J th is 490 A/cm 2 with lasing in n=4 (f-shell). For an increased QD density, J th is smaller than 100 A/cm 2 at room temperature. State-filling spectroscopy is also used to study the effects of alloy intermixing in QD ensembles having well-defined electronic shells and with various densities. Rapid thermal annealing (RTA) is performed on QD samples grown with different intersublevel energy spacings. For InAs/GaAs QDs, the intersublevel is tuned between ∼90 and 25 meV. The intense and sharp shell structures observed in photoluminescence (PL) indicate unambiguously that the QDs retained their zero-dimensional (0D) density of states after the diffusion of the potential, which also causes strong blueshifts (over ∼200 meV) and a pronounced narrowing of the inhomogeneously broadened emission (down to ∼12 meV).