Hot carrier relaxation

In terms of predictive capabilities, this distinction creates problems as most of the studies of hot-carrier relaxation have been confined to bulk processes by the virtue of the experimental limitations associated with all optical methods. The majority of the carriers are generated and probed in the bulk of the crystal. Fortunately, in the last few years significant progress has been made in the development of femtosecond photoemission spectroscopy (Bokor et al, 1986 Goldman and Prybyla, 1994 Haight, 1996 Schmuttenmaer et al, 1996) which is sensitive to the near-surface carrier relaxation processes. 

Zhou X. and Hsiang T. Y. (1990), Monte-Carlo determination of femtosecond dynamics of hot-carrier relaxation and scattering processes in bulk GaAs , J. Appl. Phys. 67,7399-7403. 

Figure 3.1 Hot-carrier relaxation/cooling in semiconductors. Source Nozik (2001b).

Rosker M. J., Wise F. W. and Tang C. L. (1986), Eemtosecond optical measurement of hot-carrier relaxation in GaAs, AlGaAs, and GaAs/AlGaAs multiple quantum well structures , App/. Phys. Lett. 49, 1726-1728. 

The electrons excited into the different levels within in the single well, could be transferred to an acceptor molecule in the electrolyte either by thermionic emission across the outer barrier layer Otherm) or tunneling through it (Jiun) (F g- 9.33). The photocurrent spectrum does not give any information about whether a hot electron was transferred. The observed structure in these spectra could in principle be caused simply by quantized absorption followed by a complete hot carrier relaxation and electron transfer from the lowest quantum level. 

Figure 9.36 Hot carrier relaxation (cooling) in a semiconductor bulk material (after [85]).

Also shown is the temperature-dependent hot-carrier relaxation for NC with an average diameter of 3.8 nm. ... 

Solar cells based on hot carrier extraction and CM rely on precise control of hot carrier relaxation were expected to be realized in nanostructured semiconductors e.g. QDs) because of enhanced carrier arrier interactions and discretized energy levels. As will be shown below, TRTS is capable of probing charge carrier dynamics at early times after photoexdtation, including intraband relaxation and CM in bulk materials and quantum dots. As such, TRTS represents a powerful technique for evaluating novel semiconductor systems that may be used in the design of more efficient solar cells. 

In the event that the optical excitation involves above-band-gap light or field-accelerated carriers, the kinetic energy of the electrons is above that of the k = 0 extremes of the valence band and conduction band and the carrier distribution includes hot carriers. The following discussion will chronicle the series of events that lead to the relaxation from this excess energy condition. 

Edelstein D. C., Tang C. L. and Nozik A. J. (1987), Picosecond relaxation of hot-carrier distributions in GaAs/GaAsP strained-layer superlattices , Appl. Phys. Lett. 51, 48-50. 

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. 

A class of futuristic solar cells, often called hot carrier solar cells, seeks to harvest the full energy of solar photons. Such cells would utilize the additional energy content of a blue photon relative to ared one.126 In present-day solar cells, equilibrated carriers are collected and hence all absorbed photons with energy greater than the bandgap contribute equally to the measured efficiency. The realization of such hot carrier solar cells therefore requires electron transfer processes that are competitive with nonradiative decay of molecules or phonon relaxation in solids.126 Literature data indicate that such relaxation occurs on a femtosecond timescale. The ultrafast... 

In a modification of this model (32), photogenerated minority carriers which have not undergone full intraband relaxation may also be injected into the electrolyte this process is called hot carrier injection (32) (see Figure 14). This process can occur if the ther-malization time (tj ) of the photogenerated carriers in the semicon-... 

From these analyses and calculations several general criteria for obtaining hot carrier injection at semiconductor-electrolyte junctions are evident. The overall criterion is that both the tunneling time of the photogenerated minority carriers and the effective relaxation time of the electrolyte are faster than the thermalization time of these carriers in the semiconductor. 

Hot carrier charge transfer processes are important in solid state devices [51]. One fundamental question concerns whether these hot carriers can be transferred across the semiconductor-liquid junction before they completely relax to the band edge [91]. This problem is not only of interest from a fundamental point... 

Two-photon time-resolved photoemission (TPTRP) spectroscopy has been developed to directly study the dynamics of optically excited electrons at metal and semiconductor surfaces. This technique has been applied to direct measurement of hot electron relaxation in noble and transition metals [27, 28], surface-state dynamics on clean and adsorbate-covered metal surfaces [29, 30], as well as charge carrier dynamics in semiconductors, where much work has been performed. 

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