Effect of Illumination

The effect of illumination on the I-V curves of both p-Si and n-Si is shown in Fig. 3.13. The anodic reaction kinetics of/ -type silicon are not affected by illumination because the reaction consumes holes which are the majority carriers and their concentration is little affected by illumination. For n-Si during anodization, tbe interface is reversely biased and in order to sustain the reaction, either holes have to be generated or electrons have to be injected from the electrolyte. Thus, in the dark, an extra voltage, Vexj above that which is required for anodization of p-Si, is needed to drive the current. For example, about 100 V is required for 7mA/cm initially in NMA. The extra voltage needed to anodize -Si diminishes with increasing illumination intensity and at sufficient light intensity the anodic current becomes identical to that for p-Si. The quantum yield of illuminated anodization is low a value as low as 1% has been found for the anodization of silicon under illumination.  

FIGURE 3.13. Comparison of constant-current anodization curves in the dark and under illumination for p-Si and n-Si. After Hasengawa et al. ° (Reproduced by permission of The Electrochemical Society, Inc.) 

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Under the effect of illumination, new phenomena arise at semiconductor electrodes, which are discussed in Chapter 29. 

For a crystal illuminated by a photoelectrically active light, the quantities j/°, rr, and 77+ have values different from those for a crystal in the dark. Thus, the effect of illumination is to change the relative content of different forms of chemisorption on a surface for particles of each particular species in other words, it changes the population of electron and holes on the surface local levels corresponding to chemisorbed particles. A change in the quanti-... 

We consider first the effect of illumination on the adsorptivity (75). For the sake of simplicity we limit ourselves to the case when the chemisorbed particles are of a purely acceptor nature. Let them correspond to the acceptor surface levels A in Fig. 24. The level FF in this figure depicts the... 

The effect of illumination on the adsorptivity of the surface has been observed by a number of authors and studied in detail on various adsorbents for different adsorbates and in different frequency intervals (e.g., 76-84 a review of the experimental work is given in Sf). In some cases photodesorption was observed, in other cases, on the contrary, photoadsorption. Why one or another of these two opposite effects takes place has not yet been experimentally elucidated. This remains a problem for a further experimental investigation. 

The experimental data on the effect of illumination on the catalytic activity are as yet extremely scarce. Nevertheless, there are some papers in which this effect seems to have been detected (86-89). 

In conclusion we stress once more that the above-considered mechanism of the effect of illumination on the adsorptivity and catalytic activity of a semiconductor holds in the case when the absorption of light increases the number of free electrons or holes (or both) in the crystal. This, however, does not always take place. The absorption of light by the crystal may proceed by an exciton mechanism. This seems to be the case in the region of intrinsic absorption, which is as a rule photoelectrically inactive. 

Effect of illuminating a larger crystal volume and focusing the beam on the detector... 

Fleet, C. F. and Siebert, K. J. (2005). Effect of illumination intensity on visual perception of turbidity. Food Qual. Pref. 16, 536-544. 

There have been a few experiments related to the effect of illumination of the growth of CdS films. Simple heating of the deposition bath by absorption of the radiation is one obvious factor that can affect the deposition [68]. However, even in this case, other effects occur, since the color of the bath was reported to darken if UV (sunlight) illnmination was employed. Based on previous studies of illuminated CdS colloids when elemental Cd was formed, both as a film and in solution [69], as well as the known tendency of ZnS to undergo reduction to metallic Zn under UV illumination, this darkening may be assumed to be caused by elemental Cd. There are several possible mechanisms that may explain snch an effect re-dnction of the CdS by photogenerated electrons is one possibility. 

Another possibility that could explain the effect of illumination is a change in the electric double layer surrounding the CdSe particles, either adsorbed on the substrate or in the solution, which could lower a potential barrier to adsorption and coalescence, as suggested previously for film formation from Se colloids under illumination [93]. Partial coalescence would reduce the blue spectral shift due to size quantization. However, the spectral shape is not expected to undergo a fundamental change in this case. The photoelectrochemical explanation therefore appears more reasonable. 

The Effect of Illumination. In an alkaline solution, an n-GaP electrode, (111) surface, under illumination shows an anodic photocurrent, accompanied by quantitative dissolution of the electrode. The current-potential curve shows considerable hysterisis as seen in Fig. 2 the anodic current, scanned backward, (toward less positive potential) begins to decrease at a potential much more positive than the onset potential of the anodic current for the forward scanning, the latter being slightly more positive than the Ug value in the dark, Us(dark). 

Comparisons of experimental I-V characteristics with those predicted by theoretical models (JJ are commonly made to analyze the effects of illumination on semiconductor junction devices. These comparisons have not normally been made for semiconductor-electrolyte (S-E) junctions, most likely due to the lack of suitable theoretical models. 

Fig. 2.26. Effect of illumination termination on the brutto-reaction quantum yield cp. Point 1 corresponds to the termination of sample illumination point 2 corresponds to the resumption of illumination after 1 hour.

The effect of illumination seen in the current/potential behavior is reflected also in capacity measurements as evaluated in the form of Mott/ Schottky-plots (Fig. 2). Illumination leads to a parallel shift of this plot in the same direction and by about the same amount as in I/E curves. The plot is shifted back to its dark position if the appropriate redox couple is added. Other minority carrier acceptors on the other hand are not able to shift the light-plot back onto the plot obtained in the dark. 

Fig. 5.28. The effect of illumination level on the dark current in polysilicon [149].

A series of experiments were carried out to observe the effect of illuminating Ce(IV) with light of wavelength 312 nm in the presence of colloidal SnC>2, sulphuric acid (pH 0) and ethanol as an electron scavenger. From Fig. 5, it can be seen that, upon illumination, the concentration of Ce(III), which was initially zero, increases with illumination time as a result of photocatal-ysed reduction of Ce4+. The concentration of Ce3+ continues to increase until it is equal to the original concentration of Ce(IV). Upon the removal of the illumination, the cerium remains in the Ce3+ state. 

When deployed on-line, the semiconductor photocatalyst may be required to photoreduce more than one type of actinide metal ion simultaneously. Figure 9 shows the effect of illuminating U(VI) with light of wavelength 350 nm in the presence of colloidal SnCh, nitric acid (pH 0) and ethanol as an electron scavenger for the semiconductor photocatalyst and Ce(IV) as a non-radioactive, thermodynamic analogue for Pu(IV). Comparison of the data in Fig. 9 with the data recorded under similar conditions as shown in Fig. 7 indicates that the presence of Ce(IV) has no effect on the rate of photocatalysed reduction of U(VI) to U(IV). Furthermore, spectroscopic analysis indicates that virtually all of the Ce(IV) has been reduced to Ce(III) over the same timescale, suggesting that the simultaneous photocatalysed reduction of two or more different types of (actinide) metal ion can be accomplished with no loss of yield for either reaction. 

Specific adsorption of anions leads to a shift of the electrode potential of germanium towards more anodic values apparently because the surface becomes less p-type. Accordingly, the effect of illumination on the potential of electrodes other than strongly p-type is decreased in the presence of anions (7). As shown in Fig. 6, a given photoeffect in the presence of 0. IN KC1 corresponds to the photoeffect exhibited by a more p-type... 

Pig. 9. Effect of illumination upon the mixed potential of n-type Ge in 4 N HNO at room temperature. 

The effect of illumination on pore formation in n-type silicon has been studied by a number of groups [117, 118]. In general, photogenerated holes appear to make the porous structure similar to the porous layers formed in p-type silicon. The structure of porous layers as a function of depth formed under illumination is strongly dependent on wavelength and whether frontside or backside illumination is used. 

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