Two-Layer PS

Two-Layer PS. Two-layer PS, with a micro PS layer on top of a macro PS layer, can form under certain conditions. For n-Si, formation of a two-layer PS is associated with front illumination, although it can also be formed with back illumination. For p-Si, two-layer PS is found to form on lowly doped substrates. For moderately or highly doped p-Si or for n-Si in the dark, formation of two-layer PS has not been observed. 

FIGURE 8.42. Etched layer thickness as a function of the substrate resistivity. (Reprinted from Al Rifai et 2000, with permission from Elsevier Science.) 

FIGURE 8.43. Three microstructural types observed in PS formed on illuminated n-Si. (a) Single-layer (SL), (b) double-layer (DL), and (c) large macropore (LMP) types. After Osaka et al. (Reproduced by permission of The Electrochemical Society, Inc.) 

The occurrence of the two-layer PS under front illumination depends on light intensity and the amount of charge passed as illustrated in Fig. 8.43. There is a correlation between the occurrence of two-layer PS and the saturation photocurrent value. Only a single micro PS layer forms at photocurrent densities below the photosaturation value whereas a two-layer PS forms at current densities above the saturation current as shown in Fig. 8.44. Also, a macro PS layer forms only after a certain amount of 

FIGURE 8.44. Voltammograms of n-Si electrodes in aqueous solution containing lOwt % HE and 35 wt % C2H5OH at 5 mV/s under different light intensities. The illumination intensity was adjusted by changing the distance between the light source and the n-Si electrode. After Osaka et o/. ° (Reproduced by permission of The Electrochemical Society, Inc.) 

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B backside illumination 4 MeCN acetonitrile MP macropores of two-layer PS d eth ethanol,... 

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. 

Two layered PS occurs only on illuminated n-Si and lowly doped p-Si. 

The results of the above analysis suggest that the formation of macro PS on lowly doped materials can be associated with a nonlinear potential distribution in the solid of a curved surface due to the high resistivity of the solid. The formation of two-layer PS on... 

Two-layer PS with a micro PS on top of a macro PS layer is formed on lowly doped p-Si or illuminated n-Si. For lowly doped p-Si two-layer PS can form when the conditions are such that the space charge layer and the resistive layer differ in dimension by several orders of magnitude. 

The formation of two-layer PS on p-Si involves two different physical layers in which the potential-current relations are sensitive to the radius of curvature. The space charge layer of p-Si under an anodic potential is thin, which is responsible for the formation of the micro PS. The non-linear resistive effect of the highly resistive substrate is responsible for that of macro PS. The effect of high substrate resistivity should also occur for lowly doped n-Si. However, under normal conditions, the thickness of the space charge layer under an anodic potential, at which macro PS is formed, is on the same order of magnitude as the dimension... 

Two-layer PS on n-Si reported, Arita Sunahama7 1 on n-Si with back illumination20 ... 

The morphology of the PS formed on n-Si also strongly depends on illumination conditions, that is, intensity, frequency, and direction (front or back). Very different morphologies are produced by front versus back illumination. Back illumination generally produces straight pores, whereas front illumination typically produces a two-layer PS as shown Fig. 8.19(6c). 

Figure 8.28 shows the pore size distributions for the PS formed on highly doped / -Si at different current densities." For the PS formed on heavily doped silicon, the pores at a given HF concentration have a narrower distribution at lower current at a given current density, the distribution is narrower at lower HF concentrations. A bimodal distribution of pore diameter is gena-ally associated with two-layer PS on lowly doped / -Si and iUmninated n-Si. The PS formed on p-Si has also been found to have a bimodal distribution of particles sizes small spherically shaped particles with a diameter of a few nanometers and large cylindrically shaped particles oriented with their axis perpendicular to the surface.The distribution of pore size with multiple peaks may be attributed to the fact that PS may have a surface micropore layer and smaller branched pores. Due to the hierarchical structure of the branched pores, the distribution of pore diameters for highly branched PS is found to be fractal-like. ° ... 

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