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

film. This quantization regime leads to discrete energy states which havedispersion in k-space [88]. This means the hot carriers can cool by undergoinginterstate transitions that require only one emitted phonon followed bya cascade of single-phonon intrastate transitions; the bottom of each quantumstate is reached by intrastate relaxation before an interstate transitionoccurs. Thus, the simultaneous and slow multiphonon relaxation pathwaycan be bypassed by single-phonon events, and the cooling rate increasescorrespondingly.More complete theoretical models for slowed cooling in QDs have beenproposed recently by Bockelmann and co-workers [86,89] and Benisty and coworkers[85,87]. The proposed Benisty mechanism [85,87] for slowed hotcarrier cooling and a phonon bottleneck in QDs requires that cooling onlyoccurs via longitudinal optical (LO) phonon emission. However, there areseveral other mechanism by which hot electrons can cool in QDs. Mostprominent among these is the Auger mechanism [90]. Here, the excess energyof the electron is transferred via an Auger process to the hole, which then coolsrapidly because of its larger effective mass and smaller energy level spacing.Thus, an Auger mechanism for hot electron cooling can break the phononbottleneck [90]. Other possible mechanisms for breaking the phonon bottleneckinclude electron–hole scattering [91], deep level trapping [92], andacoustical–optical phonon interactions [93,94].A. Experimental Determination of Relaxation/CoolingDynamics and a Phonon Bottleneck in Quantum DotsOver the past several years, many investigations have been published thatexplore hot electron cooling/relaxation dynamics in QDs and the issue of aphonon bottleneck in QDs [11]. The results are controversial, and it is quiteremarkable that there are so many reports that both support [11,95–109] andcontradict [11,92,110–122] the prediction of slowed hot electron cooling inQDs and the existence of a phonon bottleneck. One element of confusion thatis specific to the focus of this chapter is that although some of thesepublications report relatively long hot electron relaxation times (tens ofpicoseconds) compared to what is observed in bulk semiconductors, theresults are reported as being not indicative of a phonon bottleneck because therelaxation times are not excessively long and PL is observed [123–125] (theorypredicts infinite relaxation lifetime of excited carriers for the extreme, limitingcondition of a phonon bottleneck; thus, the carrier lifetime would bedetermined by nonradiative processes and PL would be absent). However,because the interest here is on the relative rate of relaxation/cooling comparedto the rate of electron transfer, slowed relaxation/cooling of carriers can beconsidered to occur in QDs if the relaxation/cooling times are greater thanCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.