Non-radiative recombination

Ej is a demarcation energy, similar to that defined in the analysis of dispersive transport (see Section 3.2.1). It is assumed that all carriers which are thermally excited recombine non-radiatively, but the same result is obtained if some fraction are subsequently retrapped and recombine radiatively. The luminescence efficiency is given by the fraction of carriers deeper than E, . An exponential band tail density of states proportional to exp (E/kf,) results in a quantum efficiency of... 

Semiconducting III-V QDs have been investigated in the past decade, particularly InP QDs, which possess a band gap of 1.35 eV. The principal attraction to these semiconductors focuses on the robustoess of the covalent bond in III-V semiconductor groups compared with the ionic bond in II-VI semiconductor groups. The formation of covalent bond enhances the optical stability of the QD systems. Thus, the reduction of the toxicity derived from the noncorrosive composition elements enables the use of QD systems in a biological field [110, 111]. The resultant defects, also called surface sites, act as traps for nonradiative decay of the QDs under the excited state [112]. Some of the excited electrons can cross to the surface states located in the intraband gap and subsequently recombine non-radiatively with the holes in the valence band, thereby decreasing the PL-QY. Several syntheses have been developed to enhance the PL-QY. To passivate surface... 

There are many ways of increasing tlie equilibrium carrier population of a semiconductor. Most often tliis is done by generating electron-hole pairs as, for instance, in tlie process of absorjition of a photon witli h E. Under reasonable levels of illumination and doping, tlie generation of electron-hole pairs affects primarily the minority carrier density. However, tlie excess population of minority carriers is not stable it gradually disappears tlirough a variety of recombination processes in which an electron in tlie CB fills a hole in a VB. The excess energy E is released as a photon or phonons. The foniier case corresponds to a radiative recombination process, tlie latter to a non-radiative one. The radiative processes only rarely involve direct recombination across tlie gap. Usually, tliis type of process is assisted by shallow defects (impurities). Non-radiative recombination involves a defect-related deep level at which a carrier is trapped first, and a second transition is needed to complete tlie process. 

Radiative recombination of minority carriers is tlie most likely process in direct gap semiconductors. Since tlie carriers at tlie CB minimum and tlie VB maximum have tlie same momentum, very fast recombination can occur. The radiative recombination lifetimes in direct semiconductors are tlius very short, of tlie order of tlie ns. The presence of deep-level defects opens up a non-radiative recombination patli and furtlier shortens tlie carrier lifetime. 

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

Core/shell-type nanoparticles ovm ated with higher band inorganic materials exhibit high PL quantum yield compared with uncoated dots d K to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe ZnS [7] have been prepared from organometaHic precursors. 

The decay on a picosecond time-scale, the so-called fast band, is understood as a quasi-direct recombination process in the silicon crystallites or as an oxide-related effect [Tr2, Mgl]. This fast part of the luminescence requires an intense excitation to become sizable it then competes with non-radiative channels like Auger recombination. The observed time dependence of the slow band is explained by carrier recombination through localized states that are distributed in energy, and dimensionally disordered [Gr7]. 

An approximation of the lifetime in PS at RT using an electron-hole pair density equal to one pair per crystallite and the radiative recombination parameter of bulk silicon give values in the order of 10 ms [Ho3]. The estimated radiative lifetime of excitons is strongly size dependent [Sa4, Hi4, Hi8] and increases from fractions of microseconds to milliseconds, corresponding to an increase in diameter from 1 to 3 nm [Hy2, Ta3], as shown in Fig. 7.18. For larger crystallites a recombination via non-radiative channels is expected to dominate. The experimentally observed stretched exponential decay characteristic of the PL is interpreted as a consequence of the randomness of the porous skeleton structure [Sa5]. 

The EE and phE mechanisms for neat polymers proposed by ourselves and others all involve the consequences of breaking bonds during fracture. Zakresvskii et al. (24) have attributed EE from the deformation of polymers to free radical formation, arising from bond scission. We (1) as well as Bondareva et al. (251 hypothesized that the EE produced by the electron bombardment of polymers is due to the formation of reactive species (e.g., free radicals) which recombine and eject a nearby trapped electron, via a non-radiative process. In addition, during the most intense part of the emissions (during fracture), there are likely shorter-lived excitations (e.g., excitons) which decay in a first order fashion with submicrosecond lifetimes. The detailed mechanisms of how bond scissions create these various states during fracture and the physics of subsequent reaction-induced electron ejection need additional insight. 

Primary energy loss pathways include radiative and non-radiative deactivation of the dye sensitizer (Process 6), recombination of the conduction band electrons by the oxidized sensitzer (Process 7), or recombination of the conduction band electrons by the the oxidized form of the redox system (Process 8). 

The absorption edge of (Ga,Mn)As is not sharp, as shown in fig. 20 (Kuroiwa et al. 1998 Szczytko et al. 1999b). This is probably due to impurity band formation caused by the high concentration of ionized Mn and compensating donors (Kuroiwa et al. 1998). Even below the fundamental absorption edge, the absorption coefficient is rather large due to free-carrier (Casey et al. 1975) and intra-Mn absorption. There is no report on the observation of exciton states or photoluminescence, which is probably due to non-radiative recombination, carrier screening, and the formation of an impurity band (Ando et al. 1999). 

The parameter cro depends on the recombination mechanism (radiative vs. non-radiative) and is typically of the order 107 or 1015 s-1, respectively [21] (see, however, Zamaraev et al. [27], who observed cro up to 1021 s 1). The other recombination parameter, r0, is nothing but half the Bohr radius of the wave-function of an electron centre and is, for example, about 0.5 A for F and Ag° centres and 1 A for shallower Tl° centre in KC1. For paramagnetic defects this parameter could be found by means of EPR and ENDOR [28-30]. 

When a semiconductor is illuminated with light of sufficient energy, electrons from the valence band are excited to the conduction band and the hole-electron pair is generated. The relaxation can take several pathways some of which are non-radiative. The radiative transition leads to photoluminescence (Fig. 9.20). This is a solid-state analogue of molecular fluorescence. The nonradiative recombination and radiative transitions are again the two competing processes. 

Here, the responses are normalized to the maximum concentration r>o of excitations. The signal evolution in a bi-exponential decay is therefore n(t) = Ani(t) + Bn2(t), where A and B are proportional to the radiative (or non-radiative) rates of the two levels. For solids, a monoexponential PL decay can be explained by the thermally activated recombination of highly mobile electrons and holes trapped onto radiative defects. Such a mechanism requires that the spatial separation of the trapped charge carriers be small. 

Figure 2. Drawing illustrating energy transfer processes within a phosphor grain. S co-activator or sensitizer A = activator P = poison center. Short wiggly arrows represent non-radiative transitions. Dashed line outlines the unknown depth to which energy from radical recombination excitation can be transferred.

The charge carriers formed upon absorption of light (reaction (7.1)) can recombine in a radiative or non-radiative way according to reactions (7.12) to (7.15). This is clearly seen from the rather rapid depletion of the transient absorption spectra recorded during laser flash photolysis studies (see Fig. 7.4). 

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