Semiconductors electron-hole recombination

Light is generated in semiconductors in the process of radiative recombination. In a direct semiconductor, minority carrier population created by injection in a forward biased p-n junction can recombine radiatively, generating photons with energy about equal to E. The recombination process is spontaneous, individual electron-hole recombination events are random and not related to each other. This process is the basis of LEDs [36]. 

In a subsequent work [182], it was shown that the photoelectrochemical performance of InSe can be considerably improved by means of selective (photo)electrochemical etching. Interestingly, whereas the cleavage vdW plane showed little improvement, the photocurrent in the face parallel to the c-axis was doubled. Note that, in contrast to InSe crystals cleaved in the plane perpendicular to the c-axis that are almost defect free, the crystals cut in the plane parallel to the c-axis contain a high density of defects on their surface which leads to a high rate of electron-hole recombinations and inferior quantum efficiency. The asymmetry in the role of electrons and holes, as manifested, e.g., in the fact that surface holes carry out the selective corrosion of the semiconductor surface in both cleavage orientations, was discussed. 

An important aspect of semiconductor photochemistry is the retardation of the electron-hole recombination process through charge carrier trapping. Such phenomena are common in colloidal semiconductor particles and can greatly influence surface corrosion processes occurring particularly in small band gap materials, such... 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

These cells operate under illumination in combination with a bias, which serves to either drive electrolytic reactions for which the photon energy is insufficient or to increase the rate of chemical energy conversion by reducing electron-hole recombination in the semiconductor bulk. Most commonly an electrical bias is provided to drive the reactions [36-41]. 

Van der Holst JJM, van Cost FWA, Coehoom R, Bobbert PA (2009) Electron-hole recombination in disordered organic semiconductors validity of the Langevin formula. Phys Rev B 80 235202... 

However, there are a number of difficulties associated with the synthesis of colloidal semiconductor particles. The preparation of stable, monodispersed, well-characterized populations of nanosized, colloidal semiconductor particles is experimentally demanding and intellectually challenging. Small and uniform particles are needed to diminish non-productive electron-hole recombinations the mean distance by which the charge carriers need to diffuse to reach the particle surface from which they are released is necessarily reduced in small particles. Monodispersity is a requirement for the observation of many of the spectroscopic and electro-optical manifestations of size quantization in semiconductor particles. Small semiconductor particles are difficult to maintain in solution in the absence of stabilizers flocculations and Ostwald ripening... 

Aqueous pools of reversed micelles have been fruitfully employed for the in situ generation of semiconductor particles. The first publication in this area described the formation of CdS in sodium bis(2-ethylhexyl)sulfosuccinate (AOT) aggregates in isooctane [611]. The preparation involved the addition of aqueous CdCl or Cd(N03)2 to isooctane solutions of AOT. Exposure to controlled ammeters of the CdS particles formed. Irradiation of degassed, AOT-reversed-micelle-entrapped, platinized CdS by visible light (450-W Xenon lamp X > 350 nm) in the presence of thiophenol (PhSH) resulted in sustained hydrogen formation. Sacrificial electron transfer occurred from thiophenol to positive holes in the colloidal CdS and, consequently, diminished undesirable electron-hole recombinations (Fig. 101) [611]. 

In many semiconductors the majority of the photoelectrons are produced by excitation from the valence band, the process thus simultaneously producing holes. Surface traps may act as recombination centers for electron-hole recombination and a change in the number or energy of these surface traps, or a change in the height of the surface barrier, may change the rate of recombination. For example Bube (9,10) has concluded that it is through this effect that the adorption of water vapor influences the photoconductivity of cadmium sulfide. 

Electron-hole recombination velocities at semiconductor interfaces vary from 102 cm/sec for Ge3 to 106 cm/sec for GaAs.4 Our first purpose is to explain this variation in chemical terms. In physical terms, the velocities are determined by the surface (or grain boundary) density of trapped electrons and holes and by the cross section of their recombination reaction. The surface density of the carriers depends on the density of surface donor and acceptor states and the (potential dependent) population of these. If the states are outside the band gap of the semiconductor, or are not populated because of their location or because they are inaccessible by either thermal or tunneling processes, they do not contribute to the recombination process. Thus, chemical processes that substantially reduce the number of states within the band gap, or shift these, so that they are less populated or make these inaccessible, reduce recombination velocities. Processes which increase the surface state density or their population or make these states accessible, increase the recombination velocity. 

By including terms to describe the rate of emission of electrons and holes from filled and empty surface states respectively, constraining the total number of surface states to Nt by N. + Ng = Nt and assuming that the states do not interact, the electron-hole recombination at. tile.surf ace has been analyzed -1— in analogy with Hall-Shockley-Read recombination. These methods are important in the study of semiconductor-... 

We explore below the hypothesis that the observed overshoot current vs. time behavior arises from this effect. In essence, the time dependent change in IT1" produces an associated change in the barrier height for electron transfer to the semiconductor surface, altering the surface electron/hole recombination rate. 

---