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Authors:
Thomas Switaiski1, Jan-Hindrik Schulze2, Tim David Germann2, André Strittmatter2, Udo W. Pohl2, Dieter Bimberg2 and Ulrike Woggon1 1 Institut für Optik und Atomare Physik, Technische Universität Berlin 2 Institut für Festkörperphysik, Technische Universität Berlin Superlattices of submonolayer depositions (SML-stack) which are spatially coupled to a Stranski-Krastanow (SK) grown quantum dot (QD) layer offer the possibility to control the charge carrier transfer between the SML-stack and the SK-QDs and thus the carrier decay dynamics. With time-resolved and time-integrated photoluminescence measurements we investigated the impact of this coupling. The studied samples contain one layer of InGaAs-SK-QDs, which is overgrown with a spacer of the matrix material GaAs of nominal thickness d (0.75 - 5nm), followed by a 10x(0.5ML InAs/ 2ML GaAs)-SML-stack. In the absence of a SK-QD layer a radiative decay time of about 500ps is measured for the SML-stack at 5K. Coupling to quantum dots results in shortening of the submonolayer photoluminescence. Its time constant is reduced with decreasing spacer d. The observed carrier decay dynamic of the PL from a SML-stack with a seed layer of SK-QDs is not simply the superposition of exponential decays, as Fermi-blocking is present. Therefore, a rate equation system (RES) is essential for the description of the dynamics. The presence of the energetically lower SK-QDs introduces an additional decay channel for carriers localized inside the SML-stack. Thus, there are mainly two concurrent decay processes for the SML carriers at low temperature: first the radiative recombination, whose time constant was measured, and second the transfer into the quantum dots. The time constant of the latter can be approximated by solving the rate equation system numerically and comparison to the experimental data. The resulting values of the transfer time constant exhibit a strong dependence on the spacer thickness d. SML-stacks based on InAs/GaAs are used as active medium in laser diodes, providing high power conversion efficiencies, low threshold current densities and high modulation frequencies. |
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