Exciton localization energies

Figure 4.2 Exciton localization energies versus the respective donor binding energies (Haynes plot) for bulk ZnO. The solid line is a fit to Equation 4.2 with 0 = —3.8 eV and b = 0.365. (After Refs [55, 56].)...

Such renormalization can be obtained in the framework of the small polaron theory [3]. Scoq is the energy gain of exciton localization. Let us note that the condition (20) and, therefore, Eq.(26) is correct for S 5/wo and arbitrary B/ujq for the lowest energy of the exciton polaron. So Eq.(26) can be used to evaluate the energy of a self-trapped exciton when the energy of the vibrational or lattice relaxation is much larger then the exciton bandwidth. 

Fig. 7.19 Influe nee of QC on the basic properties of excitons localized in Si nanocrystals. Confinement energy dependence of (a) relative strength of no-phonon and TO phonon-...

The physical phenomenon of current generation in simple D/A systems can be thought of in terms of the six chemical steps depicted in Fig. 4. (1) The absorption of a photon leads to a localized exciton with energy oo on either the donor or... 

It is discussed how the primary processes of defect formation during irradiation occur via electronic excitation. This can take the form of either the creation of electron-hole pairs, followed by trapping into localized energy states, or of exciton creation leading to the formation of stable vacancy and interstitial defects. Heating the sample after the irradiation causes the release of this stored energy in the form of phonons or photons. Photon emission, ie. luminescence, results from either electron-hole recombination or from vacancy-interstitial recombination. Several examples of both types are discussed for crystalline CaF and SiC. ... 

One can say that the obtained by us experimental results upon 2D exciton localization (taking place due to the growth of the crystal dielectric permeability anisotropy parameter) with o are very close to [27] where the behaviour of polaron excitons in parabolic quantum dots were considered and shown that the dot size decrease results in increasing the exciton binding energy. 

This coupling is assumed local, i.e., creation or absorption of vibrations occurs without exciton transfer, as in (2.15). This approximation amounts to considering only the R dependence of the local energy Dnm. We find that the xs do not depend on q. 

In Section I, the spectra of e"(ai) consist of Dirac 5 peaks (1.79). In a real crystal these peaks are broadened by static disorder, thermal fluctuations, and excitation-relaxation processes. Discarding for the moment the static disorder, we focus our attention on broadening processes due to lattice phonons, which may be described alternatively in terms of fluctuations of the local energies of the sites, or in terms of exciton relaxation by emission and absorption of phonons. These two complementary aspects of the fluctuation-dissipation theorem64 will allow us to treat the exciton-phonon coupling in the so-called strong and weak cases. The extraordinary (polariton) 0-0 transition of the anthracene crystal will be analyzed on the basis of these theoretical considerations and the semiexperimental data of the Kramers-Kronig analysis. 

We note that the notions optical and electrical gap are here used in the context of the classical band theory of solids and can be confusing in application to molecular (van der Waals bonded) solids, where they have the opposite meaning the optical gap reflects the energy of excitonic (localized) states, while the electrical gap stands for the lowest energy between free carrier states. 

Fig. 11. Total energy of an exciton in an anisotropic elastic continuum for different Pt-Pt distances Rm). The energy is calculated for Mg[Pt(CN)4] 7 H20 from Eq. (6). a(ai ct ) represents a localization parameter which describes a free exciton (FE) with a = 0 and a localized exciton (self-trapped exciton STE) with a = 1. The exciton binding energy EB is normalized to zero for different R-values...

In an excitation process, the electron and the hole can remain bound, producing an exciton state just below the conduction band. Indeed, the mass of an inner hole is considered as infinite and the exciton binding energy is thus almost zero with reference to the absorption threshold energy. If the resonance lines were excitonic type transitions, the emission spectmm should be exactly the reverse of absorption. We would see that this is not the case although a localized excited Mjy state has a large probability of existing, sometimes the resonance Mjy lines are absent, whereas the resonance My lines are the most intense of the spectrum (77). 

The FEs can bind to neutral shallow impurities and become bound excitons (BEs), with a value of Eex slightly larger than the one of the FE. The difference is called the localization energy E oc of the BE. For the P donor, it is 4 meV in silicon, but 75 meV in diamond. E oc is given approximately by Haynes empirical rule [20] as 0.1 A, where A is the ionization energy of the impurity. BEs are created by laser illumination of a semiconductor sample at an energy larger than Eg and the study of their radiative recombination by PL... 

The problem of the kinematic interaction between two paulions is similar to the problem of localized states of an exciton in the presence of a vacancy (98). Indeed, the kinematic interaction governing the relative motion of two Pauli particles is formally analogous to the one-particle potential created by a vacancy, which cannot be occupied by an exciton. In this case the equation determining the localized exciton state energy E is... 

The localized high concentration of carriers with opposite charges is favorable for exciton formation in the vicinity of the interface. This site is the so-called recombination zone (RZ). The emitted photons are then the result of the desexcitation of those excitons, with energy equal to the excitonic gap. 

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