Doping trapping energy

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. 

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

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. 

It can be seen from Figure 3.10a and d that the emission spectra of the neat BCzVB and DPVBi doped with BCzVB are essentially the same and can be attributed to an energy transfer process. The emission spectrum of CBP doped with BCzVB is quite different it comes from emission contributed from both CBP and BCzVB molecules owing to both charge trapping as well as a partial energy transfer process. 

The activation energy for oxide ion conduction in the various zirconia-, thoria- and ceria-based materials is usually at least 0.8 eV. A significant fraction of this is due to the association of oxide vacancies and aliovalent dopants (ion trapping effects). Calculations have shown that the association enthalpy can be reduced and hence the conductivity optimised, when the ionic radius of the aliovalent substituting ion matches that of the host ion. A good example of this effect is seen in Gd-doped ceria in which Gd is the optimum size to substitute for Ce these materials are amongst the best oxide ion conductors. Fig. 2.11. 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

From comparison with the results of charge trapping and anion BSD experiments on pure and doped -hexane films [32] (see as an example Fig. 13), it has been suggested that negative charging between 7 and 15 eV in XPLE is dominated by DBA to alkane chains. In contrast, charging below 5 eV is likely to be associated with molecular impurities (water, O2, and antioxidant additives) since pure alkane molecular solids are unable to trap electrons at these energies (Fig. 13). 

The active region of GaN lasers consists of of GaN containing several thin layers (30-40 A thick) of indium doped-GaN, In/lai-JM. The addition of indium reduces the band gap within the thin layers, so that the bottom of the conduction band is at lower energy than that in the bulk GaN. Electrons in this conduction band are effectively trapped because they need to gain energy from an external source to pass into the conduction band of the bulk GaN. Figure 8.13 illustrates schematically the bottom of the conduction band and the top of the valence band for a series of thin layers of In/lai JSl in GaN. 

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