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TEM cross-section images of two different tunnel injection structures: (a) barrier thickness , (b) barrier thickness , (c) high-resolved TEM of the latter. The contour was drawn over the contrast corresponding to . The letter z denotes the growth direction. (d) Level scheme of the structure (not to scale). Full (red) arrows mark ground state transitions, whereas the dashed (blue) arrow denotes excited state transitions. (e) PL spectra of a structure with GaAs barrier for above barrier excitation: full black line represents cw excitation , black symbols stand for pulse excitation , red open circles denote the PLE spectrum with the monitor set to the QD ground state energy at . The spectroscopic measurements are done at .
Transient PL from a structure with barrier for two excitation energies: (a) , (b) , corresponding to excitation energetically below and above the QW ground state transition energy, respectively. [(c) and (d)] PL transients for two structures with barrier thickness of (black full circles) and (red open). (c) QW emission at after excitation into the barrier with ; (d) QD ground state emission at after excitation into the QW ground state with . Time zero corresponds to the maximum of the fs excitation laser pulse as detected by the setup (full width at half maximum ). All measurements are done at and excitation density is .
Tunneling time between QW and QDs vs barrier thickness deduced from the decay of the QW PL (full squares) and the rise of the QD PL (full circles). Open circles mark data that can not be assigned to meaningful -values, because of the limitations set by the QD PL rise time. Triangles are taken from Refs. 6 and 7. The dashed line correspond to the WKB approximation, whereas the doted lines represent the border cases for the time constants as measured at the reference samples containing either a single QW or QD layer.
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