Formation of Porous Layer

Among many key parameters which affect the pore morphology (such as current density, voltage, time of anodization, composition of electrolyte, etc.) substrate doping concentration (or resistivity) plays a particularly significant role. However, not much data related to the effect of SiC substrate resistivity on the formation of a porous layer has been published so far. In this work, we discuss surface and pore morphology of SiC and explore its correlation with substrate doping concentration. 

It should be noted that the effect of UV illumination on the gain in current during anodization was not significant ( 0.1 mA) due to the low density of UV light ( 50 mW cm-2). 

I-V curves, the formation of porous SiC was performed in the avalanche breakdown mode. For heavily (H) doped samples an operating current point of 10 mA was at the voltage of 7 V, while for moderately (M) and low (L) doped samples the operating voltage points for the same current were established at 48 and 125 V, respectively. The transient period of time during which the current approached this operating point of 10 mA was 10 s for each experiment, following which, the anodization current was constant for a period of time. 

Therefore, a higher dopant concentration will lead to the formation of smaller pores as well as a smaller spacing between the pores, because a smaller radius of curvature on the pore bottom is required for stable pore propagation. 

The pore depth penetration in the high-doped samples was about 15 pm after 5 min of anodization at 40 mA cm-2, while the moderately and low-doped sample had pore depth penetrations of 3 and 1 pm, respectively. Since the porous layers were formed at the same conditions, i.e. the same total charge was applied to the samples, the amount of 

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Attack by alkali solution, hydrofluoric acid and phosphoric acid A common feature of these corrosive agents is their ability to disrupt the network. Equation 18.1 shows the nature of the attack in alkaline solution where unlimited numbers of OH ions are available. This process is not encumbered by the formation of porous layers and the amount of leached matter is linearly dependent on time. Consequently the extent of attack by strong alkali is usually far greater than either acid or water attack. 

The formation of porous layers in silicon by chemical etching in HF/HNO3 solutions was first reported at about the same time as electrochemical etching [101-104]. These so-called stain etch films are characterized by rough or porous surfaces, typically < 500 A in thickness. Recent work has shown that these stain etch films exhibit strong visible photoluminescence, similar to the emission observed from electrochemically etched porous silicon layers. 

S. Langa, I. M. Tiginyanu, J. Carstensen, M. Christopherson, and H. Foil, Formation of Porous Layers with Different Morphologies during Anodic Etching of n-InP, Electrochemical and Solid State Letters 3(11), 514, 2000. 

It has been proposed that the crystallization of most aluminophosphates presumably involves a chain to layer transformation process wherein the chain is initially hydrolyzed in solution, leading to the formation of other chain structure types. Condensation of the chains leads to cross-linking and then to the formation of porous layers and subsequently the open-framework structures (108). 

With many materials, agglomeration may be achieved by heating as a result of which softening occurs in the surface layers. For the formation of porous metal sheets and discs, high temperatures are required. 

Some ligands chelating Zn(II) ions were also found to be effective corrosion inhibitors [309, 310] owing to the growth of an organometallic layer strongly attached to the metal surface, which prevent the formation of porous corrosion products. 

The electrochemistry of silicon is highly important as a tool for surface treatment and the formation of porous silicon. Under the reverse bias (anodic for n-type, cathodic for p-type) of silicon immersed in an electrolyte, a space charge layer is formed near the electrode surface, in which the concentration of the charge carriers differs from that of the bulk material. The width of this space charge layer depends on the type and density of dopants in the material and the potential bias. 

The optical properties of porous silicon have given rise to renewed interest in the processes leading to pore formation and the relationship between the structure and characteristic properties of porous layers. Indeed, porous semiconductors may be considered a new class of materials, since pore formation is not limited to silicon and has been observed for a wide range of compound semiconductors. 

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. 

To account for their distinctive characteristics, several authors (notably Peter Kelemen) suggested the formation of the layered refractory peridotites by olivine-forming, melt-rock reactions, leading to the evolution of porous-flow channels ... 

Another interesting and widely studied case is the formation of porous metal oxides by anodization of metals. Here, the electrolytic procedure yields a thin layer of porous materials applicable in catalysis, in anticorrosion, batteries, and other applications. Such materials will be discussed in Chapter 6. 

Leached surface layers may form at localized sites of dissolution because of differences in the rate of detachment of silica and the other components of the mineral. If the formation of leached layers is highly localized, as hypothesized by Chou and Wollast (1985b), their existence cannot be verified by current spectroscopic techniques. Samples weathered at pH values < 4 and > 9 do show the existence of uniform, thicker (up to a 100 nm or more) altered surface layers, but these are apparently so highly hydrated and porous that they do not present a diffusional barrier to continued dissolution of the sample (Casey et al., 1989b). 

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