Illumination intensity

Figure 15-21 shows the dependence of the short circuit current and the photocurrent at -IV (reverse) bias as a function of the illumination intensity (514.5 nrn) the data show no indication of saturation al light intensities up to ca. 1 W/cm2. 

C60 has been used to produce solvent-cast and LB films with interesting photoelec-trochemical behavior. A study of solvent-cast films of C60 on Pt rotating disc electrodes (RDEs) under various illumination conditions was reported [284]. Iodide was used as the solution-phase rednctant. The open-circuit potential shifted by 74 mV per decade of illumination intensity from a continuous wave (cw) argon-ion laser. The photocurrent versus power was measured at -0.26 V under chopped illumination (14-Hz frequency, vs. SCE) up to 30 mW cm and was close to linear. The photoexcitation spectrum (photocurrent versus wavelength) was measured at 0.02 V (vs. SCE) from 400 to 800 mn and found to be... 

The basic experimental arrangements for photocurrent measurements under periodic square and sinusoidal light perturbation are schematically depicted in Fig. 19. In the previous section, we have already discussed experimental results based on chopped light and lock-in detection. This approach is particularly useful for measurement at a single frequency, generally above 5 Hz. At lower frequencies the performance of lock-in amplifier and mechanical choppers diminishes considerably. For rather slow dynamics, DC photocurrent transients employing optical shutters are more advisable. On the other hand, for kinetic studies of the various reaction steps under illumination, intensity modulated photocurrent spectroscopy (IMPS) has proved to be a very powerful approach [132,133,148-156]. For IMPS, the applied potential is kept constant and the light intensity is sinusoid-... 

This expression describes well the illumination intensity dependence of the signal indicating changes in the electrical conductivity of Au/ZnO being acted upon by RGMAs. Figure 5.26 illustrates the ex-... 

Anodic dissolution of n-Si can also proceed at a polarization under illumination. The maximum current is limited by illumination intensity when the saturation photo current density is lower than the critical current, Ji. The characteristics of i-V curves of n-Si under a high illumination intensity, when the reaction is no longer limited by the availability of photo generated carriers, is identical to that for p-Si. Similar also to p-Si, formation of PS on n-Si occurs only below the critical current, Jx 24... 

The pores at the surface are smaller than those in the bulk of PS as, for example, shown in Figure 11, 8,16,24 Such an increase in pore diameter from the surface to bulk is due to the transition from pore initiation to steady growth. Also, two-layer PS, a micro PS layer on top of a macro PS can form for on illuminated n-Si or on lowly doped p-Si. For the micro PS layer formed on front illuminated n-Si, pore diameter is less than 2 nm and thickness of PS changes with illumination intensity and the amount of charge passed. Also, the diameter of macro pores on front illuminated n-Si changes with the amount of charge Passed.20 Pore size and depth variation of PS on n-Si are very different for front and back illuminated n-Si samples. 

There is also an etched layer of Si on the surface under illumination as illustrated in Figure 18. This etched layer is mainly due to photo-induced corrosion. As a result of the photo induced dissolution the top surface of PS layer recedes with time. The rate of dissolution depends on doping, F1F concentration, current density and illumination intensity. 

Figure 17. Voltammograms of n-Si electrodes in aqueous solution containing 10 wt.% HF and 35 wt.% C2H5OH at 5 mV s 1 under different light intensities. The illumination intensity was adjusted by changing the distance between the light source and the n-Si electrode. After Osaka el a/,78...

As illustrated in Figure 26, which is a varied presentation for a single pore from the scheme shown in Figure 19, there are five possible phases in the current path in which significant potential drops may occur. The distribution of the applied potential in the different phases of the current path depends on doping type and concentration, HF concentration, current density, potential, illumination intensity and direction. The phases in the current path... 

Illumination generates holes within the material of PS and causes photo corrosion of PS that is much faster than that in the dark. Depending on illumination intensity and time, the pore walls in a PS can be thinned to various extents by the photo induced corrosion. This corrosion process is responsible for the etched crater between the initial surface and the surface of PS as illustrated in Figure 28. It is also responsible for the fractal structure of the micro PS formed under illumination. 

