Excitonic recombination

At low temperatures, donors and acceptors remain neutral when they trap an electron hole pair, forming a bound exciton. Bound exciton recombination emits a characteristic luminescence peak, the energy of which is so specific that it can be used to identify the impurities present. Thewalt et al. (1985) measured the luminescence spectrum of Si samples doped by implantation with B, P, In, and T1 before and after hydrogenation. Ion implantation places the acceptors in a well-controlled thin layer that can be rapidly permeated by atomic hydrogen. In contrast, to observe acceptor neutralization by luminescence in bulk-doped Si would require long Hj treatment, since photoluminescence probes deeply below the surface due to the long diffusion length of electrons, holes, and free excitons. 

For a critical concentration of excitons 17 = 7/. the critical radius (, below which bimolecular annihilation process predominates over singlet exciton recombination can be expressed as [5],... 

Two energies in fig. 21 are important quantities for the luminescence behavior. First, the energy Et represents the binding energy of the rare earth bound exciton with respect to the conduction band and second, bt, the necessary energy for back-transfer, corresponds to the difference between the bound exciton recombination energy and the 2Fs/2 2F7/2 transition... 

One of the advantages of lanthanide-doped semiconductor nanocrystals is that lanthanide luminescence can be efficiently sensitized by exciton recombination in the nanocrystals. The photoluminescence efficiency of Er3+ in Si nanocrystals in Si02 increases by more than five... 

Ishizumi and Kanemitsu (2005) have studied PL properties of Eu3+ doped ZnO nanorods fabricated by a microemulsion method. The PL of bound exciton recombination and ZnO defects was observed near 370 and 650 nm under 325-nm light excitation, but no emission of Eu3+ occurred. On the other hand, the sharp PL peaks due to the intra-4f transitions of Eu3+ ions appeared under nonresonant excitation below the band-gap energy of ZnO (454 and 457.9 nm) in addition to direct excitation to 5D2 (465.8 nm). Therefore the authors concluded that the energy transfer occurs from the ZnO nanorods to Eu3+ ions through ZnO-defect states. This energy transfer mechanism seems very different from the previous one and more spectroscopic evidence is required to confirm it. 

Hossain et al [15] measured die cathodoluminescence in undoped aluminium nitride (AIN) thin films at 300, 77 and 4.2 K. These films were grown on sapphire and SiC substrates by LP-MOCVD at 1473 K. As shown in FIGURE 1, two distinct peaks were observed at about 6.1 and 5.9 eV, respectively. The energy position of these peaks increases and the linewidth becomes narrower, as the temperature is decreased. They believed that these two peaks are due to exciton recombination. Recently, MacMillan et al measured cathodoluminescence in AlN-GaN superlattices [16],... 

Since GaN materials contain a high density of extended defects (10 - 1010 cm 2), it is expected that the carrier and exciton recombination dynamics vary from sample to sample. Studies have revealed that the exciton lifetime is correlated with the yellow emission line at about 2.2 eV the stronger the yellow... 

A survey of the optical bandgap, excitonic recombination properties under low excitation and electron hole plasma recombination in AlxGai.xN has been given. Demand for UV applications, i.e. gas sensors or monitors, flare sensors, medical applications, chemical and biochemical applications and light sources for phosphors increases rapidly, which will surely lead to the further improvement of the quality of the AIN containing nitrides, and thus give us much more information about their luminescence properties. 

Finally, we have measured the time-dependent behavior of the PL spectra at 77 K. At a delay time of 0-2 ns we observed strong UV PL (Fig. 7). At steady state conditions a deep-level yellow-red emission coimected with defect luminescence is observed. So it could be concluded that exciton recombination is a faster process than recombination via deep levels. 

Carter and coworkers studied how side-chain branching in PFs affects device performance with and without an additional HTL of cross-linkable polymer 2 [ 19]. They found that the device efficiency is affected more by the position of the exciton recombination zone than by variations of polymer morphology induced by side-chain branching, which mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. For double-layer devices (ITO/PEDOT PSS/2/3,4, or 5/Ca), a typical brightness of 100 cdm 2 at 0.8 MV cm-1, maximum luminance of 10 000 cd m-2 at 1.5 MV cm x, and device efficiencies between 1.3 and 1.8 cd A 1 for 3 and 5 branching can be achieved. 

Christen J. and Bimberg D. (1990), Line shapes of intersnbband and excitonic recombination in quantum wells influence of final-state interaction, statistical broadening, and momentum conservation , Phys. Rev. B 42,7213-7219. 

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