Diffusivity, free carriers

Photoelectrochemical techniques have been utilized to determine the minority (electron) diffusion length (L) and other electrical parameters of p-ZnTe [125] and p-type Cdi-jcZnjcTe alloys [126]. In the latter case, the results for a series of single crystals with free carrier concentration in the range 10 " -10 cm (L = 2-4 xm, constant Urbach s parameter at ca. 125 eV ) were considered encouraging for the production of optical and electro-optical devices based on heterojunctions of these alloys. 

There have been many investigations of photoinduced effects in -Si H films linked to material parameters. Changes have been observed in the carrier diffusion length, unpaired spin density, density of states in the gap, and infrared transmission. The transition from state A to B seems to be induced by any process that creates free carriers, including x-ray radiation and injection (double) from the electrodes. Because degradation in a solar cell is accentuated at the open-circuit voltage conditions, the A to B transition occurs upon recombination of excess free carriers in which the eneigy involved is less than the band gap. It has been pointed out that this transition is a relatively inefficient one and the increase in spin density takes place at a rate of 10-8 spins per absorbed photon. 

The mechanism by which cations are transported across a membrane is represented in Figure 18a. A cation-carrier complex is initially formed at the interface. This lipophilic species then diffuses across the membrane as an ion pair and dissociates at the other interface to water soluble ion pair and membrane-soluble carrier. The final step is back diffusion of the free carrier to the initial interface. The factors which influence transport rates and selectivity have been the subject of much research (79PAC979, B-81MI52102). 

Carrier-mediated transport (or facilitated diffusion) consists of the transfer of a substrate across a membrane, facilitated by a carrier molecule located in the membrane. It is a cyclic process comprising four steps (1) formation of the carrier-substrate complex at one interface (2) diffusion of the complex through the membrane phase (3) release of the substrate at the other interface (4) back diffusion of the free carrier. 

Fig. 2. Deuterium (D) and free carrier concentration (n) profiles of a n-type GaAs Si bulk sample exposed to a rf deuterium plasma for 90 min. at 250°C (rf power density = 0.2W/cm2). The loss of free carriers occurring only in.the deuterated region suggests that hydrogen plays a major role in the free carrier concentration decrease. The deuterium concentration drop in the near surface region is attributed to a deuterium out-diffusion during the cooling stage of the sample with the plasma off. J. Chevallier et al., Materials Science Forum, 10-12, 591 (1986). Trans. Tech. Publications.

Fig. 28. Representation of a hydrogenated field effect transistor. The carrier concentration in the active layer is controlled, before the gate deposition, by the hydrogen neutralization of the donors present in the highly silicon doped layer. The insert shows the free carrier concentration gradient from the gate, which results from the hydrogen diffusion. J. Chevallier and M. Aucouturier, Ann. Rev. Mater. Sci. 18, 219 (1988). Annual Reviews Inc.

Hamada et al. 1992 Saito et al. 1992), this increase of absorption is caused by high-frequency conductivity of the free carriers in metallic nanotubes. Relative intensities of the spectra of Fig. 11.7 we have found as a result of the diffuse reflection measurements of powders at low wavenumbers. The discussed above Drude approximations of the low-energy part of the absorption spectra are shown by dashed curves in Fig. 11.7. Comparison of the spectra 1 and 2 shows that hydrogenation decreases high-frequency conductivity of the SWNTs by one order of magnitude. 

LP-CVD ZnO Optical total and diffuse transmittance spectra (TT and DT spectra) of a temperature series of undoped LP-CVD ZnO films are shown in Fig. 6.25 TT does not vary strongly with substrate temperature. Indeed, as Fig. 6.25 is related to a series of undoped samples, the values of carrier density N are too low to produce an observable free carrier absorption effect... 

Total and diffuse transmittance spectra (TT and DT, resp.) and haze factor (i.e., DT/TT measured at 600 nm) are presented in Fig. 6.29 as a function of the H2O/DEZ ratio. As the thickness d of the ZnO samples does not vary significantly within this series, we may assume that the trends observed hereafter are not due to a variation of d. TT does not vary within this series, with the single exception of the TT curve for the ZnO sample deposited with a H2O/DEZ ratio of 0.8, i.e., the only sample deposited with an excess of DEZ. The TT of this sample is systematically lower than the TT curves of the ZnO samples deposited with an excess of water. The reduction of TT in the NIR area is similar for all the curves. This indicates that free carrier... 

In organic cells, however, the steps involved in the generation of photo-current are (1) light absorption, (2) exciton creation, (3) exciton diffusion, (4) exciton dissociation in the bulk or at the surface, (5) field-assisted carrier separation, (6) carrier transport, and (7) carrier delivery to external circuit. Assuming that only the excitons which reach the junction interface produce free carriers, if the blocking contact is illuminated [65],... 

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