Dynamics of the wetting-layer-quantum-dot interaction in InGaAs self-assembled systems

An experiment is described to study the carrier dynamics in an InGaAs quantum-dot-wetting-layer system emitting in the 1-μm band at carrier densities typical of laser-diode operating conditions. The temporal evolution of the differential luminescence spectrum generated by the dots when an ultrafast optical pulse is used to perturb the steady-state carrier population of the surrounding wetting layer is measured. This provides information about the dynamic interaction of wetting-layer and quantum-dot populations. Study of the differential luminescence signal shows a marked bottleneck in the transfer of charge carriers into the quantum dots from the two-dimensional layers that surround them when the equilibrium occupancy of the dot states is high. Recovery of the system back to steady state then takes place over a time interval in excess of 600 ps. This situation arises because the dc electrical injection produces a high carrier density in the dot ensemble, and thus a high proportion of the additional photogenerated carriers remain in the wetting layer as the system thermalises. This reservoir of carriers then feeds into the dots as states are emptied by recombination events.