Hot electron cooling

Short-wavelength photons (of energy much greater than Eg) create hot carriers. If, somehow, thermalization of these carriers can be avoided, photoelectrochemical reactions that would otherwise be impossible with the cooled counterparts, i.e., at very negative potentials for n-type semiconduetors, would be an intriguing possibility. The key issue here is whether the rate of electron transfer across the interface can exceed the rate of hot electron cooling. The observation of hot carrier effects at semiconductor-electrolyte interfaces is a controversial matter (see, e.g.. Refs. [3],... 

Rosenwaks et al. (1993) performed calculations on the PL intensity versus time and energy data to determine the time dependence of the qnasi-Fermi level, electron temperatnre, electronic specific heat, and ultimately the dependence of the characteristic hot-electron cooling time on electron temperatnre. 

Figure 3.11 Time constant for hot-electron cooling (tavg) versus electron temperature for bulk GaAs and GaAs multiple quantum wells at three excitation intensities. Source Rosenwaks et al. (1993).

Figure 3.12 Hot phonon bottleneck to slow hot-electron cooling in QWs. At high light intensity, hot electrons produce hot phonons which can reheat electrons via phonon absorption to keep them hot. Pioss is power loss per electron.

Figure 3.13 Various competing channels for hot-electron cooling. Source Nozik (2005).

K Krai, B Hejda. Long-wavelength LO-phonon generation during hot-electron cooling in polar semiconductors. Phys Rev B 48 11,461-11,464, 1993. 

Quite apart from thermolysis occurring before fragmentation, the temperature of the ion source may have a marked effect on the appearance of a mass spectrum. Comparison of mass spectra obtained with hot and cooled ion-sources and of spectra obtained by photon impact or field ionization show by the increased amount of fragmentation that a molecular ion possesses a greater excess of internal energy when formed in a hot, electron-impact source. Possible origins of this excess internal energy are collision with or radiation from surfaces. Some effects of hot and cold ion sources are discussed. 

Hot electron and hole cooling dynamics in qnantnm-confined semicondnctors... 

The cooling, or energy-loss, rate for hot electrons is determined by LO phonon emission throngh electron-LO-phonon interactions. The time constant characterising this process can be described by the following expression (Ryan et al, 1984 Cai et al, 1986 Christen and Bimberg, 1990)... 

In contradiction to the results showing slowed cooling in QDs, many other investigations are reported in the literature in which a phonon bottleneck was apparently not observed. These results have been reported for both self-organised SK QDs and II-VI colloidal QDs Nozik (2001b) gives a detailed reference list. However, in several cases (Heitz et al., 1991 Heitz et al, 1998 Sosnowski et al, 1998), hot-electron relaxation was found to be slowed, but not sufficientiy for the authors to conclude that this was evidence of a phonon bottieneck. 

The delocalised quantised 3-D miniband states could be expected to slow the carrier cooling and permit the transport and collection of hot carriers to produce a higher photopotential in a PV cell or in a photoelectrochemical cell where the 3-D QD array is the photoelectrode (Nozik, 1996). Also, MEG might be expected to occur in the QD arrays, enhancing the photocurrent (see Fig. 3.14). However, hot-electron transport/collection and MEG cannot occur simultaneously they are mutually exclusive and only one of these processes can be present in a given system. 

Photo-induced electron transfer reactions from quantum well electrodes into a redox system in solution represent an intriguing research area of photoelectrochemistry. Several aspects of quantized semiconductor electrodes are of interest, including the question of hot carrier transfer from quantum well electrodes into solution. The most interesting question here is whether an electron transfer from higher quantized levels to the oxidized species of the redox system can occur, as illustrated in Fig. 9.31. In order to accomplish such a hot electron transfer, the rate of electron transfer must be competitive with the rate of electron relaxation. It has been shown that quantization can slow down the carrier cooling dynamics and make hot carrier transfer competitive with carrier cooling. 

---