Each of the above processes has its own characteristic kinetic and rate law and, in principle, each responds differently to the process variables (illumination intensity, dopant density, presence of adsorbates, activity of surface potential determining ions, width of and potential drop in the space charge region, position of the band edges). 

If the same experiment is performed with an n-type Si electrode under identical illumination intensity the anodic photocurrent is found to be larger than for the p-type electrode under cathodic conditions. This increase is small (about 10%) for current densities in excess of JPS. Figure 3.2 shows that in this anodic regime injected electrons are also detected at p-type electrodes. This allows us to interpret the 10% increase in photocurrent observed at n-type electrodes as electron injection during anodic oxide formation and dissolution. 

For current densities below JPS the photocurrent in aqueous HF is found to be increased by a factor of 2 or even up to a factor of 4 for small photocurrent densities [Br2, Mai, Pel]. This effect is shown in Fig. 4.13. For non-aqueous HF electrolytes factors between 2 and 3 are observed. For further reduction of the illumination intensity the multiplication factor approaches infinity, because of the illu-... 

Fig. 4.14 Anodic potential scan (50mVs ) of an n-type Si electrode in 1% HF under con stant (bold broken line) and chopped illumination (solid line, illumination intensity corresponding toJP). Point A Current transients are minimal belowJPS. Points B, C Current...

Illumination is a relevant parameter in the electrochemistry of silicon because photogenerated carriers may initiate or contribute to the charge exchange at the electrolyte-silicon interface. If an electrode is illuminated, photogenerated electron-hole pairs are generated corresponding to the number of absorbed photons. This number depends on spectral distribution, total illumination intensity and losses due to optical reflection and transmission. The number of electron-hole... 

An initially flat silicon electrode surface will develop a surface topography if the photocurrent varies locally. This variation can be caused by a lateral variation of the recombination rate or by a lateral variation of the illumination intensity. The photoelectrochemical etching of a silicon electrode is related to the etch-stop techniques discussed in Chapter 3. While different etching rates for different areas of the electrode may be obtained by electrical insulation or by a different doping density, the etch rate may also be altered by a difference in illumination intensity. Basically four photoelectrochemical etching modes are possible for homogeneously doped substrates ... 

The experimentally observed parabolic increase in pore depth and linear decrease in concentration shown in Fig. 9.18 c indicate that Eq. (9.6) is valid [Le9]. The macropore growth rate decreases linearly with l according to Eq. (9.6). If, therefore, a constant pore diameter is desired for a macropore array, a decrease in etching current or illumination intensity, respectively, with time is required. 

A local variation in porosity can be produced by an inhomogeneous illumination intensity. However, any image projected on the backside of the wafer generates a smoothed-out current density distribution on the frontside, because of random diffusion of the charge carriers in the bulk. This problem can be reduced if thin wafers or illumination from the frontside is used. However, sharp lateral changes in porosity cannot be achieved. 

M NH4C1 are determined to be 0.14 V (SCE) and -0.54 V (SCE), respectively [Otl], A similar value of -0.35 V (SCE) is observed for n-type Si in 1 M HF by microwave reflectivity measurements [Na7]. Figure 10.3 summarizes values of Vh, obtained by different methods. Note that the scatter in these data is much larger for p-type silicon electrodes than for n-type. A similar scatter has been observed in the determination of the OPC potential of p-type electrodes, which is found to be more sensitive to parameters such as, for example, illumination intensity than that of n-type electrodes, as discussed in Section 3.2. 

For not too low doped samples (D W), however, the contribution of 1SCR is usually negligible. If the surface recombination velocity at the illuminated front is low, IBPC then only depends on sample thickness D, illumination intensity eP, and minority charge carrier diffusion length ID. 

Figure 13.12. Response of the phase angle and intensity in response to changes in the illumination intensity at fixed oxygen concentration.

A single-beam spectrophotometer, hke the one shown in Figure 1.5(a), presents a variety of problems, because the spectra are affected by spectral and temporal variations in the illumination intensity. The spectral variations are due to the combined effects of the lamp spectrum and the monochromator response, while the temporal variations occur because of lamp stability. 

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

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