Localised carriers

The localised carriers in those tail states may play a significant role in InGaN MQW lasing. However, further studies are required to better understand the lasing phenomena in these MQWs. 

It is important to recognise that a sub-band gap optical transition leads to a delocalised carrier of one type and a localised carrier of opposite type. Steady-state photocurrent flow requires that the localised carrier is excited subsequently to the valence or conduction band, either by absorption of a second photon (process (b) in Fig. 3) or by thermal excitation (processes (c, d)). Bandgap states localised at the semiconductor surface may be of special importance for sub-band gap photocurrent flow. In process (e), an electron (majority carrier) is optically excited into the conduction band, and the resulting empty surface state is refilled by an interfacial electron transfer process. The latter process is similar to the process of dye sensitised electron injection in the nanocrystalline Ti02 solar cell [20-26, 129). 

UV-vis ectra of DBSA doped Polyaniline solutions in m-xylene were found to be dependent on heat treatment in the case of high viscosity samples (Figure 5). During heat treatment the band at 750 nm attributed to localised carriers is gradual shifted toward low energies and its intensity is decreased e an absoiption increases at low energy leading to a more metallic (19) spectrum. 

For localised carriers, the Heikes formula states that ... 

In the presence of weak disorder, one should consider an additional contribution to the resistivity due to weak localisation resulting from quantum interference effects and/or that due to Coulomb interaction effects. A single-carrier weak localisation effect is produced by constructive quantum interference between elastically back-scattered partial-carrier-waves, while disorder attenuates the screening between charge carriers, thus increasing their Coulomb interaction. So, both effects are enhanced in the presence of weak disorder, or, in other words, by defect scattering. This was previously discussed for the case of carbons and graphites [7]. 

So, despite the very small diameter of the MWCNT with respeet to the de Broglie wavelengths of the charge carriers, the cylindrical structure of the honeycomb lattice gives rise to a 2D electron gas for both weak localisation and UCF effects. Indeed, both the amplitude and the temperature dependence of the conductance fluctuations were found to be consistent with the universal conductance fluctuations models for mesoscopic 2D systems applied to the particular cylindrical structure of MWCNTs [10]. 

Figure 7.6 Semi-quantitative analysis of an organic two-component coating on a metal surface. The classifier grades the results into within specification (light grey), warning (white), mixing ratio/coverage outside specification (dark grey) and surface defects on the metal carrier (black). A significant isolated problem, B localised constant problem, C acceptable localised random flaws, E start of massive failure, F localised application of primarily one component only due to spray nozzle obstruction.

Several studies have been performed during the last two decades on CPPs which may function as carriers for different minerals, especially calcium. Published data on the effect of CPP/casein on mineral solubility and absorption are inconsistent, partly due to the diversity of the experimental approaches. Most of the findings in the literature that deal with the mineral absorption-stimulating effect of CPP are based on in vitro, in situ, cell culture or single meal studies. Majority of the studies have been done with rats and have provided considerable evidence for the potential effect of casein-derived phosphopeptides to improve mineral absorption. This potential is not limited to calcium but is also valid for zinc and iron, and possibly other elements that have not been investigated so far (FitzGerald, 1998). Furthermore, CPPs have been shown to have anticariogenic properties, based on their ability to localise amorphous phosphate in dental plaque (Reynolds, 1998). 

InN tends always to be very highly conducting which makes it impossible to study shallow donor levels and the alloy InxGai.xN is complicated by strain effects, spinodal decomposition and carrier localisation. It has so fin been impossible to identify free excitons (which serve as a standard in PL) so little progress... 

FIGURE 1 Comparison of optical absorption and gain spectra of a GalnN/GaN double heterostructure [12], The development of the spectra from absorption to gain with increasing pump power (from top to bottom) is indicative of free-carrier lasing with no influence of localised states. 

Low-excitation, low-temperature experiments like photoluminescence or photoluminescence excitation spectroscopy tend to indicate a considerable influence of localisation effects on the optical properties of GakiN/GaN quantum wells. Under high-excitation conditions typical for lasing, however, it is clearly seen that lasing from GalnN/GaN quantum well structures is due to a free-carrier plasma. 

It is normally unnecessary for the electrochemist to be concerned with the mobility of carriers in most of the semiconductors whose properties have been studied, since the very low conductivity of "small polaron samples would normally preclude their measurement. However, a proviso must be entered here in the case of binary and, more especially, ternary samples. It may well be the case that the majority carriers in a particular material are indeed itinerant (i.e. have mobilities in excess of ca. 1 cm2 V 1s 1), but there is no guarantee that this will be true of the minority carriers generated by optical absorption. Thus, the oxide MnTi03 shows a marked optical charge transfer absorption from Mn(II) to Ti(IV), the latter being the CB. The resultant holes reside on localised sites in the Mn levels, presumably as local Mn(III) centres, and are comparatively immobile. The result is that there is... 

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