Electronic Relaxation in Semiconductors

A very interesting problem is concerned with the physical limitations of the ultimate speed of electronic computers. Since any bit corresponds to a transition from a nonconducting to a conducting state of a semiconductor, or vice versa, the relaxation time of electrons in the conduction band and the recombination time certainly impose a lower limit for the minimum switching time. This electronic relaxation can be measured with the pump-and-probe technique. The electrons are excited by a femtosecond laser pulse from the upper edge of the valence band into levels with 

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To study the regularities of photoexcited electron relaxation in the reaction of the electron transfer by the method of flash photolysis in microsecond timescale, we had to change the electron acceptor concentration in a liquid phase. The ability of the acceptor molecules to adsorb at the surface of the semiconductor colloidal particle was found to determine the character of changes in the photobleaching relaxation kinetic curves. 

Kamalov, V. F. Little, R. Logunov, S. L. El-Sayed, M. A. Picosecond electronic relaxation in CdS/HgS/CdS quantum dot quantum well semiconductor nanoparticles, J. Phys. Chem. 1996, 100, 6381. 

Klimov V. I. and McBranch D. W. (1998), Femtosecond Ip-to-ls electron relaxation in strongly confined semiconductor nanocrystals , Phys. Rev. Lett. 80, 4028-4031. 

Fig. 1. (a) Optical excitation in a simple metal. The excited electron-hole pair can relax rapidly (path 1) so that the quantum efficiency of the photoemission process (2) is small. In addition, the back reaction (4) involving electron injection from the solution may occur before the photoemit-ted electron relaxes in the solvent (3). (b) Optical excitation in an insulator or semiconductor. Here, geminate recombination is slower due to the forbidden gap so that the quantum efficiency of photoelectrochemical reactions can approach unity under optimum conditions. In principle, either conduction band (1) or valence band (2) reactions can occur, but in practice the reaction of the minority carrier is most important for extrinsic semiconductors. 

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. 

A.J. Nozik, Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots, Ann. Rev. Phys. Chem. 52 (2001) 193-231. 

Of special interest for thermally stimulated relaxation (TSR) is how to remove the system from equilibrium and physical phenomena that can be measured (monitored) during the relaxation process. We restrict ourselves by considering the physical phenomena, although these can also take place in chemical and biological objects. Further, among physical process, we consider only those that involve redistribution of electronic charge carriers in semiconductors during the relaxation process. 

The finite decoherence time is due to some inelastic scattering mechanism inside the system, but typically this time is shorter than the energy relaxation time re, and the distribution function of electrons inside the system can be nonequilibrium (if the finite voltage is applied), this transport regime is well known in semiconductor superlattices and quantum-cascade structures. 

In electronics, a well-established procedure to make statements on the sign of the electronic carriers is establishing the appropriate junctions (cf. diodes). The transformability of the semiconductor experiments to ion conductors suffers from the fact that the situation in ion conductors is more related to the situation in relaxation type semiconductors than to lifetime semiconductors note that only the latter shows the typical significant electronic effects such as in diodes or transistors. Nonetheless setting up ionic diodes and ionic transistors may be a worthwhile task for the future. (One such attempt to find out the nature of the ionic carriers (Oj or Vq ) in PbO by diode effects, viz. by a contact to the vacancy conductive YSZ, has been reported in Ref.217)... 

Electron-phonon interaction in a semiconductor is the main factor for relaxation of a transferred electron. There are two different relaxation processes that decrease the efficiency of light conversion in a solar system (1) relaxation of an electron from a semiconductor conduction band to a valence band and (2) a backward electron transfer reaction. The forward and backward electron transfer processes have been already included in the tunneling interaction, HSm-qd, described by Eq. (108). However, the effect of SM e-ph interaction is important for the correct description of electron transfer in the SM-QD solar cell system. In the previous section, we have gradually considered different types of interactions in the quantum dot and obtained the exact expression for the photocurrent (128) where the exact nonequilibrium QD Green s functions determined from Eq. (127) have been used. However, in... 

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