Carrier capture

Deep state experiments measure carrier capture or emission rates, processes that are not sensitive to the microscopic structure (such as chemical composition, symmetry, or spin) of the defect. Therefore, the various techniques for analysis of deep states can at best only show a correlation with a particular impurity when used in conjunction with doping experiments. A definitive, unambiguous assignment is impossible without the aid of other experiments, such as high-resolution absorption or luminescence spectroscopy, or electron paramagnetic resonance (EPR). Unfortunately, these techniques are usually inapplicable to most deep levels. However, when absorption or luminescence lines are detectable and sharp, the symmetry of a defect can be deduced from Zeeman or stress experiments (see, for example, Ozeki et al. 1979b). In certain cases the energy of a transition is sensitive to the isotopic mass of an impurity, and use of isotopically enriched dopants can yield a positive chemical identification of a level. 

Localized states in the bulk of a semiconductor that have energies within the bandgap are known to capture mobile carriers from the conduction and valence bands.— The bulk reaction rate is determined by the product of the carrier density, density of empty states, the thermal velocity of the carriers and the cross-section for carrier capture. These same concepts are applied to reactions at semic ijiductor surfaces that have localized energy levels within the bandgap.— In that case the electron flux to the surface, F, reacting with a surface state is given by... 

A similar situation exists for carrier capture by surface states. Relatively large capture cross sections are observed but no adequate theoretical treatment exists. Theory to describe the capture process is greatly complicated by presence of the surface. The carrier motion as well as the vibrational behavior of the crystal is perturbed by the surface. What does seem clear, however, is that the surface state should be tightly enough bound to the crystal lattice so that phonon emission is possible. In addition, the state should be close enough to the semiconductor to overlap the wave function of the semiconductor carrier. 

If the oppositely charged carriers are generated independently far away of each other (e.g. injected from electrodes) volume-controlled recombination (VR) takes place, the carriers are statistically independent of each other, the recombination process is kinetically bimolecular. It naturally proceeds through a Coulombically correlated electron-hole pair (e h) leading to various emitting states in the ultimate recombination step (mutual carrier capture) (Fig. 3 for more details, see Figs. 11 and 27 in Sec. 2.3). As a result, the overall recombination probability becomes a product of the probability of the pair formation, Pr(1) = (1 + Tm/Tt) , and the capture probability, PR(2) = (1 + tc/t(1)... 

The recombination rate constant k depends on the doping density Nj, the thermal velocity v of majority carriers and the majority carrier capture cross section a of recombination centres ... 

Usually the defects reduce the lifetime of non-equilibrium carriers and, consequently, their diffusion length in semiconductors. However, because of the presence of the closely spaced AlGaAs barriers, the carrier capture by the QDs in our samples is not diffusion-limited. That is why a difference in the quenching factor of the PL intensity at a given irradiation dose for the above- and below-bandgap excitation for all energies above the m = 2 QD excited state (Fig. lb) is not observed. Thus, the loss of carriers occurs mainly in the dots due to tunneling of caniers from the dots to adjacent non-radiative recombination centers. 

In general, the occupation of states in the gap is dominated by majority-and minority-carrier emission and majority-carrier capture. The emission rate for electrons at temperature T is usually given as... 

Thermal equilibrium requires that the capture rate equal the emission rate at Ef. Therefore, majority-carrier capture follows the same dependence on energy and temperature as Eq. (5). However, for capture the characteristic energy E denotes the height of the conduction-band edge above the bulk Fermi level, E = AEp -b w(x), where AEp = E — Ep in the bulk material. This energy is the average energy of the potentid barrier that electrons must travel up to be captured at the point x under consideration. 

In these equations, n is the density of the free charge carriers, ( the density of the charge carriers captured in shallow traps, s the static dielectric constant, the density of states at the transport level Eg, Vext the applied voltage, and G(( ) gives the density of the trapping states per energy interval dE. Ne in disordered fUms or in crystals with narrow conduction bands is equal to the number of molecules per unit volume. 

The second important mechanism of photogeneration is photostimulation of long-lived stable cation-radicals of the donor PI fragments, representing the hole (major carriers) captured by deep centers (PSC). Accumrrlation of the cation-radieals in the dark and photo processes leads to the dependenee of photovoltaic characteristics on the number of charge discharge cycles of the sample. 

There are some techniques, such as admittance spectroscopy and deep-level transient spectroscopy (DLTS), that are quite powerful in the characterization of deep levels in semiconductors [139]. These techniques have also begun to be used for the characterization of conjugated polymers such as PPV [178] and MEH-PPV [179]. These techniques may permit the determination of several trap parameters such as activation energy, concentration, charge carrier capture cross section, defect donor/acceptor character that can contribute to the chemical identification of the traps. 

After uptake of DNA complexes, escape from intracellular vesicles into the cytoplasm represents a major bottleneck. Entrapment in lysosomal or phagocytic vesicles is thought to be associated with degradation of the complexes in these compartments. The fate of the dehvered DNA strongly depends on the selected polycationic carrier. Capture in endo/lysosomes seems to be a more serious hurdle for pLL polyplexes than for dendrimer or PEI polyplexes. The following section will discuss approaches to overcome this barrier. 

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