Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

10 ps (about an order of magnitude greater than that for bulk semiconductors).This is because previous work that measured the time of electrontransfer from bulk III–V semiconductors to redox molecules (metalloceniumcations) absorbed on the surface found that ET times can be subpicosecondsto several picoseconds [36,126–128]; hence, photoinduced hot ET can becompetitive with electron cooling and relaxation if the latter is greater thantens of picoseconds.In a series of articles, Sugawara et al. [97–100] have reported slow hotelectron cooling in self-assembled InGaAs QDs produced by Stranski–Krastinowgrowth on lattice-mismatched GaAs substrates. Using time-resolvedPL measurements, the excitation power dependence of PL, and the currentdependence of electroluminescence spectra, these researchers report coolingtimes ranging from 10 ps to 1 ns. The relaxation time increased with electronenergy up to the fifth electronic state. Also, an extensive review of phononbottleneck effects in QDs was recently published, which concludes that thephonon bottleneck effect is indeed present in QDs [42].Gfroerer et al. report slowed cooling of up to 1 ns in strain-inducedGaAs QDs formed by depositing tungsten stressor islands on a GaAs QWwith AlGaAs barriers [109]. A magnetic field was applied in these experimentsto sharpen and further separate the PL peaks from the excited-state transitions,and thereby determine the dependence of the relaxation time on levelseparation. The authors observed hot PL from excited states in the QD whichcould only be attributed to slow relaxation of excited (i.e., hot) electrons.Because the radiative recombination time is about 2 ns, the hot electronrelaxation time was found to be of the same order of magnitude (about 1 ns).With higher excitation intensity sufficient to produce more than one electron–hole pair per dot, the relaxation rate increased.A lifetime of 500 ps for excited electronic states in self-assembled InAs/GaAs QDs under conditions of high injection was reported by Yu et al. [104].PL from a single GaAs/AlGaAs QD [107] showed intense high-energy PLtransitions which were attributed to slowed electron relaxation in this QDsystem. Kamath et al. [108] also reported slow electron cooling in InAs/GaAsQDs.Quantum dots produced by applying a magnetic field along the growthdirection of a doped InAs/AlSb QW showed a reduction in the electronrelaxation rate from 10 12 to 10 10 s 1 [99].In addition to slow electron cooling, slow hole cooling was reported byAdler et al. [105,106] in S-K InAs/GaAs QDs. The hole relaxation time wasdetermined to be 400 ps based on PL rise times, whereas the electronrelaxation time was estimated to be less than 50 ps. These QDs only containedone electron state, but several hole states; this explained the faster electronCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.