Radiative recombination, exciton bound

The study of the radiative recombination of excitons makes it possible to investigate the influence of the radiation-stimulated destruction of C60 fullerenes (partially of C70) on changes of the singlet states within the energy gap. It is known that the emission of excitons in this case is the result of the presence of own dimeric traps [11] and X-centers, caused by the chemically bound with fullerenes and intercalated impurities [8], and also of taking into account the corresponding phonon states. 

An exciton bound to a shallow neutral donor of interstitial zinc (Fig 1 a) and of interstitial lithium (Fig. lb) is presented, for example, in our spectra. In some instances the radiative recombination of an exciton bound to a neutral defect may not lead to the ground state of the respective defect but to an excited state of the carrier at this occupied center (2 - electron transition). In a hydrogenic model we can calculate an ionization energy of the neutral donor state of interstitial zinc to 0.05 eV and of interstitial lithium to 0.033 eV. 

Figure 1. Schematic representation of radiative recombination of an exciton bound to a neutral donor where the final state is the donor in the ground or in the excited configuration. The inset shows the initial state of the neutral-donor-bound exclton in the ground and several excited rotational states. (Reproduced with permission from Ref. 24. Copyright 1983 American Physical Society.)...

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... 

Radiative recombination of an exciton bound to a shallow impurity generally leaves this impurity in the electronic ground state, resulting in the principal BE (PBE) line, but weaker PL lines can also be observed at lower energies, where the impurity is left in an electronic excited state. These so-called two-electron or two-hole PL spectra are usually observed in their phonon-assisted form, and they mainly involve s-like excited states whose detection escapes the absorption experiments. These PL experiments are, therefore, valuable complements to absorption spectroscopy, which involves mainly the p-like excited states, and examples will be given when appropriate. 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

Another characteristic of the neutral donor-bound exciton transition is the TES transition in the spectral region of 3.32-3.34 eV. These transitions involve radiative recombination of an exciton bound to a neutral donor, leaving the donor in the excited state (2s, 2p states), thereby leading to a transition energy that is less than the DBE energy by an amount equal to the energy difference between the first excited (2s, 2p)... 

Fig. 8.16 Scheme of the individual steps in the process of intrinsic photogeneration of charge-carrier pairs, Mp - M, in a molecular C7stal. The charge carriers are polarons (p). a Sq = neutral ground state S], S2, S3 are singlet excitons. Rate constants k/ i for autoionisation, kn for radiationless and kr for radiative intramolecular recombination (fluorescence), b bound charge-carrier pairs... 

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