Recombination center energy

All TSRs involve the release of trapped charge carriers into either the conduction band or valence band and their subsequent capture by recombination centers and recapture by other traps (retrapping). Their experimental investigation is undertaken with the goal of determining the characteristic properties (parameters) of traps cap-tnre cross sections, thermal escape rates, activation energies, concentration of traps. 

Thus, the study of electrogenerated luminescence spectra may give information on the energy of recombination centers. This method was used to study recombination properties of photoelectrodes in cells for solar energy conversion (Ellis and Karas, 1980). 

In many semiconductors the majority of the photoelectrons are produced by excitation from the valence band, the process thus simultaneously producing holes. Surface traps may act as recombination centers for electron-hole recombination and a change in the number or energy of these surface traps, or a change in the height of the surface barrier, may change the rate of recombination. For example Bube (9,10) has concluded that it is through this effect that the adorption of water vapor influences the photoconductivity of cadmium sulfide. 

It is important to emphasize that the photocatalytic reactivity of the metal ion-implanted titanium oxides under UV light (A < 280 nm) retained the same photocatalytic efficiency as the unimplanted original pure titanium oxides under the same UV light irradiation conditions. When metal ions were chemically dopec into the titanium oxide photocatalyst, the photocatalytic efficiency decreased dramatically under UV irradiation due to the effective recombination of the photo-formec electrons and holes through the impurity energy levels formed by the doped metal ions within the bandgap of the photocatalyst (in the case of Fig. 10.3).14) These results clearly suggest that metal ions physically implanted do not work as electron and hole recombination centers but only work tc modify the... 

Figure 8.6 Calculated energy diagrams for the Si110H114, B6Si108Hn4, and BB6Sin08H1i4 model clusters with optimization. Unoccupied levels are denoted by broken lines and occupied levels are denoted by solid lines. Open and filled circles are unoccupied and occupied states, respectively. The B6Si108H114 has a double acceptor level located immediately above the top of the valence band. B< B6Si108Hn4 has many mid-gap levels localized to the B B6 cluster and may act as recombination centers.

Photoluminescence is the radiation emitted by the recombination process and as such is a direct measure of the radiative transition. Information about non-radiative recombination can often be inferred from the luminescence intensity, which is reduced by the competing processes (Street 1981a). The most useful feature of the luminescence experiment is the ability to measure the emission spectrum to obtain information about the energy levels of the recombination centers. The transition rates are found by measuring the transient response of the luminescence intensity using a pulsed excitation source. Time resolution to about 10 s is relatively easy to obtain and is about the maximum radiative recombination rate. The actual recombination times of a-Si H extend over a wide range, from 10 s up to at least 10- s. 

In Si crystals subjected to heat-treatments after irradiation with high energy (>2 MeV) particles or to irradiations at elevated temperatures (500-800 K), the formation of complex defect-impurity clusters have been established [1,2]. They are highly thermostable. Such clusters can cause significant changes in electrical and optical properties of the irradiated materials and devices, and, in particular, they can serve as effective recombination centers for minority charge carriers in high-speed Si-based devices. 

Deep-level states play an important role in solid-state devices through their behavior as recombination centers. For example, deep-level states are tmdesirable when they facilitate electronic transitions that reduce the efficiency of photovoltaic cells. In other cases, the added reaction pathways for electrons result in desired effects. Electroluminescent panels, for example, rely on electronic transitions that result in emission of photons. The energy level of the states caused by introduction of dopants determines the color of the emitted light. Interfacial states are believed to play a key role in electroluminescence, and commercieil development of this technology will hinge on understanding the relationship between fabrication techniques and tile formation of deep-level states. Deep-level states also influence the performance of solid-state varistors. 

